This set of Waste Water Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Types of Settling Tank”.
1. _______ is a process by which particles settle to the bottom of the liquid.
Explanation: The process by which particulates settle at the bottom of a liquid and form sediment is called settling. Particles experience a force, either due to gravity or due to centrifugal motion and tend to move in a uniform manner.
2. Settling tanks are also called as sedimentation tanks.
Explanation: Settling tanks are also referred to as sedimentation tanks. The process by which particulates settle at the bottom of a liquid and form sediment at the bottom of the vessel is called settling.
3. Which is the primary force acting on the Settleable particles?
a) Gravitational force
b) Centrifugal force
c) Mechanical force
d) Radial force
Explanation: The applied force such as gravity and a drag force that is due to the motion of the particle through the fluid. The applied force is not affected by the particle’s velocity. The drag force is a function of the particle velocity.
4. How many types of settling are present?
Explanation: There are four types of settling present. Type 1 is for non-flocculent particles, type 2 is for flocculent particles, type 3 is called as zone or hindered settling, type 4 is called as compression settling.
5. Type III settling is called as hindered settling.
Explanation: Close particles that hinder interparticle forces and the settling of neighbouring particles, settle at a constant velocity and settle in secondary clarifiers in type III settling.
6. What is type IV settling also called as?
a) Zone settling
b) Hindered settling
c) Compression settling
d) Expansion settling
Explanation: In a high concentration settling where the particles touch each other and settling can occur only by compression of the compacting mass is called compression settling.
7. What is the product obtained when a coagulant is mixed with wastewater?
Explanation: The most commonly used coagulant is alum. A coagulant combines with the impurities in wastewater and forms an end product called as flocs which is greater in diameter and can be easily removed.
8. Which mode of settling are tube settlers based on?
a) Counter current settling
b) Cocurrent settling
c) Cross flow settling
d) Flocculant settling
Explanation: Settling in tubesettler is based on counter current settling. Solids that settle down, move due to gravity. These constitute various plastic shaped geometries that enhance the settling characteristics of the sedimentation basin.
9. In countercurrent settling how is the solid suspension fed?
a) From the bottom
b) From the sides
c) From the top
d) Flow is up the plates
Explanation: In countercurrent settling the solid suspension is fed from the top. The time taken for a particle to settle in both countercurrent/co current settling is the same. The direction in which the feed is fed is different.
10. How is the length of the surface calculated in case of countercurrent settling?
a) Lp=wVƟ/ v cos Ɵ
b) Lp=(VƟ – V sinƟ)/ v cos Ɵ
c) Lp=w(VƟ – V sinƟ)
d) Lp=w(VƟ – V sinƟ)/ v cos Ɵ
Explanation: The length of the surface is calculated as Lp=w (VƟ– V SinƟ)/ v cos Ɵ. Where LƟ is the length of the surface. V is the settling velocity. W is the perpendicular distance. Ɵ is the angle of surface inclination.
11. When will the particles settle in a tank which is subjected to countercurrent settling?
a) V<(VƟ – V sinƟ)/v cosƟ
b) V>= VƟ/v cosƟ
c) V>= (VƟ – V sinƟ)/v cosƟ
d) V<VƟ/v cosƟ
Explanation: The particles will settle in a tank when V>= (VƟ – V sinƟ)/v cosƟ. Where V is the settling velocity. Ɵ is the angle of surface inclination.
12. Lamella Clarifier works on which principle?
a) Cocurrent settling
b) Countercurrent settling
c) Cross flow settling
d) Hindered settling
Explanation: Lamella Clarifier works on Countercurrent settling. The feed is fed by a feed duct to the feed box. The flow is directed downwards towards individual side-entry plate slots.
13. How is the length of the surface of a settling tank determined in case of cross- flow settling?
a) Lp=wVƟ/ v cos Ɵ
b) Lp<wVƟ/ v cos Ɵ
c) Lp= (VƟ – V sinƟ)/ v cos Ɵ
d) Lp<(VƟ – V sinƟ)/ v cos Ɵ
Explanation: The length of the settling tank in case of cross flow settling is determined as Lp=wVƟ/ v cos Ɵ .Where Lp is the length of the surface. V is the settling velocity. W is the perpendicular distance. Ɵ is the angle of surface inclination.
14. How is the area of a sludge thickening tank calculated?
Explanation: The area of the sludge thickening tank is calculated as A=QTa/H0. Where A is the area of the sludge thickening tank. Q is the flow rate. Ta is the time to reach the desired underflow concentration. H0 is the initial height of the interface column.
15. How is the critical concentration of sludge thickener tank calculated?
a) Hu= C02 H0/ Cu
b) Hu= C0 H0/ Cu2
c) Hu= Cu H0/ CoS
d) Hu= C0 H0/ Cu
Explanation: The critical concentration is calculated as Hu= C0 H0/ Cu. Where Hu depth at which the solids are under the desired underflow concentration. Cu is the desired underflow concentration.
16. What is the major assumption made when solid flux method is used to find the area of the sedimentation basin?
a) Settling basin is not operating at a steady state
b) Settling basin is operating at an adiabatic state
c) Settling basin is operating at a steady state
d) Settling basin is operating at an isobaric state
Explanation: Settling basin is operating at a steady state. This is the assumption made when the area of the sedimentation basin is to be calculated by the solid flux analysis method. The downward flux of the solids is brought about by gravity.
17. How is the height of the sludge calculated in a sludge drying bed?
Explanation: The height of the sludge in a sludge drying bed due to compression settling is calculated as Ht-Hx= (H2-Hx)eit2 where Ht is the height of the sludge at time t. Hx is the height of sludge after 24 hrs. H2 height of sludge after 2 days. i is a constant for a given suspension.
Sanfoundry Global Education & Learning Series – Waste Water Engineering.
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