A spill at a wastewater treatment plant in southern Prince George’s County sent about 1.5 million gallons of partially treated sewage onto the plant’s grounds overnight Wednesday, officials said.

No sewage reached any storm drains or waterways, said Luis Maya, a spokesman for the Washington Suburban Sanitary Commission (WSSC), which provides water and sewer services in Prince George’s and Montgomery counties.

Maya said the spill was contained on the site of the Piscataway Wastewater Treatment Plant in Accokeek. Maya said the sewage had already gone through the first step in the treatment process. The spill, he said, occurred when workers tried to change the flow of the sewage during a construction project underway at the plant.

The spill began at 3:50 p.m. Wednesday and ended at 7:15 a.m. Thursday, WSSC said.

Maya said WSSC is working to pump as much of the sewage as possible back into the plant via a drain. He said the utility notified the Maryland Department of the Environment and the Prince George’s County Health Department.