Question 1

Why do we add 35 mg/L excess lime?

Accepted Answer

Magnesium hydroxide precipitation requires pH ≥ 10.6, which takes about 35 mg/L of lime over the stoichiometric demand. Without it, Mg removal is incomplete.

Question 2

When is split treatment used?

Accepted Answer

When raw Mg is high, half the water gets full lime treatment (high pH) and the other half is blended back, then recarbonated. This cuts excess lime and CO₂ consumption.

Question 3

What is the difference between quicklime and hydrated lime?

Accepted Answer

Quicklime is CaO (56 g/mol); hydrated lime is Ca(OH)₂ (74 g/mol). Both supply the same OH⁻ but at different mass: 100 lb of CaO = 132 lb of Ca(OH)₂.

Question 4

What pH is needed for full softening?

Accepted Answer

Ca removal completes at pH ~9.4 (CaCO₃ precipitation). Mg removal requires pH ≥ 10.6 (Mg(OH)₂ precipitation).

Lime-Soda Softening Dose Calculator