Christchurch City Council staff are considering next steps in the damage assessment work after completing a trial to remove a section of the fire-damaged filter media at the Christchurch Wastewater Treatment Plant on Thursday.
An excavator removed 2.2 tonnes of melted material from one of the trickling filters that were destroyed in the fire on 1 November.
Head of Three Waters, Helen Beaumont, says the half-day operation has provided valuable information.
“We now know the condition and state of the filter media, how easily it can be removed, and how much it packs down when it’s compressed,” Ms Beaumont says.
“There’s quite a bit of open space between the filter media to provide room for the biological slime to grow and to allow air to pass up through the trickling filters. We will be compacting the media to make it easier to dispose of this material.”
The media in each of the trickling filters is estimated to be between six and seven metres deep, with a total volume of about 24,000 cubic metres. This material is expected to compact down to 3000 cubic metres, which would weigh in the region of 500 tonnes.
Ms Beaumont says damage assessments of the concrete trickling filter structures are currently under way. The results of these assessments will determine if the structures need to be completely demolished.
“Once we receive those findings we’ll have a much better idea on our next steps. If the structures need to be demolished, that will require a much different deconstruction process than if the walls can be preserved.
“If we have to remove the filter media in the same way we undertook Friday’s trial, the process will be painstaking and will need to be done very carefully so that the trickling filter walls aren’t damaged.”
Ms Beaumont says there was no notable increase in odours as they removed the media.
“It was encouraging that the filter removal trial didn’t result in any significant odours. However unfortunately we still have the ongoing smells coming from the secondary contact tanks and the oxidation ponds, and we’ll continue to treat the wastewater at these points.
“These processes are dealing with much higher loads of organic material with the trickling filters out of action. We’re optimising these parts of the process to try and get the best treatment of the wastewater and to reduce the smells as much as possible.”
“From our monitoring, the quality of our ocean outfall continues to meet the consent conditions, which we’re very pleased about.”