Brief description of ebb&flood hydroponic system
Sometimes i'm being asked how that stuff works, so i'll better give a brief description of it.
Ebb & flood system (also called "flood & drain") - one of the most popular hydroponic systems. Roots of the plant are situated in neutral porous grow medium (in my case - expanded clay pellets), which holds the fertilizer solution in it, while allowing air to the roots, throughout the layer of medium. Exposure to air dramatically increases nutrients uptake in roots, which is one of the key benefits of hydroponic approach.
Medium is flooded with mineral fertilizer solution for a very short time, just to apply fresh solution to the medium. Frequency of floods may vary, usually 3-4 times a day.
Flood starts when pump is turned on (right tube on the picture has a pump at the lower end). Ebb starts when pump is off, water pours back thru the same tube and thru the still pump.
Left tube prevents overflow.
Pump creates pressure difference by means of spinning impeller, which can raise water level to a certain height only. The height limitation needs to be taken in account when building a system and choosing a pump. Optimistic pump performance value is specified as litre/hour rate which the pump is able to reach, while liquid levels before and after she pump are the same. My 6W 300 l/h pumps can lift water up to 80cm, as its effective rate drops to zero as level difference reaches 80cm.
I was already asked a couple of times about a problem with medium emergence when it's flooded, which affects at least clay pellets and perlite. No need to worry about that, as even when medium floats, it gets flooded good enough, and it only happens with young plants, when roots are not developed enough to hold the medium. I did not take any special measures to prevent that, it had fixed itself after a while, as roots grew.