Waste Water Engineering Multiple Choice Questions on “Rapid Filtration”.
1. When was the rapid sand filer first used?
a) 1930
b) 1920
c) 1950
d) 1960
Answer: b
Clarification: The first modern rapid sand filtration plant was designed and built by George W. Fuller in Little Falls, New Jersey. Fuller’s filtration plant went into operation in 1920 and its success was responsible for the change to this technology in the U.S.
2. Rapid sand filters require large area compared to slow sand filters.
a) True
b) False
Answer: b
Clarification: Rapid sand filters were widely used in large municipal water systems by the 1920s because they required smaller land areas compared to slow sand filters.
3. Pressure flow is considered in rapid filtration.
a) True
b) False
Answer: b
Clarification: The polluted or unfiltered water flows through the filter medium under gravity or under pumped pressure and the floc material is trapped in the sand matrix.
4. Which of the following processes is not done before rapid filtration?
a) Mixing
b) Flocculation
c) Sedimentation
d) Chlorination
Answer: d
Clarification: Mixing, flocculation and sedimentation processes are typical treatment stages that precede filtration. Chemical additives, such as coagulants are often used in conjunction with the filtration system.
5. The direction of water in filter beds is reversed for ______
a) Filtration
b) Cleaning
c) Decreasing velocity
d) Decreasing flow
Answer: b
Clarification: Rapid sand filters must be cleaned frequently, often several times a day, by backwashing, which involves reversing the direction of the water and adding compressed air.
6. For a rapid sand filter, what is the ratio of length: breadth while designing it?
a) 1:2
b) 1:3
c) 2:3
d) 4:1
Answer: b
Clarification: For a rapid sand filter the ratio of length: breadth while designing it is 1:3. The area is calculated from the velocity and the flowrate of water. From this ratio and the area, the length and breadth of the filter are calculated.
7. Calculate the area of the rapid sand filter for the following details.
Flow: 2000 m3/h.
Velocity: 5m/h
Backwash time: 30 mins
a) 18 m2
b) 20 m2
c) 22 m2
d) 16 m2
Answer: a
Clarification: The total operation time equals 24 minus the back wash time. Area equals Flowrate × total hours of operation/ [velocity × total hours of operation × (total hours of operation-backwash time). Area= 2000 × 24/ (5 × 24 × 23.5) =18 m2.
8. Calculate the length of the rapid sand filter for the following details.
Flow: 1000 m3/h.
Velocity: 5m/h
Backwash time: 30 mins
a) 1.0 m
b) 1.9 m
c) 1.7 m
d) 1.2 m
Answer: b
Clarification: The total operation time equals 24 minus the back wash time. Area equals Flowrate × total hours of operation/ [velocity × total hours of operation × (total hours of operation-backwash time). Area= 1000 × 24/ (5 × 24 × 23.5) =9 m2. Length: Breadth is in the ratio 1:3. Length = 1.7 m.
9. Calculate the breadth of the rapid sand filter for the following details.
Flow: 500 m3/h.
Velocity: 5 m/h
Backwash time: 20 mins
a) 4.5 m
b) 5.7 m
c) 3.7 m
d) 7.5 m
Answer: c
Clarification: The total operation time equals 24 minus the back wash time. Area equals Flowrate × total hours of operation/ [velocity × total hours of operation × (total hours of operation-backwash time). Area= 500 × 24/ (5 × 24 × 23.7) =4.5 m2. Length: Breadth is in the ratio 1:3. Breadth = 3.7 m.
10. What is the turbidity achieved by using a rapid filtration system?
a) 0.1 NTU
b) 0.7 NTU
c) 1.5 NTU
d) 0.9 NTU
Answer: a
Clarification: The turbidity achieved by using a rapid filtration system is 0.1 NTU. This is used to operate systems which have rates greater than 100 times that of slow sand filtration. Coagulation is used to precondition the water.
11. What is the limit of turbidity which a 2 stage filtration a handle?
a) 180 NTU
b) <100 NTU
c) 50 NTU
d) 200 NTU
Answer: b
Clarification: Two stage filtration is a type of Rapid filtration. This can handle upto 100 NTU. This is followed by a rough filter after a coagulation system. The rough filter is followed by a filtration system.
12. What is the height of the anthracite maintained in a dual media filter?
a) 1.5-1.8 m
b) 2.2 -2.4 m
c) 2.5-2.8 m
d) 2.8-3.0 m
Answer: a
Clarification: The height of the anthracite maintained in dual media filter is 1.5-1.8 m. Initially, the height maintained used to be 0.45-0.6 m. GAC is sometimes used instead of anthracite in case of dual media filters.
13. What is the height maintained of a deep bed monomedia?
a) 1.5-1.8 m
b) 2.2 -2.4 m
c) 2.5-2.8 m
d) 2.8-3.0 m
Answer: a
Clarification: The height of the deep bed monomedia is maintained at 1.5-1.8 m. This is a type of rapid filtration. The media is usually GAC/Anthracite.