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?

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