Chlorine dioxide is a powerful oxidising biocide and has been successfully used as a water treatment disinfectant for several decades in many countries. Rapid progress has been made in the technology for generation of the product and knowledge of its reactivity has increased with improved analytical techniques. Chlorine dioxide is a relatively stable radical molecule. It is highly soluble in water, has a boiling point of 110C, absorbs light and breaks down into ClO3- and Cl-. Because of its oxidising properties chlorine dioxide acts on Fe2, Mn2+ and NO2- but does not act on Cl, NH4+ and Br- when not exposed to light. These ions are generally part of the chemical composition of natural water.

Because of its radical structure, chlorine dioxide has a particular reactivity - totally different from that of chlorine or ozone. The latter behave as electron acceptors or are electrophilic, while chlorine dioxide has a free electron for a homopolar bond based on one of its oxygens. The electrophilic nature of chlorine or hypochloric acid can lead, through reaction of addition or substitution, to the formation of organic species while the radical reactivity of chlorine dioxide mainly results in oxycarbonyls. Generally chlorine dioxide rapidly oxidises phenol type compounds, secondary and tertiary amines, organic sulphides and certain hydrocarbon polycylic aromatics such as benzopyrene, anthracene and benzoathracene. The reaction is slow or non-existent on double carbon bonds, aromatic cores, quinionic and carboxylic structures as well as primary amines and urea.

The oxidising properties and the radical nature of chlorine dioxide make it an excellent virucidal and bactericidal agent in a large pH range. The most probable explanation is that  in the alkaline media the permeability of living cell walls to gaseous chlorine dioxide radicals seems to be increased allowing an easier access to vital molecules. The reaction of chlorine dioxide with vital amino acids is one of the dominant processes of its action on bacteria and viruses.


Chlorine dioxide is efficient against viruses, bacteria and protozan clumps usually found in raw water.. A rise in pH level further increase its action against f2 bacteriophages, amoebic clumps, polioviruses and anterovirus. It is efficient against Giardia and has an excellent biocidal effect against  Cryptosporidia which are resistant to chlorine and chloromines.  It has been demonstrated that ClO2 has greater persistence than chlorine. In a recent report for dosages 3 times lower than those of chlorine at the station outlet, the residual of ClO2 used alone was always higher than that of Cl2 which also required 3 extra injections of chlorine in the distribution system.
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The reduction of bad tastes and odour with ClO2 is the result of the elimination of algae and on the negligible formation of organo-chlorinated derivatives. The latter formed under chlorination give rise to very unpleasant odours. By its action on dissolved organic materials, without the formation of organic halogen compounds, ClO2 limits problems of taste and colour. In addition the low dosages used in post disinfection are an advantage. When chlorine dioxide replaces chlorine in a system it may take up to 15 days for the benefits of the change to become apparent. Changes should be made gradually to avoid problems of a sudden release of slime into the system.