SVI is the volume in mL occupied by 1 gram of mixed liquor after 30 minutes of settling. 80 to 150 mL/g is typical; above 150 means slow settling.
SVI
Formula
SVI, mL/g = (SSV30, mL/L x 1,000) / MLSS, mg/L SDI, g/mL = 100 / SVI
Inputs used
- 30 minute settled volume (SSV30) (mL/L)
- MLSS (mg/L)
- Dilution factor (1 if undiluted)
Worked example (White River WRRF, fictional)
- SVI = 310 x 1,000 / 2,620 = 118 mL/g
Tips
- Use MLSS from the same sample as the settleometer.
- If SSV30 is above about 800 mL/L, dilute 1:1 and enter a dilution factor of 2.
Results are estimates for operator use. Check units and inputs, and confirm decisions with plant data and procedures.
This calculator also appears on ActivatedSludge.com, our consulting site. Results are for training and operator use; always follow your plant’s procedures.