Sewage odor complaints stop in Derry, but officials monitoring plant
DERRY — Public Works officials are fairly certain about what caused a pungent smell that recently emanated from the town’s wastewater plant and left some area residents in a foul mood.
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.
He added that a string of warmer days in early May, with temperatures in the 70s and 80s, helped exacerbate the growing stench that came from the treatment facility at 43 Transfer Lane. Humidity added to the problem, he said.
The plant is in Derry, but lies near Londonderry and Exit 4 of Interstate 93. It serves Derry and southern Londonderry, Carrier said.
In late May, the smell affected Londonderry residents living in the area near Home Depot and Market Basket, Carrier said.
Officials first received complaints from residents on May 20. The calls continued to come in until last Thursday, Carrier said.
“Fortunately, it’s over,” he said. “We haven’t received any complaints in recent days.”
Carrier said he wanted to assure councilors and the public that the wastewater facility continues to operate with its permit as is it designed.
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.
The system is aerated by mechanical means, which provides an environment for biological activity to break down and treat the waste, he 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.
When questioned by councilors, Carrier said this year was one of the worst in recent memory.
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.
Councilors agreed to have Public Works report back in a month with an update on the matter.