Calcium Hydroxide / Calcium Oxide (Lime) Dosing Equation
Lime — both hydrated lime Ca(OH)₂ and quicklime CaO — is dosed for pH correction, recarbonation, alkalinity adjustment and lime-softening. Most plants slake quicklime on-site to 10–25% Ca(OH)₂ slurry, then dose by progressing-cavity pump. Equivalent kg/hr of dry hydrate is also a valid metric.
Where:
- Flow = Main flow rate in m³/hr
- Dose_PPM = Target concentration in mg/L or ppm
- Strength_% = Percentage active ingredient of the stock chemical
- SG = Specific Gravity (density relative to water) of the stock
Related dosing calculators
Other chemical-specific dosing calculators in the same series — same formula, different defaults:
- Chemical Dosing Calculator (generic) — the universal seed page
- Caustic Soda Dosing Calculator — pH Raise · Neutralisation
- Alum Dosing Calculator — Coagulation · Jar Test Scale-Up
- Phosphate Dosing Calculator — Boiler · Cooling · Anti-Corrosion
- All Chemical Engineering Calculators →
Frequently Asked Questions
Roughly 5–15 mg/L Ca(OH)₂ per pH unit, depending on starting alkalinity and CO₂. For a typical surface water at 50 mg/L alkalinity, about 10 mg/L Ca(OH)₂ raises pH by 1 unit. The exact dose comes from stoichiometry on free CO₂ and HCO₃⁻ — best validated by titration.
Quicklime is CaO (94%+ purity); hydrated lime is Ca(OH)₂ (90%+ purity). 1 kg CaO + 0.32 kg water → 1.32 kg Ca(OH)₂. Quicklime is cheaper per kg of active CaO but must be slaked on site (heat, dust, safety risk). Hydrated lime is ready to slurry — preferred for plants <50 ML/day.
10% Ca(OH)₂ slurry sits at SG ≈ 1.07 and stays pumpable through standard PD pumps and PVC piping without settling out. Stronger slurries (15–25%) are common with progressing-cavity pumps and aggressive agitation, but risk pipework scaling.
1 mg/L of Ca(OH)₂ adds about 1.35 mg/L as CaCO₃ alkalinity. 1 mg/L of CaO adds 1.79 mg/L as CaCO₃. So 50 mg/L Ca(OH)₂ raises alkalinity by ~67 mg/L — useful when designing remineralisation or post-RO stabilisation.
Yes. Lime stabilisation of sewage sludge typically uses 200–400 kg Ca(OH)₂ per dry tonne of sludge to raise pH to >12 for >2 hours, achieving Class B biosolids. This is a much higher dose than water-treatment pH correction.