Sludge volume index

Sludge Volume Index (SVI) is used to describe the settling characteristics of sludge in the aeration tank in Activated Sludge Process. It is a process control parameter to determine the recycle rate of sludge. It was introduced by Mohlman in 1934, and has become the standard measure of the physical characteristics of activated sludge processes. It is defined as 'the volume (in ml) occupied by 1 gram of activated sludge after settling the aerated liquor for 30 minutes'.

SVI (ml/g) = settled sludge volume/sample volume (ml/l) * 1000 (mg/g)/ suspended solid concentration (mg/l)