Ammonium-nitrogen (NH4-N) removal using a vertically moving biofilm system was investigated at a temperature of 11degreesC. The biofilms in this process were grown on biofilm modules, consisting of high surface area plastic media, which were vertically and repeatedly moved LIP into the air and down into the waste-water. The maximum NH4-N removal efficiency was up to 98.8% and the areal NH4-N removal rates, based on the surface area of the biofilm modules, were within the range of 0.68-1.94 g NH4-N m(-2) day(-1). The kinetics of ammonium removal occurring after carbonaceous oxidation was completed can be simulated by using a zero-order empirical model. The ammonium removal rate was found to be proportional to the square root of the mass of biofilms on the modules. Monitoring DO concentrations in the bulk fluid showed that the vertically moving biofilm system possessed good aeration efficiency.