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The Secret of Forward Osmosis in Desalination

Updated on November 18, 2014

Unlike reverse osmosis, which uses excessive pump energy and electricity to force salt water through a membrane that separates the salt from the water (high pressure is required from the pumps to do this), forward osmosis uses a natural chemical solute to draw naturally the same process. It relies on the natural pressure difference between the salt water and draw solute to do this. If the correct system works, the cost per gallon in energy will be much less than if done in reverse osmosis, which is the ultimate goal. Clean, drinkable water, for much less. Today, it is a costly affair for most RO systems in operation. The few FO systems that exist today show that desalination costs can be greatly reduced.

Most RO and FO systems have similar components and the general schematic have similar patterns. The salt water enters and is treated to some degree before entering a series of membranes that separate the salt from the water. The brine from the water molecules is discharged back into the ocean. The water is treated further because it is not drinkable. Heat exchangers are used to either heat or cool the water as it travels through the system to or from a coalescer and through numerous filters and carbon filters. By the time the water reaches the end stage, calcium may be added or other chemicals to make it appealing for human consumption. The bigger the systems are that produce 50,000 million gallons of water per day are more complex to build and maintain costing millions. So, finding a way to create clean water cheaply (in terms of energy used to create it) is the goal.

FO systems use far less electrical pumps than RO systems because FO uses the natural pressure difference between the salt water and draw solute. No pumps force the salt water as in RO. There are some 14 draw solutions in various evaluation stages, 7 of them, show real promise. Some of these are complex concoctions of chemicals, diols, nano complexes. Some are food based, meaning a human could ingest them because the composition is found in many foods. But these have their own problems: how does the FDA become involved since it is food based? Do they? Is approval needed? What are the effects on a human if the food based solute is ingested? Do studies need to be done before used for water purification? Most of these food based solutes are tasteless because they mix well in water after salt and water molecules are separated by the membrane. The draw solute enters a coalescer to where under extreme heat of 90 degree C or so, the mixture separates and mostly pure water is filtered and treated at the end stage. Yet, even at this stage, small amounts of the solute remains. Is it dangerous to humans? Other chemicals, like boron, remain in small amounts that fail permissible water regulations. It is hard to remove completely.

The two things that FO systems uniquely have are special membranes able to filter at the micron level and the draw solution.The membranes allow for the salt water to enter on one side, while the draw solution, allows the water molecules to pass through into the solution leaving the salt behind. The membranes are usually custom made and easily can cost $4-7,000 each. One renowned FO\RO specialist membrane manufacturer is Toyobo of Japan. Their membranes are used in many such systems.The secret of the FO system is finding the right combination of membrane and draw solution and separating the water molecules from the solution in such small amounts that it does harm humans when consumed. Another element is to how to use waste heat (steam, solar etc.) most effectively to avoid the use of electrical pumps or reduce their use. This would decrease the cost in kilowatts to produce clean water. FO has been shown to work well in small systems (less than 10 mgd) when compared to RO, but the challenge is in the larger systems.


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