Drinking Water from the Sea: Polymeric Membranes for Desalination
- By Kamil Ezgin
- Category: Issue 83 (September - October 2011)
One billion people in the world live in water-stressed areas, and RO membrane technology is the leading desalination technology to overcome the problem of insufficient clean water. Today, more than 1 billion people are suffering from the lack of potable water. About 2.3 billion people (41 percent of the earth’s population) live in regions with water scarcity; this number is estimated to be 3.5 billion by 2025.1 96.5 percent of the world’s water is found in seas and oceans, and the remainder is found as ice caps, brackish water, and fresh water sources (e.g. lakes, rivers, and ground waters). To overcome water shortage problems, methods such as water conservation and dam construction have been applied for several years, but they are not enough against increasing water demand and decreasing fresh water sources.2 Water is also very important for generating energy, and vice versa. The largest portion of U.S. electric production is provided by thermoelectric power generation, where steam-driven turbine generators are used to generate electricity. In 2000, thermoelectric power plants used 39 percent of all fresh water sources in the United States.3 All these reasons make the production of drinking water a worldwide issue. Desalination Since most of world’s water supply is found in oceans and seas, desalination is the process of removing salts and minerals from either ocean or brackish water to make it safe for human consumption and use. The most widely applied desalination processes are divided into two main categories, thermal distillation processes and membrane processes. Desalination via thermal distillation methods, which separate liquid mixtures based on their boiling points, mainly fall into three categories: multi-stage flash (MSF), multi-effect distillation (MED), and mechanical vapor compression (MVC). Thermal distillation processes require the evaporation of water while leaving the salt in a concentrated brine. Middle Eastern countries mainly use thermal-based desalination plants to produce fresh water because of their easily accessible fossil fuel sources.2, 4 Membrane-based separations are the main choice of producing potable water in countries outside the Middle East. More than 50 percent of the newly installed desalination plants have been using reverse osmosis (RO) membrane technology (since 2001).2 Membrane separations A membrane is an interphase between two adjacent phases acting as a selective barrier, regulating the transport of substances between the two compartments. It is a very thin film that allows passage of some types of substances while preventing the passage of other substances, depending on their sizes. Membranes used for separation technology gave rise to an interdisciplinary area including many fields of science and engineering such as chemistry, chemical engineering, material science, process engineering, environmental science, ecology, and economics.5, 6 Today, the membrane industry is impressively large. The membrane separation technology market is quite diverse and ranges from medicine to the chemical industry, and the most important markets are medical devices and water treatment. There was a $2 billion sale of synthetic membranes worldwide in 2003.6 Water purification membranes Water treatment processes employ several types of membranes. They include microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) membranes. They are designed to remove materials of increasing sizes. MF membranes have the largest pore size and typically reject large particles and various microorganisms. UF membranes have smaller pores than MF membranes and, therefore, in addition to large particles and microorganisms, they can reject bacteria and soluble macromolecules such as proteins. RO membranes are effectively nonporous and therefore exclude particles and even many low molar mass species such as salt ions, organic substances, etc.7 NF membranes are relatively new and are sometimes called “loose” RO membranes. They are porous membranes, but since the pores are ten of angstroms or less, they exhibit performance between that of RO and UF membranes.8 Of these membranes, NF and RO membranes constitute the dominant technology for desalination of water.9 2.1 Nanofiltration Membranes Membranes for nanofiltration (NF) are usually comprised of cellulose acetate or aromatic polyamides. NF allows diffusion of organic compounds, and rejects some salts with low pressures being applied. NF itself cannot purify seawater to drinking water standards, but it is a process that can be used to produce mildly salty water, or as a water-softening technique.2, 4 When NF is coupled with RO, then it can be used to turn seawater into drinking water.10 Nanofiltration membranes usually have negative charges (e.g., carboxylate groups, sulfonate groups, etc.), and as a result, ion repulsion is a major factor in determining salt rejection. More highly charged ions, such as sulfate, are more highly rejected than monovalent ions, such as chloride, by a negatively charged nanofiltration membrane. In particular, NF membranes are used to remove divalent ions such as calcium and magnesium, which are mainly responsible for water hardness. These membranes also usually display good rejection of organic compounds with molecular weights above 200 to 500 grams.2,11,12 2.2 Reverse osmosis membranes Osmosis is a natural process in which water molecules move across a semipermeable membrane from a lower solute concentration area to the higher solute concentration area. Water flows until a chemical potential equilibrium of water is established. When equilibrium is reached, the pressure difference between the two sides of the membrane is equal to the osmotic pressure of the solution.12 Reverse osmosis (RO) is the process of forcing water from a region of high solute concentration through a membrane to a region of low solute concentration by applying a pressure that is greater than the osmotic pressure. As a result, separation of water from the solution occurs as pure water from the high concentration side to the low concentration side. The RO process includes a feed water source, feed pre-treatment, a high-pressure pump, RO membrane modules and post-treatment steps. RO membranes are capable of rejecting monovalent ions such as sodium and chloride, which makes the RO process a valuable method for desalination. Membranes used for RO processes have salt rejections of more than 99 percent. RO membranes do not have distinct pores, but rather rely on free volume within the polymer film. RO membrane separations depend highly on the properties of the polymer film such as the chemical and physical structure of the membrane material. Desired RO membranes should be resistant to chemical substances and microbial organisms, stable over a long time both mechanically and structurally, and have ideal separation properties such as high water flux, high salt rejection, chlorine, and fouling (clogging of membrane pores) resistance. Approximately one billion of six billion people in the world live in water-stressed areas, and RO membrane technology is the leading desalination technology to overcome the problem of insufficient clean water and estimated to continue its leadership in the near future.13 Scientists and engineers are extensively investigating the development of the most efficient membrane desalination technology to produce the cheapest potable water. On the other hand, cells use membranes, though scientists do not try to further develop them, since they were already designed in a perfect manner. Cellular membranes have a phospholipid structure with embedded proteins. They control many different kinds of transportations of substances in and out of cells (e.g. sugar, drugs, ions). They are so well designed that they know which substances are helpful or harmful for the cell, and decide on the passage of substances based on that. Many researchers have tried countless times for many years to produce an equally wonderful membrane technology for making clean water. But cellular membranes, consisting of hundreds of functions in living organisms, do not form spontaneously. REFERENCES 1) R.F. Service, Freshwater resources, desalination freshens up. Science, (2006). 313, 1088- 1090. 2) L.F. Greenlee, D.F.Lawler, B.D. Freeman, B. Marrot, P. Moulin, Reverse osmosis desalination: Water sources, technology and today’s challenges. Water Research (2009), 43, 2317-2348. 3) T.J. Feeley, T.J. Skone, G.J.Stiegel, A. McNemar, M.Nemeth, B. Schimmoller, J.T. Murphy, L. Manfredo, Water: A critical resource in the thermoelectric power industry.Energy (2008), 33, 1-11. 4) G. A. Tularam, M. Ilahee, Environmental concerns of desalinating seawater using reverse osmosis. J. Environ. Monit.(2007), 9, 805–813. 5) P. Vandezande, L. E. M. Gevers, I. F. J. Vankelecom, Solvent resistant nanofiltration: separating on a molecular level. Chem. Soc. Rev.(2008), 37, 365–405. 6) M. Ulbricht, Advanced functional polymer membranes. Polymer (2006), 47, 2217–2262. 7) R.H. Perry, D.W.Green, Eds., Perry’s Chemical Engineers’ Handbook, 7th ed., McGraw-Hill: New York, 1997. 8) Sagle, A., and B. Freeman, "Fundamentals of Membranes for Water Treatment," in The Future of Desalination in Texas: Volume 2, Report Number 363, Texas Water Development Board, Austin, TX, pp. 137-154 (2004). 9) H.B.Park, B.D.Freeman, Z.Zhang, M.Sankir, J.E.McGrath, Highly Chlorine-Tolerant Polymers for Desalination, Angew. Chem. Int. Ed. (2008), 47, 6019-6024. 10) N. Hilal, H. Al-Zoubi, N. A. Darwish, A. W. Mohammad, M. Abu Arabi, A comprehensive review of nanofiltration membranes: Treatment, pretreatment, modelling, and atomic force microscopy, Desalination (2004), 170, 281-308. 11) A. Gorenflo, D. Velazquez-Padron, F.H. Frimmel, Nanofiltration of a German groundwater of high hardness and NOM content: performance and costs. 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