What is the gas most commonly found in fresh wastewater?

The gas most commonly found in fresh wastewater is carbon dioxide. In the wastewater treatment process, organic matter is broken down by microorganisms, which leads to the production of carbon dioxide as a byproduct of this decomposition. This process occurs in both aerobic (in the presence of oxygen) and anaerobic (in the absence of oxygen) environments.

While carbon monoxide and hydrogen sulfide can also be present in wastewater, they are not as prevalent in fresh wastewater compared to carbon dioxide. Carbon monoxide typically originates from combustion processes, and in wastewater scenarios, it is not a primary component. Hydrogen sulfide may occur in wastewater, especially as it decomposes and anaerobic conditions are present, but it is more characteristic of older or stagnant wastewater rather than fresh wastewater.

Oxygen, on the other hand, while crucial for aerobic treatment processes, is not a significant gas released in fresh wastewater in the same way carbon dioxide is. Thus, carbon dioxide is the most commonly found gas in fresh wastewater due to the biological processes that occur, making it the correct choice.