Three geometrically different biofilms were investigated for the start-up of deammonification reactor. The planar biofilm with 4 g/L biomass could achieve 0.47 kgN/(m(3) day) nitrogen removal, compared to only 3 and 2 g/L biomass needed for cylindrical and granular biofilms, respectively. Planar biofilm was significantly affected by Dissolved Oxygen (DO) changes, whereas granular biofilm could effectively work in a wide range of DO. The maximum performance of 0.49, 0.83 and 1.27 kgN/(m(3) day) were obtained in planar, cylindrical and granular biofilms, respectively, reflecting that granular biofilm was the most capable due to its large surface area for mass transfer. Cylindrical biofilm was also effective as denitrifiers growth was intimately related to a large anaerobic zone. In addition, DO should be increased abruptly for each biofilm as the shortened HRT. This investigation indicates profound influence of biofilm geometry on deammonification process, which might serve as input for further experimental progress. (C) 2012 Published by Elsevier Ltd.