The Massachusetts Water Resources Authority (MWRA) provides wastewater treatment for 43 communities in the greater Boston area, treating 365 million gallons of sewage per day at its Deer Island Wastewater Treatment Plant, the second largest in the United States. Following primary and secondary wastewater treatment, Deer Island's 12 egg-shaped anaerobic digesters reduce the volume of sludge solids by about 60 percent and produce biogas. The biogas fuels steam boilers that supply heat and hot water for the plant's processes and facilities. The steam also powers a turbine generator that produces about 28 MWh of electricity per year. This co-generation facility supplies more than 97 percent of the treatment plant's thermal and 17.5 percent of its electrical demands, and saves MWRA approximately $15 million in fuel oil costs and $2.5 million in electricity costs annually. The digested sludge flows by pipeline to another facility where most of the water is removed (and sent back to Deer Island for treatment) and the solids are dried into pathogen-free, pelletized fertilizer. Overall, the anaerobic digestion process is key to making a valuable product and allows the treatment plant to use local renewable resources in place of fossil fuels for a significant portion of its energy needs. Starting in mid-2014, MWRA plans to pilot the co-digestion of “source-separated organic matter” (mostly food waste) with the sewage sludge in one or two of its digesters. The additional organic matter will boost the production of biogas and enable the Deer Island plant to produce more of its own energy and save even further on energy purchases.
Located on the Merrimack River, the Greater Lawrence Sanitary District (GLSD) operates a wastewater treatment facility that serves the communities of Lawrence, Methuen, Andover, and North Andover in northeastern Massachusetts, as well as the southern New Hampshire town of Salem. In 2002, GLSD built three anaerobic digesters to process wastewater sludge. Today, the three digesters process an average of 135,200 gallons of wastewater sludge and 48,000 pounds of septage per day. The resulting biogas is used in place of natural gas to heat the plant and to dry the biosolids (treated sludge) into pathogen-free pelletized fertilizer in a facility operated by the New England Fertilizer Company (NEFCO). Processing waste through anaerobic digestion reduces GLSD's energy needs and greenhouse gas emissions, all while sustainably managing wastewater sludge. The system is so successful that GLSD has received requests to process organic waste from other sources, including municipalities, food processors, and other wastewater treatment plants. GLSD is currently exploring options for upgrading its current facilities.
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