What is a common cause of oxygen deficiency in a sewer manhole?

Oxygen deficiency in a sewer manhole is often attributed to bacterial action that consumes oxygen during the decomposition of organic matter. As bacteria break down this organic material, they utilize oxygen in the process, which can lead to a significant reduction in the available oxygen in the environment. In anaerobic conditions, where oxygen levels are very low, bacteria may produce other gases such as methane and hydrogen sulfide, contributing to the hazardous atmosphere in these confined spaces.

In contrast, while chemical reactions can affect gas concentrations, they do not typically lead to the same systematic depletion of oxygen as bacterial action does. Temperature changes can influence the metabolism of bacteria but are not a direct cause of oxygen deficiency. Flooding events may introduce water and other materials into a manhole, potentially impacting gas concentrations and temporary conditions, but again, they do not directly correlate to a consistent cause of oxygen deficiency like bacterial respiration does. Thus, the unique interaction of bacteria with organic materials in sewer systems distinctly highlights why bacterial action is the correct choice in understanding oxygen deficiency in sewer manholes.