A multi-population biofilm model for completely autotrophic nitrogen removal was developed and implemented in the simulation program AQUASIM to corroborate the concept of a redox-stratification controlled biofilm (ReSCoBi). The model considers both counter- and co-diffusion, oxygen is supplied through a gas-permeable membrane that supports the biofilm while ammonia (NH4+) is supplied form the bulk liquid. On the contrary, in the co-diffusion biofilm, both oxygen and HH4+ are supplied from the bulk liquid. Results of the model revealed a clear stratification of microbial activities in both of the biofilms, the resulting chemical profiles, and the obvious effect of the relative surface loadings of oxygen and NH4+ (J(O2)/J(NH4+)) on the reactor performances. Steady-state biofilm thickness had a significant but different effect on T-N removal for co- and counter-diffusion biofilms: the removal efficiency in the counter-diffusion biofilm geometry was superior to that in the co-diffusion counterpart, within the range of 450-1,400 mu m; however, the efficiency deteriorated with a further increase in biofilm thickness, probably because of diffusion limitation of NH4+. Under conditions of oxygen excess (J(O2)/J(NH4)(+) > 3.98), almost all NH4+ was consumed by aerobic ammonia oxidation in the co-diffusion biofilm, leading to poor performance, while in the counter-diffusion biofilm, T-N removal efficiency was maintained because of the physical location of anaerobic ammonium oxidiers near the bulk liquid. These results clearly reveal that counter-diffusion biofilms have a wider application range for autotrophic T-N removal than co-diffusion biofilms.