Many design and operational parameters that can impact power generation in microbial fuel cells (MFCs), such as flow over the electrodes, can only be effectively examined in larger-scale systems. A maximum power density of 0.101 +/- 0.006 W m(-2) (0.74 +/- 0.05 W m(-3)) was obtained in an 85-L MFC with graphite fiber brushes (5.1 cm diameter, 61 cm long) and flat air cathode (0.62 m(2) exposed area; anode-cathode spacing of 1.3 cm) in batch mode. Recirculating the anolyte diagonally through the chamber (entering the top right side of the reactor and exiting the bottom left side) further improved performance by 17% to 0.118 +/- 0.006 W m(-2), at a hydraulic retention time (HRT) of 22 min (3.9 L min(-1)), compared to static flow conditions. This power density was also higher than that obtained with parallel flow through the chamber (more evenly distributed using a manifold; 0.109 +/- 0.009 W m(-2)). Reducing the diameter of the anode brushes from 5.1 cm to 2.5 cm did not improve the anode performance. These results demonstrate the importance of electrode spacing and hydraulic flow on large-scale MFC performance.