A fluid treatment assembly comprises one or more cross flow fluid treatment units positioned between opposite end pieces. The fluid treatment unit includes a permeable fluid treatment medium having a feed side and a permeate side. The fluid treatment assembly further comprises a feed inlet and feed passage, a permeate outlet and a permeate passage, and a retentate outlet and a retentate passage. The feed passage directs feed fluid from the feed inlet to the permeable medium and tangentially along the feed side of the permeable medium. The permeate passage directs permeate from the permeate side of the permeable medium to the permeate outlet. The retentate passage directs retentate from the feed side of the permeable medium to the retentate outlet. A flow restrictor is positioned in the retentate passage.