The physical and biological mechanisms of attached-biomass growth were analyzed and a steady-state model was proposed to determine the soluble carbonaceous removal in an RBC unit for different organic loading rates in the reactor. The objective of the model was the prediction of the organic loading rate corresponding to the maximum removal capacity in the system. A system of equations was solved where the influent soluble carbonaceous substrate concentration was the main variable. Monod's rate law was used for the growth of microorganism: the soluble carbonaceous substrate was the limiting substrate. Endogenous decay was neglected. The influence of disk rotational speed on the RBC removal capacity was investigated, the disk rotational speed being a parameter acting on oxygen transfer in the biofilm. The criteria for the evaluation of the kinetic parameter in the model were proposed.