What is the gas most commonly found in fresh wastewater?

The gas most commonly found in fresh wastewater is primarily carbon dioxide. In the context of wastewater, fresh wastewater is generally considered to be that which has not yet undergone significant decomposition or interaction with anaerobic bacteria. In the early stages of wastewater treatment, the biological processes that occur lead to the production of carbon dioxide as organic materials are broken down.

While oxygen is present in fresh wastewater due to its mixing with air and potential aeration, it is not the predominant gas, especially as biological activity increases, which tends to promote the generation of carbon dioxide. Anaerobic decomposition processes, which occur after the initial phase, can lead to the production of gases such as hydrogen sulfide and methane, but these are not relevant to fresh wastewater.

Thus, in fresh wastewater, the generation of carbon dioxide due to aerobic biological activity is well-recognized, making it the most common gas present in that state.