Combined L₂ -Stable Feedback and Feedforward Aeration Control in a Wastewater Treatment Plant

A nitrifying activated sludge process with ammonium feedback and inflow feedforward control is studied. The feedback and feedforward measurements are subject to time delay and the control loop includes a saturation. A Hammerstein-based model is therefore identified, including process delays and the nonlinearity. A Monod-type nonlinearity is used, as motivated by the oxygen injection efficiency reduction, with increasing dissolved oxygen concentration. The proposed feedback control strategy includes tuning of a linear lag compensator that has a limited low-frequency gain which allows global stability to be established with the Popov criterion. The feedforward controller attenuates the disturbance and provides an overall improvement of the control strategy. The performance of the combined feedback and feedforward aeration controller is evaluated with a benchmark model.