DO - oxegenation

[moose]

Found some interesting reading. I found the difernet methods of oxygenationof particular intrest

[Fwoor]

Certainly an interesting overview. I think the Downflow bubble contactor could be achievable and (fairly) easily scaled to systems. I suppose some calculations could be done based off their pure oxygen figures, and dissolved air composition being around 35.6%.

One thing that did get me thinking was the required flow rate for maximum dissolution.

[Fwoor]

For those operators that have pumps pumping back to the fish tank the downflow bubble contactor would seem to be easy to implement. Having the outlet directly in the tank as per this link (http://books.google.com.au/books?id=cXF8gzWFWEYC&lpg=PA298&ots=R7m_BMc0x9&dq=%22As%20the%20gas%20bubbles%20are%20carried%20downward%20toward%20the%20discharge% 2C%20the%20water%20velocity%20is%20reduced%20until%20it%20just%20equals%20the%20upwar d%20buoyant%20velocity%20of%20the%20bubbles.%20Thus%2C%20%22&pg=PA300#v=onepage&q=%22As%20the%20gas%20bubbles%20are%20carried%20downward%20toward%20the%20discharge,% 20the%20water%20velocity%20is%20reduced%20until%20it%20just%20equals%20the%20upward%2 0buoyant%20velocity%20of%20the%20bubbles.%20Thus,%20%22&f=false) (figure A). It certainly looks to be a efficient system in terms of dissolution as compared to airstones and one I will look at incorperating in to my future AP adventures!

[Outbackozzie]

Good read, especially regarding oxygenation.

[arachdog]

Yep its all good stuff. I don't think people give pure oxygen enough consideration. It often works out to be very cost effective for commercial RAS operations since even though its expensive it allow for far greater stocking densities. Although I guess its probably less important in aquaponics since that's not really about the fish.

[Watrout]

this is a bit off topic but another way of oxegenating the water is pumping air with out any diffuser under a water pump (http://cgi.ebay.com.au/Aquarium-Fish-Tank-Power-Flow-Pump-Wave-Maker-5000L-H_W0QQitemZ280413944308QQcmdZViewItemQQptZAU_Pet_Supplies?hash=item4149f939f4) that just has a prop that moves the water around. this prop then chops up the air and pushes it around with the water.( no stone to clog up or any air volume loss) this seems to work pretty well.

[Fwoor]

Whilst Im sure that would work Watrout (its also mentioned in the first paper), I think the benifits of the Downflow bubble contactor (DBC) is that it uses things most AP people already have (Pump and air supply) with only a minor addition of a bucket (or similar inverted cone). On top of that there is suggestion that they can achieve nearly 100% conversion to dissolved gasses (first paper)!

Personnaly I think the DBC is the most elegant, simplest and easiest to achieve (particularly for people pumping back to the fish section already!)
Edit: Also I think the reliability of the DBC is probably greater that that of the mechanical solutions =)

[arachdog]

Fwoor, I wouldn't get too tied up with dissolution efficiency. Remember you only got one pump, and the most your DBC can achieve is 100% oxygen saturation of the influent. A typical aquaponics setup doesn't have enough flow rate for that to be the only source of oxygen. Also that extra efficiency isn't exactly free. It comes from the pump. Another way to use the same energy to create more DO would be a spray bar, packed column or venturi. I'm not sure if those options are better, but they are worth considering. My gut feeling is the spray bar may actually be more efficient, since its acting on a larger volume of water.

[Fwoor]

Arachdog,

I do take your point about the typical flow rates of the aquaponics systems, Very valid. I think the scale is the solution to this issue as the DBC system can be scaled fairly easily.

This is my simplistic view of it. Bubbles that make it back to the surface are NOT achieving 100% dissolution levels. If no bubbles make it to the surface the dissolution is 100%.

I think that whilst a spray bar may contact a larger surface area (if at pressure), it is not achieving the same level of dissolution as the DBC. Also due to the spray bars design (pushing water through a resrictive hole), Increasing the pressure of the water helps but at the cost of work done by the pump.

One other point (in my ramblings) I would like to put forward is the cone design of the DBC. I have a feeling this reduces the pumps workload due to the pressure drop. This, however, may be a moot point as the gas added to the system increases the pressure.

[arachdog]

This is my simplistic view of it. Bubbles that make it back to the surface are NOT achieving 100% dissolution levels. If no bubbles make it to the surface the dissolution is 100%.

True, but not dissolving completely doesn't mean that the energy is entirely wasted. Bubbles that make it to the surface create important currents that drag new water to the surface to be oxygenated. They also remove CO2 from the water, which is particularly important if you don't have a trickle filter or degassing column.



I think that whilst a spray bar may contact a larger surface area (if at pressure), it is not achieving the same level of dissolution as the DBC.
Also due to the spray bars design (pushing water through a resrictive hole), Increasing the pressure of the water helps but at the cost of work done by the pump.
One other point (in my ramblings) I would like to put forward is the cone design of the DBC. I have a feeling this reduces the pumps workload due to the pressure drop. This, however, may be a moot point as the gas added to the system increases the pressure.

Apparently choking a centrifugal pump doesn't have the the effect you might think. It actually slightly reduces the amount of power it draws. Google it if you don't believe me there is people out there trying to flog flow restrictors to people instead of VSD's that will tell you all about it.
But of course it will reduce your flow, but then so will the DBC. It has to if you think about it, the reason the bubbles are slowing down is because they are colliding with water molecules and taking some of their energy.
Overall if your really into energy efficiency you would probably better off using neither and getting yourself a vertical aspirator. They are supposed to be 50% more efficient than a centrifugal blower/ air stone combination, which is pretty impressive.

[Fwoor]

True, but not dissolving completely doesn't mean that the energy is entirely wasted. Bubbles that make it to the surface create important currents that drag new water to the surface to be oxygenated. They also remove CO2 from the water, which is particularly important if you don't have a trickle filter or degassing column.

Apparently choking a centrifugal pump doesn't have the the effect you might think. It actually slightly reduces the amount of power it draws. Google it if you don't believe me there is people out there trying to flog flow restrictors to people instead of VSD's that will tell you all about it.
But of course it will reduce your flow, but then so will the DBC. It has to if you think about it, the reason the bubbles are slowing down is because they are colliding with water molecules and taking some of their energy.
Overall if your really into energy efficiency you would probably better off using neither and getting yourself a vertical aspirator. They are supposed to be 50% more efficient than a centrifugal blower/ air stone combination, which is pretty impressive.

Good points!
Actually, That point about degassing made me recall something relevant I read recently.
There was a study done on increasing the level of CO2 in a greenhouse and seeing whether it had any effect on the plants, IIRC it did have a positive effect with the plants growing faster than their lower CO2 environment counterparts.
Surely we can immitate this by taking low DO, High DCO2 and dumping it at the GB. If we incorperate the DBC then its a veritable cycle, dumping the parts where they do the most good.

Anyhow, I still feel the DBC is worth strong consideration in an aquaponics setup given what we need and what we probably already have.

Oh and to be honest, I am interested in efficiency ... but not as much as cost effectiveness. :)

[moose]

In the analysis you must not forget the cost of supplying the O2 to the DBC. This must come from either oxygen tanks or an oxygen generator. Unless this can be offset with increased fish production, I don’t believe that the DBC may be justified in a home system. While the packed column may be a better answer for home system as it does not require O2. Of course until I get my first system up and running not sure I know what I think I know.

[Fwoor]

In the analysis you must not forget the cost of supplying the O2 to the DBC. This must come from either oxygen tanks or an oxygen generator. Unless this can be offset with increased fish production, I don’t believe that the DBC may be justified in a home system. While the packed column may be a better answer for home system as it does not require O2. Of course until I get my first system up and running not sure I know what I think I know.

I don't believe a pure oxygen source is required for a DBC as before

dissolved air composition being around 35.6% [Oxygen].

I do think that a DBC with air being pumped in will be more effective than an airstone.

[arachdog]

If you were using pure O2 you would be crazy not to use a DBC or possibly inject it under pressure. But you certainly wouldn't tolerate it escaping to the water surface.

But with ordinary air I think situation changes. Pumping water is the most inefficient operation in aquaculture/aquaponics, more so than the aeration, so personally I don't think its the place where you want to be tapping energy unnecessarily.

Interesting idea trying to liberate the CO2 in the grow beds for enhanced plant growth. Sounds good in theory, its just a question of if you can create enough CO2 and get it to hang around long enough to significantly raise the concentration around the plant. I don't think sealing the whole system in greenhouse would work though. You would probably end up CO2 deficient as the plants use it faster than the fish create it.

[moose]

Not sure of how much of a concern it would be but have been told that over aeration can produce a dangerously high level of dissolved nitrogen in the water. Don’t know if using air in a DBC could cause same issue.

[Fwoor]

I don't think sealing the whole system in greenhouse would work though. You would probably end up CO2 deficient as the plants use it faster than the fish create it.

CO2 has a density greater than air at STP at around 1.98 kg/m3. That means it would'nt need to be a closed system, and the portion that isnt CO2 would still be air (status quo)


Not sure of how much of a concern it would be but have been told that over aeration can produce a dangerously high level of dissolved nitrogen in the water. Don’t know if using air in a DBC could cause same issue.

Just after a quick google, The only thing I found was the issue with saturation of dissolved nitrogen in water causing insufficient levels of DO. Certainly something to keep in mind though!

[arachdog]

CO2 has a density greater than air at STP at around 1.98 kg/m3. That means it would'nt need to be a closed system, and the portion that isnt CO2 would still be air (status quo)

Sure it looks good on paper and would work fine in the real world as long as there is no wind, convection currents, people walking around, or gnats ****ing.



Just after a quick google, The only thing I found was the issue with saturation of dissolved nitrogen in water causing insufficient levels of DO. Certainly something to keep in mind though!

Gas bubble disease which is caused by excessive nitrogen only really occurs when air is injected under pressure and hence becomes super saturated. The nitrogen then comes out of solution in the fishes blood forming tiny bubbles, causing all sought of problems. But it's not something you likely to ever achieve with just a DBC.

[Crusty]

If I may contribute to this thread. I would not recommend a DBC if you are not using bottle o2. Your most effective method of aeration in these small units is air stones but be sure to get yourself some quality ones, not the aquarium $2 variety.

[Fwoor]

If I may contribute to this thread. I would not recommend a DBC if you are not using bottle o2. Your most effective method of aeration in these small units is air stones but be sure to get yourself some quality ones, not the aquarium $2 variety.

Hi Crusty, I appreciate your recommendation but if you would'nt mind, could you provide some more of a basis for the recommendation? I am unsure of the effectiveness particularly.
Just guessing but I assume contact area of the bubbles (with an airstone) is why your suggesting it to be more effective.

[Crusty]

Hi Crusty, I appreciate your recommendation but if you would'nt mind, could you provide some more of a basis for the recommendation? I am unsure of the effectiveness particularly.
Just guessing but I assume contact area of the bubbles (with an airstone) is why your suggesting it to be more effective.
Hi Fwoor

It is a bit more about the gas mass transfer rate and efficiency. The downflow cones are designed for use with pure oxygen. If you inject air (not oxygen) into them, you will successfully transfer oxygen but you will also transfer the nitrogen in the air as well. The nitrogen may be of some use to the plants, but the fish will not appreciate it.

When we talk about efficiency, DFBC assume you need increased pressure to operate them, along with a predefined length (height) of the cone/column. This means you have to increase the head height and pressure of your pump. From a power consumption point of view, many will be using high flow, low head pumps that require very little energy to run in comparison to a pump that would be needed to run a DFBC.

In terms of design, it is more than likely you will end up with a foam fractionator, which operate like a down-flow bubble contractor though with air (not oxygen) in a system without micron screen filtering. Without the screen filtering, you will disolve solids and re suspend them (no vent on the top of a DFBC). This will increase the organic load of the system, which will then cause some other wonderful problems.

This is not to say, you can not give it a go and find the results for yourself (the fun of experimenting). I think you need a minimum of 10 to 14PSI to run them and keep in mind an efficient DFBC can disolve 90% of the gas injected. If you are injecting air, it contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.038% carbon dioxide.

Please let us know how you go.

[arachdog]

Crusty: I'm not sure why you are suggesting a high pressure water pump is a requirement for operation of a DFBC. Even if you wanted to inject the air under high pressures (which as we both know wouldn't be a good idea) you can create the required head of pressure by taking your pipework down (possibly into the ground) and then back up again.
But to be honest my understanding of a DFBC is that its more about increasing contact time by using the flow of the pump to slow the rate of rise of the bubbles, not increasing the injection pressures per se. Although I can see how a pure oxygen system might use a combination of increased injection pressure, in conjunction with a DFBC.

I still agree with you in general though, I think they are a bit of a waste of time with just air since the extra dissolution you get comes at a price in the form of a flow restriction.

Sidebar: Have you installed many pure O2 systems ? I'm interested to hear how widespread they have become. The economics of it look really promising to me.

Fwoor: I believe Crusty's suggestion to use a quality airstone stems from the fact that they create smaller bubbles than the cheaper variety and hence have more surface area and therefore achieve better dissolution rates. Which is yeah, basically what you said.

[Crusty]

Crusty: I'm not sure why you are suggesting a high pressure water pump is a requirement for operation of a DFBC. Even if you wanted to inject the air under high pressures (which as we both know wouldn't be a good idea) you can create the required head of pressure by taking your pipework down (possibly into the ground) and then back up again.

DFBC work at higher than atmospheric pressure to increase the diffusion efficiency. If you are simply increasing the contact time for diffusion, then perhaps a U-tube set up will be more beneficial. This would simply take your pipework down (insert air) and then up again in a U to extend the contact time. This will extend the contact time, if that is all you are after. If you want to increase the efficiency you need to increase the pressure. To do this when you are pumping down into the ground and back up again, you will need to go down at least 10meters to add 14psi (1 atmosphere), which makes them less efficient in comparison to the DFBC. IMO in a back yard culture system, I do not see any benefit in doing either.


But to be honest my understanding of a DFBC is that its more about increasing contact time by using the flow of the pump to slow the rate of rise of the bubbles, not increasing the injection pressures per se. Although I can see how a pure oxygen system might use a combination of increased injection pressure, in conjunction with a DFBC.

You are right about the increased contact time. The difference is the DFBC starts to become efficient at the 2meter high mark. But they are designed (as a cone) to reduce the velocity of the water flowing through them. EG: water enters at X velocity, is slowed down by the expansion of the bottom of the cone and then continues on its way. By this reduction of velocity, you will need to increase the flow rate considerably at the top (in addition to the added head height) for you to have the required flow rate on exiting the cone. If that makes sense. In other words, you can not use the same size pump as you would in say your CHOP system, then add a o2 contactor and not expect some significant loss in water flow. This suggests the pump will require more energy to achieve this. This is where the energy efficiency comes in. The air stones will adequately (cost efficiently) aerate your water for the kind of densities grown in your back yard.


I still agree with you in general though, I think they are a bit of a waste of time with just air since the extra dissolution you get comes at a price in the form of a flow restriction.

Sidebar: Have you installed many pure O2 systems ? I'm interested to hear how widespread they have become. The economics of it look really promising to me.

Fwoor: I believe Crusty's suggestion to use a quality airstone stems from the fact that they create smaller bubbles than the cheaper variety and hence have more surface area and therefore achieve better dissolution rates. Which is yeah, basically what you said.

When I suggested quality air stones, I mean something like the "Sweetwater" range would be more suited. I see systems being sold with 30cent aquarium airstones which really fly in the face of any design criteria for aeration in systems, back yard or not.

As we are working with shallow tanks (<1meter) air stones are not all that efficient and it is the turbulence at the surface (breaking of the surface tension) that creates the most gas exchange. In these shallow tanks, try to avoid the fine pore diffusing stones as they block up quickly and are primarily suited to bottled O2. Look for a medium pore. These will not make your air pump work as hard and will break the surface tension of the water well enough. As one of those magical guidelines, roughly 150mm medium air stone per 500 liters of water. Always buy a few spare as you will have to soak them in acid, once a month to keep them working at their best. I think they price at around 25 bucks per 300mm and can be cut in half.

You will want to keep in mind too much bubling in the culture tank will make your fish cranky, so some aeration in the culture tank and the rest elsewhere, sump or usually the bio filter (if you have one). Also you can set up your supply line to the culture tank to suck air like a venturi.

Sidebar: You will find every RAS aquaculture farm has oxygen injection to one extent or another.

Hope that helps.

[arachdog]

OK I see where your coming from now.

We've established that there is two ways to increase dissolution efficiency. Increase the contact time, or increase the pressure. Now the kind of DFBC that you describe is clearly designed to make use of both of these effects because its using expensive pure oxygen and its worth the considerable energy losses to dissolve it as much as possible. In addition there is no fear of supersaturating the water with nitrogen causing gas bubble disease you'll get with air.

However I believe the kind of DFBC that Fwoor is suggesting (correct me if I'm wrong) only uses the first effect.. ie. In the tank, no extra head at all. It will technically still increase you dissolution efficiency through increased contact time. But as I've already said earlier in the thread, I agree with you, it probably still won't have a net efficiency benefit.



The difference is the DFBC starts to become efficient at the 2meter high mark.
Are you talking about the system efficiency or dissolution efficiency here, and what exactly is significant about 2m?



Sidebar: You will find every RAS aquaculture farm has oxygen injection to one extent or another.

Wow! That surprises me. So every single install you have done has included it?
How common is it as a means of boosting production, ie. not just as backup system?
Whats more usual, bulk liquid oxygen, oxygen generators or bottled ?
I've only seem the inner workings of one commercial RAS facility and that didn't include it at all, but that was nearly ten years ago.

[Crusty]

Are you talking about the system efficiency or dissolution efficiency here, and what exactly is significant about 2m?
Similar to air stones in less than 1 meter, their efficiency drops. From memory diffusion in less than 1 meter would be lucky to be 5% efficient which is related again to the contact time and pressure. For the extra energy to run a cone, there is some minimum height, velocity, pressure etc design principles to be met for it to become efficient. For example there would be very little point in running a cone 500mm tall as the column simply is not long enough to make much of a difference to the contact time. You will get some benefit, though I doubt it will return a result that would warrant the additional cost, which is what we are talking about. Without adequate filtration, I believe it would simply run like a fractionator and cause more issues than it is worth.

The best way to know how they will respond in a back yard culture, have a go and whip one up and see what it does. I see no harm coming from the experiment. You should have a Do2 meter at least though.


Wow! That surprises me. So every single install you have done has included it?
How common is it as a means of boosting production, ie. not just as backup system?
Whats more usual, bulk liquid oxygen, oxygen generators or bottled ?
I've only seem the inner workings of one commercial RAS facility and that didn't include it at all, but that was nearly ten years ago.
Yes. As I said they all use it to one extent or another. Low density operators do not use it (only stand by), though I would think their viability would be in question if they are using Tank RAS. Some run bulk liquid oxygen in DFBC for increased production (no aeration in the culture tanks in normal production and when harvest dump diffusers) Some run small and large oxygen generators directly to their bio for greater production. Others have bottles on standby for treatments etc.

[arachdog]

Similar to air stones in less than 1 meter, their efficiency drops. From memory diffusion in less than 1 meter would be lucky to be 5% efficient which is related again to the contact time and pressure. For the extra energy to run a cone, there is some minimum height, velocity, pressure etc design principles to be met for it to become efficient. For example there would be very little point in running a cone 500mm tall as the column simply is not long enough to make much of a difference to the contact time. You will get some benefit, though I doubt it will return a result that would warrant the additional cost, which is what we are talking about. Without adequate filtration, I believe it would simply run like a fractionator and cause more issues than it is worth.

OK, so what your really talking about is cost effectiveness. Effiency isn't really the right word since its a sliding scale not an absolute measure. By which I mean 1.1m is not going to be significantly more efficient than 1m, so you can't say one is efficient and one isn't. But even if your talking cost effectivness then its would seem to imply to me that there must be a better alternative below 1m.




Yes. As I said they all use it to one extent or another. Low density operators do not use it (only stand by), though I would think their viability would be in question if they are using Tank RAS. Some run bulk liquid oxygen in DFBC for increased production (no aeration in the culture tanks in normal production and when harvest dump diffusers) Some run small and large oxygen generators directly to their bio for greater production. Others have bottles on standby for treatments etc.

Yeah right interesting, thanks for the information. To be honest I think viabilty of the RAS farm I worked on probably was in question. It was more of a richmans hobby and for some strange reason they were employing marine biologists to run it.