Development and application of a PID auto-tuning method to a wastewater treatment process

This paper presents an application regarding a recently developed extension of the widely used relay-feedback PID auto-tuner. The proposed method consists of two steps: process identification and controller design. First, a non-iterative procedure is suggested for identification of one point on the process Nyquist curve. A second-order model (SOS) is obtained and then used for PID controller design based on the internal model principle (IMC). For the identification of the point on the Nyquist curve a relay in the feedback loop (as used in standard auto-tuning) which operates on the integral of the error is used. The method is illustrated on an application to a wastewater treatment process. In the considered process, the wastewater is treated in order to obtain an effluent having the substrate concentration within the standard limits. The performances of control algorithms are illustrated by simulation results. The influence of changing a design parameter in the desired closed loop behavior is shown.