To investigate the effect of granular activated carbon (GAC) adsorption and size of microbial aggregates in inoculum on stimulating direct interspecies electron transfer (DIET) during anaerobic digestion of fat, oil, and grease (FOG), seed sludge was divided into two inocula (big (>0.85 mm)/small (0.15-0.85 mm)) for FOG digestion with/without GAC. More long-chain fatty acids (LCFAs) were adsorbed on GAC in the reactor with small aggregates than that with big aggregates, corresponding to 57 % and 10 % decreased methane production, respectively. Adsorption of unsaturated LCFAs (e.g., oleic acid) on GAC was found to reduce LCFA bioavailability, hinder DIET via GAC, and change community structure. Compared to pre-adsorption of oleic acid on GAC, pre-attachment of microbes on GAC resulted in 5.6-fold higher methane yield for oleic acid digestion. Together, competition of LCFA adsorption between GAC and microbial aggregates is essential for enhanced methane recovery from FOG digestion via GAC-induced DIET.

To investigate the combination of carbon-based conductive materials and exogenous hydrogen (EH2) on methane recovery from fat, oil, and grease (FOG), granular activated carbon (GAC) and carbon cloth (CC) were chosen to collaborate with EH2, resulting in increased methane production by 59 % and 84 %, respectively. Further digestion of long chain fatty acids (LCFAs) confirms that enhanced direct interspecies electron transfer (DIET) was achieved in the reactors with GAC/CC + EH2 than those with GAC/CC only. Other evidences (such as increased microbial population and rapid degradation of volatile fatty acids) were found to support the role of GAC/CC + EH2 in promotion of DIET. Significant change of microbial community was observed using GAC/CC + EH2, which was mainly attributed to the enrichment of electrogenic species (such as Spirochaetaceae, Syntrophomonas palmitatica, and Methanosaeta), leading to some changes in metabolic pathways during acidogenesis and methanogenesis. Together, enhanced DIET was achieved by GAC/CC + EH2, thus improving the methane recovery from FOG.

Little is known about the influence of the saturation degree of long chain fatty acids (LCFAs) on the bio-methane potential of calcium-LCFAs salts. In this study, palmitic acid and oleic acid were chosen as the model compounds to investigate the impact of saponification between calcium and saturated/unsaturated LCFAs on the methane recovery from LCFAs in anaerobic digestion. A 2.2-fold enhancement of methane yield was obtained due to the formation of calcium palmitate, which was primarily attributed to the enhanced bio-aggregation and significant change of microbial community. However, saponification between calcium and oleic acid decreased the methane recovery from oleic acid digestion. Only partial saponification with excess oleic acid led to 4% increment of methane production. The low bio-accessibility of calcium oleate and the little change of microbial community may be responsible for the small difference of methane recovery due to the formation of calcium oleate.

Though saponification between calcium and fat, oil, and grease (FOG) could reduce inhibition caused by long chain fatty acids (LCFAs) in anaerobic digestion of FOG, the role of calcium in methane recovery is not well understood. In this study, different calcium sources (CaSO4, Ca(OH)(2) and CaCl2) and mole ratios of Ca2+/LCFAs (0.25, 0.5 and 1) were chosen to investigate the role of calcium in anaerobic digestion of FOG. Compared to CaCl2/CaSO4, Ca(OH)(2) was the most suitable calcium source due to the highest fraction of saponification in the calcium soaps as well as the 22% increment of methane production. For the reactors using CaCl2/CaSO4 as calcium source, Ca2+/LCFAs = 0.25 is preferred to enhance methane production, and the adsorption of the released LCFAs from calcium soaps onto biomass might be the primary pattern of LCFA degradation. Regarding the reactors using Ca(OH)(2) as calcium source, Ca2+/ LCFAs = 0.5 is beneficial for the enhancement of methane yield, and the combination of bio-aggregation on calcium soap and adsorption of released LCFAs on biomass may be the explanation for the relatively high methane production. Different calcium source and Ca2+/LCFAs could primarily affect the composition of syntrophs and methanogens, leading to different methane recovery from anaerobic digestion of FOG. (c) 2022 Elsevier Ltd. All rights reserved.