Microbiology Procedure

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Slow Sand Filter

	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.
The purification of water is accomplished not by the screening action of the sand for the spaces arc much to large, but by a different principle. A colloidal, flocculent material composed of bacteria, algae, and Protozoa accumulates in the surface layers of fine sand. This Slimy, gelatinous film closes up the pores between the sand grains and makes the filter bed more and more effective. Since bacteria have a negative electrical charge and colloidal material on the sand grains has a. positive charge, bacteria are thus adsorbed on the particles.
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.

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