Navy

Converting seawater to fresh water with forward osmosis

Forward osmosis desalination process using ammonia bicarbonate

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“Water, water everywhere, nor any drop to drink.” This technology may prove Samuel Taylor Coleridge wrong.

Potable water for field operations is often made in large quantities from local supplies of fresh, brackish, or saltwater using the reverse osmosis (RO) process.

The military has used RO technology for water purification with great success for many years. However, reverse osmosis technology is significantly more complex and energy intensive than conventional municipal processes. Reverse osmosis requires seawater to be pressurized to 900-1,000 pounds per square inch for operation.

Forward osmosis (FO) has been used to a lesser extent to produce potable water from saltwater. This process uses a semi-permeable membrane that allows the passage of water but blocks the passage of salts. In FO, the driving force pushing water across the membrane is the difference in osmotic pressure. For FO, seawater is on one side of the membrane and a draw solution is on the other side of the membrane. The draw solution has an osmotic pressure that is significantly higher than that of seawater. This allows the draw solution to pull water molecules from the seawater through the membrane in an effort to equalize the osmotic pressure on both sides of the membrane.

One of the issues related to FO systems is the identification of draw solutions that are non-toxic and can be easily separated and reused. Meeting these needs, Navy researchers have seized upon ammonium bicarbonate (a common rising agent used in baking) as the draw solution in an FO system. The design only necessitates heating to approximately 150° Fahrenheit, a temperature at which the ammonium bicarbonate decomposes into ammonia, carbon dioxide, and water.

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2%
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$2,000

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