1.5: Disinfection with chlorine dioxide

The possible presence of microbial contaminants in drinking water poses a greater risk to public health than the possible presence of disinfection by-products (DBP). Therefore, disinfection should not be compromised in order to control DBP.

Where the concentrations of DBP consistently exceed associated health-based guideline values, the methods of water treatment, disinfection and distribution should be reviewed.

General description

Chlorine dioxide is a strong oxidant that in addition to being an effective biocide can be used to oxidise iron and manganese, and control 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:

$$\text{ClO}_2 + 5e^- = \text{Cl}^- + 2\text{O}^{2-}$$

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, methionine, 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 and Au 2004). Whilst not producing THMs, the by-products chlorate and chlorite can be produced.

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 mainly to its relative cost, and lack of a persistent residual) to control taste and odour, oxidise iron and manganese, and more recently, remove the precursors of THMs and total organic halogen (TOX). In some supplies chlorine dioxide has been used in combination with chloramination.

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 highly reactive and can be rapidly consumed.

Performance validation

Table IS1.5.1 presents published C.t values for chlorine dioxide that have been demonstrated as achieving a two and four log reduction in the target microorganism. These values are supplied for illustrative purposes only. For chlorine dioxide C.t values that achieve a greater log reduction, the cited references should be consulted. The C.t value that is applied at a particular water treatment plant should be based on the microbial risk assessment for that particular water supply system.

Table IS1.5.1 Published C.t values for 99% (2 log) and 99.99% (4 log) inactivation of various microorganisms by chlorine dioxide 1,2,3

Microorganism	Chlorine Dioxide C.t value (mg/L.min) C.t⁹⁹	Chlorine Dioxide C.t value (mg/L.min) C.t⁹⁹·⁹⁹	Reference
Escherichia coli	0.4 - 0.75	NA	USEPA 1999, LeChevallier and Au 2004
Enteric viruses	5.6	33	USEPA 1999
Giardia	17	34	USEPA 1999
Cryptosporidium	860	1720	USEPA 2010

Notes:

1. Water temperature is 5°C.

2. pH is within the range of 6-9.

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 IS1.5.1 is that the C.t values required to inactivate bacteria and viruses, and to some extent Giardia, are comparable to those for chlorine, but the C.t value required to inactivate Cryptosporidium is unlikely to be able to be achieved in most drinking water supply systems.

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 by-products during disinfection.

It has excellent oxidising ability, which reduces taste, minimises colour and oxidises iron and manganese complexes.

Turbidity at the time of disinfection should be less than 1 NTU.

The effectiveness of chlorine dioxide is also not as sensitive to changes in pH as chlorine. There is some evidence that effectiveness against protozoa increases from pH 6 to 8 (USEPA 1999).

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 by-product, in some cases residual chlorine dioxide may also be present.

Operational considerations

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 and critical limits for the dosing process (section 3.4.2);

• 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; and

• undertaking employee training (section 3.7.2) to ensure that the dosing process operates to the established target criteria and critical limits.

Operational monitoring

The table below summarises the operational monitoring that should be undertaken for chlorine dioxide, based on recommendations from the New Zealand Ministry of Health (NZ MoH 2008).

Operational Parameter	Monitoring
pH	Online monitoring
Turbidity	Online monitoring
Chlorine dioxide concentration	Online monitoring

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.

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).

United States Environmental Protection Agency (USEPA) (1999). Alternative disinfectants and oxidants guidance manual. Washington DC.

United States Environmental Protection Agency (USEPA) ( 2010). Long term 2 enhanced surface water treatment rule toolbox guidance manual. Washington DC.

White GC. (1999). Handbook of Chlorination and Alternative Disinfectants. John Wiley & Sons, Inc, New York.