Wastewater Treatment and Recycling Nptel Week 4 Answers
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Wastewater Treatment and Recycling Nptel Week 4 Answers (July-Dec 2025)
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Question 1. BOD removal through degradation is mainly a process of:
a) Mass transport
b) Phase transfer
c) Mass Transformation
d) Dispersion
Question 2. The process responsible for the self-purification of natural waters is:
a) Sedimentation
b) Filtration
c) Biodegradation
d) All of these
Question 3. In a water sample, the DO consumed at any point of time t is equal to:
a) Ultimate BOD – BOD exerted by the time t
b) Ultimate BOD – existing BOD at time t
c) BOD exerted by the time t + existing BOD at time t
d) BOD exerted by the time t existing BOD at time t
These are Wastewater Treatment and Recycling Nptel Week 4 Answers
Question 4. Match the conditions given in the first column with the zones given in the second column, for a river receiving a wastewater stream:
Conditions
(i) BOD of river water is maximum
(ii) DO in the river water is minimum
(iii) BOD is relatively much lower
Zones
A. Septic zone
B. Recovery zone
C. Decomposition zone
a) (i)-A, (ii)-B, (iii)-C
b) (i)-B, (ii)-C, (iii)-A
c) (i)-C, (ii)-A, (iii)-B
d) (i)-A, (ii)-C, (iii)-B
Question 5. The self-purification constant is relatively highest for:
a) Rivers of high velocity
b) Rivers of low velocity
c) Large lakes and impounding reservoirs
d) Small ponds
Question 6. Engineered systems for wastewater treatment primarily work based on:
a) Physics principles
b) Chemistry principles
c) Microbiology principles
d) All of these
Question 7. In the generic mass balance equation given as V.dC/dt = Qin. Cin – Qout. Cout – V. (dC/dt)reaction (in usual notations), if Qin and Qout are taken as zero, it indicates:
a) Steady state operation
b) Batch process
c) Plug-flow reactor
d) Conservation substance
These are Wastewater Treatment and Recycling Nptel Week 4 Answers
Question 8. Identify the correct statements regarding plug-flow reactors:
Statement I: Generally, there is no mixing in plug flow reactors, therefore pollutant concentration may be different at different parts (sections) inside the reactor.
Statement II: Due to varying concentrations in the reactors, plug flow reactors can never be operated under steady state.
a) Statement I is correct, while statement II is incorrect
b) Statement II is correct, while statement I is incorrect
c) Both, statements I and II are correct
d) Both, statements I and II are incorrect
Question 9. A town is discharging 38 MLD wastewater having COD as 585 mg/L in a river with a flow of 21 m³/s. The COD of the river water was found negligible (zero) in the upstream. Assume complete mixing of wastewater in the river water at the point of discharge, what would be the COD of the river water in the immediate downstream after receiving the wastewater discharge?
a) 124 mg/L
b) 76.4 mg/L
c) 39.1 mg/L
d) 12 mg/L
Question 10. The 5-day BOD of wastewater, at 20°C incubation temperature, would be (in mg/L)
Question 11. The BOD reaction rate constant k (at 27°C, and base e) would be (in day⁻¹)
These are Wastewater Treatment and Recycling Nptel Week 4 Answers
Question 12. The 3-day BOD of this wastewater, at 27°C incubation temperature, would be (in mg/L):
Question 13. If the pollutant is conservative in nature, the steady state concentration of the pollutant in the outflowing stream would be (in mg/L):
Question 14. If the pollutant is non-conservative in nature, and the pollutant in the lake is reduced (degraded) following first-order kinetics with k = 0.24 h⁻¹ the steady state concentration of the pollutant in the outflowing stream would be (in mg/L):
Question 15. Considering the pollutant is non-conservative (getting degraded following first-order kinetics with k = 0.24 h⁻¹) the steady state concentration of the pollutant in the lake water would be (in mg/L):
These are Wastewater Treatment and Recycling Nptel Week 4 Answers