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Anode Plate

Oilute Product Water

Concentrated Brine Water

Anode Waste

Figure 2. Electrodialysis process diagram.

In addition to the membrane stacks in electrodialysis units, various supporting equipment is essential. This includes pumps for circulation of concentrating and diluting flows; flushing streams for cathode and anode plates; injection systems for pH control; pressure concentration, pH alarms, and control systems and backflushing controls; feed strainers and filters; and grounding systems. Because of the high pH of the cathode stream, substances, such as carbonates and hydroxides could precipitate on the cathode surface and adjoining membrane; often sulfuric acid is injected to maintain the stream at pH of 2 or less. Also, recirculating concentrate requires an acid addition to yield a low pH for stability along with additional substances such as sodium hexametaphosphate. A key point to remember is that the separation is achieved by removing ions by passing them through a semipermeable membrane. The electric field applied across the membrane transports only ions. As noted the application of this technology is to desalting brackish water, to removing TDS from water and to the removal of certain heavy metals. The issue of concentration polarization results in an increase of the resistance of flow of ions across the membrane. The current must therefore be increased to overcome this resistance.

Brine Product

Brine Product

Figure 3. Flow through electrodialysis stack.

Suspended materials and macromolecules can be separated from a waste stream using a membrane and pressure differential, called Ultrafiltration. This method uses a lower pressure differential than reverse osmosis and doesn't rely on overcoming osmotic effects. It is useful for dilute solutions of large polymerized macromolecules where the separation is roughly proportional to the pore size in the membrane selected.

Ultrafiltration membranes are commercially fabricated in sheet, capillary and tubular forms. The liquid to be filtered is forced into the assemblage and dilute permeate passes perpendicularly through the membrane while concentrate passes out the end of the media. This technology is useful for the recovery and recycle of suspended solids and macromolecules. Excellent results have been achieved in textile finishing applications and other situations where neither entrained solids that could clog the filter nor dissolved ions that would pass through are present. Membrane life can be affected by temperature, pH, and fouling.

Ultrafiltration equipment are combined with other unit operations. The unique combination of unit operations depends on the wastewater characteristics and desired effluent quality, and cost considerations.

Like normal filtration, with ultrafiltration (UF), a feed emulsion is introduced into and pumped through a membrane unit; water and some dissolved low molecular weight materials pass through the membrane under an applied hydrostatic pressure. In contrast to ordinary filtration however, there is no build-up of retained materials on the membrane filter.

A variety of synthetic polymers, including polycarbonate resins, substituted olefins, and polyelectrolyte complexes, are employed as ultrafiltration membranes. Many of these membranes can be handled dry, have superior organic solvent resistance, and are less sensitive to temperature and pH than cellulose acetate, which is widely used in RO systems.

In UF, molecular weight (MW) cutoff is used as a measure of rejection. However, shape, size, and flexibility are also important parameters. For a given molecular weight, more rigid molecules are better rejected than flexible ones. Ionic strength and pH often help determine the shape and rigidness of large molecules. Operating temperatures for membranes can be correlated generally with molecular weight cutoff. For example, maximum operating temperatures for membranes with 5,000 to 10,000 MW cutoffs are about 65° C, and for a 50,000 to 80,000 MW cutoff, maximum operating temperatures are in the range of 50° C.

The largest industrial use of ultrafiltration is the recovery of paint from water-soluble coat bases (primers) applied by the wet electrodeposition process (electrocoating) in auto and appliance factories. Many installations of this type are operating around the world. The recovery of proteins in cheese whey (a waste from cheese processing) for dairy applications is the second largest application, where a

Ultrafiltration is a preferred alternative to the conventional systems of chemical flocculation and coagulation followed by dissolved air flotation. Ultrafiltration provides lower capital equipment, installation, and operating costs.

market for protein can be found (for example, feeding cattle and farm animals). Energy consumption at an installation processing 500,000 pounds per day of whey would be 0.1 kWh per pound of product. Another large-scale application is the concentration of waste-oil emulsions from machine shops, which are produced in association with cooling, lubrication, machining, rolling heavy metal operations, and so on. Ultrafiltration of corrosive fluids such as concentrated acids and ester solution is also an important application. The chemical inertness and stability of ultrafilters make them particularly useful in the cleaning of these corrosive solutions. Uses include separation of colloids and emulsions, and recovery of textile sizing chemicals. Biologically active particles and fractions may also be filtered from fluids using ultrafilters. This process is used extensively by beer and wine manufacturers to provide cold stabilization and sterilization of their products. It is also used in water pollution analysis to concentrate organisms from water samples. Food concentration applications can be applied to processing milk, egg white, animal blood, animal tissue, gelatin and glue, fish protein, vegetable extracts, juices and beverages, pectin solutions, sugar, starch, single-cell proteins, and enzymes.

Figure 5 conceptually illustrates how ultrafiltration works. Water and some dissolved low molecular weight materials pass through the membrane under an applied hydrostatic pressure. Emulsified oil droplets and suspended particles are retained, concentrated, and removed continuously as a fluid concentrate. The pore structure of the membrane acts as a filter, passing small solutes such as salts, while retaining larger emulsified and suspended matter. The pores of ultrafiltration membranes are much smaller than the particles rejected, and particles cannot enter the membrane structure. As a result, the pores cannot become plugged. Pore structure and size (less than 0.005 microns) of ultrafiltration membranes are quite different from those of ordinary filters in which pore plugging results in drastically reduced filtration rates and requires frequent backflushing or some other regeneration step. In addition to pore size, another important consideration is the membrane capacity. This is termed/ii« and it is the volume of water permeated per unit membrane area per unit time. The standard units are gallons per day per square foot (gpd/ft2) or cubic meters per day per square meter (mVday/m2).

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Figure 5. Ultrafiltration basics.

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Q Oil Pirticlei

Figure 5. Ultrafiltration basics.

Because membrane equipment, capital costs, and operating costs increase with the membrane area required, it is highly desirable to maximize membrane flux.

Ultrafiltration utilizes membrane filters with small pore sizes ranging from 0.015^ to 8fi in order to collect small particles, to separate small particle sizes, or to obtain particle-free solutions for a variety of applications. Membrane filters are characterized by a smallness and uniformity of pore size difficult to achieve with cellulosic filters. They are further characterized by thinness, strength, flexibility, low absorption and adsorption, and a flat surface texture. These properties are useful for a variety of analytical procedures. In the analytical laboratory, ultrafiltration is especially useful for gravimetric analysis, optical microscopy, and X-ray fluorescence studies.

All particles larger than the actual pore size of a membrane filter are captured by filtration on the membrane surface. This absolute surface retention makes it possible to determine the amount and type of particles in either liquids or gases-quantitatively by weight or qualitatively by analysis. Since there are no tortuous paths in the membrane to entrap particle sizes smaller than the pore size, particles can be separated into various size ranges by serial filtration through membranes with successively smaller pore sizes. Figure 6 shows pore size in relation to commonly known particle sizes. Fluids and gases may be cleaned by passing them through a membrane filter with a pore size small enough to prevent passage of contaminants. This capability is especially useful in a variety of process industries which require cleaning or sterilization of fluids and gases.

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Figure 6. Range of common particle sizes (diameter) over range of UF pore size.

The retention efficiency of membranes is dependent on particle size and concentration, pore size and length, porosity, and flow rate. Large particles that are smaller than the pore size have sufficient inertial mass to be captured by inertial impaction. In liquids the same mechanisms are at work. Increased velocity, however, diminishes the effects of inertial impaction and diffusion. With interception being the primary retention mechanism, conditions are more favorable for fractionating particles in liquid suspension.

In contrast to reverse osmosis, where cellulose acetate has occupied a dominant position, a variety of synthetic polymers has been employed for ultrafiltration membranes. Many of these membranes can be handled dry, have superior organic solvent resistance, and are less sensitive to temperature and pH than cellulose acetate. Polycarbonate resins, substituted olefins, and polyelectrolyte complexes have been employed among other polymers to form ultrafiltration membranes.

Preparation details for most of the membranes are proprietary. As noted earlier, molecular weight cutoff is used as a measure of rejection, however, shape, size, and flexibility are also important parameters. For a given molecular weight, more rigid molecules are better rejected than flexible ones. Ionic strength and pH often help determine the shape and rigidness of large molecules. Membrane lifetimes are usually two years or more for treating clean streams (water processing), but are drastically reduced when treating comparatively dirty streams (e.g., oily emulsions). Membrane guarantees by manufacturers are determined only after pilot work is done on the particular stream in question. In some cases as little as a 90-day guarantee may be given for oil/water waste applications. There are in fact both current and many emerging uses of ultrafilters in the areas of biological research, processing sterile fluids, air and water pollution analysis, and recovery of corrosive or noncorrosive chemicals. The technology is applicable to dewatering some sludges, but this use is highly dependent on the particular sludge itself. There are no commercial uses of UF for sludge dewatering at this time, but several sources have been found which claim that this represents a possible near-future application. Pollution of water supplies within the food industry is a significant problem, since many food wastes possess

extremely high biochemical oxygen demand (BOD) requirement. In the potato starch industry, for example, waste effluent containing valuable proteins, free amino acids, organic acids, and sugars can be processed by ultrafiltration. Reclamation of these materials, which are highly resistant to biodégradation, are providing an economic solution to this waste removal problem. For the concentration of juices and beverages, RO is preferable to evaporation due to lower operating costs and no degradation of the product. Processing by RO retains more flavor components than does heated, vacuum pan concentration. Since ultrafiltration does not retain the low molecular weight flavor components and some sugar, UF is employed as a complement to RO. A two-stage process may be used in which the first stage, UF, allows the passage of sugars and other low molecular weight compounds. This permeate is then dewatered by RO and recycled back to the main stream. High juice concentrations are possible in this manner because the UF removes the colloidal and suspended solids which would foul the RO, and helps relieve some of the high hydraulic pressure due to high osmotic pressure of the juice. In a process as shown in Figure 7, a citrus press liquor, or multiphase suspension, is ultrafiltered following a coarse prefiltration. The resultant clear permeate is processed through ion exchange and granular activated carbon adsorption units to remove low molecular weight contaminants and inorganic salts. The product is a natural citrus sugar solution suitable for reuse. The concentrated suspended solids are used in making animal feed. Pectins are a family of complex carbohydrates which are used to form gels with sugar and acid in the production of jellies, preserves, and other confections from fruit juices. The recovery of starch and other high molecular weight compounds from waste effluents is an important application for UF. The output from a 30-ton-per-day starch plant would be about 432,000 gpd with a solids content of 0.5 percent to 1.0 percent, and 9,000 to 14,000 mg/Liter COD.

Press Liquor Prefiltration Ultrafiltration

Press Liquor Prefiltration Ultrafiltration

Figure 7. Process flow scheme for sugar recovery from citrus press liquors.

Treatment systems which can both reduce the strength of this waste and recover valuable by-products, such as proteins, are an obvious advantage to this industry. Heat and acid coagulation, distillation, and freezing techniques are more costly and less efficient than UF in protein recovery. Reverse osmosis is also a competing process, but protein recovery by UF would lead to a somewhat higher purity. In the production of single-cell proteins as a food source, UF has several applications. For harvesting cells, UF can replace centrifugation in some applications since the efficiency of centrifugation decreases rapidly with particle size. Ultrafiltration is also well suited for recovering and concentrating the metabolic products of fermentation (enzymes, for example). In a related application, UF is also able to concentrate and desalt protein products, being more efficient than dialysis for this purpose. Moreover, a UF membrane module may be coupled with a fermenter so that toxic metabolites can be continuously removed from the system as fresh substrate is introduced. This permits the growth limitations of a batch fermenter to be relieved and permits a substantial increase in productivity. A membrane

IMPORTANT APPLICATIONS

«p Recovery of paint from watersoluble coat bases (primers) applied by the wet electrodeposition process (electrocoating) in auto and appliance factories.

«p Recovery of proteins in cheese whey (a waste from cheese processing) for dairy applications. This is done if a market for protein can be found, in particular for feeding cattle and farm animals. In cheese whey processing, a typical unit might process 500,000 pounds a day of whey for 300 days a year.

*sr The concentration of waste-oil emulsions from machine shops, which are produced in association with cooling, lubricating, machining, and heavy metal rolling operations. The separation of the oil from the water works well with stable emulsions, but with unstable emulsions the oil will clog the filter.

«p Biologically active particles and fractions may be filtered from fluids using ultrafilters. This process is used extensively by beer and wine manufacturers to provide cold stabilization and sterilization of their products.

«p Water pollution analysis to concentrate organisms from water samples.

Filtering cells and cell fractions from fluid media. These particles, after concentration by filtration, may be examined through subsequent quantitative or qualitative analysis. The filtration techniques also have applications in fields related to immunology and implantation of tissues as well as in cytological evaluation of cerebrospinal, fluid.

enzymatic reactor is similar to a membrane fermenter with the exception that no microorganisms are present. Instead, enzyme-catalyzed reactions take place and reuse of the enzyme is simplified. That is, purification problems and enzyme removal from end products can be eliminated.

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

HOW, WHERE AND WHY MEMBRANES FOUL

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.

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

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