Batch experiments are conducted at various concentrations of initial total ammonia nitrogen (TAN) with acetate as an electron donor to examine the effects of free ammonia (NH3) inhibition on electricity production in single-chambered microbial fuel cells (MFCs). This research demonstrates that initial TAN concentrations of over 500 mg N L-1 significantly inhibit electricity generation in MFCs. The maximum power density of 4240 mW m(-3) at 500 mg N L-1 drastically decreases to 1700 mW m(-3) as the initial TAN increases up to 4000 mg N L-1. Nitrite and nitrate analysis confirms that nitrification after complete acetate removal consumes some TAN. Ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) are also inhibited by increasing the initial TAN concentrations. Another batch experiment verifies the strong inhibitory effect of TAN with only small differences between the half-maximum effective concentration (EC50) for TAN (894 mg N L-1 equivalent to 10 mg N L-1 as NH3) and optimum TAN conditions; it requires careful monitoring of the TAN for MFCs. In addition, abiotic control experiments reveal that granular activated carbon, which is used as an auxiliary anode material, adsorbs a significant amount of ammonia at each TAN concentration in batch MFCs. (C) 2010 Elsevier B.V. All rights reserved.