Chapter Thirty-Two - Chlorine Dioxide

Abstract

Chlorine dioxide is a highly effective oxidant with good germicidal properties that also provides excellent residual disinfection action and biofilm control. The chemistry of chlorine dioxide is explained, as is chemical dosing and available process technology. The advantage of chlorine dioxide over chlorine and chloramines is based on the reduction of taste and order problems and the prevention of trihalomethanes and other chlorinated disinfection by-product (DBP) formation. Unlike chlorine, it is easy to generate on site although sodium chlorite in its dry form it is highly unstable and dangerous. It does produce two DBPs of its own, chlorite and chlorate. Also, where free chlorine is released during chlorine dioxide generation, there is a risk that halogen-substitute DBPs may be formed. The WHO has set a guideline value for chlorite in drinking water of 0.2 mg L−1. In the UK a limit for combined residual concentrations of chlorine dioxide, chlorite and chlorate has been set at 0.5 mg L−1 as ClO2 in treated water, with a corresponding maximum contaminant level (MCL) of 1.0 mg L−1 set by the USEPA. In practice, these limit values restrict chlorine dioxide doses to 0.75 and 1.5 mg L−1 in the UK and USA, respectively. Such limits mean that the use of chlorine dioxide as a main disinfectant for drinking water is no longer possible. However, it is still widely used prior to treatment for pre-disinfection or as a pre-oxidant for the removal of taste, odour, colour, phenol, iron and manganese. A pre-oxidant dose should rarely exceed 1.5 mg L−1, as 0.2–0.4 mg ClO3 L−1 produced at a chlorine dioxide dose of 1.0 mg L−1.