After my last post I think that I figured out the answers to my own questions.
I would guess that the bung holes should be kept plugged and used as clean outs. The two tanks should be connected with a pipe close to the top so that water transfers from one to the other high above the sediments. Netting should be used to reduce turbulence from water movement. I think that I would want to pump in and gravity flow it out. If I tried to siphon in and the electricity went out it would drain the tank and overflow the filter.
Am I getting warmer?
Are you attempting to do the same as the book small scale aquaculture or something different? If the same I would cut out the top of barrel bung holes and all.
If your pump is set on top of your filter material there will be minimal effluent as it will be trapped in the filter material. If your power goes out it simply stops sending water back to the biofilter.
