Anode biomass rather than soluble organic matter is fuel for electricity production in microbial fuel cell at longer hydraulic retention time

Abstract

This study presents a novel method for independently calculating organic matter (OM) removal and electricity generation in a microbial fuel cell (MFC) treating sewage. This methodology used OM concentration as the exclusive variable for input, facilitating the prediction of electricity generation with variable Coulombic efficiency. The employed MFC, of tubular design, incorporates an air core surrounded by a carbon-based cathode, an anion exchange membrane, nonwoven graphite fabric, and encircled by twelve carbon brush anodes. This setup successfully reduced chemical oxygen demand (COD) from 180 to 50 mg L−1 and biological oxygen demand (BOD) from 84 to 15 mg L−1 within a 35 h hydraulic retention time (HRT). The resulting effluent BOD satisfies Japanese wastewater treatment standards. In the early HRT phase, the MFC exhibited rapid OM reduction, followed by a stabilization of OM concentration. Despite varying HRTs and the sustained OM levels at prolonged HRTs, the MFC consistently generated 0.29 ± 0.14 A m−2. This apparent contradiction prompted the consideration of additional OM sources from the anode biomass (Cb). Calculating electricity production based on OM concentration rather than OM degradation was achieved by assessing anode resistance (RAn) across different levels of OM supplementation, employing the Butler–Volmer equation. This approach, incorporating considerations for Cb and RAn, accurately reproduced MFC performance. It was observed that OM supplementation from Cb constituted 50% at 11 h HRT and escalated to 90% at 27 h HRT. Further analysis, including 16S rRNA gene-based microbial community profiling, indicated that higher cathode resistance (RCat) at 1.0 Ω m2, compared to a minimum RAn of 0.10 Ω m2, favored methanogenesis over electrogenesis on the carbon brushes. This finding underscores the importance of reducing RCat and minimizing electrode distance, coupled with enhancing the specific surface area of both electrodes, as crucial factors for the advancement of MFC technology.