Increase in midges
22 January 2025.
The wet weather in late December 2024 and January 2025 has had a positive impact on the biological health of the ponds. The volume of rain means wastewater entering the plant wasn’t as strong, and as a result, algae are flourishing.
Algae helps the pond system operate well by generating oxygen within the ponds and preventing the anoxic conditions which can cause odour. The fragile health of the ponds is demonstrated through recent odour monitoring(external link), which shows intermittent, low levels of odour are being detected at the community odour monitoring sites. This is an improvement in odour levels in late 2024, which spiked during the transition to summer mode. Maintaining the health of the ponds is critical to ensuring odour is at a minimum.
Unfortunately, algae are also a food source for midges and the increase in algae has resulted in a surge in the number of midges on and around the oxidation ponds, with numbers much higher than the previous few summers.
We have a midge control programme in place which includes native planting around the oxidation ponds. Native plants encourage midges to stay close to the ponds rather than spreading to local residential and commercial areas. Since 2018 we have planted approximately 75,000 native plants around the oxidation ponds.
In the past, when the ponds were healthy, we dredged the bottom of the ponds and altered the flow paths through the ponds to reduce midge numbers. We’ve considered these control options but have decided against them at this time as there is a real risk of disturbing and unsettling the health of the ponds, which could generate odour. In addition to this, odour caused by a change in pond biology takes time to address, and in a worst-case scenario, an increase in odour could last until winter.
The weather has a huge part to play in how our oxidation ponds operate, and the strength of wastewater entering the ponds is now increasing due to a recent patch of reasonably dry weather. If this weather continues it could make the pond conditions less suited to algae, which could in turn result in a decrease in midges.
We know the midges are frustrating and are grateful for the ongoing patience of residents living in impacted areas. When making decisions about the next steps we’re balancing odour and midge management to try and get the best result for the community. We’ve engaged expert external advice to get a second opinion on what the impact of control measures would be, and to ensure that we’re doing what we can.
Increase in odour during summer transition
6 December 2024.
There has been an increase in odour coming from the Christchurch Wastewater Treatment Plant as the plant transitions to summer mode.
When the weather is warm and dry, the wastewater entering the plant is more concentrated. This puts more pressure on our temporary systems, which have been struggling to deal with the increase, and results in more odour.
We have plans in place to deal with seasonal changes but it’s not possible to predict all the variables that impact plant operations and odour.
Our staff are aware of the increase in odour and have made changes to our operations to stabilise the treatment process and reduce odour. We’ve increased the number of aeration lanes in service from one to three, which increases our treatment capacity. We’ll keep a close eye on this to ensure we can control the amount of activated sludge growing in the aeration basins and will continue to tweak operations as needed.
Of the 16 aerators on the ponds, 15 are operational, and we’re working on getting the 16th working again as quickly as possible.
We anticipate that the treatment process will stabilise in the next few days and odour will progressively reduce over the next week or so.
We’re doing our best to keep the plant running in its damaged state and appreciate your patience as we deal with this increase in odour.
Demolition of trickling filters underway
Demolition of the Christchurch Wastewater Treatment Plant’s fire-damaged trickling filters has started and quick progress is being made to bring the walls down.
We’re aiming to have the walls of the trickling filters down by Christmas and in the new year we’ll shift our focus to removing the foundations.
Insurance settlement secures the future of treatment plant
9 October 2024.
An activated sludge reactor will replace the fire-damaged trickling filters at the Christchurch Wastewater Treatment Plant to fix long-standing odour issues impacting local residents.
Christchurch City Council and its insurer have settled an $85 million insurance claim following the fire in 2021, which destroyed two trickling filters at the plant in Bromley.
The filters were a critical part of the sewage treatment process, and the damage impacted the treatment process causing major odour issues that have affected the local community.
The settlement was approved by Councillors unanimously and follows complex negotiations. Confidentiality was required for these negotiations so we could get the best outcome for residents and the environment.
FENZ fire investigation report published
16 December 2022.
We have just received from FENZ their Fire Investigation Report – Shuttle Drive, Bromley, Christchurch City [PDF, 5.1 MB].
Consequently, we have not had an opportunity to consider it fully. Our own investigation is ongoing and we will not be commenting further.
Snapshot of hydrogen sulphide levels decrease
2 November 2022.
This graph shows how the hydrogen sulphide levels coming from the oxidation ponds have significantly decreased over the four months from 10 June to 10 October 2022.
The activated sludge plant was made operational on 28 July, with the biology reaching full maturity on 17 August, despite the challenges of growing the biology over the middle of winter.
It took four weeks for the better-quality wastewater to flush through our network of six ponds, with the odour substantially reduced by mid-September. This was in line with the timeframes that we promised the community we'd work to.
Hold off from cleaning your house
30 September 2022.
With hydrogen sulphide levels dropping and the days warming up, it might be tempting to clean any discolouration off your house that might have been caused by a chemical reaction between lead paint and hydrogen sulphide.
We ask that you keep waiting. Before we begin encouraging people to clean their homes, we'd like to see at least four weeks of low hydrogen sulphide levels and improved weather to get an idea of how any discolouration fades.
We're about to start monitoring some 'test houses' and will provide updates on our website to show how the discolouration improves.
If you absolutely must clean your house now, make sure you're familiar with our list of dos and don'ts that we published last month.
Do:
- Read the report findings and information(external link) on house discolouration.
- Wash your walls with a commercial chlorine-based solution, or warm, soapy water. This could help remove any organic dust, common moulds and some discolouration. The presence of these common moulds is not linked to the wastewater treatment plant fire.
- Adhere to the manufacturers’ proprietary ratio recommendations if you use a commercial chlorine-based solution.
- Several washes might be required.
Don't:
- Don't use a high-pressure washer to try and clean any staining or discolouration, as this could result in flakes of lead-based paint entering the environment.
- Don't exceed the manufacturer’s proprietary ratio recommendations if you use a commercial chlorine-based solution.
Our team is keeping a register of homes that have reported paint discolouration. If your home has discolouration and you haven't let us know, or if you'd like more advice about what to do, email wastewater@ccc.govt.nz.