[0001] Embodiments presented herein relate to a separation module and more
particularly to reverse osmosis, forward osmosis, and physical filtration modules.
Physical filtration can include micro, ultra and nano filtration processes.
[0002] Membrane modules are widely used for separating fluids with dissolved and
suspended organic and inorganic solids. Processes used for this purpose can include
reverse osmosis, forward osmosis, and physical filtration. In reverse osmosis, a feed
solution such as, but not limited to brackish or impure water, sea water, and so forth, is
passed through a semi-permeable membrane at a pressure higher than the osmotic
pressure of the feed water. A permeate, for example, purified water is obtained on the
other side of the semi-permeable membrane.
[0003] In forward osmosis, water from a feed solution such as, but not limited to
brackish or impure water, seawater, and so forth, is drawn through a semi-permeable
membrane due to the osmotic pressure difference between the feed solution and a draw
solution. The draw solution therefore exits the separation module with a reduced
concentration of draw chemical due to the increased percentage of water.
[0004] Lastly, for the physical filtration processes such as micro-, ultra- and nanofiltration,
a feed solution containing suspended solids is introduced to the separation
module at higher pressure than exists in the permeate channels of the module. Water
flows through the pores of the separation membrane and exits the separation module
through a permeate channel.
[0005] In the above processes for reverse osmosis, forward osmosis and physical
filtration, the feed channels are typically defined by geometry of the module and more
typically, b adhesives that are disposed on the edges of the feed spacer materials.
Though channels defined by this method have been shown, a robust implementation has
yet to be achieved. For example, the adhesive adheres to the membrane face which in the
case of RO or FO is typically very thin on the order of lOOnm. When the feed channel is
pressurized above the pressure of the permeate channel, a stress concentration at the
membrane-adhesive joint develops and generally results in a tear in the membrane. The
tear in the membrane then results in decreased purification. In other cases, where the
feed pressures are not high enough to immediately tear the membrane at the stress
concentration, handling, pressure fluctuations or periodic cycling can have similar effect
on the membrane tearing.
[0006] In other cases, end caps, or end potting can be disposed on the ends of the
modules to define the feed solution flow path. Lastly, chemical joining of the layers can
also be used to define the feed solution flow path within the feed channel. However, such
joints may be susceptible to leaks at high pressures of the feed. Further, processes
involved in producing chemical joints may be expensive and time consuming. Variations
in the thickness of each feed channel along the flow direction may also not be possible in
such cases. The chemically joined edges may also cause damage to the layer of
membrane element. Moreover, chemical joints may not provide additional rigidity to the
layer of permeate carrier. In spiral wound modules, it may also be difficult to roll
chemically joined leaves around the core.
[0007] Therefore there is a need for a feed spacer gasket technology that overcomes
these and other shortcomings of the prior art.
BRIEF DESCRIPTION
[0008] A separation module utilizing a feed spacer and a method for forming such a
separation module are provided. A gasket comprising a flexible waterproof material is
disposed on at least part of one or more edges o the feed spacer. A membrane layer is
disposed on a first surface of the feed spacer. A permeate carrier is disposed on a surface
of the membrane element opposite the feed spacer.
[0009] Several embodiments of a separation module are provided. The membrane
module includes at least one layer of a permeate carrier, at least one layer of a membrane
element, and at least one layer of a feed spacer. The membrane module further includes
at least one layer of a feed spacer wherein edges of the feed spacer are at least partly
covered by one or more strips of a waterproof flexible material. A seal is formed
between the membrane element and the feed spacer, wherein the one or more strips of the
waterproof flexible material is compressed against the membrane element to form the
seal. The flexible waterproof material may be compressed against the membrane element
either by winding the membrane stack around a central core, or by compressing the
flexible waterproof material against the membrane element using a suitable frame and
plate assembly.
[0010] A method for fabricating a separation module is provided. The method
includes providing a feed spacer and impregnating at least part of one or more edges of
the feed spacer with a flexible waterproof material. The method further includes
providing a membrane element on one side of the feed spacer, and providing a permeate
carrier on the opposite side of the membrane element. In several embodiments, the
method further includes winding the feed spacer, the membrane element, and the
permeate carrier around a core. The flexible waterproof material compresses against the
membrane element to form a seal in the feed channel.
BRIEF DESCRIPTION OF DRAWINGS
[001 1] FIG. 1 illustrates the sequence of layers of materials, according to an
embodiment;
[0012] FIG. 2 illustrates the waterproof gasket, according to several embodiments;
[0013] FIG. 3 is a cross section view o f a membrane stack of a separation module,
according to one embodiment;
[0014] FIG. 4 is a cross section view o f a membrane stack of a separation module,
according to another embodiment;
[0015] FIG. 5 illustrates a membrane stack of a separation module, according to one
embodiment;
[0016] FIG. 6 illustrates a membrane stack of a separation module, according to
another embodiment;
[0017] FIG. 7 illustrates a membrane stack of a separation module, according to yet
another embodiment; and
[0018] FIG. 8 is a profile plot of gasket thickness against a length of the feed spacer,
according to various embodiments.
DETAILED DESCRIPTION
[00 1 ] Various embodiments presented herein will be described in detail below with
reference to accompanying drawings. It will be apparent, however, that these
embodiments may be practiced without some or all of these specific details. In other
instances, well known process steps or elements have not been described in detail in
order not to unnecessarily obscure the description of the embodiments. The following
example embodiments and their aspects are described and illustrated in conjunction with
apparatuses, methods, and systems which are meant to be illustrative examples, not
limiting in scope.
[0020] Embodiments presented herein describe a feed spacer and a separation module
employing the feed spacer. Depending on the particular embodiment, the separation
module can be used for reverse osmosis, forward osmosis or physical filtration
applications. Exemplary embodiments for the applications will become evident through
the descriptions provided with the accompanying figures.
[0021] FIG. 1 illustrates an example sequence of materials in a typical spiral wound
separation module 100 applicable for reverse osmosis, forward osmosis and physical
filtration, according to various embodiments. The separation module includes one or
more layers of membrane element 102 disposed between one or more layers of feed
spacer 104 and one or more layers of permeate carrier 106. The layers of the membrane
elements 102, the feed spacer 104, and the permeate carrier 106 are wound around a
central core 108. The central core 108 may include separate channels for the feed
solution, the permeate and the retentate. The sequence of layers may be repeated any
number of times depending on the desired geometry of the separation module.
[0022] The basic function of the spiral wound separation module 100 for reverse
osmosis, forward osmosis, and physical filtration is described in the following
paragraphs.
[0023] Reverse Osmosis
[0024] The feed solution may be pumped through the feed spacer 104 at high
pressure, usually 2-17 bar (30-250 PSI) for brackish water, and 40-70 bar (600-1000
PSI) for seawater. Due to the pressure of the feed solution, the feed solution flowing
through the feed spacer 104 is forced into the membrane element 102. The permeate, for
example, purified water, may pass through the membrane element 102 and collect in the
permeate carrier 106. The permeate carrier 106 carries the permeate to a permeate
discharge port. The retentate, for example, brine, does not pass through the membrane
element 102, and remains in the feed spacer 104. The feed spacer 104 carries the
retentate to a retentate discharge port.
[0025] Physical Filtration
[0026] The feed solution may be pumped through the feed spacer 104 at high
pressure. Due to the pressure of the feed solution, the feed solution flowing through the
feed spacer 104 is forced into the membrane element 102. The filtrate may pass through
the membrane element 102 and collect in the permeate carrier 106. The permeate carrier
106 carries the filtrate to a filtrate discharge port. The impure feed solution does not pass
through the membrane element 102, and remains in the feed spacer 104. The feed spacer
104 carries the impure feed solution to a impure feed discharge port.
[0027] Forward Osmosis
[0028] The feed solution may be pumped through the feed spacer 104, and a suitable
draw solution may be pumped through the permeate carrier 106. Due to the osmotic
pressure gradient across the membrane element 102, a net flow of permeate from the feed
solution in the feed spacer 104 to the draw solution in the permeate carrier 106 occurs.
The permeate may pass through the membrane element 102 and collect in the permeate
carrier 106. The permeate carrier 106 carries the permeate to a permeate discharge port.
The permeate may then optionally be subject to a second separation process such as
reverse osmosis, or draw solute separation techniques. The retentate does not pass
through the membrane element 102, and remains in the feed spacer 104. The feed spacer
104 carries the retentate to a retentate discharge port.
{0029] FIG. 2 illustrates flexible waterproof gaskets impregnated on a feed spacer in
the separation module, according to several embodiments. Membrane stack 200 may be
employed in several different separation module configurations for the reverse osmosis,
forward osmosis and physical filtration processes. The membrane stack 200 includes one
or more layers of membrane element 202 disposed between one or more layers of feed
spacer 204 and one or more layers of permeate carrier 206. A flexible waterproof gasket
208 is disposed on the lateral edges of the feed spacer that are perpendicular to the axis of
a cylindrical separation module. The flexible waterproof gasket 208 may preferably be
disposed on the feed spacer 204 prior to assembly of the membrane stack 200. The layers
of the membrane elements 202, the feed spacer 204, and the permeate carrier 206 are
wound around a central core 210. Due to winding of the membrane elements 202, the
feed spacer 204, and the permeate carrier 206 around the central core 210, a seal is
formed between the membrane elements 202 and the flexible waterproof gasket 208 due
to compression. The seal thus defines a feed solution channel between the membrane
elements 202 adjacent to the feed spacer 204.
[0030] FIG. 3 illustrates a cross section view 300 of an exemplary membrane stack,
according to one embodiment. The membrane stack includes a feed spacer 302. The
feed spacer 302 includes an open mesh structure 304. The lateral edges of the open mesh
structure 304 may be covered, at least partly, with one or more flexible waterproof
gaskets 306. The flexible waterproof gasket may be made ofa rubbery material having a
glass transition temperature below typical operating temperatures (5-6 degree Centigrade)
of the separation module. The flexible waterproof gasket may be made of materials such
as including thermoplastics and thermosets. Example materials include, without
limitation, hot melt adhesives such as ethylene-vinyl acetate (EVA) copolymers,
ethylene-acrylate copolymers such as ethylene-vinylacetate-maleic anhydride, ethyleneacrylate-
maleic anhydride, terpolymers, ethylene n-butyl acrylate, ethylene-acrylic acid,
and ethylene-ethyl acetate; polyolefins such as low density polyethylene (LDPE), high
density polyethylene (HDPE), polypropylene, polybutene-1, polyamides and polyesters,
polyurethanes such as thermoplastic polyurethanes and reactive urethanes; styrene block
copolymers including styrene-butadiene-styrene, styrene-isoprene-styrene, styreneethylene/
butylene-styrene, styrene-ethylene/propylene block copolymers,
polycaprolactone, polycarbonates, fluoropolymers, silicone rubbers, and thermoplastic
elastomers. In particular, ethylene-vinyl acetate (EVA) may be used to form the flexible
waterproof gaskets 306. In the embodiment illustrated in FIG. 3, the lateral edges of the
open mesh structure 304 may be covered completely with one or more flexible
waterproof gaskets 306.
[0031] The flexible waterproof gasket 306 may be disposed on the open mesh
structure 304 using any suitable technique. n one embodiment, the open mesh structure
304 is impregnated with a hot thermoplastic material, such as EVA. The feed spacer 302
is then stacked with one or more membrane elements 308, and one or more permeate
carriers 310 to form the membrane stack for a separation module. Compression of the
flexible waterproof gasket 306 against the membrane elements 308 effectively forms a
seal for the feed channel. In several embodiments (such as the embodiments described in
conjunction with FIG. 5) for spiral wound and flat module configurations, the pressure
difference between the feed channel and the applied feed solution pressure is small. In
such embodiments the flexible waterproof gaskets 306 easily seal the feed channel.
[0032] FIG. 4 illustrates a cross section view 400 of an exemplary membrane stack,
according to one embodiment. The membrane stack includes a feed spacer 402. The
feed spacer 402 includes an open mesh structure 404. The lateral edges of the open mesh
structure 404 may be covered, at least partly, with one or more flexible waterproof
gaskets 406. Example materials and techniques suitable for forming the flexible
waterproof gasket 406 are described in conjunction with FIG. 3. The membrane stack of
FIG. 4 further includes an adhesive 408 applied between the flexible waterproof gasket
406 and the adjacent membrane elements 410. The adhesive 408 may be applied on the
flexible waterproof gasket 406 and around the outer edges of the flexible waterproof
gasket 406 to improve the sealing.
[0033] In embodiments for spiral wound and flat module configurations, where the
pressure difference between the feed channel and the applied feed solution pressure is
large, the adhesive 408 may further bond the feed spacer 402 to a membrane element
410. Suitable materials for the adhesive 408 form a bond with the flexible waterproof
gasket 406 as well as with the membrane element 410. One example of a suitable
adhesive is a thermosetting urethane.
[0034] Although FIGS. 2, 3, and 4 illustrate flexible waterproof gaskets disposed on
lateral edges of the open mesh structure, in various other embodiments, flexible
waterproof gaskets may also be is disposed on the axial edges of the open mesh structure,
particularly the axial edge distal from the central core. Such embodiments are described
in conjunction with FIGS. 6 and 7.
[0035] FIG. 5 illustrates a membrane stack 500 for use in a separation module,
according to one embodiment. Membrane stack 500 may be suitable for use in a spiral
flow separation module. The membrane stack 500 includes one or more layers of
membrane element 502 disposed between one or more layers of feed spacer 504 and one
or more layers of permeate carrier 506. A flexible waterproof gasket 508 is disposed on
the lateral edges of the feed spacer 504 at the axial ends of a cylindrical separation
module. Membrane stack 500 may be employed in several different separation module
configurations for the reverse osmosis, and physical filtration processes.
[0036] The feed solution may flow spirally inwards from an inlet disposed on a
circumferential edge of the spiral flow separation module, into the central core 510.
Alternatively, the feed solution may flow spirally outwards from the central core 510 into
an outlet disposed on a circumferential edge of the spiral flow separation module. As the
feed solution flows through the feed spacer 504, the membrane element 502 recovers a
permeate. The permeate flows across the membrane element 502 into the permeate
carrier 506. The permeate then flows spirally inwards from a circumferential edge of the
permeate carrier 506, into the central core 510.
[0037] Similar to the flexible waterproof gasket 508 disposed on the feed spacer 504,
the permeate carrier 506 may also include a flexible waterproof gasket 512 disposed
thereon. The flexible waterproof gasket 512 may form a seal, the seal defining a
permeate channel between the membrane elements 502 adjacent to the permeate carrier
506.
[0038] FIG. 6 illustrates a membrane stack 600 for use in a separation module,
according to one embodiment. Membrane stack 600 may be suitable for use in a cross
permeate flow separation module. The membrane stack 600 includes one or more layers
of membrane element 602 disposed between one or more layers of feed spacer 604 and
one or more layers of permeate carrier 606. The feed spacer 604 further includes a
flexible waterproof gasket 608 disposed on the lateral edges of the feed spacer 604 at the
axial ends of a cylindrical separation module. Membrane stack 600 may be employed in
several different separation module configurations for the reverse osmosis, forward
osmosis and physical filtration processes.
[0039] The feed solution may flow spirally inwards from an inlet disposed on a
circumferential edge of the cross permeate flow separation module, into the central core
610. Alternatively, the feed solution may flow spirally outwards from the central core
610 into an outlet disposed on a circumferential edge of the cross permeate flow
separation module. As the feed solution flows through the feed spacer 604, the
membrane elements 602 recover a permeate. The permeate flows across the membrane
element 602 into the permeate carrier 606. The permeate then flows through the
permeate carrier 606 axially out towards the axial ends of the cross permeate flow
separation module. The permeate may flow out axially through one end, or both ends. In
one embodiment, the cross permeate flow separation module may be used for forward
osmosis process. The draw solution flows axially through the permeate carrier 606.
[0040] Flexible waterproof gaskets 612 disposed on the permeate carrier 606 may
form a seal, similar to the seal formed by the flexible waterproof gasket 608. The flexible
waterproof gaskets 612 defining a permeate/draw channel between the membrane
elements 502 adjacent to the permeate carrier 506, and direct the flow of the
permeate/draw solution axially through the cross permeate flow separation module.
[0041] FIG. 6 illustrates a cross permeate flow separation module having a spiral feed
flow, and an axial permeate/draw solution flow. However, it should be appreciated that
flow paths of the permeate/draw solution and the feed solution may be reversed. In other
words, the cross permeate flow separation module may have a spiral permeate/draw
solution flow and an axial feed flow. In such embodiments, the feed spacer 604 may
have disposed thereon a flexible waterproof gasket along the axial edges parallel of the
cross permeate flow separation module, similar to flexible waterproof gasket 612. On the
other hand, the permeate carrier 606 may have disposed thereon a flexible waterproof
gasket along the lateral edges of the cross permeate flow separation module, similar to
flexible waterproof gasket 608.
[0042] FIG. 7 illustrates a membrane stack 700 for use in a separation module,
according to one embodiment. Membrane stack 700 may be employed in several
different separation module configurations for the reverse osmosis, forward osmosis and
physical filtration processes. The membrane stack 700 includes one or more layers of
membrane element 702 disposed between one or more layers of feed spacer 704 and one
or more layers of permeate carrier 706. The feed spacer 704 further includes a flexible
waterproof gasket 708 disposed on the lateral edges and the distal axial edge of the feed
spacer 704. The feed spacer 704 also includes a flexible waterproof gasket 710 disposed
perpendicular to the axis of the cylindrical separation module. The flexible waterproof
gasket 710 may be disposed substantially mid way between the lateral edges of the feed
spacer 704. The flexible waterproof gasket 710 may not extend up to the distal axial
edge of the feed spacer 704. The flexible waterproof gasket 708 and the flexible
waterproof gasket 710 define a U-shaped feed channel for feed solution flow.
[0043] The feed solution may flow into the central core 712 from an inlet at one axial
end of the central core 712. The feed solution flows into the feed spacer 704 and spirally
outwards to the end of the feed spacer 704. The feed solution turns the corner at the
distal end of the flexible waterproof gasket 710 and flows spirally inwards to the central
core 712. The feed solution then drains out of an outlet at the opposite axial end of the
central core 712.
[0044] As the feed solution flows through the feed spacer 704, the membrane
elements 702 recover a permeate. The permeate flows across the membrane element 702
into the permeate carrier 706. The permeate then flows through the permeate carrier 706
axially out towards the axial ends of the cross permeate flow separation module. The
permeate may flow out through one axial end, or both axial ends. In one embodiment,
the separation module may be used for forward osmosis process. The draw solution
flows axially through the permeate carrier 706.
[0045] Flexible waterproof gaskets 712 disposed on the permeate carrier 706 may
form a seal, similar to the seal formed by the flexible waterproof gasket 708. The flexible
waterproof gaskets 712 defining a permeate/draw channel between the membrane
elements 702 adjacent to the permeate carrier 706, and direct the flow of the
permeate/draw solution axially through the separation module.
[0046] Similar to the embodiment illustrated in FIG. 6, the flow paths of the
permeate/draw solution and the feed solution may be reversed. In other words, the
separation module may have a spiral permeate/draw solution flow and an axial feed
solution flow. In such embodiments, the feed spacer 704 may have disposed thereon a
flexible waterproof gasket along the proximal and distal axial edges similar to flexible
waterproof gasket 712. On the other hand, the permeate carrier 606 may have disposed
thereon a flexible waterproof gaskets configuration similar to flexible waterproof gaskets
708 and 710.
[0047] In some embodiments, the flexible waterproof flexible waterproof gaskets
may allow variable height feed channels. Variable height feed channels may facilitate
optimal interaction of the feed water with the semi-permeable membrane, while
minimizing pressure drop through the feed channel.
[0048] FIG. 8 illustrates a profile plot 800 of thickness of the flexible waterproof
gasket against the length of the feed spacer, according to various embodiments. For
spiral wound configurations, the length of the feed spacer is the spiral length measured
from the central core. As would be apparent to one skilled in the art, the direction of the
feed flow would determine the direction of the thickness gradient. Accordingly, the
variation of the feed channel can be tailor for any of the feed flow configuration
embodiments shown in FIGS. 2, 3, 4, 5 and 6.
[0049] Profile 802 is a straight line indicating that the thickness of the flexible
waterproof gasket is constant throughout the length of the feed spacer. Thus the height of
the feed channel remains unchanged as the feed water flows from the inlet to the core.
[0050] Profile 804 is a straight line indicating a linearly increasing thickness of the
flexible waterproof gasket. The thickness is lowest at the end near the retentate outlet
from the module, and highest at the end near the feed solution inlet. In other words, the
height o f the feed channel linearly decreases as the feed water traverses from the feed
solution inlet to the retentate outlet.
[005 1] Profile 806 is a step type profile indicating that the thickness of the flexible
waterproof gasket increases in steps with the length of the feed spacer. n one example
implementation, the profile 806 may provide a feed channel having a different height for
every turn of the membrane stack. For implementations where the feed solution enters
through an axial inlet port, the height of the feed channel for the outermost turn of the
membrane stack would be highest, and the height of the feed channel for the innermost
turn of the membrane stack would be lowest. Whereas, for implementations where the
feed solution enters through central core, the height of the feed channel for the innermost
turn of the membrane stack would be highest, and the height o f the feed channel for the
outermost turn of the membrane stack would be lowest.
[0052] Profile 808 is a curve indicating that the thickness of the flexible waterproof
gasket increases non-linearly and gradually with the length of the feed spacer. In one
example implementation, the profile 808 may become substantially flat after a predefined
length of the feed spacer.
[0053] The thickness profile of the flexible waterproof gasket may be determined
using factors such as, but not limited to, the decrease in feed volume due to purification
of the feed water as it flows through the feed channel. Such a decrease in feed volume
reduces the feed solution velocity in a fixed height feed channel. Thus, the thickness
profile may be selected based on the required velocity gradient from the feed solution
inlet to the retentate discharge port, without changing the operating parameters of the
pump used to pressurize the feed water. Maintaining the feed solution velocities may
also decrease concentration polarization and maintain mass transport across the
membrane, thus improving efficiency of the spiral feed flow RO element.
[0054] The foregoing description includes various embodiments of the separation
module in a spiral wound configuration. However, the teachings of these embodiments
may equally be applied to flat-type separation modules. In particular, the embodiments
described in conjunction with FIG. 6 may readily be practiced in a separation module in
the flat-type configuration. Flat-type separation modules include a membrane stack
similar to that described in conjunction with FIG. 1. However, the membrane stack is
laid flat on a frame or plate assembly, rather than being wound around a central core.
Various arrangements of plates and frames may be used to compress the flexible
waterproof gaskets against the membrane elements to effectively seal the feed channels.
Further, as described in conjunction with FIG. 8, the flexible waterproof gaskets may
have a thickness varying along the longitudinal length of the feed spacer. Flat-type
configurations of separation modules typically include feed inlet ports and retentate
discharge ports connected to the feed carriers, and permeate discharge ports connected to
the permeate carriers.
[0055] Although specific implementations and application areas are described in
conjunction with the embodiments presented herein, such description is solely for the
purpose of illustration. Persons skilled in the art will recognize from this description that
such embodiments may be practiced with modifications and alterations limited only by
the spirit and scope of the appended claims.
CLAIMS:
. A separation module comprising:
a feed spacer;
a gasket comprising a flexible waterproof material disposed on at least
part of one or more edges of the feed spacer;
a membrane layer disposed on a first surface of the feed spacer; and
a permeate carrier disposed on a surface o f the membrane element
opposite the feed spacer.
2. The separation module of claim 1, wherein the gasket comprises a
thermoplastic polymer.
3. A separation module according to claim 1, additionally comprising a core
element, and wherein the feed spacer, the membrane element, and the permeate carrier
are radially disposed around the core element.
4. A separation module according to claim 1, wherein a seal is formed by
compression of the gasket against the membrane element.
5. A separation module according to claim 1, wherein the waterproof flexible
material is at least partly disposed on axial edges o f the feed spacer.
6. A separation module according to claim 1, wherein thickness of the
flexible waterproof material varies along a length of the feed spacer.
7. A separation module according to claim 1 further comprising an adhesive
material between the gasket and the membrane element.
8. A separation module according to claim 1, wherein the feed spacer
comprises an open mesh structure.
9. A reverse osmosis system comprising one or more separation modules
according to claim 3.
10. A forward osmosis system comprising one or more separation modules
according to claim 3.
1. A physical filtration system comprising one or more separation modules
according to claim 3.
12. A method for fabricating a separation module, the method comprising:
providing a feed spacer;
impregnating at least part of one or more edges of the feed spacer with a
flexible waterproof material;
disposing a membrane element on the feed spacer; and
disposing a permeate carrier on a surface of the membrane element
opposite the feed spacer.
13. The method of claim 12, additionally comprising winding the feed spacer,
the membrane element, and the permeate carrier radially around a core.
14. The method of claim 2, additionally comprising compressing the flexible
waterproof material to form a seal.
15. The method of claim 12, additionally comprising disposing a second
membrane element on a surface of the permeate carrier opposite the first membrane
element.
16. The method of claim 12, additionally comprising applying an adhesive
between the flexible waterproof material and the membrane element.
17. The method of claim 12, additionally comprising disposing a
thermosetting polymer between the flexible waterproof material and the membrane
element.