BACKGROUND: A circulating column microbial fuel cell (MFC) with Cu anode and Au-Cu air cathode was used for power generation and chemical oxygen demand (COD) removal from synthetic wastewater. The column was operated in repeated-fed batch mode using acclimated anaerobic sludge. The contents of the column MFC were circulated while the feed wastewater was fed to the reactor in fed-batch mode. Effects of feed COD concentration and COD loading rate on voltage difference, power density and percentage COD removal were investigated. RESULTS: The highest voltage difference (650 mV), power density (40 W m(-2)) were obtained with a feed COD of 6400 mg L-1, yielding 45% COD removal with a COD loading rate of nearly 90 mg h(-1). Low COD loadings (<90 mg h(-1)) caused substrate limitations, and high loadings (> 90 mg h(-1)) resulted in inhibition of COD removal and power generation. The highest percentage COD removal (50%) was obtained with feed COD content of 10.35 g L-1 or a COD loading rate of 145 mg h-1. CONCLUSION: The power densities obtained with the circulating column MFC were considerably higher than those reported in the literature due to elimination of mass transfer limitations by the high circulation rates, proximity of electrodes and small anode surface area used in this study. Further improvements may be possible with optimization of the operating parameters. (C) 2009 Society of Chemical Industry