Whanganui residents, particularly those who live near the airport, have been plagued by a sickening sulphur odour since before Christmas.
Mayor Annette Main said tonnes of lime slurry are being spread across the treatment pond on Wednesday to suppress the stench.
She says the method has worked at other treatment plants, although the risks include killing the good bacteria that break down the sludge.
Ms Main says if the lime doesn't work, the council will be forced to bypass the plant and dump the wastewater straight to sea.
