Part 3: Adding the peripheral structures
Being a bit more distracted than I wanted to be, the final stages of adding towers and NFT has not been completed yet. Thanks to Google Sketch-up, I can show you where I’m headed though and in this way, I can explain what I am trying to achieve at this point in time.
The core is the nutrient factory, although the media beds are also planted. The fish tank, sumps and filters all work together to give me the system nutrient levels that will hopefully drive the entire periphery that will be totally dedicated to plant production. I know that the system can handle 20 kg fish / 1000 litre fish tank as it just about managed it last year without the filter expansion. I’m still planning to stock below that and see how much fish I need to drive the whole set-up. I intend to plant only lightly in the media beds this season though – the bed with the passion fruit will not get any more plants and the three others will likely average around 10 plants each. The two red bins on top of the fish tank is also gravel filled, and acts as a frequently flooding seed germination section.
The major production will be in 5 banks of horizontal and vertical 110 mm pipe structures. All the vertical towers are filled with a mix of PVC granules and vermiculite, while the horizontal pipes are without media. The NFT that curves around the grow beds are in place, one bank of compact towers are completed and one tall tower (4 more to come) is being tested. The main 48 plant unit supported against the front of the greenhouse is nearing completion, thus I only have to complete another 8 towers and connect the whole lot to make the system look like the computer model.
All the compact and tall towers will be driven via a pump in the “clean” sump, while the 48 plant NFT array in the front of the unit will be fed from the “dirty” line. Before water reaches it, it will go through a 150 litre upflow filter that will ensure only clean water gets into the NFT. The “clean” line runs of a 60 W pump on 15 minutes on 1 hour off cycle, while the “dirty” line is a 75 W pump running 15 minutes on 45 minutes off. The pumps are not new, thus the power rating is a bit higher than what I can get currently. I need to lift water 2.8 meters though, which make the pump requirements a bit beefier than expansive units. Still, it uses a total of 0.73 kW per day. Divide that by the capacity of 227 plants you get a power use of 3.2 Watt per plant per day (Not calculating the seedling production). In South African terms, that is a cost of 10.3 cents per month per plant. I am currently working on the economics of running the unit and will then also try to retro-cost the entire system in current terms to try and get to a point where I can comfortably advise people on the monthly break-even (presently that is 5 passion fruits, a punnet of strawberries (500g), three cucumbers or 4 lettuce heads) on power and water use, total annual running costs and projected ROI depending on type of structure, although in home system terms, I weigh the benefits of the system paying for itself and then some on monthly running costs above critical ROI investigations.
Still, the basic numbers involved here is making me think that a cube design for limited space applications can be very rewarding. In the next section I will look at all the growing environments created in the unit, and how this relates to my "total basket" approach for aquaponic home production.