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[APD] Re: CO2 reactor placement, post or pre?

Lew Newcomb was kind in commenting on my rather picayune
discourse on CO2 reactor placement, especially since I was
basically saying it works fine either way.  He asked:

> Does a reactor have to be counter-current?  I thought I
> saw someone's
> positive opinion of gas/water flowing in the same
> direction - no?

Well, you can try anything and see what happens ;-) . And
there is a way it can work with the water entering the
bottom instead of the top.

Your basic reactor is essentially a box (or call it a tube
if it's cylindrical) with water fittings at either end and
an opening somehwere for CO2 to be pushed in under a slight
pressure (5, 10, 15, 20 psi, something like that).  The CO2
is often entered at the bottom but I don't think that's
really all the critical in most situations with the water
entering at the top.  The larger CO2 bubbles, freshly
entering the box climb to the top and the water pushes
through it.  Some amount might even collect at the top, but
if the water flow is adequate, that gas at the top won't
keep getting bigger and bigger -- just so big and the rest
is absorbed.  The gas has no where esle to go :-)  .

If the water flows into the bottom and out the top, the CO2
will want to go right to and through the top opening too --
even faster than the water.  You could try curving the
outlet hose from the reactor to stop the bubbles at the
"crown" of the curve, but the water flow in the tube will
probably carry the bubbles away too quickly.

However, with the CO2 and water entering at the bottom, you
might still get enough absorbtion under some circumstances.
 For each CO2 bubble that you push into the box, it's
possible that it will be absorbed so quickly that by the
time a bubble reaches the top of the box, it is very small 
-- sort of the way a diffuser works in an aquarium.  The
very small bubbles aren't all CO2 -- traveling through the
water the bubble sheds some CO2 and picks up other gases
from the water, so it contains a mix of other gases from
the water and not just CO2.  Not as big a deal to lose
those tiny bubbles.  But I think this forward flow setup is
unlikely to work in most situations.  Too much CO2 will
just race out of the reactor.

The real beauty of a reactor is that, with the water
entering at the top, the CO2 *cannot get out except by
being absorbed into the water*.  Is there an exception to
that?  Yes, definitely!  If the water flow is swift enough,
very small CO2 bubbles can be blown right out the bottom
fitting of the reactor.  Having bioballs, or mesh, or
rocks, or some such thing in the reactor helps to prevent
that spreading out the force of the flow until it reach the
bottom fitting.  If the reactor is too small for the amount
of water you put through it, even the bioballs or mesh,
etc. won't help enough -- you'll still blow out a lot of
CO2 as tiny bubbles.

I guess a sort of middle method would be to try a reactor
on its side.  Never tried that!  I suspect more CO2 would
be flushed out as bubbles than with the reverse flow setup
but maybe not as much as with the forward flow setup.

Generally you don't need a really teriffic flow of water to
make a reactor work, but if you filter or pump output is
weak enough that you want to use forward flow -- hmmm, I'd
get another pump or filter but thats not a practical option
for everyone. Once you have a reactor, all it costs to try
it the diff ways is your time and labor.

Give it a go and report back.

Hope that helps,
Scott H.

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