Benefits and Costs of Co-Digestion for a Large Utility

The Massachusetts Water Resources Authority (MWRA) operates a large wastewater treatment facility located on Deer Island in Boston Harbor. The 1,400 ML/d secondary facility has twelve, 11.3 ML egg-shaped digesters that minimize the sludge requiring further processing and generates 112 mmbtu/hr of biogas. On an annual basis, the biogas furnishes 98 percent of the plant's heating needs (including digester process heating) and 20 percent of the plant's power needs. The annual combined heat and power value of the biogas is approximately $20 million. Currently, seven of the twelve digesters are utilized.MWRA commissioned CDM Smith to explore the benefits and costs of co-digestion of wastewater sludge and other organic wastes at Deer Island. The study examined the types and characteristics of organic wastes that might be available, the logistics of transporting the wastes to the plant, the required modification of plant facilities, the expected additional biogas production, and the impact of co-digestion on the wastewater treatment process (sidestream impacts), subsequent dewatering, thermal drying and biosolids product marketability.The MWRA's objectives were to make better use of existing assets, reduce reliance on purchased power and increase revenues to help stabilize user rates. Co-digestion would also assist the State of Massachusetts in their goal of increasing recycling rates. The State intends to ban the incineration or landfilling of source-separated organics (SSO) potentially diverting up to 300,000 metric tons per year. SSO would typically include food wastes from supermarkets, institutions, and food producers.The processing of organic wastes at Deer Island is constrained by difficulty of transporting it to the facility. Although Deer Island is connected to the Town of Winthrop by a causeway, the truck route to the plant traverses narrow streets with dense residential development. MWRA has a long-standing agreement with the community that truck traffic will be kept to a minimum. Therefore, the study had to focus on marine transport by barge from one or more sites with good access.Another unique aspect of the feasibility study was to determine the method and responsibility for pre-processing of organic wastes. Since MWRA does not want to generate any residue which would require transport from the plant to a landfill, it is likely that private sector haulers would pre-process the wastes off-island and deliver an “engineered food waste” (EFW) to the plant. Pre-processing would typically include methods to screen and pulp the wastes, removing contaminants such as glass, plastic and metals to produce a uniform, pumpable material.The study examined a wide range of EFW quantities from approximately 10 percent to 30 percent of the volume of wastewater sludge. The report determined that the volume of additional biogas would increase by approximately 25-75 percent with EFW addition. However, approximately 30 percent of EFW remains after digestion and adds to the amount of sludge requiring downstream processing. Costs for additional processing and costs for expanded biogas utilization were factored into estimates of tipping fee charges necessary to sustain the co-digestion plan.MWRA has commenced laboratory studies of co-digestion, and intends to conduct full-scale trials at the plant and further refine the costs and benefits of co-digestion identified in the report. The Co-Digestion Report to MWRA will be completed in the spring of 2013 and the results will be available for WEFTEC.