Membrane Separations

Membrane separations have been playing an increasing role in wastewater treatment; this is the most evident in processing whey wastewater with ultrafiltration in the dairy industry, one of many types of membrane separation technologies, and in membrane pervaporation of volatile organic compounds (VOCs) from wastewater. Currently, a new field of membrane separations called nanomembrane technology is said to be the future nano-technology for achieving clean water with this purported type of "smart membranes." It is predicted that nanomembranes are able to separate molecules by differences between molecular weights of a mixture of compounds—a feat that current membranes (polymeric or inorganic membranes) are unable to achieve. Whether this prediction of smart membranes can really hold water in the future is anyone's guess; however, the use of membrane-based technologies will undoubtedly increase among the evolving water and wastewater treatment landscapes.

A membrane can be viewed as a discreet (or discriminating) barrier that allows some components of the wastewater feed to pass through the membrane faster than the other components. A membrane provides a third phase, mostly a solid phase that straddles between two fluid phases serving as the origin and the destination of the separation process. The membrane is most likely polymeric, though new inorganic membranes are now emerging at a rapid pace. The principal mechanisms of membrane separations are molecular diffusion in solid and tortuous viscous flows in micro-porous solids.

Membrane technology is an evolving separation technology, and because of its multidisciplinary characters it can be used to perform a large number of separations in food and agricultural wastewater treatment. The membrane processes that are commonly found in processing plants or research laboratories include microfiltration (MF), reverse osmosis (RO), ultrafiltration (UF), nanofiltration (NF), electrodialysis (ED), membrane distillation (MD), and pervaporation (PV). Membrane processes are based upon different separation principles or mechanisms and their applications in food processing range from concentration of food fluids to aromatic flavor recovery. The membrane is at the center of every membrane process. However, membrane separations can be achieved only when a driving force is applied to the underlying membrane process. A schematic diagram of a two-phase conceptual system is shown in Fig. 3.12. No perfect man-made membrane ever existed. This situation will be with us in the foreseeable future until perhaps we fully understand the mechanisms that regulate the mass transfer in the membrane, and we are able to tailor the membrane structures to the need of separation of molecules of interest by using the latest advancement in nanotechnology. In assessing membrane systems, two experimental parameters that determine the overall performance of membrane processes should be the main focus of designers' attention. The first one is selectivity, the other, permeation flux.

Figure 3.12. A schematic diagram of a conceptual two-phase membrane system.

The selectivity of a membrane toward a mixture, which characterizes the extent of separation, is customarily expressed by one of two quantities: the retention, R, and the separation factor, a. The R is more suitable for the membrane separation of a dilute binary system and given by Equation 3.19:

Cf where Cf is the solute concentration in the feed stream and Cp the solute in the permeate. The value of R varies between 100% (complete rejection or retention) and 0% (complete permeation). For most mixtures, however, the selectivity factor is more adequate (Equation 3.20):

where Ci and Cj are the concentrations of components i and j in the permeate and in the feed. The value of ajj is greater than 1 if the component i is more readily permeable than component j and if the separation occurs.

The other parameter, permeation flux, takes many forms depending upon the underlying membrane processes. It is normally expressed as the following (Equation 3.21):

' dz where K is the phenomenological coefficient and dg/dz is the driving force expressed as the gradient of g (concentration, temperature, pressure) in the z direction toward the membrane. The phenomenological coefficient K is strongly related to the driving force, module configuration, and operating conditions.

Membrane processes can be classified according to the nature of their driving forces and pore size of the membrane. Although all membrane processes are driven by the electrochemical potential gradient, one particular driving force is usually dominated in a membrane process. Three types of membrane separation processes relevant to the food industry can be considered: those driven by hydrostatic pressure difference, those driven by the partial vapor pressure gradient, and those driven by electrical potential differences. The next sections present brief general descriptions of the membrane processes used or potentially usable in various operations of the food industry.

Membrane separation by hydrostatic pressure difference

Membrane performance of a pressure-driven system is usually described by the flow rates of water (solvent) and solute (permeate). The flow of water (volume flux) through a membrane without considering concentration polarization and fouling (or/and gel layer) is expressed by the following (Equation 3.22)

where Kw is water permeability; AP is the applied pressure; An is the osmotic pressure difference; and a is the reflection coefficient of the membrane toward the solute, which is a measure of degree of solute rejection. The driving force is (AP — An) as dg/dz in Equation 3.21. Because there is no perfect membrane, we may suspect (it can be verified) that some solutes, including those undesirable, also transport across the membrane, though less freely than water (solvent) (Equation 3.23):

where Ks is the solute permeability and Cm and Cp are concentrations on the upstream side of the membrane and on the permeate side, respectively. Note that Ks has a different unit from that of Kw because the driving force in Equation 3.23 is expressed as the solute concentration difference.

Although Equations 3.22 and 3.23 are generally considered as valid phenomenological expressions, the true meanings of Cm and Cp are not what they seem to be. This is because in a pressure-driven membrane process, the retained solutes transported by convective transmembrane flux can accumulate at the membrane surface leading to high concentration of solutes near the membrane, a phenomenon called concentration polarization. This concentration gradient is encompassed in a region designated as the boundary layer (velocity has its own gradient due to the viscous effect at the water-membrane interface and the no-slip condition for a common cross-flow membrane configuration, thus a velocity boundary layer). In a steady state situation, the concentration polarization is the result of solute buildup counterbalanced by the solute flux through the membrane plus the diffusive flux of solute at the membrane surface toward the bulk flow on the upstream side of the membrane. The magnitude of the concentration polarization is expressed by the Equation 3.24 as a result of the solute mass balance based upon the concentration profile of the film model illustrated in Fig. 3.13:

where Cb is the concentration of the solute in the bulk flow and k is the mass transfer coefficient that is the ratio of diffusivity of the solute in the solvent to the thickness of the concentration boundary layer, which can be interpreted as the mass transfer coefficient when the permeation flux approaches zero. The cause of the concentration polarization phenomenon is different in reverse osmosis, as in microfiltration or ultrafiltration. In reverse osmosis, as the low molecular weight material is retained on the membrane surface (see the following section about the characteristics of all pressure-driven membrane processes), the increase in the solute concentration causes the osmotic pressure to rise, which decreases the water flux, as illustrated in Equation 3.22. In ultrafiltration, the high concentration of larger molecules accumulated on the membrane surface does not result in significant osmotic pressure increase. However, these retained molecules may lead to precipitation and possibly formation of a gel layer on the mem

Membrane Gel Layer or rouled layer

Figure 3.13. A schematic diagram of a film model.

Membrane Gel Layer or rouled layer

Figure 3.13. A schematic diagram of a film model.

brane surface. The mass transfer coefficient, k, in Equation 3.24 has to be determined experimentally because the thickness of the concentration boundary layer is usually an unknown quantity that is strongly influenced by hydrodynamics of the system. The mass transfer coefficient, k, however, can often be related to the semiempirical Sherwood number correlations with the following form of expression (Equation 3.25):

where Re is the Reynolds number, Sc the Schmidt number, and Sh is Sherwood number; and a, b, and c are all constants. In Equation 3.25, ^ and v are dynamic viscosity and kinematic viscosity, respectively while p is the density, D the diffusivity, and dh the hydrodynamic diameter. It is clear that the mass transfer coefficient k is mainly a function of the feed flow velocity, the density, the viscosity, the diffusivity of the solute, and the membrane module type. Many Sherwood relationships for different flow regimes and membrane module shape and dimensions are available in the literature (Cheryan, 1986; Rautenbach and Albreht, 1989; Mulder, 1991).

Microfiltration (MF)

Microfiltration (MF) is a form of filtration that has two common forms. One form is cross-flow separation. In cross-flow separation, a fluid stream runs parallel to a membrane. There is a pressure differential across the membrane. This causes some of the fluid to pass through the membrane, while the remainder continues across the membrane, cleaning it. The other form of filtration is called dead-end filtration or perpendicular filtration. In dead-end filtration, all of the fluid passes through the membrane, and all of the particles that cannot fit through the pores of the membrane are stopped. Cross-flow microfiltration is used in a number of applications, as either a prefiltration step or as a process to separate a fluid from a process stream. Dead-end microfiltration is used commonly in stopping particles in either prefiltration or final filtration before a fluid is to be used. Cartridge filters are typically composed of microfiltration media. MF is a pressure-driven membrane filtration process that has a membrane with a pore size typically of 0.2-2 ^m and able to retain particles with molecular weights equal or larger than 200 kDa. MF membranes are symmetric with a characteristic spongelike network of interconnecting pores. It has been successfully used in the beer brewing industry to remove bacteria in the production of long shelf-life draft beers. The dairy industry has also found MF useful in removing bacteria or particulate substances and fractionation of milk proteins. MF is the membrane process that most closely resembles a conventional filtration unit. The transport mechanism of MF is undoubtedly sieving action. Thus, the volume flux through the MF membranes is expressed with a Hagen-Poiseuille relationship (Equation 3.26):

if the membrane is perceived as a bunch of straight capillaries. When a nodular structure (the space between spheres) exists, a Kozeny-Carman equation is usually applied to the following (Equation 3.27):

= K^S2(1 -£)2 Az where S is the internal surface area, £ the volume fraction of the pores, t the pore tortuosity, ^ the viscosity, and K the Kozeny-Carman constant.

Chapter 3: Physicochemical Wastewater Treatment Processes 87 Ultrafiltration (UF)

Ultrafiltration (UF) is the most common membrane process used in the food industry and it involves the use of a membrane with a pore size ranging between 0.01-0.2 ^m. Ultrafiltration is not as fine a filtration process as nanofiltration, but it also does not require the same energy to perform the separation. Applications of ultrafiltration in food processing can most likely be found in situations that call for separating one or more desirable components from a liquid mixture. Ultrafiltration is capable of concentrating bacteria, some proteins, some fats, some colloidal minerals and constituents that have a larger molecular weight of greater than 10 kDa, but it is typically not effective at separating organic streams (Rosenberg, 1995). In UF, the chemical nature of membrane materials has only little effect upon the separation because ultrafiltration separation, like microfiltration, is based upon sieving mechanisms; thus, ultrafiltration is only somewhat dependent upon the charge of the particle and is much more concerned with the size of the particle. The mass transfer equations for UF are similar to those for MF.

Reverse Osmosis (RO)

Reverse osmosis (RO), also known as hyperfiltration, is the finest filtration known. This process will allow the removal of particles as small as ions from a solution. Reverse osmosis is used to purify water and remove salts and other impurities in order to improve the color, taste, or properties of the fluid. It can be used to purify wastewater streams that need additional treatment to remove water, which will pass through the reverse osmosis membrane, while rejecting other ions and colloids from passing. The most common use for reverse osmosis is in purifying water. It is used to produce water that meets the most demanding specifications that are currently in place. Reverse osmosis uses a membrane that is semipermeable, allowing the fluid that is being purified to pass through it, while rejecting the contaminants that remain. Most reverse osmosis technology uses a process known as cross-flow to allow the membrane to continually clean itself. As some of the fluid passes through the membrane, the rest continues downstream, sweeping the rejected species away from the membrane. The process of reverse osmosis requires a driving force to push the fluid through the membrane, and the most common force is pressure from a pump. A reverse osmosis process involves pressures 5-10 times higher than those used in ultrafiltration. As the concentration of the fluid being rejected increases, the driving force required to continue concentrating the fluid increases. Reverse osmosis is capable of rejecting bacteria, salts, sugars, proteins, particles, fats, and other constituents that have a molecular weight of greater than 0.15-0.25 kDa. The separation of ions with reverse osmosis is aided by charged particles. This means that dissolved ions that carry a charge, such as salts, are more likely to be rejected by the membrane than those that are not charged, such as organics. The larger the charge and the larger the particle, the more likely it will be rejected. The transport mechanism of RO as well as of nanofiltration is now believed to be the solution diffusion mechanism. The evaluation of RO performance can be conducted with Equations 3.21-3.24.

Nanofiltration (NF)

Nanofiltration (NF) is a form of filtration that uses membranes to preferentially separate different fluids or ions. Nanofiltration is not as fine a filtration process as reverse osmosis, but it also does not require the same energy to perform the separation. Nanofiltration also uses a membrane that is partially permeable to perform the separation, but the pores of the membrane are typically much larger than the membrane pores that are used in reverse osmosis. Nanofiltration is most commonly used to separate a solution that has a mixture of some desirable components and some that are not desirable. An example of this is the concentration of corn syrup. The nanofiltration membrane will allow the water to pass through the membrane while holding the sugar back, concentrating the solution. As the concentration of the fluid being rejected increases, the driving force required to continue concentrating the fluid increases. Nanofiltra-tion is capable of concentrating sugars, divalent salts, bacteria, proteins, particles, fats, and other constituents that have a molecular weight greater than 1 kDa. Nanofiltration, like reverse osmosis, is affected by the charge of the particles being rejected. Thus, particles with larger charges are more likely to be rejected than others. The mass transport mechanism of NF and the membrane material used are quite comparable to those of RO. In some cases, NF is grouped into reverse osmosis processes.

Membrane separations by electrical potential difference: Electrodialysis (ED)

Electrodialysis (ED) is an electrically driven membrane separation process that is capable of separating, concentrating, and purifying selected ions from aqueous solutions (as well as some organic solvents). The process is based on the property of ion exchange membranes to selectively reject anions or cations. If membranes are more permeable to cations than to anions or vice versa, the concentration of ionic solutions increases or decreases, so that concentration or depletion of electrolyte solutions is possible. Because in electrodialysis only ionic species are transferred directly, removal of ionic species from nonionic products can be accomplished so that purification is possible. Electrodialysis reversal (EDR) is an electrodialysis process in which the polarity of the electrodes is reversed on a prescribed time cycle, thus reversing the direction of ion movement in a membrane stack. The advantage of EDR is that it mitigates some of the concentration polarization and membrane fouling problems (Davis, 1990). The largest application of ED is the production of potable water from brackish water. Electrodialysis can remove salts from food, dairy, and other products, as well as concentrate salts, acids, or bases. It also finds applications in wine and juice stabilization and in removing unwanted total dissolved solids that can build up in product streams (Lopez-Leiva, 1988; Davis, 1990).

Faraday's law supplies the basis to model ion transport and affirms that the total current in an electrolytic cell is equal to the sum of the electricity conveyed by each ion species (Equation 3.28):

where I is the current density, f the Faraday's constant, Q the flow rate, AQ the concentration difference, Ji the molar flux, ei the current efficiency, and Zi the valence of ion i. Concentration polarization also severely affects the current density and the diffusive flux (the current density) through the concentration gradient over the boundary layer for a univalent ionic solution (Z = 1) (Equation 3.29):

where D is the diffusivity; Cm and Q are concentrations at the membrane surface and in the bulk, respectively; 8c is the thickness of the concentration boundary layer; and tm and tbl are the transport numbers of the ion in

the membrane and in the solution, respectively. The transport number of the ion is defined as the following (Equation 3.30):

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