Patent Publication Number: US-2005126966-A1

Title: Membrane separation device and membrane separation method

Description:
FIELD OF THE INVENTION  
      The present invention relates to a membrane separation apparatus of a flat sheet membrane type and a membrane separation process that are capable of being utilized in solid-liquid separation, deionization, soluble organic component removal, colloidal silica wastewater treatment, latex wastewater treatment, various waste liquids treatment, tap water filtration, activated sludge treatment, food wastewater treatment, domestic wastewater treatment (water reclamation), ship wastewater treatment, raw sewage treatment, COD reduction, BOD reduction, filter back wash wastewater treatment, printing ink wastewater treatment and the like.  
     BACKGROUND OF THE INVENTION  
      A membrane separation apparatus is used for separation of liquid (water to be treated), which contains various matters dissolved or mixed in water into purified water (permeate) and highly concentrated water. Of various types of the membrane separation apparatus, a cross-flow membrane separation is broadly carried out. The cross-flow membrane separation is carried out, as illustrated in  FIG. 11 ( a ), such as by sending pressurized water by a feeding pump  42  to a membrane module  41  that has a function to separate water into permeate and concentrate, retrieving the permeate from the membrane module  41  via a conduit  43 , retrieving the concentrate from the membrane module  41  via a conduit  44 , returning the concentrate into the membrane module  41  via a conduit  45  and a circulating pump  46 , and circulating the concentrate many times through the conduits  44 ,  45 , thereby increasing the concentration level, and as illustrated in  FIG. 11 ( b ), by feeding water stored in a tank  47  into the membrane module  41  by a feeding pump  48 , retrieving permeate from the membrane module  41  via a conduit  49 , returning concentrate into the tank  47  via conduits  50 ,  51 , feeding water of the tank  47  into the membrane module  41  by the feeding pump  48 , and circulating the concentrate many times through the conduits  50 ,  51 , thereby increasing the concentration level. In the meantime, the membrane modules which are classified according to the currently used module type are as follows:  
      (1) HOLLOW FIBER MODULE It is made up of a bundle of hollow fibers each having an outer diameter of about 500-1500 μm. Although it has a remarkably large membrane area per unit volume, it has shortcomings in that plugging is likely to occur in a hollow fiber which constitutes a narrow flow passage, frequent washing is required, and water to be treated is limited to relatively clean raw water and therefore it is necessary to strictly carry out a pretreatment step.  
      (2) SPIRAL WOUND MODULE It is made up of a membrane and a spacer that are wound around a centrally located water-collecting pipe in spiral fashion. In many cases, several water-collecting pipes are placed in a pressure vessel and successively connected each other in tandem. This results in a large membrane area in compact size, but has a shortcoming in that plugging is likely to occur in a flow passage of the membrane, in the same manner as the hollow fiber module.  
      (3) TUBULAR MODULE It is made up of a bundle of a few dozen support tubes each having an inner diameter of several millimeters to several tens of millimeters and a length of up to about 3 meters with a membrane mounted on an inner surface or outer surface of each tube. This is advantageous in that plugging is unlikely to occur in flow passages of the tubes, but has a shortcoming in that the value of the membrane packing density is smaller than those of the both modules and the membrane packing efficiency is inferior.  
      Japanese Examined Patent Application Publication No. Hei-03-18490 discloses a filtration and separation apparatus, as illustrated in  FIGS. 12 and 13 . In  FIGS. 12 and 13 , a tubular outer casing  61  has one end closed with a dome-like bottom  62 , and another end sealed with a lid  63 . The lid  63  has openings respectively defining a raw-water inlet connection part  64 , a salt-water outlet connection part  65  and an outlet for a permeate discharge pipe  66 . Inner casings  67 ,  67  are disposed in the outer casing  61  along the longitudinal axis thereof, each inner casing  67  being filled with a stack of diaphragm cushions  68  with spacers  69  disposed between the adjacent diaphragm cushions  68 ,  68 . A tubular bolt  70  extends through the stack of the diaphragm cushions  68  and has a plurality of radially extending holes  71 . According to the filtration apparatus disclosed in the above cited patent publication, raw water flown in the outer casing  61  through the inlet connection part  64  permeates through the diaphragm cushions  68  so that a part (permeate), which has passed through the diaphragm cushions, flows into the tubular bolt  70  through the radially extending holes  71  and is discharged via the permeate discharge pipe  66 , while a non-permeate part (retentate) is discharged through the salt-water outlet connection part  65 .  
      The filtration apparatus of the cited patent publication, which necessitates an elongated fluid passage for water to be treated, has shortcomings in that a large pressure loss is caused, and foreign components are likely to be deposited in a space  72  between the inner casings  67 ,  67  disposed along the longitudinal axis.  
      The present invention has been conceived in consideration of those problems associated with the prior arts. It is an object of the present invention to provide a membrane separation apparatus and a membrane separation process that cause less fouling and plugging and therefore are capable of achieving membrane separation even with relatively small flow rate of water to be treated, have an excellent membrane packing density, and are unlikely to cause deposition of foreign components in the apparatus.  
     SUMMARY OF THE INVENTION  
      In order to achieve the above object, according to the present invention, there is provided a membrane separation apparatus, which includes a pressure vessel with a flow-regulating plate disposed on the side of the water inlet, inner casings disposed in this pressure vessel, and stacks of membrane separation elements respectively disposed in the inner casings. The membrane separation elements have a through-hole extending therethrough, while the inner casings have permeate discharge passages extending along the longitudinal axis thereof so as to be communicated with the through-hole so that water that has been fed into the pressure vessel via the flow-regulating plate permeates through the membrane separation elements and is discharged to the outside via the through-hole and the permeate discharge passages defined in the inner casings.  
      According to the present invention, there is also provided a membrane separation apparatus, which includes a pressure vessel having a water inlet and a concentrate outlet, an inner casing disposed in the pressure vessel and a membrane separation element group. The membrane separation element group is made up of a stack of membrane separation elements and spacers disposed between the membrane separation elements. The membrane separation element group has a through-hole extending through the membrane separation elements and the spacers. The through-hole defined in the membrane separation element group is communicated with permeate discharge passages defined in the inner casing so that water that has been fed into the pressure vessel permeates through the membrane separation elements and is discharged to the outside via the through-hole and the permeate discharge passages.  
      It is preferable to employ in the membrane separation apparatus of the present invention an arrangement in which plural inner casings are disposed in the pressure vessel and membrane separation element groups disposed in the adjacent inner casings are located close to each other.  
      With the above preferable arrangement, the membrane separation element groups are located close to each other so that no space is formed therebetween unlike the prior art. Accordingly, this arrangement can solve the problem associated with the prior art, namely “deposition of foreign components” in the space.  
      It is preferable to employ in the membrane separation apparatus of the present invention an arrangement in which each of the membrane separation elements has a five-layer structure of a permeable membrane, a space retaining member, a support plate, a space retaining member and a permeate membrane laminated together in this order, and the support plate has at least one cut that has a first end communicated with the through-hole.  
      With the above preferable arrangement in which the cut is defined in the support plate to be communicated with (contact) the through-hole, it is possible to effectively transmit liquid that has permeated through the membrane separation element, to the permeate discharge passages via the cut and the through-hole. That is, the cut as defined can reduce the pressure loss of permeate and secure smooth flow of permeate into the through-hole.  
      It is also preferable to employ in the membrane separation apparatus of the present invention an arrangement in which the cut has a second end located outside of a project area of the spacers in the support plate.  
      With the above preferable arrangement in which the second end of the cut is located outside of the project area of the spacers in the support plate, even in case where the membrane separation elements and the spacers are tightly stacked together, it is possible to secure a flow-in passage for liquid that has permeated through the membrane separation elements, thus achieving proper transmission of permeate (transmission via the permeate discharge passages) via the cut or the like.  
      It is also preferable to employ in the membrane separation apparatus of the present invention an arrangement where plural membrane separation element groups are disposed in the inner casing.  
      With the above preferable arrangement in which the plural membrane separation element groups are disposed in the inner casing, it is possible to treat water in large volume with respect to an installation area than ever before.  
      It is also preferable to employ in the membrane separation apparatus of the present invention an arrangement in which the adjacent membrane separation element groups are disposed at a certain angle relative to each other. They are more preferably disposed substantially orthogonal to each other.  
      With the above preferable arrangement in which the adjacent membrane separation element groups are disposed at a certain angle (e.g., orthogonal) relative to each other, it is easy to cause turbulent flow of water to be treated. As a result, deposition of suspended solid on membrane surfaces (the surfaces of the permeable membranes) of the membrane separation elements is suppressed. More preferably, the membrane separation element groups are disposed substantially orthogonal to each other, since it produces a maximum effect in causing turbulent flow of water.  
      The “certain angle” is not limited to a specific angle, but an angle greater than 0., for example 10.-90. is preferable in light of the balance relative to the pressure loss or the like.  
      It is also preferable to employ in the membrane separation apparatus of the present invention an arrangement in which the flow-regulating plate disposed on the side of the water inlet of the pressure vessel is disposed substantially orthogonal to the adjacent membrane separation element group.  
      With the above preferable arrangement, turbulent flow of water is likely to occur between the flow-regulating plate and the membrane separation element group. Thus, deposition of suspended solid on the membrane surfaces (the surfaces of the permeable membranes) of the membrane separation elements is suppressed in the same manner as above.  
      The inner casing of the membrane separation apparatus of the present invention may be replaced with water collecting members that are capable of holding the membrane separation elements therebetween and have permeate discharge passages communicated with the through-hole defined in the membrane separation element group.  
      Specifically, the membrane separation apparatus of the present invention may include a pressure vessel having a water inlet and a concentrate outlet, water collecting members disposed in the pressure vessel, and a membrane separation element group held between the water collecting members. The membrane separation element group is made up of a stack of membrane separation elements and spacers disposed between the membrane separation elements. The membrane separation element group defines a through-hole that extends through the membrane separation elements and the spacers. The through-hole defined in the membrane separation element group is communicated with permeate discharge passages respectively defined in the water collecting members, so that water that has been fed into the pressure vessel permeates through the membrane separation elements and is discharged to the outside via the through-hole and the permeate discharge passages.  
      The water collecting members of the membrane separation apparatus each are preferably made of a plate-like member.  
      According to the membrane separation process of the present invention, in which a pressure vessel has a water inlet and a concentrate outlet; an inner casing is disposed in the pressure vessel; a membrane separation element group is disposed in the inner casing; the membrane separation element group is made up of a stack of membrane separation elements and spacers disposed between the membrane separation elements; the membrane separation element group defines a through-hole that extends through the membrane separation elements and the spacers; and the through-hole defined in the membrane separation element group is communicated with permeate discharge passages defined in the inner casing, the process includes drawing water into the pressure vessel, making said water permeate through the membrane separation elements and discharging permeate to the outside via the through-hole and the permeate discharge passages.  
      According to the membrane separation process, in which a pressure vessel has a water inlet and a concentrate outlet; water collecting members are disposed in the pressure vessel; a membrane separation element group is held between the water collecting members; the membrane separation element group is made up of a stack of membrane separation elements and spacers disposed between the membrane separation elements; the membrane separation element group defines a through-hole that extends through the membrane separation elements and the spacers; and the through-hole defined in the membrane separation element group is communicated with permeate discharge passages respectively defined in the water collecting members, the process includes drawing water into the pressure vessel, making said water permeate through the membrane separation elements and discharging permeate to the outside via the through-hole and the permeate discharge passages. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a perspective view of a pressure vessel of the present invention.  
       FIG. 2  is a perspective view of an inner casing separated into an upper part and a lower part.  
       FIG. 3  are schematic views of the pressure vessel, in which  FIG. 3 ( a ) is a cross sectional view of one example of the pressure vessel according to the present invention;  FIG. 3 ( b ) is a front view of an inlet side thereof,  FIG. 3 ( c ) is a cross sectional view illustrating a part of a flow-regulating plate as viewed in the direction of arrows C-C in  FIG. 3 ( d ); and  FIG. 3 ( d ) is an enlarged sectional view of the flow-regulating plate, as viewed in the direction of arrows D-D in  FIG. 3 ( a ).  
       FIG. 4  is a perspective view of a stack comprising a number of membrane separation elements in exploded state.  
       FIG. 5  are schematic views of an inner casing of the present invention, in which  FIG. 5 ( a ) is an exploded perspective view for explanation of a method of fastening a membrane separation element to a lower half of the inner casing; and  FIG. 5 ( b ) is a plan view of a pipe.  
       FIG. 6  is a cross sectional view of a membrane separation element.  
       FIG. 7  are schematic views of a membrane separation element, in which  FIG. 7 ( a ) is a plan view illustrating an example of cuts formed around a through-hole of a support plate of the membrane separation element; and  FIG. 7 ( b ) is a cross sectional view taken along line VII-VII in  FIG. 7 ( a ).  
       FIG. 8  are schematic views of a membrane separation element, in which  FIG. 8 ( a ) is a plan view illustrating another example of the cuts formed around the through-hole of the support plate of the membrane separation element; and  FIG. 8 ( b ) is a cross sectional view taken along line VIII-VIII in  FIG. 8 ( a ).  
       FIG. 9  is an explanatory view showing the flow of back wash.  
       FIG. 10  is a view illustrating an example of fluctuations in transmembrane pressure resulting from the back wash.  
      FIGS.  11 ( a ) and  11 ( b ) are schematic flowcharts of a cross-flow membrane separation process.  
       FIG. 12  is a vertical cross sectional view of the conventional membrane separation apparatus.  
       FIG. 13  is a cross sectional view taken along line XIII-XIII in  FIG. 12 .  
       FIG. 14  is a schematic cross sectional view of the membrane separation apparatus according to another example of the present invention.  
       FIG. 15  are schematic views of the membrane separation apparatus according to another example of the present invention, in which  FIG. 15 ( a ) is a schematic side view of an inner casing disposed in the pressure vessel;  FIG. 15 ( b ) is a schematic side view of membrane separation element groups disposed in the inner casing; and  FIG. 15 ( c ) is a view as viewed in the direction of arrow C in  FIG. 15 ( a ).  
       FIG. 16  are schematic views of the membrane separation apparatus according to still another example of the present invention, in which  FIG. 16 ( a ) is a schematic cross sectional view of the membrane separation apparatus according to another example of the present invention;  FIG. 16 ( b ) is a schematic cross sectional view as viewed in the direction of arrows B-B in  FIG. 16 ( b ); and  FIG. 16 ( c ) is a schematic cross sectional view as viewed in the direction of arrows C-C in  FIG. 16 ( a ). 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
      That is, according to the essential feature of the present invention, a membrane separation apparatus is characterized by that it comprising a pressure vessel having a water inlet at a first end and a concentrate outlet at a second end; inner casings disposed along a longitudinal axis of the pressure vessel; a flow-regulating plate disposed on the side of the water inlet of the pressure vessel; stacks of membrane separation elements respectively disposed in the inner casings; spacers disposed between the adjacent membrane separation elements, the spacers also serving as seal members; each stack of membrane separation elements and the spacers therein together defining a through-hole extending from a first side to a second side, of the stack of membrane separation elements; the inner casings having a permeate discharge passage along the longitudinal axis thereof, and the permeate discharge passage being communicated with the through-hole of each stack of membrane separation elements; wherein water that has been fed into the pressure vessel via the flow-regulating plate permeates through the membrane separation elements and is discharged to the outside via the through-holes and the permeate discharge passage.  
      In the present invention in which the flow-regulating plate is disposed on the side of the water inlet of the pressure vessel, water fed into the vessel is regulated and therefore does not cause channeling, allowing the flow rate of water to a relatively high level. Although depending on the type and behavior of water to be treated, it is possible to achieve a flow rate of 0.5-2.0 m/sec. Where less than 0.5 m/sec, sufficient permeate flux cannot be produced. Even where exceeding 2 m/sec, only insufficient energy consumption is made. In the present invention, even without taking a method involving feeding pressurized water into the pressure vessel, it is possible to carry out membrane separation of water fed into the pressure vessel by keeping the permeate discharge side under negative pressure (not higher than 0.1 MPa). For treatment of domestic wastewater (a reverse osmosis membrane), latex wastewater, filter backwash wastewater (an ultrafiltration membrane) and the like, it is preferable to feed water via a pressurizing system into the pressure vessel. For some types of high-concentration sludge (e.g., activated sludge) and the like, it is preferable to feed water via a negative pressure system into the pressure vessel.  
      Water that has been fed into the pressure vessel via the flow-regulating plate permeates through the membrane separation elements and is discharged to the outside via the through-hole and the permeate discharge passage of each inner casing. For this, a pipe is inserted into the through-hole, and the pipe has a permeate discharge groove along the lengthwise direction thereof. This permeate discharge groove is defined on the outer circumference of the pipe. The description will be made for the following embodiment (and the respective examples) by taking for example the case where the “pipe” is used as a member defining the permeate discharge groove, while the present invention is not necessarily limited to this arrangement so that a “solid rod” may be used in place of the pipe according to needs and circumstances. Where the solid rod is used, the permeate discharge groove is defined on the outer circumference in the lengthwise direction thereof.  
      This pipe serves as a permeate passage as well as serving to fix the membrane separation elements in position, thus providing an apparatus having a simplified structure for the permeate discharge. The permeate discharge groove defined on the pipe makes permeate easy to flow and suppress the occurrence of unnecessary pressure loss for permeate. In order to prevent plugging of this permeate discharge groove in a short period of time, it is preferable to define a groove having a cross sectional area of 0.5-1.0 mm×0.5-1.0 mm (d 1 ×d 2  in  FIG. 5 ( b )). It is also preferable to provide more than one permeate groove, in which several (e.g., 2-8) grooves are defined with a certain spacing.  
      The length of each inner casing is preferably set to be substantially equal along the longitudinal axis of the pressure vessel to the length of the stack of the membrane separation elements, thereby forming substantially no space between the adjacent inner casings along the longitudinal axis of the pressure vessel. That is, forming no space between the adjacent inner casings prevents deposition of suspended solid therein.  
      The description was made for this embodiment by taking for example the case where the length of the inner casing is substantially equal along the longitudinal axis of the pressure vessel to the length of the stack of the membrane separation elements, while the present invention is not necessarily limited to this arrangement. As long as substantially no space is formed between the adjacent stacks of the membrane separation elements (membrane separation element groups), the length of the inner casing can be equal or different from the length of the stack of the membrane separation elements along the longitudinal axis.  
      The ratio of short side to long side of the membrane separation elements having a rectangular flat membrane shape is 1:1-1:3, and the through-hole preferably lies on the bisector of the short side. This is because where the ratio of short side to long side exceeds 1:3, it is difficult to fabricate the membrane separation element through ultrasonic welding or the like, and even where the membrane separation element having a ratio of short side to long side exceeding 1:3 can be produced, such element has only a small membrane area, and the through-hole lying on the bisector of the short side can equally draw permeate to the through-hole. With this arrangement in which the membrane separation elements have a rectangular flat membrane shape; the ratio of short side to long side is 1:1 to 1:3; and the through-holes lie on the bisector of the short side, there is an advantage of achieving easy fabrication (cutting, welding) of the membrane separation element which can be backwashed even though it is a flat sheet membrane. It is also possible to achieve even water collection by positioning the through-holes on the bisector of the short side.  
      It is preferable to employ an arrangement where each membrane separation element has a five-layer structure of a permeable membrane, a space retaining member, a support plate, a space retaining member and a permeable membrane laminated together in this order, and the support plate has at least one cut that is located around and extends to the through-hole. Permeate in each membrane separation element tends to permeate through the permeable membranes and drain cloth (non-woven cloth or the like) of the space retaining members, which act as flow passages (drain) for permeate and reach the through-hole; however it is likely to prevent permeate from smoothly flowing from the membrane separation element to the through-hole since the spacers serving also as seal members are disposed between the adjacent membrane separation elements so that the spacers press the membrane separation elements from above and below. For this, at least one cut is defined in the support plate around the through-hole, which extends to the through-hole, can reduce the pressure loss of permeate which permeates through each membrane separation element and secure smooth flow of permeate into the through-hole.  
      Microfiltration membranes, ultrafiltration membranes, nano-filtration membrane or reverse osmosis membrane can be used as the permeable membranes. Changing the type of the permeable membranes according to the intended use produces an effect to make themselves adaptable to various applications thereof.  
      Where gas is entrained in water to have the two-phase flow of water and gas and this flow is sent to the pressure vessel, it is expectable to produce a washing effect (a fouling and plugging prevention effect) on the membrane separation elements by gas entrained in water. As this gas, air or nitrogen can be used.  
      Gas may be intermittently entrained in water rather than successively. For example, by performing the entrainment of pressurized air in water once a day, it is expectable to remove suspended solid with air bubbles, thus making membrane fouling and plugging hard to occur in the membrane separation elements.  
      Also, it is possible to remove suspended solid deposited on the membrane surface by flowing pressurized backwash water or air-entrained backwash water into the vessel from the permeate discharge side thereof. As the backwash water, permeate can be used. The pressure of the backwash water is preferably 0.05-0.2 MPa and more preferably 0.05-0.1 MPa. This is because under pressure exceeding 0.2 MPa, the membrane may be broken and a sufficient washing effect can be produced with a pressure of 0.05-0.1 MPa.  
      The membrane separation apparatus of this embodiment can be used in cross-flow filtration, dead-end filtration or the like.  
     EXAMPLES  
      The examples of the present invention will be described with reference to the attached drawings.  FIG. 1  is a perspective view of a pressure vessel  3  having a water inlet  1  at a first end and a concentrate outlet  2  at a second end. As illustrated in  FIG. 2 , a large number of inner casings  6 , each made up of an upper half  4  and a lower half  5 , are disposed adjacent to each other along a longitudinal axis of the pressure vessel.  
      As illustrated in  FIG. 3 ( a ), a flow-regulating plate  7  is mounted on the side of the water inlet  1  of the pressure vessel  3 . The flow-regulating plate  7  has plate-like members  8  extending in a circular opening defined on the side of the water inlet  1  of the pressure vessel  3  in a comb-like formation with a certain space between the adjacent plate-like members as aligned in two rows, as illustrated in  FIG. 3 ( b ). As illustrated in  FIG. 3 ( d ), an enlarged sectional view of the flow-regulating plate  7  as viewed in the direction of arrows D-D in  FIG. 3 ( a ), a length L 2  of the plate-like members  8  is 30-60 mm and preferably 40-50 mm. As illustrated in  FIG. 3 ( c ), a part of the flow-regulating plate  7  as viewed in the direction of arrows C-C in  FIG. 3 ( d ), the cross sectional size d 1  of each plate-like member  8  is 1-10 mm and preferably 2-5 mm, while d 2  is 5-20 mm and preferably 7-15 mm. A space L 1  between the adjacent plate-like members  8  is 5-20 mm and preferably 7-10 mm. The flow-regulating plate of the two-row arrangement allows water to be substantially uniformly distributed into the inlet side of the inside of the pressure vessel and introduced into the vessel with its flow regulated.  
      As illustrated in  FIG. 2 , a large number of membrane separation elements  9  are stacked together within each inner casing  6 . Also, as illustrated in  FIG. 4 , spacers  10 , which also serve as seal members, are disposed between the adjacent membrane separation elements  9 ,  9 .  
      As illustrated in  FIG. 3 ( a ), through-holes  11  are formed as extending through the membrane separation elements  9  and also the spacers from a first side to a second side, of a stack of the membrane separation elements  9 , and pipes  12  are inserted in the through-holes  11 . As illustrated in  FIG. 5 ( a ), four permeate discharge grooves  13  are defined on each pipe  12  along the lengthwise direction thereof.  
      As illustrated in  FIG. 4 , the membrane separation elements  9  each have a rectangular, flat sheet membrane shape, and in this example, the ratio of short side to long side is 1:2, and the through-holes  11  for receiving the pipes lie on the bisector of the short side. Specifically, it is preferable to have a long side of 100-300 mm and a short side of 50-150 mm. If, with the excessive length, the membrane separation elements are hard to be fabricated with ultrasonic welding and also it is unlikely to conduct back wash (because of possible break).  
      As illustrated in  FIG. 6 , the membrane separation elements  9  each have a five-layer structure of a permeable membrane  14 , a non-woven cloth  15 , a support plate  16 , a non-woven cloth  17  and a permeable membrane  18  laminated together in this order. The permeable membrane  14 , the non-woven cloth  15 , the non-woven cloth  17  and the permeable membrane  18  are welded together along their edges with ultrasonic welding. While the material of the support plate  16  may be a synthetic resin or metal, it is essential that a material used allows the membrane separation elements  9  to maintain their given shape.  
      As illustrated in  FIG. 7 ( a ), the support plate  16  has eight cuts  19  around each through-hole  11 , which cuts extending to the corresponding through-hole. A reference numeral  20  represents a ring-shaped gasket, and the cuts  19  extend to the outside of the corresponding gasket  20 . As the gaskets, oval gaskets  21  as illustrated in  FIG. 8 ( a ) may be employed. In this case, cuts  22  extend to the outside of the corresponding gasket  21 . The gaskets  20 ,  21  correspond to the spacers  10  combined with a sealing capability.  
      As illustrated in  FIG. 3 ( a ), the inner casing  6  has permeate discharge passages  23  along the longitudinal axis, while the pipes  12  inserted in the through-holes  11  are communicated with the permeate discharge passages  23 .  
      As illustrated in  FIG. 3 ( a ) or  FIG. 4 , each membrane separation element  9  has two through-holes  11  so that water flows which has passed through the permeable membrane  14  or  18  and the non-woven cloth  15  or  17  of the membrane separation element and reached through-holes  11   a ,  11   b , are joined in the permeate discharge passages  23  and discharged to the outside of the membrane module.  
      Cuts  19  or  22  which are, as illustrated in  FIG. 7 ( a ) or  FIG. 8 ( a ) formed in each support plate  16  around the through-holes  11  so as to extend to the through-holes  11 , serve as permeate passages along with the non-woven clothes, so that water permeating through the membrane separation elements has reduced pressure loss and permeate is allowed to smoothly flow into the through-holes  11 . The number of cuts  19  or  22  are not limited to specific numbers, as long as they can reduce the pressure loss of permeate and secure smooth flow of permeate into the through-holes  11 . In actual use, 4-20 cuts may be formed around each through-hole. The width of each cut is preferably about 0.5 mm.  
      As illustrated in  FIG. 2 , the length of each inner casing  6  (an accommodation section for the membrane separation elements (a membrane separation element group)) is substantially equal along the longitudinal axis of the pressure vessel to the length of the stack of membrane separation elements  9  (a membrane separation element group) and therefore substantially no space is formed between the stacks of the membrane separation elements  9  of the adjacent inner casings.  
      Thus, as illustrated in  FIG. 3 , water introduced into the pressure vessel  3  through the flow-regulating plate  7  permeates through the membrane separation elements  9  and is discharged to the outside through the pipes  12  inserted in the through-holes  11  and the permeate discharge passages  23  formed in the inner casing  6 , thereby discharging concentrate to the outside through the concentrate outlet  2 . An inlet portion and an outlet portion of the pressure vessel  3  gradually increases or decreases in diameter so as to prevent the formation of swirls or vortices in the flow. Therefore, such a diameter-increasing portion  24  or a diameter-decreasing portion  25  can reduce pressure loss of feed water as much as possible.  
      The membrane separation apparatus of the present invention can be dismantled in remarkably easy manner, so that each inner casing can easily be pulled out from the pressure vessel  3  by detaching a flange  26  of  FIG. 1  that fixes the inner casing to the pressure vessel  3 . In general, 5-10 inner casings are mounted inside the pressure vessel  3 . As illustrated in  FIG. 2 , each inner casing  6  is made up of an upper half  4  and a lower half  5 , which are fastened together with bolts  27 , nuts  28  and washers  29 , as illustrated in detail in  FIG. 5 , so that the inner casing  6  can be separated into two halves simply by loosening the structure of a bolt-nut threaded engagement. As illustrated in  FIG. 2 , a stack of the membrane separation elements  9  can be exposed to the outside by removing the upper half  4  of the inner casing  6 . As illustrated in  FIG. 4 , spacers  10 , which serve also seal members, are interposed between the vertically adjacent membrane separation elements  9 , while the pipes  12  penetrate through the stack of the membrane separation elements  9  as skewers, so that the dismantling can easily be achieved by removing the membrane separation elements  9  and the spacers  10 . The pipes  12  may be pulled out and removed subsequent to removal of the membrane separation elements  9  and the spacers  10 , or the pipes  12  may be first pulled out and removed for the subsequent removal of the membrane separation elements  9  and the spacers  10 .  
      Now, the description will be made for the effects of the characteristic structure of the present invention, based on a specific experiment. 
          (1) Effect of the Cuts        

      As illustrated in  FIG. 7  or  8 , it can be expected that the pressure loss of permeate passing through each membrane separation element is reduced by the formation of the cuts around the through-holes  11  of the support plate  16  of the membrane separation element. Therefore, an experiment was conduced to confirm its effect, of which explanation will be made below.  
      For the membrane separation element in the arrangement as illustrated in  FIG. 6 , the permeable membranes  14 ,  18  are in the form of ultrafiltration membranes; the support plate  16  is made of ABS resin (the size of the membrane separation element: 100 mm×200 mm); the spacer  10  is made of EPDM (Ethylene Propylene Diene Methylene); a total of twenty-eight membrane separation elements  9  were stacked together (100 mm×100 mm×200 mm) (corresponding to the “membrane separation element groups”) and were installed in each inner casing  6  having a diameter of 150 mm, as illustrated in  FIG. 2 ; and five inner casings thus arranged were disposed in contact with each other in the pressure vessel  3 , as illustrated in  FIG. 3 . Clear water was fed into a conduit (not shown) connected to the flow-regulating plate  7  on the inlet side of the pressure vessel  3  and filtered with a permeate flux of 100 l/m 2 /hr.  
      a. In Case Where the Cuts are Formed in the Support Plate  
      In case where the eight cuts  19  as illustrated in  FIG. 7  were formed in the support plate  16  of the membrane separation element  9 , the pressure of clear water on the inlet side of the pressure vessel  3  was 0.2 MPa, while the pressure of water discharged through the permeate discharge passages  23  was 0.18 MPa, and therefore it has been found that the pressure loss was 0.02 MPa.  
      b. In Case Where the Cuts are not Formed in the Support Plate  
      In case where the cuts  19  or  22  as illustrated in  FIG. 7  or  8  were not formed in the support plate  16  of the membrane separation element  9 , the pressure of clear water on the inlet side of the pressure vessel  3  was 0.25 MPa, while the pressure of water discharged through the permeate discharge passages  23  was 0.18 MPa, and therefore it has been found that the pressure loss was 0.07 MPa. Thus, it is apparent that permeate within the membrane separation element is discharged only through the non-woven clothes when no cuts are formed in the support plate  16  of the membrane separation element, resulting in an increased pressure loss of permeate.  
      (2) Effect Resulting from Omission of Spaces Between the Membrane Separation Elements of the Adjacent Inner Casings  
      When a space exists between the membrane separation elements of the adjacent inner casings, suspended solid is deposited in the space, which results in increased pressure loss in a flow passage. Therefore, as illustrated in  FIG. 2 , the length of the inner casing  6  is set to be substantially equal to the length of the membrane separation element  9  along the longitudinal axis of the pressure vessel so as not to substantially cause a space between the membrane separation elements  9  of the adjacent inner casings  6 , as illustrated in  FIG. 3 . This prevents deposition of suspended solid so that reduction of the pressure loss in the flow passage can be expected.  
      In consideration of the above, as illustrated in  FIG. 3 , when the operation is made with substantially no space between the membrane separation elements  9  of the adjacent inner casings  6 , the pressure loss resulting from subtracting the pressure of water discharged through the permeate discharge passages  23  from the pressure of clear water on the side of the water inlet  1  of the pressure vessel  3  was 0.01 MPa. On the other hand, when the operation was made with a space of 15 mm between the membrane separation elements of the adjacent inner casings  6 ,  6 , the pressure loss resulting from the subtraction of the pressure of water discharged through the permeate discharge passages  23  from the pressure of clear water on the side of the water inlet  1  of the pressure vessel  3  was 0.02 MPa, and it has been found that the pressure loss in the flow passage was increased. If a pretreatment step (e.g., removing deposited, suspended solid) is sufficiently performed, the membrane separation can be done even if there is a space of about 10 mm or smaller between the membrane separation elements of the adjacent inner casings.  
      With the structure having substantially no space between the membrane separation elements of the adjacent inner casings  6 , the entire length of the pressure vessel can be shortened about 7.5%.  
      (3) Effect of Back Wash  
      Activated sludge (MLSS: 5000-30000 mg/liter) was fed into the pressure vessel  3  having the structure as illustrated in  FIG. 3  and membrane separation was performed for 6 hours. The transmembrane pressure difference (the difference between the pressure of water to be treated on the inlet side of the pressure vessel  3  and the pressure of water discharged through the permeate discharge passages  23 ), which was originally about 0.1 MPa, was increased to 0.15 MPa. This increase was assumed due to adhesion or deposition of suspended solid in the structural elements of the membrane separation elements and accordingly the back wash was performed by a back wash system having a flow as illustrated in  FIG. 9 . In explanation of  FIG. 9 , reference numeral  31  represents a water tank,  32 : a pressure vessel (a membrane module equipped with the membrane separation elements of the present invention),  33 : a permeate tank,  34 : a water feeding pump,  35 : a back wash water feeding pump and  36 : a flow regulation valve of permeate.  
      That is, water to be treated (raw water) stored in the water tank  31  is fed into the membrane module  32  by the water feeding pump  34  via a conduit  37 , and permeate is fed into the permeate tank  33  via a conduit  38 . At the time of back wash, the water feeding pump  34  is stopped and the water feeding pump  35  is actuated to add back pressure (0.1 MPa) to the membrane separation elements within the membrane module  32  via a conduit  39 , and suspended solid attached to the membrane separation elements  9  was washed away with clear water and discharged to a concentrate side through the concentrate outlet  2  (see  FIG. 3 ). The back wash water feeding pump  35  was intermittently operated (several minutes every 120 minutes) by using a timer. As a result of the back wash, the transmembrane pressure difference was recovered to 0.11-0.12 MPa and therefore the operation of the back wash water feeding pump  35  was stopped, while the operation of the water feeding pump  34  was resumed and the membrane separation was continued. An example of fluctuations in transmembrane pressure difference resulting from the back wash is illustrated in  FIG. 10 . The vertical axis of  FIG. 10  represents the transmembrane pressure difference, and the horizontal axis represents the elapsed operation time of the membrane separation apparatus. The back wash water feeding pump was not continuously operated, but intermittently operated for several minutes every two hours. In a graph looking like a saw blade in  FIG. 10 , each of hills and valleys appear about every two hours, in which the valleys represent recovery of the transmembrane pressure by the back wash.  
      The timing at which the back wash water feeding pump  35  is actuated by the timer can be selectively determined according to the degree of fouling in the membrane separation elements.  
      (4) Comparison Between a Conventional Membrane Separation Equipment and the Membrane Separation Apparatus of the Present Invention  
      The prerequisites as compared for achieving treatment of domestic wastewater in the amount of 10 m 3 /day to have BOD reduced to 20 ppm or lower in water to be treated, are stated in Table 1 as follows.  
                               TABLE 1                                   Limit       Area Required           Concentration of   Required   for Installation           Suspended Solid   Pretreatment   of the Entire           of Feed Water   Equipment   Equipment                                                    Present   ≦several 100 ppm   Strainer    4 m 2         Invention       Tubular Module   ≦several 100 ppm   Strainer    6 m 2         Hollow Fiber   ≦several 10 ppm   Coagulation   10 m 2         Module       Sedimentation/               Sand               Filtration       Spiral Wound   ≦several ppm   Coagulation   12 m 2         Module       Sedimentation/               Sand Filtration/               Microfiltration                  
 
      As shown in TABLE 1, in the present invention and the tubular module, the limit concentration of suspended solid of water (feed water) for treatment is relatively high so that only a simple equipment such as a strainer is enough for pre-treatment. However, in the spiral wound module and the hollow fiber module, the limit concentration of suspended solid of water to be treated (feed water) is low so that pretreatment equipments such as a coagulation sedimentation equipment and a sand filtration equipment, and further a microfiltration equipment are required. As a result, a large area for installation is necessitated.  
      Comparing the concentration of suspended solid required to water (raw water) in TABLE 1, the present invention is on the same level as the tubular module so that a strainer or its equivalent is enough for the pretreatment. However, as shown in TABLE 2, the present invention can take a higher membrane packing density (allows the membrane separation elements to be packed in the module with high density), and the lower pressure loss, as compared with the tubular module. A large value of the membrane packing density can be taken because flat sheet membrane elements are stacked together and therefore can be tightly packed with no space therebetween.  
      On the other hand, in case where relatively-large cylindrical tubular membranes (a diameter of about 5-10 mm) are inserted into a cylindrical module, the membrane packing density is lowered because a space is consequentially caused between the adjacent cylindrical tubular membranes.  
      A high membrane packing density can allow the enlargement of the necessary flow passages when a module having the same membrane area is fabricated. Even if the flow rate is low, fluid containing a large solid content is easy to flow (the Reynolds number is increased) because of the enlarged fluid passages, and therefore it is possible to achieve membrane separation even with low flow rate, according to the present invention.  
                           TABLE 2                                   Membrane Packing   Flow Rate of           Density   Feeding Water                                                        Present Invention   10 m 2 /module    1 m/sec           Tubular Module    2 m 2 /module   2-3 m/sec                      
 
      (5) Effects of the Membrane Separation Apparatus of the Present Invention  
      The test results will be summarized as below.  
      {circle over (1)} The pressure loss of permeate can be improved by having cuts in the support plate of a membrane separation element.  
      {circle over (2)} The entire length of the pressure vessel can be shortened 7.5% by omitting the space between the membrane separation elements in the adjacent inner casings. Also, omission of the space prevents deposition of suspended solid in such a space, which contributes to reduction of the pressure loss of water to be treated.  
      {circle over (3)} A bulky pretreatment equipment is not necessary because of a moderate quality of water required to raw water, and therefore only a small area is enough for installation of the entire equipment and the membrane separation can be performed with high efficiency even with low flow rate of water to be treated. Thus, low flow rate of water is better to contribute to high efficiency, but still there is need for a minimum flow rate for the purpose of preventing membranes from being fouled and plugged.  
      The present invention is not necessarily limited to the above examples, while various modifications may be made without departing from the scope of the present invention.  
      The description will be made for another example with reference to the drawings as attached.  
       FIG. 14  is a schematic cross sectional view of the membrane separation apparatus according to another example of the present invention. The basic structural elements of the membrane separation apparatus of this example are basically the same as those as illustrated in  FIG. 3  and the like and therefore the same reference numerals are allocated thereto for omission of the description on them and the description will be mainly made for the differences from the above example.  
      As illustrated in  FIG. 14 , the membrane separation apparatus of this embodiment includes the pressure vessel  3 , the inner casing  6  disposed in this pressure vessel  3 , two membrane separation element groups  90  ( 90 A,  90 B) disposed in this inner casing  6 , the flow-regulating plate  7  disposed on the inlet side for water to be treated in the inner casing  6 . Herein, the membrane separation element groups  90  as used each may have a size of 100 mm×100 mm×200 mm.  
      Each membrane separation element group  90  is made up of a stack of the plural membrane separation elements  9  and the spacers  10  disposed between the adjacent membrane separation elements  9 . The through-holes  11  are formed in each membrane separation element group  90  as extending through the membrane separation elements  9  and the spacers  10  from a first side to a second side, of the stack. The pipes  12  are inserted in the through-holes  11 . The four permeate discharge grooves  13  (see  FIG. 5 ( a )) are defined on each pipe  12  along the lengthwise direction thereof.  
      In this embodiment, as illustrated in  FIG. 14 , the adjacent membrane separation element groups  90  (a first membrane separation element group  90 A and a second membrane separation element group  90 B) are disposed orthogonal to each other.  
      Water passing through (or permeating through) the membrane separation element groups  90  is ordinarily distributed along the surface of each membrane separation element formed into a flat shape. That is, a space between the adjacent membrane separation elements  9  defines a flow passage for water passing for treatment.  
      Accordingly, in the arrangement as illustrated in  FIG. 14 , with the membrane separation element groups  90  disposed orthogonal to each other, the flow passages of water in the membrane separation element groups  90  are necessarily oriented orthogonal to each other.  
      In this example, the flow-regulating plate  7  located on the inlet side for water to be treated is thus disposed orthogonal to the first membrane separation element group  90 A adjacent to this flow-regulating plate  7 . That is, it is so structured that the flow passages of water at the flow-regulating plate  7  (flow passages between the plate-like members  8 ) are oriented orthogonal to the flow passages of water in the first membrane separation element group  90 A.  
      In this example, as described in the aforesaid example, the first and second membrane separation element groups  90  are also disposed adjacent to each other with substantially no space therebetween (for example, disposed with the adjacent membrane separation element groups  90  held in contact with each other).  
      In this example, for the illustrative purpose, only the permeate discharge passages  23  communicated with the through-holes  11  of the second membrane separation element group  90 B are illustrated, while permeate discharge passages (not illustrated) communicated with the through-holes  11  of the first membrane separation element group  90 A are formed at positions substantially  90 . away from the permeate discharge passages  23  in the inner casing  6 .  
      The thus arranged membrane separation apparatus of this example produces the following effects in addition to the effects produced in the aforesaid example.  
      In the example of  FIG. 14 , the adjacent membrane separation element groups  90 A,  90 B are disposed substantially orthogonal to each other. With this arrangement in this embodiment, the flow of water to be treated in the membrane separation apparatus is likely to be turbulent so that it is possible to prevent suspended solid from being deposited on the membrane surfaces (permeable membrane surfaces) of the membrane separation elements  9 .  
      In the above respective examples (the examples illustrated in  FIGS. 3 and 14 ), while the description was made by taking for example the case where the two membrane separation element groups are disposed in the inner casing, the present invention is not necessarily limited to this arrangement. According to needs and circumstances, one, or three or more membrane separation element groups may be disposed in the inner casing.  
      The description will be made for the arrangement with three or more (specifically eight) membrane separation element groups disposed in the inner casing, with reference to  FIG. 15 .  
       FIG. 15  are schematic views of the membrane separation apparatus according to another example of the present invention, and more specifically schematic views of the inner casing of the membrane separation apparatus and its inside structure, etc.  
      The apparatus of this example substantially includes the same structural elements as those of the above respective examples, except for the structure of the inner casing. Accordingly, the same reference numerals are allocated to the same structural elements as the aforesaid examples, while the description will be mainly made for the differences from the above embodiments.  
       FIG. 15 ( a ) is a schematic side view of an inner casing  96  disposed in the pressure vessel (not illustrated), in which three inner casings  96  are connected to each other and disposed in the pressure vessel.  FIG. 15 ( b ) is a schematic side view of the membrane separation element groups  90  disposed in the inner casings  96 .  FIG. 15 ( c ) is a view as viewed from the arrow C in  FIG. 15 ( a ).  
      As illustrated in  FIG. 15 , in this example, the inner casings  96  each are made up of an upper half  94  and a lower half  95 , and designed to be able to accommodate eight membrane separation element groups  90 . Specifically, as illustrated in  FIG. 15 ( b ), a first membrane-separation-element block  90 X and a second membrane-separation-element block  90 Y, each block comprising four membrane separation element groups  90 , are provided and are disposed orthogonal to each other in each inner casing  96 .  
      The inner casing  96  has permeate discharge passages  23 X formed in communication with the through-holes  11  of the first membrane-separation-element block  90 X and permeate discharge passages  23 Y formed in communication with the through-holes  11  of the second membrane-separation-element block  90 Y.  
      According to the example of  FIG. 15 , the plural (eight) membrane separation element groups  90  are disposed in each inner casing  96  so that a larger volume can be treated relative to the installation area.  
      Also, according to this example, it is so structured as to have in each inner casing  96  the first membrane-separation-element block  90 X disposed substantially orthogonal to the second membrane-separation-element block  90 Y. This arrangement can also produce the same effects of the example of  FIG. 14 . The basic structural elements, which are the same as those of  FIG. 3  and the like, also produce the same effects.  
       FIG. 16  are schematic views of the membrane separation apparatus according to still another example of the present invention, in which  FIG. 16 ( a ) is a schematic cross sectional view of the membrane separation apparatus according to another example of the present invention;  FIG. 16 ( b ) is a schematic cross sectional view as viewed in the direction of arrows B-B in  FIG. 16 ( a ); and  FIG. 16 ( c ) is a schematic cross sectional view as viewed in the direction of arrows C-C in  FIG. 16 ( a ).  
      The membrane separation apparatus of this example is substantially made up of the same structural elements as those of the aforesaid examples, except that while the inner casings are disposed in the pressure vessel in the aforesaid examples, the membrane separation apparatus of this example is provided with no inner casings illustrated in the respective examples. Accordingly, the same reference numerals are allocated to the same structural elements of the aforesaid examples for omission of the description on them and the following description will be mainly made for the characteristic portions (the differences from the above examples) of this example.  
      As illustrated in  FIG. 16 , the membrane separation apparatus of this example includes a pressure vessel  103  having a rectangular tube shape, water collecting members  106  having a plate-like shape provided in the pressure vessel  103 , two membrane separation element groups  90  held between a pair of the water collecting members  106 , and the like. Herein, the membrane separation element groups  90  each having a size of, for example, 100 mm×100 mm×200 mm, are used.  
      The membrane separation element groups  90  each are made up of a stack of the plural membrane separation elements  9  and the spacers  10  disposed between the adjacent membrane separation elements  9 . The through-holes  11  are formed in each membrane separation element group  90  so as to extend through the membrane separation elements  9  and the spacers  10  from a first side to a second side, of the stack. The pipes  12  are inserted in the through-holes  11  and each pipe have the four permeate discharge grooves  13  along the lengthwise direction thereof (see  FIG. 5 ( a )).  
      The pressure vessel  103  of the membrane separation apparatus of this example has flanges  121 ,  122  respectively provided on the sides of the water inlet  1  and the concentrate outlet  2 . Disposed respectively on the sides of the water inlet  1  and concentrate outlet  2  are an inlet-side joint  130  and an outlet-side joint respectively having flanges  131 ,  142  designed to be connectable respectively to the flanges  121 ,  122 . The flanges  121 ,  122  provided in the pressure vessel  103  are connected to the flanges  131 ,  142  provided in the joints  130 ,  140  by using fastening means comprising such as bolts and nuts. The flange  142  of the outlet-side joint  140  has permeate discharge passages  143  in communication with permeate discharge passages (later described) formed in the water collecting members  106 .  
      In this example, as illustrated in  FIG. 16 , the membrane separation apparatus is made up of the membrane separation element groups  90 , each comprising the stack of the membrane separation elements  9 , the spacers  10  and the like, and the plate-like water collecting members  106  which hold the membrane separation element groups  90  together. More specifically, the membrane separation element groups  90  are held between a pair of the water collecting members  106  that are in turn fastened with plural fastening bands  116  (corresponding to “fastening means” in the present invention). The membrane separation element groups  90  which have been thus fastened together by the pair of the water collecting members  106  and the fastening bands  116  are tightly held in the pressure vessel  103  of the rectangular tube shape, thus making up the membrane separation apparatus.  
      In the membrane separation apparatus of this example, the membrane separation element groups  90 , each held between the pair of the water collecting members  106 , are each fastened together by four fastening bands  116 . These fastening bands  116  are provided at positions corresponding to the positions of the pipes  12  for the purpose of preventing leakage between the membrane separation element groups  90  and the pipes  12 .  
      The water collecting members  106 , which are designed to hold the membrane separation element groups  90  therebetween, each have mounting grooves  106   a  for mounting the fastening bands  116 . In this example, it is possible to dispose the water collecting members  106  and the membrane separation element groups  90  in the pressure vessel  103  with the fastening bands  116  mounted in the mounting grooves  106   a . These fastening bands  116  are made by using stainless steel, plastic, rubber or the like.  
      In this example, the membrane separation element groups  90  are disposed so as to have substantially no space between the adjacent membrane separation element groups  90 , in the same manner as the aforesaid examples.  
      The thus arranged membrane separation apparatus of this example produces the following effects in addition to the effects of the examples illustrated in  FIG. 3  and the like.  
      In this example, the water collecting members  106  are provided in place of an inner casing of the other examples that holds the membrane separation element groups and has the permeate discharge passages. The water collecting members  106 , in the same manner as the inner casing, have the permeate discharge passages  23  and serve to hold the membrane separation element groups  90 , but have a different structure from the inner casing. Specifically, the inner casing is of a cylindrical shape for accommodating the membrane separation element group; on the other hand, the water collecting members  106  each are of a plate-like shape. Also, as illustrated in  FIG. 16 , in this example, each membrane separation element group  90  is held between the pair of the water collecting members  106 , while the water collecting members  106  and the membrane separation element group  90  are fastened together by using the fastening bands  116 . The water collecting members  106  each have a width substantially equal to the width of the membrane separation element group  90 , as illustrated in  FIG. 16 ( c ). The fastening bands  116  fasten the pair of the water collecting members  106  in contact with end portions of the water collecting members  106  and the membrane separation element group  90  so as to fasten the membrane separation element group  90  held between the water collecting members  106 .  
      As described above, according to this example, with the membrane separation element group  90  held in position, only the thin-profile fastening bands exist on the lateral sides. On the other hand, in the aforesaid examples, the cylindrical inner casing is used and therefore there exists, on the lateral sides, a thickness substantially equal to the thickness above and below the membrane separation element group.  
      Therefore, according to this example, it is possible to make up the membrane separation apparatus without providing useless spaces on the lateral sides of the membrane separation element group  90  by using the plate-like water collecting members  106 . The pressure vessel  103  can be reduced in size to an extent equivalent to the omission of this useless space. Accordingly, with the arrangement of this example, it is possible to reduce the installation area by about 30% compared with a case where an apparatus having the same treatment capability is installed. It is a matter of course to achieve the cost-down by the reduction of the installation area. The water collecting members  106 , which are made of plate-like members, can be easily produced, thereby achieving the cost-down as well as ease of fabrication.  
      In this example, as illustrated in  FIG. 16 , the pressure vessel  103  is formed into a rectangular tube shape for matching in shape to the water collecting members  106  as well as the membrane separation element groups  90  in order to omit a dead space. This allows the overall shape of the membrane separation apparatus to be a square shape.  
      Accordingly, in this example, it is possible to easily stack plural membrane separation apparatuses together. That is, in the above examples, the membrane separation apparatus is made up of a cylindrical pressure vessel in order to resist high pressure, and therefore plural apparatuses cannot be simply stacked together but require some supporting members for stacking. According to this example in which the membrane separation apparatus is of a square shape, plural apparatuses can be more easily arranged in a stacked structure.  
      The membrane separation apparatus of this example can solely achieve the reduction of the installation area, as described above, and also when plural apparatuses are to be arranged in a stacked structure, supporting members to be equipped to the apparatuses can be omitted. As a result, the installation area can be reduced with a higher level compared to the case where a single membrane separation apparatus is solely arranged.  
      Where the pressure vessel  103  of the rectangular tube shape is to be used, it is preferable to use as a permeable membrane a membrane other than a reverse osmosis membrane in light of the balance between the manufacturing cost and pressure withstanding capability of the pressure vessel.  
      In  FIG. 16 , the pressure vessel  103  is provided with the flanges  121 ,  122 . The present invention is not necessarily limited to this arrangement. According to needs and circumstances, the pressure vessel  103  of each membrane separation apparatus may not be provided with the flanges  121 ,  122 . Therefore, it is possible to employ for example an arrangement in which a predetermined number of the membrane separation apparatuses each having no flanges in the pressure vessel  103  are stacked together to form a block of the membrane separation apparatuses, each block being thus provided with flanges, joints and the like.  
      With the above arrangement, it is possible to further reduce the installation area in a case where the membrane separation apparatuses are stacked together.  
      For the example of  FIG. 16 , the description was made for the case where the water collecting members each are formed into a plate-like shape. The present invention is not necessarily limited to this arrangement. As long as the water collecting members have the permeate discharge passages and are able to hold the membrane separation element group therebetween, their shapes and arrangements are not limited to specific ones. Therefore, the water collecting members may be formed by using, for example, pipe members having a circular or elliptical cross section. Although the description was made for the pressure vessel having a rectangular cross section, the present invention is not limited thereto. The vessel having a square cross section may be used.  
      Although  FIG. 16  illustrate an arrangement with no flow-regulating plate, the present invention is not necessarily limited to this arrangement. Still in the membrane separation apparatus having an arrangement as illustrated in  FIG. 16 , it is preferable to provide the flow-regulating plate in the water inlet  1  according to needs and circumstances.  
      Further, the adjacent membrane separation element groups  90  may be disposed substantially orthogonal to each other in the same manner as the example of  FIG. 14 , according to needs and circumstances.  
      While the above descriptions were made by taking for example the case where the flowing direction of water to be treated is identical to the flowing direction of permeate, the present invention is not necessarily limited thereto. For example, the water inlet  1  and the concentrate outlet  2  may be reversely located, through which water to be treated flows. That is, water to be treated may flow into the concentrate outlet  2  of the above examples and concentrate flows out of the water inlet  1  of the above examples. Also, flow-regulating plates may be respectively provided both in the water inlet  1  and the concentrate outlet  2 , in which the direction of water flow is changed every certain period of time.  
      In the above examples, specific reference was not made to the material of the pressure vessel  3 ,  103 . In the present invention, for example, metal such as stainless steel and carbon steel, or resin or the like designed to be capable of resisting pressure may be used. In case of using a metallic material, the pressure vessel  3 ,  103  may be formed by applying such as resin lining to the inner side thereof.  
      The present invention thus arranged produces the following effects.  
      (1) According to the invention recited in claims  1 ,  4 ,  9 ,  15 ,  16  and  17 , it is possible to provide a membrane separation apparatus and a membrane separation process that are unlikely to cause membrane fouling and plugging, capable of achieving membrane separation even when water to be treated flows at a relatively low rate, have an excellent membrane packing density, as well as preventing foreign components from being deposited in the apparatus. Specifically, according to the invention recited in claims  9  and  17 , since the water collecting members made from a plate like member are used in place of the inner casing, it is possible to provide the membrane separation apparatus of a compact size and the membrane separation process using the same, which also produce the above effects.  
      (2) According to the invention recited in claims  2  and  5 , it is possible to shorten the overall length of the pressure vessel and thus make the vessel compact, while limiting the deposition of suspended solid in the pressure vessel.  
      (3) According to the invention recited in claims  3  and  12 , it is possible to reduce the pressure loss of permeate within the membrane separation elements. Also, according to the invention recited in claim  13 , further reduction of the pressure loss can be achieved.  
      (4) According to the invention recited in claim  6 , it is possible to treat water in larger volume with respect to an installation area than ever before.  
      (5) According to the invention recited in claim  7 , it is possible to limit deposition of suspended solid or the like on membrane surfaces (the surfaces of the permeable membranes) of the membrane separation elements, since the adjacent membrane separation element groups are disposed at a certain angle relative to each other, thus making it easy to cause turbulent flow. Particularly, according to the invention recited in claim  8 , the membrane separation element groups are disposed substantially orthogonal to each other, which produces a maximum effect in causing turbulent flow of water. Hence, it is possible to more effectively limit deposition of suspended solid or the like on the membrane surfaces of the membrane separation elements. Further, according to the invention recited in claim  14 , it is possible to limit deposition of suspended solid or the like on the membrane surfaces of the membrane separation elements in the manner as above, since turbulent flow of water is easy to occur between the flow-regulating plate and the membrane separation element group.  
      (6) According to the invention recited in claim  18 , it is possible to provide the membrane separation process which is unlikely to cause membrane fouling and plugging of the membrane separation elements.  
      (7) According to the invention recited in claim  19 , it is possible to provide an effective membrane washing process.