Hydrogen sulfide formation in a wastewater collection system is usually caused by which process?

• A. Bacterial reduction of methane in the absence of dissolved oxygen
• B. Bacterial oxidation of sulfur in the presence of dissolved oxygen
• C. Bacterial reduction of methane in the presence of dissolved oxygen
• D. Bacterial reduction of sulfate compounds in the absence of dissolved oxygen

Answer: (D)

Hydrogen sulfide in wastewater collection systems is produced mainly when microbes operate in oxygen-poor conditions and use sulfate as an alternative electron acceptor. Sulfate-reducing bacteria metabolize organic matter and reduce sulfate (SO4^2−) to hydrogen sulfide (H2S) as a byproduct. This anaerobic, sulfate-reducing process is common in pipes where flow is slow and oxygen is depleted, leading to H2S formation.

If oxygen is present, other processes dominate that do not produce hydrogen sulfide in the same way. Oxidation of sulfur compounds in the presence of oxygen tends to convert sulfide back to sulfate, not create H2S. Methane-related processes aren’t responsible for H2S formation in this setting, and methane reduction isn’t a typical sewer mechanism. Thus, the sulfate reduction in the absence of dissolved oxygen best explains why H2S forms in these systems.