Frequency Limited Model Reduction Technique for a Multivariable Activated Sludge Process and its PI-controller Application

The high order of a two-input-two-output activated sludge system is reduced using Gawronski and Juang’s frequency limited reduction technique. The manipulated variables are the concentration of nitrate and dissolved oxygen (DO). A reduced fourth order system is obtained within the frequency band of 0.7 rad/s to 1.0 rad/s. The reduced system is compared with a fourth order activated sludge process obtained from Enn’s frequency weighted model reduction technique. Relative gain array (RGA) analysis is done to identify the interactions in the multivariable system hence a multi-loop control is applied. A PI-controller is utilized with optimized parameters gained from Particle Swarm Optimization (PSO) technique. Results obtained are shown in sigma plot with an improved performance in terms of steady-state error. Key-Words: Model reduction technique, multi-loop control, PI-controller, Particle Swarm Optimization, multivariable system.