SVI: The Most Valuable 30 Minutes in Your Day
The Sludge Volume Index (SVI) might seem simple, but it’s one of the most powerful indicators of biological system health in wastewater treatment. Done right, this 30-minute test can help you:
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Catch filamentous outbreaks early
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Prevent solids carryover
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Optimize wasting and RAS rates
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Stay in compliance with your discharge permit
But here’s the thing — most plants don’t run SVI tests accurately or consistently. Let’s fix that.
What Is the Sludge Volume Index?
SVI (mL/g) = Settled Sludge Volume (mL/L) ÷ MLSS (g/L)
In plain English? It tells you how well your sludge settles. Lower numbers = tighter, healthier sludge. Higher numbers = trouble ahead.
| Range | Interpretation |
|---|---|
| < 100 mL/g | Excellent settling |
| 100–150 mL/g | Normal/acceptable |
| >150 mL/g | Warning: bulking risk |
| >200 mL/g | High risk: investigate immediately |
How to Run an Accurate SVI Test
1. Collect the Right Sample
Pull your mixed liquor sample from the aeration basin, not the clarifier. Make sure it represents average conditions.
2. Measure MLSS
Use standard methods (like gravimetric drying) to get an accurate Mixed Liquor Suspended Solids (MLSS) value in g/L.
3. Settle It Right
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Use a 1L graduated cylinder or Imhoff cone
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Gently invert to mix — don’t shake
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Set it on a flat, vibration-free surface
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Let it settle for exactly 30 minutes
4. Record & Calculate
Measure the settled sludge volume in mL and plug it into the formula.
Example:
Settled Volume = 250 mL
MLSS = 3.0 g/L
SVI = 250 ÷ 3.0 = 83 mL/g
Common SVI Mistakes (That Can Cost You)
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Not mixing the sample uniformly
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Using a dirty or scratched cylinder
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Skipping the MLSS test or estimating it
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Letting the sample sit too long
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Settling in a room with vibrations (yes, even from HVAC)
How to Make SVI a Real Tool — Not Just a Test
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Run it consistently — same time, same method
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Track trends, not just daily numbers
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Pair with microscopic analysis to identify filaments
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Train every operator to run it the exact same way
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Use it to fine-tune wasting, RAS, and F:M ratios
Why SVI is Worth Taking Seriously
A reliable SVI testing program helps your facility:
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Avoid permit violations due to solids carryover
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Reduce chemical spending on filament outbreaks
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Extend equipment life by preventing overloads
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Make smarter operational decisions in real time
How BCG Water Can Help
At BCG Water, we help wastewater facilities set up accurate, operator-friendly testing protocols that give them more control over plant health.
We support:
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On-site SVI training
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SOP development
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Troubleshooting sludge issues
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Performance benchmarking
If you’re guessing your SVI — or not even running it — we’ll help you get the process dialed in.
Call to Action (CTA)
Want to run more accurate tests and make smarter decisions?
👉 Book a Lab Process Review and let’s improve your SVI game.