This set of Waste Water Engineering Questions and Answers for Campus interviews focuses on “Suspended Solids Removal & Dissolved Air Floatation – 2”.
1. What does the performance of a Dissolved air floatation depend on?
a) Air to solid ratio
b) Air to oil ratio
c) Air to flow rate of water
d) Air to the grease ratio
View Answer
Explanation: Dissolved air floatation performance depends on air to solid ratio. This ratio will vary with different type of suspension. This floatation process is dependent on the surface area of the materials.
2. What are the typical A/S ratios encountered in a Dissolved air floatation method?
a) 0.25-0.5
b) 0.005-0.06
c) 0.5-2
d) 3-5
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Explanation: The typical A/S ratios encountered in a Dissolved air floatation method is 0.005-0.06. Floatation depends on solid loading rate and particle rise velocity. It also depends on the concentration of particulate matter and the quantity of air used.
3. Which of these pollutants is removed by Dissolved air floatation?
a) Free oil
b) Emulsified oil
c) Both free and emulsified oil
d) Only grease
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Explanation: The Dissolved air floatation removes emulsified oil. This process is similar to froth floatation. This process is widely used to treat effluent from the oil industry, petrochemical etc.
4. What portion of the Dissolved air floatation effluent is recycled?
a) 70-80%
b) 80-90%
c) 30-50%
d) 15-20%
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Explanation: Around 15-20% of Dissolved air floatation effluent is recycled. The recycled flow is mixed with unpressurized main stream prior to letting it inside the floatation tank. This process is mainly used to treat industrial wastes.
5. What is dispersed air floatation used to remove?
a) Emulsified oil and suspended solids
b) Free oil and suspended solids
c) Emulsified oil
d) Free oil
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Explanation: Dispersed air floatation used to remove suspended solids. It is also used to remove emulsified oil. It doesn’t however remove free oil.
6. What is the pressure supplied for a dissolved air flotation process?
a) 1-2 bar
b) 5-7 bar
c) 2.75-3.75 bar
d) 9.75-12.75 bar
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Explanation: The pressure supplied for a dissolved air flotation process is the 2.75-3.75 bar. This is around 275-375 kPa. The entire flow for a dissolved air flotation is held under pressure in a retention tank for several minutes.
7. Which of these chemicals is not used during Dissolved air floatation?
a) Aluminium Salts
b) Ferric salts
c) Activated silica
d) Magnesium salts
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Explanation: Aluminium and Iron salts are used in dissolved air flotation in order to bind particulate matter. Activated silica is also used for the same purpose. Only magnesium salts are not used in dissolved air flotation process.
8. What is the hydraulic loading rate considered for a conventional dissolved air flotation?
a) 4 m/h
b) 5-15 m/h
c) 25-30 m/h
d) 35-40 m/h
View Answer
Explanation: The hydraulic loading rate considered for a conventional dissolved air floatation is 5-15 m/h. The mixing intensity is considered as 50-100 sec-1. Usually, the basin depth is considered 2.0-3 m.
9. What is the hydraulic loading rate considered for a high rate dissolved air flotation?
a) 10-30 m/h
b) 30-35 m/h
c) 40-55 m/h
d) 60-80 m/h
View Answer
Explanation: The hydraulic loading rate considered for a high rate of dissolved air floatation is 10-30 m/h. The mixing intensity is considered as 50-100 sec-1. Usually, the basin depth is considered 2.5-4.5 m.
10. What is the separation zone loading rate for a conventional dissolved air floatation process considered?
a) 10-30 m/h
b) 30-35 m/h
c) 4-9 m/h
d) 6-18 m/h
View Answer
Explanation: The separation zone loading rate for a conventional dissolved air floatation process considered is 6-18 m/h. In case of high rate dissolved air floatation process it is considered as 20-40 m/h. It is higher in case of high rate air floatation process.
11. What is the contact zone loading rate for a conventional dissolved air floatation process considered?
a) 100-200 m/h
b) 300-350 m/h
c) 40-90 m/h
d) 60-180 m/h
View Answer
Explanation: The contact zone loading rate for a conventional dissolved air floatation process considered is 100-200 m/h. The detention time considered while designing this is 10-20 mins. The contact zone loading rate is little higher in case of high rate floatation process.
12. What is the contact zone loading rate for a conventional dissolved air floatation process considered?
a) 120-300 m/h
b) 300-350 m/h
c) 40-90 m/h
d) 60-180 m/h
View Answer
Explanation: The contact zone loading rate for a conventional dissolved air floatation process considered is 120-300 m/h. The detention time considered while designing this is 10-15 mins. The contact zone loading rate is little higher in case of high rate floatation process.
13. What is the recycle ratio considered while designing the dissolved air floatation process?
a) 10-30
b) 1-4
c) 4-9
d) 6-12
View Answer
Explanation: The recycle ratio considered while designing the dissolved air floatation process is 6-12. This holds good for conventional type dissolved air floatation process. This is also true in case of high rate dissolved air floatation process.
14. What is the detention time considered for the contact zone while designing the conventional dissolved air floatation process?
a) 1-2.5 mins
b) 2.5-4 mins
c) 4-9 mins
d) 9-15 mins
View Answer
Explanation: The detention time considered for the contact zone while designing the conventional dissolved air floatation process is 1-2.5 mins. In case of high rate dissolved air floatation it is a little lesser. It is considered as 1-2 mins.
15. DAF separates _______ in water.
a) Suspended particles
b) Dissolved particles
c) Floating particles
d) Settleable solids
View Answer
Explanation: A dissolved air flotation (DAF) clarifier separates suspended solids in wastewater. DAF clarifiers can be round or rectangular tanks.
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