Recent episodes of wastewater overflow into the Shawsheen River have drawn attention to something mostly out of sight and taken for granted.
That would be the sanitary sewer system, “a network of collection mains, pump stations, and force mains to convey waste to the Massachusetts Water Resources Authority treatment plant at Deer Island” in Boston Harbor, explained David Manugian, Director of the Bedford Department of Public Works.
“There are 30 pump stations and approximately 85 miles of mains in the system serving some 4,500 customers,” Manugian said. “These are located both in newer neighborhoods, where they were built with the residences, and older neighborhoods, where they were built when the residences converted from septic systems.
“The town has a dedicated staff of four as well as the water/sewer operations manager to keep the system running properly,” the director noted.
Manugian explained the physics of sewage disposal: “Waste generally enters the system from the customer through a gravity service line. It then flows downhill to a low point, where it is collected at a pump station.”
The pump then drives the sewage uphill through a pressurized force main to a high point, from which it can then flow to another pump station, he said. “In this way, it makes its way across the town to the main pump station” at 299 The Great Road, across from the DPW headquarters.
At the central pump station, waste from the entire town is pumped via a dedicated line through Lexington, connecting to the metropolitan system.
Manugian explained the importance of “minimizing the amount of inflow and infiltration” that penetrates the gravity mains. “Inflow means surface water entering pipes through structures, such as manholes or illegal sump pump connections,” he said. “Infiltration is groundwater entering pipes through joints.”
“Inflow and infiltration are a critical consideration, because any water that leaks into the system must be treated at Deer Island,” Manugian said. “Thus, communities are paying to treat relatively clean groundwater and stormwater in addition to the wastewater.”
The MWRA has meters at the Great Road pump station to measure the flow for billing purposes.
He stressed that “significant in-flow and infiltration can periodically overwhelm the system and cause sanitary sewer overflows.”
Manugian said inflow and infiltration are “inevitable as pipes settle and structures start to leak, and so the town has a program of constantly reassessing the system for potential leaks, quantifying the leaks, and prioritizing the repairs.”
Currently the DPW is probing for sources of inflow and infiltration through a flow isolation program, he said, “where at night, they temporarily block upstream flow for a pipe section and look for downstream flow that has leaked in.”
“Force mains are also more susceptible to settling and cracking over time than gravity lines,” Manugian pointed out, adding that maintenance of force mains includes periodic inspections for corrosion.
Maintenance of the pump stations varies depending on the type of facility, the director said. “The major components to be maintained include the pumps, the wet wells in which they sit and where sewage is collected, generators for backup power, and communications systems.”
“Even in a properly operating system, there are opportunities for biological activity that can create a corrosive environment for all of the pump station equipment,” he pointed out, adding, “Pump stations are regularly checked to make sure they are working properly. Ultimately, capital replacement is considered on a cycle of around 30 years.”
“Regular maintenance and capital investment help keep the sewer system working properly, prevent overflows, and minimize costs,” he said. “Capital planning not only includes keeping the existing system running efficiently, but also improving it to respond to changing conditions, such as rainfall patterns, flooding, and new business infrastructure needs, as on Middlesex Turnpike.
Maintenance of force mains includes periodic inspections for corrosion.
Read about the Bedford Department of Public Works’ new tactic to identify sources of groundwater infiltration in the sanitary sewer lines.
Loading ...
