This paper proposes a multiloop decentralized control strategy for the control of a wastewater treatment plant based on an Activated Sludge (ASP) model. The ASP is a described by means of a nonlinear model and results on a Two-Input Two-Output multivariable system. Even some advanced control strategies have been presented in the literature, the proposal of this paper is to show that by appropriate tuning of decentraliced PI controllers, it is possile to get comparable performance as with other approaches. With this purpose the paper proposes a way of addressing the design of the decentraliced controllers as well as a regulation based tuning for a PI controller. Simulations are carried out on the nonlinear model showing the performance of the proposed approach.