A great discussion. The question of what happened in overflow box with a wet/dry trickle filter is one of the most misunderstood process in our hobby. My fiend that suggested this site to me looked at my tank and suggested that I put powerhead to suck the air out of the siphon to reduce siphon water power loss. This simply isn?t necessary if you have the setup correctly.
Take a look at the diagram below. If you overflow box is anything like this (mine is). You will see that during a power failure the siphon will not break. Let me explain.
As you can see the outer overflow box is in fact divided into 2 section. The outlet for water into your sump in on your left and the siphon tubes are placed on the right (section without an outlet). This important since it is that side that retain water during power failures (since it has no outlet). During normal process, the water level in the inner overflow box is slightly higher than the water level in the outer overflow box, or else the water will not flow from inner to outer overflow box. This works simply due to the pressure exerted on the bottom on the water column in your inner box is great than the pressure exerted on the outer overflow box and thus water will flow from inner to outer.
During a power loss. Your pump in the sump will stop working and water will no longer flow from the sump to the tank. But the siphon between the overflow boxes will still work. Eventually the water level in your tank will drop below the small teeth (opening) or inlets, the water from the tank will no longer flow from the tank into the inner overflow box. But this still would stop the water from flowing from the overflow box to sump until the water level in the inner overflow box and outer overflow box becomes even (leveled). When this happened the pressure exerted by the 2 water columns are equal and thus water will stop flowing.
If you siphon tube on either sides are bellow this so called critical level, the siphon is maintained, ready for water to flow when the power comes back up. The critical thing here is how far the ends of the siphon tubes reaches on either sides. How do you decide that. Simple. Look at the top of the divider that separate the outer overflow box. During normal operations, the water level in the inner overflow box is slightly higher than that. So the end of the siphon tube in the inner overflow box must reach below the top of this divider, which I call the critical level. If the siphon tube in the inner overflow box doesn?t reach that far below (critical level), the siphon will easily be broken when air get in the tube when the water level reaches the critical level.
As you can see there is really no need to place powerhead attached to the siphon to restart the siphon after a power failure.
Rohn
Edited: Forgive me for any spelling error and syntax error as I am pressed for time when I wrote this.