Multimedia backwashable filtration system

DETAILED DESCRIPTION I claim: 1.
A rotatable valve comprising: a housing having an elongate cylindrical chamber therein; first, second, third and fourth passages extending through said housing and communicating with said chamber; a cylindrical valve body mounted for rotation in said chamber, said valve body having an axially extending passage therein, said axially extending passage communicating with a source of fluid supply for the valve, said valve body having a first area adjacent its cylindrical surface which is aligned with said first and fourth passages, and a second area also adjacent its cylindrical surface which is aligned with said second and third passages, said valve body also including first and second ports communicating between said axially extending passage and said first area and a third port communicating between said axially extending passage and said second area; said cylindrical valve body being selectively rotatable between either of at least three positions including, a first position in which said first port communicates with said first passage and said second passage communicates with said third passage, a second position in which said second port communicates with said first passage, and a third position in which said third port communicates with said second passage and said first passage communicates with said fourth passage.
2.
The rotatable valve of claim 1, wherein said second passage is sealed when said cylindrical valve body is rotated to said second position.
3.
The rotatable valve of claim 2, wherein the sealing is by a seal on the surface of said valve body in said second area which aligns with said second passage when said valve body is rotated.
4.
The rotatable valve of claim 1, wherein the surface of the cylindrical valve body in said first and second areas is spaced from the surface of said chamber to define an annular passage between said surfaces, and said first and second areas are divided from each other to define first and second annular passages at said first and second areas respectively