250+ TOP MCQs on Sedimentation with Coagulation and Answers

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Environmental Engineering Multiple Choice Questions on ” Sedimentation with Coagulation”.

1. What is formed when coagulant is added to water?
a) Scum
b) Soap
c) Bubbles
d) Floc
Answer: d
Clarification: When coagulant is added to water, then white gelatinous precipitate is formed called floc.

2. The chemical composition of Alum is _____________
a) Al2 (SO4)3.18H2O
b) Al2 (SO4)2.18H2O
c) Al3 (SO4)2.18H2O
d) Al4 (SO4)3.18H2O
Answer: a
Clarification: Alum is the most common and universal coagulant used in water treatment having chemical composition Al2 (SO4) 3.18H2O.

3. The chemical compound which is insoluble in water, formed when alum is added to water is _____________
a) Al (OH)3
b) CaSO4
c) CO2
d) Ca (OH)3
Answer: a
Clarification: Al2 (SO4)3.18H2O + 3Ca (HCO3)2 = 2Al (OH)3 + 3CaSO4 + 18H2O + CO2
Where Al (OH)3 is insoluble in water.

4. Which gas is released when alum is added to water?
a) Al (OH)3
b) CaSO4
c) CO2
d) Ca (OH)3
Answer: c
Clarification: Carbon dioxide gas is released when alum is added to water, which is corrosive to metals.

5. What indicates the permanent hardness when alum is added to water?
a) Al (OH)3
b) CaSO4
c) CO2
d) Ca (OH)3
Answer: b
Clarification: The presence of calcium sulfate indicates the permanent hardness in water when alum is added to water.

6. Alum is effective when pH of water is between ____
a) 8-10
b) 6-8
c) 6.5-8.5
d) 7-9
Answer: c
Clarification: 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.

7. Why Alum is preferred over other coagulants?
a) It is easy to dewater the sludge formed
b) It imparts corrosiveness to water
c) It reduces taste and odor in addition to turbidity
d) The time required for floc formation is less
Answer: c
Clarification: 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.

8. The chemical composition of Chlorinated Copperas is __________
a) Fe2 (SO4)3 + FeCl3
b) FeSO4.9H2O
c) FeSO4.18H2O
d) Fe2 (SO4)3
Answer: a
Clarification: 6FeSO4.7H2O2 +3Cl2 = 2Fe2(SO4) 3 + 2FeCl3 + 7H2O
The ferric sulfate and ferric chloride so formed are called as Chlorinated Copperas.

9. Ferric chloride is effective over a pH range of __________
a) 3.5-6
b) 6.5-8
c) 3.5-6.5
d) 2-5
Answer: c
Clarification: 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.

10. Why Magnesium carbonate is not commonly preferred as coagulant?
a) It is expensive
b) It does not remove color
c) Formation of sludge
d) The time required for floc formation is less
Answer: c
Clarification: 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.

11. Which of the following statement is wrong regarding Iron salt?
a) Iron salt produces less floc than Alum
b) Detention time for sedimentation by using Iron salt as coagulant is less
c) Handling of Iron salt requires some skills
d) Iron removes H2S, taste and odor
Answer: a
Clarification: Iron salt produces heavy floc due to which more suspended matter is removed than with alum.