Leachate from the Bonny Glen Landfill could make Marton's wastewater sludge too toxic to be trucked back to the landfill for disposal, Geoff Mills says.

He and his wife Gretta submitted on Thursday to a hearing considering whether the landfill can be expanded. Mr Mills lives in Marton and works in the water quality and water treatment sector. For the last 15 years most of his work has been overseas.

He said it would be ironic if leachate from the landfill resulted in sludge too full of heavy metals to be put back there. The leachate from Bonny Glen is bothering him, as a ratepayer and out of concern about its effect on the environment.

Leachate is the liquid that oozes out of the waste. Mr Mills said it had a high organic content and was high in ammonia. The limited checks on its mineral content provided "more than enough information to be concerned about toxicity effects at the Marton wastewater ponds".

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Bonny Glen's leachate is stored in ponds near the tip face. It's trucked to the Marton wastewater system to be treated, under a "handshake" agreement made in 2008.

The treated water is discharged into the "tiny" Tutaenui Stream, which flows into the Rangitikei River.

The Marton system has been unable to continuously comply with its resource consent for years, Mr Mills said, despite spending to improve it. Reports have said leachate from Bonny Glen is likely to be having a major effect.

Sludge should be removed from the ponds every 10 to 15 years, but Mr Mills did not think that had ever been done. When it was, there was a chance it would be too toxic to be allowed back at Bonny Glen. Some of that toxicity could be from tannery waste, either past or present.

Mr Mills proposed two possible solutions. One was for the landfill owner, Midwest Disposals, to take full responsibility for the leachate. That could put it in a difficult position, unless the Palmerston North wastewater system is advanced enough to treat the leachate.

Or Midwest Disposals and Rangitikei District Council could take joint responsibility. The company has said it will pay the full cost for council treatment, and it could do some pre-treatment at Bonny Glen.

It would be a long and costly process to determine the right pre-treatment for "abnormal waste such as tannery wastes", Mr Mills said. He was not confident the result would work well, or that the company would be asked to pay the true cost.

He compared the situation to Wanganui's and Eltham's wastewater treatment woes - systems designed for "low to normal domestic loading" and not coping with industrial waste.

Mr and Mrs Mills asked for the resource consent hearing to be adjourned, until some of those matters could be worked out. They were told the consent only concerned effects at the actual Bonny Glen site - regardless of whether decisions on it had effects elsewhere.

"It just seems a strange anomaly that that happens," Mr Mills said.

The hearing was not adjourned and he now intends to take the matter up with Rangitikei District Council.