Update on aerators and the oxidation ponds

Despite our best efforts, the oxidation ponds at the wastewater treatment plant are again starting to produce some odours as we head into the cooler winter period. Monitoring has shown that these odours are more noticeable on calmer mornings.

With this in mind, we’ve changed Ponds 1, 2a and 2b back to orange on the Pond Tracker above, and on the Wastewaterfire website. This means the water quality in the ponds is average and odours are possible.

We’re still trying to determine exactly what’s causing the smell, but we believe a combination of factors might be at play, rather than one significant issue.

How this has affected hydrogen sulphide levels

We have strategically set up hydrogen sulphide (H2S) monitors at locations around the oxidation ponds and nearby community. These have been in place since July last year and the results are regularly published at the link below.

Our monitors continue to detect low levels of H2S from the ponds. The monitors have recently recorded an increase in H2S activity due to the aerators being out of commission, but the average levels remain below 0.01 parts per million – well below the 0.03ppm threshold of concern we have adopted in consultation with Health New Zealand.

The next steps

All five disc aerators are expected to be reinstalled and operational on Pond 1 by 9 June. Circulating the wastewater will again distribute the oxygenated water around the entire pond, which should decrease the odours.

If the odours don’t decrease, we’re looking at a number of other chemical options, such as adding hydrogen peroxide – a compound that naturally breaks down into water and oxygen – and chemical coagulant to potentially settle the sludge in Pond 1.

However, there is still a risk that odours could persist over the winter months. Last year we saw the ponds begin to recover and perform well from September onwards, as the warmer weather and longer daylight hours improved the biological health of the ponds.

Where to get help

If you need health support, we have information available on our website. This includes information on Pegasus Health's Partnership Community Workers and contact numbers for Healthline and counselling services.

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.