Membrane filtration microfiltration membranes

Depending on the size of cells and debris, and the desired clarity of the filtrate, microfiltration membranes with pore sizes ranging from 0.01 to 10 pm can be used. In cross-flow filtration (CFF see Figure 9.2b), the liquid flows parallel to the membrane surface, and so provides a higher filtration flux than does dead-end filtration (Figure 9.2a), where the liquid path is solely through the membrane. In CFF, a lesser amount of the retained species will accumulate on the membrane surface, as some of retained species is swept from the membrane surface by the... 

Physical Techniques Physical techniques include ultraviolet radiation (UV), membrane filtration [microfiltration (MF) and ultrafiltration (UF)j, and sand filtration. These techniques either modify the bacterium itself to hinder reproduction (UV) or remove bacteria via particle size filtration (MF, UF, and sand filtration). These techniques can be capital intensive and do little to address biofilm once formed. 

Fig. 28. Schematic representation of dead-end and cross-flow filtration with microfiltration membranes. The equipment used in dead-end filtration is simple, but retained particles plug the membranes rapidly. The equipment required for cross-flow filtration is more complex, but the membrane lifetime is...

Membrane Filtration. Membrane filtration describes a number of weU-known processes including reverse osmosis, ultrafiltration, nanofiltration, microfiltration, and electro dialysis. The basic principle behind this technology is the use of a driving force (electricity or pressure) to filter... 

The individual membrane filtration processes are defined chiefly by pore size although there is some overlap. The smallest membrane pore size is used in reverse osmosis (0.0005—0.002 microns), followed by nanofiltration (0.001—0.01 microns), ultrafHtration (0.002—0.1 microns), and microfiltration (0.1—1.0 microns). Electro dialysis uses electric current to transport ionic species across a membrane. Micro- and ultrafHtration rely on pore size for material separation, reverse osmosis on pore size and diffusion, and electro dialysis on diffusion. Separation efficiency does not reach 100% for any of these membrane processes. For example, when used to desalinate—soften water for industrial processes, the concentrated salt stream (reject) from reverse osmosis can be 20% of the total flow. These concentrated, yet stiH dilute streams, may require additional treatment or special disposal methods. 

Polymer Membranes These are used in filtration applications for fine-particle separations such as microfiltration and ultrafiltration (clarification involving the removal of l- Im and smaller particles). The membranes are made from a variety of materials, the commonest being cellulose acetates and polyamides. Membrane filtration, discussed in Sec. 22, has been well covered by Porter (in Schweitzer, op. cit., sec. 2.1). 

Process Description Microfiltration (MF) separates particles from true solutions, be they liquid or gas phase. Alone among the membrane processes, microfiltration may be accomplished without the use of a membrane. The usual materi s retained by a microfiltra-tion membrane range in size from several [Lm down to 0.2 [Lm. At the low end of this spectrum, very large soluble macromolecules are retained by a microfilter. Bacteria and other microorganisms are a particularly important class of particles retained by MF membranes. Among membrane processes, dead-end filtration is uniquely common to MF, but cross-flow configurations are often used. 

Filtration Cross-flow filtration (microfiltration includes cross-flow filtration as one mode of operation in Membrane Separation Processes which appears earlier in this section) relies on the retention of particles by a membrane. The driving force for separation is pressure across a semipermeable membrane, while a tangential flow of the feed stream parallel to the membrane surface inhibits solids settling on and within the membrane matrix (Datar and Rosen, loc. cit.). 

Intensive technologies are derived from the processes used for the treatment of potable water. Chemical methods include chlorination, peracetic acid, ozonation. Ultra-violet irradiation is becoming a popular photo-biochemical process. Membrane filtration processes, particularly the combination microfiltration/ultrafiltra-tion are rapidly developing (Fig. 3). Membrane bioreactors, a relatively new technology, look very promising as they combine the oxidation of the organic matter with microbial decontamination. Each intensive technique is used alone or in combination with another intensive technique or an extensive one. Extensive... 

Economics Micronltration may be the triumph of the Lilliputians nonetheless, there are a few large-industrial applications. Dextrose plants are very large, and as membrane filtration displaces the precoat filters now standard in the industry, very large membrane microfiltration equipment will be built. 

Recently, membrane filtration has become popular for treating industrial effluent. Membrane filtration includes microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse... 

Microfiltration. Microfiltration is a pressure-driven membrane filtration process and has already been discussed in Chapter 8 for the separation of heterogeneous mixtures. Microfiltration retains particles down to a size of around 0.05 xm. Salts and large molecules pass through the membrane but particles of the size of bacteria and fat globules are rejected. A pressure difference of 0.5 to 4 bar is used across the membrane. Typical applications include ... 

Membrane filtration (reverse osmosis, nanofiltration, ultrafiltration, microfiltration)... 

Membrane absorbers are continuous chromatographic supports, which circumvent some of the above-mentioned problems of particulate stationary phases. They were originally derived from membrane (filtration) technology. The immobilization of interactive (ionic, hydrophobic, or biospecific) groups on the surface of microfiltration membranes was found to increase the selectivity of certain separation procedure. Ideally such activated membranes, or membrane adsorbers, allow the selective adsorption of certain substances and substance classes, which may subsequently be eluted by means of a stepwise change of the mobile phase (elution buffer). More complete information on the various types of modern membrane technology can be found in some recent reviews [e.g., 31-33]. 

Ceraver s entry into the microfiltration and ultrafiltration field followed a completely different approach. In 1980, it became apparent that the type of product made by Ceraver for uranium enrichment, which was a tubular support and an intermediate layer with a pore diameter in the microfiltration range, might be declassified. Ceraver therefore developed a range of a-AljOj microfiltration membranes on an a-AljOs support with two key features first, the multichannel support and second, the possibility to backflush the filtrate in order to slow down fouling. 

Porous metals have long been commercially available for particulate filtration. They have been used in some cases as microfiltration membranes that can withstand harsh environments, or as porous supports for dynamic membranes. Stainless steel is by far the most widely used porous metal membrane. Other materials include silver, nickel. Monel, Hastelloy and Inconel. Their recommended maximum operating temperatures range from 200 to 650°C. Elepending on the pore diameter which varies from 0.2 to 5 microns, the water permeability of these symmetric membranes can exceed 3000 L/h-m -bar and is similar to that obtained with asymmetric ceramic microfiltration membranes. Due to the relatively high costs of these membranes, their use for microfiltration has not been widespread. 

Air and Water Permeabilities and Filtration Capacities of Various Microfiltration Membranes... 

Bubble Point Constancy. Although the exact relationship between the bubble point and the "pore size" of a microfiltration membrane is a matter of dispute (11, 12, 13, 14), nevertheless, it remains the quickest and most convenient means for demonstrating the continuing integrity of a membrane filtration system. It is consequently important that the bubble point be both reproducible (within a given range) and constant. It was, therefore, of considerable interest to discover that the bubble points of both conventional and poly(vinylidene fluoride) membranes increased with immersion time in deionized water whereas those of Tyrann-M/E and polyamide remained essentially constant (Figure 6). 

Filtration can remove fine suspended solids and microorganisms, and microfiltration membranes of cellulose acetate or polyamides are available that have pores 0.1-20 /xm in diameter. Clogging of such fine filters is an ever-present problem, and it is usual to pass the water through a coarser conventional filter first. Ultrafiltration with membranes having pores smaller than 0.1 fim requires application of pressures of a few bars to keep the membrane surface free of deposits, water flows parallel to the membrane surfaces, with only a small fraction passing through the membrane. The membranes typically consist of bundles of hollow cellulose acetate or polyamide fibers set in a plastic matrix. Ultrafiltration bears some resemblance to reverse osmosis technology, described in Section 14.4, with the major difference that reverse osmosis can remove dissolved matter, whereas ultrafiltration cannot. 

Reverse osmosis The use of pressure to force the solvent of a solution through a membrane, but not its solutes (compare with filtration, microfiltration, nanofiltration, and ultrafiltration). 

In filtration of gas-borne aerosol particles by microfiltration membranes, capture by adsorption is usually far more important than capture by sieving. This leads to the paradoxical result that the most penetrating particle may not be the smallest one. This is because capture by inertial interception is most efficient for larger particles, whereas capture by Brownian motion is most efficient for smaller particles. As a result the most penetrating particle has an intermediate diameter, as shown in Figure 2.35 [55,56],... 

Depth membrane filters are usually preferred for in-line filtration. As particles are trapped within the membrane, the permeability falls, and the pressure required to maintain a useful filtrate flow increases until, at some point, the membrane must be replaced. The useful life of the membrane is proportional to the particle loading of the feed solution. A typical application of in-line depth microfiltration membranes is final polishing of ultrapure water just prior to use. Screen membrane filters are preferred for the cross-flow microfiltration systems shown in Figure 7.1(b). Because screen filters collect the retained particles on the surface of the membrane, the recirculating fluid helps to keep the filter clean. 

Metal oxides, used for manufacturing of ceramic nanofiltration membranes, are intrinsically hydrophilic. This limits the use of these membranes to polar solvents filtration of nonpolar solvents (n-hexane, toluene, cyclohexane) usually yields zero fluxes. Attempts have been made to modify the pore structure by adding hydrophobic groups, for example, in a silane coupling reaction [38, 43]. This approach is similar to modifications of ultrafiltration and microfiltration membranes... 

Ultrafiltration and microfiltration membranes produce high porosities and pore sizes in the range of 30-100 nanometers (UF) and higher (MF), which enable the passage of larger dissolved particles and even some suspended particles. The separation-filtration mechanism is based on molecule/particle sizes. The nanofiltration membrane lies between the UF and RO membranes, combining the properties of both so that the two mechanisms coexist. In addition, the NF membrane may be... 

Mass-transport limitations are common to all processes involving mass transfer at interfaces, and membranes are not an exception. This problem can be extremely important both for situations where the transport of solvent through the membrane is faster and preferential when compared with the transport of solute(s) - which happens with membrane filtration processes such as microfiltration and ultrafiltration - as well as with processes where the flux of solute(s) is preferential, as happens in organophilic pervaporation. In the first case, the concentration of solute builds up near the membrane interface, while in the second case a depletion of solute occurs. In both situations the performance of the system is affected negatively (1) solute accumulation leads, ultimately, to a loss of selectivity for solute rejection, promotes conditions for membrane fouling and local increase of osmotic pressure difference, which impacts on solvent flux (2) solute depletion at the membrane surface diminishes the driving force for solute transport, which impacts on solute flux and, ultimately, on the overall process selectivity towards the transport of that specific solute. 

Fig. 3. SEM micrograph of a CA microfiltration membrane fouled (A) after filtration of BSA only for 3100 s, (B) after completion of SMY deposition portion of cycle 11, and (C) after completion of backflushing portion of cycle 11. The primary feed contained 2.0 g/L of BSA, and the secondary feed contained 1.34 g/L of yeast. The cycle conditions were tf = 300 s, tsf = 15 s, and tb = 3, with an average TMP of 7.5 psi maintained during forward as well as reverse filtration.

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