Today, more and more French communities have a critical point of view concerning the performance of their wastewater sewerage systems. The main reason is linked to the methodology of the studies in the design phase. The process is neither adapted to the complexity of the decision-making task, nor to a general management of the wastewater sewerage in a territory. In order to make these studies more coherent and the choices more rational, we propose a new formulation of the methodology as an alternative to the current one. Our approach relies on decision-making support which borrows concepts from expert systems and multicriteria analysis in order to structure the reasoning process and to take into account the very different criteria a real decision-making task often implies. We show that this support has to be interactive and iterative in order to ensure that coherent and relevant solutions are chosen.