Floating Rafts

View the slideshow to see the evolution of the construction of my system.

Floating rafts or Deep Water Culture (DWC) is another AquaPonics technique, developed and promoted by the University of the Virgin Islands (UVI).
UVI is one of the pioneers of AquaPonics.

see the links page for more info on UVI.

DWC uses tanks or ponds to grow plants in on floating rafts.

Plants are grown in small pots filled with media that are inserted in holes in the rafts that keep them floatin on the water surface.

The main advantage of this system is the ease of planning, sowing and harvesting of one particular fast growing crop, i.e. lettuce and herbs: very long "raceway" ponds are calculated so that the harvesting of one or more rafts coincides with the sowing of new ones.
These raceways can be as long as 100 and 200 m.

The main drawback of this system is the difficulty of bringing enough oxygen to the plant's roots.
It is doubtful if this can be reached by passive aeration if the whole water surface is covered with rafts.
This calls for extra investment in aeration, both in hardware (air pump, tubing and diffusers) and in energy consumption.

Another problem with DWC is plant support: the small pots do not give much stability. This is no problem for low plants like lettuce and herbs.

Higher growing plants need a support to grow against, especially if outside a greenhouse and exposed to wind.

A challenge to meet.

I have built a small pilot DWC system in my greenhouse to study and compare the advantages of this system to NFT and growbeds.
I intend to build longer raceways outside.

The bottom and sides of the tank are constructed out of a 5 mm multiplex board stapled to a frame made of 45 x 20 mm ribs.

The bottom of my DWC tank is made of two panels measuring 3 m x 1 m , the sides are 30 mm high.

On the inside, 90 bookshelf supports are screwed to both bottom and sides for strengthening.

A thick polyethylene foil (leftover from my greenhouse covering) is then spread over the tank and carefully folded into the corners.
Takes a bit of adjustment and patience, but the rectangular form of the tank is a big help.
Kind of wrapping a birthday present inside out. Not really difficult, just make sure the folds in the corners always take the foil up to the rim of the tank, else it will be leaking.

The rims of the foil are folded over the sides and stapled to the frame.

A 50 mm tank drain fitting is installed in each growbed.
For that I use a guide made of a 20 mm piece of board with a hole drilled in it with the corresponding hole saw. I weigh that down to hold it in place.

With an exacto knife I then first cut the corresponding hole out of the foil, helped by the guide. This is important, lest the teeth of your hole saw wrap up the foil while drilling (been there, done that).

After that, cutting the hole through the bottom with the hole saw is childs play.

The drain fitting is connected to the fish tank solids lifting overflow drain, so the DWC side is not really a drain but a fill.

The pump is located in the DWC tank and pumps the water back to the fish tank.
The fish tank (CHIFT -see the "definitions and acronyms" page) overflows into a pond type filter which then overflows into the DWC tank, from which it is pumped back to the fish tank, which then overflows ... etc ...etc ...

Insulation.

I have the intention of filling the gaps between the frame ribs with insulation and staple a second 5 mm board against them to close up, then screw a plastic or aluminum L-profile all around the rim and on the corners for finishing touch.

see more pictures of the construction principle on the "rectangular growbeds" page.
To the best of my knowledge the information I present is accurate. All pages on this website © Hygicell 2009