Report on $74m sewage plan due today

A $74.3 million plan for the scheduled stage two upgrade of Dunedin's secondary sewage treatment facilities is to be considered by city councillors today.

A report to this afternoon's Dunedin City Council infrastructure services committee meeting has recommended a preferred "option A" be accepted for the installation of secondary treatment systems at the city's Tahuna wastewater plant.

The option - including a combination of ultraviolet (UV) light disinfection, digestion systems and drying techniques - was expected to cost $7.1 million more than the original $67.2 million budget, Tahuna upgrade project manager Brian Turner said.

However, annual operating costs would be lower - down from $5.7 million to $3.7 million - because methane captured during the plant's processes, for use in drying the solid waste, could be used as an energy source rather than diesel, he said.

The committee was expected to make a recommendation, either supporting the option or an amended preference, which would then be considered for final approval at the next full council meeting on May 18.

The report's recommendation was the result of a long-running process that had seen up to 29 options for the city's secondary treatment considered, before being reduced to a shortlist of 11 initially, and then down to four earlier this year.

The progress follows the January 23 commissioning of the city's new $37 million, 1.1km outfall pipe running from the Tahuna plant out to sea from Lawyers Head, which has dramatically improved water quality, including at St Kilda and St Clair beaches.

Stage two of the upgrade was needed because, under the terms of the council's resource consent for the Tahuna plant, the practice of using chlorination to disinfect wastewater discharges had to cease by September 30, 2011.

Chlorination was linked to the formation of carcinogenic compounds that could threaten sea life and the environment, but not humans, Mr Turner said.

A new system would be needed to replace chlorination, to meet effluent standards also contained in the consents, and the upgrade would allow UV disinfection to be used instead, Mr Turner said.

Once operational, the plant would produce about 4.2 tonnes of dried solid waste each day, which was less than that already produced at the Tahuna plant because of the additional drying that would take place.

And, because the material would be drier, it could be more suitable for secondary use as a landfill cover or on golf courses, rather than being burnt at the Tahuna plant or dumped in pits at the Green Island landfill, as happened now, Mr Turner said.

"It's something we have got to develop and work on," he said.

If approved, the stage two construction was expected to begin by early next year to meet the "tight" September 30, 2011 deadline, Mr Turner said.

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