Anaerobic co-digestion of food waste and sewage sludge under mesophilic and thermophilic conditions: Focusing on synergistic effects on methane production

•Co-digestion of food waste and sewage sludge was investigated at different temperatures.•Temperature did not affect the extent but the rates of biodegradation in mono- and co-digestion.•Methane yields and extent of biodegradation increased as food waste increased.•Positive synergism was associated with digestion time. Globally, food waste (FW) and sewage sludge (SS) are among major organic wastes that needs to be properly handled. Anaerobic digestion (AD) has been a widely accepted method to treat those wastes and simultaneously recover energy and nutrients. The objective of this study was to elucidate synergistic effects in co-digestion of FW and SS by incubating at mesophilic and thermophilic conditions. The results demonstrated temperature did not significantly affect the cumulative methane yields (CMY); instead, higher temperature resulted in accelerated methane production. Although not clearly shown on CMYs (less than 10% increase), positive synergistic effects on methane production rates were characterized by a co-digestion impact factor (CIF) during the early phase of co-digestion. This early synergism (up to 24% increase) was primarily attributed to the accelerated hydrolysis due to addition of readily degradable FW, which may have practical implication for selection of retention time in order to optimize digestion process.