An expert control structure is proposed for the control of dissolved oxygen (DO) in a Nitrification Denitrification Biological Excess Phosphorus Removal (NDBEPR) plant to account for the several processes that are influenced by the DO concentration in the aerator. In the scheme a supervisory fuzzy controller determines the set point of an inner DO control loop where an Adaptive Robust Generic Model Control (ARGMC) controller is used. The fuzzy supervisory control has a hierarchical structure. Off-line measurements of biological parameters of influent and effluent streams can be used to periodically update the set points of the fuzzy controllers, The complete control scheme has been designed for an actual plant and tested by a simulation programme based on the n, 2 IAWQ model for the biological reactors and on separate models of the clarification and thickening phases for the settler, Simulation and experimental results show that good operation conditions can be obtained.