Abstract: Disclosed is a liquid separation apparatus of spiral type including a membrane module comprising a hollow mandrel having at least a hole, at least one pair of semipermeable membrane sheets and at least one pair of first and second spacing layers, said membrane sheets and spacing layers being wound about said mandrel, said first spacing layer forming, with said membrane sheets, a first passage for a permeated solution to be discharged therefrom into the interior of said mandrel through said hole, said second spacing layer forming, with said membrane sheets, a second passage for a feed solution or a nonpermeated solution having a first opening on the circumference of said module and an axial opening in the vicinity of said mandrel, said second spacing layer consisting of an inner layer portion in the vicinity of said mandrel and the remaining outer layer portion, said inner layer portion having an axial sectional area or a thickness substantially larger than that of said outer layer portion.

Abstract: Permselective membranes are prepared by crosslinking synthetic organic polyamides or polyhydrazides having --CO.sub.2 H groups and/or --SO.sub.3 H groups with a salt of metal selected from the class consisting of alkaline earth metals, transition metals, aluminum group metals and lead group metals.The membranes are useful in separating components of fluid mixtures or solutions such as water containing dissolved salts, and are especially useful in producing pure water from saline water.