Technology -- Salt-Free Solution: Desalination
The massive oil spill in the Persian Gulf has focused concern on contamination of Saudi Arabian desalination plants. It turns out that desalination technology, though historically hampered by high costs, may hold promise for such areas as drought-burdened California.
Though water is the most common substance on Earth, covering 70 percent of the globe, developments in the Persian Gulf War and in California are underlining the rarity and preciousness of the fresh variety.
About 97 percent of the world's H2O is ocean brine and undrinkable because fatal dehydration results when the kidneys try to wash out the salt.
But technical advances in the past 20 or 30 years - some involving The Boeing Co. and Battelle Pacific Northwest - are bringing mankind closer to remedying the Ancient Mariner's desperate lament, ``Water, water, everywhere, nor any drop to drink.''
More and more communities worldwide are building desalination plants, even though converting sea water into fresh water remains costly because it requires enormous amounts of energy.
The biggest plants are Saudi Arabia's huge sea-water refineries on the Persian Gulf, which pump millions of gallons of fresh water daily to serve the Gulf nations - and the Desert Storm armies.
Those Saudi desalting plants have made headlines because of fears the massive Iraqi oil spill in the Gulf will choke inlet valves. Whatever happens there, we can expect a continuing trend toward construction of desalination plants in arid localities.
For instance, California.
With that state's officials considering declaring a state of emergency because of the fifth straight year of drought, Santa Barbara has commissioned the nation's biggest municipal salt-
water conversion plant. It's scheduled to begin operating by next February to produce several million gallons of fresh water daily.
For decades California and the Southwest have sought ways to pipe fresh water from the Pacific Northwest. But, given the political and technical problems of importation, Santa Barbara water-supply development manager Stephen Mack says his city has chosen a desalination plant ``because that's doable.''
Part of that doability is linked to recent technical advances.
From ancient times, the primary desalting procedure was to boil saltwater and condense the vapor. But after World War II, scientists began seeking methods that used less energy.
U.S. research was subsidized from 1952 to 1975 through an obscure federal agency called the Office of Saline Water. Apart from supporting esoteric university research into subjects such as the salt glands of gulls, the agency's clients included Battelle Pacific Northwest Laboratories, hired to find how to remove harmful copper from waste brine and reduce calcium sulfate build-up in desalination plants.
With its airplane business slumping around 1970, even The Boeing Co. entered the desalination field by joining El Paso Natural Gas and Reading and Bates Offshore Drilling Co. on an OSW federal contract.
Boeing built a desalination test plant in 1971, using a new process called a flat-plate vapor-compression evaporator, and continued exploring the desalination market for some time. They closed shop several years ago. ``Basically, we couldn't make a profit,'' says a Boeing spokesman.
But from those early years of international research two commercially successful filtering methods did emerge:
-- Reverse osmosis, which involves pumping water through a semi-permeable membrane, often made of cellulose acetate, that mechanically filters out the salt and retains it on the membrane.
-- Electrodialysis, which uses positive and negative electricity to pull water through membranes. Electrodialysis, used chiefly to desalt brackish ground water, is based on the principle that salt in water breaks up into positively charged ions of sodium and negatively charged ions of chloride. These particles are then drawn off with electrodes.
Most of the Middle East's giant plants use flash distillation, in which sea water entering a vacuum chamber boils into a salt-free vapor. But reverse osmosis is the preferred technology in the U.S., where it's widely used to produce super-pure water vital for silicon-chip manufacturing and kidney dialysis.
Despite all the technical advances, the search continues for a dramatic design revision to cut energy costs. A Tacoma company, ABAM Engineers, has helped develop a promising alternative called OTEC, for ocean thermal-energy conversion.
The OTEC process uses a long vertical pipe to take advantage of the fact that tropical ocean water is as much as 40 degrees warmer at the surface than at the ocean floor. Warm surface water is injected into a vacuum where it vaporizes, as in flash distillation, and the steam is used to power the vacuum pump. Cold subsurface water then condenses the steam into drinkable water. A federally funded OTEC pilot plant at Keahole Point in Hawaii produces 8,500 gallons of fresh water daily.
Agricultural and industrial pollution is adding urgency to the hunt for more efficient ways to cleanse water. In the Middle East, acute water shortages and river pollution threatens to produce enormous suffering and strife, according to the Center for Strategic and International Studies.
The Washington, D.C., think-tank pinpoints the Nile basin, the Euphrates and Tigris river basins, and Israel and its neighbors as the most sensitive areas.
Closer to home, Los Angeles Mayor Tom Bradley said this month that mandatory water rationing is needed to cope with a five-year drought that has left the city's water supply at its lowest point in history. At the same time, California's Gov. Pete Wilson directed an emergency task force of water officials to report to him later this month with a list of drought-fighting options.
Frustrated by recent rainless years that have driven some homeowners to paint their dry lawns green, Santa Barbara hired a Watertown, Mass., water-treatment company called Ionics to build and operate a reverse-osmosis desalination plant for the city. Construction is due to start this summer.
Roger Martin, project manager, says the plant will produce an unusually high ratio of fresh water - 45 percent of each gallon pumped. The 55 percent salty residue will be mixed with outflow from a nearby sewage waste-water plant to adjust the salinity back to about normal level before discharge into the sea.
Curtis Gollrad, another Ionics official, says his company already owns and operates a reverse osmosis plant that for the past two years has removed salt from 1.8 million gallons of saltwater daily for a municipality in the Canary Islands. ``It's a successful project,'' he says.
Admittedly desalination success comes at a price. For example, residents of Santa Barbara will pay half a cent a gallon, about triple the Seattle rate. Yet Santa Barbara officials still foresee desalination as a growth industry in water-short communities.
``Historically water has been pretty cheap, perhaps underpriced,'' says Stephen Mack, Santa Barbara's water-development manager. ``But as we become more urbanized, paying more may not seem unreasonable.''
Indeed not. As recently as 15 years ago the total annual production of fresh water at the world's desalination plants was only around 50 million gallons. Now the figure has risen to several billion, and the figure swells each year as more plants come on line.
Still, nature continues to run the biggest desalting operation. Every day the sun evaporates millions of tons of ocean, which condenses and falls back to earth as our drinking water.
Fortunately, so far we haven't gotten a bill.
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FOR MORE INFORMATION
If you'd like to read more about desalination, here are some sources:
-- SeaFrontiers magazine, ``No Cholesterol, Salt-Free Fresh Water from Oceans,'' May/June, 1990.
-- Technology Review, ``Seawater to Drink,'' August/September, 1989.
-- World Book Encyclopedia.
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HOW SANTA BARBARA WILL DRINK THE SEA
Parched by the California drought, this coastal community has contracted Ionics, a Watertown, Mass., company to build a desalination plant based on the reverse-osmosis process of of water recovery.
1. Ocean water is sucked into a pipe inlet valve 28 feet deep and 2,500 feet from shore. The seawater pumps in 16,000 gallons a minute at 15 pounds per square inch.
Pumps pile on the pressure
2. Then booster pumps on shore force the water through silt-removing primary filters at 65 pounds per square inch.
Membranes filter salt from brine
3. the pressurized seawater is now pumped through a series of membrane tubes that are set in 16 prefabricted trailers on the plant's grounds. The water flows through the tubes, 1,000 gallons per minute at 1,200 pounds pressure. What emerges is 450 gallons of drinkable water per minute.
The reverse-osmosis membrane
4. The key to the process is the reverse-osmosis membrane tubes. Within each tube, pressure on the seawater forces water molecules through the plastic or polymeric membranes, but the salt stays behind. The resulting drinkable water is captured and the remaining salty waste water is routed to a nearby sewage treatment plant for return to the sea.
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Flash distillation plant
Flash distillation, the main water source for Persian Gulf nations, involves pumping seawater into a vacuum chamber where it flashes into steam, which is then condensed into fresh water.
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Ion exchange plant
Electrodialysis uses electricity to pull salt ions through a stack of membranes, leaving desalted water between the membranes. This method is sometimes used on brackish ground water.