Adaptive Control of a Waste Water Neutralization Process — Control Concept, Implementation and Practical Experiences

Abstract

The first part of the paper describes a new method for adaptive pH-control of a continous neutralization process in a waste water treatment plant. The waste water which is collected from different laboratories, pilot plants and production plants is fed into a neutralization reactor with a low mean residence time and a dominating time constant of about 30 sec (fast dynamics). As the composition and the concentration of the waste water changes quickly, the nonlinear titration curve alters accordingly. Consequently the gain factor of the neutralization process, depending on the derivation of the titration curve, also tends to alter quickly. In this case, neither conventional fixed PID control (due to the wide range of the process gain) nor adaptive control with recursive parameter estimation (due to the quick alterations of process parameters and nonmeasurable disturbances) can be applied. In order to overcome these problems a new adaptive control which is based on the principle of model reference adaptive control, has been developed. The gain of the main control algorithm will be altered by a superposed adaptive control loop so that in the case of a control deviation the pH-value of the effluence approaches its setpoint by a prescribed trajectory. This adaptive control system was developed in a pilot plant. The algorithms were implemented on a Personal Computer in the programming language BASIC. Many experiments have proved it being very practical for many types of disturbances.The second part of the paper describes the implementation of control scheme in the large neutralization plant. As controller hardware we used a standard programmable digital controller called IAC (Interactiv Controller) from Fisher Controls. The firmware of this controller contains blocks for control functions (e.g. PID) and in addition arithmetic and logic functions which are similar to assembler mnemonics of customary microprocessors.In order to implement the control scheme on this controller, the BASIC program which was developed in pilot operation was carried over to the syntax of the IAC. Only small modifications had to be done due to the different time behaviour of the pilot and large neutralization reactor.The adaptive control scheme is in operation in our large neutralization plant since October 1984. The control behaviour is statisfactorily, even for large disturbances. In the third part the control behaviour is described by showing some representative curves of the effluent pH (controlled variable) and influent pH.