A membrane is physical barrier through which pure solvent can pass while other molecules or particles are retained. Most membranes are made from synthetic polymers, although in recent times, interest in the use of ceramic materials has increased. These tend to be very robust and can withstand harsh operating and cleaning conditions. For ultrafiltration (UF) and microfiltration (MF) applications, polysulphones are often used. Other materials that are used commercially include polyvinylidenefluoride (PVDF), polyacrylonitrile (PAN) and polypropylene (PP). Reverse osmosis (RO) membranes are often made of polyamide while nanofiltration (NF) membranes are constructed from a variety of polymers, including polysulphone, polyimide, and ceramic materials. In the case of UF and MF, the semi-permeability of the membrane is largely a result of the relative sizes of the solute/particles and the membrane pores. This is what is known as a sieving mechanism. Solute retention is a little different in RO and is largely determined by charge effects. The RO membrane can be thought of as a matrix through which solvent and solute diffuse at different rates. NF occupies one of those awkward transitions zones that are often encountered in engineering and it has characteristics of both UF and RO. Solute retention is a highly complex process, involving both size and charge effects. [caption id="attachment_28937" align="alignright" width="300"]membrane-filtration Figure 1 Dead-end and crossflow filtration[/caption] MF is the filtration of suspensions containing particles that are generally less than 10 μm in diameter. For particles greater in size than this, the ‘micro’ prefix is dropped and it is generally understood that filtration is done in dead-end mode rather than the crossflow mode typically used in membrane filtration operations (Fig.1). Figure 1 Dead-end and Crossflow filtration UF is essentially the filtration of solutions of high molecular weight molecules. NF, by contrast, involves the filtration of solutions of molecules such as peptides, antibiotics and other compounds with molecular weights in the range 100–1000 g/mol. In RO, the solutions being filtered contain very small ionic species such as Na+ and Cl- ions. It is worth mentioning in passing that there are situations where one might use a membrane in a filtration process for which it is not actually designed. For example, it would not be unheard of to use a UF membrane to filter a microbial suspension because such a choice may lead to better long-term performance. Whether this is UF or MF is a debatable point, but it is probably more sensible to classify it as UF.

Microfiltration and ultrafiltration in industry

Microfiltration and ultrafiltration are used widely in industry. MF is sometimes used in areas like microbial and animal cell separation, often as part of the downstream processing of bioproducts. It is also used in clarification of beverages such as wine, beer and fruit juice, and in dairy processing and wastewater treatment processes, especially in submerged membrane bioreactors (MBRs). MBRs can operate over long periods and this is partly because they tend to be operated at very low transmembrane pressures, i.e. in the so-called sub-critical flux regime where the build-up of foulant on the membrane surface is minimised. Fouling is also limited by the presence of air bubbles created during sparging of the bioreactor/digester. Operation is made even more sustainable by incorporating backflushing, a process in which the flow through the membrane is periodically reversed, thus ‘blasting’ foulant from both the interior and the surface of the membrane. [caption id="attachment_28939" align="alignright" width="300"]membrane-filtration-ultrafiltration Figure 2 Batch and continuous feed-and-bleed ultrafiltration[/caption] UF is used for the concentration and purification of solutions of macromolecules of all kinds, especially proteins. It can be operated in both batch and continuous modes (Fig. 2). Multi-stage continuous systems in which the retentate from one stage forms the feed to the next are commonly used. Figure 2 Batch and Continuous Feed-and-Bleed Ultrafiltration Diafiltration is a technology that is closely related to UF and it is frequently operated in constant volume mode as shown in Figure 3. The basic idea is that by operating in the manner shown, low molecular weight impurities (e.g. metal ions) are washed from the solution while valuable, high molecular weight products are retained in the feed tank. [caption id="attachment_28940" align="alignright" width="300"]membrane-filtration-diafiltration Figure 3 Process configuration for constant volume diafiltration (CVD)[/caption] Figure 3 Process configuration for constant volume diafiltration (CVD) The main applications of RO are the desalination of water and the production of ultra-pure water for electronics industries. NF, which is an emerging technology, lies somewhere between RO and UF and has potential for use in wastewater treatment (including phosphorous removal), food processing and textile dye removal. All of the techniques mentioned above can be termed pressure-driven processes. Table 1 shows the approximate pressure ranges required for the various membrane filtration processes. Table 1: Approximate pressure ranges for membrane filtration processes
Technique Pressure Range (Bar)
MF 0.5 - 3
UF 1 - 10
NF 7 - 40
RO 25 - 100
The variety of pressures required reflects the nature of the suspension/solution in each case and the type of membrane employed. In MF and UF, the membranes have distinct pores and fluid flow theory tells us that smaller pores lead to greater pressure drops across the membrane. In the case of UF, the need for greater pressures, resulting from the smaller pores, is enhanced by the osmotic pressure of the solution adjacent to the membrane. This creates an osmotic ‘back pressure’ that opposes the applied pressure. In RO, the permeability of the solvent through the dense, pore-free, membrane is low while the osmotic back pressure created by the low molecular weight solutes is much higher than it is for the high molecular weight solutes that arise in UF. The pressures employed in NF fall between those used in UF and RO, reflecting the fact that NF has both UF and RO characteristics. The housing in which a membrane is placed is termed a module, or in some instances, a cartridge.

Membrane filtration and basic module types

The same basic module types can be used for all membrane processes ranging from MF to RO, although specific types of module tend to be preferred for different types of filtration. Flat sheet modules use multiple flat sheet membranes in a sandwich arrangement consisting of membrane sheets, attached at their edges to a backing support. Spacer screens, or meshes, provide the channel for the feed flow and are typically 0.3–1 mm in height. Although flat sheet modules are prone to having their flow channels clogged, they have still been used successfully in bacterial and yeast cell separations (MF) and find widespread use in UF. Spiral wound modules are similar to the flat sheet type except that the whole assembly is wrapped up rather like a Swiss roll. The feed flows down the retentate channels, while permeate flows radially through the membrane, spiralling into a central permeate duct. Because of the presence of the spacer screens, the flow channel can become clogged quite easily when used for MF applications. Spiral wound systems are used extensively for UF applications in dairy industries, as well as in NF and RO. The shell-and-tube configuration, used extensively in heat exchangers, is also widely used in membranes. The feed enters usually through the ‘tubes’, which are porous. The permeate flows radially through the wall of the tubes into the ‘shell’ to the exit ports. Different types of shell-and-tube module are distinguished by the size of their membrane diameters. In tubular modules, individual membrane tubes with internal diameters greater than about 6 mm are packed in small bundles, which are kept in place by two end-plates. The primary advantage of the tubular membrane module design is the turbulent flow, which leads to high fluxes. Furthermore, they are not easily plugged, something that makes them suitable for applications involving high solids concentrations. The main disadvantage of tubular modules is their very low membrane area per unit volume. Hollow fibre modules consist of large numbers of flexible, narrow-bore (≤ 200m internal diameter), porous fibres bonded at each end to a common header. They are widely used in medical applications, especially haemodialysis. The primary advantage of hollow fibre modules is their high-permeate flowrate per unit module volume. The main disadvantage of the hollow fibre module design is that it is susceptible to particulate plugging. In conclusion, membrane filtration represents a highly versatile range of technologies that potentially provide solutions to technological problems in a wide variety of industries. However they are not necessarily a panacea. For example, while the use of membranes for the removal of a pathogen like cryptosporidium from drinking water is technically feasible, real questions remain as to whether 100% rejection of this microorganism could ever be guaranteed even when nanofilters are employed. Crucially, membrane performance inevitably deteriorates over time and, even now, research is required to produce longer-lasting, high-performing membranes. Finally, nanofiltration is a technology that is receiving a huge amount of attention in the academic literature but fundamental theory is still at an embryonic stage and empirical approaches are required to determine if NF is a viable technological solution for a host of industrial problems. This means that potentially costly pilot scale trials will always be required even at the earliest stage of the research and development process.

Greg Foley BE MS PhD is associate dean for teaching and learning, Faculty of Science and Health and a lecturer in bioprocess engineering, School of Biotechnology, Dublin City University. He is also a member of the DCU Water Institute.