Wanganui Mayor Annette Main said the city's wastewater treatment plant had been badly designed and even under ideal conditions it would never cope with the city's wastewater.

And she said a decision about the future of the plant was several months away.

Ms Main was speaking after a month of a stench from the malfunctioning wastewater treatment plant wafting across the city. The malfunction - and the smell - was caused by an excess amount of protein waste being dumped into the plant by one of Wanganui's six wet industries. The company responsible has never been identified.

Two more dumps - one of sulphides and one of fat - have since entered the plant.

Ms Main told the Wanganui Chronicle as the problem progressed it had become obvious the plant would not cope with the sort of waste going into it.

"That is the advice that I've been receiving from people who provided exactly that advice to the council at the time [the plant was built].

What I can't tell you is why that advice was not taken."

Ms Main said she understood Wanganui's waste was unique because of the number of wet industries in such a small population.

She said the advice she was given is that the two-pond system in place isn't the right system for Wanganui's kind of waste, especially the trade waste.

"To put it simply, there's an aerobic layer that's working away on the top and there's sludge underneath that is supposed to be working in an anaerobic way in the dark.

"But because the sludge is so high it can't work - after five years it's at a level it should have been after 25 years."

Ms Main said she did not realise how serious the problem with the wastewater treatment plant was until after the dump in December.

"In October, [infrastructure manager] Mark Hughes provided some advice that the way the plant was designed meant that we were getting less than optimum performance from the plant and the sludge was building up, and so on. He suggested ways that that could be fixed, and that was when the bio-augmentation programme started."

Ms Main said the emphasis was on getting rid of the smell and stopping the illegal discharges.

"But in two to three months we should be able to look at what can be done with the plant, and it seems there may be options. We should be able to fix it, rather than doing a complete overhaul," Ms Main said.

- WANGANUI CHRONICLE