Membrane ultrafiltration systems

The waters in question are pumped to a membrane bioreactor equipped with an air injection system, where part of the feed is recycled, making it move across a membrane ultrafiltration system, to prevent the presence of suspended microelements in the later phase of reverse osmosis. From the ultrafiltration process, two streams are obtained a concentrated stream of salts and microbial mass, which is recycled to the bioreactor, and a permeate stream that passes to the reverse osmosis plant. 

Figure 19. Iri situ steam sterilization of the membrane ultrafiltration system.

The concept of producing clarified fruit juice by a single pass of enzyme-treated fruit puree through a tubular, metallic ultrafiltration membrane system was recently described by Thomas et al. (5, 6). The process is known commercially as the Ultrapress process, since pressing and ultrafiltration are accomplished simultaneously (7). The metallic membrane ultrafiltration system is composed of sintered stainless steel tubes of varying diameters with membranes formed-in-place within the porous matrix of the tubes by deposition of various metallic oxides. Metallic oxides in combination with polymers are also possible. 

The particle sizes covered by filtration range from the large pebbles of the mineral sector s screens to the ultrafine particles and large molecules of the membrane ultrafiltration systems. Most systems involving contaminant removal are concerned with fine particles - fine enough, for example, to have stayed suspended in atmospheric air for long periods of time. 

Ultrafiltration of micellar solutions combines the high permeate flows commonly found in ultrafiltration systems with the possibility of removing molecules independent of their size, since micelles can specifically solubilize or bind low molecular weight components. Characteristics of this separation technique, known as micellar-enhanced ultrafiltration (MEUF), are that micelles bind specific compounds and subsequent ultrafiltration separates the surrounding aqueous phase from the micelles [70]. The pore size of the UF membrane must be chosen such, that the micelles are retained but the unbound components can pass the membrane freely. Alternatively, proteins such as BSA have been used in stead of micelles to obtain similar enan-tioselective aggregates [71]. 

Figure 6.6 ULtrafiLtration separates molecules based on size and shape, (a) Diagrammatic representation of a typical laboratory-scale ultrafiltration system. The sample (e.g. crude protein solution) is placed in the ultrafiltration chamber, where it sits directly above the ultrafilter membrane. The membrane, in turn, sits on a macroporous support to provide it with mechanical strength. Pressure is then applied (usually in the form of an inert gas), as shown. Molecules larger than the pore diameter (e.g. large proteins) are retained on the upstream side of the ultrafilter membrane. However, smaller molecules (particularly water molecules) are easily forced through the pores, thus effectively concentrating the protein solution (see also (b)). Membranes that display different pore sizes, i.e. have different molecular mass cut-off points, can be manufactured, (c) Photographic representation of an industrial-scale ultrafiltration system (photograph courtesy of Elga Ltd, UK)...

Generally, a distinction can be made between membrane bioreactors based on cells performing a desired conversion and processes based on enzymes. In ceU-based processes, bacteria, plant and mammalian cells are used for the production of (fine) chemicals, pharmaceuticals and food additives or for the treatment of waste streams. Enzyme-based membrane bioreactors are typically used for the degradation of natural polymeric materials Hke starch, cellulose or proteins or for the resolution of optically active components in the pharmaceutical, agrochemical, food and chemical industry [50, 51]. In general, only ultrafiltration (UF) or microfiltration (MF)-based processes have been reported and little is known on the application of reverse osmosis (RO) or nanofiltration (NF) in membrane bioreactors. Additionally, membrane contactor systems have been developed, based on micro-porous polyolefin or teflon membranes [52-55]. 

The foregoing conclusions arising from earlier work (6b,10, 57,63-85) offer fruitful directions for a continuing program of research and development involving a wide variety of membrane materials, and reverse osmosis and ultrafiltration systems. 

Membrane reactor systems in which the enzyme is recovered by ultrafiltration of the reaction mixture after hydrolysis is complete have been developed. These systems have been pilot tested in Australia but have not been commercialized (Zadow 1984). 

Ultrafiltration is a French originated process that uses a membrane filtering system. In its raw form, whey contains protein, lactose, ash, and some minerals. This should not surprise anyone since whey is the bi-product of cheese or casein production from milk. The original ultrafiltration method separated the ash and lactose from the whey protein resulting in a product providing about 35-70% protein. As the process improved the protein, content was elevated to up to 80% -86.5% protein content. Ultrafiltration provides a decent product with... 

Ultrafiltration systems should never be taken off line without thorough flushing and cleaning. Because membrane modules are normally stored wet, the final rinse solutions should contain a bacteriostat such as 0.5% formaldehyde to inhibit bacterial growth. 

Microfiltration cross-flow systems are often operated at a constant applied transmembrane pressure in the same way as the reverse osmosis and ultrafiltration systems described in Chapters 5 and 6. However, microfiltration membranes tend to foul and lose flux much more quickly than ultrafiltration and reverse osmosis membranes. The rapid decline in flux makes it difficult to control system operation. For this reason, microfiltration systems are often operated as constant flux systems, and the transmembrane pressure across the membrane is slowly increased to maintain the flow as the membrane fouls. Most commonly the feed pressure is fixed at some high value and the permeate pressure... 

Methodological artefacts may arise for a number of reasons, most notably as a result of specific interactions of species with the filter membrane. Therefore the choice of the ultrafiltration system, the properties and influence of the membrane and the operating conditions must be carefully considered before the ultrafiltration technique is applied for the separation of different radionuclide species in environmental samples. 

The excellent review by van Reis and Zydney [1] provides a comprehensive discussion of all major uses of membranes for processing of large molecules. This chapter will be essentially focused on the use of ultrafiltration for the concentration and fractionation of proteins, and the development of membrane chromatography systems. 

Fermentation is typically conducted in dilute suspension culture. The low concentration in such systems limits reaction efficiency, and the presence of particulate and colloidal solids poses problems for product recovery and purification. By circulating the fermentation broth through an ultrafiltration system, it is possible to recover product continuously as they are generated while minimizing loss of enzyme or cells and keeping product concentration in the bioreactor below the self-inhibition level for the biocatalyst. This process is referred to as perfusion. As the ultrafiltration unit is part of the production process, the entire system is often considered a membrane reactor. 

Figure 16.4 Spiral wound ultrafiltration system. Courtesy of Koch Membrane Systems.

Al distribution in serum Ultrafiltration system Gel filtration column UF system washing with 10% HN03> 24 h membranes twice 0.1 mol L-1 NaOH gel filtration column 30 mol L-1 Na2C03) 48 h [89]... 

The steps involved in purification include harvesting of the bioreactor, followed by inactivation of cells and concentration of the starting material, which is the cells, or the medium when the product is secreted. Concentration is achieved by using centrifugation or membrane filter systems by ultrafiltration. Cells are disrupted using the physical, chemical or enzymatic method that is best suited to the particular situation (Hopkins, 1991 Papoutsakis, 1991). 

Blatt WF, Dravid A, Michaels AS, and Nelson L. Solute polarization and cake formation in membrane ultrafiltration Causes, consequences and control techniques. In Membrane Science and Technology, ed., Fhnn JE, Plenum Press, New York, 1970, pp. 47-97. 40. Blanpain-Avet P, Doubrovine N, Lafforgue C, and Lalande M. The effect of oscillatory flow on crossflow microfiltration of beer in a tubular mineral membrane system—membrane fouhng resistance decrease and energetic considerations. J. Membr. Sci., 1999 152(2) 151-174. 

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