Minimum hardness achievable by lime-soda ash process

Minimum hardness achievable by lime-soda ash process

• A. 10-20 mg/L as CaCO3
• B. 300-400 mg/L as CaCO3
• C. 30-40 mg/L as CaCO3
• D. 40-70 mg/L as CaCO3

Answer: (C)

Lime-soda ash softening removes hardness by raising the water’s pH so calcium and magnesium precipitate as calcium carbonate and magnesium hydroxide. In practice, even with careful dosing, a small amount of hardness remains after treatment because of carbonate equilibria and the need to maintain enough alkalinity for subsequent steps. With this method alone, the typical lower limit of residual hardness is about 30–40 mg/L as CaCO3. Pushing below this range generally requires additional treatment steps (like ion exchange or reverse osmosis) or much more chemical use, which isn’t economical. So, the minimum hardness achievable with lime-soda ash alone is around 30–40 mg/L as CaCO3.

Lime-soda ash softening removes hardness by raising the water’s pH so calcium and magnesium precipitate as calcium carbonate and magnesium hydroxide. In practice, even with careful dosing, a small amount of hardness remains after treatment because of carbonate equilibria and the need to maintain enough alkalinity for subsequent steps. With this method alone, the typical lower limit of residual hardness is about 30–40 mg/L as CaCO3. Pushing below this range generally requires additional treatment steps (like ion exchange or reverse osmosis) or much more chemical use, which isn’t economical. So, the minimum hardness achievable with lime-soda ash alone is around 30–40 mg/L as CaCO3.