Methane potential and metagenomics of wastewater sludge and a methane‐producing landfill solid sample as microbial inocula for anaerobic digestion of food waste

BACKGROUND Anaerobic digestion of food waste is influenced by the selection of an adequate microbial inoculum. In this study, wastewater sludge, a methane (CH4)‐producing landfill solid sample and the combination of both [1:1 volatile solid (VS)] were tested as inocula to treat food waste modelled to the Mexican diet. Hence, biochemical methane potential assays at inoculum to substrate (I:S) ratios of 1–3 and each inoculum metagenome were determined. RESULTS Methane production was optimal at I:S = 1 for sludge and the combined inoculum, whereas for landfill solid sample increasing the proportion of inoculum was beneficial up to I:S = 2. The landfill solid sample produced more CH4 than commonly used sludge (311.5 ± 11.1 and 282.5 ± 18.1 mL CH4 g−1 VS, respectively) but presented an adaptation phase (four days). Combining inocula (I:S = 1) produced a synergistic effect in CH4 yield (374.5 ± 10.5 mL CH4 g−1 VS), biogas quality (58.0 ± 0.3% CH4) and COD removal (90.0 ± 0.6%) without start‐up time. Moreover, an enriched microbial community high in Archaea and Bacteria was observed, probably because of the incorporation of predominant members of each inoculum (Bacteroidetes, Firmicutes and Euryarchaeota in sludge, and Proteobacteria and Actinobacteria in landfill solid sample). Archaea was represented mainly by the genus Methanosaeta (order Methanosarcinales), suggesting that methanogenesis occurred by the acetoclastic pathway. CONCLUSION Food waste from Mexican diet was a suitable feedstock for biogas generation. Optimal conditions were observed when combining both inocula. Further studies on population dynamics during digestion would help understand the synergistic effect for effective industrial application. © 2018 Society of Chemical Industry