The Use of ClO2 in Drinking Water Treatment: Formation and Control of Inorganic By-Products (ClO2 −, ClO3 −)

The effect of drinking water processes on the concentration of chlorite, chlorate, and chlorine dioxide has been examined. Laboratory studies indicate that under drinking water conditions (neutral pH and low concentrations of chlorite), chlorite and chlorine dioxide are oxidized rapidly by ozone or are converted into chloride by activated carbon. Chlorine (HOCl, ClO⁻) can oxidize ClO₂ ⁻ and ClO₂ into chlorate. However, taking into account the concentrations involved and the residence time, the kinetic rate constants are too tow to allow chlorate formation in the distribution system. UV irradiation of ClO₂ ⁻ and ClO₂ at 254 nm also produces chlorate. On the basis of kinetic results, the production of chlorate under UV disinfection conditions (irradiation doses ≈ 250 J/m²) is unlikely.

Analyses of samples collected from two drinking water utilities that use ClO₂ for preoxidation and for disinfection indicate that, as expected from laboratory data, chlorite ion was reduced into chloride by activated carbon or oxidized into chlorate by ozonation. The analyses also revealed the presence of ClO₂ ^-f and ClOj at the outlet of ClO₂ generators.