What is the Potomac Interceptor?

The Potomac Interceptor (PI) sanitary sewer system carries about 60 million gallons of wastewater daily from areas near Dulles Airport to the Potomac Pumping Station in Washington, DC. Flows from the pump station are then sent to the Blue Plains Advanced Wastewater Treatment Plant for treatment before discharge into the Potomac River.

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, DC sewer system.

The PI serves Fairfax, Loudoun County, Town of Vienna, Herndon, Dulles Airport and Montgomery County, MD. DC Water is performing replacement and rehabilitation of the PI.

Potomac Interceptor Segments

The Potomac Interceptor (PI) system is made up of four primary segments: the PI main trunk sewer, the Upper Potomac Interceptor (UPI), the Upper Potomac Interceptor Relief Sewer (UPIRS), and the Maryland Upper Potomac Interceptor (MUPI).

• The PI main trunk sewer runs through Maryland and Virginia and includes the Sugarland Run Extension, the Difficult Run Extension, and the Upper Maryland Spur.
• The MUPI, located in Montgomery County, Maryland, connects to the UPI and the D.C. line.
• The UPI starts at the Maryland/DC border and carries flows from the MUPI and other District connections to the UPRIS.
• The UPRIS begins at the DC border and conveys flows from the PI main trunk and additional service connections to Blue Plains.

The PI system is operated and maintained by DC Water, except for the MUPI, which is managed by the Washington Suburban Sanitary Commission (WSSC).

Potomac Interceptor Projects

PI01

Potomac Interceptor Sewer Reconstruction Manhole 31- Great Falls, Virginia

Potomac River Crossing- Click here for Project Factsheet. 

PI02- Project Webpage Coming Soon!

Helicopters at Great Falls?

From January 27 to February 2, 2025, DC Water conducted geological investigations in the Potomac River to determine the most effective strategy and design alternatives for the replacement and rehabilitation of the Potomac Interceptor.

Rock core samples were taken just below the Washington Aqueduct dam in Great Falls. This investigation will provide DC Water’s engineers a better understanding of the geology in the area. The investigation required the transport of both workers and a drilling rig by helicopter that drilled up to 50 feet in depth at five locations within the Potomac River.

More information on rock cores and what occurred during the week long investigation can be found here.  Find the latest news and photos here.

Did You Know?

Buildings in Disguise- There are multiple buildings serving as odor control facilities. These facilities draw odorous air from the Potomac Interceptor, pass the air through a media-filled vessel to remove the majority of odors, and discharge the treated air to the atmosphere. The facilities are part of the Potomac Interceptor's Odor Control Abatement Program. 

Just Facts

Over 54 Miles of Potomac Interceptor Length

11 Tunnel Sections

2 River Tunnel Crossings

36-96 Inches Diameter Pipe Size

Contact Information

Potomacinterceptor@dcwater.com

202-787-4096