Hydrogen Sulfide

Hydrogen sulfide gas, (H₂S), is a major source of odour in wastewater systems. H₂S gas has a characteristic 'rotten egg' smell. This gas results from the reduction of sulfate (SO₄^-2) to hydrogen sulfide (H₂S), by anaerobic bacteria:
SO₄^-2 + 2C + 2H₂O → 2HCO₃^- H₂S

It is a highly corrosive and toxic gas, and it is soluble in wastewater.

At a pH = ~9, around 99% of the dissolved sulfide occurs as HS^-, which is non-odorous. This means if the wastewater is maintained at around pH=9, odorous H₂S gas is not released. While at pH>9 odorous ammonia (NH₃) is released.

External Links & References

1. US EPA Design Manual: Odor and Corrosion Control in Sewerage Systems and Treatment Plants