Overall costs of the new plant have been put at more than $41 million and initial forecasts had trade waste rates at $4.5 million annually. They are currently about $2.5 million and nearly half of that sum is paid by the Imlay works alone.
But revised costs have come up with a figure of $2.8 million and council officers said even that could track downward.
Mr Lambly said his company's involvement hinged on trade waste charges.
"We're a meat company and we'd like to be in a position to use council's services but at the moment. it's an economic reality for us that we can't buy into," he said.
He said initial costings meant Affco Imlay was facing a 55 per cent increase in its annual trade waste charges (Current council financial modelling had trade waste charges increasing by 24 per cent and not 55 per cent).
"Very clearly we don't believe we can go forward purely on a cost perspective. We operate 11 plants around the country and Imlay's trade waste fees are higher than all those other plants put together," he said.
He said all those other meat plants have pre-treatment systems in place, treating waste either biologically or chemically before discharging into municipal schemes.
Mr Lambly told councillors Affco was looking at treating its own waste and was getting designs done now with costings expected this month. Once that detail was together he said the company was keen to again meet with council to discuss future treatment options.
Those options could include higher end treatment on site but in partnership with council.
But he stressed the Affco group had not formally withdrawn from being a user of the WWTP.
He said the meat industry was under pressure around the country as stock numbers reduced and competition for that stock intensified.
Mr Lambly said until they got costings he could not say if Affco would be better off fully committing to the city scheme or treating its own waste.
"There have been figures bandied around in the last few years but it would difficult for me to walk out of here today making a full and final commitment to council."
But he said it was important for council to know that Affco was open-minded and prepared to consider any review of WWTP costs.
The final stage of the two-day workshop went behind closed doors but Mayor Hamish McDouall said any decisions would not be made until the council went through its Annual Plan process.