In-situ biogas upgrading with H2 addition in an anaerobic membrane bioreactor (AnMBR) digesting waste activated sludge

Abstract

Biological in-situ biogas upgrading is a promising approach for sustainable energy-powered technologies. This method increases the CH4 content in biogas via hydrogenotrophic methanogenesis with an external H2 supply. In this study, an anaerobic membrane bioreactor (AnMBR) was employed for in-situ biogas upgrading. The AnMBR was operated in semi-batch mode using waste activated sludge as the substrate. Pulsed H2 addition into the reactor and biogas recirculation effectively increased the CH4 content in the biogas. The addition of 4 equivalents of H2 relative to CO2 did not lead to appreciable biogas upgrading, although the acetate concentration increased significantly. When 11 equivalents of H2 were introduced, the biogas was successfully upgraded, and the CH4 content increased to 92%. The CH4 yield and CH4 production rate were 0.31 L/g-VSinput and 0.086 L/L/d, respectively. In this phase of the process, H2 addition increased the acetate concentration and the pH because of CO2 depletion. Compared with a continuously-stirred tank reactor, the AnMBR system attained higher CH4 content, even without the addition of H2. The longer solid retention time (100 d) in the AnMBR led to greater degradation of volatile solids. Severe membrane fouling was not observed, and the transmembrane pressure remained stable under 10 kPa for 117 d of continuous filtration without cleaning of the membrane. The AnMBR could be a promising reactor configuration to achieve in-situ biogas upgrading during sludge digestion.