Information Sheet 1.5
Disinfection with chlorine dioxide
General description
Chlorine dioxide is a strong oxidant that in addition to being an effective biocide can be used to
control iron, manganese, and taste- and odour-causing compounds. It has also been used as a
secondary disinfectant in many European countries (Le Chevallier and Au 2004).
Chlorine dioxide is highly soluble in water (particularly at low temperatures), and is effective
over a range of pH values (pH 5–10). Theoretically, chlorine dioxide undergoes five valence
changes in oxidation to chloride ion:
ClO2 + 5e– = Cl– + 2O2–
However, in practice, chlorine dioxide is rarely reduced completely to the chloride ion (White
1999).
Chlorine dioxide is thought to inactivate microorganisms through direct oxidation of tyrosine,
methionyl, or cysteine-containing proteins, which interferes with important structural regions of
metabolic enzymes or membrane proteins (Gates 1998). In water treatment, chlorine dioxide has
the advantage of being a strong disinfectant, but of not forming trihalomethanes (THMs) or
oxidizing bromide to bromate (Le Chevallier & Au 2004). Whilst not producing THMs, the byproducts chlorate and chlorite can be produced.
Performance validation
Chlorine dioxide is roughly comparable to free chlorine for the inactivation of bacteria and
viruses at a neutral pH (White, 1999), but is more effective than free chlorine at pH 8.5 (Hoff &
Geldreich, 1981).
Chlorine dioxide is an effective disinfectant for the control of Giardia lamblia; the required Ct
values for 1-log inactivation (pH 6–9) range from 5 mg/L.min at 20oC to 21 mg/L.min at 0.5oC
(USEPA, 1989; White, 1999). The 3-log inactivation Ct values (pH 6–9) range from 19
mg/L.min at 15oC to 63 mg/L.min at 0.5oC. These values are 3–14 times less than those required
for free chlorine, but approximately 20 times more than those required for ozone (Le Chevallier
and Au 2004).
Le Chevallier and Au (2004) reviewed a range of studies that demonstrate that chlorine dioxide
can inactivate Cryptosporidium, but the listed Cts required to achieve inactivation vary widely.
The New Zealand Ministry of Health (2008) allow a 2-log inactivation credit for
Cryptosporidium by chlorine dioxide at a Ct of 858 mg/L.min at 5oC, and 357 mg/L.min at 15oC.
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Table IS1.6.1 provides a summary of Ct value ranges for the 99% inactivation of various
microorganisms by chlorine dioxide. The Ct values should be considered as indicative only, and
suitable safety factors should be applied to ensure adequate disinfection.
Table IS1.6.1 Examples of Ct values for 99% (2 log) inactivation of various
microorganisms by chlorine dioxide 1,2,3
Microorganism
Chlorine Dioxide
(mg/L.min)
Reference
Escherichia coli
0.4-0.75
USEPA 1999
Enteric viruses
5.6
USEPA 1999
Giardia
17
USEPA 2009
Cryptosporidium
357 (15oC)
NZ MoH 2008
o
Notes: (1) Temperature is 5 C unless stated.
(2) pH is within range of 6-9 unless stated.
(3) The values in the table are based on published values and should be viewed as the
minimum values necessary to achieve effective disinfection
The important conclusion to draw from Table IS1.6.1 is that the Cts required to inactivate
bacteria and viruses, and to some extent Giardia, are comparable to those for chlorine, the Ct
required to inactivate Cryptosporidium is significantly greater, and is unlikely to be able to be
achieved in most drinking water supply systems.
Given that the dosing point for chlorine dioxide will be a critical control point (CCP), other
important issues that will need to be considered to ensure the effectiveness of the process are:

establishing target criteria (section 3.4.2) and critical limits for the dosing process

preparing and implementing operational procedures (section 3.4.1) and operational
monitoring (section 3.4.2) for the process

preparing corrective action procedures (section 3.4.3) in the event that there are excursions in
the operational parameters

undertaking employee training (section 3.7.2) to ensure that the dosing process operates to
the established target criteria and critical limits
Water quality considerations
Chlorine dioxide is a reactive gas that cannot be easily stored or transported, and must be
generated on site; this is usually done by acid treatment of sodium chlorite, which generates the
gas with little or no chlorine contamination and so avoids the formation of chlorinated
byproducts during disinfection.
It has excellent oxidising ability, which reduces taste, minimises colour and oxidises iron and
manganese complexes.
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Turbidity at the time of disinfection should be less than one nephelometric turbidity unit.
Disinfection with chlorine dioxide is optimised at a pH of less than 8, and its effectiveness
increases about three-fold between pH 6 and 9. The effectiveness of chlorine dioxide is also not
as sensitive to changes in pH as chlorine.
Practical considerations
Reliable equipment is available for disinfection with chlorine dioxide. However, the technology
involved is moderately complex, but more effective controls for the process are developing.
Chlorine dioxide is rapidly consumed and volatilised, and this is a major disadvantage.
Persistence
Chlorine dioxide provides a moderately persistent residual.
By-products
By-products from the use of chlorine dioxide include chloride ions, chlorite ions, chlorate ions
(see Fact Sheet on Chlorine dioxide/ chlorate/ chlorite for more information). Whilst not a byproduct, in some cases residual chlorine dioxide may also be present.
Application
Chlorine dioxide is a suitable disinfectant for a small to medium sized water treatment plant. It
has been used mainly as a preoxidant (rather than as a primary disinfectant, due primarily to its
relative cost, and lack of a persistent residual) to control taste and odour, remove iron and
manganese, and more recently, remove the precursors of trihalomethanes and total organic
halogen (TOX). In some supplies chlorine dioxide has been used in combination with
chloramination.
Operational monitoring
As chlorine dioxide only produces a moderately persistent residual, operational monitoring is
performed by measured by measuring the concentration of chlorine dioxide being add to the
water. This can be achieved via the use of on line analyser. The concentration of chlorine
dioxide being added to the water should be continuously monitored (MoH 2008).
Regular monitoring for chlorite and chlorate should also be undertaken.
References
Gates D (1998). The chlorine dioxide handbook: water disinfection series. American Water
Works Association, Denver, CO
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Hoff JC, Geldreich EE (1981). Comparison of the biocidal efficiency of alternative disinfectants.
Journal of the American Water Works Association, January, 40–44
LeChevallier MW and Au K-K (2004) Water treatment and pathogen control. World Health
Organization, Geneva.
New Zealand Ministry of Health (2008) Drinking-water Standards for New Zealand 2005
(Revised 2008)
USEPA (1989). Guidance Manual for Compliance with the Filtration and Disinfection
Requirements for Public Water Systems Using Surface Water Sources. Washington DC, United
States Environmental Protection Agency.
USEPA (1999)Alternative disinfectants and oxidants guidance manual. USEPA Washington DC
White GC (1999). Handbook of Chlorination and Alternative Disinfectants. New York, John
Wiley & Sons, Inc.
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