Re:"Wonder Water", O2 stauration, etc...

> From: Jim Kostich <jkos at execpc_com>
> Date: Wed, 26 Jun 1996 13:38:10 +0700
> I'm still trying to grasp what I few others have been trying to
> explain about "partial pressures" and such, but it seems to me that
> I've frequently heard plant gurus state that carbon dioxide is somehow
> "driven out" by aeration, implying that somehow the additional
> dissolved gases replace the CO2.  Is this true, or is the CO2 removed
> by some other mechanism?

No, I think you are adding your own interpretation of "driven out".
Think of aeration as providing a convenient "escape route" for excess

Theoretically, a tank of water at typical aquarium temperatures in
equilibirium with the atmosphere will have roughly 0.5 mg/l of
dissolved CO2.  The chemist gurus can explain why in terms of
solubility and partial pressures.  I tend to think of it as the
"equivalent" concentrations in the air and the water are the same and
CO2 will have no reason to diffuse across the air/water interface.

If there is a higher level of CO2 in the water, either due to bioload
respiration (typically 3-5 mg/l) or CO2 injection (10-20 mg/l), then
CO2 will try to diffuse into the region of lower concentration, i.e.,
from the water to the atmosphere.  In a quiet aquarium, the surface
layer of water would be close to equilibrium and layers further down
would have increasingly higher concentrations, with the whole system
trying to reach a state where all concentrations are equal.

With aereation, three things are happening.  The bubbles increase the
surface area of the aquarium a little bit, allowing faster diffusion.
They also expose some of the higher CO2 concentration areas in the
water lower down to localized low concentrations (the CO2
concentration in the bubble).  Most importantly, the turbulence allows
the water to mix and keeps the water around the water/air interface
supplied with higher concentrations, allowing diffision to occur at a
more rapid rate.
So aeration is not "driving out" CO2 but is simply increasing the rate
at which the CO2 is able to equilibrate.  Aeration also "adds" oxygen
the same way - the bioload uses the oxygen in the water, lowering the
relative concentration and the mixing allows more rapid diffusion from
higher concentration in the air.

In the opposite sense, an efficient CO2 reactor attempts to keep the
CO2 in contact with the water as long as possible so it has time to 
diffuse from the higher CO2 concentration in the reactor to the lower 
concentration in the water. 

Fish respiration works the same way.  CO2 laden blood with a
relatively high concentration passes through the gills where it comes
into close contact with water with a relatively low CO2 concentration
and diffuses from the blood to the water.  When water has a very high
level of dissolved CO2, no diffusion occurs from the blood to the
water and the fish suffocates.