For hydrogen sulfide (H2S) production, you must have sulfate in the wastewater. When you combine sulfate and domestic sewage in an anaerobic reducing environment (normally an ORP < -200 mV) - you will give sulfur reducing bacteria an environment in which they thrive. These organisms convert oxidized sulfur species to S= and hydrogen sulfide. A very toxic and odorous compound, H2S is the focus of most odor control programs.
We often ignore the odors caused by volatile organic acids formed by other bacteria when they start decomposition of domestic sewage under anaerobic conditions. In this case various sugars, proteins, and fatty acids are converted into short chain organic acids. In an anaerobic digester with methanogenic bacteria, the short chain volatile fatty acids are converted into methane gas and carbon dioxide. However, normal collection systems and ponds do not support much methanogen activity as they are not "anaerobic enough" ~ reducing environment that is not sufficiently negative on a consistent basis. When the short chain volatile acids enter in zones of turbulence, aeration, or greater surface area, the volatile acids enter the atmosphere which the human nose detects at very low concentrations. Common wastewater organic acid and their associated odors are given below:
- Rotten eggs - mercaptans & H2S
- Sweaty feet, rancid - propionic aicd
- Unpleasant, rancid - butyric acid
- Rancid cheese - isovaleric acid
- Vinegar, sharp - acetic acid
- Ammonical - ammonia, urea