Earth's water is great, but 97.3% is that people still can not directly use the sea salt; freshwater only 2.7%, while the closest to human life, lakes, rivers and shallow groundwater and other freshwater resources, only the total fresh water reserves 0.34%, which can be another exploitation of only 0.2%. World's freshwater resources in time and space distribution is extremely uneven, so that the limited freshwater resources can not be fully utilized. According to statistics, more than 100 countries around the world short of water, serious water shortage has reached more than 40