A fluid treatment assembly comprising: first and second opposite end pieces (11, 12); at least one fluid treatment unit positioned between the first and second end pieces (11, 12), the fluid treatment unit including a permeable fluid treatment medium having a feed side and a permeate side; a manifold positioned between the first and second end pieces (11, 12) said manifold including a body having a mounting surface facing the fluid treatment unit(s) on one side or on each opposite side of the manifold; a feed inlet and a feed passage extending from the feed inlet through the fluid treatment unit to the feed side of the fluid treatment medium; a permeate outlet and a permeate passage extending from the permeate side of the fluid treatment medium through the fluid treatment unit to the permeate outlet; a retentate outlet and a retentate passage extending from the feed side of the fluid treatment medium through the fluid treatment unit to the retentate outlet; said retentate outlet being formed as a part of said manifold and said retentate passage extending from the feed side of the fluid treatment medium through the fluid treatment unit, the manifold and the manifold body to the retentate outlet; and a flow restrictor positioned in the retentate passage, the flow restrictor being configured to increase the back pressure in the retentate passage; wherein the flow restrictor is positioned in the retentate passage within the manifold body and configured as one or more restrictive flow orifices arranged serially and/or in parallel, or as one or more restrictive flow channels or capillaries arranged serially and/or in parallel; or wherein the flow restrictor is formed in the mounting surface of the manifold body in the form of a groove, said groove being sealed either by a thin impermeable sheet bonded to the surface of the manifold body over the groove to enclose the groove or by a fluid treatment unit pressed against or bonded to the mounting surface to enclose the groove; or wherein the manifold includes a restrictor layer attached to the manifold body, said restrictor layer comprising one or more sublayers (41A-41D) defining the flow restrictor, said retentate passage extending through the restrictor layer and the flow restrictor being disposed in the retentate passage in the restrictor layer. first and second opposite end pieces (11, 12); at least one fluid treatment unit positioned between the first and second end pieces (11, 12), the fluid treatment unit including a permeable fluid treatment medium having a feed side and a permeate side; a manifold positioned between the first and second end pieces (11, 12) said manifold including a body having a mounting surface facing the fluid treatment unit(s) on one side or on each opposite side of the manifold; a feed inlet and a feed passage extending from the feed inlet through the fluid treatment unit to the feed side of the fluid treatment medium; a permeate outlet and a permeate passage extending from the permeate side of the fluid treatment medium through the fluid treatment unit to the permeate outlet; a retentate outlet and a retentate passage extending from the feed side of the fluid treatment medium through the fluid treatment unit to the retentate outlet; said retentate outlet being formed as a part of said manifold and said retentate passage extending from the feed side of the fluid treatment medium through the fluid treatment unit, the manifold and the manifold body to the retentate outlet; and a flow restrictor positioned in the retentate passage, the flow restrictor being configured to increase the back pressure in the retentate passage; wherein the flow restrictor is positioned in the retentate passage within the manifold body and configured as one or more restrictive flow orifices arranged serially and/or in parallel, or as one or more restrictive flow channels or capillaries arranged serially and/or in parallel; or wherein the flow restrictor is formed in the mounting surface of the manifold body in the form of a groove, said groove being sealed either by a thin impermeable sheet bonded to the surface of the manifold body over the groove to enclose the groove or by a fluid treatment unit pressed against or bonded to the mounting surface to enclose the groove; or wherein the manifold includes a restrictor layer attached to the manifold body, said restrictor layer comprising one or more sublayers (41A-41D) defining the flow restrictor, said retentate passage extending through the restrictor layer and the flow restrictor being disposed in the retentate passage in the restrictor layer.