After the 90th day of operation polarization curves (Fig. 6) were attained by varying the external resistance from 500,000 O to 1 O and measuring the steady-state voltage across the resistors. The curves reveal that both concentration and ohmic losses affected the performances of systems 1 and 4 and contributed to their internal resistances of 500 O and 508 O, respectively. Systems 1 and 4 produced maximum power densities of 0.168 W/m3 and 0.163 W/m3 during the polarization tests while between days 50 and 90 they produced average voltages of 0.309 ± 0.012 V and 0.307 ± 0.012 V, respectively. The similarity of results between systems 1 and 4 indicates that electrode spacing and availability of dissolved oxygen may be equally important parameters in optimizing the CW-MFC.