Alum Coagulation (exp:12) - assignment - Q-A

Alum Coagulation
(1) Q: What do you understand by coagulation and flocculation? Which coagulants are most commonly used for water and wastewater treatment.
Ans:
Coagulation: The term coagulation is used to describe the process by which the charge on particles is destroyed.
Surface water generally contains wide variety of colloidal impurities that may cause the water to appear turbid and may impart color to the water. Colloidal particles that cause color and turbidity are difficult to separate from water because the particles will mot settle by gravity and are so small that they pass through the pores of most common filtration media. In order to be removed, the individual colloids must aggregate and grow in size so that they can settle by gravity. Chemical agents are used to promote colloid aggregation by destroying the forces that stabilize colloidal particles. The process of destroying the stabilizing forces and causing aggregation of colloids is referred to as chemical coagulation.
Flocculation: The term flocculation is used to determine the aggregation of particles into larger units.
The chemicals used for the purpose of coagulation is called coagulants.The most common coagulants used in water and wastewater treatment are aluminum and ferric salts as alum, ferric chloride and ferric sulfate.
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(2) Q: Why addition of alum may result in a drop in pH value. Discuss the affect of alum dose on pH from your experimental results.
Ans:
The common metal salt alum (aluminum sulfate) is a good coagulant for water containing appreciable organic matter. The chemical formula used for commercial alum is Al2(S04)3 . 14 H2O. Once dissolved in water, aluminum forms hydroxo-complexes and solids [e.g., Al(OH)3(s), Al(OH) +2, Al(OH)2+, Al(OH)4- ; Eqs. 1-5] and as a result pH of water is lowered, especially if alkalinity of water is low.
Al2(SO4)3 . 14 H2O (alum) = 2Al +3+ 3 SO4 –2 (1)
Al +3+ 3H2O = Al(OH)3(s) + 3 H+ (2)
Al +3+ H2O = Al(OH) +2 + H+ (3)
Al +3+ H2O = Al(OH)2+ + 2 H+ (4)
Al +3+ H2O = Al(OH)4- + H+ (5)
Theoretically, each mg/l of alum consume approximately 0.50 mg/l (as CaCO3) of alkalinity. For water with low alkalinity, this may result in significant reduction in pH that may interfere with formation of aluminum hydroxide flocs. In water treatment with alum coagulation, residual aluminum that may be present in water after the coagulation process is a cause of concern.
Remedy: If the alkalinity is insufficient, coagulant aids such as *lime ,Ca(OH)2, *soda ash (Na2CO3), *activated silica and *poly-electrolytes are used to provide the necessary alkalinity.
(3) Q: What is the primary mechanism by which heavy metal ions are removed during coagulation.
Ans: Coagulation with alum and ferric chloride or ferric sulfate is also widely used for removal of heavy metal ions (e.g., lead, arsenic) from water. In this process heavy metal ions are primarily removed by adsorption (and subsequent precipitation) onto coagulated flocs of metal (either aluminum or iron) hydroxides. Coagulation with alum and ferric chloride/sulfate has been successfully used for removal of arsenic from water.

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