Lime–Soda Ash Softening Dose Calculator
Lime–soda softening doses for operators and certification exam prep: enter CO2, alkalinity, and hardness — get lime and soda ash doses in mg/L and lb/day, with as-CaCO3 conversions built in.
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How the calculation works
Lime neutralizes free CO2, converts bicarbonate (carbonate hardness) to carbonate, and precipitates magnesium as Mg(OH)2. Soda ash removes non-carbonate hardness. The standard textbook formulas (all as CaCO3 unless noted):
Lime dose (mg/L): f × (CO2 + bicarbonate alkalinity + Mg removed) + excess
Soda ash dose (mg/L): 1.06 × non-carbonate hardness removed
Lime factor f: 0.56 for quicklime (CaO), 0.74 for hydrated lime (Ca(OH)2) — the mass ratios 56/100 and 74/100
Excess lime: 35 mg/L as CaO (46.25 mg/L as Ca(OH)2) when magnesium is being removed, driving Mg(OH)2 precipitation to completion; zero otherwise
Feed rates: lb/day = 8.34 × flow (MGD) × dose (mg/L) ÷ (purity ÷ 100)
Notes & limitations
- Unit clarity is everything. CO2 reported as CO2 is converted to as-CaCO3 with the 50/22 factor (2.27); magnesium reported as ion Mg is converted with the 100/24.3 factor (4.12). Entering the wrong basis is the most common mistake in softening calculations.
- Excess lime of 35 mg/L as CaO is applied whenever any magnesium removal is entered, since Mg(OH)2 needs high pH to precipitate completely.
- Purity matters. Commercial lime and soda ash are never 100% pure; feed rates here are divided by the purity fraction so you order and feed the right amount of product.
- Verify doses with jar tests. Confirm the calculated lime and soda ash doses — and the resulting pH and finished hardness — under your plant’s actual conditions before setting chemical feed rates.
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