This article aims the observer synthesis for uncertain nonlinear systems and affected by unknown inputs, represented under the multiple model (MM) formulation with unmeasurable premise variables. A proportional integral observer (PIO) is considered. In order to design such an observer, the nonlinear system is transformed into an equivalent MM form. The Lyapunov method, expressed through linear matrix inequality (LMI) formulation, is used to describe the stability analysis and for the observer synthesis. An application to a model of wastewater treatment plant (WWTP) is considered and the performances of the proposed approach are illustrated through numerical results. (C) 2011 Elsevier Ltd. All rights reserved.