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As any good accidental discovery had developed in the past, my thoughts on this topic grew out of "unfinished business" and accidental observation.  I always wanted to add a sump to the system, as I had too little fish tank volume to run all the beds in flood and drain mode.  I have raised beds thus had unused space underneath (the groups pic is of my system being described here).  I shoved a sump made from a 200 liter plastic bin underneath a bed, and coupled it to the IBC roughly at the 700 liter line with two 75 mm fittings.  Water can flow in and out freely as the system floods and drains.  The intention was to allow some of the beds to drain directly into the sump, but I got busy with other stuff.  Suddenly the IBC cleared up beautifully, and it dawned on me that I inadvertantly added a clarifier and not a sump.  Fines were floating up and into the sump, and settles in stead of flowing out again.  Now I have two options - siphon the fines out once a month and figure out if it goes back or gets tossed, or add media to the sump and turn it into a biological filter.  There are pros and cons to both options - one is more work and as we see from many raft operators, they are turning their net tanks into media beds to get better performance.  I cannot plant this sump up, thus it will become a sink for nutrients, where biological processes will flux some of it back to the water column.  This process works for wetlands, and I'm wondering what it will do to my little set-up. 

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Good point.  O2 can be consumed very rapidly and is more likely to be the cause of any apparent nutrient gradient issues.  Thanks to everyone for the confirmations!

Hi TC,

You've illustrated some great points. My media beds show no signs of iron deficiency while the same plants in the raft exhibit them at higher PH ranges. 

As there is little to no air in contact with surface water under raft and almost no water agitation, on longer raft runs the plants closer to the inflow will do better and the ones further away will show slower growth due to diminished oxygen.

In backyard AP you're limited because of the standard ratios when growing nutrient demanding plants and need to have a balance in terms of types of plants in the system. In commercial designs with high fish loads and solids filtration and other processes like added mineralization you can increase nutrients/plant ratios in the system.

Well in backyard systems you can actually get away with growing lots of plants in comparison to the fish load because generally you are not filtering and removing the solids and the mineralization is taking place right in the media beds.  These things actually seem to get trickier when you are trying to do it at really high stocking in a commercial system and hence why commercial systems need some one on site keeping an eye on things every day while a backyard system might get left unattended for a holiday.

Part of why I want to get away from DWC in my set-up is to have the "wet root" towers that I am now playing with.  having lots of air contact time between cycles can negate oxygen shortages and potentially even improve the DO of the system as compared to having the same volume of DWC.  Lots of short pipes should not have the same DO drop as those long commercial units, and having them under timed irrigation should give the plant all it needs (I am hoping)!

Hi Kobus,

You're right. In terms of hybrid bio-filtration in mixed AP systems DWC for me is at the bottom of the component list. I consider your beds,towers,and even your sump play a greater role in this respect far less if we have to look at the question of plant and system oxygenation.

Hi TC,

I agree. It always comes down to your goals/intentions. These days I'm looking at AP a bit differently. To me, AP has to crossover to do what it came here to do, become the mainstream in agriculture, to give us the alternative of feeding the world without harming it. To some commercialism means money to some it is a gift to be shared. My backyard AP has given to me great joy and sharing it even greater.

TCLynx said:

Well in backyard systems you can actually get away with growing lots of plants in comparison to the fish load because generally you are not filtering and removing the solids and the mineralization is taking place right in the media beds.  These things actually seem to get trickier when you are trying to do it at really high stocking in a commercial system and hence why commercial systems need some one on site keeping an eye on things every day while a backyard system might get left unattended for a holiday.

I'm taking the same attitude about the DWC....in fact, my approach is more about doubling up and trying to gain some grow space from the fish tank by making it a deep raft bed.  I've considered abandoning the raft idea altogether, keeping the long trough as a fish tank, but placing an arch of NFT tubes overhead instead.  However, I think the reflectivity and insulation of the white styrofoam rafts will prove essential in regulating water temperatures.

I also really like the timed flow to NFT, but I think I'll achieve it via the output from a flood/drain media bed.  In an ideal world, plants really want super foggy wet air on their roots.  They'll deal with being completely immersed given enough DO, but why not give them what they want, especially if it is easier, cheaper and more effective for the grower?

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