Microfiltration permeate flux

Fig. 6. Normalized microfiltration permeate flux vs time for different concentrations of yeast in secondary feed reservoir ( ) 1.36 g/L ( ) 0.68 g/L ( ) 0.34 g/L. The cycle conditions were tf = 280 s, = 10 s, and th = 10 s, with Pf=Ph = 7.5 psi. The primary feed contained a mixture of yeast (with the same concentration of yeast as in the secondary feed) and 2.0 g/L of BSA. The dashed line represents the normalized permeate flux during filtration of protein alone without deposition of SMY and without backflushing. Error bars represent SD for two to three repeats.

Cakl, J., Bauer, I., Dolecek, P., and Mikulasek, P. 2000. Effects of backflushing conditions on permeate flux in membrane cross-flow microfiltration of oil emulsion. Desalination 127 189-98. 

Colloidal materials present in surface waters can also plug RO membranes, causing a decrease in permeate flux. Colloidal plugging can be avoided by using one of several possible pretreatment steps. Ultrafiltration (qv) (UF) or microfiltration (MF), depending on the size of the colloid, can be used to filter out the colloidal material. Alternatively, a coagulant such as alum can be added to the water to form aggregates of the colloid, which can then be filtered in a similar manner as suspended solids. 

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]. 

Marcinkowsky et al. (16) were the first to use dynamic secondary membranes in reverse osmosis for rejection of salts. Giiell et al. (17) later investigated protein transmission and permeate fluxes in microfiltration of protein mixtures using yeast to form a predeposited secondary membrane, and they observed higher flux and protein transmission in the presence of the secondary layer. Kuberkar and Davis (18) also observed higher flux and transmission of BSA in the presence of a cake layer of yeast,... 

Fig. 5. Permeate flux during microfiltration of mixture of 2.0 g/L of BSA and 1.34 g/L of yeast with deposition of SMY and with backflushing (A), without SMY but with backflushing (x), and without deposition of SMY and without backflushing ( ) permeate flux for filtration of BSA without deposition of SMY and with backflushing...

The goal of ultrafiltration, in contrast to microfiltration, is to retain protein molecules by the membrane while passing smaller solutes through the membrane with the permeate. Ultrafiltration experiments were performed with polysulfone membranes (30,000-Dalton mol wt cutoff). Figure 9 shows a comparison of the permeate flux vs time obtained during ultrafiltration of cellulase in the presence and absence of SMY that was periodically removed by backflushing and then replaced with a new SMY. 

Fig. 8. Protein transmission during microfiltration of 2.0 g/L BSA only (O) and 2.0 g/L of BSA in presence of SMY and backflushing (A), and permeate flux vs time for...

In microfiltration, the permeate flux increases inversely with the suspension viscosity and proportionally to the applied pressure, provided that there is no membrane fouling (Belford, 1988 Ho and Zydney, 2000). To accelerate the process, it is possible to decrease the solution viscosity by increasing the temperature, although not so much as to denature the protein. 

PERMEATE FLUX BEHAVIOR DURING MICROFILTRATION OF PROTEIN-ADSORBED MICROSPHERES IN STIRRED CELL... 

High-purity WPC (i.e., 70-95% proteins or total solids) can be produced by thermocalcic aggregation, followed by microfiltration, ultra filtration and diafiltration of whey proteins. Ultraflltration has been practiced since early 1970s. It appears that zirconia membranes on carbon supports with a MWCO of 10,000 to 20,000 daltons and zirconia membranes on alumina supports with a pore diameter of 0.05 to 0.1 pm are suitable for this purpose. A permeate flux of as high as 60 L/hr-m for processing acid whey to a protein content of 25 to 37% using a zirconia membrane with a MWCO of 10,000 daltons has been reported [Merin and Daufin, 1989]. 

Both microfiltration (02 m) and ultrafiltration (4 nm) alumina membranes are very effective in removing bacterias. For example, the bacteria level of a lagoon water is reduced from 1,000-5,000/cm to 0.03-0.4/cm and 0.03-0.1/cm with the microHltration and ultrafiltration membrane, respectively [Castelas et al., 1984]. The total coliform level drops from 50-500/cm to zero for both types of membranes. The accompanying permeate flux is 600-1,200 L/hr-m for 70 hours for the microfiltration membrane when the water contains a low level of colloids and only 200 L/hr-m for 20 hours when the concentrations of colloids and organic materials are high. The ultrafiltration flux varies between 100 and 250 L/hr-m for 1,000 hours of operation. 

In another study, similar fluxes have been observed. Through the use of microfiltration grade alumina membranes, broth clarification has concenuated the bacterias to a solids content of 21 % with an average permeate flux of 120 L/hr-m [Guibaud, 1989]. 

Microfiltration and ultrafiltration are the two main filtration techniques for which ceramic membranes have been widely used to date. As described in Section 6.2.1.2, MF and UF ceramic membranes exhibit macro- and mesoporous structure, respectively, which result from packing and sintering of ceramic particles. Liquid flow in such porous media is convective in nature and the simplest description of permeation flux, J, is given by the Darcy s equation [20] ... 

Gupta BB, Blanpain P, and Jaffrin MY, Permeate flux enhancement by pressure and flow pulsations in microfiltration with mineral membranes, J. Membr. Sci. 1992 70 257-266. 

Arsenic removal from drinking water is a major problem in many parts of the world. Han et al. [60] investigated arsenic removal by flocculation and microfiltration. Ferric chloride and ferric sulfate were used as flocculants. Results showed that flocculation before microfiltration led to significant arsenic removal in the permeate. Furthermore, the addition of small amounts of cationic polymeric flocculants resulted in significantly improved permeate fluxes during microflitration. 

FIGURE 20.6 Effect of TMP on the permeate flux in beer CMF through a 0.45 p,m cellulose acetate membrane, at 7= 0°C and w = 2 m/s. (From Moraru, C.I., Optimization and membrane processes with applications in the food industry Beer microfiltration. PhD thesis. University Dunarea de Jos Galati, Romania, 1999.)... 

The relationship between permeate flux and the flow characteristics in microfiltration processes is often described using the him model, which is based on the concentration polarization concept [38] ... 

While higher process temperatures do have a positive effect on permeate flux in membrane separation (Figure 20.15), due to lower product viscosity and enhanced hydrodynamics at the membrane surface, in beer microfiltration the value of this parameter is dictated by the technological requirement that beer be filtered cold, typically at temperatures between — 1.5°C and+2°C. 

Marshall, A.D., Munro, P.A., and Tragardh, G., The effect of protein fouling in microfiltration and ultrafiltration on permeate flux, protein retention and selectivity A literature review, Desalination, 91, 65, 1993. 

---