Elucidating Microbial Community Adaptation to Anaerobic Co-digestion of Fats, Oils, and Grease and Food Waste. anaerobic digester metagenome

Despite growing interest in co-digestion and demonstrated process improvements (e.g., enhanced stability and biogas production), few studies have elucidated how co-digestion impacts the anaerobic digestion microbiome. Three sequential bench-scale respirometry experiments were conducted at thermophilic temperature (55°C) with various combinations of primary sludge (PS); thickened waste activated sludge (TWAS); fats, oils, and grease (FOG); and food waste (FW). Two additional runs were then performed to evaluate microbial inhibition at higher organic fractions of FOG (30-60% volatile solids loading (VSL; v/v)). Co-digestion of PS, TWAS, FOG, and FW resulted in a 16% increase in biogas production relative to digestion of PS and TWAS. A substantial lag time was observed in biogas production for vessels with FOG addition that decreased by more than half in later runs, likely due to adaptation of the microbial community. 30% FOG with 10% FW showed the highest increase in methane production, increasing 53% compared to digestion of PS and TWAS. FOG addition above 50% VSL was found to be inhibitory with and without FW addition and resulted in volatile fatty acid (VFA) accumulation. Biogas production was linked with high relative activity and abundance of syntrophic fatty acid oxidizers alongside hydrogenotrophic methanogens, signaling the importance of interspecies interactions in AD. Specifically, Syntrophomonas and Syntrophothermus relative activity were significantly correlated with biogas production. Further, biogas production increased over subsequent runs along with mcrA gene expression, a functional gene in methanogens, suggesting temporal adaptation of the microbial community to co-digestion substrate mixtures.