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How It Works
The Sludge Volume Index measures how well activated sludge settles in a clarifier. You let a mixed-liquor sample settle for 30 minutes, record the volume it occupies, then divide by the suspended solids concentration. A lower SVI means denser, faster-settling sludge and a healthier treatment process.
The formula is SVI = 1000 × SV / SS, where SV is settling volume (mL/L) and SS is suspended solids (mg/L). Well-settling sludge typically falls between 50–150 mL/g.
Example Problem
A 1-liter sample of mixed liquor settles to 250 mL after 30 minutes. The suspended solids concentration is 2,500 mg/L. What is the SVI?
- SV = 250 mL/L, SS = 2,500 mg/L
- SVI = 1000 × 250 / 2,500 = 100 mL/g
An SVI of 100 mL/g indicates good settling characteristics -- the sludge is neither bulking nor excessively dense.
Frequently Asked Questions
What is a good sludge volume index?
An SVI between 50 and 150 mL/g generally indicates healthy, well-settling sludge. Values above 150 mL/g suggest bulking sludge that settles poorly, while values below 50 mL/g may point to old, over-oxidized sludge.
What causes high SVI in activated sludge?
High SVI is usually caused by filamentous bacteria that form a loose, fluffy sludge blanket. Low dissolved oxygen, nutrient imbalances, or excessive organic loading can all promote filamentous growth and push SVI above 200 mL/g.
How do you measure settling volume for SVI?
Fill a 1-liter graduated cylinder with well-mixed activated sludge and let it stand undisturbed for exactly 30 minutes. Read the volume of the settled sludge blanket in mL. That reading is your SV value.
What is the difference between SVI and DSVI?
The Diluted SVI (DSVI) corrects for high sludge concentrations by diluting the sample so the settled volume stays below 200 mL/L. This avoids compression effects that make standard SVI unreliable at high MLSS levels.
Related Calculators
- Biochemical Oxygen Demand Calculator -- measure the organic load entering your treatment plant.
- Wastewater Screening Calculator -- design bar racks and fine screens upstream of the activated sludge process.
- Ideal Reactor Calculator -- size the aeration basin using residence time and flow rate.
- Flocculation Calculator -- design flocculation basins for solids removal in treatment trains.
- Volume Converter -- convert between gallons, liters, cubic meters, and other volume units.