Assemblies for use in separation of a fluid feed. Assemblies typically include an elongated housing having a fluid feed stream inlet, a residual stream outlet and a permeate stream outlet. The housing contains at least one membrane separation element to form a membrane separation element linear string that at least in part defines a linear permeate passage tube. A permeate adapter is joined with an end of the linear permeate passage tube. The permeate adapter includes a receiving opening for receiving an end of the linear permeate passage tube. The adapter defines a permeate passageway extending therethrough and includes a plurality of permeate discharge openings disposed such as to place the receiving opening and the permeate discharge openings in fluid flow communication.