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Disinfection by-product (DBP) formation in drinking water starts in the disinfection stage of a treatment plant, but can further increase or change downstream in the distribution system. If chlorine dioxide is applied in final disinfection, it is important to monitor chlorite and chlorate in the distribution system, since they can cause oxidative damage to red blood human cells. The World Health Organization recommends a Guideline Value for these parameters of 700 µg L−1 in drinking water. This work considers the drinking water distribution system of Cremona (76,000 inhabitants) in the North of Italy, fed by two equal treatment plants, that apply chlorine dioxide disinfection, located in the west and east of the city. In this work the distribution system was monitored for 6 years (2006–2011) analysing residual chlorine, chlorite and chlorate concentrations at 26 points of the network. The software Epanet 2.0 (USEPA) was applied to the distribution network in order to estimate the mixing zone of the water coming from the west and east treatment plants, and to simulate the propagation of residual chlorine, chlorite and chlorate. The measured and simulated concentrations were compared. The results of the network monitoring show a high chlorine dioxide consumption, since residual chlorine concentration is always below 0.12 mg L−1 at each point in the network. The chlorite concentration exceeds 700 µg L−1 in 12–16% of cases in the first two years, in the 48% of cases in 2008, and in 1-8% of the cases from 2009 to 2011. Predictably, chlorite exceeds the WHO GV at the points of the network more distant from the treatment plants. Conversely, the chlorate concentration is always below 200 µg L−1 at each point of the network. In fact, at the outlet of each treatment plant the chlorite concentration is high, while chlorate concentration is significantly below the GV. With regard to the software simulation with Epanet 2.0, the determination of the mixing zone for water coming from the west and east treatment plants and the comparison between the measured concentration and the values obtained from the simulation show the usefulness of the model for predicting DBP concentration in the distribution system.

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