Sludge Volume Index Calculator

The Sludge Volume Index (SVI) measures how well activated sludge settles in a wastewater treatment system — a critical check for plant operators monitoring biological treatment performance. Enter the Settled Sludge Volume (V₃₀), Mixed Liquor Suspended Solids (MLSS) concentration, and Sample Volume to calculate the SVI value in mL/g. You'll also get a Sludge Condition assessment and Settleability Rating to interpret whether your sludge is settling well or showing signs of bulking. Also try the Langelier Saturation Index Calculator.

mL/L

Volume of settled sludge after 30 minutes

mg/L

Dry weight concentration of suspended solids

mL

Volume of sludge sample used for testing

Results

Sludge Volume Index (SVI)

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Sludge Condition

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Settleability Rating

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Results Table

Frequently Asked Questions

What is Sludge Volume Index (SVI)?

SVI is a measure of the settleability and compactability of activated sludge. It represents the volume in milliliters occupied by 1 gram of sludge after 30 minutes of settling. See also our Water Activity (aw) Calculator.

How do you calculate SVI?

SVI is calculated using the formula: SVI = (Settled Volume × 1000) / MLSS Concentration. The settled volume is measured after 30 minutes of settling, and MLSS is the mixed liquor suspended solids concentration.

What is a good SVI value?

Generally, SVI values between 80-120 mL/g indicate good settling characteristics. Values below 80 may indicate pin floc or old sludge, while values above 150 suggest poor settling and potential bulking issues.

Why is SVI important in wastewater treatment?

SVI helps operators monitor and control the activated sludge process. It indicates settling problems early, helps optimize return activated sludge rates, and ensures efficient solid-liquid separation in clarifiers.

How often should SVI be measured?

SVI should be measured daily in most activated sludge systems. More frequent monitoring may be necessary during process upsets or when making operational changes to the treatment system.

What causes high SVI values?

High SVI values are typically caused by filamentous bulking, low food-to-microorganism ratio, nutrient deficiency, low dissolved oxygen, or pH imbalances. Identifying the root cause is essential for proper treatment.

How can SVI be controlled?

SVI can be controlled by maintaining proper F/M ratio, adequate dissolved oxygen levels, appropriate pH, sufficient nutrients, and proper return activated sludge rates. Chlorination may be used to control filamentous bacteria.