What gas is NOT typically found in sewage?

Chlorine is not typically found in sewage under normal circumstances. Chlorine is a chemical element often used as a disinfectant in water treatment processes to kill bacteria and other pathogens. It may be introduced intentionally during sewage treatment to sanitize the water or to prevent the spread of illness, but it is not a natural byproduct of sewage itself.

In contrast, oxygen, hydrogen sulfide, and methane are commonly associated with sewage. Oxygen can be present in varying degrees, especially in less polluted water that supports aerobic bacteria. Hydrogen sulfide, a result of anaerobic digestion, is produced when organic matter breaks down without oxygen, often giving sewage its characteristic odor. Methane is another byproduct of the decomposition of organic material in the absence of oxygen, produced by anaerobic bacteria during digestion processes in sewage.