The fine walls that separate air from water

A pet project of some researchers in the past was to try to develop an artificial gill so that people could breathe under water like fish. No scuba gear would be necessary in the Neptunian world, only a thin rubbery membrane allowing people to draw air from the water.

Today this idea is merely remembered as one of yesterday's dreams. But enough realistic uses now are evolving for synthetic membranes so that their use is expected to become a key technology of tomorrow.

Membranes long have been used to turn saltwater into tap water, and they have some biology-related applications. Now they are being developed to separate gases, recover rare metals, and purify or concentrate substances ranging from food to fouled water. The new filters also help shave energy costs, control pollution, and recycle materials.

The membranes usually are thin plastic walls or fibrous hollow tubes. Their emergence culminates a decades-old effort to mimic the biological filters that abound in nature. Seagulls, for instance, have nasal glands that extract salt from seawater.

In a forest-capped stretch of Wyoming, researchers from Bend Research and the US Bureau of Mines recently tested a tubular membrane system to recover uranium. In this system the leach is pumped through porous hollow tubes saturated with a special chemical fluid, which latches onto the metal atoms in the ore, filtering them out. Result: uranium recovered at 20 to 30 cents a pound cheaper than by conventional methods. As rich ore deposits become scarce, synthetic screens also may find use in chromate, copper, and nickel mining industries.

For now, the membrane industry is still in a chrysalis state. Annual world sales of the filters and related equipment are less than $1 billion.

Last year $170 million of membranes were sold in the US; by 1990 that should jump to $550 million annually, Anna Crull of Chemical Technology Consultants, a Houston-based consulting firm, estimates.

About 30 companies in the United States are working on the technology. The firms range from corporate giants like Monsanto and Du Pont to bantams like Bend Research of Bend, Ore., and Moleculon Research Corporation of Cambridge, Mass.

One area in which membranes increasingly are flexing their might is in separating gases. Oil and chemical companies are using synthetic filters developed in 1979 by the Monsanto Company to recover hydrogen and other gases from industrial processes.

Other systems are developing membranes to remove chemicals from waste streams. Moleculon has developed a membrane ''sandwich'' to filter acids and other chemicals from effluents, making the water reusable. Electronics firms, chemical companies, and pulp and paper mills are interested in the idea.

Better systems are being developed, too, to turn seawater into drinking water; it's a fast-growing use of membranes, particularly in parched countries. High oil prices have made membrane-desalination a favorable alternative to older energy-intensive methods, which use heat to evaporate water.

Further ahead, membrane separation systems hold major promise for biotechnology, concentrating foods (such as sugars), and a range of biomedical applications.

Japan is racing ahead to develop membrane technology. The Japanese are particularly interested in its importance to future biotechnology - for instance , in separating proteins. Though the US is still the leader in most areas of this research, Japan is funneling $42 million into membrane research and development over five years, says Harold Lonsdale, president of Bend Research and editor of the Journal of Membrane Science. That's probably ''more than the rest of the world'' combined, he adds.

Still, austere economic times are hampering commercialization of the technology worldwide. Many of the industries that would use the filters - such as chemical and mining - don't have the money to put into the often expensive systems.