FEATURE
The Promis
MARK R. WIESNER AND SHANKARARAMAN CHELLAM
he invention of synthetic membranes in the middle of tiiis century is arguably the most significant development in water treatment since rudimentary procedures for doing this by carbon filtration, UV disinfection, coagulation, and filtration were first described in Sanskrit texts roughly 5000 years ago. Today, nearly 50 years since creation of synthetic polymers and the asymmetric membrane, novel developments and refinements in membrane technology continue to be active themes of research. These advances are rapidly enlarging our capabilities to restructure production processes, protect the environment and public health, and provide new technologies for sustainable growth. Membrane technologies play an increasingly important role as unit operations for resource recovery, pollution prevention, and energy production, as well as environmental monitoring and quality control. They are also key component technologies of fuel cells and bioseparation applications. The membrane technologies market has risen from $363 million in 1987 to more than $1 billion in 1997. Approximately 40% of membrane sales is destined for water and wastewater treatment applications (i); food and beverage processing combined with pharmaceuticals and medical applications account for another 40% of sales; and the use of membranes in chemical and industrial gas production is growing. This broad range of applications and projected sales growth to $1.5 billion by 2002 (2) suggests that membrane technologies are now well accepted and cost-effective, conferring unique advantages over previous separation processes. Driving expansion in environmental applications are improvements in the underlying technology, a more competitive market, a more demanding regulatory environment, a broader range of membrane processes, and the availability of new materials from which they can be fabricated. Closely linked with these developments is a growing knowledge base of operating conditions and design practices that is in-
T
3 6 0 A • SEPTEMBER 1, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
© 1999 American Chemical Society
e of Membrane Technology A n expanded understanding of membrane technology is fostering new environmental applications.
creasing membrane performance and decreasing uncertainties at bidding and design stages. Significant progress has been made in understanding fouling of pressure-driven membranes in the treatment of liquid streams. Foulants found in water, wastewater, and industrial and hazardous wastes include colloidal materials, scale-forming solutes, bacteria, and dissolved or macromolecular organic matter. At the heart of membrane technology research is the formulation of the membranes themselves. The scope of applications is being enlarged, and costs are being reduced through the development of membranes that have reduced thicknesses, improved stability at pH and temperature extremes, and greater compatibility with oxidants such as chlorine. Although much current research is directed toward producing the ideal membrane and membrane system, little effort is being made at standardization, which is needed to increase interchangeability among membranes and membrane systems produced by different manufacturers. Given the scale of most current projects, vendors have little incentive to produce interchangeable units and systems. At some point, protocols for membrane module system design will have to be established, thereby allowing greaterflexibilityin choosing membranes over the lifetime of a water treatment facility.
Economies of scale Studies (3, 4) performed in 1993 and 1994 indicated that life-cycle costs of new, relatively small water treatment facilities uess than 20,uuu m /day) using pressure-driven membrane processes should be comparable with or less than those of new facilities using conventional processes. This appeared to be true in unit operations required for particle removal or reduction of dissolved organic materials. Subsequent design experience over the last several years has supported these predictions, and as the membrane market has grown, the scale of membrane facilities has become more ambitious. The first ultrafiltration (UF) facility for potable water treatment, inaugurated in 1988 at Aubergenville, France, had a design capacity of 160 m3/day. Today, facili-
ties exceeding 100,000 m3/day are being planned, although design and operating experience at this scale is lacking, and accurate estimates of the economies of scale and facility costs are unavailable. Moreover, estimates of incurred regulatory implementation costs, based on the use of these technologies, are subject to considerable uncertainty. The cost of membrane facilities can be expected to scale less than linearly with capacity. The leastcost design of a larger facility is unlikely to be a simple collection of smaller-capacity assemblages, as was speculated 10 years ago. Opportunities for spreading out the cost of some treatment process components increase with scale, and economies of scale for individual components may become more important in determining overall system costs expressed per unit of production over 3. facility's lifetime. As facility scale increases, off-the-shelf solutions proposed by membrane vendors will probably be replaced with more tailored designs in which larger arrays of membranes share pumps, monitoring equipment, and cleaning facilities. A move away from vendor-specific designs toward approaches that enable facilities to interchange membranes from multiple suppliers and determine which membranes are optimally suited for a given application should increase membrane market competitiveness. It is helpful to understand what kinds of economies of scale should be anticipated as the industry moves in these directions. Available information suggests capital costs for current membrane systems have economies of scale characterized by exponents between 0.4 and 0.8—reported economies of scale depend on the data (5-7) examined. Lower values derive from fits to cost estimates for smaller plants; upper values correspond to larger plants currentiy in operation. Considering membrane system capital costs as a function of components used, overall economies of scale will depend on the cost-capacity relationships for each component and the manner of system assembly. Inspection of cost curves for membrane components estimated from nonmembrane applications suggests that significant economies of scale may be realized
TABLE 1
Capital costs of membrane system components Capital cost correlations have been derived for various membrane system components. Cost estimates have economies of scale characterized by exponents ranging from 0.39 to 0.71. The notations are C (capital cost of treatment facility); Q (gallons per day (gpd) of facility throughput, except for "Pumps," where Q means gallons per minute of pump throughput); and h (foot-pounds per square inch). Correlation
Item
Pumps Pipe and valves Electrical and instrumentation Housing Excavation and site work Concrete Labor (installation)
C C C C C C C
= = = = = = =
96.75(Q * h)° 17.64(a) 0 4 2 8.15(Q) 0 - 66 628.09(Q) 0 - 32 52.16(a) 0 - 39 48.84(a) 0 - 44 4.14(a) 0 - 71
3 6 2 A • SEPTEMBER 1, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
Units 39
C = dollars; O = gal/min; C = dollars; 0 = gpd C = dollars; 0 = gpd C = dollars; 0 = gpd C = dollars; 0 = gpd C = dollars; 0 = gpd C = dollars; C = gpd
References
(5) (6,7) (6,7) (6,7) (6,7) (6,7) (6,7)
as membrane facility designs are adapted to larger scales (see Table 1 on previous page). Pumps, pipes, and valves, which together account for approximately 50% of membrane facility capital costs in current designs, follow cost-capacity relationships with exponents near 0.4. This implies a substantial opportunity for improving the economies of scale of membrane facilities, thereby extending the range of capacities for which membranes are cost-effective. Operating costs strongly influence the overall cost-effectiveness of membrane systems. Periodic replacement of membranes is an important component of the operating cost component. The lifecycle cost of a membrane facility is significantly impacted by membrane replacement frequency, or membrane life. Manufacturers usually provide a warranty for membranes for a designated period, typicallyfiveyears. Experience suggests that the membrane life for a properly operated facility may easily exceed 10 years. Sensitivity analyses on design and operating parameters for membrane systems suggests that costs are also quite sensitive to permeate flux— permeate flow per area of membrane. Accurate estimates of flux are necessary to compare membrane application cost with the cost of other technologies. Presenuy, site-specific permeate fluxes can only be obtained through pilot studies. Considerable effort has been invested in understanding processes that lead to permeate flux decline, or fouling, as membranes are operated over time. One research goal is to minimize fouling by improving membrane formulations, module geometries, and operating conditions. Development of predictive models of permeate flux in pressure-driven membrane systems may eventually allow accurate estimates of membrane performance to be calculated from knowledge of raw water quality.
Membrane fouling intricacies The transport of foulants to membranes and their accumulation on and within membranes are affected by the fluid dynamics of the membrane system, membrane chemistry, and the nature of the foulants. Some systems are designed to operate with the feed flow introduced tangential to the membrane to reduce the accumulation of materials on the membrane. Observed fouling by colloidal and larger particulate materials in these crossflow units is considerably less than that predicted from conventional solutions to the convective-diffusion equation applied to porous ducts. Additional sources of particle transport away from the membrane were proposed to explain this so-called flux paradox; these mechanisms include inertial lift on the particles, shear-induced diffusivity originating from particleparticle interactions, and effects of particlemembrane surface chemistries. Individually, no proposed mechanism fully explains higher-than-predicted permeate fluxes observed with various colloidal suspensions. Considerable variation in permeate flux is predicted with a minimum in the back transport of particles from the membrane surface (and thus a maximum po-
tential for fouling) for particles near 0.1 pm in diameter (8) when these mechanisms are considered together as functions of colloid size. Only indirect experimental evidence for this hypothesized minimum in back-transport, as interpreted from permeate flux data, has been available. Recently, however, the first direct experimental confirmation of the existence of a minimum in backtransport based on measurements of particle residence time distributions was reported (9). This work suggests that fouling should be gready reduced when membranes are operated at a permeate flux below a critical level determined by the magnitude of the sum of back-transport mechanisms. In practice, for low membrane fouling rates in constant flux, crossflow microfiltration is observed when the permeate flux is maintained below a threshold value. This critical flux has also been identified in surface water nanofiltration, in which an exponential relationship between fouling rates and permeate flux has been reported. In practical terms, this means that for a given design capacity, the balance beBenefits to tween decreased membrane area and increased chemical cleansociety extend ing frequency with increasing well beyond an permeate flux will determine the optimum design criteria. increased Consideration of multiple particle back-transport mechapotential for nisms largely resolves the flux paradox. However, highly accutapping water rate predictions of permeate flux supplies. based on first principles remain elusive, largely because of the failure to predict the specific resistance created by a layer of deposited material. It appears that during crossflow filtration of colloidal suspensions the cake-specific resistance controls the steadystate specific flux. The particle size distribution of the feed stream and module hydrodynamics determine the cake particle size distribution and specific resistance. Current mechanistic approaches to calculating specific flux in colloidal crossflow filtration have ignored the influence of cake morphology on the effective permeability of the combined membranefoulant layer system. Increased accumulation of fines (when supramicron particles are filtered) coupled to shear-induced rearrangement of particles in the cake can result in substantially reduced cake permeability, compared with cakes formed using a dead-end protocol. This phenomenon may also partly explain poor correlations obtained between membrane fouling and currently employed fouling indices such as the silt density index (SDI) and modified fouling index. There is strong evidence for a critical transmembrane pressure that should not be exceeded in operating membrane systems. Beyond this critical pressure, colloidal cakes are compressed, and cake—membrane attachment is enhanced. Many commercial UF and microfiltration systems are operated without a crossflow in a socalled dead-end filtration mode. In these cases, the SEPTEMBER 1, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 3 6 3 A
FIGURE 1
Streaming potential response In experiments with ceramic aluminosilicate membranes, adsorption of dissolved organic matter (tannic acid) within membrane pores resulted in a more negative charge within the membrane matrix in comparison with deionized water. Subsequent addition of electrolytes along with the tannic acid instantaneously reduced the magnitude of the charge. In particular, the presence of calcium in the feed stream reduced the magnitude of membrane charge to nearly zero.
Source: Faten Nazzal, Rice University, Houston, Tex.
kinetics of permeate flux decline appear to be primarily dependent on the transmembrane pressure and the efficacy of the periodic backwashing procedure used. Little research has been performed to identify the factors affecting foulant detachment in these systems during backwashing. The transmembrane pressure, time between backflushing, and concentration near the membrane are all affected by the recovery of a dead-end filtration system. When these systems are operated at a constant flux below a threshold value of recovery, low fouling rates are observed, and they do not decrease significantly by increasing backwashing frequency. However, beyond this critical recovery point, fouling rates are strongly dependent on the backwashing frequency (and therefore recovery). Increased penetration of foulants into the membrane matrix, irreversible (nonbackwashable) association of organics, and biofoulants with the membrane material beyond the critical recovery point may result in increased fouling rates and decreased backwash effectiveness. The critical recovery value is dependent on membrane material and geometry, feed water composition, and backwashing procedure.
A complicated picture The adsorption of naturally occurring organic matter (NOM) to membrane surfaces is frequently cited as the primary cause of chronic fouling of membranes used for both desalting and as pretreatment for membrane desalting in water treatment and wastewater recovery. The nature of this fouling is poorly understood and stands as a key impediment to developing improved methods for membrane cleaning. 3 6 4 A • SEPTEMBER 1, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
The characteristics of organic materials that determine their relative propensity to foul membranes appear to include their affinity for the membrane, molecular weight, functionaitty, and conformation. For example, hydrophobic membranes have been observed to be more prone to fouling by NOM than are hydrophilic membranes. Usually, greater charge density on the membrane surface is associated with greater membrane hydrophilicity. Polysulfone, cellulose acetate, ceramic, and thinfilm composite membranes used for water treatment and wastewater recovery typically carry some degree of negative surface charge. Conditions that render dissolved organic materials more hydrophobic should augment adsorptive fouling of membranes. Calcium ions and protons TTI3V associate with functional NOM molecules rendering them hydrophobic Although solution chemistry may affect the hydrophobicity of NOM, the formation of salt bridges and direct precipitation of NOM may also reduce its stability in water. These trends have been observed in the fouling of membranes by proteins. These laboratory observations support field experience wherein hard surface water has been observed to exhibit a greater tendency to foul reverse osmosis membranes than soft surface water, despite the fact that both waters produced the same SDL The transport of aqueous solutions through membranes made up of materials with charged interfaces may generate secondary driving forces that link the flows of solvent, solute, and electric current across the porous medium. Specifically, application of a pressure gradient to induce the flow of ion-bearing water through membranes with charged pores gives rise to electrokinetic phenomena, such as streaming potential and electro-osmotic backflow. The extent to which electrokinetic phenomena affect the flow of solvent or solutes through a capillary depends on the thickness of the diffuse double layer, K"1 (K is the Debye parameter), capillary radius, and surface potential of the membrane pore. A reduction in apparent permeate flux as a result of interactions between fluid in membrane pores and the migrating ions is often expressed as an increase in apparent viscosity of fluid within the pore. Although such electroviscous effects can play a significant role in determining permeate flux, measurements of the underlying electrokinetic phenomena can also be used to probe changes in membrane surface chemistry as a function of the solution chemistry and the fouling characteristics of a given feed stream In rxiciny cases membrane surfaces respond rapidly (within seconds) to the presence of dissolved organic matter (see Figure 1 above)
Analytical tools Mark Clark and coworkers [10) at the University of Illinois-Urbana examined changes in membrane surface chemistry resulting from adsorption of dissolved organic matter and subsequent fouling. They considered the adsorption of well-characterized humic and fulvic acids on ultrafiltration membranes and then characterized the fouled membranes in
terms of hydrophobicity as determined by contact angle measurements; pore zeta potential, computed from streaming potential measurements such as those shown in Figure 1; and surface chemical composition obtained from X-ray photoelectron spectroscopy. Results indicate that the adsorption capacity is smaller for fuMc acid than for humic acid and that fulvic acid competes with calcium phosphate compounds for adsorption sites on the membrane. Low pH and, in some cases, high calcium concentrations increased the adsorption of humic substances on the membranes. The adsorption capacity decreased with decreasing (more negative) zeta potential and with increasing hydrophilicity. Adsorption of humic substances on the UF membranes rendered them more hydrophilic, and the apparent pore charge became less negative, suggesting that pore adsorption sites are preferentiallyfilledbefore other sites on the membrane surface. Earlier work on organic fouling of membranes done by Joel Mallevialle and colleagues at the research labs of the Lyonnaise des Eaux in Le Pecq, France, attempted to identify the characteristics of NOM that might signal a particularly strong tendency to foul membranes. Using pyrolysis/GC/MS, they looked at classes of organic matter fragments deposited on membranes during the treatment of surface waters. On the basis of large quantities of materials in the polysaccharide and polyhydroxyaromatic classes deposited on UF membranes, they concluded that organic matter rich in these fractions should be more prone to foul membranes. We are currently investigating fouling of membranes by dissolved organic matter following the opposite approach. Rather than fouling the membrane with true NOM and then observing the pyrolysis fragments of deposited organic matter on the fouled membrane, we begin with individual compounds of similar functionality and composition to the pyrolysis fragments, observe their fouling behavior, and build a mixture of organic compounds with similar fouling properties to a solution of NOM. The fouled membranes are analyzed by atomic force microscopy and infrared spectroscopy with attenuated total reflection to determine changes in membrane surfaces produced by foulants individually and in mixtures and changes in the concentration of the foulants with depth into the membrane surface. Compounds (such as proteins) that are not as abundant on fouled membranes compared with, for example, polysaccharides, may play a critical role in membrane fouling. In addition to its direct effect on membrane fouling, organic matter in natural waters plays a determinant role in the cohesion of colloids deposited on the membrane. Analysis of organic foulants in natural waters and their relative concentrations in the deposited cake suggests that polyphenolic compounds, proteins, and polysaccharides bind together colloids that deposit on the membrane and may cement the cake to the membrane surface. Adsorptive fouling and stabilization of the cake (or formation of a gel layer) by organic matter in water im-
FIGURE 2 Modeling membrane performance Computational fluid dynamics simulation of flow near a rotating membrane disk, visible as a vertical blue band on the right. Red indicates a region of high circulation above the membrane tip near the outer wall of the vessel. (Simulation performed by Christophe Serra, Rice University, Houston, Tex.).
pair the efficiency of purely hydraulic cleaning methods such as backflushing, fast pulsing, or crossflow reversal. As a result, reagents used for chemical cleaning of the membrane must be able to efficiently break up or redissolve organic compounds. Module geometry research Module geometry plays a large role in determining the application, type of feed stream to be treated, and pretreatment requirements. Conventional membrane geometries can be poorly suited to the treatment of special feed streams such as sludges, suspensions of delicate microorganisms, or cellular material, as well as slurries of adsorbants such as powdered activated carbon. In these instances, the use of innovative geometries has been explored with the goal of maintaining membrane performance, improving separation, and increasing yields. Georges Belfort and coworkers have conducted extensive work on the fluid dynamics in membranes having a unique spiral tube geometry that promotes Dean vortices (ii). These secondary flows enhance the transport of materials from the membrane surface that might otherwise foul it. Similarly, rotating annular membranes that promote Taylor vortices that enhance transport from the membrane have been developed with applications primarily in bioseparations. Recent work on the fluid mechanics of rotating membrane disks has provided scale-up criteria for applications oriented toward treatment of streams with a high solid content such as sludges and other residuals. The rotating disk membrane filter is a high shear rotary crossflow device composed of hollow membrane-covered disks stacked on a hollow shaft, which turns inside a pressurized vessel. This liquid separation technology provides for continuous threeway separation of water, materials lighter than water (for example, oils), and heavier-than-water SEPTEMBER 1, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 3 6 5 A
FIGURE 3
Atomic force microscopy image of an alumoxanederived membrane Images obtained by atomic force microscopy together with BET adsorption isotherms and permeability data indicate thatthe alumoxane synthesis pathway can be used to readily produce membranes with pore sizes in the range of 10-20 nm.
Source: Images produced by Diane Bailey, Rice University, Houston, Tex.
materials (for example, suspended solids). Water permeates into the membrane disks and exits through the hollow shaft under pressure within the vessel to the outside of the membrane disks. Disk rotation produces a shear at the membrane surface that scours deposited materials from the membrane, thereby maintaining low resistance to flow through it; centrifugal force within the membrane disk creates a back pressure that may reduce the efficiency of this process. Studies of the fluid mechanics of this system using computational fluid dynamics and laboratory observations of particle transport and deposition have been undertaken to better understand the process and optimize design (see Figure 2).
Moving forward Membrane formulations are being developed that are tailored to avoid fouling by a given feed stream or are adapted to provide more selective transport of a targeted species—as in the use of the protonexchange membranes in fuel cells. Although most effort is concentrated on membranes derived from synthetic polymers, there is interest in the use of ceramic membranes in gas separations, as well as viable alternatives to polymeric membranes in micro-, ultra-, and nanofiltration applications. Ceramic membranes typically have greater tolerance to high t e m p e r a t u r e s , corrosive environments, and wear compared with membranes derived from organic polymers. Two common methods are used to produce ceramic membranes: sintering and the more typical solution-gelation (sol-gel) method. The latter can be energy-intensive and usually involves the use of various solvents and acids. We have recently demonstrated the feasibility of 3 6 6 A • SEPTEMBER 1, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
forming alumina membranes via a new procedure that mitigates environmental impacts, while producing membranes that are relatively difficult to fabricate by conventional sol-gel and sintering methods {12). These membranes are made of alumoxane precursors, using an entirely water-based solution of the mineral boehmite and organic acids. Alumoxane size is controlled by varying organic ligand size. This variable, in conjunction with control of the firing conditions, allows for considerable control of the resultant pore size of the membrane (see Figure 3). Controling pore size and creating narrow pore size distributions of membranes is important in specialized industrial applications, including the food and beverage processing, biotechnology, pharmaceuticals, waste oil treatment, petrochemical processing, textile, paper and pulp, and metal industries. The alumoxane method may eventually allow for highly specific control of pore size and distribution through judicious choice of organic acids and processing conditions. Future developments in membrane science that rely on a biological model for chemical transport and membrane maintenance, as well advances in materials science and nanotechnologies, should extend our capabilities to do more with less. Anticipated technological advances—and resulting benefits to society—extend well beyond an increased potential for tapping water supplies and include more efficient resource recovery, development of membranebased fuel cells for generating electricity, more compact (and less objectionable) waste and hazardous waste treatment facilities, and improved environmental monitoring.
References (1) Rogers, R. Chem. Eng. News s998, 74(34), ,8-39. (2) Membrane Separation nechnologies to 2002, Report Not 1027, Freedonia Group, Inc.: Cleveland, OH, 1998. (3) Pickering, K.;Wiesner, M. R.J. Environ. Eng. .193,119(5), 772-797. (4) Wiesner, M. R.; Sethi, S.; Hackney, J.; Jacangelo, J.; LaUmne, J. M. /. Am. Water Works Assoc. 1994, 86(12), 33-414 (5) Perry, R. H.; Chilton, C. H. Chemical Engineers' Handbook. McGraw-Hilll New York, ,191. (6) Gumerman, R. C; Culp, R. L.; Hasen, S. R Esttmattng Water Treatment Costst EPA-600/2-79-162a-d; U.S. Environmental Protection Agency, U.S. Government Printing Office: Washington, DC, ,979. (7) Gumerman, R. C; Burris, B. E.; Hansen, S. P. Estimation of Small System Water Treatment Costs; U.S. EPA Contract No. 68-03-3093; U.S. Environmental Protection Agency, U.S. Government Printing Office: Washington, DC, 1984. (8) Wiesner, M. R.; Clark, M. M; Mallevialle, M. /. Environ. Eng. 1989, 115(1)) 20-40. (9) Chellam, S.; Wiesner, M. R. Environ. Scii Technol. 1997, 31(3), 819-824. (10) Jucker, C; Clark, M. M. /. Membr. Scii.994, 97, 37-52. (11) Chung, K.-Y., Brewser, M. E.; Belfort, G. AIChEJ. .995, 42(2), 347-358. (12) Rhonda L.; Callender, C; Harlan, J.; Shapiro, N. M.; Jones, C. D; MacQueen, D. L. Chem. Mater. 1997, 9(11)1 24182433. Mark R. Wiesner is a professor in the Department of Environmental Science and Engineering at Rice University, in Houston, Tex. Shankararaman Chellam is an assistant professor in the Department of Civil and Environmental Engineering at the University of Houston.
