Although the distillation process was initially applied in seawater desalination plants to solve the current global water shortage, it has been replaced by the energy-saving type reverse-osmosis membrane process worldwide since the beginning of this century. Considering the population increase and economic development, the Mega-ton Water System' research project was conducted to identify the element technologies required for the 21st century in Japan. Enhancing the operational reliability of seawater desalination plants, these development technologies also enable the establishment of earth-friendly seawater desalination plants anticipating considerable energy savings, lowering of the environmental impact, and reduction of water production costs.