Clarifier Surface Loading Calculator

Work out clarifier surface loading instantly with clear inputs, formula shown and shareable results.

About Clarifier Surface Loading Calculator

A secondary clarifier must satisfy three criteria: surface overflow rate for clarification, typically 16-32 m3/m2/day; solids loading rate for thickening, generally under 6 kg/m2/hour; and weir loading rate to avoid drawing solids over the weir. Solids loading uses the combined influent and return sludge flow.

Formulas

Surface overflow rate

SOR = influent flow / clarifier area

Solids loading rate

SLR = (influent + return sludge) x MLSS / (area x 1000)

Worked example

With the default values below, Clarifier Surface Loading Calculator returns Surface Overflow Rate of 26.526. Change any input above and every figure here is recalculated in your browser.

Inputs used

Influent Flowminimum 0 m3/day: 12,000 m3/day
Clarifier Diameterminimum 0 m: 24 m
Return Sludge Flowminimum 0 m3/day: 6,000 m3/day
Mixed Liquor Suspended Solidsminimum 0 mg/L: 3,500 mg/L

Results

Surface Overflow Rate: 26.526
Solids Loading Rate: 139.261
Clarifier Surface Area: 452.39
Weir Loading Rate: 159.155

Step by step

1Values used
2Surface overflow rate
3Solids loading rate
4Surface Overflow Rate= 26.526
5Solids Loading Rate= 139.261
6Clarifier Surface Area= 452.39
7Weir Loading Rate= 159.155

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.

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

Frequently Asked Questions

Which criterion usually governs?

Solids loading in activated sludge plants with high MLSS, and surface overflow rate in water treatment or low-MLSS systems. Both must be checked at peak flow with the design return rate.

What causes clarifier failure?

Bulking sludge with poor settleability, hydraulic overload, density currents from temperature or solids gradients, and inadequate sludge withdrawal causing a rising blanket.