Sludge Volume Index Calculator

Work out sludge volume index instantly with clear inputs, formula shown and shareable results.

About Sludge Volume Index Calculator

Sludge volume index is the volume in millilitres occupied by one gram of settled sludge after 30 minutes. Below 100 indicates good settling, 100-150 is acceptable, and above 150 signals filamentous bulking. SVI also caps the MLSS a clarifier can handle, since settled volume cannot exceed the available depth.

Formula

Sludge volume index

SVI (mL/g) = settled volume (mL/L) x 1000 x dilution / MLSS (mg/L)

Worked example

With the default values below, Sludge Volume Index Calculator returns Sludge Volume Index of 91.429. Change any input above and every figure here is recalculated in your browser.

Inputs used

Settled Sludge Volume After 30 Minutesminimum 0 mL/L: 320 mL/L
MLSS Concentrationminimum 0 mg/L: 3,500 mg/L
Dilution Applied To The Testminimum 0: 1
Clarifier Side Water Depthminimum 0 m: 3.5 m

Results

Sludge Volume Index: 91.429
Settling Assessment: Good settling
Maximum MLSS At This SVI: 10,937.5
Indicative Settled Blanket Depth: 1.1200

Step by step

1Values used
2Sludge volume index
3Sludge Volume Index= 91.429
4Settling Assessment= Good settling
5Maximum MLSS At This SVI= 10,937.5
6Indicative Settled Blanket Depth= 1.1200

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.

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

Frequently Asked Questions

What causes bulking?

Filamentous organisms favoured by low dissolved oxygen, low F/M, nutrient deficiency, septic influent or high readily biodegradable substrate. Identification by microscopy points to the specific cause and remedy.

Why dilute the settling test?

At high MLSS the sludge cannot settle freely and the result is meaningless. Diluted SVI or stirred specific volume index gives a comparable figure across concentrations.