Disclosed are spiral-wound membrane cartridge constructions for use in ultrafiltration and reverse-osmosis separation equipment, each of which include a feed layer, a membrane layer and a permeate carrier layer. By using a feed layer in the form of a channel material having a plurality of parallel ribs extending in an axial direction, interconnected by a matrix of smaller filaments which may be generally perpendicular thereto, this construction advantageously reduces resistance to liquid frlow axially through the cartridge, registering only very low pressure drops. Such cartridges of 40 to 70 inches in length and 8 inches in diameter may be employed in series, even within a single outer casing or pressure vessel. Such constructions are especially useful in separating liquids having high levels of particulate contamination using ultrafiltration membranes where clogging of the cartridge is a common problem and also in concentrating solutions of fairly viscous water-soluble polymers.