Performance of RO membranes in correlation with membrane structure, transport mechanisms of matter and module design (fouling). State of the art

Abstract Synthetic membranes, exhibiting a coarse porous, fine porous, or dense structure , respectively, may exist with one of the following four organizations: homogeneous, asymmetric, asymmetric provided with a skin at the top surface, or composite. The different structures and organizations are shortly reviewed. Generally, each film forming material can be cast into each of the three structures but not always into each of the four organizations since the number of polymers for which an appropriate casting solution can be formulated to cast an asymmetric membrane provided with a skin is rather limited. Subsequently, the transport mechanisms of matter across synthetic membranes are correlated with the physicochemical properties of membranes such as water content, solute partition and diffusion coefficients, membrane structure, and water structure within the membrane. In case of the existence of so-called free water within the membrane, transport of matter is generally due to diffusion and, in addition, to convection when a pressure gradient is superimposed on the concentration gradient across the membrane. On the other hand, when so-called bound water is present in the membrane, it is plausible to assume that transport of matter is solely due to diffusion. Finally, the interrelation between membrane efficiency and hydrodynamic conditions of specific modules is discussed. The review is completed by a list of commercially available membranes for desalination and water purification together with their manufacturers.