FEATURE Taking a physical phenomenonosmosis; reversing it, and coming up with a water purification process for tomorrow's water supply
Reverse osmosis makes high quality water now Perhaps there is. Already there are enormous supplies of water in and around our cities that are not being used. A sizable percentage of the U.S. population lives within 100 mi of the oceans, yet the ocean is presently almost totally unavailable as a useful source of water. In every municipality, vast quantities of water are dumped into rivers, lakes, and sewers daily-sometimes polluting, but always lost as a source of potable water.
Seymour S. Kremen UOP, Roga Division, San Diego, Calif. 92101
Reverse osmosis As the nation's population surpasses 200 million and its affluence grows even more rapidly, the demands on our natural resources have created critical problems. Not the least of these problems is supplying the needed electric energy. Already the impact of the energy crisis is being felt. Society's only apparent alternative to brownouts or some kind of power rationing seems to be the construction of electric power plants, nuclear or otherwise, which are viewed by environmentalists with considerable alarm. Less well publicized, but inevitable, is a shortage of good quality water. The demands of industry and agriculture, as well as 200 million private citizens, will generate demands for water that will completely obsolete the present distribution system before the end of this decade. How is the problem solved-by continuing to compromise the environment by building more dams, laying thousands of miles of pipelines, constructing massive interbasin water transfer systems? I s there a better and, even perchance, a less costly way?
The magnitude of the future supply problem, coupled with the enormous potential of seawater and wastewater as sources of this needed water, have prodded an old industry into rapid expansion with application of new and improved technology. This industry, the water purification industry, has been working feverishly to develop practical methods to tap these water sources. It may have found a solution by applying the long-known physical principle of osmosis. From this, it has developed a system known as reverse osmosis (RO) that already is purifying tens of millions of gallons of water every day. To understand how reverse osmosis works, it is first necessary to review the natural process of osmosis. What happens in the osmotic process is illustrated in Figure 1. If pure water (without any dissolved solids) is placed on one side of a hypothetical ideal semipermeable membrane, and a salt solution is placed on the other, then pure water will tend to flow through the membrane and dilute the salt solution on the other side. As this flow continues, there will be a reduced height on the pure
Conventional (a) versus reverse osmosis (b) water reclamation system To outfall 600.650 galimin > 1000 ppm (7800 Ib solidsiday)
a
b
4-
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550 ealimin 50 ppm-2 gr hardness
(5000 Ibsolidsidayf
RO system. Such equipment is purifying miliions of gaiions of water every day
Figure 1.
water side, and an increased height on the salt water side, now somewhat diluted. At some point of dilution and difference in column heights (head), the flow will stop. The pressure represented by this differential head is the numerical equivalent of the osmotic pressure of the diluted salt solution when equilibrium is approached. While it may seem simple, osmosis is the major mechanism by which piant and animal cells exchange nourishment and waste products across their cell walls (membranes). About 20 years ago research, with the important practical goals based on reversing this common natural process, began. As shown, also in Figure 1, if a mechanical pressure is applied to the saline Soiution, water will be forced through the membrane and, ideally, all of the salt will remain behind in concentrated form. This process requires a membrane that freely allows the passage of water, while essentially preventing the passage of salt. It also requires a pressure greater at all times than the osmotic pressure of the salt solution, which increases as concentration increases. This concept marked the birth of reverse osmosis, but much developmental work ensued before RO became the practical water purification process it is today. Private firms, the Office of Saline Water, and the Federal Water Quality Administration (predecessor to the Environmental Protection Agency) invested heavily in the development of this new process. Membranes had to be developed; practical packaging methods and configurations had to be perfected; and systems with the proper plumbing, pumps, controls, and tankage had to be designed and tested. By the late sixties, the reverse osmosis process had been developed sufficiently for industrial plants throughout the world to begin using them. Some of these plants, outfitted with fixed-price RO equipment that is fully warranted as to performance and cost, use more than a million gallons per day of purified water. industrial giants in the semiconductor field now use this equipment as a part of their operations. Basically, the process has been used as a means of demineralizing a feed water while concurrently clarifying and removing other unwanted components. As would be expected, much of the initial utilization has been in industries where purified water, available on a plentiful, reliable, and relatively low-cost basis, is a major contributor to a high-value product, such as semiconductors and other sophisticated, sensitive electronic components. This also applies to large power plants using very high-pressure steam. Reverse osmosis is also used as an antipollution method of treating industrial wastewaters, and for advanced treatment of sanitary wastes.
Osmosis
Concentrated Solution
Fresh water
:\
Semipermeable Membrane
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I
Reverse osmosis
I
Pressure I
I
Concentrated solution
i:
Fresh water
0
0 : I
Semipermeable membrane
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Concurrent with these initial industrial applications, development and refinement of reverse osmosis continuedboth to improve the industrial process itself, and to find a way economically to purify local water sources to provide large quantities of drinking water. Then, in 1969, a system was developed that transformed brackish water into pure drinking water for a seaside community. Purifying water sources The effectiveness of a reverse osmosis system in purifying brackish water is shown in Figure 2. The results shown summarize data from the operation of a 350,000 gal/day RO plant in the Florida resort community of Ocean Reef; the water supply is very brackish well water, the result of seawater intrusion. The plant, put on-line in 1971, has since been uprated to 30,000 gal/day. It produces a soft, clear water of low salinity that is consistent with U.S. Public Health Service recommended standards for drinking water. The system is not limited just to seaside communities. Colorado River water, a substantial part of the water supply of southern California, has a total dissolved solids content only slightly below 800 parts per million (ppm). Since it also has a hardness of approximately 340 ppm, Figure 2
Typical reverse osmosis plant cleans up brackish water Parts Der million Feed
Calcium Magnesium Sodium Potassium Bicarbonate Sulfate Chloride Silica Alkalinity, CaCO, Hardness, CaCO, Dissolved solids
Final product
200 175 1725 94 201 677 3060 12 165 1222 6144
12 4 90 7 31 0 155 2 25 50 301
USPHS standards
...
... ..* ... ...
250 250
... ...
...
500
reverse osmosis offers a practical alternative to the softening and filtration now needed to make it usable for most municipal and industrial purposes. For the purification of brackish waters (ultimately, for potable purposes, any water where the total dissolved solids concentration is more than 500 ppm) by reverse osmosis, present costs are already competitive with pipeline wate? available or soon to be available from massive interbasin transfer systems. Both capital cost recovery and operating costs, including the relatively low energy utilization, are featured in reverse osmosis. This has important implications in terms of increased ability to use, enhance, or protect local underground water supplies that have grown unacceptably brackish with increased use and overdrafts, aggravated in many instances by seawater intrusions. Even more important are the capabilities to reclaim large quantities of municipal and industrial wastewater and to concentrate the solids for simplified disposal. Concentration usually contributes to a practical solution to the disposal problem since the value of the reclaimed water offsets the cost of RO. Moreover, the concentration of the waste solids leads to further economies in liquid waste treatment processes. Since most of these waste streams are relatively dilute, the treatment used today to remove often minute quantities of offensive materials is disproportionately expensive and inefficient. The quality of water that the Roga Division of UOP has produced by reverse osmosis treatment of a clarified municipal primary effluent in a pilot plant operation in Pomo316
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Figure 3
Quality of water resulting from treatment of clarified primary effluent Feed concn mg/lite;, range
Calcium Magnesium Sodium Potassium Ammonia-Nb Chloride Sulfate Phosphate Total COD COD. after aeration Turbidity, JTU TDSc
Product concn mg/liter, rang;
Reduction, %a
99.3 92.5 91.5 90.2 90.4 >99.8 >99.4 65.7
6.3-17 0.7-2.6 2.8-7.5 14-44 Ni1-3
