What is the role of lime (CaO) or limestone in water treatment or eutrophication management?

• A. Lime raises pH and hardness, neutralizes acidity, precipitates metals, and can help reduce phosphorus in some treatments
• B. Lime lowers pH and reduces hardness
• C. Lime reduces dissolved oxygen
• D. Lime adds nutrients

Answer: (A)

Lime is used to adjust water chemistry in treatment and eutrophication management by raising pH and hardness, neutralizing acidity, precipitating metals, and helping remove phosphorus. When lime (CaO or Ca(OH)2) is added, it releases hydroxide ions and calcium ions. The hydroxide raises the water’s pH (making it more basic) and the calcium adds to water hardness. Higher pH also makes many dissolved metals form insoluble metal hydroxides, which precipitate out and can be removed.

Crucially, the higher pH with calcium ions promotes the precipitation of phosphate as calcium phosphate, which removes phosphorus from the water. Since phosphorus is a key nutrient driving algal blooms, this helps mitigate eutrophication.

The other statements don’t fit: lime does not lower pH or reduce hardness; it doesn’t aim to reduce dissolved oxygen, and it does not add nutrients. In practice, it removes nutrients like phosphorus rather than adding them.