Enhanced anaerobic digestion of food waste by metal cations and mechanisms analysis

Anaerobic digestion (AD) is promising on food waste (FW) disposal and bioenergy production, during which long-chain fatty acids (LCFAs) inhibition is usually existing. In this study, metal cations (i.e., Ca2+ and Mg2+) were used to enhance the AD performance of FW and explore the mechanisms behind. Results showed that the Ca2+ and Mg2+ performed concentration-depended effects on the AD of FW: promotion at low concentration while inhibition at high concentration. The optimal concentrations of Ca2+ and Mg2+ for AD of FW were 100 and 5 mg/L, respectively, which obtained 29.76% and 18.00% higher methane yield than that of control respectively. Moreover, the volatile fatty acids (VFAs) yield, the degradation of long chain fatty acids, the soluble carbohydrates and protein, some enzymes related to the 8-oxidation of LCFAs were also improved by Ca2+ and Mg2+ addition. Besides, Ca2+ addition mainly acted in the sustained release of LCFAs by formation of reversible precipitates with LCFAs. The microbial structure was also reshaped by Ca2+ and Mg2+ addition, some functional bacteria that could degrade carbohydrates, protein, and LCFAs were enriched. Overall, metal cations addition could be a promising approach for improving the performance of AD systems for lipids-rich waste such as FW.