Palmitic acid accumulation limits methane production in anaerobic co-digestion of fats, oils and grease with municipal wastewater sludge

Anaerobic co-digestion of fats, oils and grease (FOG) with municipal wastewater sludge offers the opportunity to increase methane yields; yet the impact of FOG on overall process dynamics and the associated microbial communities is not well understood. This study employed lab-scale batch anaerobic co-digestion assays to advance the understanding of the co-digestion process through studying the dynamics of formation and consumption of intermediates along the anaerobic digestion pathway and correlating these to temporal analysis of methanogenic activity as well as end-point microbial community structure. Methane production was delayed during sludge co-digestion with FOG, but not during digestion of sludge alone. Palmitic acid, a long-chain fatty acid (LCFA), accumulated during co-digestion to concentrations above 16 mM, resulting in a lag in methane production. Acetate, hydrogen, and formate, carbon and/or energy sources for methanogens during methane production, did not accumulate during the lag-phase. Expression of the mcrA gene, which encodes the methyl coenzyme M reductase alpha subunit necessary for catalyzing the final step of methanogenesis, increased when co-digesters were spiked with acetate during the lag-phase, indicating no direct total inhibition from palmitic acid to methanogenic activity in assays with a 23% FOG loading (based on total g VS). End-point microbial community analyses revealed distinct community structure and composition differences between co-digestion assays and controls, and between biological replicates with variable responses. The hydrogenotrophic methanogen genus Methanoculleus dominated adapted co-digesters, but not inhibited co-digesters suggesting its importance for adapting to FOG loadings. Palmitic acid accumulation resulting from distinct microbial community composition and structure thus characterizes stalled or inhibited FOG co-digesters.