that chemical, biological, or radiological pollutants do not exert subtle, cumulative effects on the health of man. Economics of advanced waste treatment Aside from these indirect or uncertain costs and benefits of our present water-use systems, can advanced waste treatment systems be prepared to compete now with conventional water supply and waste treatment? As an estimated cost goal, advanced waste treatment costs can be equated roughly with the costs of present functions of source development, transmission, water treatment, waste treatment, and disposal. The total is about 17 cents per thousand gallons on a national average, and approximately 35 cents per thousand gallons in prevailing high cost areas of this country. Definitive cost goals have not been established for advanced waste treatment. It would be impossible to do this with current knowledge. Besides, the only realistic cost goal which can ever be stated with certainty is that the cost of water renovation must be the very minimum that science and engineering can achieve anything greater will impose an inexcusable handicap on the country's economic growth. Finally, it is appropriate to conclude that advanced waste treatment is competing for the entire municipal water market and much of the industrial. That is the core of a new policy of water conservation and management. In fact, the purpose of the advanced waste treatment research program is to insure a continuing supply of water, suitable in quantity and quality, to serve the widest possible range of human needs. TECHNICAL APPROACHES TO ADVANCED WASTE TREATMENT General Two important objectives must be achieved if the total concept of advanced waste treatment is to be feasible. First, it will be essential to concentrate the impurities in municipal waste water into small volumes and provide for permanent disposal of this concentrate. Second, it will be necessary to produce purified water of a quality suitable for direct reuse. Concentrated wastes removed by advanced waste treatment processes cannot, of course, be discarded back into surface waters. If this were done, stream pollution problems would not be diminished and downstream water users would have to remove the same contaminants again and again. The development of methods for permanently disposing of concentrated impurities is, therefore, an important part of the concept. Efficient removal of contaminants will result in the production of water of excellent quality. Provision should be made for deliberate direct reuse of this water by the community or industry since in most cases, it would be wasteful to discharge an excellent water into a stream which is less. pure, perhaps much less pure, than the treated discharge. The development of a successful advanced waste treatment technology, therefore, could lead in one step, to the alleviation of both water pollution and water supply problems. From a technical standpoint, advanced waste treatment may be looked upon as a two-step process: (1) Separation of concentrated contaminants from the purified water product, and (2) disposal of the concentrated contaminants in a way that renders them forever innocuous. Research is necessary, therefore, on both separation and permanent disposal techniques. These two elements of advanced waste treatment are related economically since, in general, the further waste is concentrated, the higher the cost of concentration but the lower the cost of disposal. Although their objectives are similar, there are a number of important differences between advanced waste treatment and saline water conversion. First, there is the problem of permanent disposal of the concentrated wastes. When a saline water conversion plant is situated on the ocean's shore, concentrated waste brine can be dumped back into the sea which serves as an infinite waste sink. However, in Kansas City, Cincinnati, or any inland city, there is no suitable place to dispose of the concentrated wastes from a water renovation plant. Therefore, the concentrated wastes must be disposed of permanently and advanced waste treatment researchers must strive for product-towaste ratios which are much higher than those considered satisfactory for saline water conversion. While a 1:1 ratio, i.e., one volume of waste brine for each volume of product, is satisfactory for desalination ratios of at least 1:100 and very possibly 1:1000 must be achieved in waste water renovation. Such a requirement makes one problem much different technically from the other. Second, the types of impurities of challenge in waterborne wastes are quite different from those found in saline waters. In waste waters there are obviously significant quantities of organic materials which are not found in large amounts in the sea. There may also appear a broad spectrum of industrial wastes as well as biological and even radioactive pollutants. Third, the physical and chemical characters of the feed streams are distinctive. Theoretically, the carrier fluids for both saline waters and waste waters are the same, H2O. Practically, however, the feed streams are quite different-one is salt water, the other fresh. This difference is reflected in the properties of the two streams; for example, the viscosity, thermal conductivity, ohmic resistance, freezing point depression, boiling point elevation, and corrosivity. These factors can have a significant bearing on the technological feasibility, engineering process design and, most important, on the economics of the various separation techniques under study. Fourth, waste water renovation is a dynamic problem, while saline water conversion is a static problem. In any one location, for example, the feed water composition to a desalination plant should remain relatively constant. It is not expected that the composition of the sea will change appreciably over the next 50 to 100 years; but in an advanced waste treatment plant, the process must be capable of accommodating a feed stream which is changing in composition and concentration from hour to hour. It also must accept changes from day to day, from season to season, and certainly long-term changes from year to year. To illustrate, 25 years ago no synthetic detergents were found in waste waters; today they are. Today there are no soluble packaging materials present, but 5 years from now, it is anticipated that such materials will be a common component of municipal wastes. Feed composition will vary geographically as well. More and more, the composition of a municipal waste stream will reflect the wastes from the area's industrial complex. Finally, the types of treatment processes which must be considered encompass not only those capable of purifying saline waters but many others based on entirely different principles of separation. This is required by the dissimilar nature of both concentration and composition of impurities. SEPARATION PROCESSES It is conceivable that almost any physical-chemical principle of separation is applicable to advanced waste treatment. Principles under active consideration include adsorption, electrodialysis, emulsion separation, evaporation, extraction, foaming, freezing, hydration, ion exchange, and chemical or electrochemical oxidation. None of these unit operations have been applied to municipal waste treatment. Adsorption One of the most promising processes in the technology of advanced waste treatment is adsorption--the concentration and collection of contaminants at the surface of a solid. The solid or adsorbent can be removed from the liquid by simple mechanical means, carrying the associated impurities along with it. Use of activated carbon adsorbents is a proven technique in water purification for removing the same soluble organic contaminants that are refractory toward conventional waste treatment. In water purification plant application, the concentration of taste- and odor-causing contaminants is extremely low, far below the concentrations of materials which must be eliminated in waste water renovation. Since an adsorbent can only collect a limited quantity of material before it becomes saturated, present methods would be economically impractical for advanced waste treatment unless the adsorbents used were very inexpensive or could be economically regenerated to allow multiple reuse. Research is required, therefore, both in low-cost adsorbents and in low-cost regeneration methods. At the other end of the scale lie the high-quality, high-capacity, high-cost adsorbents. The many varieties of activated carbons and charcoals fall in this group. Such substances will be practical only if regeneration and multiple reuse are possible. Granular activated carbon is now in use which can be regenerated by a thermal process, i.e., the adsorbed materials are baked off at high temperature. Studies are needed to determine whether existing methods and equipment for this process can be economically applied to the complete renovation of municipal wastes. Other methods of regeneration, such as steaming and extraction must be evaluated also. One advanced waste treatment contractor has shown that, under the proper conditions, two different adsorbents may be used in tandem to give better performance than is possible with either of the adsorbents individually. Methods for producing appreciably cheaper activated carbons have been suggested; also for developing high-capacity, regenerable synthetic adsorbents. Several of these are under study. Electrodialysis Electrodialysis is a fairly common laboratory technique for concentrating ionic materials in solution. For this process, electrodes are used to impress an electrical field across a cell through which the liquid to be purified is flowing. This field causes cations (positive ions) to migrate toward the cathode and anions (negative ions) toward the anode. If there is placed between the two electrodes a number of membranes which are alternately cation- and anionpermeable, then both cations and anions can be removed from alternate compartments and concentrated in the intervening compartElectrodialysis does not appear to offer economical conversion of waters containing high concentrations of ionic impurities, such as sea water, but may have appreciable promise for treatment of waters of lower ionic strength such as sewage and waste waters. ments. Electrodialytic purification of brackish waters is already an accomplished fact in a number of plants throughout the world; its potential for waste water renovation is now being assessed. Several possible innovations to conventional electrodialysis are of interest. It has recently been demonstrated that it is possible to recirculate a waste carrier stream continuously and to discharge essentially solid contaminants from this stream by precipitation. If this technique could be applied to waste waters, it would offer great advantage because of its very high, very desirable product-to-waste ratio. A limitation to conventional electrodialysis is the sharply increasing power requirement necessary to reduce ionic concentrations further and further below roughly 500 to 700 milligrams per liter. Possible methods are being investigated for overcoming this limitation. Another area requiring study is the synthesis of new membrane compositions capable of conducting even large organic ions such as are found in municipal waste streams. To make electrodialysis more generally effective for removal of both inorganic and organic contaminants, several methods have been considered for converting nonionic organic molecules to ionic form. Evaporation It is known that waste water streams can be purified by distillation. Experiments by the Public Health Service have shown that the chemical oxygen demand (COD) of secondary sewage plant effluents can be reduced from feed to distillate by at least 98 to 99 percent in a simple, single-stage vacuum evaporation. At present heat costs, single-stage evaporation cannot be accomplished economically on the large scale required for extensive waste water renovation. Through heat economization, however, the evaporation process may be best suited to handle the feeds containing complex and variable impurities required in waste water renovation. There are several ways in which heat economy may be achieved. Among these are multiple-effect evaporation, multistage evaporation and the vapor recompression still. Variations of the simple vapor recompression will be studied for their economic application to advanced waste treatment. A process of interest for centuries is solar distillation, the use of the sun's free energy to distill water. Studies of solar intensity indicate that this process may be applicable in cold weather as well as warm weather areas. Conventional solar distillation is in itself a simple process not likely to be applicable to waste water renovation. However, by certain modifications it may prove feasible. For example, by use of forced convection of water vapor and an external condenser which would provide reuse of solar energy, it may be possible to achieve the advantages of multiple-effect evaporation. If solar distil lation cannot be used as a primary separation technique, it may have use as an economical method for further concentration of wastes following some other initial process. In any evaporation process, it is important that, although a number of problems remain to be solved in developing practicable processes for waste water renovation, certain of the major difficulties encountered in evaporation of more highly concentrated solutions such as sea water, are minimized when municipal wastes constitute the feed stream to the evaporator. Because of the relatively low concentration of impurities in waste water feed, it is possible to perform most of the evaporation from rather dilute solution. This means that the bulk of the water product may be obtained in equipment which is free of appreciable boiling point rise and of corrosion and scale problems. Extraction Solvent extraction is yet another possibility. It should be theoretically possible to remove either the contaminants from the water or water from the contaminants. The better process would be to remove the contaminants from the water since there is much less material to remove. In this process a solvent would be introduced into the bottom of an extraction column and by rising up the column would come in direct, intimate contact with the feed stream passing downward countercurrently. With the proper solvent, contaminants would be removed from the feed by dissolving preferentially in the solvent. The solvent-contaminant solution or extract could then be led into a separator in which the solvent and the contaminants are parted, perhaps by a slight change in temperature. Even if all contaminants were not removed the solvent extraction may be of value for selective removal of certain classes of materials. The major concerns with extraction are that the feed would be contaminated by the solvent and create a secondary pollution problem, or that the contaminants would be more difficult to remove from the solvent than they were from the water in the first place. The second approach is to extract the water away from the contaminants. This possibility is now being studied for desalination purposes. It can be accomplished by use of secondary or tertiary amines as the solvent, but its feasibility in the treatment of wastes has not been studied. Emulsion separation Another category of separation by liquid-liquid contact involves the phenomenon of emulsion formation. Certain solutes tend to concentrate in the monomolecular layer exactly at the interface between immiscible liquids. For organic solvent-water systems, molecules having hydrophilic and organophilic ends exhibit this tendency. Therefore, an extraction can be carried out in which the extractive capacity and the rate of extraction depend not on the volume of solvent used, but on the interfacial area between liquid phases. Such a separation can be carried out using a relatively small volume of solvent if a fine emulsion is generated. The emulsion phase can subsequently be removed and broken to recover the solvent for recycle to the system. Preliminary experiments on the applicability of this technique for advanced waste treatment are now in progress. |