Re: [compost_tea] Question about dissolved oxygen and air stones

From: Robert Norsen <bnbrew_at_yahoo.com>
Date: Tue, 28 Oct 2003 22:32:09 -0800 (PST)

 
Thanks for your input David. Glad to hear your tests confirmed ours.
We run that way as our standard system. Waves that rock the tank. The top of the tank looks like a heavy storm pounding a rocky shore. Bob

David Hall <dchall8_at_texas.net> wrote:I get interesting stuff from the aquarium folks every now and then.
Recently I got this message indicating that if I took off my
bubblers I would get better oxygen infiltration to the tea. First
of all, I pointed out that a good bubbler would add the equivalent
of 4 million square feet of surface area per day. He blew that
off. Here is his rationale.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
David - You are absolutely correct on the math, if the majority of
the gas exchange happened between bubble and water. Here's the
thing - if you consider the surface area of the container (wider is
better than deep) start with it completely stagnant - say a surface
area of 4 sqft. now add a few ripples from bubbles.... much larger
surface... Now add big violent bubbles that give it that 'perfect
storm' level chop on top and you get a LOT more surface area.

In the environment of a compost tea with so many dissolved organic
compounds (which you see from the foam) the surface tension around
each bubble is not nearly as good for gas exchange as say a soft
water environment without many dissolved proteins. By violently
agitating the top you get a lot more oxygen into the mix - the
amount of air going in through the air pump is tiny compared to the
amount of air passing over the surface so keeping that tight foam
(which small bubbles make) off the top and agitating the c*** out of
it you get better gas exchange - i had the exact same problem the
first time I heard this and I must have Missouri roots somewhere
because i said 'SHOW ME!".

I found this one to be true one summer when it got to about 110
[degrees F] in my hatchery - had a few thousand dime-sized Angelfish
in a 500 gallon vat and they were not doing well - the heat had
lowered the dissolved O2 capacity drastically. So i took off the air
stones (teeny bubbles) and pumped the air straight in, the dissolved
O2 went right back up and ammonia down since there was more oxygen
for the nitrifying bacteria to filter the excrement. I ran the math
all afternoon scratching my head until i got around to modeling the
surface disturbance on the 'puter.... yeah, I'm a geek, I admit it.

Second thing - air stones clog quite a bit over time and by yanking
them off you get better airflow so while bubble surface area is down
you go from fizzy surface on top to boiling water turbulence. You'll
also get more circulation which helps eliminate dead spots that will
go anaerobic.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Okay, quote's over...back to me now. His point on clogging was that
air stones clog from the inside with dust as it gets picked up from
dusty garages where the smaller tea brewers usually are.

But his overall point is that by simply running the air hose wide
open, you develop a roiling surface with less of that nasty old air-
blocking foam and more of just circulation within and on the
surface.

Still, I believe he is implying that by removing the airstone, he
was able to inject O2 beyond the saturation point for his 110 degree
tank. Maybe he was not saturated and removing the air stone allowed
him to saturate??

I'm not equipped to test this idea. I would think the simplest test
would be to run an DO test in a batch of tea with an air stone and
then unplug the air stone, let the batch run awhile, and test
again. Has anyone run this test, or are you willing to run it?

David Hall
San Antonio, TX


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Received on Wed Oct 29 2003 - 10:45:42 EST

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