Chlorine dioxide application in drinking water disinfection avoids trihalomethanes, but it can generate other disinfection by-products (DBPs): chlorite and chlorate. This paper concerns chlorate ion formation during chlorine dioxide generation and oxidation reactions, in relation with the use of sodium hypochlorite solutions. This aspect is very important taking into account that current WHO Drinking water Guidelines suggest a limit of 700 µg/L for chlorate ion. SMAT (the drinking water supplier of the town of Turin, northern Italy) uses both chlorine dioxide and sodium hypochlorite in its three surface water treatment plants, having a whole potentiality of about 40 M m3/y. This research considered the following issues: chlorate neo-formation processes, potential precursors and influencing conditions, and process and plant minimization intervents. The three treatment lines were analyzed by monitoring for nine months chlorate concentration in significant phases of the potabilization process and in the outflow, in order to detect the most critical conditions. Chlorate formation can be bound both to the natural degradation of hypochlorite, and to different dismutation phenomena occurring during disinfection. The first contribution can be more easily controlled: a refrigerated storage of hypochlorite was evaluated on laboratory and pilot scale, and taking into account its effectiveness to comply with WHO guidelines, this improvement will be shortly adopted in full scal
