Christchurch Wastewater Treatment Plant situation

Information about the November 2021 Christchurch Wastewater Treatment Plant fire, the resulting smells, and our work to date.

On 1 November 2021, a large fire destroyed both of the trickling filters at the Christchurch wastewater treatment plant in Bromley.

The trickling filters are a critical piece of the sewerage treatment process and the damage to them made the treatment process considerably less effective.

As a consequence, Christchurch residents, particularly those downwind during the predominantly easterly winds, have experienced a significant increase in unpleasant odours.

These smells initially came from the burnt trickling filters, but are now coming from the oxidation ponds.

Despite our best efforts, the change to the treatment process without the trickling filters means there is an increase in solids and biological material entering the oxidation ponds, contributing to the stench. We've got the site to a position where it's reasonably stable in summer and autumn, but might still produce odours during the cooler months.

This adaptive recovery action plan [PDF, 164 KB] provides an overview of the different work streams we worked across during the 2022 fire response and recovery. 

Latest infographic overview

With summer on our doorstep, the ponds are showing signs of being very healthy. Our ongoing monitoring shows that Pond 1, the only one which was still classed as orange, now meets the criteria to finally turn green.

This also means that we're able to turn off a number of aerators, and we'll monitor the situation to see if they need to be re-started.

This also allows the contractor to do some regular maintenance over the summer period. This will involve bringing the aerators to the shore and taking them away for a service. We'll reinstate them back onto the ponds ready for autumn.

We've also started the midge monitoring programme. Midge numbers are currently low, which reflects how well-established our midge control measures are, which we've progressively brought in since 2017. We'll continue to monitor the midge numbers and bring in extra midge control measures if required.

How has this affected hydrogen sulphide levels?

We have strategically set up hydrogen sulphide (H2S) monitors at locations around the oxidation ponds and nearby communities. These have been in place since July last year and the results are regularly published on the air, water quality and house-testing results page(external link).

Our monitors continue to detect low levels of H2S from the ponds. The monitors have recently recorded an increase in H2S activity due to the aerators being out of commission, but the average levels remain below 0.01 parts per million – well below the 0.03ppm threshold of concern we have adopted in consultation with Health New Zealand. Even though the levels are well under the threshold the odours are still noticeable.

We’ll continue to monitor the H2S levels and readings and publish the results on our website and will let you know if the levels begin to increase.

Wind trends in Bromley

The below wind roses show the predominant wind directions in Bromley and how they differ over the seasons. These are the average readings over the past three-and-a-half years and were collated by Jacobs. 

Summer wind

The predominant wind direction over summer is from the east and east-northeast.


Autumn wind

The predominant wind direction over autumn is from the east and east-northeast, but not as strong.


Winter wind

The predominant wind direction over winter is front the southwest and west


Spring wind

The predominant wind direction over spring returns to an east and east-northeasterly.