I won’t go into extreme detail about the physics behind siphon flow. But here is some information that will give you a basic understanding of how siphons work:
- Water always flows downhill. This means that if you fill your growbeds with water, the natural tendency will be for this water to try to make its way back down into your fish tank. All it needs is a hole from which it can escape.
- If you poke a hole in your growbed–say, half-way up–your water is only going to drain only until it is immediately below that hole. If you poke a hole in the bottom of your growbed, on the other hand, your entire bed will empty of water. Although some people do use this method for their growbeds, there is a definite downside. If water flows constantly out of your growbeds, your growbeds will not fill all the way up. This will diminish the ability of your plants to soak up water and nutrients, so they will not meet their full growth potential and your water will not be as effectively strained before returning to your fish tank.
- Once a stream of water begins to flow, however, the natural tendency is for this stream to continue unbroken. This is because of water’s cohesion–or attraction to itself. Water molecules are polar molecules, meaning that they have negatively- and positively-charged ends. If one molecule starts to flow, its natural polarity will pull another molecule behind it and another molecule behind that, much like a long chain of magnets. The only way this stream can be interrupted is by an air bubble.
This is where we get to the simple and efficient concept of a siphon. I go into greater detail about different designs below, but what we are generally trying to do is start the flow of a stream of water that will drain your entire growbed while ensuring that your entire growbed is filled before this stream begins to flow. So, here are two general facts you should become acquainted with before reading about specific siphon designs:
- The apex of your siphon reach the point at which you want your water to begin draining.
- You must have some restriction in the diameter of your siphon at some point PAST the apex of your siphon–that is, on the downhill side of your siphon. This restriction ensures that air bubbles do not find their way into your siphon and interrupt your stream of water. In each of my posts about specific designs for siphons I will show you where to put your restriction.
When installing your siphons, also keep in mind that you will need to seal any holes you make in your growbeds with some sort of water-proof sealant to prevent leakage.
Loop siphons are probably the most commonly used in aquaponic systems. They are both extremely reliable and easy to install. In the case of clogging, they are very easy to disassemble and clean out. Finally, no part of the siphon has to actually intrude on the interior of your growbed, which is quite convenient. To install a loop siphon, begin by drilling a hole in the bottom of your growbed. Shove some sort of replacement drain, piece of threaded PVC, hose adaptor, or any other implement through this hole and seal it thoroughly (with the “pointy end” sticking out of your growbed). From here, simply use PVC, hose, or some other material to create a “loop”. The apex of your loop should be in line with the point to which you want your water to fill (again, keep in mind that water must fill this apex completely for your siphon to take effect–leave a bit of space between the top of your loop and the top of your growbed) and the end of your siphon should reach below the bottom of your growbed. Loop siphons made of flexible hose or tubing are especially convenient in that the height of their apex can be easily adjusted in order to fill your growbed as completely as possible without causing it to overflow. However, it is important to remember to add restriction to these hoses by shoving a short piece of smaller tubing into the end of your siphon or at some point slightly past the apex. The diameter of a PVC siphon, on the other hand, can be easily adjusted. Here is a diagram:
This short video gives you step-by-step instructions on how to build a loop siphon:
And here is a video of a loop siphon in action:
Bell siphons are, perhaps, the most elegantly simple sort of siphons. However, they are considered to be the most unreliable type of siphon, especially among beginners. However, if you install a bell siphon correctly, they are extremely convenient and almost never get clogged. To install a bell siphon, begin by drilling a hole in the bottom of your growbed. Shove some sort of replacement drain or piece of PVC through this hole (with the “pointy end” sticking into your growbed). This “pointy end” should extend up to the point at which you want your growbeds to begin draining. The next step is building a “cap”–or a “bell”–for this tube. Grab another piece of PVC that is both longer and larger in diameter than the one you used for your first tube. Cap off one end. At the other end, cut out two or three small divots so that the end of your tube looks like the ramparts on a medieval tower. These divots should be very small so that your soil medium will not slip through them and clog your drain. Place this bell over your drain. When your growbed fills, water will slip through these divots until the water is high enough to begin draining through the first tube you installed. From this point on, a siphon effect will take over. That’s all there is to it. Here is a diagram:
This short video gives you step-by-step instructions on how to build a bell siphon:
And here is a video of a bell siphon in action: