Input-Output Stability Design of an Ammonium Based Aeration Controller for Wastewater Treatment

Ammonium feedback is commonly used for controlling the aeration in wastewater treatment plants having biological nitrogen removal. The paper proposes the use of a PI controller tuning method based on a simplified identified model with Hammerstein structure. The Hammerstein model accounts for the delay of the process caused by the settler, the multiple bioreactors dynamics and the main nonlinear process effects. The controller tuning method exploits the Popov inequality for a pre-computation of (a subset of) the stability region of the closed loop system as a function of the PI controller parameters, quantified in terms of the maximum loop delay of the system. The controller parameters are selected in the computed stability region. In the numerical study, the plant is identified as a Hammerstein model using a recently published method, here extended to identify multiple bioreactor tank dynamics. The identification data is obtained from a high fidelity simulator, which is also used for evaluation of the proposed controller tuning method. The results show that the proposed procedure results in a PI-controller tuning with predictable stability properties and a good performance.