Modelling and control of potable water chlorination.

Abstract

In potable water preparation, chlorination is the last step before the potable water enters the

distribution network. Umgeni Water Wiggins Waterworks feeds the Southern areas of Durban.

A reservoir at this facility holds treated water before it enters the distribution network. To

ensure an adequate disinfection potential within the network, the free chlorine concentration in

the water leaving the reservoir at the Umgeni Water Wiggins Waterworks should be between

0.8 and 1.2 mg/L. The aim of this study was to develop an effective strategy to predict and

control the chlorine concentration at the exit of the reservoir. This control problem is made

difficult by the wide variations in flow and level in the reservoirs, together with reactive decay

of the chlorine concentration.

A Computational Fluid Dynamic study was undertaken to gain understanding of the physical

processes operating in the reservoir (FLUENT software). As this kind of modelling is not yet

applicable for real-time control, compartment models have been created to simulate the

behaviour of the reservoir as closely as possible, using the results of the fluid dynamic

simulation.

These compartment models were initially used in an extended Kalman filter (MATLAB

software). In a first step, they were used to estimate the kinetic factor for chlorine consumption

and in a second step, they predicted the chlorine concentration at the outlet of the reservoir. The

comparison between predictions and data, allowed the validation of the compartment models.

A predictive control strategy was developed using a Dynamic Matrix Controller, and tested offline

on the compartment models. The controller manipulated the chlorine concentration in the

inlet of the reservoir in order to control the chlorine concentration in the outlet of the reservoir.

Finally, the simplest compartment model was implemented on-line, using the Adroit SCADA

system of the plant, in the form of a Kalman filter to estimate the chlorine decay constant, as

well as a predictive model, using this continuously-updated decay parameter. The adaptive

Dynamic Matrix Controller using this model was able to control the outlet chlorine

concentration quite acceptably, and further improvements of the control performance are

expected from ongoing tuning.