Chlorine dioxide has emerged in the last decade as a new alternative to traditional water disinfectants. Reports suggest good microbial control in the right application. Like other treatments, reports also suggest it is ineffective if it’s installed in the wrong situation. So following on from the idea that no disinfectant is a ‘cure all’ – where will it work and where will it not? If you want to see how it might work in your application read on…

 

Chemistry Lesson

Although the name suggests that chlorine is the active part of this chemical it is not. Chlorine dioxide is a rather unstable gas that may spontaneously explode in high concentrations (greater than 10%) in air. It is very soluble in water and does not easily return to the gas phase. Once it dissolves in water it breaks down to release free oxygen. This is much the same as ozone or hydrogen peroxide. All of the oxygen based disinfectants are very powerful disinfectants and more reactive than their ‘chlorine based’ counterparts. The disinfectant acts by release of free oxy radicals (peroxides). The oxy radicals are strong oxidising agents and more reactive than chlorine. The free oxygen literally ‘burns up’ organic material it contacts, such as bacterial cells.

ClO2 is more stable than chlorine and can also provide some residual in water systems. It is also active over a wider pH range than chlorine (pH 5 – 10). It can be supplied as a stabilised solution or be generated as a gas on site. The reactivity of the oxy radicals means that residuals are very soluble, quickly consumed and may be ‘quenched’ by metal oxides in the water system. ‘Quenching’ means that the metal uses up free oxygen and it is no longer available to disinfect. This can be important when considering it as a disinfectant. Water with significant metal salt content (eg bore water) can make chlorine dioxide very difficult if not impossible to use. Naturally corroded metal (iron pipework) will also have a quenching effect.

Disinfection By-products (DBP)

End products of chlorine dioxide in disinfection in water are a range of chloride compounds. End products of concern are chlorates and chlorites. Chlorates and chlorites may cause adverse health effects. The ADWG set a limit of not more than 0.5 mg/L as the upper limit for safe drinking water. Obviously this puts some restrictions on how much chemical you can put into the water.

If the water supply has a high demand on the disinfectant then it produces more by-products. So this means that supply water quality needs considering before deciding to use the treatment. The quenching of metals is one consideration. Metal oxides, especially iron and manganese, will soak up the free oxygen. To counter this it requires higher doses of chlorine dioxide. Of course this means higher levels of chlorates and chlorites. In the end too much metal content means this disinfectant is not a safe option without pre-treatment of the supply water. The same is also true for supply water with high organic content – for instance surface water drawn from rivers.

To make things a little more difficult the ADWG recommends periodic testing for chlorate and chlorite. Whereas chlorine dioxide levels can be quickly measured on site; chlorates and chlorites need NATA accredited laboratory analysis.

History

Along with ozone and hydrogen peroxide, chlorine dioxide has been in use for paper bleaching in pulp mills for a long time. All three chemicals have the same active disinfectant the ‘oxy free radical’.  It is also in use in the gas form as a sterilant for some medical equipment, particularly clothing and materials. Food processing is another major use. The gas is in use for fumigation of fruit and vegetables. Flour processing and bleaching is another commercial use.

The use of chlorine dioxide for potable water systems is a more recent development. Chlorine dioxide has seen more use in industrial systems, especially food processing,  since the 1970’s. In systems with high organic content it will produce less quantity of harmful DBPs than chlorine. This has led to it replacing chlorine as a more ‘environmentally friendly alternative’. It’s use in drinking water systems is probably its most recent application.

Disinfection reports

A number of reports have shown chlorine dioxide to be an effective disinfectant in hospital settings. Work also shows it is more effective against protozoans, the hosts for Legionella, than chlorine. However, some reports have shown Legionella survival at exposure concentrations up to 4 mg/L (Mustapha et al). Australian Drinking Water Quality Guidelines (ADWG) suggest that ClO2 may be useful in storages as a pre-treatment to a chlorination program. This means that chlorine dioxide is compatible with chlorine based disinfection systems (such as the potable supply). So  using the two treatments together is an option. Some studies show that effective control of Legionella in building water systems using chlorine dioxide can take months to achieve. Often, supply water quality  will determine the success of the treatment. Changes in supply water quality  (eg disruptions to the supply) can rapidly reduce the effects of the disinfectant.

Conclusions

In the right application chlorine dioxide is an effective disinfectant for Legionella and protozoa. It has similar residual properties to chlorine but a better active pH range. It is usually supplied as a liquid or generated on site as the gas is spontaneously explosive under pressure or at concentrations above 10% in air.

Adverse water conditions such as high metal content or organic load will make successful disinfection difficult.

Chlorine dioxide will require more technical expertise in installation and operation than chlorine. This may mean additional cost. Routine testing for the DBPs, chlorite and chlorate, by a NATA accredited laboratory is required by the ADWG.

Further Reading

Australian Drinking Water Quality Guidelines 6 2011 Version 3.3 Updated November 2016.

Mustapha, P. et al (2015) Monitoring of Legionella pneumophila viability after chlorine dioxide treatment using flow cytometry. Research in Microbiology 111:215-19

US EPA Office of Water EPA 810-R-16-001 (2016) Technologies for Legionella Control in Premise Plumbing Systems: Scientific Literature Review