The invention discloses a normal flow filtration cassette (1) stackable with like cassettes, said cassette comprising: at least three port conduits (2,3,4)； an inlet chamber (5) in fluid communication at least with a first of said port conduits (2)； an outlet chamber (6) in fluid communication at least with a second of the port conduits (3)； and a filter medium (7) between said inlet chamber and said outlet chamber, wherein said at least three port conduits (2,3,4) are each in fluid communication with a corresponding inlet port (8,9,10) and are each in fluid communication also with one of a corresponding set of outlet ports (11,12,13), and wherein each of said set of inlet ports (8,9 10) is arranged to cooperate with a respective complementary outlet port of an adjacent like cassette when in a stack and said outlet ports (11,12,13) are each arranged to cooperate with a respective complementary inlet port of a further adjacent like cassette when in a stack.
