Electrodialysisfor industrial water cleanup An old process can be used to solve current environmental problems
Frank B. Leitz U S . Bureau of Reclamation Denver, Colo. 80225
In water a salt dissolves to produce positively charged cations and negatively charged anions. If an electrical field is placed across the solution, the cations migrate toward the negatively charged cathode while the anions migrate in the opposite direction toward the positively charged anode. Cation-exchange membranes are permeable almost exclusively to cations; while anion-exchange membranes similarly pass only anions. If we now place a stack of membranes, alternately anion and cation, in the electrical field, the solution between one pair of membranes becomes depleted in salt while the adjacent solution become enriched. The ion-exchange membrane is the unusual component in this process. Such a membrane has the appearance of a sheet of plastic. Commercial cation membranes generally consist of crosslinked polystyrene that has been sulfonated to produce --S03H groups attached to the polymer. In water this group ionizes producing a mobile counterion (H’) and a fixed charge (--SO3-). Anion membranes have quaternary ammonium groups (-NR3+0H-) attached to a similar polymer. In such membranes 90-99 % of the current is carried by the mobile counterions. By contrast, in salt solutions about 50% of the current is carried by each ion. The mobile ions are easily exchanged with ions of the same charge from the surrounding solutions. Membranes are manufactured commercially by several companies in the U S . and Japan.
Electrodialysis apparatus In commercial practice the basic apparatus for electrodialysis is a stack of rectangular membranes terminated on each end by an electrode. Flow of the process streams is contained and directed by spacers that alternate with the membranes. The assembly of membranes, spacers and electrodes is held in compression by a pair of end plates. The apparatus thus resembles a plate-and-frame filter press. Ancillary apparatus including power supply, pumps, and piping is conventional except that plastic components are used wherever possible to avoid stray electrical currents and the introduction of metal ions into the process streams. For transfer of salt from one flowing stream to another, the simplest electrodialytic process, the membranes are arranged alternately cation and anion. The compartments that have an anion membrane on the anode side are desalting compartments while the alternate compartments are concentrating compartments. The unit, about 2.54 mm (0.1 in.) thick, composed of a cation membrank!, a desalting spacer, an anion membrane and concentrating spacer is a repeating unit termed a “cell pair.” The stack is usually internally manifolded, the manifolds being formed by lining up holes cut in the membranes and spacers. 136
Environmental Science & Technology
In the U.S. the preferred spacer design is termed a “tortuous path” in which the spacer is folded back upon itself and the liquid flow path is much longer than the linear dimensions of the unit. Mixing is accomplished by means of straps spaced about 13 mm apart in the flow path. These spacers are operated at flow velocities of 0.1-0.5 m/s (0.33-1.6 ft/s). Japanese and English manufacturers use a “sheet flow” spacer that consists of an open frame with a plastic screen separating the membranes. These spacers are operated at lower flow velocities, 0.05-0.1 m/s, to achieve a degree of desalting in each pass through the stack comparable to the tortuous path spacer. Stacks of Russian manufacture use either tortuous path or sheet flow spacers. Commercial stacks range in size from laboratory stacks [0.023 m2 (0.25 ft’) effective area per spacer, 0.46 m2 of membrane area per stack] to water desalting stacks with 2 m2 per spacer and over 2400 m2 of membrane per stack. Large stacks have a desalting capacity of more than 0.011 m3/s (250 000 gal/day) at 20-50% salt removal. It is generally not feasible to operate an electrodialysis unit SO that the desired change in salt concentration is obtained in a single pass through a stack. There are three commonly used modes of operation. In continuous operation, the output from one stage is fed directly to the inlet of the next stage until the desired concentration is obtained. In batch operation,
The electrodialysis process Saline water In
Electrodialysis plant. This 650 000 gallons per day piant at Buckeye, Arizona, oegan operarron in 7 ~ 6 2 the product is recycled to a feed reservoir, which is periodically charged, demineralized to the required degree and discharged. With a feed-and-bleed operation, the output of the unit is recycled directly to the inlet in such a flow ratio that a product of desired quality is obtained at the outlet of the unit. Process equipment is still undergoing mechanical and manufacturing improvement to make the process more economical and to expand the range and variety of solutions that may be economically treated. Better pretreatment methods, more electrically conductive membranes, and high desalting capacity per stack are some of the approaches under intensive investigation. History From the turn of the century to tt,- r. pal use for electrodialysis outside the laboratory was for brackish water desalination. The first attempt at commercializing electrodialysis was made in the twenties by a German company, Elektroosmose A.G. This venture was unsuccessful, in part, because of the low level of technoiooical develODment. Eiectrodi;alysis became a practical commercial process after the development of the synthetic ion-exchange membrane in 1948. . . ... ... - . Between 1954 and 1960 the South African tiounciI for Scientific and Industrial Research designed and built a 0.126 m3/s plant in the Orange Free State. This plant was to desalinate the large quantities of brackish water being pumped out of recently developed gold mines; the brackish water constituted a serious disposal problem in a predominately agricultural district. Before the plant was fully operative, the need for demineralizingthis large quantity of water ceased. In 1954,the first lonics unit was sold to Arabian-American Oil Company for use in'oil exploration. Since then over 450 units have been sold by lonics (Table 1). There has been an increasing commercial acceptance of electrodialysis, particularly in plants of over 0.04 m3/s capacity. Details about some of these plants-the category of water use, location, date of start-up, feed-water salinity and rated capacity-appear in Table 2. These plants are located all over the world. A large number of these plants produce water for municipal supplies. The water produced generally contains 500 ppm total dissolved solids and is used for the wide variety Of purposes to which potable water is put. Of particular interest are the plants at Buckeye, Arizona. the first municipality to have its entire water supply demineralized by electrodialysis, and at
___...
-
Foss Reservoir near Clinton, Oklahoma, which is the largest operating municipal electrodialysis facility in the US. A 0.22 m3/s plant in the town of Benghazi, Libya, opened in March 1973, was designed to demineralize local well water from 1980 ppm to 600 ppm. This plant was constructed by Wm. Boby & Co.. a British manufacturer, who chose electrodialysis over ion-exchange for this application. In industrial applications, the water is usually desalinated to between 100-200 ppm total dissolved solids. Where purer water is required, electrodialysis is followed by ion-exchange. This water finds application in petroleum refining, petrochemical production, automobile manufacturing and mining; as boiler feed, cleaner for manufactured parts, cooling water and for other process purposes. The Russian government has done considerable work on electrodialysis for desalination of the large brackish aquifers that underlie much of southern and southeastern Russia. Since fresh water resources in this area are limited, electrodialysis of local underground water is being used as an alternative to the transporting of fresh water from other areas. Work done in Japan, Italy, Israel, France, the Netherlands, and India
TABLE 1.
Installed water capacity of lonics electrodialysis unitsb
Number Of
Year
1954 1960 1967 1969 197W
unitS
3 60 151 206 450
Number of "nit6 with capacity of more than 0.04 m31/s
0 0 5 9 10
Installed capacity in
0.0027 0.0526 0.158
0.285 1.1
alncluding those on o1dei. bTnis equipment i s also manufactured b y Aqua-Chem.
Inc. (MiiwauKee, Wis.1 on a much rrnailer scale; n o w Chem-
ical is marketing equipment manufactured by A I a h i Chemical Co. ( J a ~ a n l .
Volume 10, Number 2. February 1976
137
(ranked in descending order of the amount of effort devoted to electrodialysis) demonstrates the worldwide interest and applicability of this process for water desalination. The aggregate capacity of installed electrodialysis facilities was estimated at between 1-1.5 m3/s at the end of 1974. Since almost all of the large plants have been built within the past 10 years, most of this capacity is in use. A relatively new development is the production of water containing as little as 3 ppm total dissolved solids by using electrodialysis. Ultrapure water is used extensively for boiler feed make-up and for cleaning electronic components. Conventionally such water has been produced by using ion-exchange; however, a final “polishing” is still performed by using ion-exchange after electrodialysis if greater purity is required. With the new process, however, a much greater portion of the demineralization is done by electrodialysis. The achievement of greater purity is made possible by operating the electrodialysis equipment with periodic reversal of the electrical field. In this mode, the electrodialysis equipment can operate with no chemical feed, which provides demineralization with a minimum of ecological impact. Several such installations are currently operating in the U S . Some current uses Second to brackish water desalination, electrodialysis finds its largest use in whey desalination. Whey is a dilute solution of lactose, protein, salt and fat that results from the manufacture of cheese. Approximately one gallon of whey is produced per pound of cheese. In the past whey was either used
ter Production
,
capacity, m ’/s
.0009 .057 .028 .oo 12 ,0031 .13 .066
.0028 .012 ~
Port Mansfield,Texas Sanibel, Florida Siesta Key, Florida State of Kuwait. Persian Gulf ldland of Corfu, Greece
1965 1973 1969
2400 2200
.012
1300
.087
1963 1975
4000 650-2100
.010 ,175
.052
OIL EXPLORATI
.0013 .0004 ARAMCO, Dhahran Saudi Arabia
.005 .0021 .0021 .0004 1974
4500
-025
1970
2200
.012
INDUSTRIAL
SONAT RAC HI Algeria
138
1970 3500 1970 2000
.0004 .023
1974
.05
1500
Environmental Science & Technology
as an animal feed or, in the absence of a suitable market, discarded. Substantial growth in the manufacture of cheese coupled with the very considerable nutritional value of whey, the increasing acceptance of “synthetic” foods such as imitation ice cream and cake mixes, and the progressively tightening restrictions on what can be discarded have moved dairies toward developing or exploiting the markets for products from whey. At the present time there are whey electrodialysis plants in the US., Mexico, the Netherlands, Ireland, Japan and France. It is estimated that the total annual production of desalted whey solids is over 91 X l o 6 kg (100 000 tons). This is equivalent to about 0.04 m3/s of whey. The whey deashing process in general use consists of: concentration from 6 % solids to 20-30 % solids adjustment of pH and clarification to remove insoluble proteins electrodialysis to remove 25-90% of the salt (referred to as “ash” in the dairy trade) concentration and spray drying to product a free-flowing powder. Part of the lactose may be removed by crystallization either before or after electrodialysis. Electrodialysis is done in equipment similar to that used for water desalination except that stacks containing fewer cell pairs are used to limit the voltage across a stack; sanitary piping is also used, and is arranged to avoid dead spots to facilitate cleaning. In large plants with 50% or less deashing, the whey is deashed in continuous mode, otherwise batch mode is used. For 90% salt removal the total deashing costs are about $0.055/kg ($0.025/lb) of dried whey solids, $0.044/kg for operating and maintenance and $0.01 l / k g for amortization of capital equipment. For 5 0 % salt removal these costs are reduced to $0.0138, $0.01 1, and $0.0028/kg, respectively. A typical 90%-deashed whey contains 80% lactose, 14% protein, 4.5% moisture, and less than 1% each of butter fat and ash. It has a substantial vitamin content with a distribution similar to that of non-fat dry milk and a nutritionally desirable protein balance. Deashed whey is used as a major component of infant formulas and can be used as a replacement for non-fat dry milk in ice cream, ice milk, soft-serve products, sherbets and candy. The partially delactosed material can be used in protein drinks, dietary products, baked goods, pasta and cereals. Deashed whey powder is sold by Foremost Foods Co., San Francisco, California, as Nutritekm. Deashed, partially delactosed whey is sold by the same company as ForeTeinTM. Electrodialysis has also been under study for use in the dairy industry for two other operations: control of the cation balance in milk, particularly for persons requiring low sodium intake; the replacement of strontium by calcium to reduce the radioactive elements in milk and associated products. These have both been shown to be technically feasible but have not yet been brought into commercial use. In Japan, electrodialysis was originally developed for production of concentrated brine from seawater as a first step in the extraction of sodium chloride from the sea. That country has essentially no domestic salt supply and, because of climate and scarcity of land salt production by using solar evaporation is limited. Sea salt is concentrated 6- to 7-fold in the electrodialysis plant. For some applications the brine produced by electrodialysis can be used directly. For production of dry salt, water is removed by using conventional evaporators. At the present time all salt for human consumption, about one million tons per year, is produced by this method. Use of this process on waste brine from large seawater distillation plants is presently being investigated. Future applications The variety of factors responsible for the growth in applications of electrodialysis include an increasing understanding and acceptance of electrodialysis as a unit process, develop-
industrial discharges treatable by electrodialysis Whey Metal plating baths Metal finishing rinse waters Battery manufacturing wastes Wood pulp wash water Glass etching solution Process saline effluents
ment of sufficient field and manufacturing experience so that a new application will have a high probability of successful operation, and the existence of a significant manufacturing base so that equipment can be reasonably priced. A number of potential applications are of sufficiently real interest to have prompted investigations of both technical and economic feasibility. Because the costs of desalination by electrodialysis increase with increasing feed salinity, electrodialysis has been considered more expensive than distillation for the production of potable water from seawater. Recently, however, thin components suitable for use at high temperature have been developed under the sponsorship of the Office of Water Research and Technology (OWRT), U S . Department of the Interior. These new components, used at temperatures between 310-355 K (100-180 O F ) offer the possibility of desalting seawater for total costs of $0.25-0.45/m3 ($1 .OO-1.70/1000 gal). This water cost compares favorably with distillation costs in plants of moderate size and is low enough for industrial and municipal purposes, but not for agricultural use. A pilot plant is currently under test at the OWRT facility at Wrightsville Beach, N.C. Bleaching wood pulp with chlorine or hypochlorite solution yields a copious effluent stream of salty water, the disposal of which has become a significant problem in the paper industry. By the use of electrodialysis, an effluent of 4000 ppm NaCl can be separated into a water stream containing 500 ppm or less of salt and a brine stream of up to 150 000 ppm. The water stream, demineralized to the purity required by the process, is recycled as wash water. The brine stream is electrolyzed in a membrane cell to sodium hydroxide solution and chlorine; these substances may be used directly or part of each may be combined to form sodium hypochlorite solution. Thus the brine stream is also returned to the process. A very nearly closed cycle operation is obtained. Electrodialysis is particularly desirable for the salt concentration step since process costs are relatively insensitive to the concentration of the effluent brine. The total cost for the electrodialysis in this sort of application is estimated to be $0.08/m3, $0.06 for operating and maintenance and $0.02 for capital amortization. A particular advantage of the electrodialysis process is its ability to produce solutions of high concentrations of soluble salts. A combination of electrodialysis with conventional evaporation may be substantially cheaper than evaporation alone for the production of dry salt from saline solutions. Manufacture of etched glass produces a waste stream consisting of a dilute ammonium fluoride solution. Disposal of m3/s is an enormous 400 ppm NH4F at a rate over 4 X problem for one manufacturer. It has been shown that separation of this stream by using electrodialysis into a solution containing 2 ppm of NH4F, which does not constitute a serious pollutant, and a solution of 30 000 ppm, which requires further treatment but which is less than 2 % of the original solution volume is possible. In a similar process, a dilute effluent of hydrogen fluoride from quartz tube manufacture is currently being separated into a concentrated stream of hydrofluoric acid that can be returned to the process and a dilute stream whose fluoride content, below 3 ppm, meets local discharge standards.
The salt concentrating ability of electrodialysis can be put to use in reconcentrating metal plating baths. A substantial disposal problem exists in the metal finishing industry from depleted plating baths and rinse waters. Materials such as cyanides of zinc and cadmium can be reconcentrated to bath strength and can be reduced in concentration to very low levels. This approach allows recovery of metal values while minimizing the disposal problem. Irrigation of arid regions makes possible the production of food in regions where, left to nature, the land would not be productive. Since the processes of evaporation and transpiration remove only water from the land, irrigation with even slightly saline water carries the risk that the soil may be seriously damaged by accumulation of salt. If the soil is drained, however, this risk can be eliminated. The drainage is basically brackish water that can be easily desalted. A typical agricultural drainage consists of about 3000 ppm salt of which about 50% is Na+ and CI- and the rest is SO4', HCOs-, Ca++, Mg", and other materials in trace quantities. In all electrodialytical operations, the passage of ions across a membrane is accompanied by the transport of significant amounts of water, usually in the range of 5-15 molecules of water per ion. The quantity of electro-osmotic water depends on the structure of the membranes used. By using membranes of alternating high and low water transport, it is possible to remove water from a mixed solution either with or without altering the ionic balance of the solution. This process is presently being considered for protein recovery from whey. Summlng up The examples given above serve only to indicate the wide variety of possible uses for electrodialysis. The conflicting demands of a highly industrialized society that requires an unpolluted environment will no longer tolerate the waste or casual disposal of materials that may be valuable, hazardous, or both. Whenever an effluent stream contains an organic or inorganic ionic material, electrodialysis offers a possible method for recovery, separation, segregation, or concentration of that material. It is a proven, useful tool that is increasingly finding new applications. Additional reading Mason, E. A., and Kirkham, T. A,, Chem. Eng. Prog. Symp. Ser. No. 24, 55, 173 (1959).(Process Description). Shaffer, L. H., and Mintz, M. S., "Electrodialysis" in "Principles of Desalination," 2nd ed., K. S. Spiegler, Ed., Academic Press, New York, 1966. (Process Description). Eisenmann, J. L., and Leitz, F. B., "Electrodialysis" in "Physical Methods of Chemistry-Part II B: Electrochemical Methods", A. Weissberger and B. Rossiter, Wiley-lnterscience. New York. 1971 (Applications). Craig, T. W., et al., Food Prod. Dev., 4 (a), 92 (1971). (Whey Desalination). Nishiwaki, T., and Itoi, S.,Jpn. Chem. O.,5 (4), 34 (1969). (Salt Production). Leitz, F. B.,and Accomazzo, M. A., Proc. of the Symposium on Marine Electrochemistry, The Electrochemical Society, Princeton, New Jersey (1973), pp 278-295. (Seawater Desalination). Frank B. Leitz. is head, Desalting Design Group with the U S . Bureau of Reclamation. He is presently working on the 100 million gallon per day Yuma Desalting Plant. For twelve years previously he worked at lonics, Inc. (Watertown, Mass.), on membrane processes for desalination and environmental improvement, electrochemical cell development and water treatment. He has written more than fifty papers, and holds four patents. Coordinated by LRE Volume IO, Number 2, February 1976
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