Fluid treatment arrangements and methods involve a spirally wound fluid treatment pack. The fluid treatment pack includes a fluid treatment medium, a feed region, and a permeate region. The feed region extends along a feed surface of the fluid treatment medium and the permeate region extends along a permeate surface of the fluid treatment medium. The fluid treatment pack is spirally wound around a core assembly to form a plurality of windings of the feed region, a plurality of windings of the fluid treatment medium, and a plurality of windings of the permeate region.