Water Treatment Operator-in-Training (OIT) Practice Test

The deposition of solids in a sewer line is likely to generate what gas?

1. Methane

2. Carbon dioxide

3. Hydrogen sulphide

4. Ammonia

The correct answer is: Hydrogen sulphide

The generation of hydrogen sulfide gas in sewer lines is primarily due to the anaerobic decomposition of organic matter. As solids accumulate in the sewer, microbes break it down in an oxygen-deprived environment. This process leads to the production of various gases, among which hydrogen sulfide is a significant byproduct. Hydrogen sulfide is characterized by its distinct rotten egg odor, which indicates its presence, especially in locations where organic material is decomposing. Understanding the conditions that lead to its formation helps in implementing appropriate measures for odor control and safety because hydrogen sulfide is not only malodorous but also toxic in higher concentrations. While methane and carbon dioxide can also be produced during the decomposition of organic materials, they are more commonly associated with the breakdown of less sulfur-rich materials compared to the specific conditions that favor hydrogen sulfide generation. Ammonia may also be present but is generally a result of nitrogenous waste decomposition, which is a different process. Thus, the deposition of solids in a sewer line most directly correlates with the production of hydrogen sulfide gas.