Alright. Been doing some more research and ultimately decided to try to leave the sump in the stand. There's a .pdf attached showing my current plan. I'm very open to suggestions for modifications though! I tried to take into account everything people suggested and fuse it with my own research.
All of the illustrated plumbing is 1" PVC (Schedule 40) except for the return lines in the overflow (above the bulkheads) which are 3/4". Additionally, the plumbing for the UV Sterilizer is 3/4" (didn't figure it would need 1" since it isn't pushing the kind of volume the return lines are).
Color code for plumbing is that the red lines are drain lines, the green lines are return lines, and the neon/pea green lines are UV Sterilizer plumbing.
Using transparencies I attempted to make the system as easy to see as possible, showing where things cross behind other things by fading the item closest to you. Hopefully this is clear (pretty tough to make everything plainly visible in a system like this and I just wasn't willing to do full-blown section views across the system) I pointed out some specific areas that I had questions on, using little orbs with letters on them. I'll start addressing those:
A. Any opinions on whether it's better to sit the pumps on some sort of rubber mat or something, to keep them from vibrating against the glass bottom of the tank and causing noise? Necessary? No?
B. The pump I ultimately purchased is a Fluval SP4, which nominally pushes 1823 GPH at 0', 1672 GPH at 1'6", 1548 GPH at 3', 1347 GPH at 4'6", 1281 GPH at 5'9", 1135 GPH at 7'2", 805 GPH at 9'5", 499 GPH at 10'8". I really hope it's sufficient because I found out after buying it (after, mind you), that the shyster running the pet store I picked it up at won't do refunds, at all, on anything (even new merchandise). That's what I get for supporting local businesses, I guess. Anyway, it has a union fitting just above it so that I can remove it when necessary.
C. I managed to acquire a Tidepool Biowheel sump cabinet for less than 50 bucks. Figured it was worth it just for the drawer system and wheel already configured. As illustrated in the tank, I plan to cut the "sump" portion of the plastic housing away from the unit and stage it elevated in the sump tank, allowing the water to spill toward the left side of the tank, under the unit, then up and over the baffles to the return pump. Due to the mess of plumbing to the right, I figured it was best to just let the media drawers pull out to the left. I plan to build some sort of plastic tray to cover the wooden shelf there, so the drippings don't soak into the stand.
D. As illustrated, basically every joint in this system is going to be solvent welded with PVC cement (except the ones connected to pumps and accessories). I'm curious about the 1" threaded x1 slip" adapters threaded to the bulkheads though. Since I can't fit Union Joints beneath all the bulkheads I figure I'm going to have to saw everything off if I ever have to remove the plumbing anyway, so I didn't know if it was worth it to use nylon tape on these particular joints.
E. I've read that it's best to drain about 25% of water to the refugium, so I placed this ball valve with the intention of using it to control how much flow went through this particular jog in the drain plumbing. My untrained assumption is that if I leave this valve half open, it would result in half of the water from this particular drain line flowing to the refugium, and since there are two drain lines, each taking half of the drain water, taking half of half would result in 25% of the water ending up in the refugium. Sound about right?
F. Based on recommendations I decided to utilize the Hofer Gargle Blaster design of stand pipe. I placed them as low as I could in the illustration, which amounts to about 4 inches of water in the overflows. I still haven't had very many suggestions as to exactly how much water you want in the overflows at any given time. Is it better to have the overflows nearly full, nearly empty, somewhere in between?
G and H. I picked up a Coralife Turbotwist 6x 18W UV Sterilizer, as I knew the chincy ones I have been using on our smaller tanks wouldn't cut it in this larger one. There seem to be a lot of opinions on how much water should be channeled through a UV Sterilizer in an hour. I was thinking about getting something like a 500 GPH pump to run this. Does that seem reasonable?
I. Ignore I. I covered my I question along with the Hofer Gargle Blaster in F.
J. This bit of the sump is troublesome for me. Obviously it is going to contain some sort of stand to hold the biowheel cabinet, but otherwise it's empty, and it's going to be rather hard to access once everything is in place. I honestly don't know if that's a problem or not. Since nothing but water will be in this section, I can't envision any issues arising, and I can slip a net in along the sides of the thing if I had to get ahold of something like debris. While I don't like leaving an area inaccessible, I'm having a hard time working around it in the current configuration.
Note on Bulkhead Plumbing and lack of Union Joints:
I really wanted to put union joints on all of the plumbing here, just beneath the bulkheads. I tried all sorts of configurations to get them to fit, but with all of this stuff crammed into the stand, I just couldn't see how I could make it work without putting way too many additional elbows into the system. Technically, I CAN put unions on the return line plumbing, but I'm not sure I see much of a point if the drain plumbing doesn't have the same treatment. If anybody can envision a way to make this work while fitting union joints on all four bulkheads, I'd definitely be interested to hear it.
Note on sump configuration and backflow:
I haven't done any kind of volume studies on the plumbing or overflow yet, as I figure modifications will be made to the plan based on suggestions from forum contributors. I realize the baffles will probably have to be adjusted to make the compartments different sizes, and possibly the heights adjusted, to account for backflow in the event of a power outage, etc. I could certainly use some advice on that process if anybody has expertise there.
Again, thank you in advance for any advice or suggestions you can provide.
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