Charles Morrison, chief executive, now becomes chairman of the contractor, while chief operating officer Jim Arnold steps up to become chief executive.
Arnold brings considerable experience gained over the last 17 years with the business in varying roles including executive director and chief operating officer.
He will lead the business through the next phase of our growth.
Arnold said: “I very much look forward to working closely with my senior team and our new investors, First Reserve, in continuing to successfully grow the business.”
The £600m turnover firm, which employs around 4,000 staff, was bought from Anglian Water by Bregal Capital and Motion Equity Partners in March 2008 investing alongside the firm’s management team.
First Reserve bought the business two months ago for an undisclosed sum.