The operating envelope concept is applied as an activated sludge design tool. Based on chemical oxygen demand (COD) removal kinetics, oxygen transfer capability, and secondary clarifier capacity, three lines are developed to define an envelope where the operation of the activated sludge system is satisfactory. In this analysis, these lines establish the limiting envelope of influent COD mass rate as a function of sludge age that are compatible with removal kinetics, tank size, oxygen transfer system, and secondary clarifier design. The latest version of the Eckenfelder Activated Sludge Model (EASM) for COD removal is presented and used to determine the operating envelope. The state point analysis model is used to determine the maximum mixed liquor suspended solids concentration compatible with the secondary clarifier design. Model equations and computational approach are described. The determination of model parameters is discussed and literature reported values for selected parameters by industrial category are summarized. Examples are presented of the application of the EASM for designing a complete mix activated sludge system and to illustrate the operating envelope concept