The work, worth £80m, is within the previously announced framework agreement and part of Severn Trent’s Capital Delivery Framework.
It will be delivered over the next four years, straddling both the existing AMP7 and the new AMP8 regulatory periods.
A dozen of the sites will involve Costain working to improve the quality of wastewater, for example by removing phosphorus as part of the Water Industry National Environment Programme (WINEP).
Three of the sites will involve work to protect elvers (juvenile eels) from entering river water intake pipes as part of compliance with The Eels Regulations.
The final project is the design and construction of a new disinfection contact tank at the Frankley Water Treatment works which serves Birmingham and secures the resilience of this water supply.
Sam White, Costain’s managing director of its natural resources division, said: “We’re delighted to be helping Severn Trent with their sustainability improvements.
“It’s hugely important to both companies that we do everything we can to protect the environment, as well as offering reliable and secure water supplies to customers.”