Many organic compounds have ...

Many organic compounds have traces of sulfites, including foods we eat..  Many bottled wines for example have sulfites added to prevent spoilage. Those sulfites pass through the human digestive track and end up  in the wastewater as sulfates.  As long as the wastewater stays aerated, oxygen rich, they will stay as sulfates.

If the wastewater sits in lift stations or in piping connected to pumped sections of sewers too long, the oxygen will get used up by the aerobic bacteria.  At that point anaerobic bacteria will begin breaking down the sulfates in the organic matter and release hydrogen sulfide.  When pumped into oxygenated water in the your wastewater plant, some of the hydrogen sulfide (H2S) reacts with the oxygen in the water to from SO2.  

The hydrogen sulfide has an even lower odor threshold than the SO2, but both contribute to odors from wastewater plants and the piping and pumping facilities.  Keeping the wastewater from going anaerobic by adding bubblers or chemical oxidizers to the pump stations can help to reduce the odors.