Wastewater engineering — choose the correct comprehensive statement: Which of the following statements about dissolved oxygen (DO), hydrogen sulphide (H₂S), and testing methods for sewage is correct?
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AHydrogen sulphide gas in excess may cause corrosion of concrete sewers
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BAbout 4 ppm of dissolved oxygen is ensured before discharging treated sewage into a river
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CSolubility of oxygen in sewage is approximately 95% of that in distilled water
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DDissolved oxygen in sewage can be determined by Winkler’s method
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EAll the above
Answer
Correct Answer: All the above
Explanation
Introduction / Context:Sewerage design and wastewater treatment rely on understanding dissolved oxygen (DO) behavior, corrosion risks from hydrogen sulphide (H₂S), and standard laboratory testing methods. This question integrates these fundamental concepts used by civil and environmental engineers.
Given Data / Assumptions:
- Treated effluent is to be released to a natural water body.
- Concrete sewers may experience biochemical and chemical environments that produce H₂S.
- Standard wet-chemistry methods are applicable for DO determination.
Concept / Approach:
- H₂S corrosion: Sulphide generation in septic sewage leads to H₂S gas in the crown of concrete sewers; bacterial oxidation to sulphuric acid causes crown corrosion.
- Minimum DO: Maintaining around 4 mg/L DO in receiving waters is a common conservative target to protect aquatic life.
- Oxygen solubility: Sewage contains dissolved solids; its oxygen solubility is slightly lower than that of pure water (roughly 95%).
- Winkler’s method: A standard oxidation–reduction titrimetric procedure used to determine DO.
Step-by-Step Solution:Evaluate each statement for correctness based on standard practice and theory.All four statements align with accepted engineering knowledge.Therefore, the comprehensive correct choice is “All the above.”
Verification / Alternative check:These points appear across standard wastewater texts and specifications: sewer corrosion guidelines discuss sulphide control; receiving-water standards require DO safeguards; Winkler’s method is a classic test; and salinity/impurities reduce oxygen solubility relative to distilled water.
Why Other Options Are Wrong:Choosing any single statement omits equally valid others, so only the aggregate option is fully correct.
Common Pitfalls:
- Confusing effluent DO with receiving-water DO objectives.
- Underestimating sewer crown corrosion risk from H₂S in warm climates.
- Assuming oxygen solubility in sewage equals that in pure water exactly.
Final Answer:All the above