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Norwich - Last Thursday's deluge forced the city's wastewater treatment plant to send millions of gallons of untreated sewage diluted with rainwater into the Thames River for about 35 hours.
Mark Decker, water and wastewater integrity manager for Norwich Public Utilities, said overflows from the Rose Alley pump station were discharged into the river from about noon Thursday, two hours after the rainstorm began, until about 11:30 p.m. Saturday. The combined stormwater and wastewater flows decreased after the peak of the storm passed at about 5 p.m. Thursday.
"We had to control the amount of inflow so we can maintain our treatment processes," Decker said Monday.
Mike Layer, meteorologist with the National Weather Service in Upton, N.Y., said the rainstorm began at 10 a.m. Thursday and was very heavy from about 1 to 5 p.m. Measurements of the total rainfall ranged from 5.25 inches to 7.88 inches in different parts of the city, he said.
The Norwich plant has a peak capacity of 17 million gallons per day, receiving both stormwater and wastewater into its combined treatment system. During heavy rainstorms, more water flows in than the system can handle, so excess water must be discharged into the river with minimal treatment. Decker said the excess flow passes through a network of weirs that collects the largest debris before it goes into the river.
The wastewater, he said, is "highly diluted by the stormwater."
According to information on the state Department of Energy and Environmental Protection's website, untreated wastewater from combined sewer overflows can contain pathogens, excess nutrients and chemicals, and it represents a public health and environmental concern. Officials at Ledge Light Health District, which has sanitarians who test water at public beaches on the Thames River, could not be reached to comment about results of testing after the overflows last week.
Five other communities in the state - Hartford, Waterbury, New Haven, Bridgeport and Norwalk - have systems like Norwich's that combine stormwater and wastewater. Over the past four decades, Norwich and other communities have been upgrading their systems to reduce combined overflows, and a $53.2 million long-term plan is proposed for the Norwich system to eliminate all overflows over the next 20 years, said Dennis Greci, supervising sanitary engineer with DEEP. The plan would increase the plant capacity to 33 million gallons. The city will be eligible for state grants to cover about half the cost, and low-interest loans for the remainder, he added.
Greci said NPU officials promptly notified DEEP, as required by its permit, that it was forced to send overflow into the river.
The plant is currently not required to calculate the amount of overflow, but will be required to do so in the new permit set to take effect this fall, he said. He estimated that the plant has a combined overflow 10 to 20 times per year, but typically the system is able to return to normal operations more quickly.
"This was a highly unusual event," he said. "It was just way too much water in too little time."
Greci added that because of a new state law, DEEP is changing its design standards for sewage plants to make them better able to handle climate change effects, including rising sea levels and increasing frequency of intense storms like last Thursday's.