Waste Water Engineering Questions and Answers – Methods of Sludge Processing and Disposal – 2

This set of Waste Water Engineering Interview Questions and Answers for freshers focuses on “Methods of Sludge Processing and Disposal – 2”.

1. What is the percentage of dry solids for the primary sludge assumed at the outlet of aerobic digester?
a) 1-2.5%
b) 7-10.5%
c) 2.5-7%
d) 10.5-12.5%

Explanation: The percentage of dry solid assumed at the outlet of an aerobic digester for the primary sludge is 2.5-7%. In most cases, it is considered as 3.5%. This however depends on the water constituents.

2. In the case of anaerobic digestion, what is the percentage of dry solids for the primary sludge considered at the outlet?
a) 2-5%
b) 1-2%
c) 10-12%
d) 7-9%

Explanation: The percentage of dry solids for the primary sludge at the outlet of the anaerobic digester is considered as 2-5%. Generally, it is considered 4%. This however depends on the water constituents.

3. What is the concentration of polymer added to a belt filter press handling the primary sludge?
a) 2-5 kg/Mg dry solids
b) 1-4 kg/Mg dry solids
c) 5-10 kg/Mg dry solids
d) 10-12 kg/Mg dry solids

Explanation: 1-4 kg/Mg dry solids is the concentration of the polymer that is added to the belt filter press handling primary sludge. In case of centrifuge the concentration would be 1-2.5 kg/Mg dry solids. Polymer doses will vary according to molecular weight and ionic strength.

4. Heat treatment involves heating of the sludge. The supernatant from the heat treatment system is rich in ___________
a) TDS
b) TSS
c) BOD
d) Heavy metals

Explanation: The supernatant is rich in BOD. Thus this requires to be recycled to the treatment plant in order to be treated again. Heat treatment of the sludge is done in order to condition the sludge.

5. What is the maximum temperature up to which conditioning can be conducted?
a) 50 degree
b) 70 degree
c) 60 degree
d) 40 degree

Explanation: The maximum temperature up to which the sludge can be heated in order to carry out conditioning is 60 degree. By heating the sludge the soluble BOD increases which are contained in the supernatant. This sludge preheating method enhances the dewatering performance.
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6. What is the temperature at which sludge has to be frozen in order to carry out conditioning?
a) -2 to -5 degree
b) -20 to -25 degree
c) -5 to -8 degree
d) -10 to -20 degree

Explanation: The temperature at which the sludge has to be frozen in order to carry out conditioning is -10 to -20 degree. This should be frozen for at least 30 mins. This technology works on even difficult-to-dewater sludge.

7. What is the concentration of solids in the filtrate after thawing and freezing?
a) 45-50%
b) 50-60%
c) 25-40%
d) 60-65%

Explanation: The concentration of solids in the filtrate would be 25-40% after the sludge is thawed. This would contain the very low concentration of TSS. The time required to freeze/thaw the sludge depends on the concentration of solids present in the sludge.

8. Out of the below given methods which of these have the lowest capital cost?
a) Sludge drying bed
b) Filter press
c) Centrifuge
d) Sludge lagoons

Explanation: Sludge drying bed has the lowest capital cost. It has low consumption of chemicals too. However, sludge removal is labour intensive.

9. Out of the below given methods which of these have the highest cake solids concentration?
a) Sludge drying bed
b) Recessed plate Filter press
c) Centrifuge
d) Sludge lagoons

Explanation: A recessed plate filter press yields the highest cake solid formation while handling sludge disposal. However, it has low suspended solids in filtrate. The capital cost in this treatment method is high.

10. Out of the below given methods which of these is operated in batch process?
a) Sludge drying bed
b) Belt Filter press
c) Centrifuge
d) Sludge lagoons

Explanation: The belt filter press is operated in batch process. The operation and maintenance cost for these equipments is very high. It also requires high labour cost.

11. Out of the below given methods which of these requires a grit removal prior to it?
a) Sludge drying bed
b) Filter press
c) Centrifuge
d) Sludge lagoons

Explanation: A centrifuge requires a grit removal prior to it, in order to remove the grit particles. This has a low capital cost. It has moderately suspended solids in the centrate.

12. Out of the below given methods which of these produce an odour?
a) Sludge drying bed
b) Belt Filter press
c) Centrifuge
d) Recess plate filter press

Explanation: A belt filter press produces an odour. The high pressure machines produce very dry cakes. Generally, automatic operation is not advised for these equipment.

13. Out of the below given methods in which of these organic matters is stabilized?
a) Sludge drying bed
b) Filter press
c) Centrifuge
d) Sludge lagoons

Explanation: In sludge lagoons, the organic matter is stabilised. Low capital cost is involved in this method. However large space is required to carry out this process.

14. Out of the below given methods in which of these does the sludge contain 70-80% moisture?
a) Sludge drying bed
b) Filter press
c) Centrifuge
d) Sludge lagoons

Explanation: In a centrifuge the sludge cake produced contains approximately 70-80 % water. The solid concentration varies from 10-30 %. The centrate contains very low TSS.

15. What is the general size of belt available in the markets for a belt filter press?
a) 0.5 -3.5 m
b) 4-6 m
c) 7-9m
d) <0.5m

Explanation: Generally the size of the belt in the belt filter press is 0.5-3.5 m. In most cases, a belt of width 2m is used. Sludge loading rate varies from 90-680 kg/mh depending on the sludge type.

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