Anaerobic co-digestion of municipal wastewater sludge and un-dewatered grease trap waste for assessing direct feed of grease trap waste in municipal digesters

• An attractive method to integrate grease trap waste into resource recovery system.
• Successful co-digestion was observed up to 46% GTW on volatile solids basis.
• Increase in methane potential and VS reduction were 67% and 59%, respectively.
• Anaerobic co-digestion of un-dewatered GTW can reduce the inhibition risk.
• A relatively rapid and effective digester recovery method was established.

The performance of anaerobic co-digestion of municipal wastewater sludge and un-dewatered grease trap waste was investigated for assessing direct feed of grease trap waste in municipal digesters. Substrate degradation, methane potential and process inhibition were monitored in two laboratory scale, semi-continuous feed reactors under mesophilic conditions (35 °C). The methane potential increased from 384 ml CH4 g−1 VSadded to 536 and 641 ml CH4 g−1 VSadded at 0%, 25% and 46% grease trap waste feed ratios, respectively. Co-digestion failed at the 70% grease trap waste feed ratio. The results demonstrated that feeds with volatile acids to alkalinity ratios greater than 2 could cause process failure. Recovery of the digesters following upset conditions was achieved by feeding digester effluent from a full scale digester for 10 days. The performance and stability results observed in this study were compared with data from previous studies of co-digestion of dewatered grease trap waste. The grease trap waste feed ratio that yielded stable co-digestion was increased by using un-dewatered grease trap waste. The results demonstrated that using un-dewatered grease trap waste as the co-substrate instead of dewatered grease trap waste can improve digester performance and provide a more stable digestion.