CA2279766A1

CA2279766A1 - Aeration system for submerged membrane module

Info

Publication number: CA2279766A1
Application number: CA 2279766
Authority: CA
Inventors: Hamid Rabie; Pierre Cote; Manwinder Singh; Arnold Janson
Original assignee: Zenon Environmental Inc
Current assignee: Zenon Technology Partnership
Priority date: 1999-07-30
Filing date: 1999-07-30
Publication date: 2001-01-30

Abstract

An aeration system for a submerged membrane module has one or more groups of aerators that are controlled to produce aeration which varies in short cycles. Transient flow conditions result in the tank water which helps avoid dead spaces and assists in agitating the membranes. The aerators are periodically flooded to avoid fouling.

Description

B&P File No. 4320-49 BERESKIN & PARR CANADA
Title: Aeration System for Submerged Membrane Module Inventors: Hamid R. Rabie, Pierre Cote, Manwinder Singh and Arnold janson Title: Aeration System for Submerged Membrane Module FIELD OF THE INVENTION
This invention concerns the use of scouring air bubbles produced by an aeration system to clean membranes in a submerged membrane filter.
_ BACKGROUND OF THE INVENTION
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 an activated sludge aeration tank containing wastewater and to withdraw potable water from a tank filled with water from a lake or reservoir.
The membranes are generally arranged in modules, which comprise the membranes and headers attached to the membranes, and are submerged in a tank of water containing solids. A transmembrane pressure is applied across the membrane walls which causes clean 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.
Air bubbles are introduced to the tank below the membrane modules and rise through the membrane modules. The air bubbles create an air lift which recirculates tank water through the membrane module. The rising bubbles and tank water produce a cleaning effect, scouring and agitating the membranes to inhibit solids in the tank water from fouling the pores of the membranes. There is also an oxygen transfer from the bubbles to the tank water which, in wastewater applications, provides oxygen for microorganism growth.
An aerator to produce scouring air bubbles is typically attached to a lower header of a membrane module located below the membranes and connected by conduits to an air blower. 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.
With typical aeration systems, an operator increases the rate of air flow to the aerators if more cleaning is desired. This technique, however, has a number of disadvantages. In wastewater applications, simply increasing aeration may increase the total amount of oxygen transferred to the tank water but reduces the efficiency of the oxygen transfer for two reasons. Firstly, as bubble density increases the bubbles coalesce more readily into larger bubbles which have a reduced ratio of surface area to volume. Secondly, the retention time of the bubbles is reduced because a stronger air lift causes the water column around the membranes to rise faster. Increased aeration also stresses the membranes and air blower motors and increases the amount of energy used.
E'~nother concern with typical aeration systems is that they cause the tank v,~ater to move in a generally constant 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 are not effectively cleaned and quickly foul with solids. A related problem occurs in membrane modules where the membranes are installed with a degree of slack to allow the membranes to vibrate and shake off solids. The movement of tank water in the tank often causes slackened membranes to assume a near steady state position which prevents significant vibration near the ends of the membranes.

Yet another concern with current aeration systems is that they typically foul over time which either reduces the supply of bubbles into the tank water or requires the operator to clean the aeration system frequently. For example, aeration is typically stopped from time to time for backwashing, cleaning or other maintenance procedures. Unfortunately, simple aerators have no effective mechanism to seal themselves when air-flow is stopped and tank water may enter the aeration system. Some of the tank water remains in the aeration system even when the air blowers are turned back on and interferes with the flow of air. Further, tank water in the aeration system evaporates rapidly when the air supply is turned back on leaving deposits of solids in the aeration system which foul the aeration system. In wastewater applications in particular, the solids build up may plug many of the holes of the aerators over time.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an aeration system that Via) has improved cleaning ability for a given amount of aeration, (b) reduces the size of dead zones around the membranes and promotes the movement of slackened membranes and (c) more reliably produces coarse bubbles by inhibiting fouling of the aeration system.
The present invention is directed at an aeration system which produces a transient flow pattern in the tank water and in which the aerators foul less frequently, particularly when used to create large bubbles.
The aeration system has at least one but preferably two or more groups of aerators, further preferably horizontally disposed from each other. The flow of air through a group of aerators is increased or decreased in cycles of 120 seconds or less to rapidly increase or decrease the density of the tank above the group of aerators. The cycled aeration produces vertically transient flow which in turn create turbulence and some horizontally transient flow. The transient flows and turbulence reduces the size or duration of dead zones in the tank water, preferably those which typically form near the headers of membrane modules and, particularly with membrane modules comprising hollow fibre membranes, promotes movement of tank water into and out of the group of membranes to help remove solids from the membrane module. When slackened membranes are used, the transient flow also encourages movement of the membranes.
Preferably, horizontally transient flow is encouraged by modulating the flow of air to a first group of aerators while the supply of air to at least a second group of aerators is either constant or modulated in a way that is not in sync with the modulating flow of air to the first group of aerators. More preferably, the supply of air to both a first and a second group of aerators are modulated but the two supplies of air are 180 degrees out of sync. Such an aeration system may be constructed by splitting an air supply conduit from an air blower into two manifolds, at least one having a flow limiter such that one manifold carries a higher percentage or all of the total flow of air. Each manifold has a valve which can be activated to divert its air to the other manifold. By opening one valve while closing the other, an alternating supply of air is provided to the manifolds without turning the air blower motors on and off which drastically reduces their service life.
Further, the total supply of air is generally constant so that the growth of microorganisms, if present in the tank water, is not significantly disturbed.
Preferably, the varying supply of air to the aerators causes the aerators to be flooded periodically with tank water (and then at least partially re-emptied) to wet dried foulants and to wash out foulants.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the present invention will now be described with reference to the following figures.

Figure 1 is a schematic drawing of a submerged membrane reactor according to an embodiment of the invention.
Figure 2 is a plan view schematic of membrane modules and a portion of an aeration system according to an embodiment of the invention.
Figure 3 is an elevational schematic of membrane modules and parts of an aeration system according to an embodiment of the invention.
Figures 4A, 4B, 4C and 4D are elevational representations of membrane modules and part of an aeration system according to an embodiment of the invention.
Figures 5A, 5B, 5C and 5D are schematic drawings of a part of an aeration system according to an embodiment of the invention.
Figures 6A and 6B are drawings of membrane modules and a portion of an aeration system according to an embodiment of the invention.
Figures 7A and 7B are drawings of aerators according to an embodiment of the invention.
Figures 8A, 8B and 8C are charts showing the results of tests performed on embodiments of the invention having two groups of aerators.
Figure 9 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
Referring now to Figure 1, the general arrangement of a reactor 10 is shown according to an embodiment of the invention. The description of the reactor 10 in this section generally applies to various embodiments to be described below unless modified by or inconsistent with the description of any particular embodiment.
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 is likely to have increased concentrations of the various solids, particularly where the reactor 10 is used to treat wastewater.
One or more membrane modules 20 are mounted in the tank and have headers 22 in fluid communication with a permeate side of one or more membranes 23. Membrane modules 20 are available in various sizes and configurations, including hollow fibre and flat sheet, with various header configurations. For hollow fibre membranes 23, for example, the membranes 23 may be oriented vertically and held in place between an upper and a lower header 22 in fluid communication with the lumens of the membranes 23 or oriented horizontally with horizontally opposed headers 22 in fluid communication with the lumens of the membranes 23. For flat sheet membranes 23, the membranes 23 are typically oriented vertically in a spaced apart pair with headers 22 on all four sides in fluid communication with the resulting inside surfaces. A
membrane module 20 may have one or more has microfiltration or ultrafiltration membranes 23 and many membrane modules 20 may be joined together to form cassettes, but all such configurations will be referred to as membrane modules 20. The membranes 23 in the membrane _7_ modules 20 preferably have a pore size between 0.005 and 10 microns, such as those manufactured by Zenon Environmental Inc. and sold under the ZeeWeed trade mark, but many suitable membranes 23 and membrane modules 20 are sold by other manufacturers.
During permeation, the tank 12 is kept filled with tank water 18 above the level of the membranes 23 in the membrane modules 20. Filtered water called permeate 24 flows through the walls of the membranes 23 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. Permeate 24 may be periodically flowed in a reverse direction through the membrane module 20 to assist in cleaning the membrane modules 20.
During permeation, the membrane modules 20 reject solids which remain on the outside of the membranes 23. 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.
Cleaning, and aeration if desired, are provided by bubbles 36 from an aeration system 37 having one or more aerators 38 connected by an air delivery system 40 and a distribution manifold 51 to one or more air blowers 42. 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. The bubbles 36 rise upwards through the membrane modules _8_ 20 to clean and inhibit fouling of the membranes 23.
In addition to cleaning the membranes 23 in the membrane modules 20 directly, 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.
The bubbles 36 have an average diameter between .1 and 25 mm. Larger bubbles 36 are more effective in cleaning membranes, but smaller bubbles 36 are more efficient in transferring oxygen to the tank water 18. The bubbles 36 are preferably between 3 mm and 20 mm and more preferably between 5 mm and 15 mm in diameter. Bubbles 36 of the size described above provide both effective cleaning of the membranes 23 in the membrane modules 20 and efficient transfer of oxygen to the tank water 18 without causing Pxcessive 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 ire preferred.
The bubbles 36 may be larger than a hole in an aerator 38 where the bubble 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 depending on the pressure and flow rate of the supplied air.
In typical systems, there is a pressure drop of between 5 mm and 100 mm and more typically between 10 mm and 50 mm 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 blower 42 is operating, although small amounts of tank water 18 may still seep into the aeration system 37.
In a first embodiment, the aeration system 37 is operated cyclically during permeation as will be described below. Referring still to Figure 1, the air blower 42 is operated to provide a flow rate of air which varies from a full rate to a reduced rate and then back to the full rate in repeated cycles. The rate of air flow preferably varies in a step form through the cycle meaning that air flow is altered in between the full rate and reduced rate as~ rapidly or abruptly. The inventors have noticed that when the rate of air flow is rapidly increased to the full rate, there is a short burst of unusually large bubbles and significant transient flow and turbulence.
Further preferably, the step form variation in air flow rate involves generally equal time at the full rate and reduced rate. This appears to allow the air lift effect created by the full rate air flow to be reduced substantially when air flow is supplied at the reduced rate without resulting in large time periods between successive transitions to full rate aeration.
The air blower 42 can be operated to produce the variation in air flow rate most simply by turning it on and off at the appropriate times.
Alternatively, the air blower 42 can be operated continuously and completely or partially vented during the appropriate part of each cycle.
The total cycle time varies with the depth of the tank, the design of the membrane modules 20, process parameters and the conditions of the feed water 14 to be treated, but preferably ranges from 10 seconds (5 seconds at the full rate and 5 seconds at the reduced rate) to 60 seconds (30 seconds at the full rate, 30 seconds at the reduced rate).
The amount of air used is dependant on numerous factors but is preferably related to the superficial velocity of air flow. The superficial velocity of air flow is defined as the rate of air flow delivered by the air blower 42 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 aerators. Superficial velocities of air flow of between 0.015 m/s and 0.15 m/s are preferred at the full rate. 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.
At the reduced rate, the air flow rate preferably ranges from none to about one third of the full rate. For example, with a ZW500 membrane module produced by ZENON Environmental Inc., airflow of about 15 scfm per module is often used at the full rate of aeration with air flow at the reduced rate ranging from 0 scfm per module to 5 scfm per module. Within these ranges, the reduced rate of air flow is influenced by the quality of the feed water 14. A reduced rate of air flow of 0 scfm is generally preferred, but with some feed water 14, the membranes 23 foul significantly even within the short time of the reduced aeration. In these cases, it is more efficient for the reduced rate of air flow to range up to about one third of the full rate.
Referring now to Figure 2, a plan view of a portion of another embodiment of the invention is shown. Membrane modules 20 (shown in dashed lines) are arranged within a tank 12. The air delivery system 40 has a first manifold 50 and a second manifold 52 connected to aerators 38 each comprised of holes 53 in conduits 54, the holes 53 sized to create the desired diameter of bubbles 36 (bubbles 36 not illustrated). The aerators could also other types of aerators,such as cap aerators 38 be mounted the conduits54. The objects shownin Figure 2 are in not intended be drawn scale and could havevarious dimensions.
to to Conduits 54 are typically between one to two inches in diameter. Further, the number of conduits 54 and aerators 38 for a given number and size of membrane modules 20 may vary and the conduits 54 might also be placed in other orientations or spacings. For example, the conduits 54 may be perpendicular to the membrane modules 20 instead of parallel to them as ' 10 shown. Now referring to Figure 3, an elevation of a portion of this embodiment of the invention is shown.
To create transient flow, the supply of air to at least one of the first manifold 50 or the second manifold 52 is varied over time relative to the other. For example, air supply to the first manifold 50 can be cycled between high and low or off while air supply to the second manifold 52 remains constant. Preferably, air supply to the second manifold 52 is also cycled between high and low or off but such that there is a phase shift or time lag, and more preferably a 180 degree phase shift, between the supply of air to the first manifold 50 and the second manifold 52 so that both manifolds are not carrying their highest supply of air at the same time.
Since the flow rate of air determines the magnitude of the resulting reduced density of the tank water 18 above either the first aerators 38 or the second aerators 39, altering the supply of air to the first aerators 38 or the second aerators 39 produces variations in the density of the tank water 18 above one relative to the other. These relative variations in density cause variations in the vertical velocity of the tank water 18 above the first aerators 38 or the second aerators 39. Further, the relative variations in density also causes tank water 18 to flow horizontally between the areas above the first aerators 38 or the second aerators 39. When the supply of air to at least one of the first aerators 38 or the second aerators 39 is varied over time relative to the other, both the horizontal and vertical movement of tank water 18 is generally transient. The resulting transient flow of the tank water 18 reduces dead spaces in or around the membrane modules 20.
The variation in air supply preferably occurs in a regular cycle of between two seconds and one hundred and twenty seconds in length but preferably between 20 seconds and 40 seconds in length. Short cycles of 10 seconds or less (ie. 5 seconds of air flowing through an aerator and 5 seconds without air flowing through an aerator) may not be sufficient to establish regions of different densities with the tank water 18 in a deep tank 12 where such time is insufficient to allow the bubbles 236 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 described above, a step form variation in rate of air flow is preferred.
Preferably, the supply of air to each of the first aerators 38 and the second aerators 39 is varied between a value in the range of 0% and 30% of the total supply of air and a value in the range of 70% to 100% of the total supply of air. More preferably, the total air supply is switched between the first aerator and the second aerator. Also preferably, the total supply of air to the tank 12 remains constant despite the cycling of the air supply to the first aerator 238 or the second aerator 239. It is further preferred that each cycle involve aeration at the full rate for one half of the cycle and aeration at the reduced rate for one half of the cycle and that the area aerated by each of the first manifold 50 and the second manifold 52 be similar.
With this arrangement, the air blower 42 or can be operated at a constant power level while practicing the invention.
With multiple membrane modules 20, a pattern of membrane modules 220, first aerators 38 and second aerators 39 may be repeated to control the volume of tank water 18 that is accelerated or decelerated as the air supply is cycled from the first manifold 50 to the second manifold 52. In addition, the techniques described for two sets of aerators could be extended to three or more sets of one or more aerators with a variety of patterns of air supplied to each of the three aerators, ie.
air could be supplied sequentially to each of the three or more sets of one or more aerators. Further, the first aerators 38 and second aerators 39 or additional aerators may be of various types including distinct aerators, such as aerator caps, or conduit aerators of holes in conduits.
' The amount of air used is dependant on numerous factors but is preferably related to the superficial velocity of air flow. The superficial velocity of air flow is defined as the rate of air flow delivered by the air blower 42 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 aerators but reduced to account for the effect of cycling. For example, where the entire air flow is cycled between two equally sized sets of aerators for equal periods of time, the cross sectional area of aeration is the total area served by aerators divided by two. Superficial velocities of air flow of between 0.015 m/s and 0.15 m/s are preferred. Air blowers 42 for use in drinking water applications may be sized towards the lower end of the range while air blowers 42 used for waste water applications may be sized near the higher end of the range.
Referring now to Figures 5A, 5B, 5C, and 5D, alternate embodiments of the air blower 42 are shown. Referring first to Figure 5A, an embodiment is shown in which an air blower 242 comprises a first air blower 258 and a second air blower 260. The first air blower 258 is attached to the first manifold 250 and the second air blower 260 is attached to the second manifold 252.
In Figure 5B, another embodiment of a part of the aeration system 237 is shown. An air blower 242 has a blower 243 which 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 between 0% to 30% of the total air flow. A controller 268, which could be a timer or 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 the first manifold 250 and air in the high flow line flows to the second manifold 252. 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 second manifold 252 through cross conduit 274 and air in the high flow line 264 flows to the first manifold 250 through reverse conduit 276. With 3 way ball valves in particular, the change between the two conditions described above is smooth and gradual rather than abrupt. With this arrangement, there is a constant total supply of air and wear on the air blower 242 from constantly changing speed is reduced.
Referring now to Figure 5C, another embodiment of a part of the aeration system 237 is shown. An air blower 242 has a blower 243 which blows air into a connector 261 which splits the air flow into a first manifold 250 and a second manifold 252. A first manifold valve 280, preferably a butterfly valve, and a second manifold valve 282, preferably a butterfly valve are controlled by a valve controller 284. The valve controller 284 periodically opens and closes the first manifold valve 280 and second manifold valve 282 but with a phase shift, preferably of 180 degrees so that one opens while the other closes, between them. The valve controller 284 may be a microprocessor if the first manifold valve 280 and second manifold valve 282 are electrically operated. Preferably, the first manifold valve 280 and second manifold valve 282 are mechanically operable by actuators 286 connected by arms 288 to a member 290 on the valve controller 284 which is a motor turning at the required speed or a motor attached to a gear set to produce the required speed of rotation of the member 290.
Referring now to Figure 5D, another embodiment of a part of the aeration system 237 is shown. An air blower 242 has a blower 243 which blows air into a three way valve 292, preferably a ball valve, with its two remaining orifices connected to a first manifold 250 and a second manifold 252. A three way valve controller 294 periodically opens and closes air pathways from the blower 243 to the first manifold 250 and second manifold 252 but with a phase shift, preferably of 180 degrees so that the airway to the first manifold 250 opens while the airway to the second manifold 252 closes. The three way valve controller 294 may be a microprocessor if the three way valve 292 is electrically operated.
Preferably, the three way valve 292 is mechanically operable by handle 296 connected by connector 298 to a lever 299 on the three way valve controller 294 which is a motor turning at the required speed or a motor attached to a gear set to produce the required speed of rotation of the lever 299.
Referring now to Figure 4A, 4B, 4C and 4D, a set of variations of the embodiment of Figures 2 and 3 are shown wherein the membrane modules 20 are hollow fibre membrane modules 220. A hollow fibre membrane module 220 has membranes 223 suspended between headers 222. An aeration system 237 has an air delivery system 240 comprising a first manifold 250 connected to a first aerator or aerators 238 and a second manifold 252 connected to a second aerator or aerators 239 both of which introduce bubbles 236. The first manifold 250 and the second manifold 252 are connected to an air blower 242 having two independently controllable outlets, the first connected to the first manifold 250 and the second connected to the second manifold 252.

Although the first aerators 238 and second aerators 239 are shown beside the lower header 222, they can also be located in other convenient positions where they are horizontally displaced from each other. For example, first aerators 238 could be located under a membrane module 220 and second aerators 239 located beside the same membrane module 220 as shown in Figure 4B or under an adjacent membrane module 220 as shown in Figure 4D or in other convenient locations. Further, as shown in Figure 4C, first aerators 238 and second aerators 239 could both be located under a single membrane module 220. If the membrane modules 220 are rectangular in plan view, the first aerators 238 and second aerators 239, if formed of or attached to elongated conduits, are preferably oriented parallel to the longer side of the rectangle although they may also be oriented parallel to the shorter side of the rectangle or in other orientations.
Particularly regarding the embodiment shown in Figure 4C, if a membrane module 220 is very wide then the first aerators 238 and second aerators 239 may each be made of multiple conduits 54 alternately attached to the first manifold 250 and second manifold 252.
With the greater proportion of the total airflow alternating between the first manifold 250 and the second manifold 252 in cycles, there is significant transient flow in the tank water 18 and few appreciable dead zones in or around the membrane modules 220. In addition to the transient flow, such cycling or pulsing of air creates very large bubbles 236 at the start of each pulse. These very large bubbles 236 create significant amounts of local turbulence, especially when flowing into previously still tank water 18. The resulting transient flow of the tank water 18 helps the bubbles 36 or 236 penetrate into skeins of hollow fibre membranes 23 or 223 and promotes renewal of tank water 18 within the membrane module 20 or 220. In more complicated systems, additional aerators could be arranged throughout the tank 12. By cycling the air supply to these aerators, more complicated transient flows could be established.

Referring now to Figures 6A and 6B, another effect of transient flow is illustrated in more detail for membrane modules 220 made of slackened hollow fibre membranes 278 attached between headers 222.
The degree of slack of the membranes 278 is highly exaggerated for easier illustration. Further, only two membranes 278 are illustrated for each membrane module 220 although a membrane module 220 might actually be constructed of a skein 279 (illustrated with two membranes 278 each). The skein 279 may be between 3 cm and 6 cm thick and have between 2500 and 3000 hollow fibre membranes 278 at a 10-40% packing density. A ZW500 module produced by Zenon Environmental Inc. for example, has two skeins 279 each between 6 cm and 7 cm thick and having about 2500 hollow fibre membranes 278 at about 20% packing density and a total membrane surface area of about 500 square feet. With such dense packing of the membranes 278, it is difficult to encourage bubbles 236 to penetrate the skeins 279 and the fibres on the outer edge of the skeins 279 have significantly more contact with the bubbles 236.
The natural tendency of the bubbles 236 is to go through the areas with lowest resistance such as around the membrane modules 220 or through slots between the membrane modules 220. As a consequence, the tank water 18 in these areas has less density compared to surrounding tank water 18.
If there is more air supplied to the first manifold 250, the membranes 278 assume an average shape as shown in Figure 6A with a local recirculation pattern 280 as shown. If there is more air supplied to the second manifold 252, the membranes 278 assume an average shape as shown in Figure 6B with a local recirculation pattern 282 as shown. If steady state aeration is used in either configuration, the upper 10-20% of the membranes 278 may be forced into a tightly curved shape and vibrate only very little so that solids accumulate rapidly in those sections. A smaller portion at the bottom of the membranes 278 in Figure 6B may also be tightly curved and solids could rapidly accumulate in that section. When hollow fibres membranes are used in other configurations, for example in horizontal skeins, similar problems occur but in different locations in the membrane module 220 and with the membranes 278 assuming different shapes.
In the embodiments of the invention described above, unsteady non-uniform distribution of bubbles 236 is used to change the location of the regions of lower density in the tank water 18 so that a substantial portion of the skein 279 of membranes 278 moves back and forth along a substantial portion of the length of the membranes 278 and air bubbles penetrate further into the skein 279. The air supply to the first manifold 250 and the second manifold 252 is cycled as described above and the membranes 278 cycle between the two positions shown in Figures 6A
and 6B. Hollow fibre membranes 278 in other configurations such as horizontal similarly cycle between different positions. The cycling also creates a reversing flow into and out of the skeins 279 of membranes 278 which forces bubbles 236 to penetrate deeper into the skeins 279.
Now referring to Figure 7A, a conduit aerator 300 is shown which is of a design preferred for the aerators 38, 39, 238 and 239 described above. The conduit aerator 300 has an elongated hollow body 302 which is preferably a circular pipe having an internal diameter between 15 mm and 50 mm but can be made of various other shapes and sizes of conduit including non-prismatic conduits. A series of holes 304 pierce the body 302 allowing air to flow out of the conduit aerator 300 to create bubbles. The size, number and location of holes may vary but with each half of a ZW500 membrane module, 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 of between 5 scfm and 20 scfm resulting in a pressure drop through the holes of between 10 to 100 mm of water at the depth of the aerator 300 are suitable. The body 302 may be a separate component or integrated into the headers of a membrane module. When used in the aeration systems 37 and 237 described above, the conduit aerator 300 performs the function of the aerators 38, 238 and 239 as well as part of the functions of parts of the air delivery system 40 and 240, particularly the conduits 54.
Air enters the conduit aerator 300 at an inlet 306 and exits at an outlet 308. The highest point on the outlet 308 is preferably located below the lowest point on the inlet 306 by a vertical distance between the minimum and maximum expected pressure drop of water at the depth of the aerator 300 across the holes 304. The minimum expected pressure drop of water at the depth of the aerator 300 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 aerator 300 and the water surrounding the aerator 300 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 entering the conduit aerator 300 will flow to the outlet 308 and not accumulate near the holes 304 and yet air will flow out of the outlet 308.
Now referring to Figure 7B, another conduit aerator 300 is shown having holes 304 located at the bottom of the body 302. With this configuration, it is possible to cap an end of the body 302 with a cap 310 instead of having an outlet 308, although an outlet 308 as described above may still be used. With the holes 304 at the bottom of the bodv 302, tank water is discouraged from seeping into the conduit aerator 300 by gravity and the conduit aerator 300 can be emptied of water through the holes 304.
Conduit aerators 300 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 air is not flowing through a particular conduit aerator 300, air is trapped in the conduit aerators 300 at an elevated static pressure but its dynamic pressure is reduced. This reduction in dynamic allows tank water to enter the conduit aerators 300. Further, pressure relief valves can be installed and operated to depressurize the conduit aerators if needed to increase the volume of tank water entering the conduit aerators 300 or. the rate at which the tank water 18 enters the conduit aerators 300. When air flow resumes through the conduit aerator, the tank water 18 is pushed back out of the conduit aerator.
When the supply of air is switched between manifolds as described above, however, the conduit aerator 300 is alternately flooded and emptied. The resulting cyclical wetting of the conduit aerators 300 helps re-wet and remove solids accumulating in the conduit aerators 300 or to prevent tank water 18 from drying and depositing solids in the conduit aerators 300.
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. In particular, filtration systems can vary in size and complexity and one or more of any component described above may be used regardless of whether the component appears to be singular or plural in the description above.
Examples Example 1 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 120 scfm (ie. 15 scfm per module) and 80 scfm and according to various cycling regimes. In the cycled tests, a total air supply of 80 scfm was cycled between aerators located below the modules and aerators located between and beside the modules in cycles of the durations indicated in Figure 8A. Aeration at 80 scfm in 30 second cycles (15 seconds of air to each set of aerators) was approximately as effective as non-cycled aeration at 120 scfm.
Example 2 The same apparatus as described in example 1 was tested under generally constant process parameters but for the variations in air flow indicated in Figure 8B. In particular, 70% of the total air flow of 80 scfm 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 Figure 8B, 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 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 15 scfm per module (for a total system airflow 30 scfm). 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 7.5 scfm for 10 seconds and then not aerated for a period of 10 seconds (for a total system airflow of 7.5 scfm). For a subsequent period of about 10 days, the modules were aerated such that each module was aerated at 15 scfm for 10 seconds and then not aerated for a period of 10 seconds (for a total system airflow of 15 scfm). For a subsequent period of about 10 days, the initial constant airflow was restored. As shown in Figure 8C, with aeration such that each module was aerated at 15 scfm 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 10 gfd/psi whereas with non-cycled airflow at the initial total system airflow the membrane permeability stabilised at only about 5 gfd/psi.
Example 4 3 units each containing 2 ZW 500 membrane modules were operated at various fluxes in a membrane bioreactor. Unit 1 had modules operating at 15 and 30 gfd. Unit 2 had modules operating at 18 and 27 gfd. Unit 3 had modules operating at 20 and 30 gfd. The units were first operated for a period of about 10 days with non-cycled aeration at 25 scfm per module (total system air flow of 50 scfm). The permeability decreased and stabilized at between 10 and 11 gfd/psi for Unit 1, between 8 and 9 gfd/psi for Unit 2 and between 6 and 7 gfd/psi for Unit 3. For a second period of about 14 days, a total system airflow of 36 scfm 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 14 and 15 gfd/psi for Unit 1 and between 13 and 14 gfd/psi for Units 2 and 3.
Example 5 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 Figure 9. In period A, 150 scfm of air was supplied continuously and evenly to the modules. In period B, 108 scfm 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, 108 scfm 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, 120 scfm 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, 180 scfm 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, 90 scfm 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 ' 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) 12 scfm and (b) 15 scfm. After an initial decrease in permeability, permeability stabilised at (a) about 8 gfd/psi and (b) between and 12 gfd/psi respectively. In a first experiment, aeration was supplied to the module at 15 scfm 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 15 scfm for 30 seconds and then at 5 scfm for 30 seconds. In this trial, permeability again decreased initially but then stabilised at between 11 and 12 gfd/psi.

Claims

1. In an aeration system to aerate tank water in a tank containing one or more immersed membrane modules having:
(a) an air delivery network having a plurality of distinct branches;
(b) one or more aerators in fluid communication with the distinct branches of the air distribution system and mountable below the membranes;
(c) an air supply to provide an initial air flow at an initial flow rate; and (d) a valve set in fluid communication with the air supply and having distinct outlets in fluid communication with the distinct branches of the air distribution system, wherein the valve set is operable to (i) split the initial air flow such that at any point in time at least one of the distinct branches of air distribution system receives air at a higher flow rate and at least one other of the distinct branches of the air distribution network receives air at a lower flow rate, the lower flow rate being less than one half of the higher flow rate, and (ii) switch which branch or branches of the air delivery network receive air at the higher flow rate and the lower flow rate in repeated cycles, the improvement comprising, the one or more aerators are conduit aerators which admit tank water when air is supplied at the lower flow rate.

2. In an aeration system to aerate tank water in a tank containing one or more immersed membrane modules having:

(a) an air delivery network having a plurality of distinct branches;
(b) one or more aerators in fluid communication with the distinct branches of the air distribution system and mountable below the membranes;
(c) an air supply to provide an initial air flow at an initial flow rate; and (d) a valve set in fluid communication with the air supply and having distinct outlets in fluid communication with the distinct branches of the air distribution system, wherein the valve set is operable to (i) split the initial air flow such that at any point in time at least one of the distinct branches of air distribution system receives air at a higher flow rate and at least one other of the distinct branches of the air distribution network receives air at a lower flow rate, the lower flow rate being less than one half of the higher flow rate, and (ii) switch which branch or branches of the air delivery network receive air at the higher flow rate and the lower flow rate in repeated cycles, the improvement comprising, the one or more aerators are conduit aerators covered by a resilient sleeve having slits corresponding to holes in the conduit aerators.

3. In an aeration system to aerate tank water in a tank containing one or more immersed membrane modules having:
(a) an air delivery network having a plurality of distinct branches;
(b) one or more aerators in fluid communication with the distinct branches of the air distribution system and mountable below the membranes;

(c) an air supply to provide an initial air flow at an initial flow rate; and (d) a valve set in fluid communication with the air supply and having distinct outlets in fluid communication with the distinct branches of the air distribution system, wherein the valve set is operable to (i) split the initial air flow such that at any point in time at least one of the distinct branches of air distribution system receives air at a higher flow rate and at least one other of the distinct branches of the air distribution network receives air at a lower flow rate, the lower flow rate being less than one half of the higher flow rate, and (ii) switch which branch or branches of the air delivery network receive air at the higher flow rate and the lower flow rate in repeated cycles, the improvement comprising, (f) the aerators are conduit aerators and (g) the aerators associated with a first distinct branch of the air delivery system are interspersed with the aerators associated with a second distinct branch of the air delivery system such that adjacent conduit aerators are in fluid combination with different distinct branch of the air delivery system.

4. The invention of claim 3 wherein the valve set controller is operated such that the repeated cycles are less than 120 seconds in duration.

5. The invention of claim 4 wherein the valve set controller is operated such that the repeated cycles are less than 60 seconds and more than 10 seconds in duration.

6. The invention of claim 5 wherein the valve set controller is operated such that the repeated cycles are less than 40 seconds and more than 20 seconds in duration.

7. The invention of claim 3 wherein the membrane modules have hollow fibre membranes oriented vertically.

8. The invention of claim 7 wherein the membrane modules are rectangular skeins having upper and lower headers, the aerators are conduit aerators approximately as long as the headers of the rectangular skeins, and 1 or 2 rectangular skeins are associated with each such aerator.

9. In a method of filtration having the steps of providing immersed membrane modules collected in filtration zones, permeating filtered water through the membrane modules, and backwashing the filtration zones in sequence, the improvement comprising, (a) providing aeration to the filtration zones at either a higher flow rate or at a lower flow rate, the lower flow rate being between an air off condition and one half of the higher flow rate;
(b) providing aeration at the higher flow rate for a pre-selected length of time to the filtration zones sequentially in a cycle which is not longer then 12 times the pre-selected length of time; and (c) operating the backwashing means to backwash each filtration zone during a period of aeration of that filtration zone.

10. A reactor for treating water to produce a filtered permeate comprising:
(a) a tank to hold the water;
(b) one or more membrane modules mounted in the tank so as to be normally immersed in the water;
(c) means to withdraw a filtered permeate from the one or more membrane modules; and, (d) means to both (i) aerate the feed water with fouling-inhibiting bubbles and (ii) produce transient flow conditions in the water in the tank drop down a line wherein the tank water accelerates or decelerates for much of the cycle and is rarely in a steady state.

11. The invention of claim 10 wherein the one or more membrane modules comprise hollow fibre membranes.

12. A method of aerating a plurality of immersed membrane modules comprising:
providing a flow of air to aerators below the membrane modules alternating between a higher flow rate of flow and a lower flow rate of flow, the lower flow rate being less than one half of the higher flow rate, in repeated cycles of less than 120 seconds in duration.

13. The invention of claim 12 wherein the repeated cycles are between 10 seconds and 60 seconds in duration.

14. The invention of claim 13 wherein the lower flow rate is an air off condition.

15. The invention of claim 14 wherein the higher flow rate corresponds to a superficial velocity in relation to the aerators receiving the flow of air of between 0.013 m/s and .15 m/s.

16. The method of claim 12 further comprising producing larger bubbles when the rate of the flow of gas to the aerators is at the higher flow rate and producing smaller bubbles when the rate of the flow of gas to the aerators is at the lower flow rate.

17. A method of aerating a plurality of immersed membrane modules comprising:
(a) providing a first set of aerators below the membrane modules interspersed with a second set of aerators below the membrane modules;
(b) providing an initial air flow at an initial flow rate;
(c) splitting and allocating the initial air flow such that for a first period of time the first set of aerators receives air at a higher flow rate while the second set of aerators receives air at a lower flow rate and that for a second period of time the first set of aerators receives air at a lower flow rate while the second set of aerators receives air at a higher flow rate in repeated cycles of less than 120 seconds in duration.

18. The invention of claim 17 wherein the cycles are between 20 and 40 seconds in duration.

19. The invention of claim 17 wherein the lower flow rate is an air off condition.

20. The aeration system of claim 19 wherein the higher flow rate of gas flow corresponds to a superficial velocity between 0.013 m/s and .15 m/s.

21. A cyclic aeration system to aerate tank water in a tank or tanks containing one or more immersed membrane modules comprising:
(a) an air delivery network;
(b) one or more aerators in fluid communication with the air delivery network and mountable below the membranes;
(c) an air supply to provide an initial air flow at an initial flow rate;

(d) a valve set in fluid communication with the air supply and having an outlet in fluid communication with the air delivery network; and, (e) a valve set controller to control the valve set;
wherein the valve set and valve set controller are operable to alternately supply air at the outlet at a higher flow rate and a lower flow rate in cycles of 120 seconds in duration or less.

22. The invention of claim 21 wherein the lower flow rate is less than one half of the higher flow rate.

23. The cyclic aeration system of claim 22 wherein the lower flow rate is an air off condition.

24. The cyclic aeration system of claim 23 wherein the higher flow rate has a superficial velocity of between 0.013 m/s and .15 m/s in relation to the aerators receiving air at the higher flow rate.

25. The cyclic aeration system of claim 21 wherein the cycles are between 10 seconds and 60 seconds in duration.

26. The cyclic aeration system of claim 21 wherein the membrane modules have hollow fibre membranes.

27. The cyclic aeration system of claim 26 wherein the hollow fibre membranes are oriented horizontally in the tank.

28. A reactor for treating water to produce a filtered permeate comprising:
(a) a tank to hold the water;

(b) one or more membrane modules mounted in the tank so as to be normally immersed in the water;
(c) means to withdraw a filtered permeate from the one or more membrane modules; and, (d) means to both (i) aerate the feed water with fouling-inhibiting bubbles and (ii) produce horizontal transient flow conditions in the water in the tank.

29. The invention of claim 28 wherein the one or more membrane modules comprise hollow fibre membranes.