A model for a rotating biological contactor is developed that takes into account the MichaelisMenten kinetics, the submergence level of the disk, and its rotational speed. The aerobic nature of substrate processing and mass transfer between an aqueous medium and a biofilm are described. The model includes consideration of the erosion of the biofilm by flowing water. The variability of the biofilm thickness along the radius of the disk is taken into account. It is demonstrated how the steady state characteristics of the biore actor are reached under different bioreactor conditions.