Fouling Considerations

A critical consideration with UF technology is the problem of fouling._Foulants interfere with UF by reducing product rates- sometimes drastically-and altering membrane selectivity. The story of a successful UF application is in many respects the story of how fouling was successfully controlled. Fouling must be considered at every step of UF process development in order to achieve success.

When we talk about this subject, the term foulant or foulant layer comes to the forefront. Foulant, or fouling layer, are general terms for deposits on or in the membrane that adversely affect filtration. The term "fouling" is often used indiscriminately in reference to any phenomenon that results in reduced product rates. "Fouling" in this casual sense can involve several distinct phenomena. These phenomena can be desirable or undesirable, reversible or irreversible. Different technical terms apply to each of these possibilities.

You may be surprised, but fouling is not always detrimental. The term dynamic membrane describes deposits that benefit the separation process by reducing the membrane's effective MWCO (Molecular Weight cut-off) so that a solute of interest is better retained. Concentration polarization refers to the reversible build-up of solutes near the membrane surface. Concentration polarization can lead to irreversible fouling by altering interactions between the solvent, solutes and membrane.

UF fits between nanofiltration and microfiltration in the filtration spectrum and involves separations of constituents ranging from about 1-100 nanometers in size, or about 500 to 500,000 daltons in molecular weight. UF separations involve proteins, polysaccharides and other macromolecules important to the food industry, separation is primarily according to size, but surface forces are important in determining the separation as well. UF is different from conventional filtration, also called normal or dead-end filtration, in that it operates in the crossflow mode; that is, the feed stream flows parallel to the filtration media (membrane). Crossflow acts as a sweep stream to continuously cleanse the surface of the membrane from accumulated reteníate. There are two products of UF: the permeate, containing components small enough to pass through the membrane, and the concentrate, containing the reteníales.

Cake layer formation builds on the membrane surface and extends outward into the feed channel. The constituents of the foulant layer may be smaller than the pores of the membrane. A gel layer can result from denaturation of some proteins. Internal pore fouling occurs inside the membrane. The size of the pore is reduced and pore flow is constricted. Internal pore fouling is usually difficult to clean.

Fouling can be characterized by mechanism and location. Membranes can foul in three places: on, above or within the membranes (refer to the sidebar on the next page). The term agglomeration in the general sense, describes colloidal precipitates resulting from solute-solute attractions. Agglomerates can deposit on the membrane surface, reducing permeability. On the other hand, controlled aggregation of solutes can facilitate ultrafiltration.

Sorption or adsorption refers to deposition of foulants on the membrane surface resulting from electrochemical attractions. These attractions arise from non-covalent, intermolecular forces such as Van der Waals forces and hydrogen bonding. Adsorption is associated with internal pore fouling, since most of the surface area of the membrane occurs internally. The high internal surface area of UF membranes is readily apparent from photomicrographs of cross-sections of UF membranes. The photomicrographs show spongelike structures that suggest convoluted, tortuous pore pathways. Adsorption can lead to more extensive fouling. For instance, a protein might denature upon adsorbing to the surface of an ultrafilter. The denatured protein attracts other proteins, the process repeats, and a deposit builds on the membrane surface.

UF membranes are often rated by molecular weight cut-off (MWCO); solutes above the MWCO are retained and those below the MWCO permeate through the


Simple pore blockage

Simple pore blockage

membrane. MWCO can be determined by challenging a UF membrane with a polydisperse solute, such as dextran, in a crossflow filtration experiment A retention profile or curve is determined by comparing the dextran molecular weight distribution in the feed to that in the permeate using size exclusion chromatography (SEC). MWCO is typically defined as the 90 percent retention level, or the molecular weight value on the ordinate where the retention curve crosses 90 percent on the axis. MWCO ratings are relative. Membrane retentivity depends upon many factors, including the shape of the solute used, the fluid mechanics and the various interactions possible between the solvent, solute and membrane.

MWCO curves are useful for identifying membranes with appropriate selectivity for an intended separation. Predicting the best membrane MWCO is not always straightforward, however. A common assumption is that the membrane MWCO should closely match the molecular weight of the solute of interest. Remarkably, better UF performances are sometimes achieved with membranes having MWCO's significantly higher or lower than the molecular weight of the solute to be retained. For example, lower protein adsorption and flux loss are reported in the literature in the filtration of albumin with polyethersulfone membranes when membrane MWCO's were much larger and smaller than the molecular weight of albumin. How is this possible? Better performances with the low and high MWCO membranes are explained by considering the effects of fouling on the membranes. Higher product rates are sometimes realized with lower MWCO membranes because they exclude more potential foulants and internal pore fouling is reduced. Membranes with higher MWCO's will sometimes effectively separate smaller solutes because solutes aggregate into larger entities or because foulant forms an effective dynamic membrane. The dynamic membrane reduces the effective MWCO of the ultrafilter so that the solute is retained. The larger pores suffer less flow restriction due to adsorption, and the greater hydraulic permeability of the larger

A useful analytical tool for predicting and diagnosing fouling s Fourier Transform Infrared Spectroscopy (FTIR). FTIR can reveal important information that is useful for predicting and measuring foulants. The FTIR is a standard laboratory instrument for chemical analysis, and has been applied for many years in the field of membrane science. It has been successfully applied in identifying which solutes in complex mixtures may cause fouling. The ability to distinguish foulants is advantageous in applications where complex process streams predominate. Fit with an attenuated total reflectance (ATR) accessory, FTIR allows us to look quickly and easily at the chemistry of the foulant layer and membrane surface. The ATR technique can also provide quantitative estimates of fouling. There are some references at the end of this chapter that will give you more details.

FTIR can be used to screen membranes for fouling tendencies prior to the first ultrafiltration experiment. Screening can be done by means of a simple static adsorption test. Membranes showing greater static adsorption are expected to foul more during ultrafiltration and are disfavored. Figure 8 illustrates the FTIR results of a static adsorption test using a polysulfone ultrafilter as the substrate and a water extract of soy flour as the source of potential adsorbates.

FTIR can be used to diagnose fouling as well as to predict it. The techniques are similar. Among the diagnostic possibilities, one can:

• chemically identify the foulant(s) by searching spectral libraries

• estimate the thickness of foulant layer by comparing the relative size of peaks due to the membrane and foulant

• evaluate the effectiveness of various cleaners by measuring the disappearance of foulant peaks

• surmise internal pore fouling if foulant peaks persist after the surface of the membrane has been thoroughly cleansed.

Figure 8. Example of FTIR analysis of Polysulfone (PS) ultrafilter static adsorption test.


Figure 8. Example of FTIR analysis of Polysulfone (PS) ultrafilter static adsorption test.

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