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Name: Luis A. C.
Status: N/A
Age: 40s
Location: N/A
Country: N/A
Date: 10/13/2003


Question:
I need to know the steps in the process of removing salt from water to make it potable. There are a number of ways to remove salts from water:


Replies:
1. Distillation. The salt water is repeatedly evaporated and condensed in a "reflux" condenser. At some point, determined by the design, the condensed water in the reflux condenser is tapped off. If the condenser is sufficiently efficient (that's easy in the case of inorganic salts) the water is very pure (distilled water).

2. Sequential freezing. Here salty water flows over a rotating drum that is cooled below the freezing point of water. Some of the salty water freezes out on the drum. Water has a rather complicated crystal structure that prevents inclusions of most salts in the crystal lattice. So pure water crystallizes out on the drum. There are a number of mechanical/chemical devices to rinse off the layer of pure water ice that forms. This category of purification is used in restaurants and bars. In such establishments you will see that the ice cubes are crystal clear with no inclusions of air like you observe in home-made ice cubes. In addition to the salts, dissolved gases are removed from the water. The process is very efficient for water that is not too salty.

3. Reverse osmosis. If one separates salty water and pure water by a membrane that allows water to pass through it (but not any of the dissolved ions) the natural course of events is that water from the "pure" water side will diffuse through the membrane to dilute the salty water. This transport comes to equilibrium when the salt concentration on both sides of the membrane is the same. This process is called osmosis. In reverse osmosis a high pressure is applied the salty side of the water separated by the membrane that will allow water to pass, but not the ions. If the pressure is high enough the water is "squeezed" out of the salty side into the pure water side. The pressures required are rather high and the membranes are usually not 100% efficient, so in commercial systems there are some engineering design problems to make the membrane sufficiently mechanically strong to withstand the pressure difference across the membrane, and a series of membranes may be required to improve the overall efficiency.

4. There are certain polymeric filters that exchange H(+1) ions for cationic ions M(+n) and OH(-1) ions for anionic ions A(-m). These remove the salts for H(+1) and OH(-1). Of course, here one may need to be concerned about the final pH of the effluent water. I do not think that this method is widely used.

Vince Calder


Luis, Although there are relatively simple ways to destroy pathogens in fresh-water and render it potable, about the only thing one can do to separate salt from water is to distill it. I know of no non-toxic chemical that you could add to the salt water which would precipitate all the various salts dissolved therein. Careful distillation, on the other hand, will boil off the water in a relatively pure form and leave the non-volatile salts behind.

Regards,
ProfHoff 735


Luis,

Here is a partial listing of treatment methods used for removing salts from water. Distillation-or evaporation of saline water, to produce freshwater goes back to antiquity. This was the only process seriously used or considered prior to World War II. Today distillation is one of the most widely used saline water process used on seawater.

Electrodialysis(ED)-was developed after World War II and was the first process to demonstrate that there was a practical method other than distillation for purifying salt water. In ED, salts and minerals are removed from a stream of saline water through special plastic membranes by the action of direct electrical current. Freezing-if saline water is frozen in such a way that salt crystals or brine pockets are not trapped in the ice crystals, the resulting ice will be essentially free of minerals. Ion exchange-processes have been used for more than 40 years in the zeolite softening of water.

Reverse osmosis-is presently widely used in the treatment of brackish water and seawater for drinking water supplies.

I hope that this gives you a start on your project.

Sincerely,

Bob Trach



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