Tirellil, There is no ...

Tirellil,

There is no difference in how to choose a granular filter media for seawater versus fresh water.  It is based on Stokes Law to determine the terminal velocity of the media particle when backwashing.  In brief, it depends on the intended backwash flux, backwash water temperature and the media's specific gravity.  I previously worked for a seawater filter manufacture (equipment vendor).  For most of their seawater filters, they used fine garnet, not sand.  Garnet has a higher specific gravity than silica sand, thus, they can use smaller grain size which provides finer filtration.  

Ken Martins

CH2M, Global Practice Director, Industriial Water