Phosphorus Dosing Calculator

Work out phosphorus dosing instantly with clear inputs, formula shown and shareable results.

About Phosphorus Dosing Calculator

Chemical phosphorus removal precipitates metal phosphate. Stoichiometry is one mole of metal per mole of phosphorus, but a molar ratio of 1.5-2.5 is needed in practice because the metal also hydrolyses and competes with other reactions. Each kilogram of phosphorus removed produces roughly 4 kilograms of extra sludge.

Formulas

Metal dose

Metal (mg/L) = P to remove / 30.97 x molar ratio x metal atomic mass

Product dose

Product (mg/L) = metal dose / metal fraction in the product

Worked example

With the default values below, Phosphorus Dosing Calculator returns Coagulant Required of 579.651. Change any input above and every figure here is recalculated in your browser.

Inputs used

Flow Rateminimum 0 m3/day: 5,000 m3/day
Phosphorus To Removeminimum 0 mg P/L: 5 mg P/L
Metal To Phosphorus Molar Ratiominimum 0: 1.8
Coagulantone of Ferric chloride, Ferrous sulphate, Aluminium sulphate, Polyaluminium chloride: Ferric chloride

Results

Coagulant Required: 579.651
Coagulant Dose: 115.930
Metal Ion Dose: 16.2302
Additional Sludge Produced: 100.000

Step by step

1Values used
2Metal dose
3Product dose
4Coagulant Required= 579.651
5Coagulant Dose= 115.930
6Metal Ion Dose= 16.2302
7Additional Sludge Produced= 100.000

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Metal salt coagulants are corrosive and acidic, consuming alkalinity and depressing pH. Handling, containment and alkalinity control must be designed accordingly.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Metal salt coagulants are corrosive and acidic, consuming alkalinity and depressing pH. Handling, containment and alkalinity control must be designed accordingly.

Frequently Asked Questions

Why is more than stoichiometric dose needed?

Metal ions also form hydroxide precipitates and react with alkalinity, so only part of the dose is available for phosphate. The excess ratio rises steeply as the target residual phosphorus falls below 0.5 mg/L.

Where should the dose be added?

Simultaneous precipitation into the aeration tank is most common; pre-precipitation into primary settlement removes more but wastes carbon; tertiary dosing achieves the lowest residuals but needs filtration.