Simulation of Free Chlorine Decay and Adaptive Chlorine Dosing by Discrete Time-Space Model for Drinking Water Distribution System

An ideal drinking water distribution system (DWDS) must supply safe drinking water with free chlorine residual (FCR) in the form of HOCl and OCl-at a required concentration level. The FCR as a disinfectant decreases over time due to chemical reactions in the bulk phase and at the pipe wall in the DWDS. In order to supply drinking water with the FCR concentration within the safety range of 0.2-0.6 mg/l at the points of water consumption, it is important to develop a dynamic model of the FCR using a discrete time-space model (DTSM) that accounts for free chlorine transport in the axial direction by convection, diffusion and the decay kinetic. A DTSM has been developed using Finite Difference Method (FDM) to predict the FCR in single pipes in the DWDS. The DTSM has been computed using Matlab 7.0.1 and tested with step inputs and rectangular pulse inputs to estimate the FCR at any point in the pipes over time. Data found in the literature have been used to validate the DTSM. The modelling and simulation study shows that the water velocity significantly affects the FCR concentration distribution along the pipe. Due to the fluctuation of the drinking water demand, a model-based adaptive chlorine dosing scheme is proposed to control the proper injection of chlorine.