[email protected] Tel. Fax. 31-152-616-289Engineering Aspects of Reverse Osmosis Module DesignAuthors: Jon Johnson , Markus Busch Research Specialist, Research and Development, Dow Water & Process Solutions Global Desalination Application Specialist, Dow Water & Process SolutionsEmail:[email protected] the half century of development from a laboratory discovery to plantscapable of producing up to half a million tons of desalinated seawater per day,Reverse Osmosis (RO) technology has undergone rapid transition. Thistransition process has caused signification transformation and consolidation inmembrane chemistry, module design, and RO plant configuration andoperation. From the early days, when cellulose acetate membranes were usedin hollow fiber module configuration, technology has transitioned to thin filmcomposite polyamide flat-sheet membranes in a spiral wound configuration.Early elements – about 4-inches in diameter during the early 70s – displayedflow rates approaching 250 L/h and sodium chloride rejection of about 98.5percent. One of today’s 16-inch diameter elements is capable of delivering15-30 times more permeate (4000-8000 L/h) with 5 to 8 times less saltpassage (hence a rejection rate of 99.7 percent or higher).This paper focuses on the transition process in RO module configuration, andhow it helped to achieve these performance improvements. An introduction isprovided to the two main module configurations present in the early days,hollow fiber and spiral wound and the convergence to spiral wound designs isdescribed as well.The development and current state of the art of the spiral wound element isthen reviewed in more detail, focusing on membrane properties (briefly),membrane sheet placement (sheet length and quantity), the changes inmaterials used (e.g. feed and permeate spacers), element size (most notablydiameter), element connection systems (interconnectors versus interlockingsystems).The paper concludes with some future perspectives, describing areas forfurther improvement.3912.2009


THE EARLY HISTORY OF REVERSE OSMOSIS MODULE DESIGNReverse Osmosis module design and engineering emerged with membrane technologyevolution. In order to understand module design, first membrane configuration needs to beexplored, since the module design is always tailored according to the membranecharacteristics. There is a significant difference between membrane chemistries (mostimportant ones being cellulose acetate and thin film composite with polyamide barrierlayer), and more importantly, between the different membrane configurations (hollow finefiber and flat sheet). Therefore, before looking into detail on the module configuration, themembrane development needs to be considered.The invention of RO desalination and first applicationsAfter Schoenbein succeeded in the synthesis of nitrocellulose (1845) and Fick performeddiffusion tests with nitrocellulose sheets (1855), more than 100 years had to pass beforeRied and Breton succeeded in the demonstration of reverse osmosis desalination withcellulose acetate film (1959) and Loeb and Sourirajan developed asymmetric celluloseacetate membranes, which were the base for the first real world applications of reverseosmosis.North Star, the predecessor of FilmTec, initially used cellulose tri-acetate as separatinglayer in a thin film composite flat sheet configuration (1964), but then switched to apolyamide barrier separating layer. The Dow Chemical Company (“Dow”) developed acellulose tri-acetate membrane (1971) in hollow fine fiber configuration latercommercialized the DOWEX range of HF modules (1971). Toyobo followed with asimilar hollow fiber cellulose acetate membrane in 1978.DuPont used a different membrane chemistry, initially nylon (1967), later aromaticpolyamide (1969). Of the three producers of hollow fine fiber modules, the Permasep B-9and later B-10 Permeator from DuPont became the leading element in the market in the1980s and early 1990s.4112.2009

The early cellulose acetate hollow fine fiber modules were capable of withstanding thepressures required for seawater reverse osmosis and one of the key features of the CTAfiber was that it had a relatively high level of tolerance to the presence of free chlorine at atime when competitive HF products from DuPont and spiral wound elements from FluidSystems had very low, effectively zero tolerance to the presence of free chlorine.Due its market dominance in the early years of RO desalination, the Dupont Permeator isselected as typical example to illustrate the early hollow fine fiber module configurationand the performance of the initial seawater desalination systems with this concept.The early years of RO desalination – the hollow fine fiber DuPontPermeatorThe DuPont membrane was an asymmetric fiber with 42 µm inner diameter and 85 µmouter diameter, of which 0.1-1 µm was dense skin layer and remainder porous support,made from aromatic polyamide (aramide). A typical 10-inch diameter module, containedabout 4,400,000 fibers. These were built into a module by applying epoxy adhesive to oneside during bundling and after winding became the tube sheet. The other end of the fiberbundle was sealed with epoxy to form the nub which prevents short-circuiting of the feedstream to the brine outlet [DuPont, 1983]. The module and RO process is shown in Figure1.4212.2009

Figure 1: Hollow fine fiber module and process [DuPont, 1983]The first Permasep Hollow Fiber B-10 Permeators from DuPont were introducedcommercially in Europe in 1974. These were 4’’ elements which had a capacity of 5.7 m3/d(1,500 gpd) and a salt rejection of 98.5 percent under standard test conditions (30,000 mg/LNaCl, 55bar/800 psi, 30 percent recovery, 25 C) [Cicera & Shields 1997].Between 1974 and 1997 DuPont continuously improved the design and the performance oftheir HF elements. In 1992 the double-bundled B-10 TWIN Permeator was introduced.The model 6880T with an aramide HF membrane had a capacity of 60.5 m3/d (16,000 gpd)and a salt rejection of 99.55 percent (std. cond.: 35,000 mg/L NaCl, 69 bar / 1000 psi, 35percent recovery, 25 C).Shortly before DuPont terminated membrane production the Hollow-Fiber Cartridge wasintroduced. The model SW-H-8540, Single Cartridge had a nominal 8½-inch diameter by40-inch length with a capacity of 30.3 m3/d (8,000 gpd) and 99.6 percent salt rejection (std.cond.: 35,000 mg/L NaCl, 69 bar/1000 psi, 35 percent recovery, 25 C) [Eckman et al1997].Between 1983 and 1997, for a typical sea water with a temperature between 17 – 38 C anda salt content of 36,000 – 45,000 mg/L, the major design characteristics of a single passPermasep SWRO (sea water reverse osmosis) plant with a B10-Permeator were [Andrews4312.2009

& Bergman 1983, Eckman et al 1994, Pohland et al 1994, Hamida et al 1997, Barendsen &Moch 1995]: Recovery: 30 % – 50 % Feed pressure: 1,000 – 1,200 psi (69 – 82.7 bar) Permeate quality: 500 mg/L Energy consumption: 3.7 – 8.2 kWh/m3The shift to spiral wound modulesAt the time when Permasep HF-Permeators for desalination of seawater were introducedinto the market in the 1970’s they had some advantages compared to seawater spiral woundelements which explain their success in the RO-market at this time [Moch, 1992]:Permasep HF-Permeators are self supporting membranes. This simplified the hardware forfabrication compared to flat-sheet membranes which have to be assembled with spacers andsupports. In addition the hollow fibers were able to operate up to 82.7bar (1,200 psi), whichallowed to reach relatively high recoveries, like 60 percent at 25 C and 38,000 mg/L feedTDS (total dissolved solids).At a similar specific permeate flux (flow per membrane area), a conventional flat sheetmembrane needed only about 50 percent of the feed pressure of a hollow fiber. Thisrelatively low permeability of a single fiber in comparison to a flat sheet membrane wascompensated by the Permasep HF Permeator with the extremely high area per Permeator(Single Cartridge : 372 m2, (4,000 ft2)).This high area allowed working at relatively low fluxes. This reduces concentrationpolarization and the risk of scaling. The relatively low concentration polarization alsoimproved the rejection of the Permeator.A major disadvantage of the Permasep HF Permeator was its tendency to foul and plug dueto low free space between the hollow fibers and due to dead zones in the Permeator [Moch,1992].4412.2009

In addition fouling and scaling was difficult to remove due to the low cross flow velocitiesand a relatively limited pH-range (4 – 11). These constraints required a high RO-feed waterquality (SDI 3) which resulted in higher pretreatment costs and some operationaldifficulties.To keep the rejection of the Permasep HF Permeator constant it generally had to be coatedby PT-A (poly vinyl methyl ether) and PT-B (tannic acid) [Moch, 1992]. These chemicalshad to be reapplied frequently, PT-B even after every membrane cleaning cycle.The exit of hollow fine fiber modulesNotwithstanding the benefits of chlorine tolerance of the DOWEX cellulose triacetate fiber,Dow gradually became aware of other limitations and short comings of both the CTAchemistry and the hollow fiber module construction.To address these issues Dow purchased the FilmTec Corporation, in 1985 and thus gainedaccess to polyamide, thin film Composite, flat sheet membrane technology and also tospiral wound element construction, and exited the hollow fine fiber market.The DuPont hollow fiber, which had been leading the RO market in the 1980s and early1990s, started to lose ground to polyamide spiral wound modules in the 1990s. This wasdue to the increasingly fierce competition of a larger quantity of spiral wound modulesuppliers such as FilmTec / Dow, Rohm & Haas / Hydranautics, Toray, Fluid Systems /Koch, TriSep and Osmonics / General Electrics, which significantly reduced modulepricing and advanced module concepts. The DuPont concept lost its appeal and the businessbecame increasingly unattractive, which led to the exit of DuPont from hollow fine fibermodule production.In most parts of the world, plants have converted from hollow fine fiber module use tospiral wound modules. Prominent examples for seawater plants are Galilah (United ArabEmirates), Agip Gela (Italy) and Agragua Gran Canaria (Spain) [Gorenflo et al 2004,Reverberi & Gorenflo 2007, Gorenflo & Sehn 2006]. Significant cost savings have been4512.2009

achieved by retrofitting plants from hollow fine fiber to spiral wound modules [Gorenflo etal 2004].In Saudi Arabia there are still various large old plants using hollow fine fiber modules, e.g.Al Jubail, Al Birk, Jedda, Haqel, Duba and Yanbo and even new plants have been addedrecently (Shuqeiqh, Jeddah). Toyobo is the only remaining hollow fine fiber supplier, andenjoys an attractive single supplier situation in these projects.The future of spiral wound modulesIn the past 20 years considerable improvement of seawater spiral wound elements havebeen made. The capacity of an 8-inch element has been doubled whereas the salt passage isabout three times less [Busch & Mickols 2004, Garcia Molina et al 2008]. Thisdevelopment is illustrated in Figure 2 at the example of the DOW FILMTEC seawaterreverse osmosis range99.92008: FILMTECSW 30XHR-400i99.8Rejection (%)99.799.62009: FILMTEC 2009: FILMTECSW30HRLE-440i SW 30XLE-440i2003: FILMTECSW30HRLE-440i1996: FILMTECSW30HR-3802004: FILMTECSW30XLE-4002009: FILMTECSW30ULE-440i2008: FILMTECSW30ULE-400i99.599.499.31985: FILMTECSW30HR-804099.299.11985: FILMTECSW30-80401996: 01200013000Flow (gpd)Figure 2: Evolution of spiral wound module performance, illustrated by example of theDOW FILMTEC seawater desalination product range4612.2009

It has been possible to increase the active area in an 8-inch module from 300 ft² in the earlydays (1980s) to 440 ft² and further increases are possible. These increases are possiblewhile feed spacer thickness is maintained and geometry improved. The development ofelements with larger diameter (16-inch) allows a factor 4.3 increase in membrane area, to1725 ft², and by this allows significant savings.Furthermore the maximum operating pressure for spiral wound elements was 69 bar (1,000psi) in the past. Recent improvements in membrane stability and permeate spacertechnology of some manufacturers increased the maximum pressure to 82.7 bar (1,200 psi)[Gorenflo et al, 2003, Casanas et al 2003, Kurihara et al 2001, Polasek et al 2003]. Thisallows working at a relatively high osmotic pressure and thus increasing the recovery forspiral wound elements up to 60 percent and more. Improved rejection of the membranescompensates the higher system salt passage which goes along with a higher systemrecovery.There is also ongoing work with regards to the product water tubes and the elementconnection system has been significantly improved by the introduction of inter-locking endcaps.Recent achievements as well as continued development of spiral wound module design iscontributing to significant cost savings in RO technology and offers to make thistechnology even more widely available for sustainable and affordable water production inmany parts of the world.Therefore, the remainder of this paper will focus exclusively on selected engineeringaspects of the spiral wound module, as developed for the purpose of treating water throughreverse osmosis.The discussion will emphasize the module configurations used in large-scale municipal andindustrial RO systems – those with diameters of at least 8-inches. The patent documentsand technical papers mentioned in connection with specific topics are by no means4712.2009

exhaustive, but are intended to be illustrative of the work that has occurred. The documentsusually include a useful list of references for those interested in retrieving additionalinformation.CURRENT STATUS AND FUTURE DIRECTION OF SPIRALWOUND MODULE COMPONENTS AND ENGINEERINGDespite its cylindrical configuration, the spiral-wound reverse osmosis module isessentially a flat-sheet, cross flow device. The feed water passes through the moduleaxially, while permeate moves in the spiral, radial direction toward the permeate collectiontube. The membrane interposed between these streams remains the technologicalcenterpiece of the module, but other aspects of module engineering are increasingly criticalto performance.The increased focus on module engineering is driven in part by the desire for costreduction, but more often by the desire to extract the full value of the latest membranetechnologies. The promised membrane benefits can only be fully realized when moduledesigns focus on energy efficiency and the preservation of membrane salt rejection.The following discussion is organized around the five major non-membrane components ofthe spiral wound module: Feed Spacer Permeate Spacer Permeate Tube EndcapThese key components are depicted in Figure 3.4812.2009

Folded MembraneEndcapPermeateSpacerPermeateSpacerFeed FlowDirectionGlue LinePermeateTubeFeed SpacerPermeate FlowDirectionFolded MembraneGlue LineFigure 3: Configuration of spiral wound membrane module for reverse osmosis.These components will be considered in turn, with a brief overview of the role andimportance of each, followed by a discussion of recent developments.Feed spacerBy far the most common feed spacer configuration used in reverse osmosis membranemodules is the biplanar extruded net (Figure 4a).One of the earliest patents for making the net was obtained by Nalle (1962), who describedcounter-rotating die which produced a continuous, cylindrical mesh structure that wasstretched over a mandrel, quenched, and then slit to create a flat web (Figure 4b).Most RO feed spacers are made from polypropylene, which offers the preferredcombination of extrudability, low cost, and chemical inertness. Thicknesses between 0.64912.2009

and 0.9 mm are typical. The spacer is priced below 1.00 US per square meter for the mostcommonly-used varieties.(a)1 mm(b)Figure 4: (a) Biplanar extruded netting is comprised of two intersecting sets of parallel,extruded strands. (b) An early patent was obtained by Nalle (1962).Purpose of the feed spacerThe feed spacer has two functions. It provides an open channel for the flowing feed waterby maintaining separation between the membrane sheets. It also promotes mixing withinthe feed channel, moving salt and other rejected substances away from the membranesurface.Maintaining an Open Feed Channel.A key step in the fabrication of spiral-woundmembrane modules is the rolling-up of the layered membrane and spacer materials aroundthe permeate tube. The compressive forces generated during roll-up, and the consequenttightening of the spiral, cause compression of the feed spacer and nesting of adjacent feedspacer layers.5012.2009

An apparent change in thickness may be estimated from the original thickness of thespacer, obtained from a representative sample using a caliper, and the apparent thickness ofthe feed channel, measured after module fabrication:Change in Thickness Spacer Thickness Channel Thickness 100 %Spacer ThicknessEquation 1The apparent channel thickness is estimated by measuring the body diameter of thefabricated module and the thicknesses and lengths (in the spiral direction) of all of theinternal materials of construction.The materials are non-nesting and negligiblycompressible, except for the feed spacer, which allows the apparent channel thickness to beobtained mathematically.The net-type feed spacers used in RO modules provide points of contact with the membranethat support and maintain the open feed channel. As shown in Figure 4a, these points areformed by the intersection of the polymer strands. The importance of support point densityis illustrated in Figure 5, where the change in channel thickness was plotted against thesupport point density, in points per square centimeter, for a variety of different spacers. ROmodules were made from each spacer under identical fabrication conditions, and the changein thickness was determined as outlined above.Change in Thickness (%)20151050051015Support Point Density ( number / cm2 )Figure 5: Effect of support point density upon change in apparent thickness of feed spacerduring module fabrication.5112.2009

The trend in Figure 5 illustrates a significant constraint upon feed spacer optimization.Biplanar extruded nets cannot be so reconfigured that their ability to support and separatethe membrane layers is compromised. This can occur if the number of intersections isdramatically reduced. Support point densities of 10 to 12 per square centimeter are typicalof commercially-available spacers for large-scale applications.Mixing the Feed Water. The spacer mixing effectiveness, or more precisely the masstransfer effectiveness, is expressed in terms of the concentration polarization of a givenspecie, usually a dissolved salt, that is partially or entirely rejected by the membrane. Thepolarization factor, , is defined as follows: C membraneC bulkEquation 2Where Cmembrane is the specie concentration at the membrane surface, and Cbulk is the flowweighted average concentration for the channel cross-section. depends upon the localpermeate flux, the mass diffusivity of the specie of interest, the degree of rejection, and theextent of mass transfer.For sodium chloride, conventional spacers and typical operating conditions provide average in the range of 1.05 to 1.15. The osmotic barrier in many reverse osmosis applications istherefore increased by 5 to 15 percent due to imperfect feed channel mixing. This increasesby up to 10 percent the direct energy consumption in seawater desalination. Feed spacerswhich reduce concentration polarization have been proposed, but significant improvementamong known configurations leads to increased feed channel pressure drop.The Pressure Drop TradeoffAn unwanted byproduct of the mechanical support and mass transfer functions is feedchannel pressure drop.Because RO modules are typically employed several-in-serieswithin large systems, feed-side pressure drop impacts system performance by reducing thetrans-membrane pressure, and consequently the permeate production, in the downstream5212.2009

modules. This under-utilization leads to over-utilization and increased rate of fouling in theupstream modules. pFigure 6: The tradeoff between concentration polarization, , and feed-side pressure drop,p, constrains feed spacer optimizationEfforts to improve mass transfer through optimization of the biplanar extruded net andother configurations have not resulted in dramatic changes to commercial spacers, whichremain much the same as those used 20 years ago. Reasons for this include the relativelysmall magnitude of the potential benefit associated with improved mass transfer comparedto that achieved historically through ongoing improvements in membrane chemistry. Asecond reason is the mass transfer tradeoff depicted in Figure 6, which ties reducedpolarization to increased pressure drop. A third reason is the low cost of existing spacers.The tradeoff is not immovable, and spacers have been proposed which promisesimultaneous mass transfer and pressure drop improvement. For example, multi-layerspacers (Schwinge, 2004; Meindersma, 2005) place obstructions at the membrane surfacewhere they can effectively interrupt the concentration boundary layer while minimizingdisturbance of the bulk flow.Spacers with strands of non-circular cross section appear to reduce pressure drop while stillmixing the boundary layer (Guillen, 2009; Karode, 2006). Unfortunately, economicallarge-scale manufacturing methods for such configurations have not been developed.Feed spacers and foulingIn addition to the osmotic penalty, imperfect mixing reduces salt rejection, promotesscaling at the membrane surface, and increases the rate of deposition of certain foulants.Fouling mitigation may represent the most significant opportunity for operational savingsthrough improved feed spacer design. However, the magnitude of the potentialimprovement and the means by which spacers can reduce fouling through improvedhydrodynamics are not yet well understood. Examples of recent spacer research include5312.2009

investigations of biofouling (Vrouwenwelder, 2003) and particulate fouling (Neal, 2003).There appears to be less focus on the impact of spacers on other forms of fouling, such ascolloidal and adsorptive organic fouling.Current status and future directionsRecent spacer development for commercial use has focused primarily on pressure dropreduction (Bartels, 2008; Johnson, 2005; Kihara, 2003). This has been shown to reduceenergy consumption, improve hydraulic balance in low-pressure RO systems, and lengthenthe time between cleanings in applications where excessive feed-side pressure drop is thecriterion by which cleaning intervals are determined.Anti-microbial spacers are of interest. Feed spacers containing silver (Yang, 2009) andcopper (Hausman, 2009) have been formulated. A spacer which varies in thickness alongthe length of the module has been proposed for improved hydrodynamic performance(Saveliv, 2009). The spacer can be eliminated entirely if membrane-supporting structuresare applied directly to the membrane surface (Bradford, 2007).Future feed spacer development, in both fundamental research and product improvement, isexpected to emphasize fouling performance, including protocols for measuring andcomparing rates of fouling among spacers.The tradeoff between mass transfer andpressure drop will remain at the forefront. Configurations will be presented that skew toone side of the tradeoff for the benefit of specific applications.Permeate spacerThe permeate spacer provides a conduit for the collection and transport of permeate fromthe membrane to the permeate tube. Woven polyester fabric is the most common spacer incommercial use. The tricot weave is often chosen for its structural rigidity, smoothness,and fluid-channeling characteristics.The tricot is sandwiched between two sheets ofmembrane and sealed on three edges by glue, as shown in Figure 1, to create an envelopethat is often referred to as a membrane leaf.5412.2009

Pressure drop in the permeate spacer has a profound effect upon module performance. Theeffect is detrimental in two respects.First, the net driving pressure required to obtain the desired permeate flow is increased. Inother words, the element efficiency is reduced. The element efficiency,, is the ratio ofthe actual permeate flow, Q, to the expected output based upon the active membrane area,A, the membrane permeability, P, and the net driving pressure, NDP: QA P NDPEquation 3Second, for a given average flux within the element, the range of variation of the local fluxis increased. Near the root of the leaf, close to the permeate tube, the flux is higher.Further from the tube, near the tip of the leaf, the flux is lower.Consequently, themembrane furthest from the tube may be underutilized, while the membrane close to thetube may be subject to premature fouling. The smallest possible range of variation isdesired.Permeate Spacer Pressure Drop. The pressure drop within the spacer is very nearly linearwith flow rate, and may be parameterized using the following simple relationship:qdp kwdxEquation 4where dp/dx is the pressure drop in the permeate flow direction at a given distance from thecollection tube, q is the volumetric flow rate moving through the spacer at that location, wis the width of the leaf measured parallel to the permeate tube, and k is the frictionparameter for the spacer. There is a slight variation of k with applied pressure due to thesqueezing of the woven structure.Element Efficiency.The efficiency is readily estimated from standard mathematicalmodels (Incropera, 1985). A curve relating leaf length to element efficiency was calculatedand plotted in Figure 5 using a friction parameter, k, of 130 psi-s/in3, and a membrane5512.2009

permeability, P, of 0.05 gfd/psi.This permeability is and spacer performance isrepresentative of commercial seawater RO modules.Element Efficiency (%)1009590858075102030405060Leaf Length (inches)Figure 5: Effect of leaf length upon element efficiencyP 0.05 gfd/psi, k 130 psi-s/in3.Local Flux Distribution. Using the available mathematical models, a comparison was madebetween two membrane leaves, one 29-inches long and one 40-inches long. The netdriving pressures were chosen to provide the same average flux in the two leaves. Thelocal flux was then plotted as a function of the coordinate, x, in Figure 8.Local Flux, j (gfd)201540-inch Leaf Length29-inch Leaf Length100102030Distance From Tube, x (inches)40Figure 8: Variation in local membrane flux with leaf coordinate (distance frompermeate collection tube) P 0.05 gfd/psi, k 130 psi-s/in3, javg 15 gfd.5612.2009

The local flux is seen to vary from 14.5 to 16 gfd within the shorter leaf, and from 14 to 17gfd within the longer leaf. Both of these hypothetical leaves were designed and operated toprovide an average flux of 15 gfd, but the range of variation was twice as large for thelonger leaf.Future Directions. Due to the pressure drop imposed by woven permeate spacer materials,shorter membrane leaves in spiral-wound module construction provide higher moduleefficiency and reduced flux variation. Development efforts by membrane manufacturerswill continue to accommodate current permeate spacers by focusing on increased use ofautomation, which enables defect-free fabrication of modules with more, shorter leaves.Consequently, improved permeate spacers represent untapped value.They have thepotential to increase module efficiency or, if leaf counts reduced and fabrication timesshortened, to reduce membrane module cost. The challenge for future developers will be toreduce pressure drop and maintain or improve resistance to deformation by RO feedpressures.This must be done at very low cost, as woven polyester tricot for RO istypically priced below 5.00 US per square meter.Forward Osmosis. Permeate spacers for forward osmosis applications will require evengreater strides in pressure drop reduction. The presence of a sweep stream on the permeateside of the membrane will drive consideration of a permeate channel that more closelyresembles the feed channel in terms of its mass transfer and pressure drop characteristics(Foreman, 1975).5712.2009

Permeate tubeThe permeate tube collects permeate from the spacer materials inside a module. In multimodule pressure vessels, the tubes are connected in series, and serve as a conduit for thetransport of permeate to an external manifold. The permeate tube also provides importantdiagnostic access during operation, permitting conductivity sensors and sampling probes tobe inserted in search of membrane defects and leakage.Tube configurations have been largely unchanged in 20 years of RO module development,although materials and methods of tube fabrication have been updated. Tubes for standardmodules of 40-inch length are usually extruded. Secondary machining operations add sideholes and tightly-toleranced sealing surfaces. Tubes for shorter modules are sometimesinjection-molded. Although most tubes for 8-inch diameter modules have inside diametersnear 2.5 cm, a large-diameter tube has been offered in commercially available low-energybrackish water and nanofiltration elements (Dow, 2009). The 3.5 cm inside diameterreduces pressure drop, which is a significant contributor to unwanted permeatebackpressure in low-pressure RO systems.Future Directions. Future designs will likely make further use of the tube for collecting andrelaying information.Probes located inside the tube and communicating via radiofrequency with the outside world have been described (Wilf, 2009). Additional featuresthat work cooperatively with probes and sensors to ease th

outer diameter, of which 0.1-1 µm was dense skin layer and remainder porous support, made from aromatic polyamide (aramide). A typical 10-inch diameter module, contained about 4,400,000 fibers. These were built into a module by applying epoxy adhesive to one side during bundling and after winding became the tube sheet. The other end of the fiber