In system 2 the importance of these parameters was addressed: the downflow regime into the cathode ensured that sufficient DO was available for reduction while allowing the electrode spacing to be brought to minimum. This arrangement reduced the internal resistance by 67% to 300 O and increased the maximum power density by 70% to 0.268 W/m3. The concentration of organics flowing to the cathode affected the overall efficiency of the CW-MFC by inhibiting the oxygen
reduction reaction at the cathode [28,40,41]. A longer hydraulic retention time (HRT) would allow greater breakdown of organics before the wastewater arrives at the cathode. Additionally, CWMFCs operate more efficiently with COD values below 500 mg/L or even 250 mg/L [40] as the majority of organics are removed in the anaerobic region before arriving at the cathode