Facilitating solid-state anaerobic digestion of food waste via bio-electrochemical treatment

Food waste has become a global environmental concern over emissions of greenhouse gas and odorous pollutants. Anaerobic digestion is gaining increasing attention as an effective food waste disposal method combining waste minimization and bioenergy recovery. Solid-state anaerobic digestion can handle food waste containing high contents of solids and has the merits of higher disposal volumes and less parasitic energy input, however, the low energy/mass transfer impairs the treatment efficiency. This study incorporated bio-electrochemical treatment into solid-state anaerobic digestion of food waste to overcome these shortcomings for enhanced performances. Food waste liquid leachate was used as feedstock to validate the bio-electrochemical treatment effects under different conditions. At a low applied voltage of 0.7 V and initial pH of 8.13, the methane yield increased by 77.5% while the carbon dioxide yield decreased by 16.0% compared with the control without electrodes. Although higher voltages (1.1–2.0 V) contributed to higher methane and hydrogen yields, the fast consumption and breakages of anodes significantly decreased the treatment consistency and material lifespan. Thereby, 0.7 V was selected as the applied voltage when incorporating the bio-electrochemical unit into the solid-state digester treating food waste. Cathodic hydroxide generation increased the buffering capacity, thus contributing to a more stable start-up process. With an immersed electrode surface area of 25.2 cm2/L, the highest methane yield of 526.7 mL/gVS was recorded with lower carbon dioxide and hydrogen contents initially, and the peak gaseous hydrogen sulfide emission was significantly reduced by 71.5% mainly due to the ferrous ions release from low carbon steel anode for sulfide precipitation.