Lactic acid-based fermentative hydrogen production from kitchen waste: Mechanisms and taxonomic insights

Lactic acid production under anaerobic dark fermentation is inversely related to hydrogen (H₂) production. In this study, a novel pH regulation mechanism was proposed to convert lactic acid to H₂. The study also investigates the roles played by dominant microbial communities and enzymes in the degradation of kitchen waste, resulting in lactic acid accumulation and its subsequent conversion to H₂. Furthermore, a two-phase Gompertz function was applied to describe the cumulative H₂ production, with the first phase representing H₂ production from readily degradable sugars, followed by the second phase representing H₂ production from lactic acid conversion. The study finds that the optimal H₂ production from lactic acid occurred within a pH range of 5.5 to 6.0. When the pH exceeded 6.0, it led to propionic acid accumulation and a subsequent reduction in H₂ production. The observed H₂ yield was found to be 39.23 ± 2.29 mL-H₂/g of volatile solids (VS) for the pH-unregulated experiments and 28.60 ± 2.78 mL-H₂/g VS for the pH-regulated experiments. Finally, microbial community analysis reveals that Lactobacillus (relative abundance (RA): 63–66 %) and Bifidobacterium (RA: 19–21 %) are the dominant lactate producers in the initial phase, while Megasphaera (RA: 26 %), Veillonella (RA: 3.4 %), and Clostridium sensu stricto 7 (RA: 5 %) are the primary lactate utilizers and H₂ producers in the subsequent phase. These findings highlight the mechanistic understanding of lactic acid-based H₂ production from putrescible organic waste like kitchen waste and enhance the understanding of the acidogenic phase of anaerobic digestion.