A multi-million-dollar, three-year upgrade of the Marton wastewater treatment plant has been proposed.

Rangitikei district councillors will this morning discuss changes aimed at making the plant compliant with its resource consent.

The MWTP has been non-compliant for at least a decade, partly due to its acceptance of leachate (run-off) from the nearby Bonny Glen landfill. The latest compliance report found the plant's discharge into the Tutaenui Stream "significantly non-compliant". At times, ammoniacal nitrogen levels have been 35 times the recommended limit and deemed to adversely affect aquatic life.

Earlier this year, Horizons Regional Council asked Rangitikei District Council to explain why the plant consistently failed consent and how this would be remedied. RDC has since come up with a plan to bring it up to standard by 2019, when it needs to renew its consent. A report by the council utility assets manager Joanna Saywell said the council also planned to sign a formal trade waste agreement with Bonny Glen owner Midwest Disposals and establish a community focus group to liaise with council throughout the upgrade.

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RDC's assets and infrastructure committee will be asked to endorse the proposal to upgrade to the MWTP by 2018.

Midwest Disposals is developing leachate pre-treatment facilities at the landfill. The council report estimates it will have the capacity to negate discolouration and nitrogen by mid-2016. The council and Midwest will build holding tanks so leachate and septic waste can be fed into the treatment plant at a consistent rate to avoid "shock loading".

Meanwhile, the council is testing sediment to establish whether heavy metal from the leachate is affecting the Tutaenui Stream.