Backyard Aquaponics
A place to discuss aquaponics

So after reading a bunch on these forumas about dissolved oxygen and ways to obtain it, I remember reading something along the lines of: air stones don't actually do much of anything and that it requires breaking the surface tension of the water and churning in or turning over air into it in order to get higher DO levels. So my question is: has anyone done any tests to see if changing the way the water returns from the GB'S to the FT's will change DO levels? An example would be a system that put raindrop sized water drops over a large area vs. a straight dump pipe vs. a waterfall setup. I am trying to plan out my initial system and would obviously like as much DO as possible and this seemed like it would be an easy thing to possibly integrate.

This link might be useful:

http://www.fao.org/docrep/X5744E/x5744e0m.htm

My observations, the air stones will do just fine. You are not going to have too much air! The more you mix it up by means of air stones or something like water falls, the better your fish going to be.

Ask people who have lost power for a while and have lost bunch of their fish in their fish tank. As long as air contacts the water, there will be some exchange and therefore addition of oxygen to the water. I have 8 big air stones in my fish tank and fish has not complaint yet!

Thanks, that link really helped. So it isn't so much creating as much DO as possible, but regulating it to the correct quantities for each type of fish. So is there a chart or listing of ideal DO consumption somewhere for the more common AP fish?

It is about creating as much DO as possible. There are not just fish in the system that use DO.

Good link MacGyver

I think the problem with this sort of experiment is how to reliably and accurately measure the DO in the backyard setting, without paying many hundreds or thousands of dollars for equipment or paying exorbitant laboratory testing fees?

If anyone has done the required testing, then I would be interested in the results too.

Here is something Dr Wilson Lennard said a while ago about DO.

OK, DO and aeration. Aerated driven aeration is a complex issue. Bubbles do bleed air (oxygen) directly into the water column, but the efficiency of the approach is due to two major factors:

1/ The size of the bubble - the smaller the bubble, the greater the surface area to volume ratio of the bubble and so the more the exchange potential.

2/ The contact time of the bubble with the water column - the longer the contact time, the greater the potential gas exchange.

In reality, most fish tanks are 1m (3ft) deep or less. In this case, the contact time of the bubble with the water is very low, and so little chance is available for direct gas exchange from the bubble. In addition, in hobby circles, the majority of air stones used produce relatively large bubbles due to the fact that ones that produce small bubbles are more expensive.

In this situation the VAST MAJORITY of the gas exchange occurs when the bubbles break the water surface up and increase the surface area of the water surface. This greater surface area at the water surface increases the water surface area by a large amount and you get direct gas exchange at the water surface directly from the atmospheric air (oxygen).

This is why I state (as you have seen in the video) that you don't get air or gas exchange from the bubble itself, but from the water surface area increase by the bubbles chopping up the water surface. Yes, the reality is that some gas exchange does occur via the bubble itself, but as I say, the vast majority of the gas exchange is due to the water surface being chopped up and increasing the water surface area for atmospheric gas exchange. I cannot put a figure on the % that comes from the bubble and the % that comes from the surface area break up and increase, but will hazard to guess that 80% or so occurs at the surface, and as little as 20% or less directly from the bubble.

This why when you look at pond aquaculture operations, they use paddle wheels to give aeration; they don't use direct aeration. Others have done the research to show that the paddle wheels are far more efficient because of the above explanation. If the majority of the gas exchange was via the bubble, then pond farmers would use aeration via blowers and air stones; but, they don't, and these are the reasons why.

I am sorry, I don't have research papers based on this; it is enough for me to accept the knowledge of other scientists and the aquaculture industry who have worked in this field for a long time. It is a well accepted principal in aquaculture circles.

Tank systems use direct aeration over paddle wheels simply because the fish stay away from the paddle wheels that chop the water up and cause currents, eddies and noise the fish don't like, and tanks need to be efficient in space utilisation and don't want things to effect the hydraulic dynamics of water flows in them, so paddle wheels are not used much in tanks, unless they are very large.

This video of me has caused a stir in the aquaponics forums; you are not the first to contact me about it. I have never really answered it as it seems quite petty to me; really, who cares what I (Wilson) say and who really cares where the exchange occurs; it occurs and that is all that is required. Go to the established aquaculture community and see what they say!

If ALL the air difused across the bubble surface, then there would be NO bubbles breaking the water surface at the top of the bubble column because all the air would diffuse into the water and dissolve into it! In addition, if an appreciable amount of air diffused across the bubble surface, then the bubbles would get noticably smaller as they rose through the water column; I have not measured the difference in bubble diameter between when the bubble leaves the air stone and when it reaches the surface, but I see little difference in bubble size from the exit point of the air stone and the water surface in a 1m water column depth! Also, the fact that air contains only 16% oxygen means that if anything diffuses across the bubble surface, the majority of diffusion would be nitrogen (about 80% of air) and not oxygen anyway.

Thanks for that, Charlie.

My only caveat is that as air is composed of 20% O2 and 79% N2, do both gases diffuse equally, or is one more likely to diffuse than the other? Of course, AP is interested in O2, not N2 (as a stable form of nitrogen it won't be available to the plants, unlike nitrate). So if the oxygen was more likely to dissolve than N2, then the bubbles would still reach the surface (because 79% of it is nitrogen).

I do agree that smaller bubbles (with a much greater surface area for diffusion) are more efficient than large bubbles.

I'll go and do some research about the partial pressure of O2 and N2 and report back.

Perhaps the good doctor comes from a different planet to us but Earth's atmosphere comprises close to 21% Oxygen, not 16%. Also, one could expect bubbles to increase in size as they rise due to reduced water pressure allowing then to expand. Very small bubbles below a certain diameter, however, could be expected to shrink or disappear completely as they rise, depending on depth of release and temperature, due to their greater rate of diffusion into the water by virtue of their greater relative surface area.
Having said that, the information contained in that post is invaluable and I thank you for it, Charlie (and Wilson).

So what about doing something like bubbles in reverse? It was mentioned that the smaller the bubbles have higher surface area per bubble in relation to their volume. The problem being that bubblers that dispense very small bubbles are much more expensive. Why wouldn't something like an ultrasonic humidifier work to put out very fine droplets of water into the air and accomplish the same ultimate goal, but on a very affordable scale. You would obviously have to reclaim the water and you also probably wouldn't want it in the tank itself since it may aggravate the fish, but it seems like logically it ought to work.

All been discussed before.

Heres a link to a thread discussion on micro-bubbles.

viewtopic.php?f=1&t=11757&hilit=micro%2A

I wasnt talking about microbubbles(especially because that thread was the first time Id heard of them). I was thinking more of a sytem that intercepted the GB drain and had the humidifier running but in an enclosed space so the aerated wated droplets would be forced to condense(but now containing more oxygen). The condensed water would then flow into the FT as usual. Seems like its worth an experiment.

Ok, cool I gotcha

A few things to consider if you are trying/testing/experimenting or brain storming oxygen displacement and efficiency is that water can only aerate to a point of saturation.. after that your wasting energy.

Is there something you are trying to achieve? Or simply experimentation?

I love experiments

Yeah, mainly just experimenting. I am just looking at ways of optimizing the living(growing) conditions. It seems like we already
have plenty of ways of adjusting/controlling nitrates/nitrites/ph but not really a great way of manipulating DO. I wanted a way to do it without having to rely on just growbeds, the GB drains, and bubblers since they dont seem to be easily throttled up and down.

Why do you want to "throttle" your DO "up and down"?????

Just maximise it... and leave it alone...

You can't have too much DO... but you sure as heck can have too little...