The Role of Hydrochar in Promoting Methane Production from Anaerobic Digestion with Different Inocula

Carbon materials, including hydrochar, can promote methane production from anaerobic digestion, and many previous studies have investigated the mechanism by analyzing changes in the microbial community. Based on the fact that the major functional microorganisms have varied in different studies, in order to investigate the effect of inocula on the final microbial composition and to compare the promotion effects of hydrochar on different microorganisms, this study introduced inocula from three distinct sources to anaerobic sequential batch reactors and analyzed the microbial community using 16S rRNA gene sequencing. Hydrochar showed significant promotion effects for all three of the inocula, by increasing microbial activity at high acid concentrations and/or by reducing butyrate accumulation. The dominant microorganisms in all reactors were Clostridium and Methanosarcina, and hydrochar increased the proportion of acetoclastic methanogens. The bacteria which were promoted by hydrochar (e.g., Aminicenantales, Anaerolineaceae and SBR1031 in anaerobic granular sludge and sludge digestate, as well as Lachnospiraceae in food waste digestate) only accounted for around 10%. The bacteria found to be involved in DIET in earlier studies were lacking in this study. It was speculated that hydrochar acted as an electron intermediate and supported microbial aggregation, while the possibility that hydrochar promoted DIET cannot be ruled out.