Desalination using forward osmosis with nanofiltration membranes

Isabel C Escobar and Maria R Coleman, Chemical & Environmental Engineering, The University of Toledo, and Tai-Shung “Neal” Chung, National University of Singapore.

Forward osmosis (FO) is an emerging membrane technology, often used for desalination. Like reverse osmosis (RO), FO uses a semi-permeable membrane to separate water from dissolved solutes.

The semi-permeable membrane acts as a barrier that allows water to pass through while blocking salts. Generally, any dense, non-porous, selectively permeable material can be used as a membrane for FO.

Professor Chung invented a novel polybenzimidazole (PBI) nanofiltration (NF) membrane with a narrow pore size and high flux for the FO process. PBI was chosen mainly because of its robust mechanical strength and excellent chemical stability.

However, at high ionic strength (normal in FO applications) and normal operational pH values, the charge of the membrane surface is neutral, leading to a decrease in electrostatic repulsion between the membrane surface and electrolytes to be rejected. Consequently, monovalent salts rejection decreases.

Ideal NF membranes for FO would be highly negatively charged at most pH values. Additionally, these membranes are hydrophobic and have a propensity to foul during operation.

Therefore, this collaboration is performing research to improve the separation performance of the PBI FO membrane by
  • (1) lowering membrane fouling via chemical modification, and
  • (2) increasing its monovalent salt-rejection via membrane surface modifications.

  • Since these properties are given to a porous PBI NF membrane instead of the conventionally used dense RO membranes, this project has the potential to revolutionize the field of desalination using FO. Development of a desalination process technology using the FO route has the potential to achieve up to a 75% decrease in costs and energy consumption in desalination, in comparison with conventional thermal or RO processes.

    This technology is still undergoing research.

    Acknowledgments

    The source of funding was the National Science Foundation, NSF OISE 0832894.

    This article appeared in D&WR magazine's May/June 2009 issue.

    Tags

    | Forward Osmosis | Fouling | Membrane


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