State and Input Estimation of an Anaerobic Digestion Reactor using a Continuous-discrete Unknown Input Observer∗∗The authors gratefully acknowledge the support of FNRS and CONACYT in the framework of a bilateral research agreement. This paper presents research results of the Belgian Network DYSCO (Dynamical Systems, Control, and Optimization), funded by the Interuniversity Attraction Poles Programme, initiated by the Belgian State, Science Policy Office. The scientific responsibility rests with its authors.