Ozone

Ozone, whose chemical formula is O3, is a gas which is naturally found in the upper most layers of the atmosphere, where it is created from oxygen due to the effect of ultraviolet rays. Its presence along with its characteristic odour can sometimes be detected at lower altitudes during stormy weather (as a result of electric discharges in the atmosphere on the oxygen in the air). Ozone is used in the water treatment process due to its oxidizing energetic properties.

It is relatively unstable in an aqueous solution, although it will remain stable for 10 to 20 minutes under the pH conditions usually found in the water treatment process (pH 6 to 8).


This disappearance of the ozone into the water can be explained by a natural phenomenon called auto decomposition, as well as by the fact that it is consumed by substances in the water which consume oxidizing agents (organic matter, oxidizable mineral matter).
A lack of residual power reflects the instability of ozone in water.


Disinfection by ozonation must therefore be complemented in the majority of cases with the addition of another reagent to take over the action of the ozone. Adding a small amount of chlorine or chlorine dioxide is usually sufficient to ensure a bacteriostatic effect in the water distribution network.


However there are cases when only ozone is used such as with exceptionally high quality water, and short and/or well maintained distribution networks.

Numerous properties

Aside from its exceptional disinfectant abilities (destroying pathogenic germs 20 to 30 times faster than chlorine), ozone is used for its numerous other properties, such as:

  • oxidizing certain pesticides,
  • destroying very high quantities of detergents,
  • completely removing phenols,
  • destroying high amounts of algae,
  • reducing taste and odour,
  • removing the original organic colour,
  • reducing very slightly chloroform-extractable substances (which provide an overall analytical measurement for organic micropolluants).

On the other hand ozone does not have any perceptible effect on ammonia (very slow oxidation kinetics).