nergy requirement for single stage operation, a reduced potential for corrosion, and few scaling or precipitation problems. The disadvantage is that it involves handling ice and water mixtures that are mechanically complex to move and process. There are several different processes that use freezing to desalt seawater, and a few plants have been built over the past 50 years. However, the process has not been a commercial success in the production of fresh water for municipal purposes. At this stage, freeze-desalting technology probably has better application in the treatment of industrial wastes than in the production of municipal drinking water.Along with membrane and freezing purification comes solar humidification. The use of direct solar energy for desalting saline water has been investigated and used for some time. During World War II, considerable work went into designing small solar stills for use on life rafts. This work continued after the war, with a variety of devices being made and tested. These devices generally imitate a part of the natural hydrologic cycle in that the suns rays heat the saline water so that the production of water vapor (humidification) increases. The water vapor is then condensed on a cool surface, and the condensate collected as fresh water product. An example of this type of process is the greenhouse solar still, in which the saline water is heated in a basin on the floor, and the water vapor condenses on the sloping glass roof that covers the basin. By any means, salt water purification has a definite economic impact. Since desalination facilities exist in over 100 countries around the world, specifying exact costs for desalting is not appropriate. What can be said with certainty is that the capital and operating costs for desalination have tended to decrease over the years. At the same time desalting costs have been decreasing, the cost of obtaining and treating water from conventional sources has ten...
