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Sedimentation Process
Water usually undergoes some degree of purification during storage in ponds or reservoirs. Suspended particles settle and carry down most of the microorganisms. The rate of purification by sedimentation depends upon the kind and amount of suspended matter as the well as physical, chemical and biological conditions of the stored water.

The rate of sedimentation is enhanced by adding alum, iron salts, colloidal silicate, etc., which produce flocculent precipitates. Microorganisms and suspended particles are entrapped and settle rapidly. Sometimes activated carbon is also added. This adsorbs the compound responsible for objectionable colour and taste of water.