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.

[RupertofOZ]

I'll pull some notes regarding some of your queries Brian...

 

But a question.... "we are trying to get away from using airstones as they are inefficient compared to airlifts"...

 

Aren't airlifts usually "powered" by airstones?????

 

And when you say "innefficient".... are you talking in relation to aeration, or water movement in comparison to pumps??

[Brian McCormick]

airstones can be planed in the airlifts, but I was reffering to the placement of airstones in the hydroponic troughs themselves. The mixing of air and water in the piping for the airlift allows for a more thorough aeration than simply placing the airstones in the tanks or troughs. so i think that to your last question the answer is yes. I was refering to aeration not water movement. I think you are right though, it is more efficient to use one pump for both aeration and water movement.

Thanks for looking through your notes for me!

[Brian McCormick]

just reread what i wrote again. I am planning on putting airlifts throughout the raft system but the problem is I dont know how frequently to place them. I know the equation for aeration rate through an airlift and if i know the oxygen depletion rate then I can get a set distance between airlifts so that the water in the trough maintains the required DO levels.

[BenHehle Beamz]

Hi Brian,

Do you know your water volume, number and weight of fish, number of plants and their respiration rate, root diameter and length, average temperature, input of feed and % of protein of food? These points all have an impact on your DO. Wouldnt it be easier to set up the system first and measure the DO at inflow and outflow? If its too low, just hang one more bubbler in your raft, step by step until you got your required DO level?

[Brian McCormick]

Yeah that certainly does sound easier. Unfortunately when we started the design, we were given certain benchmarks that we were supposed to meet. Like feeding 1,000 people. So we went to the EPA and got a fish protein recommendation for an adult per week, which was 12 ounces per adult meaning 750 pounds of fish production per week. So that was how we started our design. But from there we started researching and got several numbers from one of our professors who is an aquaculture specialist. We know that we need 161 pounds of feed per week to produce and that much fish and the size of the biofilter and clarifier size we would need. Then from Rakocy's work we were able to establish the plant sizing and caustic/nutrient supplementation needed. The average temperature we would like to be around 70-80 degrees Fahrenheit. When we were planning on designing the systemwe hadnt even considered respiration rate and root diameter and length.

[RupertofOZ]

I think you coming at it from the wrong end Brian....

 

determine your oxygenation requirements for

 

• fish stocking levels
• conversion of feed to protein
• nitrification

 

I think you'll find that any oxygen requirements for the plants will basically be taken care of accordingly...

 

Anyway... I;ll dig up some figures for you... eventually...

[RupertofOZ]

750lb of fish per week.... OMG... that's a huge project Brian.... that's about 40,000lb of fish a year....

 

I think you need to seriously engage an aquaculture consultant/expert.....

 

And a food safety consultant... a marketing manager... a sales manager... architect... town planner... and probably a couple of state government lobbists...

 

And what's your budget... $2-5million+

 

I think you should be more worried about having an oxygen supply on hand... for yourself.... don't worry about the plants...

[Brian McCormick]

haha yeah its huge the growing area is massive as well.  Fortunately there are several professors at LSU who are aquaculture experts.  So at least I know the fish wont die. just trying to make sure the plants dont either.

We are working on the economic feasibility of it, but it is a theoretical design that we have to do as part of a design competition called WERC in New Mexico.  Unfortunately, i dont have those kinds of finances.  Ill let you know when the business side of my team comes up with a number and let you know the damage.  So far all i know is the price for the solar cells if we wanted to power the whole thing through solar power would be nearly half a million, but with the 30% federal credit its only $350,000. But seriously, I never would have imagined feeding 1,000 people only a portion of their weekly food would be so large.  The hydroponics section is going to be several thousand square feet!

[RupertofOZ]

The $2million+ budget... is just to build, grow and market/sell the fish Brian....

 

Not the "hydroponics section"... or anything else...

[TCLynx]

So Brian, If you know the difference in efficiency between airlifts and air stones and the recommendations of .5 cfm of air going into a 6" air stone every 4 feet in a 4 foot wide raft.  Then you should be able to figure out how many airlifts would meet the recommendation.  The recommendation of that much air might not be based on a scientific knowledge of exactly how much dissolved oxygen eat plant takes up but is based more on the fact that many Aquaponic raft producers have tried to get away with less but found the plants struggled until they added enough air to make the plants do better and most people trying to get away with less aeration in the raft beds are disappointed with the results.

There might be ways to lift the plant support boards up so they are not impeding air/water contact at the surface but so far none of the large operations have found that to be very feasible because it then requires stronger sturdier and heavier boards (probably costlier) and they are then resting on the sides of the troughs so it makes the assembly line option of floating the rafts from planting down to harvesting end more difficult and thus labor more costly.

Now I don't know how you would install the airlifts to provide the dissolved oxygen in a manner that wouldn't also impede the function in the troughs.

However, I might recommend you get a trough of water and an air pump some air stones and your airlift parts and actually run some tests with a raft of plants on the bed and see if the airlifts actually provide for a better dissolved oxygen level for the same amount of air pump energy use.  I have a feeling that once all the other variables are put in the way, they might not be so much more efficient.  See the trick is, the plants all want the dissolved oxygen, not just the 4 plants right next to where the airlift tube happens to be.  The air stones provide a more diffused aeration to more plants for each air stone.  So even if an airlift actually puts more dissolved oxygen into the water, it won't necessarily be more efficient at getting it to all the plants.  Plants can actually use far more dissolved oxygen in the water than the fish can tolerate.  If you could figure out how to make the entire bottom of the plant trough into a giant air diffuser so the bubbles were gently coming up under the entire plant bed you would probably get better plant growth than spacing any type of aeration at intervals.

[Brian McCormick]

Lynx,

Thanks for the help.  You are totally right, the airlifts the way I was thinking would impede the assembly line idea.  My friend and I are building a separate system in May and will try out the two methods.  I am still trying to think of an idea for how best to implement the airlifts into the troughs without ruining the assembly line option.

I also had no idea that the flow rate of the air into the air stones 0.5 cfm.  If you dont mind me prying, how long have you been working with aquaponics systems?  You seem to have a lot of knowledge and I am just curious to see how you started.

[TCLynx]

Now I am really only repeating what I have read when it comes to Raft type aquaponics.  I don't have much good real experience with rafts personally.

I'm really more into media based systems.

Anyway, I began reading about aquaponics on the BYAP forum back in 2007 and became totally addicted and back then I did manage to read the whole thing.  No longer possible I fear and I've fallen behind and can't keep up either here or there let along the other 3 Aquaponics forums.  Anyway, I build my first barrel ponic system in December 2007 and immediately started on the big system and we got our first channel catfish back in March 2008 and I think we had our first fish dinner in August or October of 2008.  We have also raised Blue tilapia but gave them up cause it gets to cold here in winter to do it without heating.  We now raise bluegill and channel catfish as our main fish.  I sell aquaponics equipment and now produce as well as system design and install and tower deliveries. 

By trade for the past 20 years or so, I've been a stagehand/ AV tech which means I could probably fake my way through doing a huge variety of things after sitting through classes for anything from being an insurance agent to urological surgery or selling drugs to cows. I might not be a specialist in any of it but after having to support the specialists in all sorts of things, one gets quick to pick up the most pertinent points.

There are lots of people here with some great info.  Keep picking brains.