Christchurch Wastewater Treatment Plant update

Friday 6 March 

We’re continuing to see gradual improvements in the ponds. The latest monitoring shows that pond 6 has moved from orange to green on our odour status tracker. This is a good sign and suggests the ponds are very slowly heading in the right direction. The other ponds remain much the same, but overall the trend is slow but steady improvement. Despite this, the pond system remains very susceptible to changes in the weather.

We’re continuing to dose hydrogen peroxide into the ponds, and our trials using percarbonate at the temporary activated sludge plant are still underway.

Our maintenance contractor has been carrying out planned checks on the pond 1 aerators. This meant each aerator needed to be briefly taken offline while the work was done. All aerators are now operating again.

Seasonal change can mean more odour 

In previous years, spikes of odour have occurred as the oxidation ponds transition from summer to winter conditions.

This seasonal change happens as temperatures drop and the natural algae and biological activity in the ponds reduce. If this shift occurs too quickly or becomes unbalanced, it can create low-oxygen conditions that generate odour.

This year, the cooler and wetter summer has meant algae levels have remained lower than usual, so the ponds have effectively been operating closer to “winter mode” for some time already.

While this reduces the likelihood of a sudden transition, our team continues to closely monitor pond performance and proactively manage aeration and treatment processes to minimise any potential odour effects on the surrounding community.

We remain committed to maintaining stable operations and keeping our local residents up to date.

What to expect this weekend 

As we mentioned earlier in the week, the mix of cold weather followed by warmer, settled conditions can make odour more noticeable. When the air is still and temperatures rise after a cold snap, odour can hang around for longer and move into nearby suburbs more easily.

Because of this, there is the potential there could be some increases in odour at times over the weekend even though most of the ponds are stable.

Our team will continue monitoring conditions closely and carrying out the daily actions that support algae and oxygen levels in the ponds.

How diverting treated wastewater to the ocean outfall would work 

We’ve heard a lot of questions about the wastewater pipe that the Council is currently investigating as one option to help reduce the stench from the oxidation ponds. While no decisions have been made, this update explains how the pipe would work, and how it fits in with the existing treatment process.

Why a pipe is being looked at

Diverting a proportion of treated wastewater around the oxidation ponds to the ocean outfall would help ease pressure on the ponds by reducing the volume of treated waste they need to handle.

Since the 2021 fire damaged parts of the wastewater treatment plant, higher loads of waste have been entering the oxidation ponds. This has made it harder for the ponds to maintain healthy oxygen levels, particularly when coupled with unexpected weather conditions. All these factors contribute to ongoing odour issues.

The process 

1. A portion of the already‑treated wastewater would be diverted around the ponds, rather than through them.
    
2. Because the ponds normally reduce bacteria, this diverted flow would instead be disinfected with chlorine to ensure it meets consent limits for bacteria.
    
3. Before entering the ocean outfall pipe, this disinfected flow would be mixed back in with the pond-treated water. 

All wastewater is treated

All wastewater would continue to be treated at the main plant before going anywhere near the pipe. This treatment process includes:

• Screening to remove larger material
• Primary sedimentation to settle solids
• Temporary activated sludge processing to break down organic material
• Final clarification to remove remaining solids

This treatment happens inside the plant – not in the ponds.

Adding chlorine to reduce bacteria 

Because the ponds normally reduce bacteria, this diverted flow would instead be disinfected with chlorine to ensure it meets consent limits for bacteria. Before entering the ocean outfall pipe, this disinfected flow would be mixed back in with the pond-treated water.

Meeting consent conditions 

If this option progresses any discharge to the outfall would need to meet the consented limits.

On Wednesday, our team had a positive meeting with Environment Canterbury staff to discuss what the consent process would involve if this option is progressed. 

Where is the ocean outfall  

The long outfall pipe discharges three kilometres off New Brighton beach. The total outfall pipe length from the oxidation ponds is 5.2 kilometres and it is 1.8 metres in diameter.  It is buried about eight metres below the sea floor.

When it would be used

If approved, the pipe would be a temporary measure until the new activated sludge plant is built and operational.

Next steps 

There are important steps we need to work through before this plan can progress. We have staff  developing a plan as quickly as possible, and we will keep you updated as this progresses.

Latest hydrogen sulphide monitoring 

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.

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.