Improving the biomethane yield from food waste by boosting hydrogenotrophic methanogenesis

Highlights: • Hydrogen addition improved biomethane yield and biogas quality from food waste. • Biogas was upgraded from 65% to 77.2% CH₄ using a gas mixture of 5%-H₂ and 95%-N₂. • No inhibition to volatile fatty acids production and decomposition was observed. • High acidification potential of food waste helped to buffer excessive pH increase. -- Abstract: Anaerobic digestion of food waste is usually impacted by high levels of VFAs, resulting in low pH and inhibited methane production from acetate (acetoclastic methanogenesis); however, this could be harnessed for improving methane production via hydrogenotrophic methanogenesis (biomethanation). In this study, batch anaerobic digestion of food waste was conducted to enhance biomethanation by supplying hydrogen gas (H₂), using a gas mixture of 5%-H₂ and 95%-N₂. The addition of H₂ influenced a temporal microbial shift in substrate utilisation from dissolved organic nutrients to H₂ and CO₂ and was perceived to have enhanced the hydrogenotrophic methanogenic activity. As a result, with the release of hydrogen as degradation progressed (secondary fermentation) hydrogenotrophic methanogenesis was further enriched. This resulted in an enhancement of the upgrading of the biogas, with a 12.1% increase in biomethane (from 417.6 to 468.3 NmL-CH₄/gVS_added) and 38.9% reduction in CO₂ (from 227.1 to 138.7 NmL-CO₂/gVS_added). Furthermore, the availability of hydrogen gas at the start of the process promoted faster propionate degradation, by the enhanced activity of the H₂-utilisers, thereby, reducing likely propionate-induced inhibitions. The high level of acidification from VFAs production helped to prevent excessive pH increases from the enhanced hydrogenotrophic methanogenic activity. Therefore, it was found that the addition of hydrogen gas to AD reactors treating food waste showed great potential for enhanced methane yield and biogas upgrade, supported by VFAs-induced pH buffer. This creates the possibility to optimise hydrogenotrophic methanogenesis towards obtaining biogas of the right quality for injection into the gas grid.