Membrane fouling in reverse osmosis _RO_ systems is as all .._1_

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					Introduction

Spiral-wound membrane elements (sepralators) were used mainly for basic water purification
until the mid to late 1970s, when broader uses began to receive attention. New applications
often required special consideration such as; sanitary construction for the dairy industry, high
temperature for food processes and waste streams, and high pressure capability for
desalination. In addition, aggressive chemicals either in the feed stream or in necessary
cleaners had to be dealt with. As the sophistication of the membrane industry grew, so did
increased use of specialty polymers, improved adhesives, more engineering thermoplastics,
and even stainless steel in sepralators and their ancillary parts. This has allowed the expanded
scope of application, which continues to grow today.

Sepralator Design

Original designs used a plastic tape outer cover with a concentrate (or "brine") seal to direct flow
through the feed channels. The permeate tube protruded from the material package on each
end; requiring this thermoplastic part to bear the entire axial compressive load for the series of
sepralators (up to six) placed in one housing. An FRP outer cover was next developed for both
handling protection and improved hydraulic load-bearing. A "close-coupled" design followed
where the sheet materials are trimmed flush with the permeate tube end. This maximizes the
available space in the housing and efficiently transfers the DP load between sepralators. Most
commercial sepralators today employ this design.

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The Full-Fit design evolved next and represents a major improvement. The outer cover and
brine seal are eliminated, allowing higher feed flows (at the limiting pressure differential) which
has been found to help reduce fouling in some applications. It also eliminated the "dead-flow"
area behind the brine seal where bacteria proliferated and from where cleaning/storage
chemicals were not readily flushed. This design has become standard in pharmaceutical and
food/beverage applications, where cleanliness and ability to sanitize are essential. Also, many
high-fouling applications, from biotech to waste treatment, have been made feasible by the Full-
Fit membrane element . The attributes of this design were further enhanced by tapering it to fit
in a filter cartridge housing, combining cleanliness, convenience and economy.

Figure 1 compares the element pressure drop for a "standard" sepralator and a Full-Fit
membrane element. This difference becomes an important advantage with multiple sepralators
per housing, stages per machine, and especially for processing high viscosity solutions.

Figure 1
Figure 2 demonstrates that even though the DP for a Full-Fit sepralator is less than the outer
cover style, the all important velocity over the membrane surface at a given flow rate is nearly
equal.

Figure 2




Figure 3 further demonstrates that a critical crossflow velocity is necessary to achieve maximum
separation. It can also be seen that above a certain velocity there is little benefit. Note this is
true even for non-fouling dissolved salts. This graph is based on Full-Fit membrane element
data but the principle applies to all sepralators, at least with standard mesh spacers.

Figure 3
Several variations of the Full-Fit design are available, including unanchored "loose-wrap"
spacer, a "net sock" or sleeve, and also a sepralator in which the tail end of one spacer sheet is
welded to an inner wrap of itself. A wrap or more of mesh spacer may also be applied over a
FRP outer cover to act as a partially restricting seal. All designs give subtle performance
differences. The choice for these will depend on the application, and on the manufacturer's
viewpoint.

Membrane Materials

The membrane is usually the compatibility limiting component of a sepralator. Thus the
advantages of the spiral-wound design make it the first configuration to consider for all four
membrane classes (RO, NF, UF, MF). Membrane choice is often governed by compatibility
considerations rather than separation performance and flux-related characteristics.

The evolution of membrane materials for RO sepralators began with the cellulose acetates
(which are still workhorses). Both homogeneous and thin-film composite polyamide membranes
followed to provide wider pH range, improved separation and biological degradation resistance.
Since these are more expensive and less tolerant of oxidizing agents than the cellulosics, they
have not replaced CA but closely rival it for total use. Sulfonated polysulfone RO membrane is
more resistant to oxidation, but must be operated on completely softened, brackish water to
maintain its salt rejection capacity, which is not as high as the polyamides to begin with.

Ultrafiltration membranes are made of a wider variety of basic polymers than RO, and are
chosen for an application based upon chemical resistance, pore size and size distribution and
the solute/membrane surface chemistry (mainly governed by hydrophobicity or hydrophibicity).
UF membranes of PVDF and acrylo-nitrile chemistries are now available, but polysulfone and
cellulosics continue to predominate.

After years of effort to achieve smaller pore membranes from the UF polymers, nanofiltration
(NF) membranes were recently developed as a result of manufacturing variations of RO
membranes. They are used for partial desalting of brackish water ("membrane softening"), in
dye desalting and for other fractionation applications. Generally made from polyamides,
different chemistries are now emerging to broaden the range of feasible applications. Most, if
not all NF membrane used today, is in the spiral configuration.

The widest variety of membrane materials are still available in the microfiltration class. These
can be manufactured by phase-inversion, thermally or mechanically formed or a combination of
these. MF membranes are commercially available in a variety of polymers from stretched PTFE
to fission particle-irradiated polycarbonate, and now sepralators are used to take advantage of
the varying and unique properties of the MF class membranes.

Membrane Supports

Membrane backing materials (substrates) are usually required for support and therefore are a
factor in sepralator performance (although often overlooked). Available in woven, non-woven or
spun-bonded form, the polymers are typically polyester or polypropylene. The fabric
manufacturers have now identified membrane supports as an attractive market, and alternate
materials and improved mechanical properties are being developed at an unprecedented rate to
overcome the chemical limits of polyester and temperature/strength/adhesion limits of
polypropylene. Wovens provide a uniform surface for coating but are expensive. Non-wovens
and spun-bondeds are much less expensive but lack of uniformity can cause quality and
manufacturing yield problems. Price, uniformity, and of course operating capabilities are three
factors the membrane and sepralator manufacturer must evaluate for every substrate.

A permeate carrier fabric (PC) is also an essential support material for spirals. Traditionally a
tricot weave, PCs are chemically treated to attain compressive strength. Treatment was initially
melamine, but the advent of epoxy sizing in the early 80s has improved heat and chemical
resistance. High pressure operation, especially at elevated temperatures, was improved by
changing weave patterns and thread density of the PCs, as well as by a few novel approaches
(both patented and proprietary) to the use of common fabrics and films. Monofilament weaves,
although sometimes limited to low pressure operation, are useful in certain applications.

Adhesives

Adhesives are frequently a problem and often are the next weak link after the membrane.
Adhesives must be somewhat flexible when set, and have certain manufacturing related
handling characteristics. These requirements have limited the available choices. Improvements
are considered proprietary. Typical adhesives have a long-term upper temperature limit below
that of some of the membranes. Solvent compatibility may also make the adhesive the weak
link. While substantial progress has been made, adhesives remain an area of focus for
sepralator improvement.

Feed Channel Spacers

Early feed spacers ("turbulence promoters") were limited to co-axially extruded, diamond-mesh
polypropylene. These fabrics were only available about 0.030 inch thick (0.8 mm) and with 8-11
strands per inch. This design has changed little over the years and still predominates in water
purification and most process applications, due to the general success of the design. However
the size and configurations available in mesh net spacers have expanded in recent years.

The results of the Levy and Earle study is just the most recent confirmation of the long
established fact that the presence of a mesh spacer over a crossflow membrane surface
maintains higher flux. On a 1% bovine serum solution, the membrane flux in a crossflow cell
was 39% higher with a diamond-mesh spacer than with no spacer at similar "wall shear rates."

Varying the thickness of the spacer is a simple way to extend the range of applications for the
sepralator. Thinner mesh spacer can be used in ultrapure water treatment and other low fouling
applications to increase membrane area. Thicker mesh spacers (up to 2 mm) reduce effective
membrane area but may also reduce fouling and channel plugging. This is important when high
concentrations of suspended solids and/or large size particulates are processed (see Figure 4).
Along with evaluation of the potential of the thicker spacers and the Full-Fit design, came a
closer and more creative engineering assessment of sepralators. Full-Fit spirals now are
replacing hollow fat-fiber elements for ED paint ultrafiltration; the classical application where
spirals were not seriously considered due to the high level of paint pigments.

Figure 4
Fat-fibers elements were traditionally turned to when a high level of suspended solids was
believed to, or sometimes actually shown to, plug the sepralators feed channels. A new open-
channel sepralator spacer is now used on these type solutions and slurries. As with thicker
mesh spacers, the disadvantage of reduction of membrane area is offset by increased fouling
resistance as well as the several advantages sepralators enjoy over hollow fibers; no fiber
breakage, multiple element housings, operation of elements and housings in series, higher
allowable crossflow rates (due to much lower DP) and much higher pressure operation.

Concentrate seals are molded in the full range of typical elastomers. Specially designed seals
allow sepralators to perform well under the high D P stresses now employed, and in housings
made of tubing with varying internal diameters.

Common sepralator housings choices are FRP, stainless steel or PVC (for low pressure). FRP
housings provide corrosion resistance, but use end caps made of different polymers and must
have fluid entry through those end caps, complicating the plumbing. Stainless steel housings
offer side-entry feed and concentrate ports, end caps of the same material as well as wider
chemical resistance. Side entry housings make sepralator changeouts and inter-housing
plumbing connections much easier and less costly.

Present Sepralator Limits

To help focus the preceding information some nominal sepralator limits are offered, related to
areas of application. Nominal must be emphasized because every application will have its own
economic limits, and sepralator life depends directly on the operating environment. For this
paper a high probability of achieving a one year useful life was chosen.

The first point to emphasize is that spirals are capable of operating on particulate-laden streams
of higher concentrations than is generally believed. This was true even before the Full-Fit, then
the tubular-spiral sepralator evolved to address this perceived area of weakness.

The capacity of sepralators to handle high particulate, high solids and high viscosity solutions
has been widely underrated in the literature. Today sepralators have been successfully
operated on slurries of 40% discrete particles in the 0.1 - 20 micron range, and on a
concentrated waste solution containing 40,000 and 30,000 mg/L of oil/grease and metal fines,
respectively. Unclarified, first-press apple juice has been concentrated several-fold with mesh-
spacer sepralators as well.
High viscosity solutions can now be processed with sepralators as well, enabled by novel
channel spacer and improved load-bearing components. Concentration of animal gelatin to 12%
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(80 - 100 cps @ 38 C) is a long proven application, and sepralators have been used to process
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a long-chain, gel-forming polysaccharide @ 60 C where its viscosity approaches 500
centipoise.

Temperature limits must be considered in context with pressure, since plastic deformation is a
function of both compressive force (hydraulic pressure) and softening from heat. The
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"traditional" materials in a water purification sepralator can withstand 500 psig @ 40 C (34.5 bar
@ 104°F). If the membrane is capable, sepralators can be operated up to the boiling point of
water @ 200 psig (13.8 bar). The present "PA family" membranes can be operated in
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sepralators to 72° @ 250 psig (162°F @ 17.2 bar), or 85 C (185°F) if shorter life can be
tolerated. Many would be surprised to learn that cellulose acetate RO membranes in standard
construction sepralators have shown satisfactory separation at 600 psig (41.4 bar) and
   o
62 C(144°F) for periods of 500 to 1000 hours of continuous operation. In the case of both PA
and CA membranes, significant flux loss will occur at these temperatures

				
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