Fresh domestic sewage entering a wastewater collection system contains proteins including organic sulfur compounds oxidizable to sulfates and may contain inorganic sulfates.  Dissolved oxygen is depleted as bacteria begin to catabolize organic material in sewage.  In the absence of dissolved oxygen and nitrates, sulfates are reduced to hydrogen sulfide as an alternative source of oxygen for catabolizing organic waste by sulfate reducing bacteria (SRB), identified primarily from the obligate anaerobic species Desulfovibrio.
Hydrogen sulfide production depends on various physicochemical, topographic and hydraulic parameters such as:
* Sewage oxygen concentration. The threshold is 0.1 mg.l; above this value, sulfides produced in sludge and sediments are oxidized by oxygen; below this value, sulfides are emitted in the gaseous phase.
* Temperature. The higher the temperature, the faster the kinetics of HS production.
* Sewage pH. It must be included between 5.5 and 9 with an optimum at 7.5-8.
* Sulfate concentration.
* Nutrients concentration, associated to the biochemical oxygen demand.
* Conception of the sewage As HS is formed only in anaerobic conditions. Slow flow and long retention time gives more time to aerobic bacteria to consume all available dissolved oxygen in water, creating anaerobic conditions. The flatter the land, the less slope can be given to the sewer network, and this favors slower flow and more pumping stations (where retention time is generally longer)