Waste Water Engineering Questions and Answers – UASB Reactor – 2

This set of Waste Water Engineering Problems focuses on “UASB Reactor – 2”.

1. What is the temperature which should not be exceeded for the reaction?
a) 20˚C
b) 32˚C
c) 38˚C
d) 45˚C

Explanation: For an optimal growth of these bacteria and thus for an optimal digestion, the temperature should lie between 35-38˚C. Below this range, the digestion rate decreases by about 11% for each 1˚C temperature decrease.

2. What is the minimum influent COD load beneficial for the process?
a) 100 mg COD/Lm
b) 200 mg COD/Lm
c) 250 mg COD/Lm
d) 350 mg COD/Lm

Explanation: Influents should have concentrations of above 250 mg COD/Lm, as for lower rates, anaerobic digestion is not beneficial. Optimum influent concentrations are above 400 mg COD/Lm and an upper limit is not known.

3. What is the minimum hydraulic retention time for UASB?
a) 1 hour
b) 2 hour
c) 3 hour
d) 4 hour

Explanation: The hydraulic retention time should not be less than 2 hours. Anaerobic micro-organisms, especially methane producing bacteria, have a slow growth rate. At lower HRTs, the possibility of washout of biomass is more important.

4. What is the COD loading rate assumed for an Up flow anaerobic sludge blanket (UASB)?
a) 4-12 Kg COD/m3
b) 12-20 Kg COD/m3
c) 2-4 Kg COD/m3
d) 20-25 Kg COD/m3

Explanation: The COD loading rate assumed for an Up flow anaerobic sludge blanket is 4-12 Kg COD/m3. In most cases it is considered as 6 Kg COD/m3. Based on this the volume of the upflow anaerobic sludge blanket is sized.

5. What is the upflow velocity considered for a UASB while treating 100% soluble COD?
a) 1-3 m/h
b) 4-6 m/h
c) 6-8 m/h
d) 8-10 m/h

Explanation: The upflow velocity considered for a UASB while treating 100% soluble COD is 1-3 m/h. In most cases, it is considered as 1.5 m/h. This is the velocity of the influent mixed with the biomass which is introduced at the bottom of the reactor.
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6. What is the upflow velocity considered for a UASB while treating partially soluble COD?
a) 1-1.25 m/h
b) 1.5-2 m/h
c) 2-3 m/h
d) 3-4 m/h

Explanation: The upflow velocity considered for a UASB while treating partially soluble COD is 1-1.25 m/h. In most cases it is considered as 1.0 m/h. This is the velocity of the influent mixed with the biomass which is introduced at the bottom of the reactor.

7. What is the upflow velocity considered for a UASB while treating domestic waste water?
a) 1-2 m/h
b) 2-3 m/h
c) 0.8-1 m/h
d) 3-4 m/h

Explanation: The upflow velocity considered for a UASB while treating domestic waste water is 0.8-1. In most cases it is 0.9. This waste water is mixed with the influent biomass from the bottom of the reactor.

8. What is the retention time considered for a UASB while designing at 16-19 degree Celsius?
a) 3-5 h
b) 10-14 h
c) 22 h
d) 18 h

Explanation: The retention time considered for a UASB while designing at 16-19 degree Celsius is 10-14 h. It is usually considered as 12 h. This is for a 4m height UASB. At lower temperatures, the Solid retention time considered is higher.

9. What is the retention time considered for a UASB while designing at 22-26 degree Celsius?
a) 3-5 h
b) 10-14 h
c) 7-9 h
d) 10-12 h

Explanation: The retention time considered for a UASB while designing at 22-26 degree Celsius is 7-9 h. It is usually considered as 8 h. This is for a 4m height UASB. At higher temperatures, the solid retention time considered is less.

10. What is the retention time considered for a UASB while designing at > 26 degree Celsius?
a) 3-5 h
b) 10-14 h
c) 7-9
d) 6-8

Explanation: The retention time considered for a UASB while designing at 16-19 degree Celsius is 6-8 h. It is usually considered as 7 h. This is for a 4m height UASB. At lower temperatures, the Solid retention time considered is higher. At higher temperatures, the solid retention time considered is less.

11. Calculate the volume for the UASB with the following details.
Flow: 120m3/h
COD: 350 mg/L
UASB load considered: 6 Kg COD/ m3
a) 7 m3
b) 8 m3
c) 8.5 m3
d) 9 m3

Explanation: COD load = COD X Flow. Volume of the UASB = COD Load/ UASB Load. In this case volume of UASB = [(120 X 350)/1000]/6=7 m3.

12. Calculate the volume for the UASB with the following details.
Flow: 240m3/h
COD: 350 mg/L
UASB load considered: 8 Kg COD/ m3
a) 7.5 m3
b) 8.0 m3
c) 10.5 m3
d) 9.5 m3

Explanation: COD load = COD X Flow. Volume of the UASB = COD Load/ UASB Load. In this case volume of UASB = [(240 X 350)/1000]/8=10.5 m3.

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