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(495b) The Ammonia-Carbon Dioxide Forward Osmosis Desalination Process: a High Recovery Alternative to Reverse Osmosis

McGinnis, R. L., Yale University
Elimelech, M., Yale University

Forward osmosis (FO) has recently been proposed as an alternative desalination method. It operates by the natural osmotic flow of water across a semi-permeable membrane from saline water to a solution of higher osmotic pressure known as the ?draw? solution. For this study, our draw solution is made up of a combination of ammonia and carbon dioxide gases, both of which can be removed from solution with moderate heating and recovered. These gases are also highly soluble and therefore can generate osmotic driving forces in excess of 250 atm, suggesting that high recovery of saline water sources is possible. Low recoveries in current generation desalination systems force the discharge of concentrated brine, limiting their use to coastal areas or risking environmental harm. Reducing the volume of this brine makes its disposal easier (and cheaper), allowing the process to be used inland on brackish groundwater. Using crossflow filtration and a commercially available forward osmosis membrane, we obtained modestly high water fluxes at recoveries of up to 75% of standard seawater concentrations of NaCl. Salt rejection was also reasonably high. Even though performance was acceptable, the obtained water fluxes suggest that concentration polarization (CP) is likely present on both sides of the membrane, as well as within the membrane support structure. The latter, referred to as internal CP, is the major hindrance to permeate water flux due to severely diminished osmotic driving force. Minimization of this phenomenon would yield a greater utilization of the available osmotic driving force and hence reduce the required concentration of the draw solution. Lower draw solution concentrations will reduce the energy load on the solute recovery system and make the process more economically viable.