Exploration of Optimum Configurations of Electro-Assisted Anaerobic Digester for Sulfate-Reducing Conditions: Significance of Membranes and Mechanistic Insights

Sulfides generated from sulfate reduction commonly act as inhibitors in anaerobic digestion (AD). This study examined the performance of three configurations of microbial electrolysis cell-assisted anaerobic digestion (MEC-AD) systems: single-chamber, dual-chamber with an anion exchange membrane (AEM), and dual-chamber with a cation exchange membrane (CEM), operated under sulfate-reducing conditions. In general, the single-chamber MEC-AD reactors exhibited significantly higher methane yields (74.5–75.0%) than the control (64.4%). Upon converting a single-chamber MEC-AD to a dual-chamber configuration using an AEM, methane yield further increased up to 80.6%. The membrane facilitated the transfer of buffer anions to the anode and sulfate to the cathode, which substantially reduced un-ionized sulfide concentrations in the anode chamber. The MEC-AD with an AEM also demonstrated increased specific methanogenic activities, current densities, and microbial diversity with the enrichment of electroactive bacteria (e.g., Geobacter, Aeromonas) and hydrogenotrophic Methanobacterium. However, the MEC-AD reactor with a CEM experienced a significant reduction in methane yield (63.2%), mainly due to anolyte acidification, which increased the un-ionized sulfide concentrations.