Long-term evaluation of an air-cathode microbial fuel cell with an anion exchange membrane in a 226L wastewater treatment reactor

Abstract

Although the treatment of municipal wastewater using microbial fuel cells (MFCs) has been extensively studied, scaling the systems up for practical use remains challenging. In this study, a 226 L sewage treatment reactor was equipped with 27 MFC units, and its chemical oxygen demand (COD) removal and electricity production were evaluated. The MFC units were tubular air cores with a diameter of 5 cm and length of 100 cm, which were wrapped with a carbon-based cathode, anion exchange membrane (AEM), and nonwoven graphite fabric. The air-cathode–AEM MFC generated 0.12–0.30 A/m2, 0.072–0.51 W/m3, and 1.7–4.6 Wh/m3 in a chemostat reactor with a COD of 140–36 mg/L and hydraulic retention time (HRT) of 9–42 h throughout a year. The decrease in the COD was represented as the first-order rate constant of 0.038. The rate constant was comparable to that of other air-cathode MFCs with cation exchange membranes, indicating the necessity of a posttreatment to meet the discharge standard. It has been estimated that the MFC operation for 24 h before post-aeration can reduce the energy required to meet the discharge standard by 70%, suggesting the potential applicability of MFC in long HRT-treatments such as oxidation ditch. The resistances of the anode, cathode, and AEM were 15, 7.0, and 0.51 mΩ m2, respectively, and surface dirt rather than deterioration primarily increased the AEM resistance. A current exceeding 0.2 A/m2 significantly increases the anode potential, indicating that the current was limited by low COD. Increasing the anode-specific surface area can improve air–AEM MFCs used for practical applications.