Hierarchical Decision Process has been applied to redesign a wastewater treatment plant to achieve phosphorus removal The Hierarchical Decision Process suggests the order in which the decisions should be taken during the development of the new process, starting with the design of the reactor due to the strong interaction with the rest of the process. The redesign task is formulated as a multicriteria problem and considers simultaneously environmental, economical, legal and technical aspects. Finally, a sensitivity analysis is made to test the robustness and support the decision making procedure.