Some discussion of the sludge volume index is warranted. Of all the process control formulas discussed in so far, the SVI is the easiest and quickest for operators to perform. And it only requires a one liter graduated beaker or settleometer. Some wastewater plants use a 1000 mL graduated cylinder, the use of which is discouraged. The depth-to-diameter ratio of a graduated cylinder does not nearly approximate the depth-to-diameter ratio of a 1000 mL beaker which much more closely approximates the dimensions of a secondary clarifier. The frequency of calculating the SVI is based on how many problems there are with solids settling in the clarifier. If the clarifier effluent total suspended solids are always low, and sludge blankets are always low, good practice would dictate calculating the SVI at least once a week. The more data available to the operator, the better that operator will be in recognizing a change in plant performance before real problems start to occur.
The SVI is the volume, in milliliters, occupied by 1 gram of settled activated sludge, and it is a measure of the settling characteristics of the sludge. The SVI is easily calculated as shown in Equation 7.
Equation 7: Sludge Volume Index Formula
Any wastewater textbook will provide a “recommended range” for the SVI. The problem is that the typical SVI range is based on experience gained from operating municipal wastewater treatment plants. Though industrial wastewater systems have much in common with their municipal counterparts, they can also be very different, and this is particularly so when it comes to biological treatment and the SVI value or range is an example of where municipal and industrial plants can really show their differences. The typical range for a well-operated activated sludge wastewater system is between 50 and 150 mL/g. But there are plenty of industrial wastewater systems operating with SVIs of 300 mL/g which consistently produce excellent effluent quality. Therefore, the idea of a recommended range for the SVI is rejected when it comes to an industrial wastewater plant. What is recommended is that SVIs be run on a daily basis. The SVI value should then be compared to the effluent TSS, perhaps using simple linear regression. In this way the optimum SVI value, specific to your wastewater plant, can be determined. Any significant increase (or decrease) from the site-specific SVI will then provide an alarm indicating a potential problem with the microorganisms in the mixed liquor.
The frequency of running the SVI is up to you. Some plants really should run this simple test every day while other plants can go for long periods without any need for determining the sludge volume index. There is another technique, closely associated with the SVI, that requires the same lab setup but the volume of the settled sludge is recorded every two minutes for the 30-minute SVI time frame and then the data is graphed. An example of this is shown in Figure 7. This graph is actually a bit remarkable in its consistency over time, a time period spanning three years. When the operation of your treatment system is this consistent, this steady, you can obviously stretch the time frame between running certain tests. I’m not recommending that testing not be done. Rather, I’m suggesting that testing match the unpredictability associated with the operation of your wastewater system.
Figure 7: MLSS Settling Rate