AU2011253685B2

AU2011253685B2 - Membrane cleaning with pulsed airlift

Info

Publication number: AU2011253685B2
Application number: AU2011253685A
Authority: AU
Inventors: Gerin James; Edward John Jordan; Wenjun Liu; Roger William Phelps; Peter Zauner; Fufang Zha; Joseph Edward Zuback
Original assignee: Evoqua Water Technologies LLC
Current assignee: Evoqua Water Technologies LLC
Priority date: 2007-05-29
Filing date: 2011-11-25
Publication date: 2013-09-19
Anticipated expiration: 2028-05-29
Also published as: AU2011253685A1; AU2013100318A4; AU2013100315B4; AU2013100315A4; AU2013202174B2; AU2013202174A1

Abstract

Aspects and embodiments of the present application are direction to systems and methods for treating fluids and to systems and methods for cleaning membrane modules used in the 5 treatment of fluids. Disclosed herein is a membrane filtration system and a method of operating same. The membrane filtration system comprises a plurality of membrane modules (5) positioned in a feed tank (36), at least one of the membrane modules (5) having a gas slug generator (11) positioned below a lower header (7) thereof, the gas slug generator (11) configured and arranged to deliver a gas slug along surfaces of membranes (6) within the at least 10 one of the membrane modules (5). The membrane filtration system may further comprise a global aeration system configured to operate independently from an aeration system providing a gas to the gas slug generator. The global aeration system may be configured and arranged to induce a global circulatory flow of fluid throughout the feed tank (36).

Description

- 1 AUSTRALIA PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT ORIGINAL Name of Applicant: Siemens Industry, Inc. Actual Inventors: Fufang Zha and Gerin James and Joseph Edward Zuback and Peter Zauner and Roger William Phelps and Edward John Jordan and Wenjun Liu Address for Service is: SHELSTON IP 60 Margaret Street Telephone No: (02) 9777 1111 SYDNEY NSW 2000 Facsimile No. (02) 9241 4666 CCN: 3710000352 Attorney Code: SW Invention Title: Membrane cleaning with pulsed airlift Details of Original Application No. 2008263139 dated 29 May 2008 The following statement is a full description of this invention, including the best method of performing it known to me/us: File: 53544AUP02 MEMBRANE CLEANING WITH PULSED AIRLIFT RELATED APPLICATIONS 5 This application is a divisional of Australian Patent Application No. 2008263139 filed on May 29, 2008, titled MEMBRANE CLEANING WITH PULSED AIRLIFT PUMP, which is an Australian national stage application of International Application No. PCT/US2008/006799 filed on May 29, 2008, titled MEMBRANE CLEANING WITH PULSED AIRLIFT PUMP, which claims priority to U.S. Provisional Application Serial No. 60/940,507, titled MEMBRANE 10 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. This application is also a divisional of International Application No. PCT/US2010/037038 designating Australia filed June 2, 2010, titled MEMBRANE CLEANING WITH PULSED GAS SLUGS AND GLOBAL AERATION, which claims priority to U.S. 15 Provisional Application Serial No. 61/183,232, titled MEMBRANE CLEANING WITH PULSED GAS SLUGS, filed on June 2, 2009, 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 20 The present disclosure relates to membrane filtration systems and, more particularly, to apparatus and methods utilized to effectively clean the membranes used in such systems by means of pulsed fluid flow and/or by scouring with gas slugs which may be accompanied by a global aeration of feed in a feed vessel in which the membranes are immersed. 25 BACKGROUND OF THE INVENTION Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field. The importance of membranes for treatment of wastewater is growing rapidly. It is now 30 well known that membrane processes can be used as an effective tertiary treatment ofsewage 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 2 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 5 systems) to the community and large-scale sewage treatment. 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, a gas, or a combination thereof, and membrane surface scrubbing or scouring using a gas in the form of bubbles in a liquid. 10 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 forces applied by the bubbles. To enhance the scrubbing effect, 15 more gas may 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. Furthermore, in an environment of high concentration of solids, the solid concentration 20 polarization near the membrane surfaces may become significant during filtration where clean filtrate passes though membranes and a higher solid-content retentate is left, leading to an increased resistance of flow of permeate through the membranes. Some of these problems have been addressed by the use of two-phase (gas-liquid) flow to clean the membranes. Cyclic aeration systems which provide gas bubbles on 'a cyclic basis are claimed to 25 reduce energy consumption while still providing sufficient gas to effectively scrub the membrane surfaces. 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 30 been found to not effectively refresh the membrane surfaces. 3 DISCLOSURE OF THE INVENTION Aspects and embodiments disclosed herein seek to overcome or least ameliorate some of the disadvantages of the prior art or at least provide the public with a useful alternative. 5 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 10 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. 15 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. 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 20 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's to biodegradable dissolved organic carbon. In some applications, for example, an 25 anaerobic biological environment or a non-biological environment where oxygen or oxidants are undesirable, nitrogen may be used, particularly where the feed tank is closed with ability to capture and recycle the nitrogen. 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 30 providing a randomly generated, pulsed fluid flow such that, in use, said fluid flow moves past the surfaces of said membranes to dislodge fouling materials therefrom. For preference, the fluid 4 flow includes a gas flow which generates bubbles which move past the surfaces ofsaid 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 5 and/or frequency and/or duration. 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 10 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 15 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. Even where the gas supply to the rack is continuous and at the same flow rate, the 20 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. 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 25 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 30 applicable to other forms of membrane such flat or plate membranes. 5 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 ofgas/liquid into the sub-modules. 5 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 10 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. For preference, gas bubble flow further includes a two phase gas/liquid flow. Preferably, 15 said pulsed two-phase gas/liquid flow is produced by a device provided with an essentially constant supply ofgas. Preferably, the pulsed gas flow is random in magnitude and/or frequency and/or duration. According to a third aspect the present invention provides a membrane module comprising a plurality of porous hollow fiber membranes, said fiber membranes being arranged 20 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. For preference, the fluid flow includes a gas flow. For further preference, the gas flow is 25 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. 6 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. 5 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 10 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. In one embodiment of the invention, the supply of gas may be provided by an external 15 tank containing pressurised gas, the tank being in fluid communication with the membrane module and having control means for providing randomly generated pulses ofgas 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 20 external tank. For example, a float device could be used to activate the control means depending on the liquid level. 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 25 reducing energy concentration at the potted ends of the fibers. 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. For preference, said openings comprise a slot, slots or a row of holes. Preferably, the 30 fiber bundles are located in the potting head between the slots or rows of holes. 7 The liquid used may be the feed to the membrane module. 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 5 range of about 10 to about 300%. 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. Preferably, the fibers within the module have a packing density (as defined above) of 10 between about 5 to about 80% and, more preferably, between about 8 to about 55%. For preference, said holes have a diameter in the range of about I 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. Typically, the fiber inner diameter ranges from about 0.1 mm to about 5 mm and is 15 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. 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 20 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. 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 25 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. 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 8 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 5 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. According to another aspect the present invention provides a water treatment system 10 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. 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 at a level below an upper 15 level of liquid in 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. 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 20 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; and a source ofgas fluidly connected to the the first compartment at a level below an upper level of liquid in the first compartment. For preference, the system includes a chamber hydraulically isolated from the membrane 25 module and a gas source connected to the chamber. 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. 9 Preferably, the water treatment system further comprises a gas chamber outlet fluidly connected to said membrane module and constructed and arranged to allow for a two-phase gas/liquid flow that is random in at least one of magnitude, frequency, and duration According to another aspect the present invention provides a method of scouring a 5 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. Preferably, the method includes breaking the hydraulic seal, wherein breaking the 10 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. 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. 15 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. 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 20 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. According to another aspect of the disclosure, there is provided a water treatment system 25 comprising: a membrane module immersed in a liquid feed in a tank; a flow generating device positioned below the membrane module and having a first inlet fluidly connected to the liquid feed and an outlet fluidly connected to the membrane module; and a source of gas fluidly connected to a second inlet of the flow generating device, the flow generating device constructed and arranged to provide an intermittent two-phase gas/liquid flow to the membrane module. 10 According to another aspect of the disclosure, there is provided a method of operating a water treatment system positioned in a tank, the system comprising a membrane module comprising a plurality of hollow fiber membranes, a chamber having a liquid feed inlet and a gas inlet and positioned below the membrane module, the method comprising: introducing a liquid 5 feed into the tank; applying a vacuum to the plurality of hollow fiber membranes; and introducing a gas to the gas inlet of the chamber at a constant flow rate to displace liquid feed within the chamber and to distribute gas into the membrane module, wherein the gas is introduced into the chamber at a level below an upper level of liquid feed within the chamber. According to another aspect of the disclosure, there is provided water treatment system 10 comprising: a plurality of membrane modules arranged in a water treatment tank; a plurality of aeration devices positioned below the plurality of membrane modules, each of the plurality of aeration devices including: a liquid chamber fluidly connected to the water treatment tank; a gas chamber fluidly connected to the liquid chamber; at least one of said plurality of membrane modules in fluid communication with the gas chamber; and a source of gas fluidly connected to 15 the liquid chamber at a level below an upper level of liquid in the liquid chamber. In some embodiments, the water treatment system further comprises a gas chamber outlet fluidly connected to said at least one membrane module and constructed and arranged to allow for a two-phase gas/liquid flow that is random in at least one of magnitude, frequency, and duration 20 In some embodiments, the pulsed two-phase gas/liquid flow to each of the plurality of membrane modules is delivered simultaneously. According to another aspect of the disclosure, there is provided a water treatment system comprising: a plurality of membrane modules arranged in a water treatment tank; a plurality of gas-lift pumps positioned below the plurality of membrane modules, each of the plurality of gas 25 lift pumps constructed and arranged to deliver a pulsed two-phase gas/liquid flow that is random in at least one of magnitude, frequency, and duration to each of the plurality of membrane modules. According to another aspect of the disclosure, there is provided a water treatment system comprising: a membrane module immersed in a liquid feed in a tank; a chamber positioned 30 below the membrane module and having a first inlet fluidly connected to the liquid feed; a source of gas fluidly connected to a second inlet in the chamber; and a chamber outlet fluidly connected 1l to the membrane module and constructed and arranged to allow for a two-phase gas/liquid flow that is random in at least one of magnitude, frequency, and duration. According to another aspect of the present disclosure, there is provided a membrane filtration system. The membrane filtration system comprises a membrane module including a 5 plurality of filtration membranes immersed in a liquid medium, a pulsed gas-lift pump positioned below the membrane module, the pulsed gas-lift pump configured and arranged to deliver a pulsed two-phase gas/liquid flow along surfaces of the plurality of filtration membranes, and an aerator provided in the liquid medium positioned below the membrane module. In some embodiments the membrane module comprises a membrane mat. 10 In some embodiments the system further comprises a plurality of membrane mats, and the pulsed gas-lift pump may be configured to deliver a pulsed two-phase gas/liquid flow comprising a gas slug to adjacent membrane mats. In some embodiments the pulsed gas-lift pump has no moving parts. In some embodiments the two-phase gas/liquid flow comprises a gas slug having a width 15 longitudinally extending substantially across a width of the membrane module. In some embodiments the system comprises a plurality of membrane modules and the pulsed gas-lift pump may be configured and arranged to deliver the pulsed two-phase gas/liquid flow to the plurality of membrane modules. In some embodiments the pulsed gas-lift pump is positioned below and apart from the 20 membrane module. In some embodiments the pulsed gas-lift pump is configured to deliver randomly timed Ila two-phase gas/liquid flow pulses while being supplied with an essentially constant supply of gas. In some embodiments the pulsed gas-lift pump is further configured to deliver two-phase gas/liquid flow pulses which are random in one of magnitude and duration. In some embodiments the pulsed gas-lift pump and the aerator are supplied with gas from 5 a common source of gas. In some embodiments the system further comprises means for breaking up scum and/or dehydrated sludge accumulation within the pulsed gas-lift pump. According to another aspect, there is provided a method of cleaning filtration membranes located in a vessel containing liquid in which the filtration membranes are immersed. The 10 method comprises providing an essentially constant supply of gas to a gas-lift pump positioned below the filtration membranes to produce pulses of a two-phase gas/liquid mixture within the vessel. In some embodiments the pulses are produced at a generally random frequency. In some embodiments the method further comprises producing the pulses with one of a 15 generally random magnitude and a generally random duration. In some embodiments the method further comprises supplementing the pulses with an essentially constant gas/liquid flow through the filtration membranes. In some embodiments the method further comprises breaking up scum and/or dehydrated sludge accumulation within the gas-lift pump. 20 In some embodiments the method further comprises producing gas bubbles in the liquid from a gas diffuser positioned below the filtration membranes. In some embodiments the gas bubbles do not contact the filtration membranes. In some embodiments the pulses of the two-phase gas/liquid mixture comprise gas slugs. In some embodiments the filtration membranes are arranged in a module and the gas 25 slugs extend substantially across a width of the module. In some embodiments the method further comprises releasing the gas slugs into the liquid at a distance below a lower extent of the membrane module. BRIEF DESCRIPTION OF THE DRAWINGS 30 The accompanying drawings are not intended to be drawn to scale. In the drawings, each identical or nearly identical component that is illustrated in various figures is represented by a 12 like numeral. For purposes of clarity, not every component may be labelled in every drawing. In the drawings: FIG. I is a simplified schematic cross-sectional elevation view of a membrane module according to one embodiment of the invention; 5 FIG. 2 shows the module of FIG. 1 during the pulse activation phase; FIG. 3 shows another embodiment of the module of FIG. I during the pulse activation phase; FIG. 4 shows another embodiment of the module of FIG. I during the pulse activation phase; 10 FIG. 5 shows the module of FIG. I following the completion of the pulsed two-phase gas/liquid flow phase; FIG. 6 illustrates a membrane module aerated with a constant flow of bubbles; FIG. 7A illustrates a pair of membrane modules prior to aeration with a gas slug; FIG. 7B illustrates the pair of membrane modules of FIG. 6A at a first time period during 15 aeration with a gas slug; FIG. 7C illustrates the pair of membrane modules of FIG. 6A at a second time period during aeration with a gas slug; FIG. 7D illustrates the pair of membrane modules of FIG. 6A at a third time period during aeration with a gas slug; 20 FIG. 8 is a simplified schematic cross-sectional elevation view of a membrane module according to another embodiment of the invention; FIG. 9 is a simplified schematic cross-sectional elevation view of a membrane module according to another embodiment of the invention; FIG. 10 is a simplified schematic cross-sectional elevation view of a membrane module 25 according to another embodiment of the invention; FIG. 11 is a simplified schematic cross-sectional elevation view of a membrane module according to another embodiment of the invention; FIG. 12 is a simplified schematic cross-sectional elevation view of an array of membrane modules of the type illustrated in the embodiment of FIG. 1; 30 FIG. 13 is a simplified schematic cross-sectional elevation view of another embodiment of an array of membrane modules of the type illustrated in the embodiment of FIG. 1; 13 FIG. 14 illustrates a computerized control system which may be utilized in one or more embodiments; FIG. 15 is a partial cut away isometric view of an array of membrane modules of the type illustrated in the embodiment of FIG. 1; 5 FIG. 16 is a simplified schematic cross-sectional elevation view of a portion of the array ofmembrane modules of FIG. 15; FIG. 17 is a simplified schematic cross-sectional elevation view of a water treatment system according to another embodiment of the invention; FIGS. 18A, 18B and 18C are simplified schematic cross-sectional elevation views of a 10 membrane module illustrating the operation levels of liquid within the pulsed gas-lifi device/gas slug generator; FIGS. 19 and 19A are simplified schematic cross-sectional elevation views ofa membrane module of the type shown in the embodiment of FIG. 1, illustrating sludge build up in the pulsed gas-lift device/gas slug generator; 15 FIGS. 20 and 20A are simplified schematic cross-sectional elevation views oFa membrane module illustrating one embodiment of a sludge removal process; FIG. 21 is a graph of the pulsed liquid flow pattern and air flow rate supplied over time in accordance with one example; FIG. 22 is a graph of membrane permeability over time comparing cleaning efficiency 20 using a pulsed gas-lift device and a gas slug generator according to an embodiment disclosed herein; FIG. 23 shows a schematic representation of the various forms of gas flow within a tube; FIGS. 24A and 24B show a side elevation representation of a gas slug moving through a tube; 25 FIG. 25 shows an isometric schematic view of the test membrane module used in the examples to demonstrate the characteristics of slug flow; FIG. 26 shows a graph of bubble diameter versus height within the test module of FIG. 25; FIG. 27 is an elevational photograph of a gas slug moving through the membrane fibres 30 in the test device of FIG. 25; 14 FIGS. 28A and 28B show test device of FIG. 25 and a plane 20 mm from the glass wall of the test module onto which experimental and numerical results at three different height (Y) locations were compared; FIGS. 29A to 29C show graphs of water velocity over time for simulation and 5 experimental values in a slug flow example; FIGS. 30A to 30C show graphs of the air bubble size distribution at different levels within a test device of FIG. 25 during a pulse of the gas/liquid flow; FIGS. 31 A to 31 C show graphs of the air bubble size versus time at different levels within a test device of FIG. 25 during a pulse of the gas/liquid flow; 10 FIG. 32 shows a graph of the air flow rate versus the average time span of each pulse of gas liquid flow in the device of FIG. 25; FIG. 33 shows a graph of inlet water rate to the gas lift device over time with camera frames during a period of observation; and FIG. 34 is a chart illustrating the results of a test comparing the efficacy of a gas slug 15 generator as compared to a continuous aeration system in achieving a particular operating flux in an exemplary filtration system. DETAILED DESCRIPTION 20 This invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of "including," "comprising," 25 "having," "containing," "involving," and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. In accordance with various aspects and embodiments disclosed herein there is provided a method of filtering a liquid medium within a feed tank or vessel. The liquid medium may include, for example, water, wastewater, solvents, industrial runoff, fluids to be prepared for 30 human consumption, or forms of liquid waste streams including components which are desired to be separated. Various aspects and embodiments disclosed herein include apparatus and methods 15 for cleaning membrane modules immersed in a liquid medium. In some aspects, the membrane modules are provided with a randomly generated intermittent or pulsed fluid flow comprising pulses of a two-phase gas/liquid mixture including slugs of gas (also referred to as "plugs" of gas) passing along surfaces of membranes within the membrane modules to dislodge fouling 5 materials therefrom and reduce the solid concentration polarisation. What is meant by "gas slug flow," as well as other types of two-phase gas liquid flow, is illustrated in FIG. 23 and will be elaborated upon herein. Accordingly, the terms "pulsed flow generating device", "pulsed gas-lift pump device", "pulsed gas-lift device" and "gas slug generator" may be used to describe the same form of device or apparatus throughout the specification. In some embodiments, in 10 conjunction with the provision of the gas slugs to scour the membrane modules, there is provided an additional aeration system, for example, a global aeration system configured to induce a global cimulation of feed liquid throughout the feed tank. Referring to the drawings, FIG. 1 illustrates a membrane module arrangement according to one embodiment. The membrane module 5 includes a plurality of permeable hollow fiber 15 membrane 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. 20 A pulsed gas-lift pump 11, also referred to herein as a gas slug generator, is provided below the distribution chamber 10 and in fluid communication therewith. The gas slug generator 11 includes an inverted gas collection chamber 12 open at its lower end 13 and 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 25 riser tube 15 is provided with an opening or openings 17 partway along its length. In some embodiments, the opening or openings 17 extend only partly around a circumference of the riser tube 15. In other embodiments, the opening or openings 17 my bifurcate the riser tube 15 into upper and lower portions. A tubular trough 18 extends around and upward from the riser tube 15 at a location below the openings 17. In some embodiments, the riser tube 15 mechanically 30 couples the tubular trough 18 to the gas collection chamber 12. 16 In some embodiments, a gas slug generator is not provided for each membrane module, and in other embodiments multiple membrane modules are supplied with gas slugs from the same gas slug generator. In some embodiments gas slug generators are located directly beneath membrane modules, and in other embodiments gas slug generators are additionally or 5 alternatively located beneath and between membrane modules. In use, the module 5 is immersed in liquid feed 19 and a source of pressurized gas is applied, essentially continuously, to gas inlet port 14. As used herein, "essentially continuously" or an "essentially constant" flow means a flow which is continuous while the module is in operation except for possible occasional momentary disruptions or reductions in the flow rate. 10 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 two-phase gas/liquid flow which flows through the distribution chamber 10 and into the base of the membrane module 5. 15 In some embodiments 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 pulse. The two phase gas/liquid flow may include one or more gas slugs. The gas slug(s) and/or 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 20 of the gas/liquid mixture for a short period after the initial pulse. In accordance with some embodiments the initial surge of gas provides two phases of liquid transfer, ejection and suction. The ejection phase occurs when the gas 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 25 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 ofthe 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. In 30 other embodiments, a gas slug is produced without an accompanying suction or siphon phase. 17 The two-phase gas/liquid flow may comprise one or more gas slugs 25, as illustrated in FIG. 3 and FIG. 4. The gas slugs 25 may travel up through the membrane module, scrubbing (scouring) filtration membranes in the module. In some embodiments, for example, as illustrated in FIG. 3, the gas slugs 25 may have a dimension, for example, a width, that is a substantial 5 portion of a width of the membrane module, for example, up to about one half or more of the width of the membrane module. In other embodiments, for example, that illustrated in FIG. 5, a gas slug 25 may have a width equal to or greater than a width and/or thickness of the membrane module, such that substantially all, or all of the membrane fibers in the module are contacted by the gas slug. The gas slug may extend substantially or completely across all membranes in a 10 membrane module, and in some embodiments may extend longitudinally beyond the outermost membrane fibers in a membrane module. In some embodiments, where gas slugs are released below and/or between adjacent membrane modules, the gas slug may extend across a distance between the membrane modules and substantially into the adjacent membrane modules, for example at least half way into each of the adjacent membrane modules or substantially 15 completely through the filtration membranes in each of the adjacent membrane modules. In some embodiments, bubbles 25A may form in the wake of a gas slug from gas separating from the main body of the gas slug 25. After release of a gas slug 25 or two-phase gas/liquid pulse, the gas collection chamber 12 then refills with feed liquid, as shown in FIG. 5, and the process begins again, resulting in 20 production of another gas slug or two-phase gas/liquid flow pulse which further cleans the membrane bundles 6 within the module 5. Due to the relatively uncontrolled nature of the process, the gas slugs or two-phase gas/liquid flow pulses are generally random in frequency and duration. A benefit of gas slug scouring as compared to steady state aeration with gas bubbles is 25 illustrated in FIG. 6 and FIGS. 7A - 7D. FIG. 6 illustrates a membrane module 110, having a plurality of hollow fiber filtration membranes 120 potted in headers 130. The membrane module is aerated by a stream of small bubbles 140 produced by, for example, an air diffuser (not shown) located beneath the module 110. Representative membranes 120 are illustrated in dotted lines to illustrate how the membrane fibers may be arranged due to slack in the membranes. The degree 30 of slack illustrated is not necessarily to scale. When aeration is supplied using a flow of small bubbles, either in a continuous or cyclic aeration mode, as illustrated in FIG. 6, fiber slack is 18 pushed to the top of the modules almost eliminating horizontal movement of the fiber during aeration. As a result this method of aeration keeps the fiber bundle tightly packed during aeration. Fluid transport into the fiber bundle in the transverse direction is important to provide 5 mass transfer of solids through and along the fiber bundles and to induce fiber movement. When fiber bundles become highly packed tranverse flow becomes more difficult due to the increased resistance of flow transversely through the fiber bundles. Either continuous or cyclic aeration methods using diffused air increase the transverse flow resistance into the fiber bundle due to the forces they apply to the fiber. Continuous and 10 cyclic aeration drive the slack in the fiber to the top of the module and limit overall fiber mobility. As a result, fibers are substantially vertical when they are operated and packing density remains relatively constant from the top to the bottom of the fiber bundle. The resultant relatively low amount of transverse flow reduces mass transfer of solids within the bundle increasing the overall fouling rate of the membranes. 15 In contrast, as illustrated in FIGS. 7A - 7D, using a membrane scouring method that creates a gas slug flow around the module fiber bundle, fiber slack can be effectively utilized to increase transverse flow into the fiber bundle, improving mass transfer of solids into and out of the fiber bundle, reducing fouling potential and increasing overall fiber system performance. When gas is provided as gas slugs instead of as a continuous or cyclic stream of bubbles from an 20 air diffuser a totally different fiber movement dynamic is created. FIGS. 7A -7D illustrate the dynamics of movement of fibers within adjacent membrane modules 110 as a gas slug 25 released between the membrane modules travels upward between and through the modules. Similar dynamics would be observed for a single module having a gas slug introduced from beneath. 25 Fig. 7A illustrates the membrane modules 110 prior to the introduction of a gas slug 25. In this figure, the dotted lines represent fiber membranes 120 which are provided with some slack between the headers 130. Before the release of a gas slug the slack in the membranes results in the membranes hanging downward and into space between adjacent modules due to gravity. The arrows fin FIGS. 7A - 7D represent forces on the fiber membranes. In FIG. 7A 30 the membranes experience a force downward due to gravity. 19 Upon introduction of a gas slug 25, the gas slug 25 travels upward through the filtration modules 110. The gas slug 25 exerts forces in three dimensions on membranes in the module and creates turbulence in fluid surrounding the membranes. As the gas slug 25 moves along the membrane fibers 120, the membrane fibers are moved in a horizontal direction outward from the 5 center of the module. At a first period in time after the release of a gas slug into the modules 110, illustrated in FIG. 7B, a gas slug enters into the space between the two membrane modules, displacing the membrane fibers outward from their position in FIG. 7A. The packing density of the membrane fibers is decreased, providing for increased transverse flow of fluid between the fibers. The membrane fibers are also lifted upward as the gas slug passes along the fibers 10 because slack is taken up by the horizontal displacement of the fibers. The movement of the fibers and the transverse flow of fluid between the fibers provides for scrubbing of the surfaces of the membrane fibers. This happens across the entire length of the module as the gas slug moves vertically. As the gas slug continues up through the modules at times illustrated in FIGS. 7C and 7D, different portions of the membranes are displaced outward from the center of the 15 modules, providing for increased transverse flow of fluid through these different portions of the membrane modules. Turbulence generated in the wake of the gas slug provides for further scrubbing of the surfaces of the membranes. FIG. 8 shows a modification of the embodiment of FIG. 1. In this embodiment, a hybrid arrangement is provided where a steady state supply of gas is fed to the upper or lower portion of 20 the riser tube 15 at port 20 to generate a constant gas/liquid flow through the module 5 supplementing the intermittent pulsed gas slug or two-phase gas/liquid flow. FIG. 9 shows another modification of the embodiment of FIG. 1. In this embodiment, a second gas inlet port 14B may be provided at a different location in the gas collection chamber than gas inlet 14, for example, at a lower periphery of the gas collection chamber 12. The gas 25 inlet 14B may be provided in addition to or as an alternative to gas inlet 14. FIG. 10 shows another modification of the embodiment of FIG. 1. In this embodiment, at least a portion of the riser tube 15 extending below the tubular trough 18 terminates at the point of intersection between the portion of the riser tube 15 and a lower wall of the tubular trough 18. The opening 17 extends from a lower edge of the portion of the riser tube 15 extending to the 30 upper wall of the inverted gas collection chamber 12 to the lower wall of the tubular trough 18. In some embodiments, the opening 17 extends only partially about a circumference of the riser 20 tube 15, and in other embodiments the riser tube is divided into two portions, an upper portion and a lower portion, by the opening 17. FIG. 11 illustrates another embodiment of the present disclosure. In this embodiment, a space 31 is proved between the bottom of the membrane module 5 and the top of the gas slug 5 generator 11. The gas slug generator 1 is positioned below and apart from the membrane module 5. The gas slug generator II releases gas slugs at a distance below the lower extent of the membrane module. In FIG. 11 the gas slug generator I 1 is shown positioned below the membrane module 5. In other embodiments, a gas slug generator may alternatively or additionally be positioned below and between membrane modules to deliver gas slugs between 10 adjacent modules, as illustrated in FIGS. 7B - 7D. FIG. 12 shows an array of modules 35 and gas slug generators II of the type described in relation to the embodiment of FIG. 1. The modules 5 are positioned in a feed tank 36. In operation, the gas slugs produced by each gas slug generator 11 occur randomly for each module 5 resulting in an overall random distribution of pulsed gas slug generation within the feed tank 15 36. This produces a constant but randomly or chaotically varying agitation of liquid feed within the feed tank 36. The series of gas slugs released by each gas slug generator is described herein as occurring periodically. The terms "periodically" produced gas slugs or pulses or "periodically" released gas slugs or pulses as used herein are not limited to meaning the production or release of gas slugs or pulses at a constant rate. A "periodic" production or release 20 also may encompass production or release events which occur at random time intervals. It has been observed that the overall random distribution of pulsed gas slug generation within the feed tank 36 will in some embodiments disrupt a global circulation of feed liquid through the feed tank 36. In some embodiments, it is preferable that feed circulate through the feed tank in an upwards direction through the array of membrane modules 35 and then 25 downward around the array of membrane modules proximate the walls of the feed tank. This global circulatory flow is illustrated by the arrows in FIG. 13. It should be noted that FIG. 13 is a partial cross section of an embodiment of a membrane filtration apparatus and that the flow of feed would in actuality circulate downward along the walls illustrated as well as other walls which are not represented in this cross sectional illustration. In some embodiments, it is 30 desirable to maintain this global circulatory feed flow such that particulates and/or other contaminants within the feed become more evenly distributed throughout the feed tank than 21 would occur without this circulatory flow. In other embodiments it is desirable to increase the velocity of an existing circulatory feed flow to facilitate better distribution of particulates and/or other contaminants within the feed tank. In some embodiments the global circulatory feed flow facilitates the removal of particles and/or other contaminants from the vicinity of the membrane 5 fiber surfaces. In some embodiments, maintaining the global circulatory feed flow becomes more important as the membrane filtration system operates at higher rates of permeate flux. At higher operating rates (higher rates of permeate flux) particles may tend to build up more quickly in the vicinity of the membrane fiber surfaces than at lower operating rates, thus making it more desirable for a mechanism such as the global circulatory feed flow to operate to remove and/or 10 redistribute these particles. As illustrated in FIG. 13, in some embodiments, a gas diffuser or other aeration system, such as an aeration tube 60 having multiple aeration openings 62 may be provided in a feed tank 36 below an array of membrane modules 5. The gas slug generator(s) and the gas diffuser or other aeration system may be provided with gas from a common source of gas. As illustrated in 15 FIG. 13, the aeration openings are provided below and between adjacent membrane modules in the rack of membrane modules illustrated. In alternate embodiments the aeration openings may be provided on a lower side of the aeration tube 60, rather than on an upper side, as illustrated in FIG. 13. Further, in alternate embodiments, the aeration tube need not be located beneath the membrane modules, but could be located above a lower extremity of the membrane modules. It 20 should be noted that in FIG. 13 only one rack of membrane modules 5 is illustrated, however in some embodiments, a plurality of racks of membrane modules 5, for example, 20 racks of 16 modules each, with an aeration tube 60 between each pair of racks, may make up a membrane module array 35 utilized to filter feed from a feed tank 36. A gas, such as air, may be provided to the aeration tube 60 from an external source such 25 as a blower or a pressurized tank (not shown). The source of gas for the aeration tube 60 may be the same as the source of gas for the gas slug generators 11. In some embodiments, valves and/or flow controllers (not shown) are utilized to provide gas to the aeration tube 60 when needed, while maintaining a constant or essentially constant flow of gas to the gas slug generators 11. In other embodiments, the aeration tube 60 and the gas slug generators I I are 30 supplied with different gasses and/or gas from different sources. In some embodiments, the aeration tube 60 is supplied with a constant flow of gas to produce bubbles which flow upward 22 around and/or through the membrane modules 5 and induce or increase the flow velocity of a global circulatory flow of feed through the feed tank 36 indicated by the arrows in FIG. 13. In other embodiments, the flow of gas to the aeration tube 60 is pulsed or applied cyclically when aeration to the aeration tube 60 is activated. In some embodiments, the gas flow to the aeration 5 tube 60 may be turned on for about 30 minutes and off for about 30 minutes, and in some embodiments, this gas flow pulsation may be performed at a higher frequency, for example, up to a frequency of about one minute on and about one minute off. The on and off times for the gas supply to the aeration tube need not be the same. In other embodiments, where it is desired that the aeration tube 60 supply the aeration gas 10 only during periods of high operating rates, a flow rate sensor 102 may be provided on a permeate withdrawal outlet 64 to measure the flow of permeate being withdrawn from the filtration modules. The flow rate sensor 102 may comprise a paddle wheel type sensor positioned in the filtrate removal tube 64, a magnetic flow sensor, an optical flow sensor, or any other form of fluid flow sensor known in the art. A controller 100 coupled to the flow rate 15 sensor 102 may be configured to cause gas to be supplied to the aeration tube 60 only during periods when the permeate flow exceeds a first or predetermined threshold level. In other embodiments, the controller 100 would be configured to activate the global aeration system (cause gas to be supplied to the aeration tube 60) after a defined amount of permeate had been withdrawn from the system subsequent to a previous global aeration cycle. In some 20 embodiments, the controller 100 may cause the supply of gas to the aeration tube 60 to be pulsed when the delivery of gas to the aeration tube 60 is activated. In other embodiments, a flow sensor 104 which measures flow of feed in a feed inlet tube 66 may be used in addition to, or as alternative to flow sensor 102 to determine when to activate a gas supply to the aeration tube 60. During periods of higher than normal feed input to the feed 25 tank, the controller 100 may be configured to activate the flow of gas to the aeration tube when the flow sensor 104 indicates a flow of feed exceeding a first or particular threshold level. In a similar manner, the controller 100 may terminate a flow of gas to the aeration tube 60 responsive to receiving a signal from one or both of sensors 102 and/or 104 indicating that a flow rate of permeate and/or feed has dropped below a second or predetermined level. 30 In some embodiments, such as in a municipal wastewater treatment facility, the flow of feed may vary by time of day. For example, during times of low wastewater production, such as 23 during the late night and early morning, feed may flow into the feed tank 36 at a low rate. During times of high wastewater production, such as during the late morning hours or the early evening, feed may flow into the feed tank 36 at a higher rate. A filtration system may be controlled accordingly. For example, a timer may be used to activate and/or deactivate the 5 delivery of gas to the aeration tube(s) 60 at specified times. These times could vary between weekdays and days of the weekend and/or holidays. In other embodiments a timer may be utilized to activate the delivery of gas to the aeration tube(s) 60 after a defined period of time had passed after a previous activation of the global aeration system. In further embodiments, a timer may be utilized to activate the delivery of gas to the aeration tube(s) 60 after a defined period of 10 time had passed after another event had occurred, such as a membrane cleaning or backwash cycle, or after a defined number of backwash cycles or other events had occurred. In even further embodiments the timer could be coupled to an intelligent control system, for example, one utilizing artificial intelligence that, during a learning period, would monitor under what conditions (including, for example, permeate flow, feed flow rate, transmembrane pressure, 15 and/or time of day) the global aeration system was activated and/or deactivated. Upon completion of the learning period, the controller and/or timer would then autonomously activate and/or deactivate the global aeration system responsive to the detection of conditions under which it had learned were appropriate. In some embodiments a "normal" permeate flux rate may be defined as, for example, 20 about 25 liters per square meter of filtration membrane area per hour (lmh). In some embodiments gas may be supplied to the aeration tube 60 when the flux exceeds this "normal" rate. In some embodiments a threshold permeate flux level for activating a gas supply to the aeration tube 60 may be set at, for example, about 30 lmh. In other embodiments, this threshold level may be set higher, such as, for example, at about 40 lmh. In some embodiments similar 25 flow rates of feed into the feed tank (for example, 25 lmh, 30 lmh, or 40 lmh) may be used as threshold levels for activating a flow of gas to the aeration tube 60. In some embodiments, the flow of gas to the aeration tube 60 may be suspended when the permeate flux rate returns to "normal." In other embodiments, the flow of gas to the aeration tube 60 may be suspended when the permeate flow rate and/or the feed supply rate drops by a defined level below the activation 30 threshold level. For example, in some embodiments, the flow of gas to the aeration tube 60 may be suspended when the permeate flux rate drops by more than about 5 lmh, or the feed supply 24 rate, from the flow rate at which the gas supply was activated; or, in other embodiments, when the permeate flux drops by more than about 10 lmh below the activation threshold level. In other embodiments, gas may be supplied to the aeration tube 60 when one or both of permeate or feed flow increased by more than a specified percentage over a baseline level (such as the "normal" 5 level.) For example, the global aeration system could be activated when one or both of permeate or feed flow increased by more than about 25%, or in other embodiments, more than about 50% from a baseline level. The global aeration system would be deactivated when one or both of the permeate or feed flow returned to the baseline level, or in other embodiments, returned to a specified percentage, for example about 5% or about 10% above the baseline level. Different set 10 points could be set depending on, for example, the size of the filtration system, the type of fluid being treated, or based on calculations of the energy trade off between supplying the gas to the aeration tube(s) 60 and the expected increase in the requirements for, for example, backwashing of the membrane modules while operating under increased permeate and/or feed flow rate conditions. 15 In other embodiments, other parameters, for example, transmembrane pressure may be utilized to trigger the initiation or cessation of flow of gas to the aeration tube 60. Over time as filtration of feed progresses, an increase in concentration of particles may build up around the filtration modules. This build up of particles may block portions of the membranes in the membrane modules, thus increasing the transmembrane pressure required to obtain a specified 20 amount of permeate flow. In some embodiments, one or more transmembrane pressure sensors are configured to monitor the transmembrane pressure of one or more of the membrane fibers in one or more of the membrane modules and provide a signal to the controller 100 when the transmembrane pressure exceeds a defined set point. Responsive to this signal from the transmembrane pressure sensor(s) the controller initiates gas flow to the aeration tube 60. Gas 25 flow from the aeration tube 60 induces or increases global circulation of feed through the vessel, removing or redistributing particles from around the membrane modules, thereby reducing the observed transmembrane pressure. The desired set points for initiating or suspending air flow to the aeration tube 60 could be set at absolute levels or at relative levels, for example, at levels defined as a percentage above the transmembrane pressure observed during filtration after a 30 membrane cleaning and/or backwashing cycle (a baseline level). For example, the set point for initiating the flow of gas to the aeration tube 60 would in one embodiment be set at about 20% 25 above the baseline level, and in other embodiments, this set point would be set at a higher level, for example about 50% above the baseline level. In one example, the gas flow to the aeration tube 60 would be suspended when the transmembrane pressure returned to about 10% above the baseline level, and in another example, when the transmembrane pressure returned to about 25% 5 above the baseline level. In other embodiments, other set points for initiating or suspending air flow to the aeration tube 60 could be used depending on, for example, an examination of the trade off in energy costs between providing the gas flow to the aeration tube 60 versus the costs associated with providing sufficient suction or pressure to enable efficient operation with a particular level of transmembrane pressure. 10 In some embodiments, gas supplied from the aeration tube 60 does not penetrate the membrane modules or contact the membrane fibers therein. This may occur because the gas supplied from the aeration tube 60 experiences less flow resistance when flowing upward in spaces between the membrane modules than when flowing through the modules. In some embodiments the gas supplied from the aeration tube 60 is utilized solely to induce or enhance a 15 global circulatory flow of feed through the feed tank 36. This may especially be true in embodiments wherein the membrane fibers are enclosed at least partially or fully within a tube in the membrane modules. In other embodiments, gas supplied from the aeration tube 60 does contact the surfaces of the membrane fibers in the membrane modules, and provides energy in addition to that provided by the gas slugs from the gas slug generators 11 for scrubbing the 20 membrane fiber surfaces. The amount of gas supplied to the aeration tube(s) 60 (when activated) may in some embodiments be comparable to the flow of gas supplied to the gas slug generators 11. In other embodiments, the flow of gas to the aeration tube(s) 60, when activated, may exceed, or in other embodiments, be less than a flow of gas to the gas slug generators. For example, in one 25 embodiment, a flow of gas to the gas slug generators 11 may be about four cubic meters per hour per module and a flow of gas to the aeration system including the aeration tube or tubes 60, when activated, may be about three cubic meters per hour per module. In some embodiments, an amount of energy utilized by a filtration system utilizing both gas slug generators 11 and aeration tubes 60 may be less thanan amount of energy utilized by an 30 equivalent filtration system producing a same amount of permeate, but operating with gas slug generators 11 in the absence of the aeration tubes 60. The aeration tubes may, as described 26 above, enhance global circulation of feed through the filtration tank, removing high concentrations of particles from the vicinity of the membrane modules. Thus, less gas would need to be supplied by the gas slug generators to provide an equivalent amount of particle removal from the membranes in systems including the aeration tubes 60 than in systems without 5 the aeration tubes 60. In some embodiments including the aeration tubes 60, the amount of gas required to be supplied to the gas slug generators I I to achieve an equivalent of membrane cleaning as in systems without the aeration tubes 60 could be reduced by approximately 25%. For example, the addition of the aeration tubes 60 to a system operating with the gas slug generators 11 could enable the gas supplied to the gas slug generators to be reduced from about 10 four cubic meters per hour per module to about three cubic meters per hour per module and achieve the same amount of membrane cleaning. To provide for initiating and suspending flow of gas to the aeration tubes 60, in different embodiments, the controller 100 may monitor parameters from various sensors within the membrane filtration system. The controller 100 may be embodied in any of numerous forms. 15 The monitoring computer or controller may receive feedback from sensors such as sensors 102 and 104 and in some embodiments, additional sensors, such as pressure, trans-membrane pressure, temperature, pH, chemical concentration, or liquid level sensors in the feed tank 36, the gas slug generators 11, or in the feed supply piping, permeate piping or other piping associated with the filtration system. In some embodiments the monitoring computer or controller 100 20 produces an output for an operator, and in other embodiments, automatically adjusts processing parameters for the filtration system, based on the feedback from these sensors. For example, a rate of flow of gas to one or more membrane modules 5, one or more gas slug generator 11, and/or one or more aeration tubes 60 may be adjusted by the controller 100. In one example, a computerized controller 100 for embodiments of the system disclosed 25 herein is implemented using one or more computer systems 700 as exemplarily shown in FIG. 14. Computer system 700 may be, for example, a general-purpose computer such as those based on an Intel PENTIUM* or Core m processor, a Motorola PowerPC* processor, a Sun UltraSPARC* processor, a Hewlett-Packard PA-RISC* processor, or any other type of processor or combinations thereof Alternatively, the computer system may include specially 30 programmed, special-purpose hardware, for example, an application-specific integrated circuit (ASIC) or controllers intended specifically for wastewater processing equipment. 27 Computer system 700 can include one or more processors 702 typically connected to one or more memory devices 704, which can comprise, for example, any one or more of a disk drive memory, a flash memory device, a RAM memory device, or other device for storing data. Memory 704 is typically used for storing programs and data during operation of the controller 5 and/or computer system 700. For example, memory 704 maybe used for storing historical data relating to measured parameters from any of various sensors over a period of time, as well as current sensor measurement data. Software, including programming code that implements embodiments of the invention, can be stored on a computer readable and/or writeable nonvolatile recording medium such as a hard drive or a flash memory, and then copied into memory 704 10 wherein it can then be executed by processor 702. Such programming code may be written in any of a plurality of programming languages, for example, Java, Visual Basic, C, C#, or C++, Fortran, Pascal, Eiffel, Basic, COBAL, or any of a variety of combinations thereof Components of computer system 700 may be coupled by an interconnection mechanism 706, which may include one or more busses (e.g., between components that are integrated within 15 a same device) and/or a network (e.g., between components that reside on separate discrete devices). The interconnection mechanism typically enables communications (for example, data and/or instructions) to be exchanged between components of system 700. The computer system 700 can also include one or more input devices 708, for example, a keyboard, mouse, trackball, microphone, touch screen, and one or more output devices 710, for 20 example, a printing device, display screen, or speaker. The computer system 700 may be linked, electronically or otherwise, to one or more sensors 714, which, as discussed above, may comprise, for example, sensors such as flux, flow rate, pressure, temperature, pH, chemical concentration, or liquid level sensors in any one or more portions of the embodiments of the filtration system described herein. In addition, computer system 700 may contain one or more 25 interfaces (not shown) that can connect computer system 700 to a communication network (in addition or as an alternative to the network that may be formed by one or more of the components of system 700). This communications network, in some embodiments, forms a portion of a process control system for the filtration system. According to one or more embodiments, the one or more output devices 710 are coupled 30 to another computer system or component so as to communicate with computer system 700 over a communication network. Such a configuration permits one sensor to be located at a significant 28 distance from another sensor or allow any sensor to be located at a significant distance from any subsystem and/or the controller, while still providing data therebetween. Although the computer system 700 is shown by way of example as one type of computer system upon which various aspects of the invention may be practiced, it should be appreciated 5 that the various embodiments of the invention are not limited to being implemented in software, or on the computer system as exemplarily shown. Indeed, rather than implemented on, for example, a general purpose computer system, the controller, or components or subsections thereof, may alternatively be implemented as a dedicated system or as a dedicated programmable logic controller (PLC) or in a distributed control system. Further, it should be appreciated that 10 one or more features or aspects of the control system may be implemented in software, hardware or firmware, or any combination thereof. For example, one or more segments of an algorithm executable on the computer system 700 can be performed in separate computers, which in turn, can be in communication through one or more networks. FIGS. 15 and 16 illustrate another embodiment of a membrane filtration system 15 according to the present disclosure. FIG. 15 is an isometric view of a bank of membrane modules including multiple racks of membrane modules 5 mounted in a feed tank 36. Walls of the feed tank are cut away to show the bank of membrane modules. FIG. 16 illustrates a cross section of a portion of the membrane module bank of FIG. 15 perpendicular to the axis of the aeration tubes 60. In these FIGS. it can be seen that the aeration tubes 60 are located 20 substantially centered below and between adjacent membrane module racks within the bank of membrane modules. In some embodiments, aeration tubes 60 are also provided between outside membrane module racks (membrane module racks closest to walls of the feed tank) and the walls of the feed tank such that the outside membrane racks have aeration tubes 60 on both sides of the lengthwise axis of the membrane module rack. 25 FIG. 17 shows an arrangement for use in a water treatment system using a membrane bioreactor. In this embodiment a pulsed gas slug or 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 125 positioned in the membrane tank 22. Wall 24 30 extends to a lower depth below the level of the liquid within the bioreactor tank 21 than does wall 125 below the level of the liquid within the membrane tank 22. The gas collection chamber 29 23 is partitioned by a connecting wall 26 between the bioreactor tank 21 and the membrane tank 22 to define two compartments 27 and 28. Gas, for example, 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 125. 5 In use, gas is provided under pressure to the gas collection chamber 23 through port 29 resulting in the level of feed liquid within the chamber 23 being lowered until it reaches the lower end 31 of wall 125. At this stage, the gas escapes rapidly past the wall 125 from compartment 27 and rises through the membrane tank 22, producing a two-phase gas/liquid flow through the membrane module 30 which may include one or more gas slugs. The surge of gas 10 also produces a rapid reduction of gas within compartment 28 of the gas collection chamber 23 resulting in further feed liquid 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 such as controller 100 discussed above. 15 It will be appreciated embodiments the gas slug generator described above may be used as or in conjunction with a cleaning apparatus in a variety of known membrane configurations and is not limited to the particular arrangements shown. A gas slug generator may be directly connected to a membrane module or an assembly ofmodules. In other embodiments a gap may be provided between a gas slug generator and a membrane module to which the gas slug 20 generator supplies gas slugs. A gas slug may be released by a gas slug generator into liquid in which a membrane module is immersed at a distance below a lower extent of the membrane module. Gas, typically air, is in some embodiments continuously supplied to the gas slug generator and a series of gas slugs is generated for membrane cleaning and surface refreshment. The pulsed flow is in some embodiments generated through the gas slug generator using a 25 continuous supply of gas, however, it will be appreciated where a non-continuous supply of gas is used a series of gas slugs may also be generated but with a different pattern of pulsing, for example, with a different frequency or with a different variation in time between production of gas slugs. In some embodiments it has been found the liquid level inside a gas slug generator 11 30 fluctuates between levels A and B as shown in FIGS. 18A, 18B and 18C. Near the top end inside the gas slug generator 11, there may be left a space 37 that liquid phase cannot reach due 30 to gas pocket formation. When such a gas slug generator I I 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 gas slug generator 11 and this eventually can lead to blockage of the gas flow channel 40, leading to reduced gas slug generation and/or two 5 phase gas/liquid flow pulsing or no gas slug or pulsed effect at all. FIGS. 19 and 19A illustrate such a scenario. 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. 18B and 18C. When the liquid level reaches the gas injection point 38 and above, the 10 gas generates a liquid spray 41 that breaks up possible scum or sludge accumulation near the top end of the gas slug generator 11. FIGS. 20 and 20A schematically show 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 gas slug generator 11. Another method is to periodically vent gas within the gas slug generator 11 to allow the 15 liquid level to reach the top end space 37 inside the gas slug generator 1 during operation. In this case, the injection of gas may be at or near the highest point inside the gas slug generator 11 so that all or nearly all the gas pocket 37 can be vented. The gas connection point 38 shown in FIG. 18A 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 20 inside the gas slug generator. In operation of the gas slug generator l Ithe liquid level A in FIG. 18A can vary according to the gas flowrate. The higher the gas flowrate, the less the gas pocket formation inside the gas slug generator 11. Accordingly, another method which may be used is to periodically inject a much higher air flow into the gas slug generator 11 during operation to 25 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 higher gas flow rate may be achieved in some plant operations by, for example, 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 30 used periodically to supply more gas flow for a short duration. 31 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 gas slug generator 11. 5 Example 1 A gas slug generator was connected to a membrane module composed of hollow fiber membranes, having a total length of 1.6m and a membrane surface area of 38 in 2 . A paddle wheel flow meter was located at the lower end of the riser tube to monitor the pulsed liquid flow rate lifted by gas. FIG. 21 shows a snapshot of the pulsed liquid flow-rate at a constant supply 10 of gas flow at 7.8 n 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 I 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. 15 A comparison of membrane cleaning effect via the gas slug generator and normal airlift devices was conducted in a membrane bioreactor. The membrane filtration cycle was 12 minutes filtration followed by one 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 gas lift device versus a gas slug generator. The membrane cleaning 20 efficiency was evaluated according to the permeability decline during the filtration. FIG. 22 shows the permeability profiles with the two different devices at different air flow-rates. It is apparent from these graphs that the membrane fouling rate is less with the gas slug generator because it provides more stable permeability over time than the normal gaslift pump. A further comparison was performed between the performance of a typical cyclic 25 aeration arrangement and the gas slug generator of the present invention. The airflow rate was 3 m 3 /h for the gas slug generator, and 6 m 3 /h for the cyclic aeration. Cyclic aeration periods of 10 seconds on/10 seconds off and 3 seconds on/3 seconds off were tested. The cyclic aeration of 10 seconds on/10 seconds off was chosen to mimic the actual operation of a large scale plant, with the fastest opening and closing of valves being 10 seconds. The cyclic aeration of 3 seconds 30 on/3 seconds off was chosen to mimic a frequency in the range of the operation of the gas slug 32 generator. The performance was tested at a normalised flux of approximately 30 lmh, and included long filtration cycles of 30 minutes. 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 5 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 more stable membrane permeability, confirming a more effective cleaning process with the pulsed airlift arrangement. 10 Table 1: Effect of air scouring mode on membrane performance Operation mode Pulsed 10s on/10s off cyclic 3s on/3 s off cyclic airlift aeration aeration Membrane permeability 1.4-2.2 3.3 - 6 Imh/bar 3.6 lmh/bar drop during 12 minute lmh/bar filtration Membrane permeability 2.5 -4.8 10 - 12 Imh/bar 7.6 lmh/bar drop during 30 minute Imh/bar filtration The above examples demonstrate that an effective membrane cleaning method may be performed 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 15 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. Example 2 20 The efficacy of a membrane scouring system including gas slug generators (also referred to herein as pulsed gas-lift pumps) was compared to that of a continuous gas bubble aeration system in a membrane filtration system including a plurality of vertically oriented planar mats of membrane fibers. For the portion of the test in which scouring with gas slug generators was 33 performed, gas slug generators were positioned below and spaced apart from the mats of membrane fibers. Similarly, the portion of the test in which scouring with continuous gas bubble aeration was performed, the gas bubble aerators were positioned below and spaced apart from the mats of membrane fibers. 5 A higher stable operating flux was achieved when the system was operated with gas slug scouring as opposed to continuous gas aeration. A stable operating flux of 90 Imh was achieved when operating the filtration system with gas slug scouring. A stable operating flux of only 62 lmh could be achieved when operating the filtration system with gas bubble aeration. Thus, an increase in operating flux of about 50% was achieved when operating with gas slug aeration as 10 compared to continuous gas bubble aeration. A higher stable transmembrane pressure was achieved when the system was operated with gas slug scouring as opposed to continuous gas aeration., A stable transmembrane pressure of55 kPa was achieved when operating the filtration system with gas slug scouring. A stable transmembrane pressure of only 46 kPa could be achieved when operating the filtration system 15 with gas bubble aeration. The results of this test are summarized in FIG. 34 in which achieved stable operating flux in lmh and achieved stable transmembrane pressure (TMP) in kPa for the filtration system operated with gas slug scouring and with continuous gas bubble aeration ("continuous style aeration") are illustrated. A common Y axis is used for both the stable operating flux and 20 transmembrane pressure measurements. These results indicate that a membrane filtration system operating with gas slug scouring can achieve significantly higher productivity that a similar system operating with aeration in the form of a continuous flow of gas bubbles. It will be appreciated that, although the embodiments described above use a pulsed 25 gas/liquid flow which may comprise a series of gas slugs, the invention is effective when using other randomnly pulsed fluid flows including gas, gas bubbles, and liquid. Membrane scrubbing accomplished using a a two-phase gas/liquid slug flow finds particular application in a membrane bio-reactor (MBR) treatment systems, though it will appreciated that such a slug flow may be used in a variety of applications requiring a gas and/or a 30 two-phase gas/liquid flow to produce a cleaning effect on membranes. As such, embodiments disclosed herein are not limited in application to MBR systems. Similarly, MBR applications 34 often require the use of a gas, typically air, containing oxygen in order to promote biological action within the system whereas other membrane application may use other gas apart from air to provide cleaning. Accordingly, the type of gas used is not narrowly critical. MBR fluid treatment is a combined process of biological oxidation with membrane 5 separation. This technology has been employed for industrial and domestic wastewater treatment. Compared to some other fluid treatment technologies, MBR has the advantages including smaller footprint, high yield and extra-purity of effluent, higher organic loading and lower sludge production. To further increase productivity and efficiency while maintaining a stable operational performance, the control of concentration polarization and subsequent 10 membrane fouling is desirable. Techniques shown to be effective include turbulence promoters, corrugated membrane surfaces, pulsating flow and vortex generation. However, it has been demonstrated that injecting air bubbles is a cheap and effective way of reducing concentration polarization and thus enhancing the permeate flux in hollow fiber membrane modules. In addition, in the process of a membrane bio-reactor, air bubbles may also be used for another 15 purpose - as oxygen supply. Depending on the air and liquid flow rates into a gas slug generator and the properties of the liquid, the mixture of air and liquid can adopt a wide spectrum of flow patterns. A number of different flow patterns are illustrated in FIG. 23. In an MBR where the applied air flow rates are relatively low, gas slug flow (also known as plug flow) has been found desirable. In these air 20 liquid two-phase flow systems, a few mechanisms have been identified to contribute to the flux increase: a) Experimental investigations on the effect of the hydrodynamic conditions and system configuration on the permeate flux in an MBR system showed that the permeate flux for two-phases (air and liquid) cross flow was 20-60% higher than that of single phase (liquid only) 25 cross flow. It is desirable to have higher superficial cross flow because at higher velocity magnitude, the activated sludge can be maintained and the membrane surface can be constantly scoured, which subsequently results in a higher filtration rate and a lower risk of membrane fouling. b) Gas slugs generate secondary flows (or wake regions) which assist in breaking up 30 cake layer and subsequently promoting local mixing near the membrane surface. Slug flow, in 35 addition, also produces a stabilized annular liquid film flowing in between the slug and the tube wall as shown in FIG. 24A. The liquid film can be a high shear region promoting mass transfer. c) Moving slugs result in pulsing pressure in the liquid around the slug, with a higher pressure at its nose and lower pressure at its tail, as best shown in FIG. 24B. This can 5 cause instability and disturbance of the onset of a concentration boundary layer near the membrane surface. To demonstrate the effectives of slug flow in a MBR system, a study was undertaken using both numerical and experimental investigations to study the hydrodynamic behaviour of a two-phase (water-air) MBR system under a slug flow patten. Particle image velocimetry (PIV) 10 was adopted for experiment and computational fluid dynamics (CFD) was chosen as the numerical tool. Experimental Measurement The experimental setup is best shown in FIG. 25. A rectangular tank 50 was constructed 15 out of transparent material. The tank 50 was provided with a water injector 51 at its base and an overflow outlet 52 near it upper end. A fiber membrane module 53 was located within the tank 50. The lower end of the module 53 was provided with a skirt 54 and a gas slug generator 55 constructed according to the embodiment described above. Porous zones 56 were provided in the module to allow fluid flow to and from the module 53. The fibre membranes were potted in 20 potting material 57. To create the gas slug flow regime, the novel gas slug generator 55 described above was used to generate the two-phase gas/liquid flow. This arrangement was capable of generating air slugs at a well-controlled time interval. Experimental measurements were conducted using the test setup shown in FIG. 25; one 25 set of which is the flow field measurement using PIV and the other set of which is air bubble size distribution and their trajectories measured by high speed camera. The former measurement was carried out in order to provide reliable and accurate flow data for CFD model refinement while the latter served as an input parameter for CFD modelling. A typical PIV experimental setup was used, which comprised of a CCD camera and a 30 high power laser. A double pulsed laser was used to illuminate a light sheet across the flow. At the same time, the flow field was seeded with particles to scatter the laser light and work as 36 tracking points. A CCD camera that could take two frames in quick successions was placed orthogonal to the plane of the light sheet. During measurement, which took place through the side window of the test device, the first pulse from the laser illuminated the flow and the light scattered from the particles is captured as the first frame by the camera. After a controlled time 5 interval, the second pulse of the laser again illuminated the flow. The light scattered by the particles was captured as the second frame by the camera. The displacement that individual particles travelled was calculated from the two captured frames. Knowing the time between exposures of the camera, the flow velocity was then evaluated. For measuring the sizes of air bubbles, a high speed camera was employed. This camera 10 has 17pm pixels and is capable of capturing up to 250,000 frames per second at reduced resolution. Numerical Modelling In order to replicate experimental observations, the CFD model integrated a Eulerian 15 multiphase model with porous medium scheme and incorporated the vertically dependent filtration flux measurements. A transient simulation for the slug flow study was performed. Model Geometry and Operating Conditions Based on an experimental prototype, the corresponding CFD model geometries were 20 generated, as shown in FIG. 28A. A transient simulation, based on the FIG. 25 model geometry was carried out to replicate the two-phase gas/liquid slug flow phenomena. From the experiment, it is known that under air scouring flow rate of 4 m 3 /hr, it takes 4.2 seconds to generate one air slug; with 3.8 seconds being the air accumulation stage and 0.4 seconds is the air pulsed stage. To simulate the process of the generation of air slugs, a time dependent step 25 function of mass and momentum source terms were employed in the transient simulation. The mass source has the value of 14.62 kg/m s and the momentum source is 8.27 N/m 3 , which were calculated from the operating conditions listed in Table 2. The conditions are the same for both simulation and experiment. 37 Table 2: Operating conditions for both numerical simulation and experiment Parameters (Unit) Slug Fibers packing density (%) 20 Water circulation flow rate (m 3 /hr/module) 2.46 5 Air scouring flow rate (m 3 /hr/module) 4 Filtration flux (1/m 2 /hr) 25 Mathematical Equations 10 To simulate the hydraulic distribution within a membrane bio-reactor unit, elements that have significant influences on the hydrodynamics were taken into consideration. The MBR system used in the experiment operated using a slug flow regime and included a membrane separation device in which was provided two-phases of state; i.e. water and air bubbles. The membrane separation device includes of a bundle of fibers, which created resistance to the flow 15 circulation. In addition, vacuum pumps were used to generate filtration on the membranes. These features are interdependent and were factored into the CFD model via the incorporation of the following schemes: i. Eulerian multiphase model is applied to account for the mixing behavior of two-phases, ii. Theoretical model of vertically dependent filtration flux, 20 iii. Porous medium model to consider the membrane module resistance to water circulation, and iv. Experimentally measured bubble diameter profile. Eulerian Multiphase Model 25 In the Eulerian multiphase model, a few sets of the coupled basic conservation equations ofmass, momentum and turbulence kinetics are applied to simulate the flow field and concentration distributions of water and air. a. Mass Continuity Equation 30 Eq. (1) indicates the unsteady mass continuity equation for phase q. 38 a a,po,) +V -(a,p =qFq rlhp -rthq) )+ S (1l) at P=I Where t is time (s), a is the volume fraction of fluid, V is the velocity (m/s) of phase q and rhm characterizes the mass transfer (kg/s) from phase p to q, rhqp characterizes the mass transfer from the q th to p phase and Sq is the source or sink term. 5 b. Momentum Conservation Equation The unsteady momentum balance for phase q gives -(aqpq , )+V -(aqpq )= -aqVp+V rq +aqpqg+ ( +h,, Jiq-r lqpV) (2) Ih 10 where is the q' phase stress-strain tensor (Pa) (see eq. (3)), Rp is an interaction force between phases, p is the pressure (Pa) shared by all phases, g is gravity (mIs), and Vp is the inter-phase velocity. ( -T 2 T = aqq(VV +VV +aq (,q _ p V -Vq I (3) Here pq and Aq are the shear and bulk viscosity (kg/ms) of phase q, respectively. 15 c. Realizable K-E Mixture Turbulence Model The K (Turbulent kinetic energy per unit mass (m 2/s2)) and E (Turbulent kinetic energy dissipation rate (m 2 /s 3 )) equations describing the realizable K-C mixture turbulence model are as follows: 20 (p,,,K)+ V - p K +G4,, + Gb,. - Pe (4) a p,,(3V-p V,,=V.I"'VKI+psSn,( at + (P V)V(Ii VJPrCmnbn (5) 2 - pC2 + C C 3 , G K+ V3C K 39 Here Gb., is the generation of turbulence kinetic energy due to buoyancy, Gkm is the generation of turbulence kinetic energy due to the mean velocity gradients, and v is kinematic viscosity (m 2 /s). 5 The mixture density and velocity, p,, (kg/m 3 ) and V,, are computed from N _ N _, aipi V IV ______ p. =yXaip; ; ,,=' -' ( aip and the turbulent viscosity, p,,, is computed from 2 p, ,n = p,, P"C In these equations, C 2 and C;, are constants and ar and a, are the turbulent Prandtl 10 numbers for K and c, respectively. Vertically Dependent Filtration Flux In the experiment where the suction pump is on, because of the pressure drop while permeate flux travels in the fiber lumens, the filtration flux is vertically dependent; with higher 15 trans-membrane pressure at the top of the fibers and lower trans-membrane pressure at the bottom of the fibers. In order to reflect this phenomenon, a vertical filtration flux is calculated from the pressure difference across the fiber. Eq. (6) shows a vertically dependent filtration flux. Filtration Flux = 0.0046*H*H - 0.00 12*H+ 0.0 13 (6) 20 where filtration flux is in the unit ofkg/s and H is height in meters. The vertically dependent filtration flux is included as volumetric mass sink, Sq of eq. (1). This mass sink is added in the porous region to represent the vertically dependent filtration flux along the fibers. 25 Porous Medium Model The porous medium model incorporates flow resistances in a region of the model defined as porous zone (see FIGS. 21 A and 211B). In other words, the porous medium model applies an 40 additional volume-based momentum sink in the governing momentum equations to simulate the pressure loss through a porous region. In this study, the following model is used to represent the flow resistances. S=-(DIpV+ pVg V. 3 (7) 5 where Si is the source term for the id' (x, y or z) momentum equation and D and K are prescribed matrices. The first term in eq. (7) represents viscosity-dominated loss and the second term is an inertia loss term. These resistances are calculated based on the tube bank assumption which is similar to fiber bundle used in MBR. 10 Experimentally Measured Bubble Diameter Profile For a better comparison between experiment and simulation, a variable bubble size was applied. The bubble size profile was determined from the high speed camera experiment, as shown in FIG. 26. But, due to the limitations of the experiment, for the slug flow regime, the bubble diameter was measured from Y=1.4m to Y=1.8m. Below Y=1.4m, the bubble diameter 15 was assumed as 3mm and above Y= 1.8m, the bubble diameter was assumed as 5mm. As shown in FIG. 27, a slug flow regime is generated using the aeration device described above. Under this flow regime, both PIV measurement and CFD simulation are conducted and the results are extracted at three different locations along cut-plane 20 mm from glass wall, as shown in FIG. 28B. 20 FIGS. 29A to 29C show the comparison between simulated and experimentally measured water Y velocity component at Y=1.532 m, Y=1.782 m and Y=1.907 m along plane 20 mm from the wall, respectively. In FIGS. 29A to 29C, the solid line represents the simulation results and the dotted line stands for experimental measurements. Both experiment and simulation show five cycles of air slug generation. Each cycle illustrates a down-flow velocity followed by an 25 upward velocity for Y=1.532 m and Y=1.782 m. For Y=1.907 m, it is a stronger down-flow velocity followed by a weaker down-flow velocity. In general, within experimental uncertainties and simulation assumptions, the comparison between simulation and experiment at these three locations can be considered as fairly good. FIGS. 30A to 30C show graphs of the measured air bubble size distribution measured at 30 the top, middle and bottom of the test device during the gas slug generation. 41 FIGS. 3 1 A to 3 1C show graphs of the number of bubbles versus time measured at the top, middle and bottom of the test device during the gas slug generation. FIG. 32 shows a graph of the average time span of each air/gas slug pulse versus airflow rate. 5 FIG. 33 shows a graph of the pulses on inlet water flow into the aerator generated by the gas slug flow within the aerator. The frames indicate measurements taken by the high speed camera. It can be seen that the inlet water or liquid flow increases rapidly with the generation of the gas slug and then falls again to a lower or zero flow until the next gas slug is produced. From this study, it is observed from experiment and simulation that operation under a slug 10 flow regime has advantages compared to operation under a bubbly flow regime: a) Slug flow is a time-dependent process. During the generation of a gas/air slug, the liquid about the membrane fibers exhibits flow instability. This can disturb the concentration boundary layer build up and the accumulation of particles near the membrane surfaces. b) The flow instability also enhances the oscillation of the fibers. This is desired 15 because the movement of the fibers in a bundle could have a number of effects including collision between fibers that could erode the cake layer on the membrane surface. c) Slug flow produces a stabilized annular liquid film flowing in between the slug and the tube wall. The liquid film can be a high shear region assisting in wearing away cake layer from the tube wall. 20 d) Gas/air slugs are larger in size than previously utilized aeration bubbles and thus could generate stronger and longer wake regions, which could disrupt the mass transfer boundary layer and promote local mixing near the membrane surfaces. e) Operation under slug flow regime requires less air to be supplied than a typical bubbly flow aeration system. For example, in some embodiments, a slug flow aeration system 25 would operate using about 4 m 3 /hr of gas per module whereas a typical bubbly flow regime which would be operated to produce similar levels of aeration would operate with 7 m3/hr of gas per module. Less gas/air consumption results in lower energy utilization, and thus lower operating costs. Utilization of a global aeration system as described herein in conjunction with the 30 apparatus described above for providing cleaning of membrane modules with a gas slug flow is expected to provide even further advantages. 42 Testing has shown that non-uniformity of particle concentration within an entire tank may be significantly reduced using a global circulation system as described herein. The global circulation system establishes up-flow regions are at the membrane module, and in the space between racks, and down-flow regions at the surrounding of the tank. By having a well 5 controlled flow fields, the particles are more evenly distributed throughout the feed tank. The increased uniformity of particle distribution within a filtration or feed vessel including filtration modules operating utilizing slug flow membrane cleaning as described above is expected to provide for lower energy operation of a filtration system comprising such a filtration vessel. This is because utilization of global aeration in conjunction with gas slug flow 10 membrane cleaning provides additional redistribution of accumulated solids away from the membrane modules than would be accomplished using gas slug flow cleaning alone. This provides for less gas to be utilized for slug flow cleaning of the membranes to achieve a same amount of membrane cleaning. For example, as described above, in a filtration system utilizing a gas slug flow cleaning mechanism using 4 m 3 /hr per module, the gas consumption of the gas 15 slug cleaning mechanism is expected to be reducible to 3 m 3 /hr per module or less if operated in conjunction with a global aeration system. In addition, the removal of solids from the vicinity of the membrane modules would increase the amount of time that the modules could be operated between backwashing or other cleaning operations. By adding a global aeration system to a filtration system operating with gas slug flow membrane cleaning it is expected that energy 20 savings may amount to up to at least about 10% or more versus systems with only gas slug flow membrane cleaning. Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be 25 part of this disclosure, and are intended to be within the spirit and scope of the invention as defined by the appended claims. Accordingly, the foregoing description and drawings are by way of example only. 43

Claims

1. A water treatment system including: a tank comprising a water to be treated; 5 a liquid chamber fluidly connected to the tank; a gas chamber fluidly connected to the liquid chamber; a membrane module fluidly connected to the gas chamber; and a source of gas fluidly connected to the liquid chamber at a level below an upper level of liquid in the liquid chamber 10

2. A water treatment system according to claim 1 further including a gas transfer system having a suction side connected to the liquid chamber and a discharge side connected to the gas chamber. 15

3. A water treatment system according to claim 1 further including a membrane module vessel containing the membrane module and the membrane module vessel is hydraulically connected to the tank.

4. A water treatment system according to any one of the preceding claims further 20 comprising a tube positioned within the gas chamber and having an outlet in fluid communication with the membrane module through which a pulsed gas flow is introduced to the membrane module and an opening in fluid communication with the gas chamber

5. A water treatment system comprising a liquid reservoir fluidly connected to a source of 25 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; and a source of gas fluidly connected to the the first compartment at a level below an upper level of liquid in the first compartment. 30

6. A water treatment system according to claim 5 further comprising a chamber hydraulically isolated from the membrane module and a gas source connected to the chamber. 44

7. A water treatment system according to claim 6 wherein the membrane module is immersed in mixed liquor contained in a membrane tank and the membrane tank is hydraulically connected to the aeration zone which is fluidly connected to the chamber. 5

8. A water treatment system according to claim 1 further comprising a gas chamber outlet fluidly connected to said membrane module and constructed and arranged to allow for a two phase gas/liquid flow that is random in at least one of magnitude, frequency, and duration. 10

9. A method of operating a water treatment system positioned in a tank, the system comprising a membrane module comprising a plurality of hollow fiber membranes, a chamber having a liquid feed inlet and a gas inlet and positioned below the membrane module, the method comprising: introducing a liquid feed into the tank; 15 applying a vacuum to the plurality of hollow fiber membranes; and introducing a gas to the gas inlet of the chamber at a constant flow rate to displace liquid feed within the chamber and to distribute gas into the membrane module, wherein the gas is introduced into the chamber at a level below an upper level of liquid feed within the chamber. 20

10. The method of claim 9, wherein introducing the gas to the gas inlet of the chamber at a constant flow rate comprises pulsing a gas/liquid flow at randomly spaced intervals in a range of about 0.1 second to 200 seconds from the chamber to the membrane module.

11. The method of claim 9, wherein introducing the gas to the gas inlet of the chamber 25 comprises introducing gas into a tube when a level of gas within the chamber reaches a first level.

12. The method of claim 11, wherein the first level is at a position below an inlet of the tube. 30

13. The method of claim 12, further comprising introducing liquid feed into the tube when the level of gas within the chamber reaches a second level. 45

14. The method of claim 13, wherein the second level is at a position above the inlet of the tube. 5 15. A water treatment system comprising: a plurality of membrane modules arranged in a water treatment tank; a plurality of aeration devices positioned below the plurality of membrane modules, each of the plurality of aeration devices including: a liquid chamber fluidly connected to the water treatment tank; 10 a gas chamber fluidly connected to the liquid chamber; at least one of said plurality of membrane modules in fluid communication with the gas chamber; and a source of gas fluidly connected to the liquid chamber at a level below an upper level of liquid in the liquid chamber.

15

16. The water treatment system of claim 15 further comprising a gas chamber outlet fluidly connected to said at least one membrane module and constructed and arranged to allow for a two-phase gas/liquid flow that is random in at least one of magnitude, frequency, and duration. 20

17. The water treatment system of claim 15, wherein the pulsed two-phase gas/liquid flow to each of the plurality of membrane modules is delivered simultaneously.

18. A water treatment system according to claim 1, claim 5 or claim 15 substantially as herein described with reference to any one of the embodiments of the invention illustrated in the 25 accompanying drawings and/or examples.

19. A method of operating a water treatment system according to claim 9 substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples. 30 46