The extent of the damage to the Christchurch Wastewater Treatment Plant will not be fully known until the fire in the trickling filters is completely out.

Fire and Emergency New Zealand still has crews working on the fire, which started yesterday afternoon on the roof of Trickling Filter No2 and then spread across to Trickling Filter No1.

Nearly 50 firefighters were working on the fire at its peak.

The blaze was brought under control about 6pm last night but firefighters are still on scene dampening down hotspots in Trickling Filter No2.

The trickling filters are circular concrete structures filled with plastic media that support the micro-organisms for biological processing of the sewage.

Sewage is introduced into the filter via rotating arms which spray the sewage across the top of the filter media.

Air is forced in through the media to maintain aerobic condition and support biochemical oxidation of the sewage, releasing carbon dioxide and water.

Christchurch City Council Head of Three Waters and Waste Helen Beaumont says despite the damage to the trickling filters, the wastewater treatment plant can still accept the full sewage flow from the city.

“We have contingency plans in place for the wastewater plant and can bring back-up systems into service,’’ Ms Beaumont says. “Staff are working on ways to optimise the efficiency of the functioning treatment processes so that we can compensate for bypassing the trickling filters.’’

Ms Beaumont says the contractors were working on the roof completing a refurbishment project when the fire started, but the cause of the blaze is still under investigation.

“We have engaged an independent investigator and are working with both Fire and Emergency New Zealand and Worksafe to establish what happened.’’

Ms Beaumont says until a full damage assessment is undertaken, it is not possible to say how long, or how much it will cost, to replace the trickling filters.

“This work will be undertaken as soon as it is safe to do.’’