Ultrafiltration membrane for advanced treatment of sewage (waste) water and preparation process and treatment method thereof

Abstract

This invention relates to an ultrafiltration membrane for advanced wastewater treatment, its preparation process, and treatment method. The ultrafiltration membrane unit of this invention employs a hollow fiber membrane module with an internal helical support structure. When wastewater is treated using this ultrafiltration membrane, the wastewater passes through the hollow membrane in a rotating flow. The shear force prevents the formation of a laminar flow layer on the membrane surface, thus avoiding concentration polarization and the formation of a gel layer. This results in strong antifouling capabilities and allows for repeated use of the ultrafiltration membrane.

Description

Technical Field

[0001] This invention relates to an ultrafiltration membrane for deep treatment of sewage (wastewater), its preparation process and treatment method, belonging to the field of water pollution control. Background Technology

[0002] Water pollutants generated during ship transportation mainly include ship sewage, oily wastewater, and chemical tank cleaning water. Photocatalytic oxidation coupled with membrane separation technology has received widespread attention for advanced wastewater treatment and enhanced removal of recalcitrant organic pollutants. It can be applied to the advanced treatment of ship sewage, oily wastewater, and chemical tank cleaning water, efficiently removing organic pollutants from the water. Photocatalysis uses oxygen and water as raw materials and solar energy as energy to degrade pollutants in water in a sustainable manner without secondary pollution. Suspended photocatalytic coupled membrane separation systems uniformly disperse the photocatalyst in the feed water or solution, while the membrane material intercepts and separates the catalyst. This method increases the probability of contact between pollutants and the photocatalyst, improving photocatalytic efficiency. However, dispersed photocatalysts are difficult to recover and reuse, and are prone to aggregation, reducing catalytic efficiency. Hollow fiber membrane modules are a commonly used component type in suspended photocatalytic coupled membrane separation systems due to their high packing density and small footprint. When the photocatalyst is transported to the membrane separation unit for recovery along with the treated solution, the catalyst may deposit on the membrane surface or in the transport pipeline, causing membrane fouling and affecting membrane separation performance, thus reducing photocatalytic efficiency. Meanwhile, frequent membrane cleaning caused by membrane fouling will also increase operating costs.

[0003] For hollow fiber internal pressure membranes, to ensure smooth liquid flow within the hollow fiber tubes, the membrane body needs to maintain high support. Furthermore, currently produced hollow fiber membranes worldwide are hollow straight filaments. The feed liquid does not change direction while flowing inside the membrane filaments. Under pressure, a laminar layer forms near the inner wall (dense layer), leading to concentration polarization and the formation of a gel layer, thus reducing membrane flux. CN114939353A discloses a hollow fiber membrane with a helical structure. In the disclosed embodiments, fiber filaments are wound in a helical form on the surface of the hollow fiber base membrane, forming a hollow fiber membrane with a helical structure. Utilizing the helical structure on the outer surface of the hollow fiber base membrane, a specific spacing can be maintained between the membranes to preserve fluid channels, prevent membrane compression, and reduce the contact area between the membrane and gas or liquid. This solves the problem of easily forming localized dense stacking areas or voids within the membrane contact reactor, leading to poor fluid flow and problems such as channeling, short circuits, and dead zones. However, in this patent, the feed liquid does not change its flow direction inside the membrane fibers. Although the shear stress generated by the cross-flow velocity can reduce the concentration polarization phenomenon inside the membrane to a certain extent, it cannot prevent the formation of filter cake layer on the membrane surface as the membrane module operates for a longer period of time.

[0004] Therefore, it is necessary to design an ultrafiltration membrane for the deep treatment of sewage (wastewater) and its preparation process and treatment method. Summary of the Invention

[0005] The present invention aims to provide an ultrafiltration membrane for advanced treatment of sewage (wastewater) and its preparation process and treatment method.

[0006] This invention provides an ultrafiltration membrane for deep treatment of wastewater. It adopts a hollow fiber membrane module with an internal spiral support structure, which allows the feed liquid to pass through the hollow membrane in a rotating flow. The turbulence generated by the constant change of the flow direction of the feed liquid generates stronger shear force on the membrane surface. The shear force prevents the feed liquid from forming a laminar layer on the membrane surface, avoids concentration polarization, and prevents the formation of a gel layer, thus preventing the deposition of pollutants on the membrane surface and exhibiting strong antifouling ability.

[0007] The filter membrane has an outer diameter of 0.5-3.0 mm, an inner diameter of 0.3-2.0 mm, an internal spiral support pitch of 0.5-2 mm, and a molecular weight cutoff of 1 kDa to 300 kDa.

[0008] The spiral support structure and the outer membrane are formed from the same polymer material in one piece; the spiral support is continuously arranged along the axial direction with a spiral angle of 15°–45°; the height of the spiral support is 5%–20% of the inner diameter of the membrane; the structure enables the fluid to form a continuous rotating turbulent flow state in the membrane cavity.

[0009] This invention provides a process for preparing an ultrafiltration membrane for advanced wastewater treatment: S1: Polysulfone, solvent, pore-forming agent and additives are heated and dissolved, and then allowed to stand to degas before the casting solution is obtained; S2: The treated spinning casting solution is extruded through a spinneret to form a film; The spinneret has an outer annular orifice and a rotatable inner helical orifice, wherein the outer annular orifice is rotatable. The interior of the annular orifice and the helical orifice contains casting fluid, while the exterior contains core fluid. S3: During the molding process, the inner side of the spinning casting liquid in the annular hole exchanges phase with the spinning core liquid to form a dense separation skin. The spinning casting liquid in the spiral hole exchanges phase with the core liquid to form a spiral support. The two parts of the spinning casting liquid simultaneously enter the air path under the action of gravity and traction to deform, eliminate some stress, and form a hollow filter membrane with an outer layer as the filter base membrane and an inner spiral support.

[0010] The ultrafiltration membrane is wound into filaments under the traction of a membrane filament winding machine.

[0011] Optionally, the membrane material of the spinning casting solution is polysulfone, polyethersulfone, or polyvinylidene fluoride.

[0012] Optionally, the solvent of the spinning casting solution includes one or more of dimethylformamide (DMF), dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), and triethyl phosphate (TEP).

[0013] Optionally, the pore-forming agent in the spinning casting solution includes one or more of polyvinyl alcohol, polyvinylpyrrolidone, vinylpyrrolidone / vinyl acetate copolymer, and vinylpyrrolidone / vinylimidazol copolymer.

[0014] Optionally, the additives in the spinning casting solution include one or more of ethylene glycol monomethyl ether, ethylene glycol dimethyl ether, propylene glycol dimethyl ether, diethylene glycol dimethyl ether, and methyltetrahydrofuran.

[0015] Optionally, the dissolution temperature of the spinning casting solution is 70~100℃, and more specifically 80~85℃.

[0016] Optionally, the spinning temperature of the spinning casting solution is controlled at 25~40℃, and further at 25~35℃.

[0017] Optionally, the operating pressure is between 0.2 and 0.6 MPa.

[0018] This invention provides a method for advanced treatment of wastewater, comprising the following steps: S1 Pretreatment: Ship sewage, ballast water or chemical tank cleaning water undergoes coagulation and sedimentation pretreatment to remove suspended solids and other impurities from the water; S2 Catalytic Oxidation: The effluent from the pretreatment unit enters the photocatalytic oxidation unit, where pollutants in the wastewater are degraded and removed by visible light photocatalytic oxidation. S3 Filtration: The effluent from the photocatalytic oxidation unit enters the membrane filtration unit for further removal of pollutants. The photocatalyst carried into the membrane unit is retained by the ultrafiltration membrane and returned to the photocatalytic oxidation unit for reuse. The effluent is reused or discharged after meeting the standards. S4 Recycling: Ship sewage, ballast water, or chemical tank cleaning water treated by the membrane unit are discharged or reused in compliance with standards.

[0019] Optionally, the photocatalytic unit uses a synthesized powdered photocatalyst, which comes into full contact with pollutants in the waste, and the pollutants are degraded and removed under visible light. Optionally, the catalyst material is a synthesized Ag / Pg-C3N4 composite material.

[0020] The present invention has the following beneficial effects: The ultrafiltration membrane of this invention has a helical support structure, which allows the feed liquid to pass through the hollow membrane in a rotating flow manner. The shear force present prevents the feed liquid from forming a laminar flow layer on the membrane surface, thus avoiding concentration polarization and the formation of a gel layer, resulting in strong antifouling ability; the membrane flux decay rate is reduced by ≥30%; the concentration polarization layer thickness is reduced by ≥25%; and the flux retention rate is ≥80% after 72 hours of continuous operation.

[0021] The advanced wastewater treatment method of this invention increases the contact frequency between the catalyst and pollutants, reduces catalyst deposition on the membrane surface, decreases catalyst agglomeration, and improves catalyst utilization efficiency and catalytic performance. The catalyst recovery rate is ≥90%, and the performance shows no significant degradation after ≥10 cycles. The COD removal rate is 10–20% higher than that of ordinary ultrafiltration membranes with the same pore size and filtration area. The transmembrane pressure rise rate is low, eliminating the need for frequent backwashing and saving system operating energy. Attached Figure Description

[0022] Figure 1 Cross-sectional view of ultrafiltration membrane fibers Figure 2 Perspective view of ultrafiltration membrane Detailed Implementation

[0023] The technical solution of the present invention will be further described and illustrated below through specific embodiments, but the present invention is not limited to the following embodiments.

[0024] Unless otherwise specified, the raw materials and other chemical reagents used in the embodiments of the present invention are all commercially available products.

[0025] Example 1 like Figure 2 As shown, this embodiment of the invention provides an ultrafiltration membrane for deep treatment of wastewater. The ultrafiltration membrane is a hollow fiber membrane module with an internal spiral support structure, allowing the feed liquid to pass through the hollow membrane in a rotating flow. The spiral support structure and the outer membrane body are formed from the same polymer material in a single molding process. The spiral support is continuously arranged along the axial direction with a spiral angle of 25°. The height of the spiral support is 10% of the membrane's inner diameter; the pitch is 1 mm. This structure creates a continuous rotating, turbulent flow pattern within the membrane cavity. The ultrafiltration membrane has an outer diameter of 3.0 mm, an inner diameter of 2.0 mm, an internal spiral support pitch of 1 mm, and a molecular weight cutoff of 10 kDa.

[0026] Example 2 A process for preparing an ultrafiltration membrane for advanced wastewater treatment: S1: At 85°C, dissolve 17 wt% polysulfone in 72 wt% dimethylacetamide, stir for 8 hours, then add 6 wt% ethylene pyrrolidone / vinyl acetate copolymer and 5 wt% diethylene glycol dimethyl ether, continue stirring until completely dissolved, and let stand to remove bubbles before use.

[0027] S2: The treated spinning casting solution is extruded through a spinneret to form a film; The spinneret has an outer annular orifice and a rotatable inner helical orifice, wherein the outer annular orifice is rotatable. The interior of the annular orifice and the helical orifice contains casting fluid, while the exterior contains core fluid. S3: During the molding process, the inner side of the spinning casting liquid in the annular hole exchanges phase with the spinning core liquid to form a dense separation skin. The spinning casting liquid in the spiral hole exchanges phase with the core liquid to form a spiral support. The two parts of the spinning casting liquid simultaneously enter the air path under the action of gravity and traction to deform, eliminate some stress, and form a hollow filter membrane with an outer layer as the filter base membrane and an inner spiral support.

[0028] The ultrafiltration membrane is wound into filaments under the traction of a membrane filament winding machine.

[0029] Example 3 The steps for advanced wastewater treatment using the aforementioned ultrafiltration membrane are as follows: S1 Pretreatment: Shipboard domestic wastewater with COD 300–500 mg / L and turbidity 50 NTU is adjusted to pH 6.8–7.2. 100 mg / L polyaluminum chloride (PAC) and 2 mg / L anionic polyacrylamide (PAM) are added to the wastewater, which is then introduced into an inclined plate sedimentation tank (inclination angle: 55–60°). After a retention time of 15–30 min, the pretreated effluent is obtained. After treatment, the turbidity is reduced to below 25 NTU. S2 Catalytic Oxidation: The effluent from the pretreatment unit enters the photocatalytic oxidation unit. The photocatalyst is Ag / Pg-C3N4, with a concentration of 0.5 g / L and a particle size of 50–200 nm. The oxidation is carried out under visible light intensity of 200 W / m². 2 Under normal operating conditions, pollutants in wastewater are degraded and removed by visible light photocatalytic oxidation, achieving a COD removal rate of 90%. S3 Filtration: The effluent from the photocatalytic oxidation unit enters the membrane filtration unit for further removal of pollutants. The influent flow rate is controlled at 50-60 L / h, the operating pressure is 0.4 MPa, the temperature is 25℃, and the average axial flow velocity is 0.6 m / s. The photocatalyst carried into the membrane unit is retained by the ultrafiltration membrane and then returned to the photocatalytic oxidation unit for reuse. The effluent COD removal rate is 92%, the turbidity is less than 1 NTU, and the photocatalyst recovery rate is 90%.

[0030] S4 Recycling: Shipboard sewage treated by the membrane unit meets discharge standards or is reused.

[0031] Example 4 Referring to the treatment method in Example 3, a hollow fiber ultrafiltration membrane system of the same area was used to replace the ultrafiltration membrane described in this invention for wastewater treatment. The COD removal rate of the effluent was 81%, the turbidity was less than 1 NTU, and the photocatalyst recovery rate was 76%.

[0032] The operation and processing effects of the ultrafiltration membrane system of the present invention (Example 3) and the hollow fiber ultrafiltration membrane system of the same area (Example 4) are compared as follows: The above embodiments were chosen to best illustrate and describe the present invention, but are not intended to exhaustively describe the precise forms disclosed herein. Many modifications and variations are possible so that those skilled in the art can best utilize the present invention, the scope of which is defined by the appended claims.

Claims

1. An ultrafiltration membrane for deep treatment of sewage (wastewater), employing a hollow fiber membrane module with an internal spiral support structure.

2. The ultrafiltration membrane as described in claim 1, wherein, The membrane has an outer diameter of 0.5-3.0 mm, an inner diameter of 0.3-2.0 mm, an internal spiral support pitch of 0.5-2.0 mm, and a molecular weight cutoff of 1 kDa to 300 kDa.

3. The ultrafiltration membrane as described in claim 1 or 2, wherein, The spiral support structure and the outer membrane are molded from the same polymer material in one piece; the spiral support is continuously arranged along the axial direction with a spiral angle of 15°–45°; the height of the spiral support is 5%–20% of the inner diameter of the membrane; the pitch is 0.5–2.0 mm; the structure enables the fluid to form a continuous rotating turbulent flow state in the membrane cavity.

4. A process for preparing an ultrafiltration membrane for advanced wastewater treatment as described in claim 1: S1: Polysulfone, solvent, pore-forming agent and additives are heated and dissolved, and then allowed to stand to degas before the casting solution is obtained; S2: The treated spinning casting solution is extruded through a spinneret to form a film; The spinneret has an outer annular orifice and a rotatable inner spiral orifice, wherein the outer annular orifice is rotatable, the interior of the annular orifice and the spiral orifice are casting liquids, and the exterior is a core liquid; S3: During the molding process, the inner side of the spinning casting solution in the annular hole exchanges phase with the spinning core solution, forming a dense separation skin layer. The spinning casting solution in the spiral hole exchanges phase with the core solution, forming a spiral support. Both parts of the spinning casting solution simultaneously enter the air path under the action of gravity and traction, undergoing deformation to eliminate some stress, forming a hollow filter membrane with an outer layer as the filter base membrane and an inner spiral support. The ultrafiltration membrane is wound into filaments under the traction of a membrane filament winding machine.

5. The preparation process according to claim 3, wherein, The membrane material of the spinning casting solution is polysulfone, polyethersulfone, or polyvinylidene fluoride; the solvent of the spinning casting solution includes one or more of dimethylformamide (DMF), dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), and triethyl phosphate (TEP).

6. The preparation process according to claim 3, wherein, The pore-forming agent of the spinning casting solution includes one or more of polyvinyl alcohol, polyvinylpyrrolidone, ethylene pyrrolidone / vinyl acetate copolymer, and ethylene pyrrolidone / vinyl imidazole copolymer.

7. The preparation process according to claim 3, wherein, The additives in the spinning casting solution include one or more of ethylene glycol monomethyl ether, ethylene glycol dimethyl ether, propylene glycol dimethyl ether, diethylene glycol dimethyl ether, and methyltetrahydrofuran.

8. The preparation process according to claim 3, wherein, The dissolution temperature of the spinning casting solution is 70~100℃, and more specifically 80~85℃.

9. A method for deep treatment of wastewater using the ultrafiltration membrane according to claim 1, comprising the following steps: S1 Pretreatment: Ship sewage, ballast water or chemical tank cleaning water undergoes coagulation and sedimentation pretreatment to remove suspended solids and other impurities from the water; S2 Catalytic Oxidation: The effluent from the pretreatment unit enters the photocatalytic oxidation unit of the ultrafiltration membrane, where pollutants in the wastewater are degraded and removed by visible light photocatalytic oxidation. S3 Filtration: The effluent from the photocatalytic oxidation unit enters the membrane filtration unit for further removal of pollutants. The photocatalyst carried into the membrane unit is retained by the ultrafiltration membrane and returned to the photocatalytic oxidation unit for reuse. The effluent is reused or discharged after meeting the standards. S4 Recycling: Ship sewage, ballast water, or chemical tank cleaning water treated by the membrane unit are discharged or reused in compliance with standards.

10. The processing method as described in claim 9, wherein, The photocatalytic unit uses a synthesized powdered photocatalyst, which comes into full contact with pollutants in the waste, and the pollutants are degraded and removed under the action of visible light.

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