7 The Coanda Discharge

The Coanda Discharge letting out below the water surface
The Achilles heel of the Bell Siphon is getting the silly thing to start.

There is an art to setting the bends in the typical 90 degree elbows of the typical discharge pipe design. If it fails to work in manner A, bend one way. If it fails to work in manner B, bend the other way. Or fuss with the rate of flow into the growbed. Or check the pipes for bio-fouling and rinse with a hose.

No wonder many people just give up on bell siphons and revert to timers!

The Coanda Discharge eliminates any need to fuss with the discharge. The Coanda Effect is that fluid likes to stay adhered to a surface even when that surface bends. It is why planes fly. It is why milk sometimes misses your cup and drips in a strange direction.

I stumbled onto the Coanda Discharge by accident. I planned to put together the kind of right-angle configuration I’d seen in videos of aquaponics systems. But I didnt have a 90 degree elbow, so I used a 45 degree elbow instead. The 45 degree elbow worked great. Because of my fluid dynamics background, I recognized why it worked so well and what it needed to be named.

When water overflows the lip inside a bell siphon, water sheets down the sides of the tube, leaving a column of air. You need to form a plug of water in a location where gravity can pull it out, sucking out the air column and starting the siphon. A 90 degree bends form that “plug” by making the water splash, but when you turn the water 90 degrees from vertical, you’re horizontal and gravity is no longer helping you.

In a Coanda discharge, though, the slug of water is formed by the stable “mounding” of the water as it encounters the pipe inserted into the 45 degree elbow. There’s little to no splashing, and when the plug of water has formed, you still have plenty of gravity to pull out that water plug and the air column behind it.

Here’s a video clip showing the difference between no elbow, the 90 degree elbow, and the 45 degree elbow cases.When the outlet of the Coanda discharge is buried below the water surface in your CHOP fish tank, the only significant noise is the burp when the siphon breaks.

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11 Media Options

Winter seedlings in river rock, transplanted to Hydroton in spring
Fish waste isn’t the perfect fertilizer. Waste is mostly ammonia, which is nearly useless to plants and deadly to fish. The perfect fertilizer is nitrate. The ammonia in fish waste becomes nitrate when enough beneficial bacteria have done their magic. "Media,” like rocks and hydroton balls, are where the beneficial bacteria like to live. It so happens that “media” does a nice job of physically supporting plants and roots.

When it comes to media, I stand on the shoulders of giants. Those who have gone before me assert that you need something that is ¾” in diameter, on average. Dirt, sand, and small pebbles will clog and develop oxygen-deprived zones when used as the predominant medium in an aquaponics system.

What is the best media? It depends. The major options are gravel, river stones, hydroton, and expanded shale /slate.

• Gravel is cheap, maybe $10 for 50 gallons. But it’s heavy and hard on the hands. Depending on where it comes from, it may not be pH neutral.
• River stones aren’t quite as hard on ones hands as gravel, and are also cheap. They are also heavy and may not be pH neutral.
• Hydroton is the lightest option, it’s pH neutral, and it’s a dream to plant in. But Hydroton is only made in Germany and China, and it costs about $200 for 50 gallons (sold in 50L bags), assuming you can find it locally and don’t have to pay shipping. One downside to Hydroton is that it will shift under pressure, like when a heavy wind blows though your garden.
• Expanded Shale and Expanded Slate are pH neutral and almost as light as Hydroton. These products are both made in the US and are primarily used in construction and landscaping. If you can find a local bulk supplier for the ¾” product, the price per unit volume can be as inexpensive as gravel. But if you have to get it shipped, it is about as expensive as Hydroton.
  
  I’ve used river rock and Hydroton, and I loved the benefits of Hydroton. When I moved to my outdoor system, I divided the river rock from my single initial growbed across the four growbeds, filling the top with Hydroton. I figured the expensive loveliness of Hydroton would be wasted in the bottom-most sludge region of my growbed. And perhaps that layer of river rock anchored plants that might otherwise have toppled.
  
  If you’ve got enough media hosting beneficial bacteria, you can consider converting one of your growbeds to constant flood (with or without floating raft). The rule of thumb I’ve seen for home systems is 3-4 media beds for each constant flood bed. Constant flood beds can be good for starting seeds, growing duckweed, or growing leafy plants like basil and lettuce.

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