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Coagulation in Water Purification


The first step in water purification is coagulation, which is a destabilizing process where tiny particles in the water coagulate into a large floc.

Metal salts, particularly trivalent metal salts such as aluminum and ferric salts, are the most commonly used coagulants. Organic polymers can also be used to coagulate water with high turbidity, but they may be carcinogenic. The higher the valence of the metal ion used as a coagulant, the more effective it is, but overdosing can lead to restabilization of the surface charge. To prevent restabilization, more metal salts can be added, which can form metal hydroxides that enmesh all the suspended solids and make the water clean. The optimum dose of coagulant is found through experiments called “Jar tests,” where different dosages of the coagulant are added and the water with the lowest turbidity is deemed the optimum dose.

Review questions:

  • What are some common metal salts used as coagulants?
  • Why is overdosing of metal salts during coagulation a problem?
  • How is the optimum dose of coagulant determined in the Jar test experiment?
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