Backyard Aquaponics
A place to discuss aquaponics

Right now I'm using gravel beds -- for biofilter and growing medium. I'm looking to build another greenhouse (30'x100', 9 x 30m), but using a floating raft system similar to the one at UVI. I notice that the depth of water in their troughs, and in most of the raft tanks, is 12"(30cm). I can't find a rationale or explanation as to how that depth was determined or why it is needed.

They apparently also run about 16-17 gallons (60L) of effluent water a minute through each trough. Again, I am wondering if that is just the way it worked out or there is some metric involved.

I'd like to build shallower troughs, say, 6" (15cm) water depth. It would seem there is plenty room for root growth and nutrient passage.

Am I missing something? Anyone have a feel for such things?

Thanks,

m

I suppose there could be plenty of reasons for that depth. I don't know for certain since I have never gone to UVI to study.
First, the more water in a system, the more stable it will be.
12 inches is a handy number for calculating with.
Remember that it is not just the roots but also the net pots and rafts that need space. I have grown hydroponically in a 6" deep container but I think the deeper containers did better, probably mainly to do with temperature stability. The shallower the container, the faster the liquid can heat up and cool off.

Depending on the plants being grown, 6" deep DWC beds may still easily get clogged with roots, the 12 inch depth probably give more space for water to circulate freely and keep things oxygenated.

The flow rate they use is most likely based on the amount of circulation needed to keep the fish tank water healthy. It is generally a good idea to circulate/filter the volume of your fish tank each hour, you might get away with less in some circumstances but even tilapia slow down when the water quality gets poor.

I'm guessing you know this since you are basing things on the UVI model but....
Don't forget the filtration and aeration when you switch to DWC. It is not enough to simply pump some fish tank water through a DWC bed and call it aquaponics. You need to assure there is enough filtration to support the amount of fish and keep water quality good and support the amount of plants desired.

Thanks, TC,

Of the concerns you raise, it seems the one that I really need to address is that of the possible clogging with roots. (How do NFT systems manage to work if that is a problem?)

I posted here earlier in the year about my system using worms. I promised to post back on the worms but have not as I farm and am just covered up in the growing season. But I do have some definitive findings and will post them with pictures.

Essentially, worms use only a fraction of the oxygen that fish do, so aeration is not a problem. Even solids, something the UVI folks seem to live in fear of, is not such a problem as the worm bed (gravel) acts as a solids filter. Not having fish, the "stability" required for fish is simply not required.

Correct me if I wrong, but the polystyrene rafts float above the water depth and the net pots are only about 1/2" into the water?

Presently, I'm converting my gravel beds to floating raft tanks just to test some other hypotheses. But the water in those beds will only be 6" deep. Obviously, if I could see a train wreck coming I wouldn't do it. I was just hoping someone here might have experience with different depths of floating raft systems. I just want to make sure that depth is not just something that was first assumed and everyone else has used it.

Thanks again,

m

I would go for the 12 in ones most raft plants the roots are over 12 long and with raft beds there is always a layer of junk on the bottom
You will always need a few air stones ,Because roots need plenty of oxygen
You mentioned nft system the neutrient film is in contact with the roots for seconds then its changed bringing fresh stuff

For NFT it is necessary to filter out the solids or the roots get totally gunked up. I tried hooking up some NFT pipe to my system and just sending the unfiltered water to the NFT pipe and ugg, the roots got all covered in fish poo slime. I have since set up some NFT pipes that are fed water that has been filtered through flood and drain gravel filled grow beds first and it is much better now. However, NFT pipes can get totally clogged with roots and I regularly have to pull the basil plants out and rip most of their roots off regularly or the pipe will be clogged and overflowing.

I highly recommend sending filtered water to the Raft grow beds, especially with shallower beds.

The shallower the DWC bed, the more the roots of the plants will spread out and weave together with other plants making it more difficult to pull them out without disturbing the other plants. Yes, in theory the foam rafts will float on the water but the more plants you have in the raft, the lower the raft is going to sit in the water, taking up a bit of that 6" depth.

TC,

I'm going to go ahead experimenting with the 6" tanks, just to see what happens. But I think I will plan do the 12" of water in the troughs for the larger greenhouse. I'll have 5, 4'x96' troughs. There's a some more expense (deeper troughs and bigger pump), but not near the expense of having to tear up and start over again if I mis-guess.

With your counsel on water clearness, part of the experiment with the tanks will be to see how clear the water will be as it comes out of the worm gravel bed (this bed is not for growing anything but worms). Maybe I'll have to do more. Or not. So far, I've not had any turbidity in the present configuration.

Thanks,

m

If there is enough filtration for the solids and surface area with aeration for the bio-filter bacteria all should be fine.

Like the recent discussion about growbed depth... it's not really a matter of actual growbed depth... or raft bed depth...

It's about oxygenation and flow rate...

In the case of growbeds this has ramifications for bio-filtration... with raft systems, with solids removal... and clarifying/settling tanks.. it's more about oxygenation...

As you're running a "worm" leachate system... it's going to take some fiddling to determine just how much oxygenation is required to process the worm leachate...

I'd err on the side of caution... and suggest the deeper beds... hence greater flow throughput... but certainly try both and report back please...

Rupert,

Thoughtful comments. And I think the considerations you mention are important.

The one thing I do know is that oxygen is not near the problem with worms as it is with fish. With simple diffusers on drain backs, my DO stays at pretty close to max levels regardless of temperature. My conclusion is that, for worms, I have far and away, plenty of 02. Worms, particularly with a flood and drain system (might be more difficult if worms were constantly submerged in water) may take only tiny quantities of O2 from the water.

One of the reasons for having greater total volume of water is the"stability" if gives fish. larger volumes, I think, tends to ameliorate some of the conflicts between fish and plants. If these conflicts exist at all with worms, they are not near as critical.

I'm preparing a post on worms for the thread I began over a year ago to fill out some of the details.

Thanks.

m

Worms may use far less oxygen than fish but they do still need it.

Remember that flood and drain provides all the oxygen possibly needed to whatever is living in the flood and drain bed (plants worms bacteria etc.) As soon as you are no longer flooding and draining, you are limited to the oxygen that is forced into the water by bubblers. Also, flood and drain generally means there are points in the system where the water is falling and splashing at least a little which provides a little more aeration.

Plants in DWC definitely do better with plenty of bubblers around.

Hey mornings... you might want to take a look at Dr. Brett Roe's work on "vermiponics"...

http://www.vermiponics.com/

TC,

This is still pretty hypothetical but the results so far support what I'm saying.

What I have in mind is a system that floods and drains in the gravel bed just for worms and worm feeding, and then uses a flow through raft system for plants. Essentially, most or whatever oxygen I entrain is for plants and bacteria.

As I said before, oxygen is just not a problem with worms. They don't compete for air. They breath in or out of water equally well, but, apparently, they have little need or opportunity to use up the oxygen in the water, certainly nothing like fish must.

I don't use bubblers. Just don't need them. I think without fish to use up the oxygen, plants and bacteria will get all they need with much less effort. And, since worms generate very little ammonia, I even have need for less biofilter.

m

Rupert,

When I first started I contacted him. I don't like being cynical but he seemed to know less than me about what I am doing. E.g, he was amazed that the worms actually reproduce in the water/gravel -- and reproduce, they do!

His website seems more to sing his own praises rather than being helpful.

On the other hand, Dr. Clive A Edwards (author of "Biology and Ecology of Earthworms", the "bible" on worms) has been really helpful and generous with his time.

m

So long as you are still running flood and drain beds for the worms (that also happens to be a great place for bacteria) then you will just have to see how the plants do in the DWC and decide if you need more depth or air as you test it out.

We here all know that worms do famously in flood and drain gravel beds. Or even in flooded beds provided there is ample aeration so they don't flee to the surface and fry in the sun. Worms can live completely under water if there is plenty of aeration but I also know worms will flee or die if they can't breath.

Let us know how your trials go, it will be interesting to hear about your findings on DWC bed depth over time.

.... no comment....