When the power went out, most of the flow backed into primary tanks, he said. But some of the wastewater overflowed into the spillway and went into the creek, he said. Shadrach said he thinks only about 18,000 gallons entered the creek.
The flow that entered the creek was completely treated except for the disinfectant process, he said.
The sewer plant began running as normal when the power resumed, he said.
As a precautionary measure suggested by the Washington County Health Department, signs were posted Thursday afternoon at Antietam Creek warning the public to avoid water contact and fishing, he said.
The signs say the amount of flow could have been as high as 175,000 gallons, he said.
A bacteria sample from the plant's affluent into the creek will be tested today. If the sample is at a satisfactory level, the signs will be removed this afternoon, Shadrach said.
It is the same plant that was partially shut down Feb. 9 after high concentrations of chemicals common to industrial cleaners and other industrial products were dumped into the sewer system.
An investigation into the source of the chemicals is continuing.
The partial shutdown in February let millions of gallons of largely untreated wastewater flow from the city plant into Antietam Creek.