DERRY — The air is smelling much better these days.
A recent odor problem at the town’s wastewater treatment plant has improved significantly, town officials reported.
Deputy Public Works Director Tom Carrier reported that a recent oxygen deficiency at the plant’s lagoon off Fordway caused a strong smell, which permeated the area off Exit 4 of Interstate 93.
The smell was evident all the way into Londonderry.
Motorists, residents and business owners reported the strong smell that lasted for days.
Carrier said the problem was a natural phenomenon.
It’s something of a spring tradition for the sewage-like smell, but this year was different.
“The odor was more potent, but not uncommon,” Carrier said. “This was an unusual year.”
Derry’s wastewater treatment lagoon is one of the largest lagoon facilities in the state, Carrier said, with lagoons 20-foot deep handling millions of gallons of untreated sewage.
At the lagoon, solid sewage waste is broken down through a natural process involving two types of bacteria: aerobic bacteria that rely on oxygen to survive and anaerobic bacteria that don’t need oxygen, Carrier said.
The bacteria — or bugs, as Carrier called them — are inactive during the winter.
As the seasons change and water heats up, sludge at the bottom of the lagoon floats toward the top. It typically gets taken care of by the aerobic bacteria in a process known as “lagoon turnover,” Carrier said.
In this case, oxygen levels were too low for the bacteria to work.
With this past winter being so harsh, the conditions were ripe for the unpleasant gases to take over.
“It gives off a pungent odor,” Carrier said. “And those prolonged winter months caused a larger-than-average sludge blanket to form.”
Derry resident Katherine Prudhomme-O’Brien said she was embarrassed for the community.