Saponification with calcium enhanced methane yield in anaerobic digestion of fat, oil, and grease: The essential role of calcium

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.