Tenders for the $24 million upgrade of Wanganui's ailing wastewater treatment scheme will be considered by the Wanganui District Council next month.
The council has drawn up a shortlist of four tenders it said were "compliant".
In preparation for the upgrade, contractors have been removing sludge built up in the main pond since the plant was commissioned in 2007.
Mark Hughes, council's infrastructure manager, said that, so far, 5000 dry tonnes of sludge had been removed and the process was ongoing. The removal was expected to be finished by the end of next month.
Sludge was initially taken out of the smaller of the two ponds and pumped into the main pond and then the de-sludging began in earnest.