Abstract:
A spiral wound membrane element and a method for cleaning a spiral wound membrane sheet are provided. The spiral wound membrane element comprising a perforated central tube; one or more membrane sleeves, each sleeve comprising two membrane sheets enclosed at two edges around a feed spacer sheet and wrapped around the central tube; a permeate carrier sheet attached to the central tube at a first edge opposite a second edge; and a seal on third and fourth edges of the permeate carrier sheet. The method comprises introducing two solution with different solute concentration to two sides of the spiral wound membrane sheet; establishing a concentration gradient between the first and second sides; communicating a first solvent from the first side to the second side to remove a foulant located on the second side of the membrane sheet and collecting a waste stream that contains the foulant.

Description:
FIELD 
       [0001]    The present disclosure relates generally to spiral wound membrane elements and modules. 
       BACKGROUND 
       [0002]    The following discussion is not an admission that anything discussed below is citable as prior art or common general knowledge. 
         [0003]    A spiral wound membrane element is typically made by wrapping one or more membrane leaves and feed spacer sheets around a perforated central tube. The membrane leaves each have a permeate carrier sheet placed between two generally rectangular membrane sheets. The membrane sheets are typically sealed together along three edges. The fourth edge of the membrane leaves is adjacent to the central tube and open to the perforations. One or more layers of permeate carrier sheet can also be wrapped around the central tube to support the membrane leaf over the perforations in the central tube and to provide a flow path between the edge of the leaf and the central tube. An anti-telescoping device (ATD) may be attached at the ends of the membrane element to prevent telescopic unraveling of the membrane element. 
         [0004]    Feedstock, also referred to as feed water, is introduced at one end of the membrane element and flows through the feed spacer sheets and along a feed-side of the membrane sheets. Some of the feedstock passes through the membrane sheets to form a permeate stream on a permeate-side of the membrane sheets. The remainder of the feedstock, referred to as the reject, retentate or brine stream, flows through the feed spacer sheets and out of an outlet end of the membrane element. The permeate stream flows along the permeate carrier in an inwardly spiraling flow. The permeate stream follows the permeate carrier until reaching and exiting the fourth edge of the membrane leaves and entering the central tube by the perforations. Within the central tube, the permeate stream is collected and transported towards an outlet end of the central tube. 
         [0005]    The throughput or collection rate of permeate in a spiral wound membrane is related to the pressure applied across the membrane. However, the pressure required to drive the permeate flow through the permeate carrier, including from the edges of the permeate carrier, towards the central tube reduces the net driving pressure for permeate flow through the membrane. 
         [0006]    Additionally, the feed-side surface of the membrane sheets may accumulate particles, also referred to as foulants, which can further decrease the net driving pressure for permeate flow through the membrane. 
       SUMMARY 
       [0007]    A spiral wound membrane element, to be described in further detail below, comprises fluid communication between a perforated central tube and a peripheral region of the spiral wound membrane element. 
         [0008]    The spiral wound membrane element includes a wrapping of one or more membrane sleeves and one or more permeate carrier sheets around the perforated central tube. For the purposes of this disclosure, the term “membrane sleeve” shall refer to a sleeve of one or more membrane sheets that surround a feed spacer, optionally with the membrane sleeve being sealed at two edges. A feedstock may flow through the unsealed or open edges of the feed spacer and along the feed-side surface of the membrane sheets. The feed-side is also referred to as the inner surface of the membrane sheets. A permeate carrier sheet is positioned between two membrane sheets. The permeate carrier sheet has two open edges and two closed edges. One open edge is open to the central tube and the second open edge is open to the cross-sectional periphery of the spiral wound membrane element. 
         [0009]    The cross-section of the spiral wound membrane element of the present invention may have an interior region, an intermediate region and a peripheral region. The interior region may comprise the central tube, optionally a layer of a base wrap, and the portion of the membrane sleeves and permeate carrier sheets that are proximal to the central tube. The intermediate region may include membrane sleeves and the permeate carrier sheets. The peripheral region may comprise a peripheral portion of the permeate carrier sheets. 
         [0010]    Optionally, the spiral wound membrane element also includes at least one anti-telescoping device (ATD) that is positioned at one end, or both ends, of the membrane element to prevent the telescopic unraveling of the membrane element. The ATD may provide fluid communication between the central tube and the peripheral region. 
         [0011]    During use, the spiral wound membrane element may accumulate particles from the feedstock on the inner surface of the membrane sheets. This accumulation is also referred to as fouling. Fouling of the membrane sheets may decrease permeate production of the spiral wound membrane element. The spiral wound membrane element may be cleaned, also referred to as de-fouled, by direct osmosis. 
         [0012]    In a typical direct osmosis processes a low-solute solution exits the perforated central tube and enters the open edge of the membrane leaves to access the permeate carrier sheets. A higher-solute solution is introduced into the feed spacers. The discrepancy in solute concentrations on the two sides of the membrane sheet creates an osmotic gradient, also referred to as a concentration gradient, between the permeate carrier sheets and the feed spacer. The concentration gradient causes solvent from the low-solute solution to flow through the membrane sheet and into the higher-solute solution. The flow of solvent through the membrane is also referred to as solvent flux. The solvent flux may dislodge, remove, or clean some, or a significant portion, of the foulants adhered on the inner surface of the membrane sheet. Direct osmosis cleaning may continue until the solute concentrations between the permeate side and the feed side of the membrane sheet equilibrate. 
         [0013]    The flow rate of the low-solute solution through the permeate carrier is typically slower than the flow rate of the higher-solute solution through the feed spacers. Due to this flow rate disparity, the osmotic gradient tends to centrally form in the interior region and portions of the intermediate region that are closest to the interior region. This centralization likely occurs because the low-solute solution has not had enough time to flow through the permeate carrier to reach further away from the central tube. The centralization of the osmotic gradient typically results in a centralized cleaning of foulants. The portions of the membrane sheet that were not exposed to the osmotic gradient often remain fouled. 
         [0014]    Introducing the low-solute solution to both the interior region and the peripheral region of the spiral wound membrane element may increase the surface area of the membrane sheets that are exposed to the osmotic gradient, which may improve the efficiency of the direct-osmosis cleaning of the spiral wound elements. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a partial cut-away, perspective view of a spiral wound membrane element. 
           [0016]      FIG. 2  is a partial cut-away, perspective view of a spiral wound membrane element. 
           [0017]      FIG. 3  is a partial cut-away, top plan view of a membrane sleeve and permeate carrier for use in the membrane element of  FIG. 1 . 
           [0018]      FIG. 4  is a cross-sectional view of a membrane sleeve taken through line  4 - 4 ′ of  FIG. 3 . 
           [0019]      FIG. 5A  is a cross-sectional, schematic of the spiral wound membrane element of  FIG. 2  taken through line  5 - 5 ′. 
           [0020]      FIG. 5B  is a close view of Box A, of  FIG. 5A . 
           [0021]      FIG. 6  is a cut-away, exploded, perspective view of an anti-telescoping device for use with the spiral wound membrane element of  FIG. 1 . 
           [0022]      FIG. 7  is a cross-sectional view of the anti-telescopic device taken through line  7 - 7 ′ of  FIG. 1 . 
           [0023]      FIG. 8  is a cross-sectional view of two spiral wound membrane elements connected in series. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    The present disclosure describes a spiral wound membrane element that provides two permeate streams. The first permeate stream flows spirally inward to be collected within a central tube of the membrane element. The second permeate stream flows in the opposite direction, spirally outward to be collected from a peripheral region of the membrane element. 
         [0025]    The present disclosure also describes a method of cleaning the spiral wound membrane element with two solutions of different concentrations for direct-osmosis cleaning of a spiral wound membrane sheet. The first solution may be introduced to both a central region and the peripheral region of a first side of the spiral wound membrane sheet. The second solution is introduced to the second side of the membrane sheet. An osmotic gradient is established between the two sides of the membrane sheet and solvent from the lower concentration solution moves across the membrane sheet to remove particles that are lodged on the side of the membrane sheet that contains the more highly concentrated solution. 
         [0026]    Referring to  FIGS. 1 and 2 , a spiral wound membrane element  10  has an input end  11  and an output end  13 . The spiral wound membrane element  10  may be formed by wrapping one or more membrane sleeves  12  and permeate carrier sheets  20  around a perforated central tube  16 . The central tube  16  may also be called a core, a permeate tube or a produced water collection tube. The central tube  16  has a feedstock end  24  and a concentrate end  26 . The perforations of the central tube  16  may comprise a plurality of holes  22 . The holes  22  may have a diameter of approximately 0.125 inches (about 3.2 mm) and provide fluid communication between the permeate carrier sheets  20  and the inside of the central tube  16 . 
         [0027]    As shown in  FIGS. 2 ,  3  and  4 , each membrane sleeve  12  includes two membrane sheets  18  with a feed spacer  14  between them. To form the membrane sleeve  12 , the membrane sheets  18  have two sealed edges  18   a  and  18   b  that may be substantially parallel to each other. Edge  18   a  may be substantially parallel to and positioned adjacent to the central tube  16 , as shown in  FIG. 3 . Two membrane sheets  18  may be sealed together at edges  18   a  and  18   b  by a seal  19 , to form closed edges  12   a  and  12   b  of the membrane sleeve  12 , as shown in  FIG. 3  and  FIG. 4 . A variety of materials known in the art are suitable to be used as the seal  19 , such as a glue line, provided seal  19  does not permit fluid communication across edges  12   a  and  12   b.  Each membrane sleeve  12  also includes two open edges  12   c  and  12   d  that are substantially perpendicular to edges  12   a  and  12   b.  The feed spacer  14  is positioned within the membrane sleeve  12  and between the two membrane sheets  18  so that an inner surface of each membrane sheet  18  is adjacent the feed spacer  14 . The feed spacer  14  is in fluid communication with the input end  11  and the output end  13  of the spiral wound membrane element  10 . 
         [0028]    The sealed edges  18   a  and  18   b  of the membrane sheets  18 , with the feed spacer  14  therebetween, form the membrane sleeve  12  with closed edges  12   a  and  12   b  and open edges  12   c  and  12   d.    
         [0029]    The feed spacer  14  acts as a conduit for a feedstock solution to flow through the membrane sleeve  12  and across the inner surface of the membrane sheets  18 . The feedstock can generally flow from input end  11  to output end  13  of element  10 , flowing between the open edges of the membrane sleeve  12 . 
         [0030]    The membrane sheets  18  have a separation layer cast onto a supporting or backing layer. The separation layer may be, for example, cellulose acetate, a polyamide, a thin film composite or other materials that may be formed into a separation membrane. The separation layer may have pores, for example, in the reverse osmosis, nanofiltration or ultrafiltration range so that the desired molecules from the feedstock may pass through the membrane sheet  18  and enter into a permeate stream. The separation layer forms the inner surface and faces the feedstock within the feed spacers  14 . Opposite to the separation layer is the backing layer, which is adjacent the permeate carrier sheets  20 . The backing layer may also be referred to as the permeate surface of the membrane sheets  18 . 
         [0031]    The permeate carrier sheet  20  is generally rectangular with open edges  20   a  and  20   b  that are substantially parallel to closed edges  12   a  and  12   b  of the membrane sleeve  12 . The permeate carrier sheet  20  also has two closed edges  20   c  and  20   d  that are substantially parallel to the open edges  12   c  and  12   d  of the membrane sleeve  12 . Closed edges  20   c  and  20   d  are sealed by a seal  23 . A variety of materials known in the art are suitable to be used as the seal  23 , such as a glue line, provided seal  23  does not permit fluid communication across the permeate carrier sheet  20  at the edges  20   c  and  20   d.  The glue may saturate between adjacent membrane sleeves  12  and seal  23  may generally extend about 1 to 5 cm from edges  20   c  and  20   d  of the permeate carrier sheet. 
         [0032]      FIG. 4  shows the permeate carrier sheet  20  positioned between two membrane sleeves  12  and a second permeate carrier  20  below. At edge  20   a,  the permeate carrier sheet  20  is in fluid communication with small holes  22  in the central tube  16 . Optionally, an additional permeate carrier sheet, which might or might not be the same material as the permeate carrier sheet  20 , or an extension of the permeate carrier sheet  20 , can be wrapped around the central tube  16  in one or more layers before the first membrane sleeve  12  is attached to the central tube  16 . This initial wrap of permeate carrier sheet  20  supports the membrane sleeve  12  over the holes  22  and provides a path to conduct permeate from the permeate carrier sheet  20  to the holes  22  in the central tube  16 . 
         [0033]    The permeate carrier sheet  20  also includes a permeate carrier sheet extension  21  that extends away from edge  20   a  such that edge  20   b  does not coincide with the closed edge  12   b  of the membrane sleeve  12  (see  FIGS. 3 and 4 ). The permeate carrier sheet extension  21  may extend distally from edge  20   a,  past edge  18   b  of membrane sheet  18 . The permeate carrier sheet extension  21  may extend sufficiently past the closed edge  12   b  of the membrane sleeve  12  to prevent two adjacent membrane sleeves  12  from coming in contact with each other. For example, the permeate carrier sheet extension  21  may extend approximately 1 cm to 10 cm, or 2 cm to 4 cm, past the edge  12   b  of an adjacent membrane sleeve  12 , as will be described further below. 
         [0034]    When forming the membrane element  10 , one or more membrane sleeves  12 , for example 1 to 40, and one or more of permeate carrier sheets  20 , are wrapped around the central tube  16 . Each membrane sleeve  12  has an associated permeate carrier sheet  20 , for example, the one or more membrane sleeves  12  may be in a one to one ratio with the associated permeate carrier sheets  20 . When the membrane element  10  is wrapped and viewed in cross-section, as in  FIGS. 5A and 5B , the area proximal to the central tube  16  generally defines an interior region  500  of the membrane element  10 . The interior region  500  includes the bore of the central tube  16 , the wall of central tube  16  and the one or more layers of permeate carrier that may form an initial wrap around the central tube  16  (as shown in  FIG. 4 ). An intermediate region  502  is formed adjacent the interior region  500  and it comprises one or more wound layers of the plurality of membrane sleeves  12  and the permeate carrier sheet  20 . Although not shown in  FIGS. 5A  or  5 B, the intermediate region typically comprises at least four membrane sleeves  12  and at least four permeate carrier sheets  20  each extending from the interior region  500  and wrapping around the interior region  500 . Adjacent the intermediate region  502  is a peripheral region  504  of the membrane element  10 . The peripheral region  504  comprises the termination point of the edges  12   b  of the membrane sleeves  12 , the termination point of the edges  20   b  of the permeate carrier sheets  20 , the permeate carrier sheet extensions  21  and an outer wrap  42 . The peripheral region  504  is generally denoted as the region between the dotted line in  FIGS. 5A and 5B  and the intermediate region  502 . 
         [0035]    After winding the membrane sleeves  12  and the permeate carrier sheets  20  around the central tube  16 , the edge  12   b  of one membrane sleeve  12  does not line up with the edge  12   b  of the adjacent membrane sleeve. As depicted in  FIG. 5B , lines A, B, C and D each approximately indicate the termination point of edge  12   b  of each of the four membrane sleeves  512 A,  512 B,  512 C and  512 D, respectively. For example, the distance between lines A and B and reflects the approximate distance between edge  12   b  of membrane sleeve  512 A and membrane sleeve  512 B along the curved surface of the spiral wound membrane element  10 . This space is caused by the membrane sleeves  12  being generally of the same dimensions, but each of the membrane sleeves  12  is attached at edge  12   a  at a different point within the interior region  500 , either connected with the initial wrap of permeate carrier sheet  20  or directly connected to the central tube  16  (not shown). 
         [0036]    When the spiral wound membrane element  10  is wrapped around the central tube  16 , each individual permeate carrier sheet  20  may be adjacent a lower membrane sleeve  12  and an upper membrane sleeve  12 . The individual permeate carrier sheet  20  is positioned on top of the lower membrane sleeve  12  and below the upper membrane sleeve  12  so that the upper membrane sleeve  12  and the lower membrane sleeve  12  do not come in contact. For example, as shown in  FIG. 5B , the permeate carrier  520  is positioned upon lower membrane sleeve  512 B and beneath upper membrane sleeve  512 A. The permeate carrier sheet extension  521  is shown as extending beyond the edge  12 B of the upper membrane sleeve  512 A (shown as line A in  FIG. 5B ) but not extending to meet the edge  12 B of the lower membrane sleeve  512 B (shown as line B in  FIG. 5B ). The permeate carrier sheet extensions  21  form an outer permeate carrier layer or series of stripes that at least partially overlie the membrane sleeves  12 . 
         [0037]    When the spiral wound membrane element  10  is wrapped around the central tube  16 , the permeate carrier sheet  20  provides a first flow path for the permeate stream that proceeds in an inward spiral fashion, around and towards the central tube  16 . Referring back to  FIG. 5B , the permeate that crosses through the membrane sheets  18  of membrane sleeves  512 A and  512 B will collect and travel within the permeate carrier sheet  520  on the first flow path towards the interior region  500 . 
         [0038]    The permeate carrier sheet  20  also provides a second flow path for the permeate stream that flows in an outward spiral fashion around, but away from, the central tube  16 . Permeate that follows the second flow path travels through the permeate carrier sheet  20  and the permeate carrier sheet extension  21  towards open edge  20   b  and generally towards the peripheral region  504  of the spiral wound membrane element  10 . In reference to  FIG. 5B , some of the permeate that crosses through the membrane sheets  18  of membrane sleeves  512 A and  512 B will collect and travel within the permeate carrier sheet  520  on the second flow path towards the peripheral region  504 . As will be discussed further below, permeate that follows this second permeate flow path is collected from the peripheral region  504  by the anti-telescoping device  40 . The glue line  23  ensures that permeate can only exit the permeate carrier sheet  20  by the open edge  20   a  to follow the first permeate stream to the central tube  16  or by the open edge  20   b  or the permeate carrier sheet extension  21  to the peripheral region  504  and ultimately to an anti-telescoping device  40 . 
         [0039]    Further, when the spiral wound membrane element  10  is wrapped around the central tube  16 , the glue line  23  ensures that there is no fluid communication between the outside of edges  11  and  13  of the spiral wound membrane element  10  and the permeate carrier sheet  20 . 
         [0040]    As shown in  FIGS. 1 and 3 , an anti-telescoping device (ATD)  40  is positioned at ends  11  and  13  of the spiral wound membrane element  10 . The ATD  40  prevents the membrane sleeve from being pushed along the length of the center tube  18  by pressure gradients within the spiral wound membrane element  10 . The ATD  40  is secured to the central tube  16  by glue, tape, the outer wrap  42  or other suitable known methods. 
         [0041]    An outer wrap  42  is secured about the spiral wound membrane element  10  to assist the ATD  40  in the prevention of unwinding during use. The outer wrap  42  is made of materials impermeable to permeate flow, for example a plastic sheet or fiber-reinforced plastics such as fiberglass embedded in epoxy. The outer wrap  42  is in contact with the exterior or outer surface of the permeate carrier sheet extensions  21  and the wrap  42  is sealed at each end to the outside of the ATD  40 . 
         [0042]    The ATD  40  includes an outer annular body  48  that is positioned proximate to the outer most layer of the permeate carrier  20 . The outer annular body  48  includes a recess  72  along its circumference that faces the peripheral region  504 , including the outer surface of the permeate carrier sheet extensions  21 . The outer annular body  48  includes a permeate receiver  58  that extends from the outer annular body  48  between the outer wrap  42  and the outer surface of the permeate carrier sheet extensions  21 , as shown in  FIG. 7 . The permeate receiver  58  includes an inner flange  60  and an outer flange  62 , defining a recess  72  therebetween. Each flange, and therefore the recess  72 , extends around the circumference of the outer annular body  48 . The inner flange  60  has an inner surface  66  and an outer surface  64 . Similarly, the outer flange has an inner surface  68  and an outer surface  70 . The outer surface  64  of the inner flange  60  is adjacent the outer surface of the permeate carrier sheet extensions  21 . The outer surface  70  of the outer flange  62  is adjacent the outer wrap  42 . 
         [0043]    The outer flange  62  of the permeate receiver  58  extends beyond the inner flange  60 , as in  FIG. 7 . For example, the outer flange  62  includes a leading, beveled edge  74  that extends beyond the inner flange  60  to make contact with the outer surface of the permeate carrier sheet extension  21  to direct the second permeate flow from the peripheral region  504  into the recess  72  of ATD  40 . The leading edge  74  also provides a greater surface area for securing ATD  40  to the spiral wound membrane element  10  with the outer wrap  42 . 
         [0044]    As shown in  FIGS. 6 and 7 , the ATD  40  also includes an inner annular body  46  with at least one elongate hollow member  51  connecting the two annular bodies  46 ,  48  of ATD  40 . For example, the ATD  40  may resemble a wagon wheel structure, as depicted in  FIG. 6 , and includes fluid flow channels through the hollow members  51  that allow fluidic communication between the outer annular body  48  and the inner annular body  46 . 
         [0045]    The inner surface  53  of the inner annular body  46  may be proximal to, connected to or affixed to the outer surface  54  of the central tube  16 . The inner annular body may include one or more ports  57  that provide fluid communication across the inner annular body  46  to the central tube  16 . The ports  57  provide fluid communication with at least one of the small holes  22  of the central tube  16 . Each port  57  is in fluid communication with an elongate hollow member  51  that in turn is in fluid communication with the recess  72  of the outer annular body  48  of ATD  40 . Thereby a fluid passage from the peripheral region  504  to the interior region  500  is formed. For example, the flow passage may include the recess  72 , elongate hollow member  51 , and ports  57  of ATD  40 . 
         [0046]    Permeate that follows the second permeate flow stream travels spirally towards edge  20   b  of the permeate carrier sheet extension  21 , generally towards the peripheral region  504  and is collected in the outer annular body  48  of the ATD  40  and conducted through the elongate hollow member  51 , through the inner annular body  46  and via ports  57 , into the central tube  16 . 
         [0047]    In reference to  FIG. 1 , a spiral wound membrane module  30  has a spiral wound membrane element  10  located inside of a pressure vessel  32 . The pressure vessel  32  can be a pressure vessel as typically used with spiral wound membrane elements. The pressure vessel  32  has a generally tubular body  34  with an inlet end  36  that is adjacent the input end  11  of the spiral wound membrane element  10 . The pressure vessel  32  also has an outlet end  38 , adjacent outlet end  13  of the spiral wound membrane element  10  and the concentrate end  26  of the central tube  16 . 
         [0048]    The inlet end  11  and outlet end  13  of the spiral wound membrane element  10  are sealed and provide fluid communication with the interior of the pressure vessel  32 . Peripheral seals may be provided between an outer wrap  42  of the element  10  and the inside of a pressure vessel  32  to prevent fluid communication past a spiral wound membrane element  10  without passing through its feed spacers  14 . Further, the glue line  23  prevents direct fluid communication from the inlet end  36  to the permeate carrier sheet  20 . 
         [0049]    In an additional optional feature, the outer annular body  48  may also include a gland  76  and seal  78 , for example an o-ring seal, that may be located opposite to the permeate receiver  58 . The seal  78  forms a seal against the inner surface of the pressure vessel  32 . For example, gland  76  and seal  78  may seal against leakage of any permeate and leakage of feedstock from passing around the spiral wound membrane element  10 . 
         [0050]    In an additional optional feature, the permeate carrier sheet extension  21  may extend to the edge  12   b  of the lower membrane sleeve  12  that is adjacent the permeate carrier sheet  20 . In reference to  FIG. 5B , the permeate carrier sheet  521 ′ includes the permeate carrier sheet extension  521 ′ that extends the distance between lines B and C to meet the outermost edge  512   b  of membrane sleeve  512 C. With this additional optional feature, the outer most layer of permeate carrier  21 , as shown in  FIG. 7 , is not a contiguous layer of permeate carrier sheet  20  in contact with the outer wrap  42 . Rather, the permeate carrier sheet extensions  21  form a set of discontinuous bands or stripes of permeate carrier sheet extensions  21  that extend from input end  11  to output end  13  of the spiral wound membrane element  10  sealed by glue line  23  at each end. 
         [0051]    In an additional optional feature, the permeate carrier sheet extension  21  may extend beyond the edge  12   b  of the lower membrane sleeve  12 . In reference to  FIG. 5B , the permeate carrier sheet  521 ″ includes the permeate carrier sheet extension  521 ″ that extends beyond the distance between lines C and D and therefore beyond the outermost edge  512   b  of the membrane sleeve  512 D. In this optional feature, the permeate carrier sheet extension  521 ″ extends to meet line E which is beyond line D. With this additional optional feature, the individual permeate carrier sheets may extend past the edge  20   b  of the lower membrane sleeve to come in direct contact with the next permeate carrier sheet. For example, in reference to  FIG. 5B , the permeate carrier sheet extension  521 ′ may extend past line C towards line D (not shown) and contact permeate carrier sheet extension  521 ″. Thereby a contiguous outer layer of permeate carrier sheet extensions  21  that have an outer surface in direct contact with the outer wrap  542  is formed, as shown in  FIG. 7 . 
         [0052]    In an additional optional feature, the permeate carrier sheet extension  21  may extend distally from edge  20   a  to approximately 1 to 5 cm past edge  12   b  of the adjacent, lower membrane sleeve  12 . 
         [0053]    In an additional optional feature, more than one spiral wound membrane element  10  may be located within a given pressure vessel  32 . Such multiple spiral wound membrane elements  10  can be connected in series. The first end  11  of the first spiral wound membrane element  10  is either sealed, directly exits the pressure vessel  32  or is connected to a fitting that exits the pressure vessel  32  to receive feedstock. If there are multiple elements  10  in a pressure vessel  32 , the second end  13  of an upstream element  10  is typically connected to the first end  11  of a downstream element. The second end  13  of the last spiral wound membrane element  10  in a pressure vessel  32  is either sealed, directly exits the pressure vessel  32  or the end  13  is connected to a fitting that exits the pressure vessel  32 . Peripheral seals may be provided between the outer wrap (not shown) of the element  10  and the inside of a pressure vessel  32  to prevent feedstock from flowing past an element  10  without passing through the feed spacers  14  of the membrane sleeve  12 . 
         [0054]    In an additional optional feature, the membrane sleeve  12  is formed by one single membrane sheet that is folded at edge  18   a  and sealed at edge  18   b,  resulting in a membrane sleeve, with the feed spacer  14  positioned between the folded membrane. The folded edge  18   a  may be reinforced with a tape or film. 
         [0055]    In an additional optional feature, the permeate carrier sheet extension  21  of the permeate carrier sheet  20  that terminates in the peripheral region  504  of the spiral wound membrane element  10  may be made of any other filler material, beside typical permeate carrier material such as netting, that is conducive to the flow of permeate fluids therethrough. With such an optional feature, a space containing the filler material within the peripheral region  504  may be formed between the outer surface of the permeate carrier sheet extension  21  and the inner surface of the outer wrap  42 . Further optionally, a filler material may be wrapped over the outer surface of the permeate carrier sheet extensions  21 . As described above, the permeate receiver of the ATD  40  may extend into this space to direct permeate from the second permeate flow path into the internal flow passage of the ATD  40  and into the central tube  16 . 
         [0056]    During filtration operations, the feedstock solution to be filtered enters through an inlet (not shown) at the inlet end  36  of the pressure vessel  32 . Feedstock meets the edge  11  of the spiral wound membrane element  10 . The feedstock cannot enter the permeate carrier sheet from either of the closed edges  20   c  and  20   d.  The feedstock enters the membrane sleeve  12  through open edge  12   c  and flows through the feed spacer  14  and across the inner surface of each membrane sheet  18 . Once inside the membrane sleeve  12 , the glue line  19  prevents the feedstock from exiting the membrane sleeve  12  at the closed edges  12   a  and  12   b.  The resulting direction of feedstock flow is from the open edge  12   c  to open edge  12   d.    
         [0057]    While feedstock flows through the membrane sleeve  12 , permeate may pass through the inner surface of membrane sheet  18  to the permeate surface of the membrane sheet  18  that is adjacent the permeate carrier sheet  20  while the passage of dissolved salts or suspended solids or other contaminants may be rejected by the membrane sheet  18  depending on its pore size and carried away in a reject stream. The reject stream stays on the same side of the membrane sheets  18  as the feedstock, thereby concentrating the feedstock in rejected solutes so that a concentrated reject stream  114  leaves the pressure vessel  32  through a discharge tube (not shown) at the outlet end  38 . 
         [0058]    During the filtration operations, the inner surface of the membrane sheet  18  may become fouled by particles in the feedstock, also referred to as foulants, which are adhered to, lodged within, or stuck on the inner surface of the membrane sheet  18 . The foulants may plug the pores of the membrane sheets  18  and decrease the flow of permeate through the inner surface of the membrane sheet  18 . The foulants may also provide a substrate for further particles to adhere to the inner surface of the membrane sheet  18 . 
         [0059]    As the feedstock moves along the inner surface of the membrane sheet  18 , permeate passes through the membrane sheet  18  and collects on the opposite side of the membrane sheet  18  from the reject stream. For example, permeate passes through the inner surface of the membrane sheets  18  and exits the membrane sleeve  12  while the reject stream remains within the membrane sleeve  12  until discharged. The permeate collects within spaces within the permeate carrier sheet  20 . The closed edges  20   c  and  20   d  of the permeate carrier sheet  20  prevent permeate from exiting the permeate carrier sheet  20  except through the open edges  20   a,    20   b  or the permeate carrier extensions  21 . As described above, the flow of permeate fluids along the permeate carrier sheet  20  may occur in one or two, or both, directions. 
         [0060]    The first permeate flow path follows the flow path typical for a spiral wound membrane element. In following the first permeate flow path, the permeate fluid may flow in a radial path that spirals inwardly towards the central tube  16 . Edge  20   a  provides fluid communication with the holes  22  of central tube  16  so that permeate may collect inside of the central tube  16  and then typically travels in a stream directed from the feedstock end  24  to the concentrate end  26  of the central tube  16 . 
         [0061]    Permeate fluid also follows the second permeate flow path that is generally in the opposite direction to the first permeate flow path. For example, permeate fluid may follow a radial flow path that spirals outwardly, away from the central tube  16  towards the peripheral region  504  of the spiral wound membrane element  10 . Permeate fluid following the second permeate flow path may travel along the permeate sheet  20 , away from edge  20   a,  towards edge  20   b.  The permeate fluid following the second permeate flow path may travel along and lengthwise, through one or more of the permeate carrier sheet extensions  21  to be collected within the peripheral region  504  of the spiral wound membrane element  10 , for example within the ATD  40 . 
         [0062]    During cleaning operations, two solutions may be introduced into the spiral wound membrane element  10 . A first solution may be introduced into the central tube  16 , preferably at the concentrate end  26 , and the first solution may access the open edge  20   a  of the permeate carrier sheets  20  by the holes  22 . Optionally, the feedstock end  24  of the central tube  16  may fluidly communicate with the outside of the pressure vessel  32  to allow introduction of the first solution at either or both ends of the central tube  16 . The first solution may flow from the central tube  16 , along the permeate carrier sheet  20  through the interior region  500 . The first solution may also flow through holes  22  to communicate with the peripheral region  504  by way of the hollow member  51  of the ATD  40 . The first solution may flow from the recess  72  of the ATD  40  and enter the permeate carrier sheet extensions  21  by the open edge  20   b.    
         [0063]    The first solution may flow within the permeate carrier sheets  20  from both the interior region  500  and the peripheral region  504  to enter into the intermediate region  502 . The interior, intermediate and peripheral regions  500 ,  502 ,  504  may contain the first solution and substantially the whole permeate-side surface each membrane sheet  18  may be in contact with the first solution. The term “contact” refers to a solution that is adjacent a surface of the membrane sheet  18  so that under the influence of osmotic gradients or fluid pressures, or both, a solvent of the solution may flow through the membrane sheet  18 . Further, the first solution may flow to the intermediate region  502  from both the interior region  500  and the peripheral region  504 . 
         [0064]    A second solution may be introduced to either the inlet end  36  or the outlet end  38 , or both, of the pressure vessel  32 . Preferably, only one end is used to introduce the second solution to the pressure vessel  32 . Similar to the flow path of the feedstock during filtration operations, the sealed edges  12   a,    12   b  of the membrane sleeve  12 , and any peripheral seals, direct the second solution to flow through the feed spacers  14 . The second solution flows along the inner surface of the membrane sheets  18  throughout the interior, intermediate and peripheral regions  500 ,  502 ,  504  of the spiral wound membrane element  10 . The second solution may be in contact with substantially the whole of the inner surface of each membrane sheet  18 . 
         [0065]    The first and second solutions may be of different solute concentrations. The solute concentration difference between the two solutions may be sufficiently large so as to cause the movement, or flux, of solvent from one solution through the membrane sheet  18  into the other solution without any fluid pressure differential between the two solutions. When the first solution is in contact with the permeate surface of the membrane sheet  18  and the second solution is in contact with the inner surface of the membrane sheet  18  an osmotic gradient, also referred to as a concentration gradient, is created, or generated, between the permeate surface and the inner surface of the membrane sheet  18 . For example, both the first and second solutions may be a saline solution and the concentration of solute in the first solution is lower than the second solution. The difference in concentrations will move solvent, for example water, from the first solution from the permeate carrier sheets  20 , across the membrane sheets  18  into the second solution. The movement of solvent through the membrane sheet due to the osmotic gradient is also referred to as solvent flux. The solvent flux may dislodge, remove or clean foulants from the inner surface of the membrane sheets  18 . The solvent flux may continue until an osmotic equilibrium is established between the first solution and the second solution. 
         [0066]    The first solution may be introduced prior to, after, or at the same time as the introduction of the second solution. Permeate remaining in the membrane element  10  may function as the first solution until it is drawn through the membrane sheets and replaced with new first solution. Preferably, the second solution is not introduced until the first solution has established contact with the permeate carrier sheets  20  through all of the interior, intermediate and peripheral regions  500 ,  502 ,  504  of each spiral wound membrane element  10  within a given pressure vessel  32 . In this way, the osmotic gradient will be established between the permeate surface and the inner surface along substantially the entire area of all membrane sheets  18 . 
         [0067]    Optionally, as shown in  FIG. 8 , a flow restriction may be located within the central tube  16  at, or near, the opposite end of the membrane module  10  where the first solution is introduced. In this option, the first solution is preferably introduced at one end of the central tube  16  only. For example, the first solution is introduced at the concentrate end  26  and the flow restriction is positioned near the feed stock end  24 . The flow restriction restricts the first solution from flowing along the longitudinal axis of the central tube  16  and directs the first solution to flow out through the holes  22 . A portion of the first solution will access the permeate carrier sheets  20  by the open edge of the membrane leaves  12  that is adjacent the central tube  16 . Another portion of the first solution may access the permeate carrier sheets  20  by flowing through the ATD  40  and contacting the distal edge  20   b  of the permeate carrier sheets  20 . The flow restriction may be a one-way valve  550  that directs the first solution through the holes  22  but does not significantly impede the flow of permeate through the central tube  16  during filtration operations. Optionally, the feed stock end  24  of the series of membrane elements  10  may be used to withdraw permeate from the membrane elements  10  while first solution is added to the concentrate end  26  so as to allow the intermediate regions  502  to be flushed of permeate and filled with first solution before the second solution is applied to the membrane elements  10 . 
         [0068]    As described above, multiple spiral wound membrane elements  10  may be within one pressure vessel  32 . The multiple spiral wound membrane elements  10  may be connected in series and share a common central tube  16  with a flow restriction for each spiral wound membrane element  10 . Preferably the section of the central tube  16  that extends between adjacent spiral wound membrane elements either has no holes  22 , or the holes  22  are plugged, to prevent the loss of fluids between adjacent spiral wound membrane elements  10 . In this option, the first solution may be directed through the holes  22  by the flow restriction and at least a portion of the first solution in the peripheral region  504  will enter the recess  72  of the ATD  40  that is located at the same end of the spiral wound membrane element  10  as the flow restriction. This portion of the first solution may flow from the peripheral region  504  through the hollow member  51  and into the central tube  16  between adjacent spiral wound membrane elements  10 . This portion of the first solution will then enter the adjacent spiral wound membrane element  10  and the flow restriction therein will direct the first solution to flow through the holes  22 . 
         [0069]    It is to be understood that references herein to spiral or radial permeate flow do not exclude the edgewise permeate flow, that is flow in the direction that connects edges  20   c  and  20   d,  through the permeate carrier sheet. The two reverse flow paths  119  and  120  may be strictly opposite only in a plane perpendicular to the length of the spiral wound membrane element  10 . 
         [0070]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art.