Anaerobic Digestion of Food Waste

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Food waste is the single largest category of municipal solid waste (MSW) in California at 5.9 million tons or 16% of total MSW as of 1999 (CIWMB, 1999). Diverting a portion of food waste from landfills can provide a significant contribution toward achieving EPA, state, and local mandated solid waste diversion goals. In addition, diverting food waste from landfills prevents uncontrolled emissions of its breakdown products, including methane—a potent greenhouse gas. Currently, only about 2.5% of food waste is recycled nationwide, and the principal technology is composting. While composting provides an alternative to landfilldisposal of food waste, it requires large areas of land; produces volatile organic compounds (smog precursors), which are released into the atmosphere; and consumes energy. Consequently, better recycling alternatives to composting food waste should be explored. Anaerobic digestion has been successfully used for many years to stabilize municipal organic solid wastes, and to provide beneficial end products, i.e., methane gas and fertilizer. In California, approximately137 wastewater treatment plants have anaerobic digesters, with an estimated excess capacity of 15-30% (Shang, et. al., 2006). This excess digester capacity could provide an opportunity to recycle post-consumer food waste while producing renewable energy and reducing greenhousegas emissions in California.At the East Bay Municipal Utility District’s (EBMUD) Main Wastewater Treatment Plant (MWWTP), food waste is currently co-digested with primary and secondary municipalwastewater solids and other high-strength wastes. Due to the limitations at full-scale, and risksof overloading digesters, it has been difficult to accurately quantify methane gas production rates, mean cell residence time (MCRT) requirements, and volatile solids reduction values that are specific to food waste digestion. Bench-scale anaerobic digesters were fed only food waste pulp from EBMUD’s food waste processing system (see Figure 2,2). The digesters were operated at both mesophilic and thermophilic temperatures, at 15-, 10-, and 5-day MCRTs. In addition, anaerobic food waste digestion was compared with anaerobic municipal wastewater solids digestion to demonstrate the benefits of food waste digestion at wastewater treatment plants.

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