Which chemical is used to remove noncarbonate hardness in chemical precipitation softening?

• A. Magnesium Noncarbonate
• B. Hydrated Lime
• C. Calcium Oxide
• D. Sodium Carbonate

Answer: (D)

Noncarbonate hardness is the portion of water hardness that comes from minerals other than bicarbonates, typically calcium and magnesium carbonates and sulfates. To remove this fraction, you need a source of carbonate ions to convert Ca2+ and Mg2+ into insoluble carbonates that precipitate out. Sodium carbonate provides carbonate ions (CO3^2−) in solution, so Ca2+ and Mg2+ form CaCO3 and MgCO3 precipitates that can be settled and removed. That makes sodium carbonate the appropriate chemical for precipitating noncarbonate hardness in this softening process. The other options don’t supply carbonate ions for this purpose or are used for different steps in the treatment.