Fluid treatment arrangements and methods involve a plurality of fluid treatment modules and a core assembly that extends through the plurality of fluid treatment modules. Each fluid treatment module has a feed region, a permeate region, and a fluid treatment medium separating the feed region from the 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 region. Feed fluid is directed along the feed region of each module, and a portion of the feed fluid passes as permeate through the fluid treatment medium, into the permeate region, and then into the core assembly. One or more control mechanisms are fluidly coupled to the permeate region of one or more fluid treatment modules to control a flow parameter in the permeate region of the module.