Lime Dose For Softening Calculator

Work out lime dose for softening instantly with clear inputs, formula shown and shareable results.

About Lime Dose For Softening Calculator

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.

Formulas

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

Worked example

With the default values below, Lime Dose For Softening Calculator returns Commercial Lime Required of 2,298.111. Change any input above and every figure here is recalculated in your browser.

Inputs used

Plant Flowminimum 0 m3/day: 10,000 m3/day
Carbonate Hardnessminimum 0 mg/L as CaCO3: 220 mg/L as CaCO3
Free Carbon Dioxideminimum 0 mg/L: 15 mg/L
Lime Purityaccepts 0–100 %: 90 %

Results

Commercial Lime Required: 2,298.111
Lime Dose: 206.830
Pure Calcium Hydroxide Required: 2,068.300
Sludge Produced: 4,400.000

Step by step

1Values used
2Lime requirement
3Sludge
4Commercial Lime Required= 2,298.111
5Lime Dose= 206.830
6Pure Calcium Hydroxide Required= 2,068.300
7Sludge Produced= 4,400.000

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Quicklime and hydrated lime are caustic and generate heat on slaking. Handle with appropriate protection, and manage softening sludge disposal under permit.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Quicklime and hydrated lime are caustic and generate heat on slaking. Handle with appropriate protection, and manage softening sludge disposal under permit.

Frequently Asked Questions

Why is sludge production so high?

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.

What about non-carbonate hardness?

Lime alone cannot remove it. Soda ash must be added to precipitate calcium associated with sulphate and chloride, at further chemical and sludge cost.