A raft is an Aquaponic System based on a hydroponic growing method called Deep Water Culture or DWC. In a raft system, roots are grown directly in the water while the plant floats on the surface in a “raft”. The raft can be made from sheets of Styrofoam or suspended in a fixed structure. Oxygen is pumped into the system to allow for proper root health. The main benefit of growing in a raft is the water added to the system. More water allows for more stability by buffering temperature, pH, and nutrient load. Another benefit is that a raft is relatively inexpensive to set up without the added cost of media to fill the grow space.
The downside to the raft system is that there is not enough surface area for the beneficial bacteria to colonize. A bio-filter is necessary to convert toxic fish waste into healthy plant nutrients. Also, a clarifying filter is necessary to keep waste solids from building up on the roots and within the system. Creating a hybrid system by adding a raft to an existing media bed is a great way to avoid the use of a bio-filter or clarifier. The expanded clay in a media bed acts as a bio-filter and a solids remover.
Creating a hybrid system by adding a raft to an existing media bed is a great way to avoid the use of a bio-filter or clarifier. The expanded clay in a media bed acts as a bio-filter and a solids remover.
In deep water culture, plants roots are submerged in a highly oxygenated nutrient solution and little to no media is used. These systems require additional solids removal to prevent buildup. Depending on the size of the system, the surface area of the tanks and rafts used is usually sufficient for bacterial colonization and nitrogen conversion. Raft style Aquaponics was pioneered by the University of the Virgin Islands and is composed of tanks for the fish, a solids removal clarifier, a mechanical filter, degassing chamber, hydroponic rafts and a sump/base addition tank. The hydroponic troughs are typically 1’ deep by 4’ wide and can be up to 100’ long. Styrofoam sheets are floated on top of the filled troughs and plants are placed in net pots in the sheets and the roots are allowed to hang suspended in the nutrient solution. Since the roots are completely submerged, oxygenation is critical and air stones are placed every 2’-3’ down the channel. Also due to the lack of media for anchorage, only smaller plants are used unless some sort of support is added. Young plants are placed in the properly spaced holes in the beginning of the channel and harvested out of the end so that the Styrofoam sheets can be floated down and another one placed in at the start.