P7.3.1.2 Regular manual Low ozone Re-calibrate checks on quality. residual (see sensor. Dosing calibration of Appendix). controller’s Ozone Increase sensor (see sensor E. coli or dose rate DWSNZ:2000, incorrectly after coliforms until Section 3.3.4.7). calibrated. contact detected in 100 recalibration period. mL of treated undertaken. water. Calibration log. Calibration schedule not signed off. 1 The consequences of the event, and therefore the level of risk, will be influenced by the quality of the source water, the effectiveness of treatment processes prior to the ozonation process, and whether a residual disinfectant is added to the water following ozonation. Preventive measures Checking preventive measures Corrective action Causes What to check that action is needed Event: OZONE CONCENTRATION TOO LOW cont’d P7.3.1.3 Adequate checks Low ozone Adjust ozone on ozone quality. residual (see dose Dosing concentration at Appendix). controller controller set Ozone the sampling point setting. to deliver the E. coli or designated in wrong ozone after coliforms DWSNZ: 2000 contact detected in 100 (sections 3.3.1.2.3 period. mL of treated and 3.3.4.7) water. especially during periods of water quality variability. P7.3.1.4 Independent Dose Frequent Re-calculate check on calculation. calculation dose rates Incorrect calculations errors found by and change dose (especially after a checks. settings. calculation. system change Calculation Train staff in and expected dose checks not making dose rates are signed off. calculations. uncertain). P7.3.1.5 Use a dose control Ozone Replace dose method that links controller High ozone dose control to with after more demand ozone residual at contact suitable unit. coupled with the sampling point period. poor dose Manual designated in control. monitoring DWSNZ: 2000 and manual (Sections 3.3.1.2.3 ozone and 3.3.4.7). control during poor water quality episode.
P7.3.1.2 Dosing controller’s sensor incorrectly calibrated. Regular manual checks on calibration of sensor (see DWSNZ:2000, Section 3.3.4.7). quality. Ozone after contact period. Calibration log. Low ozone residual (see Appendix). E. coli or coliforms detected in 100 mL of treated water. Calibration schedule not signed off. Re-calibrate sensor. Increase dose rate until recalibration undertaken. 1 The consequences of the event, and therefore the level of risk, will be influenced by the quality of the source water, the effectiveness of treatment processes prior to the ozonation process, and whether a residual disinfectant is added to the water following ozonation. Causes Preventive measures Checking preventive measures Corrective action What to check that action is needed Event: OZONE CONCENTRATION TOO LOW cont’d P7.3.1.3 Dosing controller set to deliver the wrong ozone Adequate checks on ozone concentration at the sampling point designated in DWSNZ: 2000 (sections 3.3.1.2.3 and 3.3.4.7) especially during periods of water quality variability. quality. Ozone after contact period. Low ozone residual (see Appendix). E. coli or coliforms detected in 100 mL of treated water. Adjust ozone dose controller setting. P7.3.1.4 Incorrect dose calculation. Independent check on calculations (especially after a system change and expected dose rates are uncertain). Dose calculation. Frequent calculation errors found by checks. Calculation checks not signed off. Re-calculate dose rates and change settings. Train staff in making dose calculations. P7.3.1.5 High ozone demand coupled with poor dose control. Use a dose control method that links dose control to ozone residual at the sampling point designated in DWSNZ: 2000 (Sections 3.3.1.2.3 and 3.3.4.7). Ozone after contact period. Replace dose controller with more suitable unit. Manual monitoring and manual ozone control during poor water quality episode.
treatment-ozone-disinfection-feb14-v2.docx