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Slow Sand Filter

Slow sand filtration plants require considerable area because the rate of filtration is slow. A concrete floor with drainage tiles to collect the filtered water is constructed. The tile is covered with coarse gravel, fine gravel, coarse sand and finally 2 to 1 feet of sand at the top.

Water seeps through the filter slowly., is collected by tile drain pipes at the bottom, and is pumped into a reservoir. At best five million gallons of water per acre, per day, can be filtered. Slow sand filters are clogged by turbid water. Water to be filtered is, therefore, clarified by sedimentation with or without coagulation.

Bacteria arc also injested by Protozoa that Inhabit the upper layer of the film. Metabolic activity of microorganisms also greatly reduces the chemical content of the water.
When, the gelatinous film finally become too thick, the efficiency of the filter gradually decreases. The filter is taken out of service and the surface layer is cleaned.