"It was obvious the problem got worse, not better," Thomas said.
Thomas said it could not be determined what kind of solvent was involved or where it entered the plant. There were no equipment breakdowns, he said.
Some businesses and drainage basins at Antietam Creek are being investigated in the search for the source of the contamination, he said.
The city's drinking water will not be affected and residents still can use their toilets, Thomas said.
The sewage treatment plant has two components - a settling process, which separates liquids from solids, and a treatment center, which uses microbes and oxygen to treat sewage as it is processed through the plant.
Sewage from homes and businesses in the city and some surrounding areas will continue to flow into the plant during the shutdown and go through the settling process, Thomas said.
The remaining untreated waste water will flow out of the plant into Antietam Creek and empty into the Potomac River, he said.
It is unusual for a treatment plant to completely shut down, allowing raw sewage to flow into the river, said Richard McIntire, spokesman for the Maryland Department of the Environment.
The state is working with city officials to determine how a contaminant could have gotten past the plant's checks and balances, he said.
The sewage plant treats 5.7 million gallons of water daily, so it's likely that amount has been affected by the unknown contaminant each day since Thursday, Thomas said.
The sewage plant had other solvents enter the system but none of this quantity, he said.
Today, workers will continue to investigate the source and nature of the substance contaminating the system, Thomas said, though it is possible it will not be found.
"Sometimes, the source eludes us completely," he said.
Residents shouldn't be able to detect a difference in the water in Antietam Creek or the Potomac River because the waste will have been sufficiently diluted, he said.
"It's pretty much invisible to most everybody," Thomas said.
The city's water system won't be affected by the contaminant or the partially treated waste because it is upstream from the plant, he said.
The contamination, while serious, does not appear to pose a health risk since most people don't swim or fish in the river during winter months and the substance will be diluted in the river, Washington County Health Officer William Christoffel said.
McIntire recommended that people who have been in contact with water in Antietam Creek or the Potomac River in the past three days wash themselves thoroughly.
Water intake centers downstream from Hagerstown were put on alert to conduct additional water monitoring, he said.
Once the source of the contamination is found, it should take about a day for the sewage treatment to resume, Thomas said. Restarting the biological process will cost the city only in manpower since the microbes can be regenerated at the plant with existing resources, he said.
The city's waste water treatment plant serves about 14,000 customers in the city and surrounding area.