The process of filtration involves the flow of water through a granular bed, of sand or another suitable media, at a low speed. The media retains most solid matter while permitting the water to pass. The process of filtration is usually repeated to ensure adequate removal of unwanted particles in the water (Ramstorp, 2003). This type of slow filtration over a granular bed is generally known as slow sand filtration. It is the oldest method of filtration but still widely used in municipal water treatment plants today.

More modern filtration systems use carbon as the main constituent material of the filter. This carbon is compressed into a solid block form, as opposed to the more loosely structured, granular, sand filters. Such filters often include other media substances, in addition to the compressed, solid carbon. This type of water filter is known as a multimedia filter. These filters clean water through both physical and chemical processes. Physically, they perform the same function as slow sand filters, blocking the passage of unwanted materials with molecular structures that are larger than water. Chemically, the carbon or multimedia filters perform an added filtration function. Through the process of adsorption, the atomic charge of the carbon and other media encourages unwanted particles to abandon their bond with the water and chemically attach to the media (Ramstorp, 2003).

The water then passes through the filter, cleansed of undesirable materials. The addition of extra media to the standard filter constitution of sand or carbon allows for more particles to chemically bond to the media, resulting in greater filter performance and efficiency. Water is generally directed through several stages carbon and multimedia filters to ensure the removal of all unwanted materials. The first filtration stage will remove the most concentrated chemicals, like chlorine, while subsequent stages will remove smaller and more evasive chemicals, like pesticides.

Source: http://www.historyofwaterfilters.com/

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