Origin and Development of Global Reverse Osmosis Membrane Technology

Reverse osmosis is the reverse of osmosis. Both processes center around a semipermeable membrane. Simply put, a semipermeable membrane allows water to pass through but not the salts or other solutes dissolved in the water. If the solute concentrations on both sides of a semipermeable membrane are inconsistent, water molecules will spontaneously pass through the membrane from the low-concentration side to the high-concentration side until the concentrations on both sides of the membrane are consistent, or a certain net pressure difference is established on the high-concentration side due to factors such as rising water levels. This is osmosis, and this net pressure difference is osmotic pressure.

The invention and large-scale application of reverse osmosis (RO) is a landmark achievement in the development of modern water treatment technology. As an advanced membrane separation technology developed after the 1950s, reverse osmosis has been widely used in seawater desalination, brackish water desalination, household water purification, and wastewater reuse.

Since the 20th century, numerous important scientists and engineers internationally, along with a large number of reverse osmosis membrane companies, have jointly created a rich and colorful history of reverse osmosis technology development.

In the 1950s, the Kennedy administration in the United States began to look to seawater desalination to solve the problem of water shortages in some arid regions of the United States and the nationwide overuse of groundwater.

In 1952, the U.S. Congress passed the Saline Water Conversion Act.

In 1953, funding for desalination technology research began, although funding that year was only $175,000.

In 1955, the U.S. Department of the Interior established the Office of Saline Water (OSW) to coordinate research on various seawater desalination technologies. (By 1970, the OSW's annual budget had increased to approximately $26 million).

In August 1956, Gerald Hassler first coined the term "Reverse Osmosis (RO)" in another internal report from the University of California, Los Angeles (UCLA).

In April 1957, Donald T. Bray and his colleague E. J. Breton used the term "Reverse Osmosis (RO)" in a report to the OSW.

In 1956, Professor Samuel Yuster's research group also conducted membrane desalination research with funding from the OSW. Srinivasa Sourirajan, a 33-year-old scientist of Indian descent, was the first to participate in this research. They also adopted a similar pressure-driven research approach. In the summer of 1958, 41-year-old Sidney Loeb, a scientist of Jewish descent, also joined the research group. Sourirajan and Loeb's initial work involved screening commercial membranes. During this process, they discovered that heat treatment of a commercial cellulose acetate ultrafiltration membrane could give it certain desalination properties. They also unexpectedly found that the orientation of the membrane during testing was crucial, with one side facing the feed solution significantly outperforming the other.

The heat-treated cellulose acetate membrane achieved a desalination rate of 92%, and the water permeability reached 0.00095 m³/m²·d·atm, far higher than other membranes. More importantly, they realized that the asymmetry of the membrane structure significantly affects membrane performance, and reducing the effective thickness of the membrane is key. To further improve membrane performance, the two scientists decided to make their own membranes.

In 1959, Sourirajan and Loeb, through a series of explorations, used cellulose acetate, acetone, water, and magnesium perchlorate as raw materials in a ratio of 22.2:66.7:10.0:1.1 to prepare a casting solution, and optimized factors such as temperature, evaporation time, and heat treatment to prepare an asymmetric synthetic reverse osmosis membrane for the first time. Simply put, an asymmetric structure means that a membrane consists of two parts: a support layer and a separation layer. The support layer is relatively loose in structure, while the separation layer is relatively dense. This membrane was later called the L-S membrane.

Sourirajan and Loeb's asymmetric membrane achieved a desalination rate of 99%, and the water permeability reached an astonishing 0.0048 m³/m²·d·atm, almost on the same order of magnitude as modern commercial reverse osmosis membranes. This membrane also has good mechanical stability. This breakthrough provided the most important technological foundation for the eventual large-scale engineering application of reverse osmosis technology. After this, reverse osmosis membrane technology entered a period of rapid development and gradually moved towards commercial application.

In 1963, the North Star Research Institute in Minnesota also conducted desalination technology research with funding from the OSW.

Around 1965, Dow Chemical and DuPont both invested in the development of hollow fiber reverse osmosis membranes, possibly related to their familiarity with the textile and chemical industries.

In 1966, H. I. Mahon of Dow Chemical designed the first hollow fiber membrane spinning system and developed a hollow fiber reverse osmosis membrane based on triacetate cellulose material.

In 1967, John E. Cadotte of the North Star Research Institute invented a microporous polysulfone support membrane. In the following years, he also developed several non-cellulose acetate composite membranes. But his enthusiasm for reverse osmosis was not limited to research.

In 1971, DuPont applied for a patent for a hollow fiber reverse osmosis membrane module based on polyamide material.

FilmTec Corporation, USA:

In 1977, John E. Cadotte and three other partners founded FilmTec Corporation in Minnesota. In 1979, Cadotte applied for a patent for the preparation of reverse osmosis membranes using interfacial polymerization. The reverse osmosis membrane sheet prepared by interfacial polymerization is divided into three layers: base layer (non-woven fabric) + support layer (polysulfone) + separation layer/desalination layer (polyamide). The layers can be optimized separately during preparation, further improving the performance of the reverse osmosis membrane. Interfacial polymerization has also become the basis of the standard preparation process for modern commercialized spiral-wound reverse osmosis membranes.

In 1985, Dow Chemical acquired FilmTec Corporation after abandoning hollow fiber reverse osmosis membranes. To this day, Dow Chemical reverse osmosis membrane products still use the FilmTec trademark.