Crossflow filtration Techniques to remove organic and inorganic contaminants from water
Conventional vs. crossflow filtration9 Feed water (flow perpendicular to medium}"
Feedwater (flow parallel to medium)
Hydraulic pressure forces filtrate or permeate through ι membrane pores
Impurities Separation occurs at membrane surface
Filtrate
Rejected material •
0
/.ï
Permeate
Impurities are / " separated within filter medium
Concentrate flow carrying rejected material
Conventional filtration
Crossflow filtration
"Schematic representations Source: Osmonics, Inc.. ' 1984. A d a p t e d with permission.
Crossflow filtration (CFF) has be come a major weapon in the arsenal of industrial wastewater treatment and water treatment, purification, and dis infection. C F F t e c h n i q u e s have evolved to the extent that they are quite effective in the removal of many sus pended, colloidal, and dissolved mate rials from water. They consist of such technologies as reverse osmosis (RO) and ultrafiltration (UF), and recently they have been applied to certain types of microfiltration (MF). CFF differs from conventional parti cle filtration in that fluid flow is parallel to rather than perpendicular to the filter media. Hydraulic pressure is applied to force the fluid (the filtrate or permeate) through the pores of the medium, which is normally a semipermeable membrane made of a synthetic mate rial. The membrane is called a surface medium because separation takes place in one surface layer or plane. On the other hand, conventional flow-through filter media usually con sist of sand, activated carbon, poly mers, biological fixed films, and other packed-bed or fibrous materials. In conventional perpendicular-flow filtra 0013-936X/84/0916-0375A$01.50/0
tion, separation takes place through several layers of the media. Whereas MF can work in either the perpendicular-flow or the crossflow configuration, UF and RO are strictly crossflow methods. Also, UF and RO have one influent and two effluents— one that permeates the membrane, the other consisting of rejected material that flows back to the feed or concen trate. Ultrafiltration may be used to reject or selectively retain impurities such as organics or emulsified materials. Sepa ration or rejection occurs principally because the size of the particle rejected is larger than that of the membrane's pores—in other words, the particle is screened out. Ions and other materials such as sugars, which would cause os mosis, pass through the membrane. To reject ions and sugars, for exam ple, one must resort to RO, Rick Wilson of Osmonics, Inc. (Minnetonka, Minn.), told participants at a CFF seminar that Osmonics held at its headquarters in May. With this tech nique, the membrane repels ions that are not allowed to pass through its pores, even though the ions are nor
© 1984 American Chemical Society
mally smaller than the pores. As with UF, nonionic compounds are rejected by screening out those particles that are larger than the pores. Reverse osmosis was first shown to be a viable technology for desalting wa ter in 1958 by S. Sourirajan and S. Loeb, then at the University of Califor nia, Los Angeles. (Sourirajan is now with the National Research Council of Canada in Ottawa.) Since that time, other commercial applications, such as the concentration of valuable dissolved solids, including metals used for plat ing; BOD and contaminant removal; and preparation of water for eventual ultrapure water manufacture, have come into being. After RO, ultrapure water is "finished" by deionization or distillation. Counterpressure One job of crossflow filtration is to remove from water particles that are 5 μπι or less. The materials range in size from very fine suspended particles down to ions of dissolved solids. Larger particles are generally removed before CFF by settling or conventional filtration. Crossflow MF and UF will remove and concentrate very small par ticles, such as viruses and other patho gens and proteins. But these two methods will not re move those smaller dissolved compo nents—usually ionic ones or sugars— that would cause significant osmotic pressure. To reject such materials by RO, one must overcome osmotic pres sure, Π, with a counterpressure at least 100 psi in excess of Π to remove or reject undesired materials and force water (or other solvents) through the membrane. RO is the reverse of the natural process of osmosis by which a more dilute solution, driven by the dif ferential pressure, ΔΠ, permeates a membrane toward a more concentrated solution until concentrations on both sides of the membrane are equalized. Osmosis is an essential factor in the function of living systems. Environ. Sci. Technol, Vol. 18, No. 12. 1984
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Osmonics president D. Dean Spatz observed that "one can mathematically calculate how Π must be overcome, and other pertinent factors for RO much more easily than one can for MF and UF. For example, there are empiri cal equations by which one may esti mate not only ΔΠ, but also the concen tration of solutes left in the concentrate, the percentage of rejection of material back to the concentrate stream, and the percentage of rejection of material back to the concentrate itself (those solutes and suspended solids that do not go through the membrane). To obtain those figures for RO, one needs to know only the rate of feed flow and concentration, the concentrate flow, and the osmotic pressure to be over come. For the other two types of CFF, such factors have proven difficult if not impossible to calculate or even esti mate." For example, a 10,000-ppm sodium chloride solution has an osmotic pres sure of 125 psi. One can estimate what pressures, flows, and feed and concen trate concentrations would be needed to achieve a salt rejection rate of 9 5 % . If the solution consists of 10,000 ppm each of sodium and calcium chloride, the osmotic pressure will be 125 + 9 0 psi or 215 psi. One must then design the RO system to counteract these addi tive combined pressures. Other factors to be considered in the construction of an RO system are pH adjustments and feed water recycling paths necessary for optimum membrane permeation and salt rejection. Membrane systems for CFF can be configured in several ways. One is to construct the membrane system in the form of flat plates and frames. This configuration has been used extensively in Europe, but not in the U.S.; "they are not very cost-effective," according to Spatz. A tubular configuration, sometimes known as hollow fiber, is made and marketed by such companies as Dow and Du Pont in the U.S. and by Toyobo in Japan. These elements are widely used for water desalination. The world's largest project using such mem brane systems, with a 5.3-mgd capac ity, uses Du Pont permeators to reduce the salinity of Mediterranean seawater from about 39,000 ppm to < 5 0 0 ppm. It provides potable water for Malta, and was built by Polymetrics, Inc. (San Jose, Calif.). Osmonics developed and refined a way of winding membranes into a spi ral configuration. The company says that its membranes can now remove more than 95% of ions in feedwater and filter out particles as fine as 0.0005 ^m—3-4 orders of magnitude finer than those removable by MF. The firm 376A
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CFF work abroad At the 188th American Chemical Society National Meeting (Philadel phia, Pa.), the Division of Industrial and Engineering Chemistry heard about research and practical work under way in Canada, India, Israel, Italy, Japan, South Korea, and other countries. Topics discussed included UF treatment of paper plant waste waters and the development of a polyethersulfone UF membrane (Ja pan); RO membrane performance predictability (Canada); and the long-term future of RO technology in Israel, an arid country where water cleanup and reuse are essential. lists uses for its products in such di verse fields as toxic waste treatment, inorganic and organic contaminant re moval, foodstuff concentration, blood fractionation, and preparatory steps for ultrapure water manufacture. One im portant application is the removal and recovery of metals such as nickel for reuse in such industries as electroplat ing and metal finishing. In their various configurations, most membranes are made of cellulose ace tate. Other membranes are constructed from polyamide, polysulfone, and vari ous proprietary materials. Pore sizes Microfiltration membranes have pores that can range from 500 to 20,000 A (1 A = 1 0 - ' ° m o r 10" 4 /xm) in diameter. In microfiltration, the ma terials filtered out include bacteria, paint pigment, and dust, all of which are suspended materials. One problem common to all CFF systems is that membrane pore sizes cannot be made uniform. If they could, such systems would be much easier to operate and control. Because they can not, actual filtrations must be based on an average pore size. Moreover, after a period of membrane use, clogging or fouling of many of the pores—espe cially the larger ones—occurs, so CFF systems should be equipped with facili ties for backflushing and chemically cleaning the membranes. Then some provision for proper disposal or reuse of rejected materials must be made. Ultrafiltration membrane pores range from 15 A to 500 A in diameter. UF may be employed to remove carbon black, viruses and other pathogens, to bacco smoke, silica and other particles in colloidal form, and protein. UF re quires lower pressures than does RO, "because we don't need to overcome much osmotic pressure," said Wilson. "Normally, organic molecules of molecular weight less than 1000 will
pass through, but even for larger ones, shape and size could be factors. For instance, a long, thin molecule with a molecular weight of 2000 could pass through the pores, while a bulbousshaped one weighing 800 might not. A charge on an organic particle can in crease its chances of being rejected," Wilson added. Generally, the pores in RO mem branes are 4-11 A in diameter. Al though very low molecular weight organics ( < 100-200) might still pass through, higher MW ones do not; nei ther do ions that are charged and thus rejected from the membrane surface. RO operating conditions The ideal operating temperature for an RO system is considered to be be tween 25 °C and 30 °C. When temper atures are lower, increased kinematic viscosity inhibits the process rate. Higher temperatures reduce viscosity, thereby aiding permeation; but because reaction rates are also increased, oxida tion or hydrolysis could degrade or even destroy the membrane much more rapidly. Nevertheless, RO has been successfully carried out at temperatures as high as 60 °C, and commercial in stallations that will operate at up to 85 °C are expected shortly. Theoretically, the higher the pressure applied to an RO system, the higher the permeation rate should be. One prob lem with this is that at higher pressures, membrane compaction and irreversible fouling inhibit permeation. The maxi mum pressure a system can handle is estimated at 1000 psig, because of the limitations of membrane and associated hardware. The CFF industry is working on the development of stronger membrane and hardware materials able to withstand more rigorous conditions, because higher pressures and temperatures en hance the efficiency of an RO process. Other research objectives include de vising membranes that can withstand a greater variety of chemical environ ments and finding ways to achieve more uniform pore size for MF and UF, as well as for RO media. Business volume "There are at least 30 companies in the CFF business, plus some garage shop assemblers," according to Spatz. "I believe that the total market is $70$100 million," he adds. Besides Du Pont, some other companies in the the field or closely related businesses in clude Abcor, Aqua-Media, UOP, Culligan, Hydronautics, Desalination Sys tems, Gelman, Millipore, and Romicon (Rohm & Haas). —Julian Josephson
