Oxidation pond update

We’ve noticed that Ponds 1, 2A and 2B are showing signs of change due to the reduced daylight hours and the cooler weather conditions we have experienced lately.

While the biological oxygen demand – the loading on the ponds – hasn’t increased dramatically, the colder temperatures, rain, and reduced sunlight hours are beginning to lower the dissolved oxygen levels.

We've also noticed a slight increase in odour in the immediate vicinity of Pond 1, and some haze on the water surface where we know there are greater accumulations of sludge on the bottom of the pond.

Although the plant’s temporary treatment process continues to work well, cooler, dry weather is forecast. This could lead to less ideal natural environmental conditions for the biological health of the ponds.

Taking all these indicators into account, we’ve decided to err on the side of caution and turn Pond 1 back to ‘red’. A ‘red’ pond means its water quality is poor and there’s a high probability of odours.

Ponds 2A and 2B remain at orange. An orange reading means the water quality in that specific pond is average and there’s the possibility of odours, especially on calm and settled days.

What we're doing about this

To minimise odours over the cooler months, 16 new aerators on the Christchurch wastewater treatment plant’s oxidation ponds will begin surging to life from next week.

Operations Manager, Adam Twose, says once they’re connected, the 11 vertical shaft and five disc aerators (one pictured above) will churn more than 300kg of oxygen into the first pond’s wastewater every hour.

“The higher levels of oxygen should improve the biological health of the pond and see better-quality wastewater then flow through the pond network,” Mr Twose says.

Last week's hydrogen sulphide readings

This graph displays the hydrogen sulphide (H₂S) results from monitors situated at:

• Rudds Road, which will detect hydrogen sulphide carried on an easterly wind from the direction of the wastewater treatment ponds.
• The Dog Shelter, which is located between Living Earth and the oxidation ponds, which will detect hydrogen sulphide coming from either the ponds or the estuary.
• South New Brighton School, which will detect hydrogen sulphide carried on a west or southwest wind from either the ponds or the estuary.

The graph shows that only low levels of H₂S were detected on a few occasions at the sites. 

You can see the data for all of the locations we've monitored since July 2022, and an explanation about how to read the information by clicking the link below. 

Stay updated

We want everyone to be able to stay updated on our response and recovery from the treatment plant fire, and we regularly post new information and reports on ccc.govt.nz/wastewaterfire 

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.