Modelling and control of potable water chlorination.
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.