The refugium can be part of the sump (it is usually one of the sections of the sump. For instance in my sump the water comes from the tank and goes into a section where my skimmer is. From there it goes through a bubble trap into the refugium section which contains live rock, live sand, and caulerpa algae (most like chaeto better). From there it goes into a third chamber which contains the return pump. and goes back to the tank. I have a 75-watt bulb for the refugium section and have to constantly prune the algae as it grows quite quickly.