Water quality model is a helpful management tool for water distribution system. Estimation of growth coefficients is an important and challenging component of water quality modeling. Reactions occurring in the bulk flow for both trihalomethanes (THMs) formation and chlorine decay kinetics in Zai drinking water have been studied. It was found that both THMs and chlorine kinetics follow first-order reactions for growth and decay, respectively. A numerical model for predicting THMs formation in drinking water distribution system networks has been developed and applied. THMs measurements were taken at three locations over 106 h from a northern portion of Sport City area (Kharabshe Reservoir), while the water quality time step is set to 3 min, to insure predicting accurate total trihalomethanes (TTHMs) concentrations. The bulk flow for TTHMs formation rate constant value is determined and set to 5.82 × 10−2/h. The modeled results for THMs formation in the field network were compared favorably with the observed data for three sampling points in the network. The THMs formed inside the selected network of this case study was sixfolds higher than that formed in Zai treatment plant; and 113% higher than that formed in Kharabshe Reservoir. The model proved to be an effective tool for prediction of THMs formation in drinking water distribution systems.