A robust control scheme to improve the stability of anaerobic digestion processes

In this paper, the robust regulation of volatile fatty acids (VFA) and total alkalinity (TA) is addressed to improve the stability of continuous anaerobic digestion (AD) processes. The control scheme is derived from an AD model that includes TA as state variable and it is composed of a model-based multiple-input multiple-output feedback control and an extended Luenberger observer which not only allows the estimation of the process kinetics and variations in the influent composition but also the introduction of an antiwindup structure. The wastewater flow rate is used to regulate the VFA concentration, whereas an alkali solution is added directly to the digester to regulate TA. The controller performance is evaluated via numerical simulations showing excellent responses under the influence of control input saturations, noisy measurements, load disturbances and uncertain kinetics. Finally, it is shown that well-known practical stability criteria for AD processes can be also fulfilled by the proposed control scheme.