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Final Draft Manual SSTreatment PartB
Srinivas Rao
enregistrerEnregistrer Final Draft Manual SSTreatment PartB pour plus tard
Manila Water - Water for the Poor
ASPE PSD - Cleanouts
Kee Mbbr Brochure
MS 1228 1991 -
C-3-2008
Sewagetreatmentplant Designcalculation 130528150104 Phpapp01
CIVE434-FALL20122013-Sheets 5to9.pdf
Process Design Consideration
B220HV-F
Activated Sludge
Lecture 1 sanitary
Lect 1 Appen v Micro
USD Utilities MinimumDesignConstructionStandards 05 2015
06_Sec._4_Sewer_Testing.pdf
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MANUAL ON
SEWERAGE AND SEWAGE TREATMENT
PART B : OPERATION AND MAINTENANCE
CENTRAL PUBLIC HEALTH AND
ENVIRONMENTAL ENGINEERING ORGANIZATION
JAPAN INTERNATIONAL COOPERATION AGENCY
Part B: Operation and Maintenance
Final Draft i
CHAPTER 1 INTRODUCTION...................................................................................................... 1-1
1.1 Need for O&M............................................................................................................ 1-2
1.2 Basic Considerations of O&M ................................................................................... 1-3
1.2.1 Laws and Regulations related to O&M of Sewerage System................................ 1-3
1.2.2 Effluent Standards related to Treatment Plants ..................................................... 1-3
1.2.3 Environmental Considerations .............................................................................. 1-3
1.2.4 Budget ................................................................................................................... 1-3
1.2.5 Preventive Maintenance ........................................................................................ 1-3
1.2.6 Workmanship and Quality of Equipment .............................................................. 1-4
1.3 Outlines of O&M........................................................................................................ 1-4
1.3.1 Overview and Contents of O&M .......................................................................... 1-4
1.3.2 Management of Facilities ...................................................................................... 1-4
1.3.3 Schedule of O&M.................................................................................................. 1-4
1.3.4 Response to Accidents........................................................................................... 1-5
1.3.5 Management of Buildings and Sites...................................................................... 1-5
1.4 Organization of O&M................................................................................................. 1-5
1.4.1 Description of O&M Work.................................................................................... 1-5
1.4.2 Deployment of Manpower..................................................................................... 1-6
1.4.3 Out sourcing of O&M ........................................................................................... 1-6
1.4.4 Training ................................................................................................................. 1-6
1.4.5 Monitoting through Information Control Technology........................................... 1-6
1.4.6 Database for Effective O&M................................................................................. 1-6
1.4.7 Paragraph on Problems in Existing O&M............................................................. 1-6
1.5 Community Awareness and Participation................................................................... 1-7
1.5.1 Public Relations and Public Opinion related to Sewerage Works ......................... 1-7
1.5.2 Complaint and Redressal....................................................................................... 1-7
1.5.3 Dos and Donts for Community ........................................................................... 1-7
1.6 Potential Risk with respect to Sewerage System........................................................ 1-7
1.6.1 Provision of Disaster Prevention Systems............................................................. 1-7
1.7 Sewerage Ledger ........................................................................................................ 1-8
Final Draft ii
1.7.1 Preparation of Sewerage Ledger ........................................................................... 1-8
1.7.2 Management and Use of Sewerage Ledger ........................................................... 1-8
1.8 Budget Estimation for O&M...................................................................................... 1-8
1.9 Summary..................................................................................................................... 1-8
1.10 Relationship between Part-A (Engineering), Part-B (Operation and
Maintenance), and Part-C (Management) of Manual ................................................. 1-8
CHAPTER 2 SEWER SYSTEMS.................................................................................................... 2-1
2.1 Introduction ................................................................................................................ 2-1
2.1.1 Objectives of Maintenance.................................................................................... 2-1
2.1.2 Type of Maintenance ............................................................................................. 2-1
2.1.3 Necessity of Maintenance...................................................................................... 2-1
2.2 Inspection and Examination for Sewer....................................................................... 2-2
2.2.1 Importance of Inspections and Examinations........................................................ 2-2
2.2.2 Guidelines for Inspections and Examinations ....................................................... 2-2
2.2.3 Preliminary Inspection........................................................................................... 2-3
2.2.4 Type of Inspections and Examinations.................................................................. 2-4
2.2.4.1 Direct Inspection and Examination............................................................. 2-4
2.2.4.2 Indirect Inspection and Examination .......................................................... 2-4
2.2.5 Sewer Inspection and Examination ....................................................................... 2-7
2.2.5.1 Visual Examination..................................................................................... 2-7
2.2.5.2 Inspecting Infiltration of Water ................................................................. 2-13
2.2.5.3 Inspecting Corrosion and Deterioration.................................................... 2-18
2.2.5.4 Other Examinations................................................................................... 2-19
2.2.5.5 Precautions................................................................................................ 2-20
2.2.6 Judgement of Inspection and Examination Results............................................. 2-21
2.2.6.1 Emergency Response Criteria................................................................... 2-21
2.2.6.2 Judgement based on the Results of Inspection and Examination.............. 2-21
2.2.6.3 Testing Criteria.......................................................................................... 2-23
2.2.7 Maintenance of Records and Follow up Action .................................................. 2-24
2.3 Sewer Cleaning......................................................................................................... 2-28
2.3.1 Cleaning Equipment and Procedures................................................................... 2-28
2.3.1.1 Manila Rope and Cloth Ball...................................................................... 2-28
2.3.1.2 Sectional Sewer Rods................................................................................ 2-28
2.3.1.3 Sewer Cleaning Bucket Machine.............................................................. 2-28
2.3.1.4 Dredger (Clam-shell) ................................................................................ 2-29
2.3.1.5 Rodding Machine with Flexible Sewer Rods............................................ 2-29
2.3.1.6 Scraper ...................................................................................................... 2-30
2.3.1.7 Hydraulically Propelled Devices............................................................... 2-31
Final Draft iii
2.3.1.8 Velocity Cleaners (Jetting Machines)........................................................ 2-33
2.3.1.9 Suction Units (Gully Emptier) .................................................................. 2-33
2.3.2 Notification to STP.............................................................................................. 2-33
2.3.3 Disposal of Silt and Sludge ................................................................................. 2-33
2.3.4 Cleaning Records and their Utilization ............................................................... 2-33
2.4 Sewer Rehabilitation................................................................................................. 2-34
2.4.1 Introduction ......................................................................................................... 2-34
2.4.2 Rehabilitation Method......................................................................................... 2-35
2.4.2.1 Pipe Bursting or In-line Expansion........................................................... 2-36
2.4.2.2 Slip Lining................................................................................................. 2-36
2.4.2.3 Cured-in-place Pipe................................................................................... 2-37
2.4.2.4 Modified Cross-section Lining ................................................................. 2-38
2.4.3 Maintenance of Machinery and Apparatus for Rehabilitation............................. 2-38
2.5 Protection of Sewer Systems .................................................................................... 2-38
2.6 Protection against Infiltration and Exfiltration......................................................... 2-39
2.6.1 Measures against Infiltration of Rainwater ......................................................... 2-39
2.6.2 Measures against Exfiltration of Untreated Sewage............................................ 2-40
2.6.2.1 External Sewer Rehabilitation Methods.................................................... 2-40
2.6.2.2 Internal Sewer Rehabilitation Methods..................................................... 2-41
2.7 Manholes and Appurtenances................................................................................... 2-41
2.7.1 Inspections and Examinations ............................................................................. 2-41
2.7.1.1 Manhole .................................................................................................... 2-41
2.7.1.2 Conditions Inside Manhole ....................................................................... 2-42
2.7.2 Judgement of Examination Results ..................................................................... 2-43
2.7.3 Cleaning............................................................................................................... 2-44
2.7.4 Rehabilitation ...................................................................................................... 2-44
2.8 Cross Drainage Works .............................................................................................. 2-44
2.8.1 Inspection and Examination ................................................................................ 2-44
2.8.2 Criteria for Judging Examination Results ........................................................... 2-45
2.9 Pressure Sewer.......................................................................................................... 2-46
2.9.1 Pressurized Sewers .............................................................................................. 2-46
2.9.2 Vacuum Sewer System........................................................................................ 2-46
2.9.3 O&M System....................................................................................................... 2-46
2.10 House Service Connection........................................................................................ 2-47
2.10.1 Inspection and Examination ................................................................................ 2-47
2.10.2 Criteria for Judging Examination Results ........................................................... 2-49
2.10.3 Cleaning............................................................................................................... 2-49
Final Draft iv
2.10.4 Rehabilitation ...................................................................................................... 2-49
2.11 Safety Practices......................................................................................................... 2-49
2.11.1 Accidents related to Sewer Facilities................................................................... 2-50
2.11.1.1 Need for Traffic Control............................................................................ 2-50
2.11.1.2 Safety Measure to be taken before any manhole entry ............................. 2-50
2.11.2 Measures against Accidents................................................................................. 2-52
2.11.3 Information to Prevent Accidents and Records ................................................... 2-52
2.12 Troubleshooting........................................................................................................ 2-52
2.13 Summary................................................................................................................... 2-52
CHAPTER 3 PUMPING STATION................................................................................................. 3-1
3.2 Types and Structure of Pumping Stations................................................................... 3-1
3.2.1 Dry Pit ................................................................................................................... 3-1
3.2.2 Suction Sump or Wet Well..................................................................................... 3-2
3.2.3 Lift Stations ........................................................................................................... 3-2
3.2.4 Operation and Maintenance................................................................................... 3-2
3.2.4.1 Operation of the Pumps............................................................................... 3-2
3.2.4.2 Undesirable Operations............................................................................... 3-3
3.2.4.3 Piping and Appurtenance Maintenance....................................................... 3-5
3.3 Gates, Valves and Actuators ....................................................................................... 3-5
3.3.1 Sluice Gate ............................................................................................................ 3-5
3.3.2 Valve...................................................................................................................... 3-7
3.3.2.1 Gate Valve................................................................................................... 3-7
3.3.2.2 Non-Return Valve (Check Valve)................................................................ 3-8
3.3.2.3 Non-Return Valve (Ball Type) .................................................................... 3-9
3.3.2.4 Butterfly Valve .......................................................................................... 3-10
3.3.3 Actuators ............................................................................................................. 3-10
3.4 Screen ....................................................................................................................... 3-12
3.4.1 Types of Screens.................................................................................................. 3-12
3.4.1.1 Coarse Screens .......................................................................................... 3-12
3.4.1.2 Medium Bar Screens................................................................................. 3-12
3.4.1.3 Fine Screens .............................................................................................. 3-12
3.4.2 Screenings Removal Method............................................................................... 3-12
3.4.2.1 Manual Bar Screen.................................................................................... 3-12
3.4.2.2 Mechanical Screen (Intermittent and Continuous).................................... 3-14
3.4.3 Accessories (Conveyors) ..................................................................................... 3-17
3.4.4 Disposal of Screenings ........................................................................................ 3-17
3.5 Grit Removal ............................................................................................................ 3-17
Final Draft v
3.5.1 Preventive Maintenance ...................................................................................... 3-17
3.5.2 Regular Day to Day Maintenance ....................................................................... 3-17
3.5.3 Disposal of Grit ................................................................................................... 3-18
3.6 Pump Equipment ...................................................................................................... 3-18
3.6.2 Regular Day-to-Day Maintenance....................................................................... 3-19
3.6.3 Operation and Maintenance................................................................................. 3-19
3.6.4 Accessories.......................................................................................................... 3-20
3.6.4.1 Oil and Grease........................................................................................... 3-20
3.6.4.2 Bearing...................................................................................................... 3-20
3.6.4.3 Packing Gland........................................................................................... 3-20
3.6.4.4 Mechanical Seal ........................................................................................ 3-21
3.7 Flow Measuring Devices.......................................................................................... 3-22
3.7.1 Weir Flow-Meter ................................................................................................. 3-23
3.7.2 Electromagnetic Flow-Meter............................................................................... 3-24
3.7.3 Ultrasonic Flow-Meter ........................................................................................ 3-24
3.7.4 Fluorescent Tracers.............................................................................................. 3-25
3.8 Preventive Maintenance ........................................................................................... 3-25
3.9 Troubleshooting........................................................................................................ 3-28
3.10 Record Keeping........................................................................................................ 3-28
3.11 Duties of Site Engineer in Charge and Higher Ups .................................................. 3-31
3.12 If the STP is under O&M by the Contractor............................................................. 3-31
CHAPTER 4 SEWAGE TREATMENT FACILITIES ..................................................................... 4-1
4.2 Pump Equipment ........................................................................................................ 4-1
4.3 Fine Screen and Grit Chamber ................................................................................... 4-1
4.4 Oil and Grease Removal............................................................................................. 4-1
4.4.1 Manual Process...................................................................................................... 4-1
4.4.2 Floatation Process.................................................................................................. 4-1
4.5 Equalization................................................................................................................ 4-3
4.5.1 Operation............................................................................................................... 4-4
4.5.2 Maintenance .......................................................................................................... 4-4
4.6 Primary Treatment ...................................................................................................... 4-5
4.6.1 Primary Sedimentation Tank Management ........................................................... 4-5
4.6.1.1 Preventive Maintenance.............................................................................. 4-5
4.6.1.2 Day to Day Maintenance............................................................................. 4-5
4.6.1.3 Troubleshooting .......................................................................................... 4-5
Final Draft vi
4.7 Activated Sludge Process (ASP) ................................................................................ 4-5
4.7.1 Description of Activated Sludge Process (ASP).................................................... 4-5
4.7.1.1 Biological Treatment Processes .................................................................. 4-5
4.7.1.2 Design and Operational Parameters ............................................................ 4-6
4.7.1.3 Choice between SRT and F/M as Operation Control Parameter................. 4-6
4.7.1.4 Effect of c on Settling Characteristics and Drainability of Sludge............ 4-6
4.7.1.5 Effect of c on Excess of Sludge Production .............................................. 4-7
4.7.1.6 Excess Sludge Wasting................................................................................ 4-7
4.7.1.7 Return Sludge Flow..................................................................................... 4-7
4.7.2 Conventional Activated Sludge Process................................................................ 4-8
4.7.2.1 Start Up ....................................................................................................... 4-8
4.7.2.2 Routine Operation and Maintenance........................................................... 4-9
4.7.2.3 Aeration Equipment ...................................................................................4-11
4.7.3 Extended Aeration Process.................................................................................. 4-16
4.7.3.1 Operation of Aeration Equipment ............................................................. 4-17
4.7.3.2 Operation and Maintenance ...................................................................... 4-17
4.7.3.3 Normal Operation ..................................................................................... 4-18
4.7.3.4 Abnormal Operation.................................................................................. 4-18
4.7.3.5 Countermeasures....................................................................................... 4-18
4.7.3.6 Maintenance.............................................................................................. 4-20
4.7.4 Sequencing Batch Reactor (SBR)........................................................................ 4-20
4.7.4.1 Process Control ......................................................................................... 4-21
4.7.4.2 Records...................................................................................................... 4-23
4.7.4.3 Housekeeping............................................................................................ 4-23
4.7.5 Oxidation Ditch ................................................................................................... 4-24
4.7.5.1 Operation................................................................................................... 4-25
4.7.5.2 Equipment Maintenance............................................................................ 4-27
4.7.6 Chemical Clarification......................................................................................... 4-27
4.7.6.1 Metals Removal ........................................................................................ 4-28
4.7.6.2 Phosphorus Removal................................................................................. 4-28
4.7.6.3 Suspended Solids ...................................................................................... 4-28
4.7.6.4 Additional Considerations......................................................................... 4-29
4.7.6.5 Jar Testing ................................................................................................. 4-29
4.8 Aerated Lagoon ........................................................................................................ 4-31
4.8.1 Process Control.................................................................................................... 4-31
4.8.2 Records................................................................................................................ 4-32
4.8.3 Housekeeping...................................................................................................... 4-32
4.9 Attached Growth Systems ........................................................................................ 4-33
Final Draft vii
4.9.1 Operation............................................................................................................. 4-34
4.9.2 Maintenance ........................................................................................................ 4-34
4.10 Moving Bed Bio Reactor (MBBR)........................................................................... 4-36
4.10.1 Configuration....................................................................................................... 4-36
4.10.2 Operation and Maintenance................................................................................. 4-36
4.11 Membrane Bio Reactor (MBR) ................................................................................ 4-37
4.11.1 Operation............................................................................................................. 4-38
4.11.2 Maintenance ........................................................................................................ 4-38
4.12 Up FlowAnaerobic Sludge Blanket Reactor (UASB) ............................................. 4-38
4.12.1 Plant Commissioning and Operation................................................................... 4-39
4.12.2 Daily Operation and Maintenance of UASB....................................................... 4-39
4.12.3 Routine Maintenance........................................................................................... 4-43
4.12.3.1 Quarterly Maintenance.............................................................................. 4-43
4.12.3.2 Annual Maintenance ................................................................................. 4-43
4.12.3.3 Five-Yearly Maintenance .......................................................................... 4-43
4.12.4 Decision Schemes for Sludge Removal............................................................... 4-44
4.12.5 Shut-Down and Standstill.................................................................................... 4-44
4.12.6 Operational Cautions........................................................................................... 4-44
4.12.7 Final Polishing Unit (FPU).................................................................................. 4-44
4.12.8 Duckweed Pond................................................................................................... 4-45
4.13 Waste Stabilization Pond (WSP) .............................................................................. 4-45
4.13.1 Start-up Procedures ............................................................................................. 4-45
4.13.3 De-Sludging......................................................................................................... 4-46
4.13.3.1 When to De-Sludge................................................................................... 4-46
4.13.3.2 The white towel test .................................................................................. 4-47
4.13.3.3 De-Sludge Procedure ................................................................................ 4-47
4.13.3.4 Special cautions for anaerobic pond / maturation pond ............................ 4-49
4.13.4 Process Control.................................................................................................... 4-49
4.13.5 Record Keeping................................................................................................... 4-49
4.13.5.1 Records necessary for Anaerobic Pond..................................................... 4-49
4.13.5.2 Records necessary for Facultative Pond ................................................... 4-49
4.13.5.3 Records necessary for Maturation Pond.................................................... 4-49
4.14 Farm Forestry ........................................................................................................... 4-49
4.15 Fish Pond.................................................................................................................. 4-49
4.16 Secondary Sedimentation Tank ................................................................................ 4-49
Final Draft viii
4.17 Advanced Treatment................................................................................................. 4-50
4.17.1 Sand Filtration ..................................................................................................... 4-51
4.17.2 Multimedia Filtration........................................................................................... 4-51
4.17.3 Membrane Filtration (MF, UF, NF, RO).............................................................. 4-51
4.17.5 Integrated Nutrient Removal ............................................................................... 4-54
4.17.5.1 Nutrient Removal ...................................................................................... 4-54
4.17.5.2 Nitrogen Removal ..................................................................................... 4-55
4.17.5.3 Phosphorus Removal................................................................................. 4-56
4.18 Disinfection Facility ................................................................................................. 4-57
4.18.1 Operational Variables .......................................................................................... 4-58
4.18.2 Operational Hazards ............................................................................................ 4-59
4.19 Operation & Maintenance of DEWATS and Johkasou............................................. 4-60
4.20 Preventive Maintenance ........................................................................................... 4-60
CHAPTER 5 SLUDGE TREATMENT FACILITIES...................................................................... 5-1
5.2 Sludge Thickening...................................................................................................... 5-1
5.2.1 Gravity Thickening................................................................................................ 5-1
5.2.1.1 Process Control ........................................................................................... 5-2
5.2.1.2 Maintenance................................................................................................ 5-3
5.2.2 Centrifugal Thickening.......................................................................................... 5-3
5.2.2.1 Configuration .............................................................................................. 5-3
5.2.2.2 Operation and Maintenance ........................................................................ 5-3
5.2.3 Air Floatation Thickening ..................................................................................... 5-4
5.2.3.2 Operation..................................................................................................... 5-5
5.2.4 Belt Type Thickening ............................................................................................ 5-6
5.3 Anaerobic Digestion................................................................................................... 5-9
5.3.1 Digestion Types ..................................................................................................... 5-9
5.3.1.1 Low Rate Digestion..................................................................................... 5-9
5.3.1.2 High Rate Digestion.................................................................................. 5-10
Final Draft ix
5.3.2 Configuration....................................................................................................... 5-10
5.3.2.1 Anaerobic Digestion Tank......................................................................... 5-10
5.3.2.2 Agitator ......................................................................................................5-11
5.3.2.3 Digester Gas Equipment ............................................................................5-11
5.3.2.4 Gas Scrubbers ........................................................................................... 5-14
5.3.2.5 Gas Power Generator ................................................................................ 5-15
5.3.3.1 Feeding Schedule ...................................................................................... 5-15
5.3.3.2 Withdrawal Schedule ................................................................................ 5-16
5.3.3.3 Scum Control ............................................................................................ 5-16
5.3.3.4 Precipitate Formation and Control ............................................................ 5-16
5.3.3.5 Digester Upsets and Control Strategies..................................................... 5-16
5.3.3.6 Temperature............................................................................................... 5-17
5.3.3.7 Toxicity Control ........................................................................................ 5-17
5.3.3.8 pH Control................................................................................................. 5-17
5.3.3.9 Digester Foaming...................................................................................... 5-17
5.4 Sludge Dewatering ................................................................................................... 5-18
5.4.1 Chemical Dosing Equipment............................................................................... 5-18
5.4.1.1 Coagulant .................................................................................................. 5-18
5.4.1.2 Equipment ................................................................................................. 5-20
5.4.2 Sludge Feed Pump............................................................................................... 5-21
5.4.3 Mechanical Dewatering....................................................................................... 5-22
5.4.3.1 Centrifugal Dewatering............................................................................. 5-22
5.4.3.2 Belt Filter Press Dewatering Equipment ................................................... 5-24
5.4.3.3 Filter Press................................................................................................. 5-26
5.4.3.4 Screw Press ............................................................................................... 5-28
5.4.3.5 Rotary Press .............................................................................................. 5-30
5.4.3.6 Vacuum Filter............................................................................................ 5-31
5.5 Sludge Drying Bed ................................................................................................... 5-32
5.5.1 Applicability........................................................................................................ 5-32
5.5.2 Unit Sizing........................................................................................................... 5-32
5.5.3 Area of Beds........................................................................................................ 5-32
5.5.4 Percolation Type Bed Components ..................................................................... 5-33
Final Draft x
5.8 Record Keeping........................................................................................................ 5-34
5.9 Summary................................................................................................................... 5-34
CHAPTER 6 Electrical and Instrumentation Faciities ..................................................................... 6-1
6.2 Power Supply System................................................................................................. 6-3
6.2.1 Power Receiving and Transforming Equipment.................................................... 6-3
6.2.1.1 High Tension (HT) Panel ............................................................................ 6-3
6.2.1.2 Transformer................................................................................................. 6-5
6.2.1.3 Low Tension (LT) Panel.............................................................................. 6-5
6.2.1.4 Bus-bar ........................................................................................................ 6-6
6.2.2 Power Control........................................................................................................ 6-6
6.2.2.1 Power Factor Correction ............................................................................. 6-6
6.2.2.2 Capacitor Panel ........................................................................................... 6-8
6.2.2.3 Power Factor Correction at Motor Panel..................................................... 6-9
6.2.3 Supply and Interruption (Operation of Electrical Equipment) .............................. 6-9
6.2.4 Gas Engines......................................................................................................... 6-10
6.2.5 Dual Fuel Engine................................................................................................. 6-10
6.3 Standby Power Supply System (Generator) ..............................................................6-11
6.3.1 AC Generator ...................................................................................................... 6-11
6.3.2 Diesel Engine ...................................................................................................... 6-12
6.3.3 UPS...................................................................................................................... 6-12
6.4 Prime Movers ........................................................................................................... 6-13
6.4.1 Induction Motor................................................................................................... 6-13
6.4.2 Starters................................................................................................................. 6-14
6.4.3 Characteristics of Induction Motor...................................................................... 6-15
6.4.4 Performance Assessment of Motors .................................................................... 6-16
6.4.4.1 Efficiency of Motors ................................................................................. 6-16
6.4.4.2 Motor Load ............................................................................................... 6-17
6.4.4.3 Energy Efficiency Opportunities............................................................... 6-17
6.4.5 Condition Monitoring Techniques....................................................................... 6-17
6.4.5.1 Vibration Monitoring ................................................................................ 6-17
6.4.5.2 Vibration Analysis..................................................................................... 6-18
6.4.5.3 Thermographic Analysis ........................................................................... 6-19
6.4.6 Speed Control Equipment.................................................................................... 6-20
6.4.6.1 Variable Frequency Drive (VFD).............................................................. 6-21
6.4.6.2 Advantages and Disadvantages of VFD.................................................... 6-21
Final Draft xi
6.4.7 Motor Protection Equipment ............................................................................... 6-22
6.5 Instrumentation Facilities ......................................................................................... 6-22
6.5.1 Flow Measuring Equipment ................................................................................ 6-22
6.5.2 Level Measuring Equipment ............................................................................... 6-22
6.5.2.1 Float .......................................................................................................... 6-22
6.5.2.2 Ultrasonic.................................................................................................. 6-23
6.5.2.3 Head-Pressure ........................................................................................... 6-24
6.5.3 pH and ORP Measuring Equipment .................................................................... 6-26
6.5.3.1 pH.............................................................................................................. 6-26
6.5.3.2 ORP (Oxidation-Reduction Potential) ..................................................... 6-27
6.5.3.3 DO (Dissolved Oxygen)............................................................................ 6-27
6.5.3.5 MLSS (Mixed Liquor Suspended Solid)................................................... 6-29
6.6 SCADASystem........................................................................................................ 6-29
6.6.1 Monitoring and Control Equipment .................................................................... 6-30
6.6.1.1 Signal ........................................................................................................ 6-30
6.6.1.2 HMI (Human Machine Interface) ............................................................. 6-31
6.6.1.3 PLC (Programmable Logic Controller)..................................................... 6-32
6.6.2 Automatic Control ............................................................................................... 6-32
6.7 Cables ....................................................................................................................... 6-32
6.8 Energy Audit............................................................................................................. 6-33
6.8.1 Frequency of Energy Audit.................................................................................. 6-34
6.8.2 Scope of Energy Audit......................................................................................... 6-34
6.9 Management of Records........................................................................................... 6-35
6.9.1 Record of Operation and Maintenance................................................................ 6-35
6.9.2 Record of Operation and Maintenance and its Utilization .................................. 6-35
6.10.1 Types of Planned Maintenance............................................................................ 6-36
6.10.2 Inspection Tools................................................................................................... 6-37
6.10.2.1 Multimeter................................................................................................. 6-37
6.10.2.2 Clamp-on Meter ........................................................................................ 6-38
6.10.2.3 Megger / Megohmmeter............................................................................ 6-38
6.10.2.4 Ground Resistance Testers (Earth Meggers) ............................................. 6-39
6.10.3 General Precautions for Electrical Maintenance ................................................. 6-39
6.10.5 Planned Reconstruction....................................................................................... 6-40
Final Draft xii
CHAPTER 7 Monitoring of Water Quality ...................................................................................... 7-1
7.2 Need for Sampling and Analysis ................................................................................ 7-1
7.3 Sampling..................................................................................................................... 7-1
7.3.1 Overview............................................................................................................... 7-1
7.3.2 Grab Sample.......................................................................................................... 7-1
7.3.3 Composite Sample................................................................................................. 7-1
7.3.4 Sampling Method and Precautions in Sampling.................................................... 7-2
7.3.5 Sample Volumes, Quantity and Storage of Samples.............................................. 7-3
7.3.6 Selection of Sampling Location ............................................................................ 7-3
7.4 Relevance of Parameters ............................................................................................ 7-4
7.5 Analysis Parameters and Frequency (Liquid and Sludge).......................................... 7-4
7.5.1 Items and Frequency for ASP................................................................................ 7-4
7.5.2 Items and Frequency for WSP............................................................................... 7-6
7.6 Microbiology Sampling and Testing of Treated Sewage............................................ 7-7
7.6.1 Need for Microbiological Testing.......................................................................... 7-7
7.6.2 Testing Method...................................................................................................... 7-8
7.6.2.1 Multiple Tube Fermentation Method .......................................................... 7-8
7.6.2.2 Membrane Filter Test ................................................................................ 7-10
7.6.2.3 Colilert Test ................................................................................................7-11
7.6.2.4 Recommended Testing for Treated Sewage .............................................. 7-12
7.7 Quick and Approximate Measurement Methods ...................................................... 7-13
7.7.1 Test Paper Method............................................................................................... 7-13
7.7.2 Detector Tube Method (Transparency Tube, BOD Tube (UK)).......................... 7-13
7.7.3 Cylinder Test ....................................................................................................... 7-13
7.8 Data Analysis (Accuracy and Precision) .................................................................. 7-13
7.8.1 Processing Water Quality Test Data .................................................................... 7-14
7.8.2 Accuracy of Measured Values ............................................................................. 7-14
7.9 Frequency Management ........................................................................................... 7-15
7.10 Planning of laboratory facilities ............................................................................... 7-15
7.11 Upkeep of Plant Laboratory ..................................................................................... 7-15
7.12 Disposal of Laboratory Wastes................................................................................. 7-15
7.12.1 Solid Waste.......................................................................................................... 7-15
7.12.2 Liquid Wastes ...................................................................................................... 7-15
7.12.3 Radioactive Wastes.............................................................................................. 7-16
7.13 Personnel .................................................................................................................. 7-16
CHAPTER 8 ENVIRONMENTAL CONSERVATION................................................................... 8-1
Final Draft xiii
8.2 Odour.......................................................................................................................... 8-1
8.2.1 Odour from the Sewerage System......................................................................... 8-1
8.2.2 Odour Control Methods and Technologies............................................................ 8-1
8.2.2.1 Odour Control Procedure............................................................................ 8-1
8.2.2.2 Sampling Methods ...................................................................................... 8-1
8.2.2.3 Quantitative Testing-Analytical Methods ................................................... 8-2
8.2.3 Hydrogen Sulphide (H
S)...................................................................................... 8-3
8.2.3.1 Effects on Health......................................................................................... 8-3
8.2.3.2 Locations of Sources................................................................................... 8-3
8.2.3.3 Measurement ............................................................................................... 8-3
8.2.3.4 Preventive Measures ................................................................................... 8-3
8.2.3.5 Control ........................................................................................................ 8-4
8.2.4 Ammonia (NH
) .................................................................................................... 8-6
8.2.5 General Method of Prevention of Odour............................................................... 8-7
8.2.6 Chemical Addition................................................................................................. 8-8
8.2.7 Monitoring........................................................................................................... 8-10
8.3 Epidemiological Pollution.........................................................................................8-11
8.3.1 Effects on Health................................................................................................. 8-11
8.3.2 Locations of Sources ........................................................................................... 8-11
8.3.3 Measurement ....................................................................................................... 8-11
8.3.4 Preventive Measures............................................................................................ 8-12
8.3.5 Control................................................................................................................. 8-12
8.4 Soil Contamination................................................................................................... 8-12
8.5 Water Pollution......................................................................................................... 8-12
8.6 Sewage Treatment Plant Beautification and Landscaping........................................ 8-13
8.7 Regulation of Greenhouse Gas ................................................................................. 8-13
8.7.1 Greenhouse Gas................................................................................................... 8-13
8.7.3 Effective Use of Biogas....................................................................................... 8-14
8.8 Carbon Credit Record............................................................................................... 8-14
CHAPTER 9 OCCUPATIONAL HEALTH HAZARDS AND SAFETY MEASURES.................. 9-1
Final Draft xiv
9.1.1 Diseases................................................................................................................. 9-1
9.1.2 Accidents............................................................................................................... 9-2
9.1.2.1 Confined Space Hazards ............................................................................. 9-3
9.1.2.2 Risk of Chlorine.......................................................................................... 9-5
9.1.2.3 Fall .............................................................................................................. 9-5
9.1.2.4 Slip .............................................................................................................. 9-5
9.1.2.5 Electrical Shock .......................................................................................... 9-6
9.1.2.6 Fire .............................................................................................................. 9-6
9.1.2.7 Risks in a Wastewater Quality Test Laboratory .......................................... 9-6
9.1.3 Instances of Accidents ........................................................................................... 9-7
9.2 Safety Aspects and Measures...................................................................................... 9-8
9.2.1 Preventive Measures.............................................................................................. 9-8
9.2.1.1 Hazard-specific Preventive Measures ......................................................... 9-8
9.2.1.2 Personal Protection and Protective Devices.............................................. 9-12
9.2.1.3 Workplace-specific Preventive Measures ................................................. 9-16
9.2.2 Corrective Measures............................................................................................ 9-23
9.2.2.1 Emergency Contact ................................................................................... 9-23
9.2.2.2 Emergency Measures ................................................................................ 9-23
9.2.2.3 Searching out Hazards............................................................................... 9-26
9.3 Health Aspects and Measures ................................................................................... 9-26
9.3.1.1 Personal Hygiene against Pathogen .......................................................... 9-26
9.3.1.2 Maintaining Cleanliness............................................................................ 9-26
9.3.1.3 Health Check............................................................................................. 9-26
9.4 Safety Personnel (Organisation) ............................................................................... 9-27
9.4.1 Institutional Arrangement .................................................................................... 9-27
9.4.2 Human Resources................................................................................................ 9-28
9.5 Awareness and Training............................................................................................ 9-29
9.5.1 Manager............................................................................................................... 9-29
9.5.2 Technical Staff..................................................................................................... 9-29
9.5.2.1 Mechanical ................................................................................................ 9-29
9.5.2.2 Electrical ................................................................................................... 9-30
9.5.3 Skilled Staff ......................................................................................................... 9-30
9.5.3.2 Electrical Staff........................................................................................... 9-30
9.5.4 Unskilled Staff..................................................................................................... 9-31
Final Draft xv
9.5.5 Training Assessment............................................................................................ 9-31
9.6 Emergencies.............................................................................................................. 9-31
9.6.1 What is an Emergency? ....................................................................................... 9-31
9.6.2 How to Think during Emergencies?.................................................................... 9-31
9.6.2.1 Situation 1 ................................................................................................. 9-32
9.7 The Need to resist ..................................................................................................... 9-33
CHAPTER 10 ON-SITE SYSTEMS................................................................................................ 10-1
10.1 Introduction .............................................................................................................. 10-1
10.2 On-site Facility Maintenance Systems ..................................................................... 10-1
10.3 Maintaining On-site Facilities .................................................................................. 10-2
10.3.1 Inspecting and Maintaining the Treatment Unit .................................................. 10-2
10.3.2 Cleaning............................................................................................................... 10-2
10.3.3 Water Quality Control ......................................................................................... 10-2
10.3.4 Hygienic Measures including Infection Prevention ............................................ 10-3
10.3.5 Measures against a Disaster or Accident ............................................................. 10-3
10.4 Latrine/ Toilet ........................................................................................................... 10-3
10.4.1 Pour Flush Water Seal Latrine............................................................................. 10-3
10.4.2 Public Toilet......................................................................................................... 10-4
10.4.3 Mobile Toilet ....................................................................................................... 10-5
10.5 On-site Methods ....................................................................................................... 10-5
10.5.1 Conventional Septic Tank / Improved Septic Tank ............................................. 10-5
10.5.1.1 Purifying Wastewater and Accumulating Sludge...................................... 10-5
10.5.1.2 Mechanical Cleaning of Septic Tan........................................................... 10-6
10.5.1.3 Septage Management ................................................................................ 10-6
10.5.2 Advanced Type Treatment Units ......................................................................... 10-6
10.5.2.1 Pre-treatment Process................................................................................ 10-6
10.5.2.2 Main Treatment Process............................................................................ 10-8
10.5.2.3 Advanced Treatment Process .................................................................. 10-10
10.5.2.4 Controlling the Disinfection Unit.............................................................10-11
10.6 Septage Treatment Unit ...........................................................................................10-11
10.6.1 Collecting and Delivering Sludge ..................................................................... 10-11
10.6.2 Basics of Sludge Treatment ............................................................................... 10-12
10.6.3 Operation Control of Sludge Treatment ............................................................ 10-12
Final Draft xvi
10.6.3.1 General .................................................................................................... 10-12
10.6.3.2 Controlling the Pre-treatment Unit.......................................................... 10-13
10.7 Summary................................................................................................................. 10-14
Final Draft xvii
Appendix 1.1 Monitoring Through Information Control Technology ................................................. 1
Appendix 1.2 Database for Effective O&M......................................................................................... 3
Appendix 1.3 Conclusions and Recommendations.............................................................................. 5
Appendix 2.1 Troubleshooting in Sewers ............................................................................................ 9
Appendix 2.2 T.V. Inspection Report ..................................................................................................11
Appendix 3.1 Detailed Troubleshooting for Horizontal Centrifugal Pump Sets................................ 12
Appendix 3.2 Possible Causes and Corrective Actions to Check for Pumps..................................... 16
Appendix 3.3 Troubleshooting in Sewage Pump Stations ................................................................. 20
Appendix 4.1 Troubleshooting in STPs ............................................................................................. 22
Appendix 4.2 Operational Parameter ................................................................................................. 50
Appendix 4.3 Calculation................................................................................................................... 55
Appendix 5.1 Troubleshooting in Sludge Treatment Facilities.......................................................... 60
Appendix 6.1 Typical Ledger and Records ........................................................................................ 65
Appendix 6.2 Preventive Maintenance .............................................................................................. 69
Appendix 6.3 Troubleshooting for Electrical Facilities ..................................................................... 71
Appendix 7.1 Minimum Laboratory Equipments Needed for Tests .................................................. 80
Appendix 7.2 Suggested Laboratory Service Infrastructure for Monitoring Water Quality .............. 81
Appendix 9.1 Health and Safety Policy ............................................................................................. 82
Appendix 9.2 Characteristics of Common Gases Causing Hazards................................................... 88
Appendix 9.3 Confined Space Entry Procedure................................................................................. 91
Appendix 9.4 Confined Space Pre-entry Checklist............................................................................ 93
Appendix 9.5 First Aid....................................................................................................................... 95
Appendix 9.6 Sewage Treatment Plant Accident Report ................................................................... 98
Appendix 10.1 MECHANICAL CLEANING OF SEPTIC TANKS................................................... 99
Final Draft xviii
Table 2.1 Preliminary inspection during Defect Liability Period (DLP) ............................... 2-3
Table 2.2 Preliminary inspection for Manholes & Sewers..................................................... 2-3
Table 2.3 Preliminary inspection period for other facilities................................................... 2-4
Table 2.4 Classification of inspection .................................................................................... 2-4
Table 2.5 Sewer system inspection technologies considered applicable to Indian conditions2-5
Table 2.6 Gas analysis.......................................................................................................... 2-20
Table 2.7 Testing criteria for overall sewer span.................................................................. 2-21
Table 2.8 Testing criteria for each pipe of sewer.................................................................. 2-22
Table 2.9 Testing criteria for sewer ...................................................................................... 2-24
Table 2.10 Inspection sheet .................................................................................................. 2-24
Table 2.11 Daily report......................................................................................................... 2-26
Table 2.12 Monthly report.................................................................................................... 2-27
Table 2.13 Definition of terms ............................................................................................. 2-35
Table 2.14 Inspection and examination items for manhole.................................................. 2-42
Table 2.15 Inspection record ................................................................................................ 2-43
Table 2.16 Typical inspection items for inverted siphon...................................................... 2-44
Table 2.17 Cleaning method................................................................................................. 2-45
Table 2.18 Pressurised sewer and vacuum sewer................................................................. 2-46
Table 3.1 Tasks to be addressed in day-to-day regular maintenance.................................... 3-19
Table 3.2 Annual inspection report for pumping station...................................................... 3-29
Table 3.3 Recommended maintenance for pumping equipment .......................................... 3-30
Table 4.1 Relations between Sludge Volume Index and settling characteristics of sludge .... 4-8
Table 4.2 Dosing in Jar Test ................................................................................................. 4-30
Table 4.3 Planned maintenance for fixed film synthetic media filters ................................. 4-34
Table 4.4 Typical membrane system pre-filtration requirements......................................... 4-53
Table 4.5 Chemical cleaning agents..................................................................................... 4-54
Table 4.6 Routine operational checklist and troubleshooting guide for chlorination system4-59
Table 4.7 An example of a preventive maintenance program for activated-sludge facilities
checklist................................................................................................................. 4-60
Table 5.1 Gas-flow indication and metering ........................................................................ 5-12
Table 5.2 Gas system safety and control devices ................................................................. 5-12
Table 5.3 Typical Operation Sheet of Sludge Drying Beds.................................................. 5-33
Table 6.1 Type of losses and shares for induction motors.................................................... 6-16
Table 6.2 Criteria for class of machine................................................................................. 6-18
Table 6.3 Vibration severity chart for machine vibration limits........................................... 6-18
Final Draft xix
Table 6.4 Vibration frequencies and likely causes ............................................................... 6-19
Table 6.5 Criteria for differential temperature of electrical equipment................................ 6-20
Table 6.6 Types of cables for different voltages................................................................... 6-32
Table 6.7 Recommended maintenance on electrical equipment........................................... 6-36
Table 7.1 Hourly flow pattern during composite sampling.................................................... 7-2
Table 7.2 Recommended plant control tests on a daily basis in a typical STP ...................... 7-4
Table 7.3 Recommended plant control tests on a weekly basis in a typical STP................... 7-5
Table 7.4 Recommended plant control tests on a monthly to biannual basis in a typical STP7-6
Table 7.5 Recommended plant control tests on a monthly basis in a typical WSP................ 7-6
Table 7.6 Diseases attributable to sewage pollution of drinking water .................................. 7-7
Table 9.1 Types of diseases and their causes.......................................................................... 9-1
Table 9.2 Possible hazards by locations ................................................................................. 9-2
Table 9.3 Relationship between reduction in oxygen concentration and symptoms of anoxia
................................................................................................................................. 9-3
Table 9.4 Relationship between concentration of hydrogen sulphide and its toxic effects.... 9-4
Table 9.5 Effects on the human body when subjected to electric shock ................................ 9-6
Table 9.6 Relationship between shock current and duration.................................................. 9-6
Table 9.7 Toxicity of chemical used in water quality laboratory ........................................... 9-7
Table 9.8 Acceptable entry condition..................................................................................... 9-8
Table 9.9 Record of training assessment .............................................................................. 9-31
Table 10.1 Key pathogens contained in excreta and sludge................................................. 10-3
Final Draft xx
Figure 1.1 Combined tasks and challenges with the O&M.................................................... 1-5
Figure 2.1 Mirror Test and Mirror with rode.......................................................................... 2-5
Figure 2.2 Typical CCTV equipment in action ...................................................................... 2-6
Figure 2.3 Tree roots and sewers............................................................................................ 2-6
Figure 2.4 Photographs showing structural damage and longitudinal cracked condition of the
sewer ....................................................................................................................... 2-7
Figure 2.5 Manhole visual inspection procedure ................................................................... 2-8
Figure 2.6 Pole-mounted inspectioncamera........................................................................... 2-9
Figure 2.7 Illustration of pole-mounted TV camera inspection ............................................. 2-9
Figure 2.8 Step van CCTV system....................................................................................... 2-10
Figure 2.9 Illustration of CCTV camera inspection............................................................. 2-10
Figure 2.10 Work procedure for TV camera inspection........................................................2-11
Figure 2.11 CCTV data No1 .................................................................................................2-11
Figure 2.12 CCTV data No2 ................................................................................................ 2-12
Figure 2.13 Forms of inspection record ............................................................................... 2-13
Figure 2.14 Materials for smoke test.................................................................................... 2-14
Figure 2.15 Illustrative sketches of smoke test .................................................................... 2-14
Figure 2.16 Work procedure of smoke test .......................................................................... 2-15
Figure 2.17 Illustration of echo sound test ........................................................................... 2-15
Figure 2.18 Work procedure of echo sound test................................................................... 2-16
Figure 2.19 Drawing of dye test........................................................................................... 2-17
Figure 2.20 Pumping test ..................................................................................................... 2-17
Figure 2.21 Work procedure of pumping test....................................................................... 2-18
Figure 2.22 Neutralization test ............................................................................................. 2-19
Figure 2.23 Fatal accident related to gas in Japan sewer works........................................... 2-20
Figure 2.24 Illustration of Testing criteria for sewer............................................................ 2-23
Figure 2.25 Power bucket machine setup............................................................................. 2-29
Figure 2.26 Power rodding operation................................................................................... 2-30
Figure 2.27 Rodding heads................................................................................................... 2-30
Figure 2.28 Typical setup for hydraulic cleaning using sewer ball ...................................... 2-31
Figure 2.29 Balling equipment............................................................................................. 2-32
Figure 2.30 Scooter operation.............................................................................................. 2-32
Figure 2.31 Sewer cleaning records ..................................................................................... 2-34
Figure 2.32 Pipe bursting process ........................................................................................ 2-36
Figure 2.33 Spiral wound slip lining process....................................................................... 2-37
Final Draft xxi
Figure 2.34 Cures-in-place pipe installation procedure ....................................................... 2-37
Figure 2.35 Protection method for existing sewer ............................................................... 2-39
Figure 2.36 Sewerage leaking locations............................................................................... 2-40
Figure 2.37 Wear of cover.................................................................................................... 2-41
Figure 2.38 Offset of manhole block.................................................................................... 2-41
Figure 2.39 Not coinciding with height of road surface....................................................... 2-42
Figure 2.40 Inverted siphon cleaning work.......................................................................... 2-46
Figure 2.41 Portable TV system for small-diameter pipe .................................................... 2-48
Figure 2.42 Applications of portable TV system in difficult to access locations................. 2-48
Figure 2.43 Work procedure for examination of lateral sewer by TV camera..................... 2-49
Figure 2.44 O&M cycle ....................................................................................................... 2-53
Figure 3.1 Typical dry-well and wet-well installations.......................................................... 3-1
Figure 3.2 Typical cavitation damage of an impeller ............................................................. 3-4
Figure 3.3 Sluice gate............................................................................................................. 3-6
Figure 3.4 Gate valve ............................................................................................................. 3-8
Figure 3.5 Check valve........................................................................................................... 3-9
Figure 3.6 Typical ball type check valve.............................................................................. 3-10
Figure 3.7 Electric motor driven gear actuator......................................................................3-11
Figure 3.8 Pneumatic valve.................................................................................................. 3-12
Figure 3.9 Manual bar screen............................................................................................... 3-13
Figure 3.10 An example of risky platform........................................................................... 3-13
Figure 3.11 Mechanically-cleaned bar screen...................................................................... 3-15
Figure 3.12 UNIDO type arc screen..................................................................................... 3-16
Figure 3.13 Typical grit chamber ......................................................................................... 3-18
Figure 3.14 Packing gland.................................................................................................... 3-21
Figure 3.15 Mechanical seal................................................................................................. 3-22
Figure 3.16 Typical weir; generated elevation ..................................................................... 3-23
Figure 3.17 Magnetic flow meter ......................................................................................... 3-24
Figure 3.18 Reflecting ultrasonic flow meter....................................................................... 3-25
Figure 4.1 Typical gravity type oil and grease removal unit.................................................. 4-2
Figure 4.2 DAF unit ............................................................................................................... 4-2
Figure 4.3 Schematic of DAF unit ......................................................................................... 4-3
Figure 4.4 Parallel plate separator.......................................................................................... 4-3
Figure 4.5 Typical plan layout of activated sludge plant........................................................ 4-5
Figure 4.6 Sludge settling analysis......................................................................................... 4-7
Figure 4.7 Recirculated sludge flow ratio .............................................................................. 4-8
Figure 4.8 Conventional activated sludge process ................................................................. 4-8
Figure 4.9 Schematic of centrifugal blowers........................................................................ 4-13
Final Draft xxii
Figure 4.10 Typical air distribution system in aeration tank................................................ 4-14
Figure 4.11 Types of extended aeration plants..................................................................... 4-17
Figure 4.12 Operating cycles of intermittent SBR process .................................................. 4-21
Figure 4.13 Use of a BOD tube for instantaneous assessment of the BOD at site............... 4-23
Figure 4.14 Oxidation ditch ................................................................................................. 4-25
Figure 4.15 Typical jar testing apparatus ............................................................................. 4-30
Figure 4.16 Supernatant turbidity vs. Alum dose................................................................. 4-31
Figure 4.17 Overhanging tree branches and the small area of the lined bund may be seen. 4-32
Figure 4.18 Fixed film synthetic media filters ..................................................................... 4-33
Figure 4.19 Moving bed bioreactor...................................................................................... 4-36
Figure 4.20 Configuration of membrane bioreactor system................................................. 4-37
Figure 4.21 Schematic diagram of an up flow anaerobic sludge blanket reactor................. 4-39
Figure 4.22 Cleaning of effluent gutters .............................................................................. 4-40
Figure 4.23 Cleaning of feed inlet pipes .............................................................................. 4-41
Figure 4.24 The White Towel test ........................................................................................ 4-47
Figure 4.25 Filtration spectrum............................................................................................ 4-52
Figure 4.26 Configuration of recycled nitrification/denitrification process......................... 4-55
Figure 4.27 Breakpoint chlorination curve........................................................................... 4-58
Figure 5.1 Example of a gravity thickener. ............................................................................ 5-2
Figure 5.2 Gravity belt thickener process .............................................................................. 5-7
Figure 5.3 Sludge digestion system...................................................................................... 5-10
Figure 5.4 Silicon dioxide deposits on boiler tubes ............................................................. 5-15
Figure 5.5 Belt filter press.................................................................................................... 5-24
Figure 5.6 Screw press ......................................................................................................... 5-29
Figure 5.7 Schematic of a rotary press system..................................................................... 5-31
Figure 6.1 Electrical single line diagram................................................................................ 6-2
Figure 6.2 Protective relay ..................................................................................................... 6-4
Figure 6.3 Transformer........................................................................................................... 6-5
Figure 6.4 Vector diagram of power factor ............................................................................ 6-7
Figure 6.5 Vector diagram of power factor control ................................................................ 6-8
Figure 6.6 Example of flow of power generated from digester gas ..................................... 6-10
Figure 6.7 UPS circuit.......................................................................................................... 6-13
Figure 6.8 Starters ................................................................................................................ 6-15
Figure 6.9 Motor part load efficiency .................................................................................. 6-17
Figure 6.10 Thermographic measurement............................................................................ 6-20
Figure 6.11 VFD................................................................................................................... 6-21
Figure 6.12 Counterweighted float-level indicator .............................................................. 6-23
Figure 6.13 Float switches ................................................................................................... 6-23
Final Draft xxiii
Figure 6.14 Acoustic level-sensor installation ..................................................................... 6-24
Figure 6.15 Schematic of bubbler-level system................................................................... 6-25
Figure 6.16 Typical pH sensor ............................................................................................. 6-27
Figure 6.17 Hand held DO meter with probe for field use................................................... 6-28
Figure 6.18 Nephlometer...................................................................................................... 6-29
Figure 6.19 Typical SCADAcommunication overview....................................................... 6-30
Figure 6.20 Industrial panel-mounted type HMI.................................................................. 6-31
Figure 6.21 PC based system in which the computer acts as HMI ...................................... 6-32
Figure 6.22 Automatic control system................................................................................. 6-32
Figure 6.23 Digital multimeter (Left) and Analog multimeter (Right) ................................ 6-37
Figure 6.24 Clamp-on meter ................................................................................................ 6-38
Figure 6.25 Hand-cranked megohmmeter............................................................................ 6-38
Figure 6.26 Ground resistance tester.................................................................................... 6-39
Figure 7.1 Typical sampling apparatuses used in sampling of sewage in STPs..................... 7-3
Figure 7.2 The Progress of the multiple tube testing for coliforms........................................ 7-9
Figure 7.3 The fermentation as seen from the gas in the inverted vials inside the tubes ....... 7-9
Figure 7.4 The progress of the filter technique for coliforms .............................................. 7-10
Figure 7.5 Illustrative appearance of cultured plates showing the growth of colonies .........7-11
Figure 7.6 Colony counters: automatic integrating pen type and grid plate for manual use.7-11
Figure 7.7 The progress of the Colilert technique for coliforms.......................................... 7-12
Figure 8.1 Gas sampling pump (left side) and detector tubes (right side) ............................. 8-2
Figure 8.2 Soil (Bio) deodorisation configuration ................................................................. 8-5
Figure 8.3 Mechanism of ASP-deodorization........................................................................ 8-5
Figure 8.4 Activated-carbon deodorization............................................................................ 8-6
Figure 8.5 Aeration tank (left) and Attached growth system (right) .....................................8-11
Figure 8.6 An example of plant beautification adopted in Bangalore.................................. 8-13
Figure 9.1 Confined space entry procedure............................................................................ 9-9
Figure 9.2 Portable toxic gas detector .................................................................................... 9-9
Figure 9.3 Gas mask............................................................................................................. 9-14
Figure 9.4 Self breathing apparatus...................................................................................... 9-15
Figure 9.5 Portable blower (ducting blower) ....................................................................... 9-15
Figure 9.6 Safety signs......................................................................................................... 9-16
Figure 9.7 Fluorescent jacket ............................................................................................... 9-17
Figure 9.8 Photo showing typical confined space entry....................................................... 9-17
Figure 9.9 Example of emergency contacts ......................................................................... 9-23
Figure 9.10 Stretcher ............................................................................................................ 9-25
Figure 9.11 First aid box ...................................................................................................... 9-25
Figure 9.12 Safety management organisation ...................................................................... 9-28
Final Draft xxiv
Figure 10.1 On-site sewage treatment system...................................................................... 10-1
Figure 10.2 Metering unit..................................................................................................... 10-7
Figure 10.3 Membrane separators........................................................................................ 10-9
Figure 10.4 Flow chart of sludge treatment ....................................................................... 10-13
Figure 10.5 Flow chart of a pre-treatment unit .................................................................. 10-13
Final Draft xxv
A2O Anaerobic, Anoxic and Oxic process
AC Alternating Current
ACB Air Break Circuit Breaker
ADF Average Daily Flow
APHA The American Public Health Association
ASD Adjustable Speed Device
ASP Activated Sludge Process
b.e.p Best Efficiency Point
BFP Belt Filter Press
BIS Bureau of Indian Standards
BNR Biological Nutrient Removal
BNR Biological Nitrogen Removal
BOD Biochemical Oxygen Demand
CBM Condition Based Maintenance
CCTV Closed Circuit Television Equipment
CCU Central Control Unit
CDM project Clean Development Mechanism project
CPHEEO
Central Public Health and Environmental Engineering
CI Cast Iron
CIPP Cured-in-Place-Pipe
CMWSSB the Chennai Metropolitan Water Supply and Sewerage Board
COD Chemical Oxygen Demand
CPR Cardiopulmonary Resuscitation
CT Current Transformer
DAF Dissolved Air Floatation
DC Direct Current
DG Diesel Generator
DI Ductile Iron
DO Dissolved Oxygen
EBPR Enhanced Biological Phosphorus Removal
EDI Energy Dispatching Inlet
EEBA Emergency Escape Breathing Apparatus
EMB Eosin Methylene Blue
EPA Environmental Protection Agency
Final Draft xxvi
ESP Exchangeable Sodium Percentage
F/M ratio Food to Microorganisms ratio
FC Faecal Coliform
FFT Fast Fourier Transform
FPU Final Polishing Unit
FRP Fibre Glass Reinforced Plastic Pipes
GL Ground Level
GRP Glass Fibre Reinforced Plastic
HDPE High Density Polyethylene
HMI Human Machine Interface
HRT Hydraulic Retention Time
HT High Tension
I / O Input / Output
ICMR Indian Council of Medical Research
ID Internal Diameter
IDLH Immediately Dangerous to Life or Health
IMLR Internal Mixed Liquor Recycle
IS Indian Standards
JASCOMA Japan Sewer Collection System Maintenance Association
JICA Japan International Cooperation Agency
LEL Lower Explosion Limit
LT Low Tension
MBBR Moving Bed Bio Reactor
MBR Membrane Bio Reactor
MCC panel Motor Control Centre panel
MCCB Moulded Case Circuit Breaker
MCF Membrane Cartridge Filtration
MF Micro filtration
MLd Million Litres per Day
MLSS Mixed Liquor Suspended Solids
MLVSS Mixed Liquor Volatile Suspended Solids
MOUD Ministry of Urban Development
MPN Most Probable Number
MSRT Minimum Solids Retention Time
NF Nano Filtration
NPSHA Net Positive Suction Head Available
Final Draft xxvii
NPSHR Net Positive Suction Head Required
NRV Non-Return Valve
NUSP National Urban Sanitation Policy
NTF Nitrifying Trickling Filter
O&M Operation and Maintenance
OCR Over Current Relay
ORP Oxidation Reduction Potential
OSHA Operational Safety and Health Association
OSS On-site Sanitation System
OVR Over Voltage Relay
PB fume Palmer-Bowlus flume
PC Personal Computer
PC Power Contractor
PCB Pollution Control Board
PCC Precast cement concrete
PCCP Prestressed Concrete Cylinder Pipe
PCOM Per Capita O&M Cost
PD Positive Displacement
PHA Poly-hydroxyalkanoates
PLC Programmable Logic Controller
POA Polyphosphate Accumulating Organism
PR Protective Relay
PS Pumping Station
PT Potential Transformer
PVC Polyvinyl Chloride
PWD Public Works Department
R&D Research and Development
RAS Return Activated Sludge
RBC Rotating Biological Contactor
RCC Reinforced Cement Concrete
RO Reverse Osmosis
RPM Revolutions Per Minute
RTD Resistance Temperature Detector
SAR SodiumAbsorption Ratio
SASW Spectral Analysis of Surface Waves
SBR Sequencing Batch Reactor
Final Draft xxviii
SCADA Supervisory Control and Data Acquisition
SCBA Self-Contained Breathing Apparatus
SDB Sludge Drying Bed
SDNR Specific Denitrification Rate
SF Safety Factor
SF6 Sulphur Hexafluoride
SLB Service Level Benchmarks
SLR Solids Loading Rate
SOR Surface Overflow Rate
SPDP Screen Protected Drip-Proof
SRT Solids Retention Time
SS Suspended Solids
STP Sewage Treatment Plant
SVI Sludge Volume Index
TDS Total Dissolved Solids
TEFC Total-Enclosed Fan-Cooled
TMP Transmembrane Pressure
TSS Total Suspended Solids
TV Television
UASB Up flowAnaerobic Sludge Blanket Reactor
UF Ultra Filtration
ULB Urban Local Body
UNIDO United Nations Industrial Development Organization
UPS Uninterruptible Power Supply
UPVC Unplasticized Polyvinyl Chloride
US EPA, USEPA United States Environmental Protection Agency
UVR Under Voltage Relay
VCB Vacuum Circuit Breaker
VFA Volatile Fatty Acids
VFD Variable Frequency Drive
VFM Value For Money
VSS Volatile Suspended Solids
VT Voltage Transformer
VVVF Variable Voltage Variable Frequency
WAS Waste Activated Sludge
WEF Water Environment Federation
WHO Word Health Organization
Final Draft xxix
WOR Weir Overflow Rate
WRFs Water Reclamation Facilities
XLPE Crosslinked Polyethylene Insulated Cable
Part B: Operation and Maintenance CHAPTER 1: INTRODUCTION
Final Draft 1-1
In engineering parlance, operation refers to daily operation of the components of a sewerage
system such as collection system, pumping stations, pumping mains, STPs, machinery and
equipment, etc., in an effective manner by various technical personnel, and is a routine function.
The term maintenance is defined as the art of keeping the structures, plants, machinery and 5
equipment and other facilities in optimum working order. Maintenance includes preventive
maintenance or corrective maintenance, mechanical adjustments, repairs, corrective action and
planned maintenance. However, replacements, correction of defects etc., are considered as
actions excluded from preventive maintenance. For replacements with regard to sewerage and
sewage treatment, the broad categories of infrastructure which need to be addressed are as 10
- Collection System including house service connections and manholes
- Pumping Stations
- Pumping Mains
- STPs 15
- Utilization of biological sludge and containment of chemical sludge
There are standard O&M manuals for these in developed countries. However, O&M manual is
not yet prepared in detail in India. The following conditions prevail:
- Most of the towns are only partially sewered
- Most of generated greywater continues to flow in road side drains 20
- Per capita water usage is practically only 25 to 30 % as used in advanced countries
- Water as used is mainly from local groundwater also with high TDS, sulphates etc
- The sulphates are an agent of corrosion of concrete in sewers
- Wash basins, kitchen sinks etc do not have blenders below the sink
- Detergent powders have significant grit content 25
- Volumes of water usage are only about 30 to 40 %
- Cattle are also housed inside the cities and their dung washed into sewers
- The cattle shed washing occurs during noon times after the peak flow has passed
- These cattle shed washed dung settles in sewers and builds up to choke
- The budgets of most ULBs are inadequate for purchasing sewer cleaning machines 30
- Though sewer divers are banned, still manual labour is used to rod and clean the
- Pumping stations are not connected by website to know of flooding in the station
- Removing sewer blocks takes longer times due to manual work 35
- Instrumentation based remote operation of STPs is a far away situation
- Except a few metro cities, all records continue to be in hard copies only
- There are no newsletters aimed at operators sharing their experiences
- The above position is further complicated by as many as 25 local languages
- Disposal of solid wastes in manholes 40
These issues as in Indian conditions are directly in contrast to the situations in advanced
countries make it necessary to evolve an O&M manual specific to the Indian conditions.
1.1 Need for O&M
Even though we all recognize the reasons as pointed out above, the absence of an O&M manual
based on which these defects can be step by step rectified is keenly felt. There is the O&M 45
manual for Water Supply Systems by CPHEEO, but there is no such manual for Sewerage
Systems. Moreover, unless there is an O&M manual, ULBs cannot justify budget allocations to
meet their obligations under such a manual. The net result is this lack of attention to the
important aspect of Operation & Maintenance (O&M) of sewerage systems leads to
deterioration of the useful life of the systems necessitating premature replacement of many 50
system components and also affecting overall sanitation. As such, even after creating such
assets by investing millions of rupees, they are unable to provide the services effectively to the
community for which they have been constructed, as they remain defunct or underutilized most
of the time.
Some of the key issues contributing to the poor Operation & Maintenance have been identified 55
i. Lack of finance, inadequate data on Operation & Maintenance
ii. Multiplicity of agencies, overlapping responsibilities
iii. Inadequate training of personnel
iv. Lesser attraction of maintenance jobs in career planning 60
v. Lack of performance evaluation and regular monitoring
vi. Inadequate emphasis on preventive maintenance
vii. Lack of operation manuals
viii. Lack of appreciation of the importance of facilities by the community
ix. Lack of real time field information etc. 65
Therefore, there is a need for clear-cut sector policies and legal framework and a clear
demarcation of responsibilities and mandates within the water supply sub-sector. From the
Indian experience, it has been observed that by and large, about 20 to 40% of the total annual
Operation & Maintenance cost goes towards the personnel (Operation & Maintenance Staff), 30
to 50% of the cost is incurred on power charges and the balance is utilized for consumables, 70
repairs and replacement of parts and machinery and miscellaneous charges. In most of the cities
in India, the tariffs are so low that they do not even cover the annual Operation & Maintenance
cost.
Hence, it is a felt need to bring out this O&M Manual in sewerage systems
The O&M Manual is required to encompass various issues pertaining to an effective O&M such 75
as technical, managerial, administrative, personnel, financial & social aspects etc.
The O&M Manual is a long felt need of the sector. At present, there is no Technical Manual on
this subject to benefit the field personnel and to help the O&M authorities to prepare their own
specific manuals suitable for their organizations. Therefore, CPHEEO made plans to publish the
O&M Manual for Indian Sewerage System with JICAsupport. 80
1.2 Basic Considerations of O&M
1.2.1 Laws and Regulations related to O&M of Sewerage System
In fact there are no laws directly related to O&M of sewerage systems. The laws which are
generally applicable are invariably the municipal bye-laws which are of general nature.
1.2.2 Effluent Standards related to Treatment Plants 85
The effluent standards related to STPs are very confined to BOD of less than 20 mg/L and SS of
less than 30 mg/L. These are more of historical nature.
1.2.3 Environmental Considerations
These are generally problems with Industrial Effluent which get into sewers and which are
difficult to control. A conflict exists between the prevention and control of water pollution Act 90
and the Municipal act or Jal Nigam Act of Water and Sewerage Board act. The sewer system is
to be maintained by the municipality, Jal Board or Water Supply and Sewerage Boards. But the
power to sanction an industrial effluent to be discharged in the sewers is not with these agencies.
It is with the Pollution Control Boards (PCBs). The trouble is when industries are detected to be
discharging their effluents without the necessary treatment; the said local agencies do not have 95
legislative powers to put the industry on notice. They have to write to the PCB. The PCB is in
no hurry to immediately look into this specifically because for a whole metro city the PCB may
have only a handful of engineers and they have lots of situations like these to handle. Hence the
backlog is high. Even if a legal process is initiated, the getting orders of the court to effect
disconnection of sewer connection are difficult. It looks that one simple way of getting over 100
these is to allow the industry to connect only their toilets, baths, canteen etc. But the real
problem is the industries may be surreptitiously using that connection to discharge their
effluents also. The monitoring of these is again vested with the PCBs. Thus, a very serious
situation exists here. Hence joint subcommittee of ULB and PCB have to be created with
powers to recommende interim disconnection. 105
1.2.4 Budget
Appropriate budgetary provisions for operation and maintenance of sewerage system needs to
be provided so that O&M is carried out without any constraints such as human
resources,finance,it is dealt with in part C.
1.2.5 Preventive Maintenance 110
Preventive maintenance is a set procedure whereby each component of the system goes through
a systematic check and these components are brought into dependable use. An example can be
checking the volume and consistency of oil in the gearbox after a specified number of hours of
operation and correcting the situation either by topping up or replacing fully as needed. The
preventive maintenance issues, checking parameters and timings are all given by every 115
equipment vendor as a manual. Carrying out these tasks is to be done by the respective
equipment vendor under a separate contract called preventive maintenance contract and should
be delinked from the O&M contract. Most often this is not fully recognized and what could
have been saved by preventive maintenance finally ends up breakdown repairs. This situation
needs its importance. 120
1.2.6 Workmanship and Quality of Equipment
Workmanship defines the art or skill of a workman or workwoman. This is an art or skill with
which something is made or executed. Materials and equipment shall be new and of a quality
equal to or superior to that specified or approved. Work shall be done and completed in a
thorough and workmanlike manner and in strict conformance with the plans and specifications. 125
In general, the work performed shall be in full conformity and harmony with the intent to secure
the best standard of construction and equipment of the work as a whole or in part.
No material shall be used in the work until it has been found satisfactory by the Engineer. All
material and equipment are subject to test to determine their conformity with these
specifications. Certified factory and mill tests normally will be acceptable for standard 130
manufactured items. Whenever standard specifications are referred to, they shall be the latest
revised edition of the Standard Specifications referred to and shall be considered to be a part of
these specifications in so far as they apply. All work and materials shall be subject to inspection
by the Engineer.
The Engineer may assign such assistants as he may deem necessary to inspect the materials to 135
be furnished and the work to be done under this contract, and to see that the same strictly
conform therewith. The Engineer shall be notified of the time and place of preparation,
manufacture or construction of material for work or any part of the work which he may wish to
inspect, and of the time and place of making the factory tests required under the contract. Such
notification shall be given a sufficient length of time in advance of the beginning of the work on 140
such material or part or of the beginning of such test to allow arrangements to be made for
inspecting and testing or witnessing, as the case may be, if such inspection and testing or
witnessing are deemed practicable by the Engineer.
All necessary machinery guards, railings and other protective devices shall be provided as
specified by the Industrial Safety authority which would be the Inspectorate of Factories. Before 145
final acceptance of the work, the contractor shall cause an inspection to be made by a
representative of the inspectorate of factories and shall certify that all safety requirements have
been complied with.
1.3 Outlines of O&M
1.3.1 Overview and Contents of O&M 150
Thus an overview of O&M is taking note of the above issues and suggests appropriate remedies
to the present situation which includes finances and manpower and remote control.
1.3.2 Management of Facilities
Proper housekeeping, aesthetics and gardening are the requirement here. With the lifestyle in
cities changing to fast-forward, nobody is able to find time for these. In addition, getting labour 155
to do these is also difficult due to extra costs and security issues.
1.3.3 Schedule of O&M
A proper schedule shall have to specify what things are to be attended to at what intervals and to
whom it is to be reported to in case of faults. Clearly, this is not the case and it is all emergency
repairs all the way. 160
1.3.4 Response to Accidents
Most times, the local staff may not even know how to do first aid. All the time whenever an
accident occurs, it is the fire brigade who will be called upon to the site. Thereafter, it is a
standard procedure of getting the victim to the hospital and thereafter the local agencies come
into the picture only when defending the compensation money payable to the victim. 165
1.3.5 Management of Buildings and Sites
There is the century old practice in Public Works Department (PWD) that civil works life cycle
is 30 years and equipments 15 years. Going by this, civil works in sewerage will need
strengthening and renewal as the case may be. Similarly equipments will need replacement in
15 years. However, in actual practice, this is not the case. Only when a civil work shows up a 170
crack or a leaky roof, the position is reviewed as an ad hoc repair. Similar is the case of
machineries.
1.4 Organization of O&M
1.4.1 Description of O&M Work
A simple understanding of O&M work is the relationship between human resources, equipment 175
availability, financing the O&M and career opportunities for the staff. These are within the local
body. The perception by the public and their payment of dues to the local body is the other side
of the coin as in Figure 1.1.
Source: Asset Management for Infrastructures in Fast Developing Countries by Ype C Wijnia 180
Figure 1.1 Combined tasks and challenges with the O&M
This illustrates O&M of sewerage as responding to the four sided compression of (a) limited
budget, (b) performance requirements, (c) less public acceptance and (d) higher legal
Assets Systems Data
Standards Procedures Practices
Personnel Capability Culture
Higher Legal
Less Public
requirements like staff, salaries, etc. If all these act at the same time, the system has to collapse
and if one of them increases the pressure, the ability of the other three to be resilient is needed to 185
remain stable. If this resilience is not there, the shape of the rectangle will be altered and the
system may be responding without any control. Thus the work of O&M involves the
engineering, financing and administrative interface with the public.
1.4.2 Deployment of Manpower
There are limited promotional avenues for people in the O&M sector. Non-engineers in the 190
O&M sector entering the service will retire without any promotion only if he has no academic
qualification. This does not give them any drive to do a sincere job all the time. He will merely
be doing a routine type of work. Promotions by way of number of years of experience must be
coupled with examination of his experience and given weightage. The staff may be rotated
between sewerage and water supply sectors by providing suitable training, so that he does not 195
get into sickness from working in sewerage system throughout his career.
1.4.3 Out sourcing of O&M
Recent trend is to subcontract the O&M work. In this case the contractor hires staff from local
market and deploys them on the O&M work. He will only place the staff and earn the money
and after paying to the staff earns his profit but he may not have interest in O&M. If the 200
contractor is also the same firm who has built this sewerage system, his interests will be very
sincere. In the case of exclusive O&M outsourcing not involving the potential O&M agency in
the construction activity of the system involved, the proper qualifications, experience, personnel,
etc. are to be ensured.
1.4.4 Training 205
Development of operational skill is not taught in the schools or polytechnics or colleges. It has
to be learnt. This again must be verified once a year to understand whether he has understood
correctly and if not in order he has to be put through training. Thus, the training is a continual
1.4.5 Monitoting through Information Control Technology 210
Extracting sewage treatment conditions from water quality information of effluent is very
important for operating a sewage treatment plant effectively. An example of a Japanese sewage
treatment plant implementing monitoring using control technology is introduced here. For more
details, refer to Appendix 1.1
1.4.6 Database for Effective O&M 215
It is also important to create a database of information obtained through monitoring, and to use
this database of past data for operation henceforth. For details of such a database, refer to
Appendix 1.2.
1.4.7 Paragraph on Problems in Existing O&M
The problems faced by O&M sector in sewerage are a combination of engineering, finance, staff 220
and management. The engineering problems are absence of adequate innovations of economical
design and construction of infrastructure that will reduce the costs and will still render the
project functional to the stated goals in the required geographical coverage with lesser
expenditure. Examples are possible usage of decentralized systems, twin drain systems and
incremental sewerage etc. The finance problems are inadequate revenue as compared to 225
expenditure and norms for budget allocation which is more of historical nature than based on
time and motion evaluations as also the symbiosis of the population who are willing to pay only
if they feel services are better on the one side and the ULBs who cannot bring up improvements
unless the population contributes increased revenue upfront. The staff problems are the lack of
promotional avenues for decades on end and also absence of at least the time based scale of pay 230
resulting in inordinate stagnations in posts and staleness especially the field staff like operators,
technicians, drivers and other such posts where practically they enter and retire in the same post.
The management problems are frequent transfers from one headquarters to another resulting in
disturbances to family establishment, education of wards and care of elderly at home and lack of
incentives for exceptional performers. These are very easy to attribute but are very difficult to 235
change given the service conditions, rules, regulations etc. Yet another situation which prevails
in a democratic governance is to appraise the political governance on the nuances of engineering
projects while seeking funds and establishment by putting forth the engineering components and
needs to explain them convincingly by the chief executive of the water and sewerage authority
depending on how equipped they are to comprehend the enveloping issues. There seems to be a 240
need to position the officers of these organizations who would possess a basic qualification in
related engineering whatever be their other attainments in management so that they can
effectively conceive and communicate both ways between the political governance who are to
deploy the necessary resources and the staff of these organizations who are to implement and
carry out the O&M-an aspect in which a country policy seems to be not in place. In addition 245
please also refer to Appendix 1.3 which is an extract from the Evaluation of O & M of STPs in
India - CUPS/68/2007.
1.5 Community Awareness and Participation
1.5.1 Public Relations and Public Opinion related to Sewerage Works
Meeting the public and directly answering their questions on sewerage problems is the solid 250
foundation of goodwill. Mostly officials in charge of sewerage system feel hesitant to meet the
public because they do not have the funds to rectify the defects pointed out by the public.
Reference to Figure 1.1 is important. Only when the public are met directly, the system
drawbacks will come to light. Only then, a basis for calculating the budget allocations can be
known. Only then engineers in the field can carry out the remedies needed. 255
1.5.2 Complaint and Redressal
Most local bodies have launched the internet based complaint recording by the public. It will be
good to also publish on the web the complaints received and actions taken up to solve them.
Otherwise the public will not know.
1.5.3 Dos and Donts for Community 260
The public are also responsible to maintain the system. They must not put solid wastes, solid
vegetable cutbits, meat, plastics, etc. This is even now very well advertised by many local
agencies but then the public continue to do these sometimes. Educating them continuously has
to be a part of O&M.
1.6 Potential Risk with respect to Sewerage System 265
1.6.1 Provision of Disaster Prevention Systems
Disaster by definition is something which occurs suddenly. For example if a corroded RCC
sewer is not properly attended to in time, it will collapse all of a sudden in the crown and all the
sewage upstream will get blocked. At the same time, if an earthquake occurs and the sewer
collapses by lateral movement, then also, it is the same problem. The former situation can be 270
avoided by monitoring the sewer condition once a year by camera survey. The later cannot be
avoided. The lesson is the need for programmed monitoring of sewerage system components.
1.7 Sewerage Ledger
The NUSP and SLB have been explained in Section 1.5 of the Part-A Manual. In order to keep
track of their attainment, the upkeep of ledgers is necessary as explained below. 275
1.7.1 Preparation of Sewerage Ledger
A sewerage ledger can be either a simple ledger or a complicated ledger. The staff must receive
situation reports of such activities which he can control. There is a tendency to insist on
receiving all sorts of data where it is meant for that person or not. This has to change. For
example an administrator must be more concerned about complaint redressal than how much 280
sewage is pumped out. Similarly, the field engineer must know the sewage surface elevations in
sewers and whether sewage is overflowing the roads and document the case and put up for
funds to solve the situation.
1.7.2 Management and Use of Sewerage Ledger
This is to be done by an independent team not connected with the O&M team. Only then, the 285
real problem will be known and remedies can be taken up. The sewer ledger for compliant
redressal must be put on the website to increase the consumer satisfaction in the ledger system.
1.8 Budget Estimation for O&M
Budget estimation has been explained in Part C Chapter 5.
Revenue generation to ensure self-sustainability is an issue of political and administrative will 290
be to levy and collect practicable costs and there is nothing to strategize in it.
This Manual has been prepared with the aim of offering guidelines to workers/operators of
sewerage systems on site for operation and work performed by them, and to field engineers for
passing on instructions and judgements to the workers and operators. In practice, a sewerage 295
collection system or an STP each requires its own proprietary O&M manual suitable for the
work done, the local conditions and the scale of its own facility. It is anticipated that these
facilities will refer to the contents of the present manual and prepare make effective use of its
own proprietary manual.
1.10 Relationship between Part-A(Engineering), Part-B (Operation and 300
Maintenance), and Part-C (Management) of Manual
The Manual on Sewerage and Sewage Treatment (second edition) published in 1993 which has
been revised and updated mainly gave thrust to engineering aspects of the sewerage and sewage
treatment systems. Though, it covered topics of management and operation and maintenance of
sewerage systems but these aspects were not dealt with in detail so as to create awareness 305
amongst the practicing and field engineers regarding the importance of these two topics which
are so important for the sustainability of the systems in the long run.
The present Manual on Sewerage and Sewage Treatment has been divided into three sections, as
under, which are inter-related to each other as described below:
i) Part Aon Engineering 310
ii) Part B on Operation and Maintenance
iii) Part C on Management
Part A on Engineering addresses the core technologies and updated approaches towards the
incremental sanitation from onsite to decentralized or conventional collection, conveyance,
treatment and reuse of the misplaced resource of sewage and is simplified to the level of the 315
practicing engineer for his day to day guidance in the field in understanding the situation and
coming out with his choice of approaches to remedy the situation. In addition it also includes
recent advances in sewage treatment, sludge and septage management to abate pollution of
environment. By no means, this is a text book nor it should be. It is a simple to understand
guideline for the field engineer. 320
Part B on Operation and Maintenance addresses the issues of standardizing the human
resources and financial resources that are needed to sustain a system created at huge costs
without it slipping into an edifice for want of codified requirements of these so that it becomes
possible to address these in the estimate stage itself and seek a comprehensive approval of fund
allocations and human resources besides ushering in the era of public private partnership to 325
make the projects self-sustaining. It is a simple to understand guidance for the resource seeker
and resource allocating authorities.
Part C on Management is a refreshing approach to modern methods of project delivery and
project validation and gives a continual model for the administration to foresee the deficits in
allocations and usher in newer mechanisms. It is a tool for justifying the chosen project delivery 330
mechanism and optimizing the investments on need based allocations instead of allocations in
budget that remain unutilized and get surrendered in end of fiscal year with no use to anyone. It
is straight forward and modern approach.
It is very important to mention here in the beginning of this Part B of the Manual that Trade
names and technology nomenclatures, etc., are cited only for familiarity of explanations and not 335
a standalone endorsement of these.
Part B: Operation and Maintenance CHAPTER 2: SEWER SYSTEMS
CHAPTER 2 SEWER SYSTEMS
Asewerage system consists of the following:
- House Service Connections 340
They connect the house to sewers in the road.
- Sewers
These are pipe or conduits meant for carrying sewage and are laid along the roads and flow
by gravity.
- Lift Stations 345
When sewers are at a large depth, lift stations are used which help to move liquids fromlow
elevations to the required elevation.
- Pump Stations
They transfer the sewage from one location to another
- Sewage Treatment Plants 350
They treat the sewage to meet the effluent quality.
- Safe disposal system of final effluent
Sewers are the most important part of a sewerage system. They are laid below the ground and
are difficult to repair. Hence great care is needed in their O&M.
In this section, the following is presented: 355
- Objectives of maintenance
- Type of maintenance
- Necessity of maintenance
2.1.1 Objectives of Maintenance
Quality maintenance of sewerage system consists of the optimum use of labour, equipment, and 360
materials to keep the system in good condition, so that it can accomplish efficiently its intended
purpose of collection and transportation of sewage to the treatment plant.
2.1.2 Type of Maintenance
There are three types of maintenance of a sewerage system preventive, routine and emergency.
Preventive or routine maintenance should be carried out to prevent any breakdown of the system 365
and to avoid emergency operations to deal with clogged sewer lines or over flowing manholes
or backing up of sewage into a house or structural failure of the system. Preventive maintenance
is more economical and provides for reliability in operations of the sewer facilities. Emergency
repairs, which would be very rare if proper maintenance is carried out well also, have to be
provided for. Proper inspection and preventive maintenance are necessary. 370
2.1.3 Necessity of Maintenance
Sewer maintenance functions are most often neglected and given attention only as emergency
arises. Adequate budgets are seldom provided for supervision, manpower and equipment, unlike
the case for maintenance of other utilities like electric cables, telephone cables, gas and water
rains. Such attitude towards sewer maintenance is found even in large cities. Considering the 375
health hazards that the public at large has to face, it is appropriate to provide sufficient funds to
take care of men, material, equipment and machinery required for efficient maintenance.
All efforts should be made to see that there is no failure in the internal drainage system of
premises; a serious health hazard results when sewage backs up through the plumbing fixtures
or into the basements. The householder is confronted with the unpleasant task of cleaning the 380
premises after the sewer line has been cleaned. Extensive property damage may also occur,
particularly where expensive appliances are located in the basements.
Maintenance helps to protect the capital investment and ensures an effective and economical
expenditure in operating and maintaining the sewerage facilities. It also helps to build up and
maintain cordial relations with the public, whose understanding and support are essential for the 385
success of the facility.
2.2 Inspection and Examination for Sewer
2.2.1 Importance of Inspections and Examinations
Sewer collection systems are intended to be a reliable method of conveying sewage from
individual dischargers to sewerage treatment plants. Inspection and examination are the 390
techniques used to gather information to develop operation and maintenance programs to ensure
that new and existing collection systems serve their intended purposes on a continuing basis.
Inspection and testing are necessary to do the following:
- Identify existing or potential problem areas in the collection system,
- Evaluate the seriousness of detected problems, 395
- Locate the position of problems, and
- Provide clear, concise, and meaningful reports to supervisors regarding problems.
Two major purposes of inspection and examination are to prevent leaks from developing in the
sewers and to identify existing leaks so they can be corrected.
A designers mistake and the failure in construction are directly responsible for many of the 400
sewer failures. Due to age, deterioration of the material of the sewer by attack of hydrogen
sulphide or other chemicals, settlement of foundations and leaking joints may result in the
structural failure of the sewer. It takes a very long time from the onset of the first initial defect to
the collapse of the sewer. A crack or a leaking joint will allow subsoil water and soil to enter the
sewer causing cavities around it leading to slow settlement of foundation and the eventual 405
collapse of the sewer. Very often soil with water is carried away below the bedding along the
length of the sewer. The type of break often gives a clue to the cause. A shear failure due to
faulty foundation or movement of earth, is a clean vertical break in the pipe or barrel. Excessive
loading, either internally or externally, causes horizontal breaks. Breaks caused by internal
pressure gives cracks in the sewer while external overload causes the top of the pipe to crush- 410
Regular inspection of the sewer can pinpoint the sewer that needs to be attended to before there
is a complete failure or collapse.
For preventing the above serious instances of damages to the sewer system, the maintenance
engineer should establish adequate inspection and examination program.
2.2.2 Guidelines for Inspections and Examinations 415
Documents and data that can give information on the status of sewer facilities are necessary for
operation and maintenance of the facilities. However, enormous time and costs are necessary for
examining and inspecting the overall information on sewer facilities that extend over a wide
area. It is recommended that a preliminary inspection be implemented to acquire with
comparative ease documents and data that can be used to decide the facilities to be 420
examined/inspected and their priority, and then decide the facilities to be finally examined and
inspected for effective acquisition of data.
The procedure is to first acquire the basic information through preliminary inspection for the
examination and inspection of the facilities in this section.
Preliminary inspection Inspection and examination 425
Judge from inspection result
Decide the necessary measures (repairs, modifications)
The detailed method for conducting a preliminary inspection is described in the following 430
2.2.3 Preliminary Inspection
During the preliminary inspection of the sewerage system, subsidence, collapse, and overflows
on the roads on which sewers are laid, should be confirmed. Deformation or damage to facilities,
and deposits of sand and silt are to be confirmed during observation from the manhole. If 435
damage or possibility of damage to the facility or if any of the abnormalities listed below are
confirmed during the preliminary inspection, the facility manager should examine and inspect
the relevant locations for the following:
- Corrosion, wear, damage or crack in the facility
- Water infiltration 440
- Corrosion of steps, wear of covers, deformation of manhole, buried manhole
- Abnormal odours
- Clogging and overflowing
The suggested period of preliminary inspection to be carried out is shown in Table 2.1, Table
2.2 and Table 2.3. In addition clause 3.10 of Part A manual also deals with tracer study. 445
Table 2.1 Preliminary inspection during Defect Liability Period (DLP)
Inspection stages
Manhole Sewer Inverted siphon
Initial/first inspection
During the first 3 months of start of DLP (to expose any hidden
construction defects)
During the last 3 months of DLP
Additional inspections,
if DLP is > 4 years
At a frequency of every 2 years after first inspection during DLP
Table 2.2 Preliminary inspection for Manholes & Sewers
Roads & Traffic Conditions
Manhole Sewer
Roads subjected to heavy & mixed traffic
Once a year Once in 2 years
Roads 2m - 5m wide (6-15 feet) subject to mixed traffic
Once in 2 years Once in 2 years
Roads and lanes less than 2m wide Once in 3 years Once in 3 years
Demarcated & kerbed /raised footpaths (likely along main
roads)
Table 2.3 Preliminary inspection period for other facilities
Inverted siphon Force main and their appurtenance
Inspection period Once a year Once a year
Note: Remedial measures should be implemented immediately upon finding defects/distress/dysfunction in the
components of the sewerage system. 450
2.2.4 Type of Inspections and Examinations
In order to assess the condition of the sewers inspections and examinations are necessary.
There are two basic types of inspection and examination:
- Direct
- Indirect 455
2.2.4.1 Direct Inspection and Examination
This means a person walking through a sewer and physically inspecting the condition visually.
This shall never be done once a sewer has been put into service. Even for new sewers, the inside
diameter shall be more than 2 m. All safety precautions needed for working in confined spaces
shall be taken. Hitting at the sidewall with a hammer or other devise shall be totally prohibited. 460
The only purpose it will serve will be to get a visual idea of whether the pipe joints are made
fully. Once a sewer is put into service, this is to be banned for ever.
2.2.4.2 Indirect Inspection and Examination
These can be classified by a variety of methods as in Table 2.4.
Table 2.4 Classification of inspection 465
Defect detected
Digital cameras Any 150-1500
Zoom cameras Any >150
Push-camera Any s300
In-line leak detectors Any 100
Acoustic monitoring
PCCP 450
Sonar/ ultrasonic Any 50
Electrical leak location Non-ferrous 75
Remote field eddy
Ferrous, PCCP 50
Magnetic flux leakage Ferrous 50-1400
Laser profiling Any 100-4000
Gamma-gamma Concrete Not yet defined
Ground penetrating
Any Not yet defined
Infrared thermograph Any Not yet defined
Micro-deflection Brick Not yet defined
Impact echo/SASW
/Concrete
Source: EPA/600/R-09/049 | May 2009
Even though there are so many technologies available as above, the technology to be chosen
will depend on the affordability by the user departments. A simpler and applicable technology
compilation is as shown in Table 2.5.
Table 2.5 Sewer system inspection technologies considered applicable to Indian conditions 470
No. Technology
Sewer Size Sewer Material Sewer Condition
1 Light and Mirror Up to 300 mm Any Empty
Closed Circuit Camera Any size Any Empty
3 Sonar Systems Any size Any Fully flowing
The light and mirror is the oldest of known technologies. Two successive manholes are opened
and vented sufficiently for about an hour. Thereafter a long hand-held mirror secured at 45
degrees to the handle is lowered into the bottom of the manhole and a torch light is focussed on
the mirror from the above so that the light beam is deflected by 90 degrees to travel horizontally
through the sewer pipe and the light is seen in the opposite manhole. This is easier at dusk. This 475
can tell whether the bore of the pipe is choked or clear.
Source: http://www.sankyotrading.co.jp
Figure 2.1 Mirror Test and Mirror with rode
The closed circuit camera is propelled through the sewer by a remote controlled wired power 480
supply from a van and travels through the sewer and relays the picture of the inside to a TV in
the van. The sonar system is similar. A robot is sent through the sewer and it emits high
frequency sound waves which impinge on the pipe surfaces and returns to the emitter as a
reflection. By knowing the material of construction of the sewer pipe walls, this can be
programmed to verify the structural condition of the wall of the sewers. 485
Indirect inspection is carried out by sending a camera through the sewer for taking photographs
or a closed circuit television equipment (CCTV) to send pictures which can be seen on a TV
Screen or recorded as video. The CCTV inspection can be used for sewer lines as small as 100
mm. Above 900 mm diameter there are limitations due to lighting problems and camera line
angles. 490
Continual advances are being made in the range of TV cameras and also in quality. The type of
camera selected should be robust for use in sewers and be able to give good quality pictures.
Traction of the cameras is by pulling winches, by pushing or self traction. The former two are
not used much at present. However, self traction is suitable for use in sewers above 225 mm
diameter. Other constraints in the use of self traction are the weight of the trolley and electricity 495
requirements. Heavy silting of sewers precludes the use of self traction. The cameras are
attached to trolleys or mounted on pair of skids or single flat tray. Inspection of the sewer by
CCTV is limited to the top portion only. The objects under scrutiny are parallel to the camera
and viewing is at 40 to 50 degrees. With radial scanning head, inspection normal to the sewer
wall is possible. 500
Atypical arrangement is as shown in Figure 2.2.
Figure 2.2 Typical CCTV equipment in action
A classical problem encountered in stoneware sewers laid through light forest or heavy garden
areas is the roots of trees piercing through the joints and growing there. These become like a 505
plug and choke the sewer. This is shown in Figure 2.3. (Tree roots growing into joints of stone
ware sewers when these are laid near light forests and heavy gardens. At right is the photo of the
bunch of roots taken by a CCTV camera.)
Figure 2.3 Tree roots and sewers 510
Similarly, the structural condition of old sewers like brick arch sewers and concrete pipes can be
ascertained by sonar surveys which can provide the frontal image of the wall on a 360-degree 515
vertical spiral around the horizontal axis. These images can be analyzed carefully. The system
can also provide information on the deflection and sidewall breakages of the sewer as in Figure
2.4. (The sidewall condition shown as photographs, the structural damage to the crown of the
sewer and longitudinal cracked condition of the sewer from the survey)
2.2.5 Sewer Inspection and Examination 520
If an abnormality is detected during preliminary inspection or through notification from outside,
the maintenance engineer should judge the urgency and the content of the abnormality, and then
make a proper inspection and study.
2.2.5.1 Visual Examination
Visual examination is an inspection through images or by sight to detect an abnormality. 525
Visual examination includes direct visual inspection, and indirect visual inspection using
pole-mounted inspection camera, and closed-circuit TV equipment (CCTV).
2.2.5.1.1 Manhole Visual Inspection
The visual inspection of manhole is performed by visually checking the manhole cover and its
environment, and observing the internal parts of the manhole. To inspect the internal parts of the 530
manhole, the inspector should enter the manhole and check the items listed below. However,
refer to the sub-section 2.7.1.2 for details of the inspection items.
- Status of internal surface of manhole
- Status of sewer on the upstream and downstream sides viewed from the manhole
- Status of groundwater infiltration 535
To inspect the internal parts of the sewer from the manhole, either a mirror or a strong light
should be used for observation, or a TV camera meant for inspecting conduits should be used.
- Features of manhole visual inspection
Inspection accuracy is high because the inspector actually observes and
measures the abnormality personally. 540
Economical compared to inspection using a TV camera.
The inspected results become very useful O&M data.
- Procedure for manhole visual inspection is shown in Figure 2.5.
Figure 2.5 Manhole visual inspection procedure 545
Before entering any manhole, adequate safety measures should be taken in accordance with
subsection 2.11.1.2.
2.2.5.1.2 Inspection Using a Pole-mounted TV Camera
As shown in Figure 2.6, a pole-mounted TV camera consists of an extendable operating rod at
the front of which a camera and light are fitted. This arrangement is inserted in the manhole 550
from the ground, and the inspector on the ground observes a monitor and inspects the internal
parts of the pipe through the camera. This check may also be used for pre-inspections. The
method of inspection is shown in Figure 2.7.
The features of direct visual inspection are compared with those of inspection by TV camera
and shown below. 555
- Advantages
The inspection is easy and observations can be made in a short period.
Moreover, the data of inspection can be recorded as images.
Since the inspector works above ground, there is no chance of oxygen
deficiency or accidents by fall, and the work is safe. 560
- Disadvantages
The scope of inspection is limited to the area around the mouth of the pipe.
Offset in the horizontal direction or fine cracks cannot be detected.
The condition of the side surfaces in the sewer pipe cannot be grasped (sides
cannot be viewed). 565
Inspection: Surrounding / cover of manhole
Inspection: Inside of manhole
Work completion
Source:JASCOMA, 2007
Figure 2.6 Pole-mounted inspectioncamera
Figure 2.7 Illustration of pole-mounted TV camera inspection 570
2.2.5.1.3 Inspection using Closed-Circuit Television (CCTV)
Pipes that can be inspected by CCTV have inside diameters ranging from 150 mm to 900 mm,
but large diameter pipes may also be inspected by CCTV.
a. Types of TV Camera
TV camera may be the travelling type or the towed type. Either the direct method (taking 575
panoramic shots of the overall scene) or the side view method of taking local shots of only
abnormal locations may be used. House connection TV is described in a separate section
(2.10.1).
Figure 2.8 shows a sketch of the TV camera and the vehicle on which it is loaded. Figure
2.9 shows an illustration of the TV inspection work. 580
Final Draft 2-10
Source: EPA, 2003
Figure 2.8 Step van CCTV system
Figure 2.9 Illustration of CCTV camera inspection 585
- Features of TV camera inspection
By merely opening a manhole at one location, inspection using the travelling
TV camera is enabled.
Continuous inspection up to a maximum distance of 100 to 200 m (cable length)
is possible. 590
Connections of the lateral sewer and main sewer and defective locations should
be photographed by the side view method.
- TV camera inspection work procedure is shown in Figure 2.10.
Figure 2.10 Work procedure for TV camera inspection 595
b. CCTV Camera Inspection Record
Abnormalities detected in the pipeline during the CCTV camera inspection should be
recorded on video tape or as photographs, according to the judgement criteria. Figure 2.10
and Figure 2.11 show examples of the record.
The inspected results should be recorded in the inspection record. Examples of forms of the 600
inspection record are shown in Figure 2.13.
Figure 2.11 CCTV data No1
Preparatory process
Cleaning process
Sewer inspection with
TV camera
Find out, evaluate, and record faults
Report preparation
Figure 2.13 Forms of inspection record
2.2.5.2 Inspecting Infiltration of Water
If infiltration of water is more corresponding to the planned water flow in the sewerage plant,
the pipelines and treatment facilities will be adversely affected. This also leads to an increase in 615
the treatment costs of the wastewater treatment plant.
The cause of infiltration of water is either the pipeline is inadequate or the drainage system is
inadequate. For this reason, inspection of cross connections, flow rate inspection and
waterproofing inspections need to be combined and the route of infiltration water should be
checked. Flow rate inspections help since useful data for improvements and modifications to the 620
piping facilities can be collected.
2.2.5.2.1 Inspecting Cross Connections
Inspection has to be performed to check that storm water equipment is not connected to the
wastewater system in a separate sewer system. The scope of work is from the main pipe of the
sewerage works to the house drainage facility. 625
There are three typical methods for inspecting cross connections.
a. Smoke Test
The smoke test makes use of a smoke bomb in a separate sewerage systempipeline. A cross
connection can be judged by checking for smoke rising from the house inlet or rain gutter.
This test identifies locations where storm water inlet or rain gutter drainage equipment is 630
directly connected to wastewater pipe house inlet, and locations where storm water that has
permeated the ground from the ground surface or gutter has indirectly permeated the
wastewater pipe or house inlet. Figure 2.14 shows the materials to be used. Figure 2.15
shows illustrative sketches.
Figure 2.14 Materials for smoke test
Figure 2.15 Illustrative sketches of smoke test 640
- Features of the smoke test
The status of connection of drain pipes in each space can be checked in a short
Inadequacies in the house drainage facility can be quickly detected.
- Smoke test procedure shown in Figure 2.16. 645
Manhole Smoke Blower
Figure 2.16 Work procedure of smoke test
b. Echo Sound Test
This is a method for confirming that piping facilities are correctly connected, and is also an
effective method for knowing the plumbing systems and the routes of wastewater mains 650
and lateral sewers. Ultrasonic waves are used (transmitter and receiver).
Figure 2.17 shows the test method.
Figure 2.17 Illustration of echo sound test
- Features of the echo sound test 655
Simple method to confirm that a pipe has been connected or not.
Have a meeting
Inspect the site
Prepare for starting
Working preparation
Take some security measurements
Measure gases by gas detectors
Prepare to start blowing smoke
Detect smoke rising points
Locate miss-connected pipes
Record the sewer line of problem
Go to next section
Ventilate by a blower
Notify completion to residences
Clean up the site.
Notification / greeting
to household
Main work
Start blowing
Stop blowing
Effective especially in the connections of lateral sewers.
Cannot judge clogging or trap.
- Echo sound test procedure is shown in Figure 2.18.
Figure 2.18 Work procedure of echo sound test
c. Dye Test
Harmless fluorescent dye diluted water is made to flow from the upstream side to the
treatment plant within the range of wastewater main, lateral sewer and drainage 665
equipment. The water flow route, leakage route and reaching time are examined. Outflow
route should be checked quickly if notification of foul odour due to outflow of
wastewater particularly from masonry or pump tank is received. The method can also be
used for checking the flow status in the pipe and for measuring the flow velocity. The test
method is shown in Figure 2.19. In addition clause 3.10 of Part A manual also deals with 670
dye tracer study.
Meeting with a maintenance engineer
Site inspection
Making a construction schedule
Collecting materials / documents
If necessary
Introduce guards
Measure gasses by gas detectors
Echo sound test
Investigate connecting conditions
Locate miss-connected points
Record the points of problem in a field note
Compile data collected in the test
Summarize records in charts
Make a photo album
Renew charts and records in ledger, etc.
Notification / greeting to
surrounding residents
Figure 2.19 Drawing of dye test
2.2.5.2.2 Flow Rate Inspection
- About important areas for inspecting flow rate 675
Inspection should be carried out at locations where possibility of infiltration is high, e.g.
where groundwater level is high, at a part of a river crossing, or at a part adjacent to rivers.
a. Flow Rate Measurement
Simple flow velocity meters (Palmer Bowlus flume, electromagnetic flow meter, water
level gauge, ultrasonic flow meter) should be installed tentatively at the mouth of the 680
manhole for flow measurements in the piping facilities and fixed period measurements
carried out.
For details of flow meters, refer to Sec. 3.10 of Chapter 3 of Manual Part-A.
b. Pumping Test
This is a method for measuring the flow rate of water that has infiltrated the pipeline. The 685
flow rate of infiltrated water into the space or the system can be known within a short time.
However, the flow rate of infiltrated water varies with the variation in groundwater,
therefore, precipitation and weather at the time of measurement should be confirmed.
To drain out household wastewater from the test during inspection of each space, a cut-off
plug should be installed. This should preferably be implemented during the night time 690
when the volume of household wastewater generated is small. Figure 2.20 shows the
pumping test.
Figure 2.20 Pumping test
- Features of measurements during pumping test 695
The flow rate of infiltrated groundwater for each space or system can be
measured within a short time.
The measured values differ widely depending on the variation in the
groundwater level.
During measurements of several spaces or each system, it is difficult to remove 700
household wastewater late at night.
- Work procedure for measurement of pumped water is shown in Figure 2.21.
Figure 2.21 Work procedure of pumping test
2.2.5.3 Inspecting Corrosion and Deterioration 705
The status of deterioration or corrosion within pipe should be judged by TV camera. The
materials in the piping facility are of various kinds: concrete pipe, ceramic pipe, hard polyvinyl
chloride, brick, HDPE pipe, ductile pipe, and GRP (Glass Fibre Reinforced Plastics) pipe; so the
corrosion and deterioration conditions vary.
Methods for inspecting corrosion and deterioration conditions of a sewer include the following: 710
- Inspection by TV camera of the wall surface condition
- Crack inspection
- Neutralization test
The causes of deterioration structural concrete parts of the piping facilities are mainly the
following: 715
Crack in concrete due to concentrated loads (live loads)
Deterioration of structure due to changes with aging
Deterioration of concrete structures (concrete corrosion) due to sulphuric acid
from the generation of hydrogen sulphide
2.2.5.3.1 Concrete Corrosion 720
In a facility where wastewater resides for a long period, such as a rising main or an inverted
siphon, the wastewater is likely to become anaerobic and dissolved sulphide will be generated,
which leads to concrete corrosion because of sulphuric acid. Locations where concrete corrosion
Cut off with plugs
Install measuring equipment
Inspection: Pumping test
is likely to occur in sewerage piping facilities are as given below.
Locations where corrosion is likely to occur: 725
- Piping facilities at the discharge destination of pressure pipe (including manhole
pump)
- Upstream and downstream ends of locations where sump discharge occurs
- Upstream and downstream ends of locations where discharges containing sulphide
occurs 730
- Locations downstream of inverted siphon
For details of the corrosion mechanism, refer to the Part AManual (Chap 3).
a. pH Measurement of Concrete Surfaces
For sulphuric acid corrosion of concrete, the pH on the surface of concrete is measured by
pH test paper or pH meter which is placed on concrete surface directly, and the generation 735
of sulphuric acid in the concrete structure can be confirmed. When sulphuric acid is
generated in a manhole, the pH of the concrete surface may indicate strong acidity of 1 to 2.
b. Neutralization test (neutralization depth test by phenolphthalein)
One of the indices for judging durability of reinforced concrete structures is the
neutralization depth. This is the method of judging alkalinity with pH of 10 and above as 740
un-neutralized part and uncoloured parts as neutralized parts, enabling quantitative
information to be obtained easily by simple measurement.
When neutralization reaches the vicinity of the reinforcement, the reinforcement is likely to
be corroded easily. When corrosion of reinforcement progresses, volume expansion of
corrosive products causes crack or delamination in concrete, leading to excessive loss in 745
the durability of the structure.
For details of the procedure, refer to BS-103:Guidelines on Non-destructive Testing of
Bridges.
Source:JASCOMA, 2007 750
Figure 2.22 Neutralization test
2.2.5.4 Other Examinations
Special examinations to study in detail the conditions of a facility are as given below. For more
details, please refer to relevant documents for each item. Various kinds of information relevant
to analysis for studying gas exploration are given. 755
- Sewer invert elevation examinations: Understanding pipeline conditions and
collating with sewerage facility records.
- Sediment examination: Check sediment material, such as sand and silt, which may
have entered damaged sewer or through loose joints from outside the sewer. This
sand and silt may accumulate around the sewer and form voids. 760
- Dangerous gas detection examination: Confirming gases generated in the piping
Water quality and gases encountered in a piping facility are closely related. Table 2.6 shows the
gas analysis items in a piping facility.
Table 2.6 Gas analysis 765
Analysis items Unit
Carbon monoxide CO ( % )
Carbon dioxide CO
Hydrogen sulphide H
S (ppm)
Ammonia NH
Oxygen O
Methane CH
Nitrogen oxide NO
Source: JASCOMA, 2007
Related to O&M of sewerage system, serious accidents due to gases generated from sewerage
system are illustrated in Figure 2.23. (Accidents during O&M of Japanese sewerage system: For
ten years from 2002 to 2012.)The causes of the gas-related accidents were hydrogen sulphide,
carbon monoxide, and oxygen deficiency. 770
There is no data on accidental death in India and at least there should be such monitoring in
Number of deaths on fatal accidents related
gas of sewer maintenance work in Japan
Oxygen deficiency
Figure 2.23 Fatal accident related to gas in Japan sewer works 775
2.2.5.5 Precautions
Cleaning equipment and machinery for sewers are shown in following sections:
When entering manholes, safety measures during the work should to such as giving
consideration to traffic safety, oxygen deficiency, poisoning due to toxic gas such as hydrogen
sulphide, and so on 780
For securing workers safety, manual sewer/septic tank cleaning should be avoided because
persons are likely to come in contact directly with sludge and sewage.
Therefore, cleaning machinery and equipment which avoid manpower cleaning are needed.
Furthermore, necessary safety measures before entering manholes for cleaning should be taken.
Machinery and equipment for sewer pipes are explained in the next section, and explanations 785
on Cleaning of on-site systems are in Chapter 10.
The contamination of drinking water with sewage may occur when water supply pipe passes
through sewer manholes, generally in narrow streets, especially when water supply pipe joints
are enclosed in sewer manholes and when ever water supply pipe joints leak, contamination of
drinking water supply occurs. As such, water supply pipe lines should never be enclosed in a 790
sewer manhole. If any, such situation is observed, immediately water supply pipe be made
non-functional by stopping flow of drinking water and effected public be supplied clean
drinking water by other temporary means, such as water tankers or laying G.I. pipe over the
ground/road surface and portion of water supply lines lying in sewer manholes be shifted out of
manholes. 795
Special attention should be paid to decentralized sewer system particularly when small bore
sewer system or shallow sewer system is adopted.
2.2.6 Judgement of Inspection and Examination Results
It is necessary to judge whether urgent repairs or modifications are necessary, or normal
operation and maintenance are sufficient to ensure that the functions of piping facilities are 800
maintained when an abnormality is detected by studies and analyses. The facility manager
should make the judgment considering material of the pipe, age of the pipe, location where
buried, quality of wastewater, status of groundwater, regional environment, and so on.
The criteria given below may be used as judgment criteria.
- Emergency response criteria 805
- Judgment based on results of inspection or examination
- Testing criteria
2.2.6.1 Emergency Response Criteria
Abnormalities related to piping facilities are generally detected from inspections or from outside
reports. 810
Prompt action should be taken when an accident has already occurred. Also, when the events
below are confirmed, action should be taken immediately.
- Road surface: Irregularity exists that can cause level difference leading to
subsidence or obstruction to operation.
- Manhole: Level difference exists that can lead to obstruction of operation. 815
- Inverted siphon: Water level on the upstream side is excessively high.
2.2.6.2 Judgement based on the Results of Inspection and Examination
Testing of the overall span and by each pipe should be carried out based on the results of visual
inspection. Table 2.7 and Table 2.8 show examples of testing criteria.
The testing of the overall span is divided into the three categories (A, B, and C). 820
Table 2.7 Testing criteria for overall sewer span
1) Corroded pipe
Reinforcing bars
Aggregate exposed Rough surface
2) Vertically
deflected
ID:< 700 mm >ID ID -1/2 ID <1/2 of ID
ID:7001650 mm >1/2 of ID 1/2ID-1/4 ID <1/4 of ID
ID: > 1650 mm >1/4 of ID 1/4ID-1/8 ID <1/8 of ID
Also, the testing of each pipe is divided into the three categories (a, b, and c).
Table 2.8 Testing criteria for each pipe of sewer
3)Ruptured
concrete pipes,
missing/holed pipe
Axial crack of 2 mm
width or more
Axial crack of less
than 2 mm width
Partially missed
/holed pipe
Axial crack shorter
than 1/2 of the pipe
Axial crack of 1/2 of
the pipe length and
4)Cracked
Circumferential crack
of 5 mm width or
Circumferential
crack of 2 mm width
crack of less than 2
mm width
of 2/3 of the pipe
circumference and
crack shorter than
2/3 of the pipe
5)Gap at coupling pipes Joint displacement
pipes, etc
6) Infiltration Splashing in Flowing in Soaking
Source: JASCOMA, 2007 825
Manhole No1 Manhole No2 Manhole No3
Pipe Judge = c Pipe Judge = c Pipe Judge =b
Pipe Judge =b Pile Judge = a Pile Judge = a
Pipe Judge = a
Spane Judge = A Spane Judge = A
Manhole No 1 Manhole No. 2 Manhole No.3
A +(a+b+5c) A + (2a+2b+3c)
Example of Emargency Level -I ( In Table 2.9 )
Manhole No5 Manhole No7
Pipe Judge =b
Pipe Judge = a Pipe Judge = a
Spane Judge = B Spane Judge = B
B + (a+b+5c) B + (a+3b+3c)
Example of Emargency Level -II (In Table 2.9)
Manhole No 10 Manhole No 11 Manhole No 11
Spane Judge = C Spane Judge = B
Manhole No 10 Manhole No.11 Manhole No.12
C + (2b+5c) B + (3b+4c)
Example of Emargency Level -III (In Table 2.9)
Figure 2.24 Illustration of Testing criteria for sewer
2.2.6.3 Testing Criteria
Functional degradation, deterioration and abnormalities clarified by inspection and examination
should be assessed. A maintenance engineer should judge what countermeasure are applied for 830
inspected sewers in accordance with Table 2.9, e.g. by usual operation and maintenance or by
emergency repairs and modifications.
Based on the criteria shown in Table 2.7 and Table 2.8, emergency level I is a state where
immediate response is necessary.
Emergency level II indicates that simple response may be adopted and radical measures 835
implemented within the next five years.
Furthermore, emergency level III indicates response adopted by operation and maintenance, and
implementation of simple response partially.
Table 2.9 Testing criteria for sewer
Category Testing criteria Criteria for measures
Either As are more or as are more
in the testing results
Prompt measures are necessary
As are less frequent and Bs are
more; or as are less and bs are
more in the testing results
Necessary actions may be taken
by simple measures and extended
up to 5 years, if required
No A, Bs are few, Cs are many, or
a and b are few, cs are many in the
testing results
Actions may be taken by simple
measures and extended up to 5
years, if required
As, Bs, and Cs are judgement results of Table 2.7, and as, bs, and cs are judgment results of 840
Table 2.8.
2.2.7 Maintenance of Records and Follow up Action
To reflect the inspection and testing results in appropriate operation and maintenance of piping
facilities, the testing results should be recorded and stored in the format shown here.
a. Inspection Sheet 845
When inspections and examinations are implemented, an inspection sheet should be
prepared and recorded as shown in Table 2.10.
Table 2.10 Inspection sheet
Inspection Sheet No.
(Manhole No. etc)
Inspection Date Inspector
Abrasion, backlash, difference in level, invaded pavement,
damaged, location unknown
Corrosion, damage to the floor, infiltration, metal steps corroded,
inferior pipe end, rubbish, odour
Pipe Corrosion, damage, coupling displacement, inadequate inclination,
infiltration, roots of trees, earth, sand, and mortar, road subsidence
House inlet Cover (no damage), difference in level, corrosion, damage,
damaged invert, earth and sand, location unknown, odour
Lateral Damage, displacement, earth and sand, road subsidence
Measurement Necessary
Date of order
Date of schedule
b. Log 850
Log should be used to record daily work results, which can be used in the operation and
maintenance of piping facilities. Format is shown in Table 2.11.
c. Monthly Reports
Daily record should be summarised in monthly reports. Format of monthly report is shown
in Table 2.12. 855
Part B: Operation and Maintenance CHAPTER2: SEWER SYSTEMS
Table 2.11 Daily report
Daily Report Date Weather
complaint /
Receipt No.
Location: address Work description Inspector Tool/Material Remarks
diameter (mm) Name of cleaned area Daily total
CrewA
Crew B person
Removed sand volume
System No:
Cleaned distance m
Direct works
Name of place
repaired
Name of drainage
Entrusted works
System No. Tool/Material Daily total
Work description
Table 2.12 Monthly report
Monthly Report Date Weather
Response to
Category 1.Lateral 2.Inlet 3.Manhole
4.Ground
subsidence
5.Odour 6.Others Total
Diameter (mm) Total
Direct crew
Entrusted crew
Removed sand
Cleaned distance
Manhole type 1 2 3 4 5 6 7 Special Direct Entrusted Total
Cover replaced
Ring repaired
Barrel repaired
2.3 Sewer Cleaning 860
To operate and maintain a sewer collection system to function as intended, the maintenance
engineer should try to strive towards the following objectives:
- Minimize the number of blockages per unit length of sewer, and
- Minimize the number of odour complaints.
For this purpose, sewer cleaning using hydraulic or mechanical cleaning methods needs to be 865
done on a scheduled basis to remove accumulated debris in the pipe such as sand, silt, grease,
roots, and rocks. If debris is allowed to accumulate, it reduces the capacity of the pipe and
blockage can eventually occur resulting in overflows from the system onto streets, yards, and
into surface waters. Roots and corrosion also can cause physical damage to sewers.
2.3.1 Cleaning Equipment and Procedures 870
Sewer cleaning works require usual implements like pick axes, manhole guards, tripod stands,
danger flags, lanterns, batteries, safety lamps, lead acetate paper, silt drums, ropes, iron hooks,
hand carts, plunger rods, observation rods, shovels etc.
In addition, sewer cleaning work calls for the following special equipment and devices like a
portable pump set running on either diesel or petrol engine, rope and cloth balls, sectional sewer 875
rods, a sewer cleaning bucket machine, a dredger, a rodding machine with flexible sewer rods
and cleaning tool attachments such as augers, corkscrews, hedgehogs and sand cups, scraper,
and hydraulically propelled devices such as flush hags, sewer balls, wooden bail and sewer
scooters, sewer jetting machine, gully emptiers and pneumatic plugs.
2.3.1.1 Manila Rope and Cloth Ball 880
The most common way of cleaning small diameter sewers up to 300-mm diameter is by the use
of a manila rope and cloth ball. Flexible bamboo strips tied together are inserted in the sewer
line by a person on top. If necessary, another person inside the manhole helps in pushing the rod
through the sewer line. When the front end of the bamboo strip reaches the next manhole, a
thick manila rope, with cloth ball at one end, is tied to the rear end of the bamboo splits. The 885
bamboo splits are then pulled by another person in the downstream manhole and pushed through
the sewer line. As the rope is pulled, the ball sweeps the sewer line and the accumulated grit is
carried to the next manhole where it is removed out by means of buckets. This operation is
repeated between the next manholes until the stretch of sewer line is cleaned.
2.3.1.2 Sectional Sewer Rods 890
These rods are used for cleaning small sewers. The sewer rods may be of bamboo or teakwood
or light metal usually about one meter long at the end of which is a coupling which remains
intact in the sewer but can be easily disjointed in the manhole. Sections of the rods are pushed
down the sewer. The front or the advancing end of the sewer rod is generally fitted with a brush,
a rubber ring for cleaning or a cutting edge to cut and dislodge the obstructions. These rods are 895
also useful to locate the obstruction from either manhole in case a particular portion of the sewer
has to be exposed for attending to the problem.
2.3.1.3 Sewer Cleaning Bucket Machine
The bucket machine consists of two powered winches with cables in between. For cleaning a
section of sewer; the winches are centred over two adjacent manholes. To get the cable from one 900
winch to the other, it is necessary to thread the cable through the sewer line by means of sewer
rods or flexible split bamboo rods. The cable from the drum of each winch is fastened to the
barrel on each end of an expansion sewer bucket fitted with closing device, so that the bucket
can be pulled in either direction by the machine on the appropriate end. The bucket is pulled
into the loosened material in the sewer until the operator feels that it is loaded with debris. The 905
winch is then thrown out of gear and the opposing winch is put into action. When the reverse
pull is starts, the bucket automatically closes and the dirt is deposited in a truck or a trailer. This
operation is repeated until the sewer is cleared. Various bucket sizes are available for sewers of
150 mm to 900 mm in size. The machine is also used along with other scraping instruments for
loosening sludge banks of detritus or cutting roots and dislodging obstructions (Figure 2.25). 910
Figure 2.25 Power bucket machine setup
2.3.1.4 Dredger (Clam-shell)
It consists of a grab bucket on a wire rope which is lowered into the manhole in the open 915
condition with the help of a crane and pulley. On reaching the bottom of the manhole the
segments are closed, and the accumulated silt is picked up. The bucket is then raised above
ground level where the bucket opens and the silt is automatically dropped into a truck or a trailer.
The bucket can be closed by wire ropes or by a pneumatically operated cylinder. The
disadvantage in this system is that it cannot clean the corners of the catch pits of manholes. 920
Sometimes the deposits at tire corners may become so hard that the same may be required to be
chiselled out.
2.3.1.5 Rodding Machine with Flexible Sewer Rods
This consists of a machine which rotates a flexible rod to which is attached a cleaning tool such
as auger, corkscrew or hedgehog and sand cups (Figure 2.26). The flexible rod consists of a 925
series of steel rods with screw couplings. The flexible rod is guided through the manhole by a
bent pipe. The machine rotates the rod with the tool attached to one end, the other end being
fixed to the machine. The rotating rod is thrust into the bent pipe manually with clamps with
long handles for holding the rod near the couplings. As the rod is thrust inside, the machine also
is drawn towards the manhole. The rod is pulled in and out in quick succession when the tool is 930
engaging the obstruction, so as to dislodge or loosen it. When the obstruction is cleared, the rod
is pulled out by means of clamps keeping the rod rotating to facilitate quick and easy removal.
The various tools attached to the rods are shown in Figure 2.27.
Source: EPA, 2003 935
Figure 2.26 Power rodding operation
Figure 2.27 Rodding heads
2.3.1.6 Scraper 940
This method is used for sewers of diameter larger than 750 mm. The scraper is an assembly of
wooden planks of slightly smaller size than the sewer to be cleaned. If the scraper cannot be
lowered through the opening of manhole, it has to be assembled inside the manhole. The scraper
chains, attached to a control chain in the manhole into which it is lowered, are then connected to
a winch in the next downstream manhole by means of chains. The winch is then operated to 945
push the debris ahead of the scraper. The heading up of the flow behind the scraper and the
water dropping from the top of the scraper will also assist in pushing it in the forward direction.
This-ensures that the bottom and the sides of the sewer are cleaned thoroughly. The scraped
debris is removed manually.
Circular scrapers are used on small sewers below 350 mm diameter for cleaning the body of the 950
line. They are commonly known as discs and these discs are either collapsible and made of
metal or a wooden pair separated by about 200 mm by steel rods.
2.3.1.7 Hydraulically Propelled Devices
The hydraulically propelled devices take advantage of the force of impounded water to
effectively clear sewers. Efficiency depends on the hydraulic principle that an increase in 955
velocity in a moving stream is accompanied by a greatly increased ability to move entrained
material. The transporting capacity of water varies as the sixth power of its velocity.
a. Flush Bags
A very effective tool for cleaning portions of sewers where rods cannot be used is the sewer
flusher or flush bag. The flusher is a canvas bag or rubber bag equipped with a fire hose 960
coupler at one end and a reducer at the other end. The flusher is connected to the fire hose
and placed in the downstream end from the point where a choke is located. The bag is
allowed to fill up until it expands and seals the sewer. The upstreampressure built up due to
this damming effect breaks loose the obstructions.
b. Sewer Balls 965
These are simple elastic pneumatic type rubber balls which can be blown up to varying
degrees of inflation. They are manufactured in sizes from 150 mm to 750 mm diameter
when fully inflated. When used in cleaning a sewer, the bail is first inflated and then
wrapped in a canvas cloth, the edges of which are sewed together. A trial line, little longer
than the distance between the manholes, is attached securely to the covering. The size of 970
the ball and the covering shall be such as to fit fairly snugly into the sewer. Immediately
after the ball is thrust into the sewer, sewage commences to back up in the manhole and
continues to rise until such time as its pressure is great enough to force sewage under the
ball and move it downstream through the pipe. Acting as a compressible floating plug, it
affords enough obstruction, so that a continuous high velocity jet spurts under and to some 975
extern around the ball, thereby sluicing all the movable material ahead to the next manhole.
If the ball encounters an obstruction which is immovable, the ball merely indents to the
necessary degree and moves forward. The only fixed obstruction which will stop the
forward progress of the ball is a root mass or some similar obstruction tightly wedged into
the pipe. Bricks, stones, bottles, loose metal parts, broken pieces of pipes, sand, gravel and 980
settled sludge are easily moved ahead. If the ball stops momentarily, a pull on the trial line
is usually sufficient to set it in motion again. If the pipe is very dirty, the trial line can be
tied to a step in the upper manhole and the ball's progress can be retarded to the required
degree as the lower manhole is reached, thus giving time for complete removal of
accumulated silt and debris which has piled up ahead of the ball. Equipment arrangement is 985
shown in Figure 2.28 and Figure 2.29.
Figure 2.28 Typical setup for hydraulic cleaning using sewer ball
Figure 2.29 Balling equipment
A wooden ball, also called a sewer pile, can also be used for this purpose, particularly for
cleaning large outfall sewers. It is dropped into the sewer and owing to its buoyant action
rolls along the invert of the sewer. The obstructions caused by it to the flow produce a
vigorous scouring action along the invert and the sides which has the effect of removing 995
tree growths and the deposits fromthe sewers. This method is economical and hence can be
used at frequent intervals.
c. Sewer Scooters
This arrangement is an improved version of the scraper and consists of two jacks, a
controlling rope and the scooter with a tight fitting shield. In contrast to the scraper, the 1000
scooter completely stops any flow of sewage. The scooter, attached to the control rope, is
lowered into the manhole and then into the downstream sewer line. The downstream
manhole jack is lowered into place from the road and the upper manhole jack set across the
top of the manhole.
When the scooter is introduced in the line, it stops the flow of sewage thus building up a 1005
head behind the shield. The resulting pressure causes the scooter to move through the sewer
until it accumulates enough debris to stop its movement. The head is then allowed to build
up approximately one meter before the control rope is pulled, causing the shield to fold
back, thus allowing the accumulated sewage to gush into the sewer downstream, flushing
the debris ahead to the next manhole from where it is removed. The control rope is 1010
released, clearing the shield against the sewage and causing the scooter to advance again
until the debris stops its movement. This process is repeated until the scooter reaches the
downstream manhole where it may be removed or allowed to continue through the next
The operation of the sewer scooter is shown in Figure 2.30. 1015
Figure 2.30 Scooter operation
2.3.1.8 Velocity Cleaners (Jetting Machines)
The high velocity sewer cleaner makes use of high velocity water jets to remove and dislodge 1020
obstructions, soluble grease, gut and other materials from sanitary, storm and combined
sewerage systems. It combines the functions of a rodding machine and gully emptier machine.
Basically it includes a high pressure hydraulic pump capable of delivering water at variable
pressure up to about 80 kg/cm
through a flexible hose to a sewer cleaning nozzle. The nozzle
has one forward facing hole and a number of rear ward facing holes. The high pressure water 1025
coming out of the holes with a high velocity breaks up and dislodges the obstructions and
flushes the materials down the sewer. Moreover by varying the pressure suitably, the nozzle
itself acts as a jack hammer and breaks up stubborn obstructions. (A separate suction pump or
air flow device may also be used to suck the dislodged material).
The entire equipment is usually mounted on a heavy truck chassis with either a separate prime 1030
mover or a power take off for the suction device. The high pressure hose reel is also
hydraulically driven. The truck also carries fresh water tanks for the hydraulic jet and a tank for
the removed sludge and the various controls grouped together for easy operation during sewer
cleaning. The manufacturer's operating and servicing manuals should be carefully followed for
best results in the use of the machine. 1035
2.3.1.9 Suction Units (Gully Emptier)
Suction units create the vacuum required for siphoning of mud, slurry, grit and other materials
from sanitary, storm and combined sewerage systems. The vacuum elevated is such as to siphon
the materials from the deep manholes catch-pits etc., having depth ranging from 1 m to 8 m in
normal cases with an option to suck an additional 4 m with the help of special accessories for 1040
the purpose. The unit can be vehicle or trolley mounted.
Silt and heavy particles settled at the bottom can be agitated and loosened by pressurized air
with the help of the pump and then sucked in a tank. Once the silt tank is full, the effluent is
discharged in the nearby storm water drain or manhole and the operation is repeated until the silt
is cleared off the manhole. The silt deposited in the tank is then emptied at the predetermined 1045
dumping spot.
2.3.2 Notification to STP
Before clearing a large septic stoppage, be sure to notify the operator on duty at the downstream
STP. Septic stoppages develop when the sewer has been blocked for considerable time and/or
the air temperature is hot. Under these conditions the wastewater and organic solids turn black 1050
and smell like rotten eggs. If a large diameter sewer is blocked and a large volume of
wastewater backs up in the pipes, there might not be sufficient fresh water arriving at the
treatment plant to dilute the septic waste water. When a large volume of septic wastewater
reaches a treatment plant, the treatment processes may fail to do their intended job. By notifying
the operator in advance of the location of the stoppage and approximate volume of septic 1055
wastewater flowing towards the treatment plant, the operator can be alerted and can prepare to
minimize the impact on the treatment processes.
2.3.3 Disposal of Silt and Sludge
Sludge from sewers can be disposed of along with grit and sludge of the STP (if available).
Otherwise, the sludge and silt can be co-disposed in an eco-friendly manner with municipal 1060
solid waste.
2.3.4 Cleaning Records and their Utilization
Records of all cleaning operations should be entered and filed for future reference. These
records should include the data, street name or number, line size, distance, and manhole
numbers or identification. Also the kind and amount of materials removed, wastewater flow, and 1065
auxiliary water used should be noted. If particular problems were encountered, these too should
be noted, especially the exact location of obstructions. A record form sample is shown in Figure
During the routine cleaning operations discussed in this chapter, many manholes should be
opened and used for high-velocity cleaning or flushing of sewer. Manhole Inspection form 1070
detailing its location, condition, and any problems observed should be completed. If this is done
each time a manhole is opened during cleaning operations, over time the database for these
structures will include up-to-date information on a high percentage of them and allow better
decisions to be made in regard to routine maintenance, repair, or rehabilitation.
If pieces of broken sewer are removed, a TV inspection may be needed and repairs may need to 1075
be made on the broken sections of pipe.
Recording traffic patterns at a site can be very helpful next time the equipment is set up at the
location. Car park (such as over manholes), traffic volume during rush hours, and whether
police traffic control should be called for help before going to the site, should be indicated.
Computers are being used in many aspects of operation, maintenance, and recordkeeping of 1080
collection system. Computer software packages are available for scheduling preventive
maintenance activities, issuing work orders for repairs, keeping track of where work is done,
who did the work, when, and the labour and materials required. With the correct software, any
information in the computers records can be recalled for future use. Computers are also used to
keep spare parts inventories and to order spare parts when the supply runs low and before they 1085
are needed for scheduled maintenance and repairs.
When marking out records, remember that you or someone else will be referring to them. The
more complete the record, the easier the next operation becomes since you have a history of this
sewer.
Figure 2.31 Sewer cleaning records
2.4 Sewer Rehabilitation
Deterioration of sewers proceeds over the surface as a whole, and repair takes considerable time, 1095
therefore, it is necessary to implement renewal and repair according to a plan on the basis of the
results of inspection and examinations. This practice will prevent accidents.
In older cities, most sewers though developed have already exceeded the service life. In such
cities, adequate renewal and repair may resolve urgent problems and help extend the service life
of the facilities, reducing O&M expenses. 1100
The two terms renewal and repair are clearly segregated as follows. Renewal is not included in
O & M duties but in construction because the time of implementation is the starting point of the
new service life and changes must be made to fixed assets
a. Renewal
The term means improvement and replacement of facilities not caused by expansion of 1105
drainage area.
Improvement: Reconstruction or replacement of the facility that has not yet reached the
specified service life.
Replacement: Reconstruction or replacement of the facility that has reached the specified
service life. 1110
b. Repair
This refers to partial replacement or repair of damage to the facility. Repair provides
utility but not an increase in functions, so it does not contribute to extension of the service
life of the facility. Repair simply maintains the capacity and life and does not cause a
change in fixed assets. 1115
However, making a clear distinction between O&M and construction duties is often difficult for
implementation of renewal and repair according to the plan. In certain cases, it is therefore
desirable to plan these duties as one package. Improvement of functions of existing sewers
while incorporating elements related to planning and construction projects is generally called
rehabilitation. The definition of terms related to rehabilitation is given in Table 2.13. 1120
Table 2.13 Definition of terms
Source: JICA, 2011
2.4.2 Rehabilitation Method
Under the traditional method of sewer relief, a replacement is made or additional parallel sewer
line is constructed by digging along the entire length of the existing pipeline, while these 1125
traditional methods of sewer rehabilitation require unearthing and replacing the deficient pipe
(the dig-and-replace method), trenchless methods of rehabilitation use the existing pipe as a host
for a new pipe or liner. Trenchless sewer rehabilitation techniques correct pipe deficiencies that
require less restoration and cause less disturbance and environmental degradation than the
traditional dig-and-replace method. Trenchless sewer rehabilitation methods include: 1130
Terms Definition Classification
Rehabilitation,
All concepts to improve functions of existing
sewer pipes.
All measures
Repair Repair
Repair of structural damage of partial renewal of
Renovation Renovation
Functional improvement of a certain section
while utilizing the existing pipe structures
Renewal Renewal
Renewal of new pipes, with basic functions and
capacities remaining equal to original pipes.
Replacement Replacement
Replacement with new pipes to reinforce
functions and capacities
Installation of new pipes to enhance the flow
capacity of the entire system.
- Pipe bursting or in-line expansion
- Slip lining
- Cured-in-place pipe
- Modified cross-section liner
These alterative techniques must be fully understood before they are applied. These four sewer 1135
rehabilitation methods are described in detail in the following sections.
2.4.2.1 Pipe Bursting or In-line Expansion
Pipe bursting or in-line expansion is a method by which the existing pipe is forced outward and
opened by a bursting tool. During in-line expansion, the existing pipe is used as a guide for
inserting the expansion head (part of the bursting tool). The expansion head, typically pulled by 1140
a cable rod and winch, increases the area available for the new pipe by pushing the existing pipe
radially outward until it cracks. The bursting device pulls the new pipeline behind itself. The
pipe bursting process is illustrated in Figure 2.32.
Source: JICA, 2011 1145
Figure 2.32 Pipe bursting process
2.4.2.2 Slip Lining
Slip lining is a well established method of trenchless rehabilitation. During the slip lining
process, a new liner of smaller diameter is placed inside the existing pipe. The annular space, or
area between the existing pipe and the new pipe, is typically grouted to prevent leaks and to 1150
provide structural integrity. If the annulus between the sections is not grouted, the liner is not
considered a structural liner. Continuous grouting of the annular space provides the seal.
Grouting only the end-of-pipe sections can cause failures and leaks. In most slip lining
applications, manholes cannot function as proper access points to perform the rehabilitation. In
these situations, an insertion pit must be dug for each pipeline segment. Due to this requirement 1155
in most applications, slip lining is not a completely trenchless technique. However, the
excavation required is considerably less than that for the traditional dig-and-replace method.
System and site conditions will dictate the amount of excavation spared. Methods of slip lining
include continuous, segmental and spiral wound methods. All three methods require laterals to
be re-connected by excavation or by a remote cutter. In continuous slip lining, the new pipe, 1160
jointed to form a continuous segment, is inserted into the host pipe at strategic locations. The
installation access point, such as a manhole or insertion pit, must be able to handle the bending
of the continuous pipe section. Installation by the segmental method involves assembling pipe
segment at the access point. Slip lining by the segment method can be accomplished without
rerouting the existing flow. In many applications, the existing flow reduces frictional resistance 1165
and thereby aids in the installation process. Spiral wound slip lining is performed within a
manhole or access point by using interlocking edges on the ends of the pipe segments to connect
the segments. The spiral wound pipe is then inserted into the existing pipe as illustrated in
Figure 2.33.
Figure 2.33 Spiral wound slip lining process
2.4.2.3 Cured-in-place Pipe
A typical cured-in-place pipe (CIPP) process by the water-inversion method is illustrated in
Figure 2.34. During the CIPP renewal process, a flexible fabric liner coated with a thermosetting 1175
resin is inserted in the existing pipeline and cured to form to a new liner. The liner is typically
inserted in the existing pipe through an existing manhole. The fabric tube holds the resin in
place until the tube is inserted in the pipe and ready to be cured. Commonly manufactured resins
include unsaturated polyester, vinyl ester, and epoxy, each having distinct chemical resistance to
domestic sewage. The CIPP method can be applied to rehabilitate pipe lines with defects such as 1180
cracks, offset joints, and structurally deficient segments. The thermosetting resin material bonds
with the existing pipe materials to form a tighter seal than most other trenchless techniques. The
two primary methods of installing CIPP are winch-in-place and invert-in-place. These methods
are used during installation to feed the tube through the pipe. The winch-in-place method uses a
winch to pull the tube through the existing pipeline. After being pulled through the pipeline, the 1185
tube is inflated to push the liner against the existing pipe walls. The more typically applied
inversion-in-place method uses gravity and either water or air pressure to force the tube through
the pipe and invert it, or turn the tube inside out. This process of inversion presses the
resin-coated tube against the walls of the existing pipe. During both the winch-in-place and
invert-in-place methods, heat is then circulated through the tube to cure the resin to form a 1190
strong bond between the tube and the existing pipe.
Figure 2.34 Cures-in-place pipe installation procedure
2.4.2.4 Modified Cross-section Lining 1195
The modified cross-section lining methods include deformed and reformed methods,
swagelining TM, and roll down. These methods either modify the pipes cross sectional profile
or reduce its cross-sectional area so that the liner can be extruded through the existing pipe. The
liner is subsequently expanded to conform to the existing pipe's size. During deformed and
reformed pipeline renewal, a new flexible pipe is deformed in shape and inserted into the host 1200
pipe. While the method of deforming the flexible pipe varies, with many processes referred to as
fold and form methods, a typical approach is to fold the new liner into a U shape, reducing
the pipe's diameter by about 30 per cent. After the liner is pulled through the existing line, the
liner is heated and pressurized to conform to the original pipe shape. Another method of
obtaining a close fit between the new lining and existing pipe is to temporally compress the new 1205
liner before it is drawn through the existing pipeline. The swagelining TM and roll down
processes use chemical and mechanical means, respectively, to reduce the cross-sectional area
of the new liner. During swagelining TM and a typical draw-down process, the new liners are
heated and subsequently passed through a reducing die. Achemical reaction between the die and
liner material temporarily reduces the liner's diameter by 7 to 15 percent and allows the liner to 1210
be pulled through the existing pipe. As the new liner cools, it expands to its original diameter.
The roll down process uses a series of rollers to reduce the pipe liner's diameter. As in
deform-and-reform methods, heat and pressure are applied to expand the liner to its original
pipe diameter after it has been pulled through the existing pipe. Unlike CIPP, the modified
cross-section methods do not make use of resins to secure the liner in-place. Lacking 1215
resin-coated lining, these methods do not have the curing time requirement of CIPP. Atight fit is
obtained when the folded pipe expands to the host pipe's inside diameter under applied heat and
pressure. As with the CIPP method, dimples are formed at lateral, junctions and similar methods
of reconnecting the laterals can be employed. Materials typically used for modified
cross-section linings include Polyvinyl Chloride (PVC) and High Density Polyethylene 1220
(HDPE).
2.4.3 Maintenance of Machinery and Apparatus for Rehabilitation
Emergency cleaning and a repair are required in case of an emergency response. Therefore, a
maintenance engineer should repair machinery and equipment to the original. In addition, he
should have enough maintenance and repair materials required (for example pipes, lid, the 1225
mounting tube).
Also, he should stock construction materials such as sand, rock crushing, and asphalt for the
cave-in repair of roads.
He should ensure that the materials, equipment and facilities, and necessary safety equipment
are in standby state at all times. 1230
2.5 Protection of Sewer Systems
A sewer may get damaged if other facilities such as water pipe or electric cable work are done
beside or at the cross-section of a sewer. Especially, fluctuations due to ground excavation (pile,
underground water drops, and pile method) may have a serious impact.
To avoid damage against sewer, the maintenance engineer should do the following: 1235
- Collect related information about the constructions which are planned around the
sewer location,
- Advise appropriate construction methods to minimize impact for sewer, and
- If necessary, request to adopt the protective measures for sewer prior to the work
Typical protective measures are as follows:
Protection for existing sewer (an example is shown in Figure 2.35)
Temporary laying of sewage pipe
Changing sewer material in advance
Figure 2.35 Protection method for existing sewer
2.6 Protection against Infiltration and Exfiltration
Infiltration and inflow, while overlooked in many collection systems for decades, have now
gained recognition as major defects that can cause failure of a collection system. In most cases,
this failure results in hydraulic overloads (too much water) of the collection system or the 1250
sewage treatment plant.
In the case of a collection system, hydraulic overloads result in surcharged manholes,
overflowing manholes and exposure of community to diseases and pollutants carried by the
wastewater in a collection system. This type of failure is also known as a sanitary sewer
overflow. 1255
In the case of an STP, infiltration and inflow can result in plant loads exceeding the plant
capacity. Bypassing raw sewer to the environment has been the only answer in the past, but this
practice is no longer allowed.
2.6.1 Measures against Infiltration of Rainwater
Inflow detection and collection depend upon the type and source of inflow causing the problem. 1260
Inflow is water that is not polluted and should not be in a wastewater collection system. Inflow
is water that enters a sewer as a result of a deliberate illegal connection or by deliberate drainage
of flooded areas into a wastewater collection system.
In many areas the main line portion of the collection system is relatively tight. Amajor source of
infiltration in this situation can be the house service lines. They can be tested for leaks using 1265
smoke tests and by development of small cameras and robotic equipment.
Collection or elimination of inflow/ infiltration depends on the type and location of the source
of problem. Typical solutions to inflow/infiltration problems include:
a. Manholes
Raise rim elevation by use of grade rings if not located in streets (inflow). 1270
Install watertight covers where needed (inflow).
Install inflow protection covers (inflow).
Seal covers (inflow).
Seal or repair barrels (infiltration).
b. Sewer Pipes (Infiltration) 1275
Seal segment of damaged pipes and joints.
Dig up and replace damaged pipes and joints.
Line sewer with a plastic liner and/or fibre liner material.
2.6.2 Measures against Exfiltration of Untreated Sewage
Exfiltration is the leakage of wastewater out of the collection system through broken or damage 1280
pipes and manholes. All wastewater collection systems, except some constructed in recent years,
have many leaks. These systems may exfiltrate wastewater through defective pipe joints and
cracks. The wastewater that does exfiltrate may contaminate shallow wells or open ditches
where children and pets play. To make an old collection system airtight would be extremely
expensive and not very cost-effective. Major points of infiltration or exfiltration in a collection 1285
system can be identified by the use of television or smoke testing and can then be corrected.
Source: EPA/600/R-01/034
Figure 2.36 Sewerage leaking locations
The proper selection of corrective or rehabilitation methods and materials depends on a 1290
complete understanding of the problems to be corrected, as well as the potential impacts
associated with the selection of each rehabilitation method. Pipe rehabilitation methods to
reduce exfiltration (and simultaneously infiltration) fall into one of the two following categories:
- External Rehabilitation Methods
- Internal Rehabilitation Methods 1295
Certain conditions of the host pipeline influence the selection of the rehabilitation method. It is
therefore necessary to assess these factors to prepare the pipe for rehabilitation. Rehabilitation is
proceeded by surface preparation by cleaning the pipes to remove scale, tuberculation, corrosion,
and other foreign matter.
The concerned departments, corporation, urban local bodies, town planning authority, jal nigam, 1300
etc have to participate in the total sanitation program. These departments should be part of a
co-ordination committee constituted at a local level and are required to meet half yearly to plan
appropriate co-ordination specific to total sanitation. These meetings however, can be more
frequent during specific items such as drought, floods, etc
2.6.2.1 External Sewer Rehabilitation Methods 1305
External rehabilitation methods are performed from above the ground surface by excavating
adjacent to the pipe, or the external region of the pipe is treated from within the pipe through the
wall. Some of the methods used include:
- External Point Repairs
- Chemical Grouting (Acryl amide Base Gel and Acrylic Base Gel) 1310
- Cement Grouting (Cement , Micro fine Cement, and Compaction)
2.6.2.2 Internal Sewer Rehabilitation Methods
Internal sewer rehabilitation methods are the same as infiltration measures.
2.7 Manholes and Appurtenances
Because they are part of the collection system, manholes require the same inspection and 1315
attention as the rest of sewer network. When located in streets, these structures are subject to
vibrations and pounding of vehicle traffic. Manholes may settle at a different rate than
connected sewer, creating cracks in joints. The objectives of manhole inspection are therefore,
to determine the proper elevations or grades around the lid, to confirm that the lid is not buried,
and to examine structural integrity (look for cracks) of the manhole and its functional capacity. 1320
The condition of the pipelines coming into a manhole may be known merely by observing the
content and volume of flows from a specific direction.
2.7.1 Inspections and Examinations
Manhole inspection and examination are made by visually inspecting the condition of the cover
and the internal parts. 1325
Manhole inspection should be carried out together with the inspection and examination of
sewer. It is generally carried out together with the cleaning of the sewer.
Safety measures during the work should be formulated giving consideration to traffic safety, 1330
oxygen deficiency, poisoning due to toxic gas such as hydrogen sulphide, and so on.
2.7.1.1 Manhole
Damage or wear in the manhole cover obstructs passage and is a risk. The facility manager
should inspect the manhole cover for damage, wear, play, non-coincidence of heights of cover
and road surface, offset of manhole block, and so on. 1335
Refer to Figure 2.37, Figure 2.38 and Figure 2.39.
Figure 2.37 Wear of cover
Figure 2.38 Offset of manhole block 1340
Figure 2.39 Not coinciding with height of road surface
2.7.1.2 Conditions Inside Manhole
Manhole is an essential facility for operation and maintenance of sewer pipes; it helps operation
and maintenance to be performed safely and easily. For smooth flow of wastewater through the 1345
sewer pipe, the following are to be properly inspected: scouring of sewer bottom, differential
settlement, manhole block, crack in side wall, sediments and condition of mouth of connected
sewer pipe.
Inspection should be performed on ground, while examination should be performed by the
relevant person entering the manhole and working inside. 1350
The inspection items and their descriptions are given in Table 2.14.
Table 2.14 Inspection and examination items for manhole
Item inspected Description of inspection
Exterior condition
Ground surface
a. Check for crack, subsidence, and cave-in
b. Check for overflow stream
c. Check for any invaded pavement
d. Check surrounding condition
a. Visual check for backlash, abraded surface, and
corrosion (Check if any mark on external or internal
surface of cover is erased.)
b. Check for any malfunction of float preventive
function, locking device, fall preventive function, etc.
c. Others (damage on rising spacer, difference in grade
of cover and grade ring, damaged grade ring, caved in
manhole cover, offset, etc.)
Interior condition
Flow and sediment
a. Check for stagnant water or flow
b. Check for any accumulation of sand and soil, pieces
of wood, and mortar including remains of
construction works and illegal disposals.
c. Check for appearance of inverts such as scouring,
damage, etc.
a. Check steps for corrosion, rattling, and missing items
(No.).
b. Check blocks for damage, crack, corrosion, gap, and
deteriorated caulking.
c. Check barrel and base for damage, crack, and
corrosion.
d. Check for any improper joint of main sewers and
laterals.
e. Check for any irregular subsidence.
Infiltration a. Check for infiltration
a. Check inflow for unacceptable or inferior quality.
b. Check for toxic gases or odour.
Table 2.15 shows an example of the form for recording inspections. 1355
Table 2.15 Inspection record
Manhole maintenance Manhole No. - -
Manhole type
Type no:
Specific ( )
Constructed year FY (______)
Nodular graphite cast iron (flat,
tapered), Gray cast iron, Concrete
Manhole Fair / Poor
Total no.
Fair no.
Poor no.
Water depth
No. Pipe diameter Type Depth to base Condition
Replace cover
Month / Year
M/Y
/ / / / / / / / /
(Condition)
a) Wear, b) Backlash, c) Difference in level, d) pavement hiding the manhole,
e) Damaged, f) Location unknown
g) Corrosion, h) Damage base, i) Infiltration, j) Corroded steps, k) Inferior pipe joint,
l) Rubbish, m) Odour
n) Corrosion, o) Damage, p) Coupling displacement, q) Inadequate inclination,
r) Infiltration, s) Roots of trees, t) Silt, soil, and mortar, u) Road subsidence
2.7.2 Judgement of Examination Results
Judgement criteria for inspection and examination show ranked levels of abnormal locations,
and can be used for judging the need for cleaning and repairs and for selecting repair methods, 1360
Judgement criteria are used to categorize by symptom the abnormal locations detected during
inspection and examination, to assess the risk level and their effect level on others, to judge the
need for cleaning and repairs and to select repair methods, etc.
2.7.3 Cleaning 1365
Manhole cleaning should be performed by the most appropriate work method that suits the
actual conditions of the work location.
In manholes at starting point, junction manholes, and manholes at sharp curve of sewers, sand
and silt get deposited and environmental problems such as foul odours occur. For this reason,
periodic cleaning is necessary. Moreover, when large debris flows in, it should be removed 1370
immediately otherwise there is a possibility of an overflow accident, float-off and dispersion of
Manhole inspection should be generally carried out together with the cleaning of the sewer. The
work on the silt and sand in the bottom part should be pursuant to cleaning of the sewer pipe,
while the dirt on the side wall should be cleaned by high-pressure jet washing vehicle. 1375
2.7.4 Rehabilitation
Degradation of functions due to damage should be confirmed and necessary repairs and
rehabilitation of the manhole should be carried out.
Manhole repair methods may be classified into watertight construction method, lining method,
partial repair method (open-cut method), and manhole cover replacement method. (Refer to Sec. 1380
3.37 of the Design Manual)
Before repairs, the objectives of the repairs should be clarified, work conditions studied, and
items below should be paid attention to, and then repairs should be carried out.
- If cover is worn out or damaged, it should be replaced immediately.
- If steps are corroded, and if they need to be replaced, they should be replaced with 1385
corrosion resistant fittings.
If internal parts of manhole and sewer bottom are damaged or worn out, they should be replaced
2.8 Cross Drainage Works
For sewer collection system, cross drainage work in an inverted siphon is a typical work. 1390
Therefore, in this section, maintenance work is described hereafter.
2.8.1 Inspection and Examination
Inspection of inverted siphon should be carried out by inspection methods similar to those of the
manhole. However, inspection of inverted siphon should be carried out considering the
characteristics listed below. 1395
- The inverted siphon pipe is always in full flow, and the inverted siphon chamber in
the upstream part is constructed such that suspended substances and sand/silt are
likely to accumulate and deposit easily. There are risks of corrosion or gas generation
in the facility because of the decomposition of these substances.
- The inverted siphon chamber is provided with a flashboard, and it should be checked 1400
to confirm that it is usable.
Table 2.16 Typical inspection items for inverted siphon
Item Inspected Points Inspected
Flow condition Check difference in levels at upper and lower chambers of
inverted siphon.
Check for absence of any suspended debris.
Structural condition Check for improper functioning of gate, stop log, etc.
2.8.2 Criteria for Judging Examination Results
Judgement criteria for inspection and examination show ranked levels of abnormal locations, 1405
and can be used for judging the need for cleaning and repairs and for selecting repair methods.
The judgement criteria for inverted siphon should be the same as the judgement criteria for
2.8.3 Cleaning
The construction of the inverted siphon is such that wastewater always remains in it at all times, 1410
so sand, silt and sludge is likely to deposit easily. This requires periodic cleaning so as to
prevent overflow and foul odour problems beforehand.
An effective cleaning method should be selected when cleaning the inverted siphon, and work
that gives adequate consideration to safety measures should be implemented. Cleaning should
be performed at least once a year. 1415
a. Replacing water in the inverted siphon
The submersible pump and generator used for replacing water in the inverted siphon should
be selected appropriately considering the influent flow rate and the head, and a replacement
plan with adequate margin should be formulated.
b. Cleaning of inverted siphon manhole 1420
The main cleaning methods of sand trap of inverted siphon manhole are vacuum truck
cleaning and manual cleaning. Table 2.17 gives the description.
Table 2.17 Cleaning method
Cleaning method Application
Cleaning by vacuum vehicle
Cleaning of manholes for inverted siphons by
means of vacuum vehicles.
If a cleaning by vacuum vehicle is difficult,
inverted siphons are cleaned by crew getting down
into the siphon or by using a crane equipped truck.
c. Cleaning of inverted siphon sewer 1425
Inverted siphon sewer should be cleaned together with the cleaning of general sewers.
After sucking up the wastewater in the manhole by submersible pump, the manhole on the
upstream side should be cleaned, and then the manhole on the downstream side and the
insides of the sewer should be cleaned. The ease or difficulty of work depends on the pipe
diameter, number of cables and closing equipment, but work should be performed 1430
according to sewer cleaning by combining mechanical and manual means. Figure 2.40
shows the cleaning work.
If the head exceeds 20 m, especially powerful vacuum trucks may need to be used.
Figure 2.40 Inverted siphon cleaning work 1435
2.9 Pressure Sewer
2.9.1 Pressurized Sewers
Pressure sewers are meant for collecting sewage from multiple sources to deliver to an existing
collection sewer, and/or to the STP. These sewers are not dependent on gravity.
The principle advantages are the ability to sewer areas with undulating terrain, rocky soil 1440
conditions and high groundwater tables as pressurized sewers can be laid close to the ground
and anchored well without infiltration. Exfiltration can be quickly detected and corrected.
Moreover, this sewer allows smaller diameter pipes and road crossings by CI or DI pipes with
trenchless technology to be laid inside a casing pipe and installation without disrupting traffic,
without opening trenches across paved roadways, or moving existing utilities, etc. 1445
Adisadvantage is the need to ensure unfailing power supply to the grinder pump
2.9.2 Vacuum Sewer System
The vacuum sewer collects sewage from multiple sources and conveys it to the STP. As the
name suggests, a vacuum is maintained in the collection system and when a house sewer is
opened to atmospheric pressure, sewage and air are pulled into the sewer, whereby the air forms 1450
a plug in the line, and air pressure pushes the sewage towards the vacuum station. This
differential pressure comes from a central vacuum station. These sewers can take advantage of
available slope in the terrain, but have a limited capacity to pull water uphill may be to
approximately 9 m. A disadvantage is the need to ensure unfailing power supply to the grinder
pump. 1455
2.9.3 O&M System
Neither pressurized sewer system nor vacuum sewer system is used in the public sewer systems
in India. Therefore, O&M is not explained here. Only system characteristics of pressurized
sewer and vacuum sewer are shown in Table 2.18.
Table 2.18 Pressurised sewer and vacuum sewer 1460
Vacuum sewer Pressurised sewer
Collection principle
Sewage pulled and conveyed by
vacuum force.
Sewage pressurized and conveyed by
a grinder pump.
Typical facility
Avacuum valve unit for one or some
households, vacuum sewer laterals,
and a lift station
Agrinder pump unit for one or some
households, and pressurized sewer
Pipe diameter Typically | 100 to 250 mm Typically | 32 to 150 mm
Laying depth
Can be laid in a shallow depth without
gradient.
Can be laid in a shallow depth.
Topographical Adequate for flat terrain which can Applicable for wide range of terrain
requirements hold certain degree of vacuum force
so that sewage can be pulled, and for
densely populated area to some
extent.
Power source Required for lift station Required for each grinder pump unit.
O&M cost
O&M cost and energy cost for
vacuum valve unit and lift station.
Generally, higher than that of gravity
O&M cost and energy cost for grinder
pump unit. Generally, higher than that
of gravity sewer.
2.10 House Service Connection
House connections or service connections to the public or municipal sewer should preferably be
approved by the Maintenance Engineer. It is necessary to ensure that the fittings and pipes in the
houses are according to the byelaws or rules or regulations in force. If such byelaws, rules or 1465
regulations do not exist, then reference may be made to the relevant IS code of practice. House
connections may be of minimum size of 150 mm in diameter and should preferably be
connected to the Municipal or Public sewer through a manhole. When Y or T connections are
allowed, extreme care must be taken when breaking the sewer pipe line and inserting the Y or
T saddle. 1470
Similarly, the connection to the manhole must be properly done and closed. Care has to be taken
so that the brick bats or other materials of construction are not allowed to fall and lie in the
manhole. This extraneous material is largely responsible for persistent clogging of the sewer
It should also be ensured that the house fittings are properly trapped not only to prevent the 1475
ingress of sewer gases into the houses but also to ensure that large objects do not find their way
into the sewers. Similarly, it should be ensured that any liquid or material which is likely to be
injurious to the material of the sewer line or to prejudicially interfere with its contents or be a
hazard to the workmen engaged in the maintenance of the sewer lines, like very hot water, acids,
chemicals, etc., are not allowed. 1480
2.10.1 Inspection and Examination
Inspection of lateral sewer and house inlet (household) should be carried out if deemed
necessary from documents and data, and cross connections and mains should be studied.
Clogging of lateral sewer and sedimentation of house inlet are the items to be inspected.
Examination of lateral sewer by TV camera should be carried out after high pressure washing of 1485
the lateral sewer. The insides of the pipe should be examined by TV camera, and recorded on
video tape. TV camera for lateral sewer is used as a direct view camera, and the camera head is
pushed by a rod towards the main from the public inlet. Also, there is a method of examination
by which the camera head is pushed in by a hard cab tire cable. Figure 2.41 and Figure 2.42
show working diagrams of examination of lateral sewer by TV camera. 1490
Figure 2.41 Portable TV system for small-diameter pipe
Source: EPA, 2003 1495
Figure 2.42 Applications of portable TV system in difficult to access locations
- Features of examination of lateral sewer by TV camera
If power supply is ensured, the vehicle loaded with TV camera can work even
in locations where access for humans is not possible.
By connecting the monitor to the vehicle loaded with TV camera, character data 1500
can be displayed on the monitor screen.
The standard examination distance per location during examination of lateral
sewer by TV camera is 5 m maximum.
- Procedure for examination of lateral sewer by TV camera is shown in Figure 2.43.
Figure 2.43 Work procedure for examination of lateral sewer by TV camera
2.10.2 Criteria for Judging Examination Results
These criteria should adhere to the inspection criteria for sewer pipes.
2.10.3 Cleaning
Cleaning should conform to the cleaning procedure for sewer pipes. 1510
2.10.4 Rehabilitation
Rehabilitation should conform to the rehabilitation procedure for sewer pipes.
2.11 Safety Practices
You are in an occupation that has an overall accident frequency rate that is relatively higher than
any other industry. Your employer has the responsibility of provide you with a safe place to 1515
work. But you, as the operator have the overall responsibility for your treatment plant and must
ensure its a safe place to work. This can only be done by constantly thinking of safety and
working safely.
You have the responsibility of protecting yourself and other plant personnel or visitors by
establishing safety procedures for your plant and then ensuring they are followed. Train yourself 1520
to analyze jobs, work areas and procedures from a safety standpoint. Learn to recognize
potentiality hazardous actions or conditions. When you do recognize a hazard, take immediate
steps to eliminate it through corrective action. If correction is not possible, guard against the
hazard by proper use of warning signs and devices/by establishing and maintaining safety
procedures, as an individual, you can be held liable for injuries or property damage which 1525
results from an accident caused by your negligence.
Internal sewer inspection
Remember accidents dont happen - they are caused! Behind every accident there is a chain
of events which leads to an unsafe act unsafe condition/or a combination of both.
Accidents may be prevented by using common sense, applying a few basic safety rules and
acquiring a good knowledge of the hazards unique to your job as a plant operator. 1530
2.11.1 Accidents related to Sewer Facilities
2.11.1.1 Need for Traffic Control
The primary function of streets is to provide for the movement of traffic. A common secondary
use within the right-of-way of streets is for the placement of public and private utilities such as
sanitary sewers. While the movement of traffic is very important, streets need to be constructed, 1535
reconstructed, or maintained, and utility facilities need to be repaired, modified, or expanded
Consequently, traffic movements and street or utility repair work must be regulated to provide
optimum safety and convenience for all.
Working in a roadway represents a significant hazard to a collection system operator as well as
pedestrians and drivers. Motor vehicle drivers can be observed reading books, files, and 1540
newspapers, shaving, talking on cellular phones, and changing tapes, CDs, or radio stations (and
using headsets to listen to them) rather than concentrating on driving. At any given time of the
night or day, a certain percentage of drivers can be expected to be driving while under the
influence of drugs or alcohol. Given the amount of time collection system operators work in
traffic while performing inspection, cleaning, rehabilitation, and repairs, the control of traffic is 1545
necessary if we want to reduce the risk of injury or death while working in this hazardous area.
The purpose of traffic control is to provide safe and effective work areas and to warn, control,
protect, and expedite vehicular and pedestrian traffic. This can be accomplished by appropriate
and prudent use of traffic control devices.
Most states, counties, and cities have adopted regulations to control traffic and reduce the risk 1550
under different circumstances. This section illustrates examples of traffic control which may or
may not meet the specific requirements of the laws in your geographical area, but should serve
to make you aware of various aspects of traffic control.
At any time traffic is affected, appropriate authorities in your area must be notified before
leaving for the job site. These could be state, country or local depending on whether it is a state, 1555
country or local street. Frequently, a permit must be issued by the authority that has
jurisdiction before traffic can be diverted or disrupted. In some cases, traffic diversion or
disruption may have an impact on the emergency response system in your area, such as access
by fire or police, and so these agencies may be involved as well in most cases, you will need to
plan ahead to secure permits and notify authorities. This may mean only a phone call or two or 1560
it could mean several days' or weeks' advance planning if you need to make extensive traffic
control arrangements.
Upon arrival at the job site, look for a safe place to park vehicles. If they are to be parked in the
street to do the job, route traffic around the job site before parking vehicles in the street. If
practical, park vehicles between oncoming traffic and the job site to serve as a warning 1565
barricade and to discourage reckless drivers from ploughing into operators.
2.11.1.2 Safety Measure to be taken before any manhole entry
All workers assigned to enter sewer manholes should be provided with proper safety equipment.
Following safety equipment is recommended:
- Approved gas detector (Properly calibrated) 1570
- Fresh air blower
- Safety harness, rope and tripod safety system
- Approved hard hat
Following guidelines may be adopted to ensure safety in manhole:
a. Oxygen content must be at least 19.5% in the confined space of the manhole 1575
measured at all levels (bottom, middle and top). Safe oxygen level is considered if it
ranges between 19.5% and 21%. Nobody should enter the manhole if oxygen level is
below 19.5% and more than 21%.
b. Ventilate the sewer line by opening at least two or three manholes on both sides
where work is to be carried out. This is mandatory where adequate blowers for 1580
ventilating sewers are not available. The manholes should be opened at least one
hour before the start of operation. The opened manhole must be properly fenced or
barricaded to prevent any person specially children from accidentally falling into the
sewer. Dummy cover with BRC welded fabric or wire-net may be used.
c. Fresh air blower ventilation system should be used as far as practicable. It is 1585
desirable to operate blowers for at least 30 minutes before start and during the
cleaning operation.
d. Measure gas inflammability in manholes using detector.
e. Presence of toxic gases may be tested before entry of a person in manhole/ sewer line
and also in between if the operations are for longer period. 1590
f. All workers should use safety harness and life line before entering the sewer line. At
least one support person at the top must be provided for each person entering the
manhole. The person entering the manhole/ sewer line must be monitored using
signal/camera /CCTV etc., throughout the operation period.
g. Structural safety of manhole rungs or steps must be tested before entering the 1595
manhole. Portable aluminiumladder must be available during the work period where
necessary. The portable ladder must be properly seated or fixed during use.
h. Ensure that no material or tools are located near the edge which can fall into the
manhole and injure the work-men.
i. Lower all tools to the workmen in a bucket fixed with rope and pulley. 1600
j. Lighting equipment used during sewer cleaning must be explosion and fire-proof.
k. Caution signboards must be displayed around open manholes during working
l. Smoking, lighting open flames or gadgets producing sparks must be prohibited
inside the manhole as well as in the immediate vicinity of open manholes. 1605
m. All workers entering the manhole must be provided with protective gear and proper
equipment. Use of portable gear and equipment must be monitored strictly.
n. Gas masks for respiratory protection must be available for use by the workers. The
workers must be trained to use the gas masks properly.
o. Sewer inspection and examination guidelines referred to in Section 2.2.5 may be 1610
followed as and when necessary.
(Honourable Supreme Court of India has directed the need for proper equipment, adequate
protection and safety gear to sewer workers who enter into the manhole for cleaning blocks.
Ref: Delhi Jal Board versus National Campaign for Dignity & Rights of Sewerage and Allied
Workers and others.). Please refer to Appendix 9.3 of this manual. 1615
When entering a large sewer system, you may be required to use special equipment. The type of
equipment might include atmospheric monitoring devices with alarms. In the event of a sudden
or unpredictable atmospheric change, an emergency escape breathing apparatus (EEBA) with at
least a 10-minute air supply should be worn for escape purposes.
2.11.2 Measures against Accidents 1620
Safety measures are dealt with in Chapter 9.
2.11.3 Information to Prevent Accidents and Records
Information to prevent accident and records are dealt with in Chapter 9.
Refer to appendices for troubleshooting for sewerage collection system. 1625
The purpose of maintenance of sewerage collection system should be to minimize stoppage of
functions. The following cycle should adhere to:
O&M engineers find out problems related to their sewer system based on information obtained
from inspections or examinations on the facilities. To solve the problems, they need to make a 1630
decision on rehabilitation actions considering prioritization of each facility.
When the facilities are rehabilitated, records of inspections as well as those of rehabilitation
should be kept.
The following cycle is regarded as essential to achieve the goal of sewer system O&M:
Inspection, Condition assessment, Decision making on rehabilitation actions, 1635
Rehabilitation, and Next inspection.
Figure 2.44 O&M cycle
Inventory database
Frequency of next
Rehabilitation Condition assessment
Decision-making on
rehabilitation actions
Part B: Operation and Maintenance CHAPTER 3: PUMPING STATION
CHAPTER 3 PUMPING STATION
3.1 Introduction 1640
Pumping stations handle sewage either as in-line for pumping the sewage from a deeper sewer
to a shallow sewer or for conveying to the treatment plant or for outfall pumping. They are
required where low lying development areas cannot be drained by gravity to existing sewerage
infrastructure, and/or where development areas are too far away from available sewerage
infrastructure to be linked by gravity means. Proper O&M of pumping systems applies to all the 1645
components of such systems.
3.2 Types and Structure of Pumping Stations
The type of pumping stations can be (a) Horizontal pumps in dry pit, (b) Vertical pumps in dry
pit, (c) Vertical pumps in suction well and (d) Submersible pumps in suction sump. All these
types include a sewage receiving sump which is called suction sump or wet well and various 1650
types of pump arrangements as shown in Figure 3.1.
(a) (b) (c) (d)
Source: CPHEEO, 1993
Figure 3.1 Typical dry-well and wet-well installations 1655
3.2.1 Dry Pit
The size of the dry pit should be adequate for the number of pumps planned and should be such
as to handle the sewage load at the desired pumping capacity. Allowance should also be made
for future requirements so that additional or larger pumps can be installed.
In this configuration, two pits (dry-pit and wet-pit) are required: one to hold the fluid, and one to 1660
house the pumps and appurtenances. This option is required for fluids that cannot be primed or
conveyed long distances in suction piping. It is typically used to pump large volumes of raw
sewage, where uninterrupted flow is critical and sewage solids could clog suction piping. It also
is used to pump solids in pipe galleries between digesters or other solids-handling equipment.
While construction costs may be higher and a heating, ventilation, and cooling system is 1665
necessary when installed below grade, this configuration is best for O&M activities because
operators can see and touch the equipment.
3.2.2 Suction Sump or Wet Well
Sewage sump is a compartment or tank in which sewage is collected. The suction pipe of a
pump may be connected to the wet well or a submersible pump may be located in the wet well. 1670
Sewage sump design depends on the type of pumping station configuration (submersible or
dry-well) and the type of pump controls (constant or variable speed). Wet-wells are typically
designed large enough to prevent rapid pump cycling but small enough to prevent a long
detention time and associated odour release.
Sewage sumps should always hold some level of sewage to minimise odour release. Bar screens 1675
or grinders are often installed in or upstream of the wet-well to minimise pump clogging
Instead of manually operated screens at the bottom which requires the staff to get down into the
screen sump, it is better to install mechanical bar screens which can automatically remove the
screenings and lift the same above ground level safely. There can also be two such screens one 1680
after the other for coarse screenings and fine screenings. This will require rectangular channels
to maintain longitudinal non-turbulent linear flow.
3.2.3 Lift Stations
In general, lift stations are invariably used in gravity sewer network where depth of cut poses a
problem in high water prone areas. The procedure is to sink a wet well on the road shoulder or 1685
an acquired plot after the shoulder and divert the deeper sewer there. The submersible pump set
will lift the sewage and discharge it to the next on line shallow sewer. This is a very useful
practice in such locations.
Equipment located in the wet well should be minimised, including suction and discharge valves,
check valves, or other equipment that require routine, periodic maintenance. This equipment can 1690
be located in small equipment manholes located adjacent to the wet well to facilitate
accessibility and maintenance for the operator.
3.2.4 Operation and Maintenance
Pumping machinery and pumping station are very important components of sewerage systems.
Pumping machinery is subjected to wear, tear, erosion and corrosion due to its nature of 1695
functioning, and therefore it is vulnerable to failures. Generally, failures or interruptions are
mostly attributed to pumping machinery rather than any other component. Therefore, correct
operation and timely maintenance and upkeep of pumping stations and pumping machinery are
of vital importance. Sudden failures can be avoided by timely inspection, follow up actions on
observations of inspection and planned periodical maintenance. Downtime can be reduced by 1700
maintaining inventory of fast moving spare parts.
Obviously due attention needs to be paid to all such aspects for efficient and reliable functioning
of pumping machinery.
3.2.4.1 Operation of the Pumps
The following points should be observed while operating the pumps. 1705
a. Dry running of the pumps should be avoided.
b. Centrifugal pumps should be primed before starting.
c. Pumps should be operated only within the recommended range of the
head-discharge characteristics of the pump.
If pump is operated at a point away from duty point, the pump efficiency 1710
normally reduces.
Operation near the shut-off point should be avoided, as it causes substantial
recirculation within the pump, resulting in overheating of water in the casing
and consequently, overheating of the pump.
d. Voltage during operation of pump-motor set should be within 10% of rated voltage. 1715
Similarly, current should be below the rated current shown on the name plate of the
e. When parallel pumps are to be operated, the pumps should be started and stopped
with a time lag between two pumps to restrict change of flow velocity to minimum
and to restrict the dip in voltage in the incoming feeder. The time lag should be 1720
adequate to allowthe head on the pump to stabilise, as indicated by a pressure gauge.
f. When the pumps are to be operated in series, they should be started and stopped
sequentially, but with minimum time lag. Any pump next in sequence should be
started immediately after the delivery valve of the previous pump is even partly
opened. Due care should be taken to keep open the air vent of the pump next in 1725
sequence, before starting that pump.
g. The stuffing box should allow a drip of leakage to ensure that no air passes into the
pump and that the packing gets adequate water for cooling and lubrication. When the
stuffing box is sealed with grease, adequate refill of the grease should be maintained.
h. The running of the duty pumps and the standby pumps should be scheduled so that 1730
no pump remains idle for a long period and all pumps are in ready-to-run condition.
Similarly, unequal running should be ensured so that all pumps do not wear equally
and become due for overhaul simultaneously.
i. If any undue vibration or noise is noticed, the pump should be stopped immediately
and the cause for vibration or noise should be checked and rectified. 1735
j. Generally, the number of starts per hour shall not exceed four. Frequent starting and
stopping should be avoided as each start causes overloading of motor, starter,
contactor and contact. Although overloading lasts only for a few seconds, it reduces
the life of the equipment.
k. Troubles in a sewage pumping station can be mostly traced to the design stage itself. 1740
This is all the more true when too much grit is likely to come into the sewage
pumping stations from sewages at monsoon time, which is difficult to handle. Hence
sewer collection system should not be allowed to collect any storm water from the
3.2.4.2 Undesirable Operations 1745
The following undesirable operations should be avoided.
a. Operation at higher head
A pump should never be operated at a head higher than the maximum recommended head
otherwise such operation may result in excessive recirculation in the pump, and
overheating of the water and the pump. Another problemthat arises if a pump is operated at 1750
a head higher than the recommended maximum head is that the radial reaction on the pump
shaft increases causing excessive unbalanced forces on the shaft which may cause failure
of the pump shaft. As a useful guide, appropriate marking should be made on the pressure
gauge. Efficiency at a higher head is normally low so such an operation is also inefficient.
b. Operation at lower head 1755
If a pump is operated at a lower head than the recommended minimum head, the radial
reaction on the pump shaft increases causing excessive unbalanced forces on the shaft
which may cause failure of the pump shaft. As a useful guide appropriate marking should
be made on both pressure gauge and ammeter. Efficiency at a lower head is normally low,
so such an operation is also inefficient. 1760
c. Operation on higher suction lift
If a pump is operated on suction lift higher than the permissible value, pressures at the eye
of impeller and the suction side fall below vapour pressure. This results in flashing of water
into vapour. These vapour bubbles collapse during passage, resulting in cavitation in the
pump, causing pitting on the suction side of impeller and casing, and excessive vibrations. 1765
In addition to mechanical damage due to pitting, pump discharge also reduces drastically.
Typical damage to impeller and sometimes to the casing is shown in Figure 3.2.
Source: http://greathub.hubpages.com/hub/piping-and-pipes#
Figure 3.2 Typical cavitation damage of an impeller 1770
d. Operation of the pump with low submergence
Minimum submergence above the bell-mouth or foot valve is necessary so as to prevent
entry of air into the suction of the pump, which gives rise to the vortex phenomenon,
causing excessive vibration, overloading of bearings, reduction in discharge and in the
efficiency. As a useful guide, the lowest permissible water level should be marked on the 1775
water level indicator. Usually the pump manufacturer indicates the minimum height of
submergence.
e. Operation with occurrence of vortices
If vibration continues even after taking all precautions, vortex may be the cause. Vortex
should be stopped by using anti vortex fittings as described in Part A of the manual: 1780
Pumping-station maintenance.
A well-planned maintenance program for pumping systems can reduce or prevent unnecessary
equipment wear and downtime. (The following maintenance information applies to both sewage
and solids pumping systems.)
The following is a maintenance checklist for a basic pumping-station: 1785
- Check the wet-well level continuously (whenever necessary).
- Record each pumps run time hours (as indicated on the elapsed-time meters) at
least once in a day and confirm that the pumps running hours are equal.
- Ensure that the control-panel switches are in their proper positions.
- Ensure that the valves are in their proper positions. 1790
- Check for unusual pump noises.
- At least once a week, manually pump down the wet well to check for and remove
debris that may clog the pumps.
- Inspect the float balls and cables and remove all debris to ensure that they operate
properly. Untangle twisted cables that may affect automatic operations. 1795
- If a pump is removed from service, adjust the lead pump selector switch to the
number that corresponds to the pumps remaining in operation. (This allows the lead
pump levels to govern the operating pumps starts and stops.).
3.2.4.3 Piping and Appurtenance Maintenance
Properly maintaining pumping-station pipelines and other appurtenances can minimize pump 1800
loads. Excessive head losses on either the suction or discharge side of a pump can increase
energy use and the wear rate and consequently, the O&M costs. Excessive head losses also may
lead to process or treatment problems because solids move slower, so the proper solids balance
is not maintained. Operators can monitor head losses by routinely checking the pressure gauges
on both sides of the pumps. 1805
When operators notice excessive head losses (indicated by a pressure drop on the suction side of
the pump or an increase in pressure on the discharge side), they should determine whether the
losses are a result of partial clogging, a restriction somewhere in the line, or materials built up
on the pipe wall. To find clogs, operators should start by checking the pressure at various points
in the suction and discharge piping, and look for spots with abrupt head loss (such as valves or 1810
other constrictions). If something is caught in a valve or other appurtenance, the operator should
stop the pump and physically open out the valve head, remove the blockage. In smaller pumps,
it is easier to remove the entire valve, disassemble and remove blockage, reassemble and refit.
During such time, other pumps shall be run. Scum build-up problems typically are addressed via
source control (for instance, by installing grease traps in the collection system at locations 1815
suspected or known to generate grease, such as restaurants, etc.).
3.3 Gates, Valves and Actuators
3.3.1 Sluice Gate
A sluice gate (Figure 3.3) is traditionally a wooden or metal plate which slides in grooves in the
sides of the channel. Sluice gates are commonly used to control water levels in sewerage 1820
treatment plants.
Attention should be paid to the following points for proper operation:
a. Test for proper operation
Operate inactive sluice gates by smearing grease on stem threads.
Figure 3.3 Sluice gate
b. Clean and paint
Clean sluice gate with wire brush and paint with proper corrosion-resistant paint.
c. Adjust for proper clearance 1830
For gates seated against pressure, check and adjust top, bottom, and side wedges until each
wedge applies nearly uniform pressure against gate in the closed position. This shall be
done by the manufacturer and not the operator.
d. Check for the following:
Ensure unobstructed operation of gate and headstock. 1835
Ensure that the spindle is not touching the stem guide.
Remove foreign matter like paint, concrete, etc. in the fully open position of
gate.
e. Dos for sluice gates
Operate the gate at least once in every three months. 1840
Check the nuts of all construction and foundation bolts once in a year. Tighten
the bolts, if loose.
Examine the entire painted surface for any signs of damage to the protective
paint.
f. Donts for sluice gates 1845
Do not remove lock plates until the gate has been properly installed.
Do not keep the gate out of operation for more than three months.
Do not forget to set the stop nut in the correct position.
Do not disturb the adjustment of wedge block bolts/studs.
Do not over torque the crank handle/hand wheel. 1850
3.3.2 Valve
On the delivery side of centrifugal pumps, a non-return valve is necessary to prevent
back-pressure from the delivery head on the pump, when the pump is shut off. To avoid
water-hammer, which is likely to be caused by the closure of the valve, the valve may be
provided with an anti-slam device, which may be either a lever and dead-weight type, a 1855
spring-loading type or the dash pot type.
Pumps may be run in parallel operation with different permutation of the standbys. Isolation
valves would be needed to isolate those pumps which are to be idle. Generally, the isolating
valves are gate valves, which should preferably be of the rising stem type, since this type offers
the advantage of visual indication of the valve-position. For exterior underground locations, gate 1860
valves are generally used.
3.3.2.1 Gate Valve
A gate valve is a valve that opens by lifting a round or rectangular gate/wedge out of the path of
the fluid (Figure 3.4).The distinct feature of a gate valve is that the sealing surfaces between the
gate and seats are planar. The gate faces can form a wedge shape or they can be parallel. Typical 1865
gate valves should never be used for regulating flow, unless they are specifically designed for
that purpose.
Figure 3.4 Gate valve 1870
Gate valves require the following maintenance:
a. Replace packing
Modern gate valves can be repacked without removing them from service. Before
repacking, open the valve wide. This prevents excessive leakage when the packing or the
entire stuffing box is removed. It draws the stem collar tightly against the bonnet bushing 1875
on a rising stem valve.
b. Operate valve
Operate inactive gate valves to prevent sticking.
c. Lubricate gearing
Lubricate gate valves as recommended by manufacturer. Lubricate thoroughly any gearing 1880
in large gate valves. Wash open gears with solvent and lubricate with grease.
d. Lubricate rising stem threads
Clean threads on rising stem gate valves and lubricate with grease.
e. Lubricate buried valves
If a buried valve works hard, lubricate it by pouring oil down through a pipe that is bent at 1885
the end to permit oiling the packing follower below the valve nut.
3.3.2.2 Non-Return Valve (Check Valve)
Normally, a check valve is installed in the discharge of each pump to provide a positive shutoff
from force main pressure when the pump is shut off and to prevent the force main from draining
back into the wet well. 1890
The most common type of check valve is the swing check valve which is shown in Figure 3.5.
This valve consists of a valve body with a clapper arm attached to a hinge that opens when the
pump starts operating and closes to seat when the pump is shut off.
Figure 3.5 Check valve 1895
Check valves must close before the water column in the pipe reverses flow; otherwise, severe
water hammer can occur when the clapper arm slams against the valve body seat. If this occurs,
an adjustment of the outside weight or spring is usually required. A traditional clapper type of
check valve has a lever on the extended shaft which allows adjustment of the weight on the arm
or spring to vary the closing time. Wear occurs within the valve primarily on the clapper 1900
hinge-and-shaft assemblies and should be checked annually for looseness.
Preventive maintenance (To be done only by the manufacturer)
a. Inspect Disc Facing
Open valves to observe condition of facing on swing check valves equipped with neoprene
seats on disc. 1905
If metal seat ring is scarred, dress it with a fine file and lap with fine emery paper wrapped
around a flat tool.
b. Check Pin Wear
Check pin wear on balanced disc check valve since disc must be accurately positioned in
the seat to prevent leakage. 1910
3.3.2.3 Non-Return Valve (Ball Type)
Non-return valve (Figure 3.6) is depending on a light weight and suitable coated ball moving
inside the flowing pipe to occupy an elevated angular position while the fluid is in pumping and
dropping back to close the reverse flow through the pipe. Because it is a sphere sitting over a
circular opening, it is expected to seat properly and seal the reverse flow. The material of the 1915
ball, the coating and its sturdiness against dents caused by the slide are important matters. The
ball is replaced by opening the top flange after switching off the pump. This can be installed in
any position, vertical or horizontal.
Figure 3.6 Typical ball type check valve 1920
When flow occurs, the ball is lifted into the angular piping and is held there because its weight
is lighter than the sewage and the velocity of flow. When the flow stops, it slides back and seals.
3.3.2.4 Butterfly Valve
Butterfly valves are another type of valve that have been successfully used as suction and
discharge isolation valves in pumping stations. They are frequently used in sewage plants where 1925
waste streams with a high solids content are encountered, such as in sludge pumping systems.
A butterfly valve consists of the valve body and a rotating disc plug that operates through 90
degrees.
This is usually a disc rotated by 90 degrees by external handle. In the open position, the disc is
in line with the flow. In the closed position, the disc is at 90 degrees to the flow and it stops the 1930
flow. Usually, the axis is vertical although horizontal axis arrangement may also be used in
smaller sizes. The closing and opening can be manual or mechanized. The butterfly valves
occupy less space and are generally preferred for pipe sizes larger than 150 mm.
Many agencies specify butterfly valves as opposed to gate valves because they are less
susceptible to plugging. 1935
Butterfly valves require the following preventive maintenance to be done by the manufacturer:
c. Adjust gland
The adjustable gland holds the plug against its seat in the body and acts through
compressible packing, which functions as a thrust cushion.
Keep gland tight enough at all times to hold plug in contact with its seat. If this is not done, 1940
the lubricant system cannot function properly, and solid particles may enter between the
body and plug and cause damage.
d. Lubricate all valves
Apply lubricant by removing lubricant screw and inserting stick of butterfly valve lubricant
for stated temperature conditions. 1945
Be sure to lubricate valves that are not used often to ensure that they are always in operating
condition. Leave lubricant chamber nearly full so that extra supply is available by turning
screw down. Use lubricant regularly to increase valve efficiency and service, promote easy
operation, reduce wear and corrosion, and seal valve against internal leakage.
3.3.3 Actuators 1950
These are replacements for physical operation by the operators. Actuators are used for
automation of valves. An actuator rotates the valve spindle or lifts and drops the same.
a. Electric geared motor actuator
The actuator consists of a rotor stator unit driving an output shaft through a single stage
worm reduction gear, which incorporates an automatic mechanical device for changing 1955
manual drive to power drive. The actuator includes a travel limit switch unit and a torque
switch unit, and is of totally enclosed construction. When power fails, electric motor driven
gear actuators retain their positions. When power supply returns, pay attention how the
valves move.
Electric motor driven gear actuator is shown in Figure 3.7. 1960
Figure 3.7 Electric motor driven gear actuator
b. Solenoids
Solenoids are the most common actuator components. Basically, it consists of a moving
ferrous core (a piston) that moves inside wire coil. Normally the piston is held outside the 1965
coil by a spring. When a voltage is applied to the coil and current flows, the coil builds up a
magnetic field that attracts the piston and pulls it into the centre of the coil. The piston can
be used to supply a linear force. Diaphragm valve have small holes on it. The holes should
be free from clogging by debris otherwise the diaphragm may not open.
c. Pneumatics 1970
Pneumatic systems are very common, and have much in common with hydraulic systems
with a few key differences. The reservoir is eliminated as there is no need to collect and
store the air between uses in the pneumatic system. Also because air is a gas, it is
compressible and regulators are not needed to recirculate flow; however, since the gas is
compressible, the systems are not as stiff or strong. 1975
In general, the pneumatics are liable to cause accidents such as when the air hose suddenly
pull out of the hose clamp and jets high pressure air on persons nearby, and should
normally be avoided. The electric geared motor type is preferred.
Pneumatic valve is shown in Figure 3.8.
Figure 3.8 Pneumatic valve
d. Hydraulic system
Actuator (hydraulic motor and hydraulic cylinder) is operated by hydraulic fluids
(hydraulic oil), which is pressurised by hydraulic pump driven by an electric motor.
Smooth movement and variable speed can be achieved. Moreover, the installed relief 1985
valve can prevent the system from breakdown. It should be noted that hydraulic oil leaks
as pressure increases. Check for oil leakage regularly. Hydraulic system should be kept
clean because it is vulnerable to dust or rust. Take precautions to avoid fires because the
hydraulic oil is combustible.
In all cases, preventive maintenance by manufacturer shall be done periodically and a wall chart 1990
exhibited on site.
Screenings in sewage from the incoming sewer at the depth below ground level need be
separated and lifted above ground level and removed there from either by mechanical or manual
method. 1995
3.4.1 Types of Screens
3.4.1.1 Coarse Screens
Coarse screens are usually bar screens consisting of vertical or inclined bars spaced at equal
intervals across a channel through which sewage flows. The openings are usually 25 mm.
Hand-cleaned screens are usually inclined at 45 degrees to the horizontal. 2000
3.4.1.2 Medium Bar Screens
Medium bar screens have clear openings of 12 mm.
3.4.1.3 Fine Screens
Fine screens are mechanically-cleaned devices. Fine screens may be of the drum or disc type,
mechanically cleaned and continuously operated. They are also used for protecting beaches 2005
where sewage without any treatment may be discharged into the sea for disposal by dilution.
3.4.2 Screenings Removal Method
3.4.2.1 Manual Bar Screen
Hand cleaned screens should be cleaned as often as required to prevent backing up of sewage.
Amanually-cleaned bar screen is shown in Figure 3.9. 2010
Figure 3.9 Manual bar screen
The following are important for O&M of manual bar screen:
a. Preventive maintenance for checking and repairing the following once a week 2015
Check whether the standing platform is at least 2 m wide with the first 1 m as
slotted. An example of a risky platform is presented in Figure 3.10, in which
there is no space for the operator to stand after he has lifted and dumped
screenings on it. Because of the lack of space, he may move backwards and fall
into the sewage channel. Also, screens should be inclined to the horizontal by an 2020
angle of 60 degrees or more, otherwise, the operator has to bend forward. The
rear side of the platform should have handrails. If handrails are not provided,
enter this point in the site book.
Check the condition of ladders and paint them periodically.
Verify that there are no broken metal parts that protrude outside. 2025
Once a month check the rigidity of handrails.
Verify the platform for its sturdiness by gently setting the foot on it.
Figure 3.10 An example of risky platform
Verify that the lighting is not in front or behind the operator. It should be above
the operator, at least 2.5 m high and mounted on the sidewall. 2030
These lights should not have local on-off switches.
Verify that the operator platform and slotted platform have 3-m head room and
roof so that the operator is not drenched and he can lift the cleaning rake freely.
b. Regular maintenance on a daily basis and repairs
Verify that the screen rods have not broken loose. 2035
Verify that the cleaning rake is well washed in running water after each use.
Verify that gum boots are kept inside a locker covered with mesh.
Verify that disposable gloves are available for all 3 shifts and a stock of one
month is available.
Verify that helmet is available. 2040
c. Operation
Before daily operation, verify all the above. If these points are not met, do not
enter the screen area. Enter all missing items in the site register.
If all items are in order, do the cleaning once in four hours in each shift.
Ensure that operators do not stand one behind the other. This may cause an 2045
accident because while pulling the rake backwards, the operator in the front
may hit and push the operator in the rear into the sewage channel.
Once the screens are cleaned and screenings are deposited on the slotted
platform allow them to drip dry till the next cleaning after 4 hours.
Push the screenings with the rake to the side of the platform to drop them into 2050
the tipper positioned there.
Move the tipper to the vermin compost site, dump the contents in the pit and
cover with earth as prescribed in Sec.3.4.4 Disposal of Screenings.
3.4.2.2 Mechanical Screen (Intermittent and Continuous)
Mechanically cleaned racks are generally erected almost vertically. Additional provision should 2055
be made for manual raking in case the mechanical rakes are temporarily out of order. Plants
using mechanically-cleaned screens have controls for
- Manual start and stop
- Automatic start and stop by clock control
- High level switch 2060
- High level alarm
- Starting switch or overload switch actuated by loss of head and
- Overload alarm.
There are various types of mechanisms in use, the more common being travelling rakes that
bring the debris up out of the channel and drop them into hoppers or other debris containers 2065
(Figure 3.11).
Source: WEF, 2010
Figure 3.11 Mechanically-cleaned bar screen
The rotary drum screen by United Nations Industrial Development Organization (UNIDO) 2070
(Figure 3.12) type arc screens is also included here.
In the drawing, the screening rods are in the form of arcs. The cleaning takes place when the
meshing teeth at both ends of a diametrical rotating arm plough through the screen openings and
push the screenings upwards. Upon exiting the upper end of the screen which is well above the
operating sewage level, a built in spring loaded arrangement in the diametrical rods jacks out the 2075
meshing teeth gently, which pushes the screenings gently into a collection trough. The
screenings can be manually removed or a conveyor belt can collect the droppings and drop them
into a container on the ground through a drop chute.
Source: Tamilnadu Water Supply and Sewerage Board, 2012
Figure 3.12 UNIDO type arc screen
Refer to Sec.5.6 of Part Aof the manual for details of screens.
a. Preventive Maintenance
Verify the equipment manufacturers manual for preventive maintenance 2085
instructions and carry out the same (if permitted to be done by the operator).
Switch off electrical power before doing any work on the mechanical screen.
Before you start the days work, check for any friction between metal parts. If
friction exists and the sound is disturbing, disconnect the electric supply and 2090
divert all sewage to manual screens. Enter this action in the site register. Do not
perform repairs by yourself because it is dangerous.
Check the alignment of the tipper plates. If the screenings are slipping back and
are not going up, allow the machine to work and do not stop it. Enter the
abnormality in the site register and request for visit by the manufacturers 2095
engineer. Do not perform repairs by yourself because it is dangerous.
Before you start the days work, do not approach the mechanical screen unless
you are wearing, electrical gloves, safety helmet, and safety boots.
Before you start the days work, switch off the mechanical screen and restart it. 2100
Watch for any friction or sparks. If you notice sparks, disconnect the electric
supply and divert all sewage to manual screens. Enter the abnormality in the site
register. Sometimes, these sparks can be dangerous and may cause
electrocution.
Follow the procedure for disposing screenings as described earlier. 2105
3.4.3 Accessories (Conveyors)
Belt conveyors are used in conveying the screenings to the trolley parked by the side of the
screen chamber. Generally these are meant only for mechanical screens. For manually operated
screens, the water content has to first drip out fully before the screenings can be put on the
conveyor. In the case of mechanical screens, the angle is close to vertical, the height is more and 2110
dewatering is automatic, but this is not the case with manual screens. If it is to be used, then the
conveyor belt has to be behind the operator. The operator first picks up the screenings, drops it
on the slotted platform and allows four hours for the screenings to drip fully. Thereafter, he can
lift it by the same fork and turn it around 180 degrees and place it on the conveyor belt behind
him. On the other hand, in smaller plants he can directly push the screenings to the slotted 2115
platform and into the trolley on the ground after the sidewall. All the guidelines for preventive
maintenance, regular day-to-day maintenance and operation, and site register entries by the
operator are the same as before.
3.4.4 Disposal of Screenings
Screenings generally consist of non-bio degradable stuff like plastic sachets, milk packets, 2120
shampoo packets, etc., with very little organic content. Hence, it is best disposed of as a secure
landfill, which should be prevented from direct rainfall and also flow of overland rainwater. The
procedure specified by the pollution control authority should be adhered to without fail.
3.5 Grit Removal
The different types of grit removal equipment are given in Part A of the manual. These are 2125
velocity controlled channels, detritors, aerated grit chambers, vortex type, etc.
3.5.1 Preventive Maintenance
Almost all these equipment are patented. Each manufacturer has proprietary schedules for
preventive maintenance. These schedules should be followed. Preventive maintenance should be
done only by the manufacturer or the erection contractor who has installed these equipment, and 2130
not by the operators.
3.5.2 Regular Day to Day Maintenance
The operator should hose the mechanical parts using the high pressure hose, and pump the final
treated sewage so that slime does not accumulate.
Where flap gates or turnstiles are provided, the operator should necessarily exercise these 2135
once a day.
The operator should not enter the chambers unless the sewage entry is blocked, the chamber has
been dry for at least two hours and the operator is wearing an oxygen mask.
In the case of velocity controlled channels, the trip switch controlled travelling bridge with
suspended suction hoses for each channel all connected to a vacuum pump set are standard 2140
items. If this system fails and grit accumulates in the channel, each channel should be taken out
of sewage flow. The scour valve should be opened below the chamber and the sewage after
filtering through the in-built filter port should be allowed to drain to the site drain. Thereafter,
the chamber should be allowed to air dry for at least two hours, high pressure water jetting,
draining and air drying cycle carried out at least three times. Subsequently, labourers can be 2145
deployed to scrap the grit provided that the labourers wear goggles, gloves, safety shoes and
oxygen masks.
3.5.3 Disposal of Grit
The grit is usually pre-rinsed in the grit removal chamber itself before it is evacuated from it.
Figure 3.13 shows a typical grit chamber. 2150
Clean grit is characterised by the lack of odour. Washed grit may resemble particles of sand and
gravel, interspersed with inert materials from households. Grit washing mechanism has to be
included whenever the detention time is more and flow through velocity is less. Unless washed,
it may contain considerable amount of organic matter. This becomes an attraction to rodents and
insects and is also unsightly and odorous. The grit should be contained in a secure landfill as 2155
directed by the local pollution control authority or disposed along with the municipal solid
wastes, if permitted.
Figure 3.13 Typical grit chamber
3.6 Pump Equipment 2160
The types of pumps are dealt with in Part A of the manual. These are horizontal centrifugal,
vertical shaft centrifugal, dry submersible and wet submersible pumps.
This shall be done only by the manufacturer / his authorized service agency / properly trained
staff. The operator shall not carry out preventive maintenance. 2165
3.6.2 Regular Day-to-Day Maintenance
This should include the tasks as given in Table 3.1.
Table 3.1 Tasks to be addressed in day-to-day regular maintenance
Maintenance Interval
Daily Week Month Year
Earthing Check whether earthing is proper yes
Visual Appraisal Check that safety aids and first aid are in place yes
Gland packing Check for leaks and condition of mechanical seals yes
Alignment Check alignment using computerized monitor yes
Oil & Grease Check lubrication as per manufacturers procedure yes
Motors Check vibration and temperature yes
Mountings Check for vibrations from foundation bolts yes
Bearings Check for unusual sounds yes
Pump sequence Start & stop the pumps as per duty condition yes
Foundation Check for spalling or cracking yes
Proper operation of submersible pump systems requires that minimum submergence should be
maintained always. This is for two primary reasons: 2170
- Prevention of motor overheating
- Prevention of vortexing and associated problems
The following should be inspected:
- Inspect seal for wear or leakage and repair, if required.
- Visually inspect the oil in the motor housing. 2175
Remove pipe plug from housing.
Make sure oil is clean and clear, light amber in colour and free from suspended
particles.
Milky white oil indicates the presence of water.
If the system fails to operate properly, carefully read the instructions and perform maintenance 2180
recommendations.
Before starting the pump, check the following:
- Check insulation resistance by megger at free end of cable and verify with pump
manual. 2185
- Check continuity between ends of motor in the same phase and in all phases.
- Check resistance across moisture sensing wires and verify with pump manual.
- Physically rotate the coupling joint and verify smooth movement.
- Check for leaky oil plug and fix it before starting.
- Check for the bulbs indicating the onoff status of the pump and replace fused bulbs. 2190
- Look for warning lamps for alerting the pumped liquid entering the oil chamber.
- Close the discharge valve before starting the pump. This is also taken care by check
valve.
- Open the discharge valve gradually and not all of a sudden.
- While the pump is running at full flow, check the power consumed to be within duty 2195
- If the power consumed is very high, stop the pump and inform the manufacturer.
- Switch off the pump only after the discharge valve is closed.
3.6.4 Accessories
3.6.4.1 Oil and Grease 2200
- Pumps, motors, and drives should be oiled and greased strictly in accordance with
the recommendations of the manufacturer. Cheap lubricants may often become the
most expensive in the end.
- Oil should not be put in the housing while the pump shaft is rotating because a
considerable amount of oil will be picked up and retained due to the rotary action of 2205
the ball bearings. When the unit comes to rest, an overflow of oil will occur around
the shaft or oil will flow out of the oil cup.
3.6.4.2 Bearing
- Pump bearings should usually last for many years if serviced properly and used
correctly. 2210
- There are several types of bearings used in pumps such as ball bearings, roller
bearings, and sleeve bearings. Each bearing has a special purpose, such as thrust load,
radial load, and speed. The type of bearing used in each pump depends on the
manufacturer's design and application.
- Whenever a bearing failure occurs, the bearing should be examined to determine the 2215
cause and, if possible, to eliminate the problem.
3.6.4.3 Packing Gland
- Check packing gland, which is the unit's most abused and troublesome part. (Figure
3.14)
Figure 3.14 Packing gland
- If the stuffing box leaks excessively when gland is pulled up with mild pressure,
remove the packing and examine the shaft sleeve carefully.
- Replace grooved or scored shaft sleeve because packing cannot be held in stuffing 2225
box with roughened shaft or shaft sleeve.
- Replace the packing a strip at a time, tamping each strip thoroughly and staggering
the joints. Position the lantern ring (water sealing) properly.
- If grease sealing is used, completely fill the lantern ring with grease before putting
remaining rings of packing in place. 2230
- The proper size of packing should be available in the plant's equipment files.
3.6.4.4 Mechanical Seal
Many pumps use mechanical seals instead of packing. (Figure 3.15)
- Mechanical seals serve the same purpose as packing; that is, they prevent leakage
between the pump casing and shaft. The seals have two faces that mate tightly and 2235
prevent water from passing through them.
- The different materials are selected for their best application. Some of the factors for
selection of material are:
Liquid and solids being pumped
Shaft speed 2240
- Initially, mechanical seals are more expensive than packing when installed in a pump.
This cost is recovered through maintenance savings over a period of time. 2245
- Some of the advantages of mechanical seals are as follows:
They last from three to four years without any maintenance, resulting in labour
savings.
Usually, there is no damage to the shaft sleeve at the time of their replacement.
Continual adjusting, cleaning, or repacking is not required. 2250
The construction of a mechanical seal is shown below.
Figure 3.15 Mechanical seal
- Whatever be the method used, the mechanical seal must be inspected frequently. 2255
- Grease cups must be kept full at all times and inspected to make sure they are
operating properly. When a pump is fitted with a mechanical seal, it must never run
dry or the seal faces will be burned and ruined.
- Mechanical seals should not leak from the gland. If a leak develops, the seal may
require resurfacing or it may have to be replaced. 2260
- Repair or replacement of mechanical seal requires the pump to be removed and
dismantled.
- Seals are quite delicate and special care must be taken when installing them.
Mechanical seals differ widely in their construction and installation, and the seal
manufacturer's instructions must be followed. 2265
3.7 Flow Measuring Devices
Flow, similar to water level (Refer to Sec.6.5.2 Level Measuring Equipment), is one of the
most important wastewater parameters to be measured. The various types of flow-measuring
devices have three basic criteria that determine their performance: area, velocity, and device
characteristics. The two basic types of flow measurement are open-channel and closed-pipe. For 2270
good measuring-device performance, both types require approach conditions free of
obstructions and abrupt changes in size and direction. Obstructions and abrupt changes produce
velocity-profile distortions that lead to inaccuracies.
3.7.1 Weir Flow-Meter
A weir measures the liquid flowing in open channels or partially filled pipes under atmospheric 2275
pressure. (Figure 3.16) This device causes the flow to take on certain characteristics (such as
shape and size) depending on the device used. Changes in flow-rate produce a measurable
change in the liquid level near or at the device. This level is related to flow-rate by an
appropriate mathematical formula. The specific device determines the location and accuracy of
level measurements and is extremely important for accurate performance. 2280
Measurement errors occur if the actual crest height differs from the designed height due to
accumulated matter on the channel floor. Remove sediment, if necessary. Floating matter or
surface wave may cause incorrect level measurements and lead errors in flow measurements.
Therefore, floating matter should be removed immediately.
Figure 3.16 Typical weir; generated elevation
Equation for rectangular weir is shown below.
Q = 1.85 (L 0.2H) H
(3.1) 2290
Where Q is cum/sec and H is in m
Refer to Sec.3.10 of Part A of the manual for equations to calculate flow rate with various types
of weirs.
3.7.2 Electromagnetic Flow-Meter
Magnetic flow meters are used extensively in applications ranging from filtered effluent to 2295
thickened or digested solids. (Figure 3.17) They function by electromagnetic induction, in
which the induced voltage generated by a conductor moving through a magnetic field is linearly
proportional to the conductor's velocity. As the liquid (the conductor) moves through the meter
(generating the magnetic field), the voltage produced is measured and converted to a velocity
and, thus, a flow-rate. Magnetic meters require a full pipe for proper operation. Proper 2300
grounding is important for certain brands. In applications where greasing of electrodes is likely,
additional equipment for degreasing the electrode may be required. Magnetic flow meters
provide no obstructions and are manufactured with abrasion-and corrosion-resistant liners,
which is why they are frequently used in solids metering. Repairs should be done only by
manufacturers representatives. 2305
Electromagnetic flow meters rarely break down because they have no moving parts. Dirt on
sensors should be cleaned because that may cause error in measurements.
Figure 3.17 Magnetic flow meter 2310
3.7.3 Ultrasonic Flow-Meter
Ultrasonic flow meters are based on the measurement of ultrasonic wave water-transit time or
frequency shift caused by the flowing fluid. An instrument that measures wave-transit time is
called a time-of-flight or counter-propagation ultrasonic flow meter (Figure 3.18). Ultrasonic
waves of known frequency and duration are beamed across the pipe at known angles. The waves 2315
are sensed either directly by an opposing receiver or indirectly as reflected waves. The changes
in wave transit time or frequency caused by the flowing liquid are linearly proportional to the
liquid velocity. This velocity is converted from flow and output to a display by conversion
electronics. The presence or absence of air bubbles and density of solids in the fluid being
metered affect the meters. Operators should follow the manufacturers specifications and 2320
carefully match the meters to the application.
Figure 3.18 Reflecting ultrasonic flow meter
3.7.4 Fluorescent Tracers 2325
Florescent tracer method requires the use of a tracer like Rhodomine B Dye which is injected
using a peristaltic pump from a small volume of a known concentration of dye solution. The dye
is injected into the gravity or pumping main. After travelling and getting mixed, the dye
concentration is measured there. The mass is the same in the beginning and after travelling. The
instrument used is called Fluorometer. The dye will automatically degrade. It does not affect the 2330
water body.
3.8 Preventive Maintenance
Equipment becomes more complex with the application of advanced technologies and
automation systems in recent years. Thus, high technical knowledge is required and technicians,
technical tools, and special instruments are necessary for implementing preventive maintenance 2335
of the equipment. Unlike O&M contractors, manufacturers can provide such skilled staff and
special tools. The manufactures can provide safe and secure maintenance based on their long
experience and abundant information on their products. Preventive maintenance after expiry of
warranty period should be provided by the manufacturers continuously.
A good maintenance program is essential for a pumping station to operate continuously at peak 2340
design efficiency. A successful maintenance program will cover everything from mechanical
equipment, such as pumps, valves, scrapers, and other moving equipment, to the care of the
plant grounds, buildings, and structures. For preventive maintenance, it is advisable to follow a
schedule for the maintenance of the equipment. The schedule covers recommendations for
checks and remedial actions to be observed at different intervals such as daily, monthly, 2345
quarterly, semi-annually, annually and bi-annually.
Operators should receive training to obtain more knowledge of characteristics and structure of
machinery and to improve their maintenance skill.
a. Mechanical Maintenance
Mechanical maintenance is of prime importance as the equipment must be kept in good 2350
operating condition for the plant to maintain peak performance. Manufacturers provide
information on the mechanical maintenance of their equipment. Operators should
thoroughly read manuals on the plant equipment, understand the procedures, and contact
the manufacturer or the local representative if there are any questions. The instructions
should be followed very carefully when performing maintenance on equipment. Operators 2355
also must recognise tasks that may be beyond their capabilities or repair facilities, and
should request assistance when needed.
b. Maintenance of Civil Structures
Building maintenance is another program that should be maintained on a regular schedule.
Buildings in a treatment plant are usually built of sturdy materials to last for many years. 2360
Buildings must be kept in good condition by repairs. For selecting paint for a treatment
plant, it is always a good idea to have a painting expert help the operator select the types of
paint needed to protect the buildings from deterioration. The expert also will have some
good ideas as to colour schemes to help blend the plant in with the surrounding area.
Consideration should also be given to the quality of paint. A good quality, more expensive 2365
material will usually give better service over a longer period of time than the economy-type
Building maintenance programs depend on the age, type, and use of a building. New
buildings require a thorough check to ensure that essential items are available and are
working properly. Older buildings require careful observation and prompt attention to 2370
detect leaks, breakdowns, and replacements beforehand. Attention must be given to the
maintenance requirements of many items in all plant buildings, such as electrical systems,
plumbing, heating, cooling, ventilating, floors, windows, roofs, and drainage around the
buildings. Regularly scheduled examinations and necessary maintenance of these items can
prevent many costly and time-consuming problems in the future. 2375
In each plant building, periodically check all stairways, ladders, catwalks, and platforms
for adequate lighting, head clearance, and sturdy and convenient guardrails. Protective
devices should be around all moving equipment. Whenever any repairs, alterations, or
additions are made, avoid building accident traps such as pipes laid on top of floors or hung
from the ceiling at head height, which could create serious safety hazards. 2380
Keep all buildings clean and orderly. Janitorial work should be done on a regular schedule.
All tools and plant equipment should be kept clean and in their proper place. Floors, walls,
and windows should be cleaned at regular intervals to maintain a neat appearance.
c. Valve Maintenance
Valves should be lubricated regularly (according to the manufacturers instructions), and 2385
valve stems should be rotated regularly to ensure ease of operation. These activities should
be part of a regular pump-maintenance program.
d. Electric Actuator Maintenance
- Declutch and operate the manual hand wheel.
- Check oil level and top up, if required. 2390
- Re-grease the grease lubricated bearing and gear trains, as applicable.
- Check the insulation resistance of the motor.
- Check for undue noise and vibration and take necessary rectification measures.
- Tighten limit switch cam ends. Check for setting; readjust, if necessary.
- Examine all components and wiring thoroughly and rectify as necessary. 2395
- Change oil or grease in the gear box and thrust bearing.
- Check the condition of the gears and replace them if teeth are worn out.
e. Flow Meter Maintenance
Each individual sensing meter will have its own maintenance requirements.
The single most important item to be considered in sensor maintenance is good 2400
housekeeping. Always keep sensors and all instrumentation very clean. Good
housekeeping, the act of providing preventive maintenance for each of the various sensors,
includes ensuring that foreign bodies do not interfere with the measuring device. Check for
and remove deposits that will build up from normal use. Repair the sensor or measuring
device whenever it is damaged. 2405
External connections between the sensing and conversion and readout devices should be
checked to ensure such connections are clean and connections are firm. Be sure no foreign
obstruction will interfere or promote wear. On mechanical connections, grease as directed;
on hydraulic or pneumatic connections, disconnect and ensure free flow in the internal
passage. 2410
f. Maintenance of Pumps
The maintenance schedule should list out items to be attended to at different periods, such
as daily, semi-annually, annually, and others:
i. Daily Observations
Leakage through packing 2415
Bearing temperature
Undue noise or vibration
Pressure, voltage and current readings
ii. Semi-annual Inspection
Free movement of the gland of the stuffing box 2420
Cleaning and oiling of the gland bolts
Inspection of packing and repacking, if necessary
Alignment of the pump and the drive
Cleaning of oil-lubricated bearings and replenishing fresh oil. If bearings are
grease-lubricated, the condition of the grease should be checked and replaced to 2425
correct quantity, if necessary.
An anti-friction bearing should have its housing packed with grease so that the
void spaces in the bearings and the housing are 1/2 to 2/3 filled with grease. A
fully packed housing will cause the bearing to overheat and will result in
reduced life of the bearing. 2430
iii. Annual Inspection
Cleaning and examination of all bearings for flaws developed, if any
Examination of shaft-sleeves for wear or scour. Cleaning and examination of all
bearings for flaws developed, if any
Checking clearances 2435
Clearances at the wearing rings should be within the limits recommended by the
manufacturer. Excessive clearances indicate a drop in the efficiency of the pump. If
the wear is on only one side, it means misalignment. Not only should the
misalignment be corrected, but also the causes of the misalignment should be
investigated and the clearances reset to the values recommended by the manufacturers. 2440
If the clearance on wear is seen to be 0.2 or 0.25 mm more than the original clearance,
the wearing ring should be renewed or replaced to obtain the original clearance. These
are to be done by the equipment representative.
Impeller-hubs and vane-tips should be examined for any pitting or erosion.
End-play of the bearings should be checked. 2445
All instruments and flow-meters should be re-calibrated.
Pump should be tested to ensure proper performance is being obtained.
In the case of vertical turbine pumps, the inspection can be bi-annual. Annual
inspection is not advisable because it involves disturbing the alignment and
clearances. 2450
iv. Annual Maintenance and Repairs
Consumables and lubricants
Adequate stock of such items as packing glands, belts, lubricating oils, greases should
be maintained.
Replacement of spares 2455
To avoid downtime, a stock of fast-moving spares should be maintained. A set of
recommended spares for two years of trouble-free operation should be ordered along
with the pump.
Repair workshop
The repair workshop should be equipped with tools such as bearing-pullers, clamps, 2460
pipe-wrenches, and other general-purpose machinery such as welding set grinder,
blower, drilling machine.
3.9 Troubleshooting
Refer to Appendix 3.1 to Appendix 3.3.
3.10 Record Keeping 2465
The purpose of recording data is to track operational information that will identify and duplicate
optimum operating conditions.
A record of equipment performance and repairs allow operations or maintenance personnel to
properly evaluate an equipments effectiveness and determine if the equipment meets the
objectives to justify its purchase and installation. As a minimum, the following basic 2470
information should be maintained for each equipment in the pumping station:
- Plant equipment identification number
- Manufacturer
- Model number and serial number
- Type 2475
- Dates of installation and removal from service
- Reasons for removal
- Location when installed
- Calibration data and procedures
- Hours required to perform maintenance 2480
- Cost of replacement parts
- Operations and maintenance manuals, references and their locations
- Apparatus failure history
An example of an annual inspection report for pumping station is shown in Table 3.2. Inspection
reports should be prepared for each pumping station according to the equipment installed. 2485
Table 3.2 Annual inspection report for pumping station
Pumping station annual inspection report
General condition of equipment
No.1 No.2 No.3
1.Pump
2.Gates
Gate operator (manual)
Gate operator (motor)
3.Crane and hoist
4.Siphon breaker
5.Trash racks
Drive chain
Electrical: Date:
1.Motors
2.Mortor bearing
3.Swichgear controls
4.Control panels
1.Water levels Elevation Remarks
Building and grounds: Date:
1.Sump
2.Forebay
3.Discharge chamber
4.Gatewell to river outlet
6.Fire extinguishers
7.Tools and cabinets
8.Painting
9.Caulking
10.Grating, rails and ladders
11.Water system and plumbing
12.Louvers and ventilators
13.Windows
14.Doors
Recommended maintenance/inspection tasks for equipment in pumping stations are summarised
by frequencies and are listed in Table 3.3. Because the required maintenance/inspection and
their frequencies may differ depending on the equipment installed, maintenance plans should be 2490
prepared according to manufacturers instruction manuals of related equipment.
Table 3.3 Recommended maintenance for pumping equipment
Start up Monthly 3-Mo 6-Mo 1Yr 5Yr Opr hrs
Trash rake GI,O,CL CL
Motors AL PG CL
Heaters GI
Gear reducers GI CH
Drive chain PG PG
Pillow blocks PG
Torque limit coupling PG
Shear pin and sprocket GI
Trip cam GI
Control panel GI,CL
Sub-station drainage GI
Building structure GI
Trash-rack GI
Toilet facility GI
Domestic water GI
Holding tank GI PO
Siphon breakers GI
Unit heaters GI
Fire extinguishers GI GI
Switch gear GI
Bus and connections GI,CL
Instruments and lamps GI GI,CL
Lighting panel GI GI
Control panel GI GI
Grounding GRT
Float control GI GI
Main pump motors starts
Entrance channel GI,RS
Sump GI,RS
Gates GI,O,CL
Stem GI CL,SG
Thrust nut GI CL,SG
Manual operators PG
Motor operators GI,CL
O Operate GRT Ground resistance test
CH Change TO Test oil
CL Clean GI General inspection
MR Megger and record AL Add lubricant
PG Pressure grease PO Pump out
SG Surface grease TS Test
RC Remove condensate RS Remove silt
3.11 Duties of Site Engineer in Charge and Higher Ups 2495
The site engineer should first check the entries of the operator in the previous three shifts and
take corrective action, or alert the supervisor by e-mail and make an entry in the site register. If
the site engineer cannot correct the problem within two weeks, he should directly send an e-mail
message to the plant in charge. If no action is taken even after two weeks, the complete
responsibility will rest with the plant in charge from then onwards, including the responsibility 2500
for any accidents/fatalities caused by not taking the requisite action.
3.12 If the STPis under O&M by the Contractor
The references to operator, site engineer and plant in charge inevitably apply to the staff of the
contractor. The engineer in charge of supervising the contractors work should review the site
register once a fortnight and institute such remedies as available under the contract. 2505
The most important thing for O&M of pumping stations is to minimize suspension time due to
facility failures and maximize the life of pumps.
For accomplishing these targets, the following causes of breakdown of pumps should be
eliminated: 2510
- Inflow of screenings into pumping stations
- Overloading of pumps
Preventive maintenance is also essential for early detection of abnormalities.
Part B: Operation and Maintenance CHAPTER 4: SEWAGE TREATMENT
CHAPTER 4 SEWAGE TREATMENT FACILITIES
Sewage treatment is a multi-stage process designed to clean water and protect natural water
bodies. Municipal sewage contains various wastes. If untreated or improperly collected and
treated, this sewage and its related solids could hurt human health and the environment.
A treatment plants primary objectives are to clean the sewage and meet the plants permit
requirements. Treatment plant personnel do this by reducing the concentrations of solids, 2520
organic matter, nutrients, pathogens, and other pollutants in sewage. The plant must also help
protect the receiving water body, which can only absorb so many pollutants before it begins to
degrade, as well as the human health and environment of its employees and neighbours.
One of the challenges of sewage treatment is that the volume and physical, chemical, and
biological characteristics of sewage continually change. Some changes are the temporary results 2525
of seasonal, monthly, weekly, or daily fluctuations in the sewage volume and composition.
Other changes are long-term, the results of alterations in local populations, social characteristics,
economies, and industrial production or technology. The quality of the receiving water and the
publics health and well-being may depend on a treatment plant operators ability to recognize
and respond to potential problems. These responsibilities demand a thorough knowledge of 2530
existing treatment facilities and sewage treatment technology.
4.2 Pump Equipment
Refer to Chapter 3. (Sec.3.6 Pump Equipment)
4.3 Fine Screen and Grit Chamber
Refer to Chapter 3. (Sec.3.4 Screen and Sec.3.5 Grit Removal) 2535
4.4 Oil and Grease Removal
4.4.1 Manual Process
The oil and grease removal unit consists of are simple tanks with an underflow baffle where the
floating oil and grease is detained on top of the sewage. These are fit only for small STPs of
about 1 MLd capacity or less. The floating oil and grease is removed by a rotating slotted pipe 2540
as in Figure 4.1.
In actual operation, the scum of oil and grease is removed by rotating the slit pipe so that the
scum flows over the slit, through the pipe and goes to a holding high-density polyethylene tank
below the pipe on the outside. The scum is then sold to pollution board-authorized oil
re-refining firms. The grit that settles in the trough below is drained to a sump and pumped to 2545
the beginning of the grit chamber.
The maintenance is very simple and requires periodic cleaning only.
4.4.2 Floatation Process
This process involves floating the oil and grease by either fine bubbles of compressed air or
directly by steam liberated near the floor. The same figure as in Figure 4.1 can also be used by 2550
releasing fine bubbles of compressed air at the floor or steam near the floor. Commercially, the
air is dispersed into very fine particles with the raw sewage and released under gravity in a
shallow tank where the fine bubbles take the oil along with them to the surface and are skimmed
off by a scoop pips as in Figure 4.2. The unit is typically called a dissolved air floatation (DAF)
unit. The schematic of this is shown in Figure 4.3. A parallel plate separator is shown in 2555
schematic in Figure 4.4.
Source: http://en.wikipedia.org/wiki/Industrial_wastewater_treatment
Figure 4.1 Typical gravity type oil and grease removal unit
Source: http://www.tradeindia.com/fp341764/High-Flow-Rate-Dissolved-Air-Flotation-Clarifier.html
Figure 4.2 DAF unit
Source: http://www.sciencedirect.com/science/article/pii/S0263876212002705
Figure 4.3 Schematic of DAF unit 2565
http://en.wikipedia.org/wiki/Industrial_wastewater_treatment
Figure 4.4 Parallel plate separator
All these units are almost patented types and there are no fixed O&M guidelines. Each unit has
to follow the guidelines of the respective manufacturer. 2570
4.5 Equalization
Flow equalization can be either inline or offline. With inline flow equalization, all of the flow
enters the flow equalization basin, and a constant outflow rate is maintained. With offline flow
equalization, only that portion of the flow above a given flow rate (typically the average flow) is
diverted into the flow equalization basin. The accumulated flow is then released during 2575
low-flow periods to bring the total flow to average flow for the day.
The inline flow equalization is the easiest to control. Typically, the flow is pumped out using
flow-controlled variable-speed pumps or is pumped in and flows out by gravity using a flow
control valve and flow meter. If the latter is used, careful selection of the flow control valve is
needed to prevent clogging, even if screened or primary treated sewage is to be equalized. 2580
For offline flow equalization, flow control gates or variable speed pumps can be used. If a
constant elevation side weir is used, achieving a controlled flow rate over the side weir is
difficult and is not recommended. Variable speed pumps are a better choice.
4.5.1 Operation
Fill-and-draw mode is the most efficient method of operating an equalization basin for flow 2585
dampening. The basin is filled during the day when peak flows are occurring, and then it is
drawn down at night when the plant is receiving low flows and, hence, is more capable of
treating excessive flow. If an equalization basin is not operated in fill-and-draw mode, it will act
as a mass loading equalization basin only, assuming the basin is completely mixed.
The successful operation of equalization basins requires proper mixing and aeration. Design of 2590
mixing equipment provides for blending the contents of the tank and preventing deposition of
solids in the basin.
Mechanical aerators, which offers one method of providing both mixing and aeration, have
oxygen transfer in clean water under standard conditions, but the oxygen-transfer efficiency
(OTE) in sewage is lower. Minimum operating levels for floating aerators typically exceed 1.5 2595
m and vary with the power and design of the unit. Low-level shutoff controls are needed to
protect the unit. If the equalization basin floor is subject to erosion (earthen basins), concrete
pads on the basin floor are recommended. Baffling may be necessary to ensure proper mixing,
particularly with a circular tank configuration.
Below are some of the recommended monitoring elements required in flow equalization basins. 2600
- Basin liquid level
- Basin dissolved oxygen level
- Influent pH
- Mixers and/or aeration blower status
- Influent/effluent status pumps 2605
- Influent/effluent flow
4.5.2 Maintenance
Because grit removal is rarely provided ahead of equalization, grit will accumulate in the basins.
Therefore, provisions for collecting these solids should be made in the design. If the primary
purpose of the equalization basin is flow dampening, then after the basin has been emptied 2610
following the peak flow event, primary sludge solids will be present on the basin bottom. Water
cannons or strategically placed cleaning hoses, ideally supplied with plant effluent water, will
allow for cleaning the basins. Other equalization basin types that do not operate in a fill/draw
mode still will accumulate solids after a time and will have to be emptied for cleaning. The time
between cleanings is dependent on the influent sewage characteristics and will likely have to be 2615
established by plant operation staff based on operational experience.
4.6 Primary Treatment
4.6.1 Primary Sedimentation Tank Management
This is a simple gravity controlled separation for removing the settleable solids and the
Biochemical Oxygen Demand (BOD) that is removed along with it. 2620
4.6.1.1 Preventive Maintenance
Preventive maintenance of the equipment should be done by the equipment supplier as per the
4.6.1.2 Day to Day Maintenance
The most important is the daily cleaning of the overflow weirs and the weekly scraping of the 2625
floor and walls of the launder. Also periodical checking of the walkway for corrosion is
important. In actual day to day working, the operator should not lean or put his weight on the
handrails.
4.6.1.3 Troubleshooting
Troubleshooting is as given in Appendix 4.1. 2630
4.7 Activated Sludge Process (ASP)
The activated sludge process is still the most widely used biological treatment process for
reducing the concentration of organic pollutants in sewage. Well-established design standards
based on empirical data have evolved over the years.
The activated sludge process has been designed in many different modifications. The process 2635
selected depends on the treatment objectives, site constraints, operational constraints, etc. The
process can be categorized by loading rates, reactor configuration, feeding and aeration patterns,
and other criteria including numerous biological nutrient removal (BNR) processes. Typical
plan layout is illustrated in Figure 4.5.
Figure 4.5 Typical plan layout of activated sludge plant
4.7.1 Description of Activated Sludge Process (ASP)
4.7.1.1 Biological Treatment Processes
In the biological treatment of sewage, the stabilisation of organic matter is accomplished
biologically using a variety of microorganisms, principally bacteria. Microorganisms convert 2645
the colloidal and dissolved carbonaceous matter into gases and non-degradable matter and
incorporate it into their cell tissue. The resulting cell tissue, having a specific gravity slightly
greater than that of treatment achieved, is only to the extent of the portion of organic matter that
has been converted to various gaseous and non-degradable end products because the cell tissues,
which itself is organic, and will be measured as BOD in the effluent. 2650
Conversion of organic matter can be accomplished aerobically, anaerobically or facultatively.
Oxidation of organic matter to various end products is carried out to obtain the energy required
for the synthesis of new cell tissues. In the absence of organic matter, the cell tissue is
endogenously respired to obtain energy for maintenance. In most treatment systems, these three
reactions, oxidation, synthesis and endogenous respiration occur simultaneously. 2655
The microbial mass comprises a heterogeneous population of microorganisms, mostly
heterotrophic bacteria. Various groups of organisms carry out their metabolic reactions
independently as well as sequentially. The selection of organism in the treatment process occurs
naturally, depending upon the sewage characteristics and the environmental conditions
maintained. 2660
4.7.1.2 Design and Operational Parameters
The ASP operation is commonly controlled by maintaining the design Mixed Liquor Suspended
Solids (MLSS), or sometimes, by maintaining design Food to Microorganisms (F/M) ratio. The
latter approach takes care of fluctuations in the quality of raw sewage. If actual F/M is to be
assessed, then measurement of active biomass measured as Mixed Liquor Volatile Suspended 2665
Solids (MLVSS) is needed, which is often difficult to measure and may also give an erroneous
result in case of industrial effluent containing suspended organic waste solids. The solid
retention time (SRT), which is directly related to F/M, is not being used for operational control.
Some of the important design and operational parameters are described and discussed in
following sections. 2670
Operational parameters and their formula are explained in Appendix 4.2. Examples of their
calculations are described in Appendix 4.3.
4.7.1.3 Choice between SRT and F/M as Operation Control Parameter
The evaluation of the active mass of microorganism often makes the use of F/M as a control
parameter impractical. Biological solids are commonly measured by measuring volatile 2675
suspended solids. This parameter is not entirely satisfactory because of the variety of volatile
matter not related to active cellular material.
On the other hand, the evaluation of SRT as a plant control parameter is simple. Since SRT is
the ratio of total suspended solids in the system and that wasted per day, it requires only
measurement of total suspended solids. The proportion of active biomass in solids in the system 2680
and that in solids wasted, either from the aeration tank or from the recycle line, is the same. Use
of SRT as a plant controlling parameter becomes simpler if sludge wasting is done directly from
aeration tank, as the ratio of total solids in system to solids wasting per day reduces to the
ratio of aeration tank volume to volume of sludge wastes per day, provided the mass of solids
escaped in treated effluent is negligible. 2685
4.7.1.4 Effect of c on Settling Characteristics and Drainability of Sludge
It has been established that as a system is operated at higher solids retention time, the settling
characteristics of the biological floc improve. For domestic sewage, SRTs of the order of 3 to 4
days are required to achieve effective settling. Further, it is established that drainability of waste
sludge also improves when a system is operated at higher SRT. The SRT at which a process is 2690
operated approximately represents the average age of biomass present in the process. As the
biomass ages, it contains increasing proportion of dead cells and inert matter. Presence of higher
proportion of mineralised sludge in a process operated at high SRT is responsible for better
setting characteristics and better drainability of sludge.
4.7.1.5 Effect of c on Excess of Sludge Production 2695
SRT is inversely related to F/M ratio. A higher operational SRT represents a low F/M ratio, a
condition of limiting substrate. Bacteria undergo endogenous respiration or decay under a
limiting substrate environment. More biomass undergoes endogenous respiration, resulting in
less net bacterial growth. Therefore, excess sludge production is reduced as a system is operated
at high SRT. Further, since the settling characteristic of sludge improves at high SRT, 2700
concentrated underflow can be withdrawn from the secondary sedimentation tank. This also
results in reduction in volume of excess sludge as for a fixed mass of excess sludge.
4.7.1.6 Excess Sludge Wasting
Excess bio-sludge is commonly wasted from return sludge line. It can also be wastes directly
from aeration tank. If excess bio-sludge is directly wasted from aeration tank, then increased 2705
volume of sludge is a disadvantage. However, if excess bio-sludge is mixed with influent of
primary settling tank and wasted as mixed sludge of primary settling tank, then direct wasting
from aeration tank has no influence on final volume of sludge and therefore, can easily be
adopted. The operator of a plant needs to have an idea of actual volume of excess sludge
wasting required. 2710
4.7.1.7 Return Sludge Flow
Sufficient return sludge capacity should be provided if the biological solids are not to be lost in
the effluent. However, a return flow rate higher than that is required unnecessarily increases
solids loading on settling tank and results in withdrawal of dilute sludge. The ratio of return
sludge flow to average flow can be set on the basis of sludge volume index (SVI). SVI is 2715
defined as the volume in mL occupied by one gram of activated sludge mixed liquor solids, dry
weight, after settling of 30 min. in a 1,000 mL graduated cylinder. The procedure of SVI
measurement is as follows (Figure 4.6).
a. Collect a sample of mixed liquor or return sludge.
b. Carefully mix sample and pour into 1,000 mL graduate cylinder. Vigorous shaking 2720
or mixing tends to break up floc and produces slower setting or poorer separation.
c. Record settleable solids percentage at regular intervals.
Figure 4.6 Sludge settling analysis
mg ,
) L / mg ( liquor mixed in matter Suspended
) L / ml min( in liquor mixed settled in Sludge
) g / ml ( SVI
= (4.1) 2725
Table 4.1 provides SVI values and probable indication of settling properties of activated sludge.
For all cases refer to remedies in Troubleshooting in Appendix
Table 4.1 Relations between Sludge Volume Index and settling characteristics of sludge
SVI Indication
Less than 50 ml/g Pin floc potential
50 to 100 ml/g Good range
100 to 150 ml/g Filament growth
150 to 200 ml/g Bulking at high flows
200 to 300 ml/g Bulking
More than 300 ml/g Severe bulking
Sourse: JICA, 2011
Quantity of return sludge flow is keyed to settled sludge volume. The ratio of recirculated 2730
sludge flows to the settled sludge volume. V
should be equal to the flow entering the clarifier
to the clarifier volume (Figure 4.7).
Figure 4.7 Recirculated sludge flow ratio
4.7.2 Conventional Activated Sludge Process 2735
The conventional activated sludge process typically consists of a concrete biological reactor
followed by a concrete clarifier. Sewage and return activated sludge (RAS) enter together at one
end of the reactor and leave mixed at the other end. This mixed liquor flows into the clarifier
where it is allowed to settle and the treated effluent separates from the activated sludge. The
effluent from the process flows over the clarifier weirs while the settled activated sludge is 2740
either recycled to the reactor or wasted out of the system. (Figure 4.8)
Figure 4.8 Conventional activated sludge process
4.7.2.1 Start Up
Start-up help should be available from the design engineer, vendors, nearby operators, or other 2745
specialists. The equipment manufacturers or contractor should be under contractor start-up
instruction and assistance. During start-up, they should be present to be sure that any equipment
breakdowns are not caused by improper start-up procedures.
The operator may have several options in the choice of start-up procedures with regard to
number of tanks used and procedures to establish a suitable working culture in the aeration 2750
tanks. The method described in this section is recommended because it provides the longest
possible aeration time, reduces chances of solids washout, and provides the opportunity to use
most of the equipment for a good test of its acceptability and workability before the end of the
First, start the air blowers and have air passing through the diffusers before primary effluent is 2755
admitted to the aeration tanks. This prevents diffuser clogging from material in the primary
effluent and is particularly important if fine bubble diffusers are used.
Fill both aeration tanks to the normal operating water depth, thus allowing the aeration
equipment to operate at maximum efficiency. Using all of the aeration tanks will provide the
longest possible aeration time. The operators are trying to build up a micro organism population 2760
with a minimum amount of seed organisms, and you will need all the aeration capacity available
to give the organisms a chance to reach the settling stage.
After a biological culture of aerobes is established in the aeration tanks, sufficient oxygen must
be supplied to the aeration tank to overcome the following demands:
- DO usually is low in both influent sewage and return sludge to the aerator. 2765
- Influent sewage may be septic, thus creating an immediate oxygen demand.
- Organisms in the presence of sufficient food create a high demand for oxygen.
The effluent end of the aerator should have a dissolved oxygen level of at least 1.0 mg/L. DO in
the aerator should be checked every two hours until a pattern is established.
Thereafter, DO should be checked as frequently as needed to maintain the desired DO level and 2770
to maintain aerobic conditions in the aerator. Daily flow variations will create different oxygen
demands. Until these patterns are established, you will not know whether just enough or too
much air is being delivered to the aeration tanks. Frequently, excess air is provided during early
mornings when the inflow waste load is low. Air supply may be too low during the afternoon
and evening hours because the waste load tends to increase during the day. 2775
If sewage enters the tank before air is coming out of the diffusers, the diffusers could become
plugged. If the plant is the diffused-air type with air lift pumps for return sludge, the air line
valve to the air lifts (pumps) will have to be closed until the settling compartment is filled.
Otherwise, all the air will attempt to go to the empty compartment and no air will go to the
diffusers. Once the settling compartment is filled from the overflow from the aeration tank, the 2780
air lift valves may be opened. They will have to be adjusted to return a constant stream of water
and solids to the aeration tank. This adjustment is usually two to three turns open on the air
valve to each air lift.
There may be a build-up of foam in the aeration compartment during the first week or so of
start-up. A 25-mm water hose with a lawn sprinkler may be used to keep it under control until 2785
sufficient mixed liquor solids are obtained.
Try to build up the solids or mixed liquor suspended solids (MLSS) as quickly as possible
during start-up. This can be achieved by not wasting sludge until the desired level of MLSS is
4.7.2.2 Routine Operation and Maintenance 2790
4.7.2.2.1 Aeration Tanks
The operational variables in an activated sludge plant include:
- Rate of flow of sewage,
- Air supply,
- MLSS, 2795
- Aeration period,
- DO in aeration and settling tanks, and
- Rate of sludge return and sludge condition.
The operator should possess a thorough knowledge of the type of system adopted, namely,
conventional, high rate, extended aeration or contact stabilisation so that effective control of the 2800
variables can be exercised to achieve the desired efficiency of the plant.
Inspection of mechanical aerators should be done for:
- Bearings,
- Bushes, and
- Transmission gears. 2805
and they should be lubricated as per the schedule suggested by the manufactures.
The whole unit should be thoroughly inspected once a year, including replacement of worn out
parts and painting with anti-corrosive paints to achieve the desired efficiency of the plant. A
record of operations should be maintained.
When inhibitory substance for activated sludge (such as industrial sewage) is contained in 2810
influent, treatment in reactors may be affected. To avoid such an inhibition, colour and odour of
plant influent should be checked through daily inspections such as at the grit chambers or the
primary sedimentation tanks where sewage flows in at first. If any abnormal condition is
observed, report to a person in charge of water quality or the plant manager.
4.7.2.2.2 Sewage Flow 2815
Since the activated sludge treatment is biochemical in nature, conditions in the aeration tank
should be maintained uniform at all times. A sudden increase in the rate of flow or sludge of
flow should be avoided. If supernatants from digester containing more than 3,000 mg/L of SS
are taken into the settling tank, then they should be pre-treated as otherwise heavy load will be
imposed on the activated sludge system. Measurement of sewage flow and the BOD applied to 2820
the aeration tank should be made.
4.7.2.2.3 Air Supply
Frequent checks of DO at various points in the tank and at the outlet end should be made; it
should not be less than 1 mg/L. It will help in determining the adequacy of the air supply. The
uniformity of air distribution can be easily checked by observing bubbling of the air at the 2825
surface, which should be even over the entire surface area of the tank. If the bubbling looks
uneven, clogging of diffusers is indicated. Clogging is also confirmed by the increase of 0.1 to
0.15 kg/cm
in the pressure gauge reading. Adding chlorine gas to air may help in removing
clogging of diffusers on air side if it is due to organic matter. Other methods of cleaning will
have to be resorted to if this procedure does not clear up the clogging. Air flow meters should be 2830
checked periodically for accuracy; air supply and air pressures should be recorded hourly and
daily, respectively, to avoid over-aeration or under-aeration. Mechanical or surface aerators
should be kept free from fungus or algae growths by cleaning them periodically.
4.7.2.2.4 Mixed Liquor Suspended Solids
Control of the concentration of solids in the mixed liquor of the aeration tanks is an important 2835
operating factor. It is most desirable to hold the MLSS constant at the suggested concentration.
The test of MLSS should be done at least once a day on large plants, preferably during peak
flow. As the MLSS will be minimum when the peak flow starts coming in and will be maximum
in the night hours when the flow drops, operating MLSS value would be the average hourly
value in a day; the same should be verified at least once a month. In case of very large plants, 2840
regular daily check is desirable.
4.7.2.2.5 Return Sludge
The return sludge pumps provided in multiple units should be operated according to the increase
or decrease in return sludge rate of flow required to maintain the necessary MLSS in aeration
unit, based on the SVI. The SVI should be determined daily to know the condition of sludge. A 2845
value of over 200 definitely indicates sludge bulking.
A good operation calls for prompt removal of excess sludge from the secondary tanks to ensure
that the sludge is fully aerobic. This should be measured daily and recorded. The excess sludge
is taken to the digester directly or through the primary settling tank.
4.7.2.2.6 Foaming 2850
Foaming or frothing is sometimes encountered in activated sludge plants when the sewage
contains materials which reduce the surface tension, the synthetic detergents being the major
offender. Froth, besides being unsightly, is easily blown away by wind and contaminates all the
surfaces it comes into contact with. It is a hazard to workmen because it creates a slippery
surface even after it collapses. Foam problems can be overcome by the application of a spray of 2855
screened effluent or clear water, increasing MLSS concentration, decreasing air supply or
addition of other special anti-foam agents. The presence of synthetic anionic detergents in
sewage also interferes with the oxygen transfer and reduces aeration efficiency.
4.7.2.2.7 Microscopic Examination
Routine microscopic examination of solids in aeration tank and return sludge to identify the 2860
biological flora and fauna present will enable good biological control of the aeration tanks.
4.7.2.2.8 Records
Activated sludge operation should include recording of flow rates of sewage and return sludge,
DO, MLSS, MLVSS, biota, SRT (sludge age), air, BOD, COD and nitrates in both influent and
effluent. 2865
4.7.2.2.9 Biological Uptake Rate Procedure
After deaerating the sample of at least 250 ml of mixed liquor with sodium meta bi sulphite start
the diffuser and record the dissolved oxygen with time by a dissolved oxygen probe and plot the
saturation deficit with time in semi log paper. The slope of the graph is the uptake rate.
Generally this is not for a plant control test. It is used for alpha value by comparing it with the 2870
value for tap water.
4.7.2.2.10 Nutrient Control
Nutrient control should be referred subsection 5.8.1.7.6, 5.8.1.7.7, and 5.8.1.7.8 in the Part A
4.7.2.2.11 DO Saturation 2875
DO saturation table should be referred Table 5.9 and 5.10 in the Part Amanual.
4.7.2.3 Aeration Equipment
4.7.2.3.1 Air Blowers
The blower system is designed to provide sufficient airflow to meet the system process
requirements. Blower systems are available with either positive displacement (PD) or 2880
centrifugal type units. Typically, PD units are used for plants with smaller air volume
requirements. Output airflow from a PD blower remains relatively constant with varying
discharge pressure. Centrifugal blower systems are generally equipped with additional controls
to regulate the flow as the discharge pressure varies.
a. Positive Displacement Blowers 2885
The positive displacement blower provides a constant volume (cubic meters) output of air
per revolution for a specific set of rotors or lobes. Blower output is varied by changing rotor
or lobe speed (RPMs or revolutions per minute). The higher the RPM, the greater is the air
Small positive displacement blowers ranging from 3 to 28 m3/min. are usually installed to 2890
be operated at a fixed volume output. These smaller units are directly driven by electric
motors through a direct coupling or through sheaves and belts.
If a change in air volume output is required, it is accomplished by changing the motor to
one with a higher or lower RPM or by changing sheaves to increase or decrease blower
rotor or lobe rotation (RPM), thus increasing or decreasing air output. 2895
NOTE: These small units are commonly used with package plants, pond aeration systems, small aerobic
digesters, gas mixing in digesters and gas storage compressors.
Large positive displacement blowers ranging from57 to 570 m
/min. may also be driven by
internal combustion engines or variable speed electric motors in order to change blower
volume outputs as required in activated sludge plants. By increasing or decreasing engine 2900
or motor RPM, the positive displacement blower output can be increased or decreased.
The air lines are connected to the blower through a flexible coupling in order to keep
vibration to a minimum and to allow for heat expansion. When air is compressed, heat is
generated; thus increasing the discharge temperature as much as 56 C or more.
A check valve follows next, which prevents the blower from operating in reverse should 2905
other blowers in the same system be operating while this blower is off.
The discharge line from the blower is equipped with an air relief valve which protects the
blower from excessive back pressure and overload. Air relief valves are adjusted by
weights or springs to open when air pressure exceeds a point above normal operating range,
around 0.4 to 0.7 kg/cm2 in most sewage treatment plants. An air discharge silencer is also 2910
installed to provide decibel noise reduction. Ear protective devices should be worn when
working near noisy blowers.
The impellers are machined on all exterior surfaces for operating at close tolerances; they
are statically and dynamically balanced. Impeller shafts are made of machined steel and are
securely fastened to the impellers. Timing gears accurately position the impellers. 2915
Lubrication to the gears and bearings is maintained by a lube oil pump driven from one of
the impeller shafts. An oil pressure gauge monitors the system oil pressure. An oil filter is
located in the oil sump to ensure that the oil is free from foreign materials. An oil level is
maintained in the gear housing so that gears and bearings will received splash lubrication in
case of lube oil pump failure. Air vents are located between the seals and the impeller 2920
chamber to relieve excessive pressure on the seals.
b. Centrifugal Blowers
The centrifugal blower is a motor connected to a speed-increasing gear-driven blower
(Figure 4.9) that provides a variable air output.
Minimum through maximum air output is controlled by guide vanes, which are located on 2925
the intake side of the blower. These vanes may be positioned manually by operating
personnel or may be controlled by plant instrumentation based on either DO levels in the
aeration tanks or the plant influent flows.
Figure 4.9 Schematic of centrifugal blowers 2930
The blower consists of an impeller, volute casing, shaft and bearings, speed-increasing gear
box and an electric motor or internal combustion engine to drive unit. Air enters the volute
casing through an inlet nozzle and is picked up by the whirling vanes of the impeller where
it is hurled by centrifugal force into the volute casing. Air enters the volute in its smallest
section and moves in a circular motion to the largest section of the volute where it is 2935
discharged through the discharge nozzle.
Air lines are connected to the blower through flexible couplings in order to keep variation
to a minimum and to allow for heat expansion. The air suction line is usually equipped with
a manually operated butterfly valves are usually electrically or pneumatically operated.
The impeller is machined on all surfaces for operating at close tolerances and is statically 2940
and dynamically balanced. The impeller shaft is supported in a shaft bearing stand which
contains a thrust bearing and journal bearings.
Lubrication to the bearings and gears is maintained by a positive displacement main oil
pump that is driven by the speed-increasing gear unit. An auxiliary electrically operated
centrifugal oil pump is also used to provide oil pressure in the event of failure of the main 2945
oil pump and to lubricate the blower shaft bearings before start-up and after shutdown. The
oil reservoir is located in the blower base plate. Cartridge type or disc-andspace type oil
filter is based on the degree of filtration required.
Due to the very high speeds at which these blower units operate and the resultant high oil
temperature, an oil cooler unit is installed. This unit, in most cases, is a shell and tube, oil-to 2950
water heat exchanger.
c. Air Filters
Filters remove dust and dirt from air before it is compressed and sent to the various plant
processes. Clean air is essential for the protection of blowers and downstream equipment
Large objects entering the impellers or lobes may cause severe damage on 2955
blowers.
Deposits on the impellers or lobes reduce clearances and cause excessive wear
and vibration problems on blowers.
Clean air prevents fouling of air conduits, pipes, tubing or dispersing devices on
diffusers. 2960
The filters may be constructed of a fibre mesh or metal mesh material that is sandwiched
between the screen material and encased in a frame. The filter frames are then installed in a
filter chamber. Other types of filters include bags, oil-coated travelling screens and
electrostatic precipitators.
The preventive maintenance schedule for the blowers is as follows: 2965
- Weekly
Maintain proper lubricant level
- Quarterly
Check for abnormal noises and vibration
Check if air filters are in place and not clogged 2970
Check motor bearing for rise in temperature
Check that all covers are in place and secure
Lubricate motor ball bearings
Check that electrical connections are tight
Check wiring integrity 2975
- Biannually
Lubricate motor sleeve bearing
Inspect and clean rotor ends, windings and blades
Check that electrical connections are tight and corrosion is absent
- Annually 2980
Check bearing oil
4.7.2.3.2 Air Distribution
The air distribution system (Figure 4.10) is to deliver air from the blowers to air headers in the
aeration tanks and other plant processes and consists of:
- Pipes, 2985
- Valves, and
- Metering devices.
Figure 4.10 Typical air distribution system in aeration tank
An air metering device should be located in a straight section of the air main on the discharge 2990
side of the blower.
Air headers are located in or along the aeration tank and are connected to the air distribution
system from which they supply air to the diffusers. The two most common types of air headers
are the swing header and the fixed header.
The swing header is a pipe with a distribution system connector fitting, a valve, a double pivot 2995
upper swing joint, upper and lower riser pipes, pivot elbow, levelling tee and horizontal air
headers. An air blow off leg, as an extension of the lower tee connection, is fabricated with
multiple alignment flanges, gaskets, and jack screws for levelling of the header.
The fixed header is a pipe with a distribution system connector fitting, a valve, union, a riser
pipe, horizontal air headers and header support feet. These headers are generally not provided 3000
with adjustable levelling devices; they rely on the fixed levelling afforded by the feet attached
to the bottom of the horizontal air headers. Raising and lowering the air header is commonly
found in package plants, channel aeration and grit chamber aeration. Header valves are used to
adjust the air flow to the header assembly and to block the air flow to the assembly when
servicing the header or diffusers. 3005
4.7.2.3.3 Diffusers
An air diffuser or membrane diffuser is an aeration device used to transfer air and oxygen with
oxygen into sewage. Oxygen is required by microorganisms/ bacteria resident in the water to
break down the pollutants. Diffusers use the followings to produce fine or coarse bubbles.
- Rubber membrane, or 3010
- Ceramic elements.
The shapes of the diffusers can be:
- Disc,
- Tube, or
- Plate. 3015
a. Bubble size
The subject of bubble size is important because the aeration system in a sewage treatment
plant consumes an average of 50 to 70 per cent of the energy of the entire plant. Increasing
the oxygen transfer efficiency decreases the power the plant requires to provide the same
quality of effluent water. 3020
- Fine bubble
Fine bubble diffusers produce a plethora of very small air bubbles which rise
slowly from the floor of tank and provide substantial and efficient mass transfer
of oxygen to the water.
Fine bubble diffusers evenly spread out (often referred to as a grid 3025
arrangement) on the floor of a tank and provide the operator of the plant a
great deal of operational flexibility.
This can be used to create zones with high oxygen concentrations (oxic or
aerobic), zones with minimal oxygen concentration (anaerobic) and zones with
no oxygen (anoxic). This allows for more precise targeting and removal of 3030
specific contaminants.
- Coarse bubble
There are different types of coarse bubble diffusers from various manufactures,
such as the stainless steel wide band type coarse bubble diffuser.
Fine bubble diffusers have largely replaced coarse bubble diffusers and 3035
mechanical aerators in most of the developed world and in much of the
developing world.
b. Maintenance
The preventive maintenance schedule of bubble diffusers is as follows:
- Daily maintenance 3040
Check biological reactor surface pattern.
Check air mains for leaks.
Check and record operating pressure and airflow.
- Weekly maintenance
Purge water and moisture from distribution piping. 3045
Bump diffuser system.
- Annual maintenance
Drain biological reactor.
Remove excess solids that may accumulate in the reactor.
Clean diffusers. 3050
Check that retaining rings are in place and are tight.
Check that fixed and expansion joint retaining rings are tight.
4.7.2.3.4 Surface Aerators
A surface aerator is a mechanical aeration device for various types of aerobic sewage treatment
systems. Surface aerators may be either stationary or floating. The major components of the 3055
mechanical surface aerators are motor, gear box and impeller/ aerator/ propeller. More
commonly, these components come combined; but for the purpose of maintenance, they can be
easily separated.
Floating aerators generally employ reinforced fibreglass foam filled pontoons connected to the
aerator platform by a triangular tubular structural frame. The platforms are sized to provide 3060
adequate work area around the drive. Pontoons are placed to minimise any interference with the
flow pattern and maximise stability. Each of the pontoons has a ballast compartment which can
be filled with water or other liquid or other suitable material to adjust submergence and level the
4.7.3 Extended Aeration Process 3065
This is a modification of the activated-sludge process using long aeration periods to promote
aerobic digestion of the biological mass by endogenous respiration (Figure 4.11). The process
includes stabilization of organic matter under aerobic conditions and disposal of the gaseous end
products into the air. Effluent contains finely divided suspended matter and soluble matter.
Extended aeration is similar to a conventional activated sludge process except that the 3070
organisms are retained in the aeration tank longer and do not get as much food. The organisms
get less food because there are more of them to feed. Mixed liquor suspended solids (MLSS)
concentrations are from 3000 to 5000 mg/L and F/M ratio is 0.1 to 0.18. In addition to the
organisms consuming the incoming food, they also consume any stored food in the dead
organisms. The new products are carbon dioxide, water, and a biologically inert residue. 3075
Extended aeration does not produce as much waste sludge as other processes; however, wasting
still is necessary to maintain proper control of the process.
Figure 4.11 Types of extended aeration plants
4.7.3.1 Operation of Aeration Equipment 3080
Aeration equipment should be operated continuously. In a diffused-air system, the operator
controls air flow to the diffuser with the header control valve. This valve forces excess air to the
air lifts in the settling compartment. Good treatment rarely results from interrupted operation
and should not be attempted. The operator can judge how well the aeration equipment is
working by the appearance of the water in the settling compartment and the effluent that goes 3085
over the weir. If the water is murky or cloudy and the aeration compartment has a rotten egg
(H
S) odour, not enough air is being supplied. The air supplied or aeration rate should be
increased slightly each day until the water is clear in the settling compartment. If the water is
clear in the settling compartment, the aeration rate is probably sufficient. Try to maintain a DO
level of around 2 mg/L throughout the aeration tank, if the operator has a DO probe or lab 3090
equipment to measure the DO. Try to measure the DO at different locations in the aeration tank
as well as from top to bottom.
4.7.3.2 Operation and Maintenance
Two methods are commonly used to supply oxygen from the air to the bacteria-mechanical
aeration and diffused aeration. Both methods are mechanical processes with the difference being 3095
whether the mechanisms are at or in the aerator or at a remote location. Mechanical aeration
devices agitate the water surface in the aerator to cause spray and waves by paddle wheels
mixers, rotating brushes or some other method of splashing water into the air or air into the
water so that oxygen can be absorbed. Mechanical aerators in the tank tend to be lower in
installation and maintenance costs. Usually, they are more versatile in terms of mixing, 3100
production of surface area of bubbles, and oxygen transfer per unit of applied power. Diffused
air systems use a device called a diffuser to break up the air stream from the blower system into
fine bubbles in the mixed liquor. The smaller the bubble, the greater is the oxygen transfer due
to the greater surface area of rising air bubbles surrounded by water. Unfortunately, fine bubbles
will tend to regroup into larger bubbles while rising unless they are broken up by suitable 3105
mixing energy and turbulence.
Record the pumping time and weekly waste solids for this time period if results are satisfactory.
If the extended activated sludge plant does not have an aerobic digester, applying waste
activated sludge to drying beds may cause odour problems. If odours from waste activated
sludge drying beds are a problem, consider the following solutions: 3110
- Waste the excess activated sludge into an aerated holding tank. This tank can be
pumped out and the sludge disposed of in an approved sanitary landfill. If aerated
long enough, the sludge could be applied to drying beds.
- The excess or waste activated sludge can be removed by a septic tank pumper and
disposed of in an approved sanitary landfill. 3115
- Arrange for disposal of the excess activated sludge at a nearby treatment plant.
Annually, check the bottom of the hoppers for rocks, sticks, and grit deposits. Also,
check the tail pieces of the air lifts to be sure that they are clear of rags and rubber
goods and in proper working condition.
Frequency and amount of wasting may be revised after several months of operation by 3120
examining:
- The amount of carryover of solids in the effluent
- The depth to which the solids settle in the aeration compartment when the aeration
device is off (should be greater than one-third of the distance from top to bottom)
- The appearance of floc and foam in the aeration compartment as to colour, 3125
settleability, foam makeup, and excess solids on the water surface of the tank
- Results of laboratory testing; a white fluffy foam indicates low solids content in the
aerator while a brown, leathery foam suggests high solids concentrations. If the
operator notices high effluent solids levels at the same time each day, the solids
loading may be too great for the final clarifier. Excessive solids indicate the mixed 3130
liquor suspended solids concentration is too high for the flows and more solids
should be wasted.
4.7.3.3 Normal Operation
Extended activated sludge plants should be visually checked every day. Each visit should
include the following: 3135
- Check the appearance of the aeration and final clarification compartments.
- Check the aeration unit for proper operation and lubrication.
- Check the return sludge line for proper operation. If air in the air lift is not flowing
properly, briefly close the outlet valve, which forces the air to go down and out the
tail piece. This will blow it out and clear any obstructions. Reopen the discharge 3140
valve and adjust to desired return sludge flow.
- Check the comminuting device for lubrication and operation.
- Hose down the aeration tank and final compartment.
- Brush the weirs when necessary.
- Skim off grease and other floating material such as plastic and rubber goods. 3145
- Check the plant discharge for proper appearances, grease, or material of sewage
origin that is not desirable.
4.7.3.4 Abnormal Operation
Remember that changing conditions or abnormal conditions can upset the microorganisms in the
aeration tank. As the temperature changes from season to season, the activity of the organisms 3150
speeds up or slows down. Also, the flows and waste (food as measured by BOD and suspended
solids) in the plant influent change seasonally. All of these factors require the operator to
gradually adjust aeration rates, return sludge rates, and wasting rates. Abnormal conditions may
consist of high flows or solids concentrations as a result of storms or weekend loads.
4.7.3.5 Countermeasures 3155
Extended aeration plant problems may be caused by solids in the effluent, odours, and foaming.
These problems could be caused by under-or- over aeration, too little or too much solids in the
aeration tank, improper return sludge rate, improper sludge wasting or disposal of waste
activated sludge, and abnormal influent conditions such as excessive flows or solids or toxic
wastes. When problems develop in the activated sludge process, try to identify the problem, the 3160
cause of the problem, and select the best possible solution. Remember that the activated sludge
process is a biological process and may require from three days to a week or longer to show any
response to the proper corrective action. Allow seven or more days for the process to stabilize
after making a change in the treatment process.
a. Solids in the Effluent 3165
i. If effluent appears turbid (muddy or cloudy), the return activated sludge
pumping rate is out of balance. Try increasing the return sludge rate. Also,
consider the possible presence of something toxic to the microorganisms or a
hydraulic overload washing out some of the solids.
ii. If the activated sludge is not settling in the clarifier (sludge bulking), several 3170
possible factors could be causing this problem. Look for too low a solids level
in the system, low dissolved oxygen concentrations in the aeration tank, strong,
stale, septic influent, high grease levels in influent, or alkaline wastes from a
laundry.
iii. If the solids level is too high in the sludge compartment of the secondary 3175
clarifier, solids will appear in the effluent. Try increasing the return sludge
pumping rate.If odours are present and the aeration tank mixed liquor appears
black as compared with the usual brown colour, try increasing aeration rates and
look for septic dead spots.
iv. If light-coloured floating sludge solids are observed on the clarifier surface, try 3180
reducing the aeration rates. Try to maintain the dissolved oxygen at around 2
mg/L throughout the entire aeration tank.
b. Odours
i. If the effluent is turbid and the aeration tank mixed liquor appears black as
compared with the usual brown colour, try increasing aeration rates and look for 3185
septic dead spots.
ii. If clumps of black solids appear on the clarifier surface, try increasing the return
sludge rate. Also, be sure the sludge return line is not plugged and that there are
no septic dead spots around the edges or elsewhere in the clarifier.
iii. Examine the method of wasting and disposing of waste activated sludge to 3190
ensure this is not the source of the odours.
iv. Poor housekeeping could result in odours. Do not allow solids to accumulate or
debris removed from sewage to sit around the plant in open containers.
c. Foaming/Frothing
Foaming is usually caused by too low a solids level while frothing is caused by too long a 3195
solids retention time.
i. If too much activated sludge was wasted, reduce wasting rate.
ii. If over aeration caused excessive foaming, reduce aeration rates.
iii. If plant is recovering from overload or septic conditions, allow time for
recovery. 3200
iv. Foaming can be controlled by water sprays or commercially available
defoaming agents until the cause is corrected by reducing or stopping wasting
and building up solids levels in the aeration tank.
v. Learn more about the operation of an activated sludge process under both
normal and abnormal conditions. There operator will also find a troubleshooting 3205
guide for activated sludge plants.
4.7.3.6 Maintenance
Maintenance of equipment in extended aeration plants should follow the manufacturer's
instructions. Items requiring attention include:
a. Plant Cleanliness 3210
Wash down tank walls, weirs, and channels to reduce the collection of odour-causing
b. Aeration Equipment:
i. Air blowers and air diffusion units
ii. Mechanical aerators 3215
c. Air Lift Pumps
d. Scum Skimmer
e. Sludge Scrapers
f. Froth Spray System
g. Weirs, Gates, and Valves 3220
h. Raw Sewage Pumps
4.7.4 Sequencing Batch Reactor (SBR)
In SBR operations, the cycle processes Fill-react, React, Settle and Decant are controlled by
time to achieve the objectives of the operation. Each process is associated with particular reactor
conditions (turbulent/quiescent, aerobic/anaerobic) that promote selected changes in the 3225
chemical and physical nature of the sewage. These changes lead ultimately to a fully treated
effluent. Figure 4.12 shows a typical SBR operation.
- Fill or Fill-react
The purpose of Fill-React operation is to add substrate (raw sewage) to the reactor. The
addition of substrate can be controlled either by limit switches to a set volume or by a timer 3230
to a set time period. If the volume is set, the Fill-React process typically allows the liquid
levels in the reactor to rise from 50-80 per cent to 100 per cent. If controlled by time, the
Fill-React process normally lasts approximately 25 per cent to 50 per cent of the full cycle
time. Period of aeration and/or mixing during Fill are critical to the development of
organisms with good settling characteristics and to biological nutrient removal (Nitrogen 3235
(N), Phosphorus (P)). An advantage of the SBR system of time control is its ability to
modify the reactor conditions during the phases to achieve the treatment goals. This phase
ends when the liquid level in the tank reaches a predetermined level.
Source: Nishihara Environment Co., Ltd. 3240
Figure 4.12 Operating cycles of intermittent SBR process
The purpose of SETTLE is to allow solids separation to occur, providing a clarified
supernatant to be discharged as effluent. In the SBR, this process is normally more efficient
than in a continuous flow system, because in the Settle mode the reactor contents are 3245
completely quiescent. The Settle process is controlled by time and is usually fixed between
30 minutes to an hour so that the sludge blanket remains below the withdrawal mechanism
during the next phase.
- Decant/Discharge
The purpose of the decantation is to remove the clarified, treated water from the reactor. 3250
Sludge wasting is another important step in SBR operation that greatly affects process
performance. It is not included as one of the three basin processes because there is no set
time period within the cycle dedicated to wasting. The amount and frequency of sludge
wasting is determined by process requirements, as with conventional continuous flow
systems. In an SBR operation, sludge wasting usually occurs during the Settle or Decant 3255
phases. A unique feature of the SBR systemis that there is no need for a RAS system. Since
the aeration and settling occurs in the same tank, no sludge is lost in the reaction phase and
none has to be returned from clarifier to maintain the sludge content in the aeration tank.
This eliminates the need for the hardware and controls associated with the conventional
RAS system. The sludge volume and, thus, sludge age in the reactor of the SBR system is 3260
controlled by sludge wasting only.
The manual given by the equipment supplier should be followed. Usually these units are
controlled automatically by programmable logic controllers (PLCs). The precaution
needed is to make sure that power supply is available continuously. If power supply fails,
immediately bring the genset on-line. If there is no genset or if there is no diesel, do not 3265
operate the SBR and close it. Inform the plant in charge and also report to the. official
responsible for overall O&M in the head office directly.
4.7.4.1 Process Control
The SBR has in built process control. Depending on the BOD load, it adjusts the Dissolved
Oxygen (DO) supply by sensing the residual DO and varying the speed of air compressor and 3270
hence the rate of air supply. The most important thing for day to day testing is to understand the
SBR as designed. It may have fully aerobic or anoxic and aerobic or anaerobic, anoxic and
aerobic.
If anaerobic cycle is there, check whether the floor level mixer is working and if it is out of
order, start the installed standby mixer. If both are not in order, enter in the site register and 3275
inform the plant in charge. Make sure that hydrogen sulphide gas is not sensed in the ambient
air near the SBR. If it is sensed by smell, then going near the tank is not advisable. Make sure it
is entered in the site register and it is reported directly to the plant in charge. The operator
should not try and remedy the position. The supervisor should institute and take steps to get the
designer, contractor and O&M team together and rectify the situation. There is a theory that 3280
COD to sulphate ratio is deciding the process. This needs to be checked up and corrected. A
method of correcting the imbalance will be to recycle the treated effluent from a treated sewage
sump to dilute the COD of incoming sewage. The daily tests shall be pH, COD and dissolved
phosphate measured by colorimetric method or Nessler Tubes of 50 ml with fresh standards
prepared every week. BOD can be a weekly test. 3285
In the anoxic cycle, check whether the floor level mixer is working and if it is out of order, start
the installed standby mixer. If both are not in order, enter it in the site register and inform the.
plant in charge. Daily tests will be nitrate estimated by Nesslerization procedure in 50 ml
Nessler tubes. The test is to be done in the beginning, in the mid cycle and at the completion of
the cycle of anoxic phase. If there is no reduction in the nitrate, then something is not in order. 3290
Proceed to check the MLVSS. It should be at least 75 %. If this is not so, enter the value in the
site register and inform the plant in charge. The supervisor should institute and take steps to get
the designer, contractor and O&M team together and rectify the situation.
In the aeration cycle, check the residual DO. This is to be indicated by the built in sensor. If the
sensor is not working use the Winkler method by collecting the mixed liquor and filtering it 3295
through Whatman filter paper number 4 in a BOD bottle and with the tip of the funnel
connected by a rubber tubing so that the filtrate enters the BOD bottle in the submerged
condition always and avoids additional aeration. A procedure for easy use in the field for
instantly testing the BOD is to use a BOD tube. This has been introduced in the Chennai
Metropolitan Water Supply and Sewerage Board (CMWSSB) by M/S Severn Trent of UK as 3300
part of a twinning arrangement. Details of the tube can be obtained from CMWSSB. A
photograph of the CMWSSB chemist using the tube is shown in Figure 4.13.
The principle of the test is related to the BOD caused by colloidal and suspended organics as
relatable to the BOD. The BOD related to suspended solids is inbuilt in the calibration. This
tube is developed only for sewage and not for industrial effluents. The test is performed by 3305
holding the tube as in the photo after filling the treated sewage to incremental heights and
finding out at which point, the black coloured + mark at the bottom vanishes. There is a reading
etched on the side of the tube and this is read at the sewage level when the + mark vanishes
from sight. The principle is the colloidal solids and SS have their portion of BOD. The more the
volume needed to hide the bottom + mark, the less is the colloidal solids and SS and hence, 3310
the lesser is the BOD due to this portion. It is a combination of nephelometry and theory.
Usually the results are within 90%.
- The BOD tube
The Palintest Tube is a specially calibrated plastic tube and is the implest possible
method of performing the instantaneous probable BOD and SS tests on secondary 3315
treated sewage in the field to help the operator to get a feel of these parameters
quickly. The test kit is a tube graduated at 30 to 500 turbidity units. Adouble length
tube with additional graduations from 5 to 25 turbidity units is optionally available.
These were calibrated by the Department of Public Health Engineering, University
of Newcastle upon Tyne. It has an etched black cross mark at bottom 3320
- Procedure
Hold the tube vertically over a white surface and view downwards.
Gradually pour secondary sewage and watch the cross mark
Stop pouring when the cross mark is no longer visible
Read the graduation at the top of the sample in the tube. 3325
This represents the turbidity in Jackson Turbidity units (JTU).
For secondary sewage, the graduation may also be taken as SS
Half the value of JTU plus 5 is also the probable BOD
Source: CMWSSB 3330
Figure 4.13 Use of a BOD tube for instantaneous assessment of the BOD at site.
If the DO is lesser than 20 % of the design value, enter it in the site register and inform the plant
in charge. Check the MLVSS if the above situation occurs. This can be a weekly test. Check the
In the settling cycle, check the SS of the decanted effluent and its COD. There is no need to 3335
check the BOD at the end of every cycle. Prepare a curve of BOD to COD for the treated
sewage and verify the BOD by testing for the COD. This will show the trend every two hours
itself instead of 3 days for BOD actual test. This can however be a weekly test. If the SS and
BOD varies by more than 10 % in the treated sewage, enter the values in the site register and
inform the plant in charge. The decanter cannot be subjected to preventive maintenance in a 3340
functioning SBR. The raw sewage has to be bypassed with prior permission of the supervisor
before this is carried out. The electrical drive of the decanter will require its greasing in some
equipment. Make sure there is a grease guard and grease does not fall into the SBR basin.
Where the rope and pulley method is used, change the rope every month.
4.7.4.2 Records 3345
The limited parameters as above and the flow rate and cycle times are the records.
4.7.4.3 Housekeeping
In all SBR systems, verify build up of slime on the sidewalls in the freeboard. If noticed, scrub
it down into the basin itself during the filling phase. This can be done by the operator standing
on the peripheral walkway and using a long handle wire brush. If there is no such walkway, 3350
leave the slime as it is.
4.7.5 Oxidation Ditch
An oxidation ditch is a modified activated sludge biological treatment process that utilizes long
Solids Retention Times (SRTs) to remove biodegradable organics. Oxidation ditches are
typically complete mix systems, but they can be modified to approach plug flow conditions. 3355
(Note: As conditions approach plug flow, diffused air must be used to provide enough mixing.
The system will also no longer operate as an oxidation ditch). Typical oxidation ditch treatment
systems consist of a single or multichannel configuration within a ring, oval, or
horseshoe-shaped basin. As a result, oxidation ditches are called "racetrack type" reactors.
Horizontally or vertically mounted aerators provide circulation and aeration in the ditch. 3360
Preliminary treatment, such as bar screens and grit removal, normally precedes the oxidation
ditch. Primary settling prior to an oxidation ditch is sometimes practiced, but is not typical in
this design.
Flow to the oxidation ditch is aerated and mixed with return sludge from a secondary clarifier. A
typical process flow diagram for an activated sludge plant using an oxidation ditch is shown in 3365
There is usually no primary settling tank or grit removal system used in this process. Inorganic
solids such as sand, silt, and cinders are captured in the oxidation ditch and removed during
sludge wasting or cleaning operations. The raw sewage passes directly through a bar screen to
the ditch. 3370
The bar screen is necessary for the protection of the mechanical equipment such as rotor and
pumps. Comminutors or barminutors may be installed after the bar screen or instead of a bar
screen. The oxidation ditch forms the aeration basin and here the raw sewage is mixed with
previously formed active organisms. The rotor is the aeration device that entrains (dissolves) the
necessary oxygen into the liquid for microbial life and keeps the contents of the ditch mixed and 3375
moving. The velocity of the liquid in the ditch must be maintained to prevent settling of solids,
normally 0.3 to 0.45 m/sec. The ends of the ditch are well rounded to prevent eddying and dead
areas, and the outside edges of the curves are given erosion protection measures.
The mixed liquor flows from the ditch to a clarifier for separation. The clarified water passes
over the effluent weir and is chlorinated. Plant effluent is discharged to either a receiving stream, 3380
percolation ditches, or a subsurface disposal or leaching system. The settled sludge is removed
from the bottom of the clarifier by a pump and is returned to the ditch or wasted. Scum that
floats to the surface of the clarifier is removed and either returned to the oxidation ditch for
further treatment or disposed of by burial.
Since the oxidation ditch is operated as a closed system, the amount of volatile suspended solids 3385
will gradually increase. It will periodically become necessary to remove some sludge from the
process. Wasting of sludge lowers the MLSS (mixed liquor suspended solids) concentration in
the ditch and keeps the microorganisms more active.
Figure 4.14 Oxidation ditch 3390
4.7.5.1 Operation
Process controls and operation of an oxidation ditch are similar to the activated sludge process.
To obtain maximum performance efficiency, the following control methods must be maintained.
a. Proper Food Supply For the Microorganisms
Influent flows and waste characteristics are subject to limited control by the operator. 3395
Municipal ordinances may prohibit discharge to the collection system of materials that are
damaging to treatment structures or to human safety. Control over wastes dumped into the
collection system requires a pre-treatment facility inspection program to ensure
compliance. Alternate means of disposal, pre-treatment, or controlled discharge of
significantly damaging wastes may be required in order to permit dilution to an acceptable 3400
level by the time the waste arrives at the treatment plant.
b. Proper DO Levels
Proper operation of the process depends on the rotor assembly supplying the right amount
of oxygen to the waste flow in the ditch. For the best operation, a DO concentration of 0.5
to 2.0 mg/L should be maintained just upstream of the rotors. Over oxygenation wastes 3405
power and excessive DO levels can cause a pinpoint floc to form that does not settle and is
lost over the weir in the settling tank. Control of rotor oxygenation is achieved by adjusting
the ditch outlet level control weir.
The level or elevation of the rotors is fixed but the deeper the rotors sit in the water, the
greater the transfer of oxygen from the air to the water (greater DO). The ditch outlet level 3410
control weir regulates the level of water in the oxidation ditch.
c. Proper Environment
The oxidation ditch process with its long-term aeration basin is designed to carry MLSS
concentrations of 3,000 to 5,000 mg/L. This provides a large organism mass in the system.
Performance of the ditch and ditch environment can be evaluated by conducting a few 3415
simple tests and general observations. The colour and characteristics of the floc in the ditch
as well as the clarity of the effluent should be observed and recorded daily. Typical tests are
settleable solids, DO upstream of the rotor, pH, and residual chlorine in the plant effluent.
Laboratory tests such as BOD, COD, suspended solids, volatile solids, total solids, and
microscopic examinations should be performed periodically by the plant operator or an 3420
outside laboratory. The results will aid operator in determining the actual operating
efficiency and performance of the process.
Oxidation ditch solids are controlled by regulating the return sludge rate and waste sludge
rate. Remember that solids continue to deteriorate as long as they remain in the clarifier.
Adjust the return sludge rate to return the microorganisms in a healthy condition from the 3425
final settling tank to the oxidation ditch. If dark solids appear in the settling tank, either the
return sludge rate should be increased (solids remaining too long in clarifier) or the DO
levels are too low in the oxidation ditch.
Adjusting the waste sludge rate regulates the solids concentration (number of
microorganisms) in the oxidation ditch. The appearance of the surface of the oxidation 3430
ditch can be a helpful indication of whether the sludge wasting rate should be increased or
decreased. If the foam on the surface is white and crisp, reduce the wasting rate. If the foam
on the surface is thick and dark, increase the wasting rate. Waste activated sludge may be
removed from the ditch by pumping to a sludge holding tank, to sludge drying beds, to
sludge lagoons, or to a tank truck. Ultimate disposal may be to larger treatment plants or to 3435
approved sanitary landfills.
Remember that this is a biological treatment process and several days may be required
before the process, responds to operation changes. Make operator changes slowly, be
patient, and observe and record the results.
d. Proper Treatment Time and Flow Velocities 3440
Treatment time is directly related to the flow of sewage and is controlled by an adjustable
weir. Velocities in the ditch should be maintained at 0.3 to 0.45 m/sec to prevent the
deposition of floc. With this in mind, the ditch contents should travel the complete circuit
of the ditch, or from rotor to rotor every 3 to 6 minutes. If the rotors are operated by time
clocks (30 minutes off and 30 minutes on, for example), the velocities in the ditch must be 3445
sufficient to re-suspend any settled material.
e. Proper Water/Solids Separation
MLSS that have entered and settled in the secondary clarifier are continuously removed
from the clarifier as return sludge, by pump, for return to the oxidation ditch. Usually, all
sludge formed by the process and settled in the clarifier is returned to the ditch, except 3450
when wasting sludge. Scum that is captured on the surface of the clarifier also is removed
fromthe clarifier and either returned to the oxidation ditch for further treatment or disposed
of by burial.
f. Observations
Some aspects of the operation of an oxidation ditch plant can be controlled and adjusted 3455
with the help of some general observations. General daily observations of the plant are
important to help operator determine whether or not the oxidation ditch is operating as
intended. These observations include colour of the mixed liquor in the ditch, odour at the
plant site, and clarity of the ditch and sedimentation tank surfaces.
i. Colour 3460
Operator should note the colour of the mixed liquor in the ditch daily. Mixed liquor
from a properly operating oxidation ditch plant should have a medium to rich dark
brown colour. If the MLSS, following proper start-up, changes colour from a dark
brown to a light brown and the MLSS appears to be thinner than before, the sludge
waste rate may be too high, which may cause the plant to lose efficiency in removing 3465
waste materials. By decreasing sludge waste rates before the colour lightens too much,
operator can ensure that the plant effluent quality will not deteriorate due to low
MLSS concentrations.
If the MLSS becomes black, the ditch is not receiving enough oxygen and has gone
anaerobic. The oxygen output of the rotors must be increased to eliminate the black 3470
colour and return the process to normal aerobic operation. This is done by increasing
the submergence level of the rotor.
ii. Odour
When the oxidation ditch plant is operating properly, there will be little or no odour.
Odour, if detected, should have an earthy smell. If an odour other than this is present, 3475
operator should check and determine the cause. Odour similar to rotten eggs indicates
that the ditch may be going anaerobic, requiring more oxygen or a higher ditch
velocity to prevent deposition of solids. The colour of the MLSS could be black if this
were the case. Odour may also be a sign of poor housekeeping. Grease and solids
build-up on the edge of the ditch or sedimentation tank will go anaerobic and cause 3480
odours. In an oxidation ditch, odours are much more often caused by poor
housekeeping than by poor operation.
iii. Clarity
In a properly operating oxidation ditch, a layer of clear water or supernatant is usually
visible about a meter upstream from the rotor. The depth of this relatively clear water 3485
may vary from almost nothing to as much as five or more cm above the mixed liquor.
The clarity will depend on the ditch velocity and the settling characteristics of the
activated sludge solids.
Two other good indications of a properly operating oxidation ditch are the clarity of
the settling tank water surface and the oxidation ditch surface free of foam build-up. 3490
Foam build-up in the ditch (normally not enough to be a nuisance) is usually caused
by insufficient MLSS concentration. Most frequently foam build-up is only seen
during plant start-up and will gradually disappear.
Clarity of the effluent from the secondary clarifier discharged over the weirs is the best
indication of plant performance. A very clear effluent shows that the plant is achieving 3495
excellent pollutant removals. A cloudy effluent often indicates a problem with the
plant operation.
4.7.5.2 Equipment Maintenance
Regularly scheduled equipment maintenance must be performed according to manufacturers'
instruction manuals. Operator should check each piece of equipment daily to see that it is 3500
functioning properly. There may have very few mechanical devices in the oxidation ditch plant,
but they are all important.
The rotors and pumps should be inspected to ensure that they are operating properly. If pumps
are clogged, the obstructions should be removed. Listen for unusual noises. Check for loose
bolts. Uncovering a mechanical problem in its early stages could prevent a costly repair or 3505
replacement at a later date.
Lubrication should also be performed with a fixed operating schedule and properly recorded.
Follow the lubrication and maintenance instructions furnished with each piece of equipment.
Make sure that the proper lubricants are used. Over lubrication is wasteful and reduces the
effectiveness of lubricant seals and may cause overheating of bearings or gears. 3510
4.7.6 Chemical Clarification
Chemicals are used for a variety of municipal treatment applications, such as to enhance
flocculation/sedimentation, condition solids, add nutrients, neutralize acid base, precipitate
phosphorus, and disinfect or to control odours, algae, or activated-sludge bulking.
Chemical precipitation is a widely used, proven technology for the removal of metals and other 3515
inorganics, suspended solids, fats, oils, greases, and some other organic substances (including
organophosphates) from sewage.
Precipitation is assisted through the use of a coagulant, an agent which causes smaller particles
suspended in solution to gather into larger aggregates. Frequently, polymers are used as
coagulants. The long-chain polymer molecules can be either positively or negatively charged 3520
(cationic or anionic) or neutral (non-ionic). Since sewage chemistry typically involves the
interaction of ions and other charged particles in solution, these electrical qualities allow the
polymers to act as bridges between particles suspended in solution, or to neutralize particles in
solution. The specific approach used for precipitation will depend on the contaminants to be
removed, as described below. 3525
4.7.6.1 Metals Removal
Water hardness is caused primarily by the dissolution of calcium and magnesium carbonate and
bicarbonate compounds in water, and to a lesser extent, by the sulphates, chlorides, and silicates
of these metals. The removal of these dissolved compounds, called water softening often
proceeds by chemical precipitation. Lime (calcium oxide), when added to hard water, reacts to 3530
form calcium carbonate, which itself can act as a coagulant, sweeping ions out of solution in
formation and settling.
To do this with lime alone, a great deal of lime is typically needed to work effectively; for this
reason, the lime is often added in conjunction with ferrous sulphate, producing insoluble ferric
hydroxide. The combination of lime and ferrous sulphate is only effective in the presence of 3535
dissolved oxygen, however. Alum, when added to water containing calcium and magnesium
bicarbonate alkalinity, reacts with the alkaline substances to form an insoluble aluminium
hydroxide precipitate.
Soluble heavy metal ions can be converted into insoluble metal hydroxides or carbonates
through the addition of hydroxide compounds. Additionally, insoluble metal sulphides can be 3540
formed with the addition of ferrous sulphate and lime.
Once the optimal pH for precipitation is established, the settling process is often accelerated by
addition of a polymer coagulant, which gathers the insoluble metal compound particles into a
coarse floc that can settle rapidly by gravity.
4.7.6.2 Phosphorus Removal 3545
Metal salts (most commonly ferric chloride or aluminium sulphate, also called alum) or lime,
have been used for the removal of phosphate compounds from water. When lime is used, a
sufficient amount of lime must be added to increase the pH of the solution to at least 10,
creating an environment in which excess calcium ions can react with the phosphate to produce
an insoluble precipitate (hydroxyl apatite). Lime is an effective phosphate removal agent, but 3550
results in a large sludge volume.
When ferric chloride or alum is used, the iron or aluminium ions in solution will react with
phosphate to produce insoluble metal phosphates. The degree of insolubility for these
compounds is pH dependent.
4.7.6.3 Suspended Solids 3555
Finely divided particles suspended in solution can escape filtration and other similar removal
processes. Their small size allows them to remain suspended over extended periods of time.
More often than not, the particles populating sewage are negatively charged. For this reason,
cationic polymers are commonly added to the solution, both to reduce the surface charge of the
particles, and also to form bridges between the particles, thus causing particle coagulation and 3560
settling.
Alternatively, lime can be used as a clarifying agent for removal of particulate matter. The
calcium hydroxide reacts in the sewage solution to form calcium carbonate, which itself acts as
a coagulant, sweeping particles out of solution.
4.7.6.4 Additional Considerations 3565
The amount of chemicals required for treatment depends on the pH and alkalinity of the sewage,
the phosphate level, and the point of injection and mixing modes, among other factors.
Competing reactions often make it difficult to calculate the quantities of additives necessary for
chemical precipitation. Accurate doses should be determined by jar tests and confirmed by field
evaluations. Chemicals are usually added by a chemical feed system that can be completely 3570
enclosed and may also include storage space for unused chemicals.
Choosing the most effective coagulant depends on jar test results, ease of storage, ease of
transportation, and consideration of the operation and maintenance costs for associated
Chemical precipitation is normally carried out through a chemical feed system, most often a 3575
totally automated system providing for automatic chemical feeding, monitoring, and control.
Full automation reduces manpower requirements, allows for less sophisticated operator
oversight, and increases efficiency through continuous operation.
An automatic feed system may consist of storage tanks, feed tanks, metering pumps (although
pumpless systems do exist), overflow containment basins, mixers, aging tanks, injection quills, 3580
shot feeders, piping, fittings, and valves.
Chemical feed system storage tanks should have sufficient capacity to run for some time
without running out and causing downtime. At least a one month supply of chemical storage
capacity is recommended, though lesser quantities may be justified when a reliable supplier is
located nearby, thus alleviating the need for maintaining substantial storage space. Additive 3585
chemicals come in liquid and dry form.
4.7.6.5 Jar Testing
Secondary treated sewage from STPS may sometimes carry over the microbes from the clarifier.
When chlorination of the treated sewage is to be carried out, these suspended microbes will
consume the added chlorine before the organic matter in the treated sewage can be oxidized and 3590
pathogenic faecal organisms can be killed. Hence, it may be necessary to carry out coagulation,
flocculation and sedimentation before chlorine is applied. For details of the theory of
coagulation, flocculation and sedimentation, the CPHEEO Manual on Water Supply and
Treatment may be consulted. The purpose of a jar test is to find out which chemical and at what
dosage is needed to improve the clarity of secondary treated sewage. In general, such 3595
coagulation, flocculation and sedimentation is not recommended for raw sewage because the
disposal of the resulting sludge becomes difficult due to a mix of biological and chemical sludge.
At the same time, the phosphorous present in sewage at even as low as 1 mg/l is known to form
a coating around the flocs and prevent them from settling and this in fact increases the turbidity
of raw sewages. This is the reverse of addition of phosphate to cooling waters to prevent the 3600
precipitated scales from settling out in the heat exchanged surfaces.
Jar testing entails adjusting the amount of treatment chemicals and the sequence in which they
are added to samples of raw sewage held in jars or beakers. The sample is then stirred so that the
formation, development, and settlement of floc can be watched just as it would be in the
fullscale treatment plant. (Floc forms when treatment chemicals react with material in the raw 3605
sewage and clump together.)
A typical laboratory bench scale jar test apparatus can be shown in Figure 4.15. The apparatus
allows for six samples each of 1-2 Litre in size, to tested simultaneously.
Source: http://www.neutecgroup.com/jartest.htm 3610
Figure 4.15 Typical jar testing apparatus
The procedure of jar testing is as follows;
The following jar test procedure uses alum (aluminum sulfate) a chemical for coagulation/
flocculation in sewage treatment, and a typical six jar tester.
a. First, using a 1,000 millilitre (mL) graduated cylinder, add 1,000 mL of raw water 3615
to each of the jar test beakers. Record the temperature, pH, turbidity, and alkalinity
of the raw water before beginning.
b. Prepare a stock solution by dissolving 10.0 grams of alum into 1,000 mL distilled
water. Each 1.0 mL of this stock solution will equal 10 mg/L (ppm) when added to
1,000 mL of water to be tested. 3620
c. Using the prepared stock solution of alum, dose each beaker with increased
amounts of the solution. See Table 4.2 below for an example of the increments and
d. After dosing each beaker, turn on the stirrers. This part of the procedure should
reflect the actual conditions of the plant as much as possible. Meaning, if the plant 3625
has a static mixer following chemical addition, followed by 30 minutes in a
flocculator, then 1.5 hours of settling time before the filters, then the test also
should have these steps. The jar test would be performed as follows: Operate the
stirrers at a high RPM for 1 minute to simulate the rapid mixer.
e. Rreduce the speed of the stirrers to match the conditions in the flocculator and 3630
allow them to operate for 30 minutes. Observe the floc formation periodically
during the 30 minutes.
f. At the end of the 30 minutes turn off the stirrers and allow settling. Most of the
settling will be complete after one hour.
g. Use a pipette to draw a portion from the top of each beaker, and measure its 3635
turbidity.
h. Plot supernatant turbidity versus alum dose (Figure 4.16) for the sewage sample and
comment on their shapes.
i. Find out the optimum alum dose. i.e, 25 mg/L from Figure 4.16
If none of the beakers appear to have good results, then the procedure needs to be run again 3640
using different dosages until the correct dosage is found.
Table 4.2 Dosing in Jar Test
Jar No. ml Alum Stock Added mg/LAlum Dosage
1 1.0 10.0
Figure 4.16 Supernatant turbidity vs. Alum dose
4.8 Aerated Lagoon 3645
The aerated lagoon process consists of aeration of the facultative pond of the stabilization pond
by means of an aerator.(Refer to Section 4.13 Waste stabilization pond)
Aerated lagoons are generally provided in the form of simple earthen basins with inlet at one
end and outlet at the other to enable the sewage to flow through while aeration is usually
provided by mechanical means to stabilize the organic matter. The major difference between 3650
activated sludge systems and aerated lagoons is that in the latter settling tanks and sludge
recirculation are absent.
4.8.1 Process Control
Daily tests will be for SS and COD. The BOD will be obtained from the standard curve made
out for this sewage from a curve of BOD to COD. The BOD tube is also useful. There is nothing 3655
much to do by way of process control in aerated lagoon except making sure that all surface
aerators are in working condition. Some aerated lagoons have a final section of the lagoon itself
as the settling compartment. Some other lagoons have a dedicated clarifier outside the lagoon.
In such a case, the return sludge is also provided in some STPs. This return sludge arrangement
must run continuously. The excess sludge disposal is not provided for in aerated lagoons 3660
normally. In case of clarifiers it may be used. Mechanical dewatering facilities are generally not
advised because the MLSS concentrations will be much lesser than in conventional ASPs.
Sludge drying beds with green cover to prevent direct rainfall on the beds is the answer to such
The DO concentration in an aerated lagoon is the best means to determine if the lagoon is 3665
operating properly. Typical practice is to maintain 1 to 2 mg/l DO in the lagoon. Aminimum DO
level of 1 mg/l should be maintained in the lagoon during the heaviest loading periods. Often
the heaviest oxygen demand is during the night when the algae are respiring. The pH range in
the lagoon should range from 7 to 8. The pH can exceed 9 during algal blooms, especially in
low-alkalinity sewage. Surface mechanical aerators when used, should produce good turbulence 3670
and a light amount of froth.
4.8.2 Records
The limited parameters as above and the flow rate and cycle times shall be maintained as
4.8.3 Housekeeping 3675
Keep the bunds free of any grass or weeds. Do not allow tree branches of trees to hang over the
lagoon. Follow all guidelines for motors. If high speed floating aerators are used, pull them out
of the lagoon before attending to it. Check if the power cable is having sufficient slack. Verify
that the power cable is tied at about 3-m centres to vertical secure posts. Do not enter the lagoon
unless you are wearing a life vest and are on a boat with an aide if the aerators are not connected 3680
by a platform.
In all aerated lagoons, weeds and over hanging tree branches shall be avoided. Aphoto of such a
situation is shown in Figure 4.17.
Figure 4.17 Overhanging tree branches and the small area of the lined bund may be seen 3685
- The tree roots will enter the lining and break the concrete slab joints easily.
- Once this occurs, the slabs will lose their strength and start falling down into the
lagoon itself.
- Once this sets in, the earth in the bund will be easily eroded in rains and the bund will
cave in. 3690
- This leads to the lagoon sewage running out on land and polluting the land and water
in wells and streams.
- The hanging tree branches will be dropping leaves which will support growth of
mosquitoes.
- Manual scraping shall be done from the top of bund and not by persons entering the 3695
lagoon.
- In such cases, the branches shall be cut and the cut portions sealed with cow dung.
The biggest danger is if the bund gets broken and sewage escapes; it is very difficult to control
- Reconstructing the bund is also a problem when raw sewage keeps coming daily.
- Stopping the sewage escaping from the broken bund can be done by the following: 3700
Pack cement bags with mix of 90 % clay and 10% sewage and stack them one
over the other.
These have to be dumped to form a cofferdam inside the sewage spread.
Thereafter, the reconstruction of the bund can be taken up easily.
4.9 Attached Growth Systems 3705
One of major attached growth systems adopted in sewage treatment lately is a fixed film
synthetic media filter, which consists of synthetic media such as inclined corrugated media
placed in cube sized packs and the inclinations changed to opposite directions in successive
layers as shown in Figure 4.18.
Figure 4.18 Fixed film synthetic media filters
Primary sedimentation is a pre-requirement in these applications. In Figure 4.18, the applied
sewage is distributed from the top of the media pack by stationary or hydraulically driven
reverse jet arms on opposite radii or rotated by a mechanical drive. The requirements to apply
the sewage on the entire plan are to uniformly and simultaneously allow the gas exchange by 3715
releasing at the top and fresh air automatically forcing itself from the bottom. The microbial
films develop on the fixed media and bring about the metabolism as the sewage passes over
them as a film. In due course of time, the thickness of the film increases and results in sloughing
and getting carried away to secondary settling tanks. Recirculation of the treated effluent is
sometimes practiced before sedimentation so that the enzymes released by the microbes are 3720
returned to the reactor for solubilising the sewage organicmatter.
Many operating problems may be avoided by changing one or more of the following process
control variables: distribution rates, and clarifier operation.
a. Distribution Rates 3725
As a principal process control measure, operators can control the rates at which sewage and
filter effluent are distributed to the filter media. Recirculation can serve several purposes,
- Reduce the strength of the sewage being applied to the filter.
- Increase the hydraulic load to reduce flies, snails, or other nuisances. 3730
- Maintain distributor movement during low flows.
- Produce hydraulic shear to encourage solids sloughing and prevent ponding.
- Reseed the filters microbial population.
- Provide uniform flow distribution.
- Prevent filters from drying out. 3735
b. Clarifier Operation.
The manner in which secondary clarifiers are operated can significantly affect the filter
performance. Although clarifier operation with fixed film reactors is not as critical as that
with suspended-growth systems, operators must still pay close attention to final settling.
Sludge must be removed quickly from the final settling tank before gasification occurs or 3740
denitrification causes solids to rise. Use of the secondary clarifier as a principal means of
thickening (rather than simply for solids settling) may not produce the best effluent quality,
especially during summer months, when denitrification is likely to occur. The sludge
blanket depth in the secondary clarifier should be limited to 0.3 to 0.6 m. Continuous
pumping or intermittent pumping with automatic timer control are used to accomplish 3745
solids wasting.
Planned maintenance will vary from plant to plant, depending on unique design features and
equipment installed. Although this chapter cannot address all of these items, a summary of the
most common and important maintenance tasks follows. 3750
Table 4.3 is a guide to planned maintenance.
Table 4.3 Planned maintenance for fixed film synthetic media filters
Rotary Distributors
Observe the distributor daily. Make sure the rotation is smooth and that spray nozzles are not
plugged.
Lubricate the main support bearings and any guide or stabilizing bearings according to the
manufacturers instructions. Change lubricant periodically, typically twice a year. If the bearings are
oil-lubricated, check the oil level, drain condensate weekly, and add oil as needed.
Time the rotational speed of the distributor at one or more flow rates. Record and file the results for
future comparison. A change in speed at the same flow rate indicates bearing trouble.
Flush distributor arms monthly by opening end shear gates or blind flanges to remove debris. Drain
the arms if idle during cold weather to prevent damage via freezing.
Clean orifices weekly with a high-pressure stream of water or with a hooked piece of wire.
Keep distributor arm vent pipes free of ice, grease, and solids. Clean in the same manner as the
distributor arm orifices. Air pockets will form if the vents are plugged. Air pockets will cause
uneven hydraulic loading in the filter, and nonuniform load and excessive wear of the distributor
support bearing.
Make sure distributor arms are level. To maintain level, the vertical guy wire should be taken up
during the summer and let out during the winter by adjusting the guy wire tie rods. Maintain arms in
the correct horizontal orientation by adjusting horizontal tie rods.
Periodically check distributor seal and, if applicable, the influent pipe to distributor expansion joint
for leaks. Replace as necessary. During replacement check seal plates for wear and replace if wear is
excessive. Some seals should be kept submerged even if the filter is idle or their life will be severely
shortened.
Remove ice from distributor arms. Ice buildup causes nonuniform loads and reduces main bearing
Paint the distributor as needed to guard against corrosion. Cover bearings when sandblasting to
protect against contamination. Check oil by draining a little oil through a nylon stocking after
sandblasting. Ground the distributor arms to protect bearings if welding on distributor and lock out
the drive mechanism at the main electrical panel. Adjust secondary arm overflow weirs and pan test
sewage distribution on filter as needed.
Fixed nozzle distributors
Observe spray pattern daily. Unplug block nozzles manually or by increasing hydraulic loading.
Flush headers and laterals monthly by opening end plates. Adjust nozzle spring tension as needed.
Observe condition of filter media surface daily. Remove leaves, large solids and plastics, grease
balls, broken wood lath or plastic media, and other debris. If ponding is evident, find and eliminate
the cause. Keep vent pipes open, and remove accumulated debris.
Store extra plastic media out of sunlight to prevent damage via ultraviolet rays. Observe media for
settling. After they are installed, media settle because of their own weight and the weight of the
biofilm and water attached to its surface. Settling should be uniform and should stabilize after a few
weeks. Total settling is typically less than 0.3 m for random plastic media, less for plastic sheet
media, and nearly zero for rock. If settling is nonuniform or excessive, remove some of the media
for inspection.
Observe media for hydraulic erosion, particularly in regions where reversing jets hit the media.
Underdrains
Flush out periodically if possible. Remove debris from the effluent channels.
Media containment structure
Maintain spray against inside wall of filter to prevent filter fly infestation and to prevent ice buildup
in winter.
Practice good housekeeping. Keep fibreglass, concrete, or steel outside walls clean and painted, if
applicable. Keep grass around structures cut, and remove weeds and tall shrubs to help prevent filter
fly and other insect infestations. Remember, using insecticides around treatment units may have
adverse effects on water quality or biological treatment units.
Filter pumps
Check packing or mechanical seals for leakage daily. Adjust or replace as needed. Lubricate pump
and motor bearings as per manufacturers instructions. Keep pump motor as clean and dry as
possible. Periodically check shaft sleeves, wearing rings, and impellers for wear; repair or replace as
needed. Perform maintenance of speed reducer, coupling, and other appurtenant equipment
maintenance according to manufacturers instructions.
Secondary clarifier
Lubricate drive motor bearings, speed-reducing gear, drive chains, work and spur gears, and the
main support bearing for the solids-collection equipment according to the manufacturers
instructions. Flush scum troughs and grease wells daily. Maintain solids withdrawal equipment.
Clean effluent wells and baffles at least weekly. Paint or otherwise protect equipment from
corrosion as needed.
Appurtenant equipment
Maintain piping, valves, forced draft blowers, and other appurtenant equipment according to the
manufacturers instructions.
The information provided in Table 4.3 is not equipment or plant-specific. Therefore, both the
manufacturers literature and engineers operating instructions should be consulted and followed. 3755
The frequency of maintenance procedures depends on sitespecific conditions. However, until
operating experience is gained, frequent plant inspections and maintenance should continue.
Maintenance schedules should consider the increased performance of fixed film synthetic media
filters in warm weather months, which may reduce the effect of removing process units from
service. 3760
4.10 Moving Bed Bio Reactor (MBBR)
4.10.1 Configuration
The moving bed biofilm reactor (MBBR) is based on the biofilm carrier elements. Several types
of synthetic biofilm carrier elements have been developed for use in activated sludge processes.
These biofilm carrier elements may be suspended in the activated sludge mixed liquor in the 3765
reaction tank by air from the diffusers in aerobic reactors and by means of propeller mixers in
anaerobic and anoxic reactors. The carrier elements are retained by suitably sized sieves or
plates.
These processes are intended to enhance the activated sludge process by providing a greater
biomass concentration in the aeration tank and thus offer the potential to reduce the basin size 3770
requirements. They have also been used to improve the volumetric nitrification rates and to
accomplish the denitrification in aeration tanks by having anoxic zones within the biofilm depth.
Because of the complexity of the process and issues related to understanding the biofilm area
and activity, the process design is empirical and based on prior pilot-plant or limited full-scale
results. Typical diagram of MBBR is shown in Figure 4.19. 3775
Figure 4.19 Moving bed bioreactor
4.10.2 Operation and Maintenance
There are now more than 10 different variations of the processes in which a biofilm carrier
material of various types is suspended in the aeration tank of the activated sludge process. 3780
Differently varied processes have their own characteristics and require specific O&M. Therefore,
operators should have thorough knowledge on their systems and implement daily O&M
according to the manufacturers instruction manuals. Refer to PartA 5.16.13 MBBR for system
description on the varieties of MBBR.
4.11 Membrane Bio Reactor (MBR) 3785
The membrane bioreactor (MBR) process is a combination of activated sludge process and
membrane separation process. Low pressure membranes (ultra filtration or microfiltration) are
commonly used. Membranes can be submerged in the biological reactor or located in a separate
stage or compartment and are used for liquid-solid separation instead of the usual settling
process. (Figure 4.20) 3790
Source: JSWA
Figure 4.20 Configuration of membrane bioreactor system
Basically, primary sedimentation tank, final sedimentation tank and disinfection facility are not
installed in this process. The reaction tanks comprise an anoxic tank and an aerobic tank, and 3795
the membrane modules are immersed in the aerobic tank. Pre-treated, screened influent enters
the membrane bioreactor, where biodegradation takes place. The mixed liquor is withdrawn by
water head difference or suction pump through membrane modules in a reaction tank, being
filtered and separated into solid and liquid. Surfaces of the membrane are continuously washed
down during operation by the mixed flow of air and liquid generated by air diffuser set at the 3800
bottom of the reaction tank. Permeate from the membranes constitutes the treated effluent.
4.11.1 Operation
All MBR systems require some degree of pumping to force the water flowing through the
membrane. One type of membrane systems uses a pressurized system to push the water through
the membranes. The major systems used in MBRs draw a vacuum through the membranes so 3805
that the water outside is at ambient pressure. The advantage of the vacuum is that it is gentler to
the membranes; the advantage of the pressure is that throughput can be controlled. Both systems
also include techniques for continually cleaning the system to maintain membrane life and keep
the system operational for as long as possible. All the principal membrane systems used in
MBRs use an air scour technique to reduce build-up of material on the membranes. This is done 3810
by blowing air around the membranes out of the manifolds.
The permeate from an MBR has low levels of suspended solids, i.e., the levels of bacteria, BOD,
nitrogen, and phosphorus are also low. Disinfection is easy and may not even be required,
depending on permit requirements.
The solids retained by the membrane are recycled to the biological reactor and build up in the 3815
system. As in conventional biological systems, periodic sludge wasting eliminates sludge
build-up and controls the SRT within the MBR system. The waste sludge from MBRs goes
through standard solids-handling technologies for thickening, dewatering, and ultimate disposal.
Chemical addition increases the ability of solids to settle. As more MBR facilities are built and
operated, a more definitive understanding of the characteristics of the resulting sludge will be 3820
achieved. However, experience to date indicates that conventional sludge processing unit
operations are also applicable to the waste sludge from MBRs.
4.11.2 Maintenance
The key to the cost-effectiveness of an MBR system is membrane life. If membrane life is
curtailed such that frequent replacement is required, costs will increase significantly. Membrane 3825
life can be increased in the following ways:
Good screening of larger solids before the membranes to protect the membranes from physical
Throughput rates that are not excessive, i.e., that do not push the system to the limits of the
design. Low rates reduce the amount of material that is forced into the membrane, and thereby 3830
reduce the amount that has to be removed by cleaners or that will cause eventual membrane
deterioration.
Mild cleaners - cleaning solutions most often used with MBRs include regular bleach (sodium)
and citric acid, are regularly used. The cleaning should be in accordance with manufacturers
recommended maintenance protocols. 3835
4.12 Up FlowAnaerobic Sludge Blanket Reactor (UASB)
The Up flow Anaerobic Sludge Blanket Reactor (UASB) (Figure 4.21), maintains a high
concentration of biomass through formation of highly settleable microbial aggregates. The
sewage flows upwards through a layer of sludge. Separation between gas-solid-liquid takes
place at the top of the reactor phase. Any biomass leaving the reaction zone is directly 3840
recirculated from the settling zone. The process is suitable for both soluble wastes and those
containing particulate matter. The process has been used for treatment of municipal sewage at
few locations and hence performance data and experience available presently are limited.
Figure 4.21 Schematic diagram of an up flow anaerobic sludge blanket reactor 3845
4.12.1 Plant Commissioning and Operation
Two to three months are needed to build up a satisfactory sludge blanket without the addition of
seed sludge from a working UASB. Ashorter time is needed, if seeding is done.
During the start-up period, COD removal in the UASB gradually improves as sludge
accumulation occurs. This may be called the sludge accumulation phase. The end of the sludge 3850
accumulation phase is indicated by sludge washout. At this time, the reactor is shut down to
improve the quality of the sludge. This may be called the sludge improvement phase. After
sludge improvement, blanket formation starts. Once the blanket is formed, again some surplus
sludge washout could occur and to stabilise the stable operation, the excess sludge needs to be
removed periodically. The excess sludge so removed can be sent directly to the sludge treatment 3855
The sludge accumulated in the UASB is tested for pH, volatile fatty acids (VFA), alkalinity,
COD and SS. If the pH reduces while VFA increases, new material should not be fed until the
pH and VFAstabilise.
Daily operation of the UASB requires minimum attention. No special instrumentation is 3860
necessary for control, especially where gas conversion to electric power is not practiced. As
stated, surplus sludge is easy to dry over an open sand bed. The reactor may need to be emptied
completely once in five years, while any floating material (scum) accumulated inside the gas
collector channels may have to be removed every two years to ensure free flow of gas.
4.12.2 Daily Operation and Maintenance of UASB 3865
a. Cleaning of Effluent Gutters
All V-notches must be cleaned in order to maintain the uniform withdrawal of UASB
effluent coming out of each V-notch. The irregular flow from each V-notch results in the
escape of more solids washout. Similarly, blocking of the V-notches of the effluent gutters
will lead to uneven distribution of sewage in the reactor. Therefore, the effluent gutters 3870
have to be inspected on a regular basis to remove any material blocking and even the
outflow over the V-notches in the gutters. The regular maintenance involves cleaning of
V-notches with a broom three times a day (Figure 4.22) and removing sludge with a brush
or with a water jet once a day.
Figure 4.22 Cleaning of effluent gutters
b. Unclogging Feeder Pipes
The feeder pipes should be checked regularly for clogging. Flexible iron rods can serve the
purpose. A submersible pump can be used to unclog the feeder pipes (Figure 4.23). These
feeder pipes are generally clogged due to rags and floating material. It is necessary to 3880
provide a fine screen or extra prevention at the screen to capture floating material at the
pre-treatment unit itself.
Clogged inlet pipes Submersible pump 3885
Cleaning of feed pipes by submersible pumps Clean feed pipes (equal flow distribution)
Source: PWSSB
Figure 4.23 Cleaning of feed inlet pipes
c. Removal of Floating Mat 3890
Floating mat must be removed from the top of the surface of reactor with a rake. The
removed material should be disposed at the dumping site.
d. Check of Leakage of Biogas
The gas collectors should be checked for leakage. Leakage is easily detected by applying
soap solution to the piping. This should be done on a regular basis. If the gas collectors are 3895
leaking, the valve at the end of one bay in the gas leak should be first closed and then
repaired as soon as possible. Regular maintenance includes opening of hatch boxes and
removing floating layer inside the gas collectors.
e. Scrubbing of Biogas
Waste at the Top of UASB Reactor 3900
The risk of the corrosion of dual fuel engine parts, as biogas contains H
S, can be
minimised if biogas can be scrubbed before using it as fuel for dual fuel gas engines.
f. Check for Sludge Withdrawal Ports
The ports of the sludge withdrawal must be free from any clogging which reduces the
chances of checking of sludge height in reactor. 3905
purpose. A submersible pump can be used to unclog the feeder pipes.
g. Methanogenic Activity
Successful operation of a UASB reactor depends upon maintaining a satisfactory balance
between methane and acidogenic bacteria. The methane formers are susceptible to changes 3910
in environmental conditions such as pH, temperature etc. The methanogenic activity must
be analysed monthly.
h. Proper Sludge Wasting
Sludge must be removed or transferred from the UASB reactor occasionally based on the
sludge yield or concentration of TSS or VSS. Higher sludge withdrawal points to a poor 3915
performance of the reactor in terms of treatment.
i. Biogas Analysis
The biogas analysis is used largely at sewage treatment plants where information on fuel
value of gas is important. In addition, knowledge of gas composition can be of considerable
help in the control of digestion units. Sudden changes in gas composition can signal a 3920
change either in the operation of the treatment unit or in the amount or composition of
incoming sewage. Such changes can thus be used as a warning sign to suggest the need for
closer observation and control of treatment unit.
j. H
S Determination
The determination of hydrogen sulphide will continue to be an important consideration 3925
wherever gas is used for fuel in gas engines, particularly in areas where the sulphate content
of sewage is very high.
k. Sludge Pumping Station Maintenance
After every sludge withdrawal operation, clean the pipeline by opening the top flushing
valve until all the sludge in the pipeline is washed out. The sump has to be cleaned with 3930
Never keep the sludge in the sump, it may damage the pumps. Before getting into the
sumps for any maintenance, keep the top cover open for an hour before anybody gets in so
that any accumulated biogas will vent to the atmosphere. Keep the valve chamber dry and
valves clean. Check the electrical components regularly. 3935
l. Biogas Holder Operation and Maintenance
The biogas produced in the reactors is taken in the common FRP pipes to the biogas holder.
The biogas before going to the gas holder passes through moisture trap. The gas coming to
the gas holder is measured through gas flow metres connected to FRP pipe after the
moisture trap 3940
The biogas before going to the holder is branched off. One branch is taken to the flaring
system, the other to the biogas engine. Before going to the engine, the gas is measured from
the flow meter provided on pipeline going to the engine. Sluice valves are provided on the
lines to isolate the flow which is manually operated.
In case of sudden reduction in dome levels, the reactor FRP dome connector and its 3945
connection to the gas pipe header should be checked with soap water for any leakage of
gas. This is one of the reasons for having a gas holder level trap.
The typical UASB preventive maintenance check list is below.
- Date and time
- Check and clean weir levels of division boxes 3950
- Clean-up- feed inlet points
- Cleaning of V-notches
- Removal of sludge from effluent gutter by water jet or brush
- Removal of floating layer on the top of reactor
- Cleaning and scrubbing of effluent channels 3955
- Check gas pipes for leakage
- Leakage greasing of spindle of sludge valves
- Cleaning of sludge sump
4.12.3 Routine Maintenance
4.12.3.1 Quarterly Maintenance 3960
- The spindles of the valves have to be greased every three months.
- The glands and packing of the valves have to be checked every three months and
replaced, if necessary.
4.12.3.2 Annual Maintenance
The reactor should be emptied after the first year of full operation to check the complete feeder 3965
and sludge withdrawal systems, especially the valves and the internal pipes for any
accumulation of debris, sludge etc.
- Afirst check of the complete systemincluding valves and holes should be made after
one year, or earlier when required. Routine check can be established on the basis of
the first inspection observation. 3970
- The effluent gutters should be checked for levelling and alignment once a year. Each
gutter should be horizontally levelled and all gutters in one reactor should be at the
same level.
- Electrical wiring should be checked every year.
- Corrosion on electrical connections should be removed every year. 3975
- The cement structures should be checked yearly and repaired when necessary.
- The sludge filtrate water pumps should be maintained.
4.12.3.3 Five-Yearly Maintenance
Every five years, the following maintenance should be carried out.
- Each reactor should be alternately put out of operation. 3980
- Clean the inside concrete surface.
- Apply new coating of epoxy to the concrete surface.
- Check quality of feed inlet pipes and replace when necessary.
- Check fixing of the feed inlet pipes, both at the distribution boxes and at the bottom.
Change corroded fixing material when necessary. 3985
- Check position of PVC sheets.
- Check the fixing-material of the PVC sheets and replace when necessary.
- Check the quality of gas collectors and carry out repairs where necessary.
4.12.4 Decision Schemes for Sludge Removal
The removal of sludge is subject to a number of choices-how much sludge should be removed, 3990
from where should be removed, etc.
4.12.5 Shut-Down and Standstill
At shutdown of the plant, the sludge will settle at the bottom of the reactor. The biological
activity of the sludge decreases slowly during standstill. Care should be taken to ensure that the
sludge is not exposed to aerobic conditions. This might occur, for instance, when the reactor is 3995
flushed with clean water for prolonged periods.
At shutdown of the plant, the gas production will decrease. At a prolonged period of plant
standstill, the pressure in the gas collection system can drop and air may enter into the system.
In this situation, internal parts of the gas flare, the gas metre and the pressure/vacuum release
valves that normally are not in contact with the atmospheric air start to corrode. These parts 4000
have to be protected, for instance by greasing.
If the water level in the tank is lowered during shutdown, the limited capacity of the vacuum
release valves should be kept in mind. It is possible that imploding of the gas collectors may
occur due to fast withdrawal of the reactor contents. At lowering of the water level in the reactor,
it is advised to open the manholes on the top of the gas collectors. Only after re-establishing the 4005
maximum water level, the manholes can be closed and sealed.
In general, any type of work on the gas collectors requires the opening of the manholes as the
explosive moisture of air and methane can develop in or around the gas collectors. When it is
necessary to enter the reactor while sludge is present, it should be realised that methane is being
formed continuously. A proper ventilation of the reactor is necessary. Very strict rules 4010
concerning open fire, spark emission, etc, should be followed. When entering the reactor plant,
personnel should wear respiration equipment. Measurement of explosion risk and hydrogen
sulphide concentration should be taken frequently when repair work is carried out.
4.12.6 Operational Cautions
- Do not get upon the UASB unless you have a gas mask, safety shoes, goggles and 4015
helmets.
- Do not carry any ignitable matters on your person.
- Once you reach the walking platform at top, check the H2S by hand held meter.
- Unless it registers safety, immediately climb down the UASB.
- Once all the above are ensured, proceed to check any overflows of sewage and if so, 4020
stop the UASB.
- Check for corrosion at least once in 6 months and get it rectified.
4.12.7 Final Polishing Unit (FPU)
Not much maintenance is required for this unit. The algal growth needs to be maintained, and
the dead algae floating on the top of water surface has to be periodically removed. The baffles 4025
provided at the outlet unit have to be cleaned regularly. Keep the floating material away from
the unit. See that dead algal do not pass out into the pond. These ponds should be
de-sludged/de-silted regularly depending on the depth of sludge accumulation. Maintenance
record should be maintained.
4.12.8 Duckweed Pond 4030
The bund sides shall not be grown over by weeds. The Figure 4.17 illustrates the same.
4.13 Waste Stabilization Pond (WSP)
Waste stabilization ponds are open, flow-through earthen basins specifically designed and
constructed to treat sewage and biodegradable industrial wastes. Waste stabilization ponds
provide comparatively long detention periods extending from a few to several days. 4035
There are three principal types of WSP:
- Anaerobic,
- Facultative, and
- Maturation ponds.
Anaerobic ponds and facultative ponds are designed for BOD removal, and maturation ponds 4040
are designed for faecal bacterial removal. These three types of WSP are arranged in a series
first an anaerobic pond, then a facultative pond, and finally (and if needed to achieve the
required effluent quality) one or more maturation ponds.
Apart from the above three types, there is another type of WSP called aerobic pond, which are
seldom used. When used, follow the same procedures as in facultative ponds. 4045
4.13.1 Start-up Procedures
Pond systems should preferably be commissioned at the beginning of the hot season so as to
establish as quickly as possible the necessary microbial populations to effect waste stabilization.
Prior to commissioning, all ponds must be free from vegetation. Facultative ponds should be
commissioned before anaerobic ponds: this avoids odour release when anaerobic pond effluent 4050
discharges into an empty facultative pond. It is best to fill facultative and maturation ponds with
freshwater (from a river, lake or well; mains water is not necessary) so as to permit the gradual
development of the algal and heterotrophic bacterial populations. Primary facultative ponds may
advantageously be seeded in the same way as anaerobic ponds. If freshwater is unavailable,
facultative ponds should be filled with raw sewage and left for three to four weeks to allow the 4055
microbial population to develop; a small amount of odour release is inevitable during the period.
The maintenance requirements of ponds are very simple, but they must be carried out regularly.
Otherwise, there will be serious odour, fly and mosquito nuisance. Maintenance requirements
and responsibilities must therefore be clearly defined at the design stage so as to avoid problems 4060
later. Routine maintenance tasks are as follows:
- Removal of screenings and grit from the inlet works
- Cutting the grass on the embankments and removing it so that it does not fall into the
pond (this is necessary to prevent the formation of mosquito-breeding habitats; the
use of slow-growing grasses minimises this task) 4065
- Removal of floating scum and floating macrophytes, such as. Lemna, from the
surface of facultative and maturation ponds (this is required to maximize
photosynthesis and surface re-aeration and prevent fly and mosquito breeding)
- Spraying the scum on anaerobic ponds (which should not be removed as it aids the
treatment process), as necessary, with clean water or pond effluent, or a suitable 4070
biodegradable larvicide, to prevent fly breeding
- Removal of any accumulated solids in the inlets and outlets
- Repair of any damage to the embankments caused by rodents, rabbits or other
- Repair of any damage to external fences and gates 4075
And precautions and practices are described below:
- The scum has a tendency to form at the corners of the ponds and supports mosquito
- In anaerobic ponds, during times of low pH odour is produced. In such occasions,
addition of sodium hydroxide is required to raise the pH to 7. The advantage of 4080
sodium hydroxide is it produces less sludge. In case production of sludge is not a
concern then lime can be added to raise the pH to 7. Once the pH is raised to 7
odour can be eliminated.
- Anaerobic ponds, low pH produces odours.In such cases addition of NaOH it
produces less sludge, or lime can be added to raise the pH to 7 to eliminate odour 4085
caused by H
- The scum need not be taken out of the ponds at any cost.
- When scum has accumulated to dense zones in the corners, they need to be removed.
- What is needed is beating the surface of the scum by a light long pole while standing
at the bank. 4090
- This releases the gases that are supporting the scum layer and automatically the mat
sinks back.
- These are dealt with like any other foods which are stabilized by the organisms of the
pond.
- Fish shall not be allowed to breed in any of the ponds. 4095
- The precautions of operating manually the scum removal shall be totally adopted.
- Sometimes sludge removal would become necessary.
- Thumb rule will be to verify the depth of sludge and de-sludge once it is about 30 %
of depth.
4.13.3 De-Sludging 4100
The biggest challenge to an operator in the management of pond systems is to identify when a
pond requires de-sludging, and to carry out the de-sludging thereafter; it is more important to
deal with the sludge thus taken out without giving rise to environmental problems. These issues
are addressed in this section so as to help the operator develop adequate confidence in this task.
4.13.3.1 When to De-Sludge 4105
When raw sewage without grit removal is admitted to the pond, a general rule of thumb to
calculate the grit accumulation can be taken as 0.5 meters for a ten year period. Similarly, the
accumulation of sludge can be taken as 0.7 meters for a ten year period. Thus, in the illustrative
design, it can be seen that the total depth has adequate allowance for these plus the required
minimum depth of liquid depth for the treatment. In the absence of adequate design reference, 4110
the build-up of sludge to a height wherein it will start overflowing through the outlet is the
upper limit. However, if that is allowed, there is no space for the liquid holding capacity and
treatment efficiency will suffer. Hence, the operator must clearly know the basis on which the
pond has been designed and commence de-sludging when that depth is reached by the sludge
accumulating from the bottom. If even that information is not available, the operator must make 4115
a decision.
4.13.3.2 The white towel test
The white towel test is used to understand the depth of the sludge and this will help in deciding
desludging Malan (1964). White towelling material is wrapped along one-third of a sufficiently
long pole, which is then lowered vertically into the pond until it reaches the pond bottom; it is 4120
then slowly withdrawn. The depth of the sludge layer is clearly visible since some sludge
particles will have been entrapped in the towelling material Figure 4.24. The sludge depth
should be measured at various points throughout the pond, away from the embankments, and its
mean depth calculated. Duncan Mara (2004)
Figure 4.24 The White Towel test
4.13.3.3 De-Sludge Procedure
Measure the depth of the pond from the top of the bund to the bottom. To do this, use the
effluent chamber that may be outside the pond. Usually, these chambers have the same or
slightly larger depth than the pond. If there is no effluent chamber, assemble a long casuarina 4130
sturdy pole of about some 6 meters long and start sliding it slowly on the inside slope so that the
pole follows the gradient and stops at the bottom of the pond which could be sensed by the
strong reaction for a gentle push of the pole. Mark the top of the bund elevation on the pole with
a gentle cut in the hacksaw. Pull the pole out and measure the distance of the cut from the
bottom of the pole after laying the pole flat on the ground. Record the distance. Repeat the 4135
measurement on all sides of the pond. If the readings are within plus or minus 5%, take the
average. If not repeat the measurement at different locations until you get a reading within plus
or minus 5%. Let us say this reading is 5 meters. Nail a perpendicular small pole to one end of
the longer pole such that the distance of protrusion of this short pole is a meter. Then slide the
long pole horizontally over the bund top by resting it on the bund top and checking the 4140
horizontality with a spirit level by placing it on the long pole such that the free end of the short
pole touches the inner side of the bund. Mark the location with a gentle double cut by a hacksaw
on the long pole where the inner side slope of the bund crosses the long pole. Measure this
length. Let us say this is 2 meters. The total depth of the lagoon will now be calculated as [1 / 2]
x [5] = 2.5 meters. Next, measure the inclined length of the waterline from the inner edge of the 4145
inside slope of the bund. Let us say this is 1.4 meters. The freeboard will now be calculated as
[1.4 / 2] = 0.7 meters. The design liquid depth is 2.5-0.7 = 1.8 meters. In this depth, it is now
necessary to measure the depth of sludge accumulation such that at least 1.2 meters of liquid
depth is available for treating the sewage.
This measurement requires the use of the serrated glass tube as described earlier and in addition, 4150
support from fire service personnel, life vests for all those on board the paddle boat, minimum
of two persons, a good fibreglass or wooden boat doubly checked for water tightness, an
experienced boatman and oxygen masks for all those on board the boat. Once all these are
assembled, the boat has to be rowed to the three or four random locations and the glass tube
exercise has to be carried out. After driving the glass tube into the hardness of the bottom clay, 4155
rotate it gently in-situ and lift with an unscrewing motion. Do not pull straight. Gently release
the tube from the clay by feeling it and raise by changing the grip on the tubes and allowing the
tube to go up freely. Once out of the water surface, keep it on the floor of the boat vertically and
allow the contents to settle down for 30 minutes. Thereafter paint on the glass tube with a good
paint the level at which the sludge is seen. Then return to the shore and lay the tube horizontally 4160
over the ground and use a water jet or air jet to clear the tube contents. If the chemist requires it,
collect the sludge and give it to him for analysis. On any account, do not use a mechanized boat.
The methane gas present on the surface of the pond can be ignited by the spark of the motor
with unpleasant consequences.
Another method is to take a slim bamboo pole with a height equal to the depth of the pond plus 4165
2 m as the optimum. A white fluffy towel is wrapped around end of the pole for a height equal
to the designed liquid depth of the pond. It should be tied securely and tightly in not less than
three to four wrappings. Thereafter set sail on a good manual paddle boat over the pond as
described earlier. Lower the pole with the white towel end into the pond till the pole has reached
the bottom. Hold steady for about 15 minutes. Then raise the pole. The darkish or blackish 4170
colour at the bottom end as seen on the towel is the mark of the depth of sludge. Rinse it well by
repeatedly dipping into the liquid upper layers before proceeding to the next random location
and repeat the testing.
- Repeat the above depth measurements slowly without hurry. Always do this in clear
non-rainy weather. Make sure you have at least four readings, which are fairly close. 4175
- Once the sludge depth is thus measured, consult the chemist for any tendency of
efficiency drop in the pond for BOD removal. If the chemist feels that there is a
steady decline and efficiency is going down, consult the plant superintendent.
- As a rule of the thumb, if the liquid height is less than 1.2 meters in a facultative or
anaerobic pond, it is time for de-sludging. Take the decision jointly and never by 4180
- The best method of de-sludging is to take one pond out of operation during the
beginning of summer and pump out the water portion to the other ponds. Thereafter,
it normally takes two months for a sludge depth of about 2 meters to dry out.
- Deploy manpower equipped with oxygen mask to gently turn the dried sludge upside 4185
down uniformly over the whole area so that drying is hastened. Never use a machine
during this operation as methane may get released.
- Once this is completed and the sludge is dried, deploy scraper equipment like a
Procaine and evacuate the sludge over the bund and on to the ground on the earth
side of the bund. 4190
- The sludge can be heaped into a pile by manual labourers who should wash their
hands thoroughly with soap after finishing their work.
4.13.3.4 Special cautions for anaerobic pond / maturation pond
All the points listed earlier in aerated lagoon and facultative ponds apply here also except that
the depth of sludge before de-sludging will be according to the original design. The boat ride to 4195
measure the sludge depth shall not be used in these ponds. Instead, the white towel test shall be
conducted and a long boom crate shall be used without making any person stand at the end of
the boom.
4.13.4 Process Control
There is nothing much to control in the process of purification of sewage in WSP except making 4200
sure that the sludge accumulation does not exceed 30% of the total liquid depth or the design
depth of sludge.
4.13.5 Record Keeping
4.13.5.1 Records necessary for Anaerobic Pond
- Daily tests and records will be the flow and SS 4205
- Monthly tests shall be the BOD after filtering through Whatman 42 filter paper and
4.13.5.2 Records necessary for Facultative Pond
- Daily tests and records will be the flow and SS
- Weekly tests will be identification of organisms as per Standard Methods drawings 4210
4.13.5.3 Records necessary for Maturation Pond
- Monthly tests shall be the BOD after filtering through Whatman 42 filter paper and 4215
- Yearly test of faecal and total coliforms at the peak summer and peak monsoon shall
be conducted.
4.14 Farm Forestry
Please hand over the O&M work to the local forestry department who are competent in this. 4220
4.15 Fish Pond
These ponds shall not be used henceforth except in the case of the Mudiali farm.
4.16 Secondary Sedimentation Tank
A typical plant may have clarifiers located at two different points. The one that immediately
follows the bar screen, comminutor, or grit channel is called the primary sedimentation tank, 4225
merely because it is the first sedimentation tank in the plant. The other, which follows other
types of treatment units, is called the secondary sedimentation tank or the final sedimentation
tank. The two types of sedimentation tanks operate almost exactly the same way. The function
of a primary clarifier is to remove settleable and floatable solids. The reason for having a
secondary sedimentation tank is that other types of treatment following the primary 4230
sedimentation tank convert more solids to the settleable form, and they have to be removed from
the treated sewage. Because of the need to remove these additional solids, the secondary
clarifier is considered part of these other types of processes.
The main difference between primary and secondary sedimentation tanks is in the density of the
sludge handled. Primary sludge is usually denser than secondary sludge. Effluent from a 4235
secondary clarifier is normally clearer than primary effluent.
Solids that settle to the bottom of a sedimentation tank are usually scraped to one end (in
rectangular clarifiers) or to the middle (in circular clarifiers) into a sump. From the sump, the
solids are pumped to the sludge handling or sludge disposal system. Systems vary from plant to
plant and include sludge digestion, vacuum filtration, filter presses, incineration, land disposal, 4240
lagoons, and burial.
Disposal of skimmed solids varies from plant to plant. Skimmed solids may be buried with
material cleaned off the bar screen, or pumped to the digester. Even though pumping skimmed
solids to a digester is not considered good practice because skimmings can cause operational
problems in digesters, it is a common practice. 4245
Of all the different types of clarifiers that an operator must regulate, secondary clarifiers in the
activated sludge process are the most critical and require the most attention from the operator.
- Levels of sludge blanket in the clarifier
- Concentration of suspended solids in the clarifier effluent 4250
- Control and pacing of return sludge flows
- Concentration of dissolved oxygen (DO) in the clarifier effluent
- Level of pH
- Concentration of RAS
4.16.2 Maintenance 4255
Annually, during periods of low flow, each clarifier should be shut down for inspection, routine
maintenance, and any necessary repairs. Even though the clarifier and all equipment are
working properly, an annual inspection helps to prevent serious problems and failures in the
future when harmful consequences can result.
During normal operations, operator should schedule the following daily activities: 4260
a. Inspection
Make several daily inspections with a stop, look, listen, and think routine.
b. Cleanup
Using water under pressure, wash off accumulations of solid particles, grease, slime, and
other material from walkways, handrails, and all other exposed parts of the structure and 4265
c. Lubrication
Grease all moving equipment according to manufacturer's specifications and check oil
levels in motors where appropriate.
4.17 Advanced Treatment 4270
Advanced sewage treatment processes typically are used to further reduce the concentrations of
suspended solids, nutrients (nitrogen or phosphorus) and soluble organic chemicals in secondary
treatment effluent. These processes may be physical, chemical, biological, or a combination of
these processes.
4.17.1 Sand Filtration 4275
Sand filters have influent and effluent distribution systems consisting of pipes and fittings. Head
loss is a measure of solids trapped in the filter. As the filter becomes filled with trapped solids,
the efficiency of the filtration process falls off, and the filter must be backwashed. Filters are
backwashed by reversing the flow so that the solids in the media are dislodged and can exit the
filter; sometimes air is dispersed into the sand bed to scour the media. 4280
Sand filters can be automatically backwashed when the differential pressure exceeds a preset
limit or when a timer starts the backwash cycle.
4.17.2 Multimedia Filtration
A multimedia filter operates with the finer, denser media at the bottom and the coarser, less
dense media at the top. Acommon arrangement is given as below. 4285
- Top: Anthracite
- Middle: Sand
- Bottom: Garnet
These media can be used alone, such as in sand filtration, or in a multimedia combination.
Some mixing of these layers occurs and is anticipated. During filtration, the removal of the 4290
suspended solids is accomplished by a complex process involving one or more mechanisms,
- Straining,
- Sedimentation,
- Interception, 4295
- Impaction, and
- Adsorption.
The size of the medium is the principal characteristic that affects the filtration operation. If the
medium is too small, much of the driving force will be wasted in overcoming the frictional
resistance of the filter bed. If the medium is too large, small particles will travel through the bed, 4300
preventing optimum filtration. As same as sand filtration, back wash is required to keep
adequate filtration efficiency.
4.17.3 Membrane Filtration (MF, UF, NF, RO)
Membrane filtration is used for polishing water for specific uses like industry process water, or
for aquifer infiltration. In India, membrane filtration is widely used in the water and sewage 4305
sectors. (Figure 4.25)
MF Microfiltration membranes are porous membranes with pore sizes between 0.1 and 1
micron (1 micron=1000 nanometre). They allow almost all dissolved solids to get through and
retain only solids particles over the pore size.
UF Ultra filtration membranes are asymmetric or composite membranes with pore sizes 4310
around between 0.005 and 0.05 micron. They allow almost mineral salts and organic molecules
to get through and retain only macromolecules
NF Nano filtration membranes are reverse osmosis membranes with pore sizes around 0.001
micron. They retain multivalent ions and organic solutes that are larger than 0.001 micron.
RO Reverse osmosis membranes are dense skin, asymmetric or composite membranes that let 4315
water get through and rejects almost all salts.
a. Operational Unit Processes
All membrane filtration systems have associated operational unit processes that are
essential for maintaining and optimizing system performance and therefore critical to the 4320
successful implementation of the technology. These operational processes include
backwashing, chemical cleaning, and integrity testing. For the purposes of this discussion,
pre-treatment and post-treatment are also considered operational unit processes associated
with membrane filtration. Each of these processes and its role in the operation of a
membrane filtration system are described in the following sections. Although not every 4325
membrane filtration system utilizes all of these processes, many utilize each process to
some degree.
b. Pre-treatment
Pre-treatment is typically applied to the feed water prior to entering the membrane system
in order to minimize membrane fouling, but in some cases may be used to address other 4330
water quality concerns or treatment objectives. Pre-treatment is most often utilized to
remove foulants, optimize recovery and system productivity, and extend membrane life.
Pre-treatment may also be used to prevent physical damage to the membranes. Different
types of pre-treatment can be used in conjunction with any given membrane filtration
system, as determined by site-specific conditions and treatment objectives. Pilot testing can 4335
be used to compare various pre-treatment options, optimize pre-treatment, and/or
demonstrate pre-treatment performance. Several different types of commonly used
pre-treatment for membrane filtration systems are discussed in the following subsections.
Figure 4.25 Filtration spectrum 4340
c. Pre-filtration
Pre-filtration, including screening or coarse filtration, is a common means of pre-treatment
for membrane filtration systems that are designed to remove large particles and debris.
Pre-filtration can either be applied to the membrane filtration system as a whole or to each
membrane unit separately. The particular pore size associated with the pre-filtration 4345
process (where applicable) varies depending on the type of membrane filtration system and
the feed water quality. For example, although hollow-fibre microfiltration (MF) and ultra
filtration (UF) systems are designed specifically to remove suspended solids, large
particulate matter can damage or plug the membranes fibres.
Because nanofiltration (NF) and reverse osmosis (RO) utilize non-porous semi permeable 4350
membranes that cannot be backwashed and are almost exclusively designed in a
spiral-wound configuration for municipal water treatment applications, these systems must
utilize much finer pre-filtration in order to minimize exposure of the membranes to
particulate matter of any size.
A summary of the typical pre-filtration requirements associated with the various types of 4355
membrane filtration is presented in Table 4.4.
d. Backwashing
The backwash process for membrane filtration systems is similar in principle to that for
conventional media filters and is designed to remove contaminants accumulated on the
membrane surface. Each membrane unit is backwashed separately and in a staggered 4360
pattern so as to minimize the number of units in simultaneous backwash at any given time.
During a backwash cycle, the direction of flow is reversed for a period ranging from about
30 seconds to 3 minutes. The force and direction of the flow dislodge the contaminants at
the membrane surface and wash accumulated solids out through the discharge line.
Membrane filtration systems are generally backwashed more frequently than conventional 4365
media filters, with intervals of approximately 15 to 60 minutes between backwash events.
Typically, the membrane backwash process reduces system productivity in the range of 5
to 10 percent due to the volume of filtrate used during the backwash operation.
Table 4.4 Typical membrane system pre-filtration requirements
Membrane System Pre-filtration Requirements
Classification Configuration Size (m) Type(s)
Membrane Cartridge
Filtration (MCF)
Cartridge 300 - 3,000
Strainers;
Microfiltration (MF) /
Ultra filtration (UF)
Hollow-Fibre,
Inside-Out
300 - 3,000
Nanofiltration (NF) /
Spiral-Wound 5 - 20 Cartridge Filters
Pre-filtration is not necessarily required for MCF systems 4370
Backwashing is conducted periodically according to manufacturers specifications and
site-specific considerations. Although more frequent backwashing allows for higher fluxes,
this benefit is counterbalanced by the decrease in system productivity. In general, a
backwash cycle is triggered when a performance-based benchmark is exceeded, such as a 4375
threshold for operating time, volumetric throughput, increase in transmembrane pressure
(TMP), and/or flux decline. Ideally, the backwash process restores the TMP to its baseline
(i.e., clean) level; however, most membranes exhibit a gradual increase in the TMP that is
observed after each backwash, indicating the accumulation of foulants that cannot be
removed by the backwash process alone. These foulants are addressed through chemical 4380
cleaning.
Because the design of spiral-wound membranes generally does not permit reverse flow, NF
and RO membrane systems are not backwashed. For these systems, membrane fouling is
controlled primarily with chemical cleaning, as well as through flux control and cross flow
velocity. The inability of spiral-wound membranes to be backwashed is one reason that NF 4385
and RO membranes are seldom applied to directly treat water with high turbidity and/or
suspended solids.
e. Chemical Cleaning
Chemical cleaning is another means of controlling membrane fouling, particularly those
foulants such as inorganic scaling and some forms of organic and biofouling that are not 4390
removed via the backwash process. As with backwashing, chemical cleaning is conducted
for each membrane unit separately and is typically staggered to minimize the number of
units undergoing cleaning at any time. While chemical cleaning is conducted on both
MF/UF and NF/RO systems, because non-porous, semi-permeable membranes cannot be
backwashed, chemical cleaning represents the primary means of removing foulants in 4395
NF/RO systems. Although cleaning intervals may vary widely on a system-by-system
basis, the gradual accumulation of foulants makes eventual chemical cleaning virtually
inevitable. Membrane cartridge filters are an exception, however, in that cartridge filters
are usually designed to be disposable and thus are typically not subject to chemical
cleaning. (Table 4.5) 4400
Table 4.5 Chemical cleaning agents
Category Chemicals Commonly Used
Typical Target
Contaminant (s)
Citric Acid (C
Inorganic scale
Base Caustic (NaOH) Organics
Oxidants /
Sodium Hypochlorite (NaOCl)
Chlorine (Cl
) Gas
Hydrogen Peroxide (H
Organics;
Surfactants Various
Inert particles
As with backwashing, the goal of chemical cleaning is to restore the TMP of the system to
its baseline (i.e., clean) level. Any foulant that is removed by either the backwash or
chemical cleaning process is known as reversible fouling. Over time, membrane processes 4405
will also typically experience some degree of irreversible fouling which cannot be removed
through either chemical cleaning or backwashing. Irreversible fouling occurs in all
virtually membrane systems, albeit over a wide range of rates, and eventually necessitates
membrane replacement.
4.17.5 Integrated Nutrient Removal 4410
4.17.5.1 Nutrient Removal
Sewage may contain high levels of the nutrients nitrogen and phosphorus. Excessive release of
these nutrients to the environment can lead to a buildup of nutrients, called eutrophication,
which can in turn encourage the overgrowth of weeds, algae, and cyanobacteria (blue-green
algae). This may cause an algal bloom, a rapid growth in the population of algae. The algae 4415
numbers are unsustainable and eventually most of them die. The decomposition of the algae by
bacteria uses up so much of oxygen in the water that most or all of the animals die, which
creates more organic matter for the bacteria to decompose. In addition to causing deoxygenation,
some algal species produce toxins that contaminate drinking water supplies. Different treatment
processes are required to remove nitrogen and phosphorus. 4420
4.17.5.2 Nitrogen Removal
Nitrogen is removed through the biological oxidation of nitrogen from ammonia to nitrate
(nitrification), followed by denitrification, i.e., the reduction of nitrate to nitrogen gas. (Figure
Nitrogen gas is released into the atmosphere and thus removed from the water. Nitrification 4425
itself is a two-step aerobic process, each step facilitated by a different type of bacteria. The
oxidation of ammonia (NH
) to nitrite (NO
) is most often facilitated by Nitrosomonas spp.
(nitroso referring to the formation of a nitroso functional group). Nitrite oxidation to nitrate
(NO
), though traditionally believed to be facilitated by Nitrobacter spp. (nitro referring the
formation of a nitro functional group), is now known to be facilitated in the environment almost 4430
exclusively by Nitrospira spp.
Denitrification requires anoxic conditions to encourage the appropriate biological communities
to form. It is facilitated by a wide diversity of bacteria. Sand filters, lagoons and reed beds can
all be used to reduce nitrogen, but the activated sludge process (if designed well) can do the job
the most easily. Since denitrification is the reduction of nitrate to nitrogen gas, an electron donor 4435
is needed.
Figure 4.26 Configuration of recycled nitrification/denitrification process
This can be, depending on the sewage, organic matter (from faeces), sulphide, or an added
donor like methanol. Sometimes the conversion of toxic ammonia to nitrate alone is referred to 4440
as tertiary treatment. Many sewage treatment plants use axial flow pumps to transfer the
nitrified mixed liquor from the aeration zone to the anoxic zone for denitrification. These pumps
are often referred to as Internal Mixed Liquor Recycle (IMLR) pumps.
4.17.5.2.1 Process Control
Operators of biological nitrogen removal (BNR) facilities need more process-control knowledge 4445
than those of conventional treatment facilities to keep them operating smoothly. The key
operating parameters for a BNR facility typically include:
a. SRT
SRT is the key to understanding whether the BNR process has enough time to function
effectively. When evaluating SRT, operators should answer such questions as: 4450
Is the SRT long enough to establish nitrification?
How much sludge should be wasted to maintain a desired SRT?
Can the SRT be increased by maintaining a higher MLSS?
b. F/M Ratio
The F/M ratio is a good indicator of how well selector reactors will promote the growth of 4455
floc-forming bacteria. When the F/M ratio is high, floc-forming bacteria have a
competitive advantage over filamentous bacteria. Selector loading also helps ensure that
nuisance bacteria will not cause operating problems. The selector cells should be arranged
so BOD is taken up rapidly.
c. HRT 4460
Although not used in daily BNR operations, HRT indicates whether the plant is operating
within a normal contact time. Nitrifying facilities, such as conventional activated sludge
and A2O, typically have an HRT between 5 and 15 hours.
d. Oxygen Levels
When a conventional activated sludge systemis converted into a BNR facility, its dissolved 4465
oxygen requirements typically increase, requiring changes in the aeration equipment or
diffuser layout.
e. Alkalinity and pH Control
Every time 1 mg of ammonia-nitrogen is oxidized to nitrate, 7.14 mg of alkalinity is
consumed. Likewise, every time 1 mg of nitrate is converted to nitrogen gas, 3.57 mg of 4470
alkalinity is recovered.
f. ORP (OxidationReduction Potential).
Automated control systems for the internal anoxic mixing process measure the ORP so
they can detect nitrate depletion in the mixed liquor. This variable indirectly measures
nitrate availability in an aqueous media, although there is no direct correlation between any 4475
specific ORP value and nitrate concentration.
Oxidationreduction potential measures the net electron activity of all oxidationreduction
reactions occurring in sewage. It is affected by temperature, pH, biological activity, and the
systems chemical constituents, but its response pattern to changes in a solutions oxidative
state is reproducible in a specific system. In continuous-flow suspended-growth systems, 4480
the control systems ORP breakpoints must be constantly reviewed and revised. In batch
systems (e.g., SBR or cyclic aeration systems), however, a characteristic knee (change in
ORP values) indicates when the system is changing from an oxidized state to a reduced
g. Recycle Flows 4485
For sewage facilities with either ammonia and/or nitrate limitations, it will be necessary to
adjust recycle flows (typically RAS flow) to achieve operational goals.
h. Secondary Clarification
It is essential that the secondary clarifier be able to both separate biological solids from the
treated effluent and also concentrate the solids without a build-up of sludge within the 4490
clarifier. Parameters of concern with clarification are the hydraulic loading rate (HLR) and
the solids loading rate (SLR).
4.17.5.3 Phosphorus Removal
Phosphorus removal is important as it is a limiting nutrient for algae growth in many fresh water
systems. It is also particularly essential for water reuse systems where high phosphorus 4495
concentrations may lead to fouling of downstream equipment such as reverse osmosis.
Phosphorus removal in excess of metabolic requirements can be achieved by using enhanced
biological phosphorus removal (EBPR) or chemical addition.
Phosphorus can be removed biologically in a process called enhanced biological phosphorus
removal In this process, specific bacteria, called Polyphosphate Accumulating Organisms 4500
(PAOs), are selectively enriched and accumulate large quantities of phosphorus within their
cells (up to 20 percent of their mass). When the biomass enriched in these bacteria is separated
from the treated water, these biosolids (sludge) have a high fertilizer value.
The EBPR process consists of anaerobic and aerobic zones. By definition, an anaerobic zone
contains no usable dissolved oxygen or nitrate. In this zone, PAOs do not grow, but consume 4505
and convert readily available organic material (i.e. VFAs) to energy-rich carbon polymers called
poly-hydroxyalkanoates (PHA). The energy required for this reaction is generated through
breakdown of the stored polyphosphate (poly-P) molecules, which results in phosphorus release
and an increase in the bulk liquid soluble phosphorus concentration in the anaerobic stage.
Magnesium and potassium ions are concurrently released to the anaerobic medium with 4510
phosphate. In addition, for a substantial amount of reducing power is required PAOs to produce
PHA. The breakdown of glycogen, another form of internal carbon storage, generates the
reducing power.
Phosphorus removal can also be achieved by chemical precipitation, usually with salts of iron
(e.g. ferric chloride), aluminium (e.g. alum), or lime. This may lead to excessive sludge 4515
production as hydroxides precipitate and the added chemicals can be expensive. Chemical
phosphorus removal requires significantly smaller equipment footprint than biological removal,
is easier to operate and is often more reliable than biological phosphorus removal. Another
method for phosphorus removal is the use of granular laterite. Once removed, phosphorus, in
the form of a phosphate-rich sludge, may be stored in a land fill or resold for use in fertilizers. 4520
4.18 Disinfection Facility
Disinfection of effluent from water reclamation facilities (WRFs) is required to decrease the
disease risks associated with the discharge of sewage containing human pathogens (disease
causing organisms) into receiving waters. These microorganisms are present in large numbers in
sewage effluents. 4525
The chlorine gas is controlled, metered, introduced into a stream of injector water and then
conducted as a solution to the point of application.
The primary advantage of vacuum operation is safety. If a failure or breakage occurs in the
vacuum system, the chlorinator either stops the flow of chlorine into the equipment or allows air
to enter the vacuum system rather than allowing chlorine to escape into the surrounding 4530
atmosphere. In case the chlorine inlet shutoff fails, a vent valve discharges the incoming gas to
the outside of the chlorinator building.
The operating vacuum is provided by a hydraulic injector. The injector operating water absorbs
the chlorine gas and the resultant chlorine solution is conveyed to a chlorine diffuser through
corrosion resistant conduit. A vacuum chlorinator also includes a vacuum regulating valve to 4535
dampen fluctuations and allow smooth operation. Vacuum relief prevents excessive vacuum
within the equipment.
Chlorine gas flows from the chlorine container to the gas inlet. After entering the chlorinator,
the gas passes through spring-loaded pressure regulating valve which maintains the proper
operating pressure. A rotameter is used to indicate the rate of gas flow. The rate is controlled by 4540
V-notch variable orifice. The gas then moves to the injector where it dissolved in water and
leaves the chlorinator as a chlorine solution (HOCl) ready for application.
4.18.1 Operational Variables
The process-control variables associated with chlorination systems are:
a. Detention (Contact) Time 4545
The chlorine solution is best injected into the effluent via a diffuser or, preferably, a flash
mixer. Otherwise, some of the chlorine gas could come out of solution undissolved
(stratification). This would impair the efficiency of disinfection and increase its costs.
Typically, depending on the STPs (sewage treatment plants) permit requirements or the
state or regional regulatory requirements, chlorine detention time should range from 30 to 4550
60 minutes at the average daily flow (ADF) and should equal or exceed 15 minutes at peak
flows. Such detention times allow a safety factor for possible hydraulic inefficiency of the
contact chamber, thus maximizing pathogen inactivation.
b. Chlorine Residual
Depending on the effluent-disposal method (receiving-water discharge or reclaimed-water 4555
reuse) the permit may require a chlorine residual in the contact chamber effluent. The three
types of chlorine residuals are combined, free, and total. Free and total residuals are
typically monitored.
The combined residual consists of chloramines and chloro-organic compounds that are
formed by the reaction of chlorine with ammonia and organic compounds in the secondary 4560
or tertiary effluent. Each milligram per litre of ammonia consumes 10 mg/L of chlorine.
The chlorine dose that satisfies the ammonias chlorine demand is called the breakpoint.
Note that the combined residual decreases slightly as the chloramines and chloro-organic
compounds are oxidized at a narrow range of chlorine doses less than the breakpoint
(Figure 4.27) 4565
Figure 4.27 Breakpoint chlorination curve
c. Indicator Bacteria Results
Regardless of the chlorine residual method employed enough chlorine solution must be
injected into the effluent to sufficiently destroy or inactivate the indicator bacteria that 4570
signal the likely presence of pathogens. The primary objective of chlorination is to destroy
pathogenic organisms; however, the coliform bacteria often used as indicators are not
pathogenic. The indicator bacteria inactivation concept works because coliform and other
indicator bacteria are much easier to detect than pathogens and more difficult to destroy
than most pathogens, except possibly viruses. 4575
Testing directly for pathogens is complex and costly. If the coliform count has been
sufficiently reduced through disinfection, a corresponding proportion of pathogens has
probably been inactivated.
4.18.2 Operational Hazards
a. Chlorine Hazards 4580
Chlorine is a gas, 2.5 times heavier than air, extremely toxic, and corrosive in moist
atmospheres. Dry chlorine gas can be safely handled in steel containers and piping, but
with moisture it must be handled in corrosion-resisting materials such as silver, glass,
Teflon, and certain other plastics. Chlorine gas at container pressure should never be
piped in silver, glass, Teflon, or any other plastic material. Even in dry atmospheres, the 4585
gas is very irritating to the mucous membranes of the nose, to the throat, and to the lungs;
a very small percentage in the air causes severe coughing. Heavy exposure can be fatal.
b. Warning
When entering a room that may contain chlorine gas, open the door slightly and check for
the smell of chlorine. Never go into a room containing chlorine gas with harmful 4590
concentrations in the air without a self-contained air supply, protective clothing, and help
standing by. Help may be obtained from your chlorine supplier and your local fire
Routine operations and troubleshooting 4595
Table 4.6 lists routine operational checks of chlorination equipment and remedies if these
checks indicate potential problems.
Table 4.6 Routine operational checklist and troubleshooting guide for chlorination system
Items What to check Potential problems Corrective actions
Record scale
Degrading effluent increases
chlorine demand (nitrite
demand increases use)
Monitor dose and demand. Adjust
process to improve effluent quality.
Low scale weight, chlorine
about to run out
Replace container or cylinder
scale reaches zero to prevent
sediment from entering system.*
lines,
valves, and
Erratic reading Scale not tared out properly
If necessary, retare or calibrate
scale.
chlorine leaks
Personal injury (potential
death), evacuation logistics,
and corrosion of nearby
equipment and electronics
Work with a trained assistant, wear
SCBAs ( Self-Contained Breathing
Apparatus ), and follow all
appropriate safety procedures when
closing container or cylinder main
valve and evacuating chlorination
pipe network.
Repair all leaks immediately; they
will only get worse. Notify
emergency response teams if
Iced container
or cylinder
Chlorination rate too high
Reduce chlorination rate, or
manifold containers or cylinders
together. If an evaporator is being
used, be certain liquid chlorine is
being withdrawn from the
containers bottom valve.
Leaks Chlorine evaporating
Repair all leaks immediately.
Evacuate chlorine network and
repair PVC pipes.
Follow all appropriate safety
procedures when working with any
chlorine leak.
Chlorination system
Correct all potential problems
Main manifold
Could break rupture disk?
If necessary, evacuate network and
replace rupture disk. Check all
network valves for correct
positioning.
*This will prevent sediment in the container from entering the chlorination network and possibly damaging the
process equipment. 4600
4.19 Operation & Maintenance of DEWATS and Johkasou
The package treatment plants like Johkasou and also DEWATS have to be maintained as per the
in accordance with the instructional procedures of the respective vendors of these systems.
4.20 Preventive Maintenance 4605
Preventive Maintenance addresses the civil, mechanical, electrical, instrumentation and
automation aspects.
In respect of civil works, follow the local rules, regulations and guidelines of the local Public
Works Department (PWD). These procedures are mostly annual. It will be better to hand over
such maintenance to the PWD and remit the costs to that department. 4610
In respect of mechanical equipment, it is better to enter into a contract with the contractor who
has built the STP to do this as per the directions of the equipment suppliers and retain the
equipment supplier to check and certify the work.
In respect of the electrical installations, it is better to entrust this work to the local Electricity
Department, similar to civil works. 4615
In respect of instrumentation and automation, similarly, entrust the work to the contractor who
supplied and erected these and retain a third party agency to certify the proper completion of the
The following checklist (Table 4.7) is an example of a preventive maintenance program for
activated-sludge facilities. When developing a site-specific schedule consult the service manuals 4620
that were provided with each piece of equipment.
Activated-sludge system preventive
maintenance (typical)
Aeration blower
Maintain proper motor lubricant level
Lubricate motor roller bearings 1.5months
Check that air filters are in place and not clogged
Check motor bearing rise temperature
Check motor for voltage and frequency variations
Rotate blower operation
Check wiring integrity
Lubricate motor sleeve bearing or 2000 hours
Inspect and clean rotor ends, windings, and blades
Check that electrical connections are tight and
corrosion is absent
Change blower bearing oil
Re-lubricate after checking flexible couplings As needed
Oxygen dissolution system
Lubricate motor
Change gear drive oil
Lubricate flex coupling or 2500 hours
Inspect gear tooth pattern wear, shaft and bearing
end play alignment, bolting, and seal condition
Fine bubble diffusers
Check biological reactor surface pattern
Check air mains for leaks
Check and record operating pressure and airflow
Purge water moisture from distribution piping
Bump diffuser system
Drain biological reactor
Remove excess solids that may accumulate
Clean diffusers
Check that retaining rings are in place and tight
Check that fixed and expansion joint retaining
rings are tight
Remove trash and debris
Test torque-control line switches
Test torque overload alarm
Verify torque scale pointer moves
Check drive unit for accumulated condensation
Check drive oil level and quality
Check drive overload response controls
Inspect entire mechanism above and below water
Inspect and tighten all nuts and bolts
Inspect lubrication for torque-overload protection
device, per manufacturers instructions
e.g., 18
months or 500
Return activated sludge pumps (centrifugal)
Lubricate pump bearings or 2000 hours
Lubricate motor bearings
Waste activated sludge pumps (centrifugal)
4.21 Troubleshooting 4625
Refer to Appendix 4.1.
4.22 Record Keeping
The importance of maintaining adequate O&M records cannot be overemphasized. The purpose
of recording data is to track operational information that will identify and duplicate optimum
operating conditions. Records of the volume and concentration of waste sludge fed to the 4630
digester and volume and concentration of digested solids removed from the digester should be
kept. Additional information that needs to be maintained, include DO concentration and pH.
Keep a monthly report form. In plants where the aeration system capacity is marginally
adequate in providing desirable DO concentration in the digester, record DO concentration data
on a trend chart. If chemicals are added to the digester for pH or odour control, record the type 4635
and amount of chemicals added. If mechanical aerators are used, record the power usage. In the
case of diffused-air systems, air flow records may be of interest. If airflow meters are not
available, records of power consumption may be useful. Experimenting with the aeration system
often leads to significant savings in power costs.
A record of instrument performance and repairs allow operations or maintenance personnel to 4640
properly evaluate an instruments effectiveness and determine if the instrument meets the
objectives used to justify its purchase and installation. As a minimum, the following basic
information should be maintained for each instrument in the sewage treatment plant:
- Model number and serial number 4645
- Type
- Dates placed into and removed from service
- Calibration data and procedures 4650
- Hours required to perform maintenance
- Operations and maintenance manual references and their locations
4.23 Summary 4655
This manual provides troubleshooting lists for possible problems in STPs in Appendices.
Operators should check their operational problems and study the countermeasures from the
troubleshooting lists so that they can take prompt measures against the problems when they
occur.
Part B: Operation and Maintenance CHAPTER 5: SLUDGE TREATMENT FACILITIES
CHAPTER 5 SLUDGE TREATMENT FACILITIES 4660
Sludge treatment processes are often the most difficult and costliest part of sewage treatment.
Untreated sludge is odorous and contains pathogens. Sludge stabilization processes reduce
odours, pathogens, and biodegradable toxins, as well as bind heavy metals to inert solids, such
as lime that will not leach into the groundwater. The resulting biosolids can be used or disposed 4665
of safely.
Sewage residuals include primary, secondary, mixed, and chemical sludge, as well as screenings,
grit, scum, and ash. The concentration and characteristics of chemical sludge depend on the
treatment chemicals (alum, ferric salts, or lime) used. It typically is found at treatment plants
that have tertiary treatment, such as phosphorus removal. The use or disposal method for 4670
residuals depends on how much treatment they have received. Biosolids are residuals that have
been stabilized so they can be beneficially used as a soil amendment. Combustible residuals,
such as screenings, may be incinerated or landfilled. Non-combustible residuals, such as grit,
may be landfilled.
5.2 Sludge Thickening 4675
The role of sludge thickening is to thicken the sludge of low concentration generated in sewage
treatment facilities, and to make subsequent processes such as sludge digestion and sludge
dewatering more effective. Thickened sludge may be of two kinds: primary settling tank sludge
generated in the primary settling tank and excess sludge generated in the secondary settling
tank. 4680
Sludge thickening may be broadly classified into four types by thickening method: gravity
thickening, centrifugal thickening, floatation thickening and belt type thickening.
When the thickening of sludge is inadequate, not only will the efficiency of subsequent sludge
treatment reduce, but also centrate containing large amount of suspended solids will return to
the sewage treatment facilities and degrade the water quality of treated water. 4685
For this reason, excess sludge for which gravity thickening is difficult is increasingly being
mechanically thickened using centrifugal thickening machines or floatation thickeners. When
the water content of thickened sludge is 98% or more especially, separation and thickening
should be considered.
5.2.1 Gravity Thickening 4690
Gravity thickening is the most common practice for concentration of sludge (Figure 5.1).
Gravity thickening concentrates sludge through sedimentation. The normal acceleration of
gravity provides the driving force for the separation.
This is adopted for primary sludge or combined primary and activated sludge but is not
successful in dealing with activated sludge independently. Gravity thickening of combined 4695
sludge is not effective when activated sludge exceeds 40% of the total sludge weight, and other
methods of thickening of activated sludge have to be considered.
Continuous flow tanks are deep circular tanks with central feed and overflow at the periphery.
Better efficiencies can be obtained by providing slow revolving stirrers, particularly with gassy
Sludge. 4700
It is necessary to ensure provisions for:
- Regulating the quantity of dilution water needed
- Adequate sludge pumping capacity to maintain any desired solids concentration,
continuous feed and underflow pumping
- Protection against torque overload 4705
- Sludge blanket detection
Figure 5.1 Example of a gravity thickener.
- Variable-speed drives may be used to increase rake speed to agitate the sludge
blanket and release trapped gas bubbles, and to prevent rat-holing or coning. 4710
Prolonged operation at high speeds will reduce the ultimate solids concentration and
reduce the life of the thickener drive mechanism.
- Scum removal equipment may include a skimmer and scum box. The ancillary
equipment should include positive-displacement pumps (plunger, rotary lobe,
diaphragm, or progressing cavity pumps). Process control equipment includes 4715
sludge blanket indicators (light path, sonic, or variable-height taps), online process
monitors on the feed or underflow, torque readouts on the rake drive, and timers to
vary the pump on-off (or speed) sequences. In cold weather areas or areas where
odours are a problem, thickeners are typically covered.
Gravity thickeners are either continuous flow or fill and draw type, with or without addition of 4720
chemicals. Use of slowly revolving stirrers improves the efficiency. Continuous flow tanks are
deep circular tanks with central feed and overflow at the periphery.
Greater attention to the thickener is required when thickening waste activated sludge because it
has a large surface area per unit mass, resulting in low settling rates and resistance to being 4725
compacted. Sludge tends to stratify in the gravity thickener while continuing biological activity,
which includes the production of gases that can cause accumulated sludge to float.
Gravity thickener operation responds to changes in process temperatures; therefore, loading
rates should be reduced to values at the lower end of the range when temperatures exceed 15 to
20C, depending on the ratio of primary to secondary sludges. Higher temperatures will require 4730
additional dilution.
The following should be checked before or during operation:
- Avoid starting a thickener that contains accumulated sludge. To avoid overload, the
sludge should be disposed of before starting the mechanism.
- Check and adjust the skimming mechanism to increase the amount of scum drawn 4735
into the scum box and to reduce the amount of supernatant carried with the
skimming.
- Check all condensate drains and remove any accumulated moisture.
- Visually examine the skimmer to ensure that it properly comes into contact with the 4740
scum baffle and scum box.
- Inspect skimmer wipers for wear.
- Install kick plates on the gravity thickener bridge to prevent objects from falling into
the tank.
- An object lodging in the underflow discharge pipe or under the mechanism will 4745
quickly halt operation of the thickener. If an object falls into the tank, immediately
halt thickener operation to prevent torque overload.
- During plant rounds, regularly observe and record the drive torque indicator, which
is the best indicator of mechanical problems.
- Regularly check the underflow pump capacity because pumps wear rapidly in a 4750
thickened sludge operation.
- Follow the manufacturers recommended lubrication schedule and use
recommended lubricant types. Oil should typically be changed after the first 250
hours of operation and every 6 months thereafter.
5.2.2 Centrifugal Thickening 4755
Thickening by centrifugation is chosen only when space limitations or sludge characteristics
will not permit the adoption of other methods. This method involves high maintenance and
power costs.
5.2.2.1 Configuration
Decanter centrifuges are centrifuges that have a screw conveyor inside that transports the settled 4760
sludge along the bowl and out of the centrifuge. They thicken and dewater the sludge
simultaneously.
In the centrifuge, the process is the same, varying only in degree. Centrifuges use the principle
of sedimentation to separate liquids from solids based on the same principle as the clarifiers and
thickeners in the wet end of the plant. In case of sedimentation, it is the difference in density 4765
between the solids and the surrounding liquid that drives separation.
All process devices benefit from a constant feed quality, and centrifuges are no exception.
Common problems are varying ratios of primary to secondary sludges or feed material that has
become septic. For very complicated reasons, septic sludge is more difficult to thicken than 4770
fresh sludge. Holding feed material in storage tanks under uncontrolled conditions is poor
practice. When in doubt, measure the pH drop through the tankage.
The manufacturer generally sets the bowl speed and it is rarely changed thereafter. Assuming
the present speed was the correct speed several years ago is not proof that it is the best speed
now. The plant engineer should adjust the speed periodically, to confirm that it is correct and to 4775
remind operators that it is a variable. It is good policy to consult the manufacturer before
changing the bowl speed.
a. Start-up
Most modern centrifuges have a one-button start. Manual systems take a few minutes, but
are not onerous. When the centrifuge is up to speed, the controls unlock the feed and 4780
polymer pumps, and the operator begins to put the centrifuge online. The start-up sequence
Turn on the feed and polymer to about one-third of the normal rate.
Reduce the differential revolutions per minute and/or pond to minimum.
When the cake thickness reaches the normal value, begin increasing the 4785
differential and the polymer feed rate. Some plants can jump directly to the
normal operating conditions as soon as the cake is sealed, while others have to
ramp up more slowly.
b. Shutdown
Again, modern centrifuges have a one-button stop. The shutdown sequence is as follows: 4790
Shut off the feed and polymer and turn the flushing water on.
When clear water exits both ends of the centrifuge, push the centrifuge stop
At some point, as the centrifuge slows down, flush water will come around the
feed tube or around the casing seals. Note how long it took between engaging 4795
the stop button and the water gushing out. Next time, shut the water off a
minute or two sooner.
With the flush water off, the centrifuge can usually coast to a stop without
operator intervention.
c. Sampling and Testing 4800
Sampling and testing should include TSS and/or total solids for the feed, total solids for
thickened sludge, and TSS, ammonia, and/or phosphorus (under some conditions) for
centrate.
5.2.3 Air Floatation Thickening
Air floatation units employ floatation of sludge by air under pressure or vacuum and are 4805
normally used for thickening the waste activated sludge. These units involve additional
equipment, higher operating costs, higher power requirements, and more skilled maintenance
and operation. However, removal of grease and oil, solids, grit and other material as also odour
control are distinct advantages.
In the pressure type floatation units, a portion of the subnatant is pressurised from 3 to 5 kg/ cm
and then saturated with air in the pressure tank. The effluent from the pressure tank is mixed
with influent sludge immediately before it is released into the flotation tank. Excess dissolved
air then rises up in the form of bubbles at atmospheric pressure attaching themselves to particles
which form the sludge blanket. Thickened blanket is skimmed off while the unrecycled
subnatant is returned to the plant. 4815
Flotation thickeners are equipped with both surface skimmers and floor rakes. The surface
skimmers remove floating material from the thickening tank to maintain a constant average float
blanket depth. Floor rakes are essential for removing the non-floatable heavier solids that settle
to the bottom of the flotation thickener. Most units are baffled and equipped with an overflow 4820
weir. Clarified effluent passes under an end baffle (rectangular units) or peripheral baffle
(circular units) and then flows over the weir to an effluent launder. The weir controls the liquid
level within the flotation tank with respect to the float collection box and helps regulate the
capacity and performance of the flotation unit.
The saturation system typically includes a recycle pressurization pump, an air compressor, an air 4825
saturation tank, and a pressure release valve. Although the flow through the pressurization pump
typically is recycle-flow, it can be makeup water. The pressure release valve controls pressure
loss and distributes the gas-saturated pressurized flow into the feed sludge as dissolved air
emerges from solution. The rapid reduction in pressure causes dissolved air (under pressure) to
emerge from solution or effervesce into minute bubbles. 4830
Other important equipment that forms part of an air floatation thickening system is the float
handling and pumping system. After removal of the float from the air floatation thickening unit,
float solids are deposited in a hopper and then pumped for further processing. This aspect of the
operation requires special considerations because of the following characteristics of float solids:
The parameter that is manipulated by the operator to control the performance of a thickener is 4835
the effective drainage time. This parameter is controlled by:
- Adjustment of skimmer on-time,
- Adjustment of skimmer off-time, and
- Adjustment of skimmer speed.
5.2.3.2 Operation 4840
A float total solid content of 4% represents a typical minimum for flotation thickeners handling
solids without primary sludge. Under optimum conditions, however, 5 to 6% solids content can
be expected.
Proper operation requires reducing variations of the feed rate and concentration. A feed sludge
holding and mixing tank helps with intermittent operation. Most units are operated continuously. 4845
Some are operated with a short period shut-down during weekends, while others are operated
only during certain hours of the day.
The speed and the on-off times of the float skimmers should be set to maximize the float solids
concentration, but should not be set too slow to cause excessive float-depth accumulation.
Dilution reduces the effect of particle interference on the rate of separation. Concentration of the 4850
sludge increases and the concentration of effluent suspended solids decreases as the sludge
blanket detention time increases.
- Fill the tank with screened final effluent or plant non-potable water until
overflowing. 4855
- Continue plant non-potable water flow to the air floatation unit and engage the
recycle system, including the compressor, if applicable. Adjust to proper pressures
and flow. After proper functioning, proceed.
- Ensure that the float and underflow pumps are functional by pumping some water.
- Prepare the polymer, if applicable, and engage the polymer addition at proper flowor 4860
as jar tests have indicated if starting after prolonged shutdown, start-up, or process
changes.
- Stop polymer flow, if applicable, and at the same time stop waste activated sludge
feed. 4865
- If only down for a short time (30 minutes or so), there is no need to shut down the
recycle system, including the compressor. If down for longer than this period, shut
down the recycle system.
- The float rake timer can be left on until most of the float is removed into the hopper
and pumped for further processing. 4870
- If the unit is going to be down for more than 24 hours, displace the tank contents with
non-potable water or drain and clean the tank, all troughs, and pipelines.
- In a typical operation, only the recirculation pump and retention tank discharge
valves are closed when stopping a units operation. All other valves remain open,
with the exception of valves on drain lines. 4875
- Checking all oil levels and ensuring that the oil fill cap vent is open;
- Checking all condensation drains and removing any accumulated moisture;
- Examining drive control limit switches;
- Visually examining the skimmer to ensure that it is in proper contact with the scum 4880
baffle and scum box;
- Inspecting skimmer wipers for wear;
- Adjusting drive chains or belts;
- Semi-annual inspections of major elements for wear, corrosion, and proper
adjustment include: 4885
Saturation systemseductors (if used) or nozzles should be inspected for wear
or cleaned whenever the efficiency begins to decline, or on a semi-annual basis;
Mechanical systems, including shaft bearings and bores, bearing brackets, baffle
boards, flights and skimming units, suction lines and sumps, and sludge pumps.
5.2.4 Belt Type Thickening 4890
Gravity belt thickeners (Figure 5.2) work by filtering free water from conditioned sludge by
gravity drainage through a porous belt. The gravity drainage area is usually horizontal but may
be inclined under some circumstances. Chemical conditioning is generally required to flocculate
the sludge and separate the solids from the free water. Chemical conditioning may be
accomplished by injecting the chemical through an injection ring and mixing it with the sludge. 4895
After chemical injection, the sludge velocity is reduced in a retention tank and the sludge is
allowed to fully flocculate before overflowing by gravity onto the moving belt.
Figure 5.2 Gravity belt thickener process
As the moving belt carries the sludge, plows clear portions of the belt for the filtrate to drain 4900
through and gently turn over the solids, thereby exposing more free water. Prior to the discharge,
most gravity belt thickeners have some type of dam or an adjustable ramp.
The hydraulic capacity of the equipment is determined by multiple factors including sludge type,
sludge concentration, polymer type and polymer dosage, belt speed, belt type, as well as the
obvious machine width and length. 4905
a. Polymer Addition
Polymer addition usually occurs by injection through a multiport injection ring; it is mixed
with the sludge as it flows through an inline mixing device. In some cases, the polymer may
be mixed mechanically in the retention tank. However, this generally results in higher 4910
polymer consumption.
b. Flocculation Tank
The flocculation tank allows the incoming sludge velocity to be reduced so that
flocculation can fully occur before the sludge overflows onto the moving belt. Design of
the tank is critical to prevent short-circuiting within the tank. 4915
c. Belt and Supports
Belts are typically woven from polyester fibre. High pH and other unusual conditions may
require special materials. The belt is supported on grid strips that also serve as wipers on
the bottom of the belt. This wiping action increases the drainage capacity of the belt.
d. Belt Tensioning 4920
Unlike a belt press, the performance of the belt thickener is not dependent on belt tension.
Once the dewatering belt on a thickener is tight enough to prevent slippage on the drive
roller, additional tension is unnecessary. Additionally, the belt tension on a thickener is not
dependent on the type or amount of sludge loading. As such, the requirements for belt
tension are much lower on a gravity belt thickener compared to a belt filter press. 4925
Belt tension is a result of moving one roller closer to or further away from the other
roller(s). This displacement may be through hydraulic, pneumatic, or mechanical actions.
Once the belt is tensioned, it is not necessary to relax the tension until the belt is replaced.
e. Belt Drive
All belt thickeners have a variable-speed belt drive with a typical speed range of 8 to 40 4930
m/min. The belt speed may be either mechanically or electrically varied with speed
controls at the local control panel. Typically, the belt drive is attached to a rubber-coated
drive roller.
f. Belt Tracking
During operation of a belt thickener, the belt should more or less remain centered and not 4935
move laterally on the machine. Although the belt should not move, some type of belt
tracking device is included on most machines. Comparatively, the belt on a belt thickener is
similar to a conveyor belt; all tracking devices have some roots in the conveyor or
papermaking industry.
5.2.4.2 Operation and Maintenance 4940
Start the hydraulic unit (or air compressor) and allow tension to develop in the
Start the belt drive and use an initial setting of approximately 20 m/min belt
speed. 4945
Start the wash water pump and allow the belt to pre-wet.
Start the polymer pump and allow the fresh polymer to reach the polymer
injection point.
After thickened sludge is available, start the thickened sludge pump (or
thickened sludge conveyor). 4950
After the system is running, begin fine-tuning the process by adjusting the
sludge flow, polymer dose, mixing energy, belt speed, and so on until the results
are within the desired process parameters. It is important to only adjust one item
at a time and to allow time for the adjustment to take effect before making
another change. 4955
Shut down sludge feed pump.
Shut down polymer feed pump.
As the thickened sludge hopper empties, shut down the thickened sludge pump
(or thickened sludge conveyor). 4960
Drain the flocculation tank.
Wash the machine down from top to bottom.
Allow the belts to be completely washed (this could take 15 to 45 minutes)
without sludge or polymer.
Shut down the wash water pump. 4965
Shut down the belt drive.
Shut down the hydraulic unit/air compressor.
c. Sampling and Testing
At a minimum, gravity belt thickeners should be sampled and analyzed as follows:
Sample influent feed for total solids, TSS, total volatile solids, pH, and flow. 4970
Sample wash water for TSS and flow.
Sample thickened sludge for total solids and flow.
Sample filtrate for TSS and flow.
Measure flow and quantity of polymer used.
Measure any dilution water used to make up polymer. 4975
d. Process Control
Numerous variables affect the overall thickening process. Listed below are most of these
variables.
Sludge feed, polymer, polymer dosage, mixing energy, retention time
Belt speed, belt tension, belt type, ramp angle 4980
Upstream variables, slurry pump selection, solids concentration
Biological sludge content, sludge storage time, wash water characteristics
5.3 Anaerobic Digestion
In anaerobic digestion, anaerobic bacteria thrive in an environment without dissolved oxygen by
using the oxygen that is chemically combined with their food supply. 4985
Two major types of bacteria are present in the digester. The first group starts eating the organic
portion of the sludge to form organic acids and carbon dioxide gas. These bacteria are called
acid formers. The second group breaks down the organic acids to simpler compounds and forms
methane and carbon dioxide gas. These bacteria are called gas formers. The methane gas is
usually used to heat the digester or to run engines in the plant. The production of gas indicates 4990
that organic material is being eaten by the bacteria. Sludge is usually considered properly
digested when 50% of the organic matter has been destroyed and converted to gas. This
normally takes approximately 30 days if the temperature is kept at about 35C.
Most digestion tanks are mixed continuously to bring the food to the organisms, to provide a
uniform temperature, and to avoid the formation of thick scum blankets. When a digester is not 4995
being mixed, the solids usually settle to the bottom, leaving a liquid known as supernatant above
the sludge. In many plants, however, there is no separation of solids and liquids after two days
of sitting without mixing due to the type of sludge. The supernatant is displaced from the tank
each time a fresh charge of raw sludge is pumped from the primary clarifier. The displaced
supernatant usually is returned from the digester back to the plant head-works and mixed with 5000
incoming raw wastes. Supernatant return should be slow to prevent over-loading or shock
loading of the plant.
In most new plants, sludge digestion takes place in two tanks. The first or primary digester is
usually heated and mixed. Rapid digestion takes place along with most of the gas production. In
the secondary tank, the digested sludge and supernatant are allowed to separate, thus producing 5005
a clearer supernatant and better-digested sludge.
Digester sludge from the bottom of the tank is periodically removed for dewatering.
5.3.1 Digestion Types
Two different types in anaerobic sludge digestion process namely, Low rate and High rate, are
used in practice. The basic features of these processes are shown in Figure 5.3. 5010
5.3.1.1 Low Rate Digestion
Low rate digestion is the simplest and the oldest process; essentially a low rate digester is a
large storage tank, occasionally, with some heating facility. The basic features of this process are
shown in Figure 5.3.
Raw sludge is fed into the digester intermittently. Bubbles of sewage gas are generated and their 5015
rise to the surface provides some mixing. In the case of few old digesters, screw pumps have
been installed to provide additional intermittent mixing of the contents, say once in 8 hours for
about an hour. As a result, the digester contents are allowed to stratify, thereby forming four
distinct layers: a floating layer of scum, layer of supernatant, layer of actively digesting sludge
and a bottom layer of digested sludge; essentially the decomposition is restricted to the middle 5020
and bottom layers. Stabilized sludge which accumulates and thickens at the bottom of the tank is
periodically drawn off from the centre of the floor. Supernatant is removed from the side of the
digester and returned back to the treatment plant.
Figure 5.3 Sludge digestion system 5025
5.3.1.2 High Rate Digestion
The essential elements of high rate digestion are complete mixing and more or less uniform
feeding of raw sludge. Pre-thickening of raw sludge and heating of the digester contents are
optional features of a high rate digestion system. All these four features provide the best
environmental conditions for the biological process and the net results are reduced digester 5030
volume requirement and increased process stability.
Complete mixing of sludge in high rate digesters creates a homogeneous environment
throughout the digester. It also quickly brings the raw sludge into contact with microorganisms
and evenly distributes toxic substances, if any, present in the raw sludge. Furthermore, when
stratification is prevented because of mixing, the entire digester is available for active 5035
decomposition, thereby increasing the effective solids retention time.
5.3.2 Configuration
5.3.2.1 Anaerobic Digestion Tank
Anaerobic digestion tanks may be cylindrical or cubical in shape. Most tanks constructed today
are cylindrical. The floor of the tank is sloped so that sand, grit, and heavy sludge will tend to be 5040
removed from the tank. Most digesters constructed today have either fixed or floating covers.
A fixed cover digester may develop an explosive mixture in the tank when sludge is withdrawn
if proper precautions are not taken to prevent air from being drawn into the tank. Each time a
new charge of raw sludge is added, an equal amount of supernatant is displaced because the
tank is maintained at a fixed level 5045
A floating cover moves up and down with the tank level and gas pressure. Normally, the vertical
travel of the cover is about 2.5m, with stops (corbels) or landing edges for down (lowering)
control and maximum water level for upward travel. Maximum water level is controlled by an
overflow pipe that must be kept clear to prevent damage to the floating cover by overfilling. Gas
pressure depends on the weight of the cover. The advantages of a floating cover include less 5050
danger of explosive mixtures forming in the digester, better control of supernatant withdrawal,
and better control of scum blankets. Disadvantages include higher construction and maintenance
5.3.2.2 Agitator
Maintenance requires that the condensate be drained from the lines at least twice a day, that the 5055
diffusers be cleaned to prevent high discharge pressures, and that the compressor unit be
properly lubricated and cooled.
Propeller mixers are found mainly on fixed cover digesters. Normally, two or three of these
units are supported from the roof of the tank with the propeller blades submerged 3 to 3.5 m in
the sludge. An electric motor drives the propeller stirring the sludge. Digested sludge normally 5060
contains a great deal of grit and debris. This type unit usually has reversible motors so the
propeller may rotate in either direction. In one direction the contents are pulled from the top of
the digester and forced down the draft tube to be discharged at the bottom. By operating the
motor in the opposite direction, the digested sludge is pulled from the bottom of the tank and
discharged over the top of the draft tube to the surface. Reversible motors also assist in 5065
minimizing accumulations of rags on the propeller.
If two units are installed in the same tank, an effective way to break up a scum blanket is
operating one unit in one direction and the other unit in the opposite direction, thereby creating
a push-pull effect. The direction of flow in the tubes should be reversed every day.
A limitation of draft tube-type mixers is the potential formation of a scum blanket. If the water 5070
level is maintained at a constant elevation, a scum blanket forms on the surface. The scum
blanket may be a thick layer and the draft will only pull liquid sludge from under the blanket,
not disturbing it, Lowering the level of the digester to just 7 to 10 cm over the top of the drain
tube forces the scum to move over and down the draft tube. This applies mainly to
single-direction mixers. 5075
Pumps are sometimes used to mix digesters. This method is common in smaller tanks. The tank
may or may not be equipped with a draft tube positioned in such a way that the pump suction
may be from the top or valved from the bottom of the digester. Control of scum blankets with
this method of mixing depends on how the operator maintains the sludge level and where the
pump is pulling from and discharging to the digester. 5080
Pressure gauges should be installed on the pump suction and discharge pipes. A change in
pressure could indicate that the pump is not functioning properly and the desired mixing may
not be taking place in the digester.
5.3.2.3 Digester Gas Equipment
a. Gas Tank 5085
Several types of gas storage are available. The most common means of low-pressure gas
storage is the floating gas-holder cover. Membrane storage can be installed either on the
digester, to serve both as cover and storage space, or on the ground as a standalone
structure.
b. Flow Meter 5090
Gas production is a measure of digester performance. Reliable monitoring equipment alerts
plant operators to process malfunctions and gas leaks. The flow meters used for gas
monitoring can be broadly classified as positive-displacement, thermal-dispersion, and
differential pressure flow meters (Table 5.1).
Separate flow meters are recommended for each digester because digester gas-production 5095
rates vary. Separate flow meters are also recommended to monitor gas use by the utilization
equipment. The gas may contain moisture and impurities, which may cause maintenance
problems for the metering devices.
Table 5.1 Gas-flow indication and metering
Type Application Operation and maintenance
Used in many plants. Large
devices that require bypasses
when servicing.
Accurate gas measurement unless
dirty.
Does not require upstream and
downstream straight-pipe
sections for accuracy.
Somewhat tolerant of dirty gas.
Requires periodic
cleaning and rebuilding.
Thermal dispersion insert
Increasingly used technology
because of tolerance to dirty gas
and simple installation. Accurate
for low flows.
Some accuracy problems with
zero flow, such as for flares.
Requires some upstream and
distances for accuracy. Can be
inserted and removed through
ball valve.
Smaller than most metering
devices. Requires periodic
cleaning of probe. Must not have
liquid impinging on probe. No
bypass needed, reducing need for
piping and isolation valves to
Pressure differentia
orifices and Venturis
Commonly used in older plants.
Upstream and downstream
straight piping sections needed.
Requires frequent cleaning when
used for dirty gas. Accuracy
depends on instrument
cleanliness. Can significantly
constrict gas flow when fouled.
Source: WEF, 2008 5100
c. Gas system safety and control devices (Table 5.2)
Table 5.2 Gas system safety and control devices
Flame traps
(large pipe)
and flame check
(small piping)
Used to stop flame propagation in
a pipe. Flame traps are installed
with thermal shutoff valves as
noted below. Installed on gas
supplies to engines, compressors,
boilers, flares, or other sources of
ignition. Also used on digester
covers with pressure- and
vacuum-relief valves.
Require periodic cleaning, typically
May require insulation and heating in
cold climates to prevent ice formation.
Foam will blind the devices, rendering
other devices, particularly relief valves,
ineffective.
Thermal shutoff
Spring-loaded or pressure-operated
isolation valves that trip and shut
off at flame temperatures. Must be
Replace element periodically or when
tripped.
Usually includes an indicator to show
installed between flame trap and
source of ignition.
that the valve has tripped. May require
insulation and heating in cold climates to
prevent ice formation.
Pressure- and
vacuum relief
Provided for all digester covers to
prevent over pressurization or
vacuum conditions in the digester.
Typically adjustable, weighted
valves, built as common unit.
Should be set below design limits
of cover. Typically two sets
provided for each cover to allow
servicing of one set while keeping
digester in service. Typically
discharge to area around valves.
Available with pipe to allow
discharge away from valves.
Check seals periodically; replace if leaks
develop.
Relief valves are last safety measure for
digester covers to prevent over pressure
or vacuum. These valves do not provide
liquid relief.
Typically provided with waste gas
flares. Maintain pressure upstream
by relieving overpressure to flare.
Usually uses weights to set relief
pressure; however, some use
springs. Upstream sensing line
should ideally be in gas collection
piping header, away from the
valve and large enough to reduce
the potential for clogging.
Frequent maintenance required to
prevent sticking (open or closed). These
valves regulate pressure in the
gas-collection piping system and are the
first level of protection for over
pressurization of the digester cover.
Low-pressure
Used to prevent backflow of gas.
Typically constructed with leather
or flexible flap to ensure operation
at low pressure. Not commonly
Check flap and replace if not
functioning.
Waste gas flare
or burners
Provided to dispose of surplus gas.
Usually provided with standing
pilot or electronic ignition.
Provided for all plants to serve as a
means of suitable disposal of gas.
Untreated gas should not be freely
Open pipe or
candle flare
Used for smaller plants and where
stringent air-emissions control is
not needed. Generally visible
flame, gas disposal device.
Good pilot-gas supply is needed (may
require natural gas or propane supply).
Heat and corrosion make periodic
replacement necessary.
Pilot system requires frequent servicing.
combustion flare,
ground flare, or
controlled flare
Used at plants where stringent air
emissions are required and visible
flame is not desired. Control air
supply to maintain sufficient
temperature to completely burn
gas. These flares must be operated
at a specific temperature to achieve
emission limits. If oversized,
auxiliary fuel may be required; this
increases operating costs.
Require periodic system maintenance for
controls and air blowers.
Manometers used for low pressure.
Gauges used for higher pressures,
particularly for gas utilization
Periodically clean manometers, replace
fluid, and calibrate gauges. Pressure
gauges should be provided with an
isolating diaphragm to prevent gas
contaminants from fouling the
mechanism.
5.3.2.4 Gas Scrubbers
a. Foam and Sediment Removal 5105
Many systems are equipped with sediment traps and foam separators for cleaning digester
gas. These devices provide a wide spot in the gas piping system for slowing velocities,
collecting foam and particulates entrained in the gas, and removing collected condensate.
The foam separator is a large vessel with an internal plate fitted with water nozzles that
provide a continuous spray. The foam- and sediment-laden gas enters the vessel near the 5110
top and travels down through the spray wash under the baffle wall and up through a second
spray wash, exiting the vessel through an elevated discharge nozzle. The spray wash and
the internal plate reduce foam in the gas to prevent carryover to gas utilization equipment.
b. Hydrogen Sulphide Removal
The generation of hydrogen sulphide can also be inhibited by using ferric chloride injection 5115
into the digester. But this is usually very difficult as handling ferric chloride is not easy
because it is very acidic and reactions with skin can be troublesome. Injection of ferric
chloride can however be used as a temporary measure when sulphates in raw sewage
become very high in drought situations when the population may use a lot of hard water for
many purposes other than drinking, and which may increase the sulphate in raw sewage. 5120
Iron salts can be added at the following locations in the treatment process:
The primary clarifier (helps settling and improves overall facility odour control)
The suction side of the digester sludge-recirculation pump
The suction side of a mechanical mixer
Iron salts should not be added directly upstream from the heat exchangers because this can 5125
result in deposits of vivianite on heat exchanger surfaces.
Refer to section 6.5.15.3 of the Part-A manual for the methods in use in India for removing
hydrogen sulphide from digester gas.
c. Moisture Reduction
Moisture is condensed from digester gas as it cools. Gas piping should have a slope of at 5130
least 1% toward the condensate collection point. To effectively remove the moisture, the
gas flow should not exceed 3.7 m/s countercurrent to condensate flow.
The condensate is collected in traps that should be located at low points in long pipe runs
and wherever gas is cooled. Drip taps, which can be controlled manually or automatically,
provide a convenient and safe means for removal of accumulated condensate. Manually 5135
operated drip taps are recommended for indoor applications. Float-controlled, automatic
drip taps are also available, but these require frequent maintenance to keep the valves
operating. Should the float stick, gas can escape to the surrounding atmosphere, which
limits their use to outdoor installations (where permitted by local codes and safety
considerations). 5140
d. Carbon Dioxide Removal
Carbon dioxide can be removed from the digester gas by water or chemical scrubbing,
carbon sieves, or membrane permeation; however, all of these technologies are expensive
and their use may be cost-effective only if the gas is to be upgraded to natural gas quality
and sold. 5145
e. Siloxane Removal
Siloxanes are components of toiletries and personal care cosmetics such as sprays,
deodorants, lipsticks, gels, lotions, shaving creams, cleaning fluids, and so on. Their use
is growing every year. Not much data is available on their removal in STPs. It has been
reported that siloxanes find their way in digester gas. The concern is in cold climates, if 5150
digesters are to be heated to maintain temperature of about 35C, silicon dioxide deposits
on the heat exchanger tubes, which reduces heat transmission. A typical photograph is
Figure 5.4 Silicon dioxide deposits on boiler tubes 5155
Hence it becomes necessary to protect equipment from siloxanes. This problem can occur
also when digester gas is burned in gas engines. However, this has not been reported as a
serious problem in digester gas usage in India.
f. Activated Carbon Scrubbers
Activated carbon scrubbers used to remove siloxanes from digester gas operate according 5160
to the same principles as the carbon scrubbers used for odour control in sewage treatment
plants. The digester gas is passed through a vessel filled with activated carbon, which
captures the organics, including siloxanes, hydrogen sulphide, and several other
compounds in digester gas. With proper maintenance and replacement of the carbon, the
siloxanes in the digester gas can be removed to below detection limits. However, 5165
activated carbon is not selective with regard to siloxanes and will remove other
compounds as well. Consequently, if the digester gas contains other organics, the carbon
will require frequent replacement. In India, the cocoanut shell activated carbon is locally
available and is economical. Removal of hydrogen sulphide before it passes through the
carbon scrubbers will provide better siloxane removal and extend the life of the carbon 5170
bed.
5.3.2.5 Gas Power Generator
Refer to Sec.6.4.2 Gas Engines.
5.3.3.1 Feeding Schedule 5175
Uniformity and consistency are keys to digester operation. Sudden changes in feed solids
volume or concentration, temperature, composition, or withdrawal rates will inhibit digester
performance and may lead to foaming. The ideal feeding procedure is a continuous,
24-hour-per-day addition of a blend of different types of feed solids (primary and WAS). Where
continuous feeding is impossible, a 5- to 10-min/h feed cycle is used. Smaller STPs that operate 5180
a single 8-hour shift use a schedule of at least three feedings: at the beginning, middle, and end
of the shift. And typical causes of organic overloads include the following:
- Starting the digester too rapidly,
- Excessive volatile solids loading as a result of erratic feeding or a change in feed
solids composition, 5185
- Volatile solids loadings exceeding the daily limits by more than 10%,
- Loss of active digester volume because of grit accumulation, and
- Inadequate mixing.
5.3.3.2 Withdrawal Schedule
Solids should be withdrawn from the primary digester immediately prior to feeding raw sludge 5190
to prevent short-circuiting. In digesters with surface overflow, the timing and rate of solids
withdrawal and feed are coordinated to occur concurrently. Solids should be withdrawn at least
daily to avoid a sudden drop in the active microorganism population. The primary digester may
be regulated to simply overflow to the secondary digester or to the digested sludge storage tank
as raw sludge is added. Solids may be withdrawn from the following locations: 5195
- The bottom of the digester,
- The overflow structure, and
- Any point within a well-mixed digester.
A benefit of removing solids from the bottom of the digester is that it may also remove the grit
that accumulates on the bottom of the digester. If possible, solids removal should be performed 5200
It is important to recognize that because digestion destroys volatile solids, the concentration of
the biosolids removed from the digester will be lower than the feed concentration unless the
digester is decanted.
5.3.3.3 Scum Control 5205
Scum accumulation in digesters is common. Scum is a combination of undigested grease and oil
and often contains buoyant materials, such as plastics that are not removed at the plants
headworks. Scum floats on the digester liquid surface and can accumulate, forming a dense mat.
Properly designed and operated digester mixing systems can typically blend the scum into the
tank contents. 5210
If the digester operates without mixing for longer than 8 hours, scum may rise and float on the
liquid surface. After mixing is restarted, the scum is re-suspended within the liquid. The primary
method of scum control is to keep the digester mixing system well-maintained during operation.
5.3.3.4 Precipitate Formation and Control
The digestion process can produce crystalline precipitates that affect both the digestion system 5215
and downstream solids-handling processes. The precipitates can accumulate on pipes and
dewatering equipment, causing damage and blockages and requiring costly and time-consuming
maintenance. Common precipitates include struvite, vivianite, and calcium carbonate. The
constituents that form these precipitates are present in undigested sludges and are released
during the digestion process and converted to soluble forms that can react and crystallize. Their 5220
formation varies from site to site, depending on the chemistry of the digested sludges and the
treatment processes. Because precipitates preferentially form on rough or irregular surfaces,
glass-lined sludge piping and long-radius elbows help minimize their accumulation.
5.3.3.5 Digester Upsets and Control Strategies
The four basic causes of digester upsets are hydraulic overload, organic overload, temperature 5225
stress, and toxic overload. Hydraulic and organic overloads occur when the design hydraulic or
organic loading rates are exceeded by more than 10% per day. The overload conditions can be
controlled by managing digester feeding, as well as ensuring that the effective digester volume
is not diminished by grit accumulation or poor mixing. Digester feeding is controlled by proper
operation of upstream headworks, clarifiers, and thickeners to ensure the feed sludge 5230
concentrations. In the event of a digester upset, an effective control strategy includes the
following steps:
- Stop or reduce sludge feed.
- Determine the cause of the imbalance.
- Correct the cause of the imbalance. 5235
- Provide pH control until treatment returns to normal.
If only one digester tank is affected, the loading on the remaining units can be carefully
increased to allow the upset unit to recover. If overloading is affecting several units, reducing
the feed will require a method of dealing with the excess sludge by hauling it to another facility,
providing temporary storage onsite, or chemically stabilizing and disposing of the sludge. 5240
5.3.3.6 Temperature
Temperature-related stress is caused by a change in digester temperature of more than 1 or 2C
in fewer than 10 days, which would reduce the biological activity of the methane-forming
microorganisms. If the methane formers are not quickly revived, the acid formers, which are
unaffected by the temperature change, continue to produce volatile acids, which will eventually 5245
consume the available alkalinity and cause the pH to decline.
The most typical causes of temperature stress are overloading sludge and exceeding the
instantaneous capacity of the heating system. Most heating systems can eventually heat the
digester contents to the operating temperature, but not a harmful temperature variation.
5.3.3.7 Toxicity Control 5250
The anaerobic process is sensitive to certain compounds, such as sulphides, volatile acids, heavy
metals, calcium, sodium, potassium, dissolved oxygen, ammonia, and chlorinated organic
compounds. The inhibitory concentration of a substance depends on many variables, including
pH, organic loading, temperature, hydraulic loading, the presence of other materials, and the
ratio of the toxic substance concentration to the biomass concentration. 5255
5.3.3.8 pH Control
The key to controlling the digester pH is to add bicarbonate alkalinity to react with acids and
buffer the system pH to about 7.0. Bicarbonate can be added directly or indirectly as a base that
reacts with dissolved carbon dioxide to produce bicarbonate. Chemicals used for pH adjustment
include lime, sodium bicarbonate, sodium carbonate, sodium hydroxide, ammonium hydroxide, 5260
and gaseous ammonia. Lime addition can be messy and will produce CaCO
. Although
ammonia compounds can be used for pH adjustment, they may cause ammonia toxicity and
increase the ammonia load on the liquid treatment processes through return streams.
Consequently, their use is not recommended.
During a digester upset, volatile acid concentrations may begin to rise before bicarbonate 5265
alkalinity is consumed. Because pH depression does not occur until alkalinity is depleted, it may
be observed only after the digester is well on its way to failure.
5.3.3.9 Digester Foaming
Digester foam consists of fine gas bubbles trapped in a semi-liquid matrix with a specific
gravity of 0.7 to 0.95. The gas bubbles are generated below the sludge layer and are trapped as 5270
they form. While some foaming always occurs, it is considered excessive if it plugs piping or
escapes from the digester. Excessive foaming can cause the loss of active digester volume,
structural damage, spillage, and damage to the gas-handling system, as well as being
malodorous and unsightly. The most common cause of digester foaming is organic overload,
which results in the production of more VFAs (volatile fatty acids) than can be converted to 5275
methane. The acid formers (which release carbon dioxide) work much more quickly than the
methane-forming microorganisms. The resulting increase in carbon dioxide typically increases
foam formation. Factors that can contribute to organic overload include:
- Intermittent digester feeding;
- Separate feeding or inadequate blending of primary sludge and waste activated 5280
sludge;
- Insufficient or intermittent digester mixing; and
- Excessive amounts of grease or scum in digester feed (especially problematic if the
digester is fed in batches).
Organic overload can be minimized by feeding the digesters continuously (or as often as 5285
possible), blending different feed sludges well before feeding, ensuring that the digester-mixing
system is operable, and limiting the quantities of grease or scum in the digester feed.
5.4 Sludge Dewatering
Most of the digested primary or mixed sludge can be compacted to a water content of about
90% in the digester itself by gravity but mechanical dewatering with or without coagulant aids 5290
or prolonged drying on open sludge drying beds may be required to reduce the water content
further. The dewatering of digested sludge is usually accomplished on sludge drying beds which
can reduce the moisture content to below 70%. But excess oil or grease in the sludge will
interfere with the process. Where the required space for sludge drying beds is not available,
sludge conditioning, followed by mechanical dewatering on centrifugation, belt press, filter 5295
presses, screw press, rotary press, and vacuum filters.
5.4.1 Chemical Dosing Equipment
5.4.1.1 Coagulant
Chemical conditioning is the process of adding certain chemicals to enable coalescence of
sludge particles facilitating easy extraction of moisture. The chemicals used are ferric and 5300
aluminium salts and lime, the more common being ferric chloride with or without lime.
Digested sludge, because of its high alkalinity exerts a huge chemical demand and therefore the
alkalinity has to be reduced to effect a saving on the chemicals. This can be accomplished by
elutriation. Polyelectrolytes show promise for sludge with finely dispersed sludge. The choice of
chemical depends on pH, ash content of sludge, temperature and other factors. Optimum pH 5305
values and chemical dosage for different kinds of sludge have to be based on standard
laboratory tests. The dosage of ferric chloride and alum for elutriated digested sludge is of the
order of 1.0 kg/m
of sludge. Alum when vigorously mixed with sludge, reacts with the
carbonate salts and releases CO
, which causes the sludge to separate and water drains out more
easily. Hence for effective results, alum must be mixed quickly and thoroughly. The alum floc, 5310
however, is very fragile and its usefulness has to be evaluated as compared with ferric chloride
before resorting to its application.
Feeding devices are necessary for applying chemicals; mixing of chemicals with sludge should
be gentle but thorough, taking not more than 20 to 30 seconds. Mixing tanks are generally of the
vertical type for small plants and of the horizontal type for large plants. They are provided with 5315
mechanical agitators rotated at 20 to 80 rpm.
a. Inorganic Chemicals
Inorganic chemical conditioning is associated principally with vacuum and pressure
filtration dewatering. The chemicals typically are lime and ferric chloride. Ferrous
sulphate, ferrous chloride, and aluminium sulphate are also used, although less commonly. 5320
i. Ferric Chloride
Ferric chloride solutions typically are used at the concentration received from the
supplier (30 to 40%); however, some STPs dilute the ferric chloride to approximately
10% to improve mixing and reduce the acidity and corrosivity of the material. This
can be done in day tanks or inline. Dilution may lead to hydrolysis reactions and the 5325
precipitation of ferric chloride crystals.
An important consideration in the use of ferric chloride is its corrosive nature. It reacts
with water to form hydrochloric acid, which attacks steel and stainless steel. When
diluted with sludge, the acidity is neutralized by the alkalinity of the sludge and
thoroughly diluted so that the end product is quite benign. Interlocks must be used to 5330
ensure that ferric chloride is always added to sludge in the proper ratio and is never
pumped into sludge lines or process equipment by itself.
Special precautions must be taken when handling this chemical. The best materials are
epoxy, rubber, ceramic, polyvinylchloride, and vinyl. Contact with the skin and eyes
must be avoided. Rubber gloves, face shields, goggles, and rubber aprons must be 5335
used at all times. Ferric chloride can be stored indefinitely without deterioration.
Customarily, it is stored in above-ground tanks constructed of resistant plastic and
surrounded by a containment wall. Ferric chloride can crystallize at low temperatures,
which means that the tanks must be kept indoors or heated.
ii. Lime 5340
Vacuum filters and filter presses commonly use lime and ferric chloride to make the
sludge easier to filter and improve the release of the sludge from the filter media. Lime
is available in two dry formsquicklime (calcium oxide) and hydrated lime [Ca(OH)
].
When using quicklime, it is first slurried with water and converted to calcium
hydroxide, which is then used for conditioning. Because this process (known as 5345
slaking) generates heat, special equipment is required.
Quicklime must be stored in a dry area, because it reacts with moisture in the air and
can become unusable.
Hydrated lime is much easier to use than quicklime, because it does not require
slaking, mixes easily with water with minimal heat generation, and does not require 5350
any special storage conditions.
Lime typically is used in conjunction with ferric salts. Although lime has some slight
dehydration effects on colloids, odour reduction, and disinfection, it is used because it
improves filtration and release of the cake from the filter media. The lime reacts with
bicarbonate to form a precipitate of calcium carbonate, which provides a granular 5355
structure that increases porosity and reduces compressibility of the sludge.
b. Organic Flocculants
Organic flocculants are widely used in many industries and processes involving the
separation of sludge fromliquids. These liquid- sludge separation applications may involve
processes related to the recovery of finished products, clarification or purification of 5360
liquids, and volume reduction of waste materials.
While organic polyelectrolytes are commonly used in applications involving liquid-sludge
separation, the processes of sewage sludge thickening and dewatering are completely
dependent on their use.
i. Polymer Characteristics 5365
The product characteristics of these complex and proprietary polyacrylamide
flocculants may vary according to the following:
Electronic charge (anionic, nonionic, or cationic)
Molecular weight (standard viscosity) 5370
ii. Polymer Specifications and Quality Control
Along with the product identification and type and form of product, the following
standard product specifications should be obtained to determine storage conditions,
pumping requirements, and potential hazards: 5375
Total solids
Bulk viscosity
Freezing point 5380
5.4.1.2 Equipment
a. Liquid Feeders
A typical solution-feed system consists of a bulk storage tank, transfer pump, day tank
(sometimes used for dilution), and liquid feeder. Some liquid chemicals can be fed directly
without dilution, and these may make the day tank unnecessary, unless required by a 5385
regulatory agency. Nonetheless, dilution water can be added to prevent plugging, reduce
delivery time, and help mix the chemical with the sewage. However, sometimes, the
dilution water can have adverse chemical effects. For instance, dilution water that has not
been softened can potentially cause calcium carbonate scale to build up on the piping.
Special consideration should be given to the final water chemistry of the solution before 5390
adding dilution water.
Liquid feeders are typically metering pumps. Metering pumps are generally of the
positive-displacement type using either plungers or diaphragms.
Positive-displacement pumps can be set to feed over a wide range (10:1) by adjusting the
pump stroke length. 5395
The chemical addition rate can be set manually by adjusting a valve or the stroke/speed on
a metering pump.
b. Dry Chemical Feeders
Lime and alum are typical of the kinds of chemicals used with a dry chemical-feed system.
It consists of a feeder, a dissolver tank, and a storage bin or hopper. These systems are 5400
complex because of their many storage and handling requirements. The simplest method of
feeding dry or solid chemicals is by hand. Solid chemicals may be pre-weighed and added
or poured by the bagful into a dissolving tank. This method generally applies only to small
plants where dry chemical-feed equipment is used.
Most dry feeders are of the belt, grooved-disk, screw, or oscillating-plate type. The feeding 5405
device (belt, screw, disk, etc.) is typically driven by an electric motor. Many belt feeders,
particularly the gravimetric type, also contain a material flow-control device such as a
movable gate or rotary inlet for metering or controlling flow of the chemical to the feed
Many metering pump systems handle chemicals that coat or build a layer of residue or slurries
that can settle out solids during operation. Strainers are helpful in removing large particulates,
but the operator must keep these cleaned. Periodic flushing to remove residues and deposits is
often required. Piping and valve arrangements should allow the system to be isolated so that a
clear liquid, such as water, can be used to pressurize the system for flushing the residue or solid 5415
buildup. Such flushing systems can be operated manually using hand-operated valves or can be
automatically operated using solenoid valves with a timer control system. Systems where the
metering pumps and piping are periodically shut down will require flushing connections to
remove solids.
Most feeders, regardless of type, discharge their material to a small dissolving tank equipped 5420
with a nozzle system or mechanical agitator, depending on the solubility of the chemical being
fed. The surface of each particle needs to be completely wet before it enters the feed tank to
ensure thorough dispersal and avoid clumping, settling, or floating. When feeding some
chemicals, such as polymers, into dissolvers, care must be taken to keep moisture inside the
dissolver from backing up into the feeder. 5425
Systems where the metering pumps and piping are periodically shut down will require flushing
connections to remove solids. In addition, an allowance for T- and Y-cleanouts should be
included for the piping system where longer horizontal piping runs cannot be adequately
flushed.
A metering pump will lose capacity and become erratic when the suction or discharge valves
become worn or when poor hydraulic conditions exist. These conditions will be indicated by the
cylinder test. Also, debris in the chemicals being fed may obstruct or block the check valves,
thus impeding their operation and decreasing the pumps performance.
- Check dust filters periodically. 5435
- Periodically clean and calibrate level measurement and indication instrumentation in
liquid and dry storage tanks.
- Check the level and condition of the oil in the gear reducer.
- Check the condition of all painted surfaces.
- Clean dirt, dust, or oil from equipment surfaces. 5440
- Check all electrical connections.
- Stop and start equipment, checking for voltage and amp draw and any movement
restrictions because of failed bearings, improper lubrication, or other causes.
- Check the drive motor for any unusual heat, noise, or vibration.
- Check the packing for leakage and wear. 5445
5.4.2 Sludge Feed Pump
The following operations directly affect sludge pump performance.
Positive-displacement pumps need a drive system that can operate the pump at the speed needed
to perform adequately under all operating conditions. Sometimes, this involves manually and 5450
automatically timed starts and stops, as well as variable pump discharge rates. This
variable-speed arrangement can be provided via mechanical vari-drives; variable pitch pulleys;
direct-current, variable-speed drives; alternating-current, variable-frequency drives;
eddy-current magnetic clutches; or hydraulic speed-adjustment systems. Each has various
advantages and disadvantages with respect to cost, amount and ease of maintenance required, 5455
efficiency, turndown ratio, and accuracy. Because positive-displacement pumps are constant
torque machines, operators should ensure that the output torque of variable-speed drive exceeds
the pumps torque requirement at all operating points. Although variable-speed drives are often
either a necessity or an enhancement to proper plant operation, the challenge is providing the
continued maintenance and servicing required. 5460
Operators should check the following items:
- Inlet and outlet flow rate
- Noise or vibration
- Bearing housing temperature
- Running amperage 5465
- Pump speed
- Pressure
Following is the maintenance checklist for sludge pumps:
- Check the level and condition of the oil in the gear reducer. 5470
- Check the shaft alignment.
- Visually inspect mounting fasteners for tightness.
- Clean dirt, dust, or oil from equipment surfaces.
- Check all electrical connections. 5475
- Check mechanical seals and packing for leakage or wear.
5.4.3 Mechanical Dewatering 5480
5.4.3.1 Centrifugal Dewatering
Centrifugation is the process of separating solids from liquids by the process of sedimentation,
enhanced by centrifugal force.
5.4.3.1.1 Operation
A centrifuge can thicken or dewater the sludge with only a minor change in the weir setting 5485
(also called pond setting). Likewise, it can dewater sludge to a moderate consistence at low
polymer dose or produce very dry solids using higher polymer dosages.
a. Sludge Type and Quality
The operation of the wet end of the plant determines the quality of the sludge, which, in
turn, greatly affects the dry end. 5490
b. Polymer Activity and Mixing with the Sludge
If the polymer does not react well with the sludge, performance suffers. Also, adding the
polymer closer to or further from the centrifuge will affect performance.
c. Polymer Type and Dosage
Some polymers are designed to obtain drier cakes than others. 5495
Likewise, the dosage will increase and decrease with cake dryness. Some polymers become
less effective at higher dosages. This will be apparent froma quick jar test or observing that
adding more polymer results in either poorer operation or the same operation.
Hydraulic Loading
Centrifuges are less limited by the volume of water that passes through the centrifuge 5500
than filtration devices. As a result, thinner feed sludges will have less effect on
performance than in filtration devices.
Solids Loading
The solids residence time is important. If there is more sludge to dewater, there will be
less solids residence time and therefore wetter solids, all else being equal. 5505
The solids capture is generally fixed by the plant management, and is not an operating
variable.
d. Torque Control
In recent years, nearly all centrifuges have a controller that allows the operator to choose a 5510
scroll drive load or torque set point, and the controller then adjusts the differential speed to
maintain that set point. In this manner, the torque and therefore the cake dryness is fixed.
One way of looking at the centrifuge is that it is a very expensive viscometer. The conveyor
is turning at a controlled speed immersed in the sludge. The effort or torque needed to turn
the conveyor is measured by the scroll drive device. 5515
As the cake becomes drier, its viscosity increases, which, in turn, increases the torque or
load on the scroll drive. When operating in load control, the controller automatically
adjusts the differential revolutions per minute to maintain a constant torque level, and the
operators only task is to observe the centrate quality from time to time and adjust the
polymer rate to maintain the desired centrate quality. 5520
This is a really simple control; one of its virtues is that the major operating cost - cake
-dryness is fixed, and any operator error shows up in the centrate, which is easy to see and
is not so costly if it is off slightly.
Consult the manufacturer of the centrifuge for a recommendation on operating speed
changes. 5525
e. Process Control
The following shutdown procedures are suggested:
Stop sludge and polymer feed to the centrifuge.
Add flush with water (effluent water is acceptable) until the centrate is clear and
the torque level begins to drop. 5530
Turn the centrifuge off.
Continue flushing at 25% of normal feed flow until the centrifuge reaches 7 to
800 r/min.
Turn off the lubrication system and cooling water when the unit is completely
stopped. 5535
5.4.3.1.2 Maintenance
During operation, the operator should check for the following:
- The oil level and the flow of oil to the bearings in circulating oil systems
- Flow of cooling water and oil temperature, to ensure it is operating in the proper
- Machine vibratione
- Ammeter reading on the bowl motor
- Bearing temperatures, by touching them
- System for leaks
- Centrate quality 5545
- Scroll drive torque
Because the centrifuge will shut itself down in the event of a fault, the operator typically only
looks at the mechanical parameters once per shift.
5.4.3.2 Belt Filter Press Dewatering Equipment
The operation of a Belt Filter Press (BFP) is based on the principles of filtration and comprises 5550
the following zones (Figure 5.5):
- Gravity drainage zone, where the feed is thickened
- Pressure zonee
- Shear zone
Figure 5.5 Belt filter press
a. Process Variables
There are several process variables that affect the performance of all dewatering systems.
In general, dewatering devices must run at 95% capture or better, so capture really is not an 5560
operating variable. Of the remaining parameterscake dryness, loading, and polymer
dosagewithin limits, the operator can take from one to give to another. For drier cake,
one can reduce the loading and/or increase the polymer dosage.
Cake Dryness
Increased cake dryness comes at the price of lower capacity and/or higher polymer 5565
dosage. The ability to obtain higher cake dryness is, to a great extent, a function of the
design of the press. Presses with extended gravity zones to better pre-concentrate the
feed and additional pressure rollers give longer sludge residence times and drier cake.
Polymer Type and Dosage
Some polymers are designed to obtain drier cakes than others. Likewise, the dosage 5570
will increase and decrease with the cake dryness. Some polymers become less
effective at higher dosages. This will be apparent from a quick jar test or observing
that adding more polymer results in either poorer operation or same operation.
Belt filter presses are limited by the volume of water that can go through the belts. As 5575
a result, thinner feed sludges will result in less quantity of dry solids produced, all else
being equal.
Likewise, more solids will result in less solids residence time inside the press and
therefore wetter sludge, all else being equal. 5580
The solids capture is typically fixed by the plant management and is not an operating
Belt composition and condition, speed, and tension
Size and number of rollers 5585
Wash water flow, pressure, and suspended solids concentration
b. Sequence of Operation
The sequence of operation for a BFP typically is set up in the following order:
Open wash water valve.
Start wash water pump. 5590
Start pneumatic/hydraulic belt tension system.
Start belt drive and dewatered cake conveyor.
Start polymer solution feed pump.
Start sludge feed pump.
Modern presses typically have a one-button start system, so the operator only has to 5595
manually start the feed and polymer pumps. In any event, one benefit of filling out the
operating log is that the operating conditions the last operator used are known, as are
the conditions of the previous week and month.
Rollers and bearings require frequent lubrication. Follow the manufacturers operations and 5600
maintenance manual for lubrication schedule. This extends the life of the roller bearings and
belt drive motor.
Replacement of filter belts is a common maintenance requirement.
The following procedures will extend the life of the BFP and reduce its operating cost:
Wash down the BFP every day after finishing the dewatering shift. This 5605
prevents cake from drying and accumulating in different sections of the BFP.
Confirm that all rollers are turning freely.
Check the press weekly for damaged bearings.
Check the grinder that prevents large particles from entering the press twice per
year. 5610
Clean the wash water nozzles as frequently as necessary (this depends on the
quality of the wash water). This ensures proper cleaning of the belts.
Inspect and change sludge containment and washbox seals, as necessary.
Inspect and clean doctor blades from any accumulated debris, hair, or any other
foreign materials. 5615
Clean the chicanes (plows) in the gravity section after shutting down the press.
For any other maintenance of complex mechanical parts of the BFP, contact the
manufacturer for advice.
5.4.3.3 Filter Press
Filter presses for dewatering are generally either recessed plate filters or diaphragm filter 5620
presses. With the advent of better organic polymers, belt filters and centrifuges have largely
displaced filter presses in the market. Filter presses can be attractive in unusual circumstances.
The fixed-volume recessed plate filter press consists of a series of plates, each with a recessed
section that forms the volume into which the feed enters for dewatering. Filter media or cloth,
placed against each plate wall, retains the cake solids while permitting passage of the filtrate. 5625
The plate surface under the filter media is specifically designed with grooves between raised
bumps to facilitate passage of the filtrate while holding the filter cloth. Before pumping into the
press, the feed must be chemically conditioned to flocculate the solids and release the water held
within the solid mass. Most typical conditioning systems use inorganic chemicals and organic
polymers. 5630
High-pressure pumps force the feed into the space between the two plates. The filtrate passes
through the cake and the filter media and out of the press through special ports drilled in the
Pumping continues up to a given pressure and is stopped when solids and water fill the void
volume between the filter cloths and filtrate flow slows to a minimal rate. The press then opens 5635
mechanically and the cake is removed, one chamber at a time.
a. Process Variables -Chemical Conditioning
Polymers have a narrow range of effective dosage. A dose that is too low or too high will
result in a wet cake. Lime and ferric chloride have a broader range of effective dosage. 5640
While it is desirable for an operator to reduce the chemical usage to reduce costs, if erratic
equipment operation or erratic feed qualities occur, a higher lime dose typically will protect
against a wet cake. Polymer conditioning requires much less chemical per unit mass of
solids dewatered, which results in more room in the press for organic sludges, and
increased capacities. 5645
- A torn cloth immediately results in a filtrate flow that is very dirty and heavy with
solids.
- Feed sludge concentration. Avery thin feed may blow out through the plate surfaces
during the initial high-volume fill of the press because there would be too much
filtrate flow for the drain capacity. Athin feed will at least require a longer filtration 5650
time and produce a wetter cake. Athick feed typically will produce a drier cake with
a much shorter filtration time.
For a conventional filter press, the operator can control the following machine variables:
- Feed application rate by pacing the flow to the filter press.
- Overall filtration time, including such variables as the time at each pressure level in 5655
multiple pressure level operations.
- Use and amounts of pre-coat or body feed. Typically, pre-coat is unnecessary when
inorganic chemicals, such as lime and ferric chloride, are applied. Pre-coat may be
needed if particle sizes are extremely small, filterability varies considerably, or a
substantial loss of fine solids to and through the filter media is anticipated. 5660
- Conditioning chemicals, type, dosage, location, and mixing efficiency. Polymer
addition versus lime and ferric chloride conditioning typically are not
interchangeable, as each chemical requires special mixing and flocculation energies
and reaction times. Polymers only need a quick mix before injection to the press.
Modifications to the piping and mixing systems are typically needed if a change in 5665
the type of chemical for conditioning is desired.
- Flocculation efficiency and energy vary with the type of chemical being used.
Polymer floc shears easily and remains stable for only a few minutes. Lime floc is
more durable and remains stable for a few hours.
- Filter media. Filter cloth media vary widely, with different filament composition, 5670
weave pattern, and weave tightness.
b. Operational Considerations
- Many presses are very noisy when in operation. Hearing protection may be needed.
- Never insert objects between the press plates as they are being discharged without
first shutting the unit down by tripping the light curtain or flipping the emergency 5675
shutdown switch.
- Lime treatment results in considerable ammonia fumes being released during cake
discharge. Make sure that adequate ventilation pulls these fumes away from the
operator, preferably with a high-capacity, down-draft blower system. If an adequate
ventilation system is not operational, short term exposure may be allowed, if 5680
approved, and if an approved ammonia respirator is worn by all operators assisting
with the cake discharge.
- Hydrochloric acid washing of the press releases volatile acid fumes, which should
not be inhaled or exposed to moist body tissues, such as eyes and lungs. A
high-capacity ventilation system, as previously noted, is essential. If approved, 5685
short-term exposure may be allowed with an approved respirator and complete
coverage of all exposed skin.
- Lime powder is very caustic when it comes into contact with moist body tissues.
Therefore, an approved respirator and complete coverage of all exposed skin is
necessary when working around lime. 5690
Follow all equipment manufacturers recommendations. Some typical areas that need special
attention are as follows:
- The plate handles and the frame rails require frequent grease application to prevent
binding and excessive wear. 5695
- The plate shifter chain or other plate shifting devices require frequent lubrication.
- Even if a shredder is used before the conditioning step, rags will quickly accumulate
on all mechanical mixer blades. These need to be removed frequently to prevent
damaging the mixer gears and shaft bearings from operating out of balance.
- Lime systems scale up and plug over time and are unpleasant and potentially 5700
hazardous for operators to clean. When lime is used, cloth and plate washing may
require both an acid and a water wash because lime causes scaling.
- Ferric chloride, hydrochloric acid, lime, and ammonia cause considerable corrosion
to metal surfaces, such as the plate handles with their retaining bolts, the shifter
chain, and even steel plates under a hard rubber cover. Frequent cleaning and 5705
lubrication are necessary to reduce corrosion. Powder-coated steel handles,
polypropylene plates, and polytetrafluoroethylene-coated frame rails have been used
by some facilities to reduce corrosion problems. Also, adding an inhibitor to
hydrochloric acid will reduce its metal corrosion properties.
- From time to time, the cloths and gaskets should be removed, the plates should be 5710
pressure cleaned, and new cloths and gaskets cut and installed.
5.4.3.4 Screw Press
Screw presses dewater sludges first by gravity drainage at the inlet section of the screw and then
by squeezing free water out of the sludges as they are conveyed to the discharge end of the
screw under gradually increasing pressure and friction (Figure 5.6). The increased pressure to 5715
compress the sludges is generated by progressively reducing the available cross-sectional area
for the sludges. The released water is allowed to escape through perforated screens surrounding
the screw while the sludges are retained inside the press. The liquid forced out through the
screens is collected and conveyed from the press, and the dewatered sludges are dropped
through the screws discharge outlet at the end of the press. 5720
Screw speed and configuration, as well as screen size and orientation, can be tailored for each
dewatering application.
Solids are combined with polymer and pumped into the flocculation vessel. After flocculation,
sludges are transferred to the screw press. In the horizontal screw press configuration, sludges
are fed by gravity from the flocculation tank into the screw press headbox. If a rotary screen 5725
thickener is used, sludges flow from the flocculation tank to the rotary screen thickener and then
to the screw press headbox. Sludges then flow from the headbox into the inlet of the screw
Figure 5.6 Screw press 5730
Screw presses run continuously at low speeds and do not require close operator supervision;
therefore, they are easy to maintain and have low power consumption. The manual cleaning
schedule ranges from once per week to once every 30 days.
a. Chemical Conditioning 5735
Polymer addition promotes particle flocculation and increases the dewatering and
solids-capture rates. Jar testing and pilot testing can be used to estimate the type and
quantity of polymer necessary for each application, because it may vary significantly
depending on sludge characteristics. Polymer consumption is affected by multiple
parameters (e.g., grit content of the sludge, the presence or absence of primary clarifiers, 5740
the type of biological treatment, and the type and duration of sludge digestion).
b. Cleaning System
Screw press systems have automatic cleaning systems which involve plant water and spray
nozzles. During automated wash cycles, washwater from solenoid valves sprays onto the
screw press screen to remove built-up sludge. 5745
The brushes are made of nylon with stainless steel mounting hardware. This mainly cleans
the screen to allow water to drain by gravity (especially in the lower part of the screen) and
minimize resistance to water filtration. Clean screens require less dewatering pressure,
which improves the solid capture rate.
The second cleaning process is an automatic spray wash system, which cleans the screen 5750
from the outside. It is comprised of a rotating spray-bar washing system and spray nozzles
fed by solenoid valves.
c. Rotation Speed
Typical rotation speeds range from0.1 to 2.0 rpmfor horizontal screw presses and from0.5
to 2.0 rpm for inclined screw presses. In general, an increase in screw rotation speed 5755
increases production capacity but decreases cake solids concentration. In a full-scale
application, increasing rotational speed from 1 to 1.25 rpm reduced the cake concentration
from 23 to 20%
Maintenance checkpoints are as follows: 5760
- Check the drive of the screw for abnormal sound and vibration during operation.
- Check the screen for any damage or clogging.
- Check the cleaning nozzle for clogging.
- Check the amount and the leakage of lubricating oil.
- Check the reading of the ammeter and indicator lamps. 5765
5.4.3.5 Rotary Press
Rotary presses are a relatively new technology that can achieve cake solids and solid capture
performance similar to belt presses and centrifuges. Rotary press and rotary fan press
dewatering technology relies on gravity, friction, and pressure differential to dewater sludge.
The major elements of a rotary press are the polymer feed and mixing system, parallel filtering 5770
screens, a circular channel between the screens, the rotation shaft, and a pressure- controlled
outlet (Figure 5.7).
The press screens of rotary fan consist of fabricated wedgewire with small openings and linear
gaps. The rotary press drive configuration allows up to six rotary press channels to be operated
on a single drive. Each channel has bearings, and the combined unit has an outboard bearing 5775
cantilevered on one end. The rotary fan press drive configuration uses a maximum of two rotary
press channels on a single drive with isolated bearings in a sealed gearbox.
A key feature of both rotary press and rotary fan press dewatering technology is their slow
rotational speed. Typical installations use speeds of 1 to 3 rpm. This provides low vibration, low
shear, and low noise. 5780
a. Operational Control
Operators can control the performance of the rotary press or rotary fan press by changing
polymer type and dosage, feed rate, feed pressure, wheel speed, and outlet pressure.
Both types of rotary press require minimal supervision and can be unattended between 5785
startup and shutdown.
- Hydraulic Loading Rate
The hydraulic loading rate is a function of the equipments size and number of channels.
The technology is modular, and the hydraulic loading rate of single-drive units ranges from
0.5 to 15 L/s, although a maximum hydraulic loading rate of 3 L/s per channel is typical. 5790
Rotary presses provide better performance on residuals with higher fiber content (e.g.,
primary sludges).
- Chemical Conditioning Requirements
Chemical conditioning is mandatory to attain design performance in rotary press or rotary
fan press dewatering. Polymer feed systems can be supplied by the manufacturer or can 5795
be procured independently. In both cases, the feed systems typically include a polymer
storage tank and metering pump, which feeds the polymer into the mixing or flocculation
tank, where it is blended with the sludge. Dry or emulsion polymers can be used.
- Solids Loading Rate
Because solids capture is a function of the adjustable back pressure, the solids loading rate 5800
varies with the hydraulic loading rate. At higher solids concentrations, residuals will
accumulate in the outlet zone, form cake, and extrude more quickly.
Rotary presses and rotary fan presses include a self-cleaning system that must run for 5
minutes per day at the end of use to flush all lines and equipment. The system does not 5805
require high-pressure water for flushing. Typically, the normal in-plant water source has
sufficient pressure, but in some cases, high-pressure booster pumps may be required.
Figure 5.7 Schematic of a rotary press system
5.4.3.5.2 Maintenance 5810
Maintenance checkpoints are as follows:
- Check the drive of the rotary for abnormal sound and vibration during operation.
- Check the rotary for any damage or clogging.
5.4.3.6 Vacuum Filter
The vacuum filter consists of a cylindrical drum over which is laid a filtering medium of wool,
cloth or felt, synthetic fibre or plastic or stainless steel mesh or coil springs.
The drum is suspended horizontally so that one quarter of its diameter is submerged in a tank
containing sludge. 5820
The principal variables that affect vacuum filter operation are as follows:
- Chemical conditioning
- Filter media type and condition 5825
- Drum submergence
- Drum speed
- Vacuum level
Chemical conditioning most significantly influences dewaterability, because it changes the
physical and chemical nature of the feed. Conditioning agents produce a feed that releases 5830
water more freely, thereby producing a drier cake. Conditioning agents also add to the
sludges to be discarded.
b. Sludge Conditioning
Sludge conditioning is accomplished by the addition of various coagulants or flocculating
agents such as ferric chloride, alum, lime, and polymers. The amount of chemical solution 5835
added to the conditioning tank is normally established by laboratory testing of sludge grab
samples.
The installation of a blanket may require several days' work.
A blanket will usually last from 200 to 20,000 hours, but this depends greatly on the blanket 5840
material, conditioning chemical, backwash frequency, and acid bath frequency. An improper
adjustment of the scraper blade, or accidental tear in the blanket, will usually require its
replacement.
Both cloth blankets and coil springs filters require a high pressure wash after 12 to 24 hours of
operation and, in some instances, an acid bath after 1,000 to 5,000 operating hours. 5845
5.5 Sludge Drying Bed
These are the age old practices in India and are still preferred in arid parts where land is
available and affords employment opportunities to unskilled labour. Other areas where rainfall
is frequent are not suited for drying beds.
5.5.1 Applicability 5850
This. method can be used at all locations where adequate land is available and dried sludge can
be used for soil conditioning. When digested sludge is deposited on well drained bed of sand
and gravel, the dissolved gases tend to buoy up and float the solids leaving a clear liquid at the
bottom which drains through the sand rapidly. The major portion of the liquid drains off in a few
hours after which drying commences by evaporation. The sludge cake shrinks producing cracks 5855
which accelerates evaporation from the sludge surface. In areas having greater sunshine, lower
rainfall and lesser relative humidity, the drying time may be about two weeks while in other
areas, it could be four weeks or more. Covered beds are not generally necessary.
5.5.2 Unit Sizing
The sludge drying process is affected by weather, sludge characteristics, system design 5860
(including depth of bed) and length of time between scraping and lifting of sludge material.
High temperature and high wind velocity improve drying while high relative humidity and
precipitation retard drying.
5.5.3 Area of Beds
The area needed for dewatering and drying the sludge is dependent on the volume of the sludge, 5865
cycle time required to retain sludge for dewatering, drying and removal of sludge and making
the sand bed ready for next cycle of application and depth of application of sludge on drying
bed. The cycle time between two dryings of sludge on drying beds primarily depends on the
characteristics of sludge including factors affecting its ability to allow drainage and evaporation
of water, the climatic parameters that influence evaporation of water from sludges and the 5870
moisture content allowed in dried sludge. The cycle time may vary widely, lesser time required
for aerobically stabilized sludges than for anaerobically digested sludge and for hot and dry
weather conditions than for cold and/or wet weather conditions.
5.5.4 Percolation Type Bed Components
A sludge drying bed usually consists of a bottom layer of gravel of uniform size over which is 5875
laid a bed of clean sand. Open jointed tile under drains are laid in the gravel layer to provide
positive drainage as the liquid passes through the sand and gravel.
Sludge that is drawn to the beds contains 4-10% solids depending upon the type of sludge. Wet
sludge should be applied to the beds to a depth of 20 to 30 cm. After each layer of dried sludge 5880
has been removed, the bed should be raked and levelled. Sludge should never be discharged on
a bed containing dried or partially dried sludge. It is preferable to apply the sludge at least a day
or two after the sludge cakes are removed.
Removal of dried sludge from bed surfaces should be done with shovel, taking care that as little
as possible of the sand is removed. When the sand layer is reduced to as low as 10 to 15 cm, it 5885
should be examined for clogging by organic matter and if found, the entire sand should be
removed and the bed re-sanded to the original depth of 20 to 30 cm.
The dried sludge cakes may be sold as fertiliser. Some part of the sludge should be used in the
plant itself for gardening, lawns, etc to demonstrate its fertiliser value and to develop a market
value for the digested and dried sludge. Suitable storage facilities may be provided for the dried 5890
sludge.
Records of operation of sludge drying beds should show the time and quantity of sludge drawn
to each bed, the depth of loading, the depth of sludge after drying time and the quantity of dried
sludge removed. The solids content of wet digested sludge, its volatile portion and pH should be
determined and recorded. Likewise, the moisture content and fertiliser value in terms of NPK of 5895
dried sludge should also be analysed and recorded. A typical operation sheet of sludge drying
bed is as follows (Table 5.3).
Table 5.3 Typical Operation Sheet of Sludge Drying Beds
Date and Time Check List
Cleaning of weeds
Quantity of sludge to respective SDB
Depth of the wet sludge
Depth of dried sludge
Quantity of sludge removed
Cleaning and scrubbing of splash plate, pipes etc
Volume of sand added
Cleanup-washing scrubbing of filtrate sump
5.6 Preventive Maintenance 5900
All preventive maintenance of equipment is to be done as per the equipment manufacturer only.
Preventive maintenance of process control resides only in digestion process and has been
discussed above.
Refer to Appendix 5.1. 5905
5.8 Record Keeping
There is no standard format for record keeping. Each STP has to have its own format. The
crucial parameters to be recorded are the pH and temperature of the digesters on a daily basis.
The gas analysis can be recorded once a week.
5.9 Summary 5910
Treatment of sludge, which is converted from removed organic matter in sewage, as well as
quality and quantity of sewage inflow, and quality of treated water, are quite important in STPs.
Problems encountered in the course of operation are gathered in Appendices as troubleshooting.
Operators can make use of the troubleshooting chart to take appropriate measures against each
trouble. 5915
Part B: Operation and Maintenance CHAPTER 6: ELECTRICAL AND
INSTRUMENTATION FACILITIES
CHAPTER 6 ELECTRICALAND INSTRUMENTATION
FACIITIES
Electrical systems which are to supply electrical power for an entire STP consist of power
receiving and transforming equipment, power distributing equipment, cables, drives and standby 5920
generators.
Instrumentation facilities are also installed for the purpose of measuring and collecting process
data such as flow rate, pressure, water qualities, and so on, at all times. These are utilized to
monitor and control treatment processes at optimal conditions for a stable treatment. The
instrumentation facilities consist of sensors for processes, signal converters, operating devices 5925
(actuators), controllers (PLC: Programmable Logic Controller), monitoring devices (PC:
personal computer), etc.
This chapter describes the following electrical and instrumentation facilities:
a. Power receiving and transforming equipment (Substation & transformers),
b. Standby power supply system (Generators, Engines, UPS: Uninterruptible Power 5930
Supply),
c. Prime movers and motor controllers (Motors, Starters, Cabling),
d. Instrumentation system, and
e. Supervisory control and data acquisition system (SCADA).
A typical single line diagram depicts the entire electrical power flow system of an STP. The 5935
single line diagram not only presents the type and number of equipment but also the electrical
specifications. This is an important document for an O&M person who would like to refer it in
case of any operational or maintenance need. Every STP should have:
- A single line diagram kept for record and displayed properly in the STP facility
particularly near the electrical sub-station. 5940
- Single line diagram periodically reviewed and updated suitably in case of any
- All people involved in the electrical and instrumentation work understand the single
line diagram.
Typical electrical single line diagram is shown in Figure 6.1. 5945
Part B: Operation and Maintenance CHAPTER 6: ELECTRICALAND INSTRUMENTATION FACILITIES
Figure 6.1 Electrical single line diagram
6.2 Power Supply System
Power supply systems have the following three major functions:
- Transfer power from the transmission system to the distribution system; 5950
- Reduce the voltage to the specified level (Typical voltage level is 415 volt for STPs)
suitable for connection to local loads; and
- Protect the entire network by identifying and isolating electrical faults selectively.
6.2.1 Power Receiving and Transforming Equipment
If the STP facility receives the electrical supply at high voltage i.e. 11KV, 6.6 KV or 3.3 KV, it 5955
has to be reduced to the low voltage level. A substation which is used to step-down high voltage
to low voltage, consists of the following equipment or devices:
6.2.1.1 High Tension (HT) Panel
The HT panel is composed of the following equipment and devices.
6.2.1.1.1 Disconnecting Switch 5960
Disconnecting switches are devices to open/close a high voltage circuit when high-voltage
equipment are inspected, measured, tested, or cleaned.
The devices are capable of safely breaking no load current but not load current.
For safe O&M work, be sure to open/close the disconnecting switch only after opening a circuit
breaker which is located on the secondary side, just downstream of the disconnecting switch. 5965
6.2.1.1.2 Circuit Breaker
Circuit breakers are switches that open/close electric circuits in normal and abnormal conditions
(especially in short circuit). Therefore, the circuit breakers must be capable of tripping the
circuits in conjunction with protective relays and by cutting off the short-circuit current
definitely and safely, avoiding accidents due to high current. 5970
Circuit breakers for high voltage are categorised into the following types according to their
techniques of eliminating arcs:
a. Air Break Circuit Breakers (ACB)
b. Vacuum Circuit Breaker (VCB)
c. Inert Gas Method (SF6) 5975
6.2.1.1.3 Power Fuses
The function of a power fuse is to sense and prevent flow of excess current in electrical devices
and electrical wire from by melting the fuse element and thereby breaking the electric circuit
when subjected to a short-circuit. Power fuses are typically used for smaller electrical systems
because they have the capability and speed for breaking circuit as compared with circuit 5980
breakers.
A proper O&M, practice is that even if only one fuse melts due to an accident such as a
short-circuit in a three phase switch, all power fuses including the melted one should be
replaced.
6.2.1.1.4 Voltage Transformer (VT) or Potential Transformer (PT) 5985
Voltage transformers are used mainly in high-voltage distribution equipment to step down
voltage in measurement circuit for safe measurement. Single-phase and three-phase types are
manufactured. The typical secondary voltage of the voltage transformer is 110 volt
(Phase-to-Phase). They are also applied to protective relays.
O&M issues to be observed in case of a PT are as follows: 5990
- If once short circuit occurs on the secondary side of a VT, over current flows into the
primary side and that may cause the fuse on the primary side to blow. The primary
fuse has also to be checked when there is a fault trip or metering mismatch.
6.2.1.1.5 Current Transformer (CT)
Current transformers are used for stepping down current to be measured safely. It is also applied 5995
to protective relays. The typical secondary current of the current transformer is 5 Amp or 1
O&M issues to be checked are as follows:
- If the secondary side of CT is open-circuited, all the current flowing to primary side
is excited by magnetic saturation and causes damages to the CT by over-heating. 6000
Therefore the secondary side should never be left open-circuited. Even when the
downstream instrument is removed for any repair, the secondary should be shorted.
6.2.1.1.6 Protective Relay
Protective relays should detect electrical faults promptly, isolate the faults from system, and
activate alarms when there is a faulty condition sensed in the electrical supply to the circuits or 6005
electrical equipment (short circuit, earth fault, single-phase, reverse power flow etc.)
The protective relays should have the following three characteristics:
a. Certainty: The relay should always be sensing the parameters for action when there
is a fault or specified abnormality.
b. Selectivity: The relay should be able to obey a selection of the limits beyond which a 6010
fault will be judged.
c. Promptness. The relay should sense and operate within the shortest possible time.
Categories according to protective functions are as follows:
a. Over current relay (OCR): Monitor and protect against over load and short-circuit;
b. Under voltage relay (UVR) and over voltage relay(OVR): Detect and protect under 6015
voltage (power failure) or over voltage; and
c. Earth fault relay: Protect by detecting current leakage to earth.
Protective relay is shown in Figure 6.2.
Figure 6.2 Protective relay 6020
6.2.1.2 Transformer
A transformer is the most important component in substations. Transformers receive electrical
power at high voltage and transform it to lower service voltage. They also provide isolation
between high voltage and low voltage supply.
Cooling system for oil-immersed transformer: Oil serves as direct cooling medium to disperse 6025
the heat that is generated from windings and core. The oil is in turn cooled by indirect cooling
medium such as air at the oil radiator.
Cooling system for dry transformer: Utilize surrounding air or SF6 as cooling medium.
Transformer Efficiency: The efficiency of a transformer varies between 96% and 99%. The
efficiency of transformer not only depends on design, but also on operating load. The 6030
transformer losses are mainly attributed to:
- Constant Loss: This is also called iron loss or core loss, which mainly depends upon
the material of the core and magnetic circuit of the flux path. Hysteresis and eddy
current loss are two components of constant loss.
- Variable Loss: This is also called load loss or copper loss, which varies with the 6035
square of the load current.
The best efficiency of a transformer occurs at a load when constant loss and variable loss are
equal. For distribution transformers, installed in an STP, the best efficiency would occur around
50% load.
O&M checks to be made are as follows: 6040
- Check connections of cables for looseness and overheating.
- Check the transformer for abnormal vibration and noise.
- Check oil and winding temperature regularly with respect to manufacturers manual.
- Check for moisture ingress by observing the colour of the silica gel.
- Check for level of oil in the conservator. 6045
Transformer is shown Figure 6.3.
Figure 6.3 Transformer
6.2.1.3 Low Tension (LT) Panel
LT panels or LT switchboards are designed to distribute stepped-down voltage to power 6050
equipment and control panels. They typically consist of moulded case circuit breakers (MCCBs),
power contactors (PCs), protective relays (PRs), meters, indication lamps, control switches, etc.
a. Moulded Case Circuit Breaker (MCCB)
An MCCB is designed to open/close low voltage feeder circuit or branch circuit
at normal condition. It also breaks the circuit automatically in case of abnormal 6055
condition such as overload, short circuit, etc.
b. Power Contactor
A power contactor is typically used for on / off control of motors. A relay can be
installed on the circuit for overload protection. Electromagnetic force works to
open /close the contacts. 6060
O&M checks to be made:
Check for abnormal noise or overheating of exciting coils, abnormal noise and
discolouration of contacts (carbonized or worn contact surfaces by arcing).
Check for the proper working of all display indicators like voltmeters, ammeters,
energy meters, indicator lamps. 6065
Check whether the name of the panel is written on it and it is correct as per the
single line diagram.
Check for the proper earthing of the panels.
6.2.1.4 Bus-bar
Bus-bars are conductors to carry power among the various components in the power circuit in 6070
an outdoor station or distribution board. They are to be rated to carry the maximum rated current
continuously and also short-circuit current for a short time without damage.
O&M issues to be cared for:
Check connections for looseness and overheating, and check the bus bar for
discolouration. 6075
Check that the bus bars are properly colour coded (Red, Yellow or Blue) to
represent the phases.
Check that the bus bars are properly enclosed within panels.
6.2.2 Power Control
Correcting power factor is a typical power control technique. Power factor correction is 6080
described in this section.
6.2.2.1 Power Factor Correction
Active power, measured in kilowatt (kW), is the real power (shaft power, true power) used by a
load to perform a certain task. However, there are certain loads like motors, which require
another form of power called reactive power (kVAR) to establish the magnetic field. Although 6085
reactive power is virtual, it actually determines the load (demand) on an electrical system.
Electrical capacity required for some electrical equipment is referred to as apparent power
(KVA), that is, the vector sum of active power and reactive power (lagging/leading).
Most of the power machineries in STPs are driven by three-phase induction motors, which are
inductive loads. When an inductive load is driven, the sine wave of the load current flows at the 6090
same frequency as the sine wave of the voltage, but lags the voltage wave cycle slightly. When
both current and voltage source waves cross zero and maximum value at the same time, the
power factor is said to be unity, and the entire power can be utilised as real power. The
Apparent Power-KVA is equal to the real Power-KW. When the current wave is slightly
lagging the voltage wave the power factor is said to be lagging and is less than unity. The real 6095
power is less than the apparent power due to this lag. A lagging power factor is not beneficial to
a power consumer as the billing is made based on the KVA used, while the actual utilisation is
less. It is also not beneficial for the power supplier whose system power factor is also affected.
Equipment used in most industries such as drives, controllers, etc., are inductive loads which
lower the power factor. 6100
The power factor is the ratio between active power (kW) and total power (kVA), or the cosine of
the angle between active and total power. A high reactive power, will increase this angle and as
a result the power factor will be lower.
power Total
power Active
Factor Power
Figure 6.4 Vector diagram of power factor
In a typical STP since a large number of Induction Motors are used, the Power factor will be
low and needs to be improved or corrected. The power factor can be improved by installing
power factor correction capacitors to the plants power distribution system. They act as reactive 6110
power generators and therefore reduce the amount of reactive power, and thus the total power,
generated by the utilities. To improve or correct the power factor, apparent electrical capacity
required for the electrical equipment should be decreased by cancelling the lagging reactive
power by the use of leading reactive power unit. That can also reduce energy loss in cables,
transformers, etc., before reaching to load equipment. 6115
Rating of capacitor to be required for power factor correction (Figure 6.5) can be calculated by
the following vector equation.
( ) kVA
cos -
tan tan P Qc
Where, Qc : Capacitor rating in kVAR
Active Power (kW)
Reactive Power (kVAR)
Total or Real Power
(kVA)
Where, S
: Apparent power (kVA) = P jQ
(Before correction)
(After correction)
P : Active power (kW)
: Lagging reactive power (kvar) (Before correction)
: Lagging reactive power (kvar) (After correction) 6125
: Capacity of condenser (Advancing reactive power) (kVA)
: Power factor before correction
: Power factor after correction
Figure 6.5 Vector diagram of power factor control
6.2.2.2 Capacitor Panel 6130
Capacitor panels consist of some equipment such as condensers for power factor correction,
series reactors meters, relays, etc.
6.2.2.2.1 Condenser (Capacitor)
Induction motors which are inductive loads generate lagged-phase reactive power.
Phase-advanced condensers (capacitor) have the function of compensating the lagged-phase 6135
reactive power to improve power factor.
The effects gained from the condensers vary according to the points to be installed. For example,
it is effective to install a condenser on the secondary side of a transformer if reduction in load
and loss of the transformer is targeted.
With regard to operation and maintenance, the capacitors reactive power acts during light load 6140
(when power equipment has stopped), and when the current leads the voltage in the circuit so
that leading power factor occurs, and the terminal voltage of the load increases causing adverse
effects on the equipment. To prevent this phenomenon, the capacitor may need to be isolated, or
an automatic power factor regulator may need to be installed.
Normally a capacitor unit comprises of individual capacitor elements arranged in parallel/ series 6145
connected groups within a steel enclosure. An internal discharge device is a resistor that reduces
the unit residual voltage to 50V or less in 5 minutes. Capacitor units are available in a variety of
voltage ratings from 240 V to 66,000 volts and sizes (2.5 kvar to about 1,000 kvar).
The capacitors can be with external fuses or internal fuses, or both. An internal fuse is a small
fuse wire connected to each capacitor element, encapsulated in a wrapper. When a fault occurs 6150
in a particular element, the particular fuse melts, disconnecting the affected element only, and
permitting the other elements to function without interruption. An external fuse unit typically
protects each capacitor unit in a bank. In an oil-impregnated capacitor, the internal pressure may
increase resulting in expansion due to excessive current because of the failure of internal
elements. This leads to leakage of oil from the capacitor unit and failure of the capacitor. Care 6155
should be taken to select capacitors with sufficient cooling volume.
When a capacitor circuit is switched on, there is an inrush current which is likely to damage the
capacitor. Achoke or series reactor is used to control the inrush current.
6.2.2.2.2 Series Reactor
The major functions of a series reactor are to protect capacitor by means of the following: 6160
- Limiting inrush current during switching
- Limiting resonance and protection of capacitor banks
- Harmonic filtration
- Lower loss and noise level
6.2.2.3 Power Factor Correction at Motor Panel 6165
Power factor can be increased by installing low voltage capacitor in parallel with the motor.
This enables the current to be reduced. Moreover, distortion waveform can also be stabilized by
connecting a series reactor.
6.2.3 Supply and Interruption (Operation of Electrical Equipment)
Power interruption is classified into two types: a scheduled power interruption and an 6170
unscheduled power interruption.
The former requires specific operational procedure before interrupting the power supply, which
is to open the switches from the load side (power distribution panel) to the power source
(power receiving panel) sequentially. To restart power supply, close the switches from the
power source (power receiving panel) to the load side (power distribution panel) one by one. 6175
Make sure that the personnel in charge of the interruption/ restart operation know thoroughly the
configuration of the machinery, the operational characteristics, the operational procedures, the
place or position of switches installed, the electrical scheme diagram, and the load circuit
diagram to avoid incorrect procedures.
During the work of starting, operating and stopping the load equipment, pay attention to meter 6180
readings, vibration, heat, and sound of equipment. If some abnormal state is found, report to the
related person in charge immediately, investigate the causes and take appropriate measures.
In the latter case, investigate the cause of the power failure first of all. Power failures can be
caused by the following: some failure attributed to the power company (outside power
suppliers) and some local fault in the STP. To identify the causes, read indicated values or signs 6185
on the incoming supply voltmeter, under-voltage relay, earth fault relay, over current relay, etc.
Judgement by incoming supply voltmeter or under-voltage meter
If the receiving voltmeter indicates 0 and the under voltage relay is tripped,
it implies that power is interrupted on the power source side (attributed to power
company). After confirming that the receiving circuit breaker is opened, the 6190
contact person or authority prescribed by the power company should be asked
about the causes and the estimated recovery time. However, the related
substation should try to restore power at the earliest.
If the reading of the receiving voltmeter is within a specified range, the under
voltage relay is untripped and the earth fault relay or the over current relay is 6195
tripped, that implies some failure (overload, short, earth fault, etc, in
equipment or lines) has occurred in the STP and the circuit breaker for receiving
power is opened. By studying the protective relay, circuit breakers, etc.,
which were tripped or opened, identify the line with fault and isolate the broken
down point immediately before recovering from the failure. 6200
6.2.4 Gas Engines
Generally, digester gas is used as fuel in boilers for heating sludge digestion tanks; surplus gas
is incinerated in biogas combustion units and discharged to the atmosphere. (Figure 6.6)
A sewage gas generator uses the sewage gas as fuel to a gas engine and generates electricity,
which is supplied to equipment within a STP. 6205
The digester gas engine consumes gas as fuel and employs spark-ignition method, which is the
same as one used in car engines.
Figure 6.6 Example of flow of power generated from digester gas
O&M issues to be noted for are as follows: 6210
- Typical concentration of NOx in exhausted gas from digestion gas engines is about
2,000 ppm. NOx should be reduced to prevent air pollution.
- Replace the ignition plugs in the spark-ignition system periodically.
- Periodic opening up, cleaning and inspection of a digestion gas engine is extremely
important because siloxane compound gets deposited in the combustion chamber of 6215
such an engine, which causes faults in engine parts.
6.2.5 Dual Fuel Engine
Dual-fuel (gas-diesel) engines are compression-ignition, not spark-ignition engines. To ignite,
they simultaneously burn gas and a small amount of diesel fuel as pilot fuel. Their controls also
allow automatic switchover to 100% diesel fuel operation without changing load if the digestion 6220
gas supply is inadequate or is interrupted. This capability is a beneficial feature for standby units
because they can start and operate even during power failures.
Dual-fuel engines typically use 1 to 5% diesel fuel oil, but many can, if necessary, operate on 1
to 100% diesel fuel. Such fuel flexibility is an excellent advantage, especially if the digestion
gas supply is disrupted. This option includes storage and handling equipment for diesel fuel, 6225
along with gas compressors to supply digester gas to these engines.
The same O&M checks should be made for the diesel engine and the gas engine. Please refer to
6.3.2 and 6.2.4.
6.3 Standby Power Supply System (Generator)
Standby or emergency power can be supplied through AC generator and diesel engine. 6230
Most generators of STPs are installed for the purpose of standby power supply. Therefore, they
should be highly reliable because the STPs have to keep functioning without hindrance even in
the event of unexpected power cut from the power company.
6.3.1 AC Generator
A generator is machinery that converts mechanical energy to electrical energy by 6235
electromagnetic action to generate electrical power.
Synchronous generator, the principle of which is reverse as that of an electric motor, is an AC
generator which generates electric power synchronizing to rotating rate of magnetic field
passing through armature windings.
Frequency of the synchronous generator is determined according to the rate of rotation, and 6240
accordingly, the frequency decreases as the rate of rotation becomes lower than the synchronous
rate. Therefore, the rate of the engine for the synchronous generator should be regulated to
maintain the level of synchronous rate.
O&M issues to be taken care of are as follows:
- Winding 6245
Oil or dust on windings or air vent sleeves obstructs air ventilation and leads to
overheating of the generator and deterioration of insulators which may cause
short circuit and ground fault. The dust should be blown off by a compressed air
and the oil and grease should be removed with cloth, cleansing oil, etc.
In the case of severe accumulation of oil and dust, it is recommended to have the 6250
winding cleaned and varnished with insulation paint after drying thoroughly, by a
professional cleaning company.
Inspect insulation coating over terminals should be checked for overheat or
discolouration due to cracks or slacks.
- Bearing 6255
Pay full attention to any abnormal noise from bearings during running to detect
any defect at early stage. It is the simplest way to check bearing conditions.
Grease should be supplied periodically while the generator is running through the
openings for grease. If a drain valve is provided to the grease chamber, always
open the valve during filling grease to purge out old grease. 6260
- Brush (Static excitation system)
Worn brush reduces brush pressure and causes sparks that may make the surface
of a slip ring rough. To prevent it, always check for abrasion, unsymmetrical wear
or damage to brush, and pressure of the brush holder. The brush lifting device
should be inspected to ensure that it works properly. 6265
Brushes should be replaced with new ones when the wear level reaches the
designated value. The newly installed brushes should preferably be of the same
material and shape as the currently used ones.
- Exciter (brushless)
Exciters hate moisture and dust. Dust should be blown off by low pressure 6270
compressor and wiped with dry cloth.
Check bolts and nuts on terminal area and terminal block for looseness, wires for
discolouration, and conditions of earthing and installation.
6.3.2 Diesel Engine
Diesel engines are generally used as drives for back-up power generators. . 6275
The diesel engine works by the action of high-speed diesel combustion which pushes out
pistons by the expansion based on self-ignition. The compressed air is hot enough to self ignite
when diesel fuel is injected. Piston action caused by the energy is converted in the crank shaft to
rotating energy, which drives the generator. High speed diesel is typically used as fuel.
O&M issues to be handled as follows: 6280
- For details of maintenance and inspections, follow the manufacturers manual.
- Regularly check the fault alarm to ensure it works properly.
- During inspection and maintenance, take care not to allow dust contamination
especially into fuel or lubrication system.
- Check wiring for loose connection and check piping for leakage. 6285
- Do not place anything around an inlet port that obstructs suction.
- Pay attention to abnormal noise and overheat.
- Where it is necessary to store diesel for such engines, mandatory precautions
regarding storage area fire protection, clearances, etc., should be followed.
Appropriate clearance from the jurisdictional authorities on pollution control and 6290
inflammable fuel storage should be obtained.
UPS stands for Uninterruptible Power Supply and is a power supply device which works when
the usual power source is interrupted. Typical UPS circuit is shown below. (Figure 6.7 )
Figure 6.7 UPS circuit
In the normal condition, commercial AC (alternating current) power is sourced and converted to
DC (direct current), which is then supplied to an inverter, and charges a battery.
When the commercial power source is interrupted, power charged in the battery is converted to
AC and supplied to the load. 6300
The UPS has a rectifier to convert AC supply to DC for charging the battery, a DC to AC
inverter to convert the battery output to AC voltage, and a battery to act as a source of power
during normal power interruption. Other components like protection, fuses, indication, surge
controlling circuit etc., are also built into the unit.
The following points should be checked as maintenance tasks: 6305
- Check for abnormal noise, smell, and heat in UPS.
- Check for looseness in each connection.
- Check appropriate time for battery replacement.
- Check for clogged ventilation opening.
- Ensure spare fuses are kept in stock. 6310
6.4 Prime Movers
6.4.1 Induction Motor
Three-phase induction motor is widely used as a general-purpose motor due to high reliability
and low price among driving forces for general industrial machinery. Most prime movers used
for pumps or blowers in STPs are three-phase induction motors. 6315
A three-phase induction motor rotates the rotor by a rotating electromagnetic field, which is
generated in the stator core by AC current flowing in the stator winding.
Rotating speed of the revolving magnetic field is referred to as synchronous speed and
expressed as Ns (rpm). The speed of the rotor itself is slightly lower which is expressed as N
(rpm). The ratio of Ns to the differential speed (Ns/N) is referred to slip. 6320
N / N N s
) rpm ( N s N
) rpm ( p / f N
Where, f : Frequency (Hz)
p : Number of magnetic poles
s : Slip
6.4.2 Starters 6325
An extremely large current of about five to eight times the rated current flows when a motor is
started. The power factor is at an extremely low value of about 0.2 at the start. The duration of
the starting current is short, but the motor winding coil is subjected to thermal stress load as
Joule heat. Voltage fluctuation occurs in the power system and its effect becomes more
pronounced. 6330
The starting method of three-phase induction motor includes a method of restricting current at
start as mentioned above, and other methods described below.
a. Direct-on-line Starter
Power supply voltage is applied as-is, and a starting current which is several times
the rated current flows. This starter is used in motors requiring comparatively small 6335
starting currents.
b. Star Delta Starter
1/3 of power supply voltage is applied on the Y (star) connection winding at start,
while (delta) connection is used during operation. Compared to the full voltage
starter, the starting current is one third and the starting torque is also one third. 6340
c. Reactor Starter
The voltage to be applied to the motor at start is reduced by the reactor and full
voltage is applied after the motor picks up speed, and is operated. The starting
current can be restricted to a smaller value compared to the Y- starting method.
Another starting method is to use a starting compensator. 6345
Figure 6.8 shows the circuit diagram of some of the starting methods.
Figure 6.8 Starters
6.4.3 Characteristics of Induction Motor
Theoretical analysis should be carried out to study torque characteristics and overheating of 6350
three-phase induction motor due to fluctuation in frequency and variation in voltage of the
Equation for induced electromotive force:
f n K E
= (6.4)
Where, E : Induced electromotive force (V) 6355
n : Number of active coils
: Maximum magnetic flux (wb)
f : Frequency (Hz)
K : Factor of proportionality ) . ( 44 4 2
Torque equation: 6360
f / V ' K T
Where, T : Torque (Nm)
V : Voltage of power source (V)
K` : Factor of proportionality 6365
a. When power supply voltage is greater than the rated voltage
According to the equation for induced electromotive force, the frequency is
constant, therefore the maximum magnetic flux m increases, and the
over-excitation phenomenon occurs. Heat is generated because of this excitation
current. In a submersible pump, when a thermal protector is built-in in the internal 6370
coil for protection, it may activate.
b. When power supply voltage is smaller than the rated voltage
According to the torque equation, when the frequency is constant, the power supply
voltage V reduces, so the torque reduces.
c. When the power supply voltage is unbalanced 6375
When the power supply voltage is unbalanced, reverse phase current flows, and the
temperature increases because of load loss in the coil resistance.
d. When the frequency f is higher than the rated frequency
According to the torque equation, when the power supply voltage is constant, the
torque reduces. 6380
e. When the frequency f is lower than the rated frequency
When the power supply voltage is constant, the maximum magnetic flux m
increases, and heat is generated because of the excitation current. According to the
torque equation, when the power supply voltage is constant, the torque increases.
6.4.4 Performance Assessment of Motors 6385
6.4.4.1 Efficiency of Motors
The efficiency of a motor is determined by intrinsic losses that can be reduced only by changes
in motor design and operating condition. Losses can vary from approximately two percent to 20
percent. The following shows the types of losses and their typical shares for an induction motor.
Table 6.1 Type of losses and shares for induction motors 6390
Type of loss Percentage of total loss (100%)
Fixed loss or core loss 25
Variable loss: stator I
R loss 34
Variable loss: rotor I
Friction & rewinding loss 15
Stray load loss 5
The efficiency of a motor can be defined as the ratio of a motors useful power output to its
total power output.Factors that influence motor efficiency include:
- Age - New motors are more efficient
- Capacity - As with most equipment, motor efficiency increases with the rated
capacity 6395
- Speed - Higher speed motors are usually more efficient
- Type - For example, squirrel cage motors are normally more efficient than slip-ring
- Temperature - Totally-enclosed fan-cooled (TEFC) motors are more efficient than
screen protected drip-proof (SPDP) motors 6400
- Rewinding of motors can result in reduced efficiency
- Load, as described below
There is a clear link between the motors efficiency and the load. Manufacturers
design motors to operate at a 50-100% load and to be most efficient at a 75% load.
But once the load drops below 50% the efficiency decreases rapidly as shown in 6405
Figure 6.9. Operating motors below 50% of rated loads has a similar, but less
significant, impact on the power factor. High motor efficiencies and power factor
close to 1 are desirable for efficient operation and for reducing costs down of the
entire plant and not just the motor.
Figure 6.9 Motor part load efficiency
6.4.4.2 Motor Load
Because the efficiency of a motor is difficult to assess under normal operating conditions, the
motor load can be measured as an indicator of the motors efficiency. As loading increases, the
power factor and the motor efficiency increase to an optimum value at around full load. It is 6415
necessary to see the %loading of the motor. If the motor runs at more than 70% load, then the
power factor and efficiency will be good.
6.4.4.3 Energy Efficiency Opportunities
Apart from operational point of view, the motors should be seen from energy efficiency
opportunities also. The following points may be considered : 6420
a. Replace standard motors with energy efficient motors.
b. Reduce under-loading.
c. Avoid over-sized motors.
d. Improve power quality.
e. Dont go for multiple time rewinding. 6425
f. Improving maintenance practices.
6.4.5 Condition Monitoring Techniques
6.4.5.1 Vibration Monitoring
Vibration in rotating machinery is caused by many reasons like unbalance, misalignment, loose
foundation, mechanical looseness, bearing damage etc. Vibration monitoring is the most 6430
common, versatile and powerful condition monitoring technique adopted in rotating machinery
to idetify problem areas. The severity of the vibration is specified by IS 2372 which is measured
with reference to class of machine. The criteria for class of machine are given below:
Table 6.2 Criteria for class of machine
Class- I Individual parts of engines and machines integrally connected with the
complete machine in its normal operating condition. (Electrical drives up to
15 KW are typical examples of machines in this category)
Class- II Medium sized machines (typically electrical motors with 15 to 75 KW
output) without special foundation, rigidly mounted engines or machines
(up to 300 KW) on special foundations.
Class- III Large prime movers and other large machines with rotating mass mounted
on rigid and heavy foundations which are relatively stiff in the direction of
vibration measurement.
Class- IV Large prime movers and other large machines with rotating masses mounted
on foundations which are relatively soft in the direction of vibration
measurement (such as turbo-generator sets, especially those with
light-weight structures)
Class- V Machines and mechanical drive systems with un-balanceable inertia efforts
(due to reciprocating parts), mounted on foundations which are relatively
stiff in the direction of vibration measurement.
With above class of machine, the vibration severity can be judged by the following guidelines: 6435
Table 6.3 Vibration severity chart for machine vibration limits
Range for
Severity in
(mm/sec)
Example of Quality Judgment for Separate Classes of Machines
Peak RMS Class -I Class -II Class -III Class -IV
Normal 1.58 1.12
4.0 2.8 Still Acceptable
Still Acceptable
Acceptable 25.0 18.0 Unacceptable
40.0 28.0 Unacceptable
6.4.5.2 Vibration Analysis
If the measured vibration level is more than the acceptable level, then it calls for vibration
analysis which is a captured time waveform plotted as amplitude versus time, or data can be
transformed using a fast Fourier transform (FFT) and expressed as amplitude versus frequency. 6440
Any random vibration signal can be represented by a series (a Fourier series) of individual sine
and cosine functions that can be summed to yield an overall vibration level. The amplitude of
this vibration signal defines the severity of the problem. Plotting the amplitude versus the
frequency (the Fourier spectrum) allows for identification of discrete frequencies contributing
most to the overall vibration signal, commonly referred to as a signature analysis or a 6445
frequency spectrum. Machine looseness, misalignment, imbalance, and soft foot conditions
are all fairly easily identified in the frequency spectrum generated by an analyzer.
The guidelines for vibration frequency and its likely cause are shown below.
Table 6.4 Vibration frequencies and likely causes
Frequency in
terms of RPM
Most Likely Causes Other Possible Causes & Remarks
1xRPM Unbalance - Misalignment (if high axial vibration)
- Bad belts if RPM of belt
- Resonance
- Reciprocating Forces
- Electrical Problem
2xRPM Mechanical Looseness - Misalignment (if high axial vibration)
- Bad belts if 2 x RPM of belt
3xRPM Misalignment
- Usually a combination of misalignment and
excessive axial clearance (Looseness)
Less Than Oil Whirl (Less than of
RPM)
- Bad drive belts
- Background vibration
- Sub-harmonic resonance
AC Line Freq. Electrical Problem - Common Electrical Problems
Many Times
(Harmonically
related)
- Bad Gears
- Aerodynamic Forces
- Hydraulic Forces
- Mechanical Looseness
- Gear teeth times RPM of bad Gear
- No. of fan blades time RPM
- No. of impeller vane times RPM
- May occur at 2,3,4 and sometimes higher
harmonics if severe looseness
Harmonically
- Bad antifriction Bearings - Bearing vibration may be unsteady amplitude
and frequency
- Cavitation, recirculation and flow turbulence
cause random and high frequency vibration
- Rubbing
The vibration monitoring and analysis should be done periodically, typically once in 6 months 6450
for all rotating equipment.
6.4.5.3 Thermographic Analysis
Commonly identified with electrical equipment monitoring, thermography is also a useful tool
for monitoring plant machinery. Thermography measures infrared radiation energy emissions
(surface temperatures) to detect anomalies. Infrared cameras have resolution to within 0.1 C 6455
and digitally store captured images. Both the absolute and relative temperatures can be obtained
on virtually all types of electrical equipment, including switchgear, connections, distribution
lines, transformers motors, generators, and buswork.
This technique is very popular because of the following reasons:
- It is a non-contact type technique. 6460
- Fast, reliable & accurate output.
- Alarge surface area can be scanned in no time.
- It can be easily scanned from a distance up to 50 meter.
- Presented in visual & digital form.
- Software back-up for image processing and analysis. 6465
- Requires very little skill for monitoring.
This technique can be very well used for seeing the loose contact, corrosive contact of all types
of electrical joints, body temperature of motor and transformers, panels, etc. The following
criteria may be used to know the severity of the problem.
Table 6.5 Criteria for differential temperature of electrical equipment 6470
(Differential temperature
above ambient)
Recommended Action
1 Up to 10 C Non-critical No action is needed.
2 Between 10C to 20C Less critical Regular monitoring is needed.
3 Between 20C to 40C Semi critical Close monitoring needed.
Should be attended in the next opportunity.
4 Above 40C Critical Should be attended immediately as per the
severity.
Few typical thermal images captured in thermography analysis (Figure 6.10):
Hot-spot in a Transformer
Hot-spots in a 11KV cable
Hot-spot in LT cable joints
Overheating of a Bearing Loose Contact of a MCC Panel
Choked fins of a Transformer
Figure 6.10 Thermographic measurement
6.4.6 Speed Control Equipment
The following five types of ASDs (Adjustable Speed Device) are available.
- VFD: Electronic devices to control the speed of the motor by controlling the 6475
frequency of the voltage at the motor.
- Direct current ASDs: Electronic devices to control direct-current motors by
changing the voltage applied to the motor.
- Eddy-current drives: Electrical devices that use an electro-magnetic coil on one side
of coupling to induce a magnetic field across a gap, creating an adjustable coupling. 6480
- Hydraulic Drives: Devices that operate much like an automotive hydraulic
transmission.
- Mechanical speed-control products including gearing, mechanical transmissions,
and belt drivers with variable-pitch pulleys can be used.
6.4.6.1 Variable Frequency Drive (VFD) 6485
VFD varies the revolving speed of an induction motor freely by changing the power supply
frequency and the power supply voltage. Although a power transistor is used for the main circuit
and IC, and a microcomputer is for the control circuit of VFD, further advanced controlling
technology has been applied due to improved semiconductor devices in recent years. Moreover,
generally, VFD is also called inverter control equipment or variable voltage variable frequency 6490
(VVVF) equipment.
The fundamental configuration is shown in the following Figure 6.11.
Figure 6.11 VFD
After changing an alternating power source into direct current in a converter part and making it 6495
smooth, transform the direct current inversely to variable frequency alternating current at an
inverter part.
6.4.6.2 Advantages and Disadvantages of VFD
(Advantage)
- Variable speed continuous operation over a wide range is possible. 6500
- Energy-saving possibility exists.
- Brush, slip ring and so on, used in induction motor are not required.
- Soft start and soft stop are enabled, extending the motor life.
- Settings for acceleration timing and deceleration timing can be adjusted.
- Starting current can be reduced. 6505
(Disadvantage)
- Harmonic protection measures are necessary since high frequency current is
generated.
- Generation of leakage current and noise due to high frequencies need to be
restricted. 6510
- Noise prevention for other equipment (especially measuring instruments) is
(Energy conservation effects)
One of the pump characteristics is that its load torque is proportional to the square of its
revolving speed, and this torque is called square reduction torque load. 6515
- Relationship between speed of revolution N (rpm) and flow rate Q (m
/sec.)
Flow rate is proportional to the speed of revolution. QN
- Relationship between speed of revolution N (rpm) and head H (m)
Head is proportional to the square of speed of revolution. HN
- Relationship between speed of revolution N (rpm) and power P (kW) 6520
Power is proportional to the cube of speed of revolution. PN
- Relationship between speed of revolution N (rpm) and torque T (N-m)
Torque is proportional to the square of speed of revolution. TN
6.4.7 Motor Protection Equipment
Protection equipment for three-phase induction motors includes the following: 6525
- Circuit breaker
It has overload and short circuit protection functions. The former is a thermal
function and has on-delay characteristics, while the latter is an electromagnetic
function and has instantaneous characteristics.
- Thermal relay 6530
Changes bimetal with Joule heat of overload current, opens or closes the contact,
and performs on-delay operation.
- Comprehensive motor protection relay
This unit measures the current from all the three phases and checks for single
phase, unbalance, overload. The measurement and comparison of these three 6535
factors provide short circuit, single-phase, earth fault, phase sequence and thermal
protection to the motors.
- Dry-run protection
In addition dry-run protection is also provided by water level sensors in the sump
which sense any low level of water and prevent dry running, thereby protecting the 6540
pump and motor.
6.5 Instrumentation Facilities
6.5.1 Flow Measuring Equipment
Please refer to Chapter 3 for flow measuring equipment.
6.5.2 Level Measuring Equipment 6545
6.5.2.1 Float
A float device measures a liquid level from above. Floats, one of the oldest and simplest
methods of level measurement, are used extensively in wet wells or sludge vaults that require a
discrete high- or low-level indication. They are also used for local indication of level in tanks
and open channels. (Figure 6.12 and Figure 6.13) 6550
Figure 6.12 Counterweighted float-level indicator
Figure 6.13 Float switches
Afloat level meter shown in Figure 6.12 senses water level through a slide rheostat as resistance
and converts the changed resistance into current. It transmits analog output signal (DC 4 mA to
20 mA) proportional to the water level and sends the signal representing the water level
continuously to the monitoring room. 6560
O&M issues to be cared for are as follows:
- Clean inside the stilling well regularly and keep it free from floating matter or scum
to prevent malfunctions.
- Check moving parts such as a counter weight, pulley and wires for corrosion or
damage. 6565
A level switch shown in Figure 6.13 is used to control pump and to issue alarms according to
water level. The signals from the level switch are on/off digital signals, which compromise a
sequence circuit. Float switches should be located away from the tank walls to prevent the floats
from banging against the concrete wall and internal contacts from failures.
6.5.2.2 Ultrasonic 6570
Ultrasonic level measuring device installed above the liquid surface measures the level by
generating a pulse of ultrasonic waves that bounce off the liquid surface. The instrument detects
the echo, calculates the echos travel time, and converts it to a level measurement (Figure 6.14)
Figure 6.14 Acoustic level-sensor installation
Specifically, the relation between distance from sonic transmitter/ receiver to the liquid surface
and the reaching time is expressed in the following formula:
t c H =
Where, H : Distance from transmitter/ receiver to liquid surface (m) 6580
c : Sonic velocity in air= 331.5 + 0.61 temperature in Celsius (m/s)
t : Time from transmission to receiving (sec)
- Ultrasonic level meters require little daily maintenance because they have no
moving parts and work without contacting measuring objects. However the junction 6585
boxes have to be regularly checked for any water ingress.
- Keep clear the area around transmitter / receiver.
- Keep liquid surface without scum, foam, wave, etc. Normally a perforated guard
pipe protects the sensor from any turbulence and floating matter in the water. This
may need to be checked regularly. 6590
6.5.2.3 Head-Pressure
Head pressure level measuring devices, bubbler tubes and diaphragm bulbs measure the head
pressure at the liquid level and are often used in open-channels or non-pressurized tank
6.5.2.3.1 Bubbler Tube System 6595
The bubbler tube system uses a small, regulated airflow that constantly bubbles into the liquid.
Because the airflow is small, the system produces a back pressure equal to the static head of the
liquid. A conventional pressure gauge or transmitter measures this back pressure as the height of
an equivalent water column. (Figure 6.15)
Corrections are required when the liquid's specific gravity differs significantly from water. 6600
Because air is constantly bubbling out of the bubbler tube, the system is typically self-purging.
Valves may be arranged to isolate the pressure-measuring device while providing high purge
flow through the tube for preventive maintenance blow-down if fouling occurs. Stilling wells
are often used to protect the bubbler tube from turbulence and damage. To protect pneumatic
instruments and regulators, operators should clean the air supply of excessive moisture and oils. 6605
Figure 6.15 Schematic of bubbler-level system
When bubbles are discharged into the liquid from the front end of pipe, the pressure within the
pipe becomes equal to the static pressure of liquid at the front end of the pipe. This pressure is 6610
proportional to the liquid height h. The calculation is expressed as given below.
) m ( / P h = (6.7)
Where, h : Water level from tube end (m)
P : Internal pressure of tube (Pa)
: Specific gravity of liquid (kg/m
) 6615
Precautions for operation and maintenance are as given below.
The bubble type liquid level gauge does not have moving parts or mechanisms, so it has
comparatively high accuracy. Moreover, it is suited to level gauges for corrosive liquids.
However, daily maintenance of air sources such as compressor, purging set, and air piping is
very important. 6620
6.5.2.3.2 Diaphragm Bulb System
The diaphragm bulb system operates on the principle that air sealed between the dry side of the
diaphragm (in the capillary tube) and the receiver compresses or expands with the movement of
the diaphragm. A change in the static head of the liquid being measured moves the diaphragm,
so the pressure of the trapped air is the same as the head pressure. Temperature changes because 6625
of sunlight or heat build-up, particularly along the capillary tube, can cause measurement errors
as a result of expansion of the trapped air. To reduce the effect of temperature, the capillary can
be filled with a fluid unaffected by operating temperature; however, this often affects the
measurement response time.
The differential pressure from the diaphragm is detected by piezoelectric semiconductor 6630
element. The output signal (4 mA to 20 mA DC) generated by the converter is changed to
analog data in the central monitor and transmitted.
The calculation equation for level is expressed as below.
P/ H = (6.8)
P : Differential pressure (Pa)
: Density (kg/m
The following precautions should be taken related to the use of differential pressure type level
- The installed position of differential pressure transmitter should be lower than the 6640
minimum liquid level.
- When the density of the liquid changes, correction is necessary. (Span adjustment on
the converter side is required.)
- If the liquid has pulsing motion, the output of the differential pressure transmitter
may become unstable. 6645
6.5.3 pH and ORP Measuring Equipment
6.5.3.1 pH
pH is a measure of the acidity of a process liquid. Continuous measurements of pH of incoming
sewage are frequently made, particularly in plants where drastic changes in pH (as a result of
industrial discharges) cause treatment problems. 6650
A glass electrode, which is sensitive to hydrogen ion activity, measures the pH of an aqueous
solution (Figure 6.16). The electrode produces a voltage related to hydrogen ion activity and to
pH. The pH is determined by measuring the voltage against a reference electrode. While it is
generally assumed that no other ions seriously affect the pH electrode in an aqueous system,
sodium ions can have an effect. Temperature corrections are also necessary but are typically 6655
done automatically by the meter.
- Dirt on the electrode surface should be periodically removed and the surface
cleaned.
- Since the electrodes of the pH meter are made of glass, care is necessary to ensure 6660
that they do not break.
- Due to long period use of glass electrodes, dirt sticks on them gradually, the zero
point changes, and the electromotive force by pH reduces, and it stops responding to
changes in pH, making replacement necessary.
- Standard liquid should be used in the pH meter and it should be calibrated. 6665
Calibration should include zero adjustment (standard liquid with pH7) and span
adjustment (standard liquid of pH4 or pH9).
Figure 6.16 Typical pH sensor 6670
6.5.3.2 ORP (Oxidation-Reduction Potential)
Oxidation-reduction potential is a measure of easily oxidizable or reducible substances in a
sewage sample. An operator can control the process better by knowing if there is a large
quantity of reducing substances (e.g., sulphide and sulphite) that may have an immediate, high
oxygen demand and may result in an inadequate supply of oxygen for the microorganisms in the 6675
secondary process. Although not specific, the ORP measurement is instantaneous (an electrode
is used) and can be used to help maintain dissolved oxygen in the aeration tank. Another
application is to evaluate the progress of digestion and process stability in anaerobic digesters.
- Regular cleansing of electrode surface, 6680
- Precautions against breaking an ORP electrode, which is made of glass and is fragile,
- Replacement of the electrode if it is insensitive to changes of potential difference,
because of stains accumulated over a long time usage.
6.5.3.3 DO (Dissolved Oxygen) 6685
A dissolved oxygen meter is an electronic device that converts signals from a probe placed in
the water into units of DO in milligrams per litre. Most meters and probes also measure
temperature. The probe is filled with a salt solution and has a selectively permeable membrane
that allows DO to pass from the stream water into the salt solution. The DO that has diffused
into the salt solution changes the electric potential of the salt solution. T this change is sent by 6690
electric cable to the meter, which converts the signal to milligrams per litre on a scale that
anyone can read.
If DO is a critical analytical parameter, it is recommended to calibrate at 100 percent saturated
air, or use a known dissolved oxygen concentration (determined by the iodometric method) for
the upper limit, and use a zero DO solution (even if it is not explicitly stated in a particular 6695
manufacturers manual) for the lower limit. If the DO meter does not allow for a second
calibration point, the zero DO solution can be used as a check standard when the DO meter is
set to the measurement mode. The DO meter should read less than 0.5 mg/L (or to the accuracy
of the DO meter). If the DO meter does not read less than 0.5 mg/L, then there may be a
problem with the DO membrane. 6700
If it is determined that the DO membrane needs to be replaced, consult the manufacturers
manual on conditioning the new membrane before use. It is also possible that other maintenance
may need to be performed on the DO meter or the zero DO solution may need to be replaced.
Other factors that affect the accuracy of DO measurements include: improper calibration, not
verifying calibration after use, not correcting for ambient barometric pressure/altitude, and 6705
instrument drift. (Figure 6.17)
Source: M/S YSI. (There are other firms also)
Figure 6.17 Hand held DO meter with probe for field use
- Regular cleaning of diaphragms, 6710
- Zero calibration and span calibration,
- Regular replacement of internal electrode solution, and
- Regular cleaning of an electrode and replacement if broken.
Even though most of the major sewage treatment processes are not temperature-controlled, 6715
many temperature measurements are required. Obvious applications for temperature
measurement are anaerobic digesters, chlorine evaporators, incinerators, and equipment
protection. Less obvious are temperature controls for analyzers and flow meters. Temperature
measurement devices include liquid thermometers, bimetal thermometers, pressure on liquid or
gas expansion bulbs, thermistors, resistance temperature detectors (RTDs), infrared detectors, 6720
and crystal window tapes. The RTD is typically used on lower, ambient-range temperatures,
while thermocouples provide better reliability in higher ranges. Also, gas- and liquid-filled
temperature sensors and thermistors are frequently used for equipment-protection and cooling
systems.
For continued accurate service, operators should periodically calibrate the instruments using a 6725
standard temperature measurement device with high accuracy.
6.5.3.4.1 Thermocouple
The thermocouple operates on the principle that current flows in a circuit made of two different
metals when the two electrical junctions between the metals are at different temperatures. The
various combinations of metals used are tabulated in most engineering handbooks, and the 6730
selection of metals is based on the maximum temperature to be measured. Thermocouples
measure as high as 980C, with an accuracy of 1% of the full scale.
6.5.3.4.2 Resistance Temperature
A resistance temperature detector has a temperature-sensitive element in which electrical
resistance increases repeatedly and predictably with increasing temperature. The sensing 6735
element is typically made of small-diameter platinum, nickel, or copper wire wound on a special
bobbin or otherwise supported in a virtually strain-free configuration. The detector is typically
selected for high accuracy and stability. A common RTD application is the measurement of
bearing and winding temperatures in electrical machinery.
6.5.3.5 MLSS (Mixed Liquor Suspended Solid) 6740
All solids concentration meters use indirect methods (such as optical, ultrasonic, and nuclear).
Indirect methods correlate the solids concentration with a measurable factor. The limitation of
not relating perfectly to the quantity of suspended matter does not seriously affect the analyzers'
ability to produce a repeatable signal of great value in process control.
When a light beam is directed on to liquid containing suspended particulates, the suspended 6745
particulates scatter some of the light. The nephelometer helps observers measure the amount of
light that the particulate matter scatters (Figure 6.18). The amount of scattered light relates
approximately to the amount of particulate matter, particle size, and surface optical properties.
The nephelometer is a photoelectric device that uses an incandescent light source (lamp), which
produces light in wavelengths from blue to red. The light is directed to a liquid and if the liquid 6750
contains particles, some of the light strikes the particles and scatters. By placing a photocell or
light detector at an angle to the light beam rather than directly in front of it, the detector receives
only light scattered by the suspended particulate matter. Most nephelometers have the photo
detector placed at a 90-degree angle to the incandescent light source.
Figure 6.18 Nephlometer
6.6 SCADASystem
SCADAis an acronym for Supervisory Control and Data Acquisition. This presents the data as a
viewable and controllable system on the screen of a computer. The data thus collected can be 6760
stored and also analyzed for optimization of the process and for better real time process control.
It assists plant operating personnel by monitoring and announcing abnormal conditions and
failures of equipment. It allows the operators to perform calculations based on the sensor inputs.
Daily, weekly, monthly reports can be prepared using the stored data. It also allows the operator
to know the state of a process and an alarm associated with it. Atypical SCADAcommunication 6765
overview is shown in Figure 6.19.
Source: Kruger, (there are other makes also available)
Figure 6.19 Typical SCADAcommunication overview
6.6.1 Monitoring and Control Equipment 6770
For maximum use and effectiveness, signals generated by various sensors and instruments are
transmitted from the sensor to a receiver installed at another location. Often, the sensor output is
transmitted to a control panel or computer system, which allows operators to inspect many
process variables simultaneously.
The three components of a signal-transmission system are the transmitter, receiver, and 6775
transmission medium (the connection between the transmitter and receiver). The transmitter
converts a mechanical or electrical signal from the sensor into a form that the transmission
medium can use. The transmission medium contains the signal and transfers it to the receiver.
The receiver subsequently converts it into a form that the receiving system can use.
6.6.1.1 Signal 6780
6.6.1.1.1 Analog Input Signal
Analog input signal means continuous signal such as process data which is transmitted from a
local transmitter to a central control unit (CCU).
For instance, process variables expressed in physical amount such as opening degree of a sluice
gate (0-100%), sewage flow rate (0- ***m
/hr), and water level in tanks (0-***m) are converted 6785
into electrical quantities. The electrical signals are transmitted to CCU. Standard electronic
transmission systems use 4 to 20 mADC.
There are several transmission systems. One is to connect directly with the CCU I / O
(Input/Output) device via shielded cables. Another is the so-called a link system with PLC, in
which analog signal is converted to digital signal and is transmitted via coaxial cables or optical 6790
fibres.
6.6.1.1.2 Analog Output Signal
Analog output signal means signals continuously transmitted from CUU to local control
panels or equipment to direct operational amount.
Electric operated valves and regulating valves for controlling pressure of pipe, and VFD for 6795
controlling frequency of aerators or pumps, etc., are typical examples which are controlled by
analog output signals. More specifically, there are electromagnetic valves and regulating valves
used to control pressure in the piping; there are also aeration blowers or VFD (inverter)
equipment used to control pump rpm.
Standard electronic transmission systems use 4 to 20 mA DC. One of the transmission systems 6800
for analog output signal connects directly to the I/O device of CCU via shielded cables.
6.6.1.1.3 Digital Input Signal
Digital input signal refers to a contact output signal generated and transmitted from local
equipment. For example, answerback signal to express equipment conditions, alarm signal for
abnormal conditions and remote / local switch signal of local control panels are examples of 6805
digital input signal.
Contact signals are electronically converted to an appropriate series of zeros and ones. Link
systems with PLC are widely used for transmitting the signal, with which analog signal is
converted to digital signal and is transmitted via coaxial cables or optical fibres.
6.6.1.1.4 Digital Output Signal 6810
Digital output signal means contact output signal expressed 1 / 0 transmitted from CCU to an
auxiliary relay on the control centre panel. For example, on/off signal for equipment is
regarded as digital output signal. Link systems with PLC are widely applied for transmitting the
signal, with which analog signal is converted to digital signal and is transmitted via coaxial
cables or optical fibres. 6815
6.6.1.2 HMI (Human Machine Interface)
Basically the PLC is a blind device. It does not have any provision of displaying the plant status
to the operator or to enter certain data like set points or manual mode operation. An additional
device is needed for these provisions, to communicate with PLC, which will have a display to
show the status and also the means to enter set points. Such a unit is called Human Machine 6820
Interface (HMI).
There are two basic types of HMI:
- Industrial panel mounted type HMI (Figure 6.20)
- PC based system in which the computer acts as HMI (Figure 6.21)
Figure 6.20 Industrial panel-mounted type HMI
Figure 6.21 PC based system in which the computer acts as HMI
6.6.1.3 PLC (Programmable Logic Controller) 6830
PLC is electronic equipment that senses inputs and takes the decision to change outputs
according to the set rules stored in the memory.
It is primarily an electronic controller, housed in industrial housing, which has logic
programming function and can be an interface to industrial devices.
6.6.2 Automatic Control 6835
Automatic control systems can be categorized according to their control techniques as follows:
Figure 6.22 shows the control system.
Figure 6.22 Automatic control system
Instrumentation facilities are established mainly based on feedback control in combination with 6840
feed forward control in most STPs for controlling process variables such as temperature, water
level, pressure, flow rate, etc.
6.7 Cables
The flow of power from transformer to switchgear and from there to starter and to motor and
other related equipment like capacitors are through power cables. Table 6.6 gives information on 6845
various types of cables.
Table 6.6 Types of cables for different voltages
No. Range of voltage Type of cable to be used IS Ref.
1 1-230 v / 3-415 v PVC insulated, PVC Sheathed IS 1554
2 Up to 6.6 kv PVC insulated, PVC Sheathed IS 1554
Feedforward control
Sequence control
Compare process variables with their set-points and
repeat corrective actions until the variables becomes as
By predicting influences caused by disturbances, take
actions to reduce the effects. Always combine with
feedback control.
Control proceeds according to pre-programmed
Paper insulated, lead Sheathed IS 692
11kv XLPE- Cross Linked, Polyethylene insulated, PVC sheathed IS 7098
The size of the cable should be so selected that the total drop in voltage, when calculated as the
product of current and the resistance of the cable should not exceed 3%. Values of the resistance 6850
of the cable are available from cable-manufacturers.
The following points should be considered when selecting the size of the cable:
- The current carrying capacity should be appropriate for the lowest voltage, the
lowest power factor and the worst condition of installation, that is, duct-condition.
- The cable should also be suitable for carrying the short circuit current for the 6855
duration of the fault. The duration of the fault should preferably be restricted to 0.1
sec. by proper relay setting.
- Appropriate rating factors should be applied when cables are laid in group (parallel)
and/or laid below ground.
- Suitable trenches or racks should be provided for laying cables. 6860
The following O&M tasks should be implemented:
- Measure insulation resistance between cables and the earth.
- Visually observe deterioration, corrosion, and discolouration.
6.8 Energy Audit
Among all the power consuming components, pumping installations consume a large amount of 6865
energy in STPs. Need for conservation of energy, therefore cannot be over emphasized. All
possible steps need to be identified and adopted to conserve energy and reduce energy cost so
that sewage charges can be kept as low as possible and gap between high cost of sewage
treatment and affordable charge to users can be reduced.
Some adverse scenarios in energy aspects given below are quite common in pumping 6870
installations:
- Energy consumption is higher than optimum value due to reduction in efficiency of
pumps.
- Operating point of the pump is away from best efficiency point (b.e.p.).
- Energy is wasted due to increase in head loss in pumping system, for instance, 6875
clogging of strainer, encrustation in column pipes, and encrustation in pumping
main.
- Selection of uneconomical diameter of sluice valve, butterfly valve, reflux valve,
column pipe, and drop pipe, etc., in pumping installations.
- Energy wastage due to operation of electrical equipments at low voltage and/or low 6880
power factor.
Such inefficient operation and wastage of energy should be avoided to cut down energy cost. It
is therefore, necessary to identify all such shortcomings and causes which can be achieved by
conducting methodical energy audit.
Strategies as given below should be adopted for the management of energy. 6885
- Conduct thorough and in-depth energy audit covering analysis and evaluation of all
equipment, operations and system components which have bearing on energy
consumption, and identify the scope for reduction in energy cost.
- Implement measures for conservation of energy. Energy audit as implied is auditing
of billed energy consumption and how the energy is consumed by various units, and 6890
sub-units in the installation and whether there is any wastage due to poor efficiency,
higher hydraulic or power losses etc., and identification of actions for remedy and
6.8.1 Frequency of Energy Audit
Frequency of energy audit recommended is as follows. 6895
- Large installations: Every year
- Medium installations: Every two years
- Small installations: Every three years
6.8.2 Scope of Energy Audit
Scope of energy audit and suggested methodology includes following actions, steps and 6900
processes:
a. Conducting in-depth energy audit by systematic process of accounting and
reconciliation between the following:
Actual energy consumption, and
Calculated energy consumption taking into account rated efficiency and power 6905
losses in all energy utilising equipment and power transmission system, such as
conductor, cable, panels, etc.
b. Conducting performance tests of pumps and electrical equipment if the difference
between actual energy consumption and calculated energy consumption is
significant and taking follow up action on conclusions drawn from the tests. 6910
c. Taking up discharge test at rated head if test in b. above is not being taken.
d. Identifying the equipment, operational aspects and characteristics of power supply
causing inefficient operation, wastage of energy, increase in hydraulic or power
losses etc., and evaluating the increase in energy cost or wastage of energy.
e. Identifying solutions and actions necessary to correct shortcomings and lacunas in d. 6915
above and evaluating cost of the solutions.
f. Carrying out economic analysis of costs involved in d. and e. above and drawing
conclusions on whether rectification is economical or otherwise.
g. Checking whether operating point is near the best efficiency point and whether any
improvement is possible. 6920
h. Verification of penalties if any, levied by power supply authorities, such as penalty
for poor power factor, penalty for exceeding contract demand, and so on.
i. Broad review of the following points for future guidance or long term measure:
C-value or f-value of transmission main.
Diameter of transmission main provided 6925
Specified duty point for pump and operating range
Suitability of pump for the duty conditions and situation in general and
specifically from efficiency aspects.
Suitability of ratings and sizes of motor, cable, transformer and other electrical
appliances for the load. 6930
6.9 Management of Records
Records are the key to an effective maintenance program. Records can remind the operator
when routine operation or maintenance is necessary. They help ensure that schedules are
maintained and needed operation or maintenance are not overlooked or forgotten.
6.9.1 Record of Operation and Maintenance 6935
Records must be permanent, complete, and accurate. Write entries clearly and neatly on data
sheets in ink. Apencil should never be used because notations can smudge or they can be altered
or erased.
Minimum recordkeeping that may be required for operations is listed below and shown in
Appendix 6.1: 6940
a. Operational record: Power receiving and transforming equipment
b. Monthly report: Electric power receiving
c. Ledger: Electrical equipment
6.9.2 Record of Operation and Maintenance and its Utilization
Records are utilized like the following: 6945
Review of operating records can indicate the efficiency of the plant, performance of its
treatment units, past problems, and potential problems.
Records can be used to determine the financial health of the utility, provide the basic data on the
systems property, and prepare monthly and annual reports.
Generally, preventive maintenance can be described as maintenance of equipment or system
before faults occurs. It can be divided into two subgroups:
- Planned / Scheduled Maintenance (PM)
Scheduled activities to ensure that an item of equipment is operating correctly and
to therefore avoid any unscheduled breakdown and downtime. 6955
- Condition Based Maintenance (CBM)
Activities performed after one or more indicators show that equipment is going to
fail or that equipment performance is deteriorating.
The vast majority of electrical maintenance should be predictive or preventive. This section
focuses exclusively on these activities. There are four cardinal rules to follow in any 6960
maintenance program:
- Keep it clean
Dirt build-up on moving parts will cause slow operation, arcing, and subsequent
burning. Moreover, coils can short-circuit. Dirt will always impede airflow and
result in elevated operating temperatures. 6965
- Keep it dry
Electrical equipment always operates best in a dry atmosphere, where corrosion is
eliminated. Moisture-related grounds and short circuits are also eliminated
- Keep it tight
Most electrical equipment operate at a high speed or is subjected to vibration. 6970
- Keep it frictionless
Any piece of equipment or machinery is designed to operate with minimum friction.
Dirt, corrosion, or excessive torque will often cause excessive friction.
Of the four cardinal rules, none is essentially electrical in nature. The failure of a bearing in a
motor can lead to an ultimate motor winding failure that is electrical, but the root cause of the 6975
failure could have been mechanical.
The goal of any electrical preventive maintenance program is to minimize electrical outages and
ensure continuity of operation.
6.10.1 Types of Planned Maintenance
Maintenance works can be classified as follows according to their inspection intervals. The 6980
results of maintenance can be utilized for preventing possible faults or breakdowns of
equipment in future.
- Routine maintenance
Routine maintenance consists of observation for signs of overheating, dirt, loose
parts, noise and any other signs of abnormalities. It will help grasping the state of 6985
electrical equipment.
- Periodic maintenance
Regular maintenance includes inspections of electrical conditions such as electric
current, voltage, insulation resistance, ground resistance, etc.
- Detailed examination 6990
Examinations should be preprogrammed according to manufacturers
recommendation or legislation. Recommended maintenance tasks for typical
electrical equipment are listed in Table 6.7.
Table 6.7 Recommended maintenance on electrical equipment
Monthly Quarterly Semi-annual Annual
Panel, Circuit-Breaker, Starter* GI,CL AJ CB,GRT
Transformer Substation* GI GI,RE TO,MR GRT,CL
Motors* GI,CL AL,MR PG
Standby power generator GI CL,AL
Gas engine GI CL,AL
Dual fuel engine GI CL,AL
UPS GI CL
Flow Measuring equipment CB,MR
Level Measuring equipment CL CB,MR
Other Instrumentation CL CB ADL MR
Cables MR
* Details of the maintenance tasks of the marked equipment above are given in Appendix 6.2. 6995
(Legend)
CL Clean GRT Ground resistance test
MR Megger and record TO Test oil
PG Pressure grease GI General inspection
SG Surface grease AL Add lubricant
RC Remove condensate AJ Adjustment
CB Calibration RE Replenish / Replacement
ADL Add liquid
6.10.2 Inspection Tools
A wide variety of instruments are used to maintain electrical systems. These instruments
measure current, voltage, and resistance. They are used not only for troubleshooting, but for
preventive maintenance as well. These instruments may have either an analog readout, which 7000
uses a pointer and scale, or a digital readout, which gives a numerical reading of the measured
6.10.2.1 Multimeter
A multimeter is used to measure voltage and low levels of current in a live system and
continuity in a switched-off system. There are several types available in the market. They are 7005
designed to be used on energized circuits and care must be exercised when testing. By holding
one lead on ground and the other on a power lead, a user can determine if power is available,
and also can tell if it is AC or DC, the intensity or voltage (1, 10, 220, 480, and so on) by testing
the different leads. A Clamp-on Multimeter can measure larger current typical in a motor.
A digital multimeter and an analog multimeter are shown in Figure 6.23. 7010
Source: HIOKI E.E. CORPORATION
Figure 6.23 Digital multimeter (Left) and Analog multimeter (Right)
- Only qualified and authorized persons should work on electric circuits.
- Use a multimeter or other circuit tester to determine if the circuit is energized, or if 7015
voltage is off. This should be done after the main switch is turned off to ensure that it
is safe to work inside the electrical panel. Always be aware of the possibility that
even if the unit the users are working on is off, the control circuit may still be
energized if the circuit originates at a different distribution panel.
- Check with a multimeter before and during the time the main switch is turned off as 7020
a double check. This procedure ensures that the multimeter is working and that the
users have good continuity to the tester.
- Use a circuit tester to measure voltage or current characteristics to a given piece of
equipment for checking whether the circuit is alive or not. Switches can fail and
the only way to ensure that a circuit is dead is to test the circuit. 7025
- In addition to checking for power, a multimeter can be used to test for open circuits,
blown fuses, single phasing of motors, grounds, and has many other uses.
6.10.2.2 Clamp-on Meter
The clamp-on meter measures the current or amps in the circuit (Figure 6.24). It is used by
clamping the meter over only one of the power leads to the motor or other apparatus and taking 7030
a direct reading. Therefore the measurement by the clamp-on meter is safe method in a
high-current circuit.
Each lead in a three-phase motor must be checked.
Source: HIOKI E.E. CORPORATION 7035
Figure 6.24 Clamp-on meter
6.10.2.3 Megger / Megohmmeter
A megger or megohmmeter is used for checking the insulation resistance of motors, generators,
feeders, bus bar systems, grounds, and branch circuit wiring. This device actually applies a DC
test voltage, which can be as high as 5,000 volts, depending on the megohmmeter selected. 7040
The one shown in Figure 6.25 is a hand-held that applies 500 volts DC and is particularly useful
for testing low-tension motor insulation. Battery-operated and instrument style meggers are also
available in both analog and digital models.
Figure 6.25 Hand-cranked megohmmeter
If a low reading is obtained, disconnect motor leads from power or line leads. A low reading in
the megger for motor generally indicates that the motor winding insulation has broken down. If
a good reading is obtained, meg the circuit or branch wiring. If this reading is low, the wiring to
the motor is defective. 7050
Motors and wiring should be subjected to megger test at least once a year, and if possible, twice
a year. The readings taken should be recorded and plotted to determine the deterioration of
insulation and predict its breakdown.
6.10.2.4 Ground Resistance Testers (Earth Meggers)
When electrical equipment is installed in housings, they may be charged against the ground. 7055
Therefore, they should be connected with earth (referred to as earth) to reduce the potential
difference between the terminal and the earth to as low a value as possible (ideally 0 volt).
The purposes of the earth are as follows:
a. Prevent electric shock: Discharge any electricity charged in equipment housing due
to electrical insulation failure or a transformer breakdown to prevent shock; and 7060
b. Prevent breakdown of loaded equipment: Connect a neutral line on the load side of
transformer with earth and prevent high voltage on the power source side from
intruding into the load side in case of transformer fails so as to protect the loaded
Value of ground resistance is depends considerably on the soil to be earthed, and the smaller the 7065
resistance, the better.
Ground resistance testers are devices to measure the stated resistance when the circuit is earthed.
Testers with measurable range from 0 to 1,000 ohms are widely used. (Figure 6.26)
Figure 6.26 Ground resistance tester
6.10.3 General Precautions for Electrical Maintenance
The following should be ensured for safe electrical maintenance:
- Do not touch any energized parts directly.
- Fully understand configurations and operational characteristics of related electrical 7075
facilities and equipment.
- When operating electrical equipment, follow the operational procedures, confirmthe
purposes of the operation, and predict the result of the operation.
- When overheat, abnormal noise, or vibration, etc, is detected during inspection,
report the condition to the person in charge of electrical work. 7080
- When overheat, abnormal noise, or vibration, etc, is detected during inspection, stop
the equipment and investigate the causes if necessary.
- Always keep the surrounding of electrical equipment tidy and clean. Never allow
outsiders to enter the electrical equipment site.
6.10.4 Rehabilitation 7085
If the equipment in the electrical facility is old, frequent outages and high maintenance and
repair costs are likely to occur. If some of the equipment is beyond repair, breakdowns lead to
long and extensive power outages in the STP.
To prevent such occurrences, functional degradation of electric equipment and causes for
breakdown and stoppages should be tracked at an early stage and repaired. Causes of fault also 7090
follow a certain trend. Training should be imparted on predicting faults beforehand, so that
measures and repairs can be implemented.
Spare parts and tools should be kept ready on site so that repairs can be carried out. Inventory of
spare parts and tools should be confirmed, and the required number should be stored.
6.10.5 Planned Reconstruction 7095
For reconstruction of electric facilities, plans should be formulated; therefore, scheduled O&M
should have been implemented.
Items to be studied for planned reconstruction include:
- Find whether abnormalities exist from routine inspection records, data, periodic
inspections and repair records, and judge the condition of equipment. 7100
- Analyze collected and accumulated data, and understand the long-term deterioration
trend of equipment.
The primary function of electrical system is to receive power from STP, transform it, and
distribute it stably to each facility within the plant. Instrumentation system also plays an
important role indicating operating conditions.
For realizing these functions properly, the electrical system requires periodic inspection and
maintenance for early detection of abnormal conditions. The instrumentation system should be 7110
inspected and adjusted regularly so that it can provide correct readings at all times.
Part B: Operation and Maintenance CHAPTER 7: MONITORING
OF WATER QUALITY
CHAPTER 7 MONITORING OF WATER QUALITY
Sampling and analysis is a tool to forewarn against possible and potential dangers in the areas
bordering on human health and which can lead to water borne epidemics and endemics. It aims 7115
at a quick method of estimating the deterioration of the desired quality of the treated sewage not
only at the outfall but also at each incremental stage of treatment so that the dysfunction of a
particular unit operation can be detected and set right quickly before it has a cascading effect on
downstream unit operations.
7.2 Need for Sampling and Analysis 7120
Effective operation and control of a sewage treatment plant requires that the operator possess
thorough knowledge of the composition of the influent, effluent, and internal process streams.
To acquire such knowledge, the sample should be collected and analysed representative samples
of the raw sewage and streams throughout the plant and determines their characteristics.
7.3 Sampling 7125
In general, the two major categories of sample collections are for (a) physical and chemical tests
and (b) microbiological tests. In both cases, care should be taken to avoid entry of extraneous
materials such as silt, scum and floating matters into sampling bottles.
Understanding the principles and practices of sampling to obtain a representative sample is 7130
important to get at a truly representative sample instead of randomly collection leading to
misleading results. Laboratory analyses will have little value if representative sampling is not
done. Sampling points must be located where homogeneity of the sewage or waste water with
good mixing of the materials is available. Careless collections of samples give data which may
lead to wrong conclusions. 7135
7.3.2 Grab Sample
Grab samples are collected when frequent changes in character and concentrations are likely to
occur and influence the treatment, undesirable constituents are suspected, the quality is not
expected to vary or when samples require on the spot analysis for parameters such as DO, pH
and residual chlorine. For example, the testing of the suspended solids in the clarifier overflow 7140
is an independent sample and it needs to be correlated to the time of sampling because the SS
can vary between low flows, average flows and peak flows. Invariably the SS at peak flows of a
few hours in the early forenoon may be higher. If the timing is not given, this will give the
wrong impression that the entire performance over the 24 hours is having higher SS.
Representative samples should be taken with good judgment and should be analyzed within 2 to 7145
3 hours of sampling, A well washed clean PVC or plastic bucket connected to a sturdy long
handle may be suitable for grab sampling.
7.3.3 Composite Sample
Since the sewage quality changes from moment to moment and hour to hour, the best results
would be obtained by using some sort of continuous sampler-analyser. However, since operators 7150
are usually the sampler-analysers, continuous analysis would leave little time for anything but
sampling and testing. Except for tests which cannot wait due to rapid chemical and biological
change of the sample, such as tests for dissolved oxygen and sulphide, a fair compromise may
be reached by taking samples throughout the day at hourly or two-hourly intervals.
When the samples are taken, they should be refrigerated immediately to preserve them from 7155
continued bacterial decomposition. When all the samples have been collected for a 24-hour
period, the sample from specific location should be combined or composited together according
to flow to form a single 24-hour composite sample.
To prepare a composite sample:
- The rate of sewage flow must be known, and 7160
- Each grab sample must be taken and measured out in direct proportion to the volume
of flow at that time.
Table 7.1 illustrates the hourly flow and sample volume to be measured for a 12-hour
proportional composite sample. Large sewage solids should be excluded from a sample,
particularly those greater than 6 mm diameter. A sample composited according to the following 7165
table would be total 1,140 ml.
Table 7.1 Hourly flow pattern during composite sampling
(MLd)
(ml)
6 AM 0.2 100 20 12 N 1.5 100 150
7 AM 0.4 100 40 1 PM 1.2 100 120
9 AM 1.0 100 100 3 PM 1.0 100 100
10 AM 1.2 100 120 4 PM 1.0 100 100
11 AM 1.4 100 140 5 PM 0.9 100 90
Points to be noted
During compositing and at the exact moment of testing, the samples must be vigorously 7170
remixed so that they are of the same composition and as well mixed as they were when
originally sampled. Sometimes such remixing may become lax, so that all the solids are not
uniformly suspended. Lack of mixing can cause low results in samples of solids that settle out
rapidly, such as those in activated sludge or raw sewage. Samples must therefore be mixed
thoroughly and poured quickly before settling occurs. If this is not done, errors of 25-50 per cent 7175
may easily occur. For example, on the same mixed liquor sample, one person may find 3,000
mg/L suspended solids while another may determine that there are only 2,000 mg/L due to poor
mixing. When such a composite sample is tested, a reasonably accurate measurement of the
quality of flow can be made.
7.3.4 Sampling Method and Precautions in Sampling 7180
In all cases of sampling, procedures described in 'Standard Methods for the Examination of
Water and Wastewater (APHA)' or 'Manual of Methods for the Examination of Water, Sewage
and Industrial Wastes (ICMR)' or other standard manuals should be followed.
The sampling procedure is very important and is based on the purpose of sampling and tests to
be performed. In general, sewage samples shall not be aerated during collection. Some of the 7185
manually operated sampling apparatuses are shown in Figure 7.1. Each has its preference, but
the syphonic bellow at Ais the easiest to use anywhere.
Figure 7.1 Typical sampling apparatuses used in sampling of sewage in STPs 7190
A-Syphonic tube with bellows; B- Electrically operated peristaltic pump, C- Hand operated
rotary with positive displacement, D- Hand operated rotary with circular movement
The use of the syphonic bellows tube involves the dipping of the free end of the tube into the
liquid surface and keeping the pump end below the liquid level outside the structure and
pumping the bellow which sets off a siphon action. Initially some portion of the effluent is to be 7195
discharged freely. If the sample is meant for determining the dissolved oxygen, the free end after
bellows shall be extended by rubber tubing with a standard laboratory pinch cock and the free
end of the tubing dipped into the BOD bottle to effect a submerged discharge very slowly by
using the pinch cock. A timing of 10 seconds to fill the BOD bottle is considered as optimum.
The sample shall be allowed to overflow for 5 seconds before the tube is withdrawn and the 7200
bottle is corked with the ground glass cork. This is possible only in the case of tanks with water
surfaces above ground level. If the water level is below ground level, a long handle connected
scoop can be easily used. In this case, the scoop shall have a minimum of 1000 ml volume and
the above procedure can be done. The electrically operated peristaltic pumps (B) and other hand
operated devices (C), (D) are fit only for the final treated sewage samples. In all cases, the 7205
discharge end shall be submerged in the sampling bottle and overflow of samples shall be
allowed for about 5 seconds.
7.3.5 Sample Volumes, Quantity and Storage of Samples
1 to 2 litres of grab sample would be enough to perform all the tests and repeat some tests if
required. For composite samples, a total quantity of 1 to 2 litres collected over a 24 hour period 7210
is adequate, Fractional sample at intervals of 1, 2 to 3 hours should be collected in suitable
containers, each sample being well mixed and a measured portion proportional to the flow
transferred by means of a pipette, measuring cylinder or flask and integrated to form a 1 to 2
litre sample. Hourly records of flow normally available with the Plant Superintendent would
facilitate taking representative samples. All samples should be immediately transported to the 7215
laboratory for analysis. In case there is any delay in transportation,the preservation time is to be
as short as possible and in any case not exceeding 24 hours and the ice shall not be found melted
on receipt of the sample.
7.3.6 Selection of Sampling Location
Theoretically there is no end to the number of sampling stations that can be used in a STP. But 7220
then, it should be remembered that the best monitoring can be possible only when the barest
minimum and objectively oriented sampling locations and tests are carried out instead of
accumulating all and sundry data that will only confuse the situation. This is because the sewage
passes through the STP on a time deferred scale and if samples are taken all at the same time
from inlet to outfall, the chances are it is not representative of the true performance. Asuggested 7225
set of sampling points is shown below.
a. Raw sewage samples should be collected after screens or grit chambers.
b. Samples of effluent from primary clarifier or secondary clarifier tanks should be
taken from the effluent trough or pipe or ahead of discharge weirs.
c. Influent to top feed media units should be collected below the distribution arm and 7230
the effluent from the filter from the outlet chamber or at the inlet to secondary
sedimentation tank.
d. Apoint where there is good mixing should be selected for sampling of mixed liquor
in aeration tanks in the activated sludge process.
e. Influent samples of septic tanks, Imhoff tanks, and other sole treatment units such as 7235
waste stabilization ponds, oxidation ditches and aerated lagoons should he collected
ahead of these tanks in their inlet chambers or channels leading to these units.
Effluent samples should be collected outside the units in receiving weirs or channels
or chambers.
f. Sampling within these tanks should be specified in terms of depth or distance or 7240
g. Samples of raw sludge should be taken from sludge sumps or from the delivery side
of the sludge pumps through sampling cocks.
h. Return sludge sample in activated sludge plant is collected at the point of discharge
into primary units or aeration tank. 7245
i. Samples from mixed primary and secondary sludge should be collected at the point
of delivery to the digester.
j. Digested sludge samples may be drawn from the sampling points in the digester or
from the discharge end of the delivery pipe leading to drying beds.
k. Digester supernatant may be drawn from sampling cocks provided for this purpose 7250
or through sampling wells on digester dome.
The flow chart indicating the various treatment units and the sampling points may be exhibited
prominently in the laboratory. A list of tests to be carried out daily on the samples may also be
displayed as a wall chart.
7.4 Relevance of Parameters 7255
In general, the parameters can be classified as broadly into statutory need and plant control need.
The plant control needs are mainly to understand whether the STP is responding as per the
design and to inform whether corrective measures are needed. Two separate records shall be
kept; one for public consumption in respect of statutory discharge standards and the other for
in-house plant control. These parameters are not for public consumption as they will be 7260
continually changing and the public may not be able to comprehend its nuances.
7.5 Analysis Parameters and Frequency (Liquid and Sludge)
7.5.1 Items and Frequency for ASP
For day to day plant control, various sundry data need not be accumulated. Recommended tests
to becarried out in typical STPs on a daily, weekly, and monthly to biannual basis are shown in 7265
Table 7.2, Table 7.3, and Table 7.4, respectively.
Table 7.2 Recommended plant control tests on a daily basis in a typical STP
No. Tests
1. Temperature
3. Alkalinity
4. BOD (Total)
5. COD (Total)
6. TSS
7. VSS
8. Residual Chlorine
9. Moisture Content
10. MLSS
11. MLVSS
12. DO
13. SV
14. SVI
15. Ammonia, Nessler
16. Ortho P, Nessler
17. Sulphide
Table 7.3 Recommended plant control tests on a weekly basis in a typical STP
1. BOD (Filtered)
2. COD (Filtered)
3. Microscopy*
4. Faecal Coliform
5. Total Coliform
6. Oil and grease
Total residual
chlorin
Ammonical
Total Kjeldahl
10. Nitrate Nitrogen
11. Free ammonia
Phosphates (P)
13. Sulphate
14. Chloride
15. Silica
16. Ca
17. Mg
18. TDS
19. Conductivity
*Identify the following by microscopy.
Rotifers, Crustaceans, Protozoa, Ciliates, Ciliates, Nocardia, Ceronthirix, Nematodes 7270
Table 7.4 Recommended plant control tests on a monthly to biannual basis in a typical STP
1. Mercury (Hg)
2. Lead (Pb)
3. Cadmium (Cd)
Chromium (Cr+6)
Total chromium
(Cr)
6. Copper (Cu)
7. Zinc (Zn)
8. Nickel (Ni)
9. Manganese (Mn)
10. Iron (Fe)
11. Vanadium (V)
12. Cyanide (CN)
13. Fluoride (F)
15. Arsenic (As)
16. Selenium (Se)
17. Bio-assay test
(a)Alpha emitter
(b)Beta emitter
Above tables may undergo changes based on the properties of the treatment process used.
In respect of BOD test, a graph of BOD versus COD for the raw, primary treated, secondary
treated and outfall sewage should be prepared every week and the daily COD readings used to
interpret the BOD values. 7275
7.5.2 Items and Frequency for WSP
In general parameters of testing for diurnal examinations arise only when the treatment process
is dependent on solar energy like in the case of ponds. In such cases, the tests will be as follows.
Table 7.5 Recommended plant control tests on a monthly basis in a typical WSP
Flow - Measure both raw sewage and finel effluent flows
BOD C Unfiltered samples
COD C Unfiltered samples
Susupendid solids C
Ammonia C
pH G Take two samples,
one at 8:00-10:00 h and the other at 14:00-16:00 h Temperature G
Fecal coliforms G Take sample between 08:00 and 10:00 h
Total nitrogen C Only when effluent being used (or being assessed for use)
Total phosphorus C for crop irrigation. Ca, Mg and Na are required to calculate
the sodium absorption ratio
Chloride C
Electrical conductivity C
Ca, Mg, Na C
Boron C
Helminth eggs
C=24-hour flow-weighted composite sample; G=grab sample. 7280
Also on filtered samples if the discharge requirements are so expressed.
Ascaris lumbricoides, Trichuris trichiura, Ancylostoma duodenale and Necator americanus.
SAR=(0.044Na)/[0.5(0.050Ca+0.082Mg)]
where Na, Ca and Mg are the concentrations in mg/L.
Source: Duncn Mara, 1997
Care, safety and wisdom are paramount in taking samples from ponds especially diurnal 7285
samples as chances of vermin and reptiles straying around in wet climates and high summer
cannot be ruled out. Proper clothing, safety wear, etc., and the presence of a qualified
ambulance person with tool kit is mandatory in the diurnal sampling. A better way of managing
this will be to leave a floating or other pump set erected in the daytime and operate it by remote
switch in the night and collect the sample from the outlet hose of the pump set sufficiently far 7290
away at a well lighted and safe and secure location.
7.6 Microbiology Sampling and Testing of Treated Sewage
7.6.1 Need for Microbiological Testing
Many water borne pathogenic organisms which can cause diseases as in Table 7.6 and can even
cause an epidemic, are transmitted through the water route when infected with sewage which is 7295
not fully treated. This is because these organisms enter the water from the faeces of individuals
suffering from these diseases or carriers of these organisms even after they are supposed to be
fully cured.
Table 7.6 Diseases attributable to sewage pollution of drinking water
No Diseases
1 Gastroenteritis Salmonella
Animal or
Human faeces
Diarrhoea, Vomiting
2 Typhoid
typhosa
Inflamed Intestine, Enlarged
Spleen, Step ladder temperature
3 Dysentery Shigella species Human faeces Diarrhoea
4 Cholera Vibrio cholera Human faeces
Diarrhoea, Dehydration
5 Infectious hepatitis Virus
Human faeces,
Shell fish
Yellowed Skin,
6 Amoebic dysentery
Entamoeba
histolitca
Human faeces Diarrhoea, Dysentery
7 Giardiasis Giardia lambia
Diarrhoea, Cramps, Nausea,
It is both difficult and time consuming to check the treated sewage for each of these organisms. 7300
A sterile laboratory system is required, which is admittedly not easy in many parts of India
especially outside the metro cuties. Extensive research has shown that if the coliform group of
bacteria is present, there is a probability that one or more pathogens may also be present.
Therefore, coliforms have been chosen to be the bacterial group routinely tested when there is
need or assessing the bacteriological safety of water. Presence of any of the coliform group of 7305
bacteria (total coliforms) indicates general contamination, while the presence of faecal coliforms
indicates contamination of human or animal origin. These can be differentiated from other
coliforms by incubating on selective media at 44.5C.
7.6.2 Testing Method
7.6.2.1 Multiple Tube Fermentation Method 7310
7.6.2.1.1 Total Coliform Test
The oldest test is the multiple tube fermentation test. In this test, three sequential steps are
performed as presumptive, confirmed and completed tests. A moderately selective lactose broth
medium (Lactose Lauryl Tryptose Broth), containing a Durham tube is firstly used in the
presumptive test to encourage the recovery and growth of potentially stressed coliforms in the 7315
sample. If harsher selective conditions are used, a deceptively low count may result. A tube
containing both growth and gas is recorded as a positive result. It is possible for non-coliforms
(Clostridium or Bacillus) to cause false positives in this medium, and therefore, all positive
tubes are then inoculated into a more selective medium (Brilliant Green Lactose Broth or EC
Broth) to begin the confirmed test. The confirmed test medium effectively eliminates all 7320
organisms except true coliforms or faecal coliforms, depending on the medium and incubation
conditions. If a positive result is recorded in these tubes, the completed test is begun by first
streaking a loopful of the highest dilution tube which gave a positive result onto highly selective
Eosin Methylene Blue (EMB) agar. After incubation, subsequent colonies are evaluated for
typical coliform reactions. A schematic illustration is presented in Figure 7.2. Sample of the 7325
tubes tested being either yielding gas or otherwise are shown in Figure 7.3.
7.6.2.1.2 Fecal Coliform Test
This test more reliably indicates the potential presence of pathogenic organisms than do tests for
total coliform group of organisms. After presumptive test, which is the same as for the total
coliform test, test the sample with water bath set at 44.5C0.2C in EC broth media. A 7330
schematic illustration is presented in Figure 7.2.
Figure 7.2 The Progress of the multiple tube testing for coliforms
Figure 7.3 The fermentation as seen from the gas in the inverted vials inside the tubes
1. Presumptive Test
Inoculate in lactose or lauryl tryptose; incubate 242hrs
at 37C0.5C
No gas
Incubate for 24 hrs more
No coliform present
Discard tubes
Gas produced;
continue with confirmed test
(and / or faecal test)
Gas produced
Coliform group confirmed
Calculate confirmed MPN
Coliform absent
Inoculate (with loop or applicator stick) brilliant green
bile broth, incubate 483hrs at 37 C0.5C
2. Confirmed Test
Fecal Coliform
2. Fecal Coliform Test
Inoculate (with loop or applicator stick) EC broth.
Incubate 242hrs at 44.5C0.2C
Fecal Coliform absent
Fecal Coliform confirmed
Calculate fecal MPN
Based on the dilutions used, the number of tubes adopted and the identified number of tubes
with gasification, a statistical formulation is made out called the Most Probable Number (MPN)
in 100 ml of sample. It should be noted that a confirmed test may require anywhere up to 72
hours. 7340
7.6.2.2 Membrane Filter Test
In this procedure, a given size sample, generally 100 millilitres, is filtered through a membrane,
small-pore filter, which is then incubated in contact with a selective culture agar at 37C. A
coliform bacteria colony will develop at each point on the membrane where a viable coliform
was left on the membrane during filtration. After the incubation period of 24 hours, the number 7345
of colonies per plate is counted. They represent the actual number of coliforms that were present
in the volume of samples filtered. The procedure is illustrated in Figure 7.4. The incubated
plates may appear as in Figure 7.5.
Figure 7.4 The progress of the filter technique for coliforms
Figure 7.5 Illustrative appearance of cultured plates showing the growth of colonies
The number of colonies in a dish can be counted using a colony counter which can be manual or
automatic counter or hand held digital type as shown in Figure 7.6. 7355
Figure 7.6 Colony counters: automatic integrating pen type and grid plate for manual use
7.6.2.3 Colilert Test
Recently, it has been reported upon the Colilert technique which is relatively new and is a
method has been accepted by the U.S. EPA for coliform testing. This is a presence/absence test 7360
and it does not indicate the extent of contamination. But it is reported as having been proven to
be just as accurate as the membrane filtration method. In this method, the Colilert reagent
contains a formulation of salts, nitrogen, and carbon sources that are specific to total coliform. It
contains specific indicator nutrients that create a yellow colour when total coliforms are present
and fluorescence when E. coil is present. The reagent is added to a 100-milliliter water sample 7365
in a sterile, non-fluorescent borosilicate glass container. The vessel is capped and shaken
vigorously by repeated inversion to aid in mixing of the reagent. It is incubated at 35C for 24
hours. After 24 hours, the technician compares the reaction vessels to the colour in a comparator
supplied with the test kit. If the inoculated reagent has a yellow colour equal to or greater than
the comparator, the presence of total coliform bacteria is confirmed. The sequence of testing is 7370
illustrated in Figure 7.7.
A rapid 7-hour faecal coliform (FC) test for the detection of FC in water has been developed.
This membrane filter test utilizes a lightly buffered lactose-based medium (m-7-hour FC
medium) combined with a sensitive pH indicator system. The 7-hour FC test was found to be
suitable for the examination of surface waters and unchlorinated sewage and could serve as an 7375
emergency test for detection of sewage or faecal contamination of potable water. It is
particularly useful for rapid detection of recreational water quality changes related to storm
water runoff, sanitary waste spills or bypasses, and for effluent monitoring for treatment
malfunction.
7.6.2.4 Recommended Testing for Treated Sewage 7380
Whatever be the advancements occurring elsewhere in the world, while recommending a
specific testing procedure for Indian situation, the following must be considered:
a. The fact that maintaining a sterile microbiological laboratory in a STP is still a far
cry for many local urban bodies once it migrates outside the metropolitan cities.
b. The testing skills are specialty oriented and employing such microbiologists for full 7385
time by these local urban bodies will be impractical especially as such personnel will
not have promotional avenues as compared to their employment in many R&D
institutions.
c. These tests are not mandatory testing on a daily basis and can be carried out once a
fortnight by the staff of the metropolitan laboratories by collecting and preserving 7390
the samples in suitable ice boxes for transiting overnight to their laboratories.
Figure 7.7 The progress of the Colilert technique for coliforms
Considering all the above points, the multiple tube method of MPN / 100 ml should be
continued for some more time for total coliforms and faecal coliforms. The tests can be cross 7395
verified by plate count if felt necessary. The sampling procedures and analytical procedures
shall be according to the Standard Methods (or) as in force by the concerned State Public Health
Authority, as the case may be.
24-hour Detection of
Total Coliform and E.coli
Collect Sample
Add Sample to
Pre-scored Fill-to Line
Shake Vessel
to Dissolve Reagent
Record as coliform
24hrs at 35C0.5C
Record as E. Coli
Discard Discard
YELLOW COLOUR?
FLUORESCENCE?
7.7 Quick and Approximate Measurement Methods
7.7.1 Test Paper Method 7400
Tests which can be done with this method are pH and sulphide. These are mainly qualitative and
are of only incidental value.
7.7.2 Detector Tube Method (Transparency Tube, BOD Tube (UK))
Refer to Chapter 4 (4.7.4.1 Process Control,Figure 4.13).
7.7.3 Cylinder Test 7405
(Including simplified colorimetric determination and simplified absorption spectrophotometry)
The colorimetric tests depend on the two hypotheses namely, the Beers law and Lamberts law
which in simple terms correlate the concentration of a solute in a solvent to the absorbance of
monochromatic light when passed through the solution and the path length through which such
a passage has taken place. This is the principle behind estimations using a calorimeter or 7410
spectrophotometer. There is also a possibility that these can be estimated by using Nessler tubes
in the laboratory without depending on these electrically operated meters. Standard solutions are
prepared with known concentrations of the solutes and these are stored in tightly corked Nessler
tubes of 50 ml. When a new sample is to be tested, it is put through the sample preparation and
thereafter is compared by looking down through a Nessler tube filled with the prepared sample 7415
and comparing with already prepared reference tubes. Thus, an idea of the concentration is
obtained. This test is very useful on a day to day basis.
The presence of DO in aeration tank requires the elaborate procedure of using a meter operated
electronically and keeping the probes well cleaned at all times. This is not always possible.
Moreover, typical plant control requires an answer to the question of whether residual DO is 7420
present or absent in the secondary clarifier overflow. This can be easily carried out in the field
a. Take a 10 ml well washed test tube.
b. Hold it gently against the weir overflow sideways.
c. Allow the sewage to fill the tube and overflow for a few minutes. 7425
d. Gently take the tube and pour out about 2 ml.
e. Add few drops of manganous sulphate solution.
f. Add a few drops of potassium iodide solution.
g. Close the top with the thumb and invert a few times.
h. Allow to stand for a few minutes. 7430
i. If there is a yellow precipitate, DO is present.
j. If there is a white precipitate, there is no DO.
7.8 Data Analysis (Accuracy and Precision)
All analyses carried out should be properly recorded. Routine daily analysis, periodic analysis
and special analysis should be recorded separately. Copies of these reports should be sent to the 7435
Plant Superintendent immediately after the analysis is done with explanatory notes to indicate
any unsatisfactory conditions or abnormalities. The Superintendent should study the reports and
direct the operating staff for proper corrective measures in the operation schedule. Such
measures taken should be reported to the laboratory scientists who should check the efficiency
of corrective measures by re-sampling and analysis. Corrective measures followed by sampling 7440
and analysis should be repeated till such time as satisfactory results are obtained,
Data collected over a period of time on various parameters of plant control should be analyzed
and represented on charts and graphs and displayed in the laboratory for ready reference by the
supervisory staff and visitors. These should be included in the weekly, monthly and annual
reports of the laboratory. 7445
7.8.1 Processing Water Quality Test Data
The analysis of results must be done judiciously. One should not jump to conclusions. Logic of
the results should be first verified instead of blindly taking it for granted. Some of the
fundamentals to be followed are listed below:
a. The outlet BOD of any unit cannot be higher than the outlet BOD of the upstream 7450
b. The ammonia of final treated sewage cannot be the same or higher than that in raw
sewage.
c. The ortho P of final treated sewage cannot be the same or higher than that in raw
sewage. 7455
d. The SS of final treated sewage cannot be higher than that of raw sewage.
e. Rotifers, Crustaceans, Protozoans cannot be absent if BOD reduction is at least
75 %.
f. Follow the final BOD and SS on a graph which will show any sudden lapses.
g. Once a while refer the sample discretely to another lab to keep a check on the results. 7460
h. Whenever visiting the STP, verify DO qualitatively by the Winkler method.
i. Whenever in the STP, take time to see through the oil immersion microscope for
motility.
j. It is most important that you do not hold analysts alone responsible for plant failures.
7.8.2 Accuracy of Measured Values 7465
a. If ammonia is reported as nitrified, bicarbonate alkalinity must be reduced 7 times.
b. If this is not the case, carry out a repeat test before deriving conclusions.
c. Make out an audit for BOD removed versus kWh spent on aeration system.
d. Hypothetical ionic equilibrium may not tally in all the lab results.
e. This may be a genuine case as precise chloride estimation is very difficult. 7470
f. In such case, it is better to adjust the chloride value to bring the ionic equilibrium.
g. The COD reduction in treated sewage versus raw sewage cannot be less than BOD
h. If this is the case, the results are suspect.
With the availability of personal computers and software at reasonable cost, the advantages of 7475
electronic data processing for storage, retrieval and processing of laboratory test results are
obvious. To start with, the analysis results may be entered from the daily records into computer
storage. Simple programme can be written tot retrieval and presentation of data relating to any
particular parameter. This can be in the form of display of data for a fixed period or weekly or
monthly averages or the results of analysis carried out on samples collected at a particular time 7480
of the day for the period to be studied etc. A slightly more detailed programming can be
prepared for the computer to go through the results of specified parameters entered daily and
display or print out any figures which exceed a present value. This can be immediately passed
onto the treatment plant staff for investigation and rectification. The computer can also be
programmed to display and print out graphs showing the variation in any specified parameters 7485
over a period of time.
Analytical instruments are also available for carrying out tests automatically on a large number
of samples simultaneously and electronically feed the data directly into the computer using a
data logger module.
7.9 Frequency Management 7490
Laboratory results must stop at the Plant Incharge level and should not go all the way to the
official in charge of the total O&M of the sewerage infrastructure in the head office on a daily
basis . This will only set in motion a parallel organization in detecting reporting matters and
replying to higher ups and the staff will lose interest. On the other hand, a fortnightly concise
physical reporting illustrating any specific changes in raw sewage or treated sewage and 7495
suggesting ways and means and asking for specific funds / assistance alone should be sent to the
official in charge of the total O&M of the sewerage infrastructure in the head office.
7.10 Planning of laboratory facilities
This is being explained in detail in Part A manual section 5.5.5 and also Appendix 5.9, 5.10 and
7.11 Upkeep of Plant Laboratory
Awell designed and adequately equipped laboratory under a competent analyst is essential in all
sewage treatment plants. Very small size plants such as stabilization ponds need not have their
own laboratories if the facilities of a nearby laboratory are available. The results of the
laboratory analysis will aid in the characterization of any wastewater, pinpoint difficulties in the 7505
operation and indicate improvement measures, evaluate the composition of effluents and thus
estimate the efficiency of operation and also measure the probable pollution effects of the
discharge of such effluents on the receiving water bodies. The analytical data accumulated ever
a period to time is an important document for safeguarding the treatment plant from allegations
of faulty operation. The laboratory should also engage in research and special studies for 7510
evolving improvements and innovations in plant operation. The laboratory therefore must form
an integral part of the treatment plant.
7.12 Disposal of Laboratory Wastes
Any office or other place where a number of people work, requires a proper waste disposal
system. In the case of a laboratory in a sewage treatment plant, special care has to be taken since 7515
the laboratory handles harmful chemicals and the samples themselves are capable for
transmitting pathogens.
7.12.1 Solid Waste
Solid waste may include filter residues, used cotton plugs, etc. These should be collected and
disposed scientifically in an eco-friendly way. 7520
7.12.2 Liquid Wastes
Since the laboratory is attached to a sewage treatment plant, it will be possible in most cases to
drain the laboratory wastes to the inlet chamber of the treatment plant, if necessary, by pumping.
However, since the laboratory wastes may also contain concentrated acids and alkalis, it may be
necessary to provide a small holding tank where the concentrated chemicals will be diluted and 7525
neutralized to avoid the possibility of affecting the biological activity of the treatment plant,
7.12.3 Radioactive Wastes
If radioactive materials are suspected to be present in the waste samples, special precautions
will have to be taken to protect the laboratory staff. Advice on this aspect may be obtained from
the Atomic Energy Department. 7530
7.13 Personnel
Laboratories of large plants should be under the charge of a qualified and experienced analyst
supported by junior technical staff having background in the field of chemistry, biology and
bacteriology. The analyst should assimilate the details for functioning of the plants by
experience and acquire the necessary preparedness for receiving further specialised training 7535
including performance interpretation and application of advanced techniques which enable the
analyst to participate in the efficient operation of the treatment unit.
Water quality analysis in STPs provides useful parameters for judging appropriateness of
process control and quality of the treated water. Water quality analysts should have through 7540
understanding of analysis items and frequencies prescribed for their own STPs and provide the
results of analysis to the O&M engineers.
Part B: Operation and Maintenance CHAPTER 8: ENVIRONMENTAL CONSERVATION
CHAPTER 8 ENVIRONMENTALCONSERVATION
A sewerage system including both offsite and onsite treatment is an infrastructure which 7545
contributes to the improvement and conservation of the environment by reducing pollutants
discharged to the water environment. A sewerage system is considered to have overall positive
effects on environmental conservation. However, on the other hand, since the system handles
insanitary objects, physical, mental and aesthetically negative impacts on the surrounding
environment through both construction and operation stages are unavoidable. 7550
Therefore, operational measures are required to minimize odour, epidemiological pollution, soil
contamination, and water pollution. Plant beautification and landscaping would be also required
to maximize the aesthetics.
In addition, since the sewerage system is an essential urban infrastructure which supports urban
domestic, industrial and business activities, the operation of the sewerage system is also 7555
required to withstand disasters. The concept of the global warming gas regulations is also
introduced in this chapter.
8.2 Odour
8.2.1 Odour from the Sewerage System
Odours from treatment plants are a complex combination of a wide variety of compounds; 7560
however, there are certain compounds and groups of compounds that contribute significantly to
sewage odours and also significantly affect the selection of the control technology. These
include the following:
- Hydrogen sulphide, and
- Ammonia. 7565
8.2.2 Odour Control Methods and Technologies
8.2.2.1 Odour Control Procedure
Odour control is a complex and time-consuming challenge, often requiring a combination of
methods for treating odorous gases and for removing or reducing the potential causes of the
doors. If an odour problem is severe enough to affect the community, an emergency response 7570
and solution to the problems must be carried out quickly.
The approach for selecting an odour control method or technology includes the following steps:
a. Identify the odour source and characteristics through sampling and analysis.
b. List and assign priorities to controlling a specific odour problem, recognizing
considerations such as cost, plant location, future upgrading of various sewage 7575
processes, severity of the odour problem, and the nature of the affected area.
c. Select one or more odour control method or technology for implementation to meet
the objectives of steps a and b, taking into consideration the advantages and
disadvantages of each.
d. Monitor odour emissions from the treated air for process adjustments and for 7580
feedback to evaluate the solutions effectiveness.
8.2.2.2 Sampling Methods
Solving any odour problem begins with sampling and analyzing gases to identify and
characterize the odours. The principal tools for diagnosing an odour problem are the techniques
used for odour quantification and characterization. Chemical analysis of odour constituents 7585
could be performed. This is an indirect method, because the results of a chemical analysis still
need to be related to odour concentration and intensity in some way.
8.2.2.3 Quantitative Testing-Analytical Methods
Gas chromatography can be used on many odorous organic compounds, but the analysis is
complex and expensive. Portable gas-monitoring devices are described below; 7590
The concentrations of individual compounds can be measured via standard analytical methods
(Figure 8.1). For example, a simple apparatus consisting of a gas detector tube can be used in
the field. Tubes are available for a number of compounds. For more accurate and complete
results, samples should be collected in bags, stainless steel vacuum canisters, or tubes filled with
adsorbent and analyzed by gas chromatography in a laboratory. 7595
Figure 8.1 Gas sampling pump (left side) and detector tubes (right side)
Gas detector tubes are sealed glass tubes filled with an appropriate indicator chemical which
reacts with a particular gas and gives a colour reaction. To make a determination, the seals are
broken at each end of the tube and a definite volume of the atmosphere for sampling is drawn 7600
through by a hand-operated or mechanical pump. The tubes are marked off in scale divisions
and the concentration is determined according to the length of discolouration of the indicator for
a given volume of atmosphere.
Detector tubes are simple, easy to use devices that can provide reasonably reliable, on-the-spot
measurement of gas concentrations. Their accuracy may be in the range of 70 to 90% of the 7605
mean value if sampling is done carefully according to manufacturers' directions. For taking gas
samples from difficult locations, extension tubes are available from manufacturers so that the
detector tubes can be placed at the desired site.
In making use of detector tubes some precautions should be noted:
- Tubes will deteriorate with age - some types have a shelf life of two years when 7610
stored at room temperature; above 30C, deterioration is more rapid.
- Direct sunlight can affect the properties of the tubes.
- At low temperatures, around freezing or below, tubes may not give reliable readings;
they should be warmed to room temperature for best performance.
- Tubes may have cross-sensitivity to gases other than those for which they are 7615
designed. Information on cross-sensitivity should be obtained from the
Hydrogen sulphide (H
S) is the most common odorous gas found in sewage collection and
treatment systems. Its characteristic rotten-egg odour is well known. The gas is corrosive, toxic, 7620
and soluble in sewage. Hydrogen sulphide results from the reduction of sulphate by bacteria
under anaerobic conditions.
8.2.3.1 Effects on Health
Hydrogen sulphide is considered a broad-spectrum poison, meaning it can poison several
different systems in the body. Breathing very high levels of hydrogen sulphide can cause death 7625
within just a few breaths. Loss of consciousness can result after fewer than three breaths.
Exposure to lower concentrations can result in eye irritation, a sore throat and cough, shortness
of breath, and fluid in the lungs. These symptoms usually go away within a few weeks.
Long-term, low-level exposure may result in fatigue, loss of appetite, headaches, irritability,
poor memory, and dizziness. Refer also to 9.2.2.1.2 Risk of Hydrogen Sulphide Poisoning in 7630
Confined Space of Chapter 9.
8.2.3.2 Locations of Sources
a. Onsite
Septic tank, anaerobic filters, and mini-package treatment plant
b. Conveyance 7635
Sewers, manholes, and closed drains (simplified sewers)
c. Pumping Station
Collection well
d. STP
Collection well, primary settling tank, sludge thickener, sludge digester, digested 7640
sludge sump, dewatering centrate/filtrate, sludge drying beds, UASB reactor,
anaerobic lagoons, sludge lagoons, and septage treatment facility
8.2.3.3 Measurement
- Proper measurements should be performed in accordance with IS5182 Part 7.
- Short-term detector tubes, portable gas detector, etc., can be used for simplified 7645
measurements.
- When measuring concentration at a location where odour is generated (particularly
when the concentration level is not known), take care to perform the work after
wearing a gas protection mask. If the concentration is high, toxicity may be high; this
is dangerous. Refer to Sec.9.3.1.1.1,a Measurerement method. 7650
8.2.3.4 Preventive Measures
Hydrogen sulphide production can be controlled by maintaining conditions that prevent the
build up of sulphides in the sewage. The presence of oxygen at concentrations of more than 1.0
mg/l in the sewage prevents sulphide build-up because sulphide produced by anaerobic bacteria
is aerobically oxidized. Maintaining an aerobic environment also inhibits the anaerobic 7655
degradation process contributing to the generation of hydrogen sulphide. A checklist is given
- Prevent corrosion in the collection well of the facility by blowing air through the
facility.
- Avoid storing screenings and grit generated in the grit chamber for a long time. 7660
Dispose of screenings and grit at appropriate intervals.
- Retention time of sludge in the sludge treatment facilities should be appropriate. (Do
not retain sludge for a long time)
- Maintain sewage at neutral pH range because most of the sulphide is present as
odourless bisulphide ion at pH value of 7 to 8. 7665
For septic tank
- Impossible to prevent because we cannot expect fine quality water to be used for
ablution. Therefore, it is important to ventilate.
8.2.3.5 Control
The operator can 7670
- Remove sand and grit deposited in house service connection or sewer immediately.
- Properly shut doors and windows of building where substances that become sources
of foul odours are stored.
- Dispose of scum and sludge in the sedimentation basin at appropriate intervals and
do not store them for a long period. 7675
- Thoroughly clean each facility and the areas surrounding the facility.
Measures for septic tank are as follows:
- Open ventilating shaft at the cowl to the atmosphere and provide mosquito-proof
netting. The height of the pipe should extend at least 2 m above the top of the highest
building within a radius of 20 m. Refer to IS 2470 Part I and Part A 9.3.5 7680
Conventional Septic Tank.
Operational precautions for deodorisation facilities are described below.
a. Soil (Bio) Deodorisation
This method makes use of oxidation and decomposition effects by
micro-organisms to remove substances with foul odour. In actual practice in 7685
treatment plants, the ventilated air is led to the reactor or soil bed and the odour is
In this method, substance with foul odour is delivered from the bottom part of the
fertile soil bed in highly moist condition, and the substance with foul odour is
removed because of the oxidative ability of mainly heterotrophic bacteria. 7690
The following are examples of soil deodorisation (Figure 8.2).
Figure 8.2 Soil (Bio) deodorisation configuration
Notes: Air for ventilation may concentrate in a certain part of the soil. In such cases, hole may have
formed on the surface. Dig the soil so that air is vented uniformly. 7695
b. ASP (Activated Sludge Process) -Deodorisation
This method makes use of decomposition effects by micro-organisms to remove
substances with foul odour. In actual practice in treatment plants, the ventilated air
is led to the reactor and the odour is removed.
Ventilated air is delivered to the inlet side of the blower. The substance with foul 7700
odour is oxidized and decomposed by aerobic micro-organisms in the reactor. The
mechanism of ASP-deodorisation is illustrated in Figure 8.3.
Figure 8.3 Mechanism of ASP-deodorization
Caution: Pipes used underwater are likely to clog easily; so periodically clean such pipes and remove the 7705
clogged material.
c. Activated Carbon Deodorisation
Foul odours are passed through the adsorption tower filled with activated carbon
(charcoal or coconut shell charcoal) and removed by physical adsorption. (Figure
8.4 ) The effect is more pronounced when the substance with foul odour has large 7710
molecular weight. Odours are selective and the method is more effective in case of
hydrogen sulphide and methyl mercaptan. However, it does not have any effect on
ammonias and amines. It is suitable for faint odours, and is used as a finishing
deodorising agent.
The normal activated carbons are acid, chlorine-based or halogen-attached 7715
activated carbons. These are effective in removing substances like hydrogen
sulphide and ammonia.
Figure 8.4 Activated-carbon deodorization
A differential pressure gauge is installed between the inlet and outlet of gas in this 7720
equipment. When the value indicated by this differential pressure gauge is large,
clogging has probably occurred. This is a sign that the activated carbon is to be
replaced or the pipe is to be cleaned.
Measure the concentration periodically near the outlet. When the value shows a
sudden increase, it is time for replacement. (However, breakdown in the equipment 7725
may have occurred; confirm the equipment carefully and then perform work.)
Ammonia odour is most typically encountered at alkaline-stabilization facilities. Adding
alkaline materials raises the pH, which causes the dissolved ammonia in dewatered cake to
volatilize. Although the odours tend to dissipate quickly, the ammonia levels in mixing and 7730
drying areas can be high if the gas is not collected and treated.
Ammonia is a colourless gas with specific irritant odour which when compressed liquefies at
room temperature. Effects on the human body include irritation of the mucous membrane and
breathing organs. 7735
Ammonia typically appears in the dewatering processes and in the solids created from
dewatering. This is especially true in digested solids. However, the small quantity of ammonia
in sewage off gas at neutral pH contributes little to odour emissions, because the odours
typically are dominated by sulphur compounds. Therefore, it rarely is necessary to provide an 7740
ammonia removal step in treating off gas from liquid sewage treatment processes, unless lime or
other alkaline material is used in the process to elevate the pH.
- Proper measurements should be performed in accordance with IS11255 Part 6.
measurement.
In the dewatering process, do not increase the pH.
Generally, deodorisation equipment is effective in controlling ammonia, similar to H
However, care is necessary since there is selectivity depending on the substance.
8.2.5 General Method of Prevention of Odour
Following is a short checklist of operational considerations for controlling odours of primary
treatment facilities: (May also apply in other facilities)
- Remove scum routinely, with increased frequency during warm weather. 7755
- Remove sludge before it can bubble or float.
- Wash weirs and other points where floatable and slime collect. Some facilities use
submerged pipes with holes rather than effluent troughs. The submerged pipes do not
splash the primary effluent, thereby reducing the release of hydrogen sulphide.
- Wash down all spills and grease coatings. 7760
- When draining a tank, immediately flush it completely. If sludge does not drain
quickly, spray lime, calcium hypochlorite, or potassium permanganate on the sludge
surface to reduce odours. Because even a clean tank can produce odours, flushing the
tank with a chlorine solution or keeping the tank floor covered with a low
concentration of chlorine solution will reduce odours. 7765
- If the sewage is septic, add chemicals in the collection system or at the plant, as
appropriate, to reduce sulphides.
- If tanks are covered for odour control, keep plates and access hatches in place.
- Routinely check any odour scrubbers or deodorizers for plugging, adequate supply
of chemicals, proper pressures for misting, and/or effectiveness of carbon. 7770
- The splashing of primary effluent into weir troughs and effluent channels can result
in the release of hydrogen sulphide. If possible, try to minimize the splashing of
primary effluent into the channel or weirs. If it cannot be accomplished operationally,
then installing submerged effluent pipes may be necessary. This will require tank
modifications to verify the plant hydraulics and provide proper control to avoid 7775
fluctuations in the tank levels.
- Minimize the stripping of hydrogen sulphide from the sewage when using channel
air diffuser systems.
Adoption of the following regular practices will not only increase removal efficiency, but will
provide better working conditions for the operator: 7780
- Regularly remove accumulations from the inlet baffles and effluent weirs with a
hose or a broom with stiff bristles. Only experience will determine the necessary
frequency.
- Clean scum removal equipment regularly; otherwise obnoxious odours and an
unsightly appearance will result. 7785
- Keep cover plates in place except when operations or maintenance require their
removal.
- Immediately flush and remove all sewage and sludge spills. Avoid hosing down
motors and enclosed control devices.
- Establish a housekeeping schedule for the primary treatment area, including 7790
galleries, stairwells, control rooms, and related buildings, and assign responsibility
for each item to a specific employee.
- Repaint surfaces as necessary for surface protection and appearance.
8.2.6 Chemical Addition
Chemical addition can control odours in sewage treatment plant by preventing anaerobic 7795
conditions or controlling the release of odorous substances. These chemicals fall into four basic
groups based on their mechanisms for odour control, shown in Table 8-1 to Table 8-4.
The effectiveness of chemical addition as an odour control technology depends on such
variables as cost, dosage, presence of odour-causing compounds, effects of chemical
accumulations in sludge and process waters, equipment maintenance, space limitations, and 7800
safety or toxic substance concerns. Typical odour-control applications include collection
systems, headworks, primary clarifiers, process sidestreams, aeration tanks, solids-handling
applications, and storage lagoons. In general, it is more cost-effective to treat odours in the
liquid phase than in the vapour phase. Common chemical agents used to control odours include
iron salts, hydrogen peroxide, sodium hypochlorite (chlorine), potassium permanganate, nitrates, 7805
and ozone.
Table 8-1 Chemicals used for liquid-phase odour control
Chemical Effective against
Ozone Atmospheric hydrogen sulphide only
Hydrogen peroxide Hydrogen sulphide, also acts as an oxygen source
Chlorine Hydrogen sulphide and other reduced sulphur compounds
Sodium and calcium hypochlorite Hydrogen sulphide and other reduced sulphur compounds
Potassium permanganate Hydrogen sulphide and other reduced sulphur compounds
Raising the oxidationreduction potential
Higher temperatures increase microbial action of
anaerobic bacteria and increase the release of volatile
organic compounds from the liquid to the gaseous phase.
Bactericides
Kill or inactivate anaerobic bacteria
pH modifiers
Lime Prevent offgassing of hydrogen sulphide; at a very high
Sodium hydroxide pH acts as a bactericide on sewer wall slimes
Table 8-2 Liquid process operational emissions control
Process Problems Control measures
Preliminary treatment
Coarse bar screens
Influent sulphide are stripped by
turbulence inherent to these
Upstream chemical addition. Recycle
return activated sludge (RAS) to
headworks.
Containment and ventilation to a
vapour-phase control system.
Fine bar screens
Primary clarifiers
Sulphide formed in basins
during holding. Sulphide and
Remove unneeded basins from service.
Raise water level in flume to decrease
stripped at weirs. Sulphide
forms in settled solids.
weir drop.
Pump sludge more often. Avoid
cosettling of sludge. Add iron salts
directly or upstream.
Odour from residual solids in
flow equalization basins.
Install collection and removal
equipment and flush solids with
high-pressure hoses after
each basin dewatering.
Trickling filters Influent sulphides stripped at
distributors. Sulphide formed
when overloaded or oxygen
deficient.
Add iron salts upstream. Limit loading.
Provide power ventilation. Slow
distributors or increase wetting rate to
maintain a thin aerobic film.
Rotating biological
contactors (RBCs)
Aeration basins
Influent sulphide stripped at
head of basin. Sulphides form
when oxygen deficient.
Decrease aeration at head of basin.
Fine-bubble diffusers cause less
stripping than coarse bubble. Pure
oxygen causes lowest odour emissions.
Table 8-3 Solids process operational emissions control
Gravity thickeners
Co-thickening biological and
primary sludge causes sulphide
Long detention under anaerobic
conditions is problematic.
Avoid co-thickening, if multiple basins
are available. Use direct chemical
treatment to reduce sulphide formed
during thickening.
Provide containment and ventilation to
a vapor-phase control system.
Dissolved air
Aeration strips sulphide and
odours from sludge.
Use chemical pretreatment to remove
sulphide from sludge before
Belt presses
Pressing strips sulphide from
feed sludge into belt press room.
Potassium permanganate or hydrogen
peroxide will treat sulphideand other
odorous.
The H
S formed in the process
corrodes combustion equipment.
Air quality is a concern, because
S is converted to sulphur
dioxide during combustion.
Maintain proper temperature and pH in
the process. Add iron salts directly to
the digester, at headworks, or at
primary clarifiers.
Aerobic digesters
Odorous compounds form when
process is overloaded or oxygen-
deficient
Provide adequate aeration and mixing
to maintain aerobic conditions.
Feed at uniform organic loadings.
Lime stabilization
Ammonia is released due to high
pH.
Vent ammonia to outside unless
concentrations are very high or the site
is in a sensitive area.
Table 8-4 Summary of odour control technology applications at sewage treatment facilities.
Methods Effects Problems
Operating practices
Industrial process changes
Lower waste temperature
Hydrogen sulphide evolution much
Pretreat to remove odorous
Collection system
Hydrogen sulphide reduction Aeration
Grit chamber
Daily grit washing General odour reduction
Increase frequency of solids and
scum removal
General odour reduction
Aeration tanks
Remove solids deposits
General odour reduction Increase aeration to maintain
dissolved oxygen at 2 mg/L
Trickling filters
Increase recirculation rate
General odour reduction Keep vents clear
Check underdrains for clogging
Check waste gas burner
General odour reduction Relief valves should close
tightly
Maintain constant loading
Maintain adequate aeration
Liquid-phase control alternativesChemical addition
Oxidizes water-insoluble odorants
into water-soluble projects
Requires onsite
Controls slime growth; precipitates
sulphide; enhances settling
Increases solids
Nitrates Inhibits production of sulphides Costly
pH adjustment
Alkali: NaOH
pH 8 hinders bacterial growth in
sewers and retards evolution of
hydrogen sulphide.
Acid: HCl or H
Acid combines with basic ammonia
and amines
Chlorine (gas and hypochlorite)
Inhibits growth of sulphate-reducing
bacteria in sewers; oxidizes
hydrogen sulphide and ammonia
Reacts with sulphide and other
organics to reduce odour
8.2.7 Monitoring 7815
Regular monitoring of treatment processes can prevent many odour releases as well as provide
valuable information on operating procedures.
8.3 Epidemiological Pollution
Potentially pathogenic aerosols are generated as a result of the physical processes of aeration,
trickling, and spraying sewage and sludge. 7820
The density of microorganisms in aerosols is a function of the density of a specific organism in
the sewage, aeration basin, or sludge, the amount of material aerosolized, the effect of aerosol
shock (impact), and, finally, biological decay of the organisms with distance in the downwind
In a sewage treatment plant, there are commonly either stagnant anaerobic conditions or an 7825
aerated mass of heated microbial material. With the use of activated sludge as a standard
treatment process, the operators walk above and around a cauldron of airborne aerosols. They
are often exposed to low-level aerosolized versions of microbes, some of which may be
infectious.
8.3.1 Effects on Health 7830
These aerosols may contain bacterial and viral infectious agents, and infections may result from
contact with these aerosol mists. It is impossible to eliminate all sources of aerosol
contamination in a sewage treatment plant.
The immune systems of many operators build up antibodies to a variety of bacterial and viral
infectious agents. They become what are nicknamed universal carriers because they are often 7835
in contact with low levels of infectious agents that will not make them ill, but that they can build
immunity to, much like vaccination theory. However, if operators bodies do get run down or
they come into contact with a significant infectious agent, they can easily become ill.(Refer to
9.2.1 Diseases)
8.3.2 Locations of Sources 7840
Aeration tanks, Attached growth system (Figure 8.5)
Figure 8.5 Aeration tank (left) and Attached growth system (right)
8.3.3 Measurement
Perform sampling using filter paper. Then, the samples can be collected on a petridish, and 7845
grown in the laboratory. These samples can later be tested.
The following are the studies to be carried out in an STP:
The total microorganism content of air immediately over the aeration tank liquid surfaces:
a. Decreases exponentially with height at least within the first 100 cm above the
aeration tank liquid surface; 7850
b. Approaches background concentrations by extrapolation of current data within 2.5
to 4 m above the aeration tank liquid surface; and
c. Appears to be influenced by several factors, including the mixed liquor suspended
solids concentration of the aeration tanks, bacterial die-off, fallback of larger
particles, and dispersion by wind currents. 7855
In view of the above, one should bear in mind that aerosol increases closer to aeration tanks and
also increases as one goes downwind.
8.3.4 Preventive Measures
- Cover the aeration tanks and attached growth systems. (To prevent diffusion of
aerosols) 7860
- Plant tall trees around tanks to prevent diffusion of aerosols.
- Stop using surface aerators and use diffused aerators.
8.3.5 Control
Controlling epidemic microbes in sewage is difficult. The above mentioned preventive measures
are desirable. 7865
8.4 Soil Contamination
Soil contamination has become an issue in recent years. A sewerage facility that plays an
important role in conserving the environment cannot become a source of contamination itself.
Sewage leaks sometimes become issues. To prevent leaks in an STP, the following are
necessary: 7870
a. Check for leaks in every facility. (High probability of leaks in pipe parts and
connections between facilities.)
b. Even if there are no leaks in the facility, there may be fissures or cracks. If so,
immediately repair the same on site. If this is not possible, discuss with the Plant
Engineer and make arrangements to get the defects repaired. 7875
c. Sometimes, sewage overflows. In such a case, check whether the flow rate to the
facility is greater than the design flow rate.
d. If the flow rate is below the design flow rate, there is a possibility of clogging in the
stage after the problem location. The cause of the clogging should be eliminated.
e. Since sludge is thicker than sewage, it is likely to cause clogging in the sludge 7880
treatment facilities. For this reason, care should be taken against leaks from the
sewage treatment facilities. Efforts should be made to eliminate leaks.
f. If the leak is identified clearly, and soil contamination is likely to occur, discuss with
the Plant Engineer to get accurate measurements to be made by an authorized
laboratory of the Ministry of Environment and Forest. 7885
g. It is good to maintain a record of the TDS of ground water in the well waters of
households surrounding the STP so that questions of sewage seeping into ground
water and polluting the well waters can be verified.
8.5 Water Pollution
Surface water quality considerations include compliance with treated effluent standards at the 7890
discharge point with respect to parameters like BOD, suspended and floating solids, oil and
grease, nutrients, coliforms, etc. Special consideration may be given to the presence of public
bathing ghats and intake points for water supply downstream.
Another environmental consideration is the potential for ground water pollution presented by
the treatment units proposed to be built. 7895
Necessary precautions should be taken to prevent water supply contamination due to leakage
from sewers. Appropriate distance between water supply pipes and sewer pipes shall be
maintained. On the other hand, early detection of sewage leakage and the repair is indispensable
by implementing sewer inspections described in 2.2 Inspection and examination for sewer of
this manual. 7900
8.6 Sewage Treatment Plant Beautification and Landscaping
A sewage treatment plant is a facility that handles sewage. The working environment in such a
plant is poor since foul odours are generated. It is therefore essential that a clean environment be
maintained within the sewage treatment plant through daily cleaning of the plant. Within the
boundaries of the premises, open areas should be planted with trees and foul odours should be 7905
dispersed.
Specific measures should preferably be adopted such as providing park-like spaces within the
premises to offer residents a place for relaxation and rest. The treated effluent should be reused
for watering plants and trees within the premises.
Examples of plant beautification are shown in the figures below. Landscape should be 7910
maintained in this way (Figure 8.6).
Source: BBSS Board
Figure 8.6 An example of plant beautification adopted in Bangalore
8.7 Regulation of Greenhouse Gas 7915
8.7.1 Greenhouse Gas
Carbon dioxide (CO
), methane (CH
), nitrous oxide (N
O), and greenhouse gases such as
Freon are released in large quantities to the atmosphere with the increasingly energetic activities
of human beings, and the average temperature of the entire earth has increased. This
phenomenon is known as global warming. Global warming of the earth has a serious impact on 7920
the global environment.
) emissions and methane (CH
) generated are issues in STP operation.
a. Carbon Dioxide (CO
Carbon dioxide is formed from the complete combustion of any fuel that contains
carbon (e.g., methane). Any boiler, flare, or power-generation technology that 7925
combusts methane, will produce a corresponding predictable amount of carbon
dioxide.
b. Methane (CH
Methane is the principal component of both digester gas and natural gas. It is a
light gas with a specific gravity of less than 1.0. Methane is an exceptionally 7930
important greenhouse gas; its global warming potential is 21 to 23 times that of
carbon dioxide. From a greenhouse gas perspective, it is vital to completely burn
all methane without atmospheric release.
a. Reduction in the use of fossil fuels (fossil fuels are used even for generating electric 7935
power; so use of power is linked to the use of fossil fuels)
Fuel for operating the sewerage facilities and reduced use of electricity are
essential. (For instance, change from continuous operation to intermittent operation,
if possible)
b. Measures against generation of CH
from the treatment plant (since generation of 7940
this gas cannot be inhibited, conversion of the generated gas may be considered)
Since the generation of CH
is linked to global warming, checks on the CH
collection facility
and leaks in piping are very important.
If there is damage or signs of damage, repairs should be carried out quickly.
8.7.3 Effective Use of Biogas 7945
Biogas includes organic matter made of carbon, hydrogen, sulphur, and so on, and is a potential
energy source of high value.
Presently, the main uses are as follows.
- Used in dual fuel engines. A part of the power requirements of a sewage treatment
plant can be satisfied by generating power using biogas explained in clause 6.5.15.3 7950
and its sub clauses in Part A.
8.8 Carbon Credit Record
This is a term that qualifies the holder to emit one ton of carbon dioxide into the atmosphere and
is awarded to institutions or countries that have reduced their greenhouse gases below their
emission quota, which literally means emission standards. These carbon credits can be traded in 7955
the international market at their current market price.
For details, refer to Sec. 5.15 CARBON CREDIT of Part A(Manual).
In sewage treatment plants to meet the requirements, the following are to be mainly performed:
- An example of the Clean Development Mechanism (CDM) project in sewerage
facilities is biomass power generation. This project focuses on CH
generated from 7960
the facilities.
- For the project, the baseline CO
emissions must be studied. Based on this study, the
emissions in the base year and the reduction in CO
emissions thereafter are
considered for approval of carbon credits.
- Accurate data during operation is required for specifying the baseline. 7965
- Data collection and retention of accuracy of measuring instruments are the necessary
items on site.
- STP power and flow rate data are mainly collected through SCADA. (Refer to
Sec.6.6 SCADAsystem)
- Even after the project is approved, data collection and maintenance of measuring 7970
equipment will continue.
STPs are intended primarily for improvement of water environment. They should be operated so
as not to affect surrounding area, and to avoid water pollution and odour problems. In addition
to prevention of air pollution and soil contamination, improvement of the surrounding 7975
environment such as planting or landscaping is also required.
Part B: Operation and Maintenance CHAPTER 9: OCCUPATIONAL HEALTH
HAZARD AND SAFETY MEASURES
CHAPTER 9 OCCUPATIONALHEALTH HAZARDS AND
The sanitation workers, who are engaged in operation and maintenance of sewerage system or 7980
septic tanks, are exposed to different types of occupational hazards like injuries caused by
physical actions and chemicals contacts, infections caused by pathogenic organisms in sewage
and dangers inherent with oxygen deficiency, hydrogen sulphide, and combustible gases.
The Parliament of India is considering Sanitation Workers (Regulation of employment and
conditions of service) Bill, 2012. This bill will be helpful in eliminating the risk to the health 7985
and safety of sanitation workers.
As defined in the said bill, employers are obligated to provide their employees with safety
equipment or protective gears (See 9.2.1.2 for details) as well as cleaning devices and ensure
observance of safety precautions appropriate for each hazardous condition to reduce the
employees risks to health and safety. Moreover to guard against human error and carelessness, 7990
proper safety training and adequate effective supervision by safety personnel are most essential.
Although the Government of India enacted the Employment of Manual Scavengers and
Construction of Dry Latrines (Prohibition) Act, 1993, which declared the employment of
scavengers or the construction of dry latrines to be an offence, considering these, a new bill
titled The Prohibition of Employment as Manual Scavengers and their Rehabilitation Bill, 7995
2012 was prepared and introduced in the Parliament in September 2012. The Bill aims to
eliminate manual scavenging and insanitary latrine, and also provide for proper rehabilitation of
manual scavengers in alternative occupations so that they are able to lead a life of dignity.
In addition to the Acts mentioned above, employees shall follow to Contract Labour
Regulation and Abolition Act, 1970 for secure operational health and safety at their sites. 8000
Operation and maintenance of sewerage facilities, which should not be discontinued at any
moment, requires health and safety consciousness equal to or greater than one that is needed for
construction projects.
In India, health and safety policy is defined in construction project management by BIS
(Refer to Appendix 9.1). 8005
Therefore, the same health and safety policy for construction projects may be also adopted for
operation and maintenance of sewerage facilities.
STPs and PSs are subject to safety audits which confirm the status of safety and health
organizational setup, education / training, provision / inspection of personal protection, and
records of safety, to ensure occupational safety and health at the work sites. The plant engineer 8010
should rectify failures immediately, if any. The audit shall be implemented as per IS: 14489
(1998) Code of Practice on Occupational Safety and Health Audit. Standard safety audit
procedures of the inspectorate of factories shall be at a frequency of a month and compliance
reported to that agency.
9.1.1 Diseases 8015
Workers for sewerage systems and onsite systems face the risk of various health problems by
virtue of their occupation since they are exposed to a wide variety of chemicals,
micro-organisms and decaying organic matters that comprise sewage. Table 9.1 shows types of
diseases and their causes.
Table 9.1 Types of diseases and their causes 8020
Diseases Causes
- Leptospirosis
- Hepatitis
- Helicobacter pylori
- Tetanus
- Diphtheria
Pathogen present in sewers or sewage
Dermatitis Chemicals, mineral oil and tar
Respiratory symptoms Endotoxins, Bioaerosols
Source: IJOEM, 2008
9.1.2 Accidents
Workers for sewerage systems and onsite systems such as below are exposed to risk of accidents
during work. This chapter deals with oxygen deficiency, hydrogen sulphide poisoning, and
dangers of combustible gas as confined space hazards*. 8025
* Confined spaces are locations in an STP where fatal accidents frequently occur. A confined
space is defined as a space with: (1) Cramped entry and exit; (2) Absence of broad daylight and
ventilation; (3) Access is meant for very limited persons such as one or two persons. Possible
hazards such as below are considered to be common among onsite and sewerage works.
- Confined space hazards 8030
Oxygen deficiency / Hydrogen sulphide poisoning /Combustible gas
- Chlorine poisoning
- Slip
- Electric shock 8035
- Fire
In this subsection, workplaces are categorized into the following five locations:
- On-site
- Sewer system
- Pumping station 8040
- Sewage treatment plant
- Water and wastewater quality test laboratory
Possible hazards are listed for each workplace with subdivided locations in Table 9.2 by
Table 9.2 Possible hazards by locations
Fall Slip
Anaerobic filters
Closed drains
Pumping station and STP
Wet / dry wells
Settling tanks
Biological reactor
UASB reactors
Anaerobic lagoons
Sludge thickeners
Sludge digesters
Sludge drying beds
Disinfection devices
Possible hazards at wastewater quality test laboratory include toxic substances, alkalis /acids
and glass appliances.
Details of the said accidents are described below.
9.1.2.1 Confined Space Hazards 8050
Possible hazards in confined space include oxygen deficiency, hydrogen sulphide poisoning,
and danger of combustible gases.
9.1.2.1.1 Risk of Oxygen Deficiency
- When oxygen concentration (See 9.2.1.1 for details of measurement method) drops
below 17%, shortness of breath may occur. 8055
- If the concentration reduces further, consciousness may be lost.
- When the oxygen concentration drops below 10%, death may result.
Table 9.3 shows the change in symptoms of anoxia due to drop in oxygen concentration.
Concentration (%)
Symptoms of anoxia
Human begin to suffer adverse health effects when the oxygen level of their
breathing air drops below 19.5%.
Workers engaged in any form of exertion can rapidly become symptomatic as
their tissues fail to obtain the oxygen necessary to function properly.
Increased breathing rates, accelerated heartbeat, and impaired thinking or
coordination occur more quickly in an oxygen-deficient environment. Even a
momentary loss of coordination may be devastating to a worker if it occurs
while the worker is performing a potentially dangerous activity, such as
climbing a ladder.
Concentration of 12 to 16% oxygen causes increased breathing rate,
accelerated heartbeat, and impaired attention, thinking and coordination, even
in people who are resting.
At oxygen levels of 10 to 14 %, faulty judgement, intermittent respiration, and
exhaustion can be expected even with minimal exertion.
Breathing air containing 6 to 10% oxygen results in nausea, vomiting,
lethargic movements, and perhaps unconsciousness.
Breathing air containing less than 6% oxygen produces convulsions, then
cessation of breathing, followed by cardiac arrest. These symptoms occur
immediately. Even if a worker survives the oxygen deficiency, organs may
show evidence of oxygen-deficiency damage, which may be irreversible.
Source: OHSA 8060
9.1.2.1.2 Risk of Hydrogen Sulphide Poisoning in Confined Space
Hydrogen sulphide is extremely toxic. Sometimes it may be generated in high concentration in a
sewage treatment facility, which causes immediate death.
- Hydrogen sulphide enters the body through eyes or mucous membrane of breathing
organs. 8065
- Blood seeps out from the capillaries in cavities of the lungs, causes pulmonary
oedema, leading to breathing difficulties and death by suffocation.
- In sewer facilities, it is generated in rising mains with no oxygen supply and in
inverted siphons, etc., where sludge is likely to accumulate easily.
- It is generated in grit chamber, pumping well, sedimentation basin, and sludge 8070
thickening tank in sewage treatment plants.
- Hydrogen sulphide generated in sewage and deposited sludge is sealed within and in
the static condition, so it does not disperse to the atmosphere easily. However, when
agitated, it disperses all at once to the atmosphere.
The relationship between concentration of hydrogen sulphide gas and its toxic effect is shown in 8075
Table 9.4 Relationship between concentration of hydrogen sulphide and its toxic effects
Conc. (ppm) Effects and reaction by organ
Sense of odour
Sensitive persons can
sense the odour (limit of
sense of odour)
Anybody can sense the
Foul unpleasant odour of
medium strength
10 Permissible concentration (lower limit for irritation of
the mucous membrane of the eye)
20 to 30 Although bearable, after
getting accustomed to the
odour (olfactory fatigue),
any higher concentration
cannot be sensed.
Breathing organs
Lowest limit for irritating
the lungs
Conjunctivitis,
itchiness, pain in the
eyes, feeling of sand in
the eye, glare,
100 to 300 Olfactory nerve paralysis
for 2 to 15 minutes; feels
like unpleasant odour has
reduced.
If exposed continuously for
8 to 48 hours, bronchitis,
pneumonia, and death by
suffocation due to
pulmonary oedema
170 to 300 Scorching pain in the
mucous membrane of
respiratory tract; if exposure
is less than 1 hour (limit),
serious symptoms may not
bloodshot eyes and
swelling, turbidity of
cornea, corneal
damage and separation,
bending and haziness
of field of vision,
increase in pain due to
350 to 400 Exposure for 1 hour or more
may lead to loss of life
600 Exposure for 30 minutes
hour may lead to loss of life
700 Cerebral nerves
After excessive respiration for a short period, respiratory paralysis occurs
immediately thereafter
800 to 900 Loss of consciousness, respiratory arrest, death
1000 Swoon, respiratory arrest, death
5000 Instantaneous death
Source: JSWA, 2003
9.1.2.1.3 Risk of Combustible Gas in Confined Space
- Combustible gas includes methane, gasoline, thinner, and so on. 8080
- These gases become a mix of explosive gases in locations such as sewers.
- The minimum limit of concentration for explosion to occur is 5% for methane and
1.3% for gasoline.
- Combustible gasoline, thinner and so on, float on the surface of water and volatilise
at room temperature, so they are dangerous. 8085
- If large quantity of gasoline flows into a sewer, there is a possibility of large
explosion to occur.
- At locations where sewage is likely to accumulate such as in sewers, toxic or
explosive gases or vapours may be generated.
9.1.2.2 Risk of Chlorine 8090
- 30 ppm causes coughing; 40-60 ppm is dangerous in 30 minutes.
- 1,000 ppm is apt to be fatal in a few breaths.
- Chlorine gas has specific weight that is 2.49 times heavier than air, is a yellowish
green gas and is a strong irritant.
- Although its disinfecting effect is high, its toxicity is also high. 8095
9.1.2.3 Fall
- Accidents frequently occur while climbing/descending ladders.
- Accident often occurs while working at high places.
9.1.2.4 Slip
Slippery surfaces are often encountered when working in an STP and sewers. 8100
9.1.2.5 Electrical Shock
Electric shocks occur because of the following:
- Exposure of live parts and defects such as damage to insulating sheath.
- Inappropriate work such as forgetting to use insulated protective gear, touching live
parts by mistake, etc. 8105
Table 9.5 shows the effects on the human body subjected to an electric shock, while Table 9.6
shows the relationship between magnitude of shock current and duration.
The electric shock level when a person suffers an electric shock may be quantified by the
equation below.
Electric shock level = Shock current (mA) x duration (s) 8110
Table 9.5 Effects on the human body when subjected to electric shock
a Mild sensation, but not painful
b Painful shock but muscle still in control
c Muscle control affected
d Muscle contraction , breathing affected
Rapid, uncoordinated series of contractions of heart muscle causing irregular heart beat
(fibrillation) and possible death
f Severe burns, muscle contractions, stoppage of heart, death certain
Table 9.6 Relationship between shock current and duration
Magnitude of Current,
50 Hz rms value
Below 10 mA a 10s
10 mAto 15 mA b <1s c 1s-2s d 2s-10s
15 mAto 20 mA b <0.5s c 0.5s-2.0s d 2s-10s
20 mAto 40mA b <0.15s c 0.1s-2.0s d 2s-10s
40 mAto 80 mA b <0.05s c 0.05s-0.06s d 0.6s-1s e 1s-3s f 3s-
Above 100 mA b <0.04s c 0.04s-0.05s d 0.5s-0.8s e 0.8s-1.9s f 1.9s-
Source: Khanna Publishers
9.1.2.6 Fire
Burns can be very serious and can cause painful injuries. Structural damage from fires can be 8115
very costly.
The three essential ingredients of all ordinary fires are:
- Fuel - paper, wood, oil, solvents and gas.
- Heat - the degree necessary to vaporise fuel according to its nature.
- Oxygen - normally at least 15 percent of oxygen in the air is necessary to sustain a 8120
fire. The greater the concentration, the brighter the blaze and more rapid the
combustion.
9.1.2.7 Risks in a Wastewater Quality Test Laboratory
9.1.2.7.1 Toxic Substances
Persons working in the water quality test laboratory use various chemicals including toxic 8125
substances. Inhalation of excessive steam, gas or dust, etc., in the course of their work, is
harmful to health so these persons should take adequate precautions.
Typical toxic substances used in a water quality test laboratory and their toxicity are given in
Table 9.7 Toxicity of chemical used in water quality laboratory 8130
Type Symptoms and poisons
Chemical asphyxia
Hydrogen cyanide, cyanogen compounds
- Inactivation of haemoglobin in blood and oxygen associated with
breathing leads to asphyxia.
- Likely to occur easily during a reaction; permissible quantity is very
small so care is required.
Caustic action
Arsenic compounds
- Irritates mucous membrane of breathing organs, and at high
concentration, breathing is arrested.
Systemic poisoning
- Absorbed from an alimentary canal, the skin, and respiratory organs
- Diarrhoea, vomiting, and shock in 30 minutes in serious cases
(Arsenious anhydride)
Inorganic mercury
- Sharp pain in the stomach or the abdominal region, vomiting, and
bloody urine
Metal mercury
- Easily evaporated and inhaled.
- Trembling of hand and foot, and consciousness disorder
Organic phosphorus
- Absorbed from the skin, alimentary canal, and respiratory organs
- Diarrhoea, vomiting, spasm, and consciousness disorder.
9.1.2.7.2 Alkali / Acid
Acids and alkalis used in the water quality test laboratory include:
- Hydrochloric acid
- Sulphuric acid 8135
- Nitric acid
- Sodium hydroxide, etc.
9.1.2.7.3 Glass Appliances
Glass appliances can be broken if they are not handled carefully, and may lead to an injury.
9.1.3 Instances of Accidents 8140
An instance of accident related to anoxia (due to oxygen deficiency) is described below.
- Type of work: Sewerage work
- Number of casualties: 2 dead, 1 employee in serious condition
This casualty occurred when lifting the drain pump from the manhole during sewerage work.
On the day of the occurrence, storm water pipes were being replaced. 8145
- Worker (A) and worker (B) opened the manhole cover to perform work in the
manhole, and entered the manhole through the trap.
- After a while, when Worker (C) looked into the manhole, he found the workers (A)
and (B) had collapsed.
- He notified the other workers. Worker (C) rushed into the manhole for rescue and 8150
called out but there was no response.
- Soon after, worker (C) also collapsed on the spot.
- Later, workers (A), (B) and (C) were rescued by the rescue team and taken to the
- Workers (A) and (B) lost their lives, while the rescued worker (C) was admitted to 8155
the hospital with hypoxic encephalopathy (brain damage from lack of oxygen).
- Workers (A) and (B) were diagnosed with anoxia.
The probable causes of the accident were as follows:
- Was not aware that the location had the risk of oxygen deficiency.
- Did not measure the oxygen concentration and did not ventilate the manhole before 8160
entering it.
- Did not impart education and implement rescue drills related to work at dangerous
locations with oxygen deficiency.
- No safety guard was stationed.
9.2 Safety Aspects and Measures 8165
Measures to protect workers from accidents are mentioned in 9.2.1 Preventive measures are
taken to prevent accidents, and 9.3.2 Corrective measures are adopted when accidents occur.
Preventive measures and corrective measures against accidents are described below.
9.2.1.1 Hazard-specific Preventive Measures 8170
9.2.1.1.1 Confined Space Hazards
The potential for build-up of toxic or combustible gas mixture and/or oxygen deficiency exists
in all confined spaces. Characteristics of common gases causing hazards are shown in Appendix
9.2. Refer to Table 9.2 for possible confined spaces related to sewerage works.
Follow the Confined space entry procedure shown in Figure 9.1 and Appendix 9.3. Entry into 8175
confined space should not be permitted until the space is ensured to be safe as specified in Table
Table 9.8 Acceptable entry condition
Oxygen 19.5% and more
Hydrogen sulphide Less than 10ppm
Combustible gases Less than 10%
Figure 9.1 Confined space entry procedure 8180
a. Measurement Method
An example of a portable toxic gas detector that can measure oxygen, combustible gas,
carbon dioxide, and hydrogen sulphide simultaneously is shown in Figure 9.2 and is given
- Before measuring the confined space atmosphere, perform zero error correction of 8185
the instrument at a location where there is fresh air (no gas in the vicinity).
- Measure the atmosphere within the space to confirm if any hazard exists as given
Oxygen : Less than 19.5%
Hydrogen sulphide : 10 ppm or more 8190
Combustible gases : 10% LEL(lower explosive limit) or more
- Measurement should be done at three locations top, middle and bottom of the
confined space since the oxygen concentration differs according to the position.
- Record the measured results on Confined space pre-entry checklist (Appendix 9.4)
Source: PRISM GAS DETECTION PVT. LTD.
Figure 9.2 Portable toxic gas detector
b. Ventilation Method
Check atmosphere
Monitor atmosphere
Confined space entry permit
Enter the confined space
Entrants should be thoroughly trained.
Entrants should use proper wear and be
equipped with protective devices.
Safety guard should be stationed.
Ventilation should be continued during the work.
Atmosphere should be monitored during the work.
Permit should be issued for each entry by
the work supervisor.
Evacuation procedure should be prepared.
Oxygen (%)
Combustible gas (%LEL)
Carbon dioxide (ppm)
Hydrogen sulphide (ppm)
If the measured results indicate one or more of the following hazards, be sure to ventilate
the location before starting work. 8200
- Oxygen : Less than 19.5%
- Hydrogen sulphide : 10 ppm or more
- Combustible gases : 10% LEL or more
To ensure the atmosphere is safe during the work, operate the ventilation equipment
continuously. 8205
Bring the blower outlet end close to the workplace and continue to blow air at the rate of 10
/minute per person or greater.
c. Provisions for Evacuation
The work supervisor should make the following arrangements:
- Keep ready breathing apparatus, ladder, rope, safety belt and other equipment for use 8210
in evacuating or rescuing workers in the event of an emergency.
- Inspect protective gear before start of work and ensure that they are ready for use at
all times. Repair or replace gear and equipment that are defective.
- Bear in mind that gas protection mask or dust protection mask is ineffective against
anoxia. 8215
- Non-spark tools should be used in confined spaces.
d. Stationing of Safety Guard
The work supervisor should station a safety guard to detect abnormality at an early stage
and to take immediate and appropriate action.
- The safety guard should be stationed outside the opening if the situation inside the 8220
confined space can be monitored from the outside.
- The safety guard should check access to the workplace of the workers engaged in the
9.2.1.1.2 Chlorine Poisoning
- Store chlorine containers in a cool and dry place away from direct sunlight or from 8225
heating units.
- Wear a face shield when changing chlorine containers.
- As chlorine is approximately two and half times heavier than air, vents should be
provided at floor level.
9.2.1.1.3 Fall 8230
The work supervisor and worker should take the following precautions to prevent persons from
falling into manholes, etc.:
- Ensure that nobody falls from ladders (including metal rungs) and that tools are not
dropped from ground level.
- Wear uniform suitable for the work and wear the necessary protective gear. 8235
- Check that the ladder to the manhole is not corroded or worn out.
9.2.1.1.4 Slip
- Special anti-skid shoes with metal cladding over the fingers portion should be
provided by the employer for the workers. These shoes should be used by the
workers only within the STP. 8240
- Keep floor of workplaces dry and free from grease or oil.
9.2.1.1.5 Electric Shock
- Exposure of live parts and defects such as damage to insulating sheath
- Inappropriate work such as forgetting to use insulated protective gear, touching live 8245
parts by mistake, etc.
Measures to prevent electric shock are as follows:
- Methods for safe handling of electric equipment should be drilled into the workers
and inspection and maintenance methods for electric equipment should be
established. 8250
- Special precautions should be taken to prevent electric shocks at locations where
water is likely to accumulate (grit chamber, pumping room and in pipe gallery).
Rubber-soled sports shoes may be used to prevent electric shocks.
a. Electric Room
Access to the electric room should be prohibited to all except authorized personnel. Signs 8255
should be put up indicating danger when current is flowing into the room.
The electric room should be managed by the procedure below:
- Do not place combustible items near exposed wiring and electric equipment.
- Install fire extinguishers at easily visible locations such that they can be used
immediately in the event of a fire. 8260
- If there is excessive lightning, do not approach equipment wiring and lightning
arrester.
- Periodically inspect and store disconnecting switches, operating rods, insulating
plates, etc., at their specified positions.
- Store electric torch at its specified position such that it can be used immediately 8265
during an emergency such as power failure.
- Place insulating mat on the floor in front of the MCC Panel to prevent electric
shocks.
b. Equipment Repair
Before repairs of equipment or wiring, permission should be taken from the plant engineer 8270
(Refer to 9.4.2) and the work supervisor should hold a meeting and decide the work
procedure. Repair work on electrical equipment should be prohibited to all except
authorized personnel.
- Cut off circuits of equipment to be repaired before repairs.
- Put up sign indicating not to switch on power, assign a person to monitor the power 8275
switch, and strictly enforce power ON/OFF controls.
- Before starting the repair, always detect the voltage using a voltage detector or tester.
- Electric shock due to fault in cable run may also be considered; always ground the
equipment before performing work.
- Operate switches such that you do not receive an electric shock. 8280
- If a power capacitor exists, thoroughly discharge the remaining charge before
starting work.
- If equipment breaks down at night, and if there is no emergency generator at the
workplace, the worker should performrepairs during day time when there is daylight
and not during night time. 8285
9.2.1.1.6 Fire
Every facility should develop a fire prevention plan with input from the local fire marshal, fire
chief, and/or insurance company. The plan may be very simple or very complex, depending on
the specific facility needs. Some items which may be included in any plan are:
- Regulate the use, storage and disposal of all combustible materials/substances. 8290
- Provide periodic clean-up of weeds or other vegetation in and around the plant.
- Develop written response procedures for reacting to a fire situation to include
evacuation.
- Provide required service on all fire detection and response equipment (inspection,
service, hydrostatic testing). 8295
- Routinely inspect fire doors to ensure proper operation and unobstructed access.
- Immediately repair, remove or replace any defective wiring.
- Restrict the use of any equipment which may provide a source of ignition in areas
where combustible gases may exist.
- Maintain clear access to fire prevention equipment at all times. 8300
9.2.1.2 Personal Protection and Protective Devices
9.2.1.2.1 Head Protection
- All personnel working in any areas where there may be danger from falling, flying
tools or other objects must wear approved hard hats. Such hats should be according
to the relevant BIS. Specially insulated hard hats must be worn when working 8305
around high voltage to protect from electrical shock.
- It is advisable to have detachable cradle and sweat bands for two reasons (1) to
permit easy replacement of cradles and sweat bands and (2) to make possible
assignment of one helmet to several workers each with its own cradle and sweat
band for sanitary reasons. 8310
- Once broken, the crown of a hard hat cannot be effectively repaired. It must be
9.2.1.2.2 Face and Eye Protection
Impact Goggles must be worn to protect against flying objects. They can be spectacle or cup
goggles. 8315
Spectacle goggles must have rigid frame to hold lenses in proper position before the eyes.
Frames must be corrosion resistant and simple in design for cleaning and disinfection.
Cup goggles should have cups large enough to protect the eye socket and to distribute impact
over a wide area of facial bones.
- Chemical goggles and acid hoods should be used for protection against splashes of 8320
corrosive chemicals. Ahood treated with chemical-resistance material having a glass
or plastic window gives good protection. There should be a secure joint between the
window and the hood material.
- Face shields can be used against light impact. Plastic shields should be
non-inflammable, free from scratches or other flaws, which introduce distortions. 8325
- Welding masks must be used for protection from splashes and radiation produced by
- Protective creams are used to protect the skin fromcontamination and penetration by
oils, greases, paints, dust etc.
9.2.1.2.3 Hands and Lower Arms 8330
- Protective sleeves, gloves and finger pads are used for different types of hazards and
- Rubber and asbestos gloves should be long enough to come well above the wrist,
leaving no gap between the glove and coat or shirt sleeve.
- Gloves or mittens having metal parts for reinforcements should never be used 8335
around electrical equipment.
- Linemen and electricians working on energized or high voltage electrical equipment
require specially made and tested rubber gloves.
9.2.1.2.4 Body Protection
- A good quality diver suit should be provided to the diver whose services are very 8340
necessary while plugging the sewer line or removal of some hard blockage due to
stone etc. at the mouth of the pipe in the manholes. Depending upon the site
condition, suit should have provision to connect an air line with compressor or
oxygen cylinder.
- Always use rubber aprons when working with chemicals. 8345
9.2.1.2.5 Legs and Feet
- Leggings are provided where leg protection is necessary and are in the same
category as coats, frocks and aprons. Kneepads made of cloth, padding, rubber, cork
are used on jobs where kneeling is required.
- Ordinary work shoes are acceptable for many jobs. They should have non-skid soles 8350
to prevent slips.
- Safety shoes are required where there is danger of dropping tools or materials on the
feet. Toe guards have been designed for the men to wear when operating machines as
air hammers, concrete breakers etc.
- For working on electrical equipment suitable safety shoes must be used. 8355
9.2.1.2.6 Mask
a. Gas Mask
General purpose gas masks are used for respiratory protection from low and moderately
high concentrations of all types of toxic gases and vapours present in the atmosphere in
which there is sufficient oxygen to support life (Figure 9.3). Masks afford necessary 8360
respiratory protection under many circumstances but it is most important to know the
limitations of the various types available and to be familiar with their use. Even when
masks are used properly, other precautions such as never using open flames or creating
sparks in the presence of inflammable gases must be taken. The general purpose gas masks
afford protection against organic vapours, acid gases, carbon monoxide up to 2 percent 8365
concentration, toxic dusts, fumes and smoke. The gas mask consists of a face piece, a
canister containing purifying chemicals, a timer for showing duration of service and a
harness for support. Protection against specific contaminants can be achieved by the
selection of appropriate canisters.
Persons using gas masks should practice regularly with them in order to become proficient 8370
in putting them on quickly and breathing through them. Gas masks cannot be used in
oxygen deficient atmospheres, in unventilated locations or areas where large
concentrations of poisonous gases exist.
Figure 9.3 Gas mask
b. Dust Protection Mask
This is a gas mask that takes air in which fine solid particles are suspended, removes the
fine particles and detoxifies the air after passing it through the particle filter.
c. Respiratory Equipment 8380
In all dusty areas, effective filter masks should be used to guard against specific hazards.
Hose mask should be used by men entering tanks or pits where there may be dangerous
concentrations of dust, vapour, gases or insufficient oxygen. Hose mask with blower and
the airline respirator are used where the hazard is immediate, that is, hasty escape would be
impossible or cannot be made without serious injury if there is failure of the equipment. 8385
Oxygen or air breathing apparatus, that is, self-contained oxygen breathing equipment
using cylinders or bottles of compressed oxygen or air is used where required. This is a
must when the length of the hose pipe in on-line supply of oxygen exceeds more than 45 m.
Gas masks: Canisters consist of a face piece connected by a tube to a canister. Chemicals in
the canister purify contaminated air. No single chemical has been found to remove all 8390
gaseous contaminants. It does not supply oxygen and can be used where there is sufficient
oxygen.
Various types of respirators and their suitability are as follows:
Self-contained breathing apparatus
This apparatus is equipped with a cylinder containing compressed oxygen or air which 8395
can be strapped on to the body of the user or with a canister which produces oxygen
chemically when a reaction is triggered (Figure 9.4). This type of equipment is
suitable for an oxygen deficient atmosphere. It is also suitable for high concentration
of chlorine.
Source: Stylex Fire Protection Systems
Figure 9.4 Self breathing apparatus
Air-line respirator: Air-line length 90 m (max.)
It is suitable in any atmosphere, regardless of the degree of contamination or oxygen
deficiency, provided that clean, breathable air can be reached. This device is suitable 8405
for high concentrations of chlorine, provided conditions permit safe escape if air
supply fails.
9.2.1.2.7 Ear Protection
Where noise levels are high and exceeds specified limits, effective ear-pads or earplugs must be
used. 8410
9.2.1.2.8 Safety Belt
When you work on ladders or scaffolding, use extreme caution to prevent falls. Safety belt
should be used to prevent falls.
9.2.1.2.9 Portable Lighting Equipment
The equipment normally used is portable electric hand lamps of permissible types, electric cap 8415
lamps and explosion proof flash lights.
9.2.1.2.10 Portable Blowers / Ventilating Fan
Replace the air in oxygen deficient and hazardous spaces with fresh air using exhaust fan and
exhaust ducts (Figure 9.5).Ventilation also includes the method of exhausting the air, but
generally the method of blowing in air is more effective. 8420
Source: GVT Engineering
Figure 9.5 Portable blower (ducting blower)
9.2.1.2.11 Safety Fences
Visitors including adults and children visit the STP as part of social studies. For this reason, the 8425
safety management officer should install fences in the facility and ensure a proper route for
visitors so as to prevent any accident.
9.2.1.2.12 Safety Signs
To warn of danger to workers, visitors, and other construction workers in an STP, safety signs
such as shown in Figure 9.6 should be displayed in the STP. 8430
Prohibition (Red circle)
Warning (Yellow triangle)
Obligation (Blue circle)
Source: http://www.safetysignindia.com
Figure 9.6 Safety signs
9.2.1.3 Workplace-specific Preventive Measures
Good design and the use of safety equipment will not prevent physical injuries in sewerage
works unless safety practices are understood by the entire crew and are enforced. Safety 8435
preventive measures specific for the workplace are described here.
9.2.1.3.1 On-site
- Before entering the pit or tank, follow all of the procedures required for work in
confined spaces defined in 9.2.1.1.1.
- When oxygen concentration is less than 19.5% and hydrogen sulphide concentration 8440
is more than 10 ppm, use forced ventilation to ventilate the tank before entering it.
- Wear rubber gloves to prevent wounds from infection.
9.2.1.3.2 Sewer System
a. Traffic Hazards
- Before starting any job in a street or other traffic area, study the work area and plan 8445
your work.
- Traffic may be warned by high-level signs far ahead of the job site.
- Traffic cones, signs, or barricades arranged around the work, or a flagger are
applicable to direct traffic.
oncoming traffic.
Use fluorescent jacket while working along roads (Figure 9.7).
Source: Vibgyor Industries
Figure 9.7 Fluorescent jacket 8455
b. Manhole
Before entering the manhole, follow safety entry procedure. (Refer to Sec. 2.11.1.2
Safety measures to be taken before any manhole entry)
Before entering the manhole, follow all the procedures required for work in confined
spaces defined in 9.2.1.1.1. 8460
Figure 9.8 Photo showing typical confined space entry
When oxygen concentration is less than 19.5% and hydrogen sulphide concentration
is more than 10 ppm, use forced ventilation to ventilate the tank before entering it. 8465
Manhole work usually requires job site protection by barricades and warning
Never use your fingers or hands to remove a manhole lid. Always use a tool
specifically designed for this purpose.
Be alert for loose or corroded steps. 8470
Wear a properly fitted pair of rubber gloves and boots, or an approved substitute that
will provide protection from infection.
Tools and equipment should be lowered into a manhole by means of a bucket or a
basket.
9.2.1.3.3 Pumping Station 8475
Before entering the well, follow all of the procedures required for work in confined
spaces defined in 9.2.1.1.1.
- When oxygen concentration is less than 19.5% and hydrogen sulphide concentration
- Do not work on electrical systems or controls unless you are qualified and authorized 8480
- Guards over couplings and shafts should be provided and should be in place at all
- If stairs are installed in a pumping station, they should have hand rails and non-slip
treads. 8485
- Fire extinguishers should be provided in the station, properly located and maintained.
The use of liquid-type fire extinguishers should be avoided. All-purpose A-B-C
chemical-type fire extinguishers are recommended.
- Good housekeeping is a necessity in a pumping station to prevent slip and fall
accidents. 8490
- Properly secure and lock up an unattended pumping station when you leave so as to
prevent injury to a neighbourhood child and possible vandalism to the station.
9.2.1.3.4 Sewage Treatment Plant
a. Head Works
- Bar screens or racks 8495
Remove all slime, rags, grease, etc to prevent slip and fall accidents. Never
leave rake or other tools on the floor.
Never lean against safety chains.
Always turn off, lock out and tag the main circuit breaker before you begin
repairs. 8500
The time and date the unit was turned off should be noted on the tag, as well as
the reason it was turned off. No one should turn on the main breaker and start
the unit until the tag and lock have been removed by the person who placed
- Pump rooms 8505
If the room is below ground level and provided with only forced-air ventilation,
be certain the fan is on before entering the area.
Guards should be installed around all rotating shaft couplings, belt drives, or
other moving parts normally accessible.
Remove all oil and grease, and clean up spills immediately. 8510
Be sure to provide barricades or posts with safety chains around the opening to
prevent falls.
Until the area has been checked for an explosive atmosphere, no open flames
(such as a welding torch), smoking or other sources of ignition should be
allowed. 8515
Do not work on electrical systems or controls unless you are qualified and
authorized to do so.
- Wet pits or sumps
Before entering the pits or sumps, follow all of the procedures required for work
in confined spaces such as defined in 9.2.1.1.1. 8520
When oxygen concentration is less than 19.5% and hydrogen sulphide
concentration is more than 10 ppm, use forced ventilation to ventilate the tank
before entering it.
For access ladders to pit areas, the application of a non-slip coating on ladder
rungs is helpful. 8525
Watch your footing on the floor of pits and sumps; the floor may be very
slippery.
Tools and equipment should be lowered into a manhole by means of a bucket or
a basket.
Only explosion-proof lights and equipment should be used in these areas. 8530
- Grit channels
Keep walking surfaces free of grit grease, oil, slime, or other material to prevent
slip accidents.
Before working on mechanical or electrical equipment, be certain that it is
locked out and properly tagged. 8535
Install and maintain guards on gears, sprockets, chains, or other moving parts
that are normally accessible.
Before entering the channel, pit or tank, follow all of the procedures required
for work in confined spaces such as defined in 9.2.1.1.1.
When oxygen concentration is less than 19.5% and hydrogen sulphide 8540
Rubber boots with steel safety toes and a non-skid cleat-type sole should be
worn.
b. Clarifiers or Sedimentation Basins 8545
Always turn off, lock out and tag the clarifier breaker before working on the
drive unit.
Maintain a good non-skid surface on all stairs, ladders, and catwalks to prevent
slipping.
When it is necessary to actually climb down into the launder, always wear a 8550
harness with a safety line to prevent a fall accident and have someone
accompany you.
Guards should be installed over or around all gears, chains, sprockets, belts, or 8555
other moving parts. Keep these in place whenever the unit is in operation.
c. Digesters and Digestion Equipment
Methane gas produced by anaerobic conditions is explosive when mixed with
the proper proportion of air.
Smoking and open flames should not be allowed in the vicinity of digesters, in 8560
digestion control buildings, or in any other areas or structures used in the sludge
digestion system.
All these areas should be posted with signs in a conspicuous place which forbid
smoking and open flames.
All enclosed rooms or galleries in this system should be well ventilated with 8565
forced air ventilation. Never enter any enclosed area or pit which is not
ventilated.
Before entering the digester for cleaning or inspection, follow all of the
procedures required for work in confined spaces such as defined in 9.2.1.1.1.
Explosion-proof lights and non-sparking tools and shoes must always be used
when working around, on top of, or in a digester.
When working on equipment such as draft tube mixers, compressors and 8575
diffusers, ensure that equipment is properly valved out, locked out and
appropriately tagged to prevent the gas from leaking.
If a heated digester is installed, read and obey the manufacturer's instructions
before working on the boiler or heat exchanger because there is a risk of
explosion. 8580
Sludge pump rooms should be well ventilated to remove any gases that might
accumulate from leakage, spillage or from a normal pump cleaning.
Good maintenance of flame arresters will ensure that they will be able to
perform their job of preventing a back flash of the flame.
d. Aerators 8585
An operator should never go into unguarded areas alone.
Approved life buoys with permanently attached hand-lines should be accessible
at strategic locations around the aerator.
Operators should wear a safety harness with a life line when servicing aerator
spray nozzles and other items around an aerator. 8590
Lower yourself into the aerator with a truck hoist if one is available.
Be extremely careful when using fixed ladders as they become very slippery.
Watch your footing on the floor of the aerators: the floor may be very slippery.
e. Sewage Ponds
Never go out on the pond for sampling or other purposes alone. Someone 8595
should be standing by on the bank in case of trouble.
Always wear an approved li