Waste Water Engineering Multiple Choice Questions on “Chemical Clarification – 1”.
1. Clarification removes most of the turbidity.
a) True
b) False
Answer: a
Clarification: Most of the turbidity is removed by clarification, making the water crystal clear. Disinfection is usually the final step in the treatment of drinking water which destroys pathogenic microbes.
2. What is the first step in clarification?
a) Sedimentation
b) Coagulation
c) Flocculation
d) Screening
Answer: b
Clarification: Finely divided particles suspended in surface water repel each other because most of the surfaces are negatively charged. Coagulation is the first step to neutralize the charged particles and form flocs.
3. The agglomeration of destabilized particles into large particles is called _________
a) Sedimentation
b) Coagulation
c) Flocculation
d) Disinfection
Answer: c
Clarification: Flocculation is the agglomeration of destabilized particles into large particles and can be enhanced by the addition of high-molecular-weight, water-soluble organic polymers. These polymers increase floc size by charged site binding and by molecular bridging.
4. What is the density of alum content used in water treatment?
a) 9.8 lb/gal
b) 10.1 lb/gal
c) 11.1 lb/gal
d) 9.2 lb/gal
Answer: c
Clarification: Alum is the primary coagulant. Alum is widely used because it is cheap and easily available. The chemical formula for alum is AlCl3. It is used in the form of a liquid and density is 11.1 lb/gal.
5. What is the speed of the impeller in a flash mixer?
a) 90 rpm
b) 100 rpm
c) 110 rpm
d) 120 rpm
Answer: c
Clarification: The impeller used in the flash mixer is a propeller type. The average speed of the impeller used in the flash mixer is 110 rpm. Rpm refers to the number of rotations per minute.
6. Microflocculation is brought about by which phenomenon?
a) Velocity gradient
b) Differential settling
c) Brownian movement
d) Differential mixing
Answer: c
Clarification: Microflocculation is brought about by Brownian movement. This is also known as perikinetic flocculation. In this type of flocculation, particle aggregation is brought about by random thermal motion.
7. Macro flocculation is brought about by which phenomenon?
a) Velocity gradient
b) Differential settling
c) Brownian movement
d) Differential mixing
Answer: a
Clarification: Macroflocculation is brought about by velocity gradient. This is also known as orthokinetic flocculation. In this type of flocculation, particles to be flocculated are mixed.
8. Alum precipitation occurs at which pH?
a) 1-3
b) 3-5
c) 5-7
d) 7-9
Answer: c
Clarification: Alum precipitation occurs at 5-7. Minimum solubility occurs at 6. Alum is a coagulating agent.
9. Iron precipitation occurs at which pH?
a) 1-3
b) 3-5
c) 5-7
d) 7-9
Answer: d
Clarification: Iron precipitation occurs at 7-9. Minimum solubility occurs at 8. Water containing iron usually is brown in colour.
10. Out of these which is not used as a chemical coagulant?
a) Alum
b) Calcium chloride
c) Ferric Chloride
d) Poly Iron Chloride
Answer: d
Clarification: Poly Iron chloride is not a chemical coagulant. It is a flocculant. It is used in bringing about flocculation.
11. What is the one disadvantage of adding chemicals to bring about precipitation?
a) The TSS increases
b) The TDS increases
c) BOD increases
d) It imparts colour
Answer: b
Clarification: The disadvantage of adding chemicals to bring about precipitation is that this increases the TDS. There is an increase in the dissolved constituents. Addition of chlorine increases the TDS in the effluent.
12. When alum is added to the waste water containing calcium ions which compound is formed as a precipitate?
a) Al(OH)3
b) Al2 (SO4)3
c) Ca(OH)2
d) CaCO3
Answer: a
Clarification: When alum is added to a waste water containing calcium salts Al (OH)3 is precipitated. This sis a gelatinous floc. This reaction sweeps out suspended particles.
13. Along with ferrous sulphate what other chemical is added in order to bring about precipitation?
a) Alum
b) PAC
c) Lime
d) Polyelectrolyte
Answer: c
Clarification: Along with ferrous sulfate, lime is generally added to bring about precipitation. This is done in order to increase the pH. Ferric hydroxide precipitates are formed only at very high pH.
14. Why is Ferric ammonium sulfate generally not used as a coagulant?
a) It is very expensive
b) It is dependent on the Dissolved oxygen present in the waste water
c) It is a very slow process
d) It works only at high pH
Answer: b
Clarification: Ferric ammonium sulfate is not a preferred coagulant. This is because only if oxygen is present ferrous hydroxide forms ferric hydroxide which is a precipitate. Thus this is dependent on the dissolved oxygen of the waste water.
15. What is the recommended surface loading rate in case of alum floc suspension?
a) 30-70 m3/m2.d
b) 10-30 m3/m2.d
c) 70-100 m3/m2.d
d) <10 m3/m2.d
Answer: a
Clarification: The recommended surface loading rate for alum floc suspension is 30-70 m3/m2.d. The typical value is 70 m3/m2.d. In case of iron floc suspension the recommended surface loading rate is 30-70 m3/m2.d