Pond health continues to improve

Now that the new activated sludge system is allowing better-quality wastewater to enter the oxidation ponds, we're starting to see the health of the ponds recover.

Ponds 5 and 6 reached our benchmarks to turn orange last week and are continuing to improve, while our measurements for biological oxygen demand (BOD5) and dissolved oxygen are showing encouraging progress in some of the other ponds.

However, the oxidation ponds earlier in the wastewater treatment process (pond 1, for example) will take a bit longer to recover; they have received higher organic loads since the November fire, so they're therefore in the poorest health. Pond 1 is also a large pond, so will take longer to improve than some of the smaller ponds (such as 2a, 2b and 3).

The ponds are recovering slowly and although it's now likely odours will remain for most of September, these will progressively reduce as the pond health improves. Warmer weather and increased sunlight hours will also help the ponds improve faster.

We know this is not the news people want to hear, but we're doing everything we can to improve the biology of the ponds as quickly as possible.

We change a pond's colour when its water quality meets our thresholds for biological oxygen demand (BOD5) and dissolved oxygen. We’re monitoring a range of other parameters including suspended solids, pH levels, sludge levels, nitrogen and phosphorous levels, enterococci and faecal coliforms.

As wastewater moves through each pond, any remaining sludge sinks and the clearer water  flows to the next pond, meaning the quality of the wastewater continually improves through the process until it leaves the treatment plant.

However, it will take another couple of months for the biological health of the ponds to return to a level of resilience where they’re capable of handling increased organic loads if there’s a temporary failure somewhere in the treatment plant process.

We intend to only turn a pond 'green' when there are higher levels of dissolved oxygen present and its biological health reaches this level of resilience.

Mayor thanks contractors and staff

This afternoon Christchurch Mayor, Lianne Dalziel, met and formally thanked contractors, subcontractors and staff for their hard work since the November 2021 fire that destroyed the trickling filters at the wastewater treatment plant.

"I want to say a huge thank-you for all you've done and continue to do in the wake of a fire that many have described as unprecedented," Mayor Dalziel said.

Last week's air quality testing results

Every Wednesday we upload the past week's data for the continuous air monitors that have been set up near the Christchurch Wastewater Treatment plant.

The monitors are to detect the levels of hydrogen sulphide in the Shortland Stree and Rudds Road areas, as well as at Bromley and South Brighton Schools.

Note the readings online are only up until Friday 9 September – we'll publish graphs showing this week's increased levels of hydrogen sulphide early next week once the data from all of the sites has been collated.

You can read the data for all locations, and an explanation about how to read the information by clicking the link below. You can also find past readings for each site.

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 in advance for your patience and understanding as we continue to work on minimising the unpleasant smells after the fire. We’ll be in touch again soon.