"The roof caught fire and that collapsed in and has burnt through the plastic media so we have four appliances still here at the moment to try and get the flames out."
Twose said a contractor on site suffered a minor superficial injury during the incident.
Contractors had been working on the roof when the fire broke out.
"That will form part of the investigation into the cause."
Christchurch City Council head of three waters and waste Helen Beaumont said despite the damage to the trickling filters, the wastewater treatment plant can still accept the full sewage flow from the city.
Fire staff will be meeting with council staff early this morning and working with Environment Canterbury on air monitoring. Photo / Supplied
"We have contingency plans in place for the wastewater plant and can bring back-up systems into service,'' Beaumont said.
"Staff are working on ways to optimise the efficiency of the functioning treatment processes so that we can compensate for bypassing the trickling filters.''
Beaumont said 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.''
Beaumont said 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.
