The most important part of the filter is the sand bed. Its thickness is about 1 metre.
Characteristics of sand: round, 0.2 to 0.3 mm diameter, clean and free from clay and organic matter.
The bed is supported by a layer of graded gravel (Fig. 2) , 30-40 cm deep. The gravel prevents fine grains being carried into the drainage pipes. The
Fig 2 : Section of Filter Bed
sand bed presents a vast surface area; one cubic meter of filter sand presents about 15,000 sq. meters of surface area. Water percolates through the sand bed very slowly taking more than two hours. At the same time, it is subjected to a number of purification processes - mechanical straining, sedimentation, adsorption, oxidation and bacterial action. The rate of filtration of water normally lies between 0.1 and 0.4 m3/hour/per square metre of sand bed surface.
Vital layer: New filter acts as a mechanical strainer. The surface of the sand bed gets covered with a slimy growth known as "Schmutzdecke"/vital layer/zoogleal layer or biological layer. This layer is slimy and gelatinous and consists of threadlike algae and numerous forms of life including plankton diatoms and bacteria. The formation of vital layer is known as "ripening" of the filter. It may take several days for the vital layer to form fully, and when fully formed it extends for 2 to 3 cm into the top portion of the sand bed. The vital layer is the "heart" of the slow sand filter. It removes organic matter, holds back bacteria and oxidizes ammoniacal nitrogen into nitrates and helps in yielding a bacteria-free water. Until the vital layer is fully formed, the filtrate is wasted.