Do Micro Bubbles Create Gas Exchange?

[Kasey Grohowski]

Title says it all, I often get micro bubbles in my DT, and it makes me wonder if they can increase dissolved oxygen levels overall due to added gas exchange. Any thoughts? @Randy Holmes-Farley

 

[Ron Reefman]

Excellent question!

I would think it has to help some. But then I remember reading that surface water agitation is more important than the air/water mixing that goes on in a skimmer. So I'll be interested to hear Randy's thoughts and experience.

 

[lapin]

My un-scientific ipinion
Since the bubble is gas encapsulated in the water it wont exchange gas. It will however disturb the waters surface when it rises and pops.

 

[Ron Reefman]

My un-scientific ipinion
Since the bubble is gas encapsulated in the water it wont exchange gas. It will however disturb the waters surface when it rises and pops.

That's an interesting take on gas exchange. Just what makes you think gas can't exchange inside a bubble? It's not really encapsulated in water, it's just surrounded by water and I would think that could make for easier/better gas exchange.

 

[brandon429]

rising bubbles increase gas exchange due to surface tension, they increase surface area during the boil/escape at the surface as the bubble expands then pops.

But if the bubbles are so small and conditions just right that they froth vs boil then it seems we would lose the benefit, can’t wait to hear finals on the matter. I got the first part above from an old website/ company called aquaticeco.com (out of biz now) and the second part is my guess

 

[MabuyaQ]

In general there will be gas exchange between the micro bubble and the water it is in because there is a surface area, but the low amount of gas in those bubbles and the unknown composition of the gas in the bubble makes it impossible to determine what the effect is, but it will be rather small. The unlimited supply of oxygen in the air at the surface of your aquariumwater is a far more reliable source of oxygen so increasing this surface area (with waves) in relation to volume is far more important.

 

[Gablami]

I’ve wondered this myself, and is applicable to the placement of emergency air stones in the event of power outage.

1. Does the submerged depth of the air stone matter? Is it equivalent to have it an inch beneath the surface vs at the bottom? Is all that matters the pop at the surface?

2. Do you even need an air stone as opposed to just an air line? Are microbubbles more effective at gas exchange than macro bubbles?

Does anyone know?

 

[brandon429]

from that website yes the depth of the install matters, it creates a larger upwelling column when the bubbles emanate from the bottom vs top 1/3 etc because we want a rolling laminar column of low 02/high co2 water coming into top contact with atmosphere and hopefully balancing out.

2. great question I'm not sure. the discussion always comes back to total surface area expanded compared to flat/still waters. if I had to guess multi little bubbles amount to more SA but I cannot be sure. what if they're on low, but the single outlet bubble is on high/big swell? bring the ref in ha yay.

 

[brandon429]

one thing ill never forget in studying those seminars from aquatic-eco.com was that they did extensive testing in lake and river management and found that multiple water pumps aimed however they could aim them couldn't outpace one simple bubbler from the very bottom. rising columns of air are the most efficient way to aerate bodies of water given all factors like electricity cost, maintenance, practicality, return on investment etc

 

[lapin]

That's an interesting take on gas exchange. Just what makes you think gas can't exchange inside a bubble? It's not really encapsulated in water, it's just surrounded by water and I would think that could make for easier/better gas exchange.

Hum........... Maybe

 

[lapin]

Yes they can
David Kevin Woolf

• International Centre for Island Technology
• Energy Academy
• School of Energy, Geoscience, Infrastructure and Society
• Institute for Life and Earth Sciences

Bubbles can be generated at the sea surface by many mechanisms, but the main source is by the entrainment of air in breaking waves. Bubbles will scavenge material from the surrounding water, thus contributing to the cycling of dissolved and particulate organic material. When they burst at the sea surface these bubbles generate a sea salt aerosol contaminated with material scavenged from the sea-surface microlayer and below. Gases will be exchanged between a bubble and the surrounding water while it is submerged. In addition, the breaking waves and surfacing bubble plumes disrupt the surface microlayer, and this may enhance transfer of gases directly through the sea surface.

The net transfer of a gas between a bubble and the surrounding water, from entrainment until the bubble bursts or fully dissolves, contributes to the total transfer of that gas between atmosphere and ocean. This bubble-mediated transfer has some special properties that set it apart from direct air–sea transfer of poorly soluble gases. Bubble-mediated air–sea transfer velocities depend on the solubility in addition to the molecular diffusivity of the gas in seawater. Bubble-mediated transfer is not proportional to air–sea concentration difference, but is biased toward injection and the forcing of supersaturation. The entrainment of air by breaking waves increases rapidly in intensity with higher wind speeds.

 

[Ron Reefman]

@lapin, good find!

Now, are bubbles, and in particular skimmers, better at gas transfer than water surface to air where the water surface is moving due to powerheads or wavemakers?

 

[lapin]

@lapin, good find!
Now, are bubbles, and in particular skimmers, better at gas transfer than water surface to air where the water surface is moving due to powerheads or wavemakers?

This one is beyond my old defective brain

 

[sghera64]

The answer is yes, but the problem in this specific example is that the total volume of micro bubbles is insignificant.

Those who say there is not gas exchange between the content of a gas bubble and the bulk medium (water in this case) are ‘generally’ incorrect depending on the specific gas and liquid (almost no exchange in the case of helium in water). In many practical applications and commercial operations, gases are dissolved, or scrubbed, by passing bubbles through a solution. The major portion of mass transfer takes place with the bulk fluid and not at the surface. Examples are dissolving chlorine gas in pool water or into caustic water to form bleach. I’ve bubbled HCl gas into a column of water in the lab and the bubbles disappear after only a few inches away from the frit. An aquarium example is bubbling CO2 into our reef water in a CO2 reactor.

In the case of gas exchange in our aquariums, we strive to move oxygen from air bubbles into the water and strip CO2 from the water into the bubble. In a rising column of air bubbles (think under gravel filter) the vast majority of exchange takes place while the bubbles are rising. The majority of exchange is NOT at the surface due to surface agitation; assuming the rise is appreciable. This is due to the fact that the largest concentration gradient exists when the bubbles are first introduced into the water and also due to the sheer force acting on the water-air interface boundary layer is greater on a rising bubble than the laminar tangential flow going across the bulk surface caused by the bubbles displacing water at the surface. The taller the column of bubbles the greater extent of the total gas exchange takes place between bubbles and liquid. For a very short column of bubbles (maybe less than a few inches) perhaps more gas exchange occurs at and across the surface due to eddies and the break in surface tension.

 

[sghera64]

Yes they can
David Kevin Woolf

• International Centre for Island Technology
• Energy Academy
• School of Energy, Geoscience, Infrastructure and Society
• Institute for Life and Earth Sciences

Bubbles can be generated at the sea surface by many mechanisms, but the main source is by the entrainment of air in breaking waves. Bubbles will scavenge material from the surrounding water, thus contributing to the cycling of dissolved and particulate organic material. When they burst at the sea surface these bubbles generate a sea salt aerosol contaminated with material scavenged from the sea-surface microlayer and below. Gases will be exchanged between a bubble and the surrounding water while it is submerged. In addition, the breaking waves and surfacing bubble plumes disrupt the surface microlayer, and this may enhance transfer of gases directly through the sea surface.

The net transfer of a gas between a bubble and the surrounding water, from entrainment until the bubble bursts or fully dissolves, contributes to the total transfer of that gas between atmosphere and ocean. This bubble-mediated transfer has some special properties that set it apart from direct air–sea transfer of poorly soluble gases. Bubble-mediated air–sea transfer velocities depend on the solubility in addition to the molecular diffusivity of the gas in seawater. Bubble-mediated transfer is not proportional to air–sea concentration difference, but is biased toward injection and the forcing of supersaturation. The entrainment of air by breaking waves increases rapidly in intensity with higher wind speeds.

Wish I had read your post before wasting time writing mine.

 

[sghera64]

@lapin, good find!

Now, are bubbles, and in particular skimmers, better at gas transfer than water surface to air where the water surface is moving due to powerheads or wavemakers?

My engineering experience suggests to me the answer is generally yes. However this has not been my experience. The truth is that the answer depends on many conditions: energy used to agitate the water surface, amount of air movement above the water, the concentration of gases (e.g. CO2) above and in the water and in the air going into the scrubber (in the case it is different that the room air as would be the case when externally fed air is used or a CO2 scrubber is employed).

In my case, I use a CO2 reactor, so my tank water is rich in CO2, as evident by my depressed pH. I use outside air in my skimmer. But this makes only a small difference (about 0.1-0.2 pH units). I have a lot of tank water surface area with a display tank and two large frag bins and all of it is strongly agitated. Plus I have an overhead fan moving air across the DT. I even have 10% of the total water volume filled with chaeto under very strong LED grow lights.

Given my setup, I feel like strong surface agitation and air movement is working against the chaeto and skimmer. The skimmer is probably blowing the CO2 out of the tank water into the room, and then it finds its way back into the system via the surface. I’m struggling to buy that theory, but its all I have.

I could test this by sealing off the top of the tanks with plastic wrap for a few days leaving all the gas exchange from the skimmer using outside air and measure pH difference. OR - - if someone else has done this PLEASE share your findings.

 

[Kasey Grohowski]

@Randy Holmes-Farley
Your thoughts?

 

[MabuyaQ]

Never tested this but my feeling is that since a large portion of the bubbles in the skimmer are not submerged but surrounded by other bubbles there is far less exchange than you would expect from such a mass of bubbles. Probably only the relatively small surface area wher the bubbles are created and first mixed is where exchange takes place, but continuously.

 

[Randy Holmes-Farley]

Title says it all, I often get micro bubbles in my DT, and it makes me wonder if they can increase dissolved oxygen levels overall due to added gas exchange. Any thoughts? @Randy Holmes-Farley

All bubbles create some gas exchange. The smaller a bubble is, the less fas is there to exchange, but usually you have a lot of them that may make up for the smallness.

 

[Randy Holmes-Farley]

I’ve wondered this myself, and is applicable to the placement of emergency air stones in the event of power outage.

1. Does the submerged depth of the air stone matter? Is it equivalent to have it an inch beneath the surface vs at the bottom? Is all that matters the pop at the surface?

2. Do you even need an air stone as opposed to just an air line? Are microbubbles more effective at gas exchange than macro bubbles?

Does anyone know?

Deep submersion creates water flow, which itself is critical in a power failure .