Question
I'm planning to make this overflow for my tank. the goal is to drain from bottom.
in theory it will work but in real world i'm a bit worried about the noise that "air in" is going to make and where water level is going to be when power goes off.
Answer
There should not be any noise at the point that you have marked "air in", maybe just the sound of water trickling. It's vital in this design to have that unsealed air in point at the top, otherwise your tank will siphon and drain completely.
In the event that the power goes out, the water will not drain lower than the top of the pipe that goes to the sump.
My concern here is that your "water out" bulkhead has to be 100% sealed, otherwise your tank will empty. If you're able to, I would suggest rather installing the bulkhead higher up, at the lowest point that you would like the water to reach.
This design will still suck water from the bottom of the tank and won't siphon (as long as you have the air in opening). But, if your bulkhead springs a leak, won't accidentally drain your tank.
The other concern is that you will need to be certain that your drain works at least as fast as your pump, otherwise you will have overflow.
Answered By - RichieACC