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Flood and Drain

Flood and drain grow beds (also called ebb and flow systems) consist of a watertight container that holds a growing medium. The growing medium primarily provides physical support for the plants and their roots. Many people prefer this system as it most closely ‘feels’ like traditional ‘dirt’ gardening.

Flood and drain growbeds are periodically filled with water and drained. This means that a good growing medium drains water easily so the roots get access to air as the draining of the water pulls fresh air into the growbed, around the roots. There are many different growing mediums to choose from. Each has it’s strengths and weaknesses.
  • Perlite & Vermiculite are heat extruded mineral based materials that are light weight. Think rock based puffed rice cereal. It can have a tendency to float however.
  • Gravel, usually pea gravel or larger is usually cheap and locally available at a great many places. It is heavy though, so the grow bed stand needs to be designed for the weight. A crushed rock product can be hard on the hands when transplanting plants and cleaning the growbed. Some types of rock are not suited for aquaponics as they are chemically reactive and can mess with your water chemistry.
  • Fired clay products are lighter than gravel, a good size and often easier on the hands. The larger size and lessor weight makes it popular in grow beds.

The growbed itself can be made of any watertight container. While the big outdoor systems are everything from poured concrete to industrial barrels, indoor systems need to be a bit smaller. Plastic storage containers are popular because they are often a convenient size and are readily available. The grow medium can be placed directly into the growbed, or alternatively, it can be put into plastic pots which are placed into the growbed. While I have only seen a few portable demonstration aquaponics systems use the plastic pots, I believe this is a superior way of handling your plants when indoors. You can easily re-arrage your plants for aesthetic or growth reasons without disturbing the root system. If the growbed is placed over the aquarium, it’s easier to remove the growbed by moving the pots if you ever need to service your aquarium.

There are two primary methods to making a flood and drain growbed fill and empty with water. You can actively control the filling of the growbed by turning on and off the water pump or controlling a diverter valve, or you can passively let the growbed drain on it’s on using an auto-siphon.

Turning a pump on and off is easy with an inexpensive timer. The smaller pump sizes used on aquarium sized systems means we can use lamp timers. This works great if you use a small submersible water pump. However, if you are using your aquarium filter as your pump, you want it to stay on all the time. This leads to a diverter valve which will either run water to the growbed or back to the aquarium.

Passive flood and drain grow bed water cycling can be done with an auto siphon. An autosiphon automatically starts to siphon water that reaches a certain hight due to it’s design and location. There are 2 common auto-siphon styles: a loop siphon and a bell siphon.

A loop siphon is a simple loop in a tube attached to the grow bed, usually externally. The top of the loop is positioned at the maximum height you want the water level in the grow bed. When the water reaches the selected height, water starts to flow over the top of the loop. The moving water takes bubbles of the air trapped at the top of the loop with it down and out the tube, this allows more of the tube diameter to be used to train. After a short period of time, all the water is cleared out of the tube and draining at full speed occurs. Once all the water is drained out of the grow bed, the siphon sucks in air which goes to the top of the loop. Once the top of the loop is full of air, the siphon stops flowing until the grow bed fills up again. Sizing of the tube for the loop needs to be of a larger diameter than what fills the growbed so it can drain faster than it is filled, but not so large as to prevent the movement of bubbles out of the top of the loop.

A seemingly more popular auto-siphon in aquaponics is the Bell Siphon. These can be a little bit fussier to setup and maintain, but when they are working correctly, they work quite well. A bell siphon has 2 parts, and stand pipe, and a cap, called the bell. The stand pipe is usually drilled through the bottom of the growbed. The top of the stand pipe is adjusted to the desired maximum height of the water in the growbed.
The bell is then placed over the stand pipe. The bottom of the bell is designed or placed so that water can freely flow into the bottom of the bell. When the water reaches the correct height, the water starts to overflow into the stand pipe. This sucks air bubbles from the air at the top of the bell. After a few seconds enough air bubbles are cleared out of the bell that the all the standpipe is draining water. When the water is drained out of the growbed, the siphon will start to suck air. Enough air is sucked in that the siphon will ‘burp’ and break the siphon. The water will then start refilling the grow bed. The simplest and most convenient placement is to locate your growbed above your aquarium and let it drain directly back into the aquarium. You pump water from the aquarium into the growbed, and it drains back into the aquarium. This however, may not be the most aesthetically pleasing as the water level in the aquarium will fluctuate as the growbed fills and empties into the aquarium.

How a Bell Siphon works



If you add an additional, hidden, water container, called a sump, into your system you can maintain a full aquarium while the growbed and sump water levels fluctuate. Many marine and larger freshwater aquariums already incorporate a sump into their design. The sump usually sits inside of the stand of the aquarium.

If you already have a sump system as part of your aquarium setup, and your growbed is above your aquarium, just pump the water from the sump to the growbed which drains into the aquarium which drains into the sump. If your growbed is below the top of the water in your aquarium, pump the water from your sump to the aquarium which drains to the growbed which drains into the sump.

If you are adding a sump to your aquarium setup, you need to add a way for the water to drain out of the aquarium. There are two ways of doing this. One way is to modify the aquarium to contain a standpipe, just like if you where using a bell siphon (only don’t add the siphon!). This will require drilling a hole into your aquarium. Alternatively you can buy or make a device called an overflow box to the aquarium. Either method allows the aquarium to get only so full before they overflow into the standpipe or overflow box.
Subpages (1): Bell Siphon Construction
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