How is magnesium noncarbonate hardness removed?

• A. with lime
• B. with soda ash
• C. with lime then soda ash
• D. with soda ash then lime

Answer: (C)

Magnesium noncarbonate hardness is removed best by a two-step lime-soda ash softening sequence. First, lime is added to raise the pH and precipitate magnesium as magnesium hydroxide (Mg(OH)2). This takes advantage of magnesium’s tendency to form insoluble hydroxide at higher pH, so magnesium is effectively removed from the water. After most of the magnesium has been precipitated, soda ash is added. The carbonate ions from soda ash react with any remaining calcium to form insoluble calcium carbonate (CaCO3), which then precipitates out. This sequence targets the two components of noncarbonate hardness in a logical order: remove magnesium first, then remove calcium.

If soda ash were added first, calcium would begin to precipitate as CaCO3, which can consume carbonate and alkalinity and hinder later removal of magnesium. Lime alone may remove some hardness but won’t efficiently eliminate all noncarbonate hardness. Using the lime-first, then soda-ash approach ensures both ions are removed effectively, giving the best overall reduction of noncarbonate hardness.