Information about the November 2021 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 have developed an adaptive recovery action plan [PDF, 164 KB] to provide an overview of the different work streams we're working across. This plan will be updated regularly.

Latest infographic overview

We’ve noticed that Ponds 2A and 2B are showing signs of changing due to the reducing daylight hours and the very settled weather conditions we have experienced lately.

The biological oxygen demand (BOD5) has slowly been trending upwards while the dissolved oxygen levels are slowly decreasing. We have also noticed a slight visual haze on the surface of the first two oxidation ponds, and a very faint odour when we’re within 10m of the ponds’ edge.

Taking all these indicators into account, we’ve decided to err on the side of caution and turn ponds 2A and 2B back to ‘orange’ in our pond tracker.

An orange reading means the water quality in that specific pond is average and there’s the possibility of odours, especially on very calm and settled days.

What we’re doing about this

We have anticipated that the cooler months and reducing sunlight hours could affect the performance of the oxidation ponds and we are working hard to establish a series of 16 mechanical aerators on Pond 1.

These aerators, which we’ve sourced locally and are being manufactured in Rolleston and Timaru, will churn more than 300kg/h of oxygen into the wastewater and improve the biological health of the pond.

We’re currently installing the electrical cabling that’s required for these aerators, and permanent location marker buoys will be installed next week.

Five disc aerators – which transfer lots of oxygen to the water while also creating optimum flow patterns in the pond – are on schedule to be installed around the edge of Pond 1 by the end of this month.

An additional 11 vertical shaft aerators, which can push very high levels of oxygen into the water, will begin arriving on site by mid-March. This is the style of aerator we installed on the clarifiers last year, as part of the temporary treatment process.

We’ll begin installing these across Pond 1 by the end of March, and hope to have this work completed in May.

Options around a permanent solution for the fire-damaged wastewater treatment plant will go before the Council once the insurance processes have been settled. This process could take some months.

We’ll keep you updated as this project progresses.