Biofilm Air-lift Suspension reactors can be used to treat waste water at a high volumetric loading rate combined with a low sludge loading. The biofilms are formed on small suspended particles (r = 0.1 mm). We have studied the effect of particle characteristics and hydraulic retention time on the biofilm formation. It is shown that small, rough particles show the best biofilm formation. Low hydraulic retention times benefit the formation of biofilms. This results from the fact that suspension growth is minimal under these conditions. The effect of biofilm detachment became distinct from the observation that most of the bacterial growth in the biofilm is transferred to the liquid. The biofilm formation process is concluded to be a three stage process: (i) the outgrowth of single cells to micro-colonies, this process is positively influenced by the carrier surface roughness; (ii) the outgrowth of micro-colonies to small biofilms, this process is negatively influenced by the concentration of carrier material; (iii) the outgrowth of biofilms, this occurs when the majority of particles are covered with a biofilm. At that time the influence of shear due to particle-particle interactions diminishes.