A former Wanganui district councillor says the incoming council should seriously consider treating the city's domestic and industrial waste in two separate streams as potentially the most economical method.

Stephen Palmer was a councillor from 1983-2004 and made a submission along those lines to council's Annual Plan three years ago.

He argued that if council was looking at spending upward of $30 million on a new plant - replacing the plant at Airport Rd that was shut down in 2012 - then it should consider a separate treatment stream for the waste coming from the major wet industries in Castlecliff.

Mr Palmer has now written to Mayor Hamish McDouall and the councillors urging them to consider the separate treatment option when it re-opens debate on the new treatment plant.

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He said without the industrial waste volumes, the existing plant should be able to cope with the domestic load.

"I don't believe the separate sewer option was considered during the current debate and it should be as it might offer a more economical overall solution to our problem in the long term."

He said his 2013 submission fell on deaf ears and "it was clear that council had already decided irrevocably to simply build a new and enlarged plant which would continue to treat all of the wastewater together".

He said once industrial waste is discharged into a domestic sewer it mixes with human waste and must be treated as such which means any treatment plant has to be capable of treating much greater volumes than just domestic waste.

"So if the industrial waste were to be collected separately then it could be discharged to sea without the high level of treatment needed to kill e-coli and viruses," Mr Palmer said.

He said the notion of separate treatment was not new and had been suggested by Murray Gilbertson who was then council's director of planning in the 1990s.

He said Mr Gilbertson initiated discussion with the wet industries about the possibility of laying a separate industrial sewer along Heads Rd to an independent treatment plant and then to sea using the offshore pipeline that now discharges treated waste off South Beach.

The idea was the new drain would be paid for with capital contributions from the industries who, in return, would face much lower trade waste charges.

"However, they demurred and favoured pre-treating their own waste to a level that could be accepted into the domestic sewer and treatment plant. They apparently ignored the fact that that would greatly increase the on-going operating cost of treatment, to them as well as to ratepayers as whole," Mr Palmer.

At one stage council decided to spray effluent on to land in Harakeke forest at Kaitoke which would have made the offshore discharge redundant. But more detailed investigations revealed an impervious iron pan under the forest which meant a much larger area of land was needed if the spray option went ahead.

He said that was when council abandoned the land disposal option in favour of using the pipeline with a higher level of treatment.