Abstract:
A method of cleaning a membrane surface immersed in a liquid medium with a fluid flow, including the steps of providing a randomly generated intermittent or pulsed fluid flow along the membrane surface to dislodge fouling materials therefrom. A membrane module is also disclosed comprising a plurality of porous membranes ( 6 ) or set of membrane modules ( 5 ) and a device ( 11 ) for providing a generally randomly generated, pulsed fluid flow such that, in use, said fluid flow moves past the surfaces of said membranes ( 6 ) to dislodge fouling materials therefrom.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation application of U.S. application Ser. No. 12/602,316 filed on May 29, 2008, entitled MEMBRANE CLEANING WITH PULSED AIRLIFT PUMP, which is a U.S. national stage application and claims the benefit under 35 U.S.C. §371 of International Application No. PCT/US2008/006799 filed on May 29, 2008, entitled MEMBRANE CLEANING WITH PULSED AIRLIFT PUMP, which claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application Ser. No. 60/940,507, entitled MEMBRANE CLEANING WITH PULSED AIRLIFT PUMP, filed on May 29, 2007, each of which is herein incorporated by reference in their entirety for all purposes and to which this application claims the benefit of priority. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to membrane filtration systems and, more particularly, to apparatus and related methods to effectively clean the membranes used in such systems by means of pulsed fluid flow. 
       BACKGROUND OF THE INVENTION 
       [0003]    The importance of membranes for treatment of wastewater is growing rapidly. It is now well known that membrane processes can be used as an effective tertiary treatment of sewage and provide quality effluent. However, the capital and operating cost can be prohibitive. With the arrival of submerged membrane processes where the membrane modules are immersed in a large feed tank and filtrate is collected through suction applied to the filtrate side of the membrane or through gravity feed, membrane bioreactors combining biological and physical processes in one stage promise to be more compact, efficient and economic. Due to their versatility, the size of membrane bioreactors can range from household (such as septic tank systems) to the community and large-scale sewage treatment. 
         [0004]    The success of a membrane filtration process largely depends on employing an effective and efficient membrane cleaning method. Commonly used physical cleaning methods include backwash (backpulse, backflush) using a liquid permeate or a gas or combination thereof, membrane surface scrubbing or scouring using a gas in the form of bubbles in a liquid. Typically, in gas scouring systems, a gas is injected, usually by means of a blower, into a liquid system where a membrane module is submerged to form gas bubbles. The bubbles so formed then travel upwards to scrub the membrane surface to remove the fouling substances formed on the membrane surface. The shear force produced largely relies on the initial gas bubble velocity, bubble size and the resultant of forces applied to the bubbles. To enhance the scrubbing effect, more gas has to be supplied. However, this method consumes large amounts of energy. Moreover, in an environment of high concentration of solids, the gas distribution system may gradually become blocked by dehydrated solids or simply be blocked when the gas flow accidentally ceases. 
         [0005]    Furthermore, in an environment of high concentration of solids, the solid concentration polarisation near the membrane surface becomes significant during filtration where clean filtrate passes through membrane and a higher solid-content retentate is left, leading to an increased membrane resistance. Some of these problems have been addressed by the use of two-phase flow to clean the membrane. 
         [0006]    Cyclic aeration systems which provide gas bubbles on a cyclic basis are claimed to reduce energy consumption while still providing sufficient gas to effectively scrub the membrane surfaces. In order to provide for such cyclic operation, such systems normally require complex valve arrangements and control devices which tend to increase initial system cost and ongoing maintenance costs of the complex valve and switching arrangements required. Cyclic frequency is also limited by mechanical valve functioning in large systems. Moreover, cyclic aeration has been found to not effectively refresh the membrane surface. 
         [0007]    It would be desirable to provide an energy efficient operation of the scouring process without the need to control the operation by means of complex valve switching etc. It is also preferable to provide a two-phase liquid gas flow past the membrane surfaces to provide a more effective scouring process while minimising energy requirements for such a cleaning process. 
       DISCLOSURE OF THE INVENTION 
       [0008]    The present invention, at least in its embodiments, seeks to overcome or least ameliorate some of the disadvantages of the prior art or at least provide the public with a useful alternative. 
         [0009]    According to one aspect, the present invention provides a method of cleaning a membrane surface immersed in a liquid medium with a fluid flow, including the steps of providing a randomly generated intermittent or pulsed fluid flow along said membrane surface to dislodge fouling materials therefrom and reduce the solid concentration polarisation. For preference, the fluid flow includes a gas flow. Preferably, the gas flow is in the form of gas bubbles. For further preference, the fluid flow includes a two phase gas/liquid flow. Preferably, the method includes producing the pulsed two-phase gas/liquid flow using a device supplied with a flow of pressurised gas. For further preference, the supply of pressurised gas flow is essentially constant. Preferably, the pulsed fluid flow is random in magnitude and/or frequency and/or duration. 
         [0010]    In one form of the invention, the pulsed two-phase gas/liquid flow is used in conjunction with an essentially constant two-phase gas/liquid flow. 
         [0011]    Optionally, an additional source of bubbles may be provided in said liquid medium by means of a blower or like device. The gas used may include air, oxygen, gaseous chlorine, ozone, nitrogen, methane or any other gas suitable for a particular application. Air is the most economical for the purposes of scrubbing and/or aeration. Gaseous chlorine may be used for scrubbing, disinfection and enhancing the cleaning efficiency by chemical reaction at the membrane surface. The use of ozone, besides the similar effects mentioned for gaseous chlorine, has additional features, such as oxidizing DBP precursors and converting non-biodegradable NOM&#39;s to biodegradable dissolved organic carbon. In some applications, for example, an anaerobic biological environment or a non-biological environment where oxygen or oxidants are undesirable, nitrogen may be used, particularly where the feed tank is dosed with ability to capture and recycle the nitrogen. 
         [0012]    According to a second aspect, the present invention provides a membrane module comprising a plurality of porous membranes or a set of membrane modules and means for providing a randomly generated, pulsed fluid now such that, in use, said fluid flow moves past the surfaces of said membranes to dislodge fouling materials therefrom. For preference, the fluid flow includes a gas flow which generates bubbles which move past the surfaces of said membranes. For further preference, the fluid flow includes a two phase gas/liquid flow. Preferably, said pulsed two-phase gas/liquid flow is produced by a device provided with an essentially constant supply of gas. Preferably, the pulsed fluid flow is random in magnitude and/or frequency and/or duration. 
         [0013]    Where a set of membrane modules are used, the modules are generally assembled in an array, rack or a cassette located in a feed containing vessel or tank. To clean a rack or a cassette of membrane modules, the device for providing the pulsed gas or two-phase gas/liquid flow can be connected to a distributor and the pulsed gas bubbles generated are distributed into the modules through the distributor. It is preferred to arrange one device to one module or to a small number of modules. Accordingly, there typically are a number of devices installed for one rack or cassette. Gas is preferably supplied to the rack and then distributed to each device along the rack manifold. Although the gas is supplied to individual device in a continuous mode, the eruption of gas bubbles from the devices along the rack is produced at random times, keeping the membrane tank feed essentially constantly in an unstable condition. This effect reduces solid concentration polarisation and hence the filtration resistance. When looking down from the top of a rack, gas bubbles appear randomly from different modules within the rack, forming a random distribution pattern. 
         [0014]    Even where the gas supply to the rack is continuous and at the same flow rate, the volumetric gas flow to an individual module generally fluctuates in a small range, generally in less than 15%. This is due to the variation in back pressure inside the pulsed gas-lift device. 
         [0015]    For preference, the membranes comprise porous hollow fibers, the fibers being fixed at each end in a header, the lower header having one or more holes formed therein through which the two-phase gas/liquid flow is introduced. The holes can be circular, elliptical or in the form of a slot. The fibers are normally sealed at one end (usually, the lower end) and open at their other end to allow removal of filtrate, however, in some arrangements, the fibers may be open at both ends to allow removal of filtrate from one or both ends. The sealed ends of the fibers may be potted in a potting head or may be left unpotted. The fibers are preferably arranged in mats, cylindrical arrays or bundles. It will be appreciated that the cleaning process described is equally applicable to other forms of membrane such flat or plate membranes. 
         [0016]    For further preference, the membranes comprise porous hollow fibers, the fibers being fixed at each end in a header to form a sub-module. A set of sub-modules are assembled to form a module. Between sub-modules, one or more holes are left to allow the passage or distribution of gas/liquid into the sub-modules. 
         [0017]    According to one preferred form, the present invention provides a method of removing fouling materials from the surface of a plurality of porous hollow fiber membranes mounted and extending longitudinally in an array to form a membrane module, said membranes being arranged in close proximity to one another and mounted to prevent excessive movement therebetween, the method comprising the steps of providing a generally random, uniformly distributed pulsed gas bubble flow past the surfaces of said membranes, said distribution being such that said bubbles flow substantially uniformly between each membrane in said array to scour the surface of said membranes and remove accumulated solids from within the membrane module. 
         [0018]    For preference, gas bubble flow further includes a two phase gas/liquid flow. Preferably, said pulsed two-phase gas/liquid flow is produced by a device provided with an essentially constant supply of gas. Preferably, the pulsed gas flow is random in magnitude and/or frequency and/or duration. 
         [0019]    According to a third aspect the present invention provides a membrane module comprising a plurality of porous hallow fiber membranes, said fiber membranes being arranged in close proximity to one another and mounted to prevent excessive movement therebetween, the fiber membranes being fixed at each end in a header, one header having one or more openings formed therein through which a generally random pulsed fluid flow is introduced for cleaning the surfaces of said hollow fiber membranes. 
         [0020]    For preference, the fluid flow includes a gas flow. For further preference, the gas flow is in the form of gas bubbles. For preference, gas flow includes a two phase gas/liquid flow. Preferably, said pulsed two-phase gas/liquid flow is produced by a device provided with an essentially constant supply of gas. Preferably, the pulsed fluid flow is random in magnitude and/or frequency and/or duration. 
         [0021]    Preferably, the device includes a gaslift pump apparatus operative in response to said essentially constant supply of pressurized gas from a gas source connected thereto to store and randomly release pressurized gas and use the released pressurized gas to gaslift quantities of said liquid from a reservoir of liquid to produce said pulsed two-phase gas/liquid flow. 
         [0022]    For preference, said gaslift pump apparatus includes an inverted gas storage chamber for storing said gas provided by said gas source and having a closed upper end and an open lower end positioned in said reservoir of liquid, a vertical riser tube having an inlet port in fluid communication with said reservoir of liquid and an outlet port in fluid communication with said membrane module, said riser tube having an opening in fluid communication with said gas storage chamber positioned for receiving said stored gas from said chamber when the level of gas within the chamber reaches a predetermined level and gaslifting the liquid through said riser tube for discharge into said module. Preferably, the vertical riser tube is located within the gas storage chamber. 
         [0023]    In one embodiment of the invention, the supply of gas may be provided by an external tank containing pressurised gas, the tank being in fluid communication with the membrane module and having control means for providing randomly generated pulses of gas to the module to form said two phase gas/liquid flow for cleaning the membrane surfaces. In one embodiment, the control means may comprise a device positioned in a gas/liquid inlet to the membrane module and operable in dependence on the level of liquid in the inlet to provide gas from the external tank. For example, a float device could be used to activate the control means depending on the liquid level. 
         [0024]    Preferably, the fibers may be protected and fiber movement is limited by a module support screen which has both vertical and horizontal elements appropriately spaced to provide unrestricted fluid and gas flow through the fibers and to restrict the amplitude of fiber motion reducing energy concentration at the potted ends of the fibers. 
         [0025]    For preference, the module may be encapsulated in a substantially solid or liquid/gas impervious tube and connected to the pulsed gas-lift pump device so as to retain the two-phase gas/liquid flow within the module. 
         [0026]    For preference, said openings comprise a slot, slots or a row of holes. Preferably, the fiber bundles are located in the potting head between the slots or rows of holes. 
         [0027]    The liquid used may be the feed to the membrane module. 
         [0028]    For preference, the pulse frequency of the randomly generated pulses varies in a range of generally from 0.1 to 200 seconds. It will be appreciated the pulse frequency is related to the structure of the device and with a particular structure, the pulse frequency preferably varies in a range of about 10 to about 300%. 
         [0029]    Preferably, the pulsed gas-lift pump device can be optionally connected in fluid communication with a fluid distributor to substantially uniformly distribute the pulsed gas bubbles into the filtration module or modules. 
         [0030]    Preferably, the fibers within the module have a packing density (as defined above) of between about 5 to about 80% and, more preferably, between about 8 to about 55%. 
         [0031]    For preference, said holes have a diameter in the range of about 1 to 40 mm and more preferably in the range of about 1.5 to about 25 mm. In the case of a slot or row of holes, the open area is chosen to be equivalent to that of the above holes. 
         [0032]    Typically, the fiber inner diameter ranges from about 0.1 from to about 5 mm and is preferably in the range of about 0.25 mm to about 2 mm. The fibers wall thickness is dependent on materials used and strength required versus filtration efficiency. Typically wall thickness is between 0.05 to 2 mm and more often between 0.1 mm to 1 mm. 
         [0033]    According to another aspect, the present invention provides a membrane bioreactor including a tank having means for the introduction of feed thereto, means for forming activated sludge within said tank, a membrane module according to the third aspect positioned within said tank so as to be immersed in said sludge and said membrane module provided with means for withdrawing filtrate from at least one end of said membranes. 
         [0034]    According to yet another aspect, the present invention provides a method of operating a membrane bioreactor of the type described in the above aspect comprising introducing feed to said tank, applying a vacuum to said fibers to withdraw filtrate therefrom while providing said pulsed gas flow through aeration openings within said module such that, in use, said gas flow moves past the surfaces of said membrane fibers to dislodge fouling materials therefrom. 
         [0035]    If required, a further source of aeration may be provided within the tank to assist microorganism activity. For preference, the membrane module is suspended vertically within the tank and said further source of aeration may be provided beneath the suspended module. Preferably, the further source of aeration comprises a group of air permeable tubes. The membrane module may be operated with or without backwash depending on the flux and feed condition. A high mixed liquor of suspended solids (5,000 to 20,000 ppm) in the bioreactor has been shown to significantly reduce residence time and improve filtrate quality. The combined use of aeration for both degradation of organic substances and membrane cleaning has been shown to enable constant filtrate flow without significant increases in transmembrane pressure while establishing high concentration of MLSS. 
         [0036]    According to another aspect the present invention provides a water treatment system including a tank; a liquid chamber fluidly connected to the tank; a gas chamber fluidly connected to the liquid chamber; and a membrane module fluidly connected to the gas chamber. 
         [0037]    Preferably the water treatment system includes a gas transfer system having a suction side connected to the liquid chamber and a discharge side connected to the gas chamber. For preference, a source of gas fluidly is connected to the liquid chamber. Preferably the system includes a membrane module vessel containing the membrane module and the membrane module vessel is hydraulically connected to the tank. 
         [0038]    According to another aspect the invention provides a water treatment system comprising a liquid reservoir fluidly connected to a source of water; a gas/liquid chamber enclosing a first compartment and a second compartment, the first compartment fluidly connected to the liquid reservoir; and a membrane separation vessel hydraulically connected to the second compartment. 
         [0039]    For preference, the system includes a chamber hydraulically isolated from the membrane module and a gas source connected to the chamber. 
         [0040]    Preferably the membrane module is immersed in a solid-containing liquid feed contained in a membrane tank, the membrane tank is hydraulically connected to the aeration zone which is fluidly connected to the chamber. 
         [0041]    According to another aspect the present invention provides a method of scouring a membrane module including: providing a chamber having a first compartment and a second compartment; establishing a hydraulic seal between the first compartment and the membrane module; at least partially filling the first compartment with a liquid feed and introducing a gas into the chamber. 
         [0042]    Preferably, the method includes breaking the hydraulic seal, wherein breaking the hydraulic seal releases at least a portion of the gas contained in the chamber to the membrane module and then re-establishing the hydraulic seal between the first compartment and the membrane module. 
         [0043]    For preference, the method includes re-breaking and re-establishing the hydraulic seal to produce a pulsed release of at least a portion of the gas contained in the chamber. 
         [0044]    Preferably, the method includes introducing liquid feed into the second compartment. In one form of the method, introduction of the gas into the chamber is performed continuously. 
         [0045]    According to another aspect, the present invention provides a method of cleaning filtration membranes located in a vessel containing liquid by providing generally random pulses of fluid within the liquid at a number of locations within the vessel. Preferably, the pulses of fluid are random in magnitude and/or frequency and/or duration. For preference, the fluid includes gas. For further preference, the gas is in the form of gas bubbles. For preference, fluid includes a two phase gas/liquid mixture. Preferably, said pulsed two-phase gas/liquid mixture is produced by a device provided with an essentially constant supply of gas. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0046]    Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
           [0047]      FIG. 1  is a simplified schematic cross-sectional elevation view of a membrane module according to one embodiment of the invention; 
           [0048]      FIG. 2  shows the module of  FIG. 1  during the pulse activation phase; 
           [0049]      FIG. 3  shows the module of  FIG. 1  following the completion of the pulsed two-phase gas/liquid flow phase; 
           [0050]      FIG. 4  is a simplified schematic cross-sectional elevation view of a membrane module according to second embodiment of the invention; 
           [0051]      FIG. 5  is a simplified schematic cross-sectional elevation view of a water treatment system according to third embodiment of the invention; 
           [0052]      FIG. 6  a simplified schematic cross-sectional elevation view of an array of membrane modules of the type illustrated in the embodiment of  FIG. 1 ; 
           [0053]      FIGS. 7A and 7B  are a simplified schematic cross-sectional elevation views of a membrane module illustrating the operation levels of liquid within the pulsed gaslift device; 
           [0054]      FIG. 8  is a simplified schematic cross-sectional elevation view of a membrane module of the type shown in the embodiment of  FIG. 1 , illustrating sludge build up in the pulse gaslift pump; 
           [0055]      FIG. 9  a simplified schematic cross-sectional elevation view of a membrane module illustrating one embodiment of the sludge removal process; 
           [0056]      FIG. 10  is a graph of the pulsed liquid now pattern and air flow rate supplied over time; and 
           [0057]      FIG. 11  is a graph of membrane permeability over time comparing cleaning efficiency using a gaslift device and a pulsed gaslift device according to the invention. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0058]    Referring to the drawings,  FIGS. 1 to 3  show a membrane module arrangement according to one embodiment of the invention. 
         [0059]    The membrane module  5  includes a plurality of permeable hollow fiber  5  membranes bundles  6  mounted in and extending from a lower potting head  7 . In this embodiment, the bundles are partitioned to provide spaces  8  between the bundles  6 . It will be appreciated that any desirable arrangement of membranes within the module  5  may be used. A number of openings  9  are provided in the lower potting head  7  to allow flow of fluids therethrough from the distribution chamber  10  positioned below the lower potting head  7 . 
         [0060]    A pulsed gas-lift pump device  11  is provided below the distribution chamber  10  and in fluid communication therewith. The pulsed gas-lift pump device  11  includes an inverted gas collection chamber  12  open at its lower end  13  and having a gas inlet port  14  adjacent its upper end. A central riser tube  15  extends through the gas collection chamber  12  and is fluidly connected to the base of distribution chamber  10  and open at its lower end  16 . The riser tube  15  is provided with an opening or openings  17  partway along its length. A tubular trough  18  extends around and upward from the riser tube  15  at a location below the openings  17 . 
         [0061]    In use, the module  5  is immersed in liquid feed  19  and source of 20 pressurized gas is applied, essentially continuously, to gas inlet port  14 . The gas gradually displaces the feed liquid  19  within the inverted gas collection chamber  12  until it reaches the level of the opening  17 . At this point, as shown in  FIG. 2 , the gas breaks the liquid seal across the opening  17  and surges through the opening  17  and upward through the central riser tube  15  creating a pulse of gas bubbles and feed liquid which flows through the distribution chamber  10  and into the base of the membrane module  5 . The rapid surge of gas also sucks liquid through the base opening  16  of the riser tube  15  resulting in a high velocity two-phase gas/liquid flow. The two-phase gas/liquid pulse then flows through the openings  9  to scour the surfaces of the membranes  6 . The trough  18  prevents immediate resealing of the opening  17  and allows for a continuing flow of 30 the gas/liquid mixture for a short period after the initial pulse. 
         [0062]    The initial surge of gas provides two phases of liquid transfer, ejection and suction. The ejection phase occurs when the bubble slug is initially released into the riser tube  15  creating a strong buoyancy force which ejects gas and liquid rapidly through the riser tube  15  and subsequently through the membrane module  5  to produce an effective cleaning action on the membrane surfaces. The ejection phase is followed by a suction or siphon phase where the rapid flow of gas out of the riser tube  15  creates a temporary reduction in pressure due to density difference which results in liquid being sucked through the bottom  16  of the riser tube  15 . Accordingly, the initial rapid two-phase gas/liquid flow is followed by reduced liquid flow which may also draw in further gas through opening  17 . 
         [0063]    The gas collection chamber  12  then refills with feed liquid, as shown in  FIG. 3 , and the process begins again resulting in another pulsing of two-phase gas/liquid cleaning of the membranes  6  within the module  5 . Due to the relatively uncontrolled nature of the process, the pulses are generally random in frequency and duration. 
         [0064]      FIG. 4  shows a further modification of the embodiment of  FIGS. 1 to 3 . In this embodiment, a hybrid arrangement is provided where, in addition to the pulsed two-phase gas/liquid flow, a steady state supply of gas is fed to the upper or lower portion of the riser tube  15  at port  20  to generate a constant gas/liquid flow through the module  5  supplemented by the intermittent pulsed two-phase gas/liquid flow. 
         [0065]      FIG. 5  shows an array of modules  35  and pump devices  11  of the type described in relation to the embodiment of  FIGS. 1 to 3 . The modules  5  are positioned in a feed tank  36 . In operation, the pulses of gas bubbles produced by each pump device  11  occur randomly for each module  5  resulting in an overall random distribution of pulsed gas bubble generation within the feed tank  36 . This produces a constant but randomly or chaotically varying agitation of liquid feed within the feed tank  36 . 
         [0066]      FIG. 6  shows an arrangement for use of the invention in a water treatment system using a membrane bioreactor. In this embodiment the pulsed two-phase gas liquid flow is provided between a bioreactor tank  21  and membrane tank  22 . The tanks are coupled by an inverted gas collection chamber  23  having one vertically extending wall  24  positioned in the bioreactor tank  21  and a second vertically extending wall  25  positioned in the membrane tank  22 . Wall  24  extends to a lower depth within the bioreactor tank  21  than does wall  25  within the membrane tank  22 . The gas collection chamber  23  is partitioned by a connecting wall  26  between the bioreactor tank  21  and the membrane tank  22  define two compartments  27  and  28 . Gas, typically air, is provided to the gas collection chamber  23  through port  29 . A membrane filtration module or device  30  is located within the membrane tank  22  above the lower extremity of vertical wall  25 . 
         [0067]    In use, gas is provided under pressure to the gas collection chamber  23  through port  29  resulting in the level of water within the chamber  23  being lowered until it reaches the lower end  31  of wall  25 . At this stage, the gas escapes rapidly past the wall  25  from compartment  27  and rises through the membrane tank  22  as gas bubbles producing a two-phase gas/liquid flow through the membrane module  30 . The surge of gas also produces a rapid reduction of gas within compartment  28  of the gas collection chamber  23  resulting in further water being siphoned from the bioreactor tank  21  and into the membrane tank  22 . The flow of gas through port  29  may be controlled by a valve (not shown) connected to a source of gas (not shown). The valve may be operated by a controller device (not shown). 
         [0068]    It will be appreciated the pulsed flow generating cleaning device described in the embodiments above may be used with a variety of known membrane configurations and is not limited to the particular arrangements shown. The device may be directly connected to a membrane module or an assembly of modules, Gas, typically air, is continuously supplied to the device and a pulsed two-phase gas/liquid flow is generated for membrane cleaning and surface refreshment. The pulsed flow is generated through the device using a continuous supply of gas, however, it will be appreciated where a non-continuous supply of gas is used a pulsed flow may also be generated but with a different pattern of pulsing. 
         [0069]    In some applications, it has been found the liquid level inside a pulsed gas-lift pump device  11  fluctuates between levels A and B as shown in  FIGS. 7A and 7B . Near the top end inside the gas-lift pump device  11 , there is typically left a space  37  that liquid phase cannot reach due to gas pocket formation. When such a pump device  11  is operated in high solid environment, such as in membrane bioreactors, scum and/or dehydrated sludge  39  may gradually accumulate in the space  37  at the top end of the pump device  11  and this eventually can lead to blockage of the gas flow channel  40 , leading to a reduced pulsing or no pulsed effect at all  FIG. 8  illustrates such a scenario. 
         [0070]    Several methods to overcome this effect have been identified. One method is to locate the gas injection point  38  at a point below the upper liquid level reached during operation, level A in  FIGS. 7A and 7B . When the liquid level reaches the gas injection point  38  and above, the gas generates a liquid spray  41  that breaks up possible scum or sludge accumulation near the top end of the pump device  11 .  FIG. 9   10  schematically shows such an action. The intensity of spray  41  is related to the gas injection location  38  and the velocity of gas. This method may prevent any long-term accumulation of sludge inside the pump device  11 . 
         [0071]    Another method is to periodically vent gas within the pump device  11  to allow the liquid level to reach the top end space  37  inside the pump device  11  during operation. In this case, the injection of gas must be at or near the highest point inside the pump device  11  so that an or nearly all the gas pocket  37  can be vented. The gas connection point  38  shown in  FIG. 7  is an example. Depending on the sludge quality, the venting can be performed periodically at varying frequency to prevent the creation of any permanently dried environment inside the pump device. 
         [0072]    It was also noted in operation of the pump device  11  that the liquid level A in  FIG. 7  can vary according to the gas flowrate. The higher the gas flowrate, the less the gas pocket formation inside the pump device  11 . Accordingly, another method which may be used is to periodically inject a much higher air flow into the pump device  11  during operation to break up dehydrated sludge. Depending on the design of the device, the gas flowrate required for this action is normally around 30% or more higher than the normal operating gas flowrate. This is possible in some plant operations by diverting gas from other membrane tanks to a selected tank to temporarily produce a short, much higher gas flow to break up dehydrated sludge. Alternatively, a standby blower (not shown) can be used periodically to supply more gas flow for a short duration. 
         [0073]    The methods described above can be applied individually or in a combined mode to get a long term stable operation and to eliminate any scum/sludge accumulation inside the pump device  11 . 
       EXAMPLES 
       [0074]    One typical membrane module is composed of hollow fiber membranes, has a total length of 1.6 m and a membrane surface area of 38 m 2 . A pulsed flow generating device was connected to the typical membrane module. A paddle wheel flowmeter was located at the lower end of the riser tube to monitor the pulsed liquid flow-rate lifted by gas.  FIG. 10  shows a snapshot of the pulsed liquid flow-rate at a constant supply of gas flow at 7.8 Nm 3 /hr. The snapshot shows that the liquid flow entering the module had a random or chaotic pattern between highs and lows. The frequency from low to high liquid flow-rates was in the range of about 1 to 4.5 seconds. The actual gas flow-rate released to the module was not measured because it was mixed with liquid, but the flow pattern was expected to be similar to the liquid flow—ranging between highs and lows in a chaotic nature. 
         [0075]    A comparison of membrane cleaning effect via pulsed and normal airlift devices was conducted in a membrane bioreactor. The membrane filtration cycle was 12 minutes filtration followed by 1 minute relaxation. At each of the air flow-rates, two repeated cycles were tested. The only difference between the two sets of tests was the device connected to the module—a normal gaslift device versus a pulsed gaslift device. The membrane cleaning efficiency was evaluated according to the permeability decline during the filtration.  FIG. 11  shows the permeability profiles with the two different gaslift devices at different air flow-rates. It is apparent from these graphs that the membrane fouling rate is less with the pulsed gaslift pump because it provides more stable permeability over time than the normal gaslift pump. 
         [0076]    A further comparison was performed between the performance of a typical cyclic aeration arrangement and the pulsed gas lift aeration of the present invention. The airflow rate was 3 m 3 /h for the pulsed airlift, and 6 m 3 /h for the cyclic aeration. Cyclic aeration periods of 10 s on/10 s off and 3 s on/3 s off were tested. The cyclic aeration of 10 s on/10 s off was chosen to mimic the actual operation of a large scale plant, with the fastest opening and closing of valves being 10 s. The cyclic aeration of 3 s on/3 s off was chosen to mimic a frequency in the range of the operation of the pulsed airlift device. The performance was tested at a normalised flux of approximately 30 LMH, and included long filtration cycles of 30 minutes. 
         [0077]    Table 1 below summarises the test results on both pulsed airlift operation and two different frequency cyclic aeration operations. The permeability drop during short filtration and long filtration cycles with pulsed airlift operation was much less significant compared to cyclic aeration operation. Although high frequency cyclic aeration improves the membrane performance slightly, the pulsed airlift operation maintained a much more stable membrane permeability, confirming a more effective cleaning process with the pulsed airlift arrangement. 
         [0000]    
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Effect of air scouring mode on membrane performance 
               
             
          
           
               
                   
                 Pulsed 
                 10 s on/10 s off  
                 3 s on/3 s off cyclic 
               
               
                 Operation Mode 
                 airlift 
                 cyclic aeration 
                 aeration 
               
               
                   
               
               
                 Membrane 
                 1.4-2.2 
                 3.3-6 lmh/bar 
                 3.6 lmh/bar 
               
               
                 permeability drop 
                 lmh/bar 
                   
                   
               
               
                 during 12 minute 
                   
                   
                   
               
               
                 filtration 
                   
                   
                   
               
               
                 Membrane 
                 2.5-4.8 
                 10-12 lmh/bar 
                 7.6 lmh/bar 
               
               
                 permeability drop 
                 lmh/bar 
                   
                   
               
               
                 during 30 minute 
                   
                   
                   
               
               
                 filtration 
               
               
                   
               
             
          
         
       
     
         [0078]    The above examples demonstrate an effective membrane cleaning method with a pulsed flow generating device. With continuous supply of gas to the pulsed flow generating device, a random or chaotic flow pattern is created to effectively clean the membranes. Each cycle pattern of flow is different from the other in duration/frequency, intensity of high and low flows and the flow change profile. Within each cycle, the flow continuously varies from one value to the other in a chaotic fashion. 
         [0079]    It will be appreciated that, although the embodiments described above use a pulsed gas/liquid flow, the invention is effective when using other randomly pulsed fluid flows including gas, gas bubbles and liquid. 
         [0080]    It will be appreciated that further embodiments and exemplifications of the invention are possible without departing from the spirit or scope of the invention described.