A story I wrote last week for MIT Technology Review takes a look at a new energy-efficient approach to desalination developed by a Vancouver-based startup called Saltworks Technologies. Conventional desalination relies on reverse-osmosis and costly membrane technologies. Pumping the water at high pressure through these desalting membranes takes a lot of energy, which drives up the cost of this form of desalination. Another approach is to evaporate and then condense the water, another energy-intensive approach.
Saltworks has a completely different, and quite novel approach. It starts by using the sun or industrial waste heat to evaporate one pool of seawater until it becomes concentrated with 18 per cent salt (compared to 3.5 per cent for regular seawater). This concentrated stream is pumped into a desalting unit along with three other regular seawater streams. The concentrated stream is connected by specially designed “bridges” to two regular streams, and because it has a higher concentration it is compelled to diffuse its salt content — sodium and chloride — into the weaker streams. But the bridge connecting to the one weaker stream only allows sodium ions, which are positive, to flow through; the bridge connected to the second weaker stream only allows chloride ions, which are negative, to flow through. The end result at this stage is that one of the two weaker streams now has surplus positive ions, mostly sodium, and the other has surplus negative ions, mostly chloride. The two streams, now out of balance and eager to pick up ions of opposite charge, are separately “bridged” to the third regular seawater stream. The one stream with surplus positive ions strips the third stream of all its negative ions, and the second stream with surplus negative ions strips the third stream of all its positive ions. This leaves the third stream completely stripped of all ions — i.e. it’s de-ionized, or pure drinking water.
It’s a brilliant process because most of the energy that’s required comes at the front end through evaporation, which is accomplished in a low-tech way with abundant solar energy, or waste heat from a neighbouring industrial facility. The rest is accomplished through electrochemical reactions requiring no outside energy source. If Saltworks can scale this approach up, it could bring cheap desalination to the many parts of the world that need it.