P7.2.1.2 Regular manual ClO2 residual Low ClO2 Recalibrate checks on concentration. residual (see controller Dosing calibration of Appendix). sensor. controller’s controller (see sensor quality. E. coli or Increase ClO2 DWSNZ:2000 Section incorrectly coliforms dose rate until Calibration 3.3.4.8). calibrated. detected in recalibration schedule. 100 mL sample undertaken. of water leaving the treatment plant. 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 and the effectiveness of treatment processes prior to chlorine dioxide treatment. Preventive measures Checking preventive measures Corrective action Causes What to check Signs that action is needed Event: CHLORINE DIOXIDE (ClO2) CONCENTRATION TOO LOW cont’d P7.2.1.3 Periodic manual ClO2 residual Low ClO2 Adjust checks on ClO2 concentration. residual (see controller Dosing residual Appendix). point. controller’s Flow concentration, E. coli or Recalculate Dose flow especially during incorrect or coliforms dose rates and rates. periods of water incorrect dose detected in change quality variability. Dose calculation. 100 mL sample settings. calculations. Independent check of water leaving Train staff in on calculations the treatment making dose (especially after a plant. calculations. system change Frequent when expected calculation dose rates are errors found. uncertain). Calculation Install visual flow checks not indicators to allow signed off. flow to be checked Flow rates lower by eye. than expected.
P7.2.1.2 Dosing controller’s sensor incorrectly calibrated. Regular manual checks on calibration of controller (see DWSNZ:2000 Section 3.3.4.8). ClO2 residual concentration. quality. Calibration schedule. Low ClO2 residual (see Appendix). E. coli or coliforms detected in 100 mL sample of water leaving the treatment plant. Calibration schedule not signed off. Recalibrate controller sensor. Increase ClO2 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 and the effectiveness of treatment processes prior to chlorine dioxide treatment. Causes Preventive measures Checking preventive measures Corrective action What to check Signs that action is needed Event: CHLORINE DIOXIDE (ClO2) CONCENTRATION TOO LOW cont’d P7.2.1.3 Dosing controller’s incorrect or incorrect dose calculation. Periodic manual checks on ClO2 residual 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 concentration. Flow Dose flow rates. Dose calculations. Low ClO2 residual (see Appendix). E. coli or coliforms detected in 100 mL sample of water leaving the treatment plant. Frequent calculation errors found. Calculation checks not signed off. Flow rates lower than expected. Adjust controller Recalculate dose rates and change settings. Train staff in making dose calculations. point.
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