Technology News - May 26, 2004

Desalination, desalination everywhere

By the end of this year, the city of Long Beach, Calif., expects to begin operating the largest federally funded water desalination plant in the United States, says the U.S. Bureau of Reclamation, which is helping fund the project’s construction. Like most of the growing number of desalination projects around the world, Long Beach is creating drinking water from seawater using reverse osmosis membranes. This technology’s rising popularity is mainly attributable to technical improvements that have improved its reliability and cost, and the Long Beach plant will demonstrate a new approach that promises to advance the technology even further.

Modern technology may belie the ancient mariner’s famous lament of “water, water everywhere, nor any drop to drink,” but in an era of increasing freshwater scarcity, the key to desalination’s burgeoning popularity is economics, says A. Judson Hill, managing director of the Halifax Group. Although seawater desalination using reverse osmosis, which is also known as nanofiltration, still costs two to three times more than conventional water treatment, a growing number of cities in the United States, the Middle East, Europe, North Africa, and Central Asia have no other viable source of drinking water, adds Robert Reiss, president of Reiss Environmental, Inc. The World Bank and U.S. AID help fund projects in developing nations, Hill says.

Reverse osmosis is also extending desalination’s utility by cost-effectively removing salts and other contaminants from water that is less saline than seawater, including rivers, salty groundwater, industrial effluents, and even “postconsumer reclaimed water,” according to the Bureau of Reclamation. Because its costs are on a par with other option for cleaning up with these less-salty waters, the technology is increasingly appealing to communities throughout the arid U.S. west, where states must cope with saline aquifers and irrigation drainage, says Shannon Cunniff, who dealt with desalination issues with the Bureau of Reclamation until departing for a job with the U.S. Department of Defense. The bureau is promoting a roadmap for developing new technologies that will further increase desalination’s usefulness and affordability, and bureau sources say that they hope it becomes the official map for the entire federal government.

Reverse osmosis relies on a fundamental property of solutions: Solvents, in this case water, will cross a semipermeable membrane—one that keeps the salts from crossing. However, a great deal of pressure is required to force water molecules to diffuse through the semipermeable membrane, which rejects the solid salts dissolved in that water. The water that has passed through the membrane is called the permeate.

The technology behind the reverse osmosis process is little more than a system that forces water to pass through a sheet of permeable plastic—the semipermeable membrane—by applying a high pressure, Reiss says. The recent improvements in membrane performance and techniques that reduce the amount of energy needed to run the process are nonetheless notable engineering achievements, says Chuck Martz of Dow Chemical, which installed the first U.S. desalination plant in Freeport, Texas, in 1961, making it among the first in the world. That plant uses the thermal technology that was the dominant approach to desalination until the mid-90s.

The membranes at the heart of reverse osmosis were originally made of cellulose acetate, which operated only within a very narrow pH range, Reiss says. The newer ones now consist of a non-woven polyester fiber backing with a polymer coating topped by a thin polyamid microfilm, a technology that performs well under a broader range of operating conditions, Martz says.

Improvements in the polymer chemistry and the fabrication techniques that increase the number of small pores in the film’s active area have made the polyamid membranes more permeable, Martz says. These advances allows more water to go through at lower pressures, with the same kind of salt rejection. Increasing the size and flux of the membranes, as well as their functional lifetime, also has had an impact, Hill says.

Since 1990, these improvements have helped cut the cost of treating salty water by reverse osmosis from $5–7 to $2–3 per 1000 gallons (gal), Martz says. Conventional water treatment costs about $1 per 1000 gal, Hill points out.

Martz says that he expects the price of reverse osmosis to continue to decrease by the additional 30% needed for it to approach parity with conventional treatment. “We still have quite a ways to go” in improving the technology, he says.

The advances in the reverse osmosis membrane design over the past decade have been the most important factor in reducing the amount of energy necessary to push water through them, Reiss says. Nonetheless, 40–45% of the cost of running any kind of desalination plant is on the energy side, Hill says. Although the amount of pressure needed to force seawater through the membranes has dropped over the years, the process still requires a significant amount of energy to get the seawater to pressures of 800–1200 pounds per square inch (psi), Reiss says.

There are a number of techniques for recovering at least some of that energy. Two of the most popular energy recovery technologies are the pressure exchanger (PX) devised by Energy Recovery, Inc., and the dual or duplex work exchanger energy recovery (DWEER) devices designed by Desalco and DWEER Technology Ltd.

The PX devised by Energy Recovery uses positive displacement to efficiently transfer energy from the high-pressure waste stream that contains the discharged brine to the incoming process stream. Energy Recovery likens the rotational action of its device to “a Gatling machine gun firing high-pressure bullets that is refilled with new seawater cartridges while spinning around a central axis.” The DWEER technology transfers the hydraulic energy from the brine concentrate to the seawater across a piston, where it is used to help power a recirculation pump.

Another reason why using reverse osmosis to desalinate saltwater is so costly is because the process requires ultrapure water that is much cleaner than U.S. drinking water standards require, Martz explains. The water must therefore be pretreated. An option being investigated for cutting the technology’s cost is to combine the pretreatment system with the reverse osmosis facility, Martz says. Companies like Dow are investigating ways to pretreat salty water with microfiltration or ultrafiltration technologies that use a different kind of membrane, he says.

The Long Beach plant, which is expected to produce 300,000 gal of drinking water per day, will demonstrate yet another option—that of using a series of two reverse osmosis treatments, says Tai Tseng, senior civil engineer for the project. The Long Beach Water Department is patenting its “nano-nano” process, which involves sending the seawater through two sets of membranes that each remove 90% of the salinity. By the time the permeate water exits through the second set of membranes, the process can remove enough of the sodium chloride and other salts to meet the U.S. EPA standard of less than 500 milligrams of dissolved substances per liter of water, he says.

The pressures required in the Long Beach setup are significantly lower than those of other seawater reverse osmosis processes: 500–600 psi for the first pass, and 200–300 psi for the second pass, Tseng explains. Independent analyses shows that the two-stage process can be 20–30% more energy-efficient than traditional reverse osmosis desalination, but Tseng stresses that the demonstration is meant to unequivocally quantify these savings. If all goes as expected, the Long Beach engineers will use the technology to build a larger plant capable of producing 9–10 million gallons of water per day by 2010, he says.

However, Hill and Reiss agree that technology improvements can only take desalination so far. Operating a desalination plant in the United States can involve 20 or more permits and agencies, and the permitting process can take 2–4 years, Reiss says. The biggest concern is how to dispose of the concentrated brine waste from the reverse osmosis process. “Concentrate is classified as an industrial waste by the U.S. EPA because of the nature of the regulations, not because of anything in the concentrate,” Reiss explains. “The concentrate is fairly benign, but not particularly useful.”

The most popular alternatives for disposing of the waste in Florida, where many of the U.S. desalination plants operate—including the nation’s largest seawater desalination plant in Tampa—are subsurface injection, surface water discharge, sewers, and land application, he says. In other states, particularly in inland areas like the western states coping with saline aquifers or irrigation drainage, discharge options are limited, he says. Disposal is currently a big issue in communities that are contemplating using desalination, such as Las Vegas, Nev., and Houston, Texas, adds Cunniff.

The Bureau of Reclamation’s roadmap calls for increased research into options for dealing with the concentrate, and the “holy grail” is to find a beneficial reuse for it. One possibility is to mitigate the problems associated with the “freshening” that occurs when stormwaters are discharged into the ocean by adding the salty concentrate, Reiss says.

Until the regulatory issues associated with the concentrate are worked out, the appeal of desalination will be limited in many municipalities, which usually have a 20-year planning timeframe, Reiss says. “One of the biggest issues is the lack of predictability in the permitting process,” he says. Municipalities need to have confidence that they will be able to get their projects approved, he explains. Bills addressing concentrate management have been or are about to be introduced into the U.S. Congress, says Erik Webb of the U.S. Senate energy committee. However, passage is uncertain at this time. —KELLYN BETTS