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Dr Wilson Lennard is a recognized authority on Aquaponics. Here he is speaking on Aquaponics as an Ecosystem. This clip was from Murray Hallam's Practical Aquaponics Blog

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Comment by Sylvia Bernstein on February 6, 2011 at 5:38am
Whoa.. while this discussion has been truly fascinating, I'm afraid the tone is getting more bloody than most of our gentle gardening souls can handle.  Can you guys shake hands and agree to step away from the bubbles? 
Comment by Kobus Jooste on February 6, 2011 at 3:09am
Izzy:

OK – lets take this phrase by phrase.  I do not care if you say you did not want to get into a long discussion.  You decided to come in and shoot from the hip , DUH and facepalm and then no one should respond and it does not work that way.

 

“Why do I think I know better than someone with a PhD in Applied Biology?  Well, perhaps because in my BIOLOGY 1 class we EXPERIMENTALLY did a bubble test to prove that gasses dissolve in water.  We used CO2, but the concept is the same.”

 

First up, you are not an aquatic ecologist or aquaculturalist or even a physiologist.  Gunning for a person with a PhD in some other field is, IMO, very typical of people studying MED as you stated in a later post.  I will never dream of brazenly implying that I know better than a GP, but I have so often met with the reverse.  Exactly how much time has your med studies dedicated to gas exchange in aquatic environments? Pardon me if I am not impressed.  You conveniently use the CO2 diffusion experiment, but fail to mention that CO2 diffuses into water at a rate of 134 ml/liter of water while oxygen only diffuses at a rate of 4.5 ml/liter of water – thus CO2 diffuses into water at 25.6 times the amount of O2 (Schmidt-Nielsen).  Diving insects that carry an air bubble with them conveniently exploit this – CO2 that is expired goes into the water rapidly, while the O2 in the bubble does not “leak” very fast at all.

 

“Anyway, I am hoping the rest of the information he gave was accurate because it would be helpful to know these things.”

 

Exactly what are you implying other than you think the man is not accurate?

 

“I don't care how much it is because we all know that the amount of oxygen diffuses relative to the surface area to volume ratio of the bubble, DUH!  I can prove this experimentally, and you can reproduce it.  Thanks, and I am glad we can all understand diffusion DOES occur beneath the surface as well as at the surface.  You have argued that more diffusion occurs at the surface then through the interaction of the bubbles whereas I have not stated anything to the contrary.  A basic level of reading comprehension would have revealed this oversight, sir.”

 

You should care about it because that bubble is not stationary and the system is not pressurized.  It is next to nothing in the course bubblers most people use.  Arguing that you can prove it experimentally is useless, because it is in the textbooks already thank you.  You stick to this point with passion but it is really a non-issue.  You quote fine bubble aeration when most people use air stones that cannot produce those bubbles.  Thus, your point is not relevant, very much like harping on CO2 experiments when it is really very different to the case in point.  I comprehend what you said just fine.  You, on the other hand, want to cling to theoretical diffusion rates of a rapidly rising bubble and state that Dr Lennard, and myself for that matter, should consider this a valuable source of oxygen.  Perhaps we do not.

 

“As far as the context, are you trying to imply that because his audience is so far removed from his level of education he can make false statements?”

 

Implying that people are making false statements is suggesting that they are lying to people.  It is there clear in your post.  Saying you are not accusing people of lying is splitting hairs, or is a false statement not a lie when you are clearly implying that me man should know better?

 

“P.S. I have emailed Dr. Lennard to clarify the issue since you are interpreting what he is saying outside the context of what he actually said...  /facepalm”

 

Thanks for the courtesy.  Hopefully you don’t double facepalm if he sticks to his statement.  I do not speak for Dr Lennard, but for

Comment by Izzy on February 6, 2011 at 1:44am

I find it difficult to reply when you are not comprehending what I am posting... I have not accused anyone of being a liar nor am I arguing the difference in efficiencies between above surface and below surface diffusion of oxygen into the water.  I am also not arguing the different methods for diffusing oxygen into the water below the surface. 

 

My point was very clear.  Dr. Lennard, as an authority on aquaponics, should not disseminate erroneous information regarding basic scientific concepts, namely, that oxygen can diffuse beneath the surface of the water at any efficiency.  Moreover, you and I are both aware that there are devices and methods to cause oxygen to dissolve beneath the surface of the water at high levels of efficiency.

 

Regardless, it was not my intention to have such a long debate with you since you are talking about one thing and I am talking about another.  Whether it is now or when I finish my medical doctorate, I do not intend to "over simplify" things to the point of making them wrong.  Dr. Lennard has led many to believe that oxygen will not diffuse into the water below the surface, and that simply is not true.

 

P.S. I have emailed Dr. Lennard to clarify the issue since you are interpreting what he is saying outside the context of what he actually said...  /facepalm

Comment by Kobus Jooste on February 5, 2011 at 11:14pm

When we simplify aspects of environmental science, it is not because we are trying to BS the gullible masses, have problems with cognitive ability, cannot read or do not care.  It is because if you take the core importance of different processes into account, the one that gets discounted, if put into practice by itself, is not going to have sufficient impact on the greater scheme of things.  You have a shallow tank, a course air bubbler and no increase in oxygen partial pressure.  Under these circumstances, you have the surface area of the water body, and the entire surface area of the gas bubbles moving up through the water column.  Gas diffuses VERY slowly from a bubble into water, and thus, for all intents and purposes, the presence of the gas bubble on its way to the surface is not going to save the day – it is all about the water surface area in contact with the atmosphere.

 

Now, if you were using a pressure vessel such as an add-o-mizer, or if you have extremely fine bubbles that stay in the water column for an extended period of time, you will be able to get more diffusion taking place, but as oxygen diffusion is so slow, the effect of course bubbles are highly limited.  You seem to want to make a point that a Doctorate makes a statement that you disagree with, and now it becomes a lie.  What he is stating is that in aquaculture or aquaponic aeration, the bubble travelling through the water is worth very little.  I repeat my suggestion to you to repeat the exercise I suggested in the previous post.  

 

I bet that the gas bubble experiment you did was with a stationary bubble held under water.  If not, how did you discern between the effects of the bubble rising to the surface and the bubble agitating the water air interface?  I believe that I stayed courteous in my first response to you and I’m trying to do the same with this one.  I expect the same from you if you please.  Do not attack my reading skills or my intentions as a member of the scientific community.  I’m trying to explain to you how we work, and how processes gets deconstructed in order to pull the most important bits out of it.  If your response is going to be we are lying or cannot read, we will simply stop responding to you.

Comment by Izzy on February 5, 2011 at 9:26pm

Dr. Lennard states that no diffusion of oxygen occurs beneath the surface whereas I am stating there is in fact some diffusion occurring.  I don't care how much it is because we all know that the amount of oxygen diffuses relative to the surface area to volume ratio of the bubble, DUH!  I can prove this experimentally, and you can reproduce it.  Thanks, and I am glad we can all understand diffusion DOES occur beneath the surface as well as at the surface.  You have argued that more diffusion occurs at the surface then through the interaction of the bubbles whereas I have not stated anything to the contrary.  A basic level of reading comprehension would have revealed this oversight, sir.

 

As far as the context, are you trying to imply that because his audience is so far removed from his level of education he can make false statements?  "With great power comes great responsibility."  If you've heard that before, as many children have, you are aware of the moral imperative to be held accountable, which can be applied to making false scientific statements if you hold a PhD in Applied Biology.

 

And I quote, Dr. Lennard states at 7:41 http://www.youtube.com/watch?v=iitanQgsk2I:

"A lot of people don't understand. They think that when you put an air stone in with an air bubbler that the bubbles that are rising up are putting oxygen into the water; they're not."

 

Any gas can diffuse into water if bubbled.  The rates differ based on pressure, temperature, surface area, saturation, and many other potential conditions.  Again, oxygen will diffuse beneath the surface.

 

As stated in Fine Bubble Aeration - Project funded by the U.S. Environmental Protection Agency under Assistance Agreement No. CX824652 http://www.nesc.wvu.edu/pdf/WW/publications/eti/finebubble_tech.pdf:

"fine pore diffusers exhibit high oxygen transfer efficiency and high aeration efficiencies (mass oxygen transferred per unit power per unit time)"

 

I am sure you can find a myriad of references, studies, or experiments that show what I am saying to be correct, which is contrary to Dr. Lennard's statements.

 

On a final note, the reason why this is a big deal to me is because I have helped many people build and design aquaponic systems.  In my adventures, I have discovered many a DIY'er that has believed without question that bubblers are useless because somewhere they heard or read that bubbles don't actually diffuse oxygen into the water.  I have wasted more time educating them on these basic scientific concepts than helping them get done what they needed to get done to begin with.  I hope that helps you understand the implications of disseminating erroneous information to the unwitting masses.

 

Good luck in your future endeavors.

Comment by Kobus Jooste on February 5, 2011 at 3:27pm
With all due respect to your Biology 1 class, I think you should consider the context in which an individual with a post-graduate qualification in science makes a statement.  Your air bubble contains 20% oxygen and has at best a few seconds to interact with the water around it before you get air bubble hitting surface, which is in CONSTANT contact with the atmosphere.  If you took the average stream of bubbles, and made them vanish from the water column the instant they hit the surface (thus do not disturb the surface area) vs. putting a paddle wheel on the surface that just disturbed the surface and did not have a column of riding bubble, you will soon discover that your stream of bubbles is not worth the power you put into pumping it compared to surface agitation.  Thus, while in theory oxygen can diffuse from the 20% bubble to the 8 ppm water body, it is not where the true value of the bubbler lies.  Test this by setting up your air stone at the base of a tank, and moving it up close to the surface and testing the difference in DO.  That difference is the bit you are referring to, while the balance would be thanks to surface agitation.  If a stream of bubbles were that useful, we would not be using paddle wheels in pond culture.
Comment by Izzy on February 5, 2011 at 2:49pm
With all due respect to Dr. Lennard, there is in fact oxygen exchange when the bubbles are rising.  It does not purely occur at the surface.  I have run across so many people who have believed that it only occurs at the surface, and for a long time I thought they came up with that silly notion on their own.  However, I now see that Dr. Lennard is responsible for the dissemination of this erroneous information.  Why do I think I know better than someone with a PhD in Applied Biology?  Well, perhaps because in my BIOLOGY 1 class we EXPERIMENTALLY did a bubble test to prove that gasses dissolve in water.  We used CO2, but the concept is the same.  Anyway, I am hoping the rest of the information he gave was accurate because it would be helpful to know these things.

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