Councillors were told that design faults were the root cause of the stench coming from the ponds along with the plant's failure to work properly, with one consulting engineer saying at the moment it was "a pond of raw sewage".
Mark Hughes, council's infrastructure manager, said within five weeks of the plant opening in 2007 there were problems.
Compounding that was the 2008 bylaw which demanded that wet industries apply for specific trade waste discharge consents. However, Mr Hughes said, the bylaw had never been properly implemented.
The consents set loads which those industries are allowed to discharge into the treatment system. These limits were the basis on how council charged those industries for using the system.
But odour problems and other issues sparked random sampling in February, which will continue as the council tries to work within its own resource consents.
Mr Hughes said tests on March 8 showed Affco's discharges exceeded its limits in all four categories: biochemical oxygen demand (BOD), suspended solids, oil and grease, and sulphides.
Affco's BOD limits were 1870 parts per million but on that day samples showed they were running at 4800 parts per million.
"Affco investigated this and told us that the events of March 8 should never have happened and have promised us it will never happen again," he said.
In fact, Affco breached limits more than any other wet industries between March 4 and March 25 for BODs (five times), suspended solids (three times), oil and grease (five times) and sulphide (twice).
Similar samplings taken at Tasman Tanning and Land Meats showed breaches but not to that extent.
Mr Hughes said the council was keen to maintain good working relationships with industries.
But under the present bylaw there was no liability on those industries.
Despite the limits being breached, those exceedances were not the reason for the plant malfunctioning, he said. The main problem remained the plant's design.
"In fact, the problems were obvious within five weeks of the plant opening in 2007," he said.
Mike McCoy, an engineer with wastewater consulting company Cardno BTO, told the committee remedial action continued at the ponds aimed primarily at stifling odours. But Mr McCoy said he was confident design and not trade waste was the problem.
Questioned about the odour, he said it would vary depending on wind direction, air temperatures and what was happening in the ponds themselves.
Stopping new waste going into the ponds was not going to stop the odour.
"Most of the problem is coming from the sludge that's built up in the ponds. It's now a big pond of raw sewage."
United States consultant Matt Mates, who carried out a peer review of the Cardno BTO reports to council, told councillors he had seen hundreds of treatment plants around the world but had never seen another like Wanganui's which he called "unorthodox and low-cost".
"For the amount of load it's handling it's very crude. I wouldn't have selected this process," Mr Mates said.
