The way you "backwash" a sealed pressure system sounds a lot like the way a munincipal water company filters a town's tap water. (I work for a water plant.)
It works like this... sand, anthricite, or some other similar media with the coarseness roughly of ground coffee is used. A bed of this sand will allow water to pass through while filtering out solids. These solids tend to clog up the sand reducing the flow, so periodically to clean the sand you just reverse the flow of water through the sand allowing this waste water to drain out of the system. Because the sand is heavier then the solids it's filtering, the sand will fall through the water current while the solids will be carried out of the sand & away. Looks kinda like using a gravel vac on an sandy bottom tank. Rate of flow obviously needs to be regulated for the backwash, or else even the sand can be carried away.
It's a good system generally, but requires a lot of space. Your sand bed needs to have enough surface area, or maybe I should say, a foot print, so that it doesn't clog up too fast. However, to do a decent backwash on that large of a foot print, you need to have a large pump capable of moving a large volume of water fast enough to carry the dirt away. Backwashing is easiest if it's automated. Otherwise, you need to keep a close eye on things or you could wash away the sand or drain your fish tank completely in a matter of a few minutes..