Ihab Salloum[ad_1]
The activated sludge treatment process is a long, drawn-out process. Throughout different stages of this treatment, the volume of dissolved and suspended particles must be periodically calculated. In this article, I will discuss acceptable volume and density limits, as well as two indices that aid in determining the actual volume and density of dissolved substances.
Sludge Volume Index (SVI)
The SVI informs us about the settling ability of the mixed liquor. Settling is initiated only after the mixed liquor has undergone aerobic treatment. Technically, it refers to the volume of settled activated sludge in a 30-minute period during the settling stage of the activated sludge treatment process. This volume is measured in milliliters. If the volume of settled mass is low after 30 minutes, it is considered good. However, if the SVI is high, for example around 100, it indicates a high content of suspended and volatile solids. This is believed to cause issues in the later stages of treatment. Typically, the activated sludge wastewater treatment plant operates with an SVI value between 50 and 75.
Importantly, SVI can also be used to assess the quality of the sludge. If a settleometer (sized 1000 mL) is used to measure the volume of settled solids, SVI refers to the volume of 1 gram of solids in the sludge settled during the 30-minute period. The formula for calculating the sludge volume index is as follows:
SVI = (Settled sludge volume in mL/L after 30 mins. * 1000) / (MLSS, mg/L) = ml/gram
Sludge Density Index (SDI)
SDI is used similarly to SVI to determine the settling ability of the sludge, but the calculation used differs slightly. Here is how SDI is calculated:
SDI = (MLSS % * 100) / (% Volume occupied by MLSS after 30 mins. settling)
While calculating SVI, the volume obtained after 30 minutes is divided by MLSS. Conversely, when calculating SDI, MLSS is divided by the volume obtained after 30 minutes. While SVI is preferably maintained between 50 and 75 during the operation of the activated sludge treatment process, SDI values are kept between 1.0 and 2.5. It is important to note that SDI has an inverse relationship with SVI. Thus, where low SVI values are considered beneficial for settling, sludge with high SDI demonstrates better settling ability.
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