Read information about our most frequently-asked questions.

The smell

Treatment plant 

The smell originally came from two sources – the now-emptied trickling filters where the fire occurred and the oxidation ponds. 

Oxidation ponds

The fire severely damaged the trickling filters so we lost a critical part of our wastewater treatment process. As a result, poorer quality effluent was discharged into the oxidation ponds, so the ponds began to smell. If you remember Bromley in the 1960s and 70s, that smell would be very familiar.

Churning additional oxygen into the wastewater is key to improving the quality of the water within the oxidation ponds. In May 2023 we completed the installation of the 16 aerators to provide this oxygen.

We've also installed the temporary activated sludge plate that provides the wastewater to the ponds.

Wastewater quality

We installed a series of 16 aerators on Pond 1 to continue churning oxygen and maintaining the health of the wastewater. This work was completed in May. 

Now that more, better-quality wastewater is flowing into the ponds, we are only seeing it occasionally struggle over the cooler months.

Air-quality testing

Since April 2022, we’ve taken weekly samples from multiple sites, both at the plant and in the neighbourhood.  We also added extra sample locations based on the wind direction. 

 This work clearly identified reduced sulphur compounds (RSC) as the main cause of the odours. RSC are a complex group of substances that have a strong odour, even in low amounts. The monitoring found extremely odorous compounds, such as methyl mercaptan and hydrogen sulphide, present in varying concentrations, depending on the weather and wind direction.  

In the week beginning Monday 30 May, we started using a hydrogen sulphide meter at Bromley School. These meters continuously monitor very low levels of hydrogen sulphide in the air. The trial has been successful and we are leasing two extra meters and will purchase more.

We installed hydrogen sulphide meters for deployment around the Wastewater Treatment Plant site and across the neighbouring residential areas. Now that we have these meters in place we will have a much better understanding of the different types and levels of odours that people are being exposed to over time.

We continue to publish the hydrogen sulphide results weekly on this website.

The trickling filters

During the 1 November 2021 fire, the roofs on the trickling filters were destroyed. Immediately after the fire, the burnt plastic media housed inside the round concrete trickling filters gave off an acrid smell.

That smell largely went away but over time the material inside the filters has started to putrefy. When it rains the filter media gets wet and the organic matter trapped within the media putrefies, releasing a pungent odour until it dries.

Removing the filters

We had to carry out an extensive investigation into the fire and the damage it caused and work through an insurance claim, all of which has taken time. Other parties involved have also carried out their own investigations.

We needed to fully understand the damage to inform our remedial options, but our insurer agreed that the filter media needs to be removed regardless of whether the end result is a repair or a rebuild.

Demolishing the filters

We may end up having to demolish the filters. However, the material inside the filters was classed as a hazardous material and the concrete structures are huge – each is about three storeys high and 55 metres wide. We need to make sure that whatever we end up doing is safe for nearby residents, our contractors and the environment.

The material inside the trickling filters

The bulk of the material was a mix of plastic and biomass, which was used to help treat the sewage being processed through the plant. Some of the biomass within the tricking filters was rotting and that was one of the main causes of the stench early on.

Removing the material from inside the trickling filters

Southern Demolition and Salvage Ltd started work on-site on Thursday 12 May. They worked six days a week, 12 hours per day, to remove the rotting material from the fire-damaged tricking filters at the Christchurch Wastewater Treatment Plant. They completed this work in early September.

It was a complex task as the trickling filters have 8 metre-high concrete walls and there were about 26,000 cubic metres – about the volume of 10 Olympic swimming pools – of material to remove.

The rotting material was put through a chipper and compacted while still at the treatment plant site. Once chipped and compacted, a tough plastic membrane was used to seal the material for transport. 

The material was trucked and disposed of at Kate Valley Landfill.

Kate Valley Landfill is a regional waste disposal and energy park facility in North Canterbury. The 37-hectare site operates to international standards, fully compliant with New Zealand landfill guidelines and the US Environmental Protection Agency and European Union standards for municipal waste landfills. For more information, visit link).

Kate Valley Landfill runs two separate waste streams – General Waste from several transfer stations in the region, and Special Waste, which is waste that could not be sent through transfer stations, due to either its offensive or adverse health and safety characteristics. The deliveries of material, classified as special waste, from the Wastewater Treatment Plant will not impact or compromise its operations.

Timeline to replace the trickling filters

It’s likely to be years before we’re able to fully rebuild or replace the trickling filters.

The fire

The cause of the fire

Investigations into the cause and origin of the fire have been completed and FENZ has published its report.  Due to the complexity of the investigation, and the fact other parties are involved, we are not in a position to share any further information at this stage.


We have been in constant communication with our insurer since the fire. The wastewater treatment plant was fully insured.  Our insurer has accepted our claim but a settlement has not yet been reached. Investigations are still determining whether the trickling filters are a rebuild or a repair.


Running the wastewater treatment plant without the trickling filters

We have modified the plant so that we can bypass the trickling filters.

As mentioned earlier, we have converted two of the plant’s four clarifiers into aeration basins. Once wastewater passes through these new aeration basins, submersible pumps will push the wastewater through to the remaining two clarifier tanks.

The new system is now fully commissioned. The new aeration basins will not fully compensate for the treatment work that was previously undertaken by the trickling filters. However, they will greatly improve the quality of the wastewater being discharged into the oxidation ponds and the overall biological health of the ponds.

We are closely monitoring the water quality in the ponds to assess the effectiveness of the aeration basins.  We have installed 16 aerators on oxidation pond 1, to improve the water quality leading into the cooler months.

This temporary system is based on what we could swiftly implement. It maximises our ability to treat wastewater, while using some of the existing infrastructure at the plant to deliver the best possible chance of success using proven technology.

However, it is a highly lean, temporary system that's doing the job of a $100m trickling filter treatment process. It's operating near its operational limit and has a lifespan of about five years, until a permanent solution is in place.