Reliable information water quality is essential for the proper management of a water supply. Knowledgeable and skilled staff are also essential for minimising the public health risks associated with water supplies. Please read the staff training (Guide G1) and the monitoring guides (Guide G2). While we haven’t pointed out every detail of how these documents are linked with the present document, the links are many and are important. Abbreviations: MAV – Maximum acceptable value – see Drinking-Water Standards for New Zealand:2000; NTU – nephelometric turbidity units Causes Preventive measures Checking preventive measures Corrective action What to check Signs that action is needed Event: NOT ENOUGH WATER IN POST-TREATMENT STORAGE TO MEET DEMAND Possible hazards: Low pressure in the water supply system may allow entry of germs and chemical determinands; hazards associated with poor hygiene. Level of risk: High D1.1.1 Insufficient supply of raw water. Provide sufficient raw water storage to cover seasonal variations in source water availability, periods of untreatable raw water quality, maximum limits on consent for abstraction from the source. Develop auxiliary sources that can be used when secondary sources cannot meet demand. Refer also to Guide P1 series. Pre-treatment storage levels. Raw water storage levels or source flows low. Plan development of new sources. Restrict development in distribution area until adequate water available. D1.1.2 Insufficient water treatment capacity, or output limited by one or more treatment stages not operating. Design treatment plant to meet expected demand for projected life of the plant. Identify treatment stages that are limiting capacity and undertake modification of these. Ensure appropriate water safety plans are operational for all treatment processes to avoid or limit down time. storage levels low. Calculate future water use and plan actions to meet likely demand. Identify problem treatment stage and increase throughput if possible. D1.1.3 Inadequate storage capacity. Construct additional storage. Encourage water conservation during periods when demand is likely to be high. Water consumption. Need for water use restrictions. storage levels low. Water use restrictions. Event: NOT ENOUGH WATER IN POST-TREATMENT STORAGE TO MEET DEMAND cont’d
D1.1.4 Inability to transmit water from source to plant, or plant to storage (including power failure). Ensure pumps have stand-by generators. Routine inspection and maintenance of pipelines. Inspection log. Electricity supply. Water transmission ceases. Maintenance and inspection not carried out. Re-fuel generators (if necessary). Clear blockages/ repair pump breakdowns or pipe failures. D1.1.5 Leakage from the storage facility. out routine testing of storage for leaks. Change in storage level. Change in storage does not equal water input minus water output (including Locate and fix leak. Event: INTRODUCTION OF CONTAMINATING MATERIAL INTO SERVICE RESERVOIR Possible hazards: Germs, arsenic, copper, chromium, iron, chlorine, aluminium. Level of risk: High1 D1.2.1 Access by For in general: ensure the storage is covered install mesh, grills, vents, etc undertake regular tank inspections document condition of structure, mesh, grills, vents, tank liner (for gaps) etc and take action. For wooden tanks: install metal band around wooden tanks to prevent access from ground level remove vegetation near tanks install internal liner for tank roof. Maintain an FAC residual. Inspection reports. Free available chlorine (FAC). Unexplained change in water quality. Taste and odour complaints. Visual evidence of entry of animal or bird. FAC less than 0.2 mg/L. Replace damaged screens, etc. Isolate tank, chlorinate and the quality. If there is gross contamination, dump the water, clean and refill before chlorinating. 1 The level of risk from microbiological contamination is less if a chlorine residual is maintained. Causes Preventive measures Checking preventive measures Corrective action What to check Signs that action is needed INTRODUCTION OF CONTAMINATING MATERIAL INTO SERVICE RESERVOIR cont’d
(especially after heavy rain). arsenic is more than 50% of its MAV. Install roof liners in tanks with treated timber roofs. Replace lid with more suitable material. 2 See Guide D2.1 regarding approved materials. Causes Preventive measures Checking preventive measures Corrective action What to Signs that action is needed Event: INTRODUCTION OF CONTAMINATING MATERIAL INTO SERVICE RESERVOIR cont’d D1.2.5 Entry of roof drainage. Specify and audit roof design: rainwater to drain from (not into) tank restrict access to roof top hatches designed to exclude water ingress. FAC. quality. Installation design. Annual roof inspection report. Change in water quality following rain. E. coli or coliforms detected in 100 mL of water. Flaws in the design of the roof. Repair leaky joints/cracks/ hatches promptly. Replace badly designed hatches. D1.2.6 Entry of groundwater (for tanks). Repair leaks promptly. Inspect tank 1–5 years (depending on the age of the tank and risks caused by its location). quality. Tank inspection report. Unexplained change in water quality. E. coli or coliforms detected in 100 mL of water. Repair leaks. Isolate tank, chlorinate and check the quality. If there is gross contamination, dump the water, clean and refill before chlorinating. Line the tank with a suitable waterproof material. Leaks identified by the inspection. Complaints of petrol (if near underground fuel tanks). D1.2.7 Animal entry from upstream source (animals trapped and destroyed by action of control valves) – where the tank is filled directly from the source. Ensure there are screens on the source outlet. Regular screen inspection. quality. Turbidity. Taste and odour complaints. E. coli or coliforms detected in 100 mL of water. Turbidity more than 0.2 NTU. Replace and clean screens. Isolate tank, chlorinate and the quality. If there is gross contamination, dump the water, clean and refill before chlorinating.
D1.2.8 Chemical from incorrect chemical dosing. Train staff in calculation of dose rates and in the use of methods for optimising treatment plant operation. Document treatment plant dosing procedures (especially calculations of type and quantity of materials used), and those for the reservoir if it is dosed. Treatment chemicals. Turbidity. Elevated turbidity/ treatment chemical post- treatment. Absence of written descriptions of dosing procedures. Turbidity more than 0.2 NTU. See Contingency Plan. INTRODUCTION OF CONTAMINATING MATERIAL INTO SERVICE RESERVOIR cont’d D1.2.9 Backflow from drains. Ensure that there is an airgap or other suitable backflow prevention device between tank overflow and sewer or stormwater drains. quality. Turbidity. Overflow construction. Change in water quality after rain events. Turbidity more than 0.2 NTU. E. coli or coliforms detected in 100 mL of water. Disconnect any direct connections. Isolate tank, dump the water, clean, chlorinate and check the quality before refilling. Event: DEVELOPMENT, OR RE-SUSPENSION, OF SEDIMENT WITHIN TANK OR RESERVOIR Possible hazards: Germs; manganese, disinfection by-products (trihalomethanes, haloacetic acids, chloral hydrate). Level of risk: Low–moderate D1.3.1 Sediment/ slime and release. Avoidance of rapid drain/fill. Undertake regular cleaning of reservoirs (every 1–5 years depending on the quality of the source water, flow through and degree of stagnation in the tank etc). Cleaning procedures need to consider disinfection of all equipment, satisfactory training procedures, preferably isolation during cleaning, minimisation of sediment stir-up if online during cleaning. Maintain chlorine residual. Minimise conditions leading to post-treatment floc formation. FAC. quality. Turbidity. Manganese. Audit cleaning procedures and operation. Visible slime/ sediment. Customers complain of slime/ sediment. FAC residual less than 0.2 mg/L. E. coli or coliforms detected in 100 ml of water. Turbidity more than 0.2 NTU. Review cleaning procedures and plant operation. Isolate tank, chlorinate and check the quality. If there is gross contamination, dump the water, clean and refill before chlorinating.