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Forward Osmosis (FO) Water Purification

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Organization: U.S. Bureau of Reclamation

Summary: Compared to reverse osmosis (RO) and other traditional forms of desalination, forward osmosis can significantly reduce the capital and energy costs of purifying water. The Bureau of Reclamation is engaged in collaborative research and development aimed at addressing the barriers to forward osmosis technologies. The Bureau has filed a patent for a driving solute, and has made significant progress developing a two-stage process that removes and recycles the driving solute back to the forward osmosis (FO) extractor, producing potable water that is free of salt and the driving solute. The Bureau has also developed cellulose ester hollow-fiber FO membranes that are thin, strong, and offer more desalting surface area per unit volume as compared to flat sheets.

Technology Need: Commercial desalination technologies used to desalinate seawater and inland brackish waters—including electrodialysis, reverse osmosis (RO), multistage flash distillation, and vapor compression—can be cost-prohibitive due to high capital and energy costs. Desalination using forward (direct) osmosis has the potential to significantly reduce both capital and energy costs.

Overview: Traditional RO uses high-pressure pumps to force seawater (or other contaminated water) through a semi-permeable membrane. The membrane allows only pure water to pass through as the salt and other contaminants are held back by the membrane. FO employs a membrane similar to RO; however, the saltwater is not pressurized to force it through the membrane. If two solutions are placed on either side of a semi-permeable membrane, water will move toward the side with the higher concentration of solutes. Thus, if a concentrated solution of water and fertilizer (“driving solution”) is placed opposite a volume of seawater (“source water”), the natural force of osmosis will pull pure water out of the seawater and through the membrane, resulting in a larger volume of fertilizer-charged irrigation water (“FO product”). Similarly, baby formula powder or liquid concentrate can extract fresh water from contaminated source water, producing a large diluted volume of liquid baby formula. With FO, selecting the proper driving solute determines the end use of the FO product, and the salinity of the source water determines its maximum dilution.

Advantages: Compared to pressure-driven membrane processes like RO, FO operates at low pressures, translating into less energy used. FO’s primary energy consumption occurs if the driving solute must ultimately be separated from the FO product. For example, to produce drinkable water from saltwater, a concentrated

driving solution is formulated using water and a chemical that can be efficiently removed from the resulting FO product, thus leaving pure water.

Benefits: Efficient membranes and an engineered two-stage FO process offer the potential for low-cost seawater desalination. As desalination plants become

more affordable, FO may be a new alternative to drinking water shortages in many areas.

Opportunity: The next steps consist of pilot testing and maturing the FO process and driving solution, and further development of hollow-fiber membranes. Reclamation is currently seeking a qualified industry partner to cooperate in further testing and commercialization.

Contact: Chuck Moody, Principal Investigator, or Samantha Zhang, Technology Transfer Coordinator

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