Dark-fermentative biohydrogen production from vegetable residue using wine lees as novel inoculum

This work studied wine lees as a novel source of microorganisms for biohydrogen production from vegetable residue (VR). Green tomatoes (WLGT), bell peppers (WLBP), green beans (WLGB), zucchini (WLZ), peas (WLP), and (WLE) eggplants were used as a substrate for dark fermentation, which was conducted in batch assays at 37 °C for 60 d. The cumulative hydrogen yield was approximately 350, 344, 319, 314, 302, 170, and 149 mL H₂/g VS in WLZ, WLE, WLRT, WLGT, WLP, WLGB, and WLBP, respectively. A total volatile fatty acid (VFA) accumulation of about 2059 - 4995 mg HAc/L accompanied the bio-H₂ production. From day 61 to day 147, dark-fermentative digestate was subjected to an anaerobic digestion batch process under mesophilic conditions to allow the bioconversion of VFAs into renewable methane, as confirmed by a Pearson correlation value of 0.778, final VFA concentrations ≤ 131 mg HAc/L and key functional genes (e.g., K00925). Clostridium_sensu_stricto_12 and Caproiciproducens genera accounted for 44 - 78 % of relative abundance after the dark fermentation stage. The taxonomic classification also revealed a presence of Methanosaeta archaea comprised between 45 and 98 % after the two-stage anaerobic fermentation. Finally, a rough energy comparison was performed to evaluate the feasibility of the bioprocess by including practical implications and limitations.