oxygen utilization by plant roots in hydroponic raft systems

[Brian McCormick]

Hi all,

I'm new to this site so I thought I would introduce myself with a question. I am a senior in Environmental Engineering at LSU in Baton Rouge, La. I am currently working on a system design for my senior design class. I am trying to make a mass balance for the main components in the system (i.e. nitrogen, phosphorus and oxygen). I was able to create the nitrogen and phosphorus easily enough but I am having trouble finding a rate of oxygen uptake in the roots of the plants. We are using the raft system and I am trying to figure out how often I will have to reaerate the water so as not to limit the nutrient uptake by the plants due to low oxygen levels in the water. Rakocy recommends using air stones every 4 feet in raft systems, however, we are trying to get away from using airstones as they are inefficient compared to airlifts.

Anyone know what the dissolved oxygen levels need to be for plant roots? Or how much oxygen plants use per unit area?

Thanks for the help and I look forward to being active on the site.

[Brian McCormick]

Awesome story what a better way to start an addiction than doing thorough research.  Look forward to picking your brain some more.

p.s. do you know anyone on here that is an expert in raft systems?

[TCLynx]

Gina and Tonya of Green Acres do raft Aquaponics

Chris Smith does Raft aquaponics

and Ryan (hum last name not coming to mind right now) knows Raft aquaponics.

There are many others but those three have quite a bit of experience (commercial grade) and Chris and Ryan are fairly active and if you go to the Green Acres group you can probably get Gina or Tonya's attention.

[Ryan Chatterson]

A couple things here...

When using a diffuser to saturate the water with oxygen, you want small bubbles injected as deep as possible that rise as slowly as possible allowing for a longer period of oxygen diffusion to take place. In an airlift (housed in pvc), your bubbles are confined in a small space where they can merge with one another, making larger bubbles with less surface area and a quicker rise to the surface... Equating to less oxygen diffusion from the air bubbles themselves.

If you're looking to saturate the water I would use stones and not a lift (which still act as an airlift but arent confined). PVC housed airlifts are great if you need to direct flows in a certain direction or if you want vertical lift but ime they are not as effective as a bare stone when using them as an aeration device. We arent talking pure O2 run through a venturi injecture, were talking a bare pipe with little pressure and ambient O2 levels.

Good luck with the project bud.

[RupertofOZ]

Ryan said:

When using a diffuser to saturate the water with oxygen, you want small bubbles injected as deep as possible that rise as slowly as possible allowing for a longer period of oxygen diffusion to take place.

 

 

This is a commonly held mis-conception.... yes finer bubbles are preferable.... but NOT because they allow a greater time for the air/oxygen to diffuse as they rise...

 

The oxygen diffusion/exchange... takes place at the surface of the water... as the bubble breaks... not during the "bubbles" rise through the water column....

 

Think about it... for the oxygen in a bubble to diffuse into the water column as it rises... the oxygen in the bubble would have to diffuse through the surface tension layer of the "bubble"... effectively causing the bubble to lose pressure... and break...

 

It doesn't happen, certainly not in the time it takes for a bubble to rise in most systems...

 

The benefit of lots of smaller bubbles... is the surface area of the water exposed to air/oxygen diffusion... at the water surface... when it breaks....

 

That's why paddle wheel aerators are so efficient... they toss huge amounts of water... thus surface area,,, into the air, exposing it to diffusion...

 

Air diffusers are used in deep ponds/dams... but their benefit is not primarily any direct diffusion of air/oxygen into the water column... but the fact that they lift deep water to the surface.... thus exposing it to possible surface diffusion.. and mixing and more uniformly oxygenating the pond/dam...

 

A natural pond/dam... receives it's oxygenation at the water surface... primarily by surface area disturbance by wind...

 

A still, undisturbed body of water... will de-oxygenate.. and become stagnant for this reason... as any biological oxygen demand exceeds the oxygen diffusion at the undisturbed surface...

[Ryan Chatterson]

Again, we are going to agree to disagree. In large bodies of water you are trying to mix as well as aerate to bring surface water to the bottom and de-stratify the body of water. Surface aerators (like paddle wheels) are more efficient in shallow bodies of water but as depth increases diffusers take over. To say air doesn't diffuse through the bubble wall into the body of water is again, absurd. With a raft ontop of the water you aren't going to have any device throwing water into the air like a paddle wheel and you don't have a massive surface of water exposed to the air. Your comparisons just don't make sense.

[RupertofOZ]

Ryan said:

Again, we are going to agree to disagree. In large bodies of water you are trying to mix as well as aerate to bring surface water to the bottom and de-stratify the body of water. Surface aerators (like paddle wheels) are more efficient in shallow bodies of water but as depth increases diffusers take over.

Very true Ryan... yes paddle wheels are more efficent in shallow bodies of water... and diffusers are beneficial in deeper water... as they mix the low oxygenated deep waters....

And that's exactly what I said...

 

Ryan said:

 To say air doesn't diffuse through the bubble wall into the body of water is again, absurd. With a raft ontop of the water you aren't going to have any device throwing water into the air like a paddle wheel and you don't have a massive surface of water exposed to the air. Your comparisons just don't make sense.

Indeed.. it would be absurd to suggest using a paddlewheel in a raft system... unless it was really huge....

But that's not what I said Ryan... or what I meant.... and I didn't make that comparison...

I used the example of the paddlewheel to support the concept... that it is the surface area of disturbance of water.. and exposure to air... that results in the maximum diffusion of oxygen into water....

And that's just an incontravertible scientific fact.... whether you choose to agree or not...

 

Diffusers in deep ponds... where the air is injected under pressure.... may result in diffusion of oxygen into the water.... as many/most of the bubbles may in fact burst.. releasing the air contained within them... before making it to the surface....

And as you rightly point out, in agreement with what I said.... help mix the oxygen content that might otherwise stratify... and potentially under the right conditions.. cause a "pond inversion"....

But in RAS, and/or AP systems.... the bubbles rise too quickly for anything but a miniscule amount of diffusion to occur.... and it is the surface area of the agitated water... at the surface.. where the oxygen exchange occurs...

The same thing can be seen in nature... where rushing, babbling cascades of moutain waters... are highly oxygenated... partly in relation to water temperature... but mainly due the huge amount of water distrubance at the water surface... and subsequent oxygen exchange...

"Flowforms"... that bend and fold water into itself... are likewise highly oxygenated... and I have seen them incorporated into a few AP systems.... ( the size required just doesn't make them practical for AP systems though... although trout raceway and commercial systems often employ such, or similar)...

The amount of oxygen water can hold is dependant on many factors... temperature, elevation, salinity etc.. but it's infinitessimally small in relation to oxygen in the air... and unless injected into water under pressure... can not "diffuse" beyond a certain depth...

Here's a quote from an FAO article.. that illustrates what I'm saying...

 

Oxygen is absorbed in water by direct diffusion and by surface-water agitation. Solubility of oxygen in water is so small and by diffusion process alone in still water, it was caculated that it would take 6 years for oxygen to diffuse from surface to a depth of 6 meters in quiet water.

Absorption of water is very minor, that almost all the oxygen enrichment of natural waters takes place by agitation of water.

 

The same is true of AP systems aerated by air stones....fountain heads, waterfalls etc

[Todd Sowell]

Ryan,

There are other reasons Rakocy uses airstones every 4 feet. I am not sure where I read it but there is some kind of bug/crustacean that can invade your system. I'll see if I can find the article or maybe Rupert knows.

Todd

[Todd Sowell]

Ryan,

 

Rupert's correct. Dr Wilson Lennard has made the same observations.

[Brian McCormick]

The idea that the oxygenation rate is greatest as the bubbles surface makes sense, especially in only 30 cm of water. 

Do yall know what a good flow rate through a trough system is? I have heard 2 gpm but that sounds really low.

[TCLynx]

2 gpm I believe is the bare minimum through the troughs and I've heard 5 gpm is a better minimum.

[Siggy G]

I may be a little late to this discussion, but here's my 2 cents anyways.
In response to Brian's initial question: due to the fact that there are far too many variables that influence the answer to the question of how much aeration is needed and experimentation is not an option in his project, the only reasonable method to come up with a figure would be to find a system that has system parameters as close as possible to the one he's designing and follow what works in that situation.
In response to some of the comments: Brian you stated that you wanted to produce 750 lbs of fish/week with 161 lbs of feed/week. I'm assuming it's a typo, but I figured I'd point it out.

Rupert's comments on the diffusion of oxygen throughout the outside of a bubble are false. Any water/air interfaces will exchange gases. The exchange is governed by the partial gas pressures within the air and the water and the surface area. They strive to balance. They strive to achieve saturation, which by the way is dependent on many factors including temperature and the mixture of gases in the bubble and the water. But the exchange goes both ways. If for example the water has a higher gas pressure from carbon dioxide than the air, the carbon dioxide from the water can also pass to the bubble. Smaller bubbles are better at getting the most out of your aerator. First off they have a greater surface area by volume and therefore can achieve equilibrium with the water faster. Secondly they rise slower. giving more time as well. All this together can lead to a small bubble starting it's journey upward with a relatively high concentration of oxygen and reaching the water surface the same size but with less oxygen and more Carbon Dioxide for example. Not to mention that air is mostly nitrogen which isn't even half as soluble as oxygen. So the fact that the bubbles make it to the surface apparently unchanged isn't very significant.
He also needs to learn the definition of diffusion. Water disturbance or turbulence doesn't effect diffusion besides potentially exposing water with lower oxygen concentration to the air hence increasing the rate of diffusion due to a greater difference in partial gas pressures at the air/water interface. Diffusion is the movement of molecules though a liquid (or gas) without bulk motion. It's the effect that the FAO article he refers to is talking about. Diffusion's effect is mainly important at very small scales. For example the boundary layer of water surrounding a plant's roots where very close to the root surface there is almost no flow. For an oxygen molecule to make it all the way to the root, diffusion is necessary. It's extremely slow but luckily it's only necessary over extremely short distances.
The distribution of oxygen through the a body of water through water movement is called advection, and it's responsible for getting it where it needs to go. To get the oxygen to the plants roots you need a source of oxygenated water, for example a bubbler, and enough turbulent water flow to distribute it throughout the body of water.
It would seem to me though that small venturi valves would be more economical than bubblers in the long run. Has anyone tried that, blowing a stream of venturi valve bubbles across the underside of a raft?