Technology▼Solutions Water desalination takes a step forward
K AI LOON CHEN
membrane that holds back the unwanted salts. The pressure is needed to oppose the natural tendency One-third of the world’s populaever, disposal is not as simple when of the system to run in the opposite tion lives in countries with insufthe technique is used in inland ardirection—freshwater (low solute ficient freshwater to support the eas, where salt can also be a probconcentration) crossing the barrier population, according to the UN lem in waters intended for drinking. to dilute the seawater (high concenEnvironment Programme. For that Desalinating brackish waters inland tration). reason, desalination plants that exrequires an added step of evaporatIn the forward osmosis system, tract drinking water from seawater ing the brine, because injecting it the researchers take advantage of are increasingly popular across the underground would affect groundthis natural tendency. Salt water sits world. But many water-poor counwater. on one side of the membrane, but tries cannot afford the freshwater on the conventional the opposite side is desalination techtransformed into a nology—reverse oshigh-concentration mosis—because of solution by adding its relatively high NH3 and CO2. Water cost. Menachem naturally flows from Elimelech, a profesthe salt water to what sor of chemical and is now the “draw soenvironmental enlution”, which can gineering at Yale have a solute concenUniversity, and his tration as high as 10× graduate researchers that of the salt water. Robert McGinnis and “We just let water go Jeffrey McCutcheon in the right tendency. are hoping to reduce . . . We don’t apply the cost of desalinatany pressure,” says ing water with a new Elimelech, who retechnology they have ceived the National developed that they Water Research Incall forward osmosis stitute’s 2005 Clarke desalination. Prize for this work The new technology for removing the salt from seawater developed by MenaOver the years, and other achievechem Elimelech, Robert McGinnis, and Jeffrey McCutcheon (right to left) of better membrane ments in water reYale University capitalizes on water’s natural tendencies. They have received funding to build a pilot-scale system later this year. technologies and ensearch. The diluted ergy recovery devices draw solution is then have made reverse osmosis more af“The two major resistances to reheated to ~58 °C to evaporate off the fordable and efficient, says Rhodes verse osmosis are cost and brine disCO2 and NH3 for reuse, leaving beTrussell of Trussell Technologies, an charge,” says McGinnis. “That’s one hind freshwater. environmental engineering firm. But of the things that made us think of At present, some companies are the water produced by desalination doing [forward osmosis] instead.” developing forward osmosis systems still costs at least $2/1000 gal, >2× The Yale team is the only one to to treat wastewater and to clean the cost of conventional water treathave made the technique work in leachate from landfills. But Elimement, he says. A large part of this recent years, and their results are lech says that none of them have cost comes from energy use. promising so far, says Thomas Mayapplied the process to seawater deAnd additional costs may be iner, who studies desalination at Sansalination. Only one commercial volved. Reverse osmosis typically dia National Laboratories, which membrane is available for forward recovers 35–50% of the volume of is run by the U.S. Department of osmosis, he adds. This membrane seawater as freshwater, with a leftEnergy. is used to purify water for drinking, over brine concentrate. Near the In reverse osmosis, high presbut it is “not optimized for seawater coast, the brine waste is simply sure (~1000 lb/in.2) is used to push desalination.” Using this commerdumped back into the ocean. Howseawater through a semipermeable cial system, the researchers removed
3454 n environmental Science & technology / June 1, 2006
© 2006 american chemical Society
95–99% of the salt with a water flux of 1–10 μm/s (2.1–21.2 gal/ft 2 per day [gfd]) across the membrane. According to Trussell, these results are good for a prototype system, with a flux that is comparable to the reverse osmosis numbers of 8–15 gfd. However, he says that salt removal should reach 99.9% for the water to be suitable for drinking. Nevertheless, Elimelech’s research is “one of the more important developments in the last decade,” Trussell adds. “He has come up with an idea that has brought [forward osmosis desalination] much closer to feasibility.” However, the experimentally observed flux numbers are lower than what the researchers calculate from the draw solution’s osmotic pressure. The reason for the disparity, says Elimelech, is internal concentration polarization in the porous membrane—as water crosses the membrane, it dilutes the draw solution inside the pores, gradually decreasing osmotic pressure and flux.
He says that a better membrane would give more flux and a consistent 99% salt removal, which is comparable to reverse osmosis. McGinnis adds that a better membrane could recover 75% of the water from the seawater; that would mean more freshwater from the same amount of seawater and less brine waste. This would make the process useful for landlocked regions like New Mexico, which are rich in saline groundwater. The researchers have yet to evaluate the performance of an important part of their pilot-scale system—a distillation column that will separate the draw solutes from the drinking water. In the fall, they will test the operation of the column in their prototype system. The researchers will have to be careful to ensure that most of the NH3 used in the forward osmosis process is ultimately removed, because in order to be suitable for drinking, freshwater should contain
