What is lime softening in water treatment?

• A. A chemical oxidation using chlorine to remove minerals.
• B. A filtration method using sand.
• C. A biological treatment using plants.
• D. A precipitation process that uses lime to remove hardness by forming insoluble hydroxides.

Answer: (D)

Lime softening is a precipitation-based water treatment step that uses lime (calcium hydroxide) to remove water hardness. When lime is added, the pH rises and calcium and magnesium ions react to form insoluble hydroxides (and, for waters with carbonate alkalinity, calcium carbonate as well). These solid precipitates settle out and can be removed by clarification and filtration, leaving softer water. This is different from disinfection with chlorine, simple sand filtration, or biological treatment with plants, which address other aspects of water treatment.