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

Abstract

Lactic acid production under anaerobic dark fermentation is inversely related to hydrogen (H 2 ) production. In this study, a novel pH regulation mechanism was proposed to convert lactic acid to H 2 . 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 H 2 . Furthermore, a two-phase Gompertz function was applied to describe the cumulative H 2 production, with the first phase representing H 2 production from readily degradable sugars, followed by the second phase representing H 2 production from lactic acid conversion. The study finds that the optimal H 2 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 H 2 production. The observed H 2 yield was found to be 39.23 +/- 2.29 mL-H 2 /g of volatile solids (VS) for the pHunregulated experiments and 28.60 +/- 2.78 mL-H 2 /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 H 2 producers in the subsequent phase. These findings highlight the mechanistic understanding of lactic acid-based H 2 production from putrescible organic waste like kitchen waste and enhance the understanding of the acidogenic phase of anaerobic digestion.