To enhance the MFC's denitrification performance, this study investigated three different external circuits/operation modes of the MFC: alternative charging and discharging (ACD), intermittent charging (IC) and constant external resistance (R). Results showed that the ACD and IC modes offered larger output currents as well as higher nitrate and COD removal rates than the steady R mode. The best performance was achieved with the ACD mode. At the initial [COD] = similar to 1200 mg/L and [NO3-] = similar to 140 mg/L, the ACD mode delivered an average power density of 0.91 W/m(3), an average nitrate removal rate of 15.5 mg/(L d) and an average COD removal rate of 137 mg/(L d), 268%, 207% and 168% respectively greater than those by the R mode. The enhancement by the ACD and IC modes was more pronounced at lower nitrate and COD concentrations and/or with the lack of stirring of electrolyte solutions. (C) 2013 Elsevier Ltd. All rights reserved.