Enhanced dark fermentation of kitchen waste using hydrothermal carbonization char

This study investigates the impact of char from hydrothermal carbonization (HTC) on hydrogen (H 2 ) production and fermentation efficiency. The results show that the addition of hydrochar significantly enhances H 2 production. The most favorable outcomes were observed when using hydrochar produced at 250 °C. Lower HTC temperatures (180 °C) resulted in lower H 2 yields, indicated that higher temperatures are more effective in optimizing hydrochar properties for fermentation. Washing the hydrochar with acetone did not significantly alter fermentation results. Analysis of volatile fatty acids (VFAs) and pH changes demonstrated that char produced at 250 °C promoted the production of acetic and butyric acids, which are key metabolites in H 2 production pathways, while suppressing the formation of propionic acid. These findings underscore the role of hydrochar in improving fermentation conditions and directing microbial metabolism toward more efficient H 2 production. Additionally, hydrochar influenced microbial community structure, with higher HTC temperatures promoting H 2 -producing Clostridiaceae . The distribution of carbon across the solid, liquid, and gas phases further confirmed the effectiveness of HTC hydrochar in the converting of organic matter into gaseous fermentation products. • Hydrochar addition enhanced hydrogen production during dark fermentation. • Char produced at 250 °C enhanced fermentation and metabolite distribution. • Acetic and butyric acids were promoted, while propionic acid formation was suppressed. • HTC char shaped microbial communities, favoring hydrogen-producing Clostridiaceae. • Carbon balance confirmed efficient conversion of organics into gaseous products.