The wraps have come off a previously confidential report which formed the basis of Whanganui District Council's legal action against the company that designed the city's failed wastewater treatment plant.

A report, prepared by Humphrey Archer of consulting engineering firm CH2M Beca, concluded that the MWH-designed plant was never going to work properly.

Mr Archer's report - the third independent peer review council ordered into the failed WWTP - was integral to council's legal case it took against the plant designer, MWH. The city now faces a $41 million cost to build a new plant at Airport Rd.

But while this confidential information has been released, details of the legal settlement with MWH still remain secret.

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Kym Fell, council's chief executive, said Mr Archer's report had been kept confidential because it was legally privileged and formed part of the council's case against MWH.

But he said now that the settlement had been reached council sought legal opinion and had been told "there's no reason for the information to remain confidential and it's in the public interest to release it".

"It will assist the community to understand why the council pursued a claim against MWH and the reason why council has been so meticulous in considering all viable options for building a new treatment plant," he said.

Mr Archer's report was presented to council at a confidential meeting on October 28 last year.

His report found that the "optimised lagoon process" of the Whanganui plant had not been used in other treatment plants. It tried to combine all parts of the treatment functions in one lagoon, including stormwater storage capacity, an aerobic upper layer as well as an anaerobic sludge storage layer.

"The normal process for wastewater plants comprises a fully-mixed aerated lagoon followed by one to three partially-mixed aerated lagoons," he said.

His report said "significant errors" were made in estimating the sludge storage volume in the lagoon and that meant the capacity was exceeded in about 2009-10, just two years after the plant was commissioned.

Mr Archer said the required aeration energy was significantly underestimated. As well as that, the aeration disturbed the sludge layer and that prevented full anaerobic digestions of the sludge.

Mr Archer said in 2004, peer reviewers looking at the original plant asked for the aeration calculations "but these were not provided by MWH".

"Aeration calculations were sent to the council on December 13, 2007, about six months after the plant start-up," he said.

He said a major problem was the aerators disturbed the sludge layer and that meant full anaerobic digestion of the sludge could not happen. "The concerns expressed by the 2004 peer reviewers were not appropriately addressed by MWH."

That earlier peer review expressed concern that a number of issues it raised were not adequately addressed and said five key points need further action or attention.

These included:

-A rigorous risk assessment process covering ... the sewer separation process and trade waste dischargers;

-Provision for calculations used to determine the aeration requirements;

-Provision of raw data to support the claims made;

-Confirmation of the longevity of the plant particularly in terms of the sludge;

-And a need to formalise a trade waste bylaw and trade waste agreements with significant dischargers to ensure adequate control of discharges is enforceable.

For the full Archer report, go to: www.whanganui.govt.nz/Archer-Report.