The sludge volume index is defined as ‘the volume in mL occupied by 1 g of sludge after it has settled for a specified period of time’ generally ranging from 20 min to 1 or 2 hr in a 1 — or 2 L cylinder. One-half hour is most common setting time allow the mixed liquor to settle for 30 min. (larger cylinder is desirable to minimize bridging of sludge floe and war effects). SVI is 50-150 mL/mg, the sludge settle ability if good.

SVI typically is used to monitor settling characteristics of activated sludge and other biological suspension. Although SVI is not supported theoretically, experience has shown it to be useful in routine process control. The SIV determination consisted of:

1. Place in the imhoff cone of 1000 mL, 100 mL of sludge and diluted to 1000 mL with phosphate buffer.

2. Was allowed to stand for 45 minutes and then stir the contents with a glass rod.

3. The volume occupied by the mud was measured by sedimentation after 30 minutes.

4. The SIV was calculated by dividing this volume by the present VSS g in 100 mL of sludge (Sludge/gVSS) [34].