Membrane processes are primarily used for separations. As a unit operation they have several major attributes:

• They provide a well-defined, mass transfer surface area that is mostly independent of the operating conditions

• Their surface area provides selective transport of specific components between two phases

• Membrane process units are built of modules

• As devices, membrane modules typically provide high surface area per unit volume (the highest being for hollow fiber forms) and are relatively easy to operate

• The scale-up of membrane processes is usually linear with load

The selective separating layer in the module is what is typically called 'the membrane.' Currently membranes made from a broad spectrum of polymeric and inorganic compounds are commercially available.

There are many membrane processes or unit operations. They are differentiated primarily on the basis of the driving force for mass transfer through the membrane, the predominant transport mechanism, and the phases that are present. Each membrane unit operation has its own specific nomenclature, engineering characteristics, and concerns, but it is also possible that a given membrane module can be used for different membrane unit operations. Due to their very high capital, operation and maintenance costs, they are only suitable for special applications in wastewater treatment for developing countries. For application in wastewater treatment see Chapter 3.4.