In sanitary sewer systems, which gases are most commonly encountered during inspection and operation? Select the best comprehensive choice that reflects typical sewer atmospheres (toxicity and explosibility considerations apply).

Introduction / Context:
Safe sewer maintenance requires a clear understanding of typical sewer gases. Common constituents like hydrogen sulphide, methane, and carbon dioxide affect worker safety (toxicity, asphyxiation, and explosion risks) and corrosion of infrastructure. This question tests practical environmental engineering knowledge used in confined-space entry procedures.

Given Data / Assumptions:

• Sewer conveys domestic wastewater under gravity with limited ventilation.
• Biochemical activity is ongoing (anaerobic zones are common).
• No unusual industrial discharge is assumed.

Concept / Approach:

Sewer atmospheres typically contain: hydrogen sulphide (H2S) from anaerobic sulphate reduction; methane (CH4) from anaerobic digestion; and carbon dioxide (CO2) from biodegradation. H2S is highly toxic and corrosive, CH4 is flammable/explosive, and CO2 displaces oxygen causing asphyxiation. Confined-space protocols therefore require gas monitoring for O2, H2S, CO, and combustible gases before entry.

Step-by-Step Solution:

Verification / Alternative check:

Standard sewer safety training and manuals list these gases as primary hazards; many municipalities require meters to alarm at H2S thresholds and lower explosive limits (LEL) for CH4, with oxygen deficiency checks due to CO2.

Why Other Options Are Wrong:

(b), (c), and (d) each list a single gas and ignore the others that commonly co-exist. (e) Petrol vapours are not typically present in sanitary sewers and are not considered a defining sewer gas.

Common Pitfalls:

Assuming oxygen levels are always safe; overlooking the simultaneous risks of toxicity and explosion; neglecting corrosion effects of H2S on concrete and metals.

Final Answer:

Hydrogen sulphide (H2S), methane (CH4), and carbon dioxide (CO2)