Which gas is NOT typically reduced by aeration in water treatment?

In water treatment processes, aeration is primarily used to remove gases that can be problematic in water supplies, such as hydrogen sulfide and methane, as these gases can impart unpleasant odors and taste to the water. The purpose of aeration is to enhance the transfer of gases from the water into the atmosphere, effectively reducing their concentration in the treated water.

Oxygen is a gas that is often added to water during treatment processes to support aerobic biological processes and improve conditions for beneficial bacteria that can help treat contaminants. Instead of being reduced, the oxygen level is typically increased during aeration to promote aerobic conditions in the water.

In contrast, carbon dioxide is sometimes reduced during aeration, although it's not the primary focus; it can be released from the water as the aeration process encourages gas exchange. However, its removal is not as crucial as that of hydrogen sulfide or methane.

So, the rationale for stating that oxygen is not typically reduced by aeration lies in its role in enhancing the water treatment process, contrasting with the other gases which are primarily targeted for removal.