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Christchurch City Council is set to consider options for reducing odour from the ponds at the fire-damaged wastewater treatment plant.

Since the 2021 fire, the plant’s oxidation ponds have been carrying a much higher load than usual. This ongoing pressure, combined with unexpected weather, has caused a stench that’s been affecting nearby communities.

A report going to the elected Council on Wednesday 1 April will ask them to choose between two options for reducing odour:

  • Increase aeration in the ponds to improve oxygen levels.
  • Increase aeration in the ponds to improve oxygen levels and build a diversion pipeline that sends some partially treated wastewater – after disinfection – directly to the ocean outfall, reducing pressure on the ponds.

Several other options were considered but ruled out. These included a diversion-only option, which was discounted due to its significantly higher cost than aeration alone, the potential to worsen conditions in the ponds, possible adverse ecological effects in the ocean, and the need for additional consenting requirements.

The staff recommendation is to increase aeration to a level that reduces the risk of odour 95% of the time. Staff consider this the most effective and practical approach compared to a combination of aeration and diversion because it is more straightforward to build and operate, has a low risk of failure, avoids sending treated wastewater directly to the ocean, and can be delivered more quickly.

The Council recognises the ongoing impact caused by the odour. Although the team’s top priority is to keep the damaged plant running without offensive odours, work is also well underway to get the long-term fix – the new activated sludge plant – up and running as soon as possible.

The ponds will face extra pressure at times during construction of the new activated sludge plant, which is the long-term solution to odour issues. Either option would help the ponds cope during construction.

The Council will consider the report at its meeting on 1 April.