Hydrogen sulfide (H2S) in wastewater is characterized by what?

• A. Strong chlorine odor
• B. Rotten egg smell, produced under anaerobic conditions
• C. Sweet sulfurous scent under aerobic conditions
• D. No detectable odor

Answer: (B)

Hydrogen sulfide in wastewater is produced when oxygen is absent and sulfate-reducing bacteria use sulfate as an electron acceptor, releasing H2S gas. That gas gives a very distinctive rotten-egg odor, which is detectable at extremely low concentrations and is a hallmark sign of H2S in anaerobic environments like sewers and digester beds. This odor is a practical cue for operators to identify potential sulfide buildup and odor-control needs, since H2S can be corrosive and hazardous at higher levels. The other descriptions don’t fit: a strong chlorine odor points to chlorine or chloramines rather than H2S; a sweet sulfurous scent isn’t characteristic of H2S in wastewater; and saying there’s no detectable odor contradicts the well-known rotten-egg smell of H2S.