P7.1.1.2 Dose controller’s sensor incorrectly calibrated. Regular manual checks on calibration of sensor (see DWSNZ:2000). FAC. quality. Calibration schedule. FAC concentration is less than 0.2 mg/L. E. coli or coliforms detected in 100 mL sample of water leaving the treatment plant. Calibration schedule not signed off. Recalibrate controller. Increase chlorine dose rate 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 the chlorination process. Causes Preventive measures Checking preventive measures Corrective action What to check Signs that action is needed Event: NOT ENOUGH FREE AVAILABLE CHLORINE cont’d P7.1.1.3 Dose controller’s set-point incorrect, or incorrect dose calculation. Periodic manual checks on FAC concentration, especially during periods of quality variability Independent on calculations (especially after a system change when expected dose rates are uncertain. Install visual flow meters to allow chlorine flow to be checked by eye. FAC. quality. Dose flow rates and dose calculations. FAC concentration is less than 0.2 mg/L. E. coli or coliforms detected in 100 mL sample of water leaving the treatment plant. Frequent calculation errors Flow rates lower than expected. Adjust controller set-point. Recalculate dose rates and change settings. staff in making dose calculations.
P7.1.1.12 pH too high (resulting in a lower percentage of the FAC existing in its more powerful disinfecting form). For information on causes of pH too high and preventive measures, see Guide P8.1. See the Appendix for a graph showing how the effectiveness of chlorine as a disinfectant decreases as the pH increases. Causes Preventive measures Checking preventive measures Corrective action What to check Signs that action is needed Event: TOO MUCH FREE AVAILABLE CHLORINE Possible hazards: Chlorine; possibly high levels of trihalomethanes, haloacetic acids, chloral hydrate and chlorate (hypochlorite chlorination); possible heavy metals (from corroded fittings). Level of risk: Low–moderate3 P7.1.2.1 Dosing malfunction (see Guide P10). Routine maintenance of dose controller and dosing pumps. Replacement of controller if suspect. Install an system to indicate when FAC concentration is outside designated limits. FAC. Examination of maintenance log. FAC concentration is more than 50% of its MAV.4 Maintenance log shows frequent maintenance needed. Maintenance log not signed off. Identify cause of fault and rectify. Replace controller with new unit. P7.1.2.2 Dose controller incorrectly calibrated. Regular manual checks on calibration of FAC controller. FAC. Calibration schedule. FAC concentration is more than 50% of its MAV. Calibration schedule not signed off. Recalibrate controller. Decrease chlorine dose rate P7.1.2.3 Dose controller incorrect, or incorrect dose calculation. Periodic manual checks on FAC concentration, especially during periods of water quality variability. Independent 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. FAC. Dose flow rates and dose calculations. FAC concentration is more than 50% of its MAV. Frequent calculation errors found by checks. Calculation checks not signed off. Flow rates above expected values. Adjust controller set-point. Recalculate dose rates and change settings. Train staff in making dose calculations.
P7.1.2.4 Low chlorine demand2 coupled with poor chlorine dose control. Use a control method that links dose control to FAC residual at appropriate location. FAC. FAC concentration is more than 50% of its MAV. Replace dose controller with more suitable unit. Manual monitoring and manual chlorine control when quality is variable. 2 Chlorine demand is the difference between the amount of chlorine to the water and the FAC residual remaining after the chlorine has reacted with other substances in the water. If the chlorine demand of the water increases without an increase in the dose, too little disinfectant will remain to disinfect the water properly. 3 The concentrations of disinfection by-products formed, and therefore the risk they present, will depend on the amount of natural organic matter in the water. Causes Preventive measures Checking preventive measures Corrective action What to check Signs that action is needed Event: TOO MUCH FREE AVAILABLE CHLORINE cont’d P7.1.2.5 Chlorine dose solution strength too high. Check that a satisfactory FAC level is produced when a new chlorine container is brought into use. FAC. Chlorine level in chlorine dosing solution. FAC concentration is more than 50% of its MAV. Chlorine levels in dosing solution too high. Determine the cause of the high chlorine and rectify. Provide training in the preparation of chlorine solutions (including P7.1.2.6 Spillage of chlorine compound or solution into dosing solution. Train staff in handling and labelling of chemicals. FAC. Chlorine level in chlorine dosing solution. FAC concentration is more than 50% of its MAV. Chlorine levels in dosing solution too high. Identify the reasons for the spillage and rectify if possible.
P7.1.2.7 Chlorine overdose at a chlorine booster station. Many of the causes and preventative measures in P7.1.2.1–P7.1.2.6 for chlorination at the treatment plant are also applicable to chlorine boosting in the distribution system. Refer to these. Put in place either manual monitoring or in-line monitoring of the FAC residual to provide reliable control of the booster dose rate. FAC. FAC concentration is more than 50% of its MAV. Take steps noted in as appropriate. Adjust dose setting. Event: EXCESSIVE FORMATION OF CHLORINATION BY-PRODUCTS Possible hazards: Trihalomethanes, haloacetic acids, chloral hydrate and chlorate (hypochlorite chlorination). Level of risk: Low–moderate3 P7.1.3.1 Natural organic matter present in the water being chlorinated. Provision of treatment processes upstream to reduce levels of organic matter in the TOC/colour. Elevated TOC or colour. Elevated disinfection formation. Optimise treatment parameters in upstream processes to maximise organic matter removal. 3 The concentrations of disinfection by-products formed, and therefore the risk they present, will depend on the amount of natural organic matter in the water. Contingency Plans If an event happens despite preventive and corrective actions you have taken, you may need to consult with the Medical Officer of Health to assess how serious a problem is. – FAC concentration is lower than minimum acceptable level Indicators: A detectable chlorine residual cannot be obtained in the leaving the treatment plant. In 100 ml samples of water leaving the treatment plant, E. coli or coliforms are continually detectable, or is present at elevated levels (more than 10 per 100 mL). Widespread illness in the community.