Multi-objective cascade controller for an anaerobic digester

In this work, a new multi-objective control strategy based on the concentration of volatile fatty acids (VFAs) in the effluent and the methane flow rate (Qch4) has been proposed for an upflow sludge bed-filter (USBF) reactor, which is used in the anaerobic treatment of winery wastewater. The approach presented here is novel due to the following reasons: (i) it considers two operational objectives, i.e., control of the effluent quality and control of the maximum production rate of methane; (ii) it takes advantage of the difference between the dynamics of the liquid and gas phases using variables from both phases. The control system is based on a cascade control strategy with a reference signal for the methane flow rate. The control system computes the feed flow rate for adjusting the organic load applied to the reactor. The performance of the proposed control scheme is illustrated through numerical simulations and parameter optimisation using the Anaerobic Digestion Model no. 1 (ADM1) with regards to influent disturbances. Moreover, the controller has been validated in the closed-loop control of a 1.15 m3 USBF reactor treating wastewater containing ethanol, which emulates winery effluents under different operational scenarios: restart-up, long duration organic overload, long duration organic underload and successive organic disturbances. The control system supplied adequate control action in response to the different disturbances tested, and it demonstrated high reliability in achieving the desired set-point.