Maximum anode chamber volume and minimum anode area for supporting electrogenesis in microbial fuel cells treating wastewater

During scale-up of microbial fuel cell (MFC), a proportional increment in power does not usually occur determining the importance of maximum possible anode chamber volume (V_an) to exploit electrogenesis and achieve maximum energy recovery. A systematic approach is proposed for determining the optimal single anode chamber volume and the minimum anode surface area (A_an) of an MFC. The optimal anode chamber volume was estimated based on the substrate required to produce a defined maximum current that is likely to be produced from the basic electromotive force equation. The A_an was obtained by considering the area required for biofilm formation, the substrate utilization rate by electrogens, the MFC polarization curve, charge transfer kinetics and mass transport overpotential. Based on the theoretical bio-electrochemical considerations, the maximum V_an and minimum A_an required for each anode chamber are proposed for electrogenesis to dominate. A single V_an of a few litres will only be optimal for treating wastewater. With wastewater of chemical oxygen demand (COD) of 5 g l⁻¹ and considering a Coulombic efficiency and a COD removal of 80% each, a V_an of 2.02 l is optimum for a single anode chamber to produce a current up to 750 mA; which is the maximum possible current estimated from electromotive force equation. Any additional volume provided will leave the substrate unused by electrogens and encourage methanogenesis. Adopting this volume for each anode chamber in a MFC stack is recommended for treating wastewater under the assumptions of the analysis. Charge transfer kinetics dominate the minimum A_an required, which satisfies the area required for biofilm formation, MFC polarization, and mass transfer. The minimum A_an should be provided in a MFC to ensure the dominance of electrogenesis.