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

6 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.

Jeremy

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

Erik Rumbaugh

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.

Erik Rumbaugh

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.

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

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