A recently opened 12-story underground pumping station in Alexandria, Virginia, is helping solve a problem that has existed for more than 100 years.

Alexandria’s 1800s-era combined sewer overflow system served Old Town, where stormwater and sanitary sewage flowed in the same pipe.

During heavy rains, when large amounts of runoff from streets entered the combined sewer system, the excess mixture resulted in untreated flows entering the Potomac River, Hunting Creek and Hoofs Run.

Opened in July, AlexRenew’s RiverRenew project is expected to prevent 120 million gallons of combined sewer overflows from polluting the Potomac and its tributaries each year.

In the recently opened facility, “During wet weather, when flow levels rise, it overflows into our new diversion chamber, rather than overflowing into the river,” RiverRenew Program Manager Ryan Payne said. “It flows by gravity down a drop pipe and into our waterfront tunnel.”

The 130-foot-deep, 2.2-mile-long tunnel along the waterfront has the capacity to capture and hold stormwater in the midst of the storm.

“Once the rain event is over, we screen out any remaining debris and the flows are pumped through our tunnel dewatering pumps, up to the surface, where they’re treated at AlexRenew’s wastewater treatment plant,” which is adjacent, said Matt Robertson, spokesman for the company, during a Wednesday tour.

Robertson said the new system provides flexibility in the midst of a downpour.

“During the wettest events in Alexandria, the wet weather pumping station gives us additional options for treatment,” he said. “We could redirect flow to the waterfront tunnel for storage or we can redirect them to various places in our plant where those flows can be treated.”

In 2017, Virginia’s General Assembly passed a law requiring Alexandria to solve its combined sewer overflow problem by 2025.

“We made a commitment to our community in 2017 to solve this long-standing environmental issue, and we’ve worked tirelessly every day since to stand by it,” General Manager and CEO Justin Carl said on July 1, when the project became fully operational.