Typical gases in sewers: Which of the following gases can be evolved within a sewer system due to biochemical activity?

Introduction / Context:
Sewers create environments conducive to both aerobic and anaerobic microbial processes. Gas evolution affects corrosion, odor, safety, and ventilation design (e.g., risk of explosion with methane and toxicity of hydrogen sulfide).

Given Data / Assumptions:

• Presence of organic matter, biofilms, slime layers, and variable oxygen conditions.
• Typical municipal sewage composition.

Concept / Approach:
Under anaerobic niches, methanogens produce methane; sulfate-reducing bacteria generate hydrogen sulfide; aerobic and anaerobic processes yield carbon dioxide. Identifying all likely gases helps assess hazards and corrosion potential (e.g., H2S oxidizing to sulfuric acid in moist headspace, attacking concrete).

Step-by-Step Solution:
List likely gases: CO2 (ubiquitous), H2S (from sulfate reduction), CH4 (from methanogenesis).Confirm that all three are plausible in sewer headspace.Select inclusive option recognizing multiple gas species.

Verification / Alternative check:
Empirical measurements in sewer headspaces commonly report variable CH4, H2S, and CO2 concentrations, confirming the selection.

Why Other Options Are Wrong:
Single-gas answers omit other well-known sewer gases.Oxygen alone is not “evolved” by sewage; it is consumed.

Common Pitfalls:

• Underestimating H2S dangers; even low ppm can be hazardous.
• Ignoring explosion risks where CH4 accumulates.

Final Answer:
All of these