What is the standard settling time used when calculating Sludge Volume Index (SVI)?

• A. 5 minutes
• B. 15 minutes
• C. 30 minutes
• D. 60 minutes

Answer: (C)

Sludge Volume Index is based on how much of the sludge settles in a fixed, standard period, so results are comparable between samples and plants. The fixed settling time used is 30 minutes. After mixing, the sample is allowed to settle for 30 minutes, the settled volume is read (as milliliters of settled sludge per liter of suspension), and then divided by the mixed liquor suspended solids (MLSS) in g/L to get the SVI in mL/g. This 30-minute window is chosen because it provides enough time for flocs to settle to reflect typical clarifier performance, yet remains practical for routine testing. If you used a much shorter time, the settled volume would be smaller and could underestimate settleability; a much longer time would not reflect normal operating conditions and would hinder comparability.