What is the primary method to lower pH in water treatment?

The primary method to lower pH in water treatment involves the use of carbon dioxide. When carbon dioxide is introduced into water, it reacts to form carbonic acid. This process effectively lowers the pH of the water, making it more acidic. This is particularly useful in situations where water has elevated pH levels, and it helps in optimizing conditions for various water treatment processes, especially those involving coagulation and disinfection.

In contrast, calcium hydroxide and lime are generally used to raise pH levels, as they are alkaline substances. Caustic soda, like lime, also increases pH because it is a strong base. Therefore, these options do not serve the function of lowering pH in water treatment. Using carbon dioxide creates a more controlled and effective way of adjusting pH for desired water chemistry outcomes.