Which process can increase the pH levels in water?

Aeration is a process that can effectively increase the pH levels in water. During aeration, the water is exposed to air, which allows for the exchange of gases. This process helps remove dissolved carbon dioxide from the water, leading to a reduction in carbonic acid levels. Since carbonic acid can lower pH, its removal during aeration results in a higher pH value, making the water less acidic.

In contrast, other processes such as adding chlorine might actually decrease pH levels due to the formation of hydrochloric acid or other acidic by-products. Filtration is primarily used to remove particulate matter and does not significantly impact the pH level. Stirring, while it can enhance mixing and potentially assist in processes like aeration, does not directly influence the pH unless it facilitates the removal of gases.

Thus, aeration is the effective process for increasing pH levels in water due to its role in gas exchange and acid removal.