Why is it so difficult to turn sea water in to drinkable water?
In: Chemistry
It is not difficult, reverse osmosis works exceptionally well.
But…. it takes a lot of energy to run the pumps, and it takes water to back-flush system.
Energy = $$$$. Most desalination plants are conveniently located near power plants for this reason.
The problem isn’t that it’s difficult. It’s fairly easy to do. The problem is that clean, fresh water is abundant (and thus cheap). Therefore if you want a method of converting sea water to fresh water to be commercially viable, it has to be even cheaper than that.
The easiest way to separate them is to heat the water till it evaporates then cool down the “steam” till it turns back to water again, unfortunately it takes a lot of energy to evaporate water.
Naturally occurring osmosis will take place in nature. Simply reversing that process by applying a pressure great enough to change the direction of the process (between 15-70 bar dependent on temperature of water) and for ring it through a semi permeable membrane (filter) will create demineralised water. From them just need to add some minerals to make it consumable again for humans.
You might be interested to watch Manoj Bhargava’s rainmaker project on YouTube (Sorry I can’t post the link). He can convert a thousand gallons of sea water into drinking water in one hour. It’s mildly interesting stuff.
The bigest problem is the size of thesalt molecules in sea water – they are so small that we can’t just filter them out normally. The flow would be minimal, since even the water molecules would have trouble getting trough. We counter this by forcing the water trought the filter (pressuring it) which requires a lot of energy. This is the reason obtaining drinking watter this way (which is called “reverse osmosis” by the way) is so innefficient and not wide spread allready. It works nicely, but it’s just not cheap.
ELI5: we filter seawater, but the size difference between water and salt is so small, the filter can cloth easily, we use preassure to counter this but it requires energy and money in turn
It isn’t technically difficult to turn salt water in to drinkable water, it just takes a lot of energy to do it. The reason for this is entropy, which in layman’s terms is a measure of disorder and randomness. The universe likes to be random, but it takes effort to make it orderly. Think about if you had a jar of salt, and a jar of sugar. If you pour them out into one pile, they will mix together with very little effort. But separating them back out into separate jars of sugar and salt again takes a lot of effort. When you put salt into water, the salt molecules spread out into the water molecules. Pulling them out again into separate containers takes a lot of energy.
Making salt water potable (drinkable) can be done in a number of different ways, including:
1. Distillation – Heat the water up until it boils into steam. The steam leaves the salt behind. Cool the steam back down and you have fresh water.
2. Reverse osmosis – With a lot of pressure, you can push water through a filter so fine that the salt molecules can’t go through it, and you get fresh water on the other side.
3. Chemical – You can make the salt molecules bond to chemicals added to the water, then remove those chemicals to get fresh water.
All of these methods require a lot of energy, in the form of heat, pressure, or manufacturing of special chemicals. All of these things have a high cost, and there aren’t many applications where it is cheaper to desalinate water than to just ship fresh water in from somewhere else.
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