I am setting up a DFT grow tank for lettuce in South America, first one, and then more in time. The size is 4m wide x 20m long and 25cm deep with a 1000L control tank. I have had five similar sized sub-irrigation tanks with gravel before, but this is my first DFT system. I want to use a swimming pool pump with a venturi to circulate and aerate the nutrient solution, and aim to do a total replacement of the grow tank volume every 2 hours. Where in the system would be best to place the pump? Between the control tank and the grow tank, pumping from the control tank into the grow tank with an overflow back to the control tank. Or the other way around? Answer by RICK DONNAN
My other big concern is the adjustment of the EC and pH levels. I will have to do it manually using hand-held meters for now. To have it automatically done would be great, but perhaps later on. With my sub-irrigation tanks it was easy to the extent that I could do it in between watering. But how could I adjust it in a DFT system with all the plants in the bulk of the nutrient? I understand that is the reason for a control tank, but if I start adding stock solutions into the 1000L control tank by hand, surely it will still be fairly concentrated when it reaches the grow tank, unless I add the stocks ever so slowly. Is that normally the only way one does it manually?
My last question is the procedure for dumping. With one tank, and all the plants still in it, how would one dump the nutrient solution if it became necessary? Even if I did a partial dump and top up with clean water very slowly while manually adding the stock solutions into the control tank, will that not create too much fluctuation for the plants?
In one of your articles, I read that continuously bleeding a closed system could be another way of not having to do a total dump. Perhaps that would be the way for me to go?
The volume of your grow tank is 4m x 20m x 0.25m = 20m3 = 20,000 litres.
To turn this over in 2 hours requires a pumping rate of 10,000 L/hr.
In turn, your control tank of 1000L would be turned over 10 times per hour, with a calculated residence time of 6 minutes.
Presuming that you are mainly growing leafy greens, depending on what you grow, your plant density is probably about 20 plants/m2. Hence, the total plants in your grow tank would be about 4 x 20 x 20 = 1600 plants. That is, for a total solution volume of 20,000 + 1000 = 21,000L, your volume per plant is 21,000/1600 = over 13 L/plant.
For comparison, for NFT (nutrient film technique) channel systems, the typical suggested minimum is only 0.25 L/plant, although preferably higher. Therefore, the analysis of your recirculating nutrient solution will be very much slower to change.
In order to keep the nutrient solution level steady in your grow tank, it is preferable to make it the tank with the overflow. The water make-up float valve would be in your control tank. The transfer pump takes the solution from the control tank into the grow tank with the excess overflowing back to the control tank.
To minimise short circuiting, the inlet into the grow tank should be as far as possible from the overflow. Typically, they would be at the opposite ends of the grow tank. To help improve distribution, both inlet and overflow should be branched to several inlets/overflows across the tank.
The DFT systems I know all use automatic dosing into the control tank, which is very easy and steady because changes remain slow and steady. As you suggest, in your case occasional manual addition of stock solutions does pose a problem. You are helped, in that the uptake of nutrients will be very slow to impact upon the EC in your grow tank due to the huge volume of solution held. However, because you can only treat 1/20th of this volume in the control tank and this is injected into the grow tank inlet in only about 6 minutes, a single dose would give a pulse of much stronger solution into the grow tank.
My suggestion is that for EC management, you set up your A and B concentrate solutions in containers that have finely adjustable taps so that your addition can be spread over a longer time, preferably about 2 hours. Also, do your solution make-up as many times per day as is convenient. You want both parts to flow in at about the same rate, but this doesn’t need to be precise, provided the total volumes of A and B are correct.
pH control could be more difficult, so I suggest that if your pH drifts, you change the ammonium content of your formulation to offset any drift. Alternatively, find out how much acid you need to add to stabilise pH and add this to your fertiliser mix, rather than adding separately.
The major choice you face is when to dump. This will mainly depend upon the quality of your raw water, especially its sodium chloride (NaCl) content. Because of the very high volume of solution per plant in your system, it should take a long time before it would be necessary to dump. If the sodium content is under 23 ppm, then you may never need to discard. The higher the ppm Na, the quicker it builds to unacceptable levels.
As you suggest, trying to discard the full volume of grow tank is basically unworkable. You obviously can’t lower the level in the grow tank, so you would need to pump through to displace the old contents with fresh nutrient solution (not straight water). To even be a possibility, this would require automatic EC control in the control tank. Even then, this gives a huge volume to handle, plus the flow through the grow tank would not be ‘plug flow’, but there would be some mixing of old and new solutions.
This leaves the only feasible option to be ‘bleeding’. That is, taking off a small proportion of the overflow on a continuous basis. As I have mentioned in earlier answers, this happens unknowingly in many recirculating systems through leaks.
An occasional analysis would indicate how much you need to be bleeding. Try to maintain the Na level no higher than about 100 ppm in your recirculating solution. Ω
PH&G August 2015 / Issue 158