Christchurch Wastewater Treatment Plant update

Monday 2 February 

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.

We've planned a residents meeting for tomorrow evening at 5.30pm at the Bromley Community Centre. We’ll have staff on hand to explain the current situation and answer your questions. More information is available in the newsletter below.

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.

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.

Latest hydrogen sulphide monitoring 

We’ve increased hydrogen sulphide monitoring to twice weekly. See the latest result below.

The biological health of the ponds is fragile. Significant spikes of odour are being detected at community monitoring sites. Our teams are doing everything they can to address the current situation.

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.

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.