The present application discloses a membrane stack comprising a first and second membrane layers, and a spacer layer disposed between said first and second membrane layers, said membrane stack configured such that fluid passes through said membrane stack in a direction substantially perpendicular to the plane of said membrane layers and said spacer layer. The application also discloses a module comprising a membrane as described above, said module having a fluid flow path that is substantially perpendicular to the plane of the major surface of the membrane and spacer layers in said membrane stack.