Well no I hadn't thought of putting the siphon in the center but now that you mention it that makes a lot of sense! Thanks!
I could have set up a venturi drain (one pipe inside another see illustration below) with holes drilled in the bottom and a screen to keep the fish in to periodically drain out the mulm in the center just by lifting one of the pipes, but I chose not to do it this time, but probably will in the future (but not the siphon tube in the center then). The tank is already plumbed for that, but I put a plug on both sides for now. Even more effective would be a tank with a sloping bottom.
The trout hatchery tanks I saw did have bottoms that sloped slightly to center, and also had standpipes in the center as shown in the drawing (but I think only a single standpipe, not the double-walled venturi). They would briefly pull the standpipe a few times a day to clear out the mulm.
I didn't realize you had the tank raised off the ground and drilled to allow a direct drain from the bottom. As long as you've got that, it seems like using that rather than the siphon would make sense (as long as you're confident in your plumbing connections!). I assume the original design relies on a siphon because putting a drain fitting in a flexible vinyl liner and raising it off the ground enough to allow room for plumbing would be very difficult.
Here's what I want to do with the gravity flow back to the fish tank, which you are discussing. If there is enough gravity pressure and flow, I'm thinking the holes will work fine to create circular flow as long as they don't get clogged up. If they do get clogged up I'd have an emergency overflow pipe higher than the pipe with the holes, which is much bigger in diameter, but no holes.
Obviously, you can control the pressure at which water returns to the main tank from the biodisc just by increasing the height difference between the biodisc tank and the main tank. Or, if flow/pressure isn't enough, you could use a larger pump between the clarifier and the biodisc, and then use a 'T' and valve to send only part of the flow to the biodisc while sending the rest directly back to the main tank. Or... just add another small powerhead directly in the tank.
Unless I'm missing something though I don't think I have to worry about oveflow flooding the floor as the return flow to the fish tank from the RBC will be gravity overflow. The only thing that can fail is the pump (only pump in the system) that sends water from the clarifier tank to the RBC tank. If it fails the only thing that will happen is it will run itself dry to the level it's at in the clarifier tank, which is near the top. No water going to the RBC tank, simply means the RBC will not rotate and overflow to the fish tank will stop. The 'u-tube siphon will also stop flowing, but stay full as one of it's functions via gravity is to keep the level of the water in the fish tank and clarifier drum the same.
Sounds right. When I've thought through this stuff I was considering single sump systems to provide filtration to a number of tanks. That sort of scenario with tanks at multiple levels and a sump lower than all of them lends itself more to a runaway drainage problem. Given that you've only got 3 components and they're at roughly the same level, I'd guess you can build a pretty foolproof system without much problem. Your concern would be more for a failure to compromise filtration and aeration than to cause an overflow. I'd guess you could figure out a pretty simple system to set off an alarm if water stops flowing from the biodisc container back into the main tank (if water makes it that far, obviously everything upstream is working, too).
As far as siphons side by side the original plans in the book Small Scale Aquaculture- VanGorder for the large 12 foot swimming pool call for two clarifier drums and u-tubes. Perhaps part of that is for redundancy?
I'd forgotten about that detail until I looked at it again just now. I'd assume that they found that they needed more capacity than a single 55g drum would provide, and it made more sense in terms of using easy-to-find parts and keeping the drums easy to handle to use two 55g drums rather than one larger one. I think a redundant siphon maybe makes sense, but I can't see any reason for a whole second drum clarifier just for redundancy's sake.