Microbial electrosynthesis and microbial fuel cells: Transforming CO₂ and glycerol-rich wastewater into valuable products with future integration potential

The sustainable management of industrial wastewater while reducing carbon emissions remains a key challenge in the transition towards a circular bioeconomy. This study evaluates glycerol-rich biodiesel wastewater as a substrate for two complementary bioelectrochemical technologies, microbial electrosynthesis (MES) and microbial fuel cells (MFCs), operated independently to assess their microbial, metabolic, and electrochemical compatibility. Supplementing MES with wastewater and CO₂ more than doubled total carbon recovery relative to a CO₂-only control, driving acetate production to 3495 ± 205 mg L^-1 and the maximum acetate production rate to 252 ± 2 mg L^-1 day^-1, compared to 113 ± 24 mg L^-1 day^-1 in the CO₂-only control with a matured biofilm. Formate (471 ± 78 mg L^-1), propionate (643 ± 46 mg L^-1), and butyrate (65 ± 9 mg L^-1) were produced, consistent with a mixotrophic community utilising both cathode-derived electrons and soluble organic intermediates, which were negligible under strictly autotrophic conditions. Open-circuit tests showed slower wastewater fermentation to acetate and propionate than under applied potential. In an MFC experiment, glycerol-rich wastewater was biodegraded, achieving 86 % chemical oxygen demand removal within 9 days and a maximum power density of 90.5 ± 12 mW m^-2. GC-MS detected degradation intermediates consistent with proposed pathways. Sequencing revealed diverse communities, with reactor-specific Propionicimonas and Sporomusa in MES and Aquamicrobium in MFC, alongside shared taxa such as Pseudomonas and Stenotrophomonas. The findings demonstrate that the wastewater enhanced volatile fatty acid production in MES, while MFC-generated energy and intermediates could support MES-based CO₂ conversion in future coupled systems.