Which of the following is the most common factor leading to the generation of hydrogen sulfide in sewers?

The generation of hydrogen sulfide in sewers is predominantly associated with anaerobic conditions. When wastewater flows through sewers, particularly in areas lacking sufficient oxygen, anaerobic bacteria thrive. These bacteria break down organic materials and use sulfates as a food source, which leads to the production of hydrogen sulfide gas as a byproduct. This gas has a characteristic rotten egg smell and can pose both health risks and corrosion issues within sewer systems.

In contrast, high flow velocity does not typically create the necessary conditions for hydrogen sulfide formation because it often helps to maintain oxygen levels in the wastewater, which is detrimental to anaerobic bacteria. Low concentrations of ferrous sulfide wouldn't contribute to hydrogen sulfide generation as ferrous sulfide can actually form in aerobic conditions, and the presence of highly soluble fats and oils, while they may influence other aspects of wastewater treatment, does not directly lead to the generation of hydrogen sulfide like anaerobic conditions do.