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The Role of Desalination on a Thirsty Planet
- Published on Tuesday, 12 July 2016 22:57
- Osha Gray Davidson
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As global aridification increases, technological advances in desalination can play a crucial role in ensuring access to potable water for billions of people. As Israel’s water program shows, using water wisely and efficiently needs to be part of the solution.
The story of mankind is written in water. The first civilizations grew along river banks more than 12,000 years ago. Mesopotamia was watered by the Tigris and Euphrates rivers. In Egypt, civilization flourished along the Nile, and in Asia human settlements took root along the Indus and Yellow rivers.
Today’s societies would be nearly unrecognizable to our earliest ancestors. Most of us live in houses lit, heated, and cooled by electricity. Many of us drive cars, fly in airplanes, and eat food grown by people we don’t know in places far from home. But despite these technological developments, humans are still bound by the iron-clad and ancient rule of water: With an adequate and reliable source of clean potable water, we thrive. Without it, we cease to exist.
The total human population at the dawn of civilization was around 5 million people – roughly the same number of people living in Hamburg, Germany, today. With 7.4 billion thirsty people living on Earth today, coupled with our dependence on water-intensive technologies and a world that is experiencing rapid anthropogenic aridification, we may be bumping up against our water capacity. So, perhaps it’s not surprising that advances in water use policy and the harnessing of technology to increase the supply of water are most developed in Israel, a technologically advanced society in the desert.
Less than 10 miles south of the city of Tel-Aviv and separated from the Mediterranean Sea by a thin stretch of sand dunes, sits the Sorek desalination plant.
“These are not small things,” says Edo Bar-Zeev, shouting to be heard over the massive water pumps deep inside the Sorek facility. “They are water factories.”
That description is especially apt for Sorek. With a potable water output capacity of 624,000 cubic meters a day (164,843,361 gallons), it is the world’s largest seawater desalination plant. Completed in 2013 at a cost of about $500 million, Sorek today produces 30 percent of Israel’s total water demand, says Bar-Zeev, a senior lecturer at the Zuckerberg Institute for Water Research at Ben-Gurion University of the Negev (BGU).
Sorek, which covers 100,000 square meters (roughly the area of 18 American football fields), is remarkable for a number of reasons beyond its immense size. It contains several technological innovations. It is, for example, the first reverse-osmosis desalination plant to use vertically mounted 16-inch membranes– twice the size of the largest previous ones. This combination of increased size and vertical mounting increases the water flow rate at Sorek by 4.3 times, using the same amount of pressure. It also slashes the amount of piping needed by 75 percent.
“Installing the larger membranes vertically was out of the box thinking,” says Bar-Zeev. “It makes the process more efficient and in this game, increasing efficiency translates into reduced power costs.”
Desalination is an energy-intensive process, with electrical power accounting for up to half of the variable costs of producing drinkable water from seawater. More power also means more carbon and particulates released into the atmosphere. One of the primary drawbacks to desalination is the associated environmental and health damage from burning fossil fuels. But increasing efficiency by this magnitude, Bar-Zeev says, makes desalination more environmentally friendly.
Vertical mounting also reduces the Sorek plant’s footprint, makes installation and maintenance easier, and allows air trapped in the membrane to simply move up and out. The old method of horizontally-mounted membranes required additional pressure – and so more energy – to mechanically remove air.
From beginning to end, Sorek has a more environmentally friendly profile, according to experts like Heather Cooley, director of the Water Program at the California-based Pacific Institute. Most seawater desalination plants draw water from near-shore intake pipes located at or near the surface of the ocean, which entraps and kills a significant amount of marine life. “Using subsurface intakes is an important factor in preventing this mortality,” says Cooley.
Feed water for Sorek comes from two intake heads located more than a kilometer from the shore at a depth of 18 meters. The heads also draw water using slow suction, at a rate of approximately 0.15 meters per second, which allows fish that would otherwise be sucked into the pipes to swim away (Slow suction also requires less energy). Once onshore, two concrete 2.5-meter-wide pipes carry the water 2.4 kilometers to the Sorek plant where it is pre-treated by filtering through sand. The process, which is powered solely by gravity, removes suspended solids and some of the dissolved organic matter. At the end of the desalination process, the discharge water will have a higher salinity and different temperature than the ocean it’s being pumped into.
“If the salt level is high,” explains Bar-Zeev, “but the temperature is not, the discharged water will have a negative buoyancy, causing it to rush out and down in a plume.” Mixing the discharge with less saline water and regulating the temperature minimizes impacts, but researchers including Bar-Zeev are monitoring the effects of desalination outfalls on planktonic bacteria and corals. “Fish can simply swim away from the plume, but corals cannot,” Bar-Zeev says.
In the 10 minutes it takes for water to pass through membrane tubes at Sorek, the majority of the salt is removed. The membranes also remove the remaining organic matter from the water. This is a necessary step, but one that creates one of the key problems in desalination: the deposition of bacteria and other matter that form a thin layer of cells on the membrane surface. This biofilm dramatically reduces the rate at which the water flows through the desalination process.
“Biofouling,” says Bar-Zeev, “is the Achilles heel of membranes.” To force the water through a membrane with a mat of bacteria, algae and various polymers, requires increased pressure, which translates into increased energy costs. The initial sand filtration reduces the formation of the biofilm, but says Bar-Zeev, “Even with pre-treatment, biofouling will develop over time.”
A microbiologist by training, Bar-Zeev and his colleagues are working to develop innovative solutions to slow the formation of biofilms and to clean the membranes without using harsh chemicals. “The best solution to overcoming any biologically-created problem,” he says, “should be biology.”
Although Bar-Zeev isn’t ready to announce any breakthroughs quite yet, promising techniques include programming bacteria to react differently to their environment and adding nanoparticles to membranes that would act as biocides. The stakes are enormous. Each membrane costs several thousand dollars. With roughly 50,000 membranes at the Sorek plant, it is clear that replacing biofouled membranes isn’t feasible.
“Finding ways to clean the membranes, quickly and cheaply, is critical to expanding desalination,” says Bar-Zeev.
While the adjective “game-changer” is grossly overused, many experts believe it’s a reasonable description of desalination, with one caveat. It’s true that there is enough water in the ocean to meet the needs of the roughly 700 million people who today lack dependable access to potable water, through desalination, at least in theory. But Cooley at the Pacific Institute says the most important lesson a drying world can learn from Israel’s experience of moving from a water-needy land to one with plenty of water has nothing to do with desalination.
“Israel’s water program didn’t begin with desalination,” she points out, adding that in some ways, it is really the last step in a long process. “They did the cheaper things first,” says Cooley, “like aggressively pursuing efficiency and reuse.”
Israel reuses 80 percent of its treated wastewater, the highest rate in the world (No. 2, Spain, recycles 20 percent). Drip irrigation, which was invented by an Israeli, can reduce the amount of water used in agriculture by as much as 60 percent. Flood irrigation is no longer used by Israeli farmers, but this wasteful method is still the norm on most fruit and vegetable farms around the world.
Israel managed to reduce water usage by 8 percent with an aggressive educational program, writes Seth Siegel in his 2015 book, “Let There Be Water,” an in-depth history of that country’s water policies. He continues:
“Then we use price as an incentive. Almost overnight consumers found ways to save nearly double the amount of water they had saved because of our years long education campaign. It turned out that price was the most effective incentive of all.”
The amount that nearly everyone in Israel pays for water today is based on what it costs to supply that precious commodity. Cooley calls this one of the keys to reducing water usage, but cautions, “sometimes, people think pricing is the only thing that needs to be done. It’s really just one part of a complex picture.”
Thanks to a combination of all the above innovations and policies, the Israeli per capita use of water (under 40 gallons per day), is among the lowest in the world. The key reason desalination is so successful in Israel is because it enables the country to meet a demand that has been greatly reduced by other measures. The takeaway message is clear: The best and most technically advanced desalination process can’t make up for profligate water use. When coupled with conservation, recycling, and real-world pricing, however, desalination can ensure an adequate water supply far into the future for the people of a thirsty planet.
Research for this article was funded, in part, by American Associates, Ben-Gurion University of the Negev.