Water managers have to deal with complex problems due to the intertwined characteristics of processes, in particular those that occur in wastewater systems. Existing modelling approaches are usually centred in the physical, chemical and biological aspects of the individual processes, excluding the social and organisational context that will generate the global behaviour. These also include the responsibilities and decision making of different actors in the system. This paper proposes an agent-based model with the novelty to integrate the social and organisational structure of the wastewater system from which emerges the global behaviour of the system. The modelling process allows considering the legal regulations and the technical limits that would drive the decision making. The instantiation of the model, implementing a small system, evidenced the usefulness of this approach to manage the complexity of wastewater systems and its possible contribution to prevent environmental problems.