I recommend you to read the attached paper.

thank you Mohammad

Dear Weam Mahdawi,

You should make confirm if biological activity of tank is changed.

Dear Pankaj,

I think that the Biological Activity not change.

what is the main biological activity that should I check in your opinion ??

Dear Weam Mahdawi,

For aerobic treatment, you can best monitor the SOUR (specific oxygen uptake rate).

The drop of SVI seems at an acceptable rate indicating good adaptation of the micropopulation to the substrate at medium-low F/M.

In our advanced activated sludge (AAS) process we get SVI < 35 ml/gDS at very high microbial activity (respiration, enzymatic, ...).

Dear Weam Mahdawi ,

Has the COD of effluent changed?

The MLSS being relatively stable but the SVI dropping rapidly indicates low Sludge Activity.The main reason might be the microbial toxicity of influent or high loading.

I recommend you to detect the MLVSS or OUR. In addition,to check the BOD of influent.

Dear @Bruno Peeters

yes I am agree with you , the drop of SVI seems at an acceptable rate indicating good adaptation

The F/M 0.07 in our plant ( its an advanced WWTP ( 5 stage WWTP, designed to treat BOD, Nitrogen, and phosphor)

The SVI continuously decreased for today the SVI is 50.1

What do you think about that?

Lin Yang

Yes the COD in the influent was increased , but COD in the effluent not changed

Dear Weam Mahdawi

SVI looks OK. Depending on the operating temperature (unknown), the F/M (in terms of BOD or COD ?) is low but OK. However the SRT shall be monitored and controlled as to maintain sufficient activity for C, N, P removal.

Bruno Peeters

thank you very much,

What is the Suitable SOUR for aerobic treatment ?

The SVI of any activated sludge plant will change in response to the operating conditions of the plant.

In particular, under feeding or over feeding the plant can cause a huge effect on the SVI.

Note COD is not a good control in itself. COD and BOD can differ significantly. It is possible to develop a relationship between COD and BOD but note that it will differ for each chemical compound in the feed. The reliance on COD as a measure of feed load can lead to incorrect F/M ratio.

Changes in salinity can also have profound effect on SVI as can oxygen levels and sludge age.

You should also note that certain organic components of the feed can be very volatile. The result of this is that these compounds are often stripped by ghe aeration action before the microorganisms can consume them meaning that the measured F/M and the real F/M are at odds.

Low F/M leads to filamentous growth which causes severe settling issues.

Filamentous bacteria can be easily identified under microscope.

Weam Mahdawi

Normal SOUR range for AS on municipal sewage at 20°C operating temperature would be 12 - 20 mg/(h.gVSS).

An answer in part is: the reduced SVI is generally a good sign that the sludge age has increased to a desirable value; or it could mean that a problem with filiamentous organisms is gone, or it could mean that the sludge age is getting too long if it continues to drop (resulting in poor effluent quality). How is the effluent quality?

Bruno Peeters

Thank you very much

Dear Harlan,

Harlan Glen Kelly

Yes I am agree with you low SVI means good ,

The effluent of our WWTP very good . TSS around 10 mg/l, COD below 40 mg/l, and the BOD around 4 mg/l. (very good effluent )

But my question was about the reasons of the drop , two weeks ago the SVI was 160 - 150. but during this week the SVI was continuously decreased. Note: 1) the operation not change. 2) The MLSS not change

Agree with Bruno Peeters . The sludge Volume depends on C, N and P Ratio. Decrese in Sludge quantity and unchanged effluent characteristics is an indication that all the Microbial activities are going on smoothly. The reason behind sudden sludge reduction may incresed in microbial popullation and Sufficient SRT.

I might be sampling or analytical error

Vernon Stack

The F:M ratio and the SRT which is directly related to F:M influence the settleability and compactibility of the sludge . When biomass is in the endogenous decay it tend to form polymers that result into natural floocculation . In the reactor low sludge ages the sludge tends to become populated with filamentous organisms that exhibit poor settleability and sludge does not flocculate well

In the system the settled sludge is returned from the clarifier to the aeration basin to maintain the desired food to microorganism ratio. The most accurate method of determining the returned sludge is based on SVI. The SVI test determines the settling behavior of MLSS. The high SVI in your plant showed that the sludge from a conventional aeration basin does not settle well and fills up the clarifiers.Low SVI is an indicative of sludge that settles well. Please maintain now the concentration of VSS and TSS in the mixed liquor to always getting low SVI.

The F:M ratio and the SRT which is directly related to F:M influence the settleability and compactibility of the sludge . When biomass is in the endogenous decay it tend to form polymers that result into natural floocculation . In the reactor low sludge ages the sludge tends to become populated with filamentous organisms that exhibit poor settleability and sludge does not flocculate well

At the other extreme of highly starved conditions or a very high SRT , the sludge forms needle floc and does not flocculate

Good question

Yes Sir you are absolutely right Dr K N Sheth Sir

I shall follow Prof Sheth Sir's answer

https://www.biologicalwasteexpert.com/blog/my-svi-or-sv30-is-too-low-what-is-the-cause-and-fix

The cause of low SVI being low F/M (long SRT) as explained in the blog referred by Chuck A Arize is consistent with my/our experience over many decades. However this drop in SVI occurs gradually over many weeks and even months (several SRT cycles) and does not explain the fast drop in SVI reported by Weam Mahdawi back in July 2018. According to my/our experience such fast drop in SVI (rise in biosludge density) is usually caused by the microbial reaction in the biosludge to an increased level of toxic/inhibitive compounds in the wastewater feed. This toxic shock effect is exploited as a short term measure to control bulking sludge with biosludge washout in the clarifier by adding a well quantified shot of sodium hypochlorite (bleach) in the aeration basin.

I agree with Mr. Bruno Peeters, Such a fast SVI reduction will be caused by the toxic effects. We used this before by adding chlorine solution to the return sludge cycle in order to reduce SVI. It works immediately and control filamentous types but its not the solution while the main cause of bulking dose not addressed