In an effort to be self-sufficient, GLSD has three anaerobic digesters which take in the treated sludge and produce methane. This biogas is then used to heat the digesters as well as the buildings at the District.
Sludge is taken from the Gravity Thickeners and Gravity Belt Thickeners, passed through a heat exchanger, and then fed to the digesters. Each digester holds 1.4 million gallons of sludge, and can store 48,500 cubic feet of methane gas. In total, the digester can produce 400,000 cubic feet per day of methane, which is equivalent to 260,000 cubic feet of natural gas.
The methane (biogas) or natural gas is used to power the boilers that heat the heat exchangers for the digesters, so the digesters are essentially self-sustaining. This same combination of natural gas and digester gas is used to heat the buildings at the District.
GLSD is currently building a fourth digester as well as a combined heat and power plant on site. The construction is part of the Organics to Energy Project which will bring in Source Separated Organics to add into the sludge fed to the digester. This new feedstock will increase the amount of methane produced and will be used to generate electricity for the plant, eventually making the District a Net Zero energy user facility. To learn more about this project see, Projects.
In 2010, Nexamp installed a photovoltaic array at the District under the Massachusetts Solar Stimulus initiative. The District owns and operates the 441 Kilowatt array which produces 583,000 Kilowatt hours each year, roughly the same amount of energy needed to power 37 homes for a year. The array also helps to reduce the District’s CO2 emissions by 402 tons a year.