Which substance is primarily used to increase alkalinity?

Soda ash, also known as sodium carbonate, is primarily used to increase alkalinity in water treatment processes. Alkalinity is a measure of the capacity of water to resist changes in pH, which is essential for maintaining stable water chemistry and protecting against fluctuations that can harm aquatic life and disrupt treatment processes.

Soda ash raises the bicarbonate concentration in the water, enhancing its buffering capacity. This adjustment is particularly important in processes such as lime softening and for adjusting the pH of water in various treatment settings.

The other substances listed serve different purposes: calcium chloride is primarily a source of calcium ions and is often used for water softening and to increase the hardness of water; aluminum sulfate is commonly used as a coagulant in water purification processes; and chlorine is primarily used for disinfection, not for adjusting alkalinity or pH levels.