Which of the following is most closely related to the odour of sewage under anaerobic conditions?

Question

Which of the following is most closely related to the odour of sewage under anaerobic conditions?

1. A. Oxygen
2. B. Hydrogen sulfide (H2S) (Correct answer)
3. C. Carbon dioxide (CO2)
4. D. Methane (CH4)

Correct Answer

Option B — Hydrogen sulfide (H2S)

Detailed Solution & Explanation

The correct answer is Hydrogen sulfide ($\text{H}_2\text{S}$).

Key Points

• Under anaerobic conditions, sulphate-reducing bacteria reduce sulphate in sewage to hydrogen sulphide, the gas responsible for the characteristic rotten-egg smell of stale or septic sewage.
• The alternatives do not explain the odour: oxygen is absent by definition in anaerobic conditions, and both carbon dioxide and methane are odourless, even though methane is the major product of anaerobic digestion.

Additional Information

• $\text{H}_2\text{S}$ is more than a nuisance. It causes crown corrosion of concrete sewers: the gas escapes into the sewer atmosphere, *Thiobacillus* bacteria on the moist pipe crown oxidise it to sulphuric acid, and the acid eats the concrete from above the waterline downward.
• It is also acutely toxic. At low concentrations it smells strongly, but at high concentrations it paralyses the sense of smell, which is why sewer workers have died believing the gas had cleared. Confined-space entry procedures exist for precisely this reason.
• Prevention is hydraulic before it is chemical: maintaining self-cleansing velocity so solids do not deposit and septicise, adequate ventilation, and where necessary dosing with oxidants or nitrate.
• Anaerobic digester gas is typically 65-75% methane and 25-35% carbon dioxide, with traces of $\text{H}_2\text{S}$ that must be scrubbed before the gas is burned in an engine.