What is Sludge Volume Index (SVI) and what does a high SVI indicate?

• A. SVI Is The Settled Sludge Volume Per Unit MLSS After 10 Minutes; High SVI Indicates Good Settleability
• B. SVI Is The Settled Sludge Volume Per Unit MLSS After 60 Minutes; High SVI Indicates Low Sludge Blanket Depth
• C. SVI Is The Settled Sludge Volume Per Unit MLSS After 30 Minutes; High SVI Indicates Poor Settleability
• D. SVI Is The Settled Sludge Volume Per Unit MLSS After 45 Minutes; High SVI Indicates Rapid Settling

Answer: (C)

Sludge Volume Index measures how well the sludge settles by relating the volume that settles in a fixed time to how much solids are present. You let a sample stand for 30 minutes, read the settled sludge volume (SV30) in mL per liter, and measure the MLSS concentration in g/L. The SVI is SV30 divided by MLSS, giving a value in mL per g. This index shows settling performance: lower values mean compact, rapid settling, while higher values indicate fluffy or bulking sludge that settles poorly. Therefore, the statement that SVI is the settled sludge volume per unit MLSS after 30 minutes and that a high SVI signals poor settleability is correct.