The SVI calculation measures the compressibility of settled activated sludge as the volume (in milliliters) of mixed liquor required to contain how many grams of settled mlss?

• A. Viscosity
• B. Settling rate
• C. Density
• D. Compressibility

Answer: (D)

Sludge Volume Index measures how compressible the settled activated sludge is by relating the volume it occupies after settling to the amount of solids present. It is defined as the volume, in milliliters, of mixed liquor that settles in a standard period (30 minutes) divided by the mass of MLSS in grams. In practical terms, it is the volume needed to contain one gram of settled mlss. This is why the concept being tested is compressibility—the lower the SVI, the denser the settled sludge; the higher the SVI, the more volume the same mass of solids will occupy. It’s not about viscosity, density, or settling rate, which describe other properties of the sludge.