why potash alum is used in the classification of water?
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Potassium alum has been used since remote antiquity for purification of turbid liquids. It is still widely used in the purification of water for drinking and industrial processes water, treatment of effluents and post-storm treatment of lakes to precipitate contaminants.
Between 30 and 40 ppm of alum for household wastewater, often more for industrial wastewater, is added to the water so that the negatively charged colloidal particles clump together into "flocs", which then float to the top of the liquid, settle to the bottom of the liquid, or can be more easily filtered from the liquid, prior to further filtration and disinfection of the water. Like other similar salts, it works by neutralizing the electrical double layer surrounding very fine suspended particles, allowing them to join into flocs.
The same principle is exploited when using alum to increase the viscosity of a ceramic glaze suspension; this makes the glaze more readily adherent and slows its rate of sedimentation.
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As shown, chemical coagulants act to neutralize the charges on the suspended particles to prevent electrostatic repulsion. Aluminium sulphate (also termed alum) is a commonly used water treatment coagulant that, when utilized with the associated alkalinity, produces the reaction below!!