Costain said that the move was in line with its continuing disposal of its PFI equity portfolio.
The three companies – Severn Trent Costain, Severn Trent Costain Holdings and Severn Trent Costain Services – all now become 100% owned by Severn Trent.
The business will now continue as Severn Trent Services.
As a result of the sale, Costain has banked a profit of £9.1m, which is in addition to the board's previous expectations for the year. The company said that it would use the funds for investment and working capital purposes.
Severn Trent Costain was established in June 2012 to offer water and waste water management for commercial and industrial water users. This built on their existing joint venture, C2C Service, which was set up to carry out a 25-year PFI contract, Project Aquatrine, for the Ministry of Defence.
In September, Severn Trent Costain signed a multi-million pound contract with dairy firm First Milk, to design, build and operate a new wastewater treatment plant at its Haverfordwest Creamery.
Severn Trent chief executive Tony Wray said: "We are in a position where Severn Trent commands the expertise, commitment and full-service skills to take this business forward for the long term. We look forward to continuing to work with our customers and future customers to further improve service standards and the economics of their business."