The key to the desalination of water by reverse osmosis lies in the proper selection of the membrane through which the separation of salt and water occurs. The original RO membranes were made from cellulose derivatives, but are now much more likely to be made from synthetic polymers.
Two membrane formats are most commonly used for reverse osmosis, the spiralwound type and hollow fibres. A hollow fibre module is made from a bundle of hollow fibres, each of which has the salt rejecting layer on the outside surface, and an outside diameter of 90 to 100m, with an inside diameter of about 45 um. The ends of the fibres are embedded into an epoxy sheet, which is then sealed into a cylindrical glass fibre pressure vessel. The fibres can extend the full length of the containing cylinder, but it is more usual for the bundle to be bent in the middle into a U-shape, with the two ends adjacent in the housing.
The raw water is fed into the housing and part of it permeates through the walls of the fibres to discharge from the open ends of the fibres into the end-cap of the housing. The rejected brine is discharged from the other side or end of the housing
cylinder. Hollow fibre modules provide compact systems, and the large membrane surface area resulting from the high density packing of the fibres compensates for the relatively lower water permeability of this configuration.
The spiral-wound element typically embodies a flat sheet membrane, cast on to a porous polyester support sheet. Several membrane/support sheet combinations, together with intermediary spacer sheets (for liquid flow) are then wound round a
central core (Figure 4.10). The membrane can be made from a cellulosic polymer or it may be of the thin film composite type, where the polyamide salt-rejecting layer is applied to a microporous polymer film, which in turn is bonded to the support sheet. The central product water tube, around which the membrane and its supporting layers are wound, collects the fluid that permeates through the membrane. The spiral element operates as a cross-flow membrane filter. Only a proportion of
the brine permeates the membrane to become product water, while the residual brine maintains enough turbulence to minimize the build-up of rejected salts ions that might otherwise clog the membrane surface.
The narrow passageways in reverse osmosis modules allow ready blockage by even quite small rogue particles. It is thus necessary to provide reverse osmosis plants with prefilters that ensure freedom from such particles.