Two environmental organizations filed suit against the Washington Suburban Sanitary Commission on Wednesday, alleging that one of the utility’s water-treatment plants has been illegally discharging sludge into the Potomac River.

In a lawsuit filed in U.S. District Court in Greenbelt, the groups contend that the Potomac Water Filtration Plant has been dumping sediment and purifying chemicals into the river in violation of a state permit.

The WSSC serves residents of Montgomery and Prince George’s counties. The treatment plant, near Seneca, provides drinking water primarily for Montgomery residents.

According to the lawsuit, the plant draws about 124 million gallons of water from the river each day for treatment. The permit requires that sediment, which is extracted during the treatment process, and chemicals be trucked away to a landfill, except during heavy rains when the plant must deal with exceptional quantities of water.

When the river is running high, operators may discharge some of that extracted material into the river. The environmental groups — the Chesapeake Bay Foundation and the Potomac Riverkeeper — allege in the lawsuit that the treatment plant has discharged sludge at times not covered by the high-water exception and that the excess sediment is a “major stressor to aquatic plant and animal life.”

“I have the sense that the sludge-treatment facility never worked as efficiently as it was designed,” said Mary Greene, an attorney for the Environmental Integrity Project, which is representing the two groups. “There seem to be some operational and design problems with it. I think that has largely led to their practices of discharging much more frequently than they’re allowed to under the permit.”

The WSSC maintains the discharges occurred only on days of heavy rain, according to a statement from a utility spokeswoman. The utility declined to comment on the lawsuit.

Dirt and silt dissolved in river water can make it difficult for fish to breathe and deprive aquatic plants of sunlight. In the purification process, the plant also uses aluminum, which can poison fish.

“If we think of the river as a commons, everyone downstream is subject to your decisions, or is affected by your decisions, whether you’re applying pesticides to your lawn or what you’re flushing down the toilet,” said the Potomac Riverkeeper president, Matthew Logan.

Water drawn downstream from the plant is treated at other facilities, such as the Washington Aqueduct in the District.

The suit asks the court to order that the WSSC stop any illegal discharges and remediate any ecological damage that it might have caused.

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