Filtration. The purpose of filtration is to clarify water, strain out dust, dirt and other finely suspended particles. In doing so, many micro-organisms are removed but filtration alone cannot be relied upon to remove all organisms and a very good filter plant will only remove up to 98-99%. The removal of all organisms can only be ensured by an additional process of disinfection. The suspended matter referred to can be divided into: Inert Substance: Dust Dirt Fibres Living Micro-Organisms: Virus Bacteria Fungi Algae These living micro-organisms are mostly harmless, but some are pathogenic or disease producing and the number of organisms which would be an ineffective dose would vary according to the type of organism and its route of invasion. In other words the virulence of any organism is variable.
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