What it means
Sludge volume index is calculated from a 30-minute settleometer result and MLSS concentration. It gives a rough sense of whether activated sludge settles compactly or remains bulky. SVI is a trend tool, not a complete diagnosis by itself.
What operators watch
- Low SVI can indicate dense, fast-settling sludge; high SVI can indicate bulky sludge.
- Use SVI with effluent quality, blanket depth, microscopic observations, and process history.
- Sampling consistency matters for meaningful trends.
Common exam traps
- Assuming one SVI value proves the exact cause of a problem.
- Mixing up mL/L and mg/L units.
- Ignoring trends and plant-specific normal ranges.
Field example
A rising SVI with cloudy effluent may push operators to investigate filamentous organisms, F/M changes, DO, nutrients, and sludge age.
