This study applied a microbial fuel cell (MFC) to a small sewage treatment system (Johkasou) with iron electrolysis for phosphorus removal and evaluated its impact on treatment performance. The laboratory-scale reactor consisted of a 2.1L anaerobic and 1.4L aerobic tank, with the MFC installed in the anaerobic section and the iron electrolysis device in the aerobic section. An open circuit condition was also operated to compare the performance. The MFC demonstrated a maximum power density of 31 W/m² based on the cathode area. The removal of organic matter and nitrogen did not significantly change with or without applying the MFC. However, the phosphorus removal efficiency improved by 10% when the MFC was applied. The average amount of iron valence in the anaerobic sludge was estimated by the X-ray Absorption Near Edge Structure analysis (XANES). The results indicated that the valence of iron was 2.4 with the application of the MFC and 2.6 without it, suggesting that the MFC maintained iron in more reducing conditions. Additionally, the FePO₄ generated by iron electrolysis decreased in the anaerobic tank with the application of the MFC, suggesting a change in phosphorus retention form.