They then expose the concentrated salt water to two separate solutions of regular salt water via two different ‘bridges’, one which is porous to chloride ions, the second which is porous to sodium ions.
The sodium and chloride ions migrate across the respective bridges into the salt water solutions to equalise the difference in ion concentration between the solutions.
This creates two charged solutions, one enriched with sodium ions (positively charged), the second enriched with chloride ions (negatively charged).
These two solutions are then exposed across two similar bridges to the water to be desalinated. This draws sodium ions into the chloride enriched solution and draws chloride ions into the sodium enriched solution: Net result desalination. Doing this they reckon they can produce 1m3 of water using 1kWh of electrical energy, which is used to pump fluids around the pipework.
Because the system is not under pressure, they can use plastic pipes instead of steel pipes, potentially reducing capital costs also.
I met with Saltworks about six months ago in Vancouver and I was impressed by the methodical way they have been going about technology commercialisation. Despite winning a technology innovation award in British Columbia in May 2009, they have kept this remarkably quiet. An article in the Economist provides a good review of this.