Foaming The foaming process, another possible separation technique, is similar to emulsion separation except that a gas forms the second phase of the system rather than another liquid. "Foaming" should be distinguished from "froth flotation." Froth flotation, a principle which has been used in metallurgy and in waste treatment for a number of years, operates on suspended particulate matter but not dissolved materials. It operates by causing air bubbles, introduced into the bottom of a tank, to associate with the particulate matter by adhering to it, and thus decrease its density artificially. This causes the particles to be carried upward and out of the bulk stream because of their greater buoyancy. In addition to removing suspended matter, a second fundamental separation mechanism occurs which causes dissolved materials to be simultaneously removed from the bulk liquid. This occurs because all water-soluble organic compounds are surface active to some extent; that is, they tend to concentrate at a gas-liquid interface. If the foam near the air-water interface is removed the soluble compounds present are removed. This is simple, single-stage foaming. Experiments have demonstrated that the synthetic detergent concentration of secondary sewage effluents can be reduced by 80 to 90 percent by this single-stage foaming. To increase the degree of separation, foam fractionation can be employed. There is an almost direct analogy between foam fractionation and fractional distillation. It is possible to recycle some of the collapsed foam back to a foam column as reflux. This washes out the bulk liquid held up in the column and enhances the separation possible just as reflux does in a distillation column. There are certain other possible innovations in foaming technology. Trace quantities of certain non-surface-active ions have been removed from solution through foaming by complexing them with surfactant materials. This same principle could be applied to remove non-surface-active or only slightly surface-active contaminants from waste streams. The waste stream could conceivably be seeded with some cheap, highly surface-active material such as lignosulfonic acid, a waste product of the sulfite pulp industry. It is possible that foaming and absorption would result in a highly effective team for the economic removal of organic impurities from water. Freezing Freezing is a comparatively new large-scale separation process which has received appreciable attention in conversion of saline waters. It may apply also to renovation of waste waters. In the direct freezing process, feed water is sprayed into a vacuum chamber operating at about 3 millimeters Mercury absolute pressure. Part of the feed is flash evaporated, which produces a cooling effect and causes formation of fine ice crystals in the mother liquid. The ice slurry formed is pumped into a wash column in which it rises countercurrently to a small volume flow of wash water. The washed ice is then melted by condensation of the very same vapors which were removed from the vacuum chamber. A disadvantage of the above direct freezing process is the tremendous vapor volume which must be conveyed and compressed. This is a consequence of the very low absolute pressure at which the system must operate. Use of a secondary refrigerant in direct freezing can minimize this problem. One problem in this system is contamination of the product by the secondary refrigerant and it has not been established how much residual refrigerant is permissible. Also, little is known of the fate of bacteria, virus, or even common organic contaminants within the freezing process. An interesting extension of conventional freezing has been proposed in a special report prepared in the advanced waste treatment research program. This process, termed "eutectic freezing," would be used to achieve very high waste concentration ratios by operating at the eutectic point of the feed, thereby rejecting from solution pure ice and precipitated contaminants simultaneously. It is theorized that this operation could be carried out at about -10° F., and would result in the continuous discharge of two slurry streams, one being ice to be melted as product and the other being a highly concentrated waste containing all impurities present in the feed, both organic and inorganic. The economics of such a process are not unfavorable in spite of the low process temperature because the eutectic freezer need be used on only a small fraction of the total plant throughput if it is applied to an already concentrated brine discharged from some primary concentration stage. Hydration The hydration process is very similar to freezing, involving essentially the same phenomenon. A hydrate-forming gas, such as propane, is bubbled into a reactor (rather than a freezer) in which fine crystals of a solid hydrate are formed. The slurry is then pumped into a wash column in which it is washed with a small portion of the product water. The hydrate crystals are transferred to a decomposition chamber where the hydrate is caused to decompose by a change in temperature and/or pressure. Product water is drawn off by simple decantation and the hydrate-forming material is sent back to the original reactor. Gas hydration has certain thermodynamic as well as practical advantages over freezing. These advantages may be increased by use of hydrating agents which form hydrates at temperatures closer to ambient conditions. Hydration may, in fact, be thought of as a high-temperature freezing process. Oxidation Although not strictly a separation process, oxidation is being considered because of its potential ability to destroy directly organic pollutants in aqueous solution. The problem is whether economical oxidation of organic substances can be performed in highly dilute solutions such as waste waters. Ozone, hydrogen peroxide, chlorine, chlorine dioxide, and other powerful oxidants offer possibilities. Electrolytic oxidation may also be applicable. Catalytic agents may be useful in employing atmospheric oxygen as an oxidant. Ion exchange Small inorganic ions may be removed by conventional ion exchange in packed-bed, mixed-ion exchange columns. When the ion exchange capacity of the bed gas been depleted, the feed is stopped and a regenerating solution passed through the bed. In this system provision must be made for disposing of the regenerating solution. 85974 62- -6 More effective innovations of this process are possible. Electrical regeneration of ion exchange resins is of current interest as well as the possibility of synthesizing exchange resins satisfactory for both organic and inorganic ions. Electrochemical degradation Appreciable evidence exists that many organic compounds are capable of participating in electrode reactions. The electrolysis of certain organic acids in aqueous solution can lead to complete decomposition. In addition to acids, many organic compounds including alcohols, aldehydes, carbohydrates, esters, and even hydrocarbons exhibit measurable conductivity in water. Electrochemical treatment appears to offer promise in advanced waste treatment and will be studied. The goal would be electrolytic decomposition of contaminants into simple innocuous molecules or merely conversion of relatively complex, refractory organics into a biologically soft or generally nonrefractory form. Electrolysis Electrolysis of water, as a separation process, is perhaps more than any other approach considered in the "blue sky" category. The process would involve the electrolysis of water to hydrogen and oxygen, followed by recombination of these gases chemically to create "new" water. Certainly, other electrode reactions could occur and the water generated might be contaminated, but based on exploratory experimentation and information from investigators of life-cycle systems for space vehicles, it appears that pure hydrogen and oxygen are the most likely products of electrolysis of sewage effluents. This approach has one advantage over all others in that the waste water is being destroyed and re-created which should help overcome any esthetic barrier. A number of innovations are possible which may make the electrolysis process feasible in waste water renovation. PERMANENT CONCENTRATED WASTE DISPOSAL Recovery of usable products There appear to be four general possibilities for ultimate disposal of concentrated wastes from an advanced treatment process. The first would be recovery of usable products from the waste. This is the most desirable solution but may prove the most unlikely. The conversion of a waste to a valuable product could eliminate the disposal problem and conceivably produce revenue from its sale which would help defray the expense of advanced waste treatment. A number of products suggest themselves: structural materials such as brick or building blocks, road paving, aggregate for concrete, fillers for various materials, soil conditioners, and insulation. Many cities could make use of recovered salts for street deicing during winter months. The recovery of chemicals, and even extraction of pharmaceutical products, is not impossible. Fertilizer values present in municipal wastes are now utilized in many places. Sludge particles may have adsorptive properties, or these characteristics might be developed through proper activation, and could be reused in the waste water treatment process. Concentration of surfactants from the condensed foam of a foam fractionation process could be an im portant source of industrial detergents in the future. These and many other useful products may one day make the concept of multiple reuse apply not only to the water fraction of our municipal waste streams, but to the nonaqueous material as well. Land fill and ocean disposal How The second method of ultimate disposal is discharge to some waste sink having no outlet. Included in such disposal would be: (1) Dewatering and drying of the sludge with subsequent disposal as land fill, (2) discharge of brines to evaporation lagoons or to natural or artificial reservoirs set aside for this purpose, or (3) transport to the deep ocean. Transportation of residues to the disposal area would pose an engineering and economic problem in some instances. ever, land fill of other community wastes is common practice and aqueducts are in use today which transport many millions of gallons of fresh water hundreds of miles. Therefore, concentrated wastes, composing perhaps only one one-hundredth or one one-thousandth of these volumes, could conceivably be piped similar distances economically. Subsurface disposal Subsurface disposal is a third possibility for concentrated waste disposal from an advanced waste treatment plant. Subsurface disposal may be either shallow or deep. With shallow disposal reintroduction of the water fraction into usable ground water supplies may occur. Therefore, disposal by soil-spreading or shallow-well injection could be used only when provision is made for rendering potentially harmful contaminants innocuous before they find their way into usable supplies. In deep-well injection, it is assumed that no contamination of existing or future water supplies could occur. The wastes are placed in strata so deep or so sealed that the wastes are forever removed from contact with human activity. Injection could be made into porous strata or into natural or artificial cavities such as salt domes, natural caves, or abandoned mines. The feasibility of underground injection as an important disposal method depends on the availability of suitable sites. The compatability of advanced waste treatment concentrates with various underground formations needs to be established. Conversion of refractory contaminants The final possibility for ultimate disposal lies in converting the refractory contaminants to innocuous materials. In many cases this might be done prior to, and in conjunction with, recovery of products, dumping, or shallow underground disposal. Generally, it would be a chemical conversion process which would produce an effluent stream of suitable quality for release back into the environment. Biochemical processes of anaerobic digestion or aerobic fermentation may play a role. Few organic materials are completely inert to biological degradation under optimum conditions. Biochemical fuel cells, one of science's very newest developments, might be employed to recover energy for use in the process. Treatment by high-energy radiation has been suggested as a means for sterilizing wastes and even for catalyzing their oxidation to harmless materials. Incineration cannot be overlooked as a possibility and equipment is now available in which still-wet sludge may be sprayed directly into a combustion chamber. Wet oxidation of dissolved and suspended organic matter at high pressures and in the 500° to 600° F. range shows promise. Techniques such as this, which do not require dewatering of the waste sludge, may have appreciable economic advantage. If the heat value of oxidized materials can be recovered for use the day may come in which advanced waste treatment plants perform the functions now accomplished separately by refuse disposal, garbage disposal, and sewage disposal plants. The choice of waste water separation process and of an ultimate concentrated waste disposal process will depend on the technological adequacy of the processes themselves. It is known that a number of processes are now capable of these functions but the problems of economics remain to be solved. By scientific research and application of new materials, new equipment and new principles, there is much reason to believe that economically and technically feasible advanced waste treatment processes will be developed which will permit the maximum reuse of the Nation's waters for all purposes. 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H.: Average Solar Radiation in the United States. Heating and Ventilating, vol. 46, No. 7, pp. 61-64, July 1949. (8) Grune, W. N., Thompson, T. L., and Collins, R. A.: New Applications of Thermodynamic Principles to Solar Distillation. ASME Paper 61-SA-45, presented at the ASME Summer Annual Meeting, Los Angeles, Calif., June 1961. (9) Davison, R. R., Smith, W. H., Jr., and Hood, D. W.: Structure and AmineWater Solubility in Desalination by Solvent Extraction. (10) Eldib, I. A.: Foam or Emulsion Fractionation for Removal of Soluble Organics From Waste Water Effluents. Report to the Advanced Waste Treatment Research Program, September 30, 1960. (11) Weinstock, J. J.: Progress Report to the Advanced Waste Treatment Research Program, August 7, 1961. (12) Schnepf, R. W., Gaden, F. L., Microznik, E. V., and Schonfeld, E.: Foam Fractionation: Metals. Chemical Engineering Progress, vol. 55, No. 5, pp. 42-46, May 1959. (13) Eldib, I. A.: Foam Fractionation for Removal of Soluble Organics From Waste Water. Journal Water Pollution Control Federation, vol. 33, No. 9, pp. 914-931, September 1961. (14) Barduhn, A. J.: Waste Water Treatment for Reuse by Means of Freezing or Forming Gas Hydrates. Report to the Advanced Waste Treatment Research Program, July 21, 1961. (15) Brockman, C. J.: Electro-Organic Chemistry. John Wiley & Sons, Inc., 1926. (16) Shipley, J. W., and Rogers, M. T.: The Electrolysis of Some Organic Compounds With Alternating Current. Canadian Journal of Research, vol. 176, p. 147, 1939. (17) Schwope, A. D., Private Communiacation, September 19, 1961. (18) Konikoff, J. J., Private Communication, June 13, 1961. |