Valorization of organic solid waste through HTC-AD coupling: revisiting the applicability of hydrochar in AD

Hydrochar is considered as a promising carbon material for enhancing anaerobic digestion (AD). However, inconsistent results reported in previous studies have limited its practical application. This study aimed to provide insights into these inconsistencies by elucidating the role of dissolved organic matter (DOM) leached from raw hydrochar. Results demonstrate that DOM is a decisive factor governing hydrochar performance in AD, and its characteristics vary markedly with biomass feedstock. The inhibitory effect of cornstalk hydrochar (CSH) on AD was primarily attributed to complex DOM components, especially phenols components. Removal DOM effectively alleviated inhibition, as evidenced by a 15.6% increase in the methane production rate (Rm) and a significant shortening of the lag-phase (λ) from 12.1 to 8.0 days during the AD of hydrothermal wastewater (HTWW) with CSH. Solvent washing to remove DOM altered hydrochar properties by increasing specific surface area and exposing additional oxygen-containing functional groups (OCFGs), further enhancing methane production kinetics. Microbial community analysis further indicated that phenols increased the relative abundance of acid-producing Trichococcus while decreasing methanogenic Methanosaeta and electrochemically-active bacteria (Mesotoga and Anaerolinea), thereby hindering the degradation of VFAs. In contrast, biogas residue hydrochar (BRH) had no adverse effect on AD, but it failed to significantly enhance AD performance, especially for refractory HTWW, likely due to its high ash content. Based on the two feedstocks examined in this study, the results suggest that the beneficial effects of hydrochar on AD are conditional and leachable DOM plays a critical role which has not been fully recognized in previous studies.