Chlorine disinfection in traditional water treatment plants is a popular and low-cost method for disinfecting raw water before it is distributed to consumers in Egypt. Nevertheless, the chlorination process in the presence of natural organic matter and decreased water quality due to uncontrollable population results in formation of high concentrations of carcinogenic disinfection by-products, from which trihalomethanes (THMs). In this paper, the water quality of Assiut drinking water network (ADWN) was assessed in terms of THMs studying different water quality parameters. An extended period simulation based on a modelling software WaterGEMS was employed to obtain the critical locations to be examined according to key parameters namely: water age, residual chlorine concentration, water velocity, and type of storage. The study concluded that THMs concentrations from all critical locations in Assiut drinking water network would not go over the Egyptian regulatory threshold and US Environmental Protection Agency (EPA) guidelines. Some locations in ADWN showed zero residual chlorine concentration at different times a day. It is recommended that more chlorine injection points should be installed in various locations based on water quality modelling and in-field analysis. Even more, an investigated domestic roof tank should be critically operated under a planned scheme of monitoring and maintenance due to its deteriorated water quality. A wash valve chamber should be installed in the surrounding area of El-Molimen water treatment plant, that would be periodically employed by the central operator to clean the network at those locations.