First of all, I have no experience with big sumps at all. I have a tiny one on my 20 gal tank and that's it. I have been dreaming about my future upgrade though, and I thought I could share the rough sump system I have in mind:
I want to make things as easy as possible. Not only for me, but for the people who are to look after my tank when I am not home during longer periods. Because of this, I want, like you, to have everything running through the same single sump and all the flow going from one end of the sump to the other. In addition to this, I would like to add an easy way to do water changes.
During normal operations, the valve on the top left is open, the others closed. As the flow enters the first chamber, the idea is that detritus will be trapped at the bottom because of the low flow compared to the relatively wide sump. The next chamber will house whatever tech I end up using. Then we have the fuge and frag area before ending up with the return section.
When doing water changes, the idea is that I just close the valve to the top left and open the one to the right "“ shutting off whatever water is in the left section from the rest of the system but still letting the sump and tank run completely normal. By opening the bottom valve I get to flush all this water and hopefully the detritus that has been building up down the drain. I then close the bottom valve again, and add RO water. I will then activate a pump and heater which are inside this chamber before I add salt. When this has mixed I will close the right valve and open the left one again, adding the new water to the system.
The rough sketch is missing the dry backup drain which would enter the tech section next to the right valve and a proper bubble trap before the return. Also, the dimensions for each chamber are not thought through in the drawing, but you get the basic idea.
I am sure there are huge improvements to be made, but so far I like this sump (in theory
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