This paper presents the influence of four relevant factors on the flocculation behavior in the activated sludge process: organic loading rate (COD), solid retention time (SRT), dissolved oxygen (DO), and calcium ion concentration, and links them to a selected set of process responses: sludge volume index (SVI), turbidity, organic removal rate (COD), and suspended solids (SS) removal. The "Design of Experiments" (DOE) and the "Response Surface Methods" (RSM) approaches are used to establish the operating conditions corresponding to optimal final properties of the activated sludge. Using these techniques, the results show that it is indeed feasible to locate the operating conditions which optimize the flocculation process and the sludge settling properties. The study represents a first attempt to evaluate the flocculation process in activated sludge using the DOE/RSM approach.