Water and Wastewater.com Help Forum - SBR settling question

Water and Wastewater.com Help Forum - SBR settling question

A very puzzling discussion - SBR settling question

Hello,
I run an SBR and have had a baffling situation since I began there about a year ago. Maybe someone has had something similar.
Settling in a settleometer, cylinder, bucket, square bucket and just about everything else I've tried settles to about 40% after 30 minutes with an SVI around 100-125. All that's great except that the actual settling in the SBR is always around 80%(ie an 11 foot blanket in a 14 foot volume of water). In addition, the blanket always follows the decanter down during decant. In other words, 30 minutes into decant, there's still at least 3 feet of clear water.
It's not really a major problem but it drives me nuts! I can take a square bucket worth of MLSS out of the tank(the same surface to volume ratio as the SBR) when it goes into settle, don't stir it and set it right next to the tank. The SBR settles to 80% and the bucket settles to 40% in 30 minutes.
Any ideas why or what to do? The only thing I wonder is some sort of electrical grounding issue but I'm not sure.
Thanks for any input.
Jeff

Re: SBR settling question

Dear Jeff, Dear Victor Santa Cruz,

if I got it right, you meant that settling in the SBR is always worse (only 20 % of clear water layer).
This is contrary to what one could expect comparing Sludge Volume Index and sludge settling in a real basin.
Wall effects as mentioned by Victor Santa Cruz would cause poorer settling in the measuring cylinder! Further, same surface to volume ratio does not ensure compareability. Besides wall effects "sludge excess pressure" (proportional to the mass of solids/area) has to be taken into account.
However, all this would justify more extensive settling in the basin.
I see two possible explanation.
You have a kind of upflow in your SBR not present in the bucket. However, this seems a little bit mysterious.
More likely, sampling for MLSS and SVI is not representative.
Possibly, in your SBR is a concentration gradient.
So I would suggest to do sampling at various depth and determination of MLSS and SVI.
Would be nice if you could share your findings with the help forum.

Kind regards
T. Sobisch


Re: SBR settling question

Thanks for the responses.
Yes, you're right. I have 2 to 3 feet of clear water on top of my blanket but in ANY SVI test I have about 50% to 60% of the volume as clear water after 30 minutes alone. I'm pretty certain we get a representative sample. Always at the end of react and from varying depths and locations. Also, I can put the MLSS in any container and the more it resembles the actual SBRs (dimension-wise), the better the SVI..(makes sense to me)..That is until I look at the true settling IN the SBR and then I'm lost again!
The other very baffling part is the fact that, after an hour of settling I get 2 to 3 feet of clear water on top. However, after only 30 minutes of Decant, the tank's dropped almost 2 feet and I STILL have 2 to 3 feet of clear water on top. It's almost like as soon as decant starts, then the settling really starts hauling! That, combined with the consistant SVI and blanket discrepency makes me wonder if there's some kind of static charge that is grounded out when decant starts.
I talked to one of our state's operator trainers and he said he heard of this situation only one other time and it was in an SBR. Unfortunatly, he didn't know the outcome.


Re: SBR settling question

Dear Jeff,

the static charge hypothesis is not quite clear. If it is related to wall effects it would play a minor role only. If it is related to a sedimentation or streaming potential I could not see why it should be different in a separate sample. In every case charge effects can only be important if the conductivity is low. So could you check this?
It seems to me that the problem is related to the unability of taking an 'undisturbed sample'. So far I cannot image what is the origin of the 'hindered' settling of the undisturbed sludge body. However, when doing the sampling or when starting decanting the hindering is abolished. Maybe this offers the opportunity for a specialised technique - namely slurping of the clear water from the surface at one point?
I am curious about the story being continued.

Kind regards
T. Sobisch