Desalination
There are a few processes for separating the salt out of water. Evaporate, freeze, or filter it. EvaporationThe traditional way of desalinating water was to boil it off and condense the steam. However, this wastes a huge amount of heat.
To heat 1 kg of water from 20 to 100oC = 340 kJ
To evaporate 1kg of water = 2,270 kJ
As the steam contains so much energy it makes sense to use this heat to evaporate more water. Then the vapour from that evaporation can be used again, and so on.
However, a small technical detail. To cause evaporation there must be a temperature difference. The vapour must be hotter than the liquid it is evaporating.
There are two ways of doing this.
1) Vapour recompression - Compress the vapour, to heat it, or,
2) reduce the pressure in the liquid being evaporated. This will cause it to evaporate at a lower temperature.
The equipment doing this is called multiple effect evaporators. There are normally 3,4, or 5 effects depending on the price of materials vs energy
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Multiple effect evaporators. |
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Reverse osmosisNowadays the best process is reverse osmosis. It is simply the process of pushing salt water through a sieve so fine that water molecules pass through, but not salt ions. This needs a lot of pressure because the process of osmosis would normally send the water the other way. The pressure can come from electric pumps, or wave driven pumps. This website - wave energy |
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Under developmentGrapheneA team of researchers at MIT has developed a way to use atom-thin sheets of graphene for water filtration, which could lead to an inexpensive and energy-efficient way to desalinate seawater.
US NavyThe process uses about 65 percent less energy than standard reverse osmosis.
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Engineered OsmosisOasys Water has developed "Engineered Osmosis" that uses one tenth the energy of reverse osmosis.
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Oasys Water's "Engineered Osmosis" |
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Solar powered Seawater GreenhouseA delightfully simple and clever greenhouse using seawater and the sun's heat
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The cool and humid conditions in the greenhouse enable crops to grow with very little water. When crops are not stressed by excessive transpiration, both the yield and the quality are higher. The simplicity of the process imitates the hydrological cycle where seawater heated by the sun evaporates, cools down to form clouds, and returns to the earth as rain, fog or dew. A greenhouse farm irrigated with water from salt water. |
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Desalinating Microbial Fuel CellIn a microbial fuel cell, organisms feed on available nutrients and generate an electric current as they metabolize the food. They can also be modified to desalinate water and generate hydrogen. We will hear a lot more about this in future. Geobacter, a hairy looking organism that is actually capable of generating an electric current from mud or wastewater. Professor Derek Lovley and a team of researchers at the University of Massachusetts Amherst have announced that they successfully evolved a strain of Geobacter into a superbug that is eight times more powerful than other strains. The “pili,” hairlike protruberances that festoon Geobacter like nanowires. They create a thin biofilm that conducts electrons from the organism to iron in the mud or wastewater. Other bacteria colonies also anchor themselves to a food source by attaching a biofilm to it, but Geobacter is especially skillful at electron transmission. |
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Microbial fuel cellAn entirely different approach is illustrated by the University of Colorado, where researchers are working on a microbial fuel cell that can run on seawater or wastewater. Aside from generating electricity, the new fuel cell would desalinate and purify water, and also produce hydrogen gas. |