If settled solids volume increases while MLSS remains constant, what happens to SVI?

If settled solids volume increases while MLSS remains constant, what happens to SVI?

• A. Increases
• B. Decreases
• C. Stays the same
• D. Becomes negative

Answer: (B)

SVI is the amount of settled sludge volume (in milliliters) per unit mass of MLSS, expressed as settled volume after 30 minutes divided by MLSS (mL per g). If the volume settled increases while MLSS remains constant, the numerator grows while the denominator stays the same, so the SVI value rises. A higher SVI indicates poorer settleability, meaning sludge occupies more volume for the same mass. So the result is an increase in SVI.

SVI is the amount of settled sludge volume (in milliliters) per unit mass of MLSS, expressed as settled volume after 30 minutes divided by MLSS (mL per g). If the volume settled increases while MLSS remains constant, the numerator grows while the denominator stays the same, so the SVI value rises. A higher SVI indicates poorer settleability, meaning sludge occupies more volume for the same mass. So the result is an increase in SVI.