Ozone is an unstable form of oxygen with the chemical formula O ₃. In solution it decays to O ₂, in the process producing free hydroxyl radicals. Ozone and hydroxyl radicals are the two most powerful oxidants used in water treatment. Ozone is therefore a strong disinfectant; it is also used for oxidizing many of the synthetic organic chemicals found in water from lowland rivers and the natural organic colour found in soft waters from upland rivers. Ozone is a colourless gas with a characteristic pungent smell often associated with electricity. It is sparingly soluble in water, at 20 C the solubility at one Bar partial pressure is 570 mg/l.

Ozone is believed to react in water as an oxidant both by direct oxidation of chemicals by molecular ozone and by indirect oxidation by hydroxyl free radicals produced during decomposition of ozone. ^[1] Oxidation by hydroxyl free radicals is the stronger process. At low pH value direct oxidation by ozone is the more important process. At high pH value, or where the formation of hydroxyl free radicals is encouraged by UV light or by dosing hydrogen peroxide, hydroxyl oxidation may predominate.

Up until the early 1990s ozone was little used in the UK although it was widely employed in France and some other European countries. However, the European standard of 0.1 ?g/l for pesticides introduced in the 1980 Drinking Water Directive and the widespread use of pesticides and their presence in most lowland rivers meant that...