Most of the water on Earth’s surface is too salty to drink. Dissolved ions regulate the flow of substances in and out out our cells. When someone drinks salt water, the ion balance is upset and water flows out of the cells. The cells become dehydrated. The more salt water that a person drinks, the worse the dehydration of the cells becomes. Excess salt is also harmful to many plants. The process of obtaining fresh water from salt water is called desalination. Water that is safe to drink is called potable water. (Chemistry 11 P432)

Distillation:
1)Laboratory scale distillation

Figure1:Laboratory display of distillation
Steam distillation is a method for distilling compounds which are heat-sensitive.The temperature of the steam is easier to control than the surface of a heating element, and allows a high rate of heat transfer without heating at a very high temperature. This process involves bubbling steam through a heated mixture of the raw material.( Laurence ) 2)Industrial distillation

Figure2 :Typical industrial distillation tower
In nature, this basic process is responsible for the water (hydrologic) cycle. The sun supplies energy that causes water to evaporate from surface sources such as lakes, oceans, and streams. The water vapor eventually comes in contact with cooler air, where it re-condenses to form dew or rain. This process can be imitated artificially and more rapidly than in nature, using alternative sources of heating and cooling.(U.S. Department of the Interior)
3) simple solar desalination plant

Figure3:how a solar still works http://water.usgs.gov/edu/drinkseawater.html The low-tech approach to accomplish this is to construct a "solar still" which uses heat from the sun to run a distillation process to cause dew to form on something like plastic sheeting. The diagram to the right illustrates this. Using seawater or plant material in the body of the distiller creates humid air, which, because of the enclosure created by the plastic sheet, is warmed by the sun. The humid air condenses water droplets on the underside of the plastic sheet, and because of surface tension, the water drops stick to the sheet and move downward into a trough, from which it can be consumed.(U.S. Department of the Interior)
Reverse Osmosis

Figure4:the process of the reverse osmosis A more energy-efficient process for producing potable water is reverse osmosis (see figure 4).Osmosis is the natural tendency of a solvent, such as water, to move through a semi-permeable membrane to make the concentrations of solutes on both sides of the membrane equal. Water can pass through the semi-permeable membrane, but solutes, such as salt, cannot. If a semi-permeable membrane separates two aqueous solutions with different concentrations, water will flow from the mire dilute solution into the more concentrated solution until there is no longer a difference in concentration. In reverse osmosis, high pressure is applied to the more concentrated to force water through the semi-permeable membrane in the opposite direction.(chemistry11 P432)

Municipal water treatment

Figure5: Municipalities use a combination of physical and chemical processes to purify water.
Figure5 gives an overview of the processes that are used by municipalities to treat drinking water. In general, water enters a treatment plant from a surface source, such as a lake, river, or reservoir,or from a ground water source, such as a well or spring. On its way to the storage tower, the water is filtered, treated with chemicals to remove suspended particles, and treated with chlorine or ozone to kill harmful bacteria. After treatment, the water is tested to make sure that it is sure and meets current water standards.(chemistry 11 P434)
Figure 5 illustrates that the process is working from left to right. First, salt water come into a coarse screen made of metal bars filters