The Reverse Osmosis Membrane Evolution

Advances in reverse osmosis membrane technology for seawater desalination have significantly driven down capital and operating cost.

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By Dr. Harold Fravel

Advances in reverse osmosis membrane technology for seawater desalination have significantly driven down capital and operating cost.

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Interlocking membrane units endcaps
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Just a year after the official startup of the southern hemisphere’s largest desalination plant in Perth, Australia, an even larger facility is under construction in Sydney that will produce fresh drinking water for an estimated 1.5 million residents annually. In the northern hemisphere, the world’s largest desalination plant in Ashkelon, Israel – up and running only since 2005 – will soon be surpassed in volume capacity by a larger facility under construction in Hadera, located between Tel Aviv and Haifa in Israel. In the United States, 2008 marked the ribbon cutting on a desalination plant in Tampa Bay, Florida, that supplies drinking water to 2.4 million residents.

What’s behind the boom in seawater desalination? Certainly it speaks to the serious need for more water to support a growing global population and all that comes with it – more industry, more agriculture, more infrastructure. But it’s really cost efficiency at the heart of the expansion. Once so expensive that only oil-rich nations with dire water needs would consider it, desalination has become more economically viable thanks to major cost efficiencies, many achieved through advances in reverse osmosis (RO) membrane technology.

For all the pumps, piping and flow valves essential to the design and operation of a desalination plant, it’s the RO membrane element that actually separates water molecules from dissolved salts and minerals, turning seawater into tap water. It’s an energy intensive process made more efficient and less costly through improvements in membrane technology. A reduction in carbon footprint is an ancillary benefit with growing importance, making efficient membrane technology a key element in community, regulatory and government approval of desalination facilities and for more widespread acceptance of seawater desalination.

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Tampa Bay Water Desalination facility (courtesy of tampa bay water)
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Raising standards

FilmTec Corp. discovered and pioneered use of polyamide composite for RO membranes in the mid-1970s, setting the industry standard by the time The Dow Chemical Company acquired it in 1985. Under the Dow umbrella, FilmTec upgraded from hand-glued, hand-assembled construction to a fully automated system that not only drove down the cost of producing RO membranes, but also dramatically improved membrane quality, consistency and productivity.

Today, FILMTEC™ membranes sell for half the price they did just 10 years ago and produce at least twice as much water in desalination applications. More improvements are in the pipeline, including new next-generation membranes that are now being piloted in cooperation with PUB, Singapore’s national water agency. Dow’s overarching goal is to reduce the cost of seawater desalination by 35% by 2015.

Better by design

These RO elements are based on three distinct layers wrapped in a spiral-wound design to form a cylindrical filter. The feed channel allows the seawater to pass across the element. Pressure is applied to move the water through the elements and to overcome the osmotic pressure of the salt water, forcing it through a semi-permeable polyamide layer that allow water molecules to pass through while leaving salt molecules behind. The fresh water spirals through a permeate carrier layer to the core where the desalted water moves through the product water tube, eventually finding its way to the municipal drinking supply.

Improvements in chemistry and full automation in the construction of polyamide material allow more membrane sheets to be fit into each RO element. Simply put, each sheet is able to be made thinner while maximizing durability, consistency and precision. More sheets per element translates to a larger active area. This has been a major dynamic behind recent increases in flow rate and salt rejection, two factors that figure largely into the efficiency of the latest membranes and desalination.

Recently, Dow’s introduction of the interlocking end cap, or ILEC™, has been another improvement to the previous design of membrane elements that will eliminate the need for interconnectors that have been a weak spot in the performance of RO systems. Each element is locked to the adjacent element in the system with virtually no possibility of leakage or deterioration of the permeate or treated water in the central core.

History of efficiency

In the early 1990s, a typical FILMTEC RO element had a flow rate of 4,000 gallons per day (gpd) and a salt rejection of 99.4% at standard seawater conditions. Often a second RO pass was needed to reach drinking water quality. A typical desalination plant during this time period operated at 70 bar feed pressure and 35% conversion of seawater to drinking water.

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Evolution of membrane technology
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By the mid-1990s, Dow had introduced elements with flow rates of 6,000 gpd and, at 99.6%, a significantly higher salt rejection. This reduced the need for a second RO pass in many cases, lowering capital and operating costs for the end-user. Another significant increase in productivity was achieved in 2003, with introduction of FILMTEC elements with flow rates of 7,500 and salt rejections of 99.75%. The following year, Dow introduced the first of several elements with flow rates of 9,000 gpd and salt rejection of 99.7%.

Lower cost

How do advances in flow rate and salt rejection lower the cost of desalination? One way is by improving operating energy efficiency. At 20-30%, energy is the single largest cost factor in operation of a seawater desalination system and much of it is expended in the RO stage via pumps to generate enough pressure to force water ions through the millions of microscopic pores in each RO membrane. Field tests demonstrate that using advanced membrane technology can reduce pressure requirements by 2.5 to 4.5 bar to reduce energy consumption and maximize operating cost savings when compared to membrane technology available in 1996.

Where the emphasis is on reducing capital costs, field tests demonstrate that advanced membrane technology can reduce the number of elements and pressure vessels needed to produce target water capacity by 17% to 30%.1 To put these savings into perspective, consider that the Tampa Bay, Florida, desalination plant uses 9,408 FILMTEC membranes and the Askelon, Israel, desalination plant uses more than 40,000 FILMTEC membranes.

Measured in U.S. currency, the value of these efficiencies achieved in field testing ranged from $.005 to $.024 per cubic meter of water produced. For a desalination plant with total capacity on par with Tampa Bay, extrapolate an annual cost savings of $170,000-800,000. Where capacity would be comparable to Ashkelon, extrapolate an annual cost savings of $580,000 to $2.8 million.

More difficult to measure but just as real are the operating cost savings realized by minimizing plant downtime due to biofouling or scaling or even membrane element replacement. These advanced membranes are designed to resist biofilm buildup and provide constant salt rejection throughout their operating life.

Conclusion

By increasing water throughput and salt rejection, high productivity RO membranes can be used to reduce the cost of desalination. More improvements in the pipeline from FilmTec are expected to drive down costs even further, thus making a significant contribution toward helping to enable a safe, clean and plentiful water supply for all. WWi


Author’s Note:

Dr. Harold Fravel is a senior industry manager in membrane technology at Dow Water Solutions, a business unit of The Dow Chemical Company with headquarters in Edina, Minnesota, USA. Based in Jupiter, Florida, he works for FilmTec Corp., a wholly owned Dow subsidiary. Contact: 561-745-5368 or www.dowwatersolutions.com

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