Some hydrogen sulfide gas diffuses into the headspace environment above the wastewater.  Moisture evaporated from warm sewage may condense on unsubmerged walls of sewers, and is likely to hang in partially formed droplets from the horizontal crown of the sewer.  As a portion of the hydrogen sulfide gas and oxygen gas from the air above the sewage dissolves into these stationary droplets, they become a habitat for sulfur oxidizing bacteria (SOB), of the genus Acidithiobacillus.  Colonies of these aerobic bacteria metabolize the hydrogen sulfide gas to sulfuric acid.