November 4, 2013
Welcome to the Aquaponics Blog Series. We routinely share updates, photos and fun tidbits from the CLF Aquaponics Project. If you’re interested in urban agriculture, sustainable aquaculture, or hydroponics, be sure to check back regularly for new posts.
When young school groups come to visit the CLF Aquaponics Project, I love seeing the look on their faces when I pull a raft of celery out of the water, exposing the dense mass of roots underneath. Most of us gardeners never get to see the roots of our plants, but not only does observing the roots tell you about plant health, but it’s fascinating to see how every plant variety has slightly different root characteristics: did you know orange Swiss chard has orange roots?
Not all forms of aquaponics allow for these stunning displays of roots. The concept of aquaponics has been adapted to a myriad of environments worldwide, from integrating fish into a rice paddy to growing plants in greenhouses on just a trickle of nutrient-rich water in networks of plastic tubes. Two of the most commonly practiced forms of modern aquaponics are raft aquaponics and media bed aquaponics.
In raft aquaponics, which we practice at Cylburn, the water from the fish tanks continuously circulates into plant beds that hold six to eight inches of water. The plants are potted in floating rafts, allowing their roots to grow through the bottom of the pot and into the water where they absorb nutrients from the fish waste and oxygen from underwater bubblers. Rafts are maneuverable and can be removed from the system for harvesting, or to clean out the plant troughs without disrupting the plantings, making the raft systems relatively easy to scale-up to commercial size.
However, there are a few disadvantages of the raft system. For one, the rafts that most growers use are made out of polystyrene foam products (not exactly a renewable resource). At Cylburn our rafts are up-cycled cooler lids, which would otherwise be discarded by a factory in Maryland. Moreover, the raft system does not provide enough surface area for beneficial bacteria to grow, and solids form layers of sludge on the bottom of the plant beds, if left unfiltered. This means that in raft aquaponics, we often need to build a filter system that includes settling basins for solids and surfaces for bacteria growth. Unfortunately, this means we often lose valuable nutrients, like phosphorous, that are contained in the solid waste.
Media bed aquaponics attempts to address these issues. Water from the fish tanks periodically floods and then drains a bed containing a media, like gravel or vermiculite, and the plants sit directly in the media, as if it were soil. The surface area of the media allows it to work both to filter solids and host beneficial bacteria and red worms that break down the fish waste. As the bed drains, the plant roots and bacteria are able to get oxygen they need. The only catch— media bed aquaponics can be difficult to scale-up. Gravel is heavy, and if the solids don’t break down quickly enough, the beds need to be periodically cleaned, which can be labor intensive and very disruptive to plantings. Moreover, the flood and drain systems either require a timer on the pump, or a device like a bell siphon, a low tech application of every-day physics that in my experience is very difficult to get to work correctly. For more on bell siphons, see this great resource from the University of Hawaii.
Many aquaponic growers are beginning to experiment with hybrid systems that combine both a media bed and a raft system. Over the summer, we worked on constructing our own mini-media bed system. We weren’t able to easily plumb it into our raft system, so instead, we are going to use the solid waste that we flush from our filter system every day to provide nutrients for our media bed. We just started planting in this bed a few weeks ago, so stay tuned for updates, and check out our photos on Facebook.