Sludge-derived hydrochar and hydrothermal liquor enhance methane recovery and reshape antibiotic resistance gene mobility during sludge anaerobic digestion.

Sewage sludge is a potential bioresource for methane recovery through anaerobic digestion (AD), but limited hydrolysis and antibiotic resistance genes (ARGs) dissemination constrain its efficient and safe valorisation. This study compared the effects of sludge derived hydrochar, hydrothermal liquor, and their combined addition on methane recovery, methanogenic mechanisms, and ARGs mobility during sludge AD. Compared with the control, hydrochar, hydrothermal liquor, and their combined addition increased cumulative methane production by 34.3%, 32.8%, and 57.5%, respectively, with the combined addition achieving the highest methane recovery. Hydrothermal liquor promoted methane production mainly by supplying soluble substrates, whereas hydrochar enriched direct interspecies electron transfer associated microorganisms including Geobacter and Methanothrix, and improved methanogenic functional redundancy. Hydrochar alone showed the strongest potential for ARGs mobility mitigation, with the lowest ARGs abundance, fewer high risk ARGs co-occurrence, fewer transfer related co-occurrence, and the lowest final ecological and human health risk scores. Hydrothermal liquor retained high risk ARGs mobility structures and enriched horizontal gene transfer related genes. Although the combined addition retained relatively higher ARGs abundance and conjugation associated mobility potential, its final ecological and human health risk scores were lower than those of the control. Overall, combined hydrochar and hydrothermal liquor addition maximised methane recovery, whereas hydrochar alone was more effective in reducing ARGs mobility potential.