Effect of hydrogen-feeding initiation time on methane production during membrane-assisted anaerobic digestion of kitchen waste

Hydrogen-assisted anaerobic digestion offers a promising strategy for enhancing methane production through in situ biological methanation. While previous studies have primarily focused on hydrogen-transfer technologies and reactor configurations, the influence of the initiation time of hydrogen feeding on process performance remains insufficiently understood. In this study, the effect of the timing of hydrogen supplementation during the anaerobic digestion of kitchen waste was investigated using a submerged polydimethylsiloxane (PDMS) membrane contactor for hydrogen delivery under mesophilic conditions. Hydrogen supplementation was initiated at different stages of the digestion process (days 0, 3, 5, 10, or 15), and methane production, pH, volatile fatty acid (VFA) dynamics, and microbial community composition were monitored. Because the experiments were conducted in two independent series, methane yields were normalized to the corresponding control reactors to enable direct comparison of hydrogen-feeding strategies. Hydrogen addition initiated on days 3 and 5 resulted in the greatest enhancement in methane yield, increasing methane production by 31% and 35%, respectively, relative to the corresponding controls. In contrast, hydrogen supplementation from the start of digestion provided no improvement, while hydrogen addition initiated on day 10 resulted in only a slight increase in methane yield. Early hydrogen supplementation was accompanied by a pronounced increase in pH, distinct VFA dynamics, and enrichment of hydrogen-associated microbial taxa, including Methanolinea - and Natronincola -related populations. The persistence of Methanosaetaceae across all treatments indicated that acetoclastic methanogenesis remained active alongside hydrogenotrophic pathways. The combined process and microbial analyses indicated that the effectiveness of biological methanation depends not only on hydrogen availability but also on the timing of hydrogen introduction. Under the investigated conditions, days 3-5 were the optimal operational window for hydrogen supplementation, highlighting the initiation time of hydrogen feeding as an important parameter for optimizing membrane-assisted biological methanation systems treating readily degradable organic wastes.