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The aim of this work is to design a super-twisting observer for on-line estimation of the concentration of Volatile Fatty Acids (VFA) in a class of continuous anaerobic digestion (AD) processes. Taking into account the limited availability of on-line sensors for the AD processes, in this contribution is assumed that only the methane outflow rate is available for on-line measurement. The estimation technique is based on a local coordinate transformation that lead the original two-dimensional system to an observable normal form. In order to provide robustness properties under perturbation/uncertainty conditions a super-twisting algorithm is proposed. The observer convergence is analyzed by using Lyapunov stability techniques. Numerical simulations illustrate the effectiveness of the estimation scheme for a four-dimensional AD model with uncertainties associated with unmodeled dynamics and disturbances in the inflow …