Purpose
This calculator estimates the water chemistry effects of common drinking water treatment chemicals.
It tracks pH, alkalinity, calcium, chloride, sulphate and conductivity through sequential dosing stages,
and reports corrosion/scaling indices, chemical consumption, solids production and dosing system
design parameters.
Chemical Basis
Dose units & product strength. You can enter
doses in whatever unit suits you — the dropdown next to each chemical offers mg/L product
(the chemical as supplied), mg/L active, mg/L as metal (Al³⁺ or Fe³⁺) and
ppm v/v for liquids. Whatever you pick, a grey line under the dose shows the same dose in
every unit so the conversion is explicit. Internally everything is reduced to a product dose:
- ppm v/v = product ÷ SG
- mg/L active = product × (% active ÷ 100)
- mg/L as metal = product × metal fraction × (% active ÷ 100)
The chemistry factors (alkalinity, calcium, chloride, sulphate, metal)
are all stored on a 100%-active-compound basis and then scaled to the strength of
the product you actually dose via % active. So a 50% caustic soda consumes half the
alkalinity per mg of product that neat NaOH would, and a 32% hydrochloric acid about a third. Edit
a chemical's % active (or SG) in the Chemical Properties panel to match your supplier's data sheet and every
conversion, demand and storage figure follows automatically.
Each chemical is characterised by its stoichiometric effect on alkalinity (expressed as mg/L CaCO₃
per mg/L of pure active compound), plus any contribution to calcium, chloride, sulphate or
dissolved solids. These factors derive from the reaction stoichiometry of each chemical in water.
Metal coagulants. Hydrolysing aluminium and iron(III) salts consume alkalinity as
they form hydroxide floc: M³⁺ + 3HCO₃⁻ → M(OH)₃ + 3CO₂ + 3H₂O. Each mole of metal consumes three
equivalents of bicarbonate alkalinity, i.e. 5.55 mg CaCO₃ per mg Al and
2.69 mg CaCO₃ per mg Fe. The product-basis alkalinity demand is this figure times
the metal content of the product. For aluminium sulphate the demand and sulphate release follow the
hydrate (Al₂(SO₄)₃·18H₂O → 0.45 mg CaCO₃ and 0.43 mg SO₄ per mg; the 14H₂O grade is more concentrated
at 0.51 and 0.48). Pre-hydrolysed coagulants (PACl, ACH) are partly neutralised at manufacture
(basicity B), so they consume only (1−B) of the theoretical demand — this is why aluminium
chlorohydrate (basicity ≈ 83%) consumes the least alkalinity, then PACl (≈ 55%), then alum (0%):
alum > PACl > ACH. Because PACl/ACH basicity and metal content vary between products, adjust
the % active and verify against your supplier's data sheet and jar testing.
Acids. Alkalinity demand is the strong-acid equivalent per mg of pure reactant:
hydrochloric 1 eq/mol (1.37 mg CaCO₃/mg), sulphuric 2 eq/mol (1.02). Orthophosphoric acid is
triprotic (pKa₁ 2.15, pKa₂ 7.20, pKa₃ 12.35); at treated-water pH (~7.5) it behaves as roughly a
1.75-equivalent acid between the H₂PO₄⁻ and HPO₄²⁻ forms (0.89 mg CaCO₃ per mg pure H₃PO₄),
approaching 1 eq/mol below pH 7 and 2 eq/mol above pH 8.5 — set the value to suit your dosing pH.
pH. pH is computed from a carbonate equilibrium model. The total dissolved
inorganic carbon (CT) is first derived from the source water's pH and alkalinity. Each
chemical then changes alkalinity by its stoichiometric factor (and, for CO₂ dosing or stripping,
changes CT directly while leaving alkalinity unchanged). The resulting pH is solved from
the carbonate system — the distribution between carbonic acid, bicarbonate and carbonate, plus the
water equilibrium — using temperature-corrected equilibrium constants (Harned & Davis for K₁,
Harned & Scholes for K₂, Harned & Owen for Kw), as compiled in standard water
chemistry texts (Stumm & Morgan, Aquatic Chemistry; Benjamin, Water Chemistry).
Activity corrections for ionic strength are not applied, so accuracy is best at low to moderate
ionic strength; the model reproduces conventional jar-test lime/caustic titration behaviour across
the full dosing range.
Saturation & Corrosion Indices
The Langelier Saturation Index is computed from the classic empirical formulation of saturation pH
(pHs) using temperature, dissolved solids, calcium and alkalinity terms (Langelier 1936, and as
presented in Standard Methods 2330). The solubility products of the common calcium carbonate
polymorphs (calcite, aragonite, vaterite) are user-adjustable; selecting a more soluble polymorph
lowers pHs and shifts the index accordingly (Plummer & Busenberg 1982 report the underlying
solubility data).
Calcium carbonate precipitation potential (CCPP) is estimated with empirical correlations fitted to
carbonate equilibrium behaviour across typical drinking water conditions. Rigorous CCPP calculation
requires an iterative carbonate mass-balance solution (as described in Standard Methods 2330); the
correlation used here approximates that result and is most reliable within the calibrated range
(pH 6.5–8.5, alkalinity 10–150 mg/L, calcium 10–75 mg/L as CaCO₃).
Chlorine
Inorganic chlorine demand is computed stoichiometrically: ammonia-N exerts approximately 10:1
(breakpoint chlorination), soluble iron 0.62:1 and soluble manganese 1.29:1 (mass basis, from the
oxidation half-reactions). Organic demand is site-specific and user-entered. Chlorine gas consumes
alkalinity (~1.4 mg as CaCO₃ per mg Cl₂) while hypochlorite adds a small amount; both effects are
carried through the pH model.
Solids Production
Sludge production combines coagulant hydroxide precipitate (stoichiometric from metal dose),
turbidity-derived suspended solids and removed natural organic matter, following widely published
mass-balance approaches for water treatment residuals.
Limitations
- The pH model solves the carbonate equilibrium with temperature-corrected constants but without activity (ionic-strength) corrections, so pH accuracy is best at low to moderate conductivity; it does not account for non-carbonate buffers (phosphate, silicate, borate, organic acids) where these are significant.
- Ionic strength effects are represented only through the dissolved-solids term in pHs; activity corrections are not applied to individual species.
- CCPP is a correlation, not an iterative carbonate mass balance — treat values outside the calibrated range as indicative only.
- Indices predict thermodynamic tendency, not kinetics: water with positive CCPP may not actually deposit scale, and corrosion of real pipe materials depends on many factors beyond carbonate chemistry.
- Complete mixing and full reaction of each chemical is assumed; no allowance for reaction kinetics, short-circuiting, or temperature effects on reaction completeness.
- Disinfection by-product formation, coagulation performance (charge neutralisation/floc strength) and pathogen inactivation are out of scope.
- Results are estimates for comparative and preliminary design purposes. Verify against jar testing, plant trials and laboratory analysis before committing to design or operational changes.
References (public literature)
- Langelier, W.F. (1936). The analytical control of anti-corrosion water treatment. JAWWA 28(10).
- Standard Methods for the Examination of Water and Wastewater, Method 2330 (CaCO₃ saturation).
- Plummer, L.N. & Busenberg, E. (1982). The solubilities of calcite, aragonite and vaterite. Geochim. Cosmochim. Acta 46.
- Stumm, W. & Morgan, J.J. Aquatic Chemistry, 3rd ed. Wiley.
- Benjamin, M.M. Water Chemistry, 2nd ed. Waveland.
- Harned, H.S. & Davis, R. (1943); Harned, H.S. & Scholes, S.R. (1941); Harned & Owen — temperature dependence of the carbonic acid (K₁, K₂) and water (Kw) equilibrium constants.
- Crittenden, J.C. et al. MWH's Water Treatment: Principles and Design, 3rd ed. Wiley — coagulant hydrolysis stoichiometry and alkalinity demand.
- Edzwald, J.K. (ed.) Water Quality & Treatment, 6th ed. AWWA/McGraw-Hill — coagulation, prehydrolysed coagulant basicity.
- Larson, T.E. & Skold, R.V. (1958). Laboratory studies relating mineral quality of water to corrosion of steel and cast iron. Corrosion 14(6).
- White's Handbook of Chlorination and Alternative Disinfectants, 5th ed. Wiley.