Sludge Volume Index – Is it Really Important in Wastewater Treatment?

In this brief article I discuss how to asses the quantity of the particles in the activated sludge that settle to the bottom during wastewater treatment. I also present the values that indicate proper settling of the suspended solids in the activated sludge during wastewater treatment.

The Sludge Volume Index (SVI) represents the volume occupied by the concentrated sludge that is obtained after the sludge is allowed to settle for a period of half an hour. Only the volume occupied by 1gm of the sludge is considered when calculating the index. This index is also used primarily to asses the settling ability of the particles in the activated sludge, as well as the settling capacity of other types of suspensions in wastewater. The filamentous type of the bacteria which is found in the activated sludge can increase the sludge volume index. This phenomenon by which the bacteria contributes to the increase in the index value is known as filamentous bulking. The SVI is calculated by:

SVI = V/(Vo x MLSS), where,

SVI measured in ml/g, is the Sludge Volume Index;
V measured in ml, is the volume of the sludge that has been allowed to settle for half an hour;
Vo measured in l, is the volume of the sludge initially before being allowed to settle; and
MLSS measured in g/l, represents the Mixed Liquor Suspended Solids.

Any value of the sludge volume index that lies between 35 ml/g and 100 ml/g is acceptable.

Sodium Adsorption Ratio (SAR)

The permeability of the soil can be altered by the Sodium ions present in wastewater, if the untreated wastewater is discharged over surface areas. A high value of the Sodium Adsorption Ratio can lead to problems related with infiltration in the soil. An increase in the value of the SAR can also increase the possibility that Sodium will move to the Cation exchange sites.

This can be calculated by:

SAR = [Na+] / [0.5(Ca++ + Mg++)]^0.5, where the Na++, Ca++, and Mg++ are expressed in milliequivalents/l.

When the SAR value is greater than 9, the permeability of the soil is affected. Such a high level of Sodium will also become toxic for the plants.