Hierarchical dissolved oxygen control for activated sludge processes

A hierarchical controller for tracking the dissolved oxygen (DO) reference trajectory in activated sludge processes is proposed and investigated. The removal of nitrogen and phosphorous from wastewater is considered. Typically, an aeration system itself is a complicated hybrid nonlinear dynamical system with faster dynamics compared to the internal dynamics of the DO in a biological reactor. It is a common approach to neglect these dynamics and also important operational limitations of this system, such as the limits on the allowed frequency of switching of the blowers and on their capacity. The paper proposes a two-level controller to track the DO reference trajectory. The upper level control unit generates trajectories of the desired airflows to be delivered to aerobic zones of the biological reactor. A nonlinear model predictive control algorithm is applied to design this controller. The lower-level controller forces the aeration system to follow these set-point trajectories. The predictive control is also applied to design the lower-level controller based on piecewise linearised hybrid dynamics of the aeration system. The overall hierarchical controller is validated by simulation based on real data records.