MXPA01002986A

MXPA01002986A - Apparatus and method for cleaning membrane filtration modules

Info

Publication number: MXPA01002986A
Application number: MXPA/A/2001/002986A
Authority: MX
Inventors: Fufang Zha; Edward John Jordan
Original assignee: Edward John Jordan; Usf Filtration And Separations Group Inc; Fufang Zha
Priority date: 1998-09-25
Filing date: 2001-03-22
Publication date: 2003-11-07

Abstract

A method and apparatus for cleaning a membrane module (5), the membrane module comprising a plurality of porous membranes (6), said membranes being arranged in close proximity to one another and mounted to prevent excessive movement therebetween, and means (10, 12) for providing, from within the module (5), by means other than gas passing through the pores of said membranes, gas bubbles entrained in a liquid flow such that, in use, said liquid and bubbles (18) entrained therein move past the surfaces of said membranes to dislodge fouling materials therefrom, said gas bubbles being entrained in said liquid by flowing said liquid past a source of gas to draw the gas into said liquid flow. The gas bubbles are entrained into the liquid using a venturi type device (12). The membranes (20) are preferably partitioned into discrete groups (23) to assist cleaning while maintaining high packing density.

Description

APPARATUS AND METHOD FOR CLEANING MEMBRANE FILTRATION MODULES TECHNICAL FIELD The present invention relates to an apparatus and a related method for effectively cleaning membrane modules by means of a gas-liquid mixture formed by a venturi system, jet, or the like. For membrane modules to be applied to an environment of high concentration of suspended solids, for example in bioreactors, several improved module configurations are described to reduce the accumulation of solids within a module.

BACKGROUND OF THE INVENTION The importance of membranes for waste water treatment grows rapidly. It is now well known that membrane processes can be used as an effective tertiary waste treatment and provide a quality effluent. However, capital and operating costs can be prohibitive. With the arrival of submerged membrane processes where the membrane modules are submerged in a large feed tank and the filtrate is collected through suction applied to the filter side of the membrane, the membrane bioreactors combine biological and physical processes in a stage that promises to be more compact, efficient and economical. Due to their versatility, the size of membrane bioreactors can vary from home-made (such as septic tank systems) to community-scale and large-scale waste treatment. The success of a membrane filtration process depends mainly on the use of an effective and efficient membrane cleaning method. Commonly used physical cleaning methods include backwash (retro-discharge, back-off), using a permeated liquid or gas, washing or flushing the membrane surface using a gas in the form of bubbles in a liquid. Examples of the method of the second type are illustrated in U.S. Patent No. 5,192,456 to Ishida et al., U.S. Patent No. 5,248,424 to Cote et al., U.S. Patent Number 5,639,373 to Henshaw et al., U.S. Patent No. 5,783,083 to Henshaw et al., and our PCT application number WO 98/28066. In the examples referenced above, a gas is injected, usually by means of a pressurized fan, into a liquid system in which the membrane module is submerged to form gas bubbles. The bubbles that are formed in this way then move upwards to wash the surface of the membrane to remove the scale substances that form on the surface of the membrane. The Cutting force produced is mainly based on the initial velocity of the gas bubbles, the size of the bubbles and the resultant forces applied to the bubbles. The transfer of fluid in this solution is limited to the effectiveness of the gas lift mechanism. To improve the washing effect, more gas must be supplied. However, this method has several disadvantages: it consumes large amounts of energy, possibly forms a fog or foam flow which reduces the filtration area of the effective membrane, and can be destructive to the membranes. In addition, in a high solids concentration environment, the gas distribution system is gradually blocked by dehydrated solids or simply blocked when the gas flow accidentally ceases. For most tubular membrane modules, the membranes are flexible in the middle (longitudinal direction) of the modules but tend to be tighter and less flexible towards both encapsulation heads. When such modules are used in an environment containing high concentrations of suspended solids, the solids are easily trapped within the membrane assembly, especially in the vicinity of the two encapsulation heads. Methods to reduce the accumulation of solids include an improvement of the module configurations and flow distribution when gas washing is used to clean the membranes.

In the design of a membrane module, the packing density and the tubular membranes in a module is an important factor. The packing density of the fiber membranes in a membrane module as used herein is defined as the cross-sectional area encapsulated by the fiber membranes divided by the encapsulated area and is usually expressed as a percentage. From an economic point of view, it is desirable that the packing density be as high as possible to reduce the cost of making membrane modules. In practice solid packing is reduced in a less densely packaged membrane module. However, if the packing density is too low, the washing effect between the membranes also decreases, resulting in a less efficient cleaning / washing of the membrane surfaces. It is therefore desirable to provide a membrane configuration which helps in the removal of accumulated solids and at the same time maximizes the packing density of the membranes.

DESCRIPTION OF THE INVENTION The present invention, at least in its modalities, seeks to resolve or at least lessen some of the disadvantages of the prior art or at least provide the public with a useful alternative.

According to one aspect, the present invention provides a method of washing a membrane surface using a liquid medium with gas bubbles entrained therein, which includes the steps of entraining the gas bubbles in a liquid medium by the flow of the gas. liquid medium passing a gas source, and flowing the gas bubbles and liquid medium along the membrane surface to unclog the fouling materials therefrom. Preferably, the gas bubbles are entrained in the liquid stream by means of a venturi device. For a further preference, the gas bubbles are entrained or injected into the liquid stream by means of devices which forcibly mix the gas within the liquid flow to produce a mixture of liquid and bubbles, such devices include a jet, nozzle, ejector, eductor, injector or similar. Optionally, an additional source of bubbles can be provided in the liquid medium by means of a fan or a similar device. The gas used may include air, oxygen, chlorine gas or ozone. The air is the most economical for the purposes of washing or aeration. Chlorine gas can be used for washing, disinfecting and improving the cleaning efficiency by chemical reaction on the surface of the membrane. The use of ozone, in addition to the similar effects mentioned for gaseous chlorine, has features additional, such as oxidize DBP precursors and convert non-biodegradable NOMs to biodegradable dissolved organic carbon. According to a second aspect, the present invention provides a membrane module comprising a plurality of porous membranes, the membranes are placed in close proximity to each other and mounted to prevent excessive movement between them, and a means to provide, from within the module, by means other than the gas passing through the pores of the membranes, gas bubbles entrained in a liquid flow so that, in use, the liquid and the bubbles carried therein move past the surfaces of the membranes to unblock the materials of incrustation of the same, gas bubbles are entrained in the liquid by flowing the liquid passing a source of gas to extract the gas in liquid flow. Preferably, the liquid and bubbles are mixed and then flowed past the membranes to unclog the embedding materials. According to a preferred form, the present invention provides a method for 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, the membranes are placed in close proximity to each other and mounted to avoid excessive movement between them, the method includes stages of providing, from within the arrangement, by means other than the gas passing through the pores of the membranes, evenly distributed gas bubbles entrained in a liquid flow, the gas bubbles are entrained in the liquid flow when flowing the liquid passing a gas source so as to cause the gas to be entrained or mixed in the liquid, the distribution is such that the bubbles pass substantially uniformly between each membrane in the arrangement for, in combination with the liquid flow , clean by discharge the surface of the membranes and remove the accumulated solids from inside the membrane module. Preferably, the bubbles are injected and mixed in the liquid flow. By preference, the membranes comprise porous hollow fibers, the fibers are fixed at each end in a header, the lower header has one or more holes formed therein through which the gas / liquid flow is introduced. The holes can be circular, elliptical or in the form of a groove. The fibers are normally sealed at the lower end and open at their upper end to allow removal of the filtrate, however, in some arrangements, the fibers can be opened at both ends to allow removal of the filtrate from one or both ends. The fibers are preferably placed in cylindrical arrangements or assemblies. It will be appreciated that the cleaning process described is equally applicable to other forms of membrane such as flat or plate membranes.

According to a further aspect, the present invention provides a membrane module comprising a plurality of porous hollow fiber membranes, the fiber membranes are placed in close proximity to each other and assembled to prevent excessive movement therebetween. fiber membranes are fixed at each end in a header, a header has one or more holes formed therein through which the gas / liquid flow is introduced, and a dividing medium extending at least in part between the headers to divide the membrane fibers into groups. Preferably, the dividing means is formed by a spacing between respective groups of fibers. The divisions may be parallel to each other, in the case of cylindrical arrangements of fiber membranes, the divisions may extend radially from the center of the array or they may be placed concentrically within the cylindrical array. In an alternative form, the fiber bundle may be provided with a central longitudinal passage extending along the array between the headers. According to a further aspect, the present invention provides a membrane module for use in a membrane bioreactor that includes a plurality of porous hollow membrane fibers that extend longitudinally between, and that are mounted at each end to an encapsulating head. respective, the membrane fibers are arranged in close proximity to each other and mounted to prevent movement excessive between them, the fibers are divided among a number of sets, at least in or adjacent to their respective encapsulation head so as to form a space therebetween, one of the encapsulation heads has an arrangement of aeration opening formed therein to provide gas bubbles within the modules so that, when used, the bubbles move past the surfaces of the membrane fibers to unclog encrustation materials therefrom. The fiber bundle is protected and the fiber movement is limited by a module support screen which has both vertical and horizontal elements properly spaced to provide unrestricted fluid and gas flow through the fibers and to restrict the amplitude of the fiber. the concentration of reducing energy of movement of fibers in the encapsulated ends of the fibers. Preferably, the aeration openings are placed to coincide with the spaces formed between the divided assemblies. By preference, the openings comprise a slot, grooves or a row of holes. Preferably, the fiber assemblies are located in the encapsulation head between the slots or rows of holes. For additional preference, the gas bubbles are entrained or mixed with a liquid flow before being fed through the holes or slots, althoughYou will appreciate that only gas can be used in some configurations. The liquid used can be the feed to the membrane module. Fibers or sets of fibers can cross each other between the encapsulation heads although it is desirable that they do not. Preferably, the fibers within the module have a packing density (as defined above) of between about 5 to about 70%, and more preferably between about 8 and about 55%. By preference, the holes have a diameter in the range of about 1 to 40 mm, and more preferably in the range of about 1.5 to about 25 mm. In the case of a slot or row of holes, the open area is chosen to be equivalent to that of the previous holes. Typically, the inner diameters of the fibers vary from about 0.1 mm to about 5 mm and preferably are in the range of about 0.25 mm to about 2 mm,. The wall thickness of the fibers depends on the materials used and the strength required versus the filtration efficiency. Typically, the wall thickness is between 0.05 and 2 mm and more frequently between 0.1 mm and 1 mm. According to another aspect, the present invention provides a membrane bioreactor that includes a tank that has a means for introducing feed thereto, a means for forming activated sludge inside the tank, a membrane module according to the first aspect placed inside the tank so that it is immersed in the sludge and the membrane module that is provides with means of an extraction filtrate from at least one end of the fiber membranes. According to another additional aspect, the present invention provides a method for operating a membrane bioreactor of the type described in the second aspect, which comprises introducing a feed to the tank, applying vacuum to the fibers to extract filtrate therefrom while it is supplied in a manner Periodic or continuous gas bubbles through the aeration openings into the module such that, when used, the bubbles move past the surfaces of the membrane fibers to release encrustation materials therefrom. Preferably, the gas bubbles are entrained or mixed with a liquid flow when fed through the holes or slots. If required, an additional source of aeration can be provided inside the tank to aid the activity of the microorganisms. By preference, the membrane module is suspended vertically within the tank and the additional source of aeration can be provided below the suspended module. Preferably, the additional source of aeration comprises a group of air permeable tubes. The membrane module can be operated with or without backwash, depending on the flow. It has been shown that a high mixed liquor of suspended solids (5,000 to 20,000 ppm) in the bioreactor significantly reduces residence time and improves filtering quality. The combined use of aeration in both the degradation of organic substances and membrane cleaning has been shown to allow a constant filtrate flow without significant increases in transmembrane pressure while establishing a high concentration of MLSS. The use of split fiber groups allows higher packing densities to be obtained without significantly compromising gas flushing processes. This provides that higher filtration efficiencies are acquired.

BRIEF DESCRIPTION OF THE DRAWINGS The preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a schematic side elevation of one embodiment of a membrane module and illustrates the cleaning method, according to the invention; Figure 2 shows an enlarged schematic side elevation of a shape of the jet type arrangement used to form entrained gas bubbles; Figure 3a shows a schematic side elevation of a divided membrane module, according to one embodiment of the present invention; Figure 3b shows a section through the membrane assembly of the figure Figure 4a shows a schematic side elevation of a divided membrane module, according to a further embodiment of the present invention; Figure 4b shows a section through the membrane assembly of Figure 4a; Figure 5a shows a schematic side elevation of a divided membrane module, according to another embodiment of the present invention; Figure 5b shows a section through the membrane assembly of Figure 5a; Figure 6a shows a schematic side elevation of a divided membrane module, according to another embodiment of the present invention; Figure 6b shows a section through the membrane assembly of Figure 6a; Figure 7 shows a view similar to Figure 2 of a further embodiment of the invention; Figure 8 shows a view similar to that of Figure 2 of a further embodiment of the invention; Figure 9 shows a pictorial view in sectioned perspective of the lower end of another preferred embodiment of the membrane module, according to the invention; and Figure 10 shows a pictorial view in sectioned perspective of the upper end of the membrane module of Figure 9.

PREFERRED MODALITIES OF THE INVENTION With reference to the drawings, the embodiments of the invention will be described in relation to a membrane module of the type described in our previous PCT application number WO98 / 28066 which is incorporated herein by cross reference, however, it will be appreciated that the invention is equally applicable to other forms of membrane module. The membrane module 5 typically comprises fiber, a tubular or flat sheet forming the membranes 6 encapsulated at the two ends 7 and 8 and included in a support structure, in this case, a mesh 9. Either one or both ends of The membranes can be used for permeate collection. The lower part of the membrane module has a number of through openings 10 in the container 11 for distributing a gas mixture and liquid feed past the surfaces of the membrane.

With reference to the embodiment shown in figure 1, a venturi device 12 or similar is connected to the base of the module. The venturi device 12 introduces gas through the inlet 13, mixes or entrains the gas with liquid flowing through the feed inlet 14, forms gas bubbles and diffuses the liquid / gas mixture into the module openings 10. After passing through the distribution openings 10, entrained gas bubbles wash the surfaces of the membrane while moving upwards along the liquid flow. Either the liquid feed or the gas can be a continuous or intermittent injection depending on the requirements of the system. With a venturi device, it is possible to create gas bubbles and aerate the system without a fan. The venturi device 12 may be a venturi, jet, nozzle, ejector, eductor, injector or the like. With reference to Figure 2, an enlarged view of the jet or nozzle type device 15 is shown. In this embodiment, liquid is driven through a jet 16 having a passage 17 of surrounding air to produce a flow 18 of liquid entraining gas. Such a device allows independent control of the gas medium and the liquid by adjusting the respective supply valves. The liquid commonly used to entrain the gas is feed water, waste water or a mixed liquor to be filtered. The pumping of such liquid operation through of a venturi or similar system generates a vacuum to suck the gas into the liquid, or reduces the gas discharge pressure when a fan is used. By providing the gas in a liquid flow, the possibility of blocking the distribution openings 10 is substantially reduced. The present invention is preferred at least in the embodiments and can provide numerous advantages which can be summarized as follows: 1. By using a venturi device or the like, it is possible to generate gas bubbles to wash membrane surfaces without the need of a supply of pressurized gas such as a fan. When a motive fluid passes through a venturi system, it generates a vacuum to draw the gas into the liquid flow and generate gas bubbles in it. Even if a fan is still required, the use of the above process reduces the fan discharge pressure and therefore lowers operating costs. 2. The liquid and gaseous phases mix well in the venturi system and then diffuse into the membrane module to wash the membranes. When a jet-type device is used to forcefully mix the gas within the liquid medium, a further advantage is provided in that a higher velocity of the bubble stream occurs. In wastewater treatment, such deep mixing provides excellent oxygen transfer when the gas used is air or oxygen. If the gas is injected directly into a tube filled with a liquid, it is possible that the gas will form a gas layer stuck in the wall of the tube and therefore the liquid and gas will be diverted into different parts of a module, which results in a poor cleaning efficiency. 3. The flow of gas bubbles is improved by the flow of liquid along the membrane which results in a large washing cutting force being generated. This gas / liquid delivery method provides a positive fluid transfer and aeration with the ability to independently adjust the gas and liquid flow rates. 4. The injection of a mixture of a fluid in two phases (gas / liquid) into the orifices of the air distribution device can eliminate the formation of dehydrated solids and therefore prevent the gradual blocking of the orifices by such dehydrated solids. 5. The injection distribution also provides an efficient cleaning mechanism to effectively introduce cleaning chemicals into the depths of the module and at the same time provide discharge cleaning energy to improve chemical cleaning. This arrangement, in combination with the high packing density that can be obtained with the module configuration described, allows the fibers are effectively cleaned with a minimum amount of chemicals. 6. The described module configuration allows a higher fiber packing density in one module without significantly increasing the packing of solids. This adds additional flexibility to the extent that the membrane modules can be integrated into an aerobic container or placed in a separate tank. In this last provision, the advantage is a significant saving in the use of chemical substances due to the small chemical handling in the tank and the work costs due to which the chemical cleaning process can be automated. The reduction in chemical substances used is also important because the chemicals, which can be fed back to the bioprocess, are still aggressive oxidants and therefore can have a harmful effect on the bioprocess. Consequently, any reduction in the chemical load present in the bioprocess provides significant advantages. 7. The positive injection of a gas and liquid feed mixture to each membrane module provides a uniform distribution of process fluid around the membranes and therefore minimizes feed concentration polarization during filtration. Concentration polarization is greater in a large-scale system and for process feeds that contain large amounts of suspended solids. The prior art systems have little uniformity because the process fluid often enters one end of the tank that is concentrated as it moves through the modules. The result is that some modules work with much higher concentrations than others, which results in an inefficient operation. 8. Filtration efficiency is improved due to reduced filtration resistance. The lateral feed resistance is decreased due to a reduced cross-flow passage to the membrane surface and the turbulence is generated by the gas bubbles and the flow of the two phases. 9. Such a cleaning method can be used for the treatment of drinking water, wastewater and related processes by membranes. The filtration process can be driven by suction or pressurization. With reference to Figures 3 to 5, modalities of various division arrangements are shown. Again, these embodiments are illustrated with respect to sets 20 of a cylindrical tubular or fiber membrane, however, it will be appreciated that the invention is not limited to such applications. Figure 3 shows a set of tubular membranes 20 divided vertically into several thin cuts 21 by several parallel dividing spaces 22. This division of the assembly allows the accumulated solids to be removed more easily without significant loss of packing density.

Such division can be obtained during the encapsulation process to form complete divisions or partial divisions. Another method for forming a divided module is to encapsulate several small tubular membrane assemblies 23 within each module, as shown in Fig. 4. Another improved configuration of membrane module is illustrated in Fig. 5. The central free zone of membrane forms a passage 24 to allow the injection of more air and liquid. The gas and liquid bubbles then travel along the tubular membranes 20 and pass out through the fiber arrays and into the upper encapsulation head 8, flushing and removing solids from the membrane walls. A single gas or a gas / liquid mixture can be injected into the module. Figure 6 illustrates a similar additional embodiment of Figure 5, but with a single central hole 30 in the lower container 7 for admission of a liquid / cleaning gas mixture to the fiber membranes 20. In this embodiment, the fibers are dispersed adjacent to the orifice 30 and converge in separate assemblies 23 toward the upper container 8. The large central orifice 30 has been found to provide greater liquid flow around the fibers and therefore an improved cleaning efficiency. Figures 7 and 8 show additional embodiments of the invention having a membrane configuration similar to that of Figure 6 and a jet mixing system similar to that of the embodiment of Figure 2. The use of a single central orifice 30 allows the filtrate to be extracted from the fibers 20 at both ends, as shown in the figure 8. With reference to Figures 9 and 10 of the drawings, the module 45 comprises a plurality of hollow fiber membrane assemblies 46 mounted on, and extending between an upper part 47 and a lower part of the enclosing head 8. The encapsulation heads 47 and 48 are mounted on respective encapsulation sleeves 49 and 50 for attachment to an appropriate manifold (not shown). The fiber assemblies 46 are surrounded by a mesh 51 to prevent excessive movement between the fibers. As shown in Fig. 9, the lower encapsulation head 48 is provided with a plurality of slit-type vents 52, arranged in parallel. The fiber membranes 53 are encapsulated in assemblies 46 to form a divided array having spaces 54 extending transverse to the fiber assemblies. Aeration holes 52 are positioned to generally coincide with the partition spaces, although there are generally a number of aeration holes associated with each space. The lower encapsulation sleeve 50 forms a cavity 55 below the lower container 48. A gas or a mixture of liquid and gas is injected into this cavity 55 by a jet assembly 57 (described above) before passing through the holes 52 within the membrane array. In use, the use of the division allows the high energy flow of the gas and liquid mixture for cleaning, particularly near the container ends of the fiber assemblies, which helps with the removal of accumulation of solids that can be removed. accumulate around the membrane fibers. Preferably air is introduced into the module continuously to provide oxygen for the activities of the microorganisms and to continuously clean the membranes. Alternatively, in some applications, pure oxygen or other gas mixtures may be used instead of air. The cleaned filtrate is then removed from the membranes by a suction pump attached to the membrane lumens which pass through the upper vessel described in our application mentioned above. Preferably, the membrane module is operated under conditions of low transmembrane pressure (TMP) due to the high concentration of suspended solids (MLSS) present in the reactor. The membrane bioreactor is preferably combined with an anaerobic process which helps with the additional removal of nutrients from the waste feed.

It has been found that the module system used is more MLSS-tolerant than many current systems and efficient air flushing and backwashing (when used) helps efficient operation and performance of the bioreactor module. It will be appreciated that, although the invention and the embodiments have been described in relation to an application to bioreactors and similar systems, the invention may equally be applicable to other types of applications. It will be appreciated that the invention is not limited to specific embodiments described and other embodiments and exemplary embodiments of the invention are possible without departing from the spirit and scope of the invention.

Claims

1. A method of washing a membrane surface using a liquid medium with gas bubbles entrained therein, characterized in that it includes the steps of entraining the gas bubbles in the liquid medium by flow of the liquid medium passing a gas source, and making flow the gas bubbles and liquid medium along the surfaces of the membrane to separate embedded materials from it.

2. The method according to claim 1, characterized in that the gas bubbles are entrained in the liquid flow by means of a venturi device.

3. The method according to claim 1, characterized in that gas bubbles are entrained or injected into the liquid flow by means of devices which forcibly mix the gas within the liquid flow to produce a mixture of liquid and bubbles.

4. The method according to claim 1, 2 or 3, characterized in that the gas includes air, oxygen, chlorine gas, ozone or any combination thereof.

5. A membrane module, characterized in that it comprises a plurality of porous membranes, the membranes are placed in close proximity to each other and mounted to prevent excessive movement between them, and a means to provide, from the interior of the module, by different means to the gas that passes through the pores of the membranes, bubbles of gas entrained in a liquid flow so that, when used, the liquid and the bubbles carried in it will move past the surfaces of the membranes to separate materials from Inlaying it, the gas bubbles creep into the liquid by flowing the liquid past a gas source to draw the gas into the liquid flow.

6. The membrane module according to claim 5, characterized in that the liquid and the bubbles are mixed and then flowed past the membranes to separate the fouling materials.

7. A method for 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, the membranes are placed in close proximity to each other and assembled to avoid excessive movement between them, the method comprises the steps of providing, from within the arrangement, by means different from the gas that passes through the pores of the membranes, evenly distributed gas bubbles entrained in a liquid flow, the gas bubbles are entrained in the liquid flow by flowing the liquid passing through a source of gas in a manner that causes the gas to be entrained or mixed in the liquid, the distribution is such that the bubbles pass substantially uniformly between each membrane in the arrangement to, in combination with the liquid flow, flush the surface of the membranes and remove the accumulated solids from inside the membrane module.

8. The method according to claim 7, characterized in that the bubbles are injected and mixed in the liquid flow.

9. The method according to claim 7 or claim 8, characterized in that the membranes comprise porous hollow fibers, the fibers are fixed at each end in a header, at least one header has one or more holes formed therein through the which introduces the gas / liquid flow.

10. A membrane module characterized in that it comprises a plurality of porous hollow fiber membranes, the Fiber membranes are placed in close proximity to each other and mounted to prevent excessive movement between them, the fiber membranes are fixed at each end in a header, a header has one or more of the holes formed in it through from which the gas / liquid flow is introduced, and a dividing medium that extends at least in part between the headers to divide the membrane fibers into groups.

11. The module according to claim 10, characterized in that the dividing means is formed by a spacing between respective fiber groups.

12. The module according to claim 11, characterized in that the fiber membranes are placed in cylindrical arrangements and the divisions extend radially from the center of the array or are concentrically divided within the cylindrical array.

13. A membrane module, characterized in that it comprises a plurality of porous hollow fiber membranes, the fiber membranes are placed in close proximity to each other to form an assembly and mounted to prevent excessive movement between them, the fiber membranes are fixed at each end in a header, a header has one or more holes formed in it through which the flow of gas / liquid, and the fiber bundle has a central longitudinal passage that extends along the set between the headers.

14. The membrane module for use in a membrane bioreactor that includes a plurality of porous hollow membrane fibers that extends longitudinally between and that is mounted at each end with a respective encapsulation head, the membrane fibers are placed in close proximity between if and they are assembled to prevent excessive movement between them, the fibers are divided into a number of groups, at least at or adjacent to their respective encapsulation head so as to form a space therebetween, one of the heads of The encapsulation has an aeration opening arrangement formed therein to provide gas bubbles within the module so that in use, the bubbles move past the surfaces of the membrane fibers to separate fouling materials therefrom.

15. The membrane module, according to claim 14, characterized in that the aeration openings are positioned to match the spaces formed between the divided assemblies.

16. The membrane module according to claim 15, characterized in that the openings comprise a groove, grooves or a row of holes and the fiber assemblies are located in the encapsulation head between the grooves or the rows of holes.

17. The membrane module, according to claim 10, claim 13 or claim 14, characterized in that the fibers within the module have a packing density of between about 5 and about 70%.

18. The membrane module, according to claim 10, claim 13 or claim 14, characterized in that the fibers within the module have a packing density of between about 8 to about 55%.

19. The membrane module, according to claim 16, characterized in that the holes have a diameter or an equivalent diameter in the range of about 1 to 40 mm.

20. The membrane module, according to claim 16, characterized in that the holes have a diameter or an equivalent diameter in the range of about 1.5 to about 25 mm.

21. The membrane module, according to claim 10, claim 13 or claim 14, characterized in that the inner diameter of each fiber is in the range from about 0.1 mm to about 5 mm

22. The membrane module, according to claim 10, claim 13 or claim 14, characterized in that the inner diameter of each fiber is in the range from about 0.25 mm to about 2 mm.

23. The membrane module, according to claim 10, claim 13 or claim 14, characterized in that the wall thickness of each fiber is between about 0.05 mm and about 2 mm.

24. The membrane module, according to claim 10, claim 13 or claim 14, characterized in that the wall thickness of each fiber is between about 0.1 mm and about 1 mm.

25. The membrane bioreactor, characterized in that it includes a tank having a means for the introduction of feed thereto, a means for forming activated sludge inside the tank, a membrane module according to claim 10, claim 13 or claim 14 placed inside the tank so that it is immersed in the mud and the membrane module provided with a means for extracting a filtrate from at least one end of the fiber membranes.

26. The method for operating a membrane bioreactor of the type according to claim 14, characterized in that it comprises introducing feed to the tank, applying vacuum to the fibers to extract a filtrate thereof while periodically or continuously supplying gas bubbles through the openings of aeration to the interior of the module so that, when used, the bubbles move past the surfaces of the membrane fibers to release materials for embedding thereof.

27. The method according to claim 26, characterized in that the gas bubbles are entrained or mixed with a liquid flow when it is fed through the holes or slots.

28. The membrane module, according to claim 25, characterized in that the membrane module is suspended vertically within the tank and an additional source of aeration is provided below the suspended module.

29. The membrane module, according to claim 28, characterized in that the additional source of aeration comprises a group of air permeable tubes or gas distributors.

30. A membrane module, substantially as described above with reference to any of the embodiments described and their associated drawings.

31. A method for washing a membrane surface, substantially as described above with reference to any of the embodiments described and their associated drawings. ) LO0 l l oo l ^ & and - 33 - SUMMARY A method and apparatus for cleaning a membrane module (5) is provided, the membrane module comprises a plurality of porous membranes (6), the membranes are placed in close proximity to each other and mounted to prevent excessive movement between them. and means (10, 12) for providing, from within the module (5), by means other than the gas passing through the pores of the membranes, gas bubbles entrained in a liquid flow so that, when used, the liquid and the bubbles (18) dragged therein move past the surfaces of the membranes to unblock fouling materials from them, gas bubbles are entrained in the liquid by flowing the liquid past a gas source to drag the gas into the liquid flow. The gas bubbles are entrained within the liquid using a venturi type device (12). The membranes (20) are preferably divided into separate groups (23) to aid cleaning while maintaining a high packing density.