In a brief email on Thursday, Ferson commended the clean-up effort and said the “volume of the discharge has greatly diminished and GLSD staff have made extensive efforts to ensure that the discharge is contained to the project site so that the public health impacts are minimized. We have directed the plant to determine the cause of the issue and identify a remedy.”
Hogan said the overflows may be caused by a sudden spike in the amount of inorganic material such as industrial waste that arrived at the plant about the time the overflows began, upsetting the biological process that breaks down the waste in the treatment tanks. He said plant officials surveyed the larger manufacturers in the five municipalities served by the plant to try to track the source. No source was determined.
The waste inside each of the 1.4 million-gallon tanks is in an early stage of treatment and still contains coliform bacteria and other pathogens that can cause a range of diseases, including hepatitis A, typhoid and salmonellosis, a gastrointestinal illness that sickened 92 bathers — sending 16 to hospitals — at a Rhode Island lake on July 4. Health officials in that state believe the outbreak was caused by human feces in the water.
The sewage treatment tanks are called digesters because of the process going on inside, where naturally forming bacteria feed on the sludge in an oxygen-free environment. The bacteria reduce but do not eliminate the pathogens in the waste and also reduce the volume before the waste is sent on to centrifuges and heating tumblers that further reduce it to pellets, which at that point can be safely used for fertilizer and other purposes.
The sewage treatment complex on Charles Street in North Andover serves both Andovers, Lawrence, Methuen and Salem, N.H. The facility is surrounded by runways of the Lawrence Municipal Airport on the north, east and south, and by railroad tracks and the Merrimack River on the west.