How Is Salt Water Filtered into Fresh Water?

Desalination

Desalinization is the process by which salt is removed from water so that it is drinkable. Typically used in coastal desert areas or in areas where water is otherwise scarce, desalinization is a slow, deliberate process that can be used to keep up with the constantly increasing demand of fresh water for drinking and irrigation. Unfortunately, desalinization can be expensive and the process is too slow to be used as the sole source of drinking water for a large population.

Desalinization is typically carried out by a process of reverse osmosis. Reverse osmosis is the method by which a solution of a concentration tends to equalize with the surrounding liquid, even on the other side of a semipermeable membrane. In the case of desalinization, large tanks of fresh water are separated from salt water by a membrane through which salt cannot pass. The only thing separating the two water tanks is the membrane, and this leads the seawater to seek "equilibrium." That means that the liquids on both sides of the membrane will try to even out the salt concentration, giving them the same salinity. However, as the salt from the brackish water cannot pass through the membrane, only the water passes through. Since the barrier is one way, this process will continue until the pressure on one side becomes too much for the osmosis to overcome. For this reason, high-pressure tanks are used so that the pressure on the salt water can be kept up to coerce the water molecules through the membrane while leaving the salt behind. Of course, this quickly leads to clogged membranes, which must be regularly replaced. The replacement of the membrane means that the process must come to a halt, then start over again. Having to replace the filter accounts for much of the time-consuming nature of the desalination process.

There are other desalination methods; vacuum distillation remains among the most popular. Vacuum distillation is the process by which water is boiled at below-normal pressures, which means that the water is boiled at a much lower temperature. This saves a great deal of energy in long term over heating water at normal atmospheric temperatures. Of course, this technology is older than reverse osmosis filtration and has a significantly higher energy requirement than reverse osmosis. According to The Wall Street Journal, in excess of 12 billion gallons of water is desalinated around the world every day.