Dalecarlia Backwash Water Tanks Improvements

The Washington Aqueduct (WA), a division of the Baltimore District United States Army Corps of Engineers (USACE), treats water drawn from the Potomac River to produce drinking water for Washington, DC, and portions of Northern Virginia. WA is unique in the Department of Defense, and possibly the Nation, in that the Federal government owns and operates a public utility. WA has the responsibility to provide safe, reliable, and cost effective drinking water to approximately one million consumers, through its three wholesale customers, 24 hours a day, seven days a week. WA is the sole source of potable water supply to most of these consumers in Washington DC, Arlington County VA, and Falls Church VA.

The Washington Aqueduct (WA) maintains and operates two water treatment plants (WTPs), the Dalecarlia WTP and the McMillan WTP. The Dalecarlia Water Treatment Plant operates multiple dual media filters for water filtration and purification purposes. When the filters are in use, the pressure loss across the media gradually increases and eventually the filter must be cleaned (backwashed) to continue functioning within the specified water quality limits. During the filter backwash process, high volumes of water are used to fluidize the filter media and release the trapped solids. At the Dalecarlia WTP, there are two wash water storage reservoirs which store water from the 1st High pressure zone for filter backwash purposes. These tanks are known as the Underground Tank and Elevated Tank, respectively. Age and sanitary survey of the water treatment plants identified some deficiencies regarding the wash water storage reservoir. Wash water is supplied to the wash water storage reservoirs through an altitude valve, located in the Dalecarlia Pumping Station, which simultaneously supplies water to each of the tanks through 24” and 30” cast iron water mains. Wastewater from the backwash process is recycled to the Dalecarlia Forebay (i.e. the head of the treatment process) via a subsurface Backwash Recovery Pumping Station (BWRPS). The BWRPS operates two centrifugal pumps, which are critical to the Dalecarlia WTP environmental permitting compliance. Both the supply and waste facilities require modifications to restore their infrastructure, improve reliability and address sanitary deficiencies.