Short-chain fatty acids and hydrogen production in one single anaerobic fermentation stage using carbohydrate-rich food waste

Abstract

This study assessed the feasibility of melon and watermelon residues for the co-production of short-chain fatty acids (SCFAs) and hydrogen (H2) via single-stage anaerobic fermentation. The high content of carbohydrates and readily biodegradable organic matter in the wastes, along with the acid pH (5.6–5.8) imposed in the reactors, were key factors to reach high SCFAs and H2 co-production efficiencies. Mentioned conditions promoted a specialized microbial community dominated by Ruminococcus involved in the prevalence of the reverse β-oxidation pathway. This microbial metabolism caused the accumulation of isobutyric and caproic acids (76% of total SCFAs), resulting in a SCFAs production efficiency higher than 40% with concomitant high H2 production yields (395.5 and 62.7 mL H2/g volatile solids influent for melon and watermelon fermentation). A high hydraulic retention time (HRT) of 27 days, an organic loading rate of 3 g of volatile solids per day and liter of working volume and a fermentation temperature of 25 °C were also crucial for maximizing H2 production at acid pH. Overall, melon and watermelon wastes bioconversion efficiencies into H2 and SCFAs reached 64.0 and 58.7%, demonstrating the successful co-production of different bioproducts in a single-stage fermentation by establishing proper operational conditions.