Report No DWI0295
LOOSE DEPOSITS IN WATER MAINS (TMU 9023)
Final Report to the Department of the Environment
Formerly it was common practice to flush small diameter water mains to remove loose deposits. In the last 10 years or so, in many areas, flushing has become less common, and where it is carried out it tends to be in response to consumer complaints and applied to individual mains, rather than in a systematic way. It seemed possible that accumulating deposits might affect water quality.
The objectives of this project were:
Mains deposits were found to accumulate in small diameter mains and at dead ends, especially those fed by lowland reservoir and river sources. Under normal flow conditions water quality at the tap was generally acceptable. In some zones, health-related substances (toxic organics, e.g. pesticides, toxic inorganics, e.g. lead and cadmium) were present in low concentrations in the loose deposits within the water mains. Coal tar pitch linings (used prior to 1977) could result in high concentrations of polycyclic aromatic hydrocarbons (PAH) in mains deposits. Generally, these health-related substances were conveyed to the consumer's tap only under conditions of abnormal flow. In addition deposits encouraged the presence and growth of micro-organisms, although in none of the deposits examined were species having health-implications positively identified. Mains cleaning using air-scouring and flushing reduced the potential for poor water quality during abnormal flow, but would not be effective in the long term if any current poor treatment permitted continuing deposition of residues within the water distribution system. Similarly, processes such as mains corrosion and leaching of PAH from coal tar pitch linings could reduce the long-term effectiveness of mains cleaning.
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