Anaerobic digestion of fats, oils, and greases (FOG) combined with carboxylic acids: Biogas kinetics, microbiome, and metabolic pathways

The addition of carboxylic acids (C2 and C3) in anaerobic digestion (AD) can stimulate microbial growth and improve biogas production. Thus, C2 and C3 were added separately in two phases with an increasing fat, oil, and grease (FOG) concentration. The effects of C2 and C3 on biogas and volatile fatty acids (VFAs) production were studied. The microbial community shifts among bacteria and archaea were also examined, along with the metabolic pathways and functional enzymes. Biogas production during FOG digestion was increased by 1.69- and 1.47-fold upon the addition of C2 and C3, respectively, at the end of phase I. The accumulation of VFAs and long-chain fatty acids limited the biogas production in phase II. The Clostridium (28.8–48.3 %) and Turicibacter (20.1–8.7 %) were abundant in C2 and C3 amended FOG reactors. The addition of C3 also upsurged Pseudigracilibacillus (27.5 %) and Thiopseudomonas (9.08 %). Methanosphaera was the predominant archaea accounted for 79.1 % and 84.6 % in C2 and C3 amended reactors, respectively. The major predicted metabolic pathways were carbon metabolism (ko01200) and ABC transporters (ko02010). Glucosidase and coenzyme F₄₂₀ hydrogenase were the foremost bacterial and archaeal enzymes, respectively. Digestion of complex substrates (such as FOG) could be effectively enhanced by the addition of carboxylic acids.