Abstract:
Operation of a membrane filter includes the step of selectively supplying gas into the compartments of a membrane unit  50  composed of an array of membrane elements  51  disposed within a treatment tank  31 . A skirt element  71  is disposed at a bottom portion of the membrane unit and an aerator  61  is disposed under the skirt. A partition is also disposed at the bottom of the membrane unit forming compartments within the shirt element. The gas bubbles discharged from the aerator increase their rising force upon entry into gaps between the membranes. Because of the arrangement of the partition within the skirt element the flow rate of bubbles along the opposite side edge portions of the membrane unit can be increased having the advantage of cleaning the entire surface of each membrane, thereby preveting clogging by sludge, SS, colloid, etc. within the gaps between the membranes. Therefore, filtration can be maintained over a longer period of time and the power required for filtration can be reduced, as well as, the frequency of manual periodic cleaning, chemical cleaning, etc.

Description:
This is a division of application No. 09/207,336 filed Dec. 8, 1998, now U.S. Pat No 6,284,135. The disclosure of the prior application(s) is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an immersion type membrane filter apparatus and a method for operating the membrane filter apparatus. 
     2. Description of the Related Art 
     Conventionally, solid-liquid separation has been performed in many instances of waste water treatment, collection of valuable substances in water, and like treatments. A solid-liquid separation method employs a membrane filter apparatus which uses a membrane unit composed of a plurality of membrane elements, such as precision filtration membranes or ultrafiltration membranes. In this case, in the membrane element, pressure is applied on the side of water to be treated, or a negative pressure is generated on the side of treated water with respect to each membrane, so that only water permeates through the membranes. 
     However, in the membrane element, adhesion to the membrane surface of solid matter such as suspended solid causes generation of filtration resistance in addition to resistance intrinsic to the membrane material itself. As adhesion of solid matter proceeds, the associated filtration resistance increases, impairing permeability of the membrane element. In the case of fixed-rate filtration, the pressure difference between untreated water and treated water, i.e., the differential filtration pressure, increases. As a result, energy required for filtration increases. In the case of fixed-pressure filtration, the water permeation rate, i.e., the rate of water permeating through the membrane, decreases. Thus, there is provided a membrane filter apparatus in which the velocity of water flowing along membrane surfaces is increased, or in which the membrane surfaces are cleaned by means of, for example, sponge balls or carriers, thereby minimizing adhesion of solid matter to the membrane surfaces. 
     In the case of an immersion-type membrane filter apparatus, in which a membrane unit is immersed in water, an aerator is disposed under the membrane element so as to discharge air for aeration of membranes. An air lift action of the discharged air causes a shear force to be applied to membrane surfaces, thereby cleaning the membrane surfaces by means of a mixed flow of air and water (refer to Japanese Patent Publication No. 4-70958). 
     To uniformly and efficiently supply air discharged from the aerator into gaps between membranes, the membrane unit is disposed within an enclosure, which is open upward and downward, and the aerator is disposed within the enclosure at a lower portion thereof (refer to Japanese Patent Publication No. 7-20592). Alternatively, the membrane unit is disposed within the enclosure, which is open upward and downward, the aerator is disposed within the enclosure at a lower portion thereof, and flow-smoothing means is disposed between the membrane unit and the aerator (refer to Japanese Patent Laid-Open (kokai) No. 8-281080) or a skirt member is disposed at the lower end of the membrane unit (refer to Japanese Patent Laid-Open (kokai) No. 8-281083). 
     The conventional immersion-type membrane filter apparatus can uniformly supply air discharged from the aerator into each gap between membranes, but involves a problem that, as air rises within each gap between membranes, a flow of bubbles is gradually biased toward a widthwise central portion of the membrane surface. 
     FIG. 1 shows a schematic view of a conventional immersion-type membrane filter apparatus. 
     In FIG. 1, numeral  10  denotes a treatment tank for accommodating water to be treated, which is supplied thereinto through a line L 1 ; numeral  11  denotes a membrane element; numeral  12  denotes a membrane; numeral  13  denotes a frame; numeral  14  denotes a water manifold nozzle attached to the top end of the frame  13 ; and a line L 2  for discharging treated water is connected to the water manifold nozzle  14 . A pump P is disposed in the line L 2  in order to pump out treated water. A plurality of membrane elements  11  are arrayed adjacent to each other to constitute a membrane unit. 
     An aerator  15  is disposed under the membrane unit for cleaning the surfaces of the membranes  12 , and is connected to an unillustrated air source through a line L 3 . Air discharged from the aerator  15  is supplied, in the form of bubbles  16 , to the membrane element  11  uniformly along an entire bottom end S 1  thereof. The bubbles  16 , together with water, rise within each gap between the membranes  12 . To guide air discharged from the aerator  15  upward, a skirt element  17  is disposed between the membrane unit and the aerator  15 . 
     Since the membrane elements  11  extend along a predetermined length within the treatment tank  10 , the opposite side edge portions of the frame  13  and water to be treated present in the vicinity of the edge portions produce resistance to the bubbles  16  which are rising within each gap between the membranes  12 . Accordingly, as the bubbles  16  rise within each gap between the membranes  12 , the bubbles  16  are gradually biased toward a widthwise central portion of the surface of the membrane  12 . As a result, the amount of the bubbles  16  is reduced at the opposite side portions of a top end S 2  of each membrane element  11 . That is, the bubbles  16  flow at a relatively high rate in a trapezoidal region AR 1  and at a relatively low rate in triangular regions AR 2  and AR 3 . Consequently, sludge is removed by action of the bubbles  16  from the surface of the membrane  12  in the region AR 1 , whereas sludge tends to adhere to the surface of the membrane  12  in the regions AR 2  and AR 3  due to impairment in the cleaning effect of the bubbles  16 , thus failing to clean the entire surface of the membrane  12 . Further, the gaps between the membranes  12  are clogged with sludge, SS, colloid, or a like substance, resulting in a failure to maintain good filtration over a long period of time. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to solve the above-mentioned problems in the conventional membrane filter apparatus and to provide a membrane filter apparatus capable of cleaning the entire surface of a membrane and maintaining good filtration over a long period of time, as well as to provide a method for operating the membrane filter apparatus. 
     To achieve the above object, the present invention provides a membrane filter apparatus comprising a treatment tank, a membrane unit, a tubular (rectangular-tube-like) skirt element, an aerator, a partition, and gas supply means. The membrane unit is disposed within the treatment tank and is composed of an array of membrane elements. The skirt element is disposed at a bottom portion of the membrane unit. The aerator is disposed under the skirt element for the purpose of discharging gas. The partition is disposed at a bottom portion of the membrane unit and, together with the skirt element, defines a compartment within the skirt element. The gas supply means supplies gas into the compartment. 
     The gas discharged from the aerator becomes bubbles. The bubbles are supplied into and rise within the compartment, thereby increasing their rising force upon entry into gaps between membranes. 
     Since the gas is supplied into the compartment by the gas supply means, the flow rate of bubbles along the opposite side edge portions of the membrane unit can be increased while the bubbles are flowing upward within the gaps between the membranes. 
     Accordingly, the entire surface of each membrane can be cleaned, thereby preventing clogging by sludge, SS, colloid, or a like substance within each gap between the membranes and thus preventing an increase in filtration resistance. Therefore, good filtration can be maintained over a long period of time. 
     Further, power required for filtration can be reduced, and the frequency of manual periodic cleaning, chemical cleaning, or like cleaning can be decreased. 
     Preferably, the skirt element has a rectangular cross section, and the compartment is formed along two walls of the skirt element. 
     Preferably, the skirt element has a rectangular cross section, and the compartment is formed along four walls of the skirt element. 
     Preferably, a plurality of the compartments are formed. 
     Preferably, the gas supply means supplies the gas into the compartments independently of each other. 
     The present invention provides another membrane filter apparatus comprising a treatment tank, a membrane unit, a tubular skirt element, an aerator, and a partition. The membrane unit is disposed within the treatment tank and is composed of an array of membrane elements. The skirt element is disposed at a bottom portion of the membrane unit. The aerator is disposed under the skirt element for the purpose of discharging gas. The partition is disposed at a bottom portion of the membrane unit and, together with the skirt element, defines a compartment within the skirt element. 
     The partition is slanted such that its bottom end is biased toward the center of the skirt element. 
     In this case, the compartment narrows upward, thereby increasing the rising velocity of bubbles and water to be treated within the compartment. 
     Also, when gas is discharged from the aerator, more bubbles are supplied into the compartment. Accordingly, more bubbles flow upward along the opposite side edge portions of the surface of each membrane, thereby sufficiently cleaning the surface of the membrane. 
     Preferably, the membrane filter apparatus further comprises gas supply means for supplying gas into the compartment. 
     In this case, since the gas supply means supplies the gas into the compartment, the flow rate of bubbles along the opposite side edge portions of the membrane unit can be further increased while the bubbles are flowing upward within the gaps between the membranes. 
     The present invention provides a method for operating a membrane filter apparatus, which comprises a treatment tank; a membrane unit disposed within the treatment tank and composed of an array of membrane elements; a skirt element disposed so as to extend from a bottom portion of the membrane unit and divided into a plurality of compartments; and an aerator disposed under the skirt element. 
     Gas is selectively supplied into the compartments. 
     The gas discharged from the aerator is supplied into and rise within the compartments, thereby increasing a rising force of bubbles upon entry into gaps between membranes. 
     Also, since the gas is supplied into the compartments, the flow rate of bubbles along the opposite side edge portions of the membrane unit can be increased while the bubbles are flowing upward within the gaps between the membranes. 
     Accordingly, the entire surface of each membrane can be cleaned, thereby preventing clogging by sludge, SS, colloid, or a like substance within the gaps between the membranes and thus preventing an increase in filtration resistance. Therefore, good filtration can be maintained over a long period of time. 
     Further, power required for filtration can be reduced, and the frequency of manual periodic cleaning, chemical cleaning, or like cleaning can be decreased. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The structure and features of a membrane filter apparatus and a method for operating the membrane filter apparatus according to the present invention will be readily appreciated as the same becomes better understood by referring to the drawings, in which: 
     FIG. 1 is a schematic view of a conventional immersion-type membrane filter apparatus; 
     FIG. 2 is a schematic view of a membrane filter apparatus according to a first embodiment of the present invention; 
     FIG. 3 is a perspective view of the membrane filter apparatus according to the first embodiment; 
     FIG. 4 is a perspective view of a membrane unit according to the first embodiment; 
     FIG. 5 is a perspective view of a main portion of a membrane element according to the first embodiment; 
     FIG. 6 is a view illustrating a first operation mode of the membrane filter apparatus according to the first embodiment; 
     FIG. 7 is a view illustrating a second operation mode of the membrane filter apparatus according to the first embodiment; 
     FIG. 8 is a view illustrating a third operation mode of the membrane filter apparatus according to the first embodiment; 
     FIG. 9 is a view illustrating a fourth operation mode of the membrane filter apparatus according to the first embodiment; 
     FIG. 10 is a plan view of a partition unit according to a second embodiment of the present invention; 
     FIG. 11 is a perspective view of the partition unit according to the second embodiment; 
     FIG. 12 is a plan view of a partition unit according to a third embodiment of the present invention; 
     FIG. 13 is a perspective view of the partition unit according to the third embodiment; 
     FIG. 14 is a schematic view of a membrane filter apparatus according to a fourth embodiment of the present invention; 
     FIG. 15 is a perspective view of a main portion of the membrane filter apparatus according to the fourth embodiment; and 
     FIG. 16 is a perspective view of a main portion of a membrane element according to a fifth embodiment of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Embodiments of the present invention will next be described in detail with reference to the drawings. 
     FIG. 2 shows a schematic view of a membrane filter apparatus according to a first embodiment of the present invention. FIG. 3 shows a perspective view of the membrane filter apparatus according to the first embodiment. FIG. 4 shows a perspective view of a membrane unit according to the first embodiment. FIG. 5 shows a perspective view of a main portion of a membrane element according to the first embodiment. 
     In FIGS. 2 to  5 , numeral  31  denotes a treatment tank for accommodating water to be treated which is supplied through a line L 1 . Numeral  50  denotes a membrane unit immersed in water to be treated which is contained in the treatment tank  31 . The membrane unit  50  includes a plurality of flat membrane elements  51  which are arrayed at predetermined intervals such that membrane surfaces  52  extend vertically as shown in FIG.  3 . The gap between the membrane surfaces  52  is usually 5 to 15 mm. 
     The membrane element  51  includes a membrane, such as a precision filtration membrane or an ultrafiltration membrane, and is equipped with a hollow frame  51   a  along its side edge. Treated water (permeated water), or water which has permeated the membrane, is collected into the frame  51   a . A spacer  51   c  is disposed on the treated-water side of each membrane surface  52 . 
     The top end of each frame  51   a  is connected to a line L 2  via a manifold nozzle  51   b . A pump P 1  is disposed in the line L 2 . The pump P 1  is driven under control of an unillustrated controller so as to generate a negative pressure within each membrane element  51 , thereby withdrawing water into the membrane element  51 . The filtration rate of water or the permeation rate of water is regulated through adjustment of the rotational speed of the pump P 1  under control of the controller. 
     In the thus-configured membrane filter apparatus, when water to be treated is supplied thereinto through the line L 1 , and the pump P 1  is driven, only water permeates through the membrane of the membrane element  51 . In this case, when unillustrated solid matter such as suspended matter adheres to the membrane surface  52 , the adhering solid matter causes generation of filtration resistance in addition to resistance intrinsic to the membrane material itself. As adhesion of solid matter proceeds, the associated filtration resistance increases, impairing the permeability of the membrane element  51 . 
     Thus, a mixed flow of gas and water is adapted to sweep and thereby clean the membrane surface  52 , thereby minimizing adhesion of solid matter onto the membrane surface  52 . 
     To implement the above cleaning of the membrane surfaces  52 , an aerator  61  is disposed under the membrane unit  50 . The aerator  61  is connected to an unillustrated air source through a line L 3 . A regulating valve  63  is disposed in the line L 3 . The controller regulates the opening of the regulating valve  63  in order to regulate the discharge rate of air, which serves as gas of the present invention, from the aerator  61 , thereby regulating the flow rate of a mixture of air and water along the membrane surfaces  52 . In the present embodiment, the supply rate of air into each gap between the membrane surfaces  52  as represented by linear velocity (LV) is 0.05 to 3 m/min. 
     A skirt element  71  in the form of an open-ended rectangular enclosure is disposed between the membrane unit  50  and the aerator  61  in order to guide and smooth a flow of air discharged from the aerator  61 . The skirt element  71  is open upward and downward and is attached to a bottom portion of the membrane unit  50 . The skirt element  71  may assume the form of a cylindrical enclosure, a polygonal enclosure, or any other type of enclosure according to the shape of the membrane unit  50 . 
     During filtration through the membrane element  51  of water to be treated, air discharged from the aerator  61  is supplied, in the form of bubbles  16 , into each gap between the membrane surfaces  52  and uniformly along an entire bottom end S 1  of the membrane element  51 . The bubbles  16 , together with water, rise within each gap between the membrane surfaces  52 . As a result, an air lift action of the discharged air causes a shear force to be applied to the membrane surfaces  52 , thereby cleaning the membrane surfaces  52 . Subsequently, the bubbles  16  are released into the atmosphere from the top of the treatment tank  31 , whereas water to be treated is circulated downward along walls of the treatment tank  31 . 
     Since the membrane elements  51  extend along a predetermined length within the treatment tank  31 , the frames  51   a , or the opposite side edge portions of the membrane elements  51 , and water to be treated present in the vicinity of the frames  51   a  are resistant to the bubbles  16  which, together with water to be treated, are rising within each gap between the membrane surfaces  52 . Accordingly, as the bubbles  16  rise within each gap between the membrane surfaces  52 , the bubbles  16  tend to be gradually biased toward the widthwise central portion of each membrane surfaces  52 . 
     To cope with the biasing tendency of the bubbles  16 , partitions  72  and  73  are disposed within a space defined by the skirt element  71  at two widthwise opposite positions of the membrane unit  50  such that they extend along the longitudinal direction of the membrane unit  50 , i.e., along the arraying direction of the membrane elements  51 . The partitions  72  and  73  are in parallel with side walls  71   a  and  71   b  of the skirt element  71 . The partitions extend downwardly from the membrane unit  50  a major portion of the distance between the ends of the skirt element  71 . Thus, the partitions  72  and  73  and the side walls  71   a  and  71   b  define compartments  74  and  75 , respectively, while the partitions  72  and  73  define a compartment  76 . Numeral  79  denotes a base frame. The skirt element  71  and the partitions  72  and  73  constitute a partition unit. 
     The number of compartments can be increased as needed by increasing the number of partitions. However, an increase in the number of compartments complexes the structure of the membrane filter apparatus. To maintain good state of mixing of air and water to be treated and good fluidity of the mixture, the partitions  72  and  73  are preferably disposed symmetrically with respect to the center of the membrane unit  50 . 
     The gap between the skirt element  71  and the partitions  72  and  73  is determined according to the size of the membrane element  51  and the membrane unit  50 . For example, for the membrane unit  50  having a width of about 1 m, the gap is set to 1 to 10 cm. 
     An unillustrated air source is connected to the compartments  74  and  75  through lines L 4  and L 5 , respectively, so that air can be supplied into the compartments  74  and  75  independently of each other. Regulating valves  77  and  78  are disposed in the lines L 4  and L 5 , respectively. The controller can regulate the opening of the regulating valves  77  and  78  to thereby regulate the rate of air supplied into the compartments  74  and  75 . Gas other than air can be supplied into the compartments  74  and  75 . The lines L 4  and L 5 , the regulating valves  77  and  78 , and the air source constitute the gas supply means of the invention. 
     The lines L 4  and L 5  may be disposed independently of the line L 3  or may be permitted to branch off from the line L 3 . Further, another aerator may be disposed under or within each of the compartments  74  and  75  and be connected to each of the lines L 4  and L 5 . 
     Since, in the internal space of the skirt element  71 , the compartments  74  and  75  are formed along the side walls of the skirt element  71  while the compartment  76  is interposed between the compartments  74  and  75 , air discharged from the aerator  61  rises, in the form of the bubbles  16 , within the compartments  74  to  76  while being guided by the partitions  72  and  73 . Thus, when the bubbles  16  leave the compartments  74  and  75  and enter each gap between the membrane surfaces  52 , a rising force of the bubbles  16  can be increased, thereby preventing the potential problem that, as the bubbles  16  rise within each gap between the membrane surfaces  52 , the bubbles  16  are gradually biased toward a widthwise central portion of the membrane surface  52 . The partitions  72  and  73  are disposed in close contact with a bottom portion of the membrane unit  50  in order to prevent the bubbles  16  from communicating between the compartment  76  and the compartments  74  and  75 . 
     Since air is supplied into the compartments  74  and  75 , the flow rate of the bubbles  16  along the opposite side edge portions of the membrane element  51  can be increased while the bubbles  16  are flowing upward within each gap between the membrane surfaces  52 . 
     Accordingly, the entire membrane surface  52  can be cleaned, thereby preventing clogging by sludge, SS, colloid, or a like substance within each gap between the membrane surfaces  52  and thus preventing an increase in filtration resistance. Therefore, good filtration can be maintained over a long period of time. 
     Further, power required for filtration can be reduced, and the frequency of manual periodic cleaning, chemical cleaning, or like cleaning can be decreased. 
     Since the membrane unit  50  does not need to be accommodated within an enclosure, the size of the membrane filter apparatus can be reduced. Also, the structure of the membrane filter apparatus can be simplified. 
     Next, a method for operating the above-described membrane filter apparatus will be described. 
     FIG. 6 illustrates a first operation mode of the membrane filter apparatus according to the first embodiment. FIG. 7 illustrates a second operation mode of the membrane filter apparatus according to the first embodiment. FIG. 8 illustrates a third operation mode of the membrane filter apparatus according to the first embodiment. FIG. 9 illustrates a fourth operation mode of the membrane filter apparatus according to the first embodiment. 
     When the regulating valves  63 ,  77 , and  78  are opened to thereby supply air into the compartments  74  to  76 , the bubbles  16  flow upward over the entire membrane surface  52  as shown in FIG.  6 . When the regulating valve  63  is closed and the regulating valves  77  and  78  are opened, to thereby supply air into only the compartments  74  and  75 , the bubbles  16  flow upward mainly along the opposite side edge portions of the membrane surface  52  as shown in FIG.  7 . When the regulating valves  63  and  77  are closed and the regulating valve  78  is opened, to thereby supply air into only the compartment  75 , the bubbles  16  flow upward mainly along one side edge portion of the membrane surface  52  as shown in FIG.  8 . When the regulating valves  77  and  78  are closed and the regulating valve  63  is opened, to thereby supply air into the compartments  74  to  76 , the bubbles  16  flow upward mainly along a widthwise central portion of the membrane surface  52  as shown in FIG.  9 . 
     Accordingly, through combination of the above operation modes, any portion of the membrane surface  52  can be cleaned. 
     For example, in continuous cleaning, the regulating valves  63 ,  77 , and  78  are continuously opened. In combination of continuous cleaning and additional intermittent cleaning, the regulating valve  63  is continuously opened, and the regulating valves  77  and  78  are intermittently opened. In this case, the regulating valves  77  and  78  may be opened or closed concurrently or independently of each other. 
     In periodical intermittent cleaning, while the regulating valve  63  is temporarily closed, at least either the regulating valve  77  or  78  is opened. In this case, discharge of air from the aerator  61  is temporarily halted; thus, cleaning of a central portion of the membrane unit  50  becomes insufficient. Therefore, this intermittent cleaning is preferably completed within a short period of time. For example, the regulating valve  63  is closed for a time of 5 seconds to 5 minutes in a single cycle of intermittent cleaning, and this intermittent cleaning is performed 1 to 1500 cycles per day. Symbols L 3  to L 5  denote air lines, and symbol P 1  denotes a pump. 
     Next, a second embodiment of the present invention will be described. 
     FIG. 10 shows a plan view of a partition unit according to a second embodiment of the present invention. FIG. 11 shows a perspective view of the partition unit according to the second embodiment. 
     In the second embodiment, partitions  101  and  102  are disposed within a space defined by the skirt element  71  at two widthwise opposite positions of the membrane unit  50  (FIG. 4) such that they extend along the arraying direction of the membrane elements  51 . The partitions  101  and  102  are in parallel with side walls  71   a  and  71   b  of the skirt element  71 . Thus, the partitions  101  and  102  and the side walls  71   a  and  71   b  define compartments  106  and  107 , respectively, in a symmetric manner with respect to the center of the partition unit. 
     Partitions  103  and  104  are disposed within a space defined by the partitions  101  and  102  at two opposite positions located in the arraying direction of the membrane elements  51  in such a manner as to extend in the width direction of the membrane unit  50 . The partitions  103  and  104  are in parallel with side walls  71   c  and  71   d  of the skirt element  71 . Thus, the partitions  103  and  104  and the side walls  71   c  and  71   d  define compartments  108  and  109  symmetrically with respect to the center of the partition unit. 
     Thus, the compartments  106  to  109  are formed along four walls of the skirt element  71 , whereas the compartment  105  is defined by the partitions  101  to  104 . 
     Accordingly, not only are the membrane surfaces  52  sufficiently cleaned at their opposite side edge portions as viewed in the width direction of the membrane unit  50 , but also the membrane surfaces  52  located at opposite end portions of an array of the membrane elements  51  can be sufficiently cleaned. 
     Next, a third embodiment of the present invention will be described. 
     FIG. 12 shows a plan view of a partition unit according to a third embodiment of the present invention. FIG. 13 shows a perspective view of the partition unit according to the third embodiment. 
     A partition  111  is disposed within a space defined by the skirt element  71  in parallel with the side walls  71   a  to  71   d  of the skirt element  71 . The partition  111  and the four side walls  71   a  to  71   d  of the skirt element  71  define a compartment  112  symmetrically with respect to the center of the partition unit. The partition  111  also defines a compartment  113  inside. 
     Accordingly, not only are the membrane surfaces  52  sufficiently cleaned at their opposite side edge portions as viewed in the width direction of the membrane unit  50  (FIG.  4 ), but also the membrane surfaces  52  located at opposite end portions of an array of the membrane elements  51  can be sufficiently cleaned. 
     Next, a fourth embodiment of the present invention will be described. 
     FIG. 14 shows a schematic view of a membrane filter apparatus according to a fourth embodiment of the present invention. FIG. 15 shows a perspective view of a main portion of the membrane filter apparatus according to the fourth embodiment. The same features as those of the first embodiment are denoted by common numerals, and their description will be omitted. 
     Partitions  172  and  173  are disposed within a space defined by the skirt element  71  at two widthwise opposite positions of the membrane unit  50  such that they extend along the arraying direction of the membrane elements  51  (FIG.  4 ). The partitions  172  and  173  are slanted with respect to the side walls  71   a  and  71   b  of the skirt element  71 . The partitions  172  and  173  and the side walls  71   a  and  71   b  define compartments  174  and  175 , respectively, whereas the partitions  172  and  173  define a compartment  176 . 
     Since the partitions  172  and  173  are slanted such that their bottom ends are biased toward the center of the skirt element  71 , the compartments  174  and  175  narrow upward, thereby increasing the rising velocity of the bubbles  16  and water to be treated within the compartments  174  and  175 . 
     Also, when gas is discharged from the aerator  61 , more bubbles  16  are supplied into the compartments  174  and  175 . Accordingly, more bubbles  16  flow upward along the opposite side edge portions of the membrane surface  52 , thereby sufficiently cleaning the entire membrane surface  52 . In this case, the regulating valves  77  and  78  may be closed. 
     Next, a fifth embodiment of the present invention will be described. 
     FIG. 16 shows a perspective view of a main portion of a membrane element according to the fifth embodiment of the present invention. 
     In FIG. 16, numeral  151  denotes a membrane element in a flat modular form made of a hollow yarn membrane or tubular membrane; numeral  151   a  denotes a frame; numeral  151   b  denotes a manifold nozzle formed at the top end of the frame  151   a ; and numeral  152  denotes the surface of a hollow yarn membrane. 
     The present invention is not limited to the above-described embodiments. Numerous modifications and variations of the present invention are possible in light of the spirit of the present invention, and they are not excluded from the scope of the present invention.