Previous Discussions On Oxygen Transfer From Air Stone Bubbles

[Bignose]

This thread is a record of the conversations fishguy2727 and I have had in the past. This thread is closed, it just serves as a record of what has been written in the past.

Cut from two threads:

1)http/www.fishforums.net/index.php?/topic/361325-definition-of-a-mature-tank/

2)http/www.fishforums.net/index.php?/topic/376943-no-air-pump/

 

[fishguy2727]

I don't agree with all of the above. Effectively all of your nitrifying bacteria are in the filter. If it stops running the biofilm will not do anything. IME the only thing that will save a tank if the filter stops running is an air stone. Other than that you have a few hours to a day depending on the stocking.

If the heater goes out things are actually a little easier because the metabolism of the fish will slow down. A biofilm will not effect this either way. Certain fish may not tolerate lower temps as well as others, depending on how low they get, but a biofilm will play no part in this.

IMO a mature tank is one that is cycled AND has not had any issues for many months. Even if the tank has been cycled for six months, if you are still having problems of any kind it is not mature yet. It is hard to say if there is a major difference in how a 'young' tank runs compared to a 'mature' tank independent of maintenance. I have also found that how a tank is setup significantly effects how well it runs long term.

I agree that it is more than being cycled and that exactly what it is has yet to be truly understood.

 

[Bignose]

I really think that you're underestimating just how much different things are going on in a mature biofilm. Sure, a lot of nitrifying bacteria congregate in the filter, but not all, simply because not that not where all the nutrients are. Thanks to electrostatic forces (also known a van der Waals forces) whenever a molecule gets too near to any surface, it will be held there. And once it is in that van der Waals force valley, it is very hard to move it away. Consequently, every single surface had at least some nutrients ready to be consumed. This includes ammonia and nitrite, and will naturally attract the appropriate bacteria. And since it isn't just ammonia and nitrite that sticks to surfaces, the surface attracts a wide, wide variety of microoganisms.

regarding this:

IME the only thing that will save a tank if the filter stops running is an air stone.

This may actually be more in the favor of biofilms than you think. Airstones don't really do a whole lot. The bubbles are in contact with the water for too short of a time to do any significant gas exchange. What they can do is increase circulation. And, if you increase circulation, you bring more ammonia and nitrite to a wider variety of surfaces where the biofilm can help take care of it.

regarding this:

If the heater goes out things are actually a little easier because the metabolism of the fish will slow down. A biofilm will not effect this either way. Certain fish may not tolerate lower temps as well as others, depending on how low they get, but a biofilm will play no part in this.

I didn't say anything about how a biofilm will fix the fish's problems. Of course a biofilm isn't going to suddenly make a cool water intolerant fish more tolerant. But a mature biofilm will reduce the amount of organic wastes in the water even if there is a significant change in temperature. Unless you have a great deal of circulation, no tank is completely uniform in its temperature. There are cold spots and warm spots. What I am saying is that this leads to biodiversity. The bacteria that perform better in warm spots will become more dominant in the warmer areas, and same for the cooler ones. The nice thing about this is that if the whole tank gets cooler, you already have bacteria well suited to the slightly cooler temperatures ready to shine. In order to encourage biodiversity, I actually change the heater settings on my tanks every 6 months or so, just so that no one group becomes too dominant.

This biodiversity is not going to completely save a tank that has issues like a broken heater. But it does give you a larger window to do something. It is nothing but anecdote, but I've been out of town when an ice storm killed power to my residence. Normally, when I lose power, the first thing is to float the biowheels in the tank itself. But, of course, I wasn't home. When I did get home 3 days later (and still no power for 24 more hours), the tanks were cool but tests showed only trace ammonia and nitrite. Both tanks had been up and running for many years, and every surface has a very well-established biofilm. I don't think that the tanks would have survived 10 days without power, but 4 days didn't seem to be too bad, there weren't any deaths for example.

 

[fishguy2727]

Bubbles alone spend enough time with water to create massive amounts of gas exchange, if this wasn't true then protein skimmers wouldn't work.

Turn the filter off in a newly setup tank and the air stone alone will keep the fish alive. It isn't ammonia toxicity that kills fish in stagnant tanks, it is the lack of gas exchange (not enough oxygen, too much carbon dioxide).

Effectively all of the nitrifying bacteria are in the filter. If this wasn't true then you wouldn't be able to instantly cycle a new tank by moving only the fish and fitlers together (which I have done many times) and you wouldn't completely throw off a mature tank my changing all the fitler media (which is what will happen if you remove all the filter media).

Changing the temp every six months will not give you better biodiversity, the bacteria have a lifespan of hours, not months, you will simply start favoring a different type (if that).

Biofilms are good, but they are not as powerful as you are making them out to be.

 

[Bignose]

Bubbles alone spend enough time with water to create massive amounts of gas exchange, if this wasn't true then protein skimmers wouldn't work.

Incorrect. Bubbles have very little time in the water and very, very little surface area and consequently the gas exchange rates are almost insignificant.

I actually have a calculation in a post from years ago: http/www.fishforums.net/index.php?showtopic=66913&view=findpost&p=558234 Two more posts down, I estimated 7000+ hours to saturate water just using the exchange from bubbles alone. The top of the tank is where all the significant gas exchange occurs.

An airstone can help in disturbing the top surface and in mixing the water, but the bubbles themselves have very, very little to do with gas exchange.

Effectively all of the nitrifying bacteria are in the filter.

As I wrote above, a lot are. But not --all--. You cannot ignore that electrostatic forces bind molecules -- including ammonia and nitrite to every surface -- and bacteria will accumulate anywhere there is nutrients.

the bacteria have a lifespan of hours, not months

The bacteria in fishtanks are very long lived compared to most other bacteria. They are also exceptionally slow growing. They don't live for months, but they do live longer than just hours.

Changing the temp every six months will not give you better biodiversity,

I think it does because it encourages different strains of bacteria to be dominant at different times.

 

[fishguy2727]

Just because you say it is so doesn't make it true. Oxygen and CO2 diffuse, VERY quickly. If this wasn't true than protein skimmers wouldn't work. I have read multiple books that stated that the only aeration needed in a marine tank is provided by protein skimmers, which are nothing more than bubbles in water. Even just one air stone can create a lot of surface area, exactly the same as the surface area at the top of the tank. Oxygen and CO2 don't know if they are in the air, bubbles, the water, etc. They will diffuse. There is more than enough aeration created by the bubbles themselves to create significant aeration.

I am glad your own estimate supports your idea, imagine that.

The fish tank described is a horrible example of how to keep fish. That tank WILL crash, I guarantee it. No water changes and no filtration will lead to a crashed tank. 14 months of living and even some breeding using some of the hardiest fish in the world is not proof that it is a good way to keep fish.

Obviously if there is no filter there is no way for the bacteria to grow in it. In a tank with a filter the bacteria will be primarily (not only) in the filter because the flow is the highest there. This provides oxygen and food better than any other location in the tank. This doesn't mean the rest of the tank is sterile, obviously. What it means is that if you have enough filtration effectively all the bacteria will be there, not elsewhere. If this wasn't true than you could never move a setup by moving only the filters and livestock all at once. Filter or not the bacteria will grow where conditions are best. Without a filter this may mean anywhere there is higher flow. Without any difference in flow it will be spread out a lot more than any other tank.

Greenwater does not indicate ammonia. Like other algaes it needs nitrate and phosphate (as well as light and the other essentials). In fact greenwater is a very healthy thing to have for fish like goldfish and koi, they can actually eat it. It does not require ammonia at all.

 

[Bignose]

Just because you say it is so doesn't make it true. Oxygen and CO2 diffuse, VERY quickly. If this wasn't true than protein skimmers wouldn't work. I have read multiple books that stated that the only aeration needed in a marine tank is provided by protein skimmers, which are nothing more than bubbles in water. Even just one air stone can create a lot of surface area, exactly the same as the surface area at the top of the tank. Oxygen and CO2 don't know if they are in the air, bubbles, the water, etc. They will diffuse. There is more than enough aeration created by the bubbles themselves to create significant aeration.

I am glad your own estimate supports your idea, imagine that.

A protein skimmer and an airstone are two very different things. A protein skimmer works because the polarity of the molecules comes into play near the water-air interface. At the interface, organic molecules will accumulate, and then that bubble will be harvested, removing the organics from the tank.

In other words, it has nothing to do with oxygen exchange.

It also doesn't work very well in freshwater tanks, because the surface tension of freshwater doesn't support nearly the same interface as salt water does. That is, the bubbles in freshwater pop too easily before they are harvested, not removing the organics from the tank.

Oxygen and CO2 DO care is they are in air or water -- their diffusion rates are significantly different in liquids and gases! Not to put too fine a point on it, but that is literally day one stuff in the science of mass transfer.

I cited the authors of the research (the names and date are in the link above) who actually physically measured the rate gas leaves a bubble. If you wish, I can also cite the transport phenomena text and page I pulled that research from. Transport phenomena is the study of how momentum, heat, and mass is transfered around, if you didn't know. Basically, it is the science that attempts to answer questions like: how much gas exchange can a bubble do? It isn't MY idea. It is published science. Unless you have a compelling reason above and beyond YOUR personal opinion, I'm going to stick with the published science. If there is a publication out there that refutes my above claim, I'd appreciate your citing it for me. I know the literature pretty well as this is an area I work in, but I certainly don't know all of it, and always am interested in expanding my knowledge.

Lastly, and speaking as a moderator here, please watch the snarky comments like the snide "imagine that" thrown in at the end there. It is fine for us to disagree, but disrespect, direct or implied, is unacceptable. I've been respectful to you, and you need to be the same to me and all other members. Thanks.

 

[fishguy2727]

You did not come across completely respectful to me. I am not trying to be rude but if you would like to mention that I would like you to know. When you simply say 'that is false' and the way you discussed this was belittling as if there is no other opinion or possibility.

Can you provide some of the information here so that I and others can easily read it. I would rather not have to read through other threads. I am more than capable of reading through the actual primary research article if you have it. I would like to know more.

 

[Bignose]

from that other thread then:

This is a very interesting question about how much gas really escapes the bubble. If the water is completely pure of all dissolved gases, or equivalently, saturated with pure nitrogen and no oxygen, the transfer of oxygen out of the bubble can be estimated by:

N = c*sqrt( (4*D*V)/(pi*db) ) * pi * (db/2)^2

where

N = rate of which gas leaves the bubble and enters the water per unit time
c = concentration of gas in the bubble initially (in the atmosphere)
D = diffusion constant for gas in the water
V = terminal velocity of bubble (which is achieves quick, so, really its speed)
pi = pi, you know, 3.14159 and all that
db = diameter of the bubble

Now, the bubble takes H/V time to get to the top of the tank, (H = height of tank)

So N_total = c*pi*(db/2)^2*H*sqrt(4*D/V*pi*db)

This result was published by Hammerton and Garner in 1954, and their experiments found it valid for bubbles of 3 to 5 mm.

Just throwing some estimates together, I get on the order of 10^-10 g or 0.00000000001 g of oxygen from each bubble. And, that is only if the water is completely devoid of oxygen... since the water most of the time will have some dissolved oxygen in it (hopefully for our fishes about as much as it possibly can!)the transfer of one bubble is very small. In fact, as more oxygen gets into the water, the amount transfered out of the bubble decreases exponentially.

Now, there are many bubbles... but the estimates above say it would take over a million bubbles to get one milligram of oxygen into the water. And, at 25 degrees C, the oxygen saturation is about 8.2 ppm or mg/L.

So, you would need more than 8.2*(Liters in your tank)*(1 million) bubbles to completely saturate your tank. It would take quite some time for that many bubbles -- indicating that the surface gas exchange is very significant.

and

But, the biggest thing is that I estimated a minimum... the increasing oxygen in the water would decrease the mass transfer of oxygen out of the bubble. There is a sink in the water from that consumed by fish, plants, bacteria etc. But, it should reach a steady state. Since with increasing oxygen in the water it lowers the exchange exponentially:

N_act = N_org*((cb-cw)/cb)^n

where

N_act = actual gas exchange
N_org = what was calculated above with oxygen devoid water
cw = concentration of oxygen in water
cb = concentration of oxygen in bubble
n = exponent, since we are dealing with spheres, this is probably equal to 2

So, when the water is half saturated, only 1/4 of the mass transfer out of the bubble occurs.

So, now we want to know what the concentration after any time is. The amount transfered at any instant in time is a function of the immediate concentration. So, we have to integrate from 0 to T. As it is an exponential function it will take an infinite amount of time to reach 100% saturation, but we can calculate for any T what the current saturation is.

At 73 hours the water really is: 50.2% saturated with oxygen
At 100 hours: 58.0 %
At 500 hours: 87.3 %
At 1000 hours: 93.2 %

In fact, the water is not 99% saturated until 7223 hours. Or about 100 times as long as estimated when you take pure water. And, this assumes the water is perfectly mixed, which it usually is not. Unless you have some decent currents, there will be concentration gradients that will penalize this further.

Yes, you do not have to reach 100% saturation to keep your animals alive, but this is an incredible amount of time -- the surface agitation is a very significant part. The extra surface area means everything, and since it is a planar surface, the exponent in the above equation is 1/2 or 1, not 2, which is an extraordinary penalty.

Besides, after adding bio-spira you can add fish immediately (according to it instructions, anyway) you dont have to wait 73 or 7000 hours to saturate the water... the filter return splashes the surface. And, all the people who don't use air stones or wands and never report a problem. The bubbles do add some oxygen, but the dominant mechanism is the surface agitation.

And for completionist's sake, the text it is from is Bird, Lightfoot, and Stewart's Transport Phenomena 1st edition. Chapter 17, p 541, example 17.5-1 entitled "Gas Absorption from Rising Bubbles"

 

[fishguy2727]

Anything about gas exchange at the surface? Without that something from 1954 isn't enough to go on IMO. I would like to know what the gas exchange rate at the surface is. This would be the supplemental information needed to 'prove' that bubbles are not effective aerators.

 

[Bignose]

Anything about gas exchange at the surface? Without that something from 1954 isn't enough to go on IMO. I would like to know what the gas exchange rate at the surface is. This would be the supplemental information needed to 'prove' that bubbles are not effective aerators.

What's wrong with a paper from 1954? It's not like diffusion has changed since then.

The rate at the surface is significantly greater than the rate of the bubble gas exchange by induction. Clearly the fish get enough oxygen. The math show that the bubble transfer rate is very, very small. Even if the estimate is off by 3 or 4 orders of magnitude, it is still very, very small. And we know the estimate isn't that far off, since the experiment has been done, and reported. Yes, they did experiments in 1954, and unless you have a compelling reason to believe that we were done incorrectly or the authors were falsifying data, there is nothing wrong with data from 1954.

But, even all that aside, it is a question of surface area. Oxygen diffuses into water at around 230 m^2/s. It doesn't matter is that is from a bubble or from the surface, the rate is the same. Surface area is surface area. That is all that matters.

Bubbles have an incredible small surface area, and are only in contact with the water for a short time.

Take a standard 10 gallon tank. 20 inches by 10 inches at the top is about 0.13 square meters. If the bubbles are 0.5 cm in radius, you would have to have 413 bubbles in the water every second to equal the surface area exchange at the top of the tank. If the bubbles are 0.3 cm in radius, you need to have 1149 bubbles in the water. If the bubble are 0.1 cm in radius, you need to have 10345 bubbles in the water every second to equal the surface area of the top of the tank.

Then, the bubbles are only in contact for a second or so. They won't get the chance to give up all their oxygen by the time you get to the top. Whereas, the airspace at the top of the tank is always nominal normal atmosphere unless you have a very tight fitting lid or other abnormal conditions.

All of this ads up to bubbles don't do much gas exchange.

They do other good things -- they circulate the water in the tank. As they float up, they drag water from the bottom of the tank to the top, and consequently some water goes down to the bottom. Also they break up the surface, making the to surface not a flat sheet, but rippled, which adds even more surface area to the top of the tank. A filter return can do this too. But, the bubbles themselves do not contribute very much to the gas exchange. They just don't have enough surface area or time to perform much exchange at all.

 

[Bignose]

Lastly, the flux through a finite slab can be estimated by q = 1/sqrt(pi*alpha*t)(c1-c0)

q = flux rate

pi = 3.1415926...
alpha = diffusivity
t = time
c1 and c0 are the concentrations.

If the water is completely devoid of oxygen, in 1 second 1.3*10^-3 g of oxygen enter the water. That's about 7 orders of magnitude greater than the amount of oxygen one bubble transfers. Even if you assume 1000 bubbles in the water every second, the tank top still provides over 1000 times (4 orders of magnitude) the rate of oxygen exchange.

Conclusion: the tank surface dominates.

 

[Geoff-]

Incorrect. Bubbles have very little time in the water and very, very little surface area and consequently the gas exchange rates are almost insignificant.

An individual bubble may have very little time but it is immediately replaced by another bubble, therefore it's not the time of flight of 1 bubble that matters it is how much surface area you are creating with your bubble stream. Which may or may not be significant, but that's a different point.

What's wrong with a paper from 1954? It's not like diffusion has changed since then.

The laws of physics don't change but our understanding of them improves over time. If they were only testing a simple process, fine, but anything in-depth and a more recent study would be more relevant (I haven't read the study referred to).

The rate at the surface is significantly greater than the rate of the bubble gas exchange by induction. Clearly the fish get enough oxygen. The math show that the bubble transfer rate is very, very small. Even if the estimate is off by 3 or 4 orders of magnitude, it is still very, very small. And we know the estimate isn't that far off, since the experiment has been done, and reported. Yes, they did experiments in 1954, and unless you have a compelling reason to believe that we were done incorrectly or the authors were falsifying data, there is nothing wrong with data from 1954.

Even if the bubbles themselves contribute a small amount, the overall tank mixing and exposure to surface may improve significantly as a result.

If you have a static tank with enormous rates of oxygenation at the surface, but the tank doesn't mix and only the top inch of water is oxygenated, then the rate of oxygenation at the surface is irrelevant because oxygen isn't making it down into all the tank water. Water mixing is a vital ingredient in having an oxygenated tank.

The tank water can also be viewed as a closed system. If you put "only just not enough" oxygen into the system then over time oxygen levels deplete and you have a major deficiency. If you put "only just enough" oxygen in then eventually the tank oxygenates until it is saturated and everything is peachy.

They do other good things -- they circulate the water in the tank. As they float up, they drag water from the bottom of the tank to the top, and consequently some water goes down to the bottom. Also they break up the surface, making the to surface not a flat sheet, but rippled, which adds even more surface area to the top of the tank. A filter return can do this too. But, the bubbles themselves do not contribute very much to the gas exchange. They just don't have enough surface area or time to perform much exchange at all.

I think this is the best paragraph you've written because you're looking at the overall system and not trying to win an argument on the surface area of a bubble. A filter return can mix the tank, but a poorly designed filter arrangement may be deficient. This can be band-aided with an air-stone, and therefore the air-stone appears to make a big difference because it is circulating water to the surface. Saying that air bubbles don't contribute to this is like saying an engine is what makes a car move and atomised petrol droplets in the cylinders are irrelevant

 

[StandbySetting]

only the top inch of water is oxygenated

As far as I am aware this isn't true, even if it is only the top of the tank and there is no circulation, the Oxygen will still diffuse into the lower regions of the tank, because at the top the concentration of Oxygen will be the highest so it will diffuse into areas where there is a lower concentration.

 

[Bignose]

The tank water can also be viewed as a closed system.

No, treating the tank water as a closed system would be a very poor model for most situations. Unless there is an exceptionally tight fitting lid or other very extraordinary circumstances, the tank head space gets plenty of fresh air, and since fluxes will be going in and out of oxygen and carbon dioxide, you really cannot model it as closed. If a tank were really closed, how could a tank with fake plants support life for more than a very brief period of time? Fresh air is the major source of oxygen.

only the top inch of water is oxygenated

I think that this misconception stems from the above one. Since as SbS notes, diffusion will spread oxygen throughout the water. Even it is perfectly stagnant, in about 24 hours even water completely devoid of oxygen will be pretty darn close to equilibrium. There is no reason the oxygen would diffuse in 1 inch and then stop, especially when these is a virtually limitless source in the atmosphere.

An individual bubble may have very little time but it is immediately replaced by another bubble, therefore it's not the time of flight of 1 bubble that matters it is how much surface area you are creating with your bubble stream. Which may or may not be significant, but that's a different point.

I did that estimate, see this thread. Even if you put 1000 bubbles in the water every single instant (and I've never seen an airstone that put out more than 100 or 200), that is still 0.01% of the surface area of the top of a 10 gallon tank (small). As I wrote above, surface area is surface area -- and since the top of the tank is 1000 times more than even a very generous estimate of the surface area a large numbers of bubbles -- the top of the tank surface area is dominant for gas exchange.

The laws of physics don't change but our understanding of them improves over time. If they were only testing a simple process, fine, but anything in-depth and a more recent study would be more relevant

You guys are really going to have to explain to me why more recent means more relevant? How is the topic of the study in question not relevant?!? It studied the amount of gas that diffused out of bubbles in water! That is exactly what is being discussed!

Special relativity was first formulated in 1916 -- yet is still just a relevant as ever. Have you ever used a GPS? If so, you've seen general relativity in action.

Shoot, let's go back even farther. Newton first published Principia Mathematica in 1687, outlining his three laws of motion. Those laws of motion are still just a relevant today, and Newtonian mechanics are used in calculation of every bridge, building, rocket going in space, car, weather prediction, etc. etc. etc. Newtonian mechanics are just as relevant as ever.

Sure, the exact details of gas diffusion into liquid may be known better, but the 1954 paper I cited is not completely overthrown. A newer estimate may be a little more accurate, but that older estimate is no worse than +-5%. There are many processes in production in the chemical industry today based on the estimates in that paper. Virtually any distillation tower with bubble or cap trays uses the estimate from that paper. Virtually any liquid phase reactor uses estimates from that paper. The estimate in that paper has been confirmed as pretty darn good time and time and time again, like general relativity and Newtonian mechanics. For the purposes of this discussion, and estimate +-5% or even +-10% is good enough. The estimate is not several orders of magnitude off, and gives us a number to compare to others. We're not trying to exact predict what is going on, we're trying to get a general idea. And a 70 year old correlation that studied this exact topic is good enough for this purpose.

trying to win an argument on the surface area of a bubble

it isn't a question of 'win', it is a question of describing the situation accurately, and representing the physical processes that are occurring. It is not a question of 'win', it is a question of expanding knowledge. I fully concede I could be way wrong on this, but the resources I have access to and knowledge of show that predicted amount of gas exchange from even hundreds of bubbles is very tiny compared to the exchange at the tank top surface. All I can go by are the resources I have, and my experience with how well the predictions based on these resources have worked in the past. If someone has other resources to show me, I am always eager to expand my knowledge. But at this time, I am going to stick with what I know.