The Benchmark of the European group COST 624 aims to compare control strategies of activated sludge processes in Wastewater Treatment Plants. In this paper, the design of the control strategy has been oriented so as to improve the effluent quality. An accurate reduced model of the ASM1 model has been established using singular perturbations techniques. This model has been used in a sensitivity analysis for the selection of I-O variables for the controller. The results lead to a new control strategy, which is tested with an L-A control law. The simulation results show that the effluent quality is really improved.