The final treatment process before water supply is disinfection. Disinfection is the process
of destroying or inactivating pathogenic organisms. Usually, disinfection is achieved by
chemical means. The chemicals used are typically chlorine, chlorine compounds or ozone.
Chlorine is most common in the U.K while in France and Switzerland, ozone is fairly popular.
Water can be disinfected by other means, such as boiling it or subjecting it to ultraviolet
radiation, but these are not in common use.
Disinfection is taken to mean the reduction of pathogens to such a level that no disease or
infection results when the water is used for domestic purposes. Not all of the organisms will
be removed, therefore the water is not sterile. The organisms involved are mainly bacteria,
viruses and protozoa with a few higher organisms such as worms or insect larvae. Resistance
to disinfection is dependent on a variety of factors. These include water pH, temperature
and organic matter content together with biological factors - some organisms are inherently
more resistant than others.