A volatile fatty acids adaptive observer-based hierarchical optimal controller design to maximum gas production of two-stage anaerobic digestion process

Volatile fatty acids (VFA), which influence the stability and gas production, are important state variables in anaerobic digestion (AD) process. Three main components of VFA can indicate the metabolic state of AD internal environment and be considered as a better basis for AD monitoring strategies. A VFA adaptive observer (VFA-AO) is proposed to estimate unknown gas production kinetic parameters and unmeasurable states simultaneously, including the concentration of three main components of VFA. Otherwise, using the estimated information, a VFA-AO-based hierarchical optimal controller (VFA-AO-based HOC) is designed to maximize gas production rates of AD. The proposed controller is compared with two existing control algorithms. Simulation results show that, despite the concentration of pollutants in the inlet fluctuates, VFA-AO achieves a reliable states and kinetic parameters estimation, and VFA-AO-based HOC maximizes gas production.