This paper presents an alternative method for designing rotating biological contactors (RBC) for use as a secondary treatment operation. The method uses a combination of chemical kinetics, good engineering practice, operational simplicity, and cost effectiveness to design a RBC system to treat 10 mgd having an effluent below 12 mg/L of soluble biochemical oxygen demand (sBOD). The paper uses a step by step approach to illustrate (1) the number of RBC required, (2) the number of stages necessary to achieve the required effluent concentration, (3) the number of shafts in each stage, (4) the concentration of SBOD in each stage, (5) the effect on performance when the diurnal variation in flow ranges from 50 to 200% of design flow, and (6) when oxygen transfer governs the kinetics.