Low calcium dosage favors methanation of long-chain fatty acids

Anaerobic digestion of fats, oils, and greases is a desirable approach to recover methane from wastewater and solid waste. However, the efficiency and stability are unsatisfying owing to the difficulty in long-chain fatty acids (LCFAs) methanation. To make matters worse, LCFAs easily form precipitate with Ca2+, which would further reduce the stability of anaerobic digestion. Therefore, to determine the optimal Ca2+ concentrations and investigate the mechanism that how Ca2+ affect the precipitate formation and LCFAs methanation, a series of batch reactors were established with different Ca2+ concentrations. The optimum Ca2+ concentration was proven to be 20 mg/L. It presented the fastest methane production rate and highest methane yield, which was increased by 67% than the control. Taken anaerobic digestion efficiency into account, it was nearly twice as efficient as other groups by producing about double net energy. Excessive Ca2+ (2500 mg/L) was the result of the formation of large amounts of precipitate composed of calcium phosphate (50 wt%), calcium carbonate (37 wt%), and Ca-LCFAs (13 wt%), which rendered LCFAs inaccessible and decreased the LCFA degradation rate. Further microscopic and elemental investigations revealed that Ca-LCFAs precipitate were barely formed at low Ca2+ concentrations, avoiding carbon source loss. The relative abundance and absolute amount of two syntrophic bacteria, Syntrophomonas spp. and Petrimonas spp., ensured rapid methanogenesis. Metagenome prediction confirmed promotion of two crucial enzymes involved in β-oxidation. In summary, these results indicated that low Ca2+ dosage of about 20 mg/L significantly favors LCFAs methanation.