Costain has been retained as a capital delivery partner by Dwr Cymru Welsh Water for the Asset Management Programme 5 period which begins on 1 April 2010.
The five-year framework follows on from AMP 4 where Costain has delivered the wastewater capital programme in North Wales.
It is anticipated that AMP 5 will realise an overall programme value of £60m and provide both clean and wastewater programmes for Costain to design and build.
Andrew Wyllie, chief executive of Costain, said: "We are delighted to announce that we have been retained by Welsh Water as a Capital Delivery partner. Costain is a market leader in the water sector and this award is a direct result of our strategy to focus on repeat order work with key customers."