Power to biogas upgrading: Effects of different H-2/CO2 ratios on products and microbial communities in anaerobic fermentation system

Two anaerobic reactors with and without Ca2+ were operated at 35 degrees C to investigate the effects of different H-2/CO2 ratios on products and microbial communities. Through the investigation of various parameters, it was shown that the change of pH triggered by the variations of H-2/CO2 is the decisive factor affecting the product selection in anaerobic fermentation system. During the biosynthesis of ATP for cell growth and reproduction, protons (H+) were pumped from extracellular to intracellular by proton pump, which caused an increase of intrinsic pH of fermentative system. When the pH below 9.5, the methanogenic pathway was more prevalent. While the pH above 10.0 was conducive to the homoacetogenesis. Microbial community analysis showed that with the changes of H-2/CO2 ratio, a turnover had occurred. When the ratio of H-2/CO2 was 4, the main methanogen was Methanobacterium with the dominant interspecies electron transfer bacteria (IETB) of Thermovirga and DMER64. The turnover of microbial community occurred when the H-2/CO2 ratio was 4.5 and 4.25. The dominant acetogenic microorganisms were norank_o_Clostridia_UCG-014 (homoacetogen) and Natronincola (obligately alkaliphilic acetogen). When the H-2/CO2 ratio returned to 4, the dominant methanogens were hydrotropic Methanobacterium and Methanobrevibacter with four interspecies electron transfer bacteria including DMER64, Thermovirga, Dechlorobacter and Achromobacter.