Urban Wastewater Systems (UWS) are complex and their management is a challenging issue. Each one of the three principal elements that compose the UWS (i.e. sewer system, urban wastewater treatment plant and the receiving water) has particular goals to reach. However, the elements of the UWS should be ideally considered together to perform an integrated management of the UWS. Nevertheless, this approximation, which seems to be necessary, is not easy. Each one of these elements is in practice managed by a different entity, which has specific strategies and functions to optimize that sometimes are opposed. In this communication, a well known agent-oriented methodology –GAIA– is used to model the relations that take place in the UWS. A prototype is implemented in Java using Repast in order to evaluate the possibilities of agent-oriented methodologies to model this kind of complex systems.