Wastewater often exists under anaerobic conditions in collection systems and in large facultative/anaerobic ponds. When you have organics, bacteria, and an anaerobic/anoxic environment, you can produce both hydrogen sulfide and odorous organic compounds.
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:
Erik Rumbaugh has been involved in biological waste treatment for over 20 years. He has worked with industrial and municipal wastewater facilities to ensure optimal performance of their treatment systems. He is a founder of Aster Bio (www.asterbio.com) specializing in biological waste treatment.
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