DESWARE is a subset of the Encyclopedia of Water Sciences, Engineering and Technology Resources, which is part of the UNESCO Encyclopedia of Life Support Systems (EOLSS)

Encyclopedia of Desalination and Water Resources

Distillation is one of mankind’s earliest forms of separating fresh water from a salt-water solution. When salt water is boiled, the dissolved salt remains behind as the fresh water vapor is boiled away. In a distillation process, water is first boiled and then the steam, or water vapor, is cooled. This cooling condenses the steam into water again (See the figure). Thus, distillation involves adding heat energy to salt water in order to vaporize the water and then removing the heat energy from the steam to condense it into fresh water.

In nature, this basic process is responsible for the hydrologic cycle. The sun 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.

When water is heated, its temperature increases until the boiling point is reached.  While water is boiling, the steam and the boiling water are at the same temperature. However, raising water to its boiling point is not enough to cause it to boil. More heat must be added to change the water into steam. The amount of heat required to change water at its boiling point into steam at the same temperature is called heat of vaporization of water. The heat of vaporization is of major importance in distillation. The amount of heat required to vaporize water into steam is approximately five times greater than the heat needed to raise water from its freezing point to its boiling point (at ordinary sea-level atmospheric pressure (14.7 psi) water boils at 100oc).

Distillation is a two-step process involving both evaporation and condensation, heat must be added in one step and removed in the other. If these two steps were accomplished independently, the process would be inefficient and costly. In all the distillation processes, the steam is condensed by transferring heat from the steam to salt water as part of the heat source required to convert more water into steam. In this way some of the heat energy used in one step is recovered and used in the other step.

The conversion of saline water into fresh potable water and water for industrial purposes has been practiced worldwide over the last forty to fifty years of the twentieth century. It is a technique of providing and augmenting freshwater supplies in areas deficient therein, such as arid regions close to the ocean, or to other saline water bodies. Read more..

The global water crisis

Water has always been earth's most valuable resource. All ecosystems and every field of human activity depend on water. The world's supply of fresh water is running out. Already one person in five has no access to safe drinking water. The amount of water in the world is limited. The human race, and the other species which share the planet, cannot expect an infinite supply. 97.5% of the total global stock of water is saline and only 2.5% is fresh water. Approximately 70% of this global freshwater stock is locked up in polar icecaps and a major part of the remaining 30% lies in remote underground aquifers.

Population growth and the increasing need for fresh water for industrial, agricultural uses and municipal indicate there will be no letup in the increasing demand for water in the years to come. These factors account for the concern over water shortages that exist now in some areas of the country and over the more serious shortages that are projected for the near future.

See : Desalination and the Continuity of Human Civilization

Dead vegetation in drought-stricken area, Senegal

1- Dead vegetation in drought-stricken area, Senegal.

2- Children have a special	relationship with water

2- Children have a special relationship with water (Source: UNESCO Photo bank)