The activated sludge treatment process is a long drawn out process. During different stages of this treatment the volume of the dissolved and suspended particles have to be periodically calculated. In this article I discuss what volume and density limits are acceptable, and two indices that help in determining the actual volume and density of dissolved substances.

Sludge Volume Index (SVI)

The SVI tell us how good or bad the settling ability of the mixed liquor is. This settling is initiated only after the mixed liquor has been aerobically treated. Technically it is the volume of the settled activated sludge in a period of 30 mins. during the settling stage of the activated sludge treatment process. This volume is expressed in milliliters. If the volume of settled mass is low after 30 mins. then it is considered good. But if the SVI is of a high value, for example about 100 then it is believed that there is a high content of suspended and volatile solids. This is also believed to cause problems in the later stages of treatment. For most part, the activated sludge wastewater treatment plant is operated at a SVI value between 50 and 75.

Thus it can also be used to check the quality of the sludge. If a settleometer (size 1000 mL) is used to measure the volume of the settled solids then SVI refers to the volume of 1 grm of solids in the sludge settled during the period of 30 mins. The formula used for the sludge volume index calculation is as follows:

SVI = (Settled sludge volume in mL/L after 30 mins. * 1000) / (MLSS, mg/L) = ml/gram

Sludge Density Index (SDI)

The SDI is used in a manner that is similar to the SVI, to determine the settling ability of the sludge. But the calculation used differs slightly from SVI. Here’s how SDI is calculated:

SDI = (MLSS % * 100) / (% Volume occupied by MLSS after 30 mins. settling)

While calculating SVI, the volume obtained after a period of 30 mins. is divided by MLSS, it is the reverse in calculating SDI. While SVI is preferably kept in the range of 50 to 75, during operation of the activated sludge treatment process, the SDI values are kept between 1.0 to 2.5. It has to be noted that SDI has an inverse relationship with SVI. Hence where low SVI values are considered good for settling, sludge that has high SDI has better settling ability.