Activated Sludge · Settling Tests and Sludge Volume
What Is Sludge Volume Index (SVI)?
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.
Key point: SVI compares the 30-minute settled volume with the MLSS concentration; the math box shows the formula. A rising SVI means poorer compaction.
Keep learning
Activated Sludge Process Control for Wastewater Operators
This page comes from the course. It covers aeration, settling tests, solids inventory, return and waste flows, sludge age, microscopy, process upsets and daily records, one step at a time, with short knowledge checks and a certificate of completion.
Take the course