Enhancing Hydrogen Production by Zeolite Addition in the Dark Fermentation Process of Urban Organic Waste

Abstract

One of the objectives of the European Union (EU) 2030 strategy is the sustainable growth through a series of measures focused on the application of the “Circular Economy” (ec.europa.eu, 2020) with many benefits also for climate, environment, and society. Food waste and sewage sludge management is part of this actions plan. Both streams are ideal source for microbial valorization processes to produce biofuels in a sustainable way. Anaerobic digestion (AD) is the benchmark robust technology for biogas production; however, the dark fermentation (DF) is a higher-rate process which offers the possibility to accumulate important building blocks (volatile fatty acids; VFA) and hydrogen from renewable resources, at reduced volumetric impact compared to AD. Focusing on this goal, the effectiveness of the hydrogenase enzymes needs to be favoured; some strategies have been already adopted: maintaining pH between 5.0-6.0; limiting the activities of methanogenic bacteria, not responsible for VFA and H2 accumulation, by decreasing the hydraulic retention time (HRT). Mesophilic and thermophilic bio-H2 and VFA production by DF process from food waste (alone or mixed with sewage sludge) has been studied in this work through batch assays. Pursuing the interest of studying interactions between adsorbents materials and bacteria, some trials were amended with zeolite (Z; chabazite type) at zeolite/inoculum ratio (Z/I) of 0.20 g Z/g VSinoculum. The zeolite’s presence brought the H2 yield up to 0.04 m3 H2/kg VS in thermophilic trials, in parallel to the increased acetic acid production (close to 50% of the organic acids, including lactic and VFA). The rate of H2 production also increased with Z addition, up to 27.3 mmol/(L d), 23% higher than control without Z. The positive effect of zeolite could be related to its associated surface area (available for bacterial adhesion), or ability to adsorb/exchange protons. These results open novel perspectives not only in bio-H2 production but also in the study of interactions between bacteria and inorganic-porous materials.