Water quality deteriorates within distribution systems. To prevent regrowth of microorganisms, and simultaneously avoid customers' complaints about taste and odor or excessive disinfection byproducts concentrations, an important objective in water treatment is the removal of natural organic material: NOM is the cause of rapid chlorine decay, together with the source of disinfection byproducts formation and bacterial regrowth, and generally, a poor water quality. Therefore, particular care is taken in the Suburb of Paris to remove NOM. The treatment (initial TOC between 2 and 4 mg C/l) includes coagulation, flocculation, settling, sand filtration, advanced oxidation, granular activated carbon filtration and post-chlorination. The treatment objectives are less than 2 mgC/l of total organic carbon and a biodegradable organic carbon level inferior to 0.3 mgC/l in the finished water. Under these conditions, 0.2 mg/l of free chlorine is still efficient enough in the network to prevent High Plate Counts, as well as coliform regrowth problems. Due to the length of the pipe systems and the number of storage facilities, it is important to compensate for the disinfectant decay without increasing the disinfectant dose at the treatment plant. Chlorine is introduced at some 57 points in the distribution system. This treatment strategy gives satisfactory results, and the total trihalomethane concentrations remain below 35 ug/l at the consumers' tap.