What factors contribute to the existence of hydrogen sulfide in a system?

Hydrogen sulfide (H₂S) is a gas that can accumulate in wastewater systems, and its formation is intricately linked to the availability of oxygen within the environment. In aerobic conditions, where oxygen is present, bacteria primarily metabolize organic matter, leading to the production of less harmful byproducts. However, in situations where oxygen is limited or absent (anaerobic conditions), specific bacteria, especially sulfate-reducing bacteria, become active and convert sulfates in the water into hydrogen sulfide.

In environments rich in organic matter and low in oxygen, the anaerobic digestion process, often occurring in sewers or treatment facilities, facilitates the growth of these sulfate-reducing bacteria, thus contributing to increased levels of hydrogen sulfide. When oxygen is present, it acts as a more favorable electron acceptor for microbial metabolism, preventing the conditions that favor the production of hydrogen sulfide.

Therefore, the presence of oxygen plays a critical role in controlling the formation of hydrogen sulfide, which is why it is indicated as a factor that does not contribute to its existence. In contrast, the other choices mainly involve conditions or substances that can exist without directly influencing the gas's production under aerobic circumstances.