In a previous article (“What the Heck is SVI?”), I discussed sludge volume index (SVI) and its value as a process control tool. In this installment, I want to answer a few questions I received from readers, clarify the formula result, and discuss SVI a little more in depth. 

There are many methods of process control at an activated sludge treatment plant. Some of the acronyms sound like something found in today’s text messages. MCRT, SRT, F/M and SVI are all acronyms used by activated sludge plant operators and all mean something specific in process control lingo. 

For instance, mean cell residence time (MCRT) and solids retention time (SRT) mean roughly the same thing. These are expressed as days of sludge age and represent the amount of time that a pound of mixed liquor suspended solids (MLSS) or mixed liquor volatile suspended solids (MLVSS) resides in the process before it is wasted from the system or escapes over the secondary clarifier weir. 

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Food to microorganism ratio (F/M) is the amount of food, in pounds, available to 1 pound of MLVSS. Normally, F/M ratios calculate to less than 1 pound of food (as BOD or COD) per pound of MLVSS. 

Sludge volume index is considered to be the volume, in milliliters, of 1 gram of suspended solids after 30 minutes of settling. Put into simple terms, sludge volume index is the result of a calculation where two other test results are combined into one. 

SVI calculation 

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A sample of mixed liquor suspended solids is collected and allowed to settle in a large, wide-mouth container for at least 30 minutes. This settleability test result and the actual suspended solids (MLSS) test result are divided into each other, multiplied by 1,000 and presto! A number (or index) that describes the density of the sludge and its ability to compact is produced. 

The formula for SVI is written: 

SVI, mL/g = 30-minute settleability test result, mL/L     x 1,000 mg

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                         MLSS, mg/L                         gram 

According to most laboratory manuals like Standard Methods for the Examination of Water and Wastewater, the standard sludge volume index test requires a 1-liter graduated cylinder for the MLSS settling test. A separate aliquot of the mixed liquor is used for a total suspended solids test. A fresh sample of mixed liquor should be used for the test, and is normally collected from the effluent end of the aeration system just before entering the secondary clarifier. It is important to allow the sludge to settle in a quiet area where it won’t get bumped or disturbed and kept out of direct sunlight. 

I know what some of you are thinking: “You just said to use a 1-liter (or 1,000 mL) container for the settleability test. We use a settleometer and it holds 2 liters (2,000 mL). What’s up with that?” 

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

Do not use tall graduated cylinders for the settling test. The friction created by the close walls can slow the settling, change settling velocities and give false readings (Figure 2). Wide containers that are almost as wide as they are tall, give better results than tall, narrow cylinders. As sludge settles, it also compacts, squeezing clear water out of its blanket. These channels of water can be seen from the side of a settleometer and give the compacting sludge a ‘cottage cheese-like’ appearance. As the sludge thickens and compacts, water rises above it to form the supernatant. If a skinny cylinder is used for the settling test, the rising water velocity interferes with the sludge’s downward velocity, slowing the settling. Wide-mouth containers that hold at least 1 liter are acceptable; however, 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 45 minutes; the operator takes his final settleability reading at 45 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. 

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The SVI number gives a more accurate picture of the sludge’s characteristics than settleability or MLSS alone. Imagine the volume, or space that 1 gram of sludge can fit into. Think of the characteristics of that gram of sludge, the density or fluffiness of it. If filamentous bacteria are present in the sludge, that 1 gram of sludge would be light, fluffy and spread out, much like a cotton ball soaked in water. Now imagine a gram of dense, somewhat granular sludge, like mud. The same gram of sludge would take up less space than the same gram of fluffy, filamentous sludge. That’s what SVI helps describe. 

Trending SVI can help indicate changes happening in the activated sludge treatment plant, preventing settling problems before they occur. Many textbooks give guideline SVI numbers, but since every plant operates differently, the best SVI for each plant will be different and should be determined when the facility is running at optimum and used as a benchmark. Some general SVI guidelines are given below: 

SVI = 80 mL/g or less

An SVI of 80 or less usually indicates a sludge that is dense and has rapid settling characteristics. 

SVI = 100 to 200 mL/g

Most activated sludge plants seem to produce a clear, high-quality effluent with an SVI in the range of 100 to 200. The sludge typically settles slower and traps more particulate matter as it forms a uniform blanket before settling. 

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. 

How is SVI changed in a WWTP?

Since part of the SVI calculation is MLSS (the denominator), an operator can change the SVI by changing the inventory of mixed liquor suspended solids. Raising the amount of MLSS (reducing waste rates) changes the density of the floc, creating a heavier sludge particle. The more dense the particle, the more likely it will settle faster. The higher milligram per liter MLSS reduces the SVI result. 

To increase the SVI, an operator would increase the waste sludge rate, effectively creating a less dense particle that settles slightly slower. This same particle might also trap more fine suspended solids as it settles, clarifying the effluent even more. The lower MLSS mg/L results in a higher SVI calculation. 

An interesting note about how SVI relates to other process control parameters: SVI and F/M Ratio follow similar trend lines. As SVI increases, so does F/M ratio. However, SVI trends the opposite of MCRT and SRT. As SVI increases, MCRT and SRT decrease. Remember that SVI and these other parameters use the amount of MLSS (or MLVSS) in their calculations too. Some operators have come up with a colorful way to remember how SVI, F/M ratio and Waste Sludge Rates go along with each other by calling them “the three amigos.” 

Three amigos work together

SVI is a very useful tool when using chlorine or other oxidizers to control sludge bulking conditions. Combining SVI with microscopic exam results can give the operator a great way to monitor the effect of oxidizers on overall sludge characteristics. For instance, at certain SVI values, the operator knows the plant experiences sludge bulking and solids washout of the secondary clarifiers. Using an oxidizer, such as chlorine feed into the RAS flow, can prevent the SVI from climbing too high, then shut the chlorine off when SVI numbers return to normal. 

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.

Knowing what the SVI is for a given condition at a wastewater treatment plant and plotting the data on a trend chart, process control adjustments can be made before problems get out of hand. Many treatment plants run both the MLSS test and settleometer daily and calculating the SVI using those test results is not really an extra burden. 

With today's suspended solids meters, it is even faster and easier to calculate the SVI since MLSS can be directly read from the aeration tanks. 

So, as you can see, SVI is another one of the many useful tools available to the activated sludge treatment plant operator, not just another acronym!

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 


Clifton, J. “Wastewater Treatment Plant Operation”, second edition. Univ. of Florida DCE: Kendall Hunt Publishing, 1998. 

Jenkins, D., Richards, M., Daigger, G. “Manual on the Causes and Control of Activated Sludge Bulking, Foaming and Other Solids Separation Problems”, third edition, CRC Press, 2004. 

Sacramento State University, Office of Water Programs, online glossary. “SVI”,

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