My first AP system is based on an in ground recycled jacuzzi that holds 330 gallons. The grow bed is a converted above ground planter bed made of cement block, 2 high. The planter bed is lined with EDPM and filled with pea gravel. There is a 1200 GPH pump in the tank/jacuzzi that runs for 15 minutes every hour. In the grow bed there is a small, 200 GPH pump to get the water up and over the wall of the raised bed. This was also set for 15 minutes every hour, 5 minutes behind the tank pump. This system worked well from 11/10 to 6/12. The drainage in the plant bed got slower and slower, until even the mint drowned. The fixes I tried, in order, are replaced the 200GPH pump, removed the timer on the 200GPH so it ran non-stop, turned over all the pea gravel a shovel full at a time spaying water from the hose through it. It still is not draining well.
What I am thinking of trying next is to remove the pea gravel and liner, change the slope under the liner, replace the liner and then build a hybrid AP/wicking bed on top. This will be a thin, 1 inch? layer of the pea gravel, topped by a 6 inch layer of river rock ranging in size from 3 to 6 inches, covered with landscape fabric and soil. The tank pump will deliver water to the higher end, and the 200 GPH will send water back to the tank on the same 15 minutes per hour scheduled.
For more information on wicking beds, see Liz' description here http://www.phoenixpermaculture.org/profiles/blogs/wicking-bed-experiment or another description here http://www.urbanfoodgarden.org/main/wicking-beds/wicking-beds.htm
Thoughts please? What problems can you see?
Replies
The pea gravel might be part of the issue - Murray Hallam is pretty adamant about using 3/4" gravel as a grow bed medium (or the clay balls). I don't know but gravel that size (instead of pea size) might have larger pore spaces and as such be less likely to clog up over time. I think the gravel in an aquaponic gravel bed needs to be exposed to oxygen in order to keep it aerobic not anaerobic. I would be concerned that the bacteria in buried gravel with no oxygen would not be the right mix of bacteria to keep your system healthy and in balance in the nitrogen cycle. My opinion would be that your beds were probably clogged by an overload of fish and the bacteria couldn't keep up with breaking things down. Brady is probably right that worms would help with the mineralization process and in a bed with more pore space they could move around easier...
You may want to try introducing worms into your grow bed. I had a small system going for way longer than I expected and believe that they were a contributing factor in keeping the growbed "cleaner". I think the periodic large input of effort is proof that we're imitating nature, not actually cloning it. Let us know if you find a suitable solution.
I think you will need to have more grow beds for your system for adequate water cleaning. The wicking bed from Hallam's set up is in addition to gravel beds that perform solids mineralization and the gravel beds are also where the majority of the nitrogen fixing bacteria live. At least that is my understanding of what's going on. Not that your existing beds aren't a good candidate for wicking beds, but maybe consider adding more beds that would be traditional gravel beds flood and drain. The ratio for grow beds to fish tank is at least one to one - and because your fish tank is pretty densely stocked, I would probably want to err on the side of more gravel beds for water quality. I don't think that the wicking bed will perform the same as the gravel bed, for instance the gravel under the bed is constantly flooded so it would be an anaerobic environment. I'm curious about what others would think about it...
Liz,
Good thoughts, thanks. I did the math, and the bed is 374 gallons, 25'x2'x about 1' deep. So dividing by 4, 4 inches of gravel and 8 inches of soil, would make the gravel part 93.5 gallons. That would be way to far out of balance, even though my fish load has been decreased drastically. I don't want to spend the money for Hydroton-like material. I am thinking about taking out the pea gravel (Oh boy!) putting down a 2 to 3" layer of 1 to 3" river rock, a sheet of plastic hardware cloth, and putting the pea gravel back on top. Would really prefer a solution that didn't require a redo in 2 years.