Desalination

Desalination

There are a few processes for separating the salt out of water. Evaporate, freeze, or filter it.

Evaporation

The 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

 

Multiple effect evaporators.

Reverse osmosis

Nowadays 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

More detailed explanation
   

Under development

Graphene

A 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 Navy

The process uses about 65 percent less energy than standard reverse osmosis.
Clean Technica

 

 

Larger image

 

Engineered Osmosis

Oasys Water has developed "Engineered Osmosis" that uses one tenth the energy of reverse osmosis.

The Engineered Osmosis™ process is a patented membrane-based desalination platform that can turn up to 15% salt water (approximately five times the salinity of seawater) into fresh water. The EO™ process uses a proprietary draw solution comprised of thermolytic salts to naturally draw fresh water across a patented semi-permeable membrane leaving unwanted salts and contaminants behind. 

 

The mixture of draw solution and fresh water is then heated to change the phase of the draw solution solute from liquid to vapor, leaving behind fresh, potable water. No feed water boiling. No high pressure pumping.

The EO™ process can utilize steam or heat from a variety or sources, allowing Oasys to address the growing fresh water supply gap without the negative impact and expense of fossil fuels. 

The system can achieve up to 85% water recovery, discharging brine of concentrations up to 25% salt.

Oasys Water

Oasys Water's "Engineered Osmosis"

Solar powered Seawater Greenhouse

A delightfully simple and clever greenhouse using seawater and the sun's heat

Seawater is evaporated at the front of the greenhouse to create cool humid conditions inside.  A proportion of the evaporated seawater is then condensed as fresh water that can be used to irrigate the crops.   Excess freshwater created in the Seawater Greenhouse can be used to irrigate additional crops grown outside the greenhouse.

 

The air going into the greenhouse is first cooled and humidified by seawater, which trickles over the first evaporator. This provides good climate conditions for the crops. As the air leaves the growing area, it passes through the second evaporator over which seawater is flowing. This seawater has been heated by the sun in a network of pipes above the growing area, making the air much hotter and more humid. It then meets a series of vertical pipes through which cool seawater passes. When the hot humid air meets the cool surfaces, fresh water will condense as droplets that run down to the base where they can be collected.

 

 

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.

Desalinating Microbial Fuel Cell

In 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.

Clean Technica

Gas2

MIT

  

Microbial fuel cell

An 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.