A fluid treatment assembly comprising: one or more fluid treatment units, each fluid treatment unit including a permeable fluid treatment medium (531a, 531b) having a feed side and a permeate side; a feed inlet communicating with a fluid flow path extending from the feed inlet to the feed side of the permeable fluid treatment medium (531a, 531b); a permeate outlet communicating with a permeate fluid flow path extending from the permeate side of the permeable fluid treatment medium (531a, 531b) to the permeate outlet; a retentate outlet communicating with a fluid flow path extending from the feed side of the permeable fluid treatment medium (531a, 531b) through the fluid treatment unit to the retentate outlet; first and second end plates (501, 502), wherein each fluid treatment unit is positioned between the first and second end plates and wherein the first end plate includes an upper face; and a plurality of adjustable tensioning rod assemblies extending between the first and second end plates and arranged to press the one or more fluid treatment units and the first and second end plates together, each adjustable tensioning rod assembly comprising a threaded tensioning rod extending between the first and second end plates, the tensioning rod having a first threaded end extending beyond the first end plate, and a tensioning nut and a locking nut threadably attached to the first threaded end of the tensioning rod, the tensioning nut having a tensioning nut upper face, a tensioning nut lower face, and a first external diameter and the locking nut having a locking nut lower face, and a second external diameter, wherein the first external diameter is greater than the second external diameter, and wherein the locking nut lower face is directed toward the tensioning nut upper face, and the tensioning nut lower face is directed toward the upper face of the first end plate; the tensioning nut _being rotatable on the threaded tensioning rod without rotating the locking nut. one or more fluid treatment units, each fluid treatment unit including a permeable fluid treatment medium (531a, 531b) having a feed side and a permeate side; a feed inlet communicating with a fluid flow path extending from the feed inlet to the feed side of the permeable fluid treatment medium (531a, 531b); a permeate outlet communicating with a permeate fluid flow path extending from the permeate side of the permeable fluid treatment medium (531a, 531b) to the permeate outlet; a retentate outlet communicating with a fluid flow path extending from the feed side of the permeable fluid treatment medium (531a, 531b) through the fluid treatment unit to the retentate outlet; first and second end plates (501, 502), wherein each fluid treatment unit is positioned between the first and second end plates and wherein the first end plate includes an upper face; and a plurality of adjustable tensioning rod assemblies extending between the first and second end plates and arranged to press the one or more fluid treatment units and the first and second end plates together, each adjustable tensioning rod assembly comprising a threaded tensioning rod extending between the first and second end plates, the tensioning rod having a first threaded end extending beyond the first end plate, and a tensioning nut and a locking nut threadably attached to the first threaded end of the tensioning rod, the tensioning nut having a tensioning nut upper face, a tensioning nut lower face, and a first external diameter and the locking nut having a locking nut lower face, and a second external diameter, wherein the first external diameter is greater than the second external diameter, and wherein the locking nut lower face is directed toward the tensioning nut upper face, and the tensioning nut lower face is directed toward the upper face of the first end plate; the tensioning nut _being rotatable on the threaded tensioning rod without rotating the locking nut.