Hydrochar-Promoted Methane Production in Mesophilic and Thermophilic Anaerobic Digestion of Hydrothermal Pre-Treated Sludge

Hydrochar produced during the hydrothermal conversion of organic solid waste could enhance the anaerobic digestion (AD) efficiency of hydrothermal pre-treated sludge. However, there was still a lack of systematic research on the effect of hydrochar on improving the methane production and microbial communities of the AD of hydrothermal pre-treated sludge under different temperature conditions. This study explored the effect of hydrochar on methane production from the mesophilic and thermophilic AD of hydrothermal pre-treated sludge and the mechanism of microbial action based on metagenomics analysis. Hydrochar could improve the methane production efficiency of mesophilic and thermophilic AD at different initial concentrations of hydrothermal pre-treated sludge. However, the effect of hydrochar in promoting AD varied under different AD temperatures. Both temperature and hydrochar were crucial factors that could influence the microbial community. Moreover, hydrochar increased the relative abundance of archaea in the AD system, resulting in an increment of 4.99% to 15.30% compared to the control group. Mesophilic reactors exhibit greater microbial diversity. Hydrochar resulted in the significant enrichment of Synergistota in the thermophilic AD system and the enrichment of Firmicutes in the mesophilic AD system, thereby promoting the hydrolysis of proteins and polysaccharides during AD. This study has practical significance for the resource treatment of excess activated sludge.