SVI Calculator (Sludge Volume Index)
Sludge volume index (SVI) shows how well activated sludge settles and compacts. It is the volume in milliliters that one gram of solids takes up after 30 minutes of settling.
Result
Sludge volume index
100 mL/g
Your SVI is 100 mL/g: one gram of your sludge takes up about 100 mL after 30 minutes of settling.
Many conventional activated sludge plants run in the roughly 80–150 mL/g range. Always compare with your plant's own trend.
Running this daily? Get a free trending log
We'll email you a spreadsheet log for tracking this number over time, plus today's result and inputs. Or see how ComplyFlo calculates it automatically from your daily readings.
The formula
SVI (mL/g) = SV30 × 1,000 ÷ MLSS- SV30
- Settled volume after 30 minutes (mL/L)
- MLSS
- Mixed liquor suspended solids (mg/L)
Good to know
- Every facility has its own normal SVI range. Today's number means most next to your recent trend.
- SVI is a settling indicator. It does not identify which organisms are causing a change.
Read the full guide: How to Calculate Sludge Volume Index (SVI)
Frequently asked questions
What does SVI measure?
How well activated sludge settles and compacts: the mL that one gram of solids takes up after 30 minutes of settling.
What SVI is considered high?
Many plants treat above roughly 150 mL/g as a bulking or poor-settling warning, and above roughly 250 mL/g as severe. Your plant's own normal range matters most.
Can SVI tell me what is causing poor settling?
No. SVI is a settling indicator. It does not identify which organisms are causing a change.
Stop running this math by hand
ComplyFlo for wastewater plants can calculate and trend SRT, F/M, HRT, RAS, SVI and removal efficiency from your daily readings, next to your plant dashboard and DMR-ready reports.
See ComplyFlo for wastewater plantsFree checklist
TPDES Permit Compliance Checklist
Review permit-specific DMR submissions, sampling, SSO reporting, renewal milestones, and applicable pretreatment responsibilities.
Want to see it on your plant's data?
Book a free 1-hour demo with a ComplyFlo team member.