Activated sludge treatment process: what volume and density limits are ideal

The activated sludge treatment process is a lengthy process. During the different stages of this treatment, the volume of dissolved and suspended particles must be calculated periodically. In this article, I discuss what volume and density limits are acceptable, and two indices that help determine the true volume and density of dissolved substances.

Sludge Volume Index (SVI)

The SVI tells us how good or bad the settling ability of the mixed liquor is. This settling starts only after the mixed liquor has been aerobically treated. Technically it is the volume of activated sludge settled in a period of 30 min. 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 minutes. then it is considered good. But if the SVI is of a high value, for example around 100, then it is believed that there is a high content of suspended and volatile solids. This is also thought to cause problems in the later stages of treatment. For the most part, the activated sludge wastewater treatment plant operates at an SVI value between 50 and 75.

Therefore, it can also be used to check the quality of mud. If a sedimentometer (size 1000 mL) is used to measure the volume of settled solids, SVI refers to the volume of 1 g of solids in the settled sludge over a period of 30 minutes. The formula used to calculate the sludge volume index is as follows:

SVI = (Sludge volume settled in mL/L after 30 min. * 1000) / (MLSS, mg/L) = mL/gram

Sludge Density Index (SDI)

SDI is used in a similar way to SVI to determine the settling capacity of mud. But the calculation used differs slightly from SVI. This is how the SDI is calculated:

SDI = (MLSS % * 100) / (% Volume occupied by MLSS after 30 minutes of sedimentation)

When calculating SVI, the volume obtained after a period of 30 minutes. is divided by MLSS, it is the other way around in the SDI calculation. While SVI is preferably kept in the range of 50 to 75, during the operation of the activated sludge treatment process, SDI values ​​are kept between 1.0 and 2.5. It should be noted that SDI has an inverse relationship with SVI. Therefore, where low SVI values ​​are considered good for settling, mud that has a high SDI has better settling ability.