Membrane filtration layers

Another very successful development for reverse osmosis is the energy-saving membrane series produced by Nitto Denko (Tab. 4.2) [37]. The membrane filtrating layer is also an aromatic polyamide. Due to its irregular surface, the actual membrane area available for the permeation is much larger than it would be in the case of a smooth surface on the same porous support. High fluxes are therefore obtained. 

S. Yao, A. G. Fane, J. M. Pope 1997, (An investigation of the fluidity of concentration polarisation layers in crossflow membrane filtration of an oil-water emulsion using chemical shift selective flow imaging), Mag. Reson. Imag. 15, 235. 

To measure the water permeation of various diffusion layer samples, Benziger et al. [245] placed the sample DL in a pressurized membrane filtration cell. Then water was slowly added to the cylinder in the filtration cell, and the amount of water that flowed through the DL was measured as a function of time. Once all the water was drained, the sample was weighed to determine... 

Voigt A, Lichtenfeld H, Sukhomkov GB, Zastrow H, Donath E, Baumler H, Mohwald H. Membrane filtration for microencapsulation and microcapsules fabrication by layer-by-layer polyelectrolyte adsorption. Ind Eng Chem Res 1999 38 4037-4043. 

Pitfalls of the different water treatment processes are the formation of extensive amounts of sludge, which has to be deposited off, as is the case with flocculation, the formation of fouling layers during membrane filtration, or DBP formation after disinfection of NOM-containing waters. 

FIGURE 8.1 Schematics of the concentration polarization boundary layer for membrane filtration. 

Pope J.M., Yao S., and Fane A.G., Quantitative measurements of the concentration polarization layer thickness in membrane filtration of oil-water emulsions using NMR micro-imaging. Journal of Membrane Science 118 1996 247-257. 

Recent research efforts brought about new and exciting developments in membrane technology, some with direct implications for the membrane filtration of beer. For example, Stopka et al. [21] reported flux enhancement in the microfiltration of a beer yeast suspension when using a ceramic membrane with a helically stamped surface. A relatively simple modification of the ceramic membrane surface resulted in modified hydrodynamic conditions and disturbance of the fouling layer. As compared with a regular, smooth ceramic membrane of the same nominal pore size, the stamped membrane leads to higher flux and lower power consumption per unit volume of permeate at the same velocity of the feed. 

During membrane filtration, some components (dissolved molecular species or particulates) of the feed are rejected by the membrane and are transported back into the bulk by diffusion. If the concentration of the solute(s) at the surface is above the solubility limit, a gel layer is formed. 

Prevention or minimization of fouling and concentration polarization represents one of the main challenges that confronts membrane processing in general and membrane filtration of beer in particular. Various approaches have been developed to control membrane fouhng and increase the permeate flux in CMF, including membrane selection and modification, boundary layer control, use of turbulence inducers, or pretreatment of the feed. The two main strategies that are currently used in beer CMF are proper membrane selection and boundary layer control. 

Field et al. [157] introduced the concept of critical flux in membrane filtration. They proposed that upon start-up, there exists a flux below which a decline of flux with time does not occur. Although a concentration polarization layer is present, solid deposition on the membrane that gives rise to cake layer formation does not take place, so that a nonfoufing or cake-free operation is achieved. This flux is the critical flux and it may either be in strong form, in which flux is identical to the clean water flux at the same TMP, or in weak form, in which flux varies linearly with TMP but the slope of the fine differs from that of clean water [6,157,161]. 

Bacchin, P., Si-Hassen, D., Starov, V., Clifton, M.J., and Aimar, P., A unifying model for concentration polarization, gel-layer formation and particle deposition in cross-flow membrane filtration of colloidal suspensions, Chem. Eng. Sci., 57, 77, 2002. 

Ultrafiltration (UF) and microfiltration (MF) membranes can be made on less sophisticated supports. The simplest MF tubular membrane consists of an extruded porous tube (layer 1) as a support coated on the inside or outside with a macroporous layer (layer 2) which serves as the functional filtration layer. The support system shown in Fig. 6.3 is in fact a sophisticated UF or Knudsen gas separation membrane. For less demanding applications a 2-layer support could also be used. 

These NMR images prove that oil polarization layers form after feedstock flow ha,s been turned on. The layers dissipate rapidly when the flow of feedstock is turned off. This particular investigation illustrates the importance of NMR-imaging techniques for analy.sis of technologically relevant membrane filtration processes. 

The main purifying factor is a 1.5-2 cm thick biological membrane formed on the surface of the filtration layer after a certain time of operation. Thus, the resulting purification effect in slow-rate filtration is a sum of the mechanical effect of the porosity of the filter beds and the activity of microorganisms. A cross-section of a slow-rate filter is shown in Fig. 3.52. 

Capillaries in the Kidney. Filtration in the kidney occurs across a specialized capillary in the glomerulus. The glomerular capillary has several layers a monolayer of endothelial cells that interface with the flowing blood, an acellular basement membrane, and a filtration layer formed by glomerular epithelial cells. The endothelium is fenestrated that is, there are numerous... 

It is evident that many of the sampling devices described in the previous section in fact collect thin layers of water adjacent to and including the air/ sea interface itself. These type of surface film samples have become known as surface microlayers (Liss, 1975). Although such layers are in fact operationally defined by the devices used for their collection, there is the understanding that something happens to the properties of ordinary, bulk seawater at and near the air/sea interface which is contained in such microlayer samples. In this way, the microlayer becomes a real phenomenon in much the same way that the particulate state, understandable in a common-sense sort of way, is also usually defined by operational methods such as membrane filtration. 

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