How to use this calculator
- Choose softening or neutralisation.
- Enter the flow and water analysis values.
- Set lime purity and slurry strength.
- Read the dose in mg/L and kg per day.
How do you calculate lime dosage?
Softening (quicklime as CaO):
CaO = (A + B + C + D) × 1.15 ÷ purity
- A = CO₂ × 56/44
- B = bicarbonate alkalinity removed × 56/100
- C = hydroxide alkalinity in effluent × 56/100
- D = Mg removed × 56/24.3
Neutralisation:
Ca(OH)₂ = acidity (as CaCO₃) × 74/100 ÷ purity
Hydrated lime = CaO × 74/56. The 1.15 factor is 15% excess lime.
Worked example
With purpose: Lime softening (carbonate hardness and magnesium), flow 1,000 m³/day, carbon dioxide 10 mg/L as CO₂, bicarbonate alkalinity removed 200 mg/L as CaCO₃, hydroxide alkalinity left in softened water 0 mg/L as CaCO₃, magnesium removed 10 mg/L as Mg, lime purity 90 %, slurry strength for dosing 10 % w/w:
Hydrated lime Ca(OH)₂ per day: 249.5 kg
The calculator also gives hydrated lime dose 250 mg/L; quicklime cao dose 189 mg/L; quicklime per day 188.8 kg.
Assumptions and limits
- Softening formula from the California State University Sacramento Water Treatment Plant Operation manual.
- Slurry density taken as 1.07 kg/L at 10%.
Frequently asked questions
What is the difference between quicklime and hydrated lime?
Quicklime is calcium oxide (CaO) and must be slaked with water. Hydrated lime is calcium hydroxide (Ca(OH)₂), already slaked and easier to handle. 1 kg of CaO equals about 1.32 kg of Ca(OH)₂.
How much lime is needed to neutralise sulphuric acid?
Each kg of H₂SO₄ needs about 0.76 kg of pure hydrated lime (74/98). Adjust for lime purity and reaction efficiency.
Does lime softening reduce TDS?
Yes, partly. Lime precipitates calcium carbonate and magnesium hydroxide, so carbonate hardness and alkalinity fall along with TDS.
References
- California State University Sacramento, Water Treatment Plant Operation, Vol. 2
- AWWA, Water Quality and Treatment, 6th ed.