Hydrogen Sulfide (H2S)
The most common and most regulated odorant at wastewater plants — a rotten egg smell detectable at very low concentrations.
Hydrogen sulfide (H2S) is the primary odorant associated with wastewater; it's detectable by humans at concentrations as low as 0.0005 ppm, well below any health threshold.
H2S forms in collection systems and at plant headworks whenever septic conditions occur — sulfates are reduced to sulfide under anaerobic conditions, and turbulence releases dissolved H2S as a gas.
H2S is dangerous: exposure above 100 ppm causes olfactory fatigue (you stop smelling it just as it's becoming dangerous); exposure above 700 ppm can be fatal.
Continuous H2S monitoring at headworks, wet wells, and digester areas is essential — both for odor complaint investigation and for confined space entry safety.
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