A 121 gas-liquid-solid three-phase fluidized bed bioreactor in which biomass supported on activated charcoal was used to remove nitrate-nitrogen from wastewater and its performance was considered. The effects of temperature, pH, C/N ratio, gas flow rate and hydraulic residence time (HRT) on nitrate-nitrogen reduction were investigated and discussed. The optimum operated conditions such as temperature of 20-35 degreesC. pH value of 6.5-7.5, HRT of 3 h, C/N of 0.95-1 and gas influx of 0.3 m(3)/h were found. Under optimum operated conditions, the average removal ratios of COD and NOx-N are higher than 92% and 96%, respectively. In addition, the radial and axial positions have little influence on the local profiles of COD and NOx-N. (C) 2003 Elsevier Science B.V. All rights reserved.