Raft vs. Media filled Grow Bed (Sustainability)

[James McCullough]

Lots of people seem to advocate an Ebb & Flow system for hobbyists and a raft or DWC system for larger commercial applications. So what's wrong with a large scale E&F system? Okay so the media could get expensive, solids loading may be too intense at large scales and the total ratio of fish weight to system water volume is lower.  However, in favor of Ebb & Flow is the potential for greater mineralization of solids to provide more and different kinds of nutrients (supposedly eliminating the need for nutrient supplementation), better electrical efficiency by reducing active aeration requirements to the hydroponic/nitrification side of the system along with non-continuous pumping, the elimination of clarifiers and degassers and potentially better water usage efficiency (water required to handle solids from filters/clarifiers vs. lower evaporation rates in raft system).   Has anyone tried or considered a commercial sized system that uses a 0.5 lb/gal fish to tank ratio (at max) and just increases the tank to bed volume from 1:2 to 1:3 or greater?  Seems like if you kept the plant to fish ratio the same while making such an increase in bed size then this would handle the issue of solids in a large E&F (just till thoroughly between harvests and keep up the worm population in the bed). Any thoughts? I'm trying to design a system that uses primarily solar power and collected rainwater while still being scaled to a small commercial size, so an effective E&F system would be ideal if possible.

[Sahib Punjabi]

Hello James,

Check out  www.sahibaquaponics.com or come and pay us a visit. I promise you, you will not be disappointed and will be able to achieve your stated goals. No charge for you or your Professor.

 

God bless,

James McCullough said:

So let me clarify (no pun intended) the goal of the design. I'm a senior Environmental Engineering student at Georgia Tech working with a team of students under a professor to design the system. The system will be built in downtown Atlanta on Truly Living Well's farm in MLK's old neighborhood. Their mission is to bring together the community through education and the availability of organic produce (not sure if it's certified), thus what they want  us to deliver is a "closed loop" design (not entirely because they'll be providing a consistent input of food waste from their small farm and local restaurants/farmers markets) that kids and adults from the neighborhood can come see to learn about the processes that go into the production of food. Thus, we aren't necessarily worried about going heavy on the fish or produce side so much as making a robust system that doesn't demand extra inputs except for in emergencies. As far as "commercial" it doesn't need to be a money making juggernaut, but rather stay afloat (they've already got a huge network of restaurants, markets and neighbors who are willing to pay a premium for a product they can feel good about). On the other hand, as engineers/engineering students we would love some hard numbers, but as you said I think that the relative infancy of aquaponics is responsible for the general lack of such numbers other than from the big time systems and anecdotes from experimental set ups. That's why my professor is leaning toward a UVI xerox but downsized, the numbers are there and the system is proven. Ultimately though I feel that a straight UVI copy won't be delivering quite what the folks at the farm are hoping for. We've been considering a hybrid system but to convince my prof. I need some hard numbers or sizing info from an extremely reputable source. If I can't get this, would you suggest that I try to convince him to start with a small media bed setup as a proof a concept and learning experience and then proceed to move to a hybrid setup? No matter what option we go with, we'll be recording a TON of data and I look forward to sharing the results

Vlad Jovanovic said:

James, I firmly believe that the (near) future of small commercial aquaponics should (and probably will) be geared towards an integrated or 'hyrbid' approach. Media beds (along with sane stocking densities and composting worms) are already being utilized in lieu of clarifiers, net tanks, solids settling tanks etc...to pre-filter water for DWC rafts. This just seems like the logical evolutionary path for small scale plant based operations for many reasons. Crop diversity for customers, lower maintenance, not removing nutrients from the system, and some other things that you touched on in your post are but a few reasons why.

Unfortunately, there are no proven hardcore numbers or a step by step instruction manual (ppl like that sort of thing I guess) at the moment for people to latch onto and run with, neither from a university or nor a research center who publishes their data. Instead we have a few individuals experimenting, or running analysis on various hybrid system designs based on some small 'backyard' hybrid systems 'experiment' wondering if it will scale up. Or scaling it up and having success with it after having already gained an understanding of the AP basics as well as some in's and outs of what happens in an AP system when you...and why that happens...

Up until now it seems like most folks have chosen one of two paths in the AP dichotomy. Either the 'American/UVI/Raft based system (this includes Nelson Pade, Friendly's and too many others to list), or the 'Australian/IBC Tote/Media' route. The reasons people have done this, although wholly understandable within the context of the infant/toddler stages of mainstream awareness of aquaponics, are very soon, not going to be valid as we're fast approaching 'puberty'. 

It seems obvious that media beds and rafts (depending on the operators goals of course) belong in the same system, harnessing the benefits of both. Again, it is my opinion (as well as others) that this can be done by respecting a few 'hybrid system' principles, much the same way people respect a few basic ratios and/or principles of "standard" AP  (media based, or raft based) in order for either to work well.

Your thoughts on increasing media bed volume to deal with solids is valid, (though might not be the most efficient) but if I understood your drift correctly and the direction of your thoughts, I have to say I think you would be doing yourself a dis-service by boxing yourself into thinking that one must follow, or "choose" between one school or the other.

And yes, I fully expect to catch a bit of a grilling for a newbie in such an "experimental" direction, but there, I said it 

[Vlad Jovanovic]

I think that you could learn much more from people like Sahib, Chris Smith, Kobus Jooste (who is an haitus here, as he's taken a job at the South African Ministry of Environment and Land Use and had to relocate), Gina Cavaliero  and a host of others who are already doing such things and may have numbers or advice for you based on actual production or research time. I am still building, but would be more than happy to share the basics of my plan/design with anyone. To a large degree it is based on their and others experiences and suggestions (modified for my environment, needs, wants, species etc)... But if I had to condense in 100 word or less, and were building a proof of concept hybrid design, I would:

Keep a minimum ratio of 1:4 Media beds to DWC

Stock about 0.3 lbs of fish per total sq. foot of all combined grow space (then see what your nitrates tell you for your specific situation)

Use time flood and drain with an indexing valve to stagger filling/draining of multiple grow beds

Definitely without a doubt employ the services of E.fetida (red worms) in all grow beds

I would avoid a very small system (under 300-500 gallons total water volume) for the same reasons that it is not recommended in "traditional" AP 

Think about using a swirl filter after the media beds, before DWC troughs. The contents of which could be dumped back into a worm filled media bed like some folks already do. The worms seem to make short work of the swirl filter contents. (I opted not to add the swirl filter for now, but can always 'plug in it' if need be.

The above is just really general starting point for some ratios for such a system, but as Sahib very eloquently stated there are many considerations to be addressed that are site specific.

There is a diagram and a very short description of my system in this discussion: here...http://aquaponicscommunity.com/forum/topics/growbed-with-hydroton-e...

I know that none of this is going to help you much with convincing your professor...just figured I'd throw it out there.

I wish you the best of luck and success with your project James.

 

[matthew ferrell]

James, check out Rob Nash's system.  He might be able to give you more number input.

[steve]

I've heard the raft system can run with smaller pumps and water flow rates. Whereas grow beds with constant pumping seem to be heavy on electric costs. Maybe worth consideration?

[RupertofOZ]

Steve, current raft style systems are usually gravity feed from the tank > clarifier/swirl filter > raft...

With a single continuously running  pump transferring water back to the fish tank from the end of the raft or sump...

 

Hybrid systems, incorporationg media beds in place of, or in conjunction with clariers etc.... are generally configured with the Chift Pist/Chop... or ChopII methodology....

 

Both however still only use a single continuously running pump....

 

The "head" requirement to pump to the grow beds, and/or tank wont be significantly greater than that required to pump back from the raft end/sump...

 

Although, if configured with siphons... some increased flow might be required... just another reason to go "constant flood"...

 

There might be some extra restrictive forces with some extra plumbing/pipes.... but the increased "head" shouldn't be of any great significance...

[RupertofOZ]

I'm trying to design a system that uses primarily solar power and collected rainwater while still being scaled to a small commercial size, so an effective E&F system would be ideal if possible

 

 

Trying to power an aquaponics system.. that requires continuous aeration, and/or pumping.... by solar... means that the costs of the solar capacity required basically doubles the cost of the system itself...

 

Commercially, frankly I think it's almost a contradiction... at least in terms of a 2-5 year ROI...

[James McCullough]

Agreed on the cost side of things for most people, however we have access to the materials at a lower rate and we will be custom constructing the array and supports ourselves rather than buying from a commercial dealer. The start-up money is provided by a grant so the delay on the return on investment isn't a huge deal though we still want to be as efficient as possible. This was why I initially was looking at an E&F system, non-continuous pumping and aeration only on the fish side. With progress in manufacturing and harnessing efficiency, a solar powered system could become a feasible reality for most people in the near-ish future, thus a working model at current efficiencies would not only meet our goal of a sustainable system but also potentially bolster efforts of others who want to make aquaponics as sustainable as possible. This system is supposed to be more of a demonstration that isn't a money pit rather than my own personal revenue generating system.

RupertofOZ said:I'm trying to design a system that uses primarily solar power and collected rainwater while still being scaled to a small commercial size, so an effective E&F system would be ideal if possible

Trying to power an aquaponics system.. that requires continuous aeration, and/or pumping.... by solar... means that the costs of the solar capacity required basically doubles the cost of the system itself...

Commercially, frankly I think it's almost a contradiction... at least in terms of a 2-5 year ROI...

[James McCullough]

Definitely, when i get the sizing down I'll pin down the piping design and pump sizing, from there I could share the theoretical energy requirements of the different design alternatives we're working on.

steve said:

I've heard the raft system can run with smaller pumps and water flow rates. Whereas grow beds with constant pumping seem to be heavy on electric costs. Maybe worth consideration?

[James McCullough]

@ Vlad Jovanovic : So when you use 1:4 ratio of bed to raft what is your fish tank to bed volume? My guess is that you would keep that at roughly 1:2 to ensure proper solids management resulting in a total ratio of 1:2:8 (fish tank:media bed:DWC). Either way did you reduce the plant/sf density in your media area so as to provide the raft area with adequate nutrient concentrations?

P.S. Thanks to you and everyone else on here for being so helpful and amiable!!!

[Vlad Jovanovic]

I was meaning surface area, using 12" deep (which seems to have evolved into some sort of quasi-standard) for both media beds and DWC rafts. Sorry should have been more clear on that...

But just to avoid further potential confusion, are you meaning the total volume of the bed, or the volume of water that the bed will actually hold after being filled with media?

Since Archimedes let us all know that once you fill the bed with rocks, we can expect it to only hold about 40% of its total capacity in water... We also need to account for the fact that it wont be filled to the brim with media, (so an inch or so comes off that dimension) and that the standpipe will keep the water line at least two inches below the surface of the media (so yet another couple of inches off). All this will "significantly" lower the actual volume of water that the bed will be holding...

This is why it's sometimes easier to use surface area, (of course knowing the depth would help too

Well, yes and no. I did plan on leaving a number of beds empty or possibly rotating which (6) get used and which (2) don't at any given time. Not so much for nutrients sake, as this past spring/summer I was able to run a small system using only nylon socks filled with worm castings (worked remarkably well to my surprise). So adding ready to use nitrates shouldn't really a problem... More for the solids issue. Though this may not be necessary at all, I'd rather play it on the safe side...monitor/observe, and go from there. Even if I were to plant all 8 beds, I would definitely plant 'relatively' lightly or do so in a way as to contain unruly root mass from interfering much with bio-filtration. Having a good, staggered production/harvesting schedule would help some too I imagine.

At worst I'll have to add that swirl filter to the mix, but will try my best for it not to come to that...

As far as the fish tank volume...I first work out how much bio-mass (fish) I'd need for the plants, then built the tank from there. Using about 7 gallons per pound. This gives me a little leeway.

Also my media beds are a bit deeper than the 'average' 12". My space was limited in the GH, so I went deeper to add more bio-filtration capacity within the same sq. footage of real-estate... 

It's great that you are able to go solar in your situation, very cool.

[Vlad Jovanovic]

Oh yeah, and since you are going solar this may help you a bit with sizing the pipes/elbows/sweeps...in terms of energy loss due to friction blablabla...http://www.aquaticeco.com/pages/full_width/87/Friction-Loss-in-PVC-...

[TCLynx]

You might also want to do some research into how Growing power does things seeing as you note you have a huge supply of "green" waste for the project.  Composting and worm bins would be a benefit to any food growing project even if it isn't directly connected to the aquaponics system.

Now I don't really agree with some of Growing Power's advertizing or sound bites since they seem to lead the unwary into silly high stocking densities with sometimes tragic results, however, they are a good example of a urban food production center helping neighborhoods.