Hi all,
I can't quite get my head around the veeery slow rate of atmospheric CO2 diffusion in to water and plant demand...surely demand would soon outstrip supply...regardless of surface area : volume ratio and trickle filters etc...
No it doesn't, CO2 is actually quite soluble in water, so even though there is only ~400ppm in the atmosphere if you have a large enough diffusion surface you can get the levels of CO2 fairly close to equilibrium values. The other great thing about a large gas exchange surface is that it is a negative feedback circuit, when the concentration gradient is steep, gas will diffuse in/out more quickly, as levels approach equilibrium rates slow.
The same applies to oxygen as CO2, but with the difference that you have a lot more in the atmosphere (22%), but O2 is a lot less soluble in water than CO2. I'll stick with dissolved oxygen, but by definition the same processes apply to all gases that have differing concentrations at the gas exchange surface.
If you use pollution as your measure of oxygen demand (so we are talking about BOD, Biochemical Oxygen Demand) you can ameliorate a huge bioload with a trickle filter. This is a waste water example, but landfill leachate and sewage are really just uber-polluted tank water <
http://water.me.vccs.edu/courses/ENV149/trickling.htm>.
High-rate trickling filters have been used advantageously for pre-treatment of industrial wastes and unusually strong wastewaters. When so used they are called "roughing filters". With these filters the BOD loading is usually in excess of 110 pounds of BOD per 1000 cubic feet of filter medium. Generally, most organic wastes can be successfully treated by trickling filtration. Normally food processing, textile, fermentation and some pharmaceutical process wastes are amenable to trickling filtration.
If I'm not talking to the converted, I'd add plants into the equation. I think people should have access to this paper <
http://www.feng.unimas.my/ujce/images/article/volume12013/paper%203encrypted.pdf>, and it has an interesting take on a free self-produced filter media (unless you are thinning on top like me).
Biological filtration capacity is a measure of oxygen exchange, and what applies to oxygen also applies to the other atmospheric gases. You can maximize gas diffusion in various ways, including having laminar flow <
Feature Article: Water Flow is More Important for Corals Than Light, Part V — Advanced Aquarist | Aquarist Magazine and Blog> which is very effective, but technically more difficult than having a large gas exchange surface in the filter. If you have a wide shallow tank you have advantages in terms of area to volume ratio, but deep tanks can still be aerated by laminar flow, as long as you have enough water turn over.
cheers Darrel