Composite membrane, preparation method thereof and membrane bioreactor

Abstract

The invention provides a composite membrane, a preparation method thereof and a membrane bioreactor, and belongs to the technical field of membrane preparation and membrane bioreactors. The composite membrane provided by the invention comprises a base membrane with a first average membrane pore size, a macroporous membrane with a second average membrane pore size and a compact layer, wherein the compact layer comprises a polymer formed by cross-linking and curing polydimethylsiloxane and tetraethyloxysilane, and is used for bonding the base membrane and the macroporous membrane; the second average membrane pore size is greater than the first average membrane pore size.

Description

Technical Field

[0001] The present invention relates to the technical field of membrane bioreactor composite membrane preparation and membrane bioreactor, and in particular to a composite membrane and a preparation method thereof and a membrane bioreactor, and more specifically to a composite membrane suitable for a membrane bioreactor and a preparation method thereof and a membrane bioreactor. Background Art

[0002] Membrane bioreactor is a sewage treatment process that combines membrane separation technology with traditional biological treatment technology. This process has significant advantages such as excellent effluent quality, small footprint, and low sludge production. It is widely used in urban sewage treatment and industrial wastewater treatment. With the continuous integration and development of membrane technology and biological water treatment technology, a variety of membrane bioreactor types have gradually evolved, including: submerged membrane bioreactor, separation membrane bioreactor, anaerobic membrane bioreactor, membrane biofilm reactor, membrane aerated biofilm reactor, etc., to meet different treatment scenarios and needs. In the current industry context of "reducing carbon and reducing consumption", membrane aerated biofilm reactors have gradually attracted widespread attention from researchers due to their low energy consumption aeration method and the ability to achieve simultaneous nitrification and denitrification in a single reaction system.

[0003] The membrane aeration biofilm reactor provides bubble-free oxygenation to the water through membrane components. Oxygen and pollutants flow on both sides of the membrane respectively. Driven by concentration differences and adsorption by microorganisms, pollutants are decomposed and utilized by the biofilm, thereby achieving water purification.

[0004] The membranes of membrane aerated biofilm reactors are usually microporous membranes and dense membranes. Microporous membranes are prone to membrane clogging during long-term operation, while dense membranes can better avoid clogging of microporous membranes. However, dense membranes have the problems of high cost and low gas permeability.

[0005] To this end, in view of the advantages and disadvantages of microporous membranes and dense membranes, it has been proposed to prepare a composite membrane containing both micropores and dense structures, which can effectively prevent the clogging of the membrane pores of the microporous membrane while ensuring good oxygen permeability. However, the preparation method of the composite membrane is usually achieved by preparing a dense layer on the surface of the microporous membrane by methods such as surface coating and interfacial polymerization. Although a composite membrane with a dense structure is prepared on a microporous membrane by the aforementioned method, the composite membrane is prone to separation of the microporous membrane and the dense membrane or damage to the dense membrane under a higher gas supply pressure, and the microporous membrane pores may still be blocked by microorganisms and their secretions during long-term operation, resulting in a decrease in the performance of the microporous membrane. In addition, the surface of the dense membrane is relatively flat and smooth, which makes it difficult for microorganisms to form the initial colonization of the biofilm, and it is easy to cause large pieces of the formed biofilm to fall off, seriously affecting the overall processing efficiency of the membrane biofilm reactor. Summary of the invention

[0006] In view of the above technical problems, the present disclosure provides a composite membrane and a preparation method thereof and a membrane bioreactor, in order to at least partially solve the above technical problems. In this regard, the technical solution provided by the present disclosure is as follows.

[0007] As the first aspect of the present disclosure, the present disclosure provides a composite membrane, comprising: a base membrane having a first average membrane pore size, a macroporous membrane having a second average membrane pore size, and a dense layer; wherein the dense layer comprises a polymer cross-linked and cured by polydimethylsiloxane and tetraethylsiloxane, which is used to bond the base membrane and the macroporous membrane; the second average membrane pore size is larger than the first average membrane pore size.

[0008] As a second aspect of the present disclosure, a method for preparing the above-mentioned composite membrane is provided, comprising: uniformly coating a coating agent on the surface of a base membrane having a first average membrane pore size; arranging a macroporous membrane having a second average membrane pore size on the base membrane coated with the coating agent, and drying the membrane, wherein during the drying process, the coating agent undergoes a cross-linking reaction to form a dense layer having a network structure, and the coating agent simultaneously bonds the base membrane and the macroporous membrane to form a composite membrane; wherein the coating agent comprises polydimethylsiloxane, tetraethylsilane, an organic solvent and a catalyst; and the second average membrane pore size is larger than the first average membrane pore size.

[0009] As a third aspect of the present disclosure, a membrane bioreactor is provided, comprising: the composite membrane in the above embodiment.

[0010] Based on the above technical solution, a composite membrane and a preparation method thereof and a membrane bioreactor provided by the present disclosure have at least one of the following beneficial effects.

[0011] (1) In the embodiments of the present disclosure, the composite membrane provided by the present disclosure has a "sandwich" structure, with a thin film having a first average membrane pore size as the base membrane, a dense layer having a cross-linked network structure formed by cross-linking polymerization of polydimethylsiloxane and tetraethylsilane as the middle layer, and a macroporous membrane having a second average membrane pore size as the outer layer. By connecting the base membrane and the macroporous membrane with a dense membrane and setting the average membrane pore size of the macroporous membrane to be larger than the average membrane pore size of the base membrane, on the one hand, the outer macroporous membrane is used to provide a support structure for the dense layer, effectively preventing the separation of the dense layer and the base membrane or the damage of the dense membrane during long-term operation; the suitable pore structure of the macroporous membrane is conducive to increasing the initial attachment speed of microorganisms, so that the structure of the formed biofilm can be maintained and the stability can be enhanced; on the other hand, the difference in pore size is used to ensure good gas permeability. Therefore, compared with the composite membrane formed by the traditional microporous membrane and dense membrane, the composite membrane with a "sandwich" structure provided by the present disclosure has significantly improved biological attachment efficiency and load capacity, enhanced biofilm structure stability, and no clogging of the base membrane pores occurs, which is conducive to improving the treatment efficiency and stability of the membrane bioreactor.

[0012] (2) In the embodiments of the present disclosure, polydimethylsiloxane and tetraethylsiloxane are used as coating agents and adhesives, and are uniformly coated on the surface of a base film having a first average membrane pore size to obtain a coating agent-base film. Subsequently, a macroporous membrane having a second average membrane pore size is arranged on the surface of the coating agent and dried. During the drying process, the coating agent between the base film and the macroporous membrane undergoes a cross-linking reaction to form a dense layer having a cross-linked network structure, and at the same time, the base film and the macroporous membrane are bonded by the adhesiveness of the coating agent, thereby obtaining a composite membrane having a "sandwich". This preparation method is relatively simple and does not require the use of an additional adhesive. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1A This is a model structure diagram of a "sandwich" composite hollow fiber membrane in Example 2 of the present disclosure;

[0014] Figure 1B This is a physical picture of the "sandwich" type composite hollow fiber membrane in Example 2 of the present disclosure;

[0015] Figure 2A This is a physical picture of the polydimethylsiloxane-polytetrafluoroethylene composite hollow fiber membrane in Comparative Example 1;

[0016] Figure 2B Physical picture of the polytetrafluoroethylene hollow fiber membrane in Comparative Example 2. DETAILED DESCRIPTION

[0017] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of the present disclosure. In the following detailed description, for ease of explanation, many specific details are set forth to provide a comprehensive understanding of the embodiments of the present disclosure. However, it is apparent that one or more embodiments may also be implemented without these specific details. In addition, in the following description, descriptions of known structures and technologies are omitted to avoid unnecessary confusion of the concepts of the present disclosure.

[0018] The terms used herein are only for describing specific embodiments and are not intended to limit the present disclosure. The term "comprising" used herein indicates the existence of features, steps, operations, but does not exclude the existence or addition of one or more other features.

[0019] In the case of using expressions such as "at least one of A, B, and C, etc.", it should generally be interpreted in accordance with the meaning of the expression generally understood by those skilled in the art (for example, "a system having at least one of A, B, and C" should include but is not limited to a system having A alone, B alone, C alone, A and B, A and C, B and C, and / or A, B, C, etc.). In the case of using expressions such as "at least one of A, B, or C, etc.", it should generally be interpreted in accordance with the meaning of the expression generally understood by those skilled in the art (for example, "a system having at least one of A, B, or C" should include but is not limited to a system having A alone, B alone, C alone, A and B, A and C, B and C, and / or A, B, C, etc.).

[0020] In the process of implementing the inventive concept of the present disclosure, it was found that the traditional composite membrane formed by a microporous membrane (i.e., base membrane) and a dense membrane (i.e., dense layer) has the problem of separation of the microporous membrane and the dense membrane or damage of the dense membrane during long-term operation, which leads to blockage of the membrane pores of the microporous membrane, thereby affecting the overall treatment efficiency of the membrane bioreactor. The present disclosure proposes a new composite membrane, which arranges a layer of macroporous membrane on the base membrane and the dense layer through the design of the preparation process, and utilizes the supporting effect of the macroporous membrane on the dense layer to avoid damage to the dense layer or separation of the dense layer and the base membrane.

[0021] Specifically, the composite membrane provided by the present disclosure includes: a base membrane with a first average membrane pore size, a macroporous membrane with a second average membrane pore size, and a dense layer; wherein the dense layer contains a polymer cross-linked and cured by polydimethylsiloxane and tetraethylsiloxane, which is used to bond the base membrane and the macroporous membrane; the second average membrane pore size is larger than the first average membrane pore size.

[0022] In the embodiments of the present disclosure, the composite membrane provided by the present disclosure has a sandwich structure, a thin film with a first average membrane pore size is used as a basement membrane, polydimethylsiloxane and tetraethylsiloxane are cross-linked to form a dense layer with a cross-linked network structure as an intermediate layer, and a macroporous membrane with a second average membrane pore size is used as an outer layer. The formation of a dense layer on the surface of the basement membrane avoids the blockage of the basement membrane pores during long-term operation. By connecting the basement membrane and the macroporous membrane with a dense membrane and setting the average membrane pore size of the macroporous membrane to be larger than the average membrane pore size of the basement membrane, the pore structure of the macroporous membrane in the outer layer is used to provide support for the dense layer, and a rich pore structure is provided for the biofilm biofilm start-up and the structural stability of the biofilm, which is conducive to increasing the initial attachment speed of microorganisms, and maintaining the structure and stability of the biofilm, while being able to effectively prevent the separation of the dense layer and the basement membrane or the damage of the dense membrane during long-term operation. In addition, the pore size difference between the basement membrane and the macroporous membrane is used to ensure good gas permeability.

[0023] According to an embodiment of the present disclosure, the base membrane may be a flat membrane or a hollow fiber membrane. In the case where the base membrane is a flat membrane, the macroporous membrane is laid flat on the surface of the composite layer formed by the base membrane and the dense layer, wherein the macroporous membrane is a flat membrane. In other words, the composite membrane of the present disclosure includes: the inner layer is a flat membrane base membrane, the middle layer is a dense layer, and the outer layer is a macroporous flat membrane. In the case where the base membrane is a hollow fiber membrane, the macroporous membrane may be a strip-shaped flat membrane and is spirally wound on the surface of the composite layer formed by the base membrane and the dense layer. In other words, the composition of the composite membrane of the present disclosure includes: the inner layer is a hollow fiber membrane base membrane, the middle layer is a dense layer formed on the surface of the base membrane, and the outer layer is a macroporous membrane spirally wound on the surface of the composite layer composed of a hollow fiber membrane base membrane and a dense layer. In the composite membrane disclosed in the present invention, a base membrane (such as a flat membrane, a hollow fiber membrane) is used to ensure strong gas permeability. The dense layer prevents the biofilm formed by microorganisms and their secretions from clogging the membrane pores of the hollow fiber membrane. The spiral structure of the macroporous membrane or the stress generated by the macroporous flat membrane is used to provide stable support for the dense layer, providing a rich pore structure for the start-up and stability maintenance of microbial biofilm formation. At the same time, the separation of the hollow fiber base membrane and the dense membrane can be effectively avoided to accelerate the start-up cycle of the membrane bioreactor, which is expected to improve the total nitrogen removal capacity and stability of the membrane bioreactor.

[0024] According to the embodiments of the present disclosure, the material of the base membrane is selected from any one of polytetrafluoroethylene, polyvinylidene fluoride, and polypropylene, and the first average membrane pore size is 0.2~0.5μm; the material of the macroporous membrane is selected from any one of polytetrafluoroethylene and polyvinylidene fluoride, and the second average membrane pore size is 0.8~2.0μm; the thickness of the base membrane is 0.2~1.0mm; the thickness of the dense layer is 20~40μm; the thickness of the macroporous membrane is 80~150μm.

[0025] In the embodiments of the present disclosure, the base membrane is preferably a hollow fiber membrane, wherein the outer diameter of the hollow fiber membrane is 0.5~2.5mm, the inner diameter is 0.3~1.2mm, and the first average membrane pore size is 0.2~0.5μm, so as to ensure that the composite membrane has good gas permeability. The width of the macroporous membrane is 2.0~8.0mm, the second average pore size is 0.8~2μm, the spiral pitch of the macroporous membrane winding is 1~7mm, the width of the macroporous membrane and the length of the spiral intercept of the winding control the support or restraint degree of the macroporous membrane to the dense membrane. If the spiral intercept is too long, the dense layer and the base membrane are easily separated, which easily leads to the blockage of the membrane pores of the hollow fiber membrane and affects the processing efficiency of the membrane assembly. The thickness of the macroporous membrane should not be too thick. Too thick will easily cause the biofilm to be too far away from the surface of the dense membrane, resulting in an increase in gas transmission resistance. At the same time, too thick a biofilm is likely to reduce the efficiency of material transmission.

[0026] For example, the base membrane is selected from a polytetrafluoroethylene hollow fiber membrane, wherein the outer diameter of the polytetrafluoroethylene hollow fiber membrane is 0.5-2.5 mm, the inner diameter is 0.3-1.2 mm, and the average membrane pore size is 0.2-0.5 μm. The macroporous membrane is selected from a strip macroporous polytetrafluoroethylene flat membrane, wherein the strip macroporous polytetrafluoroethylene flat membrane has a width of 2.0-8.0 mm, an average pore size of 0.8-2 μm, and a thickness of 80-150 μm.

[0027] As a second aspect of the present disclosure, the present disclosure provides a method for preparing a composite membrane, comprising: uniformly coating a coating agent on the surface of a base membrane having a first average membrane pore size; arranging a macroporous membrane having a second average membrane pore size on the base membrane coated with the coating agent, and drying the membrane, wherein during the drying process, the coating agent undergoes a cross-linking reaction to form a dense layer having a network structure, and the coating agent bonds the base membrane and the macroporous membrane to form a composite membrane; wherein the coating agent comprises polydimethylsiloxane, tetraethylsilane, an organic solvent and a catalyst; and the second average membrane pore size is larger than the first average membrane pore size.

[0028] In the embodiments of the present disclosure, polydimethylsiloxane and tetraethylsiloxane are used as coating agents and binders to coat the coating agent on the surface of a base membrane having a first average membrane pore size. On this basis, a macroporous membrane having a second average membrane pore size is arranged on the base membrane coated with the coating agent by utilizing the adhesion of the coating agent. During the drying process, the polydimethylsiloxane and tetraethylsiloxane in the coating agent are cross-linked and cured to form a dense layer having a cross-linked network structure, thereby obtaining a composite membrane having a "sandwich" structure. In the composite membrane, the dense structure of the dense layer avoids the blockage of the base membrane pores by the biofilm during long-term operation, and the pore structure of the macroporous membrane provides support for the dense layer, while facilitating the attachment of microorganisms to form a biofilm and maintaining the stability of the biofilm structure. In addition, the preparation process disclosed in the present disclosure is relatively simple, and single cross-linking and film formation are performed, without the need for additional adhesives and cross-linking processes.

[0029] According to the embodiments of the present disclosure, the base film can be a flat film or a porous film, and the macroporous film is a flat film. Therefore, in the process of preparing the composite film, the macroporous film can be laid flat on the base film coated with the coating agent to prepare a composite flat film with a "sandwich" structure.

[0030] In other words, when the base membrane is a flat membrane, a coating agent is coated on the surface of the flat membrane base membrane, and then a macroporous membrane is laid flat on the flat membrane base membrane coated with the coating agent to obtain a composite flat membrane with a "sandwich" structure. When the base membrane is a hollow fiber membrane, a coating agent is coated on the surface of the hollow fiber membrane base membrane, and then a strip of macroporous flat membrane is spirally wound on the surface of the hollow fiber membrane base membrane coated with the coating agent, with a spiral pitch of 1 to 7 mm, to obtain a composite hollow fiber membrane with a "sandwich" structure.

[0031] For example, the method for preparing a membrane suitable for a membrane bioreactor includes: coating a coating agent on the surface of a polytetrafluoroethylene hollow fiber membrane to obtain a coating agent-polytetrafluoroethylene hollow fiber membrane; then, using the coating agent as an adhesive, spirally winding a strip of macroporous polytetrafluoroethylene flat membrane on the coating agent-polytetrafluoroethylene hollow fiber membrane, and drying. During the drying process, polydimethylsiloxane and tetraethylsiloxane undergo cross-linking polymerization to form a network structure, and after cooling, a composite hollow fiber membrane with a "sandwich" structure is obtained.

[0032] In the embodiment of the present disclosure, the coating agent uses polydimethylsiloxane and tetraethylsiloxane as solutes, which are cross-linked and polymerized under thermal initiation to form a dense layer with a cross-linked network structure. The average membrane pore size of the obtained dense layer is less than 5nm, and the porosity is less than 5%, which is approximately non-porous compared to the basement membrane and the macroporous membrane. The organic solvent in the coating agent is selected from n-heptane, and the catalyst is selected from dibutyltin dilaurate. Further, the mass fraction of polydimethylsiloxane in the coating agent is 5.0~20.0wt%, preferably 10.0~15.0wt%, and the concentration of polydimethylsiloxane affects the thickness and density of the dense layer. At this concentration, a dense layer with moderate thickness and relatively dense can be obtained, and the permeation efficiency of the gas is guaranteed, thereby ensuring the startup of the biofilm and the ability to treat pollutants. The higher the concentration of polydimethylsiloxane, the thicker the dense layer, and the higher the density of the dense layer, the worse the air permeability of the composite membrane, resulting in slow startup of the biofilm and low processing capacity of the membrane bioreactor. Among them, the mass percentage of polydimethylsiloxane and tetraethylsilane is 6:1~12:1, preferably 8:1~10:1; the mass percentage of polydimethylsiloxane and catalyst is 30:1~80:1, preferably 40:1~60:1; the mass percentage of polydimethylsiloxane and organic solvent is 1:6~1:12, preferably 1:8~1:10.

[0033] According to an embodiment of the present disclosure, the drying temperature is 50-100° C., preferably 60-80° C.; the drying time is 60-180 min, preferably 100-150 min.

[0034] As a third aspect of the present disclosure, the present disclosure provides a membrane bioreactor, comprising: the composite membrane in the above-mentioned embodiment. In the embodiment of the present disclosure, the composite membrane prepared by the method provided by the present disclosure is applied to the membrane bioreactor. Compared with the membrane bioreactor composed of the base membrane without coating agent or the polydimethylsiloxane-polytetrafluoroethylene hollow fiber composite membrane, the composite membrane with a "sandwich" structure provided by the present disclosure has significantly improved attachment efficiency and load of microorganisms on its surface, and the stability of the biofilm structure formed by the microorganisms is enhanced, and there is no problem of membrane pore clogging, which is conducive to the improvement of the treatment efficiency and stability of the membrane bioreactor.

[0035] According to an embodiment of the present disclosure, the membrane bioreactor of the present disclosure may be a membrane aeration bioreactor.

[0036] The technical scheme of the present disclosure is further illustrated by embodiments and related test experiments below. In the detailed description below, for ease of explanation, many specific details are set forth to provide a comprehensive understanding of the embodiments of the present disclosure. However, it is obvious that one or more embodiments may also be implemented without these specific details. Moreover, in the absence of conflict, the details in the following embodiments may be arbitrarily combined into other feasible embodiments. All instruments, consumables and reagents in the following embodiments, if not otherwise specified, can be obtained from commercial sources.

[0037] It should be noted that the base membrane used in Example 2, Comparative Example 1 and Comparative Example 2 is a polytetrafluoroethylene hollow fiber membrane with an outer diameter of 2.0 mm, an inner diameter of 1.0 mm, an average membrane pore size of 0.30 μm, and a porosity of 55.3%. The base membrane used in Example 1 is a polytetrafluoroethylene flat membrane supported by a non-woven fabric, with an average membrane pore size of 0.22 μm, a porosity of 60.8%, and a thickness of 180 μm.

[0038] The macroporous polytetrafluoroethylene flat membrane used in Example 1-2 had an average membrane pore diameter of 1.5 μm and a thickness of 100 μm.

[0039] Example 1

[0040] A method for preparing a composite flat membrane having a "sandwich" structure comprises the following steps:

[0041] (1) The non-woven fabric-supported polytetrafluoroethylene flat membrane was washed with deionized water for 10 min and then dried, and fixed on a glass plate with the polytetrafluoroethylene surface facing upward.

[0042] (2) At 25°C, 10 wt% of polydimethylsiloxane, 1 wt% of tetraethylsilane, 89 wt% of n-heptane and 0.2 wt% of dibutyltin dilaurate were placed in a magnetic stirrer and stirred for 1 h to obtain a uniformly dispersed polydimethylsiloxane coating agent.

[0043] (3) Pour the evenly dispersed polydimethylsiloxane coating agent onto the clean non-woven fabric-supported polytetrafluoroethylene flat membrane base film, and use a roller to roll the non-woven fabric-supported polytetrafluoroethylene flat membrane base film horizontally at a certain speed twice.

[0044] (4) The macroporous polytetrafluoroethylene flat membrane is covered on the upper layer of the polytetrafluoroethylene flat membrane supported by a non-woven fabric coated with a polydimethylsiloxane coating agent to obtain a composite flat membrane with a "sandwich" structure.

[0045] (5) The composite flat membrane with the "sandwich" structure is placed in an oven at 80°C for cross-linking polymerization and drying to obtain a "sandwich" composite flat membrane. The surface area of ​​the obtained composite flat membrane is 125.6 square centimeters.

[0046] The "sandwich" composite flat membrane prepared by the method in Example 1 was tested and found that its air flux could reach 168.5 L / (m 2 ·h·kPa), and under the condition that the gas supply pressure is lower than 150kPa, the dense layer formed by the polydimethylsiloxane coating agent will not bubble or detach, indicating that the composite membrane layer prepared by the method provided by the present invention provides a good supporting structure, so that it can maintain the integrity and stable operation of the dense layer under higher pressure.

[0047] Example 2

[0048] A method for preparing a composite hollow fiber membrane having a "sandwich" structure comprises the following steps:

[0049] (1) Seal the ends of the polytetrafluoroethylene hollow fiber membrane with hot melt adhesive, wait for the hot melt adhesive to completely cool down, then rinse with deionized water (e.g., 10 minutes), and dry for later use.

[0050] (2) At 25° C., 10 wt % of polydimethylsiloxane, 1 wt % of tetraethylsilane, 89 wt % of n-heptane and 0.2 wt % of dibutyltin dilaurate were placed in a magnetic stirrer and stirred for 1 h to obtain a uniformly dispersed polydimethylsiloxane coating agent.

[0051] (3) Soak a clean polytetrafluoroethylene hollow fiber-based membrane with closed ends in a polydimethylsiloxane coating agent for 5 minutes to allow the coating agent to evenly cover the surface of the polytetrafluoroethylene hollow fiber-based membrane; slowly take out the polytetrafluoroethylene hollow fiber-based membrane coated with the polydimethylsiloxane coating agent in the vertical direction at a certain speed and hang it vertically.

[0052] (4) A strip-shaped macroporous polytetrafluoroethylene flat membrane (width: 6 mm) was spirally wound with a spiral pitch of 5 mm on the outer layer of a vertically suspended polytetrafluoroethylene hollow fiber membrane coated with a polydimethylsiloxane coating agent to obtain a composite hollow fiber membrane with a "sandwich" structure.

[0053] (5) The composite hollow fiber membrane with the "sandwich" structure is placed in an oven at 80°C for cross-linking polymerization and then dried to obtain a "sandwich" composite hollow fiber membrane. The surface area of ​​the obtained composite hollow fiber membrane is 125.6 square centimeters.

[0054] Figure 1AThis is a model structure diagram of the "sandwich" composite hollow fiber membrane in Example 2 of the present disclosure, Figure 1B This is a physical picture of the "sandwich" type composite hollow fiber membrane in Example 2 of the present disclosure.

[0055] like Figure 1A-1B As shown, it can be clearly seen that the strip-shaped macroporous polytetrafluoroethylene flat membrane is spirally wound on the surface of the composite layer of the base membrane and the dense layer.

[0056] Furthermore, the "sandwich" composite hollow fiber membrane prepared by the method provided in Example 2 was tested. The test results showed that the air flux of the membrane could reach 152.8 L / (m 2 ·h·kPa), and under the condition that the gas supply pressure is lower than 200kPa, the dense layer formed by the polydimethylsiloxane coating agent will not bubble or detach, indicating that the composite membrane layer prepared by the method provided by the present invention provides a good supporting structure, so that it can maintain the integrity and stable operation of the dense layer under higher pressure.

[0057] The filling rate of the spirally wound "sandwich" composite hollow fiber membrane in the unit volume shell side is relatively high, and the strip-shaped spiral outer layer has a high binding site, making the formed outer layer structure relatively stable. The filling rate of the flat membrane "sandwich" composite hollow fiber membrane in the unit volume shell side is relatively low, and there is a risk of membrane detachment when the large-area (membrane surface area is 125.6 square centimeters) sheet structure is bonded and stacked together. Therefore, it is preferred to use a spirally wound "sandwich" composite hollow fiber membrane to construct a membrane bioreactor.

[0058] Example 3

[0059] A method for preparing a composite hollow fiber membrane having a "sandwich" structure comprises the following steps:

[0060] (1) Seal the ends of the polytetrafluoroethylene hollow fiber membrane with hot melt adhesive, wait for the hot melt adhesive to completely cool down, then rinse with deionized water (e.g., 10 minutes), and dry for later use.

[0061] (2) At 25° C., 5 wt % of polydimethylsiloxane, 0.5 wt % of tetraethylsilane, 94.5 wt % of n-heptane and 0.1 wt % of dibutyltin dilaurate were placed in a magnetic stirrer and stirred for 1 h to obtain a uniformly dispersed polydimethylsiloxane coating agent.

[0062] (3) Soak a clean polytetrafluoroethylene hollow fiber-based membrane with closed ends in a polydimethylsiloxane coating agent for 5 minutes to allow the coating agent to evenly cover the surface of the polytetrafluoroethylene hollow fiber-based membrane; slowly take out the polytetrafluoroethylene hollow fiber-based membrane coated with the polydimethylsiloxane coating agent in the vertical direction at a certain speed and hang it vertically.

[0063] (4) A strip-shaped macroporous polytetrafluoroethylene flat membrane (width: 6 mm) was spirally wound with a spiral pitch of 5 mm on the outer layer of a vertically suspended polytetrafluoroethylene hollow fiber membrane coated with a polydimethylsiloxane coating agent to obtain a composite hollow fiber membrane with a "sandwich" structure.

[0064] (5) The composite hollow fiber membrane with the "sandwich" structure is placed in an oven at 80°C for cross-linking polymerization and then dried to obtain a "sandwich" composite hollow fiber membrane. The surface area of ​​the obtained composite hollow fiber membrane is 125.6 square centimeters.

[0065] Furthermore, the "sandwich" composite hollow fiber membrane prepared by the method provided in Example 3 was tested. The test results showed that the air flux of the membrane could reach 223.4 L / (m 2 ·h·kPa), but under the condition that the gas supply pressure is higher than 25kPa, the dense layer formed by the polydimethylsiloxane coating agent will bubble, indicating that the dense layer thickness of the 5wt% polydimethylsiloxane membrane prepared by the method provided in the present invention is low, or it is difficult to evenly cover the membrane surface, and it is difficult to maintain the integrity and stability of the dense layer under higher pressure.

[0066] Example 4

[0067] A method for preparing a composite hollow fiber membrane having a "sandwich" structure comprises the following steps:

[0068] (1) Seal the ends of the polytetrafluoroethylene hollow fiber membrane with hot melt adhesive, wait for the hot melt adhesive to completely cool down, then rinse with deionized water (e.g., 10 minutes), and dry for later use.

[0069] (2) At 25° C., 15 wt % of polydimethylsiloxane, 1.5 wt % of tetraethylsilane, 83.5 wt % of n-heptane and 0.3 wt % of dibutyltin dilaurate were placed in a magnetic stirrer and stirred for 1 h to obtain a uniformly dispersed polydimethylsiloxane coating agent.

[0070] (3) Soak a clean polytetrafluoroethylene hollow fiber-based membrane with closed ends in a polydimethylsiloxane coating agent for 5 minutes to allow the coating agent to evenly cover the surface of the polytetrafluoroethylene hollow fiber-based membrane; slowly take out the polytetrafluoroethylene hollow fiber-based membrane coated with the polydimethylsiloxane coating agent in the vertical direction at a certain speed and hang it vertically.

[0071] (4) A strip-shaped macroporous polytetrafluoroethylene flat membrane (width: 6 mm) was spirally wound with a spiral pitch of 5 mm on the outer layer of a vertically suspended polytetrafluoroethylene hollow fiber membrane coated with a polydimethylsiloxane coating agent to obtain a composite hollow fiber membrane with a "sandwich" structure.

[0072] (5) The composite hollow fiber membrane with the "sandwich" structure is placed in an oven at 80°C for cross-linking polymerization and then dried to obtain a "sandwich" composite hollow fiber membrane. The surface area of ​​the obtained composite hollow fiber membrane is 125.6 square centimeters.

[0073] Furthermore, the "sandwich" composite hollow fiber membrane prepared by the method provided in Example 4 was tested. The test results showed that under the condition of air supply pressure lower than 260 kPa, the dense layer formed by the polydimethylsiloxane coating agent did not bubble or separate, but the air flux was only 32.5 L / (m 2 ·h·kPa), indicating that the 15wt% polydimethylsiloxane membrane prepared by the method provided in the present invention has a high dense layer thickness, which significantly increases its gas mass transfer resistance. Even if a higher gas supply pressure is provided, it is difficult to achieve an ideal gas flux to meet the oxygen demand of microorganisms.

[0074] The dense layer formed by 10wt% polydimethylsiloxane coating agent has better gas flux and more suitable bubble point pressure; the dense layer formed by 5wt% and 15wt% polydimethylsiloxane coating agent is too thin or too thick, resulting in too low bubble point and too low gas flux, respectively. Therefore, 10wt% polydimethylsiloxane is preferably used as a coating agent to construct a "sandwich" composite membrane system.

[0075] Comparative Example 1

[0076] Comparative Example 1 provides a method for preparing a polydimethylsiloxane-polytetrafluoroethylene composite hollow fiber membrane, comprising the following steps:

[0077] (1) Seal the ends of the polytetrafluoroethylene hollow fiber membrane with hot melt adhesive, wait for the hot melt adhesive to cool completely, then rinse with deionized water for 10 minutes and dry.

[0078] (2) At 25°C, 10 wt% of polydimethylsiloxane, 1 wt% of tetraethylsilane, 89 wt% of n-heptane and 0.2 wt% of dibutyltin dilaurate were placed in a magnetic stirrer and stirred for 1 h to obtain a uniformly dispersed polydimethylsiloxane coating agent.

[0079] (3) Soak the clean polytetrafluoroethylene hollow fiber-based membrane with sealed ends in polydimethylsiloxane coating agent for 5 minutes to make the coating agent evenly cover the surface of the polytetrafluoroethylene hollow fiber-based membrane; slowly take out the polytetrafluoroethylene hollow fiber-based membrane in the vertical direction at a certain speed and hang it vertically.

[0080] (4) placing the vertically suspended polytetrafluoroethylene hollow fiber membrane coated with polydimethylsiloxane coating agent in an oven at 80° C. for cross-linking polymerization and drying to obtain a polydimethylsiloxane-polytetrafluoroethylene composite hollow fiber membrane (such as Figure 2A as shown).

[0081] The polydimethylsiloxane-polytetrafluoroethylene composite hollow fiber membrane prepared by the method provided in Comparative Example 1 was tested. The test results showed that the air flux of the membrane could reach 156.5 L / (m 2 ·h·kPa), and under the condition that the gas supply pressure is lower than 100kPa, the dense layer formed by the polydimethylsiloxane coating agent will not bubble or separate.

[0082] Comparative Example 2

[0083] The polytetrafluoroethylene hollow fiber membrane provided in Comparative Example 2 is the polytetrafluoroethylene hollow fiber membrane base membrane in Example 2 and Comparative Example 1 (such as Figure 2B It needs to be washed with deionized water for 10 minutes and dried before use.

[0084] The polytetrafluoroethylene hollow fiber membrane in comparative example 2 was tested. The test results showed that the air flux of the membrane could reach 19100 L / (m 2 ·h·kPa), and when the air supply pressure is lower than 15kPa, bubbling in the water will not occur.

[0085] Application Examples

[0086] The "sandwich" composite flat membrane of Example 1, the "sandwich" composite hollow fiber membrane of Example 2, the polydimethylsiloxane-polytetrafluoroethylene composite hollow fiber membrane of Comparative Example 1, and the polytetrafluoroethylene hollow fiber membrane of Comparative Example 2 were respectively used for biofilm startup of the membrane aeration biofilm reactor, and the changes in the amount of biofilm and the total nitrogen effluent concentration over time were recorded. The specific experimental results are shown in Tables 1 and 2, respectively.

[0087] Table 1. Record of biomass on membrane surface during the start-up period of membrane aeration biofilm reactor using different composite membranes

[0088]

[0089] As shown in Table 1, the amount of biofilm in the "sandwich" composite hollow fiber membrane (Example 2) and the "sandwich" composite flat membrane (Example 1) at the biofilm startup stage is significantly higher than that of the polydimethylsiloxane-polytetrafluoroethylene composite hollow fiber membrane in Comparative Example 1 and the polytetrafluoroethylene hollow fiber membrane in Comparative Example 2, and the stability of the biofilm amount is excellent over time. This shows that the outermost macroporous polytetrafluoroethylene flat membrane in the "sandwich" composite hollow fiber membrane and the "sandwich" composite flat membrane provides a carrier for the initial attachment of microorganisms to form biofilms, and the more suitable pore structure of the macroporous membrane provides support for the proliferation of the biofilm and the stable maintenance of the structure, thereby accelerating the formation of the biofilm, significantly shortening the cycle required for the reactor startup, and having excellent stability.

[0090] The biofilm start-up speed of the polydimethylsiloxane-polytetrafluoroethylene composite hollow fiber membrane of Comparative Example 1 is slow, and the biofilm falls off in 60-100 days. This may be because the long-term adhesion of the biofilm and the gas supply pressure weaken the bonding force between the polydimethylsiloxane dense layer and the polytetrafluoroethylene hollow fiber membrane, causing damage to the dense layer, resulting in the biofilm falling off with the separation of the dense layer, which seriously reduces the stability of the biofilm.

[0091] Table 2. Total nitrogen removal efficiency of different composite membranes applied to membrane aerated biofilm reactor

[0092]

[0093] As shown in Table 2, the ammonia nitrogen treatment efficiency of the "sandwich" composite hollow fiber membrane (Example 2) and the "sandwich" composite flat membrane (Example 1) is significantly higher than that of the polydimethylsiloxane-polytetrafluoroethylene composite hollow fiber membrane in Comparative Example 1 and the polytetrafluoroethylene hollow fiber membrane in Comparative Example 2, and the total nitrogen removal rate is excellent in stability over time. This shows that the macroporous polytetrafluoroethylene flat membrane possessed by the "sandwich" composite hollow fiber membrane (Example 2) and the "sandwich" composite flat membrane (Example 1) can provide a continuous and stable oxygen supply to the biofilm. The porous structure of the polytetrafluoroethylene hollow fiber membrane base membrane in the "sandwich" composite hollow fiber membrane and the polytetrafluoroethylene flat membrane base membrane in the "sandwich" composite flat membrane ensure that the composite membrane has excellent gas permeability; the middle polydimethylsiloxane dense layer prevents the biological membrane and its secretions from clogging the membrane pores of the base membrane, so that the gas transmission capacity remains stable; the outer macroporous polytetrafluoroethylene flat membrane provides structural support for the polydimethylsiloxane dense layer, ensuring that it maintains excellent integrity in an environment where it is subjected to long-term biological membrane load and gas supply pressure.

[0094] In addition, the macroporous polytetrafluoroethylene flat membrane accelerates the start-up cycle of the biofilm, improves the stability of the biofilm structure, and makes the total nitrogen removal rate have a good effect on the 10th day, and the stability is excellent. However, the polydimethylsiloxane-polytetrafluoroethylene composite hollow fiber membrane in Comparative Example 1 has a small amount of biofilm, and the total nitrogen removal rate is lower than that of the "sandwich" composite hollow fiber membrane and the "sandwich" composite flat membrane, and due to the phenomenon of biofilm shedding, the total nitrogen removal rate decreases in 60-100 days. The polytetrafluoroethylene hollow fiber membrane in Comparative Example 2 has a low amount of biofilm compared to the "sandwich" composite hollow fiber membrane and the "sandwich" composite flat membrane, so the total nitrogen removal rate is also low, and because the basement membrane pores are blocked by the biofilm and its secretions, the total nitrogen removal rate is seriously reduced in 40-100 days.

[0095] The specific embodiments described above further illustrate the purpose, technical solutions and beneficial effects of the present disclosure. It should be understood that the above description is only a specific embodiment of the present disclosure and is not intended to limit the present disclosure. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present disclosure should be included in the protection scope of the present disclosure.

Claims

1. A composite film comprising: a base membrane having a first average membrane pore size and a macroporous membrane having a second average membrane pore size, and a dense layer; Wherein, the dense layer comprises a cross-linked and cured polymer of polydimethylsiloxane and tetraethylsiloxane, and is used for bonding the base film and the macroporous film; and the second average film pore size is larger than the first average film pore size.

2. The composite membrane according to claim 1, wherein: The base film is a flat film, and the macroporous film is a flat film and is laid flat on the surface of the composite layer formed by the base film and the dense layer.

3. The composite membrane according to claim 1, wherein: The base membrane is a hollow fiber membrane, and the macroporous membrane is in a strip shape and spirally wound on the surface of the composite layer formed by the base membrane and the dense layer; The outer diameter of the hollow fiber membrane is 0.5-2.5 mm, and the inner diameter is 0.3-1.2 mm; the spiral pitch of the macroporous membrane winding is 1-7 mm.

4. The composite film according to any one of claims 1 to 3, wherein: The material of the base film is selected from any one of polytetrafluoroethylene, polyvinylidene fluoride, and polypropylene, and the first average membrane pore size is 0.2-0.5 μm; The material of the macroporous membrane is selected from any one of polytetrafluoroethylene and polyvinylidene fluoride, and the second average membrane pore size is 0.8-2.0 μm; The thickness of the base film is 0.2-1.0 mm; The thickness of the dense layer is 20-40 μm; The thickness of the macroporous membrane is 80-150 μm.

5. A method for preparing a composite film, comprising: uniformly coating a coating agent on a surface of a base film having a first average film pore size; Arranging a macroporous membrane having a second average membrane pore size on the base membrane coated with the coating agent, and drying the membrane, wherein the coating agent undergoes a cross-linking reaction during the drying process to form a dense layer having a network structure, and the coating agent simultaneously bonds the base membrane and the macroporous membrane to form a composite membrane; Wherein, the coating agent comprises polydimethylsiloxane, tetraethylsilane, an organic solvent and a catalyst; and the second average membrane pore size is larger than the first average membrane pore size.

6. The method according to claim 5, wherein: The macroporous membrane is laid flat or spirally wound on the base membrane coated with the coating agent.

7. The method according to claim 5, wherein: The drying temperature is 50~100℃ and the time is 60~180min.

8. The method according to claim 5, wherein: In the coating agent, the mass fraction of the polydimethylsiloxane is 5.0-20.0wt%, the mass percentage of the polydimethylsiloxane and tetraethylsilane is 6:1-12:1, the mass percentage of the polydimethylsiloxane and the organic solvent is 1:6-1:12, and the mass percentage of the polydimethylsiloxane and the catalyst is 30:1-80:

1.

9. The method according to claim 8, wherein: The organic solvent is selected from n-heptane; The catalyst is selected from dibutyltin dilaurate.

10. A membrane bioreactor comprising: The composite membrane according to any one of claims 1 to 4.

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