ENOUGH raw sewage to fill 400 average-sized backyard tanks has spilled into the Mary River from the Aubinville wastewater treatment plant on the outskirts of Maryborough.
An estimated four million litres of black water spewed into the river system over a 24-hour period from 11am Wednesday after primary and back-up pumping systems failed.
Staff discovered the crisis at 6am yesterday and they manually activated pumps to begin to move the waste material into the treatment plant.
"The receiving wet well for the screened raw sewage is fitted with pumps that are triggered by an electronic system," plant operator Wide Bay Water's chief executive Peter Scott said.
"Rats chewed through cabling and this basically took out the ability of pumps to understand
that the sewage had risen to a level where they had to work.
"Over and above that, you have a back-up system that is a ball system, a float system, so if a well is not pumping and it gets to a certain height it should kick in.
"Both systems failed. One because of a rat and the other cause we don't know yet as it was a brand new system."
The Department of Environment and Resource Management said after investigations yesterday it does not propose to fine WBW for the discharge.
WBW must now undertake a full risk assessment at the plant and report back to DERM.
Mr Scott advised that as of yesterday the plant was again fully operational.
It is the second time in recent weeks that WBW systems failures have resulted in raw sewage discharging into local waterways.
In late January, illegal plumbing hook-ups diverted heavy rainfall into the sewerage system in Hervey Bay.
This overloaded the network, popping sewage manholes and allowing raw effluent to escape from four pumping stations.
WBW was forced to pump raw sewage into Pulgul and Eli Creeks over several days.
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