Dear Messaoud

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.

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.

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Best Wishes

Randa Osman

60-120mg/L

From my experiences, the recommended value range of SVI is 85-105 mg/L.

It's easy to find literature values. Our publication can help you

The range for optimal SVI can be between 50-150. However actual optimal number you will have to determine from operating experience as every wastewater plan can vary from region to region and also the composition of the raw wastewater. You can see from the responses SVI range does vary and all of them are correct. You should also study the sludge under a microscope to see the types like Randa has detailed in her response. Check out some links below and the first link which has a range of design parameters too:

http://www.wrights-trainingsite.com/ActSlud1onb.html

http://www.wastewatersystem.net/2010/10/correlation-between-sludge-volume-index.html

http://waterfacts.net/Formulas/SVI/svi.html

http://www.tpomag.com/editorial/2010/03/what-the-heck-is-svi

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&cad=rja&uact=8&ved=0CEMQFjAGahUKEwjjmYHumo3IAhWC1YAKHeg1CyM&url=https%3A%2F%2Fwww.michigan.gov%2Fdocuments%2Fdeq%2Fwrd-ot-activated-sludge-process_445196_7.ppt&usg=AFQjCNFV6XrE7TGTIubIhLcdhE9E5O-Z4g

http://www.ragsdaleandassociates.com/WastewaterSystemOperatorsManual/Chapter%208%20-%20Activated%20Sludge.pdf

http://onlinelibrary.wiley.com/doi/10.1002/047147164X.app1/pdf

I had good results with agroindustrial effluents (from wineries) with SVI values between 80-120 mL/g. you ca see my work in https://repositorio.utad.pt/handle/10348/61 

Thanks to all reaserchers for answers and comments.

Best regards

50-150 mL/g is the normal range, though below 100 is usually preferred.

The sludge volume index SVI depends on the concentration of total suspended solids, the depth of settling tank , the sludge volume, the stirring velocity, the effect of temperature and the floc structure and dimension.  is a usual parameter to estimate the activated sludge settling capacity. The sludge sample was conducted in a 1 L graduated cylinder and the settlement time of sludge is 30 minutes (Experimental standard method 2710D). Average 135 mL/g and Range ± 65 for urban wastewater treatment.

The sludge volume index SVI depends on the concentration of total suspended solids, the depth of settling tank , the sludge volume, the stirring velocity, the effect of temperature and the floc structure and dimension.  is a usual parameter to estimate the activated sludge settling capacity. The sludge sample was conducted in a 1 L graduated cylinder and the settlement time of sludge is 30 minutes (Experimental standard method 2710D). Average 135 mL/g and Range ± 65 for urban wastewater treatment.

The SVI depends on the amount of solids in sludge. It typically ranges between 100-200 ml/g. Any value coming under this range will exhibit a better WAS. 

50-150 mlLg

Thanks to all reaserchers for answers and comments.

Best regards

Complete answer extended by Randa M. Osman

I THINK BETWEEN 150-200 ML IS THE BEST

The common range for an SVI at a conventional activated sludge plant should be between 50 and 150. Optimum SVI must be determined for each experimentally. Sludge Density Index is used like the SVI to determine sludge settling characteristics and return sludge pumping rates. The common operational range for SDI is 1.0 - 2.5. The SVI and SDI indexes relate the weight of sludge to the volume that the sludge occupies and attempts to show how well the activated sludge separates from the mixed liquor. Sludges with a low SVI (high SDI) have good settling and compaction characteristics.

What is the appropriate range of SVI index for chicken slaughterhouse wastewater?

It depends on several factors, like temperature, O2 level, HRT, nutrients, etc. Generelay I have good results between 2 and 3 g/L VSS.