Metropolitan water district The Metropolitan Water District of Southern California, together with Interior and AEC, entered into a contract with the Bechtel Corp. for a detailed study to determine the engineering and economic feasibility of a combined nuclear-fired desalting and electric generating plant, producing water in the range of 50 150 million gallons per day with 200 to 750 megawatts of electric power. The scope of work called for in the study is divided into three phases: Phase 1 consists of a preliminary survey of possible sites on the southern California coast suitable for producing desalted water and with a means for introducing such water into metropolitan water district's distribution system. This phase will cover the economic feasibility of the several sites. Phase 2 requires that a detailed investigation be made of such sites, selected under phase 1, which will include a determination of power-water production ratios, operating flexibility, and powerplant parameters based on the system requirements. Phase 3 calls for a complete conceptual design of the optimum desalting and powerplant as determined from investigations of phase 2. This phase will provide detailed capital cost estimates, operating costs, and costs of water and power. This phase of the report will make recommendations for use and distribution of the water and the sale of power, and will provide data for use in negotiating with the utilities. Israel Under an agreement with the State of Israel, the Department of the Interior, and the Atomic Energy Commission, a contract was let with Kaiser Engineers and the Catalytic Construction Co. These two contractors, operating as a joint venture, will consider the feasi bility of a dual-purpose water desalting and powerplant on the coast of Israel to provide 70 to 110 million gallons per day of desalted water together with 175 to 200 megawatts of electric power. In contrast to the metropolitan water district study, only one site will be considered. The scope of work calls for a review and evaluation of reactors now commercially available and an investigation of more advanced reactors. The study also calls for the formulation of a development program to insure that there will be no unreasonable risk in meeting performance and that there will be no need for an unnecessary margin of overdesign required in the full-scale plant. The study will compare nuclear and fossil fuel costs and make recommendations as to the final fuel source. The final report will provide detailed designs, capital costs, operating costs, and costs of water and power from the proposed plant. Both the metropolitan water district and Israel studies should be completed by the end of the year. Both studies contemplate commercial operation of the plants by 1971-73. · STATEMENT of Dr. GEORGE F. MANGAN, JR., CHIEF, MEMBRANE DIVISION, OFFICE OF SALINE WATER INTRODUCTION With existing conversion technology, the most economical methods for converting the majority of inland brackish water supplies to potable water is via membrane processes. A membrane may be defined as a physical barrier which allows the separation of salt from water by preferentially permitting the passage of salt while restricting the passage of water or which permits the passage of water while restricting the passage of salt. The energy or driving force for the separation may be a difference in concentration of the solutions separated by the membrane, an applied current, pressure, temperature, or a combination of these forces. Membrane processes possess a number of attractive features which hold promise of significantly reducing the cost of converting brackish water and, with additional development, may find application for the conversion of sea water. Among the attractive features of membrane processes is that no phase change is required to bring about the separation of salt and water. That is, water does not have to be converted to steam and then condensed back to the liquid state. Therefore, energy costs are held to an extremely low value. Also, since membrane processes operate at ambient temperatures, corrosion problems are minimal and less costly materials of construction may be used. Maintenance costs are also reduced. The research and development activities in memebrane systems, supported and encouraged by the Office of Saline Water have been extremely successful. They have resulted in the electrodialysis process emerging as the most efficient and economical process for the conversion of brackish water in the range of 1,000 to 10,000 parts per million of dissolved salts, and they have resulted in the discovery of an eitirely new membrane concept which holds promise for becoming an efficient and economical process for converting the whole range of saline waters from 1,000 parts per million up to, and including, sea water. In addition, a number of new membrane processes are in the laboratory stage of development and show sufficient promise to justify additional bench-scale development work. They are the "transport depletion" process and the "electrogravitation transport depletion" process. I would like to describe, briefly, each of the membrane processes to the committee their advantages and limitations-and the approaches we propose to follow to realize the full potential of the membrane separation processes. I. ELECTRODIALYSIS The electrodialysis process differs from other conversion processes in that salt is removed from water rather than the water being separated from the salt. When salts dissolve in water, they divide into equal numbers of positively charged particles called cations and negatively charged particles called anions. Electrodialysis may be defined as a process in which separation of these ions is accomplished by ion-selective membranes where the energy for separation is an applied electric current. The membranes used have a special property known as "ionic selectivity," i.e., they can be made so as to allow the passage of either positively charged ions or negatively charged ions but not both. A simplified diagram of an electrodialysis stack is shown in figure 1. In the diagram (fig. 1), an electric field attracts anions to the right and the cations to the left. The "ion-selective" membranes allow ions to pas sthrough in one direction only: permeable anion membranes (A) allow anions to pass through from left to right whereas permeable cation membranes (C) allow cations to pass through from left to right. Concentrated brine and fresh water are formed in alternate cells. In the electrodialysis separation process, the quantity of electric current, the membrane area required and, therefore, the cost of the process depends on the amount of salt removed. Electrodialysis has an inherent economic advantage for desalting brackish water since the electrical energy requirement for separating salt from brackish water is much less than that which would be required to separate salt from sea water. Electrodialysis became a practical reality with the development and introduction in 1948 of stable ion-selective membranes. Prior to that time, ionselective membranes had low ionic selectivity, high electrical resistance, and extremely short lifetime. After 1948, efficient membranes were able to be synthesized for the first time as a result of new developments in plastic technology. Following the development of practical ion-selective membranes, commercial development of the electrodialysis system came about rapidly and the first production installation went on stream in mid-1954. At the present time, there are over 100 electrodialysis plants in operation throughout the world. The largest plant currently in operation is the installation in the city of Buckeye, Ariz., which has a design capacity of 65,000 gallons per day. The economic potential of electrodialysis for desalting brackish water resulted in its selection in 1959 as the process to be demonstrated in the first brackish water demonstration plant. This 250,000-gallon-per-day plant, built at Webster, S. Dak., by the Austin Co. using the Asahi Chemical Industry Co., Ltd.. stack design, became operational in the fall of 1961. Since that time, the plant has continuously supplied water to the city of Webster. The plant has generated much useful information, particularly with respect to the economics of the system and the operational problems that can arise during longterm usage of electrodialysis equipment. A. Electrodialysis limitations: Potential areas of improvement While today there is relatively widespread application of the electrodialysis process. not all brackish waters can be converted economically. Significant gains in economy by making improvements in the technology of the process are possible. Potential areas of improvement include development of more efficient membranes and also improved hydraulic system. To understand where these improvements can be made, it becomes necessary to describe the basic mechanism of ion transport through ion-selective membranes. When immersed in an aqueous solution, the structure of an ion-selective membrane can be considered to consist of a solid phase and a liquid phase. The solid phase or body of the membrane is an insoluble polymerized hydrocarbon matrix to which are attached fixed ionic charges. These fixed charges are positive for anion selective membranes and negative for cation selective membranes. The membranes are porous and within the pores, or liquid phase, are an equal number of mobile ions of opposite charge. As an example, a cation membrane of the sulfonated polystyrene tyne immersed in a solution of sodium chloride is shown in figure 2. Within the body of the membrane, sulfonic acid groups are chemically bound to the insoluble polystyrene network. In aqueous solution. the sulfonic acid group dissociates to form negatively charged fixed ions. At equilibrium, due to the requirement for electro-neutrality. there is an equivalent number of mobile positively charged ions or sodium cations within the membrane. These sodium cations occur within the pores or liquid phase of the membrane and are capable of migration. If a direct current potential is applied to the system, both the mobile sodium and mobile chloride ions will take part in the transport of electricity through the bulk solution. In the cation membrane, however. it is found that almost all current is carried by the sodium ions. The electrostatic repelling forces between the fixed negatively charged sulfonate ions prevent the chloride ions from entering and migrating through the membrane phase. In a similar manner. fixed positive ions in an anion selective membrane prevent sodium ions from entering into the anion selective membrane. This “ion exclusion effect" is largely responsible for the ion selective action of these type membranes. This description of ion transport, as giver above, is based on an idealized system. In actual operation, electrodialysis is more complex since a number of transport processes occur simultaneously. The major transport processes which occur in ion-selective membranes are shown in figure 3. Counter-ion' transport constitutes the maior electrical ion movement in the process. Co-ion transport which also occurs is undesirable and should be made as small as possible: this is dependent upon the selectivity of the membrane and upon the concentration gradient across the membrane. Water transfer, resulting from ion transport, will occur. This can be in the form of water of hydration 1 All ions in a lon-selective membrane having a charge opposite from the fixed fons ; i.e.. In the example given previously, sodium cations within the cation selective membrane. |