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Originally implying a thin, microporous or semi-permeable plastic sheet, the term ‘ membrane ’ is now applied to any filter medium that is capable of removing particles below 0.1m. The membrane represents probably the fastest growing part of the filtration media market (especially if ceramic membranes for hot gas filtration are included).

Modern membrane technology began with the development of the first high performance membrane for the desalination of salt water by reverse osmosis, as reported in Loeb and Sourirajan’s seminal paper of 1963. From these beginnings as a thin, flexible semi-permeable sheet of regenerated cellulose material, developed to separate species at the molecular and ionic level, the membrane has expanded enormously, to embrace solid inflexible ceramic and sintered metal, and an everincreasing group of polymeric materials, and to applications that now extend well into the microfiltration range. The existence of the membrane as a very effective filtration medium led to the development of the whole field of cross-flow
filtration, described above, which also now extends well beyond its reverse osmosis origins.

To many people, a membrane remains a thin flexible material, but in filtration terms the word now covers any medium that can achieve separations at 0.1m or below (down to molecular and ionic sizes), and which may be thick or thin, flexible
or rigid, organic or inorganic. In addition, many membranes are now employed in microfiltration applications at cut sizes well above 0.1 um.

The membrane is essentially a surface filtration device, with depth filtration intentionally not involved in its use. In practice many membranes are of asymmetric structure and effectively comprise two layers. The active, surface layer is a very thin skin, the permeability of which is of critical importance. The lower, thicker layer is of more open structure, its role being to serve as a mechanical support for the active layer.

The irregularity of the pores of most membranes, and the often irregular shape of the particles being filtered, results in there not being a sharp cut-off size during filtration. With symmetric membranes some degree of depth filtration could occur
as smaller particles move through the tortuous flow path. To counteract this effect, asymmetric membranes, which have surface pore sizes much less than those in the bulk of the membrane material, are used to trap the particles almost exclusively at one surface (the membrane skin) whilst still offering low hydrodynamic resistance.

The fine surface structure of all membranes implies the need for significant pressure drops across the medium in order to achieve adequate fluid fluxes. As a result, membranes need to be contained in pressure tight housings, and considerable
ingenu ity is required to achieve sound and efficient operation.

The main processes in which membranes are used in industry are the:

● filtration of fine particles, down to less than 0.1m in effective diameter, from suspension in liquids or gases (microfiltration)
● removal of very large molecules and colloidal substances from liquids (ultrafiltration)
● selective removal of some ionic species from solution (nanofiltration)
● removal of effectively all dissolved and suspended matter from water and other solvents (reverse osmosis)
● selective transport of ionic species only (electrodialysis)
● separation of mixtures of miscible liquids (pervaporation), and
● separation of gas mixtures, including mixtures of gases and vapours (gas and vapour permeation).

Most membrane processes operate by means of cross-flow filtration, in which only part of the fluid passes through the membrane as filtrate (or, more correctly, permeate, since some membrane processes operate by permeation rather than filtration); the retained part, the concentrate or retentate, consequently becomes more concentrated in particulate or solute species. Membrane systems are frequently operated in a closed loop, with the retentate recycled, and final concentrate is taken from the loop in proportion to the added feed suspension. Whereas microfiltration utilizes both throughflow and cross-flow filtration, cross-flow is the usual mode for the other membrane filtration processes, and has thereby grown to its present level of importance.

Depending on the properties of the material used, membranes may be produced in the following geometrical forms:

● flat sheets, which are self-supporting or backed by a supporting substrate, and can be paper-like in format and so be pleated and made into cylindrical cartridges
● spiral wound, which is made by laying a series of membrane sheets and spacer sheets alternately, and rolling this array up into a cylinder
● tubes – self-supporting or backed by a supporting substrate, typically 12–24 mm in internal diameter
● perforated blocks, circular or hexagonal in cross-section, perforated parallel to the axis of the block by a set of channels, on the surface of which the membrane is laid (as in Figure 3.77)
● hollow fibres – typically 40 um internal diameter and 80m outside diameter, used in bundles sealed into plates at each end of the module.

These formats are then made up as modules, and enclosed in a suitable cylindrical housing. These modules are very often employed side-by-side in multi-housing arrays to provide adequate filtration area, and these arrays can reach very large sizes ( Figure 3.78 ). Polymeric membranes are used in all these forms except the perforated block, which is very largely restricted to inorganic materials.

Because of the very fine nature of membrane media, it is normal practice to employ a filter, ahead of the membrane unit, which is intended to remove any particulate material that might interfere with the membrane process. This is especially necessary where the flow passages are very narrow, such as in hollow fibre membranes. In fact, some membranes themselves are used as prefilters to membranes operating at a finer

degree of separation. Thus there will normally be a microfilter ahead of an ultrafiltration or reverse osmosis membrane, but there may also be an ultrafiltration membrane ahead of a reverse osmosis step.

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