Backyard Aquaponics
A place to discuss aquaponics

I've got six air stones on the outside of my slo, half way down. This creates turbulence in the water and seems to draw the solids to the slo. I have no solids on the tank bottom and none of the fish have complained about the lack of air.
K.I.S.S.
Oxygen will be exchanged at anytime it comes in contact with the water surface above or below the surface.

Oh no.



The information is out there and it is clear once you take the time to sort through it all.

You really cant say it this way or that way because it depends on so many conditions but in most AP systems with airstones on the bottom of the tank a major part of the gas transfer occurs under water and the other major part occurs at the surface.

In answer to the original question:

Placing an air stone near the surface will reduce the amount of gas transfer that occurs underwater without significantly increasing the amount of gas transfer that occurs at the surface.

So if you have the choice have them as low as possible if your pump can handle it (most can be dinky aquarium pumps often can not).

Mine are off the bottom but only because air was being sucked out the drain and causing air restriction that slowed the flow out to the GBs. Raising them a bit allowed the drain flow to flush the air out and get things working again (took me a while to figure that one out).

If you had them on the bottom you might have the same problem I did. You could still probably have them a bit lower than half way though.

Molecules transfer between the air bubble and surrounding water at all points of the bubbles surface. O2 would be coming into the bubble near the top and bottom but would also be going out near the top and bottom at the same time. Also at the same time water vapour, CO2, N2 and other gasses would be doing the same thing and at the sides as well. The rate at which the transfers in versus out are taking place is dependent on the relative concentrations and percentages of saturation of each gas.



This is true but as the pressure decreases the solubility of the gasses decreases which will decrease the rate of transfer into the water.



Once the bubble burst all the air is released to the atmosphere but the water droplets traveling through the air and the disturbed surface of the water are no longer containing an air mass that has a lower concentration of O2 but are exposed to atmospheric air with a higher concentration of O2. This is pretty much going to guarantee that any water droplets expelled into the air that were not at O2 saturation when they were created will be, near as damm, when they return to the water.



I don't think this is an issue to be worried about but another reason that fish hang out near the bottom is because they may be subjected to N2 super saturated water. Fish cope with this problem by getting as deep as possible so that they don't get the bends. This is the reason why you should never have air injection below the level of the FT.

Whether the bubble has a surface or not is probably a philosophical discussion.

Bubbles shapes in water change with air mass and density (pressure). Small air bubbles are spherical and if they are small enough they will stay spherical all the way to the water surface. Air bubbles that are a bit bigger will start with a spherical shape but as they rise and the pressure decreases they will form an anulus (sp?). This means that really small air bubbles, from something like a micro pore diffueser for example, will have a higher surface area because of their spherical shape, small size and high surface area to volume ratio. However as bubble size increases a point is reached where larger bubbles have a higher surface area to volume ratio because rather than a sphere they form an anulus. As bubbles get bigger the surface area to volume ratio would reach another point such the surface area to volume ratio would go down again.

While there have been a number of studies showing that really small bubbles result in increased gas transfer rates versus larger bubbles I know of no studies that have investigated gas transfer while paying particular attention to bubble shape. The results from such studies do show increased DO efficiency but there are other factors that don't appear to be considered. In particular to sizing bubbles to produce a size that results in an anulus rather than a sphere. Since many of these experiments have been done by manufacturers of micro pore diffusers the cynic in me wonders if they increased the bubble size beyond the sweet point (smallest anulus posisble) to make comparison that was destined to be favorable to the micro pore diffuser.




Yes water has a greater surface tension than air but whether the air has a surface or not is debatable. What is not is that the gas tranfer occurs at the boundary of the air and water. Whether this boundary is an air volume surrounded by water or a water volume surrounded by air gas transfer will still occur. One of the reasons baki showers, trickle filters, LHO and the like work is because they create a large number of water droplets that travel through air and allow gas transfer to occur between air and water droplets.



Rain drops hit the earth because their rate of evaporation is slow enough that they still retain a volume by the time they hit the ground. Weather conditions often exist where this is not the case and it results in weather that to me is very unpleasant (to me) being often hot and humid. Fortunately it rarely happens around here. Further north is another matter though.

So are you saying that you woudnt aerate below the waters surface or that you woudnt run a stone, etc right at the bottom?

No. Don't aerate below the bottom of the FT.

To put it better don't aerate at a pressure that is greater than the pressure at the bottom of the FT.

If you had a FT that was 1m deep and a sump that was below the FT and 1m deep. Then you could safely aerate the sump at the bottom.

If you had a sump that was 1.5m deep (water level not the structure) then it would be a bad idea to aerate at the bottom of the sump.

In these discussions the use of airlifts often comes up and this in one of the configurations that has been suggested in the past. It vastly increases the amount of DO so much so that the surface exchange becomes relatively minor (the deeper the air injection the less significant the gas exchange at the FT surface) in fact the DO can exceed saturation so O2 can actually be lost rather than gained for the FT surface but it results in dead fish from the bends.

Far out Stuart, that's excellent information. Thanks for taking the time.

Just for interest sake, many years back, before Internet, one of the journals (think it was SciAm but only because that was the one I got monthly) showed the two most extreme places on earth - the direst and wettest. (based on moisture content of the air)

The driest was Antarctica because any moisture is ice very quickly so the air is incredibly dry. The wettest was the Sahara, but only above a certain level (think it was 1000 ft, perhaps 1000m) because although it had the one of the heaviest rainfalls and humidity in the world, the heat reflecting from the desert means it is all water vapour and driven back upwards.

Thanks heaps stuart, i was thinking you mean aerating a sump originally, but then i thought "nah he cant mean that".

I was actually going to experiement with the airlift thing one day, to lower pumping costs. is there a way to stop that? e.g multi stage airlift? sending to nft to lower DO and rebalance before entering the FT?

Stopping it no. Dealing with it maybe.

More pressure increases gas solubility of water and since air is 70% nitrogen hard to get around. I've thought about using deep airlifts to pump out of the FT and hoping that by the time the water got back to the FT the excess N2 would have escaped but I don't know how bacteria and other creepy crawlies would handle the super saturation of N2 and I wouldn't want to break up solids and so make them harder to filter.

You could do it in a series of airlifts but makes for more complexity and expense. Plus I am not convinced that airlifts are that great a low cost pumping method even when you factor in the added O2. They might be, I'm just not convinced. The more I learn about air pumps the less convinced I am. I'm still trying to understand the theories behind compressible flow and multi phase flow.

This would have to be one of the more amusing threads I have read in a while.

I have always thought that fish are tickled by the bubbles and like the movement. Its a bit like stroking a cat or tickling your wife's back (if she likes that sort of thing).

I've seen pictures of the deserts in Antartica where there are a range of mummified animals (seals, penguins, a whale) with the water completely stripped from their corpses. Some of them were hundreds of years old.

From what Stuart says, there's nothing wrong with airlifts as such - provided the airlift is occurring from the same depth as the FT is. You could put your ST 100m underground and add air to the rising tube, provided your air is being pumped into a ST only as deep as your FT.

i.e. If I understand what Stuart is saying, it is NOT the relative heights that matters, it is the pressure under which the air is entering the water column.
Caveat - going by Stuart's diagram, adding the air into the tube heading up is NOT what I am talking about - that tube would have a water column depth of 100m.

e.g. 1m deep FT on the surface has exactly the same pressure as a 1m deep ST 100m below the ground. (for the more pedantic physicists, yes there would be a difference due to distance from the centre of the Earth, but the distance has to be vastly more than 100m before it become significant in terms of what is being talked about here )

Jah, jah, jah!

Atmospheric pressure at surface level is 1 bar, at 1 meter deep it is 1.1 bar, at 2 meter it is 1.2 bar etc.

An air bubble injected at 1 meter deep and a size of, let's say, a diameter of 5 millimeter would expand with 0.45 millimeter when it reaches the surface. A diameter of 10 millimeter would expand with 0.9 millimeter. Wonder who is able to observe that with the naked eye in a turbulent water column?

Of course, this increase would only happen if non of the air would be absorbed by the water. In case there is absorption of part of the air by the water, the increase of the bubble size would even be smaller. Hence, even more difficult to observe with the naked eye.

About fish getting the bends of air injected at high pressure in a fish tank because of nitrogen saturation; sorry, but that one had me lose control of the bladder... Getting more air into a water column in order to raise the DO level is not a case of pressure but a matter of air flow in volume (liters of air). The air is made out of 78.084% N2 (nitrogen), 20.9476% O2 (oxygen) and 0.9684% of other gasses.
If air is injected at 1 meter below the surface and the fish would take it in at the same depth, the nitrogen bubbles that comes with the air mixture would increase in size with 9%. Taken into consideration that most fish (and certainly the ones grown in AC and AP) are by nature used to various depths of a couple of meters, I find it hilarious to believe my fish would get the bends when approaching the tanks surface. A trout coming from 1 meter deep and jumping out of the water would explode in the air, if such a story would be true.

So please stop with spreading these fairy tales. It's all easy to calculate and quite logical to understand.

Without getting into the calculations for it, it's about 2 third of the oxygen that is released from the air into the surrounding water column while it rises to the surface. When it pops at the surface, it increases slightly the surface area of the water, hence increasing the possibility of oxygen exchange between air and water. If at the moment of popping the air pressure is high, exchange will be larger than with low pressure.

Maybe it helps to observe a soda bottle and understand the way that works and how the carbon dioxide is added to the liquid in the manufacturing process?