# Environmental Engineering Questions and Answers – Comparison Between Slow and Rapid Sand Filter

This set of Environmental Engineering Interview Questions and Answers focuses on “Comparison Between Slow and Rapid Sand Filter”.

1. In which of the following filters, the rate of filtration is highest?
a) Slow sand filter
b) Rapid sand filter
c) Pressure filter
d) Roughing filter

Explanation: The rate of filtration is highest in pressure filter and its filtration rate lies between 6000 to 15000 litres per hour per m2 of filter area while in rapid sand and roughing filter, it is between 6000 to 8000 litres per hour per m2 of filter area.

2. The final loss of head in slow sand filter is ______ than rapid sand filter.
a) Higher
b) Lower
c) Slightly Lower
d) Equivalent

Explanation: The loss of head in slow sand filter lies between 15-100cm and of rapid sand filter lies between 30-300cm.The final loss of head in slow sand filter is 100cm while in rapid sand filter is 300cm.

3. The turbidity of water applied to rapid sand filter is less than _______
a) 35-40ppm
b) 10-20ppm
c) 30-40ppm
d) 10-15ppm

Explanation: As before filtration, sedimentation and coagulation process is done, so the turbidity of water should be less than 35-40ppm.

4. The period of cleaning of a slow sand filter is ______ than rapid sand filter.
a) Smaller
b) Larger
c) Slightly smaller
d) Slightly larger

Explanation: The period of cleaning of a slow sand filter is 1-2 months and that of the rapid sand filter is 2-3 days.

5. The bacterial removal efficiency of the rapid sand filter is ______ in comparison to slow sand filter.
a) More
b) Less
c) Equal
d) Better

Explanation: The bacterial removal efficiency of the rapid sand filter is90-99%, whereas of a slow sand filter is 98-99%.
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6. How is the grain size distribution of sand in rapid sand filter?
a) Stratified
b) Unlayered
c) Layered with heaviest grains at the top
d) Layered with lightest grains at the top

Explanation: The grain size distribution of sand in rapid sand filter is stratified with lightest grains at the top and heaviest grains at the bottom.

7. Consider the following statement.
i. The penetration of suspended matter is deep in the slow sand filter
ii. The penetration of suspended matter is superficial in the rapid sand filter
iii. Coagulation is not required in the slow sand filter
Which of the above statement is/are correct?
a) Only iii
b) i, ii
c) i, ii, iii
d) ii, iii

Explanation: The penetration of suspended matter is deep in rapid sand filter, whereas the penetration of suspended matter is superficial in slow sand filter.

8. Consider the following statement.
i. The slow sand filter is not efficient in removing turbidity
ii. The slow sand filter has a high initial cost
iii. Skilled supervision is required in slow sand filter
Which of the above statement is/are correct?
a) Only iii
b) i, ii
c) i, ii, iii
d) ii, iii

Explanation: Skilled supervision is not required in the slow sand filter but it is essential in the case of the rapid sand filter.

9. The filter run in rapid sand filter should not be less than _____ hours.
a) 1
b) 6
c) 12
d) 24

Explanation: The filter run in rapid sand filter should not be less than 24 hours and its head loss should not exceed 2m.

10. Consider the following statement.
i. Slow sand filter is not flexible in variation in demand
ii. The rapid sand filter is flexible in fluctuations in demand
iii. The method of cleaning of a slow sand filter involves agitation and back-washing
Which of the above statement is/are correct?
a) i, ii
b) i, ii, iii
c) ii, iii
d) Only iii

Explanation: The method of cleaning of a slow sand filter involves scrapping of top layer to 15 to 25mm whereas in rapid sand filter involves agitation and back-washing.

11. Rapid sand filters are very efficient in removal of color.
a) True
b) False

Explanation: Rapid sand filters are very efficient in the removal of color and the intensity of color can be brought down below 3 on cobalt scale.

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