What the Heck Is SVI?

Sludge Volume Index is a valuable measure of sludge settleability characteristics and can be monitored to help prevent process problems

Sludge Volume Index (SVI) is an extremely useful parameter tomeasure in a wastewater treatment process. In simple terms, SVI is the result of a mathematical calculation. It takes into account the 30-minute settleability test result and the activated sludge mixed liquor suspended solids (MLSS) test result to come up with a number (or index) that describes the ability of the sludge to settle and compact. SVI gives a more accurate picture of the sludge settling characteristics than settleability or MLSS alone.

SVI can indicate changes occurring in the activated sludge treatment process. By trending SVI data over a period of time, operators are able to prevent problems. Many textbooks give guideline SVI numbers, but since every plant operates differently, the best SVI for each plant will be different. The SVI should be determined when the facility is running at optimum, and should be used as a benchmark.

Testing for SVI
The standard SVI test requires a 1-liter graduated cylinder for the MLSS settling test. A separate aliquot of mixed liquor is used for a total suspended solids (TSS) test. A fresh sample of mixed liquor should be used for the tests and is normally collected from the effluent end of the aeration system, just upstream of the secondary clarifier. The formula for SVI is written:

It is important to allow the sludge to settle in a quiet area where it won’t get bumped or disturbed. The sample should also be kept out of direct sunlight.

There are various containers on the market for performing the settleability test (Figure 1). In the description of the test noted above, a 1-liter (1,000 mL/L) graduated cylinder is used. Other settling containers may be used in daily plant operation. These include the 1.4-liter polycarbonate settlometers, the Mallory settlometer, and 1,000- or 2,000-mL beakers. The Mallory settlometer can hold 2 liters but it is marked with graduations up to 1,000, and the results are read as mL/L (Figure 2).

Refrain from using tall graduated cylinders for the settling test, as the friction created by the close walls can slow the settling, change settling velocities and give false readings. Wide-mouth containers that hold at least 1 liter are acceptable, but 2-liter containers are preferred.

Operators of sequencing batch reactor (SBR) facilities may take the final settleability reading in correlation to the settle time of the SBR. For example, an SBR has a settle time of 50 minutes; the operator takes the final settleability reading at 50 minutes instead of 30 minutes. The SVI is then calculated using the settle time of the SBR. This is common, but should be noted on process control bench sheets for each SBR.

Using results
Here are some general guidelines for SVI:

SVI = 80 mL/g or less. This usually indicates a sludge that is dense and has rapid settling characteristics. This is most often attributed to an old, over-oxidized sludge typically seen in an extended aeration facility. The floc particles would be dense and granular in appearance (like a BB). As this type of sludge settles, it may leave a cloudy appearance in the supernate above the settled sludge blanket. This turbidity is called pinpoint floc (pin-floc). The sludge usually begins settling quickly after the start of the sludge settleability test, and it does not form larger particles before settling. Effluent BOD results may be below requirements, but TSS levels can still be high.

SVI = 100 to 200 mL/g. Most activated sludge plants seem to produce a clear, good-quality effluent with an SVI in this range. The sludge typically settles more slowly and traps more particulate matter as it forms a uniform blanket before settling. Microscopic examination of this MLSS would show an irregularly shaped floc particle with some filaments forming a backbone for floc-forming bacteria to attach and colonize.

In the settleability test, the sludge first forms a blanket and seems to flocculate together before starting to settle. This usually happens in the first five minutes of the test. As the particles come together, they form larger particles that have a specific gravity greater than water. As the sludge settles, you will notice channels through the sludge that are formed by the liquid being squeezed out of the sludge as it compacts.

SVI = 250 mL/g or higher. At this elevated SVI, the sludge settles very slowly and compacts poorly in the settleability test. The MLSS looks light and fluffy, not very dense. There are several reasons the SVI may be high.

If the treatment plant is new and undergoing startup, the sludge age is considered young and the floc particles are just forming. The MLSS result is usually low (less than 1,000 mg/L), and the supernatant above the sludge blanket will be cloudy, sometimes grayish/ green. This type of sludge usually leaves behind straggler floc particles that either settle slowly or not at all. Effluent BOD and TSS may still be above regulatory requirements. The term Classic Sludge Bulking has been used to describe this young sludge condition.

A high SVI may also indicate filamentous sludge bulking. In this case, a microscopic exam is recommended and might show light floc particles that contain long filaments extending out of the particle and touching filaments from other particles. Or, the filaments may be contained within the floc, causing a dispersed, open floc structure. In these cases, the liquid above the sludge blanket is usually very clear. The sludge can sit in the settleability test container for long periods and settle very little, or not at all.

When choosing a method of filamentous control (chlorination or other oxidizer), SVI should be calculated and used in trend charts to show the effectiveness of control.

Test examples
The following examples show various MLSS and settleability test results and how they can affect the SVI result.

Example 1. The settleability test is 875 in 30 minutes and the MLSS is 3,000 mg/L. The SVI calculates to 292. If the supernatant is very clear, then filamentous sludge bulking may be the cause of the high SVI. In the actual clarifier, the sludge blanket might be seen below the surface.

Example 2. The settleability test is 700 after 30 minutes and the MLSS is 1,200 mg/L. The SVI is over 580. The supernatant in the settleability test and in the clarifier looks very cloudy, with a green/gray appearance. Look for a young sludge condition, the result of toxic influent loading, or a clarifier solids washout event.

Example 3. The settleability test result is 255 in 30 minutes and the MLSS test result is 4,200 mg/L. This SVI is 61 and indicates a rapid-settling sludge condition. The clarifier may be somewhat cloudy, and pinpoint floc particles might be seen in the settleability test container. An old, over-oxidized MLSS may be the cause.

Example 4. The settleability test result is 400 and the MLSS is 3,000 mg/L. The SVI is 133. This might be a good SVI for a plant providing a good-settling sludge that first forms a blanket, and then traps fine particles as it slowly settles and compacts.

Calculating the SVI for each MLSS sample and settleability test gives the operator of an activated sludge plant a valuable tool that can help prevent problems before they begin. By knowing the SVI for a given condition at the treatment plant and plotting the data on a trend chart, an operator can make process control adjustments before problems get out of hand.

About the author
Ron Trygar, Certified Environmental Trainer, is a senior training specialist, water and wastewater programs, at the University of Florida TREEO Center in Gainesville, Fla. He can be reached at 352/392-9570 or rtrygar@treeo.ufl.edu.


Jenkins, D., Richards, M., Daigger, G. (2004). Manual on the Causes and Control of Activated Sludge Bulking, Foaming and Other Solids Separation Problems, 3rd Edition. CRC Press.

Clifton, J. (1988). Wastewater Treatment Plant Operation, 2nd Edition. Univ. of Florida DCE: Kendall Hunt Publishing.


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