What would it take to make fresh water in the desert? Well, one solution would be a solar still; a very simple device, it is essentially some sort of greenhouse with water inside it. The water evaporates and then condenses either on the sloped glass which it will then run down or by pumping the air to a cool chamber stored underground.
You could do this with all sorts of water, of course, but I was wondering what would it take to do this with sea water. You might think that this is straight-forward. Just use sea water and evaporate it off; but it is actually more complex than this, because you would end up with lots of gunk left behind.
Last year I visited a salt museum. The purpose of this is actually to produce salt, rather than water. Before I went there, I thought about how it might work quite hard, because there is a lot of stuff in the sea that you don’t want; the surprising thing is that it works entirely without filters or any others chemicals at all; just a series of ponds.
The way it works is this. The first pond contains what is essentially sea water. It is open to the sea at one point but only at high tide, which means the salt works does need to worry about tidal levels. As it is essentially a lagoon, it is full of life and even fish, and is just a bit saltier than the sea. The water is then tapped through a series of ponds. The next is more saline and kills of all the plant life, just leaving a bright red algal growth. The next is really, really salty and is sterile; the salt levels are so high that all the other ions – magnesium and so forth, deposit out, leaving a relatively pure sodium cholride solution for the final pool which are the salt pans where the water is finally driven off and salt left.
In the desert, of course, it is hotter and drier, so the process would be much more efficient. If you did all of this in a series of big greenhouses, you would have a solar still also. You would be able to harvest the plants, organic detritus, and even fish from the first pool. Salt would come from the final pool. And every pool would contribute to the production of fresh water.
All of these are useful products. The water, salt and fish are obvious. The plant and organic detritus perhaps less so. It would be too saltly for the use in agriculture. You might be able to use it in biogas digestion which should work with salty water. If not it could be taken back to the sea and sunk; spread over a wide enough area, it should act as a form of carbon capture without having detrimental effects on the sea life.
And how would you get the sea water to the desert and products back again? A Slow Flow train of course.