Hydrogen Sulphide in Sewers - Waste Water Engineering Questions and Answers

This set of Waste Water Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Hydrogen Sulphide in Sewers”.

1. H₂S is intoxicant in nature.
a) True
b) False
View Answer

Answer: b
Explanation: H₂S gas can be toxic even at low concentrations. Under certain conditions H₂S gas which can corrode the internal walls of sewers, manholes, pump stations and other concrete and steel structures by getting converted to sulphuric acid.

2. _______ corrodes internal walls of sewers.
a) Hydrochloric acid
b) Sulphuric acid
c) Sulphur dioxide
d) Carbon dioxide
View Answer

Answer: b
Explanation: Under certain conditions H₂S gas which can corrode the internal walls of sewers, manholes, pump stations and other concrete and steel structures by getting converted to sulphuric acid.

3. H₂S gas is denser than air.
a) True
b) False
View Answer

Answer: a
Explanation: H₂S gas is denser than air so it may sit at the bottom of maintenance structures such as tanks, wells, enclosures, pits, buildings, storage areas etc.

4. H₂S gas is _______ to generate sulphuric acid.
a) Oxidised
b) Reduced
c) Oxidised and reduced
d) Sulphonated
View Answer

Answer: a
Explanation: H₂S gas can be oxidised and converted into sulphuric acid within the sewer headspace on the sewer pipe wall.

5. Hydrogen sulphide is formed under ______ conditions.
a) Aerobic
b) Anaerobic
c) Anaerobic and aerobic
d) Any
View Answer

Answer: b
Explanation: Hydrogen sulphide is formed under anaerobic conditions at low flow velocities and warm temperatures. The rate of release is increased at points of high turbulence and at the outlets of inverted syphons and pressure mains.

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6. What is the flow velocity condition required for the formation of hydrogen sulphide?
a) Low
b) Medium
c) High
d) Rapid
View Answer

Answer: a
Explanation: Low flow velocity condition is required for the formation of hydrogen sulphide. Sulphuric acid can corrode the internal walls of sewers, manholes, pump stations and other concrete and steel structures.

7. What is the temperature condition required for hydrogen sulphide formation?
a) Warm
b) Cold
c) Extreme cold
d) Intermediate
View Answer

Answer: a
Explanation: Warm temperature conditions are required for hydrogen sulphide formation. Hydrogen sulphide is formed under anaerobic conditions at low flow velocities and warm temperatures.

8. The rate of release of H₂S is high at the points of ______
a) Low velocity
b) Low flow
c) Low turbulence
d) High turbulence
View Answer

Answer: d
Explanation: The rate of release of H₂S is high at the points of high turbulence. Warm temperature conditions are required for hydrogen sulphide formation.

9. What is the exposure limit of H₂S in a day?
a) 2 ppm
b) 1 ppm
c) 0.5 ppm
d) 0.1 ppm
View Answer

Answer: d
Explanation: The exposure limit of H₂S in a day is 0.1 ppm. The exposure limit of H₂S is 2 ppm for 30 minutes. The exposure limit of H₂S is 0.014 for 90 days.

10. What is the limit of H₂S exposure for 30 minutes?
a) 0.5 ppm
b) 1 ppm
c) 1.5 ppm
d) 2 ppm
View Answer

Answer: d
Explanation: The exposure limit of H₂S is 2 ppm for 30 minutes. The exposure limit of H₂S in a day is 0.1 ppm. The exposure limit of H₂S is 0.014 for 90 days.

11. What is the odour threshold limit of H₂S gas?
a) 0.0008 ppm
b) 0.008 ppm
c) 0.08 ppm
d) 0.8 ppm
View Answer

Answer: b
Explanation: The odour threshold limit of H₂S gas is 0.008 ppm. 4 ppm causes increased eye problems.

12. What amount of H₂S causes increased eye problems?
a) 1 ppm
b) 2 ppm
c) 3 ppm
d) 4 ppm
View Answer

Answer: d
Explanation: 4 ppm causes increased eye problems. 20 ppm causes neurological effects including memory loss and dizziness.

13. What is the chemical used to prevent H₂S gas formation from Magnesium hydroxide?
a) Calcium nitrate
b) Ferric nitrate
c) Oxygen injection
d) Carbon injection
View Answer

Answer: a
Explanation: Calcium nitrate is the chemical used to prevent H₂S gas formation from Magnesium hydroxide. Ferric chloride is the chemical used to prevent H₂S gas formation from Sodium hydroxide.

14. What is the chemical used to prevent H₂S gas formation from Sodium hydroxide?
a) Ferric chloride
b) Magnesium hydroxide
c) Ferric nitrate
d) Calcium nitrate
View Answer

Answer: c
Explanation: Ferric chloride is the chemical used to prevent H₂S gas formation from Sodium hydroxide. Calcium nitrate is the chemical used to prevent H₂S gas formation from Magnesium hydroxide.

15. What is the chemical used to prevent H₂S gas formation from Ferric chloride?
a) Magnesium hydroxide
b) Sodium hydroxide
c) Oxygen injection
d) Ferric nitrate
View Answer

Answer: c
Explanation: Oxygen injection is done to prevent H₂S gas formation from Ferric chloride. Calcium nitrate is the chemical used to prevent H₂S gas formation from Magnesium hydroxide.

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