Christchurch City Council is investigating pumping partially treated wastewater directly to its existing ocean outfall pipe in a bid to reduce the ongoing stench from the fire damaged Wastewater Treatment Plant.

Cooler temperatures and high rainfall have reduced oxygen and algae levels in the treatment ponds, leading to renewed spikes in odour across parts of the city.

Christchurch Mayor Phil Mauger said the council had been in discussions with Environment Canterbury and mana whenua about an interim solution.

“Because the Wastewater Treatment Plant has been running on a knife edge since the fire, the ponds have been overloaded and are now in poor health,” Mauger said.

“We can’t sit back and expect conditions to improve on their own.”

“Anyone who lives nearby knows the odour issues have become more intense, and we’ve been looking at all ways to help alleviate the stench.”

He said reducing the amount of wastewater entering the ponds would ease pressure on the system and allow them to recover.

“Reducing the amount of wastewater entering the ponds will take the load off, give them space to recover, and most importantly address the current odour,” he said.

“If we can reduce how much wastewater is going into them, it will help stabilise pond conditions while the longer term rebuild of our new treatment plant continues.”

Under the proposal, some partially treated wastewater would bypass the ponds and be diverted to the ocean outfall. The council said chlorine would be added to ensure bacteria levels were safe before discharge into the ocean.

The move would also provide additional resilience while construction continues on a new activated sludge reactor and while essential maintenance is carried out on one of the plant’s clarifiers.

Council staff were working through the operational and regulatory requirements needed to implement the plan and said residents would be kept updated as it progressed.

Mauger said the council would continue exploring further actions to improve pond health and reduce odour while the long term rebuild continued.