The effects of electrode spacing on the performance of microbial fuel cells under different substrate concentrations

In this study, the electricity generation and organic removal in microbial fuel cells (MFCs) were examined for electrode spacing (ES) covering 5.8, 10.2, 15.1, and 19.5 cm, and for each ES the MFCs were discharged with a series of influent substrates (COD_in). Results indicate that organic removal was related to COD_in but not to ES. Best chemical oxygen demand (COD) removals of 64–71% could be achieved at COD_in around 100 mg COD/L (0.11–0.14 kg COD/m³-day). Best power output 3.32 mW/m² occurred at ES 5.8 cm and nominal COD_in 300 mg COD/L. For every ES, the relationship of electricity generation to local substrate near anode (COD_ad) could be adequately modeled by Monod-type kinetics. The estimated kinetic constants involve maximum current production, I_max, 15.3–19.6 mA/m²; maximum attainable power output, P_p,max, 4.0–2.5 mW/m²; half-saturation constant of current, K_si, 22–30 mg COD/L; and half-saturation constant of power, K_sp, 24–90 mg COD/L. This study reveals that the control over ES for improving electricity generation is dependent on the level of COD_ad, which profoundly affects the optimal design of electrode placement.