or iron, or polymer. turbidity is more than 0.2 NTU colour is more than 10 TCU aluminium residual is more than 0.15 mg/L elevated residual levels of other coagulants. Routine monitoring of coagulant residuals in filtered water. Start monitoring programme. 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 after coagulation, flocculation and flotation, ie, filtration and disinfection. Causes Preventive measures Checking preventive measures Corrective action What to check that action is needed Event: PARTICLES NOT REMOVED cont’d P5.2.1.2 Dosing rates set incorrectly or Determine and document relationships between source water quality and coagulant type and demand, flocculant type and demand, and optimum pH. Make sure that chemical dosing is able to respond to changing source water quality and quantity, ie, appropriate choice of manual, flow proportional or automatic dose control (eg, streaming current detector, colour, turbidity). Plant flow rate. Chemical dosing flows. Visual inspection for floc stability. Source and post-filter: turbidity colour pH alkalinity coagulant and/or No jar test records. No dosing control based on source water quality. Elevated turbidity, colour and residual coagulant in filtered water. Determine relationships and document. Install appropriate dosing control equipment. Adjust dose rates. P5.2.1.3 Power failure. Stand-by generator. Electricity supply. Poor continuity of power supply. Obtain a generator or alternative power source. Refuel generator if necessary.
P5.2.1.4 Chemical supply exhausted. Place an alarm on the chemical supply tanks to indicate when they are close to running out. Maintain records of chemical use to provide a guide to the length of time each chemical supply is likely to last. Maintain a reserve supply adequate to cover the time before resupply. Supply levels of coagulant, flocculant and chemical. Usage rates of coagulant, flocculant and chemical. Chemical supply exhausted. Elevated turbidity and colour in filtered water. Install or repair alarm system. Urgent order chemicals. Start keeping record of chemical use. Event: PARTICLES NOT REMOVED cont’d P5.2.1.5 Incorrect chemical used. Make sure chemicals are supplied to the correct storage tanks. Clearly label supply tanks; ensure operator onsite when chemicals delivered. Delivery records In source water: turbidity colour pH alkalinity coagulant and/or flocculant zeta potential jar test. No delivery records. Elevated turbidity and colour in filtered water. Staff absent during chemical delivery. No jar test records. No dosing control based on source water quality. Ensure quality of treated water is safe. If not, dump treated water. Empty, clean and refill storage tank. Introduce and/or ensure delivery procedure. Require supplier to ensure that operator is present when chemicals are delivered. Carry out jar tests and zeta potential measurements to determine the best coagulants and flocculants and their optimum doses every time raw water characteristics change. several coagulants and flocculants. Consider (beware of disinfection by-product formation) (see Guide P4.3). Determine relationships for the most appropriate coagulant and flocculant and means of pH control.
P5.2.1.6 Chemicals of poor quality. Check strength of chemicals supplied and calculate dilutions required for dosing. Supplier’s certificate of analysis. Coagulant, flocculant and chemicals. Dilution records. Supplier’s certificate of analysis. Chemicals of possible health significance following dilution. No supplier’s certificate. Elevated turbidity and colour in filtered Calculation checks not signed off. No supplier’s certificate for chemical composition. Chemicals of potential health significance exceed 50% of their MAVs in the treated water. Check calculations. Adjust dilutions and/or dose as necessary. Check chemicals supplied for presence of chemical contaminants. Change supplier. PARTICLES NOT REMOVED cont’d P5.2.1.7 Inadequate mixing of chemicals. that mixing of coagulant, flocculant and pHadjusting chemicals is adequate over the range of flow rates through the plant. Coagulant, flocculant and chemicals. Plant flow rate. Turbidity. Colour. Visual inspection of floc stability. Turbidity of clarified Frequency of filter backwash. Elevated turbidity and colour in filtered water. Turbidity of water leaving the clarifier in the range of 25 NTU. Post filter: turbidity is more than 0.2 NTU colour is more than 10 TCU aluminium residual is more than 0.15 mg/L elevated residual levels of other coagulants. Review treatment plant hydraulics to provide improved mixing of chemicals. P5.2.1.8 Insufficient contact time for floc formation. mixing times are adequate for effective coagulation and flocculation over the range of flow rates through the plant, and that carry over to the filter is not excessive. (See P5.2.1.7.) (See P5.2.1.7.) Review treatment plant hydraulics to provide improved contact time during coagulation and flocculation. significant
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 – Turbidity higher than maximum acceptable level Indicators: A turbidity of less than 0.2 NTU cannot be maintained in the water leaving the treatment plant. Required actions: Follow the actions given in Figure 3.4 of DWSNZ:2000. Identify the reason for the failure and rectify. Record the reason for the failure and the steps taken to rectify. Modify the water safety plan if necessary. Responsibility: Manager designated responsible for water supply. Event – Excessive levels of treatment chemicals carried into the distribution system Indicators: Consumer complaints of discoloration, poor sensation, odour. Excessive chemical concentrations measured in the leaving the treatment plant or in the distribution system. Notification of error by operator or chemical delivery contractor. Required actions: Follow Section 4.4 of DWSNZ:2000. Identify the reason for the failure and rectify. Monitor chemical until it reaches acceptable limits. Record the reason for the failure and the steps taken to rectify. Modify the water safety plan if necessary. Responsibility: Manager designated responsible for water supply. Water Safety Plan Performance Assessment To make sure that your supply’s water safety plan (formerly known as a Public Health Risk Management Plan, PHRMP) is working properly, periodic checks are needed. The overview document outlines what needs to be done. The following table provides the detailed information for checking this particular supply element.