Nitrate reduction using an electrode as a direct electron donor in a biofilm-electrode reactor was investigated to treat an aqueous solution containing high concentrations of nitrate. In the absence of organic substances, nitrate was reduced by micro-organisms consuming electrons from the electrode. The maximum nitrate reduction rate was 98% at an applied current of 200 mA. As nitrate concentrations decreased, the oxidation-reduction potential (ORP) gradually decreased from -80 to -260 mA. The maximum volumetric nitrate removal rate obtained from the biofilm-electrode reactor was 0.17 mg NO3--N/(cm(2) (biofilm surface area) day). (c) 2005 Elsevier Ltd. All rights reserved.