A methodology for the modelling of wastewater treatment plants by activated sludge is proposed. It takes into account the strong coupling between hydrodynamics and biological reactions. Hydrodynamical modelling makes use of residence time distributions obtained by tracing the different units (primary settler, aeration tank and secondary clarifier) with lithium chloride. The biological model is based on IAWPRC Task Group Model 1. Simulated time and space profiles are in good agreement with experimental data, taken on an existing full-scale plant.