Reverse osmosis (RO) technology has been widely applied to water treatment such as seawater desalination, and lots of large plants with RO are in operation in the world. However, global water problems are still getting more serious, and much larger plants will be required to secure sufficient water resource in the near future. Mega-ton Water System project was carried out with the aim of developing key 21st century water treatment technologies for sustainable management of water environment and for low-carbon path. Low pressure RO membrane for seawater desalination has been studied in the project as a part of the core technologies to realize mega plant which is capable of producing 1,000,000 m(3) of freshwater per day. Fundamental and scientific research for RO membranes based on fine structure analyses by means of transmission electron microscopy with a special technique for sample preparation was conducted, and practical tools for designing new innovative RO membrane were acquired by the structure analyses to quantify the physicochemical and chemical properties of RO membranes. As the result of studying on structural design of RO membrane, low pressure SWRO membrane which enables to reduce energy consumption compared to conventional ones in the past of SWRO was obtained. (C) 2015 Elsevier B.V. All rights reserved.