This article investigates trihalomethanes (THMs) production and simulates water age in Assiut drinking water system using WaterCAD software. Prechlorinated water samples were collected from Nazlet Abdellah water treatment plant and post-disinfected with chlorine/chloramines with different chlorine-to-nitrogen ratios (Cl2/N). Experiments have examined varying residence times, ratios of Cl2/N, pH conditions, and storage containers’ type on THMs formation. The results showed that as the residence time increased, THMs concentrations increased. Water age in Assiut drinking water distribution network reaches more than 10 h. Using chloramines instead of free chlorine for post-disinfection resulted in lowering THMs concentrations to 58.9 % after 48 h of disinfection. Ratios of Cl2/N (2:1–6:1) were comparable and effective on lowering THMs concentrations, and the most effective ratio was 4:1. Also, as the pH increased, the THMs increased. The measured THMs concentrations in chloraminated water stored in glass and plastic bottles were approximately the same.