Maximum power densities of wastewater-fed microbial fuel cells (MFCs) are limited by low buffer capacities and conductivities. To address these challenges, a continuous flow MFC was constructed using a thin flow channel and an anion exchange membrane (AEM) in a novel configuration. The electrodes were separated only by a thin AEM (∼100 μm), reducing the solution resistance while facilitating transport of hydroxide ions from the cathode into the anolyte (no catholyte). The flow-MFC produced 1.34 ± 0.03 W m–2 using an artificial wastewater specifically designed to have a low buffer capacity (alkalinity of 360 mg L–1), compared to only 0.37 ± 0.01 W m–2 using a more typical cubic-shaped MFC. Internal resistance (Rint = 34 ± 1 mΩ m2) was 83% lower than that of the cubic MFC (202 ± 2 mΩ m2) due to the better mitigation of pH imbalances between the electrodes by using the AEM and zero-gap electrodes. Performance was benchmarked against a higher buffer concentration (50 mM) solution which showed that the maximum power density with additional buffering increased to 2.88 ± 0.02 W m–2. These results show that MFCs designed for selective hydroxide ion transport will enable improved power production even in low conductivity and poorly buffered solutions such as domestic and industrial wastewater.