Organic loading (weight per unit time per volume) is useful for the design of rotating biological contactors (RBC) and for comparison with the other processes such as activated sludge or oxidation ponds. The present study puts emphasis on the significance of this control or design parameter because it allows direct comparison of the RBC system's performance when operated under various circumstances and with different kinds of wastewater. The results of the paper proved that, the COD removal in rotating biological contactor systems is a function of the organic loading rate. However, each of the wastewater concentration and flow rate are also influence on the system efficiency but theirs impact can be combined by the effect of organic loading. The majority of COD removal (40-85 % of the total removal depending on the organic loading applied) occurs in the first stages of the system. There is a strong correlation between the organic loading and the concentration of the suspended solids in the rotating biological contactor basin. At higher loadings higher concentrations noted. At a loading of about, (24 g/m(2).d) suspended solids were 225, 125, 35, and 25 mg/L in the first, second, third and, the fourth stage respectively. To achieve an effluent quality of (BOD <or= 25 mg/L, COD <or= 60 mg/L), the system must be operated on organic loadings of about (22 gBOD/m(2).d and 65 gCOD/m(2).d) respectively. For nitrification process, the system must be designed to operate at organic loading of about (10 g/m(2).d) or less and, the reactor or basin volume should be designed to achieve a hydraulic loading of about (40 L/m(2).d) or less.