Question

potas alum is added to water in the loading tank and is allowed to stand for some time

Answer 1

Alum (aluminum sulfate) is added to the water to destabilize natural fine particulate matter suspended in water. This process in known as coagulation. These particles enter water through land erosion, airborne contaminants, decay of natural organic matter such as plants and animal wastes, and discharges resulting from human activities. The impurities found in water suspensions consist of charged colloids (colloid - a very small solid suspended in water which varies in size, density and electric charge) ranging in size from 5 nm to 1 um and particulates greater than 0.5 mm. Most of these particles will not settle by gravity due to their charge and because they are lighter than water (their density is less than water). The addition of alum causes these particles and colloids to clump together to form heavier particles which will settle in water.