Water and Wastewater.com Help Forums - Carriers mixing in a mbbr

Tecnico2

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Carriers mixing in a mbbr
Jan 20^th, 2012, 5:24am

Hi everybody,

We are just starting-up a mbbr pilot plant and we are having some problems with the mixing of the carriers. Before feeding the wastewater we want to check how to get the best movement of them. But no matter what we do about the aeration grid or air flow, our carriers will acumulate on the reactor surface. Here are the main parameters:

Tank dimension: 1X1X0.6 (m) (length x height x wide)
Type of carriers: anox kaldnes K3
Fill fraction: 27%
Carrier density: 0.95 g/cc
Blower capacity: 0-300 Nl/min
Aeration grid: PVC tube, holes of 2 mm diameter every 30 mm.

He have tried with different aeration grid configuration: just on the longest sides of the reactor, perimetrical aeration, perimetrical and central aeration...

I've read in many articles, commercial information and even in this forum that mixing of the carriers needs unevenly aeration, some time to develop (from 12 hours to 2-3 days!!) and it improves with biofilm on them but I guess we should see some kind of mixing before the biofilm develops itself. The only thing we see is a top layer of them on the surface.

I will appreciate very much if someone can help us with this issue as soon as possible.

Have a nice weekend!

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grrun

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Re: Carriers mixing in a mbbr
Reply #1 - Jan 20^th, 2012, 5:06pm

I haven't had any direct experience with MBBRs, but the density of the carriers is less than water and the biofilm hasn't established itself on the surface of the carriers. Secondly, I'm wondering if the geometry of your reactor is adequate to allow the carriers to be well mixed. Have you contacted anox kaldnes to determine if your geometry and air flow system is adequate? I would go ahead and start feeding the reactor to see if the growth on the carriers improves the density and allows for better circulation.

grrun

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DS God Member Offline sapere aude Posts: 1583 Company or Organization: Indept. Freelance Consultant	Re: Carriers mixing in a mbbr Reply #2 - Jan 21^st, 2012, 8:22am Hi Julian, I think this tank is too shallow to make the bed moving. Besides that, this tank is very small (0.6 m³), what is the reason for that? is it a pilot or does it have to be that small?
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Tecnico2

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Re: Carriers mixing in a mbbr
Reply #3 - Jan 22^nd, 2012, 7:12am

Hi,

I've talked with the carriers supplier and they told me the same that Grrun: go ahead with the process and wait to the biofilm develops itself. Besides that, they told me aeration grid must be evenly distributed on the bioreactor bottom, wich is different that I've read in differents posts in this forum (??).

DS, the bioreactor is that small (0.6 m³) because is a pilot plant on wich we want to understand and test the MBBR process. I've read in many papers about MBBR's of just 50 liters volume or less. So, mine may be one of the largest?

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DS

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Re: Carriers mixing in a mbbr
Reply #4 - Jan 22^nd, 2012, 9:04am

Hi Julian,

biofilm development will help in making the carriers a bit more heavy, but I would not bet on it that this will breng the final solution.

In my experience, uneven aeration gives a great mixing effect and even ungrown (and much larger, so much stonger floating) carriers can be taken down till the bottom (5.5 m) with this type of aeration. The deeper you inject or bubble the air in the tank, the stronger this effect is.

very important in mbbr is the hydrodynamic movement and shear rate of the biofilm. this way (low shear du to low movement of cariers), your pilot will not be representative. Bigger not allways is better...

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Gustavo Duarte

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Re: Carriers mixing in a mbbr
Reply #5 - Feb 2^nd, 2012, 10:42am

Hi Julian,

I've never operated a MBBR plant, but I've read a lot about it. Have you tried to decrease the fill fraction? I was wondering about the comment made by DS that the deeper you inject or bubble the air in the tank, the stronger the mixing effect is. Descreasing the filling factor can be a way to increase the distance between the aeration grid and the carriers.

The fill fraction can be another variable of testing.

Regards!

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Regards,
Gustavo Duarte

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Tecnico2

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Re: Carriers mixing in a mbbr
Reply #6 - Feb 2^nd, 2012, 11:14am

Hi everybody,

The problem about the mixing of the carriers is solved already. I followed grrun and my carriers suplier instructions and I went ahead with the process.

As soon as the carriers got wet with the raw water the mixing was perfect (with a 67% fill fraction). No matter how the aeration was since we changed the aeration grid with a set of valves which allow us to aerate in several ways.

Deepness of aeration is also important in order to get a good oxygen transfer .This factor is very important when coarse bubble diffusers are used. Nevertheless, I've read that carriers break coarse bubbles into fine bubbles improving oxygen transfer.

Now we are waiting for the biofilm develops itself, which seems to need some time. We started the process one week ago.

So thank you Grrun, DS and Gustavo. This forum is not just really useful but a school itself.

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