Midge season about to begin

Christchurch City Council
 

Christchurch Wastewater Treatment Plant update

Tuesday 14 October 

Midge season approaches

As temperatures rise, so does the likelihood of midges returning to the Christchurch Wastewater Treatment Plant. The oxidation ponds provide an ideal breeding ground, especially when algae levels increase.

Algae thrive in warmer conditions and serve as a primary food source for midges. Last summer’s heavy rainfall created perfect conditions for algae growth, which in turn led to a significant spike in midge numbers - much higher than in previous years.

Here's an overview of what causes midge numbers to increase, what we can do to control their numbers and how we make decisions that balance midge and odour control. 

How weather and wastewater impact algae

The performance of our oxidation ponds is closely tied to the weather and the concentration of wastewater entering the system. Rainfall dilutes the wastewater, which encourages algae growth. While algae are a sign of a healthy pond system and help reduce odour, they also contribute to midge proliferation.

Striking a balance: odour vs. midge management

Managing odour and midge populations is a delicate balancing act. More algae typically means less odour, but it can also mean more midges. Last year, we sought expert external advice to assess the impact of various control measures and to be sure of our decision-making.

Monitoring and control measures

This summer, we’re enhancing our midge monitoring efforts. We’re trialling equipment that monitors pond water quality, which we'll use to help us make operational decisions.

A dedicated PhD student will also be researching our ponds and midge numbers throughout the warmer months, helping us respond quickly and effectively.

Our long-standing midge control programme includes native planting around the ponds. These plants help keep midges close to the ponds and away from nearby residential and commercial areas. Since 2018, we’ve planted approximately 100,000 native plants to support this initiative.

Is dredging an option?

Before the fire at the Wastewater Treatment Plant, we used dredging and flow path adjustments to help control midge populations.

While these methods are effective, they also carry risks. Disturbing the pond’s biology can lead to increased odour, which can take months to resolve – potentially even lasting until winter 2026.

Our focus will always be prioritising approaches that maintain pond health while minimising both odour and midge impacts.

Find out more
 

Activated sludge reactor project update

We’ve now received the tenders for the construction of the project and our team is considering  the proposals. We're working through the evaluation process and expect to award the contract towards the end of November or start of December. 

The design is complete and ready to be developed into construction drawings. This is an important step and we're focused on making sure everything is lined up so we can begin work as soon as possible.

As mentioned in previous updates, once a contractor is confirmed, we’ll hold a community meeting. This will be a chance to meet the team, hear more about the project, and ask any questions you may have.

Site establishment and construction are expected to start by the end of this year or early in the new year.

Learn more about the treatment plant process
 
 

Next stage of clarifier trial about to start 

Our team is about to start trialling the installation of walls that will make it possible to isolate the treatment plant's clarifiers. These cutoff walls are a key preparatory step for critical repair works scheduled to begin February. If the trial is a success we’ll complete the installation of the walls towards the end of this year.

The walls will enable faster and more efficient dewatering, helping to streamline the upcoming works. We don’t expect the trial will impact operations at the plant.

 
 

Successful aeration basin maintenance shutdown 

A planned maintenance shutdown of the temporary aeration basin on Tuesday 8 October was successfully completed without any issues, thanks to the hard work and dedication of the team.

The high-voltage cable repairs were carried out smoothly, and operations resumed as expected. During the shutdown, all primary-treated wastewater was safely bypassed to the ponds.

 
 

Hydrogen sulphide (H2S) monitoring 

The latest graph from our hydrogen sulphide monitors shows the biological health of the ponds is still fragile. Intermittent low levels of odour are being detected at community monitoring sites. 

See the latest data
 
 

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.

FacebookTikTokYouTubeInstagramLinkedIn
 

You received this email because you are subscribed to updates from Christchurch City Council. 

 Privacy policy.

 
 
Christchurch City Council
53 Hereford St, Christchurch Central City, Christchurch 8013
Preferences  |  Unsubscribe