(757d) Simultaneous Biogas Upgrading and Hydrogen Sulfide Removal through Enhanced Electrochemically Assisted Anaerobic Digestion of Food Waste

Currently, one of the most sustainable methods to dispose of food waste is anaerobic digestion, which exhibits the highest primary energy production and leaves the smallest environmental footprint. The biogas generated through anaerobic digestion of food waste typically contains 50-70% methane, 30-50% carbon dioxide, and small amounts of impurities such as hydrogen sulfide. The high carbon dioxide reduces the heating value of biogas, whilst the hydrogen sulfide is extremely toxic to personnel, corrosive to facilities, and inhibitive to methanogenesis. Therefore, measures to reduce carbon dioxide and hydrogen sulfide in the biogas are essential to increase its further applications. Current methods typically require separate units to reduce the carbon dioxide and hydrogen sulfide in the biogas stream after anaerobic digestion, which are very costly and cannot offset the inhibitory effects.

To tackle these issues, an innovative system (Fig. 1) incorporating multifunctional bioelectrochemical treatment into the anaerobic digestion of food waste was established for in-situ biogas upgrading and hydrogen sulfide removal. Food waste was hydrolyzed in the upper layer of reactor, and volatile fatty acids were produced and washed down through the percolation layer into the leachate in the lower chamber. A cyclic flow between the lower and upper chambers was created to soak the solids and increase the mass transfer using a peristaltic pump. A pair of electrodes were submersed in the leachate and connected with a direct current power supply. A voltage of 0.3-1.0 V below the requirement of water electrolysis (~1.23 V) was applied. The low carbon steel on the anode was oxidized to release ferrous ions into the leachate, which further combined with aqueous sulfide to generate ferrous sulfide precipitate, thus significantly reducing the gaseous hydrogen sulfide evolution. Meanwhile, on the cathode, some carbon dioxide was reduced to methane through both hydrogenotrophic methanogenesis and homoacetogenesis/acetoclastic methanogenesis pathways. Different combinations of electrode materials (carbon rod, low carbon steel, etc.) and conductive materials (biochar, activated carbon, etc.) were tested to enhance the experimental performances, achieving a high hydrogen sulfide removal efficiency while maintaining minimal material sacrificing rate and electrical charge consumption. Consequently, a hydrogen sulfide removal efficiency over 75% was secured, and the methane content in the biogas was improved to over 80%.