The biofilter is a 55 gallon drum with the plastic irrigation pipe media.  There is a 3" drain pipe that goes down to the bottom with notches cut in the end for water flow.  The water splashes down to the the water level and gets lots of air mixed in. Then it flows up through the media and out a drain into the pool.  The drain is set in such a way as to cause a circular flow in the pool.  This is called an up flow filter I think.