The bacteria used to remove those nutrients and the bacteria used to break down complex bacteria and chemicals during the sewage treatment process were killed when at least hundreds of gallons of chemicals, common to industrial cleaners, insecticides, and other manufacturing products, were dumped into the sewer system. This prompted the partial shutdown of the plant off Frederick Street on Feb. 9.
The screening and settling steps of the treatment process, which remove solid material such as sludge, continued running. But the ozone disinfection and bacteria treatments of the waste water were shut down, allowing 5.6 million gallons of largely untreated waste water to flow daily from the plant into Antietam Creek.
The plant began using a chlorine disinfection process on Feb. 12, which killed most bacteria and essentially ended the flow of contaminated waste water into the creek, Maryland Department of the Environment spokesman Richard McIntire has said.
On Monday, the plant switched back to using ozone to disinfect the waste water.
The bacteria used in the treatment processes is regrowing, and will do a better job of treating the sewage as it reproduces, Barton said.
But Barton said the waste water coming from the plant now is almost at pre-shutdown quality.
Even though the biological nutrient removal process was down, tests of the waste water taken last week showed that nitrogen and phosphorous levels were below state limits, he said.
Bacteria levels in samples of waste water taken Monday were below state limits, he said.
The public was still being advised not to come into contact with Antietam Creek.
Hagerstown and state officials have reported no ill effect on creek wildlife as a result of the discharge of contaminated waste water.