Work out lime dose for softening instantly with clear inputs, formula shown and shareable results.
Lime softening removes carbonate hardness by precipitating calcium carbonate. Lime is consumed by free carbon dioxide first, at 74/44 by mass, then by the carbonate hardness at 74/100, with an excess of about 10% to drive the reaction. Every mg/L of hardness removed produces roughly 2 mg/L of sludge.
Lime requirement
Lime dose = CO2 x 74/44 + carbonate hardness x 74/100, plus excess
Sludge
Sludge (kg/day) = flow x hardness removed x 2 / 1000
Quicklime and hydrated lime are caustic and generate heat on slaking. Handle with appropriate protection, and manage softening sludge disposal under permit.
Both the calcium in the raw water and the calcium added as lime end up in the precipitate, so softening sludge volumes are large and disposal is often the dominant operating cost.
Lime alone cannot remove it. Soda ash must be added to precipitate calcium associated with sulphate and chloride, at further chemical and sludge cost.