Waste Water Engineering Multiple Choice Questions on “Sewage Filters”.
1. Rapid filtration is used as the _______ clarifying step in municipal water treatment plants.
a) First
b) Second
c) Intermediate
d) Final
Answer: d
Clarification: Rapid filtration is used as the final clarifying step in municipal water treatment plants. Flocculation and sedimentation are provided as pre-treatment units to the rapid filter if the raw water has turbidity in excess of 10-20 NTU for an efficient process.
2. Biological action takes place in a rapid filtration unit.
a) Tue
b) False
Answer: b
Clarification: There is practically no biological action in rapid filtration although some nitrification occurs where the filtration velocity is relatively slow, where the oxygen content is adequate, and where the nitrifying bacteria finds favourable nutrients in the water.
3. How many hours does it take for filters to attain turbidity breakthrough?
a) 12-16 hours
b) 18-22 hours
c) 23-36 hours
d) 38-42 hours
Answer: c
Clarification: The filter operates usually 24-36 hours before attaining turbidity breakthrough. The influent water passes through the filter medium, the supporting layer and the underdrain where it experiences an energy loss due to frictional resistance referred to as head loss.
4. What is the maximum head loss at which the filter should be cleaned?
a) 1.5 m
b) 2 m
c) 2.5 m
d) 3 m
Answer: c
Clarification: When the head loss becomes excessive and beyond 1.5-2.5 m, the filter should be cleaned. During cleaning, the water above the filter is drained until the water lies a few centimetres above the top of the bed when air from a compressed air unit is blown in reverse to the normal flow direction at a rate of about 1-1.5 m3 free air/m2 of bed area, at a pressure of 20-35 kN/m2, for about 2-3 minutes.
5. Air is blown in the reverse direction during cleaning of filter beds.
a) True
b) False
Answer: a
Clarification: During cleaning, the water above the filter is drained until the water lies a few centimetres above the top of the bed when air from a compressed air unit is blown in reverse to the normal flow direction at a rate of about 1-1.5 m3 free air/m2 of bed area.
6. At what pressure is the air blown during cleaning?
a) 5-10 KN/m2
b) 10-15 KN/m2
c) 15-30 KN/m2
d) 20-35 KN/m2
Answer: d
Clarification: Air from a compressed air unit is blown in reverse to the normal flow direction at a rate of about 1-1.5 m3 free air/m2 of bed area, at a pressure of 20-35 kN/m2, for about 2-3 minutes.
7. What is the specified minimum time for backwash of rapid filters?
a) 2 minutes
b) 5 minutes
c) 10 minutes
d) 3 minutes
Answer: b
Clarification: The entire process of backwashing the filters and restarting the water supply takes about 15 minutes. The specified minimum backwash time for a rapid filter is 5 minutes.
8. What is the rate of backwash water?
a) 0.3-1 m/min
b) 1-1.5 m/min
c) 0.03-0.3 m/min
d) 15-3 m/min
Answer: a
Clarification: The wash water required for a rapid filter may vary from 3-6% of the total amount of water filtered. Upward wash water rates are usually of the order of 0.3-1.0 m/min.