What is a good SVI value?

SVI below 80 mL/g indicates excellent settling. Values between 80 and 150 mL/g are acceptable for most treatment plants. SVI above 150 mL/g suggests poor settling, often from filamentous bulking, and may cause solids loss in the clarifier.

How is the settled sludge volume measured?

Fill a 1-liter graduated cylinder with mixed liquor from the aeration tank and allow it to settle for 30 minutes without disturbance. Read the volume of settled sludge at the bottom in mL. Divide by the cylinder volume (1 L) to get mL/L — in a 1-L test this equals the mL reading directly.

What is Diluted SVI (DSVI)?

DSVI dilutes the mixed liquor sample before the settling test, typically to a target concentration of 1,500–2,000 mg/L MLSS. This corrects for the effect of concentration on settling velocity and gives a more consistent measurement when MLSS exceeds 3,500 mg/L.

What causes a high SVI?

High SVI is most commonly caused by filamentous bacteria overgrowth (sludge bulking), low dissolved oxygen, high organic loading, nutrient deficiency, or low sludge age (SRT). Foam on the aeration tank surface combined with high SVI often points to Nocardia or Microthrix.

What MLSS value should I use?

Use the MLSS concentration from the aeration basin at the time the settling test sample was collected. Typical values range from 2,000 to 4,000 mg/L in conventional activated sludge systems. Higher MLSS in MBR or high-rate systems can make standard SVI less meaningful — use DSVI instead.

Sludge Volume Index Calculator

Name: Sludge Volume Index Calculator
Availability: InStock
Author: Nham Vu

Calculate the Sludge Volume Index (SVI) and Diluted SVI from settled sludge volume and MLSS concentration to assess activated sludge settleability.

Settling Test Inputs

Settled Sludge Volume (SSV₃₀)

mL/L

Volume of settled sludge after 30 min in a 1-L cylinder.

MLSS Concentration

mg/L

Mixed Liquor Suspended Solids in the aeration tank.

Diluted SVI (optional)

Diluted Settled Volume

mL/L

Settled volume after diluting sample to ~1,500–2,000 mg/L MLSS.

Diluted MLSS Concentration

mg/L

SVI Interpretation Guide

< 80 mL/g — Excellent settling

80–150 mL/g — Acceptable settling

> 150 mL/g — Poor settling / bulking

Formula: SVI (mL/g) = SSV₃₀ (mL/L) × 1000 / MLSS (mg/L)

Enter settled volume and MLSS, then click Calculate.

Sludge Volume Index

mL/g

0 80 150 300+

Calculation Details

Settled Volume (SSV₃₀)

MLSS

SVI Formula SSV × 1000 / MLSS

SVI Result

Summary

Calculate the Sludge Volume Index (SVI) and Diluted SVI from settled sludge volume and MLSS concentration to assess activated sludge settleability.

How it works

Enter the 30-minute settled sludge volume in mL/L (from a standard settling test in a 1-liter graduated cylinder).
Enter the MLSS concentration in mg/L (Mixed Liquor Suspended Solids from the aeration tank).
Click Calculate to compute SVI = (Settled Volume × 1000) / MLSS.
Optionally enter a diluted settled volume for Diluted SVI (DSVI), which corrects for bulking sludge at high concentrations.
Read the quality indicator: SVI below 80 is excellent settling, 80–150 is acceptable, above 150 indicates poor settling or bulking.

Use cases

Monitoring activated sludge health in municipal wastewater treatment plants.
Diagnosing sludge bulking problems causing solids carryover to effluent.
Optimizing return activated sludge (RAS) rates based on settling performance.
Tracking daily or weekly sludge settleability trends for process control.
Benchmarking performance after changes to aeration, loading, or sludge age.

Frequently Asked Questions

Last updated: 2026-07-24 · Reviewed by Nham Vu