This paper deals with setting up the parameters of a PI controller to be applied in a system for the regeneration of potable water. Because of the presence of a saturation, the system to be controlled is a nonlinear one and its nonlinearity consists of the positiveness of its output. Moreover, an anti-windup control structure is considered to manage saturation effects. In order to analyse the stability and the dynamic performance of the controlled system a Lyapunov approach is proposed. Through this approach the condition of the three parameters which characterise the controller (PI and anti-windup scheme) are calculated to obtain a compromise between a stability and fast dynamics. Simulations using real parameters of the system are shown.