 per cubic foot of water) [7.48 US or 6.23 UK gallons].Introduced in the early 1900s was another type of sand filter; the 'Rapid Sand' filter, whereby water was pumped into the top of a large volume tank (3' 0" or more cube) (1 cubic yard/200US gal/170UK gal/770 liters) containing filter grade sand, and returning to the pool through a pipe at the bottom of the tank.As there is no pressure inside this tank, they were also known as 'gravity filters'.