Which gas is most commonly associated with sewage systems?

Hydrogen Sulfide (H2S) is the gas most commonly associated with sewage systems due to its production during the decomposition of organic matter in anaerobic conditions, which are prevalent in sewage environments. When sewage breaks down, particularly in the absence of oxygen, bacteria convert sulfur-containing compounds into hydrogen sulfide. This gas is known for its characteristic rotten egg smell, making it a notable indicator in sewage and wastewater contexts.

While methane is another gas produced during the anaerobic digestion process and can be found in sewage systems, it is less prominently associated due to its odorless nature. Carbon dioxide is generated as a byproduct of various processes but lacks the distinct connection to sewage systems that hydrogen sulfide has. Ammonia, while present, arises more from nitrogenous waste than the decomposition of organic matter that produces hydrogen sulfide. Therefore, in terms of both production and association with the characteristics of sewage, hydrogen sulfide is the most relevant choice.