Self Priming Siphon System
 

As a plumber I have installed a few septic tanks with self priming siphon systems. In the image below where the pipe touches the base of the root tank you can notch the pipe so it is not sucked in to the base and restricting the flow, breaking the siphon. Where the drain pipe leaves the bottom of the root tank it can be reduced to one pipe size smaller to assist in the siphon action.

I am still an amateur so would like input in to this by the pros here...


http://writers-voice.com/Photo/siphon.jpg

Well I am not an expert but because water seeks its own level, siphoning that way should work. Provided that air bubbles don't get stuck in the system keeping it from working. As a plumber You would have more experience with knowing if it is susceptible to getting air bubbles than I do. I would think that the water level would need to get high enough to provide enough water pressure to push all the air out. If there is any air left in the system it wouldn't be a true siphon but rather a drain. Not siphoning but allowing draining of the water until the water level reaches the level of the air bubble. Then the draining would stop because with any air in the system it won't actually be siphoning. Because siphoning is a sucking action that involves pressure and draining only involves gravity (not pressure). As I am sure you know.

I also would think that the water inlet would need to be faster than the water could drain or the system would never reach full siphoning action. This could cause a problem unless you also had an overflow back to the nutrient tank, or the growing chamber would overflow spilling all over and not get back into the nutrient tank. If you had the right kind of timer that allowed you to time it just right you could probably keep this from happening without the overflow. But because of water loss through the plants drinking it and evaporation, it would be more important to check the water level in the nutrient tank daily to make sure that the pump doesn't run dry. Because if it does, it could ruin the pump. Not filling the growing chamber completely, it wouldn't siphon back at all causing the pump to run dry all the time.

Although as long as the inlet is below water level, when shut off most pumps will automatically siphon back. But depending on how you built your system this could be useful, provided it worked flawlessly. Example: if in your system was large enough that the pump couldn't overcome the back pressure due to the weight of the water above the pump pushing back. That is why most pumps have ratings that show how much water they can pump at a particular height. You may need to fill the system from closer to the top and not the bottom to relieve the water pressure. Anything below that point wouldn't get siphoned back. The self priming siphon system could be a better alternative than buying another larger more expensive pump.