Deep tunnels help DC Water manage sewer overflows

With the completion of its Northeast Boundary Tunnel (NEBT), DC Water recently finished an extensive series of tunnels designed to alleviate sewer overflow problems impacting the Anacostia River.

Washington, D.C., receives a little more than 40 inches of precipitation annually, which is a lot for a 69-square-mile city. Periodic deluges would overwhelm the city’s old sewers, dumping more than two billion gallons of a mixture of sewage and stormwater into the Anacostia River.

In February 2000, a group of environmental organizations sued DC Water over the issue. DC Water agreed to fix the problem and launched its Clean Rivers Project, a program to reduce combined sewer overflows (CSOs) and divert them into the waterways surrounding the District.

“The focus was on the Anacostia River, because before we started Clean Rivers, it was the most significantly impacted by CSOs,” said Moussa Wone, vice president of the Clean Rivers Project.

The NEBT is five miles long, has a diameter of 23 feet and sits 100 feet beneath the city. During its construction, DC Water utilized a massive Earth Pressure Balance Tunnel Boring Machine — which Wone compared in size to a factory — to nearly simultaneously cut the ground, remove the spoils and place a concrete segmental lining along the walls.

The new tunnel connects the Anacostia River Tunnel and the First Street Tunnel, which have been in service since 2018. The CSO is diverted from the existing outfall to a drop shaft, where the flow spirals down vertically to the tunnel 100 feet below ground.

The combined sewage captured by the tunnel is pumped to the Blue Plains Advanced Wastewater Treatment Plant, where it is cleaned and discharged into the Potomac River, the city’s drinking water source.

“The goal for the District is to make the Anacostia River fishable and swimmable,” Wone said. “We do post-construction monitoring to confirm the performance of the CSO controls and the resulting water quality improvements. The capture rate is 98% for the Anacostia (River), which allows us to meet the water quality standard for the Anacostia.”

The next step in the Clean Rivers Project is an $819 million tunnel project on the Potomac River.

The city’s ratepayers have made a significant investment into improving the area rivers. DC Water paid more than $1.8 billion to help keep the Anacostia River clean.

“So far, every Clean Rivers project has met or exceeded the consent decree deadlines, and we came in under budget,” Wone said.

DC Water provides more than 600,000 residents, 16.6 million annual visitors and 700,000 people employed in the District of Columbia with water, sewage collection and treatment. The agency also provides wholesale wastewater treatment for 1.6 million people in Montgomery and Prince George’s counties in Maryland, and Fairfax and Loudoun counties in Virginia.