# Waste Water Engineering Questions and Answers – Settling of Discrete Particles

This set of Waste Water Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Settling of Discrete Particles”.

1. In Newton’s law for the settling of discrete particles, the particles size is _______
a) 1 mm
b) Greater than 1 mm
c) Less than 1 mm
d) 0.5 mm

Explanation: The particle size is greater than 1mm for the specific gravity of 2.65 and temperature is 20˚C for Newton’s law for the settling of discrete particles. The Reynolds number is in the range of 103-104.

2. Newton’s law is used for_______
a) Turbulent flows
b) Non-turbulent flows
c) Laminar flow
d) Spiral flow

Explanation: Newton’s law is used for turbulent flows. The Reynolds number is in the range of 103-104. The particle size is greater than 1mm for a specific gravity of 2.65 and temperature is 20˚C for Newton’s law for settling of discrete particles.

3. For laminar flow Reynolds number is 1.
a) True
b) False

Explanation: Stock’s law is used for laminar flow conditions and the Reynolds number is 1. The particle size is up to 0.1 mm for a specific gravity of 2.65 and temperature of 20˚C for Stoke’s law.

4. The individual particle settling is hindered when the volumetric concentration is high.
a) True
b) False

Explanation: When the volumetric concentration is high, the rate of individual particle settling is hindered by the upward movement of water displaced by the downward movement of suspended particles.

5. Non-spherical particles will settle _______ spherical particles.
a) Slower than
b) Faster than
c) Equal as
d) Same as

Explanation: Non-spherical particles will settle more slowly than spherical particles of the same volume and density. For all flow conditions other than laminar the drag coefficient is also a function of the shape of the particle and must be determined experimentally.
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6. Grit and sand particles are removed from waste water by which type of settling?
a) Flocculent settling
b) Discrete particle settling
c) Hindered settling
d) Compression settling

Explanation: Sand and grit particles are removed from the wastewater by discrete particle settling. This process generally involves settling of particles in a suspension of low solids. These particles settle as individual entities.

7. In the discrete settling type which of these factors is a constant?
a) velocity
b) acceleration
c) time taken to settle
d) weight of the particles

Explanation: In the case of discrete settling process acceleration is constant. Particles suspended in a low solid concentration settle down due to gravity in a constant acceleration field. Particle settles down as individual entities and doesn’t interact with neighboring particles.

8. Which of these process is involved in the removal of chemical flocs?
a) Flocculent settling
b) Ballasted flocculent settling
c) Hindered settling
d) Compression settling

Explanation: Flocculent settling is involved in the removal of chemical flocs from the waste water. This also removes a portion of the TSS from the untreated waste water in primary settling tanks. By coalescing, the mass increases and thus the particles settle faster.

9. Which of these process is involved in the settling of particles in the secondary settling tanks?
a) Flocculent settling
b) Ballasted flocculent settling
c) Compression settling
d) Hindered settling

Explanation: Hindered settling brings about the settling of particles in the secondary settling tanks. This type of settling occurs in the biological treatment tanks. In this type of settling the inter-particle forces are sufficient enough to hinder the settling of neighbouring particles.

10. Which of these process is involved in the removal of BOD and phosphorous?
a) Flocculent settling
b) Accelerated gravity settling
c) Ballasted flocculent settling
d) Compression settling

Explanation: The Ballasted Flocculent settling is involved in the removal of BOD and phosphorous. An inert ballasting agent is added in order to bring about rapid settling. A portion of the recovered ballasting agent is recycled.

11. How concentrated are the suspensions which are subjected to hindered settling?
a) Low
b) High
c) Very high
d) Medium

Explanation: The suspension of intermediate concentration is usually subjected to hindered settling. The particles tend to remain in their fixed positions with respect to each other. The mass of the particles settles as a unit.

12. Which of these process is involved in the settling process in solid thickening facilities?
a) Compression settling
b) Hindered settling
c) Accelerated gravity settling
d) Flocculent settling

Explanation: Compression settling is involved in the settling of solid thickener facilities. This usually occurs in the lower layers of deep solids. Compression occurs due to the weight of the particles, which are constantly being added to the structure.

13. Which of these process are involved in the removal of oil and grease?
a) Accelerated gravity settling
b) Floatation
c) Flocculent settling
d) Compressed settling

Explanation: The floatation process is involved in the removal of oil and grease. This process is involved in the removal of material that floats on water. This is also involved in the thickening of solid suspensions.

14. For a discrete particle settling which of these mathematical equations is true?
a) Q= A Xv
b) Q=A/V
c) Q= A X t
d) Q=A/t

Explanation: For a discrete particle settling process Q=AX v. Where Q is the flow rate. A is the area of the sedimentation basin and v is the velocity.

15. How is the design velocity expressed mathematically for a discrete particle settling process?
a) Vc = depth/detention time
b) Vc = depth x detention time
c) Vc = depth/area
d) Vc = area /detention time

Explanation: The design velocity is expressed as Vc = depth/detention time. Where Vc is the critical velocity. This depends on both the detention time and depth of the tank.

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