2 Dec 2021

The 1 November fire at the Christchurch Wastewater Treatment Plant was one of the biggest in the city for several years and its impacts could be felt for a long time. Here, we answer some of your questions.

We are a month on from the fire. Why are there still odours coming from the treatment plant?

The fire on 1 November was centred on the two big trickling filters at the wastewater treatment plant. Because we are now unable to use the trickling filters, we have had to make modifications to the wastewater treatment process. The wastewater entering the oxidation ponds isn’t as thoroughly treated as it was before the fire – there are more solids and organic material in the ponds - so we are getting more odour issues.

During and after the fire, wastewater was flushed through the trickling filters to help put out hot spots and any smouldering areas. At a result, a lot of soot and a much reduced level of dissolved oxygen flowed through into the second half of the treatment process, harming the treatment biomass.  

How long will the smell last?

Unfortunately the smell could last for some time. It could take several months for the biomass to regrow. Until the treatment biomass is healthy again, odours will occur.

What are you doing to reduce the smell?

We have been using water misters to help suppress the smell from the material in the trickling filters. We have also been adding poly aluminium chloride – an odourless powder that dissolves in water – at two key points in the wastewater treatment process. This ‘poly dosing’ settles more of the suspended solids in the wastewater, improving the quality of the wastewater as it progresses through the plant.

We are dosing the oxidation ponds with hydrogen peroxide and installing a number of aerators to help increase the oxygen levels in the ponds. The aerators will improve the quality of the wastewater in the ponds and reduce the smell.

Are the odours likely to affect my health?

While the smell from the plant may be unpleasant, there is no evidence to suggest that it is harmful. We engaged a contractor to collect and analyse a number of dust and debris samples from across the site, particularly the fibrous material. This has confirmed that there was no asbestos material present in the fire debris.

Where did the fire start and how much damage did it cause?

The fire started in the roof of the Number 2 trickling filter and then jumped across to the roof of the Number 1 trickling filter. The roofs on both trickling filters subsequently collapsed.

We have not yet been able to inspect the interior of the trickling filters – each filter is filled with about 12,000 cubic metres of material that will need to be removed before we can do a proper inspection – so we don’t yet know the full extent of the damage.

However, we do know that the wastewater distribution system for the trickling filters has been destroyed.

The fire has also destroyed the air extraction system for the initial wastewater screening and primary settlement tanks at the plant.

What was the cause of the fire?

Multiple investigations are under way into the cause of the fire. Because these investigations are ongoing, we cannot comment on the likely cause at this stage.

Will you release the findings of the investigations?

Yes, we expect the key findings will be made public in due course.

Is the treatment plant still operating?

Yes, the plant continues to operate and capacity has not been affected so you can continue to use toilets bathrooms and kitchens as usual. However, a key step in the treatment process – the trickling filters - has been lost as a result of the fire.

What exactly is a trickling filter and what function do they serve?

The trickling filters are circular concrete structures, about eight metres high and 55 metres in diameter. They are filled with plastic media, around six metres deep, that support micro-organisms (a bacterial slime) for biological processing of the wastewater.

Wastewater drawn from the primary sedimentation tanks is introduced into the trickling filter via rotating arms which spray the wastewater across the top of the filter media. Air is forced into the bottom of the filter assembly to maintain aerobic conditions and support biochemical oxidation of the organic material and consumption of nutrients in the wastewater, releasing water and carbon dioxide.

The bacterial slime that sloughs off the plastic media is collected at the bottom of the trickling filters for further processing.

When will the trickling filters be replaced?

Until a full damage assessment can be undertaken, it is not possible to say how long it will take or how much it will cost to replace the trickling filters.

While it is obvious that the roof structure and wastewater distribution systems for the trickling filters have been destroyed, we do not yet know how much of the filter media has been damaged and whether or not the concrete housing has been structurally compromised.

To undertake a full internal inspection of the trickling filters, we need to remove most, if not all, of the filter media. This is a challenging logistical exercise that will involve removing several thousand cubic metres of filter material.

We are planning to trial the removal of a small amount of the filter material in early December. The outcome of this trial will help us establish the best way of removing the material and the timeframes for doing it.

What are you going to do with the plastic media from the trickling filters?

Our resource recovery team is looking at options for recycling the plastic material. If we can’t find ways to recycle it, it will have to be disposed of at landfill.

With the trickling filters out of action, are you now discharging raw sewage into the sea?

No. The wastewater at the plant is still is going through a comprehensive treatment process - the primary settlement tanks, the aeration tanks, the clarifiers and the oxidation ponds - before it is discharged into the ocean. We are optimising these treatment processes to compensate for by-passing the trickling filters.

We regularly test the levels of nutrients, particles and microbes in the treated effluent. This testing shows there has been a change in levels since we have modified the treatment process, however, we are still operating within our resource consent to discharge into the ocean.

The weather is warming up and I want to go for a swim at the beach. Is it still safe?

Yes. The wastewater being discharged from the plant is still being treated and it enters the ocean a long way off shore - about three kilometres from New Brighton beach.

Was the wastewater treatment plant insured?


The oxidation ponds are a wildlife reserve. Will there be any effects on the wildlife?

No, there should not be any adverse effects on the wildlife.

Is Environment Canterbury monitoring what is happening at the plant?

Yes, they are closely monitoring what we are doing at the treatment plant and visiting the site regularly.

If you have any issues with the smell from the wastewater treatment plant, please call the Council's Contact Centre on 941-8999.  If you wish to receive e-newsletter updates on the situation at the wastewater plant, email us at wastewater@ccc.govt.nz with Subscribe in the subject heading.

*Pictured above, the Christchurch Wastewater Treatment Plant before the fire. The two trickling filters are the large round structures on the right side of the photo with the domed roofs on them.