Today, finding a source of fresh water is becoming more and more difficult. Many of our streams, rivers and lakes have been contaminated with sewage, and many towns and cities obtain their drinking water from these same streams, rivers and lakes. To prevent this constant contamination, sewage treatment plants are being built in many places.

Fresh water on the world
Availibility of fresh water

These are capable of converting sewage into pure drinkable water. Another way to solve the problem of fresh water is to make use of the most abundant source of water we have: the sea. If we could learn  to get potable water from sea water easily and cheaply, we would solve the problem.

Fresh Water sTRESS
Fresh Water is in risk

Man cannot live on sea water directly because of the high proportion of minerals (mainly salt) in it. More than 2% of salt in a solution is dangerous for the human body. Sea water contains 3.5% of salt. Such a high quantity causes dehydration in human body; that is, the body loses the liquids necessary for life. Thus, it is necessary to reduce the percentage of salt in sea water to an acceptable level before using n.

Global Fresh Water per capital
Fresh Water Per Capital

How to reduce the salt in the  water or fresh water or sea water ?


A number of methods can be used to do this. The most common method is distillation. Sea water is heated until the water evaporates and the salt is left behind. The steam then condenses into pure water. Another method is freezing. When this is done, the water freezes first, leaving the salt behind. The ice is then removed and pure water is obtained.

Water in the atmosphere
Water storage in the atmosphere


A third method is called reverse osmosis. Pure water molecules are separated from the salt molecules under great pressure.  Recently scientists have been working on a completely new idea: The idea of obtaining fresh water from the air. Winds coming from the sea carry a lot of water vapour. This vapour condenses into water if it strikes something cold. If scientists can build a large condenser, then they can collect and store fresh water easily. Unfortunately, the main 30 problem with all of the mentioned methods is their high cost. That’s why scientists are looking for ways of reducing the cost.

Fresh water cycle
The renewable fresh water cycle

A. What do the following refer to?
1. These’ (line 6):
2. ‘it’ (line 17):
3. ‘to do this’ (line 17):

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