Will next-generation membranes rise to the water challenge?
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The ideal membrane would let water quickly flow through and hold contaminants back. But there is a tradeoff between a membrane’s selectivity and permeability. Membranes that are more porous to water typically also allow other contaminants to pass through.
Will next-generation membranes rise to the water challenge? By Prachi Patel Feature Editor: Laura Biedermann
W
ater is life. Yet for a substance that covers 70% of the surface of the Earth, it is becoming increasingly precious. The problem is that only 2.5% of the water on our planet is fresh water. And an exploding world population, growing industrialization, and climate change all threaten this scarce resource. One in nine people lack access to sufficient, safe drinking water. The world’s population could reach 9 billion by 2030, driving up water demand by 30%. Analysts believe current water supplies will meet 60% or less of global water needs by 2030. Providing safe water to everyone will require desalinating, purifying, or treating water on a large scale, at low cost, and in a sustainable fashion. All eyes are on membranes as a possible solution. Membranes can clean, desalt, and treat water. Unlike the large tanks and columns used for traditional water treatments, membranes have a small footprint. “They are scalable and very versatile,” said Isabel Escobar, professor of chemical and materials engineering at the University of Kentucky. “They can be packed into fairly small surface areas, and it’s fairly easy to increase their size to treat more water.” Membranes have been used to desalinate seawater and treat wastewater on an industrial scale since their invention in the 1960s. Yet, Escobar said, “as excellent as they are, they are still a relatively new technology and have a lot of drawbacks.” They are expensive, energy-intensive, and easily soiled. They can suffer from low water flow or lack of selectivity. But by borrowing concepts from biological systems, drawing on novel two-dimensional (2D) materials, and applying advances in chemical synthesis and molecular-level design approaches, researchers hope to crack some of these problems. Polymer membranes are the current standard technology. They are typically made of a very thin charged polyamide layer deposited on a porous thin-film composite. Depending on their charge and size of their pores, they can remove suspended particles and pathogens (microfiltration); organic matter and smaller pathogens (ultrafiltration); and salts, metals, and ions that cause scaling (nanofiltration and reverse osmosis). New large-scale desalination plants in Israel, Saudi Arabia, Singapore, and California rely largely on reverse osmosis (RO), in which seawater is forced through semipermeable polymer membranes that block salt and other inorganic impurities. RO requires 3–5 kilowatt hours to produce 1000 liters of drinking water. Much of that energy is needed to push water through the membranes, and it accounts for half the cost of desalination: The US Department of Energy has a goal to decrease the co
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