The five-month project in the east of the city will aim to reduce the operation of storm overflows by easing the pressure on the sewer system during storms.
A new storage tank, to be built below ground in the car park of Bath RFC's Lambridge rugby ground, will also help prevent flooding by hosting increased flows from combined sewers, which carry both wastewater from homes and businesses and rain run-off from buildings and surfaces, during periods of heavy downpours.
Overflows are designed as a relief valve to protect homes from flooding. Currently, if there is too much rainfall in the system, the overflow automatically discharges into watercourses.
However, the extra storage means that once the weather recedes, the stored water can then be returned to the sewer system for its journey to a nearby water recycling centre, where it is treated before being safely returned to the environment.
The Lambridge work is part of Wessex Water's £3 million a month investment to reduce how often storm overflows operate.
It follows earlier environmental protection work carried out by the water company on the same stretch of river several years ago, when new equipment was installed at Kensington Meadows, within the car park of supermarket Morrison’s further down London Road, to improve the screening of wastewater from a nearby storm storage tank.
Wessex Water project manager Jonathan Barker said: "The new storage will allow more than 125,000 litres of storm water to be held when there are periods of intense rainfall and then gradually returned to the sewer system for onward travel to a water recycling centre for treatment.
"By installing it we are aiming to reduce the potential for flooding and cut the automatic operation of a nearby storm overflow by around three-quarters as we continue our drive to protect the ecology and health of the waterways in our communities."
The scheme, which is taking place within the car park of the Lambridge site, is expected to be completed by the start of January next year.
The company is spending £150 million to help complete nearly 100 projects to reduce the operation of storm overflows by a quarter, while supporting the environment.
Wessex Water's Storm Overflows Improvement Plan also includes measures to beef up the monitoring of overflows across the region and upgrades more than 40 water recycling centres to increase capacity, introducing more nature-based and low-carbon methods.