Which gas produced in some wastewater environments is known for a strong odor and toxicity?

• A. Oxygen
• B. Nitrogen
• C. Hydrogen sulfide
• D. Carbon dioxide

Answer: (C)

Hydrogen sulfide is the gas produced in some wastewater environments that has a strong, rotten-egg odor and is highly toxic at relatively low concentrations. In anaerobic conditions, where organic matter decomposes without oxygen, sulfate-reducing bacteria convert sulfates into H2S, which can accumulate in confined spaces like sewers, manholes, and digesters. Its distinctive odor is noticeable even at very low levels, but as concentrations rise it becomes a serious health hazard, impairing respiration and causing other toxic effects. Oxygen, nitrogen, and carbon dioxide don’t share that combination of a strong, recognizable odor and significant toxicity at typical wastewater concentrations, so they’re not the gas of concern in this context.