Mary Stone, age 9, of Rockford, Illinois, for her question:
Can we get fresh water from the sea?
The water of the salty sea is poison to the human body. A shipwrecked sailor on a raft sees oceans of water on every side. But if he has no fresh water, he may perish from thirst, or so it was in olden days. Nowadays, a life raft has stashed away food and medical supplies. And most likely it has a little kit to change salty sea water into drinkable fresh water.
The salty chemicals in sea water are dissolved in tiny particles. The particles are too small for our eyes to see. But there are a lot of them and the mixture is downright dangerous. Drinking sea water is like drinking deadly poison. The water itself is fine, just like fresh rain water. The problem is the large amount of assort¬ed chemicals dissolved in it. The trick, of course, is to separate the water from these salty chemicals. Then we have fresh, pure drinking water. And our clever scientist know many ways to do this job. Some of their tricks are being used right now to separate fresh water from salty sea water. Others, we hope, will do the job even better in the future.
Desalination is a very grand word. If you know that it means taking out the¬ salty chemicals, you can use it to astonish your friends. The job of getting fresh water from the sea is desalination. In Freeport, Texas, they have a desalination plant that looks like a regiment of giant tanks, with stairways and catwalks. Seawater is piped in and heated above boiling point. It is sent through various chambers to turn it into steam. Then the steamy vapor is cooled into drops of fresh water that drip into storage tanks.
When water changes to steam it evaporates. It goes off and leaves the chemicals dissolved in the mixture. The salty chemicals in the sea water are left behind when the water changes to steam. The cooled steam that drips into the storage tanks is pure, fresh water. In fact, it is so fresh that it tastes flat. A little salty water from brackish wells may be added to give it some flavor. The desalination plant in Freeport uses salty water from the Gulf of Mexico. It produces more than a million gallons of fresh water every day.
At Wrightsville Beach, North Carolina, they have a different sort of desalination plant. There they freeze the salty water from the sea. They have special tricks to make the ice separate from the salty chemicals. Then the salt free ice is melted and it becomes pure water. In Webster, South Dakota, they need to separate fresh water from briny wells. This water is not nearly as salty as sea water. So the job can be done with electricity. The water is put into tanks that are fitted with metal plates called electrodes. The electrodes are connected to an electric current. The elect¬ricity draws out the salt through two layers of skin type material called membrane. This desalination process is not strong enough to separate fresh water from the salty sea. But the plant in Webster yields 250,000 gallons of fresh water a day from briny well water.
Many desalination plants already are working fine in many parts of the world. But scientists dream of even better ones. Pretty soon we expect to have several nuclear power plants working to take the salt out of sea water. They could give us lots of low cost fresh water. Scientists thing that propane also could be used to get fresh water from the sea. And they are still searching for better plastic membranes, strong enough to take all the chemicals from sea water.