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Chlorine Dioxide – A Water Treatment Biocide

Chlorine Dioxide (ClO2) is not a new chemical, but rather an old one and it was first discovered by Sir Humphrey Davy nearly 200 years ago. A yellowish-green gas in its natural state, it is used increasingly in our modern world as a more effective oxidising biocide than traditional chemicals such as chlorine or bromine.

So what’s so special about this well known chemical and why is it used so widely in industrial water treatment?

It possesses a number of key properties that differentiate it from other oxidising biocides and determine how it can be used. For instance, at room temperature, it is a water-soluble gas that dissolves in water BUT does not react with the water. This compares with traditional biocidal chemicals such as chlorine or bromine which react with water giving solutions containing a range of ionic species.

As a dissolved non-ionic gas, solutions of chlorine dioxide are particularly effective in controlling the formation of bio-films. The effectiveness of ClO2 in penetrating and controlling bio-film has been demonstrated in a laboratory environment where bio-film growth from a contaminated water is compared to that of the same water with either ClO2 or chlorine (Cl2), added at 1ppm for 1hour/day or 1ppm for 15mins x4 /day. The biocidal effects of chlorine dioxide are unaffected by system pH in the range 4 -10. Most water systems are in this band.

Chlorine dioxide selectively oxidises organic material without chlorination – does not give rise to chlorinated by-products. In other tests relating to chlorine dioxide water treatment, the selectivity of ClO2 compared to Cl2 is has also been proven. The addition of ClO2 to a polluted water immediately produces a reserve, whereas a considerable concentration of Cl2 has to be added before a reserve is generated. Because Cl2 reacts with the pollutants, ClO2 does not. It is so reactive that it cannot be compressed or liquefied. In practice it is produced on-site for immediate water treatment usage.

So to summarise, chlorine dioxide is a gas that is both highly water soluble and volatile. Due to its unstable nature, it cannot be compressed or liquefied and is always produced on-site. It is a selective oxidising agent and broad spectrum biocide which does not produce chlorinated by-products. It is often the biocide/disinfectant of choice in food washing applications and is particularly effective in controlling legionella. For these reasons and more, it has become an extremely important element of the water treatment industry.