An operator is MOST likely to be exposed to which of the following hazardous gases in a plant operation?

• A. Hydrogen Sulfide, Oxygen, Nitrogen
• B. Methane, Sodium Chloride, Nitrate
• C. Ammonium Sulfide, Nitrogen, Oxygen
• D. Chlorine, Hydrogen Sulfide, Methane

Answer: (D)

Wastewater plant operators routinely face gases produced during treatment and digestion processes. Hydrogen sulfide forms from anaerobic decomposition in sewer lines and digesters; it’s highly toxic and heavier than air, so leaks in pits or enclosed spaces pose a serious hazard. Chlorine gas is used for disinfection and can be released from storage or handling systems, presenting a choking and irritant risk to workers. Methane comes from anaerobic digestion as biogas and can accumulate in digester areas or gas collection systems, where it is flammable and can create unsafe concentrations.

This combination—chlorine, hydrogen sulfide, and methane—reflects the gases most likely to be encountered in a plant setting, covering disinfection, sewer gas, and digestion byproducts. Other options include gases or substances not typically present as routine airborne hazards in a plant (solids like sodium chloride or nitrate; inert gases), so they don’t fit the common exposure profile as well.