Serving a Thirsty World: Trends in DesalinationEd Brown | January 08, 2015
The question is clear: How will the growing global demand for water be met? Although many proposals exist, the answers are not so clear.
According to UNESCO’s Encyclopedia of Desalination and Water Resources (DESWARE), “In addition to water scarcity, chemical and biological contamination of drinking water supplies is a major global problem." A large proportion of the world's population do not have access to good quality drinking water, and around 80% of the world's diseases are attributable to inadequate water supplies, sanitation and water treatment.”
Although about 71% of the earth’s surface is covered with water, more than 96% of that is saline, or salty. As a result, effective and practical desalination technology could make great strides to solving the world’s water supply problems. As of 2013 there were more than 17,000 desalination plants in 150 countries, producing more than 21 billion gallons of water per day, according to the International Desalination Association.
Obstacles exist, however, to increasing those numbers.
The U.S. Geological Survey (USGS) defines fresh water—that is, safe for drinking—as having less than 1,000 ppm of dissolved salt. The concentration in seawater is 35,000 ppm. Water with less than 10,000 ppm of dissolved salts is defined as brackish and is found in both surface water and in underground aquifers. Even with relatively small concentrations of salt, brackish water offers an unpleasant taste, which makes it unacceptable as drinking water.
The technologies for desalination of seawater and brackish water vary, although there is some overlap.
Seawater: Reverse Osmosis
Seawater desalination using reverse osmosis (RO), a process available since the 1970s, is among the most prevalent technologies worldwide. The advantage of RO is that it can produce large quantities of treated water. For example, the largest RO plant in the Western Hemisphere is under construction in Carlsbad, Calif., and is expected to deliver 50 million gallons of drinking water a day for San Diego County beginning in 2016.
But RO requires significant energy input. What's more, because it only desalinates about 50% of the input seawater, the other 50%, which has a high concentration of salts, must somehow be discharged as waste.
RO requires a pump to pressurize the input water in order to force it through a semi-permeable plastic membrane. An electric pump typically is used to maintain that pressure. The electricity costs for running the pump can be a major factor in the technology's overall cost. Since the early 1980s research has been ongoing to develop new membrane materials that require less pressure and also reject a greater percentage of salt. Modern RO membranes, made from thin-film composite polyamide, have enabled a 30-times reduction in energy efficiency and a roughly equal reduction in salt passage compared to the early cellulose acetate membranes.
Research is being done to see if the one-atom thick carbon material called graphene, discovered in 2002, could be used to produce a perforated carbon membrane that would reduce the required pressure still further. This approach, however, is unlikely to be viable in the near future.
“You can talk about some of the other technologies,” says Menachem Elimelech, director of Yale University’s Environmental Engineering Program in New Haven, Conn., “but if you need to produce water for the drinking water supply, I still think RO is the gold standard.” Research that he is working on would use nanotechnology to develop bacteria-resistant membranes. A breakthrough could help solve the problem of biofilms that develop on membrane surfaces over time. These films increase the load on the pump and raise the amount of energy required to run it.
A recently opened desalination plant in Hadera, Israel, ranks among the world’s largest operating seawater RO desalination plants. In its third year of operation, it has produced an average of more than 100 million gallons per day of fresh water. It also serves as a working laboratory for investigating methods to reduce costs.
A major part of that effort is the ability to shift production rates throughout a 24-hour period to coincide with rising and falling costs of electricity. This shift in production can be done because the system includes a combination of units that run in parallel, each one of which can be separately controlled. The facility also uses an energy recovery system, as is typical for modern RO plants, in which the high-pressure saline effluent operates a pressure exchanger to harness a significant portion of the waste energy to assist the high-pressure pump.
The cost of desalinating seawater using RO varies widely by location, depending on the cost of electricity. Industry experts such as Gary Crisp, design engineer of the Perth, Australia desalination plant, advocate using alternative energy sources.
The second major difficulty with RO desalination is more difficult to overcome: how to dispose of the highly saline wastewater, called brine, that is the inevitable byproduct. Typical brine has around twice the salt concentration as the input seawater. Although experts disagree as to the environmental effects of dumping large amounts of brine into the oceans, some (for example, Sabine Lattemann of the German Federal Environmental Agency) have conducted research that shows a negative effect of increased salinity on marine organisms.
Because of the potentially high energy and environmental costs of reverse osmosis, efforts are underway to develop technologies to treat lower-salinity brackish water that occurs in subsurface aquifers and surface water. Water in an aquifer occupies the spaces between soil particles and fractured rock beneath the earth’s surface and tends to flow toward natural discharge sites such as springs, rivers, lakes, lagoons, swamps and the sea.
According to an article in the American University International Law Review, more than half the world’s population depends on groundwater for their basic needs. It is pumped to the surface for residential, industrial and agricultural use.
However, in the U.S. alone, according to the USGS, “In many parts of the country, groundwater withdrawals exceed recharge rates and have caused groundwater-level declines, reductions to the volume of groundwater in storage, lower streamflow and lake levels, or land subsidence.” The USGS continues, “Development of brackish groundwater as an alternative water source can help address concerns about the future availability of water.”
According to the USGS, mineralized groundwater underlies most of the United States. A UNESCO paper, Groundwater and Global Change: Trends, Opportunities and Challenges, echoes this assessment but extends it worldwide.
Brackish Groundwater Desalination
A number of desalination technologies in addition to RO are in various stages of development to treat brackish groundwater. Generally, these systems are smaller and more localized. Because it contains a lower concentration of salt—it’s often defined simply as water that tastes too salty to drink—brackish water can be purified with a lower energy input than seawater.
Treated water can be useful in rural areas that have no access to public water supplies. For example, a recent MIT study developed a plan for a self-contained solar-powered desalination system, based on the technique of electrodyalisis, to treat enough brackish water to supply the needs of a village in India with 2,000 to 5,000 people.
“The factors that point to the choice of electrodialysis in India include both relatively low levels of salinity — ranging from 500 to 3,000 milligrams per liter, compared with seawater at about 35,000 mg/L — as well as the region's lack of electrical power,” says researcher Amos Winter. His associate, Natasha Wright, says, “Even if the water is technically safe to drink, that doesn't solve the problem if people refuse to drink it because of the unpleasant salty taste.” The researchers chose solar electrodyalisis (ED), because it could produce fresh water at roughly half the energy usage of an RO system.
The desalination mechanism of electrodyalisis is opposite to that of RO. In RO, the saltwater flows through a membrane and the dissolved solids stay behind. In electrodyalisis, by contrast, the dissolved solids are pulled through the membranes and the water—now desalinated—flows out of the system.
The process works by passing brackish water between two oppositely charged electrodes. Since most salts dissolved in water are ionic, positively charged (cationic) and negatively charged (anionic), the ions are attracted to electrodes of the opposite polarity. Interposed between the electrodes are membranes designed to pass only cations or only anions. The membranes are stacked with spacers that separate the flows of brine that have negatively or positively charged ionic particles. The two streams combine and flow out of the system as brine.
Electrodyalisis produces a much greater ratio of desalinated water to brine than RO. It is also less energy intensive, since there is lower backpressure for the pumps to overcome, and the amount of electrical energy used in the ionic current flow is directly proportional to the amount of salts removed. Rectifiers to convert alternating current to the direct current needed for the electrodes are typically a major part of the system. However, because the unit being proposed for the Indian pilot project will feed the electrodes with direct current produced by solar photovoltaic cells, the rectifier stage proved unnecessary.
A paper in the Feb. 11, 2014 edition of the MIT Technology Review proposes a modification to the basic ED system by adding a filter made of sintered glass particles to the de-salted output water flow. This would remove any microscopic dirt particles and bacteria that were not eliminated in the dialysis stages.
The DESWARE encyclopedia lists a variety of ED installations, ranging from a rural water source in India that produces 30 m3 per day, to a system producing 400 m3 per day of potable water from a bore hole in Riyadh, Saudi Arabia, to one providing 500 m3 per day for a chemical plant in western Germany.
A new company, WaterFx, has installed a demonstration Concentrated Solar Still (CSS) at the Panoche Water District in California’s Central Valley, one of the world’s most productive agricultural regions. Obtaining fresh water for irrigating crops and then having to dispose of waste brine is a major expense for farmers. The CSS aims to address both problems by recycling irrigation runoff to produce fresh water and using the brine to produce solid salt and other minerals as byproducts that can be a source of revenue.
The technology uses one of the oldest desalination technologies—distillation—but is powered by a 525-foot-long, 400-kilowatt solar reflector. The reflector concentrates sunlight onto a pipe filled with oil. In turn, the heated oil is delivered to a multiple effect evaporator to generate steam, for condensing and recovering fresh water. The final stage is a thermal storage system that can run the process overnight when there is no sun.
In a video, Aaron Mandell, a company cofounder, says that what distinguished his system from traditional distillation systems is that it has increased the efficiency and accelerated the rate of evaporation by about 30 times. In addition, since the volume of brine that needs to be processed is around 7% of the overall process flow, the brine concentration exceeds 210,000 ppm of total dissolved solids (TDS). This enables commercially available crystallization equipment to be used for producing solid byproducts that can be sold rather than disposed of as waste.
The installation at Panoche is a single unit and when, operating at full capacity, will produce about 65,000 gallons of salt- and mineral-free water per day. It is projected that units of this size can be combined virtually without limit to linearly scale up the output.
Worldwide, demand for fresh water is growing and available supplies increasingly are strained. This is stimulating a turn toward desalination as a way to produce fresh water, rather than withdrawing it from naturally occurring sources. Both public and private initiatives are under way to develop new technologies as well as to improve older ones. For coastal areas, this typically means large installations for converting seawater. For inland areas, the answer is often in decentralized systems for desalinating brackish groundwater.
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