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OXIDATION AND DISINFECTION 10.35

Switzerland, regions where source water is of lower quality. In these locations, chlorine
dioxide is used for disinfection, often as an adjunct to ozonation.

Chlorine Dioxide Disinfection
Chlorine dioxide does not dissociate or disproportionate as chlorine does at normal drink-
ing water pH levels. Like chlorine, chlorine dioxide exerts a demand when it is first added
to a water supply, which must be overcome if a persistent residual is to be maintained.
Like chlorine, chlorine dioxide is photosensitive (light-sensitive), and because it is a gas
at temperatures above 1 l° C, its residuals are easily removed by aeration.
Although chlorine dioxide has recently been used at more treatment plants in the United
States because it does not form trihalomethanes or haloacetic acids, there is still concern
that there are other organic by-products of chlorine dioxide that are not yet well under-
stood, and it may have other undesirable reaction products. Information presently avail-
able indicates that the reaction products include aldehydes, carboxylic acids, and ketones.
It is generally considered that chloroorganic by-products are not produced by reaction
between chlorine dioxide and organic compounds, but may be present in practical appli-
cations as a result of free chlorine present in the chlorine dioxide solution. The principal
inorganic by-products of chlorine dioxide reactions within water treatment are chlorite ion
(C102-), chloride ion (C1-), and chlorate ion (C103-), in the order listed. Both chlorate
and chlorite ions, particularly the chlorite ion, have been implicated in the formation of
methemoglobin. Consequently, most European countries limit the level of chlorine diox-
ide that can be used. The recent Stage 1 Disinfectant Disinfection By-products Rule
(DDBPR) (1998) sets limits of 0.80 mg/L for chlorine dioxide and 1.0 mg/L for the chlo-
rite ion. No maximum contaminant level has yet been proposed for the chlorate ion.

Chlorine Dioxide Generation
Chlorine dioxide cannot be stored once it is generated because it is not safe. Numerous
stimulants may cause the pure gas to explode, including an increase in temperature, ex-
posure to light, changes in pressure, and exposure to organic contaminants. As a result,
chlorine dioxide is usually generated on-site.
All chlorine dioxide for drinking water treatment is generated from sodium chlorite.
Most generation techniques use the two-chemical oxidative process, in which chlorine,
either as a gas or in solution, is mixed with a sodium chlorite, NaC102. Chlorine dioxide
can also be generated by direct electrolysis of sodium chlorite.
The goal in generating chlorine dioxide from chlorine and sodium chlorite is to max-
imize the chlorine dioxide yield, defined as the molar ratio of chlorine dioxide produced
to the theoretical maximum. The term conversion is also used in referring to chlorine diox-
ide generation reactions; this is the molar ratio of the amount of chlorine dioxide formed
to the amount of sodium chlorite fed to the system. For other reactions that produce chlo-
rine dioxide, such, as the hydrochloric acid-sodium chlorite reaction, yield and conver,
sion will have different values.
Studies of the mechanism and kinetics of the chlorine-sodium chlorite reaction have
shown that conditions favoring the formation of chlorine dioxide are those in which the
reactants are present in high concentrations and the chlorine is present as either hypochlor-
ous acid or molecular chlorine (C12). Three methods for the generation of chlorine diox-
ide from chlorine and sodium chlorite are commercially available: the aqueous chlorine-
sodium chlorite system, the gas chlorine-aqueous sodium chlorite system, and the solid
sodium chlorite-gas chlorine system.