Performance and microbial community dynamics in full-scale anaerobic co-digesters fed with fats, oils and grease

Anaerobic co-digestion of fats, oils, and grease (FOG) with municipal sludge offers water resource recovery facilities a pathway to increase methane yields and improve volatile solids (VS) reduction. However, operational risks associated with long-chain fatty acids (LCFAs) accumulation have overshadowed these benefits. While recent efforts have identified requirements for stable FOG co-digestion, full-scale studies linking these findings to operational performance are scarce. To address this gap, two 1.36-million-gallon anaerobic digesters fed primary sludge, thickened waste activated sludge, and FOG were monitored for 10 months. Chemical oxygen demand, VS, short-chain fatty acids, LCFAs, and biogas methane content, microbial community and Syntrophomonas gene copies were measured weekly, monthly, and twice monthly, respectively. Throughout the study acetate remained 7.0, methane % in the biogas remained > 64%, and LCFAs remained < 3 mM, indicating healthy digestion. A strong positive correlation between VS destruction and FOG VS loading suggested that FOG behaves as a readily biodegradable substrate. The relative abundances of Cloacimonadaceae W5 and Synergistaceae increased with LCFA loading, from 0.4 to 1.5% and 0.4 to 1.6%, respectively, while Methanospirillum and Smithella increased at elevated in-digester LCFA concentrations, from 5 to 14.5% and from 1.6 to 5.6%, respectively. Syntrophomonas gene copies were approximately an order of magnitude higher in co-diesters vs. non-co-digesting reactors. Our findings demonstrate that full-scale FOG co-digestion can be operated stably with measurable benefits to VS destruction and microbial community adaptation, providing actionable guidance for utilities considering or operating FOG co-digestion systems.