In this work, a robust control strategy is proposed for maintaining the oxygen concentration in the aerobic tank and the pollutant, i.e., ammonium, nitrate, nitrite, concentrations at acceptable levels in the effluent water at the outlet of the activated sludge process. To this end, the Activated Sludge Model no. 1 (ASM1) is first reduced using biological arguments and a singular perturbation method, and a simplified model of the secondary settler is included. In contrast with previous studies that make use of piecewise linear models, all average operating point is evaluated using available data (here data front the COST Action 624) and the reduced-order model is linearized around it using standard techniques. Finally, a H-2 robust control strategy acting oil the oxygen injection and the recirculated flow rate is designed and tested in simulation. (C) 2009 American Institute of Chemical Engineers Biotechnol. Prog., 25: 701-708, 2009