Selective microfiltration

In addition, bacterias need to be removed from well, river or lake water before water can be rendered potable. A properly selected microfiltration ceramic membrane is effective for bacteria decontamination. When used preceding a reverse osmosis or an ion-exchange unit, the microflltiaiion membrane protects the downstream separation process from bacteria contamination and possible fouling due to colloids. 

Bio-fouling on the membrane can reduce both the flux and selectivity. Microfiltration of the feed to the membrane unit can diminish this inconvenience. Alternatively, modules designed to establish hydrodynamic conditions that minimize the fouling can be utilized. However these solutions increase the cost and are not completely trouble-free. 

The seminal discovery that transformed membrane separation from a laboratory to an industrial process was the development, in the early 1960s, of the Loeb-Sourirajan process for making defect-free, high flux, asymmetric reverse osmosis membranes (5). These membranes consist of an ultrathin, selective surface film on a microporous support, which provides the mechanical strength. The flux of the first Loeb-Sourirajan reverse osmosis membrane was 10 times higher than that of any membrane then avaUable and made reverse osmosis practical. The work of Loeb and Sourirajan, and the timely infusion of large sums of research doUars from the U.S. Department of Interior, Office of Saline Water (OSW), resulted in the commercialization of reverse osmosis (qv) and was a primary factor in the development of ultrafiltration (qv) and microfiltration. The development of electro dialysis was also aided by OSW funding. 

Pores Even porous membranes can give very high selectivity. Molecular sieve membranes exist that give excellent separation factors for gases. Their commercial scale preparation is a formidable obstacle. At the other extreme, UF,3 separations use Knudsen flow barriers, with aveiy low separation factor. Microfiltration (MF) and iiltrafiltra-tion (UF) membranes are clearly porous, their pores ranging in size from 3 nm to 3 [Lm. Nanofiltration (NF) meiTibranes have smaller pores. 

Pretreatment For most membrane applications, particularly for RO and NF, pretreatment of the feed is essential. If pretreatment is inadequate, success will be transient. For most applications, pretreatment is location specific. Well water is easier to treat than surface water and that is particularly true for sea wells. A reducing (anaerobic) environment is preferred. If heavy metals are present in the feed even in small amounts, they may catalyze membrane degradation. If surface sources are treated, chlorination followed by thorough dechlorination is required for high-performance membranes [Riley in Baker et al., op. cit., p. 5-29]. It is normal to adjust pH and add antisealants to prevent deposition of carbonates and siillates on the membrane. Iron can be a major problem, and equipment selection to avoid iron contamination is required. Freshly precipitated iron oxide fouls membranes and reqiiires an expensive cleaning procedure to remove. Humic acid is another foulant, and if it is present, conventional flocculation and filtration are normally used to remove it. The same treatment is appropriate for other colloidal materials. Ultrafiltration or microfiltration are excellent pretreatments, but in general they are... 

The most common membrane systems are driven by pressure. The essence of a pressure-driven membrane process is to selectively permeate one or more species through the membrane. The stream retained at the high pressure side is called the retentate while that transported to the low pressure side is denoted by the permeate (Fig. 11.1). Pressure-driven membrane systems include microfiltration, ultrafiltration, reverse osmosis, pervaporation and gas/vapor permeation. Table ll.l summarizes the main features and applications of these systems. 

The physical characterisation of membrane structure is important if the correct membrane is to be selected for a given application. The pore structure of microfiltration membranes is relatively easy to characterise, SEM and AFM being the most convenient method and allowing three-dimensional structure of the membrane to be determined. Other techniques such as the bubble point, mercury intrusion or permeability methods use measurements of the permeability of membranes to fluids. Both the maximum pore size and the pore size distribution may be determined.1315 A parameter often quoted in manufacturer s literature is the nominal... 

Mietton-Peuchot, M., Condat, C., and Courtois T. 1997. Use of gas-liquid porom-etry measurements for selection of microfiltration membranes. Journal of Membrane Science 133 73-82. 

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]. 

The membrane processes of cross-flow microfiltration, ultrafiltration, and reverse osmosis offer excellent potential for continuous removal of these contaminants. The selection of the optimum process is a function of the form of the contaminants present as well as several other factors. 

Cross-section structure. An anisotropic membrane (also called asymmetric ) has a thin porous or nonporous selective barrier, supported mechanically by a much thicker porous substructure. This type of morphology reduces the effective thickness of the selective barrier, and the permeate flux can be enhanced without changes in selectivity. Isotropic ( symmetric ) membrane cross-sections can be found for self-supported nonporous membranes (mainly ion-exchange) and macroporous microfiltration (MF) membranes (also often used in membrane contactors [1]). The only example for an established isotropic porous membrane for molecular separations is the case of track-etched polymer films with pore diameters down to about 10 run. All the above-mentioned membranes can in principle be made from one material. In contrast to such an integrally anisotropic membrane (homogeneous with respect to composition), a thin-film composite (TFC) membrane consists of different materials for the thin selective barrier layer and the support structure. In composite membranes in general, a combination of two (or more) materials with different characteristics is used with the aim to achieve synergetic properties. Other examples besides thin-film are pore-filled or pore surface-coated composite membranes or mixed-matrix membranes [3]. 

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. 

A membrane is usually seen as a selective barrier that is able to be permeated by some species present into a feed while rejecting the others. This concept is the basis of all traditional membrane operations, such as microfiltration, ultrafiltration, nanofil-tration, reverse osmosis, pervaporation, gas separation. On the contrary, membrane contactors do not allow the achievement of a separation of species thanks to the selectivity of the membrane, and they use microporous membranes only as a mean for keeping in contact two phases. The interface is established at the pore mouths and the transport of species from/to a phase occurs by simple diffusion through the membrane pores. In order to work with a constant interfacial area, it is important to carefully control the operating pressures of the two phases. Usually, the phase that does not penetrate into the pores must be kept at higher pressure than the other phase (Figure 20.1a and b). When the membrane is hydrophobic, polar phases can not go into the pores, whereas, if it is hydrophilic, the nonpolar/gas phase remains blocked at the pores entrance [1, 2]. 

Microporous membranes can be used in a manner similar to reverse osmosis to selectively allow small solute molecules and/or solvents to pass through the membrane and to prevent large dissolved molecules and suspended solids from passing through. Microfiltration refers to the retention of molecules typically in the size range from 0.05 to 10 pm. Ultrafiltration refers to the range from 1 to 100 nm. To retain even smaller molecules, reverse osmosis, sometimes called hyperfiltration, can be used down to less than 2 nm. 

The selection of the most suitable enzyme for a certain purpose mainly depends on its biocatalytic characteristics. Once a correct choice has been made, it is important to minimize the expenses associated with the enzyme use, as the economic feasibility of enzymatic processes is likely to depend on the cost of the enzyme production. In this context, several authors showed that the performance of various peroxidase processes was independent of enzyme purity [1,2], even suggesting that the crude enzyme was protected from inactivation [3, 4]. Microfiltration and subsequent ultrafiltration stages are sufficient to separate biomass and concentrate the enzyme for an economically viable operation [2, 5]. 

Micro-, ultra-, and nano-filtration can separate smaller particles using media with defined porous sizes (i.e., 10 1—1 pm in microfiltration, 10 2—10 pm in ultrafiltration, and 10 3— 10 2 pm in nanofiltration). Residual colloidal and suspended solids can be removed by microfiltration. Selected salts, most organic compounds, bacteria, protozoan cysts, oocysts and viruses are removed by nanofiltration, so that the treated water will be disinfected. This advanced filtration is used for the treatment of effluents for indirect potable reuse applications such as groundwater injection, water softening, decoloriza-tion, or removal of micropollution. 

Ceramic membranes are quite important since microporous ceramics are the principal barrier in UFe separation. Similar devices are used for microfiltration membranes and to a lesser extent for ultrafiltration. Homogeneous films are transformed into microporous devices by irradiation followed by selective leaching of the radiation damaged tracks, by stretching (Cortex is one welldmown example), or by electrochemical attack on aluminum. A few membranes are made by selective leaching of one component from a solid, as in membranes derived from glass or by selective extraction of polymer blends. 

Liquid separation. Separation can take place between solvents and solutes, macromolecules or particles or between species in liquid media by the effect of size exclusion. That is, those molecules or colloids larger than the size of the membrane pores will be retained or rejected while those smaller ones can pass through the membrane. The size exclusion mechanism predominates in pressure driven membrane processes such as microfiltration, ultrafiltration and even nanofiltration which has a molecular selectivity on the order of one nanometer. 

Microfiltration processing for clarification and defatting of cheese whey, for selective separation and concentration of micellar caseins from milk for various purposes, for fractionation of caseins and their peptides, for recovery of native whey proteins from milk, for gentle sterilization of milk to produce extended shelf fife liquid milk and cheese milk, for fractionation of globular milk fat and its components, for the reduction of microorganisms in cheese brine, and for the removal of colloidal particles in membrane cleaning solutions. 

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