On the Earth, the sea water occupies about 97%, the fresh water is only about 3%, and the most fresh water exists in the form of glacier on earth's surface. The fresh water used by humanity only accounts the 0.26% total quantity of global water resources. According to the statistics of United Nations, at present, the whole world surpasses 10 hundred million populations to live in the water resource lacking area, and to 2025, the populations will increase to 18 hundred million. Along with the climatic change, the drought, the requirements of populations and factories, the demanded fresh water is getting more in different countries, causing the supply of fresh water to become large deficiency. In 1965, a small reverse osmosis process desalination factory was constructed in California Coalinga. Until the later stage of 1960, a daily output 8,000 ton seawater desalination factory using the thin-film method was constructed and operated in business. Nowadays, approximately 133 nations had used seawater desalination system to create approximate 32.4 million tons fresh water in one day. Until today, the whole world had over 13 thousand seawater desalination factories in operation, which could produce 0.5% global consumption fresh water. There are two major method are developed to get the fresh water, the evaporation method and the thin-film method. The evaporation method includes the multiple effect distillation (MED, desalination efficiency is about 8.5 %), multistage flash distillation (MFD, desalination efficiency is about 14 %), and vapor compression evaporation (VCE, desalination efficiency is about 38 %). The thin-film method includes reverse osmosis (RO, desalination efficiency is about 35 %), Electro Dialysis (ED, low efficiency), and nanometer-filtering method (still in the lab-researches). At present, the reverse osmosis process is the main seawater desalination method, the amount of daily output is over 14 million tons. Multistage flash distillation is the next process, the amount of daily output is over 12 million tons, and the daily output of Electro Dialysis method is over 1.46 million tons. The reverse osmosis method and multistage flash distillation method are the two major seawater desalination methods. The used methods of the multiple effect distillation, multistage flash distillation, vapor compression evaporation, and reverse osmosis have many shortcomings. The shortcomings are large space needed, long factory constructed time needed, high factory constructed fee needed, low flexibility for factory construction, and high electrical power needed. For that, finding a method to improve the shortcomings mentioned above is necessary, especially for the important key points of high factory constructed fee and short factory constructed time. |
