A refugium is an area where you can grow macro algae and pods (copepods and amphipods) without worrying about predation. I.E. A refuge. Normally the second section of a sump is the refugium.
Some people put sand in their second section in an attempt at a DSB (deep sand bed) without really knowing what it is supposed to do, or the dangers of a DSB. A DSB is designed to be deep enough that an anaerobic (oxygen free) area of water is created. Normal sand takes at least 6" to create this zone while miracle mud can do it in about 3". In this oxygen free area a type of bacteria called anaerobic bacteria consume nitrate and produce nitrogen gas (78% of our air). Unfortunately they also produce H2S gas, a HIGHLY toxic nerve gas that can kill. In the pulp mill I used to work H2S gas was the biggest threat to our safety and my dad was almost killed by a leak. In the tank, disturbing a DSB (a true DSB, not a 3"deep normal sand bed) can kill everything in the tank. Many people can smell the effects of anaerobic bacteria when tearing down a tank by the disgusting rotten egg smell it gives off. Fortunately, if you can smell it, it is not toxic enough to kill a human (but it can kill fish) because the first thing H2S attacks is the sense of smell (nose nerves).
OK, enough of that, no DSB in the sump. I will attach a pic of my sump. It is a much simpler 2 section sump. I don't know why the norm is to divide the first 2 sections (drain pipes to sump and skimmer section as well as the refugium section). Perhaps people think the skimmer will suck in pods from the refugium, but I never have seen any indication of pods in my skimmer or skimmate.
My first section goes;
drain hoses - reactor pump (reactor is external and not required) - skimmer - chaeto (lit by a 17w CFL bulb 20 hrs/day) - rock rubble to keep chaeto from getting to the return section. That's it. A big section with a bunch of crap thrown in. then the smaller return section, drilled with an external DC Speedwave 2640 gph 6 speed pump. But the pump can just as easily be internal (ok, easier actually).
just my $0.02
