WO2020005858A1

WO2020005858A1 - Anti-fouling and self-cleaning electrically conductive low-pressure membranes submerged in reactors for water treatment

Info

Publication number: WO2020005858A1
Application number: PCT/US2019/038775
Authority: WO
Inventors: David Jassby
Original assignee: The Regents Of The University Of California
Priority date: 2018-06-25
Filing date: 2019-06-24
Publication date: 2020-01-02

Abstract

A membrane filtration system includes a frame, an electrically conductive membrane having a permeate side affixed to the frame, and a vacuum pump connected to the frame to apply a negative pressure to the permeate side of the electrically conductive membrane.

Description

ANTI-FOULING AND SELF-CLEANING ELECTRICALLY CONDUCTIVE LOW-PRESSURE MEMBRANES SUBMERGED IN REACTORS FOR

WATER TREATMENT

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 62/689,699, filed June 25, 2018, the contents of which are incorporated herein by reference in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR

DEVELOPMENT

[0002] This invention was made with government support under grant number 2017-67022-26135, awarded by the U.S. Department of Agriculture, and grant number FA8903-13-C-0009, awarded by the U.S. Air Force, Air Force Office of Scientific Research. The government has certain rights in the invention.

TECHNICAL FIELD

[0003] This disclosure generally relates to a membrane filtration system for treatment of wastewater.

BACKGROUND

[0004] Anaerobic bioreactors are an attractive treatment option for wastewater. Comparative membrane bioreactor technology relies on the use of passive polymeric membranes immersed in bioreactors. Fouling prevention is one of the most challenging aspects of waste water treatment with membranes. During filtration, suspended and dissolved materials in water deposit on a membrane surface, which leads to decreased process performance, increased energy demand, and reduced membrane lifetime. In aerobic membrane bioreactors with submerged membranes, intensive air scouring is continuously applied to the membranes to mitigate against membrane fouling; this intensive air scouring accounts for about 45% of operating costs of such membrane bioreactors. Furthermore, because of the intensive air scouring, immersing the membranes in anaerobic reactors is inadvisable, as the added oxygen would impede an anaerobic biological process. [0005] It is against this background that a need arose to develop the embodiments described herein.

SUMMARY

[0006] In some embodiments, a membrane filtration system includes a frame, an electrically conductive membrane having a permeate side affixed to the frame, and a vacuum pump connected to the frame to apply a negative pressure to the permeate side of the electrically conductive membrane.

[0007] In additional embodiments, a method of treating wastewater includes directing influent wastewater into a reactor in which an electrically conductive membrane is immersed, and filtering the wastewater by directing the wastewater through the electrically conductive membrane, wherein the filtering is performed while applying a negative pressure to a permeate side of the electrically conductive membrane, and while applying an electrical potential to the electrically conductive membrane.

[0008] Other aspects and embodiments of this disclosure are also contemplated. The foregoing summary and the following detailed description are not meant to restrict this disclosure to any particular embodiment but are merely meant to describe some embodiments of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] For a better understanding of the nature and objects of some embodiments of this disclosure, reference should be made to the following detailed description taken in conjunction with the accompanying drawings.

[0010] Figure 1 is a schematic of an electrically conductive membrane of some embodiments.

[0011] Figure 2 is a schematic of a membrane filtration system including a set of electrically conductive membranes of some embodiments.

[0012] Figure 3 is a schematic of a frame included in a membrane filtration system of some embodiments.

DETAILED DESCRIPTION

[0013] Embodiments of this disclosure are directed to a membrane filtration system for treatment of wastewater. In some embodiments, the membrane filtration system includes a reactor and a set of one or more electrically conductive membranes immersed in the reactor. Particular advantages of the membrane filtration system include the electrically conductive membranes’ anti-fouling and self-cleaning properties. Because of these properties, the electrically conductive membranes may omit or may allow reduced occurrence of physical or chemical cleaning. For example, use of other immersed membranes involves continuous air scouring and periodic chemical cleaning. These processes can dramatically increase the energy demand and cause operational disruptions. By applying an external electrical potential to surfaces of the electrically conductive membranes, electrochemical reactions (e.g., peroxide generation, hydroxyl radical generation, chlorine generation, acid/base production, direct oxidation/reduction of aqueous species, gas generation, and so forth) and electrostatic repulsive forces are generated at a membrane/water interface, which can dramatically reduce or eliminate membrane fouling.

[0014] In some embodiments as shown in Figure 1, an electrically conductive membrane 100 includes a porous support 102 that is coated with a percolating network of nanostructures 104 (e.g., carbon nanotubes (CNTs)) and cross-linked with a polymer 106 to form a robust, porous, and electrically conductive coating or layer 108 on the porous support 102. The porous support 102 can be polymeric (such as formed of, or including, poly(sulfone), poly(ether sulfone), poly(vinylidene fluoride), poly(tetrafluoroethylene), poly(propylene), poly( acrylonitrile), another polymer, or a combination of two or more thereof) or inorganic (such as formed of, or including, alumina, zirconia, stainless steel, nickel, another ceramic, another metal, another metal alloy, or a combination of two or more thereof). For example, the porous support 102 can be a filtration membrane, such as an ultrafiltration polymeric membrane having a pore size in a range of about 2 nm to about 100 nm or a microfiltration polymeric membrane having a pore size in a range of greater than about 100 nm and up to about 10 pm.

[0015] In the electrically conductive membrane 100 of some embodiments, the nanostructures 104 provide electrical conductivity, while the cross-linking polymer 106 provides a matrix that links the nanostructures 104 and affixes the nanostructures 104 to the porous support 102, as well as being used to control a pore size between the nanostructures 104. The nanostructures 104 can be formed of, or can include, an electrically conductive material, such as carbon, a metal, a metal alloy, a metal oxide, or a combination of two or more thereof. In some embodiments, at least a subset of the nanostructures 104 corresponds to high aspect ratio nanostructures, such as nanotubes, nanowires, or a combination of nano tubes and nano wires. High aspect ratio nanostructures can increase the occurrence of junction formation between neighboring nanostructures, and can form an efficient charge transport network. For example, the nanostructures 104 can be CNTs, such as single-walled CNTs, multi-walled CNTs, or a combination thereof. It is also contemplated that nanoparticles can be used in combination with, or in place of, high aspect ratio nanostructures. In some embodiments, the nanostructures 104 are functionalized with, for example, carboxyl groups (-COOH), hydroxyl groups (-OH), amine groups (e.g., -NH₂), or other functional groups to allow cross-linking with the polymer 106. Likewise, the polymer 106 can include functional groups to allow cross-linking, such as carboxyl groups, hydroxyl groups, amine groups, or other functional groups. Examples of the polymer 106 include poly(vinyl alcohol), poly(aniline), and polysiloxanes (e.g., poly(dimethylsiloxane)). Upon cross-linking of the nanostructures 104 and the polymer 106, the resulting electrically conductive layer 108 on the porous support 102 can have a pore size that is about the same as or larger than the pore size of the porous support 102. A thickness of the electrically conductive layer 108 can be in a range of about 100 nm or greater, about 200 nm or greater, about 300 nm or greater, about 400 nm or greater, or about 500 nm or greater, and up to about 1 pm or greater, up to about 5 pm or greater, or up to about 10 pm or greater. An electrical conductivity of the layer 108 can be about 500 S/m or greater, about 800 S/m or greater, or about 1000 S/m or greater, and up to about 1500 S/m or greater, up to about 2000 S/m or greater, or up to about 2500 S/m or greater.

[0016] To form the electrically conductive membrane 100 of some embodiments, the nanostructures 104 are first coated on the porous support 102. To achieve this, the nanostructures 104 are processed into a stable suspension, also referred to as an ink composition. This process involves mixing the functionalized nanostructures 104 (e.g., in the form of a powder) with a surfactant in water, followed by agitation (e.g., sonication with a horn sonicator). A resulting mixture is then centrifuged (e.g., three times) to remove amorphous carbon and residual nanostructure bundles. A resulting ink composition can be stable for multiple months with no visible aggregation or sedimentation. The ink composition is then deposited on the porous support 102 using spray-coating or another deposition method. In the case of spray- coating, the ink composition is sprayed onto the porous support 102 together with the cross-linking polymer 106. The amount of the nanostructures 104 sprayed onto the porous support 102 can determine a thickness and an electrically conductivity of the resulting electrically conductive layer 108, while the amount of the cross- linking polymer 106 sprayed onto the porous support 102 can determine the pore size of the layer 108. Once the nanostructures 104 and the cross-linking polymer 106 are deposited on the porous support 102, the deposited material is immersed into, or otherwise exposed to, a cross-linking solution (e.g., including a cross-linker, such as glutaraldehyde, together with any catalyst, such as hydrochloric acid). The resulting membrane 100 is washed with water and dried in an oven. The membrane 100 can be stored dry at room temperature.

[0017] Figure 2 is a schematic of a membrane filtration system 200 including a set of electrically conductive membranes 202 of some embodiments. As shown, the membrane filtration system 200 is a vacuum-assisted membrane filtration system, where the electrically conductive membranes 202 are affixed to a set of frames 204 and immersed or submerged in a reactor 206 (e.g., in the form of a tank) containing contaminated water, such as a bioreactor containing industrial wastewater, river water, or groundwater, along with microorganisms to affect an anaerobic biological process. A negative pressure (e.g., low vacuum in a pressure range of about 25 Torr to about 760 Torr, medium vacuum in a pressure range of about 10³ Torr to about 25 Torr, or another reduced pressure relative to a pressure in the reactor 206) is applied, through a vacuum pump 208, to a backside or a permeate side of each membrane 202, which pulls the water through the membrane 202, thereby allowing treatment of the water.

[0018] As shown in Figure 3, each frame 204 of some embodiments is a rigid polymeric frame including a porous plate 210 to which an electrically conductive membrane 202 is laminated, and an enclosure 212 affixed to, or integrally formed with, the porous plate 210 and which serves as a conduit for application of the negative pressure and to permit flow of treated water through the membrane 202 to a permeate line or conduit connected to the frame 204. Multiple such frames 204 can be included in the membrane filtration system 200, thus increasing an available membrane surface area. Also, double-sided frames also can be included, in which a pair of electrically conductive membranes are laminated to opposite sides of each frame.

[0019] During the treatment of water, the electrical conductivity of the membranes 202 allows for the application of an electrical potential to surfaces of the membranes 202, which allows the membranes 202 to be self-cleaning and anti-fouling. Referring back to Figure 2, each membrane 202 is connected to an electrical power source 214. To achieve this connection, an electrical lead, such as in the form of a thin metallic strip or wire, is included adjacent to an edge of each membrane 202 (and above a corresponding frame 204), and is sealed in place under a polymeric strip or another sealant. In this way, the electrical lead comes in contact with a nanostructure-coated membrane surface, but not with surrounding water. Electrical leads of the membranes 202 are connected to the electrical power source 214, which can be an adjustable power source that provides an electrical potential across the surfaces of the membranes 202. The electrical potential can be, for example, a negative potential. To complete an electrical circuit, a counter electrode 216 is provided adjacent to a surface of each membrane 202, and is connected to the same electrical power source 214.

[0020] As shown in Figure 2, an air scouring unit 218 is optionally included to provide intermittent aeration to further mitigate against membrane fouling. The air scouring unit 218 includes an air blower immersed in the reactor 206 to direct a flow of air. Rather than a continuous operation, operation of the air scouring unit 218 can be activated in an intermittent manner (e.g., periodically every about 20 minutes, about 30 minutes, or about 50 minutes, or another periodic or non-periodic manner), thereby reducing energy demand. Control of the air scouring unit 218 (as well as other components of the membrane filtration system 200) can be achieved via a controller 220, such as including a processor and an associated memory storing processor-executable instructions.

[0021] Advantageously, embodiments of this disclosure address the demand for suitable membranes for anaerobic membrane bioreactors that can be used treat contaminated water, including municipal and industrial wastewater. Specifically, other membranes face challenges because of extensive membrane fouling experienced during the treatment of such contaminated water. The anti-fouling and self-cleaning properties of electrically conductive membranes of some embodiments allow the membranes to be deployed in processes not otherwise feasible. This has the potential of dramatically reducing the cost of treatment of heavily contaminated wastewater, and, in addition, open up additional treatment options.

Example Embodiments

[0022] First aspect

[0023] In some embodiments, a membrane filtration system includes a frame, an electrically conductive membrane having a permeate side affixed to the frame, and a vacuum pump connected (e.g., fluidly connected) to the frame to apply a negative pressure to the permeate side of the electrically conductive membrane. [0024] In some embodiments, the electrically conductive membrane includes a porous support and an electrically conductive layer disposed on the porous support, and the electrically conductive layer includes nanostructures.

[0025] In some embodiments, the nanostructures are electrically conductive.

[0026] In some embodiments, the nanostructures form a percolating network.

[0027] In some embodiments, the nanostructures include nanotubes, nanowires, or both.

[0028] In some embodiments, the nanostructures include carbon nanotubes.

[0029] In some embodiments, the electrically conductive layer further includes a polymer.

[0030] In some embodiments, the polymer is cross-linked with the nanostructures.

[0031] In some embodiments, the electrically conductive layer is porous. In some embodiments, the electrically conductive layer has a pore size that is about the same as or larger than a pore size of the porous support.

[0032] In some embodiments, the porous support is a filtration membrane.

[0033] In some embodiments, the membrane filtration system further includes a counter electrode disposed adjacent to the electrically conductive membrane and an electrical power source, and the electrical power source is connected to the counter electrode and the electrically conductive membrane.

[0034] In some embodiments, the membrane filtration system further includes a reactor, and the frame and the electrically conductive membrane are disposed in the reactor.

[0035] In some embodiments, the membrane filtration system further includes an air blower disposed in the reactor.

[0036] In some embodiments, the frame includes a porous plate to which the electrically conductive membrane is laminated.

[0037] Second aspect

[0038] In some embodiments, a method of treating wastewater includes directing influent wastewater into a reactor in which an electrically conductive membrane is immersed, and filtering the wastewater by directing the wastewater through the electrically conductive membrane, wherein the filtering is performed while applying a negative pressure to a permeate side of the electrically conductive membrane, and while applying an electrical potential to the electrically conductive membrane. [0039] In some embodiments, the electrically conductive membrane includes a porous support and an electrically conductive layer disposed on the porous support, and the electrically conductive layer includes nanostructures.

[0040] In some embodiments, the nanostructures include nanotubes, nanowires, or both.

[0041] In some embodiments, the electrically conductive layer further includes a polymer that is cross-linked with the nanostructures.

[0042] In some embodiments, the method further includes providing intermittent aeration using an air blower disposed in the reactor.

[0043] As used herein, the singular terms“a,”“an,” and“the” may include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to an object may include multiple objects unless the context clearly dictates otherwise.

[0044] As used herein, the term“set” refers to a collection of one or more objects. Thus, for example, a set of objects can include a single object or multiple objects. Objects of a set also can be referred to as members of the set. Objects of a set can be the same or different. In some instances, objects of a set can share one or more common characteristics.

[0045] As used herein, the terms“connect,”“connected,” and“connection” refer to an operational coupling or linking. Connected objects can be directly coupled to one another or can be indirectly coupled to one another, such as via one or more other objects.

[0046] As used herein, the terms“substantially” and“about” are used to describe and account for small variations. When used in conjunction with an event or circumstance, the terms can refer to instances in which the event or circumstance occurs precisely as well as instances in which the event or circumstance occurs to a close approximation. For example, when used in conjunction with a numerical value, the terms can refer to a range of variation of less than or equal to ±10% of that numerical value, such as less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to ±1%, less than or equal to ±0.5%, less than or equal to ±0.1%, or less than or equal to ±0.05%. For example, a first numerical value can be“substantially” or“about” the same as a second numerical value if the first numerical value is within a range of variation of less than or equal to ±10% of the second numerical value, such as less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to ±1%, less than or equal to ±0.5%, less than or equal to ±0.1%, or less than or equal to ±0.05%.

[0047] In the description of some embodiments, a component provided or disposed “on” or“over” another component can encompass cases where the former component is directly on (e.g., in physical or direct contact with) the latter component, as well as cases where one or more intervening components are located between the former component and the latter component.

[0048] As used herein, the term“nanometer range” or“nm range” refers to a range of dimensions from about 1 nm to about 1 pm. The nm range includes the“lower nm range,” which refers to a range of dimensions from about 1 nm to about 10 nm, the“middle nm range,” which refers to a range of dimensions from about 10 nm to about 100 nm, and the “upper nm range,” which refers to a range of dimensions from about 100 nm to about 1 pm.

[0049] As used herein, the term“micrometer range” or“pm range” refers to a range of dimensions from about 1 pm to about 1 mm. The pm range includes the“lower pm range,” which refers to a range of dimensions from about 1 pm to about 10 pm, the“middle pm range,” which refers to a range of dimensions from about 10 pm to about 100 pm, and the “upper pm range,” which refers to a range of dimensions from about 100 pm to about 1 mm.

[0050] As used herein, the term“nanostructure” refers to an object that has at least one dimension in the nm range. A nanostructure can have any of a wide variety of shapes, and can be formed of a wide variety of materials. Examples of nanostructures include nanowires, nanotubes, and nanoparticles.

[0051] As used herein, the term“nanowire” refers to an elongated nanostructure that is substantially solid. Typically, a nanowire has a lateral dimension (e.g., a cross- sectional dimension in the form of a width, a diameter, or a width or diameter that represents an average across orthogonal directions) in the nm range, a longitudinal dimension (e.g., a length) in the pm range, and an aspect ratio that is about 5 or greater.

[0052] As used herein, the term “nanotube” refers to an elongated, hollow nanostructure. Typically, a nanotube has a lateral dimension (e.g., a cross-sectional dimension in the form of a width, an outer diameter, or a width or outer diameter that represents an average across orthogonal directions) in the nm range, a longitudinal dimension (e.g., a length) in the pm range, and an aspect ratio that is about 5 or greater.

[0053] As used herein, the term“nanoparticle” refers to a spheroidal nanostructure. Typically, each dimension (e.g., a cross-sectional dimension in the form of a width, a diameter, or a width or diameter that represents an average across orthogonal directions) of a nanoparticle is in the nm range, and the nanoparticle has an aspect ratio that is less than about 5, such as about 1.

[0054] Additionally, concentrations, amounts, ratios, and other numerical values are sometimes presented herein in a range format. It is to be understood that such range format is used for convenience and brevity and should be understood flexibly to include numerical values explicitly specified as limits of a range, but also to include all individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly specified. For example, a range of about 1 to about 200 should be understood to include the explicitly recited limits of about 1 and about 200, but also to include individual values such as about 2, about 3, and about 4, and sub-ranges such as about 10 to about 50, about 20 to about 100, and so forth.

[0055] While the disclosure has been described with reference to the specific embodiments thereof, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the true spirit and scope of the disclosure as defined by the appended claims. In addition, many modifications may be made to adapt a particular situation, material, composition of matter, method, operation or operations, to the objective, spirit and scope of the disclosure. All such modifications are intended to be within the scope of the claims appended hereto. In particular, while certain methods may have been described with reference to particular operations performed in a particular order, it will be understood that these operations may be combined, sub-divided, or re-ordered to form an equivalent method without departing from the teachings of the disclosure. Accordingly, unless specifically indicated herein, the order and grouping of the operations are not a limitation of the disclosure.

Claims

What is claimed is:

1. A membrane filtration system comprising:

a frame;

an electrically conductive membrane having a permeate side affixed to the frame; and a vacuum pump connected to the frame to apply a negative pressure to the permeate side of the electrically conductive membrane.

2. The membrane filtration system of claim 1, wherein the electrically conductive membrane includes a porous support and an electrically conductive layer disposed on the porous support, and the electrically conductive layer includes nanostructures.

3. The membrane filtration system of claim 2, wherein the nanostructures are electrically conductive.

4. The membrane filtration system of claim 2, wherein the nanostructures form a percolating network.

5. The membrane filtration system of claim 2, wherein the nanostructures include nanotubes, nanowires, or both.

6. The membrane filtration system of claim 2, wherein the nanostructures include carbon nano tubes.

7. The membrane filtration system of claim 2, wherein the electrically conductive layer further includes a polymer.

8. The membrane filtration system of claim 7, wherein the polymer is cross-linked with the nanostructures.

9. The membrane filtration system of claim 2, wherein the electrically conductive layer is porous.

10. The membrane filtration system of claim 9, wherein the electrically conductive layer has a pore size that is about the same as or larger than a pore size of the porous support.

11. The membrane filtration system of claim 2, wherein the porous support is a filtration membrane.

12. The membrane filtration system of claim 1, further comprising a counter electrode disposed adjacent to the electrically conductive membrane and an electrical power source, and wherein the electrical power source is connected to the counter electrode and the electrically conductive membrane.

13. The membrane filtration system of claim 1, further comprising a reactor, and wherein the frame and the electrically conductive membrane are disposed in the reactor.

14. The membrane filtration system of claim 1, further comprising an air blower disposed in the reactor.

15. The membrane filtration system of claim 1, wherein the frame includes a porous plate to which the electrically conductive membrane is laminated.

16. A method of treating wastewater, comprising:

directing influent wastewater into a reactor in which an electrically conductive membrane is immersed; and

filtering the wastewater by directing the wastewater through the electrically conductive membrane, wherein the filtering is performed while applying a negative pressure to a permeate side of the electrically conductive membrane, and while applying an electrical potential to the electrically conductive membrane.

17. The method of claim 16, wherein the electrically conductive membrane includes a porous support and an electrically conductive layer disposed on the porous support, and the electrically conductive layer includes nanostructures.

18. The method of claim 17, wherein the nanostructures include nanotubes, nanowires, or both.

19. The method of claim 17, wherein the electrically conductive layer further includes a polymer that is cross-linked with the nanostructures.

20. The method of claim 16, further comprising providing intermittent aeration using an air blower disposed in the reactor.