Re: desalinization

Jennifer,

As I do not know which desalinization process your are asking about, I will answer your question in general terms about the more common desalinization processes.

Desalinization, as you probably know, is the process of removing salt. I will talk about removing salt from water (like sea water) and then I will also discuss salt removal from soils. I will also try to describe the natural and the man-made processes involved in each.

I guess, first off, I will need to define salt not only as the salt that you use to season your food (which is mostly sodium chloride, NaCl) but also other types of naturally occurring salts. These include salts made with potassium, calcium, and magnesium. Realize that when I say salt in your answer, I am actually talking about all of these.

Desalinization of sea water the natural way, occurs simply when the water is evaporated off and leaves behind salt. Sea water only contains about 3.5% of salt, so you have to evaporate off a lot of water to get a small amount of salt. The rate of evaporation depends on a number of factors. These include (but are not limited to): air temperature, humidity, and depth of water. We will assume that the water is stagnant, that is no water is being added by tides and such. Generally, as air temperature goes up, evaporation is faster. Humidity, which is a measurement of the amount of water vapor in the air, has an inverse effect on evaporation. Generally, as humidity goes up, meaning the air has more water vapor in it, evaporation goes down. In other words, the air cannot hold more water therefore no more water is evaporated. Water depth is pretty easy to illustrate. Let us say that you have two glasses of water, equally filled. One glass you pour into a large baking dish, so that the water is only a thin film on the surface, the other glass you pour into a narrow necked wine bottle. You will find that the water in the baking dish will evaporate faster (given temperature and other factors remain the same for both). This has to do with surface area. That is, the amount of area that is exposed to air where water molecules can move to is larger in the baking dish than in the narrow necked bottle. This is in effect the same way that salt is naturally deposited in wide shallow pools faster than in deeper narrower pools.

Artificially we can remove salt from water using two ways. One is called distillation and the other is by osmosis. Both of these systems require external power sources to operate (although many desert desalinization plants probably employe solar panels to generate heat and electricity to operate the equipment). In distillation, the water is heated and the steam produced is channeled off so that it cools and recondenses elsewhere in the system and is collected. Impurities, including salt, are left behind in the boiling chamber. Osmosis employs a filter membrane that will allow pumped water to pass through it, but not impurities. As you can see, both of these systems require external pumps and devices. Rates of desalinization of water will depend on the size of the operation, the rate of pumping and the capacity of the systems for the collection of impurities before it needs to be cleaned. Take a trip to here for a list of ten web sites that deal with desalinization. Go to here to learn some things that can be done with sea water and sea salt.

Now, so far we have talked about salt in water. Desalinization can also mean removal of salts from soil. Salts can occur naturally or artificially in soils. In nature, salts come from rain falling, surface water or the degradation of bedrock. As the water evaporates either in or on top of the soil, the salt is deposited. Mankind has also responsible for putting salt into soil In the arid regions of the US. where irrigation is used extensively for farming, salt poisoning of crops and fields has become a big problem. Basically, here we have water pumped out of rivers or from the ground, sprayed or allowed to flow over land to water plants. As the water, which, unless filtered, has lots of salts dissolved in it, evaporates, salt is deposited on the top of and in the soil. Almost all plants are very intolerant to salt and will die. After the salt is on and in the soil, no more plants will grow there and the fields can become large dusty flat areas covered with a crust of salt. You can desalinate the soils.

One way to remove salt from soil is to leach it out. Here, a drain system is installed in the field. Large amounts of fresh water are added to the field and the salt dissolves in the water which is moved off the field by the drain system. The collected water can then be treated further to remove the dissolved salt. Another way to make the field productive again is to chemically change salt that is more harmful to plants to chemicals that plants can easily tolerate. Finally, the soil can be dug up, literally washed like in a washing machine and then put back into place. Obviously, the last alternative is not very realistic because of costs, however it could be done. The sort of method is used to remove hydrocarbons (gasoline spills, leaking gas tanks, oil spills) from contaminated soils. Go to here to learn more about this.

For the actual rates and natural laws that govern the rate of evaporation of the water, I will have to ask you to resubmit your question to the physics category of the Mad Scientist Network.
Hopes this helps.
Ted