Reverse osmosis (RO) membranes have been widely applied to seawater desalination and wastewater reclamation. Since 2000, large RO plants (100,000 m(3)/day plants) have been constructed. However, much larger plants will be required in the near future because of its worsening situation of water problems. Energy efficiency is indispensable for the larger plants, especially mega plant (1,000,000 m(3)/day plant). "Mega-ton Water System" project which consists of eight themes believes that there is plenty of room for improvement as well as optimum layout of each component used in the mega plant. In order to reduce energy demand in RO operation, high efficient RO membrane is studied in the project. The relationships between RO membrane structure and its performance were investigated by transmission electron microscope-tomography. The utilizing of pressure retarded osmosis (PRO) energy recovery is one of a solution for energy-saving and defusing the environmental impact caused by concentrated brine. A PRO hollow fiber membrane module was newly developed and a practical continuous operation has been examined at a prototype plant for 1 year. The concept of the project will be established by combining the results of eight themes as competing with other membrane and its process studies. (C) 2012 Elsevier B.V. All rights reserved.