Bryanston, South Africa - February 10, 2009
Located just off the shore of Saudi Arabia, the Shoaiba Barge Seawater Reverse Osmosis plant is a unique desalination project. It is one of Saudi Arabia’s largest seawater reverse osmosis plants and the world’s largest sea-based desalination plant. Just one year after the official start-up 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 United States, 2008 marked the ribbon cutting on a desalination plant in Tampa Bay, Florida that supplies drinking water to some 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 is really cost efficiency that is at the heart of the expansion. 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 that are essential to the design and operation of a desalination plant, it is 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 that has been 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.
Raising standards
FilmTec Corporation discovered and pioneered the use of polyamide composite for reverse osmosis membranes in the mid 1970s, setting the industry standard by the time The Dow Chemical Company acquired FilmTec 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 of membranes which 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 35percent by 2015.
Better by design
FILMTEC™ RO elements are based on three distinct layers wrapped in a spiral-wound design to form a cylindrical filter. The feed channel allows the sea water 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 semi-permeable polyamide layer that allow water molecules to pass through while leaving salt molecules behind. The fresh water spirals though 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 Dow to fit more membrane sheets into each RO element. Simply put, Dow is able to make each sheet 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 FILMTEC membranes and desalination.
Recently the introduction of the interlocking endcap 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 no possibility of leakage and deterioration of the permeate or fresh 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 99.4percent 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 35percent conversion of sea water to drinking water.
By the mid 1990s, Dow had introduced elements with flow rates of 6,000 gpd and, at 99.6 percent, significantly higher salt rejection. This reduced the need for a second RO pass in many cases, lowering both capital and operating costs for the end-user. Another significant increase in productivity was achieved in 2003 with the introduction of FILMTEC elements with flow rates of 7,500 and salt rejections of 99.75 percent. The following year, Dow introduced the first of several FILMTEC elements with flow rates of 9,000 gpd.
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 to 30 percent, energy is the single largest cost factor in the operation of a seawater desalination system and much of it is expended in the RO stage to generate enough pressure to force water ions through the millions of microscopic pores in each RO membrane. Field tests demonstrate that using advanced FILMTEC™ membrane technology can reduce pressure requirements by 2.5 to 4.5 bar to reduce energy consumption and maximize operating cost savings1 when compared to membrane technology available in 1996.
Where the emphasis is on reducing capital costs, field tests demonstrate that advanced FILMTEC membrane technology can reduce the number of elements and pressure vessels needed to produce target water capacity by 17 percent to 30 percent.1 To put these savings into perspective, consider that the Tampa Bay desalination plant uses 9,408 FILMTEC™ membranes and the Sidney desalination plant will use approximately 36,000 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 Sidney, extrapolate an annual cost savings of $456,000 to $2.2 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. FILMTEC membranes are designed to resist biofilm buildup and to provide constant salt rejection throughout the operating life of the membranes.
Affordable water for all
By increasing water throughput and salt rejection, high productivity FILMTEC™ RO membranes can be used to reduce the cost of desalination. More improvements in the pipeline 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.
1 Reducing energy consumption in seawater desalination, M. Busch, W.E. Mickols, Desalination 165 (2004) 299 – 312. (Specific reference may be found on page 311.)
About Dow Southern Africa
Dow is a world leader in science and technology solutions, delivering a broad range of products and services to customers in 160 countries. Dow traces its South African origins back to 1959 when its first sales office was established. The company is a significant local industry player and operates through a group of wholly owned subsidiaries namely Sentrachem Limited, Dow Southern Africa (Pty) Limited (including the Chemicals and Automotive businesses) and Dow AgroSciences Southern Africa (Pty) Limited. As a company that has a passion for the betterment of the human condition, Dow continues to be a proactive participant and supporter of Corporate Citizenship programs aimed at conservation and providing support, including education, to orphaned, vulnerable and underserved children. Dow is also supportive of economic transformation in South Africa as well as the Responsible Care® Guiding Principles initiative. More information about Dow Southern Africa can be found at: http://www.dow.com/imea/ssa/about/locations.htm
Dr. Harold Fravel is a Senior Industry Manager with Dow Water Solutions. He can be reached at +1 561 745 5368.
For Editorial Information:
Martti Lehtinen
Dow Water Solutions Nordic, Baltics & Sub Saharan Africa
+358 9 5845 5316
