In this study, the factors that leading to loss of chlorine residual in water distribution systems were investigated. A series of experiments performed on real water samples collected from effluent of water treatment plant before chlorination. The results showed that bulk chlorine decay rate varied significantly with the initial chlorine concentration and the organic content of water. Further, this paper explores the performance and the potential applicability of the parallel first order model and second order model to represent the bulk chlorine reactions. The modelling and simulation study lead to define the bulk chlorine coefficients as a function of initial chlorine concentration (Co) and COD by developing mathematical equations related to these parameters.