Why is seperating salt from salt water so hard and expensive?

Question

Why is seperating salt from salt water difficult? What theories are out there for new ways to do it? What about evaporation, but then how will we take the gaseous H2O back to liquid state?

Answer 1

This is basic thermodynamics.

Dissolving salt in water causes a net increase in "Entropy." Therefore, according to the "Second Law of Thermodynamics", this process cannot spontaneously reverse itself, but it requires an *external* source of energy in order to be separated again.

This is, in a sense, similar to a boulder at the top of a hill. The boulder can roll *down* the hill by itself, but requires someone to *push* it back up, to return to it's original state

More specifically, there is at least a *minimum* amount of energy, that is required in order to separate the salt and the water. The amount of energy needed is proportional to the original change in entropy.

Since I am guessing you are not familiar with thermodynamics, you'll just have to trust me on this one; removing salt from seawater is always going to take a reasonable amount of energy.

More on this later, have to leave now...

Answer 2

It is not difficult, but it requires a lot of energy, so much so that it is not feasible to supply municipal water systems from distilled sea water. The process itself is simple: boil the sea water. The water will boil away and leave the salt (brine) behind. The water vapor is then collected and condensed by passing it over tubing with cold liquid flowing through it. The condensate is then collected and stored as water. You need energy to boil the water, pump the brine back to the sea, cool the condensing medium, pump the condensing medium, and to pump the condensate to where it will be stored. It is also a relatively slow process and you need to make sure the water is boiled for a sufficient amount of time to kill bacteria.

Answer 3

Its easy to collect salt from sea water ...if you don't want to keep the water.
The hard part is to collect potable water from brine.
Distillation is the most effective but requires a lot of energy to boil the brine.
De-salination plants are the modern way of doing it. Salt water is pumped past a membrane which will pass water but not salt molecules (because they are bigger). It requires less energy to run pumps than heaters.
They don't take all the water because its easier to pump saline sludge to ocean outlets than dig out solid masses.
It has been observed that marine life thrives in areas where sufficient currents prevent the ocean from becoming too salty in a local area.

Answer 4

In the desert regions like SaudiaArabia, the salt resides on the sea side due to evaporation and the just need to pick that salt up. But in cold countries it is mostly done through distillation and salt mines.

Answer 5

It is not hard and expensive to remove salts from water, it is done all the time. The process is called Reverse Osmosis. Where I live they just opened the largest inland RO plant in the world, and it will eventually supply 25% of our drinking water.

Answer 6

One family uses 500 l. of water per day
One city with 10,000 families consumes 5,000,000 l. of water per day.
How much energy you need to evaporate that amount of water every 24 hours?
Liquid again is not the issue.

Answer 7

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Answer 8

Below is a link to a similar question of yours, I posted one year ago.