An optimization model of the activated sludge wastewater treatment systems was presented. Such a system was relatively complex, containing a series of unit processes, each designed to achieve a specific goal. The design basis of each unit in the system, which consists of primary settling, aeration and secondary settling tanks was given. The model was run at the varying dissolved oxygen concentration. A relationship was developed between sludge volume index and sludge age using data from literature. The relationships between sludge volume index and coefficients of solids flux model, V0 and n, were also found separately. An optimum design model was developed for the system. In the optimization model, relationships mentioned above were considered. The model was solved by using the Box-Complex Algorithm. The results obtained from the model were discussed.