Exploring RO's Inner Net
By Harold G. Fravel Jr. and Karen Lindsey
A deep dive into reverse osmosis (RO) elements reveals the importance of feed channel spacers for optimal membrane filtration system performance.
Understanding the construction and purpose of spiralwound reverse osmosis (RO) elements and their components gives original equipment manufacturers, engineers, and especially system operators a better idea of the flow path through the element, mechanics of fouling, and challenges in cleaning. Since few people have actually dissected an RO element or seen it manufactured, allow us to eliminate the mystery of one of its primary components, the feed channel spacer.
RO and nanofiltration (NF) spiral-wound membrane elements are constructed of multiple leaves consisting of a membrane barrier layer cast on a polysulfone layer bonded to a backing material, a permeate carrier fabric, and a plastic netting known in the industry as a feed channel spacer or several trade names including Vexar®. The membrane leaves are glued to the permeate carrier on three sides and bound to a central product tube, which collects the filtered water and passes it through the pressure vessel. This spiral-wound configuration allows a high surface area of the membrane layer in a compact module.
It’s necessary to keep the stacked membrane leaves separated in order to allow water to flow between them. This is one of the primary purposes of the feed channel spacer. The spacer netting material lies on top of each membrane leaf, allowing an open pathway for the feedwater to flow across the membrane surface from one end of the element to the other. As the feedwater is separated into permeate and concentrate, the permeate is collected within the central product tube, and the concentrate exits the element.
Feed Channel Spacer Variation
While some aspects of the material have changed over the years, the polypropylene plastic net used in constructing the original 1960s RO element prototype remains prolific. Its diamond pattern is a legacy that continues, while the specific mil thickness is selected based on each RO element manufacturer’s preference, desired flux rates, and the site-specific application.
While the spacer material itself can seem fairly low-tech, its role in optimal RO production is critical. It can directly impact system feed pressures, overall energy consumption, and degrees of membrane fouling. The spacer creates a turbulent feed flow across the membrane leaves, helping to keep minerals, organics, and inorganics from settling on the membrane surface. Stressed operating conditions that result in interruptions or reductions in flow rates or pressures can make the spacer netting an ideal haven for these same materials to migrate and flourish, to the detriment of overall operation.
As the feedwater passes through the membrane and the resulting permeate is routed to the central tube, the remaining flow becomes more and more concentrated at the membrane surface interface as minerals are rejected. This can result in what the industry knows as concentration polarization — a thin layer of high-salinity, low-turbulent flow at the membrane surface. Effective flow across the membrane aids in minimizing this phenomenon, and increased turbulence narrows the polarization band and increases system efficiency.
Originally, all RO membrane elements were manufactured by hand with one employee required for rolling 2” and 4” membranes and two employees needed for the larger 8” diameter elements. The manual production resulted in inconsistencies in production, especially in the width, depth, and symmetry of glue lines. It was an insider joke that elements manufactured on Mondays and Fridays should be avoided as they were likely to have sloppy glue lines and subsequently lower flux rates. Glue lines placed far from the outer edge of the membrane effectively reduced the active square footage of the membrane and flux rates accordingly.
This concern was eliminated when manufacturers invested in automated rolling equipment. Automated rolling improved manufacturing consistency, and more leaves could be added to 8” x 40” configurations. Glue line placement was optimized to achieve maximum membrane-active areas for increased flux rates. As the element manufacturing and rolling improved, the industry standard 28-mil spacer was replaced by a 34-mil spacer in some element models that could accommodate the change. The wider feed channel spacer contributed to increased energy efficiency and reduced pressure drop during operation, while also improving cleaning results.
The thickness of the feed channel netting is extremely important. Manufacturers have found that a thicker feed channel spacer offers less flow resistance. However, a thinner spacer allows additional leaves to be added to the same element configuration, increasing the overall element production. As a result, some elements have a 34-mil spacer while others have thinner, 28-mil spacers. Some in the industry believe that the elements with 28-mil spacers are more prone to fouling, as there is less space between membrane leaves. Others believe this narrower space enhances the flow velocity across the surface and effectively aids in cleaning. Clearly more work needs to be done in evaluating the advantages and disadvantages of each depending on specific conditions and operational goals.
Particulates in the RO feedwater can be quite detrimental to overall element performance, which is why media, sand, and cartridge filters are typically applied upstream of the RO system. However, for a variety of reasons, some particulates still make their way into the RO feedwater and can accumulate in the RO element feed channel spacer. Biological growth can also flourish in any lower-turbulent havens within the spacer netting. Fouling can occur at any point along the feed spacer, but typically begins at the feed entry point and proceeds from there as the fouling itself begins to negatively affect turbulence, further compounding the problem. In contrast, concentrated scale minerals including calcium, sulfate, and carbonate accumulate in the back end and, in some cases, can completely fill each hollow in the netting material. In our experience, extensive calcium carbonate scale in particular has resulted in RO elements weighing over 90 lbs (40 kgs) as compared to the 35 lb (15 kg) weight when new. As the spacer material becomes clogged with foulant, more and more pressure is required to pass water through the membrane layer beneath, resulting in reduced energy efficiency.
Innovations In Spacer Design
Over the years, innovative membrane manufacturers and their material vendors have studied fluid dynamics to investigate how alternative patterns in spacer materials affect flow, turbulence, and areas of low flow during operation.
Computational fluid dynamics have been studied at length using three-dimensional geometry representation. X-ray computed tomography scans and modeling have also been applied to the study of flow turbulence within RO elements. Correlations between RO system pressure drops and feed channel spacer orientation and thickness have been identified and analyzed.
Recent innovations in feed channel spacers have included incorporating a biostat into the material with the goal of inhibiting biological growth within the element. While typical spacer netting is white or tan, the biostatic spacer was a distinct new color and could be immediately identified by its exposed bands at each end of the element. The manufacturers’ research and pilot testing indicated a discernable reduction in biogrowth over time as compared to conventional spacer materials, though some believe a light fouling layer might undermine the biostat advantage over time.
Another manufacturer used an alternating strand design (ASD), where strands of different thicknesses improved performance with respect to fluid flow, pressure drop, and energy loss. Biplanar orientation of the filaments have also been investigated as well as changing the surface roughness of the material, effectively making it smoother to reduce the ability of bacteria to adhere. Still another inventor has eliminated the feed spacer netting altogether by embossing the membrane itself.
Site- or application-specific element configurations can be manufactured when volumes justify the off-spec investment and wider feed channel spacers are one customization option. RO and ultrafiltration (UF) spiral-wound elements for the dairy industry contain 30-mil to 46-mil feed channel spacers, accommodating a potentially more viscous liquid and a greater need for effective and more frequent cleaning. In some applications, the same spacer material is used for an entirely different purpose. While typical 8” RO elements have a fiberglass outer wrap, this material is not suitable for high-temperature sanitation applications which have adapted by using spacer netting as the exterior element wrap.
Configurations and mil thickness continue to evolve to meet the demands of unique applications, while time and testing may discover alternative materials and methods in the very near future. Until then, understanding the critical role of feed channel spacers in RO element performance, energy efficiency, and cleaning efficacy offers system operators an edge in troubleshooting RO performance and optimizing element cleaning.
About The Authors
Harold Fravel accepted the position of executive director for the American Membrane Technology Association (AMTA) after working for Dow Chemical/FilmTec Corporation for 36 years. He has a Ph.D. in organic chemistry from the University of North Carolina and a B.S. in chemistry from Florida State University. He resides in Jupiter, FL.
Karen Lindsey is an executive member of the American Membrane Technology Association (AMTA) board of directors. She is the VP and cofounder of Avista Technologies and has 30 years’ experience in the water treatment industry, working with companies that cast cellulose acetate membrane, produced polyamide elements, and formulated specialty chemicals.