What is my ideal Sludge Volume Index (SVI) number?

11/12/2015

We often generate SV30 and SVI numbers in our monitoring - but rarely think about how to interpret the number. Remember the SV30 test is designed to predict secondary clarifier performance in suspended growth systems (normally activated sludge). While usually overlooked, I as operators questions about supernatant turbidity & fines remaining after the sludge settles. Additionally, does the sludge float after sitting for 2 hours due to denitrificiation? Another issue could be fats, oils, and grease causing a scum (floating sludge) layer on the top after 30 minutes.

For general guidance on how SVI relates to sludge settling characteristics:

SVI (ml/g)                         Sludge Settling & Compacting Characteristics
< 80                                    Excellent - but can leave "fines" if sludge age is too long
80 - 150                              Normal zone - I usually suggest targeting 100 - 120 for best water
> 150                                  Poor settling - need to check for filaments or less likely viscous bulking

14 Comments

Jeremy

3/14/2017 06:53:52 am

I am having a problem every spring in my oxidation ditch plant. I have an extended sludge age and an anoxic selector. In the spring my SVI drops dramatically into the 60s and I am having to add flocculent to keep from violating tss limits. There appears to be excess polysaccharide in the ML but I don't think it is from low nutrients. Any ideas?

Erik Rumbaugh

3/15/2017 11:24:26 am

I have had similar questions every spring. First, I have a few questions
(1) Is turbidity increased in SV30 supernatant/clarifier effluent?
(2) F/M ratios - did influent BOD/COD decrease in mg/L.

I am thinking about a potential "old sludge" or low F/M issue. I have seen this with spring rains causing increased stormwater infiltration and lowering influent BOD.

3/16/2017 06:31:31 am

The turbidity is horrible. Running a Carrousel plant with f:m in .03 range. I looked back and when we get a decent rain, the BOD does decrease quite a bit. We sample BOD twice per week so if we do it right after a rain there is usually a spike and then following that they decrease. We have quite a bit of infiltration and it can take as long as a week for flows to return to normal after a good rain.
Thanks

3/21/2017 05:02:24 am

Based on turbidity, low SVI, and known low side F/M, I am convinced that the sludge is on the "old side". Besides low bio polymers for capturing small particles (turbidity), the system becomes susceptible to filaments under very low F/M. In this case, I would recommend wasting to increase F/M until the SVI and turbidity issues are resolved. You should also notice a color change in the floc to a more healthy brown vs dark color.

Ben Barcinas

1/23/2018 01:46:29 am

I Manage, a newly build an Oxidation Ditch secondary activated sludge treatment. our Oxidation MLSS is about 2500 mg/l, effluent TSS-5 mg/l, BOD 10 mg/l, turbidity 5 ntu, fecal coliform at <10/100ml. flow at 1.6 mgd to 4.5 mgd.

1/23/2018 06:01:36 am

I have always been a fan of Oxidation Ditch systems. With monitoring and proper wasting, they should give excellent treatment results. They are also very good for use with nutrient permits for nitrogen & phosphorus - given a few modifications.

Jatin

8/14/2018 10:13:05 am

What happens if sludge volume index is low?
Why we need sludge volume index in the range of 80 - 150.

8/14/2018 03:26:41 pm

SVI being too low can mean you do not have enough biological solids to treat influent waste - this is also very low MLSS or high F/M. It can also be seen with older sludge age - long MCRT, Low F/M. At very low SVI your clarifier does not work as well as it should. Effluent turbidity and/or high effluent TSS are the result.

site at link

4/7/2019 08:02:31 am

It’s hard to find knowledgeable people on this topic, but you sound like you know what you’re talking about! Thanks

amira

4/8/2020 12:30:52 am

hi, I am having problem with biological system. Sludge Volume for 30 min shown that sludge are not settled. The texture is light and not compact. 99% of sludge and only 1% of unclear water of sv30. I have been done wastage of sludge (WAS) of aeration but still showing same reading of SV30. Can suggest any other option/ action need to be done to avoid aeration become worst. I already checked bacteria activity showing there is paramecium sp. and Rotifers

Thank you.

4/8/2020 08:50:17 am

Do you have filamentous bacteria when doing the microscopic exam? Another thing to do is calculate SVI = (SV30/MLSS (grams)) - The SVI takes the amount of MLSS into consideration.

There are ways to control filaments if that is the cause.

Amira

4/9/2020 11:59:25 pm

Yes, there is filamentous bacteria and it shape like thread.

MLSS of aeration are
7/4/2020- 4850mg/L (SVI-205.98)
8/4/2020 - 5270mg/L (SVI - 189.56)
9/4/2020 - 5040mg/L (SVI- 5040)

*note: SV30 = 999ml/1000ml (because there is unclear water. Maybe carryover from biological clarifier (I have install bag filter to reduce the carryover into next process but unsuccessful to filter the carryover)

Based on range of SVI stated above. it shows that i have poor settleability of activated sludge. What is action/ control measure to reduce filaments in aeration?

Thank you in advance :)

4/10/2020 02:58:26 am

Hi, there is typing error on SVI value

MLSS of aeration are
7/4/2020- 4850mg/L (SVI-205.98)
8/4/2020 - 5270mg/L (SVI - 189.56)
9/4/2020 - 5040mg/L (SVI- 198.21)

Kriss

3/3/2021 06:30:55 am

SVI showing result of sedimentation after 30 min, not longer. After this time usually about 1 – 2 h sludge flotate. But SVI probe don’t show this. The same flotation is in clarifier, because sludge is in it longer than 30 min. This results higher TSS in outlet. Mostly when SVI is high. Many people don’t know about it. Generally, SVI depends on many issues, eg. MLSS, type of bacteria, process parameters, F/M ratio and many others. This parameter still change during year, months, days. I have pleasure work with good technology uses vacuum degassing of MLSS. So, I don’t have many problems resulting from high SVI. For me is not important parameter. Unfortunately, many wwtp using other technologies regularly have this problem. Eg. Amira, in my plants should compact to 90% too but than MLSS is ~double (10,000 mg/L). My operator must only remove more WAS and nothing else. Amira must add costly iron sulface to kill filamentous bacteria or find another reason.But it’s another long story.

What is my ideal Sludge Volume Index (SVI) number?

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