Sludge Volume Index (SVI) is the weight of activated sludge solids in grams per liter of settled sludge.

• A. True
• B. False
• C. It is the volume of settled sludge in milliliters per gram of suspended solids
• D. It is the mass per liter of the settled sludge

Answer: (B)

Sludge Volume Index measures how well sludge settles by relating the volume of settled sludge to the mass of suspended solids in the mixed liquor. After letting a fixed amount of mixed liquor settle, you take the volume of the settled sludge in milliliters and divide it by the mass of suspended solids present (in grams per liter). This gives SVI in units of milliliters per gram (mL/g). The statement you’re evaluating describes mass per volume (grams per liter), which is not how SVI is defined. Therefore, the correct description is that SVI is the volume of settled sludge in milliliters per gram of suspended solids. Lower values indicate better settleability, higher values indicate poorer settleability.