Wastewater sludge leaks into Manawatu River
Dark sludge from the wetland pond at Palmerston North's wastewater treatment plant oozed into the Manawatu River a week ago during a malfunction.
The pond and the rock passage to the river have been closed down while the problem is resolved, and fully treated effluent from the Totara Rd plant is being discharged directly into the river.
Waste and water services manager Rob Green said the pond became anaerobic (lacking oxygen) due to an accumulation of sludge and a period of warm weather.
The discharge of some sludge-like material from the ponds continued for a short time, coinciding with low flows in the river.
Horizons staff are working with the city council to establish whether the failure amounts to a breach of the discharge consent.
The city's sewage is still being treated to the full tertiary standard required by its consents.
After it goes through the aeration ponds, it is going through the phosphorus removal plant where alum treatment began recently as river levels dropped, and it also passes through a UV plant to kill bugs.
"The discharge is being monitored and we will be happy to share the results of monitoring with Horizons Regional Council and the public," said Mr Green.
The wetland pond and associated rock passage to the river were installed as part of the 2007 upgrade to meet cultural concerns.
Mr Green said the recovery plan was to pump the water out of the pond into the adjacent sludge lagoon, and then remove the sludge from the wetland pond and dry it out.
Horizons was reviewing the plan.
Mr Green hoped the wetland pond would be back in service within a few weeks, but it could take longer.
Meantime, a work plan is still being put together to address ongoing issues with the quality of the city's discharge.
Joint water quality testing last summer showed the discharge was having a significant adverse effect on aquatic life downstream, which is a breach of the discharge consent.
High levels of phosphorus were identified as the cause of the problem, however, there was more phosphorus in the river than what the discharge was pouring in at the time.
Staff of both councils were working to understand the problem.
A "sponge effect" was suspected, in which phosphorus discharged when the river was in high flow was somehow locked in the river bed, and was squeezed out as the river level dropped.
The work plan will be reported to the city council for approval by the end of the month.
Mr Green said a decision on whether alum dosing would continue for longer than usual once the river levels rose again would be made by councillors.
Cr Chris Teo-Sherrell said it was disturbing to witness "yet another problem" with the city's wastewater treatment.
But the councillor, who last year made a $21,000 Official Information Act request to find out what he believed council staff had not been telling councillors about the plant, said it was a "vast improvement" that councillors had been notified by email this time.