A meeting in Wellington today will hopefully explain why a peer review of Wanganui's planned wastewater treatment plant upgrade is at odds with the new design.

In a surprise setback for the district council's wastewater plant project, the reviewers have said they cannot rely on earlier estimates of the amount of sludge that will be "digested" in the primary pond. On their estimates, the volume of sludge could make the plant unaffordable.

Cardno BTO designed the new plant at Airport Rd and it was peer-reviewed by AECOM and CH2Beca, both of which have told council they cannot rely on those initial estimates and both have predicted sludge quantities would increase.

Russell Bell, appointed project manager in September, said he and council senior wastewater engineer Arno Benadie would "eyeball" Cardno representatives in the capital this afternoon.

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"We need to understand why there are differences," Mr Bell said.

"What you've got is two engineers with different opinions.

"We have to determine if the issue with the sludge might sit somewhere in the middle."

Solutions could include adding more mechanical kit to the design to speed up the digestion process to reduce the solids in the pond.

"There have been a number of things suggested but we need to understand whether that will get the sludge down to a level we want," Mr Bell said.

He said the information Cardno gave council in June looked good financially in terms of the quantity of sludge and associated costs.

"But the peer review has seen the sludge quantity go up considerably - and information we're getting from landfill operators at Bonny Glen is that their prices have gone up as well. So we've got two increases working in tandem.

"We need to verify that and come back to council."

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Mr Bell said Cardno had used Beach Rd as the basis for its calculations on waste volumes going into the treatment plant.

"They put that information into their model as assumptions of what loads will be and then added in the peak loads created in summer. Out of that, they calculated the by-products going on the pond and one of those is sludge."

The next step was testing the assumptions Cardno had made in its design.

He said the key was operational costs and they needed to narrow costs down so council knew what ongoing costs would be.

"Sludge is an issue for all plants but there are mechanisms used around the world which could reduce the sludge or at least manage it."

Mayor Annette Main said they needed to sort out why there was a difference of opinion.

"What makes one side think the pond is going to digest and what makes the other think it's not going to?" she said.

Ms Main said until the design issues had been nailed down council could only consider a range of operational costs.

"The industries that discharge into the system need to know, so they know what costs they will be faced with."

She said this approach was "completely sensible".

"The peer review is what didn't happen with the first wastewater plant - or, if they did happen, they weren't taken notice of. We can't afford that risk. We can't have a peer reviewer say there are risks and then council do nothing about that.

"I'm comfortable with that design but it's those ongoing costs. And if those costs are unaffordable, then how do we reduce them?" she said.

The sludge created by the treatment process was the problem in the wastewater treatment plant commissioned only seven years ago.

The council decided an upgrade was essential, with the upgrade tagged at $24 million.