Reverse Osmosis for Water Deminera Iization D. H. MICHAEL BOWEN
he extent to which reverse osmosis is presently and
Tpotentially useful for demineralization of brackish water was plainly evident in the number and variety of pieces of equipment using this principle which were displayed at the International Conference on Water for Peace, held recently in Washington, D. C. The phenomenon of reverse osmosis relies on the ability of certain semipermeable membranes to pass water molecules while rejecting others. When such a membrane is placed between pure water and brine in a closed system, water passes into the brine, diluting it, and establishes an osmotic pressure over the brine. I n reverse osmosis, pressure (greater than osmotic) is applied to the brine side and water flows from the brine through the membrane into the pure water. I n principle, reverse osmosis offers a simple method to desalinate water, one in which the only energy needed is that used to overcome osmotic pressure and one which can be carried out at ambient temperature. (Common desalination techniques involve phase change and inefficient heat transfer.) However, a suitable membrane material with the required high water permeability and low salt permeability is difficult to find. Extensive research work conducted under contract for the Office of Saline Water has revealed no material better than cellulose acetate. I n general, materials which are insoluble in water but which are nevertheless hydrophilic in character show some promise of demonstrating the required selectivity. Unfortunately, as in the case of nylon resins, more marked hydrophilic tendencies are often accompanied by decreased strength. Cellulose acetate has thus been by far the most common membrane material in commercial reverse osmosis systems. Since economics have shown that reverse osmosis is competitive with more conventional techniques only when long membrane life is assured (and then only for brackish water, not seawater), much work has been directed at problems concerned with membrane degradation and failure. A serious problem is an early increase in salt permeability. This has been shown to be due to hydrolysis of the cellulose acetate and can be prevented by acidifying the brine. Another trouble area is mechanical breakdown of the membrane, which is more prevalent at high flux rates and driving pressures. Naturally this problem is intensified in seawater systems where the osmotic pressure to be overcome is much higher than in brackish water systems. Manufacturers of reverse osmosis equipment have 40
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tried to get around these problems by paying attention to membrane support and by using large surface areas to ensure low flux rates. These considerations have led to the interesting diversity of physical arrangements in the equipment of the large number of firms in the reverse osmosis business. At the Water for Peace Conference all equipment manufacturers, with the exception of Du Pont, used membranes of cellulose acetate supported in one form or another. The most noteworthy contribution to the field of reverse osmosis equipment appears to be the Permasep permeator introduced by the Du Pont Co. This permeator resembles a shell-and-tube heat exchanger, the tubes of which are millions of small hollow fibers, each fiber being a semipermeable membrane. Brackish water is introduced into the permeator shell and pure water passes through the fiber walls into the hollow core of each, whence it moves to an end manifold (of undisclosed design). These hollow fibers are 25-250 p outer diameter and have wall thicknesses in the range 5-50 p . I n this very small size the fibers are extremely strong and can withstand the normal pressures used for reverse osmosis-from a few hundred to a few thousand p.s.i. Du Pont claims that the fibers are noncellulosic in nature, without being more specific, and the company holds a number of patents covering the spinning and composition of synthetic hollow fibers. To judge from the research results obtained by General Dynamics for OSW, some type of nylon is the most probable material used by D u Pont. I t is, however, worthwhile to add that a General Atomics Division engineer considers the Du Pont permeator performance data to be entirely compatible with cellulose acetate membranes. A modular approach has been adopted by most manufacturers so that capacity can be increased at constant driving pressure merely by providing additional membrane area. The economics of reverse osmosis militate at present against its use for desalination of seawater, since more than one desalination step is needed for high salt concentrations. However, large plants of more than 50,000 gal./day capacity have been operated satisfactorily with brackish waters, and water costs of less than $1.00/1000 gal. can be achieved even in small plants (about 1000 gal./day). Naturally the use of very large plants for demineralization of brackish waters will further reduce the cost of pure water. Perhaps the best hopes for the success of reverse osmosis lie in further research into membrane function and selectivity.
