Sea pollution: New sewer to reduce overflows

A giant pipeline is the new solution to sewage contamination of beaches and streams in central Auckland. Photo / Supplied

A giant pipeline is the new solution to sewage contamination of beaches and streams in central Auckland.

In an $800 million project, Watercare Services wants to bore a hole and install a 4.5m-diameter pipe - 110m underground at its deepest point - for 13km from Western Springs to the Mangere sewage treatment plant, along with smaller sewers and connections to existing local sewerage networks.

The so-called "central interceptor" sewer, for which the environmental consent and land designation process has begun, would double as a gigantic storage tank to collect 80 per cent of the diluted sewage which now overflows into waterways from combined wastewater/stormwater pipes. The normal and storm flows would go to Mangere for purification and discharge to the Manukau Harbour.

Watercare predicts that by 2030, without the project, there would be several thousand overflows each year from the 122 combined drainage overflow points in the Whau, Oakley, Motions and Meola catchments.

The interceptor is expected to reduce that to just six to 12 spills a year resulting from heavy rain.

The interceptor will duplicate part of the ageing western interceptor and provide additional capacity to cater for the growing population.

Watercare also intends to build a smaller "waterfront interceptor", at a cost estimate of $135 million, to reduce spills from the 50 overflow points in Grey Lynn and the waterfront suburbs from Coxs Bay to Freemans Bay.

Separation of combined wastewater/stormwater systems is still done in small, localised projects, but it is no longer seen as the main solution to overflows, a mantle which has shifted to the interceptors and, in some areas, large storage tanks.

A Watercare spokesman said Metrowater and the council completed many sewer separation projects, but wastewater sewers in those areas "still exhibit continuing features of combined sewers ... It is difficult to effectively and fully separate combined sewers".

Because of the huge and complex nature of the central interceptor project, it is expected to take until 2023 to complete and its associated works until 2027 - more than a generation after communities began expressing their distaste for waterway pollution.

Friends of Oakley Creek chairwoman Wendy John said the reduction in overflows would be welcome, but did not deal with the decreasing amount of land surface in the city that could absorb rainwater.