P7.2.2.3 Periodic manual ClO2 ClO2 residual Adjust checks on ClO2 residual concentration controller Dosing concentration, more than 50% of controller’s especially during its MAV. set-point Recalculate periods of water incorrect or Flow rates. Frequent dose rates and quality variability. incorrect dose calculation errors change Dose calculation. Independent check found by checks. settings. calculations. on calculations Calculation Train staff in (especially after a checks not signed making dose system change off. calculations. when expected Flow rates below dose rates are expected values. uncertain). Install visual flow indicators to allow flow to be checked by eye. P7.2.2.4 Use a control ClO2. ClO2 residual Replace dose method that links concentration controller with Low ClO2 dose control to more than 50% of more suitable demand ClO2 residual at its MAV. unit. coupled with appropriate poor ClO2 Manual location. dose control. monitoring and manual ClO2 control when water quality is variable. P7.2.2.5 Check that a ClO2. ClO2 residual Determine the satisfactory ClO2 concentration cause of the ClO2 solution ClO2 level in concentration is more than 50% of high ClO2 strength from the ClO2 produced when new its MAV. concentration generator too solution. supply of chemicals and rectify. high. ClO2 for generation is concentration in Provide first brought into dosing solution is training in the use. too high. preparation of solutions (including calculations). EXCESSIVE FORMATION OF BY-PRODUCTS FROM CHLORINE DIOXIDE TREATMENT Possible hazards: Chlorite, chlorate. Level of risk: Moderate P7.2.3.1 Carry out pilot tests Chlorite. Elevated levels of Modify to determine chlorite and chemical feed ClO2 Chlorate. optimum chemical chlorate. until ClO2 generation feed rates for ClO2 production chemistry not production. maximised, optimised and chlorine Checks to ensure and chlorite chlorite solution minimised. ratio, concentration is correct. Identify staff or pH). training needs Provide staff and provide training in chlorite training. solution preparation. Event: EXCESSIVE FORMATION OF BY-PRODUCTS FROM CHLORINE DIOXIDE TREATMENT cont’d
P7.2.2.3 Dosing controller’s set-point incorrect or incorrect dose calculation. Periodic manual checks on ClO2 concentration, especially during periods of water quality variability. Independent check on calculations (especially after a system change when expected dose rates are uncertain). Install visual flow indicators to allow flow to be checked by eye. ClO2 residual Flow rates. Dose calculations. ClO2 residual concentration more than 50% of its MAV. Frequent calculation errors found by checks. Calculation checks not signed off. Flow rates below expected values. Adjust controller Recalculate dose rates and change settings. Train staff in making dose calculations. P7.2.2.4 Low ClO2 demand coupled with poor ClO2 dose control. Use a control method that links dose control to ClO2 residual at appropriate location. ClO2. ClO2 residual concentration more than 50% of its MAV. Replace dose controller with more suitable unit. Manual monitoring and manual ClO2 control when water quality is variable. P7.2.2.5 ClO2 solution strength from generator too high. Check that a satisfactory ClO2 concentration is produced when new supply of chemicals for generation is first brought into use. ClO2. ClO2 level in the ClO2 solution. ClO2 residual concentration more than 50% of its MAV. ClO2 concentration in dosing solution is too high. Determine the cause of the high ClO2 concentration and rectify. Provide training in the preparation of solutions (including calculations). EXCESSIVE FORMATION OF BY-PRODUCTS FROM CHLORINE DIOXIDE TREATMENT Possible hazards: Chlorite, chlorate. Level of risk: Moderate P7.2.3.1 ClO2 generation chemistry not optimised chlorine and chlorite ratio, or pH). Carry out pilot tests to determine optimum chemical feed rates for ClO2 production. Checks to ensure chlorite solution concentration is correct. Provide staff training in chlorite solution preparation. Chlorite. Chlorate. Elevated levels of chlorite and chlorate. Modify chemical feed until ClO2 production maximised, and minimised. Identify staff training needs and provide training. Event: EXCESSIVE FORMATION OF BY-PRODUCTS FROM CHLORINE DIOXIDE TREATMENT cont’d
treatment-chlorine-dioxide-treatment-feb14-v2.docx