EP2558420A1

EP2558420A1 - Membrane bioreactor having mixed liquor and air conduits in a filtration tank

Info

Publication number: EP2558420A1
Application number: EP11769328A
Authority: EP
Inventors: Hong Zhao; Martin Brockmann; Heribert Moeslang; Pierre Girodet
Original assignee: Veolia Water Solutions and Technologies Support SAS
Current assignee: Veolia Water Solutions and Technologies Support SAS
Priority date: 2010-04-12
Filing date: 2011-04-07
Publication date: 2013-02-20
Also published as: WO2011130089A1; JP2013534458A; AU2011240935A1; CA2796043A1

Abstract

A method and system is provided for treating wastewater. In one process, wastewater is directed into a treatment tank and biologically treated. The biologically treated wastewater is directed as mixed liquor from the treatment tank to a bottom portion of a downstream filtration tank having at least one submerged membrane module that extends across substantially the entire cross sectional area of the filtration tank. Mixed liquor is directed from the bottom of the filtration tank upwardly into the membrane module such that substantially all of the mixed liquor received in the bottom of the filtration tank flows through the membrane module. As the mixed liquor flows vertically through the membrane module, the method includes inducing at least some of the mixed liquor through walls of one or more membrane filters that form a part of the membrane module, producing a permeate stream.

Description

MEMBRANE BIOREACTOR HAVING MIXED LIQUOR AND

AIR CONDUITS IN A FILTRATION TANK

CROSS-REFERENCE TO RELATED APPLICATION

[0001] The present application is a continuation-in-part of U.S. Patent

Application Serial No. 12/135,394 filed June 9, 2008, the disclosure of which is hereby expressly incorporated by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to wastewater treatment, and more particularly, to a wastewater treatment process that utilizes membrane filtration.

BACKGROUND OF THE INVENTION

[0003] in recent years, membrane bioreactors have become popular for treating wastewater. Membrane bioreactors combine biological treatment processes with membrane filtration to generally provide an advanced level of organic and suspended solids removal. These systems typically provide an advanced level of nutrient removal. Such membranes typically have pore sizes ranging from about 0.035 microns to 0.4 microns. This level of filtration provides high quality effluent to be transported through the membranes and generally eliminates the sedimentation and filtration processes typically used for wastewater treatment. Because the need for sedimentation is eliminated, the biological process can operate at much higher mixed liquor suspended solids concentrations. This can reduce the size of tanks required to carry out wastewater treatment.

[0004] One type of system includes at least one biological reactor and a membrane filtration tank disposed downstream from the reactor. A membrane module or cassette is typically submerged in the filtration tank. Mixed liquor is transferred from the reactor to the downstream filtration tank. The membrane module or cassette typically includes an array of submerged individual membrane filters. Mixed liquor is induced into the open space between the individual membrane filters, resulting in the mixed liquor being filtered and producing a permeate. The permeate is pumped or is flowing by gravity from the individual membrane filters and the filtration tank.

[0005] Typically the filtration tank is relatively large compared to the size of the membrane modules or cassettes. This means that when the membrane module or cassette is placed in the filtration tank, it is surrounded by mixed liquor or non- permeated mixed liquor. The term "non-permeated mixed liquor" means mixed liquor in the filtration tank that has passed through the membrane module or modules in the filtration tank. Practically, the non-permeated mixed liquor in the filtration tank tends to be recirculated multiple times through the membrane module or cassette. That is, the mixed liquor or non-permeated mixed liquor tends to move upwardly through the membrane module and exits the top of the module and then returns downwardly outside of the module, and then is induced back upwardly through the membrane module.

[0006] Typically, an air diffuser is disposed below the membrane module or cassette. The air diffuser tends to accelerate the non-permeated mixed liquor upwardly through the membrane module. The air bubbles created by the air being diffused gives rise to an air-lift effect within the membrane module or cassette. The air bubbles, after exiting the membrane module, tend to move upward in the direction of the water surface in the filtration tank while the non-permeated mixed liquor tends to turn and move downwardly in the opposite direction. The non- permeated mixed liquor is now flowing mainly outside of the membrane module towards the bottom of the fiitration tank. In some cases, the non-permeated mixed liquor exiting the top of the membrane module has a velocity that is relatively high. When the velocity of the water exiting the top of the module is relatively high, the non-permeated mixed liquor tends to retain the air bubble and does not release the air bubbles to move to the top of the water surface in the fiitration tank. The entrapped air bubbles reduce the velocity of the down flowing non-permeated mixed liquor, and by doing so the volume of the mixed liquor flowing upwardly through the membrane module is limited. This reduces the turbulence of the mixed liquor passing through the membrane module and tends to reduce finally the efficiency of filtration. Typically a portion of the non- permeated mixed liquor is coming from the treatment tank and another portion is flowing back into the treatment tank. This limits the increase of the mixed liquor suspended solids in the filtration tank.

[0007] Moreover, air diffused through the membrane modules functions to scour or clean the membrane filters within the respective modules. One of the challenges in designing a membrane bioreactor is providing a design where the air and mixed liquor that moves vertically through the filtration tank is generally uniformly distributed across the cross-sectional area of the membrane modules. Experience indicates that in many membrane bioreactors that air through the membrane modules is not uniform and that this impacts the cleaning or scouring efficiency of the air. When the air does not efficiently clean the membrane filters, this leads to increased down time for backwashing and cleaning the membrane filters. SUMMARY OF THE INVENTION

[0008] A method of treating wastewater where wastewater influent is directed into a treatment tank and treated. From the treatment tank the mixed liquor is directed to a filtration tank having at least one membrane module disposed therein. The filtration tank includes a surrounding wall structure, and the membrane module and the filtration tank are relatively sized such that

substantially all of the mixed liquor passing through the filtration tank is

constrained to move through or into the membrane module. A portion of the mixed liquor passing into the membrane module is filtered by an array of membrane filters that comprise the membrane module to form a permeate stream. The remaining portion of the mixed liquor passes from the membrane module and is recirculated back to the treatment tank. The spacing of the membrane module relative to the surrounding wall structure of the filtration tank prevents substantial recycling of mixed liquor within the filtration tank itself.

[0009] In one embodiment, the present invention comprises a membrane bioreactor having a treatment tank and a downstream filtration tank. Disposed in the downstream filtration tank are one or more membrane modules. The membrane modules and the filtration tank are sized such that the one or more membrane modules when disposed in the filtration tank occupy substantially the entire cross-sectional area of the filtration tank. This results in substantially the entire flow of mixed liquor passing from the treatment tank into the filtration tank being required to move vertically through the one or more membrane modules. That is because of the spacing relationship between the membrane modules and the filtration tank, there is little opportunity for the mixed liquor to bypass the one or more membrane modules. In one embodiment the one or more membrane moduies is spaced above the bottom of the filtration tank. Disposed in the bottom of the filtration tank is one or more mixed liquor conduits that extends throughout the filtration tank and includes an array of orifices formed in the outer wall of the one or more mixed liquor conduits. Mixed liquor is pumped from the treatment tank into the one or more mixed liquor conduits and the mixed liquor is disbursed from the conduits via the array of orifices. Also disposed in the filtration tank underneath the membrane modules are one or more air dispersing conduits. Compressed air is directed into the one or more air dispersing conduits and air is disbursed upwardly along with the mixed liquor through the one or more membrane modules.

[0010] Other objects and advantages of the present invention will become apparent and obvious from a study of the following description and the

accompanying drawings which are merely illustrative of such invention.

BRIEF DESCRIPTION OF THE DRAWINGS

10011] Figure 1 is a schematic illustration of a membrane bioreactor.

[0012] Figure 2 is a top plan view of the filtration tank and membrane module shown in Figure 1.

[0013] Figure 3 is an alternative membrane bioreactor where the filtration tank includes a plurality of stacked membrane modules.

[0014] Figure 4 is a simple schematic illustration showing an alternative membrane bioreactor having a series of filtration tanks.

[0015] Figure 5 is a schematic illustration showing another alternative membrane bioreactor having a multiplicity of membrane modules disposed in a filtration tank. [0016] Figure 6 is a schematic illustration of a hollow fiber type membrane module disposed in the filtration tank of the membrane bioreactor.

[0017] Figure 6A is a schematic illustration of a second embodiment for the membrane bioreactor.

[0018] Figure 7 is a schematic illustration of the air distribution system for the alternative design for the membrane bioreactor.

[0019] Figure 8 is a schematic illustration of the mixed liquor distribution system for the alternative design of the membrane bioreactor.

[0020] Figure 9 is a fragmentary schematic view showing portions of both the air distribution system and the mixed liquor distribution system of the alternative design for the membrane bioreactor.

[0021] Figure 10 is a fragmentary schematic view of the alterative design for the membrane bioreactor showing the filtration tank broken away to illustrate the orientation of the membrane modules disposed over the air distribution system and the mixed liquor distribution system.

[0022] Figure 1 1 is another fragmentary perspective view showing the filtration tank of the alternate design for the membrane bioreactor with the array of membrane modules disposed over the air distribution system and the mixed liquor distribution system in the filtration tank of the alternative design for the membrane bioreactor.

DESCRIPTION OF THE INVENTION

[0023] With further reference to the drawings, a wastewater treatment system is shown therein and indicated generally by the numeral 10. In the embodiment illustrated herein, the wastewater treatment system is a membrane bioreactor and is used to treat various types of water and wastewater. As used herein the term "wastewater" or "mixed liquor" includes water. As will be appreciated from subsequent portions of the disclosure, the wastewater treatment system disclosed herein is a membrane bioreactor which biologically treats wastewater. The biological treatment can include anaerobic, anoxic or aerobic treatment and may be directed at numerous types of treatment including, for example, nitrification- denitrification, removal of phosphorus or other nutrients, or BOD removal, etc.

[0024] Viewing wastewater treatment system 10 in more detail, the same includes a treatment tank 12. In the embodiment illustrated in Figure 1 , the wastewater treatment system includes a filtration tank indicated generally by the numeral 14, which is located downstream from the treatment tank 12. Disposed within the filtration tank 14 is one or more submerged membrane modules or cassettes indicated generally by the numeral 16. Generally, wastewater treated in the treatment tank 12 is directed in the form of mixed liquor from the treatment tank 12 to the filtration tank 14. In the filtration tank 14, the mixed liquor is filtered by the submerged membrane module 6 and the filtered mixed liquor results in a permeate that is removed from the membrane module 16 and from the filtration tank 14. In describing the bioreactor, reference is made in some cases to a membrane module 16. It is understood and appreciated that the filtration tank 14 may include one or more membrane modules. As discussed below, not all of the mixed liquor passing through the filtration tank 14 is filtered. This mixed liquor is sometimes referred to as non-permeated mixed liquor. The non-permeated mixed liquor is returned or recirculated back to the treatment tank 12 for further treatment.

[0025] Turning now to a discussion of the treatment tank 12, note in the drawings where there is provided a wastewater influent line 20 that is directed to the treatment tank 12. Wastewater is directed through influent line 20 into the tank for treatment. Typically, biological treatment utilizes air. Hence there is provided an air diffuser 22 disposed within treatment tank 12 and a blower 24 that is operative to generate a system of air that is diffused from air diffuser 22 into the wastewater contained in the treatment tank 12. Aeration is needed for some biological degradation, while it may be switched off for other biological processes.

[0026] Treatment tank 12 (shown in Figures 1-6) includes an outlet 26. In the case of the embodiment shown in Figure 1 , outlet 26 happens to be formed in a wall or in a pair of walls that separates treatment tank 12 from the filtration tank 14. In this case, outlet 26 is formed in the separating wall or walls such that mixed liquor is transferred from the treatment tank 12 into the lower portion of the filtration tank 14. A pump 28 or the air lift pump induced by operating diffuser 42 is provided to pump mixed liquor from the treatment tank 12 into the filtration tank 14. A valve 30 is provided downstream of the pump 28 and a feed line 29 interconnects the valve with the outlet 26. Thus, as seen in Figures 1 and 3, mixed liquor in tank 12 is pumped via pump 28 through valve 30 into feed line 29 that connects with outlet 26. This effectively transfers mixed liquor from the treatment tank 12 into the bottom portion of the filtration tank 14.

[0027] Turning to filtration tank 14, the filtration tank includes a surrounding wall structure 40. The shape and size of the filtration tank and the surrounding wall structure 40 can vary. In some embodiments the surrounding wall structure is square or rectangular in cross section. The wall structure can be built jointly together with treatment tank 12 or it can be a separate construction

[0028] In some embodiments, disposed in the lower portion of the filtration tank 14 is an air diffuser 42. In this case a blower 44 is operatively connected to the air diffuser 42 for generating a system of air and directing the air into and through the air diffuser 42. As those skilled in the art will appreciate, the air diffuser 42 may serve numerous functions. The air diffuser 42 may be utilized to disperse scouring air upwardly through the membrane module 16 for cleaning the individual membrane filters forming a part of the membrane module. In addition, the blower 44 along with the air diffuser 42 can assist in moving mixed liquor vertically through the filtration tank 14 and through the membrane module 16. The vertical movement of the non-permeated mixed liquor can be induced by aeration, by aeration plus pump 28 or by pump 28 alone without aeration.

[0029] Filtration tank 14 is also provided with a recirculating line 45. The non- permeated mixed liquor can flow by gravity and through line 45 back into treatment tank 12.

[0030] Filtration tank 14 may also be provided with a discharge line 46.

Connected to the discharge line 46 is a discharge pump 48. A valve (not shown) is typically disposed in the discharge line 46 between the filtration tank and the pump 48. This prevents the filtration tank from leaking. From time-to-time it may be advantageous to empty the filtration tank 14 in order to clean or perform

maintenance on the membrane module 16 contained therein. The discharge pump 48 along with the discharge line 46 facilitates the emptying of the filtration tank 14. [0031] As discussed above, one or more membrane modules 16 are mounted or disposed in the filtration tank 14. Each membrane module 16 basically comprises a frame structure that supports a plurality of individual membrane filters. The membrane filters are suspended and supported in the frame structure and are spaced such that the non-permeated wastewater passes by the membrane filter. The structure and type of individual membrane filters that form the membrane module 6 can vary. For example, the individual membrane filters may be of the p!ate or sheet type such as manufactured by Kubota Corporation, Toray, Microdyn- Nadir, A3 and others. Another type of membrane filter is the hollow fiber type such as manufactured by GE-Zenon, Koch-Puron, Mitsubishi-Hydronautics and others. The pore sizes of these individual membrane filters can vary. In some applications the pore size will range from about 0.01 to 0.4 microns and covers ultrafiltration and microfiltration. In addition, some of the membrane filters could be incorporated into a backwash system for washing and cleaning the filters periodically.

[0032] The term "membrane module" or "membrane cassette" used herein means a group or array of individual membrane filters that are grouped together or supported in a frame. In addition, it should be pointed out that the membrane module 16 as employed in the system and processes discussed is a submerged membrane module. This, of course, means that the membrane module 16 is submerged in mixed liquor in the filtration tank 14.

[0033] Each membrane module 16 is provided with a main permeate line or manifold 52. Permeate line 52 is operative!y connected to a network of pipes or tubes that ultimately are communicatively linked to the interior or permeate side of the individual membrane filters. The permeate is drawn either by gravity using a siphon effect or by pumping. A permeate pump 54 or a siphon is operatively connected to the permeate line 52 and is effective to create a vacuum in the individual permeate filters. This induces or draws mixed liquor through the walls of the individual membrane filters to produce the permeate. Thus, the permeate pump 54 or the siphon is effective to produce permeate from individual permeate filters of the membrane module 16 and ultimately to the permeate line or manifold 52, thereby removing the permeate from the filtration tank. In the embodiment illustrated in Figure 1 , there is provided one membrane module 16. Consequently, there is one permeate line 52 and one permeate pump 54. However, in Figure 3, there is shown four separate stacked membrane modules 16 disposed in the filtration tank 14. In this case, there is provided one main permeate line and one permeate pump for all membrane modules. As an option, in the Figure 3 embodiment, there could be provided four permeate lines and four pumps.

[0034] Filtration tank 14 and membrane module 16 are sized relative to each other. As seen in the drawings, particularly Figure 2, the membrane module 16 occupies substantially the entire cross sectional area of the filtration tank 14. That is, the surrounding wall 40 of the filtration tank 14 is spaced closely adjacent the frame of the membrane module 16. The objective is to size the filtration tank 14 such that it extends substantially entirely across the flow of the membrane module 16. That is, the filtration tank is sized such that when the membrane module 16 is placed within the filtration tank, that substantially the entire flow path of mixed liquor passing through the filtration tank will pass into and through the membrane module. During the filtration process, mixed liquor is pumped vertically through the membrane module 16. The spacing of the filtration tank 14 with respect to the membrane module 16 assures that substantially all of the mixed liquor must flow into and through the membrane module 16. The positioning of the membrane module 16 and the spacing of the filtration tank 14 is designed to minimize mixed liquor bypassing the membrane module 16 and the individual membrane filters contained therein.

[0035] Thus, the system and process prevents substantial downfiow of non- permeated mixed liquor through the filtration tank 14 and the membrane module 16 therein. That is, once the non-permeated mixed liquor moves through the membrane module 16, the system dictates that most, or substantially all, of the non- permeated mixed liquor be recirculated to the treatment tank 12 and not be permitted to flow back downwardly through the filtration tank 14 and membrane module 16 before being recirculated to the treatment tank. Thus, the downfiow of non-permeated mixed liquor is limited. In a preferred process, the downfiow of the non-permeated mixed liquor is 20% or less than the incoming mixed liquor flow to the filtration tank 14.

[0036] In one embodiment, the membrane module 16 includes hollow fiber membrane filters. These hollow fiber membrane filters are sometimes referred to as out-to-in membrane filters. This is because the mixed liquor that is filtered moves from an area outside of the hollow fiber membrane filters through a wall thereof and into an interior area within the hollow fibers. The portion of the mixed liquor that is filtered and ends up inside the hollow fiber is the permeate. The permeate pump 54 is effective to create a vacuum or a low pressure area in the various hollow fiber membrane filters. This induces a portion of the mixed liquor into the interior areas of the hollow fiber membrane filters.

[0037] Not all of the mixed liquor passing vertically through the membrane module 16 is filtered. Some of the mixed liquor exits the membrane module 16 without being filtered. Mixed liquor in the filtration tank 14 that has passed through the membrane module 6 without being filtered is referred to as non-permeate or non-permeated mixed liquor.

[0038] The non-permeated mixed liquor exiting the membrane module 16 is recirculated to the treatment tank 12. Various provisions can be made for recycling non-permeated mixed liquor to the treatment tank 12. For example, non- permeated mixed liquor in the filtration tank 14 may flow through an opening in the intervening wall separating the filtration tank from the treatment tank. Alternatively an opening may be provided in the intervening wall above the membrane module 16 such that non-permeated mixed liquor will flow from the filtration tank 14 back to the treatment tank 12. The amount of non-permeated mixed liquor recirculated to the treatment tank 12 can vary. However, in one embodiment, the ratio of non- permeated mixed liquor recirculated to the treatment tank to the permeate is approximately 2 - 100 to 1. That is, approximately 50 - 99 % of the mixed liquor pumped or moved through the filtration tank 14 is recirculated to the treatment tank 12 while approximately 1 - 50% of the mixed liquor passing through the filtration tank 14 is captured as permeate, in one design embodiment, the amount of non- permeated mixed liquor recycled from the filtration tank 14 to the treatment tank is approximately five to ten times the flow of influent wastewater into the treatment tank. The flow of influent wastewater into the treatment tank would generally, on average, be equal to the flow of permeate from the filtration tank 14. A minor portion of the mixed liquor is withdrawn as excess sludge.

[0039] The membrane modules are generally standard products. Therefore, in most cases the filtration tanks 12 are sized to accommodate the membrane modules in accordance with the system and process described above. [0040] in the embodiment illustrated in Figure 1 , the membrane module 16 is a single module or cassette. However, the system and process of the present invention may utilize a series of stacked membrane modules 16. This embodiment is illustrated in Figure 3. Here the membrane modules 16 are stacked one over the other. However, the same principles as discussed above apply with stacked membrane modules. That is, the stacked membrane modules 16 are sized with respect to the filtration tank 4 such that each occupies substantially the entire cross sectional area of the filtration tank 4. This assures that substantially all of the mixed liquor being pumped or moved vertically through the filtration tank 14 is constrained or required to move into or through the membrane modules 16. It follows that the non-permeated mixed liquor is constrained to move through each of the stacked membrane modules 16. The resulting permeate, on the other hand, can be found in any one of the stacked membrane modules 16. That is, some of the mixed liquor being filtered may end up as permeate prior to reaching one or more of the upper disposed membrane modules 16 of the stack.

[0041] In some embodiments, there may be provided a multiple number of single or stacked modules in the filtration tank 14. See Figures 3 and 5 for example. The sizing of the filtration tank follows the criteria mentioned before. In this case the feeding pipe 29 distributes the incoming mixed liquor evenly under the stack of modules while effluent line 45 is collecting the non-permeated mixed liquor and directing it back to treatment tank 12. See Figure 3. The permeate manifolds are connected to at least one permeate pipe 52 and a pump 54 or a siphon line. In the Figure 5 embodiment, it is seen that the filtration tank includes four separate membrane modules 16. The four separate membrane modules 16 together occupy substantially the entire cross-sectional area of the single filtration tank 14. This is compared to the embodiment shown in Figure 3 where there is provided four stacked membrane modules 16 that are disposed in a single filtration tank 14.

[0042] In some embodiments, there may be provided multiple filtration tanks 14. See Figure 4 for example. In this case there is provided three downstream filtration tanks 14 with each including one or more membrane module 16 or module stacks, mixed liquor in the treatment tank 12 is pumped or otherwise moved from the treatment tank via feed line 29 into the filtration tanks 14 and moved vertically through the one or more membrane modules 16 contained in each filtration tank. Each filtration tank 14 includes a return line 45 for recycling non-permeated mixed liquor back to the treatment tank 12. In designs such as shown in Figure 4, one filtration tank 14 can be completely shut down in order to clean or perform maintenance on the membrane module or modules contained therein without having to shut down the entire wastewater treatment system.

[0043] Figure 6 is a schematic illustration similar to Figure 1 but shows a certain type of membrane module. In the case of the Figure 6 embodiment, disposed within the filtration tank 14 is a hollow fiber type membrane module 16. The membrane module 16 includes a frame structure 60 and an array of generally vertically extending hollow fiber membrane filters 62. Note that the hollow fiber membrane filters 62 extend substantially across the entire cross-sectional area of the filtration tank 14. This means, of course, that as the mixed liquor moves upwardly in the filtration tank 14, that the mixed liquor will move adjacent to the hollow fiber membrane filters 62 and some of the mixed liquor will be induced into the interior area of the hollow fiber membrane filters 62 to produce the permeate.

[0044] With reference to Figures 6A-1 1 , an alternative embodiment for the membrane bioreactor is shown therein. Many of the principles discussed above with respect to the embodiments shown in Figures 1-6 apply to this alternative embodiment. The systems are similar and the processes are also similar. The basic difference between the designs shown in Figures 6A-1 l and those discussed above, is in the manner of transferring mixed liquor from the treatment tank 2 to the filtration tank 14. Further, because of how the mixed liquor is dispersed in the filtration tank and how the air is dispersed in the filtration tank, the system shown in Figures 6A-1 1 tends to cause the air and mixed liquor in the filtration tank to interact and this has the capability of more uniformly distributing the air through the membrane modules 16 contained within the filtration tank 14.

[0045] With reference to Figures 10 and 11 , the membrane modules 16 are shown contained within the filtration tank 12. In this particular embodiment, membrane modules 16 are stacked one over the other, and there is a plurality of membrane modules disposed transversely across the filtration tank 16. In the alternative embodiment the filtration tank 14 includes a dividing wall 14A that basically divides the filtration tank into two compartments. Partition wall 14A extends from the bottom of the filtration tank upwardly. The partition wall could be spaced off the bottom of the filtration tank 14 a selected distance. As discussed subsequently herein, there are structures that extend substantially the entire distance across the width of the filtration tank 14 which are located at the bottom of the filtration tank. In cases where the partition wall 14A extends to the bottom of the filtration tank 4, as shown herein, there are openings formed in the partition wall enabling such structures to pass therethrough. In any event, as illustrated in Figures 10 and 11 , the membrane modules 16 are spaced above the bottom of the filtration tank 14. This forms an open space 100 between the bottom of the filtration tank 14 and the bottom of the membrane modules 16. [0046] As seen in the drawings, the membrane modules 16 generally fill the filtration tank 14. In the embodiment illustrated herein, there are a series of membrane modules with one horizontal upper row being stacked on a lower row of modules. The individual membrane modules 16 can assume various forms. For example, the membrane module 16 may include hollow fiber membranes or plate type membranes. In the embodiment illustrated herein, each membrane module 16 includes an array of vertically oriented plate-type membranes. Each p!ate-type membrane is identified by the reference numeral 16A. A frame structure surrounds and supports the plurality of membrane plates 16A. The frame structure includes a series of side panels 16B that generally enclose the membrane module 6 around the sides. The bottom and top portions of each membrane module 16 is open. Each membrane plate 16A includes a pliable tube 16C that is communicatively connected to the interior of the plate membrane 16A for receiving and channeling permeate from the membrane plate. Each tube 16C is connected to a membrane manifold 16D that extends transversely across the module. Note in Figure 11 where the individual membrane manifolds 16D extend in parallel relationship. One end of the membrane manifolds 16D extends through a wall structure to connect to a main manifold 17. It is appreciated that all of the membrane modules include a membrane manifold 16D. Thus, the membrane modules 16 disposed about the lower horizontal row of membranes would include membrane modules 16D that are communicatively connected to membrane manifold 17. Main manifolds 16E are typically connected to a pump or vacuum source for inducing mixed liquor into the interior areas of the membrane plates 16A. [0047] As viewed in Figures 10 and 11 , mixed liquor delivered to the lower portion of the filtration tank 16 rises vertically upwardly through the membrane tank and through the individual membrane modules 16 and between the respective membrane plates 16A disposed in the respective modules. As the mixed liquor moves upwardly through the membrane modules 16, filtering takes place as the membrane plates 16A effectively filters the mixed liquor to produce a permeate within the interior of the respective membrane plates. This permeate within the plates 6A is pumped from the interior of the plates through the tubes 16C to the individual membrane module manifolds 16D.

[0048] Extending transversely across the bottom of the filtration tank 14 is a pair of mixed liquor conduits 102. As shown in Figures 8 and 9, mixed liquor conduits 102 are disposed in close proximity to the bottom of the filtration tank 14 and extend transversely through the bottom of the filtration tank. An array of orifices 104 is formed in the outer wall of each mixed liquor conduit 102. The location, size and number of orifices 104 can vary. In one embodiment it is anticipated that there would be an array of orifices 104 formed about the upper portion of each mixed liquor conduit 102. A pump 106, such as a submersible pump is disposed in the treatment tank 12. Pump 106 is operatively connected to a main feed 108. Main feed 108 extends towards the filtration tank 14 and branches into branch 108A and 108B. Both branches 108A and 108B are operatively connected to both mixed liquor conduits 102. In the embodiment illustrated herein, the connection point in each case is approximately halfway between the end of each mixed liquor conduit 102 and a mid point of the mixed liquor conduit. It is appreciated that supply pipes or conduits for supplying mixed liquor to the mixed liquor conduits 102 can be connected at various locations. Sizes of the orifices 104 can vary. The object of a preferred design is to distribute the mixed liquor generally uniformly throughout the filtration tank 14 and to do so in a manner that would provide a generally uniform mixed liquor upward velocity profile across the filtration tank 14. For example, in the embodiment shown in Figure 8, the orifices 104 that are located in close proximity to where the branches 108A and 108B connect might be of a smaller size than orifices spaced outwardly from either side thereof. This may have the effect of causing a generally uniform flow of mixed liquor to occur along the length of the mixed liquor conduits 102.

[0049] Membrane bioreactor 10 shown in Figures 6A-11 also include air dispersing conduits {sometimes referred to as air conduits) disposed in the filtration tank 14. These air conduits, like the mixed liquor conduits 102, are disposed in the bottom portion of the filtration tank 14. More particularly, as shown in Figure 10, the air dispersing conduits are disposed between the mixed liquor conduits 102 and the bottom of the membrane modules 16.

[0050] In the particular embodiment shown in Figures 6A-1 1 , the air conduits includes two sets of air conduits, a first set 1 10 and a second set 1 12. Each set of conduits includes a series of conduits or pipes that extend in parallel relationship transversely across the bottom portion of the filtration tank 14. Openings or orifices are formed in each of the air conduits for dispersing air and directing the air upwardly through the filtration tank 14.

[0051] Air is supplied to the air dispersing conduits via a compressor 114.

Compressor 1 4 is operatively connected to a main feed 116 that branches into two branches, 1 16A and 1 16B. Branch 116B is connected to the air conduits of the first set 1 0 while branch 1 16A is operatively connected to the air conduits of the second set 1 12. [0052] It is believed that alternating the air supplied to the two sets of air dispersing conduits 1 10 and 112 may have advantages. Therefore, each branch 1 16A and 1 6B includes an on/off control valve 118 or 120, Valves 118 and 120 can be connected to a controller or a mechanical actuating device for controlling the on/off status of each valve. In one design regime, it is possible to have one valve "on" for a time period of, for example, one to ten seconds, while the other valve is "off. Then during a succeeding like time period the one valve is closed and the other valve is open. Thus, at any one time there would only be air dispersed from one set of air conduits 110 or 112.

[0053] Other mechanical means can be employed to shift the valves 118, 20 between "on" and "off" positions and to alternate the "on" and "off" positions of the two valves such that in this particular control regime the two valves would not be "on" or "off" at the same time, it is appreciated by those skilled in the art that various other control devices and schemes can be employed. While alternating the air supply to the first and second sets of air conduits may have certain advantages, it is appreciated that in some embodiments this control regime might not be followed. There may be cases when it is desirable to supply air from both sets of conduits 1 10, 112 at the same time. It is appreciated by those skilled in the art that various other control devices and schemes can be employed.

[0054] The filtration tank 14 also includes various supporting structures for supporting the membrane modules 16, mixed liquor conduits 102 and the air conduits. Various types and forms of frame structures can be provided. As discussed above, in the embodiment shown in Figures 6A-11 , the frame structure provided would be designed to support the air conduits in the open space 100 generally between the bottom of the membrane modules 16 and above the mixed liquor conduits 102.

[0055] As seen in Figures 10 and 1 1 , the treatment tank 12 is separated from the filtration tank 14 by a wall. An openable gate 122 is disposed in the lower portion of the separating wall. This gate can be designed to not interfere with the branches 108A, 108B, 1 16A, and 116B, Gate 122 can be moved back and forth between open and close positions. There may be occasions when it is desirable to open the gate 122 to enable mixed liquor to flow therethrough.

[0056] As discussed previously with respect to the embodiments disclosed in Figures 1-6, it is desirable to maintain the downcomer flow through the filtration tank 14 at a minimum level. That is, once the non-permeated stream exits the upper portion of the top most disposed membrane modules 16, it is desirable for that non-permeate stream to be recycled directly back to the treatment tank 12 without flowing downwardly through the filtration tank. To assure that downcomer flow is minimized, the present invention appreciates that a seal 124 can be extended and positioned around certain membrane modules 16. Note in Figures 10-1 1 , that seal 124 extends around a top portion of upper disposed membrane modules 16. The seal 124 is contained between the framework of the membrane modules 16 and the filtration tank wall structure. This tends to seal the outer edges around the membrane modules 16 and generally prevents the flow of mixed liquor back downwardly through the filtration tank. In cases where there are membrane modules 16 disposed in side-by-side relationship, it may be beneficial to provide a seal 124 that extends between adjacent membrane modules so as to prevent downcomer flow from moving downward between respective membrane modules. [0057] One of the advantages of orienting the mixed liquor conduits 102 and the air dispersing conduits in the manner shown in Figures 6A-11 is that the mixed liquor dispersed from the mixed liquor conduits 102 and its upward movement through the membrane modules 16 may facilitate and help maintain a generally uniform air pattern in the filtration tank 14 across the cross-sectional area of the filtration tank. Initial studies suggest that in some cases the air in a membrane filtration tank tends to result in the air having a tendency to move inwardly from the sides of the filtration tank and upwardly through the filtration tank. In some cases the outer areas of the membrane modules 16 are not exposed to the same quantity of air as the more central areas of the membrane modules. It is postulated that by providing the mixed liquor conduits 102 and strategically positioning them underneath the air dispersing conduits that the mixed liquor will have a tendency to cause the air that is moving upwardly with the mixed liquor to be more uniformly distributed across the entire cross-section of the filtration tank 14.

[0058] The present invention may, of course, be carried out in other specific ways than those herein set forth without departing from the scope and the essential characteristics of the invention. The present embodiments are therefore to be construed in all aspects as illustrative and not restrictive and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.

Claims

1 . A membrane bioreactor wastewater treatment system for treating wastewater, comprising:

a. a wastewater treatment tank for receiving wastewater influent and biologically treating the wastewater in the wastewater treatment tank to form mixed liquor;

b. a filtration tank disposed downstream from the wastewater treatment tank for receiving mixed liquor treated in the wastewater treatment tank;

c. one or more membrane filtration modules disposed in the filtration tank and adapted to be submerged in the mixed liquor contained therein, and wherein the one or more membrane filtration modules filter at least some of the mixed liquor in the filtration tank to produce a permeate;

d. each membrane module including an array of membrane filters

adapted to be submerged in the mixed liquor within the filtration tank;

e. the filtration tank and the one or more membrane modules sized relative to each other such that the one or more membrane modules occupies substantially the entire cross-sectional area of the filtration tank such that substantially all the mixed liquor passing through the filtration tank is constrained to move through the one and more membrane filtration modules; one or more permeate lines operatively connected to the one or more membrane modules in the filtration tank for directing permeate from the filtration tank;

a recycle outlet for recycling mixed liquor from the filtration tank to the treatment tank;

wherein the membrane bioreactor wastewater treatment system is adapted to recycle back to the treatment tank at least 50% of the mixed liquor directed from the treatment tank to the filtration tank; one or more mixed liquor conduits extending through a portion of the filtration tank, each mixed liquor conduit including an outer wall and disposed below the one or more of the membrane modules and including a series of orifices formed in the outer wall through which mixed liquor is dispersed into the filtration tank;

one or more mixed liquor transfer conduits for directing mixed liquor from the treatment tank to the one or more mixed liquor conduits; one or more pumps for pumping mixed liquor from the treatment tank, through the one or more mixed liquor transfer conduits into the one or more mixed liquor conduits;

one or more air conduits for dispensing air into the filtration tanks and extending through the filtration tank and disposed below the one or more membrane filtration modules;

each air conduit having an array of orifices formed in an outer wall thereof for dispersing air into the filtration tank; and

a source of compressed air for supplying air to the air conduits.

2. The membrane bioreactor of claim 1 wherein the filtration tank includes a bottom and wherein the one or more membrane modules are elevated relative to the bottom of the filtration tank so as to define an open space between the one or more membrane modules and the bottom of the filtration tank; and wherein both the one or more mixed liquor conduits and the one or more air conduits are disposed in the open space below the one or more membrane modules and wherein the one or more air conduits are disposed above the one or more mixed liquor conduits.

3. The membrane bioreactor of claim 2 wherein the filtration tank is elongated and includes a width and a length and wherein the width is greater than the length.

4. The membrane bioreactor of claim 1 wherein the one or more air conduits includes first and second sets of air conduits, and wherein air is provided to the first and second sets of air conduits in alternating fashion such that in one period of time air is supplied to the first set of air conduits and not the second set of air conduits and in another period of time there is supplied to the second set of air conduits and not the first set of air conduits.

5. The membrane bioreactor of claim 1 including a seal surrounding the one or more membrane modules and disposed generally between the filtration tank and the one or more membrane modules.

6. The membrane bioreactor of claim 1 wherein the mixed liquor conduits includes two spaced apart mixed liquor conduits, and wherein mixed liquor is fed into each mixed liquor conduit at two spaced apart locations.

7. The membrane bioreactor of claim 1 wherein the series of orifices formed in the outer wall of the one or more mixed liquor conduits vary in size along the length of the respective mixed liquor conduits.

8. The membrane bioreactor of claim 1 wherein the one or more mixed liquor conduits is disposed below the one or more air conduits in the filtration tank; and wherein the mixed liquor conduits and the air conduits are oriented with respect to each other such that the mixed liquor flowing upwardly from the one or more mixed liquor conduits tends to uniformly distribute air moving upwardly from the one or more air conduits across the cross-sectional area of the filtration tank.

9. A method of biologically treating wastewater in a membrane bioreactor, comprising:

a. directing an influent stream of wastewater into a biological treatment tank and biologically treating the wastewater and forming mixed liquor in the treatment tank;

b. transferring the mixed liquor from the treatment tank to a

downstream filtration tank;

c. submerging one or more membrane modules in mixed liquor

contained within the filtration tank;

d. positioning the one or more membrane modules in the filtration tank such that the membrane modules occupy substantially the entirety of the cross-sectional area of the filtration tank;

e. transferring the mixed liquor from the treatment tank to the filtration tank including pumping mixed liquor from the treatment tank into one or more mixed liquor conduits extending through the filtration tank underneath the one or more membrane modules; f. directing mixed liquor from the one or more mixed liquor conduits through orifices formed in an outer wall of the one or more mixed liquor conduits;

g. after passing through the orifices in the mixed liquor conduits, the mixed liquor moves vertically through the filtration tank and through the one or more membrane modules in the filtration tank;

h. constraining the flow of vertically moving mixed liquor in the filtration tank such that substantially the entire flow of vertically moving mixed liquor in the filtration tank is constrained to move through the one or more membrane modules;

i. as the mixed liquor moves vertically through the one or more

membrane modules, passing a first portion of the mixed liquor through individual membrane filters forming a part of the one or more membrane modules to produce a permeate;

j. directing the permeate from the individual membrane filters and from the filtration tank;

k. recycling a second portion of the mixed liquor from the filtration tank to the treatment tank after the second portion of the mixed liquor has passed through the one or more membrane modules;

I. wherein at least 50% of the mixed liquor passing into the filtration tank is recycled to the treatment tank; and

m. dispersing air from openings in one or more air conduits extending through the filtration tank underneath the membrane modules such that both the air and mixed liquor move upwardly through the membrane modules.

10. The method of claim 9 wherein the one or more mixed liquor conduits is disposed below the one or more air conduits, and both the one or more mixed liquor conduits and the one or more air conduits are disposed below the membrane modules.

1 1. The method of claim 10 including utilizing the upwardly flowing mixed liquor in the filtration tank to generally uniformly distribute the upwardly moving air.

12. The method of ciaim 10 including utilizing the upwardly flowing mixed liquor in the filtration tank to generally uniformly distribute the upwardly moving air in the filtration tank such that the air is generally uniformly distributed across

substantially the entire cross-sectional area of the one or more membrane modules.

13. The method of claim 9 wherein the one or more air conduits includes first and second sets of air conduits; and wherein the one or more mixed liquor conduits includes at least two spaced apart conduits extending through the filtration tank with each mixed liquor conduit having an array of the orifices formed therein to permit mixed liquor to be dispersed from the mixed liquor conduits throughout the filtration tank.

14. The method of claim 3 including directing air to the first and second sets of air conduits in an alternating pattern such that during one period of time air is dispersed by the first set of air conduits and not the second set of air conduits and in another period of time air is dispersed by the second set of air conduits and not the first set of air conduits.

15. The method of claim 9 including placing a seal around the one or more membrane modules such that the seal lies between the one or more membrane modules and the filtration tank.

16. The method of claim 9 wherein both the treatment tank and the filtration tank include a tength and a width, and wherein the length of the filtration tank is less than the length of the treatment tank; and wherein the length of the filtration tank is substantially less than the width of the filtration tank.

17. The method of claim 9 including stacking a series of membrane modules one over the other in the filtration tank such that the stacked membrane modules occupies substantially the entire cross-sectional area of the filtration tank.

18. The method of claim 9 wherein each of the one or more membrane modules includes an array of out-to-in membrane filters, and wherein the mixed liquor enters the membrane filters from an outside area and passes into an interior area of the membrane filters as the permeate.

9. The method of claim 9 wherein the mixed liquor passing through the one or more membrane modules but not filtered is referred to as a non-permeate stream; and wherein the method includes restricting the movement of the non-permeate stream so as to substantially preclude the recirculation of the non-permeate stream back through the one or more membrane modules prior to the non- permeate stream being recirculated back to the treatment tank.