Classical observers are constructed on the basis of the nature of the measurement signals, namely, a continuous observer requires continuous output measurements. In this work, a novel observer which estimates continuous states when continuous and discrete measurements are available is presented. By resetting the initial condition of the observer at each sample instant, the convergence of the continuous states is guaranteed. The application to the the estimation of substrate and biomass concentrations in an anaerobic wastewater treatment process in which continuous and discrete measurements usually appear, shows the feasibility of the proposed scheme.