The works have been budgeted at around $140 million and Dunedin City Council is issuing a call for proposals.
Initially, the focus would be on upgrading the water treatment plant at Waikouaiti and upgrading the wastewater treatment plants at Mosgiel, Green Island and Tahuna.
Work is also needed at other water treatment plants, such as introducing fluoridation at Port Chalmers and Outram, replacing filter systems and improving instrumentation.
The initial work for the wastewater projects was estimated at around $25 million and the initial cost for the water projects would be around $15 million.
The bulk of the council’s $140 million budget has been allocated to 2025-2031.
An upgrade to the Waikouaiti water treatment plant has been reported for some time and attention has been drawn to it following last year’s lead contamination scare.
The council said the upgrade would include treatment to improve taste and odor issues.
The upgrade will include two new treated water tanks, new sewage settling ponds and a new backwash discharge pipeline.
Intake work will also be required, such as the installation of new raw water pumps, sump pumps and air blast cleaning equipment.
The board said in the tender documents that the aim of the upgrade was to address drinking water compliance issues and improve treated water quality, resiliency and factory operation.
Beca has completed a concept-level design for the water treatment plant upgrade.
The Mosgiel wastewater treatment plant must have a new sludge tank and a new gas flare system.
Screens will be replaced at Tahuna and electrical upgrades may be required for an ultraviolet treatment system at Green Island.
The council said two different contractors could lead the water and sewage works, but it is possible for one company to lead both.
The deadline for proposals is November 30 and contracts could be awarded in December or January.