Creating freshwater by purifying ocean water is called desalinization. Two of the most common ways to do this are reverse osmosis and distillation .
Why is this important? "Water scarcity impacts 40% of the world’s population, and as many as 700 million people are at risk of being displaced as a result of drought by 2030."- WHO
So why aren't we just purifying the ocean water then? Well, the short answer is, it's just too expensive.
The process of desalinization requires an immense load of energy, if we want to make it a viable option, we'd need to make use of renewable energy sources.
Currently, researchers are trying to use solar energy to power the process which makes sense as we often find that areas that need water tend to have a lot of sunlight available, but for some reason that I haven't been able to determine, the progress on this is very slow.
How else can we approach the water scarcity problems?
What other alternatives can be used to power the desalinization process?
All you need to desalinize water is a simple distillation (by any means). The simplest way to distill water is to make it evaporate by heating and then collect the vapor on a slightly cooler surface placed over the evaporating water source. Even though it's the simplest way, it's also the least efficient, because you lose a lot of energy and water vapor (given you don't use special equipment) in the process. However, when talking about huge amounts of water ( in this case seawater), naturally high temperatures (deserts or other arid locations), and the need to reduce external energy input, this method might become quite efficient.
Such passive distillation process is pretty much described in this creative contribution. The water can also be harvested without additional equipment, directly from the walls and roof of the dome. All you need is to shape the construction in a way that the vapor condensed on its walls and roof would drip down along the flat surface and be collected into separate vessels. Such collecting vessels could also be placed in the middle of the dome, above the pool of salty water to catch the condensed water dripping from the roof.
What it would take in the case of seawater is to dig canals from the coast inland in deserted coastal areas like Sahara, for seawater to flood them and construct "distillation greenhouses" over those canals. The soil is very hot in the daytime in such areas and this would make water evaporate fairly quickly. The temperature inside the greenhouse would be even more extreme. If it were necessary to reduce the temperature of the roof and walls of such greenhouses to speed up water condensation, white or reflective materials could be used to minimize the heat they absorb from the sun. The bottom half of the greenhouse could be made transparent to let the sunshine heat the water.
However, it's not clear how efficient such a method would be, also desalinization might be not the only process required (depending on what the water is used for). To purify it further to the level good for drinking, additional methods will be necessary. Increased salinity of the remaining seawater is another problem to solve as Darko pointed out.
What can we do with the salt to prevent it from going back into the water or soil
Darko SavicApr 07, 2021
I remember we touched upon this topic in the comments area under this idea.
Could salt somehow be processed to make it insoluble in water? There is plenty of water on our planet, it's just that the salt makes it unfit for us.
Membrane distillation technology and light-harvesting nanophotonics
Darko SavicMay 12, 2021
This video shows a desalination system that uses a combination of membrane distillation technology and light-harvesting nanophotonics.