Trap Rock Water Treatment Facility

Part of the ambitious and innovative Potomac Water Supply Program, the state-of-the-art facility will turn 40 million gallons of raw water a day, pumped directly from the Potamac River, into potable water supply.


  • Leesburg, Virginia, USA


  • Loudoun Water

Project Value

  • $207M Construction

Project Status

  • Completed May 2019

By 2040, fast-growing Loudoun County, Virginia anticipates needing up to 90 million gallons a day (MGD) of safe, clean drinking water. To meet this significant increase in demand, Loudoun Water initiated the Potomac Water Supply Program (PWSP) in 2011, which involves withdrawing raw, non-potable water from the Potomac River, storing it in retired quarries, and processing finished drinking water at a new treatment facility.

The two-phased program, developed over decades, is designed to provide a safe and sustainable supply of drinking water; minimize impacts to the Potomac River using an advanced intake system; and provide flexibility to bank raw water for use during future droughts or water supply emergencies. Phase one of the project yields a treated water supply capacity of 20 MGD, and phase two will yield an ultimate supply capacity of 40 MGD.


The $207 million Trap Rock Water Treatment Facility, a centerpiece of the PWSP, treats the raw water using a two-step ozone filtration process. The facility includes a 400-foot-long tunnel with two 50-foot drop shafts; pre-ozone contactor facility and operations building; residuals handling facilities; finished water pump station and 3,500 feet of 42-inch finished water transmission main; and sanitary pump station and 10,000 feet of 12-inch force main. The treatment plant also incorporates the supporting infrastructure of a raw water intake and pumping station, and raw water transmission main, as well as a 16,400-square-foot administrative office building designed to achieve LEED Gold certification.



WSP USA has provided program and construction management services on phase one of the program since 2008. The future phase two will bring the facility processing capacity to up to 40 MGD, which will require re-rating and addition of new pump stations and clear wells.