Sewage odor complaints stop in Derry, but officials monitoring plant
The strong odor was likely caused by an excessive buildup of sludge at the bottom of two lagoons over the unusually frigid winter, Tom Carrier, deputy director of Public Works, said during the Town Council meeting Tuesday night.
In late May, the smell affected Londonderry residents living in the area near Home Depot and Market Basket, Carrier said.
“Fortunately, it’s over,” he said. “We haven’t received any complaints in recent days.”
Derry’s facility is somewhat unique in the state in that it incorporates a system of two large, open lagoons to break down and treat millions of gallons of untreated sewage. Each lagoon is 20 feet deep and contains 42 million gallons. The facility is one of the largest of its kind in the country, Carrier said.
Over the winter, the biological activity decreases, and a layer of sludge builds up and falls to the bottom of the ponds to lie dormant. In the spring, as it becomes warmer, bacteria become active again and begin breaking down the waste. As this process occurs, gases such as hydrogen sulfide, which has a rotten egg-like smell, are produced, Carrier said.
While they are fairly certain about what caused the odor, administrators will continue to review the data to see if there could have been another cause, such as a possible shock overload to the system, Carrier said.