Abstract:
A method of air scouring an immersed membrane is described in this specification. The method comprising a step of adjusting one or more aeration parameters: between successive permeation, back pulse or relaxation cycles; during a permeation cycle; or, between a permeation cycle and a backpulse or relaxation cycle.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    For the United States of America, this application claims the benefit of U.S. Provisional Application No. 61/726,092, filed Nov. 14, 2012, which is hereby incorporated by reference. 
     
    
     FIELD 
       [0002]    This specification relates to a gas delivery device for use, for example, in supplying bubbles to inhibit fouling of an immersed filtering membrane. 
       BACKGROUND 
       [0003]    International Publication Number 2011/028341, Gas Sparger for a Filtering Membrane, describes a gas sparger that produces an intermittent flow of bubbles even if provided with a continuous gas flow. The sparger has a housing to collect a pocket of gas and a conduit to release some of the gas from the pocket when the pocket reaches a sufficient size. A large sparger can be divided into a plurality of units each having a conduit. A gas supply pipe has at least one hole aligned with each unit to deliver air to each of the units. International Publication Number 2011/028341 is incorporated by reference. 
       INTRODUCTION 
       [0004]    A method of air scouring an immersed membrane is described in this specification. The method comprises a step of adjusting one or more aeration parameters during a permeation cycle, or between a permeation cycle and a back pulse or relaxation cycle, or between successive cycles. The method may be used with a gas delivery device described in this specification in which a supply of gas is provided to a manifold with multiple ports connected to multiple conduits. The method may further comprise bringing a flow of pressurized gas into a tank to near or below the bottom of a membrane module. At about this elevation, the flow of pressurized gas is split into multiple flows of pressurized gas. Each of the multiple flows of pressurized gas is directed to a different lateral position and then released as bubbles. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0005]      FIG. 1  is a top view of a gas delivery device. 
           [0006]      FIG. 2  is a bottom view of the gas delivery device of  FIG. 1 . 
           [0007]      FIG. 3  is a side view of the gas delivery device of  FIG. 1 . 
           [0008]      FIG. 4A  is an isometric view of the bottom of the gas delivery device of  FIG. 1 . 
           [0009]      FIG. 4B  is an isometric view of the top of the gas delivery device of  FIG. 1 . 
           [0010]      FIG. 5  is a side view of the gas delivery device of  FIG. 1  in combination with an intermittent gas sparger. 
           [0011]      FIG. 6  is an isometric cross sectional view of the bottom of an alternative intermittent gas sparger. 
           [0012]      FIG. 7  is a schematic cross section of a tank having a suction driven membrane module and an aeration system immersed in the tank. 
           [0013]      FIG. 8  illustrates a method of aerating an immersed membrane module. 
           [0014]      FIGS. 9-13  illustrate alternative methods of aerating an immerse module. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    In a gas sparger as described in International Publication Number 2011/028341, a unit of the sparger that receives a larger flow rate of input gas will produce pulses of bubbles at a higher frequency. In order to uniformly clean a membrane cassette, it may be desirable to have each unit operate at near the same frequency. The holes of the gas supply pipe are made small to help equalize the gas flow rate between holes feeding different units of the sparger. However, if the gas supply pipe is installed out of level by as few as 6 mm over a length of about 500 mm, the holes at higher elevation will have a noticeably larger gas flow rate. In addition, solids entering the gas supply pipe during maintenance periods when the gas supply is turned off can dry out or agglomerate when the gas is turned back. Occasionally, a solid particle is formed in the gas supply tube that is large or rigid enough to be lodged into one of the holes and to restrict or block the hole. A partially or completely blocked hole will in turn lead to poor distribution of gas to the membranes and allow solids to accumulate on the membranes. A gas delivery device will be described below that can be used as an alternative to such a gas supply pipe either with or without a further gas sparger. 
         [0016]      FIGS. 1 to 4  show different views of a gas delivery device  10 . Alternatively, the gas delivery device  10  may be called an aerator or a sparger. In use, the gas delivery device  10  is immersed in a liquid, typically water, for example activated sludge. Pressurized gas is supplied to an inlet  12  of the gas delivery device and is emitted as bubbles from a plurality of outlets  14 . The gas is typically air, but in some applications another gas, for example biogas, nitrogen, ozone or oxygen may be used. The gas delivery device  10  shown has four outlets  14 , but there may alternatively be more or less outlets  14 . 
         [0017]    The inlet  12  is separated from the outlets  14  by a plurality of ports  16 . Each outlet  14  communicates with a port  16  through a channel  18 . Part of the gas delivery device  10  from the inlet  12  to the ports  16  functions as a manifold  15 , alternatively called a plenum, to distribute the gas entering through the inlet  12  among the channels  18 . The inlet  12 , ports  16  and outlets  14  are located at generally the same elevation but spaced horizontally. The gas flows generally horizontally in the channels  18 . 
         [0018]    The area of the ports  16  is less than the area of the channels  18 , or less than the area of the smallest of the channels  18  if they have different areas. For example, the channels  14  may have a cross sectional area that is three times or more than the cross sectional area of the ports  16 . The ports  16  restrict the flow of gas into the channels  14 . The restriction provided by the ports  16  helps to distribute the total airflow more nearly equally among the channels  18 . Decreasing the area of the ports  16  produces a more nearly equal flow in the channels  18  but also increases head loss through the ports  16 . The ports  16  may be made all of the same area. The area of the ports  16  may be reduced until the flow is adequately distributed among the channels  18 . Optionally, a port  16  opening into a long or narrow channel  18  may be larger than a port  16  opening into a short or wide channel  18  to help equalize the flow among the channels  18 . Alternatively, one or more ports  16  may be made larger than other ports  16  to intentionally increase the relative airflow through one or more channels  18 . This may be done, for example, to provide more air to the extremities of an immersed membrane cassette to counteract a tendency for water to be lifted preferentially through the center of a cassette. 
         [0019]    As shown in  FIG. 1  and  FIG. 2 , the ports  16  are located close to each other in the horizontal direction. In this way, if the gas delivery device  10  is mounted a few degrees out of level, there is very little difference in elevation between the ports  16 . In particular, the largest horizontal distance between two ports  16  is less than the average horizontal distance between adjacent outlets  14 , or less than half of the average horizontal distance between adjacent outlets  14 . The largest horizontal distance between the ports  16  is also less than 25%, or less than 10%, of the largest distance from a port  16  to an outlet  14 . This helps produce a more nearly equal distribution of the gas among the channels  18  compared to an ordinary aerator in the form of a tube with holes when the gas delivery device  10  is mounted out of level. Because the ports  16  are primarily responsible for equalizing flow between channels  18 , the outlets  14  can be made larger, for example as large as the cross sectional area of the channels  18 , so that any solids that accumulate in a channel  18  are unlikely to block the outlet  14 . 
         [0020]    The gas delivery device  10  has its outlets  14  spaced generally in a line. Alternatively, other configurations may be used. For example, channels  18  could extend along a line but in both directions from the inlet  12 . In another example, the channels  18  could radiate from the inlet  12  like spokes from a wheel hub. 
         [0021]    Optionally, the top of the channels  18  may be pointed slightly upwards. In this way, if the gas delivery device is inadvertently mounted with a slightly downwards slant, then gas will not be trapped in the channels  18  when the supply of gas is off. A slight upwards slant may also help compensate for differences between the lengths of the channels  18 . 
         [0022]    Referring to  FIG. 7 , the gas delivery device  10  may be used, for example, to provide bubbles for scouring an immersed membrane module  50 . The membrane module  50  contains a plurality of ultrafiltration or microfiltration membranes. Permeate is withdrawn from the membrane module by way of a suction pump in communication with the inside of the membranes. A typical filtration cycle comprises periods of filtration interrupted by backwashing, alternatively called backpulsing, procedures or relaxation periods in which there is no filtration. A device with a line of outlets  14  is particular suited for providing bubbles to membrane modules with rectangular elements such as flat sheet modules or ZeeWeed™ hollow fiber elements sold by GE Water &amp; Process Technologies. 
         [0023]    The gas delivery device  10  is immersed in a tank  52  containing one or more membrane modules  50 . The gas delivery device  10  may be mounted separately in the tank  52  or attached to the membrane modules  50 . Gas may be brought down into the tank from a riser pipe  54  and then spread horizontally through as header  56 . Saddles  58  attached to the header  56  receive gas from the header and carry the gas to a line of gas deliver devices  10  oriented perpendicularly to the header  56  in a generally horizontal plane. Optionally, a gas delivery device  10  may be connected directly to a header  56  or riser pipe  54 . Streams of bubbles  30  are discharged from the outlets  14  at various lateral positions relative to a membrane module  50 . The gas flowing to each lateral position bypasses any intermediate lateral positions. The bubbles  30  may be allowed to rise directly to the membranes to clean them or inhibit fouling. Alternatively, a transducer may be placed above the gas delivery device  10  to modify its output before the bubbles reach the membranes. For example, a diffuser may be placed over an outlet to disperse the bubbles over a wider area. 
         [0024]      FIG. 5  illustrates another transducer option in which an intermittent gas sparger  20 , for example of the type shown in International Publication Number  2011 / 028341 , is associated with the gas delivery device. Pressurized gas  28  is split in the gas delivery device into four bubble streams  30 . Each bubble stream  30  rises into a different cavity  32  of the intermittent air sparger  20 . Gas flowing through a conduit  18  to a particular cavity  32  bypasses any intervening cavities  32 . 
         [0025]    Each cavity  32  has a discharge conduit  34 , in the form a J-shaped tube in the example of  FIG. 5 , which acts like an inverted siphon to discharge intermittent pulses of air from the cavity  32 . Bubbles emitted from the gas delivery device  10  first collect in the cavity  32  forming a pocket of gas in the top of the cavity  32 . No gas is emitted from the cavity  32  until the pocket of gas expands to reach the low point of the discharge conduit  34 . At that time, the pocket of gas empties out of the cavity  32  through the conduit  34  and the process repeats. In this way, a continuous stream of bubbles  30  from the gas delivery device  10  is converted into an intermittent flow of bubbles from the intermittent gas sparger  20 . 
         [0026]    In  FIG. 5 , the gas delivery device  10  is shown mounted separately and below the intermittent gas sparger  20 . Alternatively, the gas delivery device  10  may be mounted to the intermittent gas sparger  20 . In the example shown, the inlet  12  may be fitted into a receptacle  26  of the intermittent gas sparger  20 . A fastener (not shown) is then placed through an eyelet  22  on the gas delivery device  10  and into an abutment  24  on the intermittent gas sparger  20 . This results in the gas delivery device  10  being located partially within the intermittent gas sparger  20 . However, the outlets  14  are below the conduits  34  and still discharge into water below the lower limit of the pockets of gas in the cavities  32 . 
         [0027]      FIG. 6  is an isometric cross sectional view of the bottom of an alternative intermittent sparger  40 . In this example, multi-port conduits  42  provide two or more outlet paths extending upwards from the low point of each multi-port conduits  42 . A divider  44  between adjacent multi-port conduits  42  has a slot  46  extending from the bottom of the divider  44  to above the low point of the multi-port conduits  42 . Each cavity with a multi-port conduit  42  replaces two cavities with a single outlet conduit and so avoids a need to balance the supply of gas between the two replaced cavities. The slot  46  in the divider  44  helps equalize the air supply to the cavities. Gas may flow in either direction through the slot  46  but the net flow will be from a cavity that receives a larger gas flow to a cavity that receives a lower air flow. 
         [0028]    The gas delivery device  10  is preferably an open-bottomed structure. For example, the channels  18  are formed by side walls and a top. The channels  18  are open at the bottom and, preferably, at their ends. The outlets  14  may be defined by the open end of the channels  18 . The manifold  15  between the inlet  12  and the ports  16  is preferably also open at the bottom. The ports  16  are preferably slots also open at the bottom of the gas delivery device  10 . In this way, solids caught anywhere in the gas delivery device  10  beyond the inlet  12  can fall or be expelled downwards out of the gas delivery device  10 . Having such a short and simple pathway for solids to leave helps prevent fouling in the gas delivery device  10 . In the event that solids somehow still accumulate in the gas delivery device, the open-bottomed structure makes it easy to locate and remove the solids, for example by spraying water into the bottom of the gas delivery device  10 . 
         [0029]    The open-bottomed construction of the gas delivery device  10  also helps accommodate a range of input gas flow rates. At low flow rates, water enters into the gas delivery device  10  and reduces the size of the ports  16  and channels  18 . At higher gas flow rates, less water enters into the gas delivery device  10  and the ports  16  and channels  18  increase in size. The gas delivery device  10  can be made to provide a well distributed flow of air at supplied air flow rates that vary, for example, from a low flow rate to a high flow rate that is two or more times as large as the low flow rate. The gas delivery device  10  may also be operated at one or more distinct intermediate flow rates or at flow rates that vary smoothly over time. This is done without the gas delivery device  10  fouling rapidly or providing excessive back pressure at either flow rate. In comparison, aerators in the form of a horizontal tube with a series of holes can foul and provide a poor distribution of air at low flow rates and excessive back pressure at high flow rates. 
         [0030]      FIG. 8  shows a method of operating a membrane filtration system having a membrane module  50  immersed in a tank  50  as shown in  FIG. 7 . The filtration system may be part of a membrane bioreactor (MBR). The operation consists of periods of permeation followed by periods of either relaxation or backpulse, alternatively call backwash. The periods of permeation may be from 10 to 50 times as long as the periods of backwashing or relaxation. The membranes are scoured with bubbles from a gas delivery device  10 , optionally in combination with an intermittent gas sparger  20 . Scouring is used during both the permeation and the backpulse/relaxation cycles to control the accumulation of solids on the membranes and to reduce membrane fouling.  FIG. 8  illustrates of a method of air scouring in which the air flow rate remains the same throughout the permeation cycle and between permeation and relaxation cycles. During the permeation cycle, transmembrane pressure (TMP) slowly builds until the backpulse/relaxation cycle where at least some solids fouling the membrane are removed from the membrane module  50 . When the next permeate cycle is begun, the TMP is reduced but begins to rise throughout the permeate cycle as more solids foul the membranes. 
         [0031]      FIGS. 9 to 13  show alternative methods of operating a membrane filtration system having a membrane module  50  immersed in a tank  50  as shown in  FIG. 7 . When the gas delivery device  10  is used alone, the aeration flow rates in  FIGS. 8 to 13  represent both the air flow input to the gas delivery device  10  and the output of the gas delivery device  10 . When the gas delivery device  10  is used in combination with an intermittent gas sparger  20 , the aeration flow rates in  FIGS. 8 to 13  represent the air flow input to, and output from, the gas delivery device  10  and the time-averaged output of the intermittent gas sparger  20 . However, the instantaneous flow rate of the output of the intermittent gas sparger  20  does not change much, or at all, with the input flow rate. Instead, the frequency at which bursts of bubbles are released from the intermittent gas sparger  20  increases with the input gas flow rate. Accordingly, in a system having intermittent gas spargers  20 , the aeration flow rates shown in  FIGS. 8 to 13  may alternatively be thought of as representing the frequency of bursts of bubbles being released from the intermittent gas sparger  20 . 
         [0032]    In at least some situations, air scouring during backpulse/relaxation cycles is more effective in preventing solids accumulation and controlling membrane fouling compared to air scouring during permeation cycles. In one method, illustrated in  FIG. 9 , the aeration flow rate remains constant at a first aeration flow rate during the permeation cycle. The aeration flow rate is increased during the backpulse/relaxation cycle to a second aeration flow rate which is greater than the first aeration flow rate. After the completion of the backpulse/relaxation cycle and at the beginning of a new permeation cycle, the aeration flow rate is decreased to the first aeration flow rate. In some cases, the amount of energy consumed by a membrane filtration system may be reduced by using the method of  FIG. 9  rather than the method of  FIG. 8 . 
         [0033]    Solids build up in the module during the permeation cycle. The aeration flow rate can also be increased during the permeation cycle so that the aeration rate is higher during a later part of the cycle.  FIG. 10  illustrates a method wherein the aeration flow rate at the beginning of the permeation cycle is gradually increased from a first aeration flow rate to a second aeration flow rate over the course of the permeation cycle and as solids build up. Alternatively, the aeration flow rate can go through one or more abrupt or step form changes from the first aeration flow rate to the second aeration flow rate within the permeation cycle. At the end of the permeation cycle and at the beginning of the backpulse/relaxation cycle, the aeration flow rate is increased to a third aeration flow rate. After the completion of the backpulse/relaxation cycle and at the beginning of a new permeation cycle, the aeration flow rate is decreased to the first aeration flow rate. 
         [0034]      FIG. 11  illustrates a method wherein the aeration flow rate varies one or more times within a permeation cycle from a first aeration flow rate to a second aeration flow rate. During the backpulse/relaxation cycle, the second aeration flow rate or an even higher third aeration flow rate may be used. In the example illustrated, the aeration flow rate remains constant at a first aeration flow rate from the beginning of the permeation cycle. After a predetermined period of time during the course of the permeation cycle, the flow rate is increased from the first aeration flow rate to a second aeration flow rate for a predetermined period of time. After the completion of the predetermined period of time, the aeration flow rate is reduced to the first aeration flow rate. At the end of the permeation cycle and at the beginning of the backpulse/relaxation cycle, the aeration flow rate is increased to a third aeration flow rate, wherein the third aeration flow rate is greater than the second aeration flow rate. After the completion of the backpulse/relaxation cycle and at the beginning of a new permeation cycle, the aeration flow rate is decreased to the first aeration flow rate. 
         [0035]    While  FIG. 11  illustrates two occurrences of an increase of the aeration flow rate from the first flow rate to the second flow rate, any number of occurrences of an increase of the aeration flow rate may be applied during the permeation cycle. The first aeration flow rate illustrated in  FIG. 11  may be any flow rate. For example, the first aeration flow rate of the air scouring method illustrated in  FIG. 11  may be zero. 
         [0036]      FIG. 12  illustrates a method wherein aeration is provided in distinct periods of time during the permeation cycle. The aeration flow rate increases from one distinct period of time to the next and then the aeration flow rate may be further increased during the backpulse/relaxation cycle. Optionally, a lower aeration flow rate, or no air flow, may be provided between the distinct periods of time. In the example shown, the aeration flow rate remains constant at a first aeration flow rate from the beginning of the permeation cycle. After a predetermined period of time during the course of the permeation cycle, the flow rate is increased from the first aeration flow rate to a second aeration flow rate for a predetermined period of time. After the completion of the predetermined period of time, the aeration flow rate is reduced to the first aeration flow rate. After another predetermined period of time, the flow rate is increased from the first aeration flow rate to a third aeration flow rate for a predetermined period of time, wherein the third aeration flow rate is greater than the second aeration flow rate. At the end of the permeation cycle and at the beginning of the backpulse/relaxation cycle, the aeration flow rate is increased to a fourth aeration flow rate, wherein the fourth aeration flow rate is greater than the third aeration flow rate. After the completion of the backpulse/relaxation cycle and at the beginning of a new permeation cycle, the aeration flow rate is decreased to the first aeration flow rate. 
         [0037]    While  FIG. 12  illustrates two occurrences of an increase of the aeration flow rate during the permeation cycle, any number of occurrences of an increase of the aeration flow rate may be applied during the permeation cycle. The first aeration flow rate illustrated in  FIG. 12  may be any flow rate. According to one embodiment, the first aeration flow rate of the air scouring method illustrated in  FIG. 12  may be zero. 
         [0038]      FIG. 13  illustrates a method wherein air scouring is only used during the backpulse/relaxation cycle and all aeration is discontinued during the permeation cycle. As illustrated, an aeration flow rate of zero is applied during the permeation cycle. At the end of the permeation cycle and at the beginning of the backpulse/relaxation cycle, the aeration flow rate is increased to a first aeration flow rate. After the completion of the backpulse/relaxation cycle and at the beginning of a new permeation cycle, the aeration flow rate is decreased to zero. 
         [0039]    In the description above, one or more aeration flow rates are said to begin with the beginning of a backwash/relaxation cycle and to end with the completion of a backwash/relaxation cycle. This is meant to be approximate. The stated aeration flow rate preferably includes at least the period of time occupied by the backwash or relaxation but may begin before the backwash or relaxation, or continue after the backwash or relaxation, or both. 
         [0040]      FIGS. 8 to 13  do not have specific time scales. However, permeation cycles typically last for 15 minutes or more, sometime 30 minutes or more. Backwash/relaxation cycles are typically at least one minute long and sometimes more than two minutes long. Changes in aeration flow rate between discrete aeration flow rates during the permeation period are preferably applied for at least one minute, more preferably for at least two minutes or five minutes. In contrast, an intermittent gas sparger  20  typically releases bursts of bubbles once every 4 to 30 seconds, more often once every 4 to 15 seconds. Accordingly, the changes in aeration flow rate shown in  FIGS. 8 to 13  do not represent individual bursts of bubbles from an intermittent gas sparger  20 . Instead, if an intermittent gas sparger  20  is used, it typically releases a plurality of bursts of bubbles within a period of time shown as having a particular aeration flow rate. 
         [0041]    The aeration flow rate (or frequency of bursts of bubbles from an intermittent gas sparger  20 ) could also be controlled considering one or more properties of feed water, of water in a process tank of a bioreactor or the tank  52  containing a membrane module  50 , or considering the performance of the membrane module  50 . Viscosity, mixed liquor suspended solids (MLSS) concentrations, extracellular polymer (ECP) concentrations, soluble microbial product (SMP) concentrations, a fouling index, temperature, and a rate of membrane fouling or recovery of flux after backwashing are some of the properties that could be used to control aeration flow rate, to determine when to change between one or more of the methods shown in  FIGS. 8 to 13 , or to control the timing of changes between aeration flow rates in a method shown in  FIGS. 9 to 13 . For example, a higher aeration flow rate, or more time at a higher aeration flow rate, or a change to a more intensive aeration process may be applied as the viscosity of water being fed to the filtration system of the tank  52  increases or its temperature decreases. In another example, a higher aeration flow rate, or more time at a higher aeration flow rate, or a change to a more intensive aeration process may be applied at higher MLSS concentrations. In another example, a higher aeration flow rate, or more time at a higher aeration flow rate, or a change to a more intensive aeration process may be applied when electricity may be obtained at a lower cost. 
         [0042]    Optionally, to reduce the frequency of changes in the speed of a blower proving gas to a plurality of gas delivery devices  10  (alone or in combination with intermittent gas spargers  20 ), a valve set may be provided between a blower and the gas delivery devices  10 . The valve set distributes the flow provided by the blower between two or more distinct branches of an air delivery system connected to the gas delivery devices such that flow in one distinct branch may vary in a time period during which the blower output does not vary. For example, at one period of time a higher aeration flow rate is applied to a first distinct branch and a lower aeration flow rate is applied to a second distinct branch, while at a second period of time the higher aeration flow rate is applied to the second distinct branch and the lower aeration flow rate is applied to the first distinct branch. Alternatively or additionally, the blower may provide air at a generally constant rate to a pressure tank or other accumulator while a valve between the pressure tank and the gas delivery device  10  is modulated to provide the required variation in aeration flow rate. 
         [0043]    Optionally, gas delivery devices  10  may also be connected to a cyclic aeration system as used in some ZeeWeed MBRs sold by GE Water and Process Technologies. In this case, even without an intermittent gas sparger  20 , the gas delivery device  10  produces a flow of bubbles for a period of about 2 to 20 seconds followed by a period in which the gas delivery device  10  produces a lesser flow of bubbles or no bubbles for a period of about 2 to 60 seconds, these periods alternating in repeated cycles over time. For example, one gas delivery device  10  can produce bubbles for 10 seconds and then be off for 10 seconds while a second gas delivery device is off for 10 seconds and then produces bubbles for 10 seconds. In this case, in the methods of  FIGS. 8 to 13 , the aeration flow rate can be interpreted as a time averaged aeration flow rate. 
         [0044]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.