# NPTEL Wastewater Treatment and Recycling Week 4 Assignment Answers 2024

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## NPTEL Wastewater Treatment and Recycling Week 4 Assignment Answers 2024

1. Identify the correct statements regarding natural purification (or Self-Cleansing) in rivers:
Statement I: Natural purification process only happens in day time, when organic
matters present in wastewater are photo-degraded.
Statement II: Natural purification process in rivers are able to remove only decomposable organic waste.

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

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2.

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3. DO Deficit in a river water means:

a. the difference between the saturated DO and the minimum DO
b. the difference between the saturated DO and the actual DO
c. the difference between the actual DO and the minimum DO
d. the difference between the actual DO and the DO required for sustaining fish population

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4. In the septic zone of a river receiving wastewater discharge:

a. BOD is zero
b. BOD is maximum
c. BOD is minimum
d. BOD is neither minimum nor maximum

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5. In a river receiving wastewater discharge, the Streeter Phelps Equation may be uses to determine:

a. How DO vary with the downstream distance
b. At which river stretch, DO would be minimum
c. What is the minimum value of DO likely to be obtained at the downstream
d. All of the above

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6. DO changes due to de-oxygenation is proportional to the:

a. Organic matter present
b. DO Deficit
c. Both, organic matter present and DO deficit
d. Neither organic matter present nor DO defici

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7. In the generic mass balance equation given as V.dC/dt = Q. Cin – Q. Cout – V.(dC/dt)reaction (in
usual notations), the last term in right hand side, V.(dC/dt)reaction, are considered as zero, if:

a. The contaminant is conservative
b. The system has attained steady state
c. The reactor is operating in batch mode
d. The reactor is a plug flow type

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8. If a conservative contaminant is released at inlet concentration Cin and discharge Q, in a completely mixed reactor of volume V, the steady state concentration of the contaminant in the outlet would be:

a. Zero
b. Cin
c. Cin / [1+V/Q]
d. Cin [I-exp{-(Q/V)t}]

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9. Rate of change of concentration of a pollutant is found to be directly proportional to the concentration of the pollutant (i.e. dC/dt a C). The type of the reaction kinetics would be:

a. Cth order
b. Second order
c. First order
d. Zero order

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10. In a plug-flow reactor operating under steady state:

a. Pollutant concentration may be different at different parts (sections) inside the reactor, however concentration does not vary with time at any particular section
b. Concentration inside the reactor is same everywhere, however, concentration may change with time at any particular section
c. Concentration may be different at different parts (sections) inside the reactor. and concentration may also change with time at any particular section
d. Concentration inside the reactor is same everywhere, at all times

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11. A batch reactor is processing 2 kL wastewater containing 20 mg/L methylene blue dye.
which undergo first order decay at rate constant of 0.25/hour. What should be the retention of
the batch if treated water should have dye concentration less than 1 mg/L.

a. 2 hours
b. 6 hours
c. 9 hours
d. 12 hours

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12. A 500 m3 CSTR receives wastewater from a single inlet at a 50 m3/h flow containing 600 mg/L COD. What would be the approximate steady state COD in the outlet, assuming COD decay constant as k=0.36 h.:

a. 25 mg L
b. 78 mg/L
c. 130 mg/L
d. 245 mg/L

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13. The ultimate BOD of the river water, just downstream of the wastewater discharge point
(assuming complete mixing), would be (in mg/L)

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14. The 5-day BOD of the river water, just downstream of the wastewater discharge point, at 20°C
incubation temperature, is (in mg/L)

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15. The 5-day BOD of the river water, sampled from 100 km downstream of the wastewater
discharge point, at 20°C incubation temperature, is (in mg/L):

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