A Rational Discussion On Oxygen Transfer From An Airstone Bubble

[Bignose]

Stemming from previous conversations (see http/www.fishforums.net/index.php?/topic/377183-previous-discussions-on-oxygen-transfer-from-air-stone-bubbles/ ), this thread will serve as a straightforward discussion about the oxygen transfer from an airstone bubble.

All rules of the forum apply, and all rules of the scientific section apply. In particular, well-accepted science cannot simply be rebutted by disliking it -- other scientific evidence must be presented to rebut it, or demonstrate that it is not as well accepted as originally thought. A lot of what was posted in the above thread

Lastly,

fishguy2727, this is the chance to try to discuss this rationally and scientifically, if you want to. I want to start with a definition of the common equations in mass transfer, to make sure we start at a common place. But, I want to know if you want to participate in this thread and the rules of this section. If not, I'll just delete this thread and junk the other one. The choice is yours. Bignose

 

[Caprichoso]

I've just read these threads in their entirety and I have never had my brain hurt like it does right now. Interesting and informative to say the least but it stressed me out to read it....I don't think it should be deleted, even though it felt like a fist fight with words lol. I learned loads from it, which is kind of the point isn't it?

 

[Bignose]

well, there is a lot of thinly veined insults, from both me and FG in those posts. And I am not sure how salvageable they are. That's why I wanted to start anew in one thread where neither one of us will be allowed to even get close to crossing a line. I'll be leaving this here for some time to see what happens, for now.

 

[Juggler75]

I'll admit I tried to read the threads, as I had been reading the threads where the 'discussion' started, but couldn't get my head round some of the ideas.
I partly understand and will accept the 1954 research on surface area transfer (I think), and that gas exchange is not as efficient within a bubble, however I keep thinking that the air above the tank is relatively static, whereas the airstone is constantly producing new bubbles (of various sizes).

So my question would be in a tank with dimensions of 24x12x15 (for sake of argument) what would be the required airstone/air pump combo to produce the same gaseous exchange from the bubbles as there would be from the surface? I don't use an airpump/airstone but before I read these threads I thought maybe I was missing a beat now not so sure.

Has there been any research done on these factors? If so what was the result?

Sorry if these questions have been asked and answered, like I said my brain was hurting and unfortunately my concentration slipping as I read on.

 

[Mikaila31]

While I completely agree that more O2 exchange occurs at the surface. There was something about the previous thread that got me thinking. O2 levels should never be an issue in the aquarium even without any bubbles/surface disruption, this is common sense. But shouldn't surface diffusion or diffusion through bubbles alone provide enough O2 for fish to survive without an issue? Fish only need 5-8ppm of O2 in the water. I inject CO2 into my tanks, which is what got me thinking. This injection is just basically diffusion from bubbles of CO2 rising through the water. Though an actual diffuser puts out very tiny bubbles compared to an air stone whose primary purpose is to circulate water. I understand the smaller bubbles will have a larger surface area to volume ratio compared to the huge air bubbles. But these bubbles raise my CO2 levels from the normal 3-5ppm to around 30ppm. CO2 is also less soluble then O2. So at least from my understanding of this an air pump should provide enough O2 for fish to live happily if there was no other O2 input. I still completely agree that you are going to get more diffusion through the surface then from a normal airpump. I've got no idea if I'm actually right, this was just something that came to my mind.

If you actually used a proper diffuser for O2 I wonder how far from equilibrium you could go... A air pump disrupts the surface with the huge bubbles it produces, this disruption is going to prevent any real gain in O2 compared to the normal equilibrium.

end of my ramblings

 

[DrRob]

Of course, all of this depends on the surface film of the water not being contaminated in any way. Films of algae and such like will throw the numbers completely.

 

[Prime Ordeal]

What a lot of waffle about something that can be fairly easily be proved one way or another by experiment. I don't have an O2 test but if I did I'd fill a bucket with water, affix an airstone to the bottom, pour some immiscible substance onto the surface - white spirit maybe(not perfect) - and do some real science!

 

[grimbeny]

Just to add a little to the discussion.

The surface area of bubbles generated by an air pump is a small fraction of the surface area covered by the top of the tank.

using the terminal velocity of a gas bubble to be 32cm/s (http/www.bubbleology.com/Hydrodynamics.html) and the equation D=V/t, it can be calculated that the average 3mm bubble will be in the water column of a 2ft tank for 0.5s.

So using calculations by fishaholic the average airpump will produce approximately 8.5m bubbles per hour (http/www.fishforums.net/index.php?/topic/66913-thoughts-on-an-airstone-and-lack-of-biowheel/page__p__558234#entry558234)there should be 2400 bubbles in the water column at any one time. So a bubble of 3mm will have a SA of 28mm^2.

So Bubbles x SA accounts for a total surface area of the bubbles being 66cm^2.

If our tank was a 2ft cube then the total surface area of the waters surface will be 3600cm^2.

There for all asside the surface area of the bubbles is approximately only 2% of the top surface of the water.

 

[CezzaXV]

I'm not into the science as much, but presumably where there aren't bubbles there will be water rippling, which is where I've been led to believe the oxygen transfer actually happens - the bubbles are just there to facilitate water movement. Tell me if I'm wrong because I don't claim to know a great deal on the subject, but even though the surface area the bubbles are on are 66cm2, the surface area that is rippling due to the bubbles is going to be much greater than this?

 

[Squidward]

Not scientfic proof, but more practical experience.

A few years ago, I check on my fish one morning to find most of them gasping at the surface. At the time I had an overstocked Malawi tank.
Fortunately it was the weekend, so I rushed to my lfs, bought an airpump and airstone and fitted it to my tank. Within an hour all my fish were behaving normally again.

A few months later the plastic sticking disc holding the airstone in place came loose, and the airstone floated to the top of the water surface, and all the bubbles were being released into the air above. Once gain fish were at the surface. And once again, within a short time of my refixing the airstone, they were fine again.

As I say, not scientific proof, but from my experience, an airstone does improve the Oxygen levels in the water.

 

[afremont]

Just to add a little to the discussion.

The surface area of bubbles generated by an air pump is a small fraction of the surface area covered by the top of the tank.

using the terminal velocity of a gas bubble to be 32cm/s (http/www.bubbleology.com/Hydrodynamics.html) and the equation D=V/t, it can be calculated that the average 3mm bubble will be in the water column of a 2ft tank for 0.5s.

So using calculations by fishaholic the average airpump will produce approximately 8.5m bubbles per hour (http/www.fishforums.net/index.php?/topic/66913-thoughts-on-an-airstone-and-lack-of-biowheel/page__p__558234#entry558234)there should be 2400 bubbles in the water column at any one time. So a bubble of 3mm will have a SA of 28mm^2.

So Bubbles x SA accounts for a total surface area of the bubbles being 66cm^2.

If our tank was a 2ft cube then the total surface area of the waters surface will be 3600cm^2.

There for all asside the surface area of the bubbles is approximately only 2% of the top surface of the water.

Wait a minute. You said the terminal velocity of the bubble was 32cm/sec. The web link says that the bubble "rapidly accelerates to the terminal velocity". I'll give you the benefit of the doubt and assuming that the bubble emerges already traveling at terminal velocity, it will be in the tank for approx 2 seconds, not .5 seconds. Therefore there should be four times as many bubbles in the column. Unless I'm making a mistake somewhere?

I did a few quick calcs and it look like a puny 2L/min pump would supply 400,000 3mm bubbles per hour, so a million sounds reasonable for a decent sized pump. So once we adjust the 66sqcm by a factor of four, the number starts to become an appreciable fraction of the surface area at 7.5%. If not for natural circulation of the water column, surface diffusion to the bottom would take a very long time. The airstone provides the added benefit of circulating the water quite well and breaking the surface tension which is a requirement to get any real diffusion anyway, right? Isn't that why filters are designed to disturb the surface by spraying it or pouring a constant stream of water thru it?

EDIT: Source for my claim on diffusion in still water:

http/www.fao.org/docrep/field/003/AC183E/AC183E04.htm

Oxygen is absorbed in water by direct diffusion and by surface-water agitation. Solubility of oxygen in water is so small and by diffusion process alone in still water, it was culculated that it would take 6 years for oxygen to diffuse from surface to a depth of 6 meters in quiet water. Absorption of water is very minor, that almost all the oxygen enrichment of natural waters takes place by agitation of water.

 

[Tymburton]

Hi, fairly new around here. But this is an interesting subject and one which i might be able to provide some input into.

A while ago i was doing experiments on oxygen content in water, from my personal experience an air stone does vastly increase the O2 conc. within a volume of water.

I was super saturating 50l of water which was held at 20 +- 0.5 C and normal atmospheric pressure. The O2 conc. of the water measured at about 10cm from the bottom of the tank was at the highest possible conc given the atmospheric conditions for argument sake lets say that on average it was 9.2 Mg/l O2. With the air pump switched off (read broken) this would drop to around 6mg/l O2.

Temperature and pressure play a huge part in the oxygen conc of water, the higher the atmospheric pressure and the lower the temp the more O2 is possible to be contained within the volume of water. Lower atmospheric pressure and higher temp will mean that less O2 is stored in the volume of water.

I also don't think that the bubbles created from an airstone are as big as 3mm, they are more like 1mm, smaller bubbles have a greater surface area and diffusion properties than larger bubbles.

When I get my tank setup I might be able to provide some evidence of this as I currently have access to a proper DO meter and a few other tasty bits of kit so I should be able to conduct a few experiments.

 

[Bignose]

Tymburton, Your description of the experiment there is incomplete at best. The discussion always centered about whether the surface of the tank is more dominant than the bubbles themselves. In your experiment, how did you prevent the surface of the tank from not adding any oxygen? Did you compare just pump on or off, or did you also compare pump off but with agitation? I have always agreed that the bubbles serve a good function of agitation and thus mixing the water up, which helps transfer gas from the surface as well.

Lastly, you best check your statement: "smaller bubbles have a greater surface area" because that certainly is not true. You probably intended "smaller bubbles have a greater surface area per unit volume of the gas" which is true -- if you have 1 L of volume of gas, 1 large bubble with all 1 L in it will have less surface area than 2 bubbles with 0.5 L each, assuming the bubbles are spherical, which isn't always true.

 

[atlas101]

Just to add a little to the discussion.

The surface area of bubbles generated by an air pump is a small fraction of the surface area covered by the top of the tank.

using the terminal velocity of a gas bubble to be 32cm/s (http/www.bubbleology.com/Hydrodynamics.html) and the equation D=V/t, it can be calculated that the average 3mm bubble will be in the water column of a 2ft tank for 0.5s.

So using calculations by fishaholic the average airpump will produce approximately 8.5m bubbles per hour (http/www.fishforums.net/index.php?/topic/66913-thoughts-on-an-airstone-and-lack-of-biowheel/page__p__558234#entry558234)there should be 2400 bubbles in the water column at any one time. So a bubble of 3mm will have a SA of 28mm^2.

So Bubbles x SA accounts for a total surface area of the bubbles being 66cm^2.

If our tank was a 2ft cube then the total surface area of the waters surface will be 3600cm^2.

There for all asside the surface area of the bubbles is approximately only 2% of the top surface of the water.

Hi, I am just trying to understand your post and I am a little confused? firstly if a bubble travels at 32cm/s in a 2ft tank then surely a bubble is in the water column for 2 seconds not 0.5, and secondly if the surface area of a 3mm bubble is 28mm squared and there are 2400 in the water at any given time then isnt the total bubble surface area 2.8 cm* 2400 = 6720 cm squared at any given time?


and taking into account the 4 times as many bubbles would that not equate to 2.8cm*9600 = 26880 cm squared,an equivalent of roughly 700% of the top surface area rather than 2%

 

[DrRob]

I've always thought, possible incorrectly, that one of the most important things about bubbles in a tank, is that, if you get a film on the water surface, then gas is trapped, exchanging, under it, in a fairly proportional fashion to the degree that the same film would interfere with gas transfer at the surface.

Also the activity of the bubbles would help to break up any blockages. Calculations start to get even more complex if you take into account the degree of water surface disturbance that the bubbles will also cause and the factor of water movement aiding gas exchange at the gas/liquid barrier.