Volume 43 Number 8
Spiral-Wound Elements for Potable Water Treatment
Summary: Crossflow membrane technology is an application with broad uses in the drinking water treatment industry. Separated into four classes, membranes have various capabilities when removing contaminants. Here, we recognize pros and cons of each and how the technology has changed over three decades.
Any new technology must overcome inaccurate perceptions that inevitably occur in the marketplace. As that technology moves into new areas of application, misconceptions can arise. Crossflow membrane technology is no exception.
For example, the first manmade, pressure-driven, "crossflow" membrane polymers were made in flat sheet form. This membrane had to be configured in a practical device to be commercialized and was first incorporated in a "plate-and-frame" design. The spiral-wound design resulted as a response to the high cost, large size, and impractical sealing arrangements inherent to plate-and-frame devices. The necessary use of mesh feed channel spacer in the spiral-wound element created the perception that it couldn't process waters with high-suspended solids levels. Additionally, complicated flow paths through these mesh spacers weren't amenable to simple fluid dynamics modeling. The general perception grew that such elements—also referred to as spirals—weren't feasible for many difficult applications, so both hollow fiber and tubular element configurations (both with round, open channels) were considered preferable.
Since 1975, spiral-wound elements were demonstrated to be feasible and indeed more favorable for many high solids applications. They were proven in both their original design and incorporating feed channel design improvements. The cost-effectiveness of these elements drove their acceptance and still dominates most process and water treatment applications, including high fructose corn syrup purification, dairy whey concentration, dye and ink desalting, and commodity-scale fermentation product purification. They even replaced hollow-fiber elements in many electric deposition (ED) paint recycle systems.
Too costly at first
Initially, membrane technology was deemed too expensive for potable water treatment, except in extreme cases of need. Trihalomethanes (THMs) and their precursor molecules, naturally occurring color and taste molecules and synthetic organic compound (SOC) pollutants drove municipalities, the American Water works Association (AWWA) and the U.S. Environmental Protection Agency (USEPA) to closely evaluate membranes in the 1980s. Actual installations to remove such pollutants, though, were rare.
Beginning in the late 1980s, and accelerating in the early 1990s, waterborne disease outbreaks and realization of widespread surface water contamination by protozoa (Cryptosporidium and Giardia) fueled more intense evaluation. Municipal systems installed for microbe removal have now surpassed those for inorganic and SOC removal. Today, few doubt membranes’ role in treating difficult water sources for wide potable use and costs have dropped dramatically as advancements in membrane system materials, engineering and manufacturing have made production more efficient and heightened removal capabilities. Still, municipal water treatment operators must continually assure that their operation’s performance is adequate. The need to test the membrane element's integrity on a routine basis, preferably in-situ, is now seen as without substitute. Most membranes are a barrier of thin, polymeric material, and rely on elastomeric, adhesive seals, both within the element configurations and larger systems that contain them. Therefore, the relative ease that a system may be integrity tested can heavily influence purchase decisions.
Reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF) and microfiltration (MF) class membranes for potable water have been applied in the United States in response to increasing USEPA mandates for water treatment objectives. Most membrane technologists know the Safe Drinking Water Act (SDWA) and subsequent rules, most notably the Surface Water Treatment Rule (SWTR) and its revisions. Pressure-driven membranes present an inherently effective barrier against waterborne microbes, and are widely accepted by those who design, study, and use them.
Table 1 summarizes the need and accepted capability of membrane processes to meet the various USEPA requirements.
Microbes are only one category of contaminants that must be removed, and membranes in all four classes (RO, NF, UF and MF) have the potential for removing several variety of contaminants. The strong and weak points of each configuration have been debated over the past two decades.2,3
The history of membrane use in potable water treatment is useful in the comparison of spiral-wound to hollow fibers elements (see Table 2). The first use was on seawater, where DuPont's hollow fine fibers (flow from the outside in) initially dominated the market. These weren't economical for other applications, however, because their tendency for severe fouling required expensive pretreatment schemes. Then John Cadotte's composite flat sheet membrane polymer was commercialized, allowing spirals to economically treat seawater. As membranes were applied to other potable applications (so-called "brackish" water desalting, color removal, sulfates and alkalinity removal for taste enhancement, etc.), spirals dominated these new applications.
When THMs and other disinfection by-products (DBPs) became a concern, the various classes of membrane were tested against them and their organic "precursor" molecules. These tests were predominantly against the spiral-wound configuration. Intense study by Florida municipalities, consulting engineers, and universities soon led to the conclusion that UF-class membranes couldn't adequately remove precursor molecules (or certain color and taste contaminants); therefore, an NF-class membrane was required. The requirements for NF improved the stature and focus on spiral-wound membranes, and precluded fat hollow fiber use.
With the 1990s came increased concern about microbial contamination. The 1993 Cryptosporidium outbreak in Milwaukee heightened interest in the United States for increased use of membrane technology in potable water treatment. Where only UF, or more likely MF, separation for microbe removal was required, the ability to backflush the hollow-fiber membranes for cleaning became attractive on high-fouling surface waters where microbes are of greatest concern. Then, concern for integrity testing the elements grew. A test technique—based on gas diffusion rates and the bubble point theory—developed for pleated MF membrane filter cartridges used in pharmaceutical sterilization was adapted. The hollow-fiber design lends itself to a simple application of this test technique. The dual advantage of backflush cleaning and simplified integrity testing has made the hollow-fiber UF and MF configurations appealing.
Previously, the spiral design raised concerns regarding confirmation of integrity; however, a review of literature and discussion with manufacturers and users who've tested membranes over decades doesn't support concern for actual system integrity loss. Actually, the industry consensus is that hollow-fiber elements are more likely to develop leaks; the capacity to easily apply the gas diffusion integrity test, however, gives hollow-fiber membranes a perceived advantage for microbial removal applications.
Often, UF- and MF-class membrane separation doesn't provide sufficient purification, so hollow-fiber UF and MF followed by spiral-wound NF/RO membranes is gaining favor as a treatment approach.4,5 Since NF and RO membranes are inherently capable of performing all the separations effected by the preceding UF and MF units, one naturally questions the need for both. Performing all separations with one step would clearly be a benefit, so sufficient rationale is required to justify the cost of adding a second step.
What can spiral-wound elements—and the systems they're put into—accomplish with assured reliability? The answer is they can remove all categories of microbes to a degree that allows them to help meet the Enhanced SWTR—unaided or, in practice, more likely as a key component of a series of treatment steps.
Controlled tests by independent microbiological laboratories show that a spiral-wound ultrafilter can attain 5-log endotoxin reduction.6 Since endotoxins are only a portion of the cell wall of gram-negative bacteria, they're more difficult to remove than the actual bacteria. Another independent laboratory test demonstrated a 9-log reduction of bacteria (P. diminuta, now Brevamundi) with a similar spiral-wound ultrafilter.7
Of course, controlled laboratory tests can only demonstrate short-term performance and don't account for potential problems inherent in large commercial systems. So how do spiral-wound systems perform for microbial removal in the field? A review of field systems, trade journals and research literature shows they perform quite well.9,10 For example, crossflow membrane systems are well accepted in beverage plants, where stringent attention to water quality is the norm. During the 1993 Cryptosporidium outbreak, the Milwaukee Coca-Cola plant was allowed to continue bottling soft drinks because it had a spiral-wound NF system.8
There's considerable precedence that spirals are a suitable technology for municipal use. Several studies involving on-site pilot testing in California and Florida have shown that single-pass, spiral-wound systems meet the microbiological criteria requirements. 5,9,10,11 Additionally, several states are reviewing field tests to allow spiral-wound membrane configurations for use in meeting the SWTR. For example, California regulations were met during a pilot test of a spiral-wound NF membrane element. Their effectiveness was demonstrated on canal water based on turbidity reduction, particle count reduction, a virus challenge, and with a Cryptosporidium oocyst challenge with a greater than 5-log reduction.12
Use of membrane technology for treating potable water is growing in the United States and the rest of the world. There are several categories of contaminants that require removal, and RO-NF classes of membrane can satisfactorily remove most of the contaminants. The actual choice of treatment technologies, including the choice of membrane configurations, will continue to vary with the needs at each site.
1. Taylor, "Drinking Water Regulation and Membrane Application," Desalination & Water Reuse Quarterly, January-March, Volume 4/4, 1995.
8. Sfiligoij, E. (Editor), "Tales from the Cryptosporidium," Beverage World, May 1994.
About the authors
Dave Paulson is corporate technical services director at Osmonics Inc. of Minnetonka, Minn. He has 28 years experience in the water treatment industry, with a focus on product design and industrial applications. He can be reached at (952) 988-6113 (fax) or email: email@example.com
Gary Davis works in Osmonics Research & Development Department in Minnetonka, Minn. He has a bachelor's degree in chemistry from St. Cloud State University, St. Cloud, Minn.
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