Three-chamber microbial electrolysis cell as a post-treatment step to refine both biogas and liquid effluent from anaerobic digestion

Present research is focused on the performance of a fully biocatalyzed three-chamber microbial electrolysis cell in the perspective of a self sustainable post-treatment process to enhance both digestate and biogas quality from the anaerobic digestion (AD). The rationale of this study was to couple carbon oxidation and ammonia removal in the anodic compartment to carbon dioxide removal and additional methane formation in the cathode compartment. To accomplish this objectives a bench-scale three-chamber continuous-flow MEC was developed, where a biomass-free accumulation chamber was inserted between anodic and cathodic compartments and separated by either protonic or anionic exchange membrane, respectively. The expected role of the intermediate chamber was to receive both ammonium and bicarbonate ions from the anodic and cathodic compartments respectively. The MEC was continuously operated in a potentiostatic mode with the typical three electrode configuration, after having inoculated the anodic and cathodic compartments by an activated sludge and an anaerobic sludge, respectively. The anode was operated under continuous flow feeding of organic carbon while the cathode was operated in batch condition, but for a 70/30 % N2/CO2 gas mixture (simulating typical methane/CO2 ration in the biogas) was continuously bubbled inside the chamber, to ensure both pH control and inorganic carbon supply for methanogens. The MEC performance was assessed through COD and electron balance as well as through removal efficiency of ammonia and carbon dioxide.