Fluid treatment arrangements having at least first and second radially displaced sets (12,13) of fluid treatment elements and methods for making and using them are disclosed: A ribbon including a permeable fluid treatment medium may be spirally wound in a plurality of windings to form a fluid treatment element of either set. At least two and as many as fifty or more fluid treatment elements (14′, 14″) may be positioned along a hollow core assembly (11) in each set. Fluid may be directed to or from the interior of the core assembly through a fluid treatment element of the first set and a fluid treatment element of the second set. In each fluid treatment element, fluid flows generally edgewise through the permeable fluid treatment medium.