sequence of sheet elements may be utilized in construction of filters comprising a plurality of filtration cassettes wherein the compressible retentate sheet is utilized to seal the top and bottom elements of a plurality of filtration cassettes of a sequence comprising compressible retentate sheet u201c cr u201d filter sheet u201c f u201d foraminous permeate sheet p filter sheet u201c f u201d non compressible retentate sheet u201c r u201d filter sheet u201c f u201d foraminous permeate sheet p filter sheet u201c f u201d and compressible retentate sheet u201c cr u201d an illustrative stacked cassette filter according to the invention may for example feature the sheet sequence cr f p f r f p f r f p f cr as shown in fig2 comprising a first compressible retentate sheet two base sequences of sheets fpfrfpf in a repetitive sequence and a second compressible retentate sheet in all repetitive sequences other than a single cassette base sequence the following relationship is observed where x is the number of filter sheets f the quantity 0 5 x u2212 1 is the number of non compressible retentate sheets r and the quantity 0 5 x is the number of foraminous permeate sheets p with two compressible retentate sheets being utilized to seal the top and bottom extremities of the intervening sequence.