The Collaborative International Dictionary of English v.0.48:

Filtration \Fil*tra"tion\, n. [Cf. F. filtration.]
   The act or process of filtering; the mechanical separation of
   a liquid from the undissolved particles floating in it.
   Fimble

WordNet (r) 3.0 (2006):

filtration
    n 1: the process whereby fluids pass through a filter or a
         filtering medium
    2: the act of changing a fluid by passing it through a filter

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "filtration":
   bolting, clarification, colature, destructive distillation,
   distillation, edulcoration, effusion, elution, elutriation,
   essentialization, excretion, exfiltration, extraction,
   extravasation, exudation, filtering, leaching, lixiviation, ooze,
   oozing, percolating, percolation, purification, refinement,
   riddling, screening, seep, seepage, separation, sieving, sifting,
   spiritualization, straining, sublimation, transudation, weep,
   weeping, winnowing