from sea to sink

With supplies of water under stress, the prospect of rendering saltwater drinkable is growing more appealing and more affordable.

A combination of need and cost is making desalination of saltwater more attractive in the United States. And reverse osmosis is the overwhelming choice when it comes to desalination methods.

Desalination, the removal of salt from either brackish or seawater to render it potable, is nothing new. By some accounts, ancient Greek sailors used simple evaporation to purify seawater. In the United States, as early as 1861, the first desalination "plant" was turning seawater into drinking water at Fort Zachary Taylor in Key West, Fla.

According to the American Water Works Association, there are more than 12,500 desalination plants in 120 countries; 60 percent of these plants are in the Middle East. The world's largest plant, in Saudi Arabia, produces 128 million gallons per day of desalted water; in all, desalination provides 70 percent of the country's drinkable water. In the British Virgin Islands, desalination provides 100 percent of the fresh water for Tortola and 90 percent for Virgin Gorda.

The association forecasts that the world market for desalinated water will grow by more than $70 billion in the next 20 years.

In the United States, overdrawing of groundwater, extended periods of drought, and continuing population growth have created a need for southwestern states like Nevada and New Mexico, as well as for coastal population centers, like California and Florida, to come up with new sources of water for municipal use.

The San Diego Water Authority has plans for a desalination plant that will be co-located with the Encina Power Plant in Carlsbad, Calif.

"Cities like Phoenix and Las Vegas are desperate for new sources of water," said Mike Hightower, distinguished member of the technical staff for Sandia National Laboratories, and co-chairman of the executive planning and review committee heading up the Tularosa Basin National Desalination Research Facility in Alamogordo, N.M. Construction of this national facility began in June this year.

"They have very limited water resources and record population growth. Brackish water desalination is their best choice for providing a long-term water supply," Hightower said.

According to Tom Hinkebein, manager of Sandia's geochemistry department and author of the Desalination and Water Purification Technology Roadmap released in 2003, "It's been obvious for years that the U.S. would outstrip its water supply. The long-term survivability of our nation relies on new ways to deal with water shortages. We need to get to a sustainable future, and desalination helps us get there."

While the need for new sources of water has been growing for some time, it's only in the past 10 years that the cost of desalination has dropped enough to make it a practical option for water-hungry municipalities.

"The San Diego Water Authority has been looking at desalination since 1992, when we were in the midst of a six-year drought," said Bob Yamada, seawater desalination program manager for the San Diego County Water Authority, and president of the American Membrane Technology Association. "But desalination was too expensive then. We had other water resources, such as agricultural-to-urban water transfers, that were much more affordable."

By all accounts, from the early 1990s until the present day, desalination technology has improved in efficiency and cost. The membranes used in the reverse osmosis desalination process cost half what they did 10 years ago, last twice as long as they did in the 1990s, and are twice as productive, Yamada said.

The lower cost and increased efficiency has put "seawater desalination in the ballpark with other new sources of water in terms of cost," Yamada said.

According to the U.S. Desalination Coalition, in 1992, the cost to desalinate an acre-foot of water was about $2,000. Today, that cost is less than $800 per acre-foot, while the cost of importing water has risen to about $500. An acre-foot, the volume that would fill an acre of reservoir to a depth of one foot, equals 325,851 gallons.

The U.S. Desalination Coalition is a national organization made up of water agencies and utilities interested in encouraging the development of seawater and brackish groundwater desalination projects.

For the San Diego County Water Authority, a need to diversify its water supply, and to create new sources of water, is driving the move to develop desalination facilities. Currently, the county imports up to 90 percent of its water from the Metropolitan Water District of Southern California. Diversifying the county's water supply is a crucial component of the Water Authority's long-term strategy for meeting the needs of the nearly 3 million people it serves.

"Desalination is the centerpiece of our strategy to diversify our water supply by 2015," said the Water Authority's Yamada. "By 2015, our goal is to have desalination provide 6 to 15 percent of our water supply."

GETTING THE SALT OUT

Desalination processes are generally divided into two methods: thermal and membrane. Either process can be used for seawater or brackish water. Brackish water is saltier than fresh water, but typically not as salty as seawater. It may result from the mixing of sea and fresh water, as in estuaries, or it may occur naturally, as in underground aquifers.

Thermal desalination has been in use for centuries, and is still the method of choice in many parts of the Middle East. Thermal desalination basically uses evaporation and distillation to remove the salt from water. The biggest drawback is the tremendous need for energy to operate a thermal desalination plant, which will run at temperatures from 35° to 120°C. The high cost of energy has made thermal desalination uneconomical for most parts of the world. The exception is the Middle East, where the dire need for water combines with the low cost of energy to make thermal desalination the technology of choice.

Thermal-method plants account for roughly 21 percent of the desalination facilities worldwide, according to a report from the California Desalination Task Force.

The technology most frequently referred to when talking about membrane desalination is reverse osmosis. In reverse osmosis, feedwater (which can be either sea or brackish water) is pumped at high pressure, generally around 1,000 psi, through semi-permeable membranes that have pores roughly 0.0001 micrometer, or a tenth of a nanometer. These membranes allow the tiny water molecules to pass through, while the much larger mineral salts are trapped and held by the membrane. The feedwater first passes through a pretreatment system, typically a variety of filters, to remove particles, such as bits of seaweed and other organic matter, which would clog the membranes.

The end result is clean, drinkable water that typically will undergo post-treatment before being stored or delivered. What's left over is a brine that's roughly twice as salty as the original feedwater. This brine, known as concentrate, is typically discharged back into the ocean, in the case of seawater, or buried in a deepwater well, in the case of brackish water. Other options for inland concentrate disposal include the use of evaporation pools and landfills.

The need for a steady source of energy and a coastal location makes proper siting a critical component to the success of a seawater desalination plant.

"The most cost-effective location for a seawater desalination plant is right on the coast, next to a power plant," said Hal Furman, executive director of the U.S. Desalination Coalition. According to Furman, coastal desalination facilities are easier to permit when co-located with power plants, especially in areas such as California, where coastal facility development is viewed as "a blight on the environment."

Building desalination facilities next to coastal power plants offers a few other cost-saving advantages, including the ability of energy-hungry desalination facilities to buy a power plant's excess energy capacity at a bargain price "inside the fence," Furman said.

"The economies of scale offered by colocating a seawater desalination facility with a coastal power plant can't be underestimated," said Steve Duranceau, vice president and national director of water quality and treatment in the Orlando, Fla., office of Boyle Engineering, which consults on desalination technology worldwide. "Half of the operating budget of a desalination facility is spent on power."

In addition, a power plant's seawater intake and discharge facilities, which provide water to cool the plant's turbines, can be used to deliver water to the desalination facility and convey concentrate back to the ocean. Additional cooling water from the power plant can be used to mix with the brine in order to dilute the concentration of salt before it's discharged into the ocean.

That approach is being used at the Tampa Bay Seawater Desalination Plant in Florida. The 25 MGD-capacity desalination facility draws about 44 million gallons a day of water that's passed through the cooling condensers of a neighboring power plant, according to Ken Herd, engineering and projects manager for Tampa Bay Water, which owns and operates the desalination facility.

Pressure vessels at Tampa Bay Water (above) hold the reverse osmosis membranes. Below, a worker loads cartridge filters into the cartridge filter housing. The filters backstop the pretreatment process.

The San Diego County Water Authority is following a similar approach. Its largest seawater desalination plant, and the one furthest along in its development, will be built adjacent to the Encina Power Station in Carlsbad, Calif. That facility will be able to produce 50 MGD when completed. The environmental review process is already under way, and should be completed by 2005, according to the Water Authority's Yamada. The project should be operational by 2010.

Like the Tampa Bay facility, the Carlsbad plant will use reverse osmosis to provide fresh water to the coastal cities of Carlsbad and Oceanside, and to the Water Authority's regional aqueduct system.

The Water Authority is also conducting feasibility studies in other coastal locations to see if they are suitable for siting a regional seawater desalination facility. The South Bay Power Plant in Chula Vista and the San Onofre Nuclear Generating Station at the north end of San Diego County are two locations under consideration, Yamada said.

"The Tampa Bay facility proves that desalination works. The problems there aren't in the desalination, but in the pretreatment," Yamada said. "We're watching that situation closely, and hope to learn from it."

The Tampa Bay plant, currently the largest in the United States, has been troubled from the start, and has yet to operate at full capacity for any length of time. The problems here lie largely in the facility's pretreatment process, by all accounts.

"Pretreatment is one of the toughest parts of the desalination process to get right," said Duranceau. "If you don't filter out the right things in the right way, your membranes will foul prematurely, and you'll lose capacity quickly."

Indeed, Tampa Bay's Herd said that pretreatment has been a problem at his facility since before it came nominally online in October 2003. "The more robust your pretreatment system, the longer your membranes last," he said. "But we have high organic loading in our feedwater and fluctuating water quality, which our dual sand pretreatment process isn't handling adequately." The end result is that the cartridge filters upstream of the sand filters, which are designed to act as a safety net that captures anything that passes through the pretreatment, are removing the suspended particles that the pretreatment system should be catching. And, those cartridge filters are lasting only about two weeks, instead of the three to four months they should last, according to Herd.

The pretreatment problems are causing even more trouble for the reverse osmosis membranes. These membranes currently need to be cleaned monthly, rather than the once every four to six months that's expected, Herd said. Though the membranes can be cleaned in place using a chemical treatment, the time spent in cleaning is time that the system isn't producing water. And the frequent cleaning shortens the lifespan of the membranes.

Membrane fouling of the type happening in Tampa Bay is caused by plugging, which happens when sizable particles get through the pretreatment filters. This is considered one of the easiest types of fouling to fix, according to Duranceau. Other fouling mechanisms include scaling, where mineral salts build up on the membranes, which is a frequent problem for brackish water desalination, and bio-fouling, where organisms create a gooey film on the reverse osmosis membranes.

Bio-fouling is the least understood type of membrane fouling, and the toughest to remedy. "We don't understand what causes bio-fouling," Sandia's Hinkebein said. "The typical biological creatures in water are hundreds of times bigger than the pore size in reverse osmosis membranes, so it makes sense that they wouldn't pass through. But we don't understand the process by which they create a film on the membrane, or how best to clean that film," he said.

BRINGING ON THE BRINE

Disposing of the highly concentrated brine that's left over from the desalination process may or may not be a major concern, depending on who you ask and whether or not you're asking about sea or brackish feedwater.

According to Furman of the U.S. Desalination Coalition, "Concentrate disposal is an overblown issue. Putting the concentrate back in the ocean has no impact on the environment."

Tampa Bay's Herd said that environmental models of the effects of concentrate disposal done in his area concluded that the effects of pumping the brine back into Tampa Bay were minimal and acceptable. He said that Tampa Bay Water has monitored the salinity of the bay, and the local biological system in the vicinity of the discharge has not changed significantly.

Tampa Bay's facility creates roughly 19 million gallons per day of concentrate that gets blended back with cooling water from the adjacent energy plant at a 70:1 ratio (concentrate to cooling water).

Brackish water concentrate disposal poses more of a problem, largely because those facilities are typically located inland, so there's no nearby ocean to send the brine back into. Instead, these facilities pump the concentrate into deep wells.

Still, Sandia's Hinkebein believes there's more to the concentrate disposal issue than is currently known. "We need to get to a much better understanding of the environmental impact of concentrate release back into the sea, and into deep wells," he said. "We need to know under what conditions it's acceptable, and when it's going to pose a problem. Instead of defining acceptable results by rules of thumb, as we do now, we need a definitive study on concentrate disposal."

FURTHERING THE CAUSE

While desalination has gotten more affordable, its advocates still have work to do to make it more efficient, more economical, and more ecologically sound.

In order to make the process affordable for municipalities, the U.S. Desalination Coalition has put forth a legislative proposal, H.R. 3834, the Desalination Energy Assistance Act of 2004, which would establish a grant program to support state or publicly owned facilities that are actively desalinating sea or brackish water for municipal or industrial use. The bill was originally introduced by Rep. Jim Davis (D-Fla.) and now has 30 co-sponsors. The U.S. Desalination Coalition's Furman hopes that the legislation will be voted upon by the next Congress, if not sooner.

"We need something that will get us over the hump of the next 10 years, where desalination isn't as cost-effective as other means of providing water," Furman said. "As more desalination projects get built, the technology will improve further, and the costs will come down until it's competitive. This bill can help us get there."

Sandia's Tom Hinkebein believes there is a lot more work to be done in terms of membrane design and function, as well as in energy recovery and beneficial reuse of concentrate, all of which will help make desalination more cost-effective.

The Tampa Bay Seawater Desalination Plant, which has a 25 MGD capacity, has experienced problems with its pretreatment process.

Some of the ideas he thinks need to be explored are using CFD to design a better membrane spacer that reduces resistance and fouling. He also mentions the need for membranes with embedded sensors that can adjust to changing characteristics of feedwater. This, too, would help reduce fouling and increase a membrane's output.

"We need to look at the energy requirements in the membrane. A high flux membrane that could operate at lower pressure would cut down on energy costs," Hinkebein said.

Beyond membranes, concentrate disposal and reduction also need further study. "We need to develop beneficial uses for the salts that are returned," Hinkebein said. "Studies suggest that if we could efficiently recover those salts, it would pay for the desalination process." Hinkebein estimates that finding a market for the salts removed through desalination would bring the cost of the process down from its current $2.50 per thousand gallons.

Research into these areas and many others will be carried out at the Tularosa National Desalination Research Facility. The facility, which is being funded by Congressional appropriation, will be used by researchers from the United States and around the world, according to Sandia's Mike Hightower.

Hightower and Tom Jennings of the Bureau of Reclamation are heading up the project. Hightower said the facility will focus on research and development of technologies addressing the technical, economic, and environmental issues associated with the treatment and use of inland brackish groundwater.

"We're going to look at novel treatment technologies to improve inland water," Hightower said. "We're also interested in ways to use renewable energy for desalination, concentrate management and reuse, and enhanced evaporation. One of the biggest costs of desalination is energy. For that reason, it is important that alternate ways to power these processes be developed."

The Tularosa Basin in south-central New Mexico is the ideal location for the facility because it contains a range of brackish water—from almost fresh to twice as salty as seawater—all within a five-mile radius. The location also offers geothermal, solar, and wind as renewable energy supplies. And the water is located only 20 to 30 feet below the surface, making it easier to pump up, Hightower said.

The desalination facility will consist of six indoor bays and three outdoor test pads, all pumped with three qualities of water.

Several entities that do desalination research, including Sandia, the Bureau of Reclamation, the Office of Naval Research, and a variety of others, will use the facility for testing and development of new desalination technologies. Hightower expects the facility to be operational by January, with full treatment facilities up and running by May or June.

"We need to make novel water technologies usable and affordable for water agencies, municipal and small town needs," Sandia's Hinkebein said. "Our population is going to keep growing, and we've got to come up with new ways to provide them with water. Desalination can keep us from a full-fledged water crisis."

home | features | breaking news | marketplace | departments | about ME | back issues | ASME | site search

© 2004 by The American Society of Mechanical Engineers