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Chlorine Dioxide

Figure 1: Chlorine dioxide application points during treatment.
Figure 1: Chlorine dioxide application points during treatment.

Chlorine dioxide (ClO2) can be applied at several points during treatment: to the raw water as a preoxidant, to the clarification tank, to post-clarification, or to the filtered water as primary disinfectant. Chloramine or chlorine may be used for secondary disinfection following chlorine dioxide application. Figure 1 shows possible ClO2 application points during treatment. Downstream residual concentrations make ClO2 concurrent with other treatment processes.

Chlorine dioxide is a chlorine compound in the +IV oxidation state. As such, it is a powerful oxidant and disinfectant. Chlorine dioxide is frequently used to improve the removal of taste and odor compounds, oxidation and removal of iron and manganese, removal of color, and inactivation of chlorine-resistant microorganisms such as Cryptosporidium. Pathogen inactivation with chlorine dioxide is much less affected by pH in the 6.0 to 8.5 range than with chlorine. However, the inactivation of Cryptosporidium oocysts and Giardia cysts using chlorine dioxide occurs more rapidly and is more efficient at higher pH. Iron concentration, manganese concentration, sunlight exposure, and aeration are among the parameters that exert additional chlorine dioxide demand.

The concentration (C), contact time (T), pH and temperature are key parameters in ClO2 used for oxidation and disinfection. The product of concentration and time (CT) is the most important operation parameter in disinfection and in activation.

Chlorine dioxide yields lower levels of organic disinfection byproducts (DBPs) in comparison to free chlorine. Total organic carbon (TOC) and ultraviolet absorbance (UV) are two measures of DBP-reactive organic materials and of ClO2 demand. However, approximately 70% of the chlorine dioxide applied in water treatment is converted to chlorite - a regulated inorganic DBP with a maximum contaminant level (MCL) of 1 mg/L. As such, the chlorine dioxide dose applied in drinking water is typically limited to approximately 1.4 mg/L. Chlorate is also an inorganic DBP of chlorine dioxide decay. Chlorite can be removed by ferrous iron or sulfite ion oxidation.

Chlorine dioxide gas is explosive under pressure and must be generated on-site. The generation process, chemicals, and capacity vary depending on the application. However, chlorine dioxide is typically formed in the reaction of sodium chlorite (NaClO2) solution with gaseous chlorine (Cl2) or with hypochlorous acid (HOCl). Improper generation conditions can lead to feeding of unreacted chlorine at the application point and the potential formation of regulated organic DBPs, and to the feeding of unreacted chlorite and problems with MCL compliance. New generators have been developed that replace the solution sodium chlorite with a solid form for minimized DBP formation; and electrolysis of sodium chlorite has recently been introduced in the U.S. for low-dose applications.