Investigating Anaerobic Co-Digestion of Sewage Sludge and Food Waste Using a Bench-Scale Pilot Study

A two-phase bench-scale study under mesophilic conditions was conducted to evaluate the anaerobic co-digestion of food waste (FW) and sewage sludge (SS) from the Massa chusetts Water Resources Authority’s (MWRA) Deer Island Treatment Plant (DITP). The study’s objectives were to determine the following for several FW to SS mixtures:• Anaerobic digestibility.• Biogas production.• Volatile solids reduction.• Sidestream impacts for nutrient load.Daily monitoring was performed for digester biogas production, biogas composition, total and volatile solids, pH, alkalinity, volatile acid alkalinity (VAA) and nutrients over a 150-day study period. Weekly characterization of FW and SS was also performed for the extent of the study.The semi-continuous digestion results indicate an increase in biogas production, volatile solids reduction (VSR) and digester stability with the addition of FW up to 50% (w/w feed total solids), in comparison to the two control digesters that were fed only SS. In comparison to the duplicate control digesters, the average biogas production was about 30% greater for the 20% FW and 80% SS mixture, and about 24% greater for the 10% FW and 90% SS mixture. The addition of FW also increased VSR in the range of 1-17% for the various FW mixtures in comparison to control digesters. The study also found that semi-continuous co-digestion of 100% (w/w feed total solids) FW was infeasible with a 28-day SRT. The 100% FW digester had an average biogas production comparable to the 10% FW and 90% SS mixture for 51 days of digestion. At 31 days of digestion, the VAA to Alkalinity ratio increased significantly (>1.0) showing initial signs of failure followed by a decrease in pH, alkalinity and biogas production at day 36.The total nitrogen (TN) and total phosphorus (TP) concentrations in the FW digesters were found to be comparable to the two control digesters, indicating no impact to sidestreams regarding nitrogen and pho sphorus loading from the addition of FW. The study concluded that FW is a highly desirable substrate for co-digestion with SS at DITP with regards to increased biodegradability and methane yield, while maintaining digester stability, TN, and TP sidestream loading.The results of the bench-scale study were also applied to estimate full-scale biosolids production at DITP for the various SS and FW mixtures, and for a planned full-scale pilot plant investigation.