The Environmental Integrity Project released this video of water being discharged from WSSC's water purification plant along the Potomac River in Potomac, Md. Two environmental groups filed a federal lawsuit against WSSC alleging that pollutants in the water releases violate the Clean Water Act. WSSC agreed to settle the lawsuit and spend millions to improve the plant. (Mary Greene of the Environmental Integrity Project)

Maryland’s largest water utility agreed in a legal settlement Wednesday to significantly reduce the amount of sludge and chemicals it releases into the Potomac River from one of its water purification plants.

The Washington Suburban Sanitary Commission, which provides drinking water to nearly 2 million people in Montgomery and Prince George’s counties, also agreed to pay a $100,000 state penalty and implement up to $8.5 million worth of short-term pollution control projects at the plant.

The plant, in Potomac, Md., northwest of the District, treats water from the river before distributing it to homes and businesses in Montgomery and parts of Prince George’s.

The legal agreement, known as a consent decree, settles a 2014 federal lawsuit that Potomac Riverkeeper and the Chesapeake Bay Foundation filed against the utility, alleging that pollution discharged from the plant violates the federal Clean Water Act. The groups alleged that after treating the river water, the plant illegally released millions of pounds of aluminum, phosphorus and sediment back into the river, hurting fish and smothering underwater grasses.

The Maryland Department of the Environment joined in the settlement negotiations.

Mary Greene, deputy director of the Washington-based Environmental Integrity Project, which represented Potomac Riverkeeper, said WSSC’s plant is a “significantly worse” polluter than other water purification plants in the Washington and Baltimore areas.

“The biggest victory is for the river,” Greene said of the settlement. “There will be millions of pounds less of pollutants in it every year. . . . I think [WSSC] is very focused on providing clean drinking water. I don’t think they’ve given enough attention to the pollution impacts of providing that service.”

Under the agreement approved unanimously by the WSSC’s six-member board of commissioners, the utility also will pay $1 million for sediment control projects in other parts of the Potomac’s drainage area. That could include farms in Washington and Frederick counties, upstream from the plant.

The unknown — and biggest — cost of the agreement will go toward long-term improvements required at the 1960s-era plant at 12200 River Rd. Under the settlement, the WSSC must hire an independent engineering firm to determine how the utility can build additional facilities to better treat and dispose of sludge and chemicals. A long-term solution must be in place within 10 years, or WSSC risks an additional $1 million penalty.

Jerry N. Johnson, the WSSC’s general manager, said such upgrades could cost well more than $100 million. He said it will be “very, very difficult” to build additional facilities on the plant’s constricted site, and expanding the plant in Potomac, which has some of the highest real estate prices in the region, will be expensive.

The WSSC is already struggling to pay for $1.5 billion of work required by a 2005 consent decree stemming from a lawsuit filed by other environmental groups. That agreement requires it to reduce sewer spills into Maryland streams. Meanwhile, the WSSC recently stepped up efforts to replace aging water pipes that continue to break.

“It’s going to be expensive,” Johnson said of implementing the new settlement.

Johnson said the sediment that the plant releases back into the river is less than what it takes out, and the aluminum and other chemicals released are at “benign” levels. Even so, he said, the settlement saves the utility legal expenses, and it will make the necessary changes.

“It’s obvious a plant constructed in the 1960s needs some improvements,” Johnson said.

The WSSC’s filtration plant is particularly tricky, both sides in the lawsuit agree, because it draws water directly from the Potomac, which becomes particularly muddy after heavy rains. Many water treatment plants, such as the WSSC’s other plant in Laurel, Md., and the one that serves D.C. Water, draw river water from reservoirs or large basins, where dirt and other solids can settle before entering the facility.

At the Potomac plant, the WSSC adds chemicals to help fine particles of dirt in the river water coagulate so they can settle on the bottom and be drawn off as sludge. Much of that sludge-chemical mixture is then hauled away via trucks to a landfill or to be used as landscaping topsoil.

However, some of it gets discharged back into the river when muddy water overwhelms the plant, officials said. The plant also discharges about 10 million gallons daily from cleaning filters and from water leaking from the plant itself, said Robert Buglass, a WSSC unit coordinator for environmental sciences.

The problem, Greene said, was that the plant routinely violated its 1997 state permit, which expired in 2002. Such permits are typically updated every five years to improve water quality, Greene said. However, the state merely extended the 1997 permit without updating it. Johnson of the WSSC said changes to the plant’s design made it impossible to comply with some of the permit’s requirements.

Jay Apperson, spokesman for the Maryland Department of the Environment, said the agency will sign the agreement. He said the WSSC’s permit is “complicated” because of the “challenges” the utility faces in treating particularly muddy river water after storms to comply with safe drinking water standards.

The consent decree will take effect after the U.S. attorney general and a U.S. District judge have reviewed it.