The force of the water going backwards through the filter lifts and separates the media which frees the debris and washes it out through a flush valve. Because a small amount of media is often washed out too, it is necessary to periodically add some more to the filters. Because sand is not easily flushed out of them, media filters are not good for situations where the water contains a lot of sand. The sand will not flush out and soon the filter will be completely filled with sand which you will have to remove by hand. Media filters must be carefully matched to the system flow rate for proper operation.
Disk filters are good at removing both particulates, like sand, and organic matter.
A disk filter consists of a stack of round disks. The face of each disk is covered with various sized small bumps. A close up view of the bumps reveals that each has a sharp point on the top of it, somewhat like a tiny pyramid. Because of the bumps, the disks have tiny spaces between them when stacked together. The water is forced between the disks, and the particulates are filtered out because they won't fit through these gaps.