The £156m utility job will be broken up into two regions covering north and south Wales.
Three other lots will cover trunk main cleaning and renewals and meter installation also divided between regions.
Firms have to meet a set of strict requirements including carrying out at least 80% of the work with a direct labour force.
Welsh Water has been under mounting pressure to improve the water main, particularly after a burst last month which left 1,000 homes in Swansea temporarily without water.
Contractors have until 18 October to fill in PQQs. Further details are available from David Barnsley at Welsh Water. Email: [email protected]