Whanganui's wet industries will bear the brunt of increasing wastewater disposal rates as a result of the district council's decision to build a new $41.2 million treatment plant.

Trade waste rates for the old plant totalled $2.25 million, whereas the council's projected trade waste rates for the new plant are $3.5 million. That's an increase of 56 per cent.

Residential ratepayers pay $350.89 for wastewater disposal, but that's set to rise by 33 per cent to $468 under the new scheme.

As the Chronicle reported in June, Affco plant manager Troy Lambly wrote to district councillors to "emphatically" state that the company would not use the new plant.

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Before the council voted to proceed with the Cardno BTO plant, council chief executive Kym Fell had been talking to the city's wet industries to find an alternative treatment solution for them.

The council is required by law to provide a wastewater treatment scheme that accommodates all users, unless they opt out.

Affco, Open Country Dairy and Land Meats, all owned by Talley group, produce in excess of 60 per cent of the city's wastewater.

They were interested in building their own plant using the council's pipes and consents to discharge treated waste into the sea.

That would have meant a revision of the design of the new plant. However, while the cost of the new plant would have been less, it would have also needed to do without the trade waste rates paid by those companies.

But just two days before the council approved the construction of the $41.2 million plant last month, Mr Fell told councillors discussions with the city's wet industries regarding a separate treatment scheme had failed.

At the time, he said the council had "bent over backwards" to accommodate what the wet industries wanted but no agreement had been reached.

"Affco have told me they cannot agree to our terms, so we have to assume they will remain in the municipal scheme," he said.