Abstract: The weight of a fluid is used to drive a plurality of semi-permeable membranes or other filter material to produce a permeate, and in at least some level of the apparatus more than 30% of the permeate produced is collected within a single filter casing. In other aspects, the filter material is at least partially contained within series production modules, which may contain transport zones for transporting feed or flushing fluid. In other aspects the ends of adjacent production modules may be designed to mate with one another using a slip fit, and the production modules may be maintained in mating relationship through connections to supporting cables or rods. In still other aspects of the inventive subject matter a submerged pump may be used to raise permeate towards the surface, and the pump may advantageously operate at least partially using centrifugal and/or air lift principles.