Original prototype bell siphon
There are two types of aquaponic growbeds:

• floating raft (plants floating with their roots in a constant stream of water)
• media-based growbeds (plants growing in some sort of rock/sand/gravel/beads)

Media-based systems are recommended for home hydroponicists because they are simpler and more reliable. Media-based systems are also referred to as flood and drain. The idea is you flood the growbed with the fish water (delivering nutrients and, um, water), then let it drain out (bathing the roots in air/oxygen).

Of the various ways to flood/drain a media-based growbed, the one that is easiest on the checkbook is a bell siphon. All a bell siphon needs are simple plumbing bits available at any hardware store. Oh, and a small pump. I loved the way the folks at EcoFilms explain it in their post about How an Aquaponics System Works:

"If the pump is the heart of an aquaponics system, then the auto-siphon are it’s lungs. A vital part of kit. Remember when you were a little kid and the teacher told you about the regular flooding of the Nile river and how fertile the Nile delta was to early farmers. Well think of the auto siphon as a kind of similar concept. It’s main purpose is to flood the grow bed drawing rich oxygen into the depths of the trough, oxygenating the plant roots and turbo charging the bacteria to do their thing."

Below is EcoFilms' animation of how a bell siphon works.
[The red button toggles the animation on and off.]

A real-life system takes many times longer to fill than the time to drain (my initial prototype system with a single growbed took 10 minutes to fill and 1 minute to drain, ignoring the dribbly parts at the beginning and end of the siphon). I found the growbed in my system only needs 10 gallons to fill the spaces between the rocks, so the change in the level of the water in the fish tank is only 2-3 inches, about 10%.

Basically, when water reaches the top of the siphon, water quickly drains out of the grow bed, sucking air down around the roots and oxygenating everything. You can have a small pump running continuously, rather than a big pump turning on for only a few minutes once an hour or so. Since the pump is on continuously, the water in the total system is also cycling continuously, which my fish and plants love.

I didn’t invent the bell siphon, but I have developed a design that doesn’t require solvents, a design that can be manufactured with just plumbing bits and a mitre saw.

I'll show the bell siphon working in tomorrow's post about the coanda discharge - for today the video just covers the parts and assembly of the bell siphon.