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REVERSE OSMOSIS A Pleasant Inversion
Robert E. Gentry, Jr. Aerojet-General, Washington, D. C. 124 Environmental Science and Technology
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Econom ically attractive, technologically feasible, and highly efective, reverse osmosis is coming on strong as a leading contender among the new technologies of water purification
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he process of separation of water from dissolved impurities is one of the oldest chemical processes. It had its beginning when water was evaporated from the earth by solar energy. As the chemical industry developed, other methods of separation evolved: distillation, freezing, solvent extraction, clathration, and the like. These methods were developed to satisfy specific needs, depending upon the material to be recovered and the process economics for a particular separation. Until very recently, distillation was the process of choice to recover large quantities of water from dilute solutions. However, within the past decade public attention has focused on the recovery of potable water from sea or brackish water and more recently on the reclamation of publicly polluted water. This attention has generated a large R&D effort on the development of processes other than distillation. Of these, reverse osmosis is one of the most promising. It holds the potentials of low energy costs, low capital equipment costs, and selective removal of dissolved matter. Reverse osmosis is a new term in the technology of water purification-a subject of worldwide interest and enterprise. This article describes the reverse osmosis process, the current status of
FEATURE
demonstration equipment, and gives an indication of plant and operating costs as well as areas of application. Reverse osmosis and desalination
The process of osmosis is as old as life itself and, in fact, the osmotic membranes are one of the primary systems of living matter. They provide for transport of water-as well as selected chemicals-into and out of cells in both animals and plants. Osmosis, the process of transport through these membranes, has been studied for many years and the laws governing it are well known. However, it has been only within the past eight or 10 years that much attention has been given to the process of reverse osmosis. And only in the past six years has serious consideration been given to the use of reverse osmosis in desalination. The principles of desalination by reverse osmosis are essentially the same as those involved in the osmotic process -except, as the name states, the process is reversed. The process consists of forcing water under pressure over a semipermeable membrane which allows water to pass through with the exclusion of dissolued materials. As an example of the selectivity of the process, 95 to 98% of the sodium chloride in a salt
solution can be excluded by the membranes. Most other salts are excluded to a greater extent. For example, calcium sulfate is excluded to the extent of 99 to 99.9%. Organic material, proteins, bacteria, viruses, and the like, are excluded to an even greater extent than are typical inorganic salts. The basis of this process is the membrane. But it was not until recently that a synthetic membrane was developed which allowed selective permeation of water with an economically feasible rate or throughput. Because the membrane is the heart of reverse osmosis process, let us consider it in detail.
Pilot. Tie-bolt Constructions (aboue) in which the support plates are clamped together at both their centers and peripheries are frequently usedfar laboratory or pilot plant operations. Spaces between the plates serue as miniature pressure vessels
Cellulose acetate membranes
During the past few years many materials have been screened to determine their usefulness as membranes. However, the only general class of materials that exhibit the high fluxes and low salt permeation required for an economical process are those based upon cellulose. In this category the most useful one by far is cellulose acetate, specially processed and specially cast. Indeed, only cellulose acetate is used in commercial or research and development reverse osmosis units today. The cellulose acetate membranes in use are very thin films-in the order of Volume 1, Number 2, February 1967 125
thin in order to yield the high flux rates required for economical operation. In most processes, support is given by what is called, not unreasonably, a support material. This support material is a hack-up plate on which the membrane is supported. M a n y technical approaches
tion by reverse osmosis, R & D workers have taken the following approaches to date: the plate-and-frame approach, the tubular approach, and the spiral-wound approach. These approaches represent different configurations of membrane support surfaces. The plate-and-frame configuration is similar to a filter press design in the chemical industry-the membrane is supported on a plate and the plates are stacked on top of each other. The tubular approach requires that the membrane be supported on the inner surface of tubes-the water moves over the membrane and flows through perforations in the tubes. In the spiralwound process the membrane supports itself, essentially, and is wound into a long spiral tube. All three processes are
iluatioi.n 2nd ""I" -..., further R & D will reveal which will be the most economic and, therefore, the system of choice. It may he that for different applications different designs will be more applicable than others in hand or yet to be developed. In the plate-and-frame design the membrane is attached to a support plate. To begin with, one plate is used to support the membrane which is glued to the periphery of this plate. This plate is then glued to a second plate to form a sandwich, the membrane being applied to the hack side of this second plate. In effect, then, two plates are glued together with a membrane on each side. Water passes through the membranes and into the grooves on the plate through holes in the plate to the area between the glued plates. Then the water flows down the radial grooves to the center where it is collected as product water in the central shaft. " 0 rings in the central portion separate feed water from product water. Baffles provide a longer flow path for the water and induce turbulence so that boundary layer concentration is not too high at the membrane surface.
Operating. Reuerse osmosis unifs have been operating under actualfield condifionsfor seueral years. This 18,000pound skid-mounfed unif wifh a capacity of 20,000 to 40,000 gallonsper day has been purifying oil well brine for six monfhs.Such units contain membranes and supporf plates stacked to form a column within a pressure vessel. Supporrs (similar f o fhose right, above) hold the membranes and acf both as baffles to increase feed water turbulence and as treafmenfpafh ex. " .. fenders, tor examme exfenamp me rrearmenf pa membra
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These baffles are placed on top of the membrane and channel the water spirally, inward or outward, depending upon where the feed water enters the system. The marked increase in path length is illustrated by the fact that the path length of each 24-inch diameter plate is approximately 22 feet. After the membranes have been applied to the support plates and the baffles have been added, the plates are stacked on each other like phonograph records, and the stack is mounted on a central shaft. The stack is then inserted into a pressure vessel. The feed stream enters the top of the pressure vessel, under the operating pressure, and flows across the membrane surfaces in series. Product water comes out in one stream through the central shaft and the reject stream comes out of another valve, both of which are at the bottom of the unit. Volume 1, Number 2, February 1967 127
Bigger. Much of the R& D on reverse osmosis is directed toward developing ever larger units capable of handling ever larger quantities of brackish water. Under construction on contract with OS W is this trailer-mounted 50,000gallon-per-day plant slated for operation by the end of this summer
For laboratory evaluation and general R&D use, a tie-bolt construction plant may be used. The 1000-gallon-per-day pilot plant in our test facility does not use a pressure vessel. Instead, the support plates are clamped together at their centers and peripheries, and the space in between serves as the pressure vessel. Product water comes out at the edge of the plates through grooves and can be collected from each plate individually. Such units can be completely automated. Indeed, in southern California one such unit with a well-water feed has run continuously for more than six months. This unit has been used to demonstrate membrane life, pretreatment requirements, feed recovery ratios, and the many other variables that must be determined before actual plant design for any particular application can be started. Pilot plants
A 10,000-gallon-per-day plant has also been built. This plant is mounted on a 5- X 7-fOOt skid with all the attendant equipment. All that is required for operation is a line for the feed water, one reject line for the product, and one for the reject stream. The unit weighs close to 6700 pounds as mounted and, therefore, is easily transportable. 128 Environmental Science and Technology
A 20,000- to 40,000-gallon-per-day capacity plant has also been built and is in operation. This plant is also skidmounted and weighs close to 18,000 pounds. The unit has been used on various natural waters and is described later in this article. A conceptional design of a 50,000-gallon-per-day reverse osmosis plant has been prepared. In fact, such a plant is under construction on contract from the Office of Saline Water (OSW) and should be on stream within six months. Reverse osmosis units with capacities of 30,000 to 40,000 gallons per day are being evaluated under a variety of conditions as demonstration plants. Data are being collected by industry and OSW on many different types of feed waters, in various locations, and with various operating parameters. The primary objective is to obtain the necessary preliminary data that will allow design refinements and give a sound basis for cost projections for future plants. The reverse osmosis process can be used to desalinate water ranging from slightly brackish (750 to 1000 p.p.m.) to sea water (35,000 p.p.m.). The process yields potable water (500 p.p.m. salt content or less). The conversion of sea water to potable water, although not economically feasible today, is a real
possibility within a year. One of the primary problems with sea water conversion is rapid decay of flux with time. This decay may be caused by compaction of the membrane under the high pressures (1500 p.s.i.) required for sea water conversion. Attempts to alleviate this decay have failed to date. Tailored membranes
Technology in the membrane area has advanced to the state that membranes can be tailored for specific applications. This is a very important development because each application requires special consideration. For instance, in the food processing industry, the end product is not water but a concentrated solution containing suspended and dissolved organic materials. For this application, membranes have been designed to pass water and some inorganic salts at a very rapid rate while retaining the organic materials necessary for food value and flavoring. Another example from the opposite end of the spectrum is the desalination of sea water for drinking purposes-the concentration of sodium chloride in the product stream must be reduced from 35,000 p.p.m. to 500 p.p.m. or less. There are membranes which can accomplish this, but currently they can do this for only limited periods. Thus, each application must be examined both on the basis of the quality of the feed stream and the quality of the product desired. Membranes can be tailored for a specific use. Furthermore, pretreatment processes can be developed and experimental data can be obtained from which units for particular applications can be designed. Water t h r o u g h p u t
The economics of water renovation by reverse osmosis depends to a great extent upon the flux of the membrane. Flux is defined as the flow of water through the membrane, measured in gallons per square foot of membrane area per day. Although flux depends upon many variables, probably the two most important are feed concentration and operating pressure. Fluxes in the range of 20 to 40 gallons per square foot per day are required to make the process of reverse osmosis economically attractive for the recovery of water itself. Although in industries such as the food industry, flux may be of less economic importance. At a typical pressure of 750 p.s.i.
(used for brackish water containing from 2 to 10,000 p.p.m. total dissolved solids), the flux ranges from about 30 gallons per square foot per day to less than 10 gallons per square foot per day, as the feed stream concentration increases from ’/* to 3 l / * z . As the feed concentration varies from about to 3 and using a pressure of 1000 p.s.i., which would be used for Feed water quality affects membrane performance in absolute units brackish water, the corresponding fluxes are 40 gallons per square foot per day dropping to less than 10 gallons per square foot per day. The effects are similar at 1500 p.s.i. The best flux rates are obtained at the higher pressures. However, design trade-offs then must be considered. It is more expensive to construct a 1500-p.s.i. plant than it is to construct a 750-psi. plant, and the capital costs must be weighed against increase in flux. Flux rate affects capital cost as well as operating cost and, therefore, has a Feed water salinity (p.p.m.) major impact on total plant economy. Insofar as total costs are concerned theoretically, fluxes of 40 to 60 gallons Regardless of operating pressures, per day are needed for economical membrane flux falls as feed operation. concentration builds In addition to flux, feed concentration, membrane life, and product-tofeed ratio are of paramount importance to the economics of reverse osmosis. With regard to membrane life, it is probably sufficient to point out that demonstration plants have been operated continuously for up to six months on natural well waters with no appreciable decline in membrane efficiency. I However, much more work is required --_3 1 2 in this technological area to accurately Feed coqcent-atic? ( D e r cent, predict membrane life. Another element of cost is the amount of water pretreatment needed. For some uses, pH adjustment is required. In Treatment costs drop as membrane other cases, chlorination may be necesflux rises sary. For feed streams with high iron Syci e 1 itel.. Hiq'
We are still high up on the learning curve for costs involved in membrane change; automated procedures for making such changes have not yet been developed. This is an area where real
improvements and real contributions can and will be made. To some extent this parameter will affect basic design, because one of the important operations to be considered in determining optimum designs is the one in which membranes can be changed when necessary. An amortization of 20 years is somewhat arbitrary, but it is the usual practice. However, because we are speaking, generally, of funding by municipal bonds it is not unreal to consider a 30year amortization schedule, as is often done with other water projects. A 30year schedule would materially reduce the advertised cost of desalinated water by reverse osmosis and would markedly improve the relative advantage of such desalination processes. The design data and operating experiences that have been used to develop these cost figures have been accumulated during the past three to four years in plants ranging in size from small laboratory units to 20,000 to 40,000 gallons per day plants, using highly specialized water sources. Better data on operating units that can be applied to more sophisticated designs for the larger scale operations are now required. The 50,000 gallon per day plant now under construction for OSW should provide some of the operating data that will allow us to
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Capital costs for a one quarter million g.p.d. plant
5.000 20 60 12,100 1.0
Operating costs for a 1million g.p.d. plant
S O Fuel Electric power 0.068 (S0.007 kw.h.1 0 Steam R a w materials membranes 0.008 0 006 Supplies Operating a n d maintenance labor 0.060 0 "008 Payroll extras General and administrative 0 "022 overhead 0.095 Amortization 0.025 Taxes a n d insurance Interest on working 0.002 capital T O T A L OPERATSO. 288 I N G COST 130
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Environmental Science and Technology
T O T A L ESSENTIAL P L A N T COSTS R a w water supply Product storage Service facilities Contingencies Engineering interest on investment d u r i n g construction Site preparation TOTAL PLANT INVESTMENT Working capital T O T A L C A P I T A L COST
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make better cost estimates for plants in the size of 250,000 to 500,000 gallons per day. Because the reverse osmosis process does not require a change of state of water, fuel cost and, therefore, total operating cost will always be considerably lower than such other processes as distillation and freezing. In these processes, energy is used to change the state of water. Where energy transfers are poor much of this energy is lost from the process. Special needs
When one considers the use of reverse osmosis one must compare the overall process with other processes such as distillation, freezing, extraction, ion exchange, electrodialysis, and others still in investigatory stages. It is probable that each of these systems will have special value for specific applications. For instance, it is difficult to remove the last bit of hardness demanded for some uses, butthis particular step can be accomplished well by ion exchange methods. Furthermore, at present, none of the processes employing membranes is economically practical for desalination of sea water. Next it is necessary to consider materials which can he rejected by the membranes. Certainly one of the first examples is salinity itself-sodium chloride. For most purposes, one can say that the membranes reject sodium chloride to a lesser extent than other solutes. (Possible exceptions are low molecular weight amines, alcohols, and acids, and certain materials, such as urea, which pass through the membrane more readily than sodium chloride.) Rejected to much greater degree than sodium chloride is hardness or scale, by a factor of between five and 20. Because the reverse osmosis system works at ambient temperatures there are indications that even when hardness might be a problem because of salt precipitation, the hardness can be held in solution so that it does not precipitate. Alkalinity is rejected. Organic matter in the form of wastes, from the chemical industry for instance, is also rejected. Viruses are rejected by the membrane, as are bacteria. There is good indication that radioactivity is also rejected. Suspended solids are rejected in some instances, though they may present a problem, in which cases pretreatment of the water is required.
Considering these rejection criteria, it is obvious that, in general, the technique has potential value wherever water must be removed, either for the recovery of water itself or for the recovery of the dehydrated product. Reverse osmosis can be used to treat sea water, brackish water, sewage and waste water, radioactive water, and mine drainage water. Further, it has wide military and industrial potential for such things as drinking water, boiler feed, photographic, and laundry water supply. A special application is in the food processing industry where the product is not water hut the suspended solids. With respect to demonstration plants, OSW is investigating the process as applied to sea water, acid mine water, and various well waters. Reverse osmosis is also being investigated with demonstration units in ihe field for such diverse uses as purification of brine well water in the oil fields for stream flooding operations, and for purification of chemical industry waste streams for reuse of process water. Big steps forward
We have come a long way in the development of the reverse osmosis proccess within the past five or six years despite the fact that comparatively little money is being spent on R&D in this area-especially when one compares it to the money spent on distillation processes during the same period. The process is on the verge of attaining much wider use within industry. Probably the most promising areas are in the chemical processing industry and the food and drug industry where reverse osmosis, as a unit chemical process, offers extremely attractive economics. Distillation, when used by the chemical and pharmaceutical industry, is more expensive than one would like to admit. Also, it is apparent from the hearings held before the Daddario Suhcommittee on Science Research and Development and elsewhere that an increase in water reuse is going to be needed throughout the nation-and the world. Such a need creates a vast potential for reverse osmosis. Since reverse osmosis can remove organic material, viruses, and bacteria, and because it can lower total dissolved solids, application of the process to tertiary sewage treatment for total water reuse is an extremely attractive possibility.
Dr. Robert E. Gentry, Jr., is technical represenfafive of Aerojef-General Carp., a posifion he has held in Washingfon, D.C., since 1961. Previously (1958-60) he worked wifh Aerojef-General as a development chemist in fhe field of solid propellants, later (1960-61) becoming head of the solid propellant informafion group. H e received his B.S. from Stanford University (1950) in chemistry and spent one year in graduate school af the University of Michigan. He returned f o Stanford where he received his Ph.D. (1954) in organic chemistry,' having studied under D r . H. S. Mosher. From Stanford he went to G. D. Searle & Co. (1954-55) where he was inuolued in process developmenf for enovid and research operations pointed foward the total synrhesis of sferoids. D r . Genfry then joined Daw Cliemical Co., Western Division (1955-58), where he did development work on vinyl polymerizafions, later moving to the post of assisfanf purchasing agent, chemicals. D r . Genfry is a member of ACS, Sigma Xi, American Insfifuteof Aeronautics and Astronautics, Army Ordnance Association, and Phi Lambda Upsilon.
Volume 1, Number 2, February 1967 131 Circle No. 13 on Readers' Service c a r d 4
