Potomac Interceptor

The Potomac Interceptor (PI) sanitary sewer system conveys approximately 50 million gallons per day (MGD) of wastewater by gravity from several service areas starting near the Washington Dulles International Airport, along the Potomac River to the Potomac Pumping Station (PS) in Washington, D.C.. Flows from the PS are sent to the Blue Plains Advanced Wastewater Treatment Plant for state-of-the-art treatment before discharge into the Potomac River. Several jurisdictions discharge into the PI system, including Loudoun and Fairfax counties in Virginia, Montgomery County in Maryland, and the District of Columbia.

The PI was built as a result of the enactment of Public Law 86-515 (the Act), by the 86th Congress, on June 12, 1960. The Act authorized the District of Columbia to plan, construct, operate, and maintain a sanitary sewer to connect Dulles to the Washington, D.C. sewer system. The intent was to safeguard the Potomac River against wastewater discharges from designated sewersheds not already connected to adequate sewage disposal facilities. The Act stipulated that the sewer should be of sufficient capacity to provide service for Dulles and for the expected growth and development in the adjacent areas in Virginia and Maryland.

The PI system consists of four primary interceptor segments including the PI main trunk, the Upper Potomac Interceptor (UPI), the Upper Potomac Interceptor Relief Sewer (UPIRS), and the Maryland Upper Potomac Interceptor (MUPI). The PI main trunk is located in Maryland and Virginia and includes the Sugarland Run Extension, the Difficult Run Extension, and the Upper Maryland Spur. The MUPI is located in Montgomery County, Maryland and conveys flows into the UPI at the D.C. line. The UPI starts at the Maryland/D.C. border and currently conveys flows from the MUPI and other service connections in Washington, D.C. to the UPIRS. The UPIRS begins at the D.C. border and conveys flow from the PI main trunk and other service connections to Blue Plains. The District of Columbia Water and Sewer Authority operates and maintains the PI system with the exception of the MUPI, which is operated and maintained by the Washington Suburban Sanitary Commission.

The PI varies in size from 30-inch to 96-inch diameter round, reinforced concrete pipe in the main trunk to 13-foot by 7.75-foot rectangular, reinforced concrete pipe in the lower reaches of the sewer system. The sewer design included provisions for interceptor venting at the manholes and access shafts along most of the sewer system to promote the exhaust of sewer gases or the intake of air as needed. Venting is generally accomplished through ventilated manhole covers or 12-inch cast iron vent pipes that extend from the manholes.

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