This set of Waste Water Engineering Questions and Answers for Entrance exams focuses on “Sludge Characteristics, Thickening and Concentration – 2”.
1. _________ is an aerobic process of mixing sewage sludge with agricultural by-product sources.
a) Aerobic digestion
b) Anaerobic digestion
c) Composting
d) Incineration
View Answer
Explanation: Composting is an aerobic process of mixing sewage sludge with agricultural by-product sources of carbon such as sawdust, straw or wood chips. Bacteria digesting both the sewage sludge and the plant material generate heat to kill disease-causing micro-organisms and parasites.
2. Which of the following processes is rarely used?
a) Aerobic digestion
b) Anaerobic digestion
c) Composting
d) Incineration
View Answer
Explanation: Incineration of sludge is less common because of air emissions concerns and the supplemental fuel (typically natural gas or fuel oil) required to burn the low calorific value sludge and vaporize residual water.
3. What is the thickened concentration of the primary sludge?
a) 1-2%
b) 2-3%
c) 3-5%
d) 5-10%
View Answer
Explanation: The unthickened primary sludge is around 2-6%. In most cases, it is considered 2%. The thickened sludge is around 5-10%. In most cases, it is considered 5%.
4. What is the thickened concentration of the trickling filter sludge?
a) 3-6%
b) 1-3%
c) 6-9%
d) 0.5%
View Answer
Explanation: The unthickened sludge from the trickling filter is around 1-4%. In most cases, it is considered 2%. The thickened sludge is around 3-6%. In most cases, it is considered 5%.
5. What is the thickened concentration of the thickened sludge from the rotating biological contractor?
a) 1-2%
b) 3-5%
c) 5-7%
d) 7-9 %
View Answer
Explanation: The unthickened sludge from rotating biological contractor is around 1-3.5%. In most cases, it is considered 2%. The thickened sludge is around 3-5%. In most cases, it is considered 5%.
6. What is the thickened concentration of air activated sludge?
a) 1%
b) 5%
c) 2-3%
d) 3-6%
View Answer
Explanation: The unthickened air activated sludge is around 0.5-1%. The thickened sludge is around 2-3%. In most cases, it is considered 2%.
7. What is the thickened concentration of extended aeration sludge?
a) 1%
b) 2-3%
c) 5%
d) 5-7%
View Answer
Explanation: The unthickened sludge from the extended aeration is around 0.5-1%. The thickened sludge is around 2-3%. In most cases, it is considered 2%.
8. What is the thickened concentration of high purity oxygen activated sludge?
a) 1%
b) 10%
c) 2-3%
d) 3-5%
View Answer
Explanation: The unthickened sludge from the high purity oxygen activated sludge is around 0.5-1%. The thickened sludge is around 2-3%. In most cases, it is considered 2%.
9. What is the thickened concentration of anaerobically digested primary sludge from primary digestor?
a) 1-3%
b) 5-7%
c) 10%
d) 12%
View Answer
Explanation: The unthickened anaerobically primary digested sludge from the primary digestor is around 8%. The thickened sludge is around 12%. In anaerobic digestion, the amount of sludge produced is very high.
10. What is the thickened concentration of primary sludge combined with trickling filter humus?
a) 1-3%
b) 4%
c) 5-9%
d) 10%
View Answer
Explanation: The unthickened primary sludge combined with trickling filter humus is 2-6 %. The thickened sludge is around 5-9%. In this case, the trickling filter humus is recycled back to the primary clarifier which gets combined with the primary sludge formed.
11. What is the thickened concentration of primary sludge combined with rotating biological contractor?
a) 1-2%
b) 3-4%
c) 5-8%
d) 9-11%
View Answer
Explanation: The unthickened primary sludge combined with rotating biological contractor is 2-6 %. The thickened sludge is around 5-8%. In this case, the sludge from rotating biological contractor is recycled back to the primary clarifier which gets combined with the primary sludge formed.
12. What is the thickened concentration of chemical tertiary sludge( High lime)?
a) 1-2%
b) 3-4%
c) 6-8%
d) 12-15%
View Answer
Explanation: The unthickened of the chemical Sludge (high lime) is 3-4.5 %. The thickened sludge is around 12-15%. The thickened low chemical Sludge is also 12-15%.
13. What is the solid loading rate in kg/ m2.d considered while designing a thickener to treat the primary sludge?
a) 40-50
b) 50-100
c) 100-150
d) 150-200
View Answer
Explanation: The solid loading rate is considered as 100-150 kg/m2.d for a thickener handling primary sludge. When a thickener is handling sludge from both the primary clarifier and rotating biological contractor then the solid loading rate is considered as 50-90 kg/m2.d. This solid loading rate is used to calculate the volume of a thickener to be designed.
14. What is the solid loading rate in kg/ m2.d considered while designing a thickener to treat the trickling filter humus?
a) 40-50
b) 50-100
c) 100-150
d) 150-200
View Answer
Explanation: The solid loading rate is considered as 40-50 kg/m2.d for a thickener handling trickling filter humus. When a thickener is handling sludge from both the primary clarifier and trickling filter then the solid loading rate is considered as 60-100 kg/m2.d. This solid loading rate is used to calculate the volume of a thickener to be designed.
15. What is the solid load rating in kg/ m2.d considered while designing a thickener to treat the sludge from the rotating biological contractor?
a) 100
b) 80
c) 35-50
d) 100-120
View Answer
Explanation: The solid loading rate is considered as 35-50 kg/m2.d for a thickener handling sludge from the rotating biological contractor. When a thickener is handling sludge from both the primary clarifier and rotating biological contractor then the solid loading rate is considered as 50-90 kg/m2.d. This solid loading rate is used to calculate the volume of a thickener to be designed.
16. What is the solid loading rate in kg/ m2.d considered while designing a thickener to treat the sludge from air- activated sludge process?
a) 100
b) 80
c) 20-40
d) 10-15
View Answer
Explanation: The solid loading rate is considered as 20-40 kg/m2.d for a thickener handling sludge from air- activated sludge process.The solid loading rate is considered as 20-40 kg/m2.d also for a thickener handling sludge from high purity oxygen activated sludge process. This solid loading rate is used to calculate the volume of a thickener to be designed.
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