Membrane biofouling control method based on high ferrate pre-oxidation and synergistic conductive membrane filtration

Abstract

The application provides a membrane biological pollution control method based on high ferrate pre-oxidation and conductive membrane filtration. First, ferrate is added to water to be treated for pre-oxidation treatment, and then the pre-oxidation effluent is directly introduced into a conductive membrane filtration unit without precipitation. In the conductive membrane filtration unit, a direct current power supply is applied to the conductive membrane, and a cathode potential is applied. The conductive membrane is a PVDF composite conductive membrane modified by blending carboxylated carbon nanotubes and graphene oxide. The method effectively inhibits the proliferation and diffusion of bacteria on the membrane. After multiple backwashes, the flux recovery rate is always maintained at more than 90%, the membrane flux is maintained well, the attenuation amplitude of the membrane specific flux is significantly slowed down, the water production rate is improved, and the operation stability is improved while the operation cost is reduced.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of water treatment, and particularly relates to a membrane biological pollution control method based on ferrate pre-oxidation and conductive membrane filtration. BACKGROUND

[0002] Membrane separation technology has been widely used in drinking water purification, sewage advanced treatment and reuse due to its advantages of high efficiency, energy saving and simple operation. However, membrane biological pollution is an inevitable problem in the long-term operation of the membrane system. In the ultrafiltration process of drinking water, the proportion of membrane biological pollution can be as high as 56%. The components of membrane biological pollution are complex, and especially the active components such as bacteria can proliferate and spread on the membrane to form a high-viscosity dense biofilm, causing serious irreversible pollution. It is difficult to restore the membrane flux by cleaning, and in severe cases, the service life of the membrane can be shortened.

[0003] As a new membrane technology, conductive membrane has been widely recognized in controlling membrane pollution. In terms of controlling membrane biological pollution, the conductive membrane as an anode can effectively kill bacteria, and as a cathode can significantly reduce the activity of bacteria reaching the membrane surface and inhibit the proliferation of bacteria on the membrane surface. However, the inhibition of biological pollution by the conductive membrane mainly occurs on the membrane surface and cannot prevent the proliferation and metabolism of bacteria in the water phase.

[0004] Ferrate, as a green oxidant with strong oxidizing property and environmental friendliness, has been widely used in water treatment. Its standard potential (+2.2V) is higher than that of conventional oxidants, and it can efficiently degrade various organic pollutants, including phenols, aromatic compounds and drug residues. Ferrate ions are reduced to Fe 3+ In an alkaline environment, Fe(OH)3flocs are further formed to achieve in-situ flocculation. Fe(OH)3also has good adsorption performance and can remove small suspended solids and dissolved organic matter in water. The reaction products are non-toxic and environmentally friendly, and do not introduce secondary pollution. The control of biological pollution by ferrate mainly relies on the pre-oxidation process of ferrate, but cannot prevent the further proliferation and spread of active bacteria reaching the membrane surface. SUMMARY

[0005] Therefore, the application aims to provide a membrane biological pollution control method based on ferrate pre-oxidation and conductive membrane filtration. Through the synergistic effect of multiple mechanisms such as pre-oxidation killing, floc capture, improvement of filter cake layer structure, enhancement of electrostatic repulsion and inhibition of membrane surface reduction potential, the proliferation and spread of bacteria are inhibited from the water inlet to the membrane surface, the irreversible accumulation of various biological components on the membrane is reduced, and the biological pollution of the membrane is effectively controlled.

[0006] To achieve the above purpose, the technical scheme of the application is as follows:

[0007] A membrane biofouling control method based on ferrate pre-oxidation and conductive membrane filtration, first, ferrate is added to the water to be treated for pre-oxidation treatment, and then the pre-oxidation effluent is directly introduced into the conductive membrane filtration unit without precipitation; in the conductive membrane filtration unit, a direct current power supply is applied to the conductive membrane and a cathode potential is applied, and the conductive membrane is a PVDF composite conductive membrane modified by blending carboxylated carbon nanotubes and graphene oxide; wherein the ferrate dosage is 130-160 μmol / L, and the mass ratio of carboxylated carbon nanotubes and graphene oxide is 100: (15-25).

[0008] The conductive membrane applies a cathode voltage with the following effects:

[0009] 1) Forming an alkaline environment near the membrane surface, which will enhance the electronegativity of both the pollutants in the water and the membrane surface, increase the electrostatic repulsion between them, and inhibit the irreversible accumulation of biological components on the membrane surface.

[0010] 2) The alkaline environment near the membrane surface will promote the further growth of Fe(OH)3 floc particles produced by ferrate pre-oxidation, forming a more loose and porous filter cake layer on the membrane surface, improving the water permeability of the membrane, enhancing the easy peeling of the filter cake layer during cross-flow and backwashing, effectively preventing the irreversible attachment of live bacteria on the membrane, and the irreversible accumulation of biological components such as extracellular polymeric substances on the membrane.

[0011] 3) H2O2 is generated near the membrane surface, and at the same time, the Fe 3+ will occur on the surface of the conductive membrane. 3+ Fe 2+ transfers electrons to Fe 2+ . Fe 2+ and H2O2 undergo electro-Fenton reaction to produce strong oxidizing ·OH, which forms a synergistic effect with ferrate pre-oxidation and high current density on the membrane surface, greatly reducing the live bacteria reaching the membrane surface, significantly reducing the bacterial activity on the membrane surface, and effectively inhibiting the rapid proliferation of bacteria on the membrane surface.

[0012] The pre-oxidation effluent cannot be precipitated and must be directly subjected to conductive membrane filtration, because if it is precipitated first, there are not enough iron hydroxide flocs in the filtered water to form a loose filter cake layer on the conductive membrane, which is not conducive to the control of membrane fouling. Ferrate cannot be directly added to the membrane tank, otherwise the strong oxidizing properties of Fe (VI) and free radicals will damage the conductive membrane.

[0013] Further, the ferrate is potassium ferrate or sodium ferrate, and the pre-oxidation reaction time is 20-40 min.

[0014] The pre-oxidized ferric salt cannot be added in too high amount, otherwise, a large amount of bacteria will be killed, resulting in a large amount of release of intracellular substances of bacteria, and aggravating membrane pollution; on the other hand, since the pre-oxidation product iron hydroxide is positively charged, if the addition amount is too high, the negatively charged pollutants in the water will be neutralized, the overall electronegativity of the filter cake layer will be greatly weakened, and the electrostatic repulsion between the filter cake layer and the conductive membrane surface will be reduced, which is not conducive to the control of membrane pollution; but the addition amount cannot be too low, otherwise the bacteria and other biological components in the water cannot be effectively captured and retained in the filter cake layer.

[0015] Further, the conductive membrane is prepared by a two-step phase inversion method, and the specific method is as follows: the carboxylated carbon nanotubes and graphene oxide are dispersed in anhydrous ethanol, and a dispersion liquid is formed by ultrasonic; then the obtained dispersion liquid is scraped to form a film, and a conductive layer is formed after drying; then PVDF casting solution is scraped on one side of the formed conductive layer, and a double-layer structure conductive membrane is formed by phase inversion.

[0016] Further, the ultrasonic time is 30-60 min, the drying temperature is 40-60℃, and the drying time is 5-10 min.

[0017] Further, the aspect ratio of the carboxylated carbon nanotubes cannot be less than 1000, the specific surface area is greater than 210m 2 / g, and the resistivity is less than 1500Ω•m.

[0018] The aspect ratio of the carbon nanotubes is to avoid agglomeration, and can better build a continuous electronic path in the conductive layer, the greater the aspect ratio of the carboxylated carbon nanotubes (CNTs), the closer the contact between the carbon nanotubes, and the more easily the conductive network is formed, thereby improving the overall conductivity.

[0019] The specific surface area will indirectly affect the conductivity of the carboxylated carbon nanotubes by affecting the construction efficiency, dispersibility and contact resistance of the conductive network.

[0020] The carboxylated carbon nanotubes make the carbon nanotubes charged, and avoid agglomeration of the carbon nanotubes through electrostatic repulsion between each other, and if agglomeration occurs, it is not conducive to the formation of a continuous electronic path.

[0021] Further, the graphene oxide is single-layer, the thickness is less than 1nm, the diameter is between 0.5-2μm, the carbon content is not less than 60%, and the oxygen content cannot be higher than 30%.

[0022] The single-layer graphene oxide (GO) has a shorter electron scattering path, a smaller interlayer contact resistance, and a better theoretical conductivity than multi-layer graphene oxide, and the smaller the thickness, the fewer the defects.

[0023] The monolayer graphene is too small in size, has a high proportion of edge defects (more oxygen-containing groups on the edge), and the conductive network formed by the lamination between the sheets is discontinuous, with large contact resistance and poor conductivity. If the size is too large, the sheets are prone to aggregation or wrinkling, the dispersibility decreases, and the continuity of the conductive network is destroyed.

[0024] The higher the carbon content of graphene oxide, the better the conductivity, and the higher the oxygen content, the worse the conductivity.

[0025] The conductivity of graphene oxide is the best, and the conductivity of other types of graphene is not as good as that of graphene oxide.

[0026] Carboxylated carbon nanotubes and graphene oxide are selected to prepare a conductive film, which fully utilizes the complementary properties of the three-dimensional space characteristics of carbon nanotubes and the two-dimensional space characteristics of graphene, and is conducive to better forming an electron path, thereby improving the conductivity. The improved conductivity of the conductive film can improve the anti-pollution performance of the film itself.

[0027] Further, the conductivity of the conductive film is greater than 25 S / m, the electrode spacing is 1 cm, and the membrane filtration mode is cross-flow filtration.

[0028] The conductivity of the conductive film reflects the conductivity of the film. Good conductivity can promote water electrolysis in the membrane pores and on the membrane surface at a lower applied voltage, which is conducive to energy saving and the formation of hydroxyl ions to form an alkaline environment. At the same time, oxygen in the incoming water will gain electrons on the surface of the conductive film to form H2O2. If the conductivity of the film is too low (poor conductivity), water electrolysis is weak, and the amount of hydroxyl ions and H2O2 generated is small, and the above synergistic mechanism between ferrate and the conductive film will not occur or will be very weak.

[0029] Further, the current density of the conductive film is 1.5-2.5 mA / cm 2 .

[0030] The current density of the conductive film is too low, which will not produce H2O2, and will not form an electro-Fenton reaction. At the same time, under a too low current density, water electrolysis will not occur significantly, and an alkaline environment will not be formed near the membrane surface, which cannot promote the further growth of ferric hydroxide floc, the reduction product of ferrate, and cannot improve the structure of the filter cake layer. On the contrary, too high a current density will stimulate microorganisms too strongly, directly killing bacteria and causing bacteria to release a large amount of intracellular substances on the membrane, which will exacerbate irreversible membrane fouling due to the high viscosity of these intracellular substances. Therefore, both the current density and the conductivity of the conductive film need to be considered, and the better the conductivity, the better the effect. Further, when treating wastewater, the membrane flux of the conductive film is 50-100 L / (m 2 •h•bar).

[0031] Furthermore, the conductive membrane filtration unit has a filtration cycle of 30-60 minutes, with a hydraulic backwash flux of 2-3 times the filtration flux and a backwash time of 1-2 minutes.

[0032] Compared with existing technologies, the membrane biofouling control method based on ferrate pre-oxidation synergistic conductive membrane filtration described in this invention has the following advantages:

[0033] (1) This invention uses a combination of pre-oxidation and conductive membrane to control pollution. Pre-oxidation is used to initially kill bacteria in the influent, and more importantly, it generates ferric hydroxide flocs and a small amount of Fe. 3+ Ions. Ferric hydroxide flocs further flocculate and grow in the alkaline environment near the conductive membrane surface, forming a more porous filter cake layer on the membrane surface during filtration. This filter cake layer has better water permeability, which helps to slow down membrane flux decay. Simultaneously, in the alkaline environment formed near the conductive membrane surface, the electronegativity of both pollutants in the filter cake layer and the membrane surface is further enhanced, increasing the electrostatic repulsion between them. This facilitates the easier removal of the filter cake layer from the membrane surface during cross-flow filtration and backwashing. As the filter cake layer leaves the membrane surface, it also carries away captured bacteria and biological components, thus reducing membrane fouling. Meanwhile, Fe2+ generated during the pre-oxidation process of ferrate... 3+ Ions gain electrons from the conductive film surface and are reduced to Fe. 2+ Ions react with H₂O₂ generated on the conductive membrane surface via an electro-Fenton reaction, producing highly oxidizing hydroxyl radicals. These hydroxyl radicals further act on bacteria and biological components near the membrane surface and accumulated on the membrane, inhibiting the rapid proliferation of bacteria on the membrane. 2+ The ions are converted back to Fe after the electro-Fenton reaction. 3+ The ions then gain electrons from the membrane and revert to Fe. 2+ Ions cause the electro-Fenton reaction to occur continuously in the near-membrane region. In summary, the synergistic effect between ferrate pre-oxidation and the conductive membrane is mainly reflected in three aspects: improving the filter cake layer structure, enhancing the electrostatic repulsion between the filter cake layer and the conductive membrane, and forming an electro-Fenton reaction to inhibit the activity of bacteria on the membrane surface and control their rapid proliferation.

[0034] (2) By means of the present invention, the number of live bacteria reaching the membrane surface is significantly reduced, the amount of irreversibly attached bacteria on the membrane surface after backwashing is significantly reduced, the bacterial activity is significantly reduced, and the proliferation and spread of bacteria on the membrane is effectively inhibited.

[0035] (3) The method of the present invention significantly reduces the rate of irreversible accumulation of biological components on the membrane, and the flux recovery rate remains above 90% after multiple backwashes.

[0036] (4) The method of the present invention maintains good membrane flux and significantly slows down the decline in membrane specific flux, which is significantly better than the coupling process of ferrate and ordinary membrane filtration.

[0037] (5) The application can be applied to practical engineering, can significantly reduce the frequency of hydraulic backwashing, prolong the service life of the membrane, improve the water production rate and operation stability, and reduce the operation cost. BRIEF DESCRIPTION OF DRAWINGS

[0038] The accompanying drawings, which form a part of the present application, are intended to provide further understanding of the present application, and are used to interpret the illustrative embodiments of the present application and their descriptions, and do not constitute improper limitations to the present application. In the drawings:

[0039] Figure 1 The device diagram of the membrane biofouling control method based on high ferrate pre-oxidation and conductive membrane filtration according to the embodiment of the present application;

[0040] Figure 2 The preparation process of the conductive membrane according to the embodiment of the present application;

[0041] Figure 3 The electron microscope morphology of the section of the CNTs / GO-PVDF conductive membrane prepared in the embodiment 1, wherein the upper to lower part in the figure is the separated skin layer, the finger-shaped hole support layer and the conductive layer;

[0042] Figure 4 The multi-cycle filtration-backwashing membrane flux decay curve of the embodiment 1 and the comparative examples 1-3;

[0043] Figure 5 The membrane filtration irreversible resistance of the embodiment 1 and the comparative examples 1-3 at the end of filtration;

[0044] Figure 6 The membrane flux recovery rate (FRR) comparison of the embodiment 1 and the comparative examples 1-3 after backwashing;

[0045] Figure 7 The SEM diagram of the filter cake layer section of the embodiment 1 and the comparative examples 1-3; a is ultrafiltration; b is conductive membrane; c is Fe(VI) pre-oxidation-ultrafiltration; d is Fe(VI) pre-oxidation-conductive membrane; the magnification of the filter cake layer in the figure is ×20k; the local magnification of the filter cake layer is ×80k;

[0046] Figure 8 The H2O2 production amount of the surface of the conductive membrane of the embodiment 1;

[0047] Figure 9 The membrane surface DMPO-•OH EPR spectrum of the pre-oxidation-conductive membrane of the embodiment 1 and the conductive membrane of the comparative example 1;

[0048] Figure 10Laser confocal images of distribution of live and dead bacteria on membrane surface after five cycles of filtration-backwashing for Example 1 and Comparative Examples 1-3; a1-a3: ultrafiltration, b1-b3: electrically conductive membrane, c1-c3: Fe(VI) pre-oxidation-ultrafiltration, d1-d3: Fe(VI) pre-oxidation-electrically conductive membrane; a1, b1, c1, d1: live bacteria, a2, b2, c2, d2: dead bacteria, a3, b3, c3, d3: comprehensive distribution of live and dead bacteria;

[0049] Figure 11 Comparison of total biomass on membrane surface for Example 1 and Comparative Examples 1-3;

[0050] Figure 12 Comparison of membrane specific flux decay curves for Example 1 and Comparative Examples 4-8;

[0051] Figure 13 Comparison of membrane filtration irreversible resistance for Example 1 and Comparative Examples 4-8;

[0052] Figure 14 Comparison of membrane flux recovery rate after backwashing for Example 1 and Comparative Examples 4-8. DETAILED DESCRIPTION

[0053] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict.

[0054] In the description of the present application, it should be understood that the terms "center", "longitudinal", "transverse", "upper", "lower", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the purpose of facilitating the description of the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application. In addition, the terms "first", "second" and the like are only for the purpose of description and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the technical features indicated. Therefore, the features defined with "first", "second" and the like can explicitly or implicitly include one or more of the features. In the description of the present application, unless otherwise stated, the meaning of "a plurality of" is two or more.

[0055] In the description of the present application, it should be noted that unless otherwise explicitly specified and limited, the terms "mounting", "connection", "linking" should be understood broadly, for example, it can be fixed connection, or detachable connection, or integrally connected; it can be mechanical connection, or electrical connection; it can be directly connected, or indirectly connected through intermediate medium, or internal communication of two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0056] The technical solutions in the embodiments of the present application will be described clearly and completely in the following with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0057] Example 1 Fe (VI) pre-oxidation-conductive membrane

[0058] A membrane biofouling control method based on high ferrate pre-oxidation and conductive membrane filtration, a water treatment device composed of a pre-oxidation reactor and a conductive membrane cross-flow filtration device is built, as shown in Figure 1 .

[0059] First, ferrate K2FeO4 is added to the water body to be treated in the pre-oxidation reactor for pre-oxidation treatment, the K2FeO4 dosage is 130 μmol / L, and the pre-oxidation time is 30 min after slow stirring and mixing in the pre-oxidation reactor.

[0060] After pre-oxidation, it is directly introduced into the conductive membrane filtration unit without precipitation. In the conductive membrane filtration unit, a direct current power supply is applied to the conductive membrane and a cathode potential is applied. The anode is a titanium plate, the electrode spacing is 1 cm, and the direct current voltage is applied to make the current density of the conductive membrane 1.5 mA / cm 2 .

[0061] Among them, the conductive membrane is a PVDF composite conductive membrane modified by blending carboxylated carbon nanotubes and graphene oxide. The carboxylated carbon nanotubes (CNTs) have an aspect ratio of 1500-2000 and a specific surface area of 280 m 2 / g, and a resistivity of 1000 Ω•m. The graphene oxide (GO) is single-layered, with a thickness of less than 0.7 nm, a diameter of 0.5-1 μm, a carbon content of 65%, and an oxygen content of 28%. The preparation method of the conductive membrane is as follows Figure 2As shown, specifically: the carboxylated carbon nanotubes and graphene oxide with a mass ratio of 100:15 were dispersed in anhydrous ethanol, stirred at 60°C for 8h, vacuum degassed for 6h, ultrasonic for 60min to form a dispersion, scraped to form a film and dried at 60°C for 5min to form a conductive layer; then PVDF casting solution was scraped on the formed conductive layer, and a double-layer structure conductive film was formed by phase inversion. Figure 3 The electron microscope morphology of the section of the prepared CNTs / GO-PVDF conductive film is shown in the figure, from top to bottom in turn is the separation skin layer, the finger-shaped hole support layer and the conductive layer, and the conductivity of the CNTs / GO-PVDF conductive film is 26S / m.

[0062] The initial flux of the membrane is 100L / (m 2 ·h·bar), the cross-flow filtration is carried out at 0.1MPa, the filtration period is set to 60min, and then hydraulic backwashing is carried out, the backwashing time is 1min, and the backwashing flux is 2 times of the filtration flux.

[0063] At the end of filtration-backwashing, the specific flux of the membrane decays to 67.3% of the initial value, and the irreversible resistance caused by irreversible pollution of the membrane is 0.11783×10 12 m -1 After backwashing, the recovery rate of the membrane flux is 94.3%.

[0064] Example 2 Fe (Ⅵ) pre-oxidation-conductive film

[0065] A membrane biological pollution control method based on high ferrate pre-oxidation and conductive membrane filtration, a water treatment device composed of a pre-oxidation reactor and a conductive membrane cross-flow filtration device is built, as shown in Figure 1 .

[0066] First, ferrate K2FeO4 is added to the water body to be treated in the pre-oxidation reactor for pre-oxidation treatment, the K2FeO4 addition amount is 160μmol / L, the pre-oxidation time is 30min.

[0067] After pre-oxidation, it is directly introduced into the conductive membrane filtration unit without precipitation, in the conductive membrane filtration unit, a direct current power supply is applied to the conductive membrane and a cathode potential is applied, the anode is a titanium plate, the electrode spacing is 1cm, and the direct current voltage is applied to make the current density of the conductive membrane be 2.5mA / cm 2 .

[0068] Among them, the conductive membrane is a PVDF composite conductive membrane modified by blending carboxylated carbon nanotubes and graphene oxide, the carboxylated carbon nanotubes (CNTs) have an aspect ratio of 3000-4000 and a specific surface area of 500m 2The graphene oxide (GO) is single-layered, with a thickness of less than 0.6 nm and a diameter of 1-2 μm, and has a carbon content of 75% and an oxygen content of not more than 22%. The conductive film is prepared as follows: carboxylated carbon nanotubes and graphene oxide with a mass ratio of 100:20 are dispersed in anhydrous ethanol, ultrasonic treatment is performed for 60 min to form a dispersion liquid, the dispersion liquid is coated to form a film, and the conductive layer is formed by drying at 60 ℃ for 5 min; then, PVDF casting solution is coated on the formed conductive layer, and a conductive film with a double-layer structure is formed through phase inversion. The conductivity of the prepared CNTs / GO-PVDF conductive film is 28 S / m.

[0069] The initial flux of the membrane is 60 L / (m 2 •h•bar), cross-flow filtration is performed at 0.1 MPa, the filtration period is set to 60 min, and then hydraulic backwashing is performed, the backwashing time is 1 min, and the backwashing flux is twice the filtration flux.

[0070] At the end of filtration-backwashing, the membrane specific flux decays to 68.5%, the irreversible resistance is 0.09834×10 12 m -1 , and the membrane flux recovery rate after backwashing reaches 95.1%.

[0071] Comparative Example 1: Filtration of a conductive film

[0072] The difference from Example 1 is that the method only uses a conductive film cross-flow filtration device for water treatment, that is, the water does not need to be pre-oxidized, but directly enters the conductive film filtration device for filtration, and the conductive film filtration parameters are completely the same as those in Example 1.

[0073] At the end of filtration-backwashing, the membrane specific flux decays to 56.5%, the irreversible resistance is 0.20386×10 12 m -1 , and the membrane flux recovery rate after backwashing reaches 89.1%.

[0074] Comparative Example 2: Ultrafiltration

[0075] The difference from Example 1 is that the method only uses a common ultrafiltration membrane filtration device for filtration, that is, the water does not need to be pre-oxidized, but directly enters the common ultrafiltration membrane filtration device for filtration, and no external voltage is applied, and the membrane filtration parameters are completely the same as those in Example 1.

[0076] At the end of filtration-backwashing, the membrane specific flux decays to 42.9%, the irreversible resistance is 0.41844×10 12 m -1 , and the membrane flux recovery rate after backwashing reaches 82.9%.

[0077] Comparative Example 3: Fe (Ⅵ) pre-oxidation-ultrafiltration

[0078] The difference from Example 1 is that the conductive membrane filtration device is replaced by a common ultrafiltration membrane filtration device, i.e. the water inlet is pre-oxidized by K2FeO4, and the water outlet enters the common ultrafiltration membrane filtration device. The K2FeO4 pre-oxidation conditions, membrane initial flux, filtration mode, filtration period, and backwashing conditions are completely the same as those in Example 1.

[0079] At the end of filtration-backwashing, the membrane specific flux decays to 47.8%, and the irreversible resistance is 0.25083 x 10 12 m -1 After backwashing, the membrane flux recovery rate reaches 87.7%.

[0080] In Comparative Example 4, the current density reaches 4 mA / cm 2

[0081] The difference from Example 1 is that the water inlet is pre-oxidized by K2FeO4, and the water outlet enters the conductive membrane filtration device. A direct current voltage is applied to make the current density of the conductive membrane 4 mA / cm 2 , and the rest of the conditions are completely the same as those in Example 1.

[0082] At the end of filtration-backwashing, the membrane specific flux decays to 45.9%, and the irreversible resistance is 0.17533 x 10 12 m -1 After backwashing, the membrane flux recovery rate reaches 86.1%.

[0083] In Comparative Example 5, the current density is 0 mA / cm 2

[0084] The difference from Example 1 is that the water inlet is pre-oxidized by K2FeO4, and the water outlet enters the conductive membrane filtration device. The conductive membrane is not electrified (i.e. the current density of the conductive membrane is 0 mA / cm 2 ), and the rest of the conditions are completely the same as those in Example 1.

[0085] At the end of filtration-backwashing, the membrane specific flux decays to 46.1%, and the irreversible resistance is 0.20048 x 10 12 m -1 After backwashing, the membrane flux recovery rate reaches 88.2 %.

[0086] In Comparative Example 6, the K2FeO4 dosage is high

[0087] The difference from Example 1 is that the water inlet is pre-oxidized by K2FeO4, and the water outlet enters the conductive membrane filtration device. The K2FeO4 dosage is 200 μmol / L, and the rest of the conditions are completely the same as those in Example 1.

[0088] At the end of filtration-backwashing, the membrane specific flux decays to 50%, and the irreversible resistance is 0.29386 x 10 12 m -1The membrane flux recovery rate reached 84.2% after backwashing.

[0089] The K2FeO4 dosage was low in Comparative Example 7

[0090] The difference from Example 1 is that the influent is pre-oxidized by K2FeO4, and the effluent enters the conductive membrane filtration device. The K2FeO4 dosage is 100 μmol / L, and the other conditions are exactly the same as in Example 1.

[0091] At the end of filtration-backwashing, the membrane specific flux decays to 42.94%, and the irreversible resistance is 0.3107 x 10 12 m -1 The membrane flux recovery rate reaches 80.3% after backwashing.

[0092] Comparative Example 8 K2FeO4 pre-oxidation-precipitation-effluent conductive membrane filtration

[0093] The difference from Example 1 is that the pre-oxidized effluent is first precipitated, and then enters the conductive membrane filtration unit, that is, the influent is pre-oxidized by K2FeO4, and is precipitated for 1 h, and the supernatant of the precipitation enters the conductive membrane filtration device, and the other conditions are exactly the same as in Example 1.

[0094] At the end of filtration-backwashing, the membrane specific flux decays to 53.45%, and the irreversible resistance is 0.1765 x 10 12 m -1 The membrane flux recovery rate reaches 86.5% after backwashing.

[0095] Comparative Example 9 Different proportions of graphene oxide and carboxylated carbon nanotubes

[0096] The difference from Example 1 is that the proportion of carboxylated carbon nanotubes to graphene oxide used in the preparation of the conductive membrane is 100:10, and the other conditions are exactly the same as in Example 1.

[0097] The conductivity of the prepared conductive membrane is only 20 S / m. Although the pure water flux of the conductive membrane is increased by 5% compared with Example 1, the pollutant interception performance is decreased by 15%, and the conductive membrane filtration performance is significantly decreased compared with Example 1. Since the basic filtration performance of the conductive membrane is significantly reduced at this ratio, there is no need to do further membrane pollution control experiments.

[0098] The experimental results show that:

[0099] By comparison Figure 4 , Figure 6 , Figure 12 , Figure 14It can be found that the membrane flux of embodiments 1-2 attenuates less and the recovery rate is also high, the flux recovery rate remains above 90% after multiple backwashing, which shows that when the pre-oxidation is combined with the conductive membrane, and the voltage applied to the conductive membrane and the K2FeO4 dosage meet the use requirements, the membrane flux remains good, the membrane flux attenuation amplitude is significantly slowed down, which is significantly better than other comparative examples. Moreover, the pre-oxidation effluent is first precipitated, which is not conducive to the recovery of the membrane.

[0100] The irreversible resistance describes the membrane resistance caused by the irreversible pollution of the membrane (which cannot be removed by hydraulic backwashing), the larger the value, the worse the recovery effect, by comparing Figure 5 and Figure 13 It can be found that the irreversible resistance of embodiments 1-2 is significantly better than comparative examples 1-8, which shows that the method of the application is more conducive to the recovery of the membrane and also more conducive to water treatment.

[0101] Figure 7 The middle red small box represents the filter cake layer on the membrane surface, and the lower right corner shows the morphology of the enlarged section of the filter cake layer. Figure 7 a is a common ultrafiltration membrane (referred to as ultrafiltration), Figure 7 b is a conductive membrane, Figure 7 c is pre-oxidation-ultrafiltration, Figure 7 d is pre-oxidation-conductive membrane. Under the operating conditions of cross-flow filtration, the filter cake layer on the membrane surface of the pre-oxidation-conductive membrane filtration process in the four processes is the thinnest, and the filter cake layer structure formed is also the most loose and porous. The loose and porous filter cake layer structure not only can improve the water permeability of the filter cake layer and slow down the attenuation of the membrane flux, but also is more easily washed away from the membrane surface by the water flow, thereby reducing the membrane pollution.

[0102] Figure 8 The amount of H2O2 generated on the surface of the conductive membrane of embodiment 1, which shows that the application of direct current makes the current density of the conductive membrane 1.5 mA / cm 2 , near the membrane surface, there will indeed be hydrogen peroxide generated, which provides the necessary conditions for the electro-Fenton reaction.

[0103] Figure 9 It shows that the hydroxyl radical signal in the ferrate pre-oxidation-conductive membrane process is significantly stronger than that in the conductive membrane, which proves that the electro-Fenton reaction indeed occurs in the coupling process, and more hydroxyl radicals are generated.

[0104] Figure 10 As can be seen in the middle, the number of active bacteria reaching the membrane surface of embodiment 1 is significantly reduced, the amount of irreversible attached bacteria on the membrane surface after backwashing is significantly reduced, and the bacterial activity is significantly reduced, effectively inhibiting the proliferation and spread of bacteria on the membrane. The number of active bacteria reaching the membrane surface of the other three conditions is obviously more.

[0105] Figure 11The phosphorus content on the membrane surface of the membranes of Example 1 and Comparative Examples 1-3 was determined by the lipophosphorus method, and the biomass was inversely calculated from the phospholipid content. It can be proved by the figure that in the ferrate pre-oxidation-conductive membrane process, the total amount of residual biomass (including live and dead bacteria) on the membrane surface after backwashing is the least. Together with Figure 10 it is jointly proved that in the coupling process, the irreversible cumulative biomass on the conductive membrane surface is the least, and the membrane fouling control effect is the best.

[0106] As can be seen from the above, only when the ferrate pre-oxidation dosage is appropriate, the pre-oxidation effluent directly enters the conductive membrane, the conductive property of the conductive membrane is good enough and the electric field strength is appropriate, can the ferrate pre-oxidation and the conductive membrane form a synergistic mechanism to jointly slow down the biological pollution of the membrane.

[0107] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A membrane biofouling control method based on ferrate pre-oxidation combined with conductive membrane filtration, characterized in that: First, the ferrate is added to the water to be treated for pre-oxidation treatment, and then the pre-oxidation effluent is directly introduced into the conductive membrane filtration unit without precipitation; in the conductive membrane filtration unit, a direct current power supply is applied to the conductive membrane and a cathode potential is applied, and the conductive membrane is a PVDF composite conductive membrane modified by blending carboxylated carbon nanotubes and graphene oxide; wherein the ferrate dosage is 130-160 μmol / L, the mass ratio of carboxylated carbon nanotubes to graphene oxide is 100: (15-25), the aspect ratio of carboxylated carbon nanotubes cannot be less than 1000, the graphene oxide is single-layer, the thickness is less than 1 nm, and the diameter is between 0.5-2 μm; The conductive membrane is prepared by a two-step phase inversion method, specifically: carboxylated carbon nanotubes and graphene oxide are dispersed in anhydrous ethanol to form a dispersion liquid by ultrasonic; then the obtained dispersion liquid is coated into a film, and after drying, a conductive layer is formed; then PVDF casting solution is coated on one side of the formed conductive layer, and a double-layer structure conductive membrane is formed by phase inversion. The current density of the conductive film is 1.5-2.5 mA / cm 2 .

2. The membrane biofouling control method based on the ferrate pre-oxidation and the conductive membrane filtration according to claim 1, characterized in that: The ferrate is potassium ferrate or sodium ferrate, and the pre-oxidation reaction time is 20-40 min.

3. The membrane biofouling control method based on ferrate pre-oxidation and synergistic conductive membrane filtration according to claim 1, characterized in that: The ultrasonic time is 30-60 min, the drying temperature is 40-60℃, and the drying time is 5-10 min.

4. The membrane biofouling control method based on ferrate pre-oxidation and conductive membrane filtration according to claim 1, characterized in that: The specific surface area of the carboxylated carbon nanotubes is greater than 210 m 2 / g and the resistivity is lower than 1500 Ω-m.

5. The membrane biofouling control method based on ferrate pre-oxidation and conductive membrane filtration according to claim 1, characterized in that: The carbon content of graphene oxide is not less than 60%, and the oxygen content cannot be higher than 30%.

6. The membrane biofouling control method based on ferrate (VI) pre-oxidation in combination with conductive membrane filtration according to claim 1, characterized in that: The conductivity of the conductive membrane is greater than 25 S / m, the electrode spacing is 1 cm, and the membrane filtration mode is cross-flow filtration.

7. The membrane biofouling control method based on ferrate pre-oxidation in combination with conductive membrane filtration according to claim 1, characterized in that: The membrane flux of the conductive membrane is 50-100 L / (m 2 •h•bar) when treating sewage.

8. The membrane biofouling control method based on ferrate (VI) pre-oxidation in combination with conductive membrane filtration according to claim 1, characterized in that: The conductive membrane filtration unit takes every 30-60 min as a filtration cycle, the hydraulic backwash flux is 2-3 times the filtration flux, and the backwash time is 1-2 min.

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