Water is an important "resource", not only for sustaining human life but also for facilitating daily life and economic activities. With the rapid population growth and economic development of the world, water problems such as water shortages and water pollution are becoming more serious. With time, these issues are expected to worsen and might even trigger international disputes. The biggest issue is that approximately 900 million people in developing countries, particularly Africa, do not have easy access to safe water. As the Sustainable Development Goals (SDGs) adopted in the 2015 United Nations Summit prioritize the resolution of water issues, "securing a stable supply of safe water" is recognized as an important issue for all of humanity.

The Earth, with approximately 70% of its surface covered with water, is also called the "water planet." However, most of this water is seawater; only 0.01% is freshwater that can be directly consumed by humans. To resolve the global water issue, the utilization of different types of water, including seawater and brackish water in inland areas, must be explored.

In regions where freshwater is scarce, such as the Middle East, seawater desalination is used to produce fresh water from seawater. "Desalination methods" include "evaporation methods" (in which fresh water is produced by heating seawater and cooling the water vapor) and "reverse osmosis methods" (in which fresh water is produced by filtering seawater through a special membrane called a reverse osmosis membrane). A reverse osmosis membrane is covered with numerous pores ( ≤1 nm in diameter) that allows the passage of only water (not impurities such as salts). Therefore, these membranes are used to produce fresh water by removing impurities from seawater. Evaporation method requires a large amount of energy, making water production expensive. In contrast, the reverse osmosis method exhibits a relatively high energy efficiency, and is therefore gaining worldwide attention. This method exhibits high potential for use in economically disadvantaged countries.

However, reverse osmosis method involves the application of high pressure onto seawater, which requires some energy. Additionally, seawater contains organic substances (such as plankton and plankton carcasses) and inorganic substances other than salts that clog the membrane. This hinders both the passage of water through the membrane and the removal of salt; therefore, the membrane must be cleaned regularly. Fresh water cannot be obtained while the membrane is being cleaned, and cleaning agents damage the membrane on repeated cleaning, thereby shortening the life of the membrane and increasing the cost of water production.

Toray Industries Inc. has been developing reverse osmosis membranes for more than 40 years. The applications of these membranes have been extended from the production of ultrapure water for the semiconductor industry to the desalination of brackish water and seawater and the reuse of sewage and wastewater. Toray has embarked on the development of a high-performance reverse osmosis membrane with the aim of contributing to water issues through application of the technologies they have accumulated over their long history.

Toray began their development of reverse osmosis membranes with the aim of utilizing them in the desalination of seawater. Development was extremely difficult and it was difficult to put it to practical use. The reason for this was that it was difficult to achieve both water permeability and increased removal of impurities such as salt.

Enlarging membrane pores increases water permeability, but also allows salt to pass through more easily. Contrarily, membranes with small pores exhibit excellent salt removal and produce high-quality water; however, the efficiency of water treatment declines. Moreover, because pressure is applied, pressure resistance is also necessary. To resolve these tradeoff issues, Toray experimented with various materials and surface structures. One such development is a cross-linked aromatic polyamide composite membrane produced by interfacial polycondensation. The protuberant surface structure of these membranes enables an increase in surface area that facilitates the purification of large amounts of seawater. Thus, despite a small pore size, these membranes efficiently remove salt from sea water to produce freshwater.

With the increased use of membranes, improvements in energy efficiency, water quality, and operational stability are required to facilitate water treatment (including the desalination of seawater and treatment of brackish water, river water, and sewage wastewater). Increasing water permeability is imperative for saving energy. This is due to the fact that if the more water flow through the membrane, obtaining the amount of water will require less energy. Furthermore, to improve water quality, it is essential to increase the impurity-removal ratio. If the surface of the membrane is resistant to fouling, not only can the decrease in water permeability be controlled, but also the frequency of membrane cleaning can be reduced. It was for the above reasons that Toray decided to develop a membrane with these numerous functions.

The development team at Toray believed that the performance of reverse osmosis membranes could be improved by precisely controlling the structure of the protuberances and pores of conventional cross-linked aromatic polyamide composite membranes. They therefore decided to analyze thoroughly the surface structure of membranes. However, the analysis of surface structures of 1 nm or less in size was challenging.

An electron microscope was used to analyze the size of the protuberances on the membrane surface. A conventional scanning electron microscope could only provide information on the shape of the protuberances. Moreover, although they could observe finer structures using a transmission electron microscope, they did not have the technology to quantify even finer structures.

Therefore, to facilitate analysis, Toray developed an original analysis technology through joint research with Toray Research Center, Inc. (TRC). The new technology enabled a detailed analysis of the internal structure and surface area of membrane surface protuberances and the membrane thickness. As a result, they were able to clarify the relationship between the structure of the protuberances and the properties of reverse osmosis membranes. This clarification facilitated their design of the reverse osmosis membrane structure.

Although Toray endeavored to quantify the sizes of the pores in the membrane, they had no means of measuring the structure of the fine pores at that time. After much brainstorming, they hit on the idea of using positron annihilation lifetime measurement, a method used to calculate the time between the injection of positrons into a sample and their annihilation through interaction with electrons. As positrons trapped in small pores are more likely to collide with surrounding electrons and exhibit short lifetimes, these measurements can be used to estimate the pore size of a membrane.

Although small-measurement apparatus are currently available, no such instruments were available in 2015. Toray collaborated with TRC and various other internal and external organizations to develop a small-measurement apparatus. They then combined use of this apparatus with molecular dynamics calculations to establish a method for measuring pore size. They were able to confirm that the size of pores in reverse osmosis membranes is 0.5 to 0.7 nm, and also found a correlation between the pore size and boron removal ratio. (Boron and boron compounds are regulated substances in drinking water.)

Based on these analysis results, Toray designed a three-dimensional model of the membrane' s structure utilizing computer simulation technology, and used the model to determine how to control the size of the protuberances and pores in order to attain high permeability, removal performance, and durability. In addition, after experimenting with the interfacial polymerization method and surface control technology, they finally achieved the ideal membrane structure through a process of trial and error. By controlling the surface of the membrane, they were also able to create a surface that is resistant to fouling.

The reverse osmosis membrane is formed into a flat sheet and rolled up like a Swiss roll cake together with the spacer to be used as a spiral-type element. One cross section is the supply side where seawater is introduced, and salt is removed as pressure is applied to permeate the water through the reverse osmosis membrane.

The salt separated from water by the membrane accumulates on the surface of the membrane, causing a concentration- polarization phenomenon (in which the ion concentration on the surface increases above the ion concentration in the feed seawater). This phenomenon changes the permeation performance of the membrane. Therefore, it is vital to reduce the ion concentration on the surface of the reverse osmosis membrane by increasing the flow velocity of the supplied water. To do so , Toray improved the performance of feed spacers. Using fluid analysis to make the feed spacer thinner and optimize the network structure, Toray developed a technology that doubled the flow velocity of the supplied water, reducing the ion concentration on the membrane surface by 30% or more.

Additionally, the spacer typically comprises a densely structured tricot (a fabric with a mesh connected in the vertical direction) that can withstand the high applied pressure. However, this dense structure impedes the flow of water, reducing the permeation performance of the membrane. Toray therefore applied their fiber processing technology in addition to fluid analysis and structural analysis, and were able to mold the molten resin with ultra-precision into dots and stripes, successfully expanding the flow path while maintaining pressure resistance.. Consequently, the flow resistance was reduced by 50% and the water permeation was increased by up to 20%. In this way, Toray improved the structure of the spacer for both the supply water and permeated water in order to maximize the membrane's performance.

By thoroughly analyzing the chemical structure and physicochemical data of membrane materials and establishing a technology for controlling the membrane-surface structure (such as optimizing the pore size and protuberance structure), Toray developed reverse osmosis membranes with high permeability and high removal performance. Owing to increased water permeability, the new membranes enabled water treatment at lower pressures than ever before, saving large amounts of energy. Moreover, owing to their fouling- and chemical-resistant properties, these membranes require less frequent cleaning and show a long lifetime. Thus, the reverse osmosis membranes developed by Toray enable the stable removal of harmful substances, such as boron, and provide a long-term high-quality water supply.

As a result, their waste water treatment calculations found that a 60% reduction in water production costs can be expected using reverse osmosis membranes. Moreover, the energy-saving properties of RO membranes are estimated to reduce carbon dioxide emissions by approximately 8 million tons over five years (from 2010 to 2014).

These RO membranes enable the stable and inexpensive production of high-quality water from different types of water (including seawater, brackish water, and sewage).

Furthermore, water purification by RO membranes is estimated to involve a significantly lower energy consumption compared to conventional methods of water purification (such as evaporation). Currently, the RO membrane technology is used in many regions of the world, and is contributing toward the resolution of global water issues through widespread application.

RO membrane applications have expanded into the food and pharmaceutical industries. In the future, RO membranes are expected to be applied in a wide range of fields related to resources and energy, including petroleum and gas mining; the production of lithium, rare metals, and other valuable resources; and biorefining. Additionally, they are attracting attention in the chemical industry as innovative low-energy technologies.