Test: Sedimentation with Coagulation - Civil Engineering (CE) MCQ

Alum is the most common and universal coagulant used in water treatment having chemical composition Al₂ (SO₄) 3.18H₂O.

When coagulant is added to water, then white gelatinous precipitate is formed called floc.

Carbon dioxide gas is released when alum is added to water, which is corrosive to metals.

Al2 (SO₄)₃.18H₂O + 3Ca (HCO₃)₂ = 2Al (OH)₃ + 3CaSO₄ + 18H₂O + CO₂
Where Al (OH)₃ is insoluble in water.

The presence of calcium sulfate indicates the permanent hardness in water when alum is added to water.

Alum is effective when pH of water is between 6.5-8.5. Its dose depends on turbidity, color, taste, pH and temperature of water.

6FeSO₄.7H₂O₂ +3Cl₂ = 2Fe₂(SO₄)₃ + 2FeCl₃ + 7H₂O
The ferric sulfate and ferric chloride so formed are called as Chlorinated Copperas.

Alum is preferred over other coagulants because it reduces the taste and odor in addition to turbidity and produce lighter flocs that can be broken easily.

When magnesium carbonate and lime are dissolved in water, magnesium hydroxide and calcium carbonate are formed which are soluble in water and sludge is formed in slurry form, so they are not commonly used.

Ferric chloride is effective over a pH range of 3.5-6.5 and above 8.5 whereas if Ferric sulfate is used then, it is effective over a pH range of 4-7 and above 9.