Abstract:
An aeration system for a submerged membrane module has a set of aerators connected to an air blower, valves and a controller adapted to alternately provide a higher rate or air flow and a lower rate of air flow in repeated cycles. In an embodiment, the air blower, valves and controller, simultaneously provide the alternating air flow to two or more sets of aerators such that the total air flow is constant, allowing the blower to be operated at a constant speed. In another embodiment, the repeated cycles are of short duration. Transient flow conditions result in the tank water which helps avoid dead spaces and assists in agitating the membranes.  
     This is a continuation of U.S. application Ser. No.  09/814,737,  filed Mar.  23, 2001,  which is a continuation-in-part of U.S. application Ser. No.  09/488,359,  filed Jan.  19, 2000,  issued as U.S. Pat. No.  6,245,239  on Jun.  12, 2001,  which is a continuation of international application number PCT/CA 99/00940,  filed Oct.  7, 1999.  The disclosures of U.S. application Ser. Nos.  09/814,737; 09/488,359  and international application number PCT/CA 99/00940  are incorporated herein by this reference to them.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to filtering liquids and particularly to using scouring air bubbles produced by an aeration system to clean or inhibit the fouling of membranes in a submerged membrane filter.  
         BACKGROUND OF THE INVENTION  
         [0002]    Submerged membranes are used to treat liquids containing solids to produce a filtered liquid lean in solids and an unfiltered retentate rich in solids. For example, submerged membranes are used to withdraw substantially clean water from wastewater and to withdraw potable water from water from a lake or reservoir.  
           [0003]    The membranes are generally arranged in modules which comprise the membranes and headers attached to the membranes. The modules are immersed in a tank of water containing solids. A transmembrane pressure is applied across the membrane walls which causes filtered water to permeate through the membrane walls. Solids are rejected by the membranes and remain in the tank water to be biologically or chemically treated or drained from the tank.  
           [0004]    Air bubbles are introduced to the tank through aerators mounted below the membrane modules and connected by conduits to an air blower. The air bubbles rise to the surface of the tank water and create an air lift which recirculates tank water around the membrane module. When the rate of air flow is within an effective range, the rising bubbles and tank water scour and agitate the membranes to inhibit solids in the tank water from fouling the pores of the membranes. Further, there is also an oxygen transfer from the bubbles to the tank water which, in wastewater applications, provides oxygen for microorganism growth. The air blower generally runs continuously to minimize stress on the air blower motors and to provide a constant supply of air for microorganism growth if desired.  
           [0005]    With typical aeration systems, an operator increases the rate of air flow to the aerators if more cleaning is desired. This technique, however, stresses the membranes and air blower motors and increases the amount of energy used which significantly increases the operating costs of the process. Conversely, an operator typically decreases the rate of air flow to the aerators if less cleaning is desired. With this technique, however, the rate of air flow is often below the effective range, which does not provide efficient cleaning. Alternately, some operators reduce the average rate of air flow by providing air intermittently. This method allows for an air flow rate in the effective range but at the expense of the air blowers which wear rapidly when turned off and on frequently. In many cases, the warranty on the air blower is voided by such intermittent operation.  
           [0006]    Another concern with typical aeration systems is that they cause the tank water to move in a generally steady state recirculation pattern in the tank. The recirculation pattern typically includes “dead zones” where tank water is not reached by the recirculating tank water and bubbles. The membranes in these dead zones, or the parts of the membranes in these dead zones, are not effectively cleaned and may be operating in water having a higher concentration of solids than in the tank water generally. Accordingly, these membranes, or the affected parts of these membranes, quickly foul with solids.  
           [0007]    A related problem occurs in modules where hollow fibre membranes are installed with a small degree of slack to allow the membranes to move and shake off or avoid trapping solids. The movement of tank water in the tank encourages slackened membranes to assume a near steady state position, particularly near the ends of the membranes, which interferes with the useful movement of the fibres.  
           [0008]    Yet another concern with current aeration systems is that the aerators themselves often foul over time. Even while the air supply is on, the local air pressure near the perimeter of the aerator holes is low and often allows tank water to seep into the aerator. When aeration is stopped from time to time, for example for backwashing, cleaning or other maintenance procedures, more tank water may enter the aeration system. A portion of the tank water entering the aeration system evaporates there, leaving deposits of solids in the aeration system. In wastewater applications in particular, the deposited solids can significantly reduce the efficiency of the aeration system or cause an operator to periodically shut down filtration to clean or replace the aerators.  
         SUMMARY OF THE INVENTION  
         [0009]    It is an object of the present invention to provide a cyclic aeration system that may be used for aerating ultrafiltration and microfiltration membranes modules immersed in tank water in a tank. The cyclic aeration system uses a valve set and a valve set controller to connect an air supply to a plurality of distinct branches of an air delivery network. The distinct branches of the air delivery network are in turn connected to aerators located below the membrane modules. While the air supply is operated to supply a steady initial flow of air, the valve set and valve set controller split and distribute the initial air flow between the distinct branches of the air distribution system such that the air flow to each distinct branch alternates between a higher flow rate and a lower flow rate in repeated cycles.  
           [0010]    In an embodiment, the valves in the valve set open or close in less than about 5 seconds, preferably less than about 3 seconds. The valve or valves associated with each distinct branch of the air delivery network begin to either open or close, or both, automatically with or in response to the opening or closing of a valve or valves associated with another distinct branch of the air delivery system. For example, the valve or valves associated with each distinct branch of the air delivery network begin to either close automatically with or in response to the opening, preferably to a fully open state, of the valve or valves associated with another distinct branch of the air delivery system. Additionally, position sensors may be fitted to the valves and the valve set controller configured such that the failure of a valve or valves to open as desired prevents closure of the valve or valves associated with another distinct branch of the air delivery system.  
           [0011]    In another embodiment, the cyclic aeration system is used to provide intermittent aeration to membrane modules arranged in a plurality of filtration zones, each associated with a distinct branch of the air delivery network. The cyclic aeration system is configured and operated to provide aeration for a predetermined amount of time to each filtration zone in turn. In other embodiment, the cyclic aeration system is used to provide intense aeration to a group of membrane modules. In one such embodiment, the cyclic aeration system is configured and operated to provide air to a branch of the air delivery network alternating between a higher flow rate and a lower flow rate in cycles of 120 seconds or less. In another such embodiment, aerators associated with a first branch of the air delivery network are interspersed with aerators associated with a second branch of the air delivery network. Air flow at a higher flow rate is alternated between the first and second branches of the air delivery network in cycles of 120 seconds or less. Where two distinct branches of the air delivery system are provided, air preferably flows at the higher rate in each distinct branch for about one half of each cycle. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    Preferred embodiments of the present invention will now be described with reference to the following figures.  
         [0013]    [0013]FIG. 1A is a schematic drawing of a submerged membrane reactor.  
         [0014]    [0014]FIGS. 1B, 1C and  1 D are drawings of membrane modules according to embodiments of the present invention.  
         [0015]    [0015]FIG. 2 is a plan view schematic of an aeration system according to an embodiment of the present invention.  
         [0016]    [0016]FIG. 3 is a series of graphs showing the effect of operating an embodiment of the present invention.  
         [0017]    [0017]FIGS. 4A, 4B,  4 C and  4 D are schematic drawings of valve sets and valve controllers according to embodiments of the invention.  
         [0018]    [0018]FIGS. 4E and 4F are diagrams of valve position over time.  
         [0019]    [0019]FIG. 5 is a plan view schematic of membrane modules and an aeration system according to an embodiment of the invention.  
         [0020]    [0020]FIG. 6 is a plan view schematic of membrane modules and an aeration system according to another embodiment of the invention.  
         [0021]    [0021]FIG. 7A is a plan view schematic of membrane modules and an aeration system according to another embodiment of the invention.  
         [0022]    [0022]FIGS. 7B, 7C and  7 D are elevational representations of membrane modules and parts of an aeration system according to alternatives to the embodiment of FIG. 7A.  
         [0023]    [0023]FIGS. 8A and 8B are elevational representations of membrane modules and parts of an aeration system according to an embodiment of the invention under the influence of a cyclic aeration system.  
         [0024]    [0024]FIGS. 9A, 9B,  9 C and  9 D are drawings of aerators according to an embodiment of the invention.  
         [0025]    [0025]FIGS. 10A, 10B and  10 C are charts showing the results of tests performed on embodiments of the invention having two groups of aerators.  
         [0026]    [0026]FIG. 11 is a chart showing the results of tests performed on embodiments of the invention having a single group of aerators. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]    General Description  
         [0028]    Referring now to FIG. 1A, the general arrangement of a reactor  10  is shown. The description of the reactor  10  in this section applies generally to various embodiments to be described below to the extent that it is not inconsistent with the description of any particular embodiment.  
         [0029]    The reactor  10  has a tank  12  which is initially filled with feed water  14  through an inlet  16 . The feed water  14  may contain microorganisms, suspended solids or other matter which will be collectively called solids. Once in the tank, the feed water  14  becomes tank water  18  which may have increased concentrations of the various solids, particularly where the reactor  10  is used to treat wastewater.  
         [0030]    One or more membrane modules  20  are mounted in the tank and have one or more headers  22  in fluid communication with a permeate side of one or more membranes  6 . The membranes  6  in the membrane modules  20  have a pore size in the microfiltration or ultrafiltration range, preferably between 0.003 and 10 microns.  
         [0031]    Membrane modules  20  are available in various sizes and configurations with various header configurations. For example, the membranes  6  may be hollow fibres potted in one or more headers  22  such that the lumens of the hollow fibres are in fluid communication with at least one header  22 . The headers  22  may be of any convenient shape but typically have a rectangular or round face where they attach to the membranes  6 . Alternatively, the membranes  6  may be flat sheets which are typically oriented vertically in a spaced apart pair with headers  22  on all four sides in fluid communication with the resulting interior surface. A membrane module  20  may have one or more microfiltration or ultrafiltration membranes  6  and many membrane modules  20  may be joined together to form larger membrane modules, or cassettes, but all such configurations will be referred to as membrane modules  20 .  
         [0032]    [0032]FIGS. 1B, 1C and  1 D illustrate preferred membrane modules  20  having rectangular skeins  8 . In each rectangular skein  8 , hollow fibre membranes  23  are held between two opposed headers  22 . The ends of each membrane  23  are surrounded by potting resin to produce a watertight connection between the outside of the membrane  23  and the headers  22  while keeping the lumens of the hollow fibre membranes  23  in fluid communication with at least one header  22 . The rectangular skeins  8  may be oriented in a horizontal plane (FIG. 1B), vertically (FIG. 1C) or horizontally in a vertical plane (FIG. 1D). A plurality of rectangular skeins  8  are typically joined together in a membrane module  20 .  
         [0033]    Although a single row of hollow fibre membranes  23  is illustrated in each rectangular skein  8 , a typical rectangular skein  8  has a mass of hollow fibre membranes  23  between 2 cm and 10 cm wide. The hollow fibre membranes  23  typically have an outside diameter between 0.4 mm and 4.0 mm and are potted at a packing density between 10% and 40%. The hollow fibre membranes  23  are typically between 400 mm and 1,800 mm long and typically mounted with between 0.1% and 5% slack.  
         [0034]    Referring again to FIG. 1A, the tank  12  is kept filled with tank water  18  above the level of the membranes  6  in the membrane modules  20  during permeation. Filtered water called permeate  24  flows through the walls of the membranes  6  in the membrane modules  20  under the influence of a transmembrane pressure and collects at the headers  22  to be transported to a permeate outlet  26  through a permeate line  28 . The transmembrane pressure is preferably created by a permeate pump  30  which creates a partial vacuum in a permeate line  28 . The transmembrane pressure may vary for different membranes and different applications, but is typically between 1 kPa and 150 kPa. Permeate  24  may also be periodically flowed in a reverse direction through the membrane modules  20  to assist in cleaning the membrane modules  20 .  
         [0035]    During permeation, the membranes  6  reject solids which remain in the tank water  18 . These solids may be removed by a number of methods including digestion by microorganisms if the reactor  10  is a bioreactor or draining the tank  12  periodically or by continuously removing a portion of the tank water  18 , the latter two methods accomplished by opening a drain valve  32  in a drain conduit  34  at the bottom of the tank.  
         [0036]    An aeration system  37  has one or more aerators  38  connected by an air delivery system  40  and a distribution manifold  51  to an air source  42 , which is typically one or more air blowers, and produces bubbles  36  in the tank water. The aerators  38  may be of various types including distinct aerators, such as cap aerators, or simply holes drilled in conduits attached to or part of the distribution manifold  51 . The bubbles  36  are preferably made of air but may be made of other gasses such as oxygen or oxygen enriched air if required.  
         [0037]    The aerators  38  are located generally below the membrane modules  20 . If the membrane modules  20  are made of rectangular skeins  8  having vertical hollow fibre membranes  23 , the aerators  38  are preferably located to produce bubbles near the edges of the lower header. With rectangular skeins  8  having hollow fibre membranes  23  in a vertical plane, the aerators  38  are preferably located to produce bubbles in a line directly below the vertical plane. With rectangular skeins  8  having hollow fibre membranes  23  in a horizontal plane, the aerators  38  are preferably located to produce bubbles evenly dispersed below the plane.  
         [0038]    The bubbles  36  agitate the membranes  6  which inhibits their fouling or cleans them. In addition, the bubbles  36  also decrease the local density of tank water  18  in or near the membrane modules  20  which creates an air-lift effect causing tank water  18  to flow upwards past the membrane modules  20 . The air lift effect causes a recirculation pattern  46  in which the tank water  18  flows upwards through the membrane modules  20  and then downwards along the sides or other parts of the tank. The bubbles  36  typically burst at the surface and do not generally follow the tank water  18  through the downward flowing parts of the recirculation pattern  46 . The tank water  18  may also flow according to, for example, movement from the inlet  16  to the drain conduit  34 , but such flow does not override the flow produced by the bubbles  36 .  
         [0039]    The bubbles  36  have an average diameter between 0.1 and 50 mm. Individual large bubbles  36  are believed to be more effective in cleaning or inhibiting fouling of the membranes  6 , but smaller bubbles  36  are more efficient in transferring oxygen to the tank water  18  and require less energy to produce per bubble  36 . Bubbles  36  between 3 mm and 20 mm, and more preferably between 5 mm and 15 mm in diameter, are suitable for use in many wastewater applications. Bubbles  36  in the ranges described immediately above provide effective cleaning of the membranes  6  and acceptable transfer of oxygen to the tank water  18  without causing excessive foaming of the tank water  18  at the surface of the tank  12 . If the reactor  10  is used to create potable water or for other applications where oxygen transfer is not required, then bubbles between 5 mm and 25 mm are preferred.  
         [0040]    The bubbles  36  may be larger than a hole in an aerator  38  where the bubble  36  is created according to known factors such as air pressure and flow rate and the depth of the aerators  38  below the surface of the tank water  18 . If the aerators  38  are located near the bottom of a large tank  12 , such as those used in municipal treatment works, an aerator  38  with holes of between 2 mm and 15 mm and preferably between 5 mm and 10 mm might be used. The air pressure supplied (relative to atmospheric pressure) is typically determined by the head of water at the depth of submergence of the aerators  38  (approximately 10 kPa per metre) plus an additional pressure required to get the desired rate of air flow through the aerators  38 . There is typically a pressure drop of between 5 mm and 100 mm of water, and more typically between 10 mm and 50 mm of water, across the holes of the aerators  38 . Parts of the aeration system  37  located at a distance below the bottom of the holes of the aerators  38  equal to the pressure drop are generally free of tank water when the air source  42  is operating, although small amounts of tank water  18  may still seep into the aeration system  37 .  
         [0041]    Cyclic Aeration System  
         [0042]    Now referring to FIG. 2 a cyclic aeration system  237  is shown having an air supply  242  in fluid communication with a valve set  254 , the valve set  254  controlled by a valve controller  256 . The valve set  254  is in fluid communication with an air delivery network  240  having a plurality of distinct branches each in fluid communication with distinct manifolds  251  in fluid communication with conduit aerators  238 . Other types of aerators may also be used with suitable modifications to the manifolds  251  or air delivery network, but conduit aerators  238  are preferred. The third branch of the air delivery network  240  and the third manifold  251  are shown in dashed lines to indicate that the number of distinct branches of the air delivery network  240  and manifolds  251  may be two or more, but preferably not more than 15.  
         [0043]    The air supply  242  is a source of pressurized air, typically one or more air blowers, and provides a flow of a gas at an initial rate to the cyclic aeration system. The gas is most often air, but may also be oxygen, oxygen or ozone enriched air, or nitrogen in which cases the air supply  242  will include oxygenation or ozonation equipment etc. in addition to an air blower. In this document, however, the term “air” will be used to refer to any appropriate gas. The amount of air provided by the air supply  242  is best determined by summing the amount of air provided to all conduit aerators  238  (to be described below) serviced by the air supply  242 . It is preferred that the air supply  242  supply a constant amount of air over time.  
         [0044]    The valve set  254  and valve controller  256  will be described in more detail below. In general terms, however, the valve set  254  and valve controller  256  (a) split the air flow from the air supply  242  between the branches of the air delivery network  240  such that, at a point in time, some of the branches receive air at a higher rate of air flow and some of the branches receive air at a lower rate of air flow and (b) switch which branches of the air delivery network  240  receive the higher and lower rates of air flow in repeated cycles.  
         [0045]    An example is illustrated in FIG. 3. In each of parts a), b), and c) of FIG. 3, Rh indicates a higher rate of air flow; Rl indicates a lower rate of air flow; and, the time from  0  to t 3  indicates a cycle which would be repeated. The cycle is divided into three substantially equal time periods,  0 -t 1 ; t 1 -t 2 ; and, t 2 -t 3 . In each of these periods, one branch of the air delivery system  240  and its associated manifold  251  receive air at Rh while the others receive air at Rl. Similarly, each branch of the air delivery system  240  and its associated manifold  251  receives air at Rh for one third of the cycles and at Rl for two thirds of the cycle.  
         [0046]    Many of the valves sets  254  to be described below can be used to produce smooth variations in air flow rate to a manifold  251 , but it is preferred if the variation is fairly abrupt as suggested by FIG. 3. The inventors have noticed that such an abrupt change produces a short burst of unusually large bubbles  36  which appear to have a significant cleaning or fouling inhibiting effect. The abrupt changes often also produce a spike in air flow rate shortly after the transition from Rl to Rh which produces a corresponding pressure surge. This pressure surge must be kept within the design limits of the cyclic aeration system  237  or appropriate blow off valves etc. provided.  
         [0047]    The amount of air provided to a manifold  251  or branch of air delivery network  240  is dependent on numerous factors but is preferably related to the superficial velocity of air flow for the conduit aerators  238  services. The superficial velocity of air flow is defined as the rate of air flow to the conduit aerators  238  at standard conditions (1 atmosphere and 25 degrees Celsius) divided by the cross sectional area of aeration. The cross sectional area of aeration is determined by measuring the area effectively aerated by the conduit aerators  238 . Superficial velocities of air flow of between 0.013 m/s and 0.15 m/s are preferred at the higher rate (Rh). Air blowers for use in drinking water applications may be sized towards the lower end of the range while air blowers used for waste water applications may be sized near the higher end of the range.  
         [0048]    Rl is typically less than one half of Rh and is often an air off condition with no flow. Within this range, the lower rate of air flow is influenced by the quality of the feed water  14 . An air off condition is generally preferred, but with some feed water  14 , the hollow fibre membranes  23  foul significantly even within a short period of aeration at the lower rate. In these cases, better results are obtained when the lower rate of air flow approaches one half of the higher rate. For feed waters in which the rate of fouling is not significant enough to require a positive lower rate of air flow, Rl may still be made positive for other reasons. With some aerators or air delivery systems, a positive lower rate of air flow may be desired, for example, to prevent the aerators from becoming flooded with tank water  18  at the lower rate of air flow. While periodic flooding is beneficial in some aerator designs, in others it causes unwanted foulants to accumulate inside the aerator. A positive lower rate of air flow may also be used because of leaks in the valves of the valve set  254  or to reduce stresses on the valve set  254  or the air delivery network  240 . Regarding leaks, the lower rate of air flow may typically be as much as about 10%, but preferably about 5% or less, of the higher rate of air flow without significantly detracting from the performance achieved with a completely air off condition. Continuing to use valves (which are typically butterfly valves) even after they have developed small leaks decreases the operating expense of the cyclic aeration system  237 . Regarding stresses on the valves in the valve set  254  or the air delivery network  240 , such stresses can be reduced by purposely not closing the valves entirely. As in the cases of leaks, the lower rate of air flow may be as much as about 10%, but preferably about 5% or less, of the higher rate of air flow typically without significantly detracting from the performance achieved with a completely air off condition.  
         [0049]    Referring now to FIGS. 4A, 4B and  4 C, alternative embodiments of the valve set  254  and valve controller  256  are shown. In FIG. 4A, an air supply  242  blows air into a three way valve  292 , preferably a ball valve, with its two remaining orifices connected to two manifolds  251 . A three way valve controller  294  alternately opens an air pathway to one of the manifolds  251  and then the other. Preferably there is a phase shift of 180 degrees so that the air pathway to one of the manifolds  251  opens while the airway to the other manifold  251  closes. The three way valve  292  may be mechanically operated by handle  296  connected by connector  298  to a lever  299  on the three way valve controller  294 . The three way valve controller  294  may be a drive unit turning at the required speed of rotation of the lever  299 . Preferably, however, the three way valve controller  294  is a microprocessor and servo, pneumatic cylinder or solenoid combination which can be more easily configured to abruptly move the three way valve  292 .  
         [0050]    In FIG. 4B, the air supply  242  blows air into a connector  261  which splits the air flow into a low flow line  262  and a high flow line  264 . A valve  266  in the low flow line  262  is adjusted so that flow in the low flow line  262  is preferably less than one half of the flow in the high flow line  264 . A controller  268 , preferably a timer, a microprocessor or one or more motors with electrical or mechanical links to the valves to be described next, controls a low valve  270 , which may be a solenoid valve or a 3 way ball valve, and a high valve  272 , which may be a solenoid valve or a 3 way ball valve, so that for a first period of time (a first part of a cycle) air in the low flow line  262  flows to one of the manifolds  251  and air in the high flow line flows to the other manifold  251 . For a second period of time (a second part of a cycle), the low valve  270  and high valve  272  are controlled so that air in the low flow line  262  flows to the a manifold  251  through cross conduit  274  and air in the high flow line  264  flows to the other manifold  251  through reverse conduit  276 .  
         [0051]    In FIG. 4C, air supply  242  blows air into a blower header  260  connected by slave valves  284  to manifolds  251 . Each slave valve  284  is controlled by a slave device  280 , typically a solenoid, pneumatic or hydraulic cylinder or a servo motor. The slave devices  280  are operated by a slave controller  282  set up to open and close the slave valves  284  in accordance with the system operation described in this section and the embodiments below. The slave controller  282  may be a microprocessor, an electrical circuit, a hydraulic or pneumatic circuit or a mechanical linkage. The slave devices  280  and the slave controller  282  together comprise the valve set controller  256 . The valve set controller  256  of FIG. 4C may also be used with the other apparatus of FIG. 4B.  
         [0052]    In FIG. 4D, air supply  242  blows air into a blower header  260  connected by slave valves  284  to manifolds  251 . Each slave valve  284  is controlled by a valve set controller  256  which consists of a plurality of cams  281 , driven by a motor  279 . The cams  281  may drive the slave valves  284  directly (as illustrated) or control another device, such as a pneumatic cylinder, which directly opens or closes the slave valves  284 . The shape of the cams  281  is chosen to open and close the slave valves  284  in accordance with the system operation described in this section and the embodiments below. The valve set controller  256  of FIG. 4D may also be used with the other apparatus of FIG. 4B.  
         [0053]    With the apparatus of FIGS. 4B, 4C or  4 D, the opening and closing times of the slave valves  284  are mechanically (preferably by a pneumatic circuit) or electrically (preferably with a programmable logic controller—PLC) interconnected such that each slave valve  284  either opens or closes, or both, automatically with or in response to the opening or closing of a slave valve or slave valves  284  in another distinct branch of the air delivery network  240 . This occurs naturally, for example, in the embodiment of FIG. 4D by virtue of the cams  281  being linked to a common motor  279 . If the motor  279  fails or turns at an improper speed, the opening and closing times of the slave valves  284  relative to each other is preserved. Where the valve set controller  256  incorporates a slave controller  282 , the slave controller  282  may incorporate a timer, but preferably does not open and close slave valves  284  based solely on inputs from the timer. For example, an acceptable set up for the slave controller  282  is to have the opening of the slave valves  284  of a distinct branch determined by time elapsed since those slave valves  284  were closed, but the closing of those slave valves  284  is determined by the slave valves  284  of another distinct branch having opened to a selected degree.  
         [0054]    The opening and closing movements of the slave valves  284  are preferably overlapped to minimize the spike in air flow rate and pressure surge shortly after the transition from Rl to Rh mentioned above. Preferably, the opening and closing times of the slave valves  284  are arranged such that the slave valve or valves  284  to any distinct branch of the air delivery network  240  do not start to close until the slave valve or valves  284  to any other distinct branch of the air delivery system  240  are fully open. Further preferably, where the valve set controller  256  includes a slave controller  282 , position sensors are fitted to the slave valves  284 . The slave controller  282  is configured such that the failure of a slave valve or valves  284  to open as desired prevents the closure of the slave valve or valves  284  of another distinct branch of the air delivery network  240 . In this way, in addition to minimizing and possibly substantially eliminating any spike in air flow, damage to the cyclic aeration system  237  is avoided if the slave valve or valves  284  to a distinct branch of the air delivery network  240  fail to open.  
         [0055]    Despite the concern for controlling any spikes in air flow rate or pressure, the overall goal of the valve set controller  256  is to produce rapid changes between Rl and Rh. The time required to open or close (partially or fully as desired) a slave valve  284  from its closed (fully or partially) or opened position respectively is preferably less than about 5 seconds and more preferably less than about 3 seconds when used with very short cycle times of 40 seconds or less. For example, FIGS. 4E and 4F show suitable slave valve  284  positions over time for an air distribution network  240  having two distinct branches where Rl is an air off condition, the cycle time is 20 seconds and the valve opening time is 3 seconds. In FIG. 4E, the start of the closing times of the slave valves  284  are interconnected such that each slave valve  284  begins to close when the other is fully open. In FIG. 4F, the start of the closing times of the slave valves  284  are interconnected such that each slave valve  284  begins to close when the other begins to open. In either case, where a positive Rl is desired, the fully closed position of the slave valves  284  illustrated can be replaced by a partially closed position, or the apparatus of FIG. 4B can be used.  
         [0056]    As an example of the interconnection of slave valves  284  described above, the regime of FIG. 4E will be discussed further below. Referring again to FIG. 4C, two manifolds  251  are controlled by two slave valves,  284   a  and  284   b,  through two slave devices,  280   a  and  280   b.  Each slave valve  284  has a limit switch  285  which provides a signal to the slave controller  282  indicating whether that slave valve  284  is open or at a desired partially or fully closed setting. Where the slave controller  282  is a PLC and the slave devices  280  are servo motors or pneumatic cylinders, the following PLC programming control narrative can be used to obtain air cycling as described in relation to FIG. 4E:  
         [0057]    1. At start-up, slave controller  282  sends a signal to slave devices  280   a  and  280   b  to open slave valve  284   a  and close slave valve  284   b  respectively.  
         [0058]    2. After 3 seconds, slave controller  282  checks for an “open” signal from limit switch  285   a  and a “closed” signal from limit switch  285   b.    
         [0059]    3. If both valves are confirmed in their correct positions, slave controller  282  sends a signal to start blower  242 .  
         [0060]    4. Seven seconds after the blower  242  is started, slave controller  282  sends a signal to slave device  280   b  to open slave valve  284   b.    
         [0061]    5. Three seconds after the preceding step, slave controller  282  checks for an “open” signal from limit switch  285   b;  if an “open” signal is received, proceed to step 6; if an “open: signal is not received, sound alarm and go to a continuous aeration mode, for example, by operating bypass valves to provide air to all manifolds  251  direct from the blower  242 .  
         [0062]    6. Slave controller  282  sends a signal to slave device  280   a  to close slave valve  284   a.    
         [0063]    7. Three seconds after step 6, slave controller  282  checks for a “closed” signal from limit switch  285   a;  if a “closed” signal is received, proceed with step 8; if a “closed” signal is not received, sound alarm and go to a continuous aeration mode.  
         [0064]    8. Four seconds after step 7, slave controller  282  sends a signal to slave device  280   a  to open slave valve  284   a.    
         [0065]    9. Three seconds after step 4, slave controller  282  checks for an “open” signal from limit switch  285   a;  if an “open” signal is received, proceed with step 10; if an “open” signal is not received, sound alarm and go to a continuous aeration mode.  
         [0066]    10. Slave controller  282  sends a signal to slave device  280   b  to close slave valve  284   b.    
         [0067]    11. Three seconds after step 10, slave controller  282  checks for a “closed” signal from limit switch  285   b;  if a “closed” signal is received, proceed with step 12; if a “closed” signal is not received, sound alarm and go to a continuous aeration mode.  
         [0068]    12. Four seconds after step 11, slave controller  282  sends signal to slave device  280   b  to open slave valve  284   b.    
         [0069]    13. Repeat steps 5-12 until unit is shut down or other control regime activated.  
         [0070]    The regime of FIG. 4E provides the advantage discussed above of having at least one distinct branch of the air delivery network  240  fully open at all times but the total time for the transition between Rl to Rh is extended to twice the valve opening time. This method is preferred for cycle times of 20 seconds or more. The regime of FIG. 4F produces a faster transition from Rl to Rh but at the risk of over stressing the cyclic aeration system  237  if the valve or valves  262  to a distinct branch start to open but then fail to open completely. This risk must be addressed with other system fail safes known in the art. The regime of FIG. 4F is preferred for cycle time less than 20 seconds and when valve opening/closing times are greater than about 3 seconds. Modifications to the narrative above can be used to produce other regimes of air cycling.  
         [0071]    Use of Cyclic Aeration to Provide Efficient Intermittent Aeration  
         [0072]    Use of the cyclic aeration system  237  to provide efficient intermittent aeration will now be described with reference to the following embodiment, it being understood that the invention is not limited to the embodiment. Referring to FIG. 5, an aeration system  237  is shown for use in providing intermittent aeration to six membrane modules  20  (shown with dashed lines) in a filtration tank  412 . The filtration tank  412  has six filtration zones (also shown with dashed lines) corresponding to the six membrane modules  20 . Alternately, the filtration zones could be provided in separate tanks with one or more membrane modules  20  in each tank. The membrane modules  20  will be used to filter a relatively foulant free surface water such that intermittent aeration is suitable.  
         [0073]    The air delivery network  240  has six distinct branches each connected to a header  251  in a filtration zone. Each header  251  is in turn connected to conduit aerators  238  mounted generally below the membrane modules  20 . The valve set  254  and valve controller  256  are configured and operated to provide air from the air supply  242  to the air delivery network  240  in a 7.5 minute cycle in which air at the higher rate is supplied for about 75 seconds to each branch of the air delivery network  240  in turn. While a branch of the air delivery network  240  is not receiving air at the higher rate, it receives air at the lower rate. Accordingly, each header  251  receives air at the higher rate for 75 seconds out of every 7.5 minutes. Operation of the air supply  242 , however, is constant and an air supply sized for one manifold  251  is used to service six such manifolds.  
         [0074]    It is preferable if backwashing of the membrane modules  20  is also performed on the membrane modules in turn such that backwashing of a membrane module  20  occurs while the membrane module  20  is being aerated. The membrane modules  20  can be backwashed most easily when each membrane module  20  is serviced by its own permeate pump  30  and associated backwashing apparatus. In large municipal systems, for example, the permeation and backwashing apparatus are typically limited to about 8 to 11 ML/d capacity. Accordingly a medium size plant (ie. in the range of 40 ML/d) will have several membrane modules  20  serviced by sets of permeation and backwashing apparatus which can be individually controlled. In some plants, backwashing is performed on the membrane modules  20  in turn to produce an even supply of permeate  24  regardless of aeration.  
         [0075]    In a pilot study conducted with feed water having turbidity of 0.3 NTU and colour of 3.9 TCU, for example, the inventors were able to achieve acceptable sustained permeability of a membrane module using 75 seconds of aeration at a higher rate of 0.035 m/s superficial velocity every 15 minutes and 15 seconds. For the remainder of the cycle there was no aeration. Each cycle involved 15 minutes of permeation through the membrane modules  20  and 15 seconds of backwashing. The 75 seconds of aeration was timed so that there was 30 seconds of aeration before the backpulse, aeration during the backpulse, and 30 seconds of aeration after the backpulse. The test suggests that if cycled aeration is timed to coincide for each manifold  251  with the backwashing of the associated membrane module  20 , then about 12 membrane modules  20  could be serviced by a single air supply  242  as part of the cyclic aeration system  237 .  
         [0076]    Use of Cyclic Aeration to Provide Intense Aeration  
         [0077]    Use of the cyclic aeration system  237  to provide intense aeration will now be described with reference to the following embodiment, it being understood that the invention is not limited to the embodiment. Referring to FIG. 6, an aeration system  237  is shown for use in providing aeration alternating between two sets of membrane modules  20  (shown with dashed lines) in a filtration vessel  512 . The filtration vessel  512  has two filtration zones (also shown with dashed lines) corresponding to the two sets of membrane modules  20 . Alternately, the filtration zones could be provided in separate tanks with one or more membrane modules  20  in each tank. The membrane modules  20  will be used to filter a relatively foulant rich surface water or a wastewater such that intense aeration is suitable.  
         [0078]    The air delivery network  240  has two distinct branches each connected to headers  251  in a filtration zone. Each header  251  is in turn connected to conduit aerators  238  mounted generally below the membrane modules  20 . The valve set  254  and valve controller  256  are configured and operated to provide air from the air supply  242  to the air delivery network  240  in a short cycle in which air at the higher rate is supplied for one half of the cycle to each branch of the air delivery network  240 . While a branch of the air delivery network  240  is not receiving air at the higher rate, it receives air at the lower rate.  
         [0079]    The preferred total cycle time may vary with the depth of the filtration vessel  512 , the design of the membrane modules  20 , process parameters and the conditions of the feed water  14  to be treated, but preferably is at least 10 seconds (5 seconds at the full rate and 5 seconds at the reduced rate) where the filtration vessel  512  is a typical municipal tank between 1 m and 10 m deep. A cycle time of up to 120 seconds (60 seconds at the full rate, 60 seconds at the reduced rate) may be effective, but preferably the cycle time does not exceed 60 seconds (30 seconds at the full rate, 30 seconds at the reduced rate) where the filtration vessel  512  is a typical municipal tank.  
         [0080]    The inventors believe that such rapid cycling creates transient flow within the tank water  18 . In particular, an air lift effect is created or strengthened when the rate of airflow changes from Rl to Rh causing the tank water  18  to accelerate. Shortly afterwards, however, aeration and the air lift effect are sharply reduced causing the tank water  18  to decelerate. With very short cycles, the tank water  18  is accelerating or decelerating for much of the cycle and is rarely in a steady state. It is believed that formation of quiescent zones in the tank water  18  is inhibited and that beneficial movement of the hollow fibre membranes  23  is enhanced. For example, horizontal hollow fibre membranes  23 , as shown in the rectangular skeins  8  of FIGS. 1B and 1D, assume a generally concave downward shape under steady state aeration and experience limited movement at their ends. With cyclic aeration as described above, however, tension in the hollow fibre membranes  23  is released cyclically and, in some cases, local currents which flow downward may be created for brief periods of time. The ends of the horizontal hollow fibre membranes  23  experience more beneficial movement and foul less rapidly. Since the beneficial effects may be linked to creating transient flow, it is also believed that factors which effect acceleration of the water column above a set of conduit aerators  238 , such as tank depth or shrouding, could modify the preferred cycle times stated above.  
         [0081]    Use of Cyclic Aeration to Promote Horizontal Flow  
         [0082]    Use of the cyclic aeration system  237  to promote horizontal flow in the tank water  18  will now be described with reference to the following embodiment, it being understood that the invention is not limited to the embodiment. Referring to FIG. 7A, an aeration system  237  is shown for use in aerating membrane modules  20  in a process tank  612 . The membrane modules  20  will be used to filter a relatively foulant rich surface water or a wastewater such that intense aeration is suitable.  
         [0083]    The air delivery network  240  has two distinct branches each connected to two distinct headers  251 , both in a single filtration zone. The headers  251  will be referred to as header  251   a  and  251   b  where convenient to distinguish between them. Headers  251  are connected to conduit aerators  238  such that the conduit aerators  238  attached to header  251   a  are interspersed with the conduit aerators  238  attached to header  251   b.  One such arrangement is shown in FIG. 7A in which header  251   a  is connected to conduit aerators  238  directly beneath the membrane modules  20  while header  251   b  is connected to horizontally displaced conduit aerators  238  located beneath and between the membrane modules  20 . Referring now to FIGS. 7B, 7C and  7 D, a set of variations of the embodiment of FIG. 7A is shown. In FIG. 7B, header  251   a  and header  251   b  are connected to alternating horizontally displaced conduit aerators  238  located beneath the membrane modules  20 . In FIG. 7C, header  251   a  and header  251   b  are connected to alternating horizontally displaced conduit aerators  238  located directly beneath alternating membrane modules  20 . In FIG. 7C, header  251   a  and header  251   b  are connected to alternating horizontally displaced conduit aerators  238  located directly beneath and between alternating membrane modules  20 . In each of these cases, the pattern may be repeated where more membrane modules  20  are used.  
         [0084]    Each of header  251   a  and header  251   b  are connected to a distinct branch of the air delivery network  240  in turn connected to a valve set  254 . The valve set  254  and a valve controller  256  are configured and operated to provide air from an air supply  242  to the air delivery network  240  in a short cycle in which air at a higher rate is supplied for one half of the cycle to each branch of the air delivery network  240 . While a branch of the air delivery network  240  is not receiving air at the higher rate, it receives air at the lower rate. The lower flow rate is preferably one half or less of the higher flow rate and, where conditions allow it, the lower flow rate is preferably an air-off condition.  
         [0085]    The total cycle time may vary with the depth of the process tank  612 , the design of the membrane modules  20 , process parameters and the conditions of the feed water  14  to be treated, but typically is at least 2 seconds (1 second at the full rate and 1 second at the reduced rate), preferably 10 seconds or more, and less than 120 seconds (60 seconds at the full rate, 60 seconds at the reduced rate), preferably less 60 seconds, where the process tank  612  is a typical municipal tank between 1 m and 10 m deep. More preferably, however, the cycle time is between 20 seconds and 40 seconds in length. Short cycles of 10 seconds or less may not be sufficient to establish regions of different densities in the tank water  18  in a deep tank  12  where such time is insufficient to allow the bubbles  36  to rise through a significant distance relative to the depth of the tank  12 . Long cycles of 120 seconds or more may result in parts of a membrane module  20  not receiving bubbles  36  for extended periods of time which can result in rapid fouling. As discussed above, the beneficial effects of the invention may be linked to creating transient flow and it is believed that factors which effect acceleration of the water column above a set of conduit aerators  238 , such as tank depth or shrouding, could modify the preferred cycle times stated above.  
         [0086]    In this embodiment, having the conduit aerators  238  connected to header  251   a  interspersed with the conduit aerators  238  attached to header  251   b  creates varying areas of higher and lower density in the tank water  18  within a filtration zone. As described above, the inventors believe that these variations produce transient flow in the tank water  18 . Where the effective areas of aeration above conduit aerators  238  attached to distinct branches of the air delivery network  240  are sufficiently small, however, the inventors believe that appreciable transient flow is created in a horizontal direction between areas above conduit aerators  238  attached to different branches of the air delivery network  240 . Referring to FIGS. 7A, 7B,  7 C,  7 D the membrane modules  20  shown are preferably of the size of one or two rectangular skeins  8 .  
         [0087]    As an example, in FIGS. 8A and 8B second membrane modules  220  made of rectangular skeins  8  with hollow fibre membranes  23  oriented vertically aerated by a cyclic aeration system  237  with conduit aerators  238  located relative to the second membrane modules  220  as shown in FIG. 7D. In FIGS. 8A and 8B, the degree of slack of the hollow fibre membranes  23  is highly exaggerated for easier illustration. Further, only two hollow fibre membranes  23  are illustrated for each vertical rectangular skein  8  although, as discussed above, a rectangular skein  8  would actually be constructed of many hollow fibre membranes  23 .  
         [0088]    With steady state aeration, it is difficult to encourage bubbles  36  to penetrate the vertical rectangular skeins  8 . The natural tendency of the bubbles  36  is to go through the areas with lowest resistance such as around the second membrane modules  220  or through slots between the second membrane modules  220  and the hollow fibre membranes  23  on the outer edge of the vertical rectangular skeins  8  may have significantly more contact with the bubbles  36 . Further, the upper 10-20% of the hollow fibre membranes  23  is often forced into a tightly curved shape by the air lift effect and moves only very little. A smaller portion at the bottom of the hollow fibre membranes  23  may also be tightly curved by the current travelling around the lower header  22 . In these tightly curved areas, the hollow fibre membranes  23  foul more rapidly.  
         [0089]    With cyclic aeration, however, air at the higher rate is alternated between header  251   a  and header  251   b.  When more air is supplied to header  251   a,  the hollow fibre membranes  23  assume an average shape as shown in FIG. 8A with a first local recirculation pattern  380  as shown. When more air is supplied to header  251   b,  the hollow fibre membranes  23  assume an average shape as shown in FIG. 8B with a second local recirculation pattern  382  as shown. Under the influence of a cyclic aeration system  237 , the hollow fibre membranes  23  alternate between the positions shown in FIGS. 8A and 8B. Accordingly, the portion of the hollow fibre membranes  23  which moves only very little is decreased in size. The cycling also creates a reversing flow into and out of the vertical rectangular skeins  8  which the inventors believe encourages bubbles  36  to penetrate deeper into the vertical rectangular skeins  8 .  
         [0090]    Conduit Aerators  
         [0091]    Now referring to FIG. 9A, a conduit aerator  238  is shown. The conduit aerator  238  has an elongated hollow body  302  which is a circular pipe having an internal diameter between 15 mm and 100 mm. A series of holes  304  pierce the body  302  allowing air to flow out of the conduit aerator  238  to create bubbles. The size, number and location of holes may vary but for a rectangular skein  8 , for example, 2 holes (one on each side) of between 5 mm and 10 mm in diameter placed every 50 mm to 100 mm along the body  302  and supplied with an airflow which results in a pressure drop through the holes of between 10 to 100 mm of water at the depth of the conduit aerator  238  are suitable.  
         [0092]    Air enters the conduit aerator  238  at an aerator inlet  306 . At the opposite end of the conduit aerator  238  is an outlet  308 . The highest point on the outlet  308  is located below the lowest point on the aerator inlet  306  by a vertical distance between the minimum and maximum expected pressure drop of water at the depth of the conduit aerator  238  across the holes  304 . The minimum expected pressure drop of water at the depth of the conduit aerator  238  across the holes  304  is preferably at least as much as the distance between the top of the holes  304  and the interior bottom of the body  302 . An air/water interface  309  between the air in the conduit aerator  238  and the water surrounding the conduit aerator  238  will be located below the interior bottom of the body  302  but above the highest point on the outlet  308 . In this way, tank water  18  entering the conduit aerator  238  will flow to the outlet  308  and not accumulate near the holes  304 .  
         [0093]    Now referring to FIG. 9B, another conduit aerator  238  is shown which is preferred for use with relatively clean tank water  18 . The body  302  has a rectangular cross section but is open on the bottom. The conduit aerator  238  may be a separate component or integrated into the headers  22  of a membrane module  20  in which case the bottom of a lower header  22  may serve as the top of the body  302 . The end of the body  302  is capped with a cap  310  which again may be a part of a header  22 . With the bottom of the body  302  open to the tank water  18 , tank water  18  which seeps into the conduit aerator  238  flows back to the tank water  18 . To prevent bubbles  36  from forming at the bottom of the conduit aerator  238 , the sides of the body  302  extend below the bottom of the holes  304  by a distance greater than the expected pressure drop through the holes  304 .  
         [0094]    Now referring to FIG. 9C, another conduit aerator  238  is similar to the conduit aerator  238  of FIG. 9A except as will be described herein. A rubber sleeve  400 , shown partially cut away, covers the body  302  and has slits  402  corresponding with the holes  304 . The slits  402  open when air is flowed into the conduit aerator  238  opening to a larger size when a higher rate of air flow is used. Accordingly, the slits  402  produce larger bubbles  36  at the full rate of air flow and smaller bubbles  36  at the reduced rate of air flow. In wastewater applications, the reduced size of the bubbles  36  provides improved oxygen transfer efficiency at the reduced rate of air flow.  
         [0095]    Now referring to FIG. 9D, another conduit aerator is shown which is preferred for use with relatively solids rich tank water  18 . The body  302  is a tube 32 mm in diameter. The holes  304  are 8 mm in diameter and mounted 30 degrees upwards of horizontal. Drainage holes  410 , at the bottom of the body  302  and typically 16 mm in diameter, allow tank water  18  seepage to drain from the body  302 . A cap  411  covers the end of the body  302 .  
         [0096]    Conduit aerators  238  such as those described above may admit some tank water  18 , even with air flowing through them, which dries out leaving an accumulation of solids. When the supply of air is switched between manifolds as described above, however, the conduit aerator  238  is alternately flooded and emptied. The difference in water elevation within the body  302  corresponds to the air pressure loss across the holes  304  between the high and low air flow conditions. The resulting cyclical wetting of the conduit aerators  238  helps re-wet and remove solids accumulating in the conduit aerators  238  or to prevent tank water  18  from drying and depositing solids in the conduit aerators  238 . If necessary, this flooding can be encouraged by releasing air from the appropriate manifold by opening a valve vented to atmosphere.  
         [0097]    Embodiments similar to those described above can be made in many alternate configurations and operated according to many alternate methods within the teachings of the invention.  
       EXAMPLES  
       [0098]    The following examples refer to ZW 500 membrane modules produced by ZENON Environmental Inc. Each ZW 500 has two rectangular skeins of vertical hollow fiber membranes. For the purposes of calculating superficial velocities, the cross sectional area of aeration for each ZW 500 membrane module is approximately 0.175 m2. All air flow rates given below are at standard conditions.  
       Example 1  
       [0099]    A cassette of 8 ZW 500 membrane modules were operated in bentonite suspension under generally constant process parameters but for changes in flux and aeration. A fouling rate of the membranes was monitored to assess the effectiveness of the aeration. Aeration was supplied to the cassette at constant rates of 204 m3/h (ie. 25.5 m3/h per module) and 136 m3/h and according to various cycling regimes. In the cycled tests, a total air supply of 136 m3/h was cycled between aerators located below the modules and aerators located between and beside the modules in cycles of the durations indicated in FIG. 10A. Aeration at 136 m3/h in 30 second cycles (15 seconds of air to each set of aerators) was approximately as effective as non-cycled aeration at 204 m3/h.  
       Example 2  
       [0100]    The same apparatus as described in example 1 was tested under generally constant process parameters but for the variations in air flow indicated in FIG. 10B. In particular, 70% of the total air flow of 136 m3/h was cycled in a 20 second cycle such that each group of aerators received 70% of the total airflow for 10 seconds and 30% of the total airflow for 10 seconds. As shown in FIG. 10B, cycling 70% of the air flow resulted in reduced fouling rate at high permeate flux compared to constant aeration at the same total air flow.  
       Example 3  
       [0101]    2 ZW 500 membrane modules were operated to produce drinking water from a natural supply of feed water. Operating parameters were kept constant but for changes in aeration. The modules were first operated for approximately 10 days with non-cycled aeration at 25.5 m3/h per module (for a total system airflow 51 m3/h). For a subsequent period of about three days, air was cycled from aerators near one set of modules to aerators near another set of modules such that each module was aerated at 12.8 m3/h for 10 seconds and then not aerated for a period of 10 seconds (for a total system airflow of 12.8 m3/h). For a subsequent period of about 10 days, the modules were aerated such that each module was aerated at 25.5 m3/h for 10 seconds and then not aerated for a period of 10 seconds (for a total system airflow of 25.5 m3/h). For a subsequent period of about 10 days, the initial constant airflow was restored. As shown in FIG. 10C, with aeration such that each module was aerated at 25.5 m3/h for 10 seconds and then not aerated for a period of 10 seconds (ie. one half of the initial total system airflow), the membrane permeability stabilized at over 250 L/m2/h/bar whereas with non cycled airflow at the initial total system airflow the membrane permeability stabilised at only about 125 L/m2/h/bar.  
       Example 4  
       [0102]    3 units each containing 2 ZW 500 membrane modules were operated at various fluxes in a membrane bioreactor. Unit 1 had modules operating at 26 L/m2/h and 51 L/m2/h. Unit 2 had modules operating at 31 L/m2/h and 46 L/m2/h. Unit 3 had modules operating at 34 L/m2/h and 51 L/m2/h. The units were first operated for a period of about 10 days with non cycled aeration at 42.5 m3/h per module (total system air flow of 85 m3/h). The permeability decreased and stabilized at between 250 and 275 L/m2/h/bar for Unit 1, between 200 and 225 L/m2/h/bar for Unit 2 and between 150 and 175 L/m2/h/bar for Unit 3. For a second period of about 14 days, a total system airflow of 61.2 m3/h was applied for 10 seconds to aerators below the modules and then for 10 seconds to aerators beside the modules. Under these conditions, permeability increased and stabilized at between 350 and 375 L/m2/h/bar for Unit 1 and between 325 and 350 L/m2/h/bar for Units 2 and 3.  
       Example 5  
       [0103]    A cassette of 6 ZW 500 modules was used to treat sewage. While holding other process parameters generally constant, aeration was varied and permeability of the modules was measured periodically as shown in FIG. 11. In period A, 255 m3/h of air was supplied continuously and evenly to the modules. In period B, 184 m3/h of air was applied for 10 seconds to aerators below the modules and then for 10 seconds to aerators beside the modules. In Period C. the same aeration regime was used, but shrouding around the modules was altered. In period D, 184 m3/h of air was applied for 10 seconds to aerators near a first set of modules and then for 10 seconds to aerators near a second set of modules. In period E, 204 m3/h of air was applied to all of the modules evenly for 10 seconds and then no air was supplied to the modules for 10 seconds. In Period F, 306 m3/h was applied to all of the modules evenly for 10 seconds and then no air was supplied to the modules for 10 seconds. In Period G, 153 m3/h was applied to aerators near a first set of modules and then for 10 seconds to aerators near a second set of modules.  
       Example 6  
       [0104]    A single ZW 500 membrane module was used to filter a supply of surface water. While keeping other process parameters constant, the module was operated under various aeration regimes and its permeability recorded periodically. First the module was operated with constant aeration at (a) 20.4 m3/h and (b) 25.5 m3/h. After an initial decrease in permeability, permeability stabilised at (a) about 200 L/m2/h/bar and (b) between 275 and 300 L/m2/h/bar respectively. In a first experiment, aeration was supplied to the module at 25.5 m3/h for two minutes and then turned off for 2 minutes. In this trial, permeability decreased rapidly and could not be sustained at acceptable levels. In another experiment, however, aeration was supplied to the module at 25.5 m3/h for 30 seconds and then at 8.5 m3/h for 30 seconds. In this trial, permeability again decreased initially but then stabilised at between 275 and 300 L/m2/h/bar.