The enhancement of biological denitrification using a biofilm-electrode reactor (BER) is demonstrated. Removal efficiencies greater than 98% can be achieved. At electric currents in excess of 20 mA, the rate of denitrification within the BER is reduced due to hydrogen inhibition. A detailed steady-state biofilm model is developed taking into account hydrogen inhibition kinetics, the diffusion of neutral and ionic species, the effects of the applied current on the diffusion of ions, gas production within the biofilm, and the presence of a phosphate buffer and a mass transfer boundary layer. The biofilm model is coupled to a model of a completely stirred tank reactor (CSTR). The models are in excellent agreement with the experimental results when the applied electric current is between 0 and 20 mA. The results of the reactor model qualitatively predict the performance of the BER at currents greater than 20 mA using a hydrogen inhibition constant of 10−4 M.