Ultrafiltration concentration factor

On the basis of the state of the art for concentration methodologies, lyophilization is recommended for sample volumes <100 L when concentrates with high concentration factors are necessary. Ultrafiltration may also be used. For sample volumes >100 L when medium-range concentration factors are required, successive ultrafiltration and dialysis, or reverse osmosis, are currently feasible. 

Because the application of NMR spectroscopy to environmental samples is relatively new, we focused our studies on the identification and characterization of DOP by 31P FT-NMR spectroscopy. Ultrafiltration and reverse osmosis concentration techniques were employed to increase the dissolved organic phosphorus concentrations to the detection level of 31P FT-NMR techniques (approximately 10-20 mg of P/L). With these concentration methods a DOP concentration factor of up to 2000 is obtainable. This chapter reports the use of 31P FT-NMR spectroscopy in the analysis of DOP. In... 

Figure 6.24 Ultrafiltration flux in apple juice clarification as a function of the volumetric feed-to-residue concentration factor. Tubular polysulfone membranes at 55 °C [27]. Reprinted from R.G. Blanck and W. Eykamp, Fruit Juice Ultrafiltration, in Recent Advances in Separation Techniques-III, N.N. Li (ed.), AIChE Symposium Series Number 250, 82 (1986). Reproduced by permission of the American Institute of Chemical Engineers. Copyright 1986 AIChE. All rights reserved...

Whey concentration, both of whole whey and ultrafiltration permeate, is practiced successfully, but the solubility of lactose limits the practical concentration of whey to about 20 percent total solids, about a 4x concentration factor. (Membranes do not tolerate solids forming on their surface.) Nanofiltration is used to soften water and clean up streams where complete removal of monovalent ions is either unnecessary or undesirable. Because of the ionic character of most NF membranes, they reject polyvalent ions much more readily than monovalent ions. NF is used to treat salt whey, the whey expressed after NaCl is added to curd. Nanofiltration permits the NaCl to permeate while retaining the other whey components, which may then be blended with ordinary whey. NF is also used to deacidify whey produced by the addition of HCl to milk in the production of casein. 

The most widely used nominal pore size for ultrafiltration is 1 nm, which is estimated to retain compounds with MWs >1000 Da. The 1 nm pore-sized membrane isolates 20% of the total DOC in surface and deep ocean waters and up to 55% of the DOC in coastal and estuarine environments (Benner et ai, 1997 Carlson et ah, 1985 Guo and Santschi, 1996). Ultrafiltration membranes with a smaller pore size are rare and do not show reproducible retention characteristics filters with a larger pore size retain only a small fraction of total DOC and they are not widely used. In general, the actual MW retained and the isolation of reproducible quantities of DOC by ultrafiltration depends strongly on the membrane (e.g. construction material, manufacturer), sample type (e.g. river, coastal, open ocean), total DOC concentration, concentration factor, extent of desalting and operating conditions (Buesseler et al, 1996 Guo and Santschi, 1996 Guo et ai, 2000). Losses to the ultrafiltration membrane can also be significant (Guo et al., 2000) and depend primarily on the physiochemical characteristics of the particular molecule. 

Milk protein standardization for continuous cheese making can also be done by ultrafiltration using ceramic membranes. Zirconia membranes with an average molecular weight cut-off (MWCO) of 70,000 daltons on carbon supports have been used for this purpose. The objective for this application is to concentrate either the whole volume of the milk to a volume concentration factor of 1.3 to 1.6 or just a fraction of the feed volume to a volume concentration factor of 3 to 4 followed by mixing the concentrate with raw milk to reduce the requirement of milk storage space [Merin and Daufin, 1989]. 

Cunha, C.R., Viotto, L.A., and Viotto, W.H., Use of low concentration factor ultrafiltration retentates in reduced-fat Minas Frescal cheese manufacture Effect on yield and sensory properties, Int. J. Dairy Technol., 57, 231, 2004. 

During UF membrane operation, there is a volume rejection. Thus, ultrafiltration data can be presented in terms of volume concentration ratio (VCR) or concentration factor (CF) as shown by the following equation ... 

Filterable organic phosphorus isolated from several sites in and adjacent to the ENR were analysed by this method, using off-line phosphorus-specific mass spectrometry detection. The organic phosphorus was isolated and concentrated by tangential ultrafiltration and lyophilization to produce concentration factors of —25 and final total organic phosphorus concentrations of —1 mg P/1. Column effluent was collected in 1 ml fractions after ultraviolet detection at 214 nm. The chromatographic fractions, along with a matrix blank and 1 and 10 mg/1... 

Figure 3.10 Mass balance for an integrated UF-RO membrane process. The process is based on a concentration factor of 20 for UF and three for RO. Adapted from Ultrafiltration Handbook, Technomic, 1988.

A key factor determining the performance of ultrafiltration membranes is concentration polarization due to macromolecules retained at the membrane surface. In ultrafiltration, both solvent and macromolecules are carried to the membrane surface by the solution permeating the membrane. Because only the solvent and small solutes permeate the membrane, macromolecular solutes accumulate at the membrane surface. The rate at which the rejected macromolecules can diffuse away from the membrane surface into the bulk solution is relatively low. This means that the concentration of macromolecules at the surface can increase to the point that a gel layer of rejected macromolecules forms on the membrane surface, becoming a secondary barrier to flow through the membrane. In most ultrafiltration appHcations this secondary barrier is the principal resistance to flow through the membrane and dominates the membrane performance. 

Two other major factors determining module selection are concentration polarisation control and resistance to fouling. Concentration polarisation control is a particularly important issue in liquid separations such as reverse osmosis and ultrafiltration. Hollow-fine-fibre modules are notoriously prone to fouling and concentration polarisation and can be used in reverse osmosis applications only when extensive, costly feed solution pretreatment removes all particulates. These fibres cannot be used in ultrafiltration applications at all. 

Ultrafiltration of heterogenous colloidal suspensions such as citrus juice is complex and many factors other than molecular weight contribute to fouling and permeation. For example, low MW aroma compounds were unevenly distributed in the permeate and retentate in UF in 500 kd MWCO system (10). The authors observed that the 500 kd MWCO UF removed all suspended solids, including pectin and PE. If PE is complexed to pectate in an inactive complex, then it is conceivable that release of PE from pectin with cations will enhance permeation in UF. At optimum salt concentration, less PE activation was observed at lower pH values than at higher pH (15). In juice systems, it is difficult to separate the effect of juice particulates on PE activity. Model studies with PE extracts allows UF in the absence of large or insoluble particulates and control of composition of the ultrafilter. In... 

The final parameter in Equation (4.9) that determines the value of the concentration polarization modulus is the diffusion coefficient A of the solute away from the membrane surface. The size of the solute diffusion coefficient explains why concentration polarization is a greater factor in ultrafiltration than in reverse osmosis. Ultrafiltration membrane fluxes are usually higher than reverse osmosis fluxes, but the difference between the values of the diffusion coefficients of the retained solutes is more important. In reverse osmosis the solutes are dissolved salts, whereas in ultrafiltration the solutes are colloids and macromolecules. The diffusion coefficients of these high-molecular-weight components are about 100 times smaller than those of salts. 

For treating water containing VOCs with separation factors of more than 500, for which concentration polarization is a serious problem, feed-and-bleed systems similar to those described in the chapter on ultrafiltration can be used. For small feed volumes a batch process as illustrated in Figure 9.16 is more suitable. In a batch system, feed solution is accumulated in a surge tank. A portion of this solution is then transferred to the feed tank and circulated at high velocity through the pervaporation modules until the VOC concentration reaches the desired level. At this time, the treated water is removed from the feed tank, the tank is loaded with a new batch of untreated solution, and the cycle is repeated. 

As the filtrate flows into the descending limb of this loop, the NaCl concentration in the fluid surrounding the tubule increases by a factor of four, and osmotic processes cause water to be reabsorbed. At the same time, salts and metabolic products are secreted into the tubular fluid. In the ascending limb, in contrast, the tubular wall is nearly impermeable to water. Here, the epithelial cells contain molecular pumps that transport sodium and chloride from the tubular fluid into the space between the nephrons (the interstitium). These processes are accounted for in considerable detail in the spatially extended model developed by Holstein-Rathlou et al. [14]. In the present model, the reabsorption l rmh in the proximal tubule and the flow resistance Rum are treated as constants. Without affecting the composition much, the proximal tubule reabsorbs close to 60% of the ultrafiltrate produced by the glomerulus. 

Many models have been published to calculate the microfiltration process. One important factor is the concentration polarization, which represents the most important limiting physical obstacle. At high particle concentration and with time, a layer is formed on the membrane. This layer is responsible for the flux reduction. A comprehensive overview on this technique is given by Ripperger52 and Staude.53 Often similar or identical module types are used in microfiltration and ultrafiltration. 

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