Synergistic bioaugmentation with Clostridium thermopalmarium and Caldibacillus thermoamylovorans improved methane production from thermophilic anaerobic digestion of food waste

Microbial bioaugmentation is a promising strategy to increase methane production in thermophilic anaerobic digestion (TAD). In this study, Clostridium thermopalmarium HK1 and Caldibacillus thermoamylovorans QK5 were combined to form a combined microbial inoculant, and the influence of this synergistic bioaugmentation strategy on the TAD of food waste was explored in batch and semi-continuous experiments. The cumulative methane production reached 512.74 ± 8.12 NmL g−1 VS with the addition of the combined microbial inoculant, which was increased by 24.77 % ± 1.98 % compared to the control group, and the bioaugmentation effect was significantly better than that of the group inoculated with C. thermopalmarium HK1 (458.44 ± 4.80 NmL g−1 VS, 11.56 % ± 1.17 %) or C. thermoamylovorans QK5 (478.74 ± 5.73 NmL g−1 VS, 16.50 % ± 1.39 %) individually. The addition of the combined microbial inoculant particularly enriched carbohydrate-degrading bacteria and protein-degrading bacteria. In terms of metabolic functions, bioaugmentation with the combined microbial inoculant enhanced carbohydrate metabolism, amino acid metabolism, and methane metabolism, promoting the complete oxidation of organic matter into CO2 via the tricarboxylic acid cycle (TCA cycle) and driving the conversion of CO2 to methane. In particular, the synergistic effect of C. thermopalmarium HK1 and C. thermoamylovorans QK5 in the combined microbial inoculant resulted in notably increases in the bioaugmentation of carbohydrate and protein degradation, the TCA cycle, and hydrogenotrophic methanogenesis.