Before you can pace a pump you need the target dose, and for a reacting system that target comes from chemistry, not guesswork. This page sets out how to calculate reagent demand from first principles for the four big dosing reactions — acid/alkali neutralisation, coagulant charge demand, metals precipitation and disinfection — using equivalents, molar ratios and solubility.
The single idea that unifies neutralisation, coagulation and precipitation dosing.
A reagent reacts mole-for-mole (or equivalent-for-equivalent) with the species it targets. Convert the load you must treat into moles or equivalents per unit time, multiply by the reaction's stoichiometric ratio, convert back to mass of reagent, and add a real-world excess factor. Stoichiometry gives the theoretical minimum; jar/bench tests give the practical dose. The gap between them is the over-dose you are paying for — see dosing strategy.
Load = mass rate of the species being treatedMW = molar mass of reagent and targetn = stoichiometric mole ratio (reagent per target)OF = over-dose factor from bench testing (typically 1.1–1.5)Dose set by acidity or alkalinity load, in equivalents.
Neutralisation is an equivalents balance: one equivalent of base neutralises one equivalent of acid. Express the stream's acid (or alkali) load as equivalents per hour and match it.
EW = equivalent weight = MW ÷ number of reactive H⁺/OH⁻pH vs reagent is strongly non-linear near neutral — the same small over-dose that does nothing at pH 4 can swing pH past 10. This is why neutralisation needs the tightest control of any dosing duty; see pH correction and over-dosing troubleshooting.
Dose set by the colloidal charge to be neutralised, confirmed by jar test.
Coagulant dose neutralises the negative surface charge of colloids. It scales with turbidity, colour (organics) and alkalinity, and is most reliably set by jar testing — but a charge-demand estimate gives the starting dose.
a, b, base = site coefficients from jar testing across the seasonal rangeDose set by molar stoichiometry to a solubility minimum.
Hydroxide and sulphide precipitation convert dissolved metal to an insoluble solid. Dose the precipitant to stoichiometry plus the alkali needed to hold the pH at the metal's solubility minimum.
Dose = demand + target residual.
An oxidant is consumed by the water (its “demand”) before any residual remains. Dose to satisfy demand and leave the target residual for the required contact time (CT).
Demand measured by a demand test on representative waterEquivalent and molar weights for the everyday dosing reagents.
| Reagent | Formula | MW (g/mol) | EW (g/eq) | Typical use |
|---|---|---|---|---|
| Sodium hydroxide | NaOH | 40.0 | 40.0 | pH up |
| Hydrated lime | Ca(OH)₂ | 74.1 | 37.0 | pH up, precipitation |
| Sulphuric acid | H₂SO₄ | 98.1 | 49.0 | pH down |
| Hydrochloric acid | HCl | 36.5 | 36.5 | pH down |
| Ferric chloride | FeCl₃ | 162.2 | 54.1 | coagulant |
| Alum | Al₂(SO₄)₃·14H₂O | 594 | 99 | coagulant |
| Sodium hypochlorite | NaOCl | 74.4 | — | disinfection |
Send us your stream analysis — acidity/alkalinity, metals, turbidity/TOC or oxidant demand — and we will calculate the theoretical dose, recommend a bench-test programme and set the over-dose factor.
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