CN102325583A - Immersed membrane cassette and method of operation - Google Patents

Abstract

A module of vertical membranes has a lower header with integral air holes. Modules are mounted in line on upper and lower beams. A skirt is formed under the cassette. Adjustable side members between the beams allow for membrane slack adjustment and bottom beam levelling. A flat aerator assembly can be inserted into spaces between the cassettes and provide bubbles into the skirts, the spaces between cassettes, or both. An aeration method involves producing bubbles primarily or only to one side of the module, alternating from one side of the module to the other, while also producing bubbles within the module or between the membranes, optionally continuously. A cleaning method involves flowing a chemical cleaning solution by force of gravity through a membrane module, optionally by injected a concentrated solution into a vented potion of a permeate withdrawal system located above the water level in a tank holding the module.

Description

Immersed membrane cassette and method of operation

For the united states, the present application claims, pursuant to 35USC 119(e), to U.S. application serial No.61/144,723 filed on 14.1.2009; 61/249,847 filed on 8/10/2009; and 61/249,853 filed on 8.10.2009, which are hereby incorporated by reference in their entirety.

Technical Field

The present description relates to submerged membrane systems, e.g., suction-driven submerged microfiltration or ultrafiltration systems for generating usable water or treating wastewater, and membrane system operating methods, including methods to prevent scaling, such as aeration (or bubble purification) methods and chemical cleaning methods.

Background

Submerged membrane systems, such as membrane bioreactors, may use hollow fiber ultrafiltration or microfiltration membranes submerged in a reservoir of water (including wastewater) to be treated. A number of hollow fibers may be mounted vertically between the upper and lower potting heads to form a module. The module is typically maintained in a size that can be manipulated by a human. To provide the membrane surface area required for large systems, the modules are joined together into larger assemblies, sometimes referred to as cassettes. The construction of the cassette and the arrangement of conduits around the cassette affect the cost of installation, the ability to package the membrane regions into the reservoir, the fluid flow in the reservoir, and the efficiency of operation of the system.

U.S. Pat. No. 5,639,373 describes a submerged membrane module. In one example, the membrane is oriented vertically between solid upper and lower rectangular potting heads, while the tubular aerator is placed on the sides of the lower potting head. In another example, a lower potting head of the module has a skirt extending below the potting head and a tube extending through the potting material. Air provided via a port in one side of the skirt flows through the tube to create a bubble at the top of the lower potting head.

U.S. Pat. No.6,245,239 describes a cyclic aeration system for submerged membrane modules. In one example, a set of rectangular modules with membranes oriented vertically between solid upper and lower potting heads have a set of aerators below the modules. The flow of air to the aerator is switched on and off in a repeated cycle.

Keep clean for maintaining operation of submerged membranes (e.g., ultrafiltration or microfiltration membranes in a membrane bioreactor). In the example described in U.S. Pat. No.6,547,968, maintenance cleaning involves frequent (e.g., 1-7 times per week) periods of contact with cleaning chemicals in order to "condition" the fouling layer, rather than attempting to clean it. The active chemical in the cleaning solution is typically chlorine, but other oxidizing agents, bases or acids may be used. The efficiency of maintaining cleanliness is related to the chlorine concentration and the contact time. When NaOCl is used to provide chlorine, the concentration may be between 100mg/L and 500 mg/L. The contact time may be from a few minutes to an hour.

Disclosure of Invention

The following description is intended to introduce the reader to a detailed description, and not to limit or define any claimed invention. The invention can be attributed to combinations or subcombinations of apparatus elements or process steps described in any portion of this document, including the figures.

The modules of vertically oriented membranes have an upper permeate header and a lower dead end header with integral air holes. The headers are not fixed separately from each other in the module itself. In plan view, the module preferably has a square cross-section, but with a permeate cap that provides a round permeate connection. The modules are mounted in line on upper and lower cross members to form a cassette. The cassette is elongated rectangular in plan view. A skirt is formed below the module or cassette to provide an open-bottomed chamber below the lower header in communication with the air vent. Adjustable side members between the beams allow for slack adjustment of the membrane and bottom beam leveling. A permeate header is provided above and coincident with the upper beam. The cartridge can be inserted from above into a receptacle mounted on the upper side of the reservoir. The aerator grid is provided separately. The main components of the aerator grid are flat assemblies of conduits and structural members that can be inserted vertically down into the spaces between the cassettes. The air holes in the aerator may be positioned to provide air bubbles into the skirt and optionally also into the space between the cassettes. The top cross beam of each cassette is attached to the reservoir and carries the weight of the cassette.

The module can also be described as having a membrane extending upward from the encapsulation head. The potting head is positioned between two opposing walls of a skirt that extends below a bottom of the potting head. There is a passage for air to flow vertically through the encapsulation head. Aerators are provided on each side of the module. Each aerator has one or more holes and bubbles are generated both between the walls of the skirt and outside the skirt. The air stream is provided to only or primarily one aerator at a time and to only or primarily the other aerator at another time. The gas flows through the encapsulation head to generate bubbles during two periods, optionally continuously. The aeration process involves generating bubbles primarily or only for one side of the module, alternating from one side of the module to the other, while also generating bubbles within the module or between membranes, optionally continuously.

One or more modules may be connected to the permeate header above the membrane surfaces of the modules. The permeate header communicates with an isolation valve to isolate the permeate header from other conduits in the permeate recovery system. The permeate header also communicates with a bleed valve on the module side of the isolation valve, wherein the isolation valve is operable to vent the permeate header to atmosphere. The chemical injection conduit allows for injection of chemicals into the permeate header. To clean the module, the isolation valve is closed. Optionally, the level of water (mixed liquid in the case of a membrane biomass generator) in the reservoir may be lowered. The cleaning chemical is injected into the permeate header where it mixes with the water in the permeate header to achieve the desired concentration. In the event that the permeate header is above water level, the bleed valve opens, allowing the chemical to flow across the membrane surface. To restore permeation, the reservoir is refilled, the relief valve is closed, and the isolation valve is reopened. The cleaning method involves flowing a chemical cleaning solution through the membrane module by gravity, optionally injecting a concentrated solution into a bleed portion of a permeate recovery system positioned above the water level in the reservoir of the retention module. By this method, only a small amount of chemicals is used. The chemical may be evenly distributed among the many modules without high flow rates. The chemicals remain in high concentration near the module and dilute to a small amount into the water outside the module.

Brief Description of Drawings

Fig. 1 is a cross-section of a module.

Fig. 2 is a side view of a cartridge in a reservoir, the reservoir being shown in cross-section.

Fig. 3 is a side view of an aerator assembly.

Fig. 4 is a schematic top view of a reservoir with a cassette and aerator assembly installed.

Fig. 5 shows a partial end view of the cassette in the reservoir with the lower portion of the set of modules in fig. 1 as a section and an illustrative aeration system for one session.

Fig. 6 shows a partial end view of the cassette in the reservoir with the lower portion of the set of modules in fig. 1 as a cross-section and an exemplary aeration system at another time period.

Fig. 7 is a longitudinal cross-section of a portion of the permeate header of fig. 2.

Fig. 8 is a section taken across the diameter of a portion of the permeate header in fig. 2.

Detailed Description

Referring to fig. 1, a module 10 has a lower potting head 12 and an upper potting head 14. A plurality of hollow fiber membranes 16 are potted in the potting heads 12, 14. The potting heads 12, 14 are sometimes also referred to as headers or tube sheets. To simplify the illustration, only some of the membranes 16 are shown. The membrane 16 is plugged at its lower end into a block of encapsulation resin within the lower encapsulation head 12. The membrane 16 passes through the upper potting head 14 to open a permeate collection cap 18 sealed to the upper surface of the upper potting head 14. The cap 18 is connected to a permeate header 20, which in turn is connected to a suction source operable to recover permeate that passes through the membrane 16. The potting heads 12, 14 may be attached to a frame 26 (only partially shown) to space the potting heads 12, 14 apart and allow the module 10 to be lowered into a reservoir of liquid to be filtered. The module 10 is intended to be immersed in an open reservoir with the membrane 16 oriented vertically.

The module 10 may be, for example, circular or square in plan view, with a diameter or width of between 100mm and 200mm or 100mm and 150 mm. A plurality of modules 10 may be arranged side by side to create a rectangular assembly. The height of the module 10 may be 1m to 2 m. The overall film surface area may be 15 square meters to 25 square meters. The lower potting head 12 has one or more (e.g., 1 to 10) holes 22 therethrough between the membranes 16. Each hole 22 may have a diameter of 5mm to 10 mm. One or more of the side walls of the lower potting head 12, portions of the frame 26 holding the modules 10, and the skirt walls 28 extend downwardly at the sides of the lower potting head 12 to define the sides of an open bottom chamber 30 (sometimes referred to as a skirt) below the lower potting head 12. The lower potting head 12 or portions of the frame 26 holding the modules 10 may define the top of the chamber 30, or an additional top plate may be employed. The extension tube 24 may extend from the bore 28 into the chamber 30. If a plurality of modules 10 are arranged side-by-side to form a rectangular assembly, the skirt wall 28 may extend along the length of the entire assembly to form one long chamber 30 under a plurality of modules 10. Alternatively, additional partition walls may be placed between each pair of modules 10 to provide a separate chamber 30 below each module 10.

Referring to fig. 2, a group of modules 10 are held in a common frame 26 by their potting heads 12, 14 to form a cassette 60. The modules 10 are arranged together in a row as tightly as possible. The frame 26 includes horizontal cross members 42 and vertical uprights 44. The permeate collection header 20 runs parallel to the framework 26 and is connected to the cap 18 of each module 10. The permeate collection header 20 also communicates with one or more larger permeate collection conduits 50 via one or more isolation valves 52. In the example shown, one end of the permeate header 20 is closed, while the other end of the permeate header 20 is attached to a common permeate collection conduit 50 via an isolation valve 52 associated with only one permeate collection header 20. Isolation valve 52 allows for isolation of one or more crossbar-cassettes for maintenance without interrupting operation. A permeate collection conduit 50 runs along the side of the reservoir 48 at a right angle to the permeate header 20 and is connected to the permeate headers of other groups of modules positioned in the reservoir 48 than the illustrated group of modules 10. The permeate collection conduit 50 is also attached to a source of suction (not shown) operable to recover permeate from the module 10. The upper cross member 42 is typically immersed in the water in the reservoir 48, and the permeate header 20 may typically be above or in the water.

Each cassette 60 is held in a pair of guides 46 connected to a reservoir 48. A pair of guides 46 are opposed to each other on opposite sides of the reservoir 48. The cassette 60 slides vertically downward into the guide 46. The upper cross beam 42 of the cassette 60 bears on the seat of the guide 46 so that the overall weight (or buoyancy) of the cassette 60 is resisted by the upper cross beam 42. Alternatively, the guide 46 may laterally constrain the lower cross member 42 or may not contact the lower cross member 42. The distance between the top and bottom cross beams 42 is set by adjusting the connection between the vertical posts 44 and the cross beams 42. The upper cross beam 42 spans the width of the immersion tank 48 and is attached on both sides to the walls of the tank 48 via guides. Although the upper cross beam 42 (and the upper potting head attached thereto) is generally submerged, the attachment point on the guide 46 or between the guide 46 and the reservoir 48 may be above the water surface.

The vertical columns 44 are rigid structural members (tubes or beams) that connect the top and bottom beams 42 and hold them at a fixed and adjustable distance. Each cassette 60 has two vertical posts 44, one at each end of the cassette 60. The distance between the top and bottom beams 42 should be slightly less than the fiber length between the potting heads 12, 14 to provide some hollow fiber slack. The amount of fiber relaxation can be adjusted for performance. The vertical posts 44 may be fixed into the bottom cross member 42 (allowing rotation), but have adjustable sliding type joints in the top cross member 42 to adjust the spacing of the cross members 42. When the bottom cross member 42 becomes buoyant, the vertical columns 44 and guides 46 maintain the bottom cross member 42 in a fixed vertical position during operation.

The vertical posts 44 may be used to change the position of the bottom cross beam 42 while the cassette 60 is in the reservoir 48 in order to adjust slack and ensure a uniform air flow rate through the holes in the lower encapsulation head. First, the top cross-beam 42 is roughly leveled by adjusting the attachment points on the reservoirs 48 or guides 46. Next, the bottom cross-beam 42 is pushed downward until the hollow fibers 16 are tensioned. The vertical post 44 is then moved back upward a distance that will provide the desired fiber slack. Again, the air flow was turned on at a low value and the bubble pattern on the surface was observed. The vertical posts can then be moved up and down until the air flow is uniform, making sure that the required adjustment is evenly split between the two vertical posts 44 (one moving up and the other moving down) to avoid significantly changing the slack. The vertical post 44 may then be locked in place.

Fig. 3 shows a side view of the aerator assembly 70. The aerator assembly 70 is separate from the cassette 60. Aerator assembly 70 is inserted between pairs of cassettes 60 and optionally in the vicinity of the outer cassettes. Each aerator assembly 70 slides vertically into an aerator guide 72 attached to the wall of the tank 48. The aerator guide 72 can extend down to the reservoir 48 (rather than up as shown), similar to the guide 46 for the cassette 60. Each aerator module 70 is made up of an aerator manifold 74, an aerator 32, and a number of downcomers (or risers) 76. The aerator assembly 70 is generally flat. The aerator 32 is sometimes referred to as an air, gas or bubble shower, or simply a shower.

The aerator manifold 74 extends between each pair of cassettes 60. The downcomers 76 are connected on each side thereof to the aerator manifold 74. Optionally, additional downcomers 76 may be provided every 200mm to 500 mm. The downcomer 76 may be long enough to position the aerator 32 under the skirt of the cassette 60 when installed. The aerator assembly 70 described herein primarily occupies space in the reservoir 48 that is in any case required for clearance for water to flow between the cassettes 60 and thereby promotes high reservoir strength (membrane surface area per unit volume or square meters of reservoir surface area).

Referring to fig. 4, the reservoir 48 is generally rectangular in plan view. The cassette 60 is placed across the width or length of the reservoir. A useful feature of the beam-cassette structure described herein is that the length of cassette 60 can be configured to increase the width or diameter of module 10 so that the length of the cassette can be approximately equal to (and include slightly less than) the width or length of reservoir 48. For retrofit situations, a custom length cassette can be constructed using standard dimensions for the module 10 by merely changing the length of the cross beam 28. The cassette 60 and aerator assembly 70 can be positioned side-by-side across the remaining dimensions of the reservoir 48 to effectively fill the reservoir area to high reservoir strength. The permeate header 20 is connected to a main permeate header 50 on one side of the reservoir 48. The aeration assemblies 70 are connected in an alternating pattern to two separate aeration headers 34 on the other side of the reservoir 48, or alternatively to a single header if air is to be supplied to all of the cassettes 60 in the reservoir 48 simultaneously.

The reservoir 48 may be 2m to 3m deep. In membrane bioreactor applications, the reservoir 48 also contains a layer of mixed liquid distribution conduit at the bottom (not shown), and a back flow activated dirt outlet or overflow (not shown). Desirably, the membrane reservoirs 48 are completely filled by the cartridge 60 to ensure a uniform flow pattern in the reservoirs 48.

Referring to fig. 5 and 6, a number of modules 10 may be immersed side-by-side in a reservoir of water to be filtered (not shown in fig. 5 and 6), such as a recirculating mixed liquor in a wastewater treatment plant. Each module 10 shown in fig. 5 and 6 may be part of a cassette 60 that extends lengthwise perpendicular to the page. The groups of modules 10 are spaced apart, for example, 200mm to 500mm center to center, to provide a gap therebetween. Aerators 32 are positioned between each pair of spaced apart modules 10 and are optionally located near but outside of the modules 10 at the group edge of the modules 10. The aerator 32 may be a conduit positioned 100mm to 500mm below the lower potting head 12 with 5mm to 15mm air holes 40 per 50mm to 100mm on each side of the aerator 32. The air holes 40 may be oriented radially so as to point 30 to 60 degrees below horizontal. The aerator 32 is attached to a header 34, and the header 34 is connected via a valve 36 to a blower 38 or other source of pressurized gas to be used for generating the bubbles. The process of membrane aeration is sometimes also referred to as air, gas or bubble sparging, or simply as bubbling.

As air or other gas flows to the aerator 32, bubbles are generated at the air holes 40. A portion (e.g., between 25% and 75%) of the bubble gas stream is trapped in the chamber 30 of the module 10, forms an air pocket below the lower potting head 12, and flows through the holes 22 in the lower potting head 12 to create a bubble within the module 10. The remaining portion of the bubbling airflow rises through the gaps between the modules 10. The bubbles rising in the gap entrain water in the reservoir, causing water to also rise through the gap. The portion of the bubble stream trapped in chamber 30 can be varied by changing the design, location or positioning of aerators 32, by changing the width of the gap between modules 10 or by changing the width of the bottom of skirt wall 28. The aerator 32 and the lower potting head 12 within the cassette 60 are preferably leveled to promote even distribution of air flow from the aerator vents 40 or from the apertures 22 of one or more lower potting heads 12 of the cassette 60.

The aerators 32 may be connected to the headers 34 such that each header 34 supplies gas to every other aerator 32. For example, if the aerators 32 in the tank are counted from left to right, an even number of aerators 32 are attached to the first header 34a and an odd number of aerators 32 are attached to the second header 34 b. The flow from the fan 38 may be switched from the first manifold 34a to the second manifold 34b by closing the valve 36a while opening the valve 36 b. After a period of time, the flow may be switched back to the first manifold 34a by opening valve 36a while closing valve 36 b. The air flow can be switched back and forth, while the permeation and backwash or relaxation cycles of the filtration operation will continue. Fig. 6 shows the gas flow with valve 36a closed and valve 36b open, while fig. 5 shows the gas flow with valve 36a open and valve 36b closed.

By the above method, bubbles are provided in the gap near the module 10, first on one side of the module 10 and then on the other side of the module 10. This promotes horizontal water flow through the membrane 16. However, because there are always bubbles entering into one side of the chamber 30, the airflow rate of bubbles generated within the module 10 through the aperture 22 is substantially constant. Thus, dirt in the water is difficult to deposit within the module 10. Specifically, the method prevents solids from accumulating in the module 10 near the lower potting head 12. It is important to avoid fouling directly above the lower potting head 12 because it is an area that is generally prone to fouling in vertical hollow fiber membranes and is difficult to clean. The dead-end encapsulation of the membranes 16 in the lower encapsulation head, although optional, also helps prevent fouling in the vicinity of the lower encapsulation head 12, since the permeable membrane pressure decreases with distance from the permeate header due to head loss of permeate flow in the lumen of the membranes 16.

Optionally, an extension catheter 24 may be inserted into the bottom end of the bore 22. Extension conduit 24 extends into chamber 30, for example 10mm to 30 mm. The air pocket is formed in the top of the chamber 30, which chamber 30 is always at least as thick as the extended length of the extension conduit 24. The air pocket is typically thicker than this and the air overflows into the extension duct 25 and passes through the holes 22. The additional pocket thickness provided by the extension duct 24 allows gas to be distributed more quickly across the chamber 30 as the gas flow switches from one aerator 32 to another and thus promotes a more nearly uniform gas flow between the holes 22 spaced across the width of the module 10.

During the maintenance cleaning operation, the cleaning solution is preferably distributed uniformly to all modules. The concentration of the cleaning solution should be high (but within the limits of the membrane material tolerances) and it is preferable to avoid excessive dilution of the water entering the reservoir outside the module. The cleaning solution is preferably delivered to the membrane surface and allowed to react there with minimal negative impact on the biomass in the membrane reservoir. The keep clean is preferably performed in the whole or almost the whole reservoir. The remaining cleaning can be done in an empty reservoir to avoid dilution into the water in the reservoir, but in this case most of the solution is lost by permeation near the bottom of the hollow fibers where the static pressure of the cleaning solution inside the module is highest. In the filtration systems described herein, fouling near the bottom of the membrane is reduced by both the aeration process and the dead-end encapsulation of the fiber bottom. In this case, it is desirable to promote the penetration of the chemical solution to the extent possible across the upper end of the membrane, but keeping the empty reservoir clean can result in further loss of cleaning solution at the bottom of the membrane, since there is relatively no fouling near the bottom of the membrane.

A permeate pumping system is typically used to deliver the retention cleaning solution to the membrane module. However, only a large volume of chlorine solution is required to fill the permeate piping network even before any cleaning solution comes into contact with the membrane. In addition, a larger flow rate is required to deliver the cleaning solution uniformly to all modules in order to take advantage of the balancing effect of the pressure losses in the modules. The combined effect of these constraints is that large amounts of low concentration chlorine solution penetrate the membrane, dilute too much, and may kill a significant portion of the biomass in the reservoir.

Referring to fig. 2, 7 and 8, the permeate header 20 is connected to a bleed conduit 54 with a bleed valve 56. Opening the bleed valve 56 exposes the inside of the permeate header 20 to atmospheric pressure. The chemical injection tubes 58 have sections that run inside the permeate header 20 with small injection holes 60 spaced along their length. Another section of the chemical injection pipe 58 is positioned outside of the permeate header 20 and is typically connected to a chemical pump 62 via an intermediate conduit and a valve, not shown, while the chemical pump 62 is connected to a chemical reservoir 64.

To effect the maintenance clean, the permeate header 20 is isolated from the permeate pumping network by closing the isolation valve 52. Alternatively, isolation valves may be provided and closed further downstream in the permeate network so that groups of modules 10 connected to the permeate conduit 50 (e.g., all modules 10 in the reservoir 48) may be kept clean simultaneously. Closing the isolation valve 52 isolates a known volume of permeate in communication with the one or more permeate headers 20.

A total amount of concentrated chlorine or other cleaning chemicals (which may optionally be predetermined based on the known volume and desired final chemical cleaner concentration described above) is injected into the permeate header 20 via the chemical injection tube 58. The chemical cleaner flows out of the injection holes 60 and is quickly mixed into the permeate in the permeate header 20 to achieve the desired final concentration. At this stage, the chemical solution remains in the permeate header 20, but a small amount of permeate is transferred to the membrane reservoir 48. The membrane aeration is preferably turned off to minimize drainage of the cleaning solution in subsequent steps.

The level of the mixed liquid in the reservoir 48 may optionally be partially reduced to create or increase a potential driving force in a direction opposite to normal osmosis. This reverse osmosis driving force can be around 10cm or a few 10cm, but is preferably less than 50 cm. If the permeate header 20 is positioned far enough above the normal water level in the reservoir 48, then sufficient potential reverse osmosis driving force may always be available without reducing the mixed liquid level. Generally, the permeate header 20 should be at least completely above the water level before the chemical solution begins to flow from the permeate header. Alternatively, since fouling typically occurs in the first 10cm or 20cm below the upper header of the vertical module, the water level may be lowered to 10cm or 20cm below the bottom of the upper header to facilitate the flow of cleaning chemicals through the upper portion of the membrane. The water level may be lowered by partially draining the reservoir 48 at any time prior to opening the bleed valve 56. Alternatively, the water level may be lowered by stopping the flow of water into the reservoir while continuing to recover permeate before closing the isolation valve 52.

The chemical flow is activated by opening the bleed valve 56 to connect the interior of the permeate header 20 to atmosphere. This allows the contents of the permeate header 20 to permeate back by gravity. The bleed valve 56, or the degree to which it is opened, may be selected so that the reverse osmosis (chemical discharge) time of the cleaning solution provides a desired contact time for the cleaning chemicals. Optionally, a waiting time of up to about 5 minutes may be provided after the reverse osmosis is substantially complete, to allow time for the chemical cleaner to react further with the soil.

When the cleaning solution has been reverse osmosis in substantially all, and any waiting time has elapsed, then the water level in the reservoir 48 is increased to its normal set point, and the permeate header 20 is filled with water by forward osmosis, while some air is bled off via the bleed valve 56. The bleed valve 56 is preferably positioned at a high point in the isolation zone, either in the isolated permeate header 20 or in communication with the isolated permeate header 20. The bleed valve 56 may then be closed, the membrane bubble purge resumed, and the isolation valve 52 opened to return the module 10 to operation. Any air that remains trapped in the permeate header 20 may be purged via the ordinary air collector of the permeate system. The invention protected by this document is defined by the appended claims. The claims are not limited to the specific examples of equipment or processes described herein.

Claims (15)

1. An immersion membrane apparatus comprising:

a) a plurality of cassettes each having in-line membrane modules, each membrane module having an upper permeate header and a lower header with integral air holes, the cassettes being disposed in a reservoir in a side-by-side spaced-apart arrangement;

b) an open-bottomed chamber communicating with air holes in a lower header of the module; and

c) a plurality of substantially planar aerator assemblies positioned in the spaces between the cassettes, the aerator assemblies including an aerator having apertures to discharge bubbles, the apertures positioned to provide bubbles into the skirt.

2. The apparatus of claim 1, wherein the plurality of aerators comprises:

a) a first aerator configured and positioned on one side of the module to provide bubbles simultaneously within the skirt and outside of the skirt;

b) a second aerator configured and positioned on the other side of the module to simultaneously provide bubbles within the skirt and outside of the skirt; and

c) an aeration system to provide air flow to the first aerator and the second aerator at different times.

3. The apparatus of claim 1 or claim 2, wherein each cassette comprises a cassette frame comprising upper and lower cross beams that retain upper and lower headers of the membrane modules in the cassette; and side members positioned on opposite sides of the cassette frame to space the upper and lower cross members apart, wherein a distance between the upper and lower cross members is individually adjustable at each side member.

4. The apparatus of any one of claims 1 to 3, wherein the apertures of the aerator are positioned to also provide bubbles into the space between the cassettes.

5. An immersion membrane apparatus comprising:

a) a plurality of cassettes each having a single linear membrane module, each membrane module having an upper permeate header and a lower header with integral air holes; and the number of the first and second groups,

b) an open-bottomed chamber communicating with the air holes in the lower header of the module,

wherein,

c) each cassette comprising a cassette frame comprising upper and lower cross beams that retain upper and lower headers of the membrane modules in the cassette; and side members positioned on opposite sides of the cassette frame to space apart the upper and lower cross members, wherein a distance between the upper and lower cross members is individually adjustable at each side member; and

d) the cassette is mounted into a reservoir, wherein the weight of the cassette is supported by a joint between a top cross member and the reservoir.

6. The apparatus of any one of claims 1 to 5, comprising:

e) a permeate header over the membrane area of at least one membrane module in communication with the permeate header of the module;

f) an isolation valve in communication with the permeate header;

g) a bleed valve in communication with permeate on the module side of the isolation valve; and

h) a chemical injection conduit having one end in communication with an inside of the permeate header on the module side of the isolation valve and another end in communication with a chemical pump and a cleaning chemical supply.

7. A membrane filtration system comprising:

a) a membrane module having a membrane filtration zone;

b) a permeate header located above the membrane filtration zone;

c) an isolation valve between the permeate header and another permeate collection conduit;

d) a bleed valve in communication with the permeate header on the module side of the isolation valve; and

e) a chemical injection conduit having one end in communication with an inside of the permeate header on the module side of the isolation valve and another end in communication with a chemical pump and a cleaning chemical supply.

8. A method for aerating a submerged suction driven module of substantially vertical hollow fibre membranes, the module having two opposite sides, the method comprising the steps of:

a) providing a bubble within the module and at a side of the module for a first period of time;

b) providing a bubble within the module and at the other side of the module for a second period of time after step a); and

c) repeating steps a) and b).

9. The method of claim 8, wherein the first period of time and the second period of time are substantially continuous so as to substantially constantly provide bubbles from within the module during steps a) and b).

10. A method according to claim 8 or claim 9, wherein a bubble is provided into the module through a potting head via a hole, wherein the bottom of the membrane is potted in the potting head.

11. The method according to any one of claims 8 to 10, wherein the gas bubbles in step a) are provided by a single aerator and the gas bubbles in step b) are provided from different single aerators.

12. A method for preventing fouling of a membrane module comprising generating gas bubbles primarily or only to one side of the module, alternately from one side of the module to the other, while also generating gas bubbles within the module or between the membranes.

13. The method of claim 12, wherein gas bubbles are generated substantially continuously within the module or between the membranes.

14. A method for cleaning a submerged membrane module in a reservoir, comprising the steps of:

a) isolating a permeate volume in communication with and above the module;

b) positioning a water surface in the reservoir at a level below the isolated permeate volume;

c) injecting a cleaning chemical into the isolated permeate volume; and

d) venting the isolated permeate volume to atmosphere.

15. A film cleaning method comprising: the chemical cleaning solution is caused to flow by gravity through the membrane module, optionally by injecting a concentrated solution into a bleed portion of a permeate recovery system positioned above the water level in a reservoir holding the module.

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