Assessment of the impact of mains cleaning to improve distribution water quality

This paper outlines Sydney Water’s experience in implementing mains cleaning as part of an integrated approach to water quality improvement. Targeted flushing of water mains is generally used as a first approach to mains cleaning, while swabbing is used as a remedial action where flushing has failed to improve water quality performance. To show the benefits of flushing/swabbing two approaches were taken. The first approach looked at the quality of the discharge water, while the second looked at the compliance and operational data before and after mains cleaning. Using this data, the distribution systems were ranked in terms of water quality improvements. The rankings show that six of the ten Distribution Systems that received the most swabbing now rank in the top ten performers, while four of the ten Distribution Systems that received the most flushing now rank in the top ten performers. The discharge water was found to contain suspended solids, total coliforms, HPC20C, iron, aluminium, manganese and biofilm/biomass. Some coliform species were found to be similar to the bacteria found in compliance coliform failures. An integrated mains cleaning strategy that includes flushing, swabbing and water main rehabilitation is essential to prevent water quality deterioration in the distribution system.