Which chemical is used for pH adjustment when the raw water pH is 6.2?

• A. Sodium hypochlorite
• B. Lime
• C. Sodium hydroxide
• D. Carbon dioxide

Answer: (A)

Raising the pH of acidic raw water is done by adding an alkaline chemical that provides hydroxide and increases buffering (alkalinity). Lime, which is calcium hydroxide, is commonly used for this purpose because it raises pH and boosts alkalinity, helping downstream processes such as coagulation and sedimentation work more effectively. Carbon dioxide would lower pH by forming carbonic acid, so it’s not used to increase pH. Sodium hypochlorite is a disinfectant and is not chosen for pH control, though it can slightly affect pH; it’s not the appropriate choice for adjusting pH. Sodium hydroxide can raise pH as well, but lime is the typical bulk pH adjuster in many treatment plants due to its cost and added buffering.