Aquaponics plumbing - the Bell Siphon

My 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:

"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."

They created a great animation of how a bell siphon works. A real-life system takes many times longer to fill than the time to drain (my system takes 10 minutes to fill and 1 minute to drain, ignoring the dribbly parts at the beginning and end of the siphon). Also, the pump never turns off. 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%.

Though I love the idea of "constant height of pond" (CHOP), the added complexity and space to achieve CHOP didn't make sense for my 3x5 system. Let me tell you the bits I used to create the system I've got.

First is the pump - 140 gallon Pondmaster that I got from a local hardware store for under $40. It has only one moving part and consumes just 10 watts. It's like having a night light plugged into a wall socket, as far as energy usage is concerned.

Next are the plumbing bits (I presume the kind of handyman attempting this will already have saws and drills, etc.). The plumbing bits are:

1) The outer sheath or media guard. It's the white tube poking up through the gravel in this picture.

This is 3" (ID) pipe roughly 1' long with dozens of 1/4" holes to allow water (but not rocks) to reach the siphon. I drilled down 12 lines, putting a hole every inch and precisely staggered the holes on adjacent lines because I'm OCD. But the regular holes also serve as a crude measuring stick, helping me determine critical distances. Turns out a 15.5 oz can fits nicely inside this sheath, to prevent rocks from falling in from above. Plus a can adds iron to your system as it rusts - useful as most fish food lacks the kind of iron levels we'd like to have in foods we eat.

Update - Turns out you can simply cut slices in the media guard - slices also work just fine for the base of the bell. FWIW, I use a crochet hook to pull the bell out of the media guard when I want to check the siphon.

2) The bulkhead. I used 3/4" electrical PVC fittings here, because the electrical fittings are dirt cheap and adequate to this low-pressure task. I was inspired by Richard Kinch's post"An Improvised PVC Bulkhead Fitting forLiquid Storage Tanks. I only used the PVC male and female electrical conduit adapters - I didn't need to modify them in any way. This part of my system drove me to 3/4" piping, because the 1" adapters have seams running up through the threads and don't seat nearly as well. As 3/4" is the size of standard bilge pump line, I figured it was large enough for my purposes.

I used a 1" bit to drill the hole through the bottom of my grow bed, and a doorknob hole cutter to drill a hole in the shelf for the drain. You can improve the seal through the grow bed with silicon or you can simply use a #18 o-ring. I used aluminum foil tape to seal the edge of the hole through the shelf just in case there were a leak, to mitigate moisture getting into the wood and turning it soft.

3) The 1" to 3/4" reducer and standpipe. I struggled long and hard on this bit, inspired by Affnan's Bell Siphon video:

My problem is that I'm in the US, so I can't get the metric bit parts he uses for his system. I settled on a 1" slip to 3/4" male threaded reducer/adapter, followed by a female threaded to female slip coupler.

The two bits together cost about $1. Turns out the 1" female slip adapter has an inner diameter of 1-5/16" (to accommodate the added wall thickness of 1" pipe). A 1-5/16" diameter circle is three times the area of a 3/4" diameter circle - meaning your siphon thinks the pump is putting out 3 times as much flow as it actually is. So 3 times as much water hits your 3/4" pipe to start the siphoning action with the 1" to 3/4" reducer than the pipe would see if it was just a flat cut. This is a good thing. Trust me.

If you like math and physics, you can go read up on Bernoulli's principle, which comes into play here. Just don't tell the helpful people at the hardware store you are putting together a plumbing fixture that creates a Bernoulli suction. My limited sample shows consistent brow-furrowing and eyebrow raising.

You'll need a short length of 3/4" pipe to connect the reducer to the bulkhead so the top of the reducer is about 1-2" below the top of your gravel. Since your bed will flood to the top of the reducer and no higher, the 1-2" layer of dry gravel keeps algae from growing on the surface of your gravel. Here's a picture of the assembled upstand (reducer through bulkhead) next to the bell.

4) The Bell. This is a 2" pipe with an end cap.

I cut six 1/2" holes in a ring around the bottom, leaving about 1/4"-1/2" of material between the edge of the holes and the end of the pipe. Then I used a hack saw and 10" miter saw to remove all but three "legs." Just above each leg, I drilled a 1/4" hole, then added a fourth 1/4" hole above these. These four 1/4" holes help break the siphon when the grow bed is drained by allowing air to fill the top of the bell.

Update - I've found that cutting slices in the bottom of the bell about an inch above the bottom works just fine. It's also quieter than my bell that had the drilled holes.

5) The gravity run. This is the part that I fiddled with the most. If the gravity run has no kink, it's possible that water will just sheet down the sides of the pipe, leaving a tube of air straight from the bottom of the run to the top of the bell that will prevent the siphon from ever forming. If the gravity run has too much kink, you'll never get enough air to the top of the bell to break the siphon. Below is the gravity run that has worked best for me so far - with a 45 degree kink:

Update - the 45-degree angle is great. You do need to insert a bit of pipe in the bottom, even if it's only 1/2-inch long, because the upper edge of the lower pipe is part of what kicks the water and creates the siphon. Here's another picture, showing a siphon drain in my constant-height-of-pond system. I've got the exit coming out at or below water level, so the slices in the lower tube allow air in to break the siphon when the growbed has drained completely.

I'm creating a pdf of instructions with photos for the whole assembly, and will update this post with a link when I've got it done.

Ciao!