What gas is most commonly associated with septic wastewater?

Hydrogen sulfide is the gas most commonly associated with septic wastewater due to the anaerobic decomposition of organic matter in environments with low oxygen levels, such as septic tanks. During this process, bacteria break down waste, producing hydrogen sulfide as a byproduct. This gas is notable for its characteristic rotten egg odor and can be toxic at higher concentrations, making it a significant concern for safety in sewage systems.

While methane is also produced during the breakdown of organic materials, it is not as distinctive to septic systems as hydrogen sulfide. Ammonia is frequently present in wastewater but does not have the same association with the decomposition process in septic tanks as hydrogen sulfide does. Nitrogen dioxide is primarily related to combustion processes and is not commonly associated with wastewater, making hydrogen sulfide the most relevant answer in the context of septic wastewater.