RO Membrane Technology
As a general rule of thumb, reverse osmosis membranes have been widely utilized as a barrier for contaminants and pathogens within water, but what material are they made from, and how do they differ?
Osmosis is a naturally occurring phenomenon where water from a region of lower solute concentration crosses a semi-permeable membrane to a region of higher solute concentration. This process is crucial in the life of plants and occurs continuously within our cells. Reverse Osmosis (RO) essentially “reverses” this natural migration by using applied pressure to force water to flow from a region of higher solute concentration across a semi-permeable membrane to a region of lower solute concentration. As such, RO can be used to concentrate salts and produce water with a lower salt concentration. Reverse osmosis is a membrane-based separation technology used to remove dissolved solids from a liquid. The first recorded synthetic membrane was prepared in 1867 by Moritz Traube as he successfully created a precipitated film membrane of copper ferrocyanide while studying osmosis. From cellulose acetate (CA) hollow fiber to advanced thin-film composite membranes in a spiral-wound configuration, developments in RO membrane technology have allowed the desalination industry to tackle a larger range of applications than ever before.
Cellulose Acetate Membrane
In 1959, researchers Sidney Loeb and Srinivasa Sourirajan created the first asymmetric RO membrane out of cellulose acetate which was able to selectively reject sodium chloride and total dissolved solids under pressure while allowing water to pass through. Following several successful desalination pilot studies, President Kennedy’s administration set goals to further develop large-scale potable desalination methods to combat shortages. In the 1960s and 70s, more and more desalination plants installed cellulose acetate RO membrane systems, followed then by power plants. As the initial leader in RO membrane technology, cellulose-based membranes were able to exhibit sodium chlorine rejection values up to 99.5 percent. Cellulose is a naturally occurring polymer found in plants such as cotton. The cellulose structure is a rod-like material that is relatively rigid, which offers its robustness. CA membranes are relatively easy to produce and are somewhat resistant to chlorine attack compared to others in the industry (<5ppm free chlorine). One disadvantage is that CA membranes require higher driving pressures to produce water than do newer types of membrane material. In addition, CA membranes tend to hydrolyze over time and are sensitive to changes in temperature and pH.
Figure 1: Chemical Structure of cellulose acetate
Thin Film Composite Membrane
Thin-film composite (TFC) membranes made their debut in 1972 and started leading the way for RO technology. The thin-film composite structure differs from the CA membrane structure as shown in Figure 2 below. While cellulose acetate membranes have a support layer and a cellulose acetate layer, thin-film composite membranes feature a support layer, usually made of polyester, a substrate layer, usually made of polysulfone, and a polyamide active layer.
Figure 2: Comparison of CA membranes (left) with TFC membranes (Right)
TFC is known to demonstrate greater water flux than the CTA membranes. Similar to CTA membranes, the membrane water permeability of TFC is also affected by the salinity gradient where the water permeability normally increases when a greater net driving force is created across the membrane. Additionally, higher power density can also be achieved with the TFC membrane compared to CTA membranes (Goh et. al.). While producing permeate with less pressure, the TFC technology is still prone to chlorine hydrolysis, biological fouling and organic scaling, and water must still be dechlorinated prior to the RO.
Future of Membranes
Advancement in the RO membrane technology continues, providing higher quality permeate with less pressure. In 1995, Hydranautics introduced the first low-energy polyamide RO membrane (Kucera, 2015). This membrane featured higher fluxes than standard brackish water RO membranes at the time, which could produce the same amount of permeate but at a lower pressure. TriSet Corporation was able to produce a low-fouling membrane featuring a more neutral surface change than previous polyamide materials. Recent research has been focusing on the density of membranes to achieve greater efficiency. It was recently found that the density of water filtration membranes, even at the atomic scale, can greatly affect the volume of RO permeate produced. The study demonstrated that precise control of density can increase membrane efficiency by 30–40% resulting in more water filtered with less energy — potentially making water purification and desalination processes more sustainable, productive, and affordable. As consumers become more aware of water costs and shortages, RO technology will surely advance to compensate, providing better chlorine resistance, lower pressures, and more permeate!
Jed Kosch
List of Resources:
[1] Sagle, A., & Freeman, B. (2004). Fundamentals of Membranes for Water Treatment. N/A. Published.
[2] Wiles, L., & Microdyn-Nadir, E. P. (2018). Reverse Osmosis: A history and explanation of the technology and How It Became So Important for Desalination. International Water Conference. Published. https://eswp.com/product/iwc-18-49/
[3] Basile, A., Gensini, M., Allegrini, I., & Figoli, A. (2021). Current Trends and Future Developments on (Bio-) Membranes: Membrane Technologies in Environmental Protection and Public Health- Challenges and Opportunities (1st ed.). Elsevier.
[4] Shenvi, S. S., Isloor, A. M., & Ismail, A. (2015). A review on RO membrane technology: Developments and challenges. Desalination, 368, 10–26. https://doi.org/10.1016/j.desal.2014.12.042
[5] Duarte, A.P., Cidade, M.T. and Bordado, J.C. (2006), Cellulose acetate reverse osmosis membranes: Optimization of the composition. J. Appl. Polym. Sci., 100: 4052-4058. https://doi.org/10.1002/app.23237
