The upflow bioreactor system without biomass-liquid separation unit was evaluated for its efficacy in sustaining autotrophic nitrification and denitrification (AND). The bioreactor system was capable of sustaining AND by means of carefully controlled oxygenation to achieve the maximum NH4+-N removal rate of 0.054 g N g VSS-1 day(-1) (38% removal efficiency) at the oxygen influx and nitrogen loading rate of 3.68 mg O-2 h(-1) L-bioreactor(-1) and 182 mg N day(-1) L-bioreactor(-1), respectively. Additional nitrogen removal was achieved in a two-stage bioreactor configuration due to endogenous denitrification under long mean cell residence time. Quiescent conditions maintained in the bioreactor provided stable hydrodynamic environments for the chemoautotrophic biomass matrix, which revealed porous, loosely-structured, and mat-like architecture. More than 95% of the total biomass holdup (1.3-1.5 g VSS) was retained, thereby producing low biomass washout rate (similar to 40 mg VSS day(-1)) with VSS < 11 mg VSS L-1 in the effluent. (C) 2007 Elsevier Ltd. All rights reserved.