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Filtration Process

This process removes the remaining microorganisms from the water. Although, different types of filter material are available, mostly a layer of sand and gravel is utilised to trap the microorganisms.

A slow sand filter uses several feet deep layer of fine sand particles. Within the sand there forms a layer of microorganisms which acts as an additional filter. This layer is called a Schmutzdecke or dirty layer.

To cleanse the filter, the top layer is removed and replaced with fresh sand. A rapid sand filter contains coarser particles of gravel. The schmutzdecke does not develop; the filtration rate is higher (over 200 million gallons per acre per day). This type of filter is generally used in municipal supplies.