A novel separator assembly for a spiral flow reverse osmosis apparatus is provided. In one embodiment, the separator assembly comprises a central core element comprising at least two permeate exhaust conduits and not comprising a concentrate exhaust conduit. Each permeate exhaust conduit defines an exhaust channel and one or more openings allowing fluid communication between an exterior surface of the permeate exhaust conduit and the exhaust channel, said permeate exhaust conduits independently defining a cavity between said conduits. The cavity is configured to accommodate a first portion of a membrane stack assembly comprising at least one feed carrier layer, at least two permeate carrier layers, and at least two membrane layers. A first portion of the membrane stack assembly is disposed within the cavity, and a second portion of the membrane stack assembly is wound around the central core element and forms a multilayer membrane assembly disposed around the central core element.