For the conversion of seawater in 50 million gpd plants or greater, OSW will combine two reliable distillation systems in an experimental unit.

For purifying brackish water in multi-million gpd plants, OSW will construct and operate a 250,000 gpd test bed plant using a pressure technique applied to membranes.

These tests are expected to provide operation and performance data that will serve as guidelines for large-scale technology. The improved techniques will become available for commercial use in large production plants, thus further reducing the cost of desalting saline waters.

Approximately $4 million has been earmarked for construction of a test module that will combine the two distillation systems—the vertical tube evaporator (VTE) and the multi-stage flash (MSF) processes. Distillation involves boiling salt water and condensing the vapor, or steam, into fresh water.

With data from the module, the technological gap between present plants in the 2.5-7.5 million gpd range and the projected plants of 50 million gpd or greater will be spanned during the 1970's.

Desalting costs consist mainly of the capital investment for material (evaporator shells) and the energy (steam) used for operating conversion plants. Thus far, the lowest cost for desalting seawater-65 to 75 cents per 1,000 gallons-has been reported by the newly built 7.5 million gpd distillation plant near Tijuana, Mexico. Plants of 1 million gpd produce fresh water for about $1.00 per 1,000 gallons.

In the area of membrane technology, OSW awarded four contracts for the beginning of work under a four-phase program of construction and operation of a 250,000 gpd brackish water reverse osmosis test bed plant. (The reverse osmosis process utilizes permeable membranes and simple mechanical pressure to separate water from salt in producing fresh water.)

Significant progress has been made in the development of membrane technology, including both reverse osmosis and electrodialysis, for desalination of brackish waters. In electrodialysis, an electrical potential is used to pull the salt ions out of solution.

Several reverse osmosis plants with capacities up to 100,ooo gpd are presently providing suitable water supplies for industrial and municipal uses at costs ranging from 50 to 75 cents per 1,000 gallons. A new 1.5 million gpd electrodialysis plant at Siesta Key, Florida, is converting brackish waters for a reported cost of 35 cents per 1,000 gallons.

The membrane processes have been playing a key role in a comprehensive field test program. Tests have been conducted on a wide variety of saline waters at Yuma, Arizona; Fort Morgan, Grand Junction and La Junta, Colorado; Laverkin, Utah; Las Vegas Wash, Nevada; Gillette, Wyoming; and Fairview Beach, Virginia.

These tests have indicated the feasibility of desalting broad range of brackish waters to potable levels. Such waters include seawater, polluted saline surface streams, irrigation return flows, saline mineral springs and community water supplies with a high level of undesirable contaminants, such as selenium.

With the information obtained from these tests, as well as others conducted at OSW test sites in five States, guidelines can be established to determine what part desalting can play in cleaning up the Nation's lakes, streams and municipal water supplies.

In addition, OSW is focusing more attention on studies that determine potential needs and uses for desalted water to meet future water supply requirements. Cooperative studies have been undertaken with Federal, State and municipal water planning and supply agencies on the feasibility and economics of various desalting processes. This is a practical way for making current technology available to those who can benefit by its use. Ways of integrating desalting plants with conventional water supply sources are being developed also through such feasibility studies.

As concern mounts for heading off water shortages in the long-range future, particularly in the arid southwest Pacific region, the use of desalting as an alternative source for large regional areas is assuming added importance. For instance, major Federal-State cooperative water resources assessment studies in the western United States will include desalination as a potential alternative solution to basinwide problems.

Along with engineering development and specialized studies of water problems, OSW conducts a broad basic and applied research program in pursuit of its goals for low-cost fresh water. Solutions to water problems can be realized only if desalting specialists understand the de

tailed nature of the problems and the saline substances with which they work.

The oceans, covering three-fourths of the earth's surface, offer an almost unlimited source of water for conversion processes. Seawater normally contains 3.5 percent salt, or 35,000 parts per million (ppm) of total dissolved solids. Bodies of water such as the Great Salt Lake and the Dead Sea have much higher levels of salt, running as much as 250,000 ppm.

A not-so-visible, but equally important, source of water is the underground brackish supply. Brackish water ranges from 1,000 to 35,000 ppm. Generally, water containing less than 1,000 ppm is considered fresh, but the U.S. Public Health Service sets 500 ppm as the recommended top limit for drinking water; some industrial processes require almost zero ppm.

Since ground water is such an integral part of desalting's scope and potential, its characteristics are significant.

About 10 percent of the rain that falls on the earth soaks into the ground and is held there in immense, sponge-like subterranean reservoirs called aquifers. These aquifers have been filled over the centuries. Presently, they contain-within a half mile of the earth's surface—a quantity of water estimated at 35 times as great as the amount in all of the world's fresh-water lakes and rivers at any one time. Ground water is usually free of turbidity and harmful bacteria-a definite advantage-but many aquifers are brackish in nature. In fact, one-half of the

land area of the continental United States alone is underlain by ground waters containing between 1,000 and 3,000 ppm.

Significantly, ground water is usually found close to the point of use. Consequently, the desalting of brackish ground water can be an economical solution to water shortages. Deep, saline wells, which have been protected from short-term fluctuations of the weather, evaporation and surface pollution, are an attractive water source. This is especially true in the many regions where surface and shallow well water supplies have been reduced to dangerously low levels by drought conditions.

In keeping with the Presidential mandate for preserving the environment, OSW stepped up its studies of brinewaste disposal at desalting plants. Five contracts, totaling more than $126,000, were simultaneously awarded to support studies of the best approaches for obtaining solid (dry) effluent from desalination plants at inland locations.

To obtain a dry effluent, which promises more efficient disposal, contractors have been investigating the use of such processes as crystallizing evaporators, spray dryers, solvent extraction, immiscible liquid direct heat evaporators and unique forms of multi-stage flash evaporators.

In summary, OSW continues to strive for a quantum jump in accomplishment and, hopefully, a breakthrough in technology. With advanced desalting techniques in hand, man will be assured of fresh water when and where he wants it and at a reasonable price.