pH Control for a Waste Water Neutralization Process

A study has been conducted to improve a control system for a process which neutralizes waste water from boiler feed water demineralizing units. To meet the enviromental requirements, the pH of the waste water must be regulated within a specified range before it is discharged from the manufacturing plant. The pH control treated here is difficult, because the volumes of the tanks are small considering the peak load, and mixing by the agitators is not sufficient. Dynamic response tests were conducted on the neutralizers to better understand the process characteristics and to obtain a process model. The process was found to contain transportation delay. A control system was designed by a feedback realization of linear optimal control for a quadratic performance index. The performance of the control system designed, together with possible alternatives that would be easier to understand and implement, was tested by simulation. It was shown that the control system would provide the required neutralization without installing larger tanks or increasing mixing capabilities.