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| 78.3 - Spring 2005 | ||||||||||||
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Seeking Water
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Printable Version |
By Hannah Yoon
With an ever-increasing population, clean freshwater is becoming scarcer in many parts of the world as the problem of surface and ground water contamination persists. Referring to the quality of freshwater from the Mississippi River, Menachem Elimelech, professor of environmental and chemical engineering notes, “The people in Louisiana drink water that has been filtered through the kidneys of millions of people living in the states upstream of the Mississippi River.”
As a result, some communities are turning to the ocean as a primary source of freshwater. However, present desalination methods not only are costly and energy-intensive but also generate waste streams that are difficult to discard. These shortcomings have led Elimelech to research and develop a less expensive method of desalination in hopes of popularizing the technology.
In the February issue of Desalination, Elimelech delineates his method to separate salt from seawater by forward osmosis, in which water flows across a semi-permeable membrane from seawater to a solution of higher osmotic pressure, known as the “draw” solution. Although the concept of forward osmosis (FO) is not new, the major obstacle, to date, for commercial use of FO in desalination has been the lack of an appropriate draw solution that would allow economical separation of freshwater product from the draw solution.
This schematic diagram shows forward osmosis desalination process of the novel ammonia-carbon dioxide. (Credit: Menachem Elimelech)
Elimelech proposes ammonium bicarbonate as an ideal solute for the draw solution due to its ready decomposition into ammonia and carbon dioxide gas upon heating. The heat requirement is low enough to be harnessed as a side product from other industrial processes. Best of all, the separated ammonia and carbon dioxide gases can combine with water to be recycled in another round of desalination. Trace amounts of remaining ammonium bicarbonate are also not toxic to the human body.
The next step in perfecting this novel desalination process is developing a semi-permeable FO membrane with higher water fluxes and salt rejection. The researchers are working with the Office of Naval Research and Osmotic Technologies, Inc. to commercialize their new method.
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