The process of filtration involves the flow of water through a granular bed, sand, activated  carbon or another suitable media, at a low speed. The media retains most solid matter while permitting the water through filter. This type of 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 & for industrial application as well.

More modern filtration systems use activated carbon with filtration media. This carbon is compressed into a solid block form, Such filters often include other media substances, in addition to the compressed solid carbon and 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 particular size contuminats materials with molecular structures that are larger than permeable water. Chemically, the carbon or multimedia filters perform an added filtration function and added advantages.