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Activated Sludge Process Control for Wastewater OperatorsPage 23 of 72

Module 3 · Settling Tests and Sludge Volume

Sludge Volume Index

Sludge volume index, or SVI, estimates the volume occupied by one gram of mixed-liquor suspended solids after 30 minutes of settling. It allows settling volume to be compared while accounting for MLSS concentration.

To find SVI, compare the settled sludge volume after 30 minutes with the MLSS concentration of the same sample. EPA and operator training manuals present this relationship, and the math box shows the formula step by step.

The math box also includes a worked example. The sludge density index, or SDI, is 100 divided by the SVI, so an SVI of 100 mL/g equals an SDI of 1.0 g/mL. When you report an SVI, also report the sampling locations, times, and methods.

A rising SVI means each gram occupies more settled volume, but it does not identify the cause. As a typical guide, an SVI of about 80 to 150 mL/g settles well, and an SVI above about 150 mL/g often signals bulking. Evaluate the trend with floc appearance, filaments, loading, DO, and effluent quality.

Exam Need to Know

  • SVI = settled mL/L × 1,000 ÷ MLSS mg/L.
  • Example: 300 × 1,000 ÷ 3,000 = SVI 100 mL/g.
  • A rising SVI = each gram occupies more volume.

Exam area: Math and Calculations