Which chemical is used to raise alkalinity in water treatment?

• A. Calcium chloride
• B. Soda ash
• C. Lime
• D. Calcium hydroxide

Answer: (C)

Raising alkalinity means increasing the water’s buffering capacity to neutralize added acids, which mainly comes from bicarbonate, carbonate, and hydroxide in the water. To achieve this, you add a reagent that supplies hydroxide or carbonate species. Lime, which is calcium hydroxide, fits this role well. When lime is added, it releases hydroxide ions, pushing the carbonate system toward higher buffering forms and thereby increasing total alkalinity. It also helps raise pH and provides calcium for related treatment benefits like precipitation of hardness as calcium carbonate. While other additives like soda ash can raise alkalinity by introducing carbonate, lime is the standard choice in many treatment setups because of its buffering effect plus the added softening benefits. Calcium chloride, on the other hand, does not raise alkalinity.