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
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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.
Lime Dosing in Water Treatment
Lime — in its two common forms quicklime (CaO) and hydrated lime (Ca(OH)₂) — is a workhorse chemical in water and wastewater treatment, used principally for pH adjustment, alkalinity correction, water softening (lime-soda process), and as a coagulation aid alongside alum or ferric chloride. It is cheap and locally available in most countries (ground from limestone), but handling and feeding it is more demanding than liquid coagulants because the dry powder is hygroscopic, the slaked slurry deposits, and the high pH of lime solutions (> 12) is corrosive to skin and aluminium components.
Quicklime vs Hydrated Lime: Which to Specify
Quicklime (CaO, calcium oxide) is the more concentrated form: each kg of CaO neutralises 1.32 kg of equivalent acidity (as CaCO₃). It must be slaked on site — mixed with water in a controlled exothermic reaction CaO + H₂O → Ca(OH)₂ — before it can be dosed as a slurry. Slaking generates substantial heat and requires dedicated slakers and grit separators; quicklime is therefore typical for medium and large plants (> 50 ML/day) where the capital cost of slaking equipment is justified by the lower per-kg chemical cost.
Hydrated lime (Ca(OH)₂, calcium hydroxide) is pre-slaked, ready to use as a slurry, and avoids the slaking infrastructure but costs more per kg of equivalent neutralising capacity (~0.74 kg CaCO₃ equivalent per kg of Ca(OH)₂). Smaller plants and intermittent dosing applications generally use hydrated lime.
The Dosing Equation
Lime feed rate follows the master dosing formula: Feed rate (kg/day) = Dose (mg/L) × Flow (ML/day) × (100 / purity %). Quicklime purity is typically 90–95% CaO; hydrated lime is 92–96% Ca(OH)₂. So a 30 ML/day plant raising pH with a 25 mg/L dose of 92% hydrated lime needs: 25 × 30 × (100/92) = 815 kg/day. As a 10% slurry that is 8,150 L/day → ~340 L/h pump throughput. The calculator above performs the conversion, including slurry concentration and pump sizing.
Common Applications and Typical Doses
Coagulation pH correction (alum-dosed plants): 5–15 mg/L lime to compensate for alum’s acidifying effect, holding coagulation pH in the 6.0–6.5 sweet spot. Final pH stabilisation (Langelier Saturation Index): 5–20 mg/L to push treated water towards LSI = 0 (calcium-carbonate equilibrium) so the distribution-system pipes neither corrode (LSI < 0) nor scale (LSI > 0). Lime-soda softening: 1.5 × (calcium hardness as CaCO₃ / 100) kg of CaO per m³ for the lime portion (typically 100–300 mg/L as CaO for hard groundwater); soda ash supplements for non-carbonate hardness. Phosphorus removal in tertiary wastewater treatment: lime to pH 11–11.5 to precipitate hydroxyapatite Ca₅(PO₄)₃OH; typically 200–400 mg/L lime depending on inlet phosphorus.
Worked Example: Plant Sizing
A 60 ML/day plant treats moderately hard surface water and uses lime to raise the alum-coagulated water back to pH 7.2 for distribution. Average dose 18 mg/L, hydrated lime purity 94%: feed rate = 18 × 60 × (100/94) = 1,150 kg/day. As a 10% slurry: 11,500 L/day = 480 L/h. Specify dosing pumps for 600 L/h with stroke control. Bulk silo for 30-day storage of dry hydrated lime: 1,150 × 30 × 1.1 (reserve) = 38 tonnes; at bulk density 600 kg/m³, silo volume ~ 65 m³ (sized at 80 m³ for headroom). The calculator drives this calculation directly.
Practical Pitfalls
Lime slurry settles fast — design dosing lines with no horizontal runs and continuous-recirculation loops to prevent blockage. Use food-grade rubber, polyethylene, or stainless steel for piping (avoid aluminium and copper, which corrode at high pH). Locate the slurry-feed point at a turbulent zone for rapid dispersion. After lime addition, a contact time of 5–30 minutes ensures complete reaction before downstream processes. Always wear PPE (gloves, eye protection, dust mask) when handling dry lime — dust contact with moist skin or eyes causes burns due to the exothermic slaking reaction.
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