Variable-speed drives have been fitted to two aerators in Wanganui's wastewater treatment plant to try and sort out a problem with dragging sludge to the top of the pond.
The modification is part of efforts to rid the ponds - and parts of Wanganui - of a nasty smell.
Wanganui District Council infrastructure manager Mark Hughes said the aerators had been fitted with the variable speed drives and were due to be craned into the top pond late yesterday, with trials beginning today.
"Last week we switched on a few aerators, but they dragged the sludge from the bottom of the pond back to the surface" Mr Hughes said.
He said treatment of the sludge and upper aerobic zones continued, with a process called bio-augmentation being used to treat the sludge.
"This involves growing 'good bugs' and injecting them into the pond where they reduce the level of sludge.
"We are treating the upper zone with Oxygain to establish a highly oxygenated buffer layer," he said.
"We are continuing to try to neutralise the odour from the ponds with an atomiser system, which sprays a fine mist into the air around the ponds," he said.