Abstract:
An assembly of immersed filtering membrane modules on the order of a cassette or larger are aerated in a series of air-on or high flow rate periods, each in the range of 0.5 to 20 seconds long, separated by longer air-off or low flow rate periods, each in the range of 5 to 40 seconds long. A manifold is in fluid communication with one or more aerators. The manifold and aerators are located in association with the modules. The assembly of immersed filtering membrane modules are provided with a burst of air from the plurality of aerators. The aerators may be fed with a continuous supply of air. The modules may be connected together in a cassette or rack. Bubbles from the aerators may be redirected or dissipated with a deflector before they contact the modules.

Description:
FIELD 
       [0001]    The present disclosure relates to submerged membrane filtration and particularly to using scouring air bubbles produced by an aeration system to clean or inhibit the fouling of membranes in a submerged membrane filter. 
       BACKGROUND 
       [0002]    The following discussion is not an admission that anything discussed below is common knowledge or citable as prior art. 
         [0003]    Immersed membranes are used, for example, for filtering water in municipal water supply treatment plants and in wastewater treatment plants. The membranes may be microfiltration or ultrafiltration membranes made in the form of modules of hollow fiber membranes. In large plants, many modules are immersed in the water to be filtered in an open tank and filtered water, also called permeate, is withdrawn through the membranes by suction. In a typical filtration cycle, a period of permeation, for example for 15 minutes to an hour, is followed by a relatively short period of backwashing or relaxation, and then the cycle repeats. In wastewater plants, and also in some water supply plants, the membranes are scoured with bubbles during permeation to clean or inhibit fouling in the membranes. The bubbles are provided according to an aeration regime that is applied throughout most, and possibly all, of the permeation part of the filtration cycle, and possibly during the backwash or relaxation part of the cycle as well. The aeration regime uses a significant amount of energy, adding to the cost and environmental impact of the plant. Accordingly, there have been various attempts to reduce the energy required by the aeration regime while still maintaining an acceptable fouling rate in the membranes. 
         [0004]    A cyclic aeration system is described in International Publication Number WO 2000/021890, published Apr. 20, 2000. In a version of this system used by GE Water and Process Technologies with its ZeeWeed™ hollow fiber membrane modules, air from one or more blowers is split through a set of controlled valves into two or four sets of aerators. The valves distribute the air to the sets of aerators in turn. For example, with two sets of aerators the air may be sent first to one set of aerators and then to the other set of aerators. An assembly of filtering membrane modules are connected together in a frame to provide a larger unit called a rack or cassette. There are typically several aerators below a cassette, but all of these aerators are part of the same assembly of aerators. In this case, the cassette experiences an aeration regime wherein bubbles are provided for a period of time, and then not generally provided for a period of time, in repeated cycles. For example, with two sets of aerators, bubbles may be provided to a cassette for 10 seconds out of every 20 seconds. This is also called a 10 seconds on, 10 seconds off cycle, or simply a 10, 10 cycle. With four sets of aerators, bubbles may be provided to a cassette for 10 seconds out of every 40 seconds, or in a 10, 30 cycle. In some plants, the valve set allows the system to switch between these two regimes depending, for example, on variations in the fouling qualities of the water or the feed flow rate. 
         [0005]    In the Mempulse™ system by Siemens Water Technologies Corp., each module is provided with an integrated pulsing air lift pump. Each module in a rack or cassette has an air tube connecting its air lift pump to a common air supply pipe. The air tube discharges air into an inverted cup in the pulsing air lift pump. A pocket of air grows inside the cup until it extends downwards to a critical elevation, at which point most of the air in the pocket discharges into the adjacent vertical passageway. The passageway is open at its bottom end to draw in water in the tank, and its top is open to gaps in a lower potting head of the module. An air lift is formed in the passageway as the air rushes through, and bubbles and water are discharged through the gaps in the lower potting head into the module. The module experiences an aeration regime that, as in the cyclic aeration system, consists essentially of an air on-air off cycle. The precise timing of the cycle in commercial systems is not known to the inventor, but the related International Publication Number WO 2008/153818, published Dec. 18, 2008 suggests a cycle of roughly 2 seconds of bubbles, with entrained water, followed by 8 seconds without bubbles. 
       INTRODUCTION TO THE INVENTION 
       [0006]    The cyclic aeration system has proven to be generally at least as effective as continuous aeration in inhibiting membrane fouling while requiring only one half the amount of air. The Mempulse™ system avoids using controlled valves, but the pulsing air lift pumps might sometimes be fouled by solids in the wastewater. The inverted cup shape makes the airlift pump susceptible to fouling. Their location at the bottoms of the modules would make the air lift pumps difficult to monitor and service if they did foul. The 2-8 cycle appears to use less air than a 10-10 or 10-30 cycle because the air is on for a lesser percentage of the entire cycle time but the air flow rate in the air on time, which is unknown to the inventor, would also need to be considered in determining how much air is used in each regime. The air pressure in the inverted cup is limited to hydrostatic pressure. 
         [0007]    In International Publication Number WO 2008/153818, the inventors of the air cycling system noted that a burst of large bubbles produced at the start of an air on period might be particularly effective at cleaning the membranes. The Mempulse™ system might seem to take advantage of such a burst of bubbles, but if so the effect is diminished by other aspects of the system. In particular, some of the energy of the bubbles is spent in drawing water through the air lift pumps. 
         [0008]    In a process described herein, an aeration regime comprising bursts of bubbles is applied to an immersed assembly of filtering membrane modules, such as a rack or cassette of membrane modules. The bursts are provided by accumulating gas under pressure in one or more aerators and then releasing a burst of the gas from the one or more aerators to the assembly of filtering membrane modules above. The aeration regime may comprise, for example, a series of air-on or high flow rate periods, each in the range of 0.5 to 4 seconds long, separated by longer air-off or low flow rate periods, each in the range of 5 to 40 seconds long. Alternatively, longer air-on periods of 4 to 20 seconds may also be provided. The aerators are located inside a tank holding the rack or cassette, and may be fed with a continuous supply of gas. 
         [0009]    In an apparatus described herein, one or more aerators are attached to and in fluid communication with one or more manifolds. Each of the manifolds are associated with one or more immersed assemblies of filtering membrane modules. Filtering membrane modules may be connected together in a cassette or rack. The one or more aerators and the manifolds to which the aerators are attached, are all located inside a tank of water with the assembly of filtering membrane modules. The one or more aerators may be fed with a continuous supply of gas through their corresponding manifold from one or more air blowers operating at a generally continuous speed and in fluid communication with the manifold. The release of gas from the aerators may be provided from a pressure sensitive automatic valve. The valve is located between the aerator and the water in the tank. Upon closing, the valve prevents sludge from entering and fouling the aerator or the manifold. The aerators, the manifolds, and the air blowers form a closed aeration system such that no sludge or wastewater may enter when the aerator valves are closed. After a threshold pressure in the aerator is reached, an aerator valve opens automatically to release the pressurized gas from the aeration system. The force of the pressurized gas as it exits from the aerators through the valve prevents wastewater from entering the aeration system when the valve is open. The valve closes automatically before the pressure in the aerator drops below the static pressure of the surrounding water to prevent water from entering the aerator. If a valve becomes fouled, the burst of the pressurized gas exiting the valve when the valve opens on its following cycle removes the fouling without having to alter the normal operation of the aeration system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    One or more embodiments of the invention will now be described, by way of example, with reference to the attached Figures. 
           [0011]      FIG. 1  shows a water treatment system in top view. 
           [0012]      FIG. 2  shows the water treatment system of  FIG. 1  in end view. 
           [0013]      FIG. 3A  shows an aeration device of the water treatment system of  FIG. 1 . 
           [0014]      FIG. 3B  shows a deflector connected to an aerator of the aeration device of  FIG. 3A . 
           [0015]      FIG. 3C  shows a plurality of aerators aligned on a branch of the aeration device of  FIG. 3A . 
           [0016]      FIG. 3D  shows an alternative arrangement to  FIG. 3C  of aligning a plurality of aerators on a branch of the aeration device of  FIG. 3A . 
           [0017]      FIG. 4  shows an assembly of filtering membrane modules connected in a cassette. 
           [0018]      FIGS. 5A to 5D  show a device for discharging bursts of a pressurized gas. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 1  shows a water treatment system  2  from a top view. The water treatment system  2  has one or more filtration tanks  4 . If there are multiple tanks  4 , they may be aligned in parallel and separated by a partition wall  3 . Each tank  4  accepts feed water through an inlet  6 . Water may be removed from a tank  4  through one or more of a permeate withdrawal system, a drain and a recycle conduit, which are not shown to simplify  FIG. 1  since they are conventional parts of a water treatment system using immersed filtering membrane modules. 
         [0020]    The tank  4  has one or more assemblies of filtering membrane modules  8  shown in dashed lines in  FIG. 1 . During filtration, the one or more assemblies of filtering membrane modules  8  are used to withdraw permeate from water in the tank  4  while rejecting solids, which remain in the tank  4  until they are digested, drained or removed in a recirculation line. A manifold  10  in fluid communication with one or more aerators  30  is associated with one or more of the assemblies of filtering membrane modules  8 . The manifold  10  delivers a gas to the one or more aerators  30  located inside the tank  4  and beneath the one or more assemblies of filtering membrane modules  8 . The one or more aerators  30  each independently release pressurized gas as bubbles which rise through the one or more associated assemblies of filtering membrane modules  8  to scour or clean the membranes. 
         [0021]    To provide air to the manifolds  10  and then to the one or more aerators  30 , air is drawn in through an air inlet  12  to a set of blowers  14 . The blowers  14  may operate at a generally constant speed, thus providing a generally constant flow rate of air. By turning off one or more of the blowers  14 , or reducing the speed of one or more of the blowers, a different flow rate of air can be provided. However, turning a blower  14  on and off frequently, or varying its speed often, can reduce the life of the blower  14 . Accordingly, it is preferable to leave the blowers  14  either on or off, and when on, operate the blowers  14  at a constant speed for at least an hour or more at a time. Alternatively, systems can be provided to supply other gases, such as nitrogen, oxygen or oxygen enriched air or biogas. 
         [0022]    The gas travels through a blower outlet pipe  16  to one or more distribution pipes  22  and then to one or more feeder pipes  33 . Each feeder pipe  33  provides the gas to the one or more manifolds  10  which are connected to one or more aerators  30 . Gas accumulates in the aerators  30  by increasing in pressure, optionally in combination with expansion of the aerators  30 . When gas is released from an aerator  30 , the gas flows out of the aerator  30  into the tank  4  and rises as bubbles through the one or more associated assemblies of filtering membrane modules  8  above. As will be described further below, gas is released from the aerators  30  in bursts, causing corresponding bursts of bubbles to be emitted. 
         [0023]      FIG. 2  shows the water treatment system  2  of  FIG. 1  in a cross sectioned end view. As shown, an assembly of filtering membrane modules  8  is positioned above one or more aerators  30  connected in fluid communication to the manifold  10 . The aerators  30  receive gas from the gas blowers  14  shown in  FIG. 1  by way of the blower outlet pipe  16  and distribution pipe  22  shown in  FIG. 1 , the feeder pipe  33 , and the manifolds  10 . Bubbles released from the aerators  30  float towards the surface of the water in the tank  4 . As they do so, the bubbles pass through the one or more assemblies of filtering membrane modules  8  to scour the membranes, thus cleaning them or decreasing their rate of fouling. 
         [0024]    In operation, gas is emitted from one or more aerators  30 , and bubbles are released into the water in the tank  4  at a rate that varies in repeated cycles. Each cycle may include a period during which gas accumulates in the aerator  30 , followed by the release of a burst of the gas. Alternatively, a cycle may be described as having an “air-on” time and an “air-off” time. The flow rate during the burst, or air on time, could be generally constant or might rise to a peak flow rate and then decline again. There may be an abrupt transition to the accumulation stage, or air-off time, during which the flow of gas may be completely stopped. Alternatively, the flow rate in the air-on time may decrease gradually and the air air-off time may be deemed to occur when the rate of gas flow from an aerator  30  is 10% or less of the peak gas flow rate in the air on-time. The duration of the burst, or air-on time, may be between 0.5 and 20 seconds or between 0.5 and 4 seconds. The duration of the accumulation stage, or air-off time, may be between 5 and 40 seconds. 
         [0025]    In the system  2  shown in  FIG. 1 , the timing of bursts of gas from the various aerators  30  is not intentionally synchronized. Although aerators  30  might emit their first burst of gas at the same time, over time the cycles of different aerators  30  may diverge from each other. 
         [0026]      FIG. 3A  shows a manifold  10  and the aerators  30  in greater detail. The manifold  10  has a header pipe  34 . The header pipe  34  is connected to and receives gas from the feeder pipe  33  shown in  FIG. 2  which is connected to and receives gas from the distribution pipe  22  shown in  FIG. 1 . A plurality of individual branches  36  extend from the header pipe  34 . Each of the branches  36  may be a section of cylindrical pipe with one end connected to and in fluid communication with the header pipe  34 , and the other end closed. Attached and in fluid communication with an individual branch  36  are one or more aerators  30 . 
         [0027]      FIG. 3B  shows a deflector  28  connected to, and spaced vertically from, the top of the aerator  30 . The deflector  28  is an inverted cup which is affixed to the aerator  30  by two arms  29  which extend upwardly from opposite sides of the body of the aerator  30  to the rim of the deflector  28 . The deflector  28  is located above each aerator  30  but below the assembly of filtering membrane modules  8  (not shown). As pressurized gas is released from the aerator  30  as bubbles  31 , and the bubbles rise and come in contact with the deflector  28 , the deflector  28  may dissipate large bubbles into smaller bubbles and/or redirect the bubbles. 
         [0028]      FIG. 3C  shows a branch  36  of aerators  30  in greater detail. The aerators  30  are circumferentially aligned and longitudinally spaced lengthwise along the branch  36 . 
         [0029]      FIG. 3D  shows an alternative arrangement of the aerators  30  along the branch  36  of  FIG. 3C . In the alternative arrangement, the aerators  30  are longitudinally spaced along, and circumferentially around, the top half portion of the branch  36 . 
         [0030]      FIG. 4  shows the assembly of filtering membrane modules  8  in greater detail. The assembly of filtering membrane modules  8  has one or more filtering membrane modules  40  arranged side by side. The filtering membrane modules  40  may be connected to each other or to a common frame (not shown). Each of the plurality of filtering membrane modules  40  has hollow fiber membranes  42  oriented generally vertically between two potting heads  44 . The ends of the hollow fiber membranes  42  are connected to potting heads  44  by a watertight connection that allows permeate to be collected in at least one of the potting heads  44 . 
         [0031]    A potting head  44  may have a spigot  46  which is connected to one or more permeate collection pipes  48 . The permeate connection to only one of the filtering membrane modules  40  is shown to simplify  FIG. 2  but typically all of the filtering membrane modules  40  in a membrane assembly  8  would be connected at some point to a common permeate collection pipe  48 . The permeate collection pipes  48  are connected to a permeate pump  50 . When the permeate pump  50  is operated, a negative pressure is created in the hollow fiber membranes  42  relative to water in the tank  4  surrounding the hollow fiber membranes  42 . The resulting transmembrane pressure draws water through the hollow fiber membranes  42 . 
         [0032]    An assembly of filtering membrane modules  8  can also be made according to other configurations. For example, filtering membrane modules  40  of vertically oriented hollow fiber membranes  42  may be round or square and connected together in rows or grids to form an assembly of filtering membrane modules  8 . Filtering membrane module  40  may also have hollow fiber membranes  42  oriented horizontally. Filtering membrane modules  40  with horizontal membranes may be arranged into an assembly of filtering membrane modules  8  by placing them side by side or in grids, or by stacking them on top of each other, or both. Filtering membrane modules  40  may also have flat sheet membranes, and may be arranged into an assembly of filtering membrane modules  8  by placing them side by side or in grids, or by stacking them on top of each other, or both. 
         [0033]      FIGS. 5A to 5D  show an example of an aerator  30  in sequential stages of operation. The aerator  30  of  FIGS. 5A to 5D  has a flexible membrane  52  with a first end  54  connected to an inlet  26 . A second end  56  of the flexible membrane  52  is connected to an outlet  32 . A stopper  58  is provided at the second end  56 . The stopper  58  and the second end  56  cooperate to form a valve which prevents gas from exiting from the flexible membrane  52  until a threshold pressure within the flexible membrane  52  is reached. When the threshold pressure is reached, the second end  56  expands so that the seal between the stopper  58  and the second end  56  is released. 
         [0034]    As shown in  FIG. 5B , the flexible membrane  52  expands as gas accumulates therein. In  FIG. 5C , the gas pressure in the flexible membrane  52  reaching the threshold pressure causes the second end  56  to expand and release the seal with the stopper  52 . When this happens, a burst of pressurized gas  60  is released from the flexible membrane  52 , past the stopper  58 , and through the outlet  32 . As the gas exits from the flexible membrane  52 , the flexible membrane  52  contracts to its initial unpressurized state as shown in  FIG. 5D . The stopper  58  again forms a seal with the second end  56  and the process of filling the flexible membrane  52  with gas repeats. 
         [0035]    Alternatively, an aerator  30  with a rigid reservoir and another automatic valve between the aerator  30  and the outlet  32  can be provided. The valve may be configured to open automatically when the threshold pressure is reached, and to stay open until a lower threshold pressure is reached. 
         [0036]    Optionally, the flexible membrane  52  may be contained within a shell  62 . The shell  62  can be filled with a resilient material. For example, the shell  62  can be sealed to the inlet  26  and the outlet  32  and filled with compressed air. The compressed air may have a higher pressure than the surrounding water. In this way, the compressed air helps the flexible membrane  52  to close on the stopper  58  at a pressure above the surrounding hydrostatic pressure and so enhances the ability of the aerator  30  to produce a higher pressure burst of pressurized gas  60 . 
         [0037]    This written description uses examples to disclose the invention and also to enable any person skilled in the art to practice the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art.