Abstract:
A module of vertical membranes has a lower header with integral air holes. Modules are mounted in line on upper and lower beams. A skirt is formed under the cassette. Adjustable side members between the beams allow for membrane slack adjustment and bottom beam levelling. A flat aerator assembly can be inserted into spaces between the cassettes and provide bubbles into the skirts, the spaces between cassettes, or both. An aeration method involves producing bubbles primarily or only to one side of the module, alternating from one side of the module to the other, while also producing bubbles within the module or between the membranes, optionally continuously. A cleaning method involves flowing a chemical cleaning solution by force of gravity through a membrane module, optionally by injected a concentrated solution into a vented portion of a permeate withdrawal system located above the water level in a tank holding the module.

Description:
[0001]    For the United States of America, this is an application claiming the benefit under 35 USC 119(e) of U.S. Application Ser. Nos. 61/144,723 filed Jan. 14, 2009; 61/249,844 filed on Oct. 8, 2009; 61/249,847 filed on Oct. 8, 2009; and, 61/249,853 filed on Oct. 8, 2009, all of which are incorporated herein in their entirety by this reference to them. 
     
    
     FIELD 
       [0002]    This specification relates to immersed membrane systems, for example suction driven immersed microfiltration or ultrafiltration systems for producing usable water or treating wastewater, and methods of membrane system operation including methods to inhibit fouling such as aeration (or gas bubble scrubbing) methods and chemical cleaning methods. 
       BACKGROUND 
       [0003]    Immersed membrane systems, for example membrane bioreactors, may use hollow fiber ultrafiltration or microfiltration membranes immersed in a tank of water (including wastewater) to be treated. Many hollow fibers may be mounted vertically between upper and lower potting heads to form a module. The module is typically kept to a size that can be handled by a person. In order to provide the membrane surface area necessary for large systems, modules are connected together into larger assemblies, sometimes called cassettes. The configuration of the cassette and the arrangement of pipes around the cassettes can affect the cost of the installation, the ability to pack membrane area into a tank, the flow of fluids in the tank and the operational efficiency of the system. 
         [0004]    U.S. Pat. No. 5,639,373 describes a module of immersed membranes. In one example, the membranes are oriented vertically between solid upper and lower rectangular potting heads and tubular aerators are placed on the sides of the lower potting head. In another example, the lower potting head of a module has a skirt extending below the potting head and tubes extending through the potting material. Air provided through a port in the side of the skirt flows though the tubes to create bubbles at the top of the lower potting head. 
         [0005]    U.S. Pat. No. 6,245,239 describes a cyclic aeration system for submerged membrane modules. In one example, a set of rectangular modules with membranes oriented vertically between solid upper and lower potting heads has a set of aerators below the modules. A flow of air to the aerators is switched on and off in repeated cycles. 
         [0006]    Maintenance cleaning is used to sustain the operation of immersed membranes, for example ultrafiltration or microfiltration membranes in a membrane bioreactor. In an example described in U.S. Pat. No. 6,547,968, maintenance cleaning involves frequent, for example 1-7 times per week, contact periods with cleaning chemical(s) to “condition” the fouling layer rather than attempt to remove it. The active chemical in the cleaning solution is often chlorine, but other oxidants, bases or acids can also be used. The efficiency of maintenance cleaning is related to the chlorine concentration and contact time. When NaOCl is used to supply chlorine, the concentration may be between 100-500 mg/L. The contact time may be several minutes to one hour. 
       INTRODUCTION 
       [0007]    The following introduction is intended to introduce the reader to the detailed discussion and not to limit or define any claimed invention. An invention may reside in a combination or subcombination of apparatus elements or process steps described in any part of this document including the Figures. 
         [0008]    A module of vertically oriented membranes has an upper permeating header and a lower dead end header with integral air holes. The headers are not fixed apart from each other in the module itself. The module preferably has a square cross section in plan view, but with a permeate cap that provides a round permeate connection. The modules are mounted in line on upper and lower beams to form a cassette. The cassette is an elongated rectangular shape in plan view. A skirt is formed under the modules or cassette to provide an open bottomed chamber under the lower headers in communication with the air holes. Adjustable side members between the beams allow for membrane slack adjustment and bottom beam levelling. A permeate header is provided above and in line with the upper beam. The cassette can be inserted from above into receivers mounted to the upper sides of the tank. An aerator grid is provided separately. The primary components of the aerator grid are flat assemblies of pipes and structural members that can be inserted vertically downwards into spaces between the cassettes. Air holes in the aerators can be located to provide bubbles both into the skirts and optionally also into the spaces between cassettes. The top beam of each cassette is attached to the tank and bears the weight of the cassette. 
         [0009]    The module may also be described as having membranes extending upwards from a potting head. The potting head is located between two opposed walls of a skirt extending below the bottom of the potting head. There are passages for air to flow vertically through the potting head. An aerator is provided on each side of the module. Each aerator has one or more holes and creates bubbles both between the wall of the skirt and outside of the skirt. Gas flow is provided at one time only or primarily to one of the aerators and at another time only or primarily to another of the aerators. Gas flows through the potting head to produce bubbles during both periods of time, optionally continuously. An aeration method involves producing bubbles primarily or only to one side of the module, alternating from one side of the module to the other, while also producing bubbles within the module or between the membranes, optionally continuously. 
         [0010]    One or more modules may be connected to a permeate header above the membrane surfaces of the modules. The permeate header is in communication with an isolation valve to isolate the permeate header from other pipes in the permeate withdrawal system. The permeate header is also in communication with a vent valve on the module side of the isolation valve operable to open the permeate header to atmosphere. A chemical injection pipe allows a chemical to be injected into the permeate header. To clean the modules, the isolation valve is closed. Optionally, the water (mixed liquor in the case of a membrane bioreactor) level in the tank may be reduced. A cleaning chemical is injected into the permeate header where it is mixed with water in the permeate header to a desired concentration. With the permeate header above the water level, the vent valve is opened allowing the chemical to flow through the membrane surfaces. To resume permeation, the tank is re-filled, the vent valve is closed, and the isolation valve is re-opened. A cleaning method involves flowing a chemical cleaning solution by force of gravity through a membrane module, optionally by injected a concentrated solution into a vented portion of a permeate withdrawal system located above the water level in a tank holding the module. By this method, only a small amount of chemical is used. The chemical may be evenly distributed among a number of modules without a high flow rate. The chemical remains at high concentration near the module, with little dilution into the water outside of the module. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a cross section of a module. 
           [0012]      FIG. 2  is a side view of a cassette of modules in a tank, the tank shown in section. 
           [0013]      FIG. 3  is a side view of an aerator assembly. 
           [0014]      FIG. 4  is a schematic top view of a tank with cassettes and aerator assemblies installed. 
           [0015]      FIG. 5  shows a partial end view of cassettes in a tank with the lower parts of a set of modules as in  FIG. 1  in cross section and a schematic aeration system at one period of time. 
           [0016]      FIG. 6  shows a partial end view of cassettes in a tank with the lower parts of a set of modules as in  FIG. 1  in cross section and a schematic aeration system at another period of time. 
           [0017]      FIG. 7  is a longitudinal cross section of a part of the permeate header of  FIG. 2 . 
           [0018]      FIG. 8  is a cross section cut across the diameter of a part of the permeate header of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Referring to  FIG. 1 , a module  10  has a lower potting head  12  and an upper potting head  14 . A large number of hollow fiber membranes  16  are potted in the potting heads  12 ,  14 . The potting heads  12 ,  14  are also sometimes called headers or tube sheets. Only a few of the membranes  16  are shown to simplify the drawing. The membranes  16  are plugged at their lower ends in a block of potting resin within the lower potting head  12 . The membranes  16  pass through the upper potting head  14  so as to be open to a permeate collector cap  18  sealed to the upper surface of the upper potting head  14 . The cap  18  is connected to a permeate header  20  which is in turn connected to a source of suction operable to withdraw permeate through the membranes  16 . The potting heads  12 ,  14  may be attached to a frame  26  (only part shown) to space the potting heads  12 ,  14  and allow the module  10  to be lowered into a tank of liquid to be filtered. The module  10  is intended to be immersed with the membranes  16  oriented vertically in an open tank. 
         [0020]    In plan view, the module  10  may be, for example, round or square with a diameter or width of between 100-200 mm or 100-150 mm. Several modules  10  may be arranged side by side to create a rectangular assembly. The height of the module  10  may be 1-2 m. The total membrane surface area may be 15-25 square meters. The lower potting head  12  has one or more, for example 1-10, holes  22  passing through it between the membranes  16 . Each hole  22  may be 5-10 mm in diameter. One or more of side walls of the lower potting head  12 , parts of a frame  26  holding the module  10 , and skirt walls  28 , extend downwards at the sides of the lower potting head  12  to define the sides of an open bottom chamber  30  (sometimes called a skirt) below the lower potting head  12 . The lower potting head  12  or parts of a frame  26  holding the module  10  may define the top of the chamber  30  or additional top plates may be used. Extension tubes  24  may protrude from the holes  28  into the chamber  30 . If several modules  10  are placed side by side to form a rectangular assembly, a skirt wall  28  may extend along the length of the entire assembly to form one long chamber  30  below several modules  10 . Alternately, additional dividing walls may be placed between each pair of modules  10  to provide a separate chamber  30  below each module  10 . 
         [0021]    Referring to  FIG. 2 , a set of modules  10  are held by their potting heads  12 ,  14  in a common frame  26  to form a cassette  60 . The modules  10  are placed as close together as possible in a row. The frame  26  comprises horizontal beams  42  and vertical posts  44 . The permeate collection header  20  runs parallel to the frame  26  and is connected to the cap  18  of each module  10 . The permeate collection header  20  is also communicates with one or more larger permeate collection pipes  50  through one or more isolation valves  52 . In the example shown, one end of the permeate header  20  is capped and the other end of the permeate header  20  is attached to a shared permeate collection pipe  50  through an isolation valve  52  associated with only one permeate collection header  20 . The isolation valve  52  allows isolation of one or more beam-cassettes for maintenance without interruption of operation. The permeate collection pipe  50  runs along the side of the tank  48  at a right angle to the permeate header  20  and is connected to the permeate headers of other sets of modules located in the tank  48  beside the set of modules  10  shown. The permeate collection pipe  50  is also attached to a source of suction (not shown) operable to withdraw permeate from the modules  10 . The upper one of the beams  42  is normally immersed in water in the tank  48  while the permeate header  20  may be normally above or within the water. 
         [0022]    Each cassette  60  is held in a pair of guides  46  connected to the tank  48 . The guides  46  of a pair face each other on opposite sides of the tank  48 . The cassette  60  slides vertically downwards into the guides  46 . An upper beam  42  of the cassette  60  bears on abutments of the guides  46  such that the weight (or buoyancy) of the cassette  60  as a whole is resisted via the upper beam  42 . The guides  46  may optionally restrain the lower beam  42  laterally or have no contact with the lower beam  42 . The distance between the top and bottom beams  42  is set by adjusting connections between the vertical posts  44  and the beams  42 . The upper beam  42  spans the width of the immersion tank  48  and is attached to the walls of the tank  48  on both sides via the guides. While the upper beam  42  (and the upper potting heads attached to it) is normally immersed, the attachment points to the guides  46  or between the guides  46  and the tank  48  can be above the water surface. 
         [0023]    The vertical posts  44  are rigid structural pieces (pipe or beam) that connect the top and bottom beams  42  and maintain them at a fixed and adjustable distance. There are two vertical posts  44  per cassette  60 , one at each end of the cassette  60 . The distance between the top and bottom beams  42  should be slightly less than the length of the fibers between the potting heads  12 ,  14  to provide some hollow fibre slack. The amount of fiber slack can be adjusted for performance. Vertical posts  44  may be fixed into the bottom beam  42  (rotation allowed) but have an adjustable slide-type connection in the top beam  42  to make adjustments to the spacing of the beams  42 . The vertical posts  44  and guides  46  maintain the bottom beam  42  in a fixed vertical position during operation when the bottom beam  42  becomes buoyant. 
         [0024]    The vertical posts  44  can be used while the cassette  60  is in the tank  48  to change the position of the bottom beam  42  in order to adjust slack and ensure even air flow rate through the holes in the lower potting head. First, the top beam  42  is roughly levelled by adjusting the attachment points to the tank  48  or guide  46 . Second, the bottom beam  42  is pushed down until hollow fibres  16  are taut. The vertical posts  44  are then moved back up by a distance that will provide the desired fibre slack. Third, the air flow is turned on at low value and the bubble pattern at the surface is observed. The vertical posts can then be moved up and down until air flow is even, making sure that the required adjustment is split evenly between the two vertical posts  44  (one is moved up, the other is moved down) to avoid changing slack significantly. The vertical posts  44  are then locked in place. 
         [0025]      FIG. 3  shows a side view of an aerator assembly  70 . The aerator assembly  70  is separate from the cassettes  60 . An aerator assembly  70  is inserted between pairs of cassettes  60  and optionally beside outer cassettes. Each aerator assembly  70  slides vertically into an aerator guide  72  attached to the tank  48  walls. The aerator guide  72  may extend downwards into the tank  48  (rather than upwards as shown) like the guides  46  for the cassettes  60 . Each aerator assembly  70  consists of an aerator header  74 , an aerator  32  and a number of down-pipes  76 . The aerator assembly  70  is generally planar. Aerators  32  are also sometimes called air, gas or bubble spargers, or simply spargers. 
         [0026]    An aerator header  74  runs between each pair of cassettes  60 . A down-pipe  76  is connected to the aerator header  74  on each side of it. Optionally, additional down-pipes  76  may be provided every 200-500 mm. The down pipes  76  may be long enough to position the aerators  32  below the skirts of the cassettes  60  when installed. The aerator assembly  70  described herein primarily occupies spaces in a tank  48  that would be required in any event for gaps for water flow between cassettes  60  and thereby facilitates a high tank intensity (square meters of membrane surface area per unit volume or surface area of a tank). 
         [0027]    Referring to  FIG. 4 , the tank  48  is typically rectangular in plan view. Cassettes  60  are laid across the tank width or length. A useful feature of the beam-cassette structure described herein is that the length of the cassettes  60  may be made in increments of the width or diameter of the modules  10  such that the length of a cassette may be generally equal to, through slightly less than, the width or length of the tank  48 . For retrofitting cases, custom-length cassettes can be built using a standardized size of module  10  merely by changing the length of the beams  28 . Cassettes  60  and aerator assemblies  70  may be placed side by side across the remaining dimension of the tank  48  to efficiently fill the tank area to a high tank intensity. The permeate headers  20  are connected to a main permeate header  50  on one side of the tank  48 . The aeration assemblies  70  are connected in an alternating pattern to two separate aeration headers  34  on the other side of the tank  48 , or to a single header if, optionally, air will be supplied to all cassettes  60  in the tank  48  at the same time. 
         [0028]    The tank  48  may be 2-3 m deep. In a membrane bioreactor application, the tank  48  also contains a layer of mixed liquor distribution pipes at the bottom (not shown) and a return activated sludge outlet or overflow (not shown). It is desirable that the membrane tank  48  be completely filled with cassettes  60  to ensure a uniform flow pattern in the tank  48 . 
         [0029]    Referring to  FIGS. 5 and 6 , a number of modules  10  may be immersed side by side in a tank (not shown in  FIGS. 5 and 6 ) of water to be filtered, for example re-circulated mixed liquor in a wastewater treatment plant. Each module  10  shown in  FIGS. 5 and 6  may be part of a cassettes  60  extending in length perpendicular to the page. A group of modules  10  are spaced apart, for example at 200-500 mm center to center, to provide gaps between them. An aerator  32  is located between each spaced pair of modules  10 , and optionally beside but outside of the modules  10  at the edges of the group of modules  10 . The aerators  32  may be pipes located 100-500 mm below the lower potting heads  12  with 5-15 mm air holes  40  every 50-100 mm on each side of the aerator  32 . The air holes  40  may be oriented radially pointing 30-60 degrees below horizontal. The aerators  32  are attached to headers  34  connected through valves  36  to an air blower  38  or another source of a pressurized gas that will be used to make gas bubbles. A process of membrane aeration is also sometimes called air, gas or bubble sparging, or simply bubbling. 
         [0030]    When air, or another gas, flows to an aerator  32 , bubbles are created at the air holes  40 . A fraction of the bubble gas flow, for example between 25% and 75%, is captured in the chambers  30  of the modules  10 , forms a pocket of gas below the lower potting heads  12 , and flows through the holes  22  in the lower potting heads  12  to create bubbles within the module  10 . The remainder of the bubbled gas flow rises through the gaps between the modules  10 . Bubbles rising in a gap entrains water in the tank causing water to also rise through the gap. The fraction of the bubble gas flow captured in the chambers  30  may be varied by the varying the design, position or location of the aerators  32 , by varying the width of the gaps between the modules  10 , or by varying the width of the bottom of the skirt walls  28 . The aerators  32  and lower potting heads  12  within a cassette  60  are preferably leveled to promote an even distribution of air flow from the air holes  40  of an aerator or from the holes  22  of the one or more lower potting heads  12  of a cassette  60 . 
         [0031]    The aerators  32  may be connected to the headers  34  such that each header  34  feeds gas to every second aerator  32 . For example, if the aerators  32  in a tank are numbered from left to right, the even numbered aerators  32  are attached to a first header  34   a  and the odd numbered aerators  32  are attached to a second header  34   b . The flow of gas from the blower  38  may be switched from first header  34   a  to second header  34   b  by closing valve  36   a  while opening valve  36   b . The flow of gas may be switched back to the first header  34   a  after a period of time by opening valve  36   a  while closing valve  36   b . The gas flow may be switched back and forth repeatedly while permeation and backwash or relaxation cycles of the filtration operation are on going.  FIG. 6  shows the gas flow with valve  36   a  closed and valve  36   b  open while  FIG. 5  shows the gas flow with valve  36   a  open and valve  36   b  closed. 
         [0032]    By the method described above, bubbles are provided in the gaps beside a module  10  first on one side of a module  10  and then on the other side of the module  10 . This promotes horizontal water flow through the membranes  16 . However, since there are always bubbles coming into one side of the chamber  30 , the rate of gas flow of bubbles produced within the module  10  through the holes  22  is substantially constant. In this way, it is difficult for foulants in the water to settle within the module  10 . In particular, the method inhibits solids accumulation in the module  10  near the lower potting head  12 . Avoiding fouling just above the lower potting head  12  is important because it is an area that is often prone to fouling in vertical hollow fiber membranes and a difficult area to clean. Dead end potting of the membranes  16  in the lower potting head, though optional, is also helps inhibit fouling near the lower potting head  12  since transmembrane pressure decreases with distance from a permeating header due to head losses to permeate flow in the lumens in the membranes  16 . 
         [0033]    Optionally, extension pipes  24  may be inserted into the bottom ends of the holes  22 . The extension pipes  24  protrude into the chamber  30 , for example by 10-30 mm. A gas pocket forms in the top of the chamber  30  that is always at least as thick as the length of protrusion of the extension pipes  24 . The gas pocket is usually thicker than that, with air overflowing into the extension pipes  25  and through the holes  22 . The additional gas pocket thickness provided by the extension pipes  24  allows gas to distribute across the chamber  30  more quickly as gas flow is switched from one aerator  32  to another and so promotes a more nearly even gas flow among holes  22  spaced across the width of a module  10 . 
         [0034]    During a maintenance cleaning operation, the cleaning solution is preferably distributed evenly to all modules. The concentration of cleaning solution should be high (though within the limit of the membrane material tolerance) and excess dilution into water in the tank outside of the modules is preferably avoided. The cleaning solution is preferably delivered to the membrane surface and allowed to react there with minimal negative impact on biomass in the membrane tank. Maintenance cleaning is preferably performed in a full or nearly full tank. Maintenance cleaning can be done in an empty tank to avoid dilution into the water in the tank, but in that case most of the solution is lost by permeation near the bottom of the hollow fibres where the static pressure of a cleaning solution inside the module is highest. In the filtration system described herein, fouling near the bottom of the membranes is reduced both by the aeration method and dead end potting of the bottom of the fibres. In this case, it is desirable to encourage the chemical solution to permeate to the extent possible through the upper ends of the membranes whereas maintenance cleaning into an empty tank may cause a further loss of cleaning solution at the bottom of the membranes due to the relative lack of fouling near the bottoms of the membranes. 
         [0035]    The permeate pumping system is often used to deliver maintenance cleaning solution to membrane modules. However, a large amount of chlorine solution is needed just to fill the permeate piping network even before any cleaning solution is contacted with the membrane. Further, a large flow rate is needed to deliver the cleaning solution evenly to all modules to make use of the equalizing effect of pressure loss in the modules. The combined impact of these constraints is that a large amount of low concentration chlorine solution permeates the membrane, dilution is excessive and a significant part of the biomass in the tank may be killed. 
         [0036]    Referring to  FIGS. 2 ,  7  and  8 , the permeate header  20  is connected to a vent pipe  54  with a vent valve  56 . Opening the vent valve  56  exposes the inside of the permeate header  20  to atmospheric pressure. A chemical injection tube  58  has a section running inside of the permeate header  20  with small injection holes  60  spaced along its length. Another section of the chemical injection tube  58  is located outside of the permeate header  20  and connected, typically through intermediate pipes and valves not shown, to a chemical pump  62  connected to a chemical tank  64 . 
         [0037]    To perform a maintenance cleaning, the permeate header  20  is isolated from the permeate pumping network by closing isolation valve  52 . Alternatively, an isolation valve could be provided and closed further downstream in the permeate network so that multiple sets of modules  10  connected to permeate pipe  50 , for example all of the modules  10  in a tank  48 , can be maintenance cleaned at the same time. Closing the isolation valve  52  isolates a known volume of permeate in communication with one or more permeate headers  20 . 
         [0038]    An amount of concentrated chlorine or other cleaning chemical, the amount optionally pre-determined based on the known volume mentioned above and a desired final chemical cleaner concentration, is injected in the permeate header  20  via the chemical injection tube  58 . The chemical cleaner flows out of the injection holes  60  and rapidly mixes into permeate in the permeate header  20  to the desired final concentration. The chemical solution remains in the permeate header  20  at this stage although a small amount of permeate is displace into the membrane tank  48 . Membrane aeration is preferably turned off to minimize dispersion of the cleaning solution in the following steps. 
         [0039]    The mixed liquor level in the tank  48  is optionally partially lowered to create or increase a potential driving force in a direction opposite to normal permeation. This reverse-permeation driving force may be around one or more 10 s of cm, but preferably less than 50 cm. Sufficient potential reverse-permeation driving force may already be available without lowering the mixed liquor level if the permeate header  20  is located sufficiently far above the normal water level in the tank  48 . In general, the permeate header  20  should at least be completely above the water level before a flow of chemical solution from the permeate header is initiated. Optionally, since fouling often occurs in the first 10 or 20 cm below the upper header of a vertical module, the water level may be lower to 10 or 20 cm below the bottom of the upper header to encourage flow of cleaning chemical through the upper parts of the membranes. The water level can be lowered by partially draining the tank  48  any time before opening the vent valve  56 . Optionally, the water level can be lowered by shutting of flow of water into the tank while continuing to withdraw permeate before closing isolation valve  52 . 
         [0040]    Chemical flow is initiated by opening vent valve  56  to connect the interior of the permeate header  20  to atmosphere. This allows the contents of the permeate header  20  to reverse-permeate by gravity. The vent valve  56 , or the extent to which it is opened, can be chosen so that the reverse-permeation (chemical discharge) time of the cleaning solution provides the desired contact time for the cleaning chemical. Optionally a wait time of up to about 5 minutes may be provided after the reverse-permeation is substantially completed to allow time for the chemical cleaner to further react with foulants. 
         [0041]    When the cleaning solution has substantially all reverse-permeated, and any wait time has elapsed, the level in the tank  48  is increased to its normal set point and the permeate header  20  fills with water by forward-permeation while some air is evacuated through vent valve  56 . Vent valve  56  is preferably located at a high point of the isolated area in or in communication with the isolated permeate header  20 . Vent valve  56  can then be closed, membrane bubbles scouring resumed, and isolation valve  52  opened to put the modules  10  back into operation. Any air still trapped in the permeate header  20  may be removed through the ordinary air collector of the permeate system. The invention protected by this document is defined by the following claims. The claims are not limited to the specific examples of apparatus or process described herein.