Is there any relationship between SV 30 and MLSS values ? 2

[RuwanR]

We are having a MBR treatment plant dedicated for a dairy industry.We are getting a 1500 ml SV 30 value and 6000 mg/l MLSS in aeration tanks. Our daily BOD load is 800 kg.I cant figure out any relationship between these two.

 

[ashtree]

These values are somewhat related but not directly linked. MLSS is a measure of the amount of solids in the tank. The SV30 is a measure of the volume to which those particular solids will settle in 30 mins. If you were able to maintain a fairly constant MLSS value over a long period of time the SV30 value would tell you if settle-ability was improving or worsening.

If you sampled today and got a certain SV30 value, then went and pumped out half the MLSS your SV30 value should be half of what it earlier. However unless your MLSS is relatively constant then the SV30 value is not much use.
Typically above 4000 MLSS the settling can be quite slow anyway as there is to much water entrained in the system to compact quickly.

You might want to try using Sludge Volume Index (SVI) as a reference tool although at higher MLSS values the results are less useful. This is defined as the volume in ml that one gram of sludge occupies.

The formula is SVI= Settled volume of sludge (in percent of the measuring cylinder) after 30 mins/ MLSS % (10000mg/l=1%)

So as an example where you had 6000 mg/l MLSS and your SV30 was 60%.

SVI= 60%/0.6%MLSS
SVI=100ml/g

The other question i have is this. With an MBR the solids liquid separation is performed by the membranes not by a sedimentation process. In this circumstance the settling performance of the MLSS is not very important because the solids do not need to be settled for the system to work. This is actually one of the benefits of the MBR system is that effluent water quality is independent of settleability.
So why the concern?




Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[RuwanR]

Please make me understand the following scenario also.If I happened to run the plant in the conventional gravity clarifier mode(in case of MBR breakdown) , what are the operational changes that I have to perform ,relative to these above mentioned conditions.

 

[ashtree]

Okay but you have not given enough information.

You say you have 1500ml of sludge when you are doing the SV30 test. Is that 1500ml in a 2 litre settleometer which should be 75%?

If this is correct this would give an SVI of 125.

Typical SVIs

Less than 80 excellent
80-150 good
150-200 fair
above 200 poor.

So at 125 your SVI is good. However at 6000mg/l MLSS you simply may have too much sludge and will overload your clarifier regardless of how well it settles. Clarifiers have a hydraulic load limit and a solids load limit. If you exceed either you will most likely get poor performance and solids in the effluent.

I would guess that at 6000 MLSS you will have too much sludge.

Solids loading will depend upon the following factors:

1) MLSS
2) Clarifier surface area and volume
3) Weir length
4) Return activated sludge flow rate(RAS).
5) Influent flow rate average and peak
6) Sludge settling rate.

You have some control over MLSS as it is dependent upon your wasting rate.
You may have control over your return activated sludge flow rate.
And you can over time influence your sludge settling rate.
All the other factors are fixed or beyond your control.

For now you need to understand what capacity you have, and what you need. You need to know what your RAS flow is and how much wasting you are doing. Likewise you need to know what your daily flow is , and what the peak is. Likewise you will need to know how big the clarifier is to be able to calculate the solids loading.






Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[RuwanR]

Sir this is regarding our SV 30 , Our activated sludge doesn't settle at all in a settlometer.So what we have advised to do is ,get 250 ml activated sludge sample ,dilute with permeate up to 1 L ,then let it settle for 30 min ,multiply the value by 4.That's how we get the 1500 ml (150 %).
According to this our actual SVI exceeds 200.

 

[ashtree]

The test that has been described to you is normally done to see whether or not the sludge has good settle ability but there is simply too much to settle. In other words with so much sludge is it just the fact that there is no room to settle.

Using your numbers and back calculating gives an SVI of something like 246 but under the circumstances i am not really sure that it is of any value. The sludge is obviously "poor settling".

Unless you have a very large area settling tank you and you operate at conservative surface overflow rates you will probably have problems at your current MLSS. You will need to waste a considerable volume before using the settling tank.

Another problem is that MBRs can actually select poor settling floc. In a conventional settling tank poor settling floc can be carried over with the effluent thus reducing the population. However this does not happen with an MBR so poor settling floc is maintained and may eventually predominate.





Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[bimr]

Regarding: "Please make me understand the following scenario also. If I happened to run the plant in the conventional gravity clarifier mode(in case of MBR breakdown) , what are the operational changes that I have to perform ,relative to these above mentioned conditions."

MBR wastewater treatment plants operate at much higher MLSS concentrations than wastewater treatment plants that use gravity clarifiers. You would have to reduce the MLSS down to the range of 2,000 to 4,000 mg/L in order to produce a floc that will settle in a gravity clarifier. Higher MLSS concentrations do not settle and this is a process limit for gravity clarifiers.

The need to produce a good settling floc is not a consideration in MBR design.