Patent Publication Number: US-2019168165-A1

Title: Membrane bioreactor system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of the Korean Patent Application No. 10-2017-0164523 filed on December 1, 2017, which is hereby incorporated by reference for all purposes as if fully set forth herein. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a membrane bioreactor system, and more specifically, to a membrane bioreactor system which does not require a separate site for a membrane separation tank since a filtration apparatus for solid-liquid separation is provided in an aerobic tank and, when cleaning the membrane of the filtration apparatus, makes use of a portion of the fine bubbles supplied in the aerobic tank to increase the dissolved oxygen of the wastewater introduced into the aerobic tank, thereby minimizing the energy consumption while guaranteeing excellent membrane cleaning effect. 
     Discussion of the Related Art 
     A membrane bioreactor system (hereinafter, “MBR system”) is a wastewater treatment system which combines a biological treatment process and a membrane separation process in order to remove contaminants from the wastewater. 
     A conventional MBR system comprises a flow control tank, an anoxic tank, an anaerobic tank, an aerobic tank, and a membrane separation tank. The anoxic tank and anaerobic tank may be omitted when the contaminants to be removed are mainly organic materials. 
     One of drawbacks of such conventional MBR system is that it requires a separate site for the membrane separation tank. 
     In order to solve the problem, it has been suggested, for example by Korean Patent No. 10-0422211 (hereinafter, “Prior Art 1”), that the solid-liquid separation should be carried out by submerging a filtration membrane in the aerobic tank rather than by intalling the membrane separation tank at a separate site. 
     As the solid-liquid separation is performed by the filtration membrane, the solids are accumulated on the surface of the filtration membrane, thereby causing a membrane fouling. The membrane fouling decreases a filtration efficiency. Thus, a membrane cleaning should be carried out so as to inhibit the membrane fouling. 
     Generally, there is provided a fine bubble generator in the aerobic tank in order to increase the dissolved oxygen of the wastewater so that the wastewater treatment by microorganism can be performed well. The fine bubbles supplied from the fine bubble generator are required to have a diameter small enough to be able to easily dissolve in water so as to substantially increase the dissolved oxygen of the wastewater. Due to such a small size, the fine bubbles themselves are of little help to the membrane cleaning. 
     In Prior Art 1, in order to inhibit the membrane fouling, the air continuously supplied from a blower is strongly erupted through the holes of a aeration pipe, thereby forming coarse bubbles. While rising in the feed water, the coarse bubbles scrub the membrane surface, thereby removing the solids adhered to the surface. 
     The energy consumption of the blower in the continuous aeration, however, is enormous to such an extent as to account for most of the entire energy consumption of the MBR system. 
     In order to decrease the energy consumption of the blower, alternative methods (e.g., a cyclic aeration) to control the air supply timing of the blower and/or the amount of the air supplied by the blower have been suggested. If average air supply per unit time is excessively reduced only for the energy-saving purpose, the membrane is contaminated by the impurities more quickly and the filtration efficiency drops rapidly. Besides, further energy saving is still required in such alternative methods. 
     Korean Patent Laid-Open Publication No. 10-2017-0121738 (hereinafter, “Prior Art 2”) discloses a MBR system which performs solid-liquid separation by submerging a filtration membrane in an aerobic tank and cleans the membrane by mechanically reciprocating the filtration membrane in the aerobic tank rather than by performing an aeration cleaning using a blower and an aeration pipe. 
     Even though the cleaning method of Prior Art 2 requires relatively low energy consumption compared to that of Prior Art 1, however, it cannot delay the decline of cleaning efficiency as long as required in this field because the cleaning effect thereof is also relatively low. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a MBR system which substantially obviate one or more problems due to limitations and disadvantages of the related art. 
     An aspect of the present invention is to provide a membrane bioreactor system which does not require a separate site for a membrane separation tank since a filtration apparatus for solid-liquid separation is provided in an aerobic tank and, when cleaning the membrane of the filtration apparatus, makes use of a portion of the fine bubbles supplied in the aerobic tank to increase the dissolved oxygen of the wastewater introduced into the aerobic tank, thereby minimizing the energy consumption while guaranteeing excellent membrane cleaning effect. 
     Additional aspects and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     According to one aspect of the present invention, there is provided a MBR system comprising: an aerobic tank configured to receive a wastewater from at least one of a flow control tank, an anoxic tank and an anaerobic tank; a fine bubble generator disposed in the aerobic tank to increase a dissolved oxygen of the wastewater introduced into the aerobic tank; and a filtration apparatus submerged in the wastewater introduced into the aerobic tank, the filtration apparatus disposed directly above the fine bubble generator, wherein the filtration apparatus comprises: a filtration membrane module; and a bubble-converting device disposed below the filtration membrane module and configured to convert a portion of fine bubbles provided by the fine bubble generator into coarse bubbles suitable for filtration membrane cleaning, wherein the bubble-converting device comprises: a case having a lower opening and a collecting space configured to confine the portion of the fine bubbles introduced therein via the lower opening; a main pipe extending downwardly from a circumference of a first hole formed in an upper plate of the case; and a branch pipe extending from a circumference of a second hole formed in a side of the main pipe, and wherein the main pipe is in fluid communication with the collecting space via the branch pipe. 
     A diameter of the first hole may be 10 to 50 mm. 
     The fine bubble generator may be configured to provide the fine bubbles having a diameter of 1 to 3 mm. 
     The filtration apparatus may be disposed opposite a wastewater inlet of the aerobic tank in the aerobic tank. 
     An opening of the branch pipe opposite to the second hole may be positioned closer to the upper plate of the case than the second hole. 
     The opening of the branch pipe may face the upper plate of the case. 
     The MBR system may further comprise a vibration apparatus configured to enable a rectilinear reciprocating movement of the filtration apparatus submerged in the wastewater. 
     The vibration apparatus may comprise: a motor; a rotor rotatable by the motor; a shaft configured to convert a rotary motion of the rotor into a rectilinear reciprocating motion of the filtration apparatus; and a rail configured to guide the rectilinear reciprocating motion of the filtration apparatus. 
     The filtration apparatus may further comprise a frame, and the filtration membrane module and the bubble-converting device may be respectively installed in the frame. 
     The MBR system may further comprise a rod for coupling the bubble-converting device to the frame, the rod configured to enable the bubble-converting device to rotate on an axis of the rod within a predetermined angle range. 
     The frame may comprise a lower horizontal member having a third hole, one end of the rod may be connected to the bubble-converting device, the other end of the rod may be inserted in the third hole of the lower horizontal member, and a lower part of the rod may have a curved cross section so that the rod can rotate in the third hole of the lower horizontal member within the predetermined angle range 
     According to the present invention, a portion of the fine bubbles supplied in an aerobic tank to increase the dissolved oxygen of an wastewater can be used for filtration membrane cleaning. Therefore, the MBR system of the present invention does not require separate aeration instruments such as a blower and an aeration pipe, and thus can minimize the energy consumption for the membrane cleaning without causing deterioration of the filtration efficiency. 
     Furthermore, according to one embodiment of the present invention, the cleaning effect can be maximized by, in addition to the membrane cleaning using the portion of the fine bubbles, vibrating the filtration apparatus when the filtration is performed. 
     It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings: 
         FIG. 1  schematically shows a MBR system according to an embodiment of the present invention; 
         FIG. 2  schematically shows a filtration apparatus according to an embodiment of the present invention; 
         FIG. 3  schematically shows a bubble-converting device according to an embodiment of the present invention; 
         FIG. 4  shows the cross section of the bubble-converting device along the IV-IV′ line of  FIG. 3 ; 
         FIGS. 5(A) to 5(C)  illustrate the operation of a bubble-converting device according to an embodiment of the present invention; 
         FIG. 6  illustrates a vibration apparatus according to an embodiment of the present invention; and 
         FIG. 7  illustrates a way how a lower horizontal member of a frame and a bubble-converting device are coupled to each other according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the exemplary embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
       FIG. 1  schematically shows a MBR system  200  according to an embodiment of the present invention. 
     As illustrated in  FIG. 1 , a MBR system  200  according to an embodiment of the present invention comprises a flow control tank  201 , an anoxic tank  210 , an anaerobic tank  220 , and an aerobic tank  230 . 
     Alternatively, either of the anoxic tank  210  or the anaerobic tank  220  or both of them may be omitted depending on the objects and/or functions of the MBR system  200 . 
     The flow control tank  201  is generally for equalization of flow and water quality and, in some cases, may further function as a bioreactor. 
     In the anoxic tank  210 , nitrous acid and/or nitric acid are/is reduced into nitrogen gas by denitrifying microbes and removed. 
     In the anaerobic tank  220 , organic materials are decomposed into methane gas and/or carbon dioxide by anaerobes and removed, and phosphorus accumulating bacterium releases phosphorus while performing intracellular synthesis of organic materials. 
     In the aerobic tank  230 , organic materials are decomposed into carbon dioxide and water by aerobes, nitrification of ammoniac nitrogen into nitrous acid and/or nitric acid is carried out by nitrifying microbes, and phosphorus accumulating bacterium accumulates phosphorus again while decomposing the organic materials produced through intracellular synthesis. 
     According to the present invention, the wastewater from at least one of the flow control tank  201 , anoxic tank  210 , and anaerobic tank  220  is introduced into the aerobic tank  230 . For example, (i) as shown in  FIG. 1 , the wastewater passing through the flow control tank  201 , anoxic tank  210 , and anaerobic tank  220  sequentially is introduced into the aerobic tank  230 , (ii) the wastewater passing through the flow control tank  201 , anaerobic tank  220 , and anoxic tank  210  sequentially is introduced into the aerobic tank  230 , (iii) the wastewater passing through the flow control tank  201  and anoxic tank  210  sequentially is introduced into the aerobic tank  230 , (iv) the wastewater passing through the flow control tank  201  and anaerobic tank  220  sequentially is introduced into the aerobic tank  230 , or (v) the wastewater passing through the flow control tank  201  is directly introduced into the aerobic tank  230 . 
     The MBR system  200  of the present invention further comprises a fine bubble generator  241  disposed in the aerobic tank  230 . The fine bubble generator  241  receives an air from a blower  242  and generates fine bubbles FB, thereby increasing the dissolved oxygen of the wastewater in the aerobic tank  230 . In order to substantially increase the dissolved oxygen of the wastewater, the fine bubbles FB supplied by the fine bubble generator  241  into the wastewater have a diameter small enough to be able to easily dissolve in the water, for example a diameter of 1 to 3 mm. 
     The MBR system  200  of the present invention further comprises a filtration apparatus  100  which is submerged in the wastewater introduced in the aerobic tank  230  and carries out solid-liquid separation therein. The filtration apparatus  100  is disposed directly above the fine bubble generator  241  in the aerobic tank  230 . 
     Therefore, according to the present invention, since the filtration apparatus  100  is provided in the aerobic tank  230 , a separate site for a membrane separation tank is not additionally required. 
     The filtration apparatus  100  of the present invention comprises a filtration membrane module  110  and a bubble-converting device  120  disposed below the filtration membrane module  110  and configured to convert a portion of fine bubbles FB provided by the fine bubble generator  241  into coarse bubbles CB suitable for membrane cleaning. The filtration membrane module  110  and the bubble-converting device  120  may be respectively installed in a frame  130 . 
     According to the present invention, instead of using separate instruments such as a blower and an aeration pipe to produce coarse bubbles necessary for aeration cleaning, a portion of the fine bubbles supplied by the fine bubble generator  241  in the aerobic tank  230  in order to increase the dissolved oxygen of the wastewater are converted into the coarse bubbles CB by the bubble-converting device  120 . 
     Thus, according to the present invention, a proper aeration cleaning can be performed using a portion of the fine bubbles FB supplied in the aerobic tank  230  without requiring installation and operation of a separate blower for an aeration cleaning, and thus the energy consumption can be remarkably reduced without deteriorating the filtration efficiency for the solid-liquid separation. 
     The coarse bubbles CB suitable for membrane cleaning cannot make a substantial contribution to the increase of the dissolved oxygen of the wastewater because they rise rapidly in the wastewater. Therefore, according to one embodiment of the present invention, as illustrated in  FIG. 1 , the filtration apparatus  100  may be disposed opposite the wastewater inlet IL of the aerobic tank  230  in the aerobic tank  230  (i.e., at the latter part of the aerobic tank  230 ). 
     In other words, since the biological reaction such as nitrification reaction occurs mainly at the former part of the aerobic tank  230 , the fine bubbles FB generated by the fine bubble generator  241  and supposed to be supplied to the wastewater positioned at the former part of the aerobic tank  230  are supplied thereto in their entirety so that the dissolved oxygen of the wastewater can be sufficiently increased and the wastewater treatment by microbes can be performed well. On the other hand, the fine bubbles FB supposed to be supplied to the wastewater positioned at the latter part of the aerobic tank  230  may be used for producing the coarse bubbles CB suitable for membrane cleaning. 
     Hereinafter, a filtration apparatus  100  according to the present invention will be described in more detail referring to  FIG. 2  to  FIG. 5 . 
     As illustrated in  FIG. 2 , the filtration apparatus  100  of the present invention comprises at least one filtration membrane module  110  and at least one bubble-converting device  120  disposed below the filtration membrane module  110 . The filtration membrane module  110  and the bubble-converting device  120  may be respectively installed in a frame  130 . 
     The filtration membrane module  110  comprises first and second headers  111  and  112  and a filtration membrane  113  positioned therebetween, the filtration membrane  113  being in fluid communication with at least one of the first and second headers  111  and  112 . Although a hollow fiber membrane is illustrated in  FIG. 1  as the filtration membrane  113 , a flat sheet membrane may be adopted as the filtration membrane  113  in stead of a hollow fiber membrane. 
     The filtration membrane  113  may be formed of polysulfone, polyether sulfone, sulfonated polysulfone, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, polyamideimide, polyesterimide, or any combination thereof. 
     The hollow fiber membrane which can be used as the filtration membrane  113  may be a single-layer membrane type or a composite membrane type. The hollow fiber membrane of a composite membrane type comprises a tubular braid and a polymer thin film coated on the outer surface thereof. The tubular braid may be made of polyester or polyamide (e.g., nylon), and the polymer thin film may be formed of polysulfone, polyether sulfone, sulfonated polysulfone, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, polyamideimide, polyesterimide, or any combination thereof. 
     One end of the filtration membrane  113  is fixed to the first header  111  through a first fixing layer (not shown), and the other end thereof is fixed to the second header  112  through a second fixing layer ( 112   a ). 
     At least one of the first and second headers  111  and  112  has a water collecting space therein, and the filtration membrane  113  is in fluid communication with the water collecting space. 
     As illustrated in  FIG. 2 , the frame  130  may comprise two vertical members  131   a  and  131   b,  two vertical pipes  132   a  and  132   b,  upper and lower cross bars  133   a  and  133   b,  upper and lower cross pipes  134   a  and  134   b,  two upper horizontal members  135   a  and  135   b,  and two lower horizontal members  136   a  and  136   b.    
     One ends of the first and second headers  111  and  112  are coupled to and in fluid communication with the upper and lower cross pipes  134   a  and  134   b,  respectively, and the other ends thereof are coupled to the upper and lower cross bars  133   a  and  133   b,  respectively. 
     The filtrate produced by the filtration membrane  113  flows into the upper and lower cross pipes  134   a  and  134   b  of the frame  130  via the first and second headers  111  and  112 , and then is discharged out of the filtration apparatus  100  via at least one of the two vertical pipes  132   a  and  132   b.    
     Alternatively, the vertical pipes  132   a  and  132   b  may be replaced with vertical members without a fluid passage, and the filtrate introduced into the upper and lower cross pipes  134   a  and  134   b  may be discharged out of the filtration apparatus  100  via a separate pipe. 
     As illustrated in  FIG. 2 , both ends of the bubble-converting device  120  disposed below the filtration membrane module  110  may be directly or indirectly coupled to the lower horizontal members  136   a  and  136   b,  respectively. 
       FIG. 3  schematically shows a bubble-converting device  120  of the present invention, and  FIG. 4  shows the cross section of the bubble-converting device  120  along the IV-IV′ line of  FIG. 3 . 
     As illustrated in  FIG. 3  and  FIG. 4 , the bubble-converting device  120  of the present invention comprises a case  121  having a lower opening LO. The case  121  comprises an upper plate  121   a  and side plates  121   b  extending downwardly from the circumference of the upper plate  121   a.  A collecting space CS surrounded by the upper plate  121   a  and side plates  121   b  is formed in the case  121 . That is, the case  121  has a collecting space configured to confine the fine bubbles introduced therein via the lower opening LO. Accordingly, a portion of the fine bubbles FB supplied by the fine bubble generator  241  in the aerobic tank  230  may be confined in the collecting space CS of the case  121  and exploited for the aeration cleaning of the filtration membrane  113 . 
     The upper plate  121   a  of the case  121  has at least one first hole H 1 . When the upper plate  121   a  has a plurality of first holes H 1 , as shown in  FIG. 3 , the first holes H 1  may be formed in the upper plate  121   a  along the longitudinal direction of the case  121 . 
     As illustrated in  FIG. 4 , the bubble-converting device  120  of the present invention further comprises a main pipe  122  extending downwardly from the circumference of the first hole H 1  formed in the upper plate  121  and a branch pipe  123  extending from a second hole H 2  formed in a side of main pipe  122 . The main pipe  122  is in fluid communication with the collecting space CS via the branch pipe  123 . 
     The opening PO of the branch pipe  123  disposed opposite to the second hole H 2  may be positioned closer to the upper plate  121   a  of the case  121  than the second hole H 2 . Further, the opening PO of the branch pipe  123  may face the upper plate  121   a.    
     Hereinafter, referring to  FIGS. 5(A) to 5(C) , the operation of the bubble-converting device  120  of the present invention will be described in detail. 
     When the filtration apparatus  100  of the present invention is submerged in the feed water to perform a filtering operation, not only the inside of the case  121  of the bubble-converting device  120  but also the insides of the main pipe  122  and branch pipe  123  are filled with the feed water. 
     As shown in  FIG. 5(A) , when the fine bubbles FB are supplied by the fine bubble generator  241  to increase the dissolved oxygen of the feed water, a portion of the fine bubbles FB are confined in the collecting space CS of the case  121  of the bubble-converting device  120 , thereby forming an air layer. 
     As the fine bubbles FB are continuously supplied by the fine bubble generator  241 , the water surface WS in the case  121  continuously gets lower. When the water surface WS reaches the same level as the second hole H 2  formed in the side of the main pipe  122 , the air confined in the collecting space moves into the main pipe  122  via the second hole H 2  as shown in  FIG. 5(B) , comes out of the bubble-converting device  120  via the first hole H 1  of the case, and rises toward the filtration membrane module  110 . 
     As mentioned above, the opening PO of the branch pipe  123  disposed opposite to the second hole H 2  of the main pipe  122  may be positioned closer to the upper plate  121   a  of the case  121  than the second hole H 2 . Accordingly, once the air confined in the collecting space CS starts to come out of the bubble-converting device  120 , as shown in  FIG. 5(C) , the water surface WS in the case  121  rises and, until the water surface WS rising inside the case  121  reaches the same level as the opening PO of the branch pipe  123 , the air is continuously discharged out of the bubble-converting device  120  in accordance with siphonage and forms coarse bubbles CB suitable for cleaning of the filtration membrane  113 . Therefore, according to the present invention, the coarse bubbles CB for cleaning of the filtration membrane  113  can be provided without any additional supply of air by a blower, and thus the energy consumption for aeration cleaning can be minimized while guaranteeing the suitable cleaning effect. 
     In order for the air discharged out of the bubble-converting device  120  via the first hole H 1  to be able to form coarse bubbles CB suitable for cleaning of the filtration membrane  113 , the first hole H 1  may have a diameter of 3 to 30 mm. Preferably, in order to maximize the cleaning effect, the diameter may be 10 to 50 mm which is larger than that of the aeration hole of the conventional aeration pipe (generally, about 7 mm). 
     As illustrated in  FIG. 6 , the MBR system  200  of the present invention may further comprise a vibration apparatus  250  configured to enable a rectilinear reciprocating movement of the filtration apparatus  100  submerged in the wastewater of the aerobic tank  230 , thereby further maximizing the membrane cleaning efficiency. In other words, in addition to the membrane surface scrubbing by the coarse bubbles CB, the filtration is vibrated in the wastewater so that the solids adhered to the surface of the filtration membrane  113  can be removed more easily. 
     The vibration apparatus  250  according to one embodiment of the present invention may comprise a motor  251 , a rotor  252  rotatable by the motor  251 , a shaft configured to convert a rotary motion of the rotor into a rectilinear reciprocating motion of the filtration apparatus  100 , and a rail  254  configured to guide the rectilinear reciprocating motion of the filtration apparatus  100 . 
     Alternatively, in order to control the rate of the rectilinear reciprocating motion, the motor  251  and the rotor  252  may be coupled to each other with a pulley and a belt. 
       FIG. 7  illustrates a way how the lower horizontal member  136   a  of the frame  130  and the bubble-converting device  120  are coupled to each other according to an embodiment of the present invention. 
     As illustrated in  FIG. 7 , the filtration apparatus  100  of the present invention may further comprise a rod  124  for coupling the bubble-converting device  120  to the frame  130 . The rod  124  may be configured to enable the bubble-converting device  120  to rotate on an axis of the rod  124  within a predetermined angle range. 
     Particularly, one end of the bubble-converting device  120  is connected to one end of the rod  124 , and the other end of the rod  124  is inserted in the third hole H 3  formed in the lower horizontal member  136   a  of the frame  130 . Although not shown in  FIG. 7 , the other end of the bubble-converting device  120  is also indirectly coupled to the opposite lower horizontal member  136   b  of the frame  130  similarly. 
     According to one embodiment of the present invention, the rod  124  and third hole H 3  are configured to enable the bubble-converting device  120  to rotate on an axis of the rod  124  within a predetermined angle range. For instance, the lower part of the rod  124  may have a curved cross section so that the rod  124  can rotate in the third hole H 3 . Further, the third hole H 3  may have a cross section of such shape that the rod  124  can rotate therein only within a predetermined angle range (e.g., as illustrated in  FIG. 7 , a circular cross section with the top part thereof cut out). 
     Accordingly, when the filtration apparatus  100  reciprocates, the bubble-converting device  120  can rotate on the axis of the rod  124  with the predetermined angle range. Consequently, even before the air layer is formed within the collecting space CS in a volume large enough for the water surface WS in the case  121  to reach the same level as the second hole H 2  formed in the side of the main pipe  122 , a portion of the air layer can escape from the collecting space CS and form the coarse bubbles CB to contribute to the cleaning of the filtration membrane  113 . 
     Hereinafter, the present invention will be described in more detail with reference to the following Examples and Comparative Examples. The following Examples are only given for better understanding of the present invention and should not be construed as limiting the scope of the present invention. 
     EXAMPLE 1-1 
     A filtration apparatus illustrated in  FIG. 2  (No. of filtration membrane modules:  38 , membrane surface area of filtration membrane module: about 22.1 m 2 , total membrane surface area of filtration apparatus: about 840 m 2 ) was submerged in the wastewater of the aerobic tank and operated with permeate flux of 25 LMH(L/m 2 /h) for 14 days. The filtration was carried out by repeating a unit process (12 hours) consisting of the steps of repeating filtration (9 min.) and backwashing (0.5 min., backwashing flux: 37.5 LMH) 76 times and performing a maintenance cleaning using 200 mg/L of NaOCl solution for 1 minute. The diameter of the aeration hole (i.e., ‘the first hole (H 1 )’) of the bubble-converting device was 50 mm. The aeration rate per unit area of the filtration membrane (hereinafter, ‘aeration rate’) was 0.001 Nm 3 /min/m 2 . During the operation of 14 days, transmembrane pressure (TMP) rise of 0.007 bar was observed. 
     EXAMPLE 1-2 
     A filtration was carried out in the same manner as in Example 1-1, except that the diameter of the aeration hole of the bubble-converting device was 30 mm. During the operation of 14 days, TMP rise of 0.008 bar was observed. 
     EXAMPLE 1-3 
     A filtration was carried out in the same manner as in Example 1-1, except that the diameter of the aeration hole of the bubble-converting device was 10 mm. During the operation of 14 days, TMP rise of 0.009 bar was observed. 
     EXAMPLE 1-4 
     A filtration was carried out in the same manner as in Example 1-1, except that the diameter of the aeration hole of the bubble-converting device was 7 mm. During the operation of 14 days, TMP rise of 0.01 bar was observed. 
     EXAMPLE 1-5 
     A filtration was carried out in the same manner as in Example 1-1, except that the diameter of the aeration hole of the bubble-converting device was 3 mm. During the operation of 14 days, TMP rise of 0.03 bar was observed. 
     EXAMPLE 2-1 
     A filtration was carried out in the same manner as in Example 1-1, except that the filtration apparatus was vibrated (amplitude: 10 cm, reciprocating 0.6 times per 1 sec.) during the filtration operation. During the operation of 14 days, TMP rise of 0.005 bar was observed. 
     EXAMPLE 2-2 
     A filtration was carried out in the same manner as in Example 2-1, except that the diameter of the aeration hole of the bubble-converting device was 30 mm. During the operation of 14 days, TMP rise of 0.006 bar was observed. 
     EXAMPLE 2-3 
     A filtration was carried out in the same manner as in Example 2-1, except that the diameter of the aeration hole of the bubble-converting device was 10 mm. During the operation of 14 days, TMP rise of 0.007 bar was observed. 
     EXAMPLE 2-4 
     A filtration was carried out in the same manner as in Example 2-1, except that the diameter of the aeration hole of the bubble-converting device was 7 mm. During the operation of 14 days, TMP rise of 0.008 bar was observed. 
     EXAMPLE 2-5 
     A filtration was carried out in the same manner as in Example 2-1, except that the diameter of the aeration hole of the bubble-converting device was 3 mm. During the operation of 14 days, TMP rise of 0.022 bar was observed. 
     Comparative Example 1 
     A filtration was carried out in the same manner as in Example 1-1, except that the bubble-converting device of the filtration apparatus was replaced with a convention aeration pipe (aeration hole diameter: 7 mm) and an air was continuously supplied by a blower to the aeration pipe in such an amount that the aeration rate could be 0.004 Nm 3 /min/m 2 . During the operation of 14 days, TMP rise of 0.01 bar was observed. 
     Comparative Example 2 
     A filtration was carried out in the same manner as in Comparative Example 1, except that the aeration rate was 0.003 Nm 3 /min/m 2 . During the operation of 14 days, TMP rise of 0.06 bar was observed. 
     Comparative Example 3 
     A filtration was carried out in the same manner as in Comparative Example 1, except that, instead of the continuous aeration, a cyclic aeration was performed by repeating air supply for 10 seconds and air non-supply for 10 seconds (i.e., the aeration rate was 0.002 Nm 3 /min/m 2 ). During the operation of 14 days, TMP rise of 0.012 bar was observed. 
     Comparative Example 4 
     A filtration was carried out in the same manner as in Comparative Example 3, except that the aeration rate was 0.001 Nm 3 /min/m 2 . During the operation of 14 days, TMP rise of 0.07 bar was observed. 
     Comparative Example 5 
     A filtration was carried out in the same manner as in Example 2-1, except that the bubble-converting device was removed from the filtration apparatus (i.e., the aeration rate was 0 Nm 3 /min/m 2 ). During the operation of 14 days, TMP rise of 0.015 bar was observed. 
     The comparison of the membrane cleaning efficiencies of the Examples and Comparative Examples is shown in the following Table 1. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Aeration  
                   
                   
               
               
                   
                   
                 Hole Diameter 
                 Aeration Rate 
                 TMP Rise 
               
               
                   
                 Cleaning Method 
                 (mm) 
                 (Nm 3 /min/m 2 ) 
                 (bar) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Ex. 1-1 
                 bubble-converting 
                 50 
                 0.001 
                 0.007 
               
               
                 Ex. 1-2 
                 bubble-converting 
                 30 
                 0.001 
                 0.008 
               
               
                 Ex. 1-3 
                 bubble-converting 
                 10 
                 0.001 
                 0.009 
               
               
                 Ex. 1-4 
                 bubble-converting 
                 7 
                 0.001 
                 0.01 
               
               
                 Ex. 1-5 
                 bubble-converting 
                 3 
                 0.001 
                 0.03 
               
               
                 Ex. 2-1 
                 bubble-converting &amp; 
                 50 
                 0.001 
                 0.005 
               
               
                   
                 vibration 
                   
                   
                   
               
               
                 Ex. 2-2 
                 bubble-converting &amp; 
                 30 
                 0.001 
                 0.006 
               
               
                   
                 vibration 
                   
                   
                   
               
               
                 Ex. 2-3 
                 bubble-converting &amp; 
                 10 
                 0.001 
                 0.007 
               
               
                   
                 vibration 
                   
                   
                   
               
               
                 Ex. 2-4 
                 bubble-converting &amp; 
                 7 
                 0.001 
                 0.008 
               
               
                   
                 vibration 
                   
                   
                   
               
               
                 Ex. 2-5 
                 bubble-converting &amp; 
                 3 
                 0.001 
                 0.022 
               
               
                   
                 vibration 
                   
                   
                   
               
               
                 Comp. Ex. 1 
                 continuous aeration 
                 7 
                 0.004 
                 0.01 
               
               
                 Comp. Ex. 2 
                 continuous aeration 
                 7 
                 0.003 
                 0.06 
               
               
                 Comp. Ex. 3 
                 cyclic aeration 
                 7 
                 0.002 
                 0.012 
               
               
                 Comp. Ex. 4 
                 cyclic aeration 
                 7 
                 0.001 
                 0.07 
               
               
                 Comp. Ex. 5 
                 vibration 
                 — 
                 0 
                 0.015 
               
               
                   
               
            
           
         
       
     
     As can be seen in Table 1, the TMP rises during the operation for 14 days were 0.06 bar and 0.07 bar in Comparative Example 2 and Comparative Example 4, respectively, which means that the filtration membranes were not cleaned properly and sufficiently. 
     Generally, when TMP of a filtration membrane is critical TMP (0.6 bar) or higher, a recovery rate achievable by a recovery cleaning is considerably reduced. Therefore, before TMP of a filtration membrane reaches the critical TMP (0.6 bar), filtration needs to be stopped and recovery cleaning should be performed. Generally, the recovery cleaning is performed every 6 months in this industry. That is, TMP of a filtration membrane is required to be lower than the critical TMP (0.6 bar) even after six-month operation. 
     Assuming the initial TMP of a filtration membrane is about 0.2 bar, if TMP rise during the operation for 14 days is 0.06 bar as is the case of Comparative Example 2 or 0.07 bar as is the case of Comparative Example 4, it is more than likely that TMP of the filtration membrane after six-month operation would be higher than the critical TMP (0.6 bar) (theoretically, not less than 0.92 bar and not less than 1.04 bar, respectively). Therefore, the recovery cleaning cycle is required to be shorter than 6 months, which is problematic. 
     Comparative Examples 1, 3, and 5 the TMP rises of which during 14-day operations were 0.01 bar, 0.012 bar, and 0.015 bar, respectively, are the cases in which the contamination of the filtration membranes was sufficiently delayed. In Comparative Examples 1 and 3, however, relatively large amount of air (0.004 Nm 3 /min/m 2  and 0.002 Nm 3 /min/m 2 , respectively) was used, and thus such a large amount of energy was consumed for the aeration cleaning. Further, considering the fact that a recovery rate achievable by a recovery cleaning gets lower as the recovery cleaning is repeated and Comparative Example 5 showed TMP rise of 0.015 bar which is greater than those of Comparative Examples 1 and 3, it is more than likely that, after a long term operation of more than 5 years, TMP of the filtration membrane of Comparative Example 5 reaches the critical TMP before the next recovery cleaning cycle comes, thereby requiring additional energy consumption. 
     On the other hand, from the fact that only small TMP rise of no more than 0.005 to 0.022 occurred in spite of the relatively small amount of the air (0.001 Nm 3 /min/m 2 ) and the fine bubbles originally supplied in the aerobic tank were used for the membrane cleaning without an additional air from any other source, it can be seen that the energy consumptions of the Examples for the aeration cleaning were minimized without deterioration of the cleaning efficiency. 
     Particularly, in Examples 1-1, 1-2, 1-3, 2-1, 2-2, and 2-3 where the diameters of the aeration holes were 10 mm or longer, the cleaning effects were so excellent that TMP rises of less than 0.01 bar were only observed. 
     Comparison of Example 2-5 with Comparative Example 5 shows that the bubbles produced by a relatively small aeration hole may be a hindrance to the membrane cleaning. It seems to be because the bubbles produced by a relatively small aeration hole cannot provide sufficient shear force on the membrane surface and, by providing oxygen, cause a biofilm contamination by microbes on the membrane surface.