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Volume 85 Issue 17 | pp. 22-24
Issue Date: April 23, 2007

Cover Stories: Keeping It Clean

Filtering Out the Bad Stuff

Polymeric membranes are increasingly being used to clean up water for drinking and industrial applications
Department: Science & Technology
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Tube City
Siemens set up this plant in Singapore that uses reverse-osmosis membranes, packed in tubes, to transform wastewater into drinking water.
Credit: Siemens
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Tube City
Siemens set up this plant in Singapore that uses reverse-osmosis membranes, packed in tubes, to transform wastewater into drinking water.
Credit: Siemens

WATER MAY BE UBIQUITOUS, but salt, brackishness, dirt, and pollution can render it unfit to drink. Bacteria such as Escherichia coli, parasites such as cryptosporidium, and other infectious agents also can make drinking water hazardous.

Municipal water authorities around the world use a variety of techniques to make water drinkable, including ion-exchange resins, sand filtration, chlorination, and distillation. But more and more, municipal water systems depend, at least in part, on membrane filtration to economically clean up less-than-pristine water.

Recent improvements in the design and chemistry of membrane filters based on polymers such as polvinylidene fluoride, polyamide, and polypropylene have made it possible to economically screen out contaminants. To make unsafe, even deadly water potable, these polymeric filters remove a range of impurities, salts, and suspended organic matter that otherwise impart poor taste, smell, and appearance.

According to Pall, a manufacturer of water purification systems that incorporate membrane filters, the size of the U.S. market for water-treatment membranes is about $4 billion and growing at 7-8% per year. Estimates vary for the rest of the world. But Jeffrey M. Seibert, president of Pall's water processing division, says the market in the European Community is about the same size as in the U.S. and that the rest of the world is fast becoming a consumer of water purification systems incorporating membranes.

Imran Jaferey, a business manager for Koch Membrane Systems, part of petroleum and chemical giant Koch Industries, says membrane filtration is in demand in Florida, Texas, California, and other locations that need to purify and treat brackish water. Overseas, opportunities are growing in Spain, the Middle East, and China to make seawater drinkable. "Our sales guys are exploring opportunities in Calcutta and Mumbai, where seawater has infiltrated wells," Jaferey says.

Koch extrudes membranes based on polymers such as polyamide and polyvinylidene fluoride and also assembles membrane systems for customers.

The growing market for water treatment, and for membrane technology in particular, has caught the eye of a number of high-profile firms. Last September, Dow Chemical formed a $350 million-per-year water treatment technologies unit. Dow Water Solutions includes the firm's Filmtec reverse-osmosis membranes, used to desalinate seawater, and ultrafiltration membrane technology acquired when Dow bought China's Omex Environmental Engineering in July 2006.

When Dow formed the water solutions unit, Chief Executive Officer Andrew N. Liveris said the company was backing a pledge, part of Dow's sustainability goals, to provide higher quality water for industrial applications and for drinking. Another global firm with a similar pledge is General Electric. Increasing clean water supplies is part of GE's pledge to exercise what it has dubbed "ecomagination," which it defines as a dedication to bringing to market technologies that lower harmful emissions, promote energy efficiency, and reduce the use of fossil fuels.

In June 2006, GE purchased Zenon Environmental, a Canadian manufacturer of hollow-fiber ultrafiltration membranes, for $689 million. At the time, David Calhoun, a GE vice chairman, pointed out that "water quality, water scarcity, and infrastructure challenges are growing around the globe." The acquisition of Zenon enhanced GE's ability to provide clean industrial, agricultural, and potable water.

The Zenon deal capped a string of purchases GE made to beef up its water treatment capabilities. The firm's spending binge took off in 2002 when it acquired BetzDearborn, a company specializing in water treatment chemicals, for $1.8 billion. A year later, GE purchased Osmonics, a maker of water treatment machines and equipment, for $240 million, and in 2005, it bought Ionics, a specialist in reverse-osmosis water desalination membranes, for $1.1 billion.

fiber power
Hollow-fiber membranes filter pollutants and bacteria from water.
Credit: Siemens
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fiber power
Hollow-fiber membranes filter pollutants and bacteria from water.
Credit: Siemens

GE's move into water treatment made it a supplier not only of membrane and chemistry technology but also of original equipment and systems to treat water. Another firm, Siemens, which claims to have installed more than one-third of all low-pressure membrane water treatment systems globally, got into the membrane business when it purchased the water services firm U.S. Filter in 2004 for $993 million. That acquisition included Memcor Products, an Australian maker of hollow-fiber micro- and ultrafiltration membranes.

Much of the growth in membrane use in the U.S. has been driven by increasingly stringent federal regulations covering drinking water drawn from surface waters such as rivers, lakes, and reservoirs, says Pall's Seibert. Older systems rely on treating water in large holding tanks with flocculants to separate out impurities and then passing the water through a sand filter, he explains. But newer systems incorporating polymeric membranes can be more cost-effective.

Seibert argues that membrane systems are the most effective screen for both bacteria and chlorine-resistant parasites such as cryptosporidium and giardia. Pores in the polyvinylidene fluoride microfiltration membranes made exclusively for Pall by Japan's Asahi Kasei are so small that they exclude the parasites and other waterborne contaminants.

The firm has also developed a microfiltration system capable of reducing arsenic levels in drinkable water below the new limit of 10 parts per billion set by the Environmental Protection Agency. Arsenic can cause bladder, lung, and skin cancer and has been linked to kidney and liver cancer.

Pall's system requires the operator to add an iron-based coagulant, such as ferric chloride, to the water. Positively charged ferric hydroxide particles adsorb the arsenic, which then can be removed by membrane microfiltration. Seibert says this system is a less costly long-term solution than alternatives such as adsorption media alone or energy-intensive reverse-osmosis filtration.

Industrial water treatment is the "bread and butter" of the reverse-osmosis filtration market, says Karen Dobson, global market manager for Dow's water solutions business. Industry sources place Dow as the number one maker of reverse-osmosis filters worldwide.

About 60% of the reverse-osmosis membranes now in use treat water destined for boilers and industrial processes where contaminant-free water is a priority, Dobson says. Another 30% of the membranes go to drinking water applications, and the balance goes to specialty industrial uses, she says.

Reverse-osmosis membranes are the highest performance membranes money can buy. Such filters can remove arsenic as well as heavy metals and bacteria, but one of their most important uses is to make seawater drinkable, Dobson points out. Working at pressures as high as 1,200 psi, they excel at removing up to 99.9% of dissolved minerals, she says.

Other membrane filters operate at low water pressure of as little as 5 psi and can more cost-effectively remove suspended particles and dissolved organic material that would quickly clog a reverse-osmosis filter. Polymeric micro-, nano-, and ultrafiltration membranes allow water to pass through pores in the membrane. Continuously cross-linked reverse-osmosis membranes allow water molecules only to pass through by diffusion and exclude almost everything but pure water.

Reverse osmosis is most attractive for desalination jobs because it is less costly than the alternative: distillation technology. According to the Texas Water Development Board, which is considering underwriting a $150 million seawater desalination plant in Brownsville, Texas, the cost per cubic meter of water treated via distillation is between 95 cents and $1.04, while the cost via reverse osmosis is 82 cents.

Dow's reverse-osmosis filter is a three-layer composite film. A cross-linked polyamide film performs the actual screening out of undesirable minerals. Dow casts the polyamide film on a supporting polysulfone substrate, which is itself cast on a polyester-based web for added strength.

The "magic" of reverse-osmosis technology is in optimizing the chemistry of the polyamide layer, Dobson points out. For brackish water, "we fine-tune the chemistry to allow the system to operate at lower feed pressures because of lower salt content," she says. Dow, like its competitors, winds the membrane into a tubular cartridge called an element module.

"Automated module fabrication techniques and chemistry improvements have reduced the price of a reverse-osmosis element to half of what it was 20 years ago," Dobson says. Moreover, she notes, "the elements yield twice as much potable water that is four times purer" than two decades before.

IMPROVEMENTS IN membrane chemistry and in the manufacture of spiral-wound elements have really helped advance membrane filtration productivity and cost-effectiveness, says Rich Franks, manager of reverse-osmosis applications at Hydranautics. The California-based filtration firm and its parent, Nitto Denko of Japan, stand as the second largest makers of reverse-osmosis membranes worldwide, according to industry sources.

Japan's Toray, the third largest reverse-osmosis membrane maker, also has seen the market for its membranes increase dramatically in recent years. According to Terry Smith, vice president of Toray Membrane USA, the firm has an edge in polyamide reverse-osmosis membrane elements for seawater desalination markets because its membrane is especially designed for removing boron contamination. Boron is known to cause reproductive and developmental problems in animals.

While seawater has 4 to 7 mg/L of boron, most reverse-osmosis membranes can reduce boron levels only to 1 to 3 mg/L, Smith explains. To meet the international standard of 0.5 mg/L or less, conventional reverse-osmosis desalination plants put water through the membrane twice. Toray's membrane can do it in one pass, he points out.

Membranes have an important and growing place in the arsenal of technologies used to bring drinking water up to acceptable standards. As populations grow, membrane makers say, municipal authorities will increasingly rely on those that can supply well-designed, cost-efficient polymers to remove the bad stuff.

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