Poor conditions starting to stabilise
As you know, over the past few weeks we’ve experienced significant challenges with the Christchurch Wastewater Treatment Plant's oxidation ponds – unexpectedly, conditions are the worst they've been since the fire.
Usually at this time of year the ponds are very healthy, but several compounding factors have contributed to the stench.
How we got here
Each year, the wastewater load which arrives over the holiday period varies significantly as industry shuts down for the summer holiday and our population varies. The temporary activated sludge plant, which is a biological process, doesn't manage this variation very well.
This can result in the temporary plant struggling and passing additional load to the ponds, however usually at this time of year the ponds are very healthy and are able to pick up the slack.
On top of this, we expected the high levels of rain in January to help flush wastewater through the system and improve pond health – however, this didn't happen, as it appears to have set back the algae growth in the ponds.
While we know the weather and high load are contributing factors, our team is reviewing all data to understand whether other factors also played a role in the current situation.
We’ve always known we're operating the temporary plant with a narrow margin for error, as it has no extra capacity. This makes the system more vulnerable to sudden changes. While the ponds are now showing signs of improvement, much of the recovery depends on natural processes such as algae growth and oxygenation.
Despite these challenges, we're using every tool available and are seeing progress. Increasing aeration in the ponds remains a key focus.
What we’re doing
- Balancing flow between ponds to better distribute load and support recovery.
- Regular jetboat oxygenation to increase dissolved oxygen.
- Ongoing monitoring of key pond health indicators, including dissolved oxygen levels, drone imagery, and lab samples (with a focus on algae levels, a major oxygen source).
- Updating our odour mitigation plan to reflect the current event.
- Doubling our odour assessment frequency, with an external contractor now reporting twice weekly (these graphs will be updated to our website).
- Dosing the ponds with hydrogen peroxide to support pond oxygenation.
- Monitoring the weather ahead of the incoming cold, wet system to help determine the best way to manage flows through the ponds.
Health, wellbeing and support services
We’re sorry for the impact this continues to have on you and your whānau. Please be assured we are doing everything we can to support the system’s recovery and reduce odour where possible.
Dr Annabel Begg, Medical Officer of Health for the National Public Health Service has provided the advice below:
"Exposure to hydrogen sulphide odour arising from the Christchurch wastewater treatment plant can cause nausea, headaches, eye and throat irritation, skin irritation, sleep disturbance, and worsening asthma symptoms at relatively low concentrations.
"If people exposed to the odour don't experience physical health effects, continued exposure to unpleasant or nasty, noxious odours can still have an adverse effect on people's mental wellbeing.
"While exposure to hydrogen sulphide is unpleasant and may result in health effects, it doesn’t accumulate in the body. Any health effects would be expected to resolve when the odours are brought back to normal levels, and long-term health effects are highly unlikely.
"People experiencing health issues as a result of the odour coming from the treatment plant should seek health advice from their GP, doctor, Hauora or healthcare provider, or call Healthline free on 0800 611 116, anytime 24/7 for advice as soon as possible. In an emergency call 111."
There are also free services across Ōtautahi Christchurch available to support and enhance overall health and wellbeing. These include:
Health Improvement Practitioners (HIPs)
HIPs are qualified mental health professionals based in General Practices (GPs). They offer free, 15–30 minute sessions focused on practical strategies to support mental and physical wellbeing. There is no limit to the number of sessions you can access.
You can book directly through your GP.
Pae Ora ki Waitaha Support
Pae Ora ki Waitaha is a free healthy lifestyles service developed with Māori and Pasifika providers.
More information is available on their website.
Healthline
Healthline is a free over-the-phone health service available 24 hours a day, seven days a week. If you or someone in your family is unwell, you can call Healthline and speak to experienced nurses and paramedics for health advice and information about what to do next.
Call 0800 611 116.
Other options we’re considering
· Methods to boost oxygen in the temporary activated sludge plant.
· Specialist algae sampling to help us further understand the situation.
What to expect
The dissolved oxygen levels have stabilised and are beginning to improve. While algae numbers remain low, results confirm algae is present in Ponds 1, 2A and 2B, with encouraging rebounds in Ponds 4, 5 and 6. The pond colour is also changing, which is another sign that conditions are starting to turn.
We remain focused on stabilising and improving the ponds before conducting a full review to identify any additional triggers behind this event.
Stay updated
We want everyone to be able to stay updated on what's happening at the Christchurch Wastewater Treatment Plant, and we regularly post new information on ccc.govt.nz/wastewaterfire
You're welcome to send this update to your networks. If you know someone in the community who doesn’t have email or internet access, please print this newsletter off and pass it to them.
If you know of anyone who would like to receive this e-newsletter they can subscribe online.
If you have any questions, or would like further information on the project, please call us on 03 941 8999 or 0800 800 169 or email wastewater@ccc.govt.nz
Thanks for your patience and understanding. We’ll be in touch again soon.
Positive signs with recent weather
Our teams have been onsite throughout the weekend closely monitoring pond conditions, odour levels and system performance. At this stage, pond conditions remain largely unchanged since our last update. Dissolved oxygen levels are still low across much of the system, so the stench is likely to continue in the short term.
The recent stretch of sunshine is helpful. Warm, bright conditions support algae growth, which plays an important role in restoring dissolved oxygen in the ponds. As algae activity increases, we expect this to gradually improve pond health. While this won’t deliver an immediate change, it is a positive factor supporting ongoing recovery.
What we’re doing
Our staff continue to:
• Monitor pond health and odour in real time.
• Review results and make small, controlled adjustments to support system stability.
• Keep aeration equipment and flow diversions operating as effectively as possible.
We know the stench is unpleasant and disruptive, and we’re sorry for the impact this continues to have on you and your whānau. Please be assured we are doing everything we can to support the system’s recovery and reduce odour where possible.
There are free services across Ōtautahi Christchurch available to support and enhance overall health and wellbeing. These include:
Health Improvement Practitioners are qualified mental health professionals based in General Practices (GPs). They offer free, 15–30 minute sessions focused on practical strategies to support mental and physical wellbeing. There is no limit to the number of sessions you can access.
You can book directly through your GP.
Pae Ora ki Waitaha is a free healthy lifestyles service developed with Māori and Pasifika providers – www.waitaha.health.nz/health-professionals/pae-ora/
Healthline is a free over-the-phone health service available 24 hours a day, seven days a week. If you or someone in your family is unwell, you can call Healthline and speak to experienced nurses and paramedics for health advice and information about what to do next. Call 0800 611 116.
We want everyone to be able to stay updated on what's happening at the Christchurch Wastewater Treatment Plant, and we regularly post new information on ccc.govt.nz/wastewaterfire
View the full newsletter here: https://createsend.com/t/r-CC104E14CC4A84162540EF23F30FEDED and subscribe to receive this newsletter online here: https://confirmsubscription.com/h/r/822D5D6D9FECF8772540EF23F30FEDED
What to expect over the weekend
Our staff are onsite monitoring the situation in real time, taking action to stabilise the ponds, and working to minimise odour where possible. Odour is likely to continue for now, but we are doing everything we can to manage and reduce its impact.
We know the odour is frustrating and unpleasant, especially for residents living close to the plant. We want to reassure you that our staff are working hard to manage the situation.
A recap on what's causing the odour
The recent heavy rainfall has disrupted the biological processes within the oxidation ponds. Monitoring shows dissolved oxygen levels have dropped across the system. Low‑oxygen conditions make odour more likely, especially in Ponds 1, 2 and 3, which are currently the most affected.
In past wet‑weather events, additional rainfall has supported pond recovery. This time, the ponds have not responded in the usual way. Increased inflow and load into the system have added further pressure, increasing the likelihood of odour.
We know the smell is unpleasant and disruptive, and we are sorry for the impact this is having on you and your whānau.
What we’re doing
- We’re monitoring pond health and odour continuously across the plant and in the surrounding community. Staff are reviewing results and making adjustments where possible. Recent and ongoing actions include:
- Actively monitoring the temporary activated sludge system, which is performing as well as it can under current conditions.
- Adjusting flows through the oxidation ponds to reduce load and odour pressure on the most affected areas, including redirecting more flow from Pond 1 into Pond 4 to take pressure off Ponds 2A and 2B. We’re monitoring this diversion closely while gradually lowering pond water levels to improve flushing and refresh the system.
- Deploying a jet boat on Ponds 2A and 2B to introduce additional oxygen without disturbing settled sludge.
- We recently installed two new aerators on Pond 2, which are performing well, and we’re installing another aerator in mid-February to further support pond health.
- We also continue a range of monitoring including pond sludge surveys, external odour specialist reviews, checking dissolved oxygen levels, upcoming weather conditions and a range of other influencing factors.
We are carefully managing all adjustments to avoid unintended impacts from changing too many factors at once.
What to expect over the weekend
Dissolved oxygen levels across the ponds remain low as of this morning. This means odour is likely to continue in the short term. Conditions may fluctuate depending on weather, temperature and system recovery. We will continue to monitor the situation closely and take further actions as needed.
We want everyone to be able to stay updated on what's happening at the Christchurch Wastewater Treatment Plant, and we regularly post new information on ccc.govt.nz/wastewaterfire
View the full newsletter here: https://createsend.com/t/r-57E575C0BB5288DF2540EF23F30FEDED and subscribe to receive this newsletter online here: https://confirmsubscription.com/h/r/822D5D6D9FECF8772540EF23F30FEDED
What we're doing to monitor and respond to odour
We know the increase in odour is unpleasant and our teams are working hard to manage the situation. There hasn't been a significant change in the situation since yesterday and we're expecting odour will continue for at least the next week.
Our operations team is actively tracking odour at multiple points across the plant and surrounding community, using our odour monitoring tools. Staff are reviewing results in real time and making adjustments where possible to minimise impacts.
Here are some of they key actions we're taking:
Temporary Activated Sludge Plant
- Our temporary activated sludge system is performing as well as it can under current conditions and we've maintained treatment capacity.
Pond flow adjustments
- We’re adjusting flow through the oxidation ponds to reduce load and pressure on the section struggling most with odour.
- Pond water levels will be temporarily lowered to allow more effective flushing and to help refresh the system.
Oxygenation
- Regular monitoring of dissolved oxygen levels continues.
- We'll deploy a jet boat to introduce additional oxygen into the ponds where it can be effective; this will be carefully balanced to avoid inadvertently stirring up settled sludge.
Aerator maintenance and upgrades
- One of the 17 aerators on Pond 1 failed during routine electrical maintenance. Our contractor is working hard to get it returned to service as quickly as possible.
- The two newer aerators installed on Pond 2 in recent months are performing well.
- An additional aerator is scheduled to be installed in February to further support pond health.
If you’re concerned about the impacts of the odour there is a range of information on our website about odour and health, and what support is available for impacted residents.
Stay updated on what's happening at the Christchurch Wastewater Treatment Plant, and we regularly post new information on ccc.govt.nz/wastewaterfire
Heavy rain causing odour
The recent period of heavy rain has significantly affected the health of several oxidation ponds at the Wastewater Treatment Plant which is causing the increased odour we’re noticing.
During previous wet weather events, the additional rainfall has generally supported pond recovery, improving overall pond health and preventing odour issues. However, this time the ponds have not responded in the same way.
Our staff are continuing to collect and analyse data to understand why these conditions have developed on this occasion.
We’ve also seen increased loading to the ponds, which also put more pressure on the ponds, increasing the likelihood of odour.
Our operations teams are working hard to closely monitor conditions and respond wherever possible to improve pond health and minimise odour. This includes:
• Adjusting flows through the ponds to stabilise loading
• Regular monitoring so we can react quickly to changes
• Using all available operational tools to support oxygen levels and improve pond performance
Based on current pond health, we expect higher than normal odour levels may continue for at least the next week. We’ll keep providing updates as conditions change.
We know odour impacts are disruptive, and we want to reassure our community that reducing them is a priority for us.
Find out more: ccc.govt.nz/services/water-and-drainage/wastewater/treatment-plants/christchurch-wastewater-treatment-plant/news-updates-and-meetings
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