The air we breathe contains 21% oxygen. When enough energy is applied to oxygen gas molecules, they may split and form ozone.
This process can either be done artificially in an ozone reactor or naturally by sunlight in the outer atmosphere or during thunderstorms.
Ozone is a highly reactive gas.
This makes ozone a very good tool for breaking down any undesirable pollutants in air or water.
The bacterial cell membrane, for instance, will be broken and thus destroying the bacteria.
Ozone can, however not be stored. If you would fill up a tank with ozone it would react with itself and only leave pure oxygen behind.
If we want to use the power of ozone reduction in a process, we must create the ozone just before we use it.