Effects of different function-oriented hydrochars on anaerobic digestion of hydrothermal wastewater: Focusing on microbial community function and organic degradation.

To elucidate the coupling relationships among hydrochar characteristics, microbial responses, and organic matter removal during anaerobic digestion of hydrothermal treatment wastewater (HTTWW-AD), raw hydrochar (HC), alkali-modified hydrochar (AHC), and iron-modified hydrochar (IHC) were prepared. Excessive microbial anabolic metabolism and limited hydrolysis-acidification efficiency were identified as the main causes of the low methane yield in HTTWW-AD. HC, AHC, and IHC increased methane yield by 115.97%, 148.25%, and 135.42%, respectively, and the methane content also increased by 9.86% - 12.50%. Metagenomic analysis revealed that microorganisms in the control (CK) system were under higher stress, whereas hydrochar addition promoted the enrichment of hydrolytic and acidogenic bacteria (HAB) and alleviated microbial stress. AHC further enriched Methanothrix and Methanobacterium, thereby enhancing both acetoclastic and hydrogenotrophic methanogenesis. The enhanced reductive methanogenesis was likely associated with its high electron-donating capacity (EDC). IHC enriched exoelectrogenic HAB, suggesting that Fe/N-related active sites may facilitate extracellular electron transfer. Gas chromatography-mass spectrometry analysis showed that HC favored the removal of ketones, N-containing heterocycles, and alcohols, whereas AHC was more effective for acids, N-containing heterocycles, and alcohols. IHC promoted the removal of diverse organic compounds, particularly ketones, phenols, and esters. These differences were associated with the enrichment of potential degraders (Hydrogenophaga, Sphaerochaeta, Mesotoga, Bacteroides, and Paludibacter), possible adsorption at surface-active sites, and Fe(III)/Fe(II)-cycle-mediated redox activation. Overall, hydrochars effectively promoted hydrolysis-acidification during HTTWW-AD. Hydrochars enriched with electron-donating functional groups favored methanogenic conversion, whereas Fe/N-related active sites were more beneficial for the removal of recalcitrant organic compounds.