Microfiltration separation method

The cells and cell lysates (fragments of disrupted cells) can be separated from the soluble components by using microfiltration (Chapter 8) with membranes. This separation method offers following advantages ... 

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. 

Production of the color involves centrifugal separation of the biomass, cell breakage, and extraction. Use of a salt solution rather than water as an extraction medium increases stability of the color during extraction. Methods for partial exclusion of the polysaccharide from the color extract in order to enhance resolubilization of the dried color were developed. These processes include either microfiltration or co-precipitation of the polysaccharide with an added positively charged polysaccha-... 

When planning an industrial-scale bioprocess, the main requirement is to scale up each of the process steps. As the principles of the unit operations used in these downstream processes have been outlined in previous chapters, at this point we discuss only examples of practical applications and scaling-up methods of two unit operations that are frequently used in downstream processes (i) cell separation by filtration and microfiltration and (ii) chromatography for fine purification of the target products. 

A practically useful predictive method must provide quantitative process prediction from accessible physical property data. Such a method should be physically realistic and require a minimum number of assumptions. A method which is firmly based on the physics of the separation is likely to have the widest applicability. It is also an advantage if such a method does not involve mathematics which is tedious, complicated or difficult to follow. For the pressure driven processes of microfiltration, ultrafiltration and nanofiltration, such methods must be based on the microhydrodynamics and interfacial events occurring at the membrane surface and inside the membrane. This immediately points to the requirement for understanding the colloid science of such processes. Any such method must account properly for the electrostatic, dispersion, hydration and entropic interactions occurring between the solutes being separated and between such solutes and the membrane. 

Microfiltration using glass fiber filter disks is an efficient alternative to centrifugation for the separation of free antigen from antigen-antibody complexes. When small numbers of samples are involved in manual methods or for mechanized aparatus, microfiltration has advantages over centrifugation. 

However, the methods applied to obtain membranes with hydrophUic and chemically modifiable surfaces via physical or chemical posttreatment of hydrophobic membranes often result in unwanted and irreproducible inhomogeneities [95]. Thus the main limitation for supports in affinity separation presently encountered lies in the availabiUty of membranes with functional groups suitable for ligand coupling. This imposes an increased need for the development of hydrophilic microfiltration membranes with suitable fimctionalizable groups. 

Today the majority of polymeric porous flat membranes used in microfiltration, ultrafiltration, and dialysis are prepared from a homogenous polymer solution by the wet-phase inversion method [59-66]. This method involves casting of a polymer solution onto an inert support followed by immersion of the support with the cast film into a bath filled with a non-solvent for the polymer. The contact between the solvent and the non-solvent causes the solution to be phase separated. This process involves the use of organic solvents that must be expensively removed from the membrane with posttreatments, since residual solvents can cause potential problems for use in biomedical apphcations (i.e., dialysis). Moreover, long formation times and a limited versatihty (reduced possibUity to modulate cell size and membrane stmcture) characterize this process. 

Membrane processes, in general, are very attractive for their simplicity and flexibility. They are capable of achieving separations at a molecular level. Membrane modules are often compact and easily scaleable. For clarification and concentration, microfiltration, ultrahltration, and reverse osmosis are the current methods of choice. RO has been widely used in the food industries as an attractive alternative to classical evaporahon the only hmitahon being its dependence on osmotic pressure, which practically limits concentration of fluid streams to 25°Bx-30°Bx. Hence, currently it is used more as a preconcentration step. In recent years, membrane processes, notably pervaporahon, membrane dishUahon and osmotic membrane distillation (OMD) [21], have been used either by themselves or in combinahon with other membrane processes to overcome the problems associated with thermal processes. 

The literature describes numerous manufacturing methods for synthetic membranes. A recent review by Pusch and Walch (1) considers membranes from a number of techniques for manufacturing membranes and discusses applications ranging from microfiltration to desalination to gas separation. In this paper, a thermal phase-separation technique of preparing membranes Is presented. The method Is a development of an Invention described In US Patent 4,247,498 by Anthony J. Castro (,2). This technique Is similar In many respects to the classical phase-inversion methods however, the additional consideration of thermal solubility characteristics of the poly-mer/solvent pair offers new possibilities to membrane production. 

Membranes are particularly suited for bioprocesses involving the cultivation of microorganisms or cells as biocatalysts, in which the product of interest is produced extracellularly. Such processes are becoming increasingly attractive when compared to those in which the products accumulate intracellularly. Some of the reasons for this include the use of novel expression systems which favor higher product concentrations, and the ease of purification as compared to an intracellular bioproduct route. One of the drawbacks remains that extracellular protein products are produced in dilute concentration. Extracellular-product based-processes require cell separation, product recovery and concentration. The use of ultrafiltration and microfiltration membranes has become a method of choice in such process schemes. 

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