Multivariable PID control design for activated sludge process with nitrification and denitrification

This paper investigates the application of multivariable PID (MPID) controllers to wastewater treatment plants (WWTPs) with pre-denitrifying process. WWTPs have usually a number of interacting control loops. The controller's parameters are often left unchanged since the plant commissioning and hence poor performance is often observed due to inevitable changes in the plant and its influent characteristics. The model-based MPID tuning methods are complicated, cumbersome and expensive, and require a high level of control expertise to design, and implement. In this paper, a simple and novel tuning method is proposed along with three existing methods, which require only step or frequency response tests. The performance of these methods is compared and the parameters are tuned using a proposed optimisation technique to meet standard quality effluent at a minimum cost. This is achieved using a nonlinear ASM1 model to determine the optimal values of the controller parameters. The results show that by effective and proper tuning of the MPID controllers, the closed-loop performance of WWTP improves and it could potentially lead to energy saving and improvement in the removal of nitrogen.