Editor’s note: This article first appeared in the November/December 2015 edition of Water Efficiency.
At least a dozen different desalination technologies exist. Distillation was the first—used thousands of years ago, and the most common through the 1990s. The process involves evaporating water by thermal or vacuum methods, thereby leaving solids behind, and condensing the desalted vapor into liquid form. Multi-stage flash, multiple-effect distillation, vapor distillation, and vapor compression are examples of such processes. Thermal distillation produces about half of all desalinated water in the world, but is being outpaced by more efficient methods.
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Electrodialysis reversal (EDR) arose in the 1960s. During EDR, feed water passes through an electric field. Dissolved positive and negative ions (e.g., sodium and chloride) migrate through a stack of alternating ion-exchange membranes until completely separated from the water. Periodically, the direction of ion flow is reversed by switching the polarity of the applied electric current. While water does not pass through the membranes, EDR effectively removes a variety of constituents. Not all feed water can be converted to permeate, or water cleaned of most dissolved solids; some water is needed to dilute and flush the brine before discharge. Recovery from EDR can exceed 90% depending on the water chemistry.
An EDR facility in Magna, UT, treats slightly saline groundwater that is also contaminated with arsenic and perchlorates. The system, engineered by GE, produces 6 million gallons per day with an 85% recovery rate and a salinity reduction of 70%.
Reverse osmosis (RO) is the most common desalination technique today. Though not new—it was developed in the 1960s—the technology has improved in recent years to the point of being competitive with other options.
In RO, feed water is driven under high pressure through a semi-permeable membrane that separates salt and water. The membrane pores allow only water molecules to pass through while holding back larger salts, minerals, and other solids. The permeate is collected; the rejected brine, or concentrate, is routed through the next membrane unit to produce more permeate or is collected and discharged.
RO membranes are manufactured in sheets, which are wrapped around a collector tube to produce a cylindrical unit a few feet long. Feed water travels between the membrane layers along the cylinder, and the permeate migrates in a spiral direction to the core collector tube. Membranes are loaded into pressure vessels and arranged in racks with connecting tubing. The type, size, and number of RO units depend on the flow rate and water quality. Typical water recovery is about 80% in RO systems, though brine discharge may require dilution and therefore lower water recovery.
A $1 billion project in Carlsbad, CA, the flagship of modern desalination technologies and the largest desalination plant in the western hemisphere, was set to open in fall 2015. The Carlsbad desalination project will produce and deliver 50 million gallons per day of high-quality water to San Diego County. At least 15 other desalination projects are proposed along the California coast.
Where 80% of the county’s water is imported, the project aims to diversify its water sources, a long-term need underscored by four consecutive years of severe drought throughout the state. Some find it ironic that a state with more than 3,400 miles of coastline could be so dry.
“California is in a serious drought right now, and any new water supplies are important to the region,” says Bob Yamada, water resources manager for San Diego County Water Authority. “Unlike water that comes from rainfall or water that comes from snowpack, we’re utilizing what essentially is the world’s largest reservoir—the Pacific Ocean.”
The public–private partnership between the San Diego County Water Authority and development specialist firm Poseidon Water, employing IDE Technologies as a joint venture with Kiewit-Shea Desalination, will provide 7% of
the county’s water by 2020, or enough for 300,000 people. The desalinated water will be mixed with other sources and become part of the overall supply. “The Carlsbad project revolutionizes desalination in the United States,” says an IDE spokesperson.
More than 20 years in the making, the project includes an intake pump station and pipeline, desalination plant, product water storage, brine discharge pipeline, waste stream treatment facilities, product water pump station, and 10-mile delivery pipeline. Some of theinfrastructure overlaps with the adjacent Encina Power Station, which helps save costs.
Up to 100 million gallons per day of seawater used for once-through, non-contact cooling at the power station will be routed to the new desalination plant. The source water will be filtered to remove large particles and bacteria.
After pre-treatment, chemicals will be added and the feed water will undergo reverse osmosis through thousands of tubular membranes. Half of the water will become drinkable, and the remaining brine will be flushed out. The entire treatment process will take about 20 minutes. After chlorination and conditioning for taste, product water will be stored and delivered to a regional distribution system via a 10-mile pipeline.
An RO pilot plant has been onsite to gauge public acceptance, and over 99% of taste testers rated the quality as “good” to “excellent.”
The rejected brine must be handled carefully. Its discharge has required a 63-inch-diameter, specially designed high-density polyethylene (HDPE) pipeline, provided by ISCO Industries, to accommodate high velocities, steep slopes, and corrosive chemistry. Since the brine will have twice the salt content of the original seawater, it will be diluted with the power station’s cooling water return flow before being discharged to the ocean.
The project will use 40% less energy than conventional desalination processes, thanks to efficient pumps, advanced membranes, and energy recovery systems. “The Carlsbad plant will be the most energy-efficient and technologically advanced plant in the Western hemisphere when online this fall,” says Jessica H. Jones, community outreach manager at Poseidon Water.
While the product is more expensive—the desalinated water will cost $1,900–$2,200 per acre-foot depending on how much is purchased—it is much more dependable than current supplies. “The Carlsbad plant is the only local, reliable water supply in San Diego that is not dependent on rainfall or the snowpack,” says Jones. “It is drought proof.”