Sumps are primarily used to house filtration, heating, dosing systems etc.
Refugiums are just that, a "refuge" for "something". In our case, it's
macro algae, pods, mysid etc. that would be quickly consumed (preyed upon) in the display tank.
With that said, some folks, like myself, design and use
sumps with a
refugium section built in. In my picture below, the entire left section of the box is what a traditional
sump would be. I've added the right side, with
refugium as a part of the design.
Refugiums need not be above the tank. Some folks feel that they must, so you can "gravity" feed the water and such from the
refugium to the tank. They feel that a pump would chop the mysid, copepods, amphipods, isopods and other critters to shreds. I've done my own (sick) testing, by pumping the micro crustaceans caught in a filter sock, from a bucket, into another bucket. I found losses only in the huge gammarus amphipods. Like the one below (actually just a molt). Otherwise, the smaller critters made it through unharmed.
In actuality, you want these larger monsters to stay behind in the
fuge, as breeders, and let the smaller versions thereof into the display as seed. I've NEVER had a problem with a
refugium pumped up, from below the tank.
Dave
