Christchurch Wastewater Treatment Plant update

Thursday 2 April 

Council approves additional aerators 

Christchurch City Council has approved the installation of additional aerators in the ponds at the fire-damaged wastewater treatment plant to reduce the risk of odour.

The plant’s oxidation ponds have been operating under increased pressure since a major fire in 2021, forcing them to function well above their usual capacity. Combined with periods of unusual weather, this sustained strain has at times resulted in offensive odours impacting nearby communities.

To help reduce the odour, the Council has approved increasing aeration in the ponds to increase oxygen levels to a level that reduces the risk of odour. The new aerators are expected to be operating within six to ten weeks.

“We’ve been committed to finding a solution for the people of the east who have been affected by the stench,” Mayor Phil Mauger said.

“We needed to look at all possible solutions, so people know we are doing the very best for the community. Now we’ve made a decision we can get on with it as quickly as possible.”

At a workshop last month Councillors were presented with a range of potential solutions with Staff then doing a full cost and risk assessment of these options.

Increasing aeration was considered the most effective and practical solution. The option is simpler to build and operate, carries a low risk of failure, avoids discharging treated wastewater directly into the ocean, and can be delivered more quickly than alternative approaches.

Staff have also been given authority to add more aerators straight away if pond conditions worsen and they are required.

Work is also well underway on the long-term solution, the activated sludge plant, which is expected to permanently resolve odour issues once operational.

During construction of the new plant, the oxidation ponds are expected to face periods of additional pressure.

Increased aeration will also help the system cope during this transitional phase, reducing the likelihood of odour while the new plant is built.

Pond health update 

Overall pond conditions remain stable, with monitoring continuing to show the system is coping well as we move through seasonal change.

Staff have noticed the pond colour changing, which reflects changes happening within the pond system. These shifts are most likely linked to cooler temperatures and the transition toward winter conditions, rather than any deterioration in pond health.

The ponds are currently around 17 degrees celcius, which is the temperature that algae and biological activity in the ponds starts to slow down.

For the moment, the ponds are continuing to support a healthy algae population, although as mentioned above, we’re seeing some changes in colour and distribution. Our team is closely monitoring these changes and will respond as required. 

Community odour monitoring continues to show only low levels of hydrogen sulphide, which aligns with the generally stable conditions we’re seeing across the system.

Pond odour tracker update

The Christchurch Wastewater Treatment Plant has recently moved from red to orange on the pond odour tracker, which is also reflective of the cooler temperatures, as our internal monitoring has shown fewer and less intensive odours onsite. The ponds remain the same. 

What to expect this weekend

Looking ahead to the long weekend, conditions are expected to remain settled and stable. The forecast for sunny weather over the next few days should help support algae activity and assist with maintaining oxygen levels in the ponds.

While we don’t expect any significant changes over the weekend, it’s important to note that some variation in pond conditions is normal at this time of year. As we move further into winter mode, fluctuations in algae behaviour and oxygen levels can occur, and this can occasionally lead to short‑term odour changes.

Our operations team will continue close monitoring and will make any operational adjustments if needed.

Chemical trial outcome 

Since last year we’ve been undertaking a trial at the Christchurch Wastewater Treatment Plant to assess four products intended to help reduce odour.

An independent assessment of the trial has now been completed by an engineering consultancy and university. 

The results 

The assessment concluded that none of the four products trialled are suitable for operational use at plant.

The results showed that:

• The products did not provide a measurable reduction in odour, and/or
• The products resulted in negative impacts on dissolved oxygen levels, pH levels, or algae.

Based on these findings, we will not be progressing with any of the products trialled.

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.