Alum Dose For Coagulation Calculator

Work out alum dose for coagulation instantly with clear inputs, formula shown and shareable results.

About Alum Dose For Coagulation Calculator

Aluminium sulphate hydrolyses to aluminium hydroxide floc, consuming alkalinity and depressing pH — about 0.5 mg of alkalinity as calcium carbonate per mg of alum. If residual alkalinity falls below about 20 mg/L the coagulation pH drops out of the optimum window and lime or soda ash must be added.

Formulas

Alum dose

Alum (kg/day) = flow (m3/day) x dose (mg/L) / 1000

Alkalinity balance

Residual alkalinity = raw alkalinity - dose x consumption factor

Worked example

With the default values below, Alum Dose For Coagulation Calculator returns Alum Required of 350.000. Change any input above and every figure here is recalculated in your browser.

Inputs used

Plant Flowminimum 0 m3/day: 10,000 m3/day
Alum Doseminimum 0 mg/L: 35 mg/L
Alkalinity Consumption Factorminimum 0 mg CaCO3 per mg alum: 0.5 mg CaCO3 per mg alum
Raw Water Alkalinityminimum 0 mg/L as CaCO3: 60 mg/L as CaCO3

Results

Alum Required: 350.000
Alkalinity Consumed: 17.500
Residual Alkalinity: 42.500
Lime Supplement Required: 0.000

Step by step

1Values used
2Alum dose
3Alkalinity balance
4Alum Required= 350.000
5Alkalinity Consumed= 17.500
6Residual Alkalinity= 42.500
7Lime Supplement Required= 0.000

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 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Residual aluminium in treated water is regulated. Coagulation pH must be controlled and residual aluminium monitored.

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

Residual aluminium in treated water is regulated. Coagulation pH must be controlled and residual aluminium monitored.

Frequently Asked Questions

How is the optimum alum dose found?

By jar testing on the actual raw water across a dose and pH matrix, repeated as raw water quality changes. Dose requirements vary several-fold between clear and turbid seasons.

Why does pH matter for coagulation?

Aluminium hydroxide solubility is minimum around pH 6.0-6.8, where floc forms best and residual dissolved aluminium is lowest. Outside that window both treatment performance and residual aluminium compliance suffer.