While the exact cause of an offensive odor from the Derry sewage treatment lagoons remains under study, the official in charge of wastewater treatment said his department is doing the best it can to make sure it doesn’t happen again.
A strong odor from the lagoons, which are located near Londonderry’s Exit 4 off Interstate 93, caused motorists and shoppers to complain this past May. The odor made the WMUR Evening News, a fact that did not amuse Derry Councilors. But Tom Carrier, Deputy Director of Public Works, said there have been no issues since June 3, the date of the last complaint.
Carrier told the Council at its Tuesday, July 15 meeting, that he has reviewed all the data from the treatment facilities, had members of the New Hampshire Department of Environmental Services come by for a look, and talked with consulting engineers. The consensus, he said, was that the odor was “nothing but the natural biological process.”
One way of mitigating the odor in the future is by better predicting a possible outbreak and adding extra oxygen to the pools, he said. Another is removing the sludge at the bottom of the lagoons. The sludge was supposed to break down due to the anaerobic process, but that obviously failed this spring, he said. However, removing the sludge is a costly venture. He has it in the Capital Improvement Plan for 2020, he said, adding, “We may have to move it forward.”
Chemical treatment of the sludge layers is also expensive, he said, and not necessarily cost-effective for a relatively brief outbreak. They would have to stockpile the chemicals, and the May incident only lasted nine days, he pointed out.
“What do you usually do with the sludge?” Councilor Tom Cardon asked.
There are different options, Carrier said. Derry’s usual procedure is to press it dry, removing all water, and then place it in a landfill.
May’s event may have been due to the unusual amount of rain in the spring, but that’s not something he can hang his metaphorical hat on, Carrier said. “There is nothing specific we can point to,” he explained. “We’ve been in compliance. I wish I could point a finger and say, ‘This is what it is.’”
“We are trying to focus on things we can control,” Carrier added, “and we can’t control the weather.”
Councilor Joshua Bourdon pressed for a solution. “Derry was a joke on the TV news and in social media,” he said. He also expressed concern about property values on homes in the area and the impact on businesses.
“We are doing the best we can and are sensitive to the issue,” Carrier said.