Three years after its wastewater treatment plant failed and was shut down, the Whanganui District Council has finally given the green light for a replacement plant, estimated to cost $38 million.

Councillors made the decision yesterday, but a number of key issues still need to be resolved before work starts on a new facility at Airport Rd. They include sorting out how much domestic and trade waste users will pay and securing a resource consent for the plant's marine outfall off South Beach.

The new plant, designed by Cardno BTO, will cost an estimated $38 million with construction spread over three years. An initial $16 million will be spent in 2016-17 and the remaining $22.7 million in 2017-18.

Council officers estimate that when the plant is operational (2018-19) rates funding that year will need to be $11.9 million. How much that will mean to households and the city's industries is still to be finalised and discussions with the main trade waste users is ongoing.

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The previous plant only started operating in 2007 and since it closed (2013) council has spent about $10 million on removing sludge from the existing ponds along with litigation (against the designers MWH), as well as getting a new plant designed and engaging various experts to advise on a new system.

Some trade waste users have already cut back their loads going into the waste stream. Tasman Tanning had committed capital to reduce sulphates and chromium in its discharges, while Affco had installed a larger rendering plant to reduce its loads.

The new design was initially costed at $30 million but a later review added a dryer to cut down sludge, a major byproduct of the treatment process. While that raises construction costs to $38 million, it cuts annual operating costs by $2.3 million.

Mayor Annette Main said the decision was the most important council had had to make in its term.

"It triggers a change to our Annual Plan and 20-15-25 10-Year Plan but in essence we've on our way to getting a new wastewater treatment plant," Ms Main said.

She said council would press on to get a three-year resource consent because it couldn't continue to operate under emergency provisions currently in place.

"We think we can build the plant within three years but if problems arise we'll be talking with Horizons Regional Council. They've been really tolerant so far and I doubt very much that they'd be difficult if we were slightly over that timeframe," she said.

Ms Main said she was "very pleased" to hear the peer reviewers say there was not another plant that could do the job at a lesser cost.

"I'm completely satisfied we've done everything we can in terms of the community to ensure we've got a solution that will satisfy both our businesses and our community."

She said the new plant gave council the balance of taking care of its waste affordably and sustainably.

"Our waste has been delivered into the ocean for far too long now and our community and regional council have been tolerant but we know it can't continue."

She said the best thing for her was that council would be building the new plant without the burden of debt hanging over from the previous plant.

Councillors were almost unanimous in their support for the project with Charlie Anderson the only dissenting voice. Voting for the recommendations were Ms Main, Sue Westwood, Jenny Duncan, Helen Craig, Jason Granville, Ray Stevens, Martin Visser, Rob Vinsen, Hamish McDouall and Rangi Wills. Absent yesterday were Philippa Baker-Hogan and Jack Bullock. Mr Vinsen, who has been tenacious in his examination of the scheme, voted for the plant, satisfied with an answer council received from Humphrey Archer from CH2M Beca, the company which peer-reviewed Cardno's design.