1 Membrane-based water treatment to increase water supply

In separation applications using membranes, the goal is to allow one or more components of a mixture to permeate the membrane, while hindering permeation of other components. A nor-mal filter meets this definition of a membrane, but, by convention, the term filter is usually lim-ited to structures that separate components larger than roughly 1–10 μm. The term membrane is used for separation of smaller compounds. Membrane types. Most of the membranes used currently are anisotropic, see figure above [1,2]. These membranes comprise a dense layer formed over a thicker porous support layer. The separation properties and permeation rates of the membrane are determined exclusively by the surface layer, while the substructure functions as a mechanical support. When the material of the top layer and that of the porous sublayer are the same, the membrane is called an asym-metric membrane. Asymmetric polymeric membranes are usually fabricated by a phase separa-tion process [3,4]. On the other hand, if the material of the top layer is different from the mate-rial of the porous sublayer, the membranes are called composite membranes, such as those cur-rently used in nanofiltration (NF) and reverse osmosis (RO). The advantage of the composite membrane over the asymmetric membrane is that the material for the top skin layer and the po