On-line estimation of VFA concentration in anaerobic digestion via methane outflow rate measurements

•Robust nonlinear observer for on-line estimation of VFA in continuous anaerobic digesters.•Only the methane outflow rate is available for online measurement.•The proposed observer is able to the estimation of VFA in different operation conditions.•The observer convergence is analyzed by using Lyapunov stability techniques. This paper deals with the design of a robust nonlinear observer as a software sensor to achieve the on-line estimation of the concentration of Volatile Fatty Acids (VFA) in a class of continuous anaerobic digesters (AD). Taking into account the limited availability of on-line sensors for AD process, in this contribution is assumed that only the methane outflow rate is available for on-line measurement. The estimation method is based on a modified version for a two-dimensional mathematical model of AD process. From the differential algebraic observability approach it is shown that the VFA concentration is detectable from the methane outflow rate measurements. The observer convergence is analyzed by using Lyapunov stability techniques. Numerical simulations illustrate the effectiveness of the proposed estimation method for a four-dimensional AD model with uncertainties associated with unmodeled dynamics and disturbances in the inflow composition.