This paper presents an application of the Robust Control Theory, concretely the H-infinity approach, to the control of the activated sludge process with the aim of proving the feasibility of these types of techniques to the control of biotechnological processes. A generic non-linear model of the process is considered. The design of the H-infinity controller is carried out based on a linearization of such a model around prescribed operating conditions. In order to evaluate the performance of the design, simulation calculations with input profiles taken from experimental data have been made. The results show the effectiveness of the H-infinity design.