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.
P7.2.1.11 Exposure to sunlight resulting in of ClO2. Ensure ClO2 dosing solution is protected from sunlight. ClO2 residual concentration. Low ClO2 residual (see Appendix). E. coli or coliforms detected in 100 mL sample of water leaving the treatment plant. Cover open channels or reticulate in pipes. P7.2.1.12 ClO2 monitoring samples taken incorrectly or incorrectly recorded (see Guide D4). Provide staff training for sample analysis and record keeping. Develop monitoring schedule and roster. Analysis records. Independent checks show monitoring inaccuracies. Identify staff training needs and provide training. P7.2.1.13 Method of ClO2 measurement incorrect, incorrectly calibrated, or reagents used in analysis have deteriorated. Provide staff training for sample analysis and record keeping. Analysis records. Independent checks show monitoring inaccuracies. Identify staff training needs and provide training. Event: CHLORINE DIOXIDE (ClO2) CONCENTRATION TOO HIGH Possible hazards: Chlorite, chlorate, chlorine dioxide. Level of risk: Low–moderate P7.2.2.1 Dosing malfunction (see Guide P10). Routine controller and dosing pumps maintenance. Replacement of controller if suspect. Fit an alarm to indicated incorrect CIO2 concentration. ClO2 residual Maintenance log. ClO2 residual concentration more than 50% of its MAV. Frequent repair needed. Maintenance log not signed off. Identify cause of fault and rectify. Replace controller with new unit. P7.2.2.2 Dosing controller’s sensor incorrectly calibrated. Regular manual checks on calibration of controller (see DWSNZ:2000 Section 3.3.4.8). ClO2 residual ClO2 residual concentration more than 50% of its MAV. Calibration schedule not signed off. Recalibrate controller sensor. Manually monitor and adjust ClO2 dose rate until recalibration undertaken. Event: CHLORINE DIOXIDE (ClO2) CONCENTRATION TOO HIGH 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
P7.2.3.2 Natural organic matter present in the water being chlorinated (reduction of ClO2 to chlorite). Provision of treatment processes upstream to reduce levels of organic matter in the TOC/colour. Elevated TOC/colour. Elevated chlorite formation. Optimise treatment parameters in upstream processes to maximise removal of organic matter. 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. Event – ClO2 concentration is lower than minimum effective level Indicators: A detectable ClO2 residual cannot be obtained in the water leaving the treatment plant. In 100 ml samples of leaving the treatment plant, E. coli or coliforms are continually detectable, or E. coli is present at elevated levels (more than 10 per 100 mL). Widespread illness in the community. Required actions: Follow the actions in Figure 3.2 of the DWSNZ:2000. Identify the reason for the failure and rectify. Record cause of system failure and steps taken to correct. Modify your safety plan if necessary. Responsibility: Manager designated responsible for the water supply. Event – ClO2 concentration is higher than maximum acceptable value Indicators: Knowledge of a major overdose of ClO2 into the water. Inability to obtain pink colour from DPD indicator despite high ClO2 dose rates. (NB: This indicates ClO2 levels well in excess of the MAV – very high ClO2 levels bleach the pink colour that normally develops in the presence of ClO2.) Change in the odour or taste of the water. Widespread levels of taste and odour complaints, or illness, in the community.