• Sludge Volume Index defined as the volume occupied in ml by 1gm of solids in the mixed liquor after settling for 30 minutes .
• Its represent the degree of concentration of the sludge in the system and hence Decide the rate of recycle of sludge require to maintain the desired MLLS (mixed liquor suspended solids) & F/M ratio in the aeration tank to achieve the desired degree of purification.

• SVI (mL/g) = $\frac{Settled -sludge-volume (mL/L)}{ Mixed-liquor-suspended-solids (MLSS) (mg/L)} X 1000$.

  Range: 50 to 150mL/gm (its indicate good settling sludge).

Significance :

• SVI gives a more accurate picture of the sludge settling characteristics than settleability or MLSS alone.

• Sludge Volume Index (SVI) is an extremely useful parameter to measure in a wastewater treatment process.

• SVI used to indicate the physical state of sludge produced in a biological aeration system.