A permeate adapter is provided. The permeate adapter may be used within a pressure vessel. The permeate adapter includes a front portion and an oppositely disposed back portion. The front portion includes a permeate opening for receiving an end of a permeate passage tube of a membrane cartridge and for being in fluid communication with a permeate reservoir. The front portion and the back portion each include one or more fluid openings. Fluid passageways are connected between the one or more front portion fluid openings and the back portion fluid openings. The fluid passageways receive an end of a membrane cartridge at the front portion. At the back portion, the fluid passageways are in fluid communication with a fluid reservoir adjacent the back portion of the permeate adapter.