Preparation of magnetic flocculant and application method of magnetic flocculant in purification of combined overflow pollution

Abstract

The invention discloses a preparation method of a magnetic flocculant and an application method of the magnetic flocculant in purification of combined overflow pollution, and is characterized in that cellulose is used as a matrix to graft an acrylamide monomer and a cationic monomer to form a flocculant skeleton structure, and a magnetic core is grafted in the flocculant skeleton structure; the synergistic effect of the cationic monomer, the acrylamide and the magnetic seed enhances the electrical neutralization flocculation effect, increases the branch chain length of the flocculant, improves the rapid precipitation capability of flocs, and ensures the reuse efficiency of the flocculant; when the magnetic flocculant is applied to on-site treatment of combined system overflow pollution, and the adding amount of the single magnetic flocculant is 100-600mg / L, the SS removal rate of combined system overflow sewage is gt; compared with a conventional water treatment process, the sedimentation efficiency is as high as more than two times; 80% of SS can be removed after 0.5 min of sedimentation, and the recovery efficiency of the magnetic seeds and the removal rate of the SS are both gt when the magnetic seeds are reused for 10 times; 80%. When being applied to primary enhanced treatment of a combined system overflow pollution water plant, the composite flocculant is compounded with an inorganic flocculant, and when the dosage of the inorganic flocculant is 120mg / L, the removal rate of SS (suspended solids) can reach 96.1%, the highest removal rate of COD (chemical oxygen demand) is 44.7%, and the highest removal rate of TP (total phosphorus) is 93.3%; the magnetic flocculant provided by the invention realizes efficient and rapid removal of combined system overflow pollution in different treatment modes.

Description

Technical Field

[0001] The invention relates to a preparation method of a magnetic flocculant and its application in purifying water treatment of combined sewer overflow pollution, belonging to the field of water pollution control. Background Art

[0002] In the early days of my country, cities were limited in size. Combined sewer systems were widely used to save on pipes and reduce maintenance costs. However, with the acceleration of urbanization, the limitations of the combined sewer network have gradually become apparent. During heavy rainfall, combined sewers are prone to overflow pollution due to overload. Untreated overflow pollution directly discharged into the receiving water will consume a large amount of dissolved oxygen in the water, affecting the survival of aquatic plants and animals and the water ecology. Due to the limitations of space conditions and the requirements of historical and cultural protection in most old urban areas, it is extremely difficult to renovate combined sewers. In order to reduce the risk of pollution, efficient control of combined sewer pollution has become a major issue related to urban water safety and human health.

[0004] Overflow sewage has an initial effect, a high content of suspended solids, and a relatively serious pollution load. According to literature research, the SS in overflow sewage is as high as 300-500 mg / L, and the COD is as high as 200-400 mg / L. In addition, the characteristics of non-continuity and timing also make it difficult to control the combined system overflow pollution. Although my country does not have a direct discharge standard to stipulate its specific water quality discharge limit, the overflow water quality should meet the local or national standards as much as possible, mainly referring to the relevant requirements in the "Water Quality Standard for Sewage Discharge into Urban Sewers" (GB / T31962-2015) and the "Comprehensive Sewage Discharge Standard" (GB 8978-1996). The current control technology for overflow pollution is reviewed. In order to achieve efficient and comprehensive control of combined system overflow pollution, the terminal treatment of combined system overflow pollution is very necessary. In the current terminal treatment process, chemically enhanced primary treatment with "magnetic flocculation" as the core is often used to control overflow pollution due to its short residence time and small footprint. Considering the water characteristics and disposal requirements of combined sewer overflow pollution, new challenges have been raised for the treatment of overflow pollution by magnetic flocculation, that is, when treating it, it is necessary to achieve faster and more efficient removal at a lower cost within a smaller dosage range as much as possible.

[0005] Nowadays, magnetic flocculation technology has been applied to a certain extent. The magnetic flocculant technology has gradually developed from magnetic flocculation to the preparation and development of magnetic flocculants. Patent CN 116062939A achieves good removal of mine water by adding 100-500 mg / L polyaluminium chloride, 5-10 mg / L polyacrylamide and 10-30 mg / L magnetic seeds when treating coal slime water in mines; Patent CN 111470575A adopts a one-pot method to add PAC or PAM during the preparation of magnetic ferroferric oxide to achieve efficient removal of phosphorus; Patent CN 118439706A synthesizes a new composite magnetic flocculant with both flocculation characteristics and magnetism by adding silane coupling agent to connect ferroferric oxide with methacrylic acid, and can achieve efficient removal of kaolin by settling for 8 minutes; Patent CN 115124120A also reduces the phenomenon of floc separation caused by external forces such as stirring through the modification of silane coupling agent, thereby promoting the improvement of its pollutant purification efficiency.

[0006] It can be seen that the conventional magnetization process is gradually being replaced due to its large dosage, complex dosing process, limited magnetic seed recovery efficiency in the formed magnetic sludge, limited sedimentation efficiency and other shortcomings. The magnetic flocculants that are gradually being developed are also developed from the ionic bond force of simple stirring to the current covalent bond connection. These patents have proven that connecting magnetic seeds with flocculants can reduce the phenomenon of floc separation caused by external forces such as stirring, enhance the tolerance of magnetic flocculants under external force interference, and promote the improvement of their pollutant purification efficiency, but they still have the following disadvantages: when removing highly polluted water bodies, the dosage is still >1g / L (the dosage of patent CN 115124120A and patent CN 110354815 A is 20g / L), and the treatment efficiency is maintained at 85%; the sedimentation time is generally maintained at about 3-10 minutes, such as patent CN114873705A, but some patents even require standing under the magnetic field for 12 hours (CN 112142174A); the recovery efficiency is generally maintained at 70% to 90%. If it is used for on-site treatment of overflow pollution, it still cannot meet the requirements of rapid sedimentation (<2min) at a relatively low dosage, and it has not yet demonstrated the removal of multiple pollutants. In addition, the preparation cost and complex process still restrict its promotion in practical applications.

[0007] Based on the above patent review, on the basis of adopting magnetic seeds and flocculants for grafting, in order to effectively improve the flocculation performance and application cost, this patent adopts natural polymer flocculant cellulose as the skeleton structure to synthesize organic flocculants. Cellulose, as one of the most abundant organic compounds on the earth, is widely present in plant cell walls and can be extracted from various plant materials (such as wood, cotton, straw). Its raw material cost is lower, the access to it is more extensive, and the production cost is lower. The patent has shown that CN106477732A uses cellulose as the skeleton structure, which not only has excellent removal efficiency, but also reduces the production cost and is pollution-free to the environment. In addition, in order to make it have a better removal effect on the most critical pollutant suspended particles in overflow pollution, monomers that can interact with it are selected for grafting. Cationic monomers can not only remove pollutants by electrical neutralization, but also, to a certain extent, broaden the pH adaptability range of the flocculant and improve the removal efficiency of pollutants, such as the cationic cellulose-based flocculant synthesized by homogeneous dissolution system and free radical reaction in patent CN 117285681A; and acrylamide, as a common coagulant, can also enhance the removal of pollutants through its bridging and net-catching functions after synthesis (patent CN 108996647 B). In addition, there is no patent for synthesizing magnetic flocculants using cellulose grafted acrylamide monomers and cationic monomers as the skeleton, and its efficiency in combined sewer overflow pollution has not been explored.

[0008] Therefore, this patent uses cellulose, which is widely present in nature, as the matrix, grafted with cationic monomers and long chains of acrylamide, fully exerting its electrical neutralization and net-catching bridging capabilities to enhance its treatment efficiency. At the same time, a modifier is used to graft the magnetic core into the flocculant skeleton structure, and a magnetic flocculant suitable for combined sewer overflow pollution is synthesized from the aspects of sedimentation rate, removal efficiency, and recovery efficiency. The process conditions under different treatment modes, including on-site treatment and primary enhanced treatment in water plants, are explored to achieve efficient and rapid removal of overflow pollution. Summary of the invention

[0009] The present invention provides a method for preparing a magnetic flocculant and applying the same to purify combined sewer overflow pollution. The magnetic flocculant uses cellulose as a matrix to graft acrylamide monomers and cationic monomers to form a flocculant skeleton structure, and a magnetic core is grafted into the flocculant skeleton structure. In addition, the synergistic effect of the cationic monomer, acrylamide and magnetic species enhances the electrical neutralization effect during the flocculation process, enhances the branch chain length of the flocculant, improves the ability of flocs to precipitate quickly, ensures the reuse efficiency of the flocculant, and realizes the efficient removal of combined sewer overflow pollution under different treatment modes.

[0010] The present invention provides a method for preparing a magnetic flocculant and applying the same to purify combined sewer overflow pollution, wherein the magnetic flocculant can be prepared according to the following method:

[0011] ① Preparation method of double-bond loaded Fe3O4 nanoparticles: weigh 2-4g Fe3O4 nanoparticles, add them to a mixed solution of ethanol and water, wherein the ratio of ethanol to water is 4-5:1, add 9-12mL ammonia water thereto, ensure that the reaction is carried out under alkaline conditions, and then ultrasonically treat the mixed suspension for 30min to make the Fe3O4 nanoparticles uniformly dispersed in the alkaline solution. After the ultrasonic treatment, add 8-10mL of vinyl triethoxysilane, a silane coupling agent containing double bonds, to the reaction container, place the reaction container in a water bath at a temperature of 50°C, and react at a stirring speed of 250-300rpm / min for 6h. The entire reaction needs to be carried out in a nitrogen atmosphere. After the reaction is completed, the product is separated by a magnet, and repeatedly washed with ethanol and deionized water until the supernatant is transparent and the pH is maintained at a neutral level.

[0012] ② Dissolution of cellulose: weigh 7g of sodium hydroxide (NaOH) and 12.0g of urea (Urea), add them to 81mL of deionized water, stir at 800rpm / min to completely dissolve them to obtain an alkaline solution, put it in a 4℃ refrigerator to cool for 30min after dissolution, measure 30mL of the cooled alkaline solution, add 1.5g of microcrystalline cellulose, stir at 800rpm / min to dissolve it into a milky white mixed solution, and then put it in a -12℃ refrigerator for precooling for 1.5 to 2h to completely dissolve it into a transparent and uniform cellulose dispersion;

[0013] ③ Grafting of magnetic flocculant: Weigh 4g of the loaded double-bond Fe3O4 nanoparticles obtained in step ①, add them to 80-100mL of deionized water, then add 30mL of cellulose solution obtained in step ② and mix, ultrasonicate for 30min to make it uniformly dispersed, after the ultrasonicate is finished, gradually add 2-4g of acrylamide monomer and 9-11g of dimethyldiallylammonium chloride cationic monomer to the reaction container, stir at a speed of 500rpm / min for 15min to make it fully mixed, and then introduce nitrogen for 30m in, to exclude oxygen from the reaction solution, slowly drop a potassium persulfate solution with a concentration of 0.166-0.184 mol / L at 45°C, and ensure that the entire reaction is carried out in a nitrogen atmosphere for 6-8 hours. After the reaction is completed, the product is separated by a magnet, and the product is repeatedly washed with ethanol and deionized water until the supernatant of the solution is transparent and the pH is kept neutral. The product after washing is dried in a vacuum oven at 60°C, and the dried product powder is ground to control the particle size to be less than 150 microns, thereby completing the grafting of the magnetic flocculant;

[0014] ④ Surface characteristics of magnetic flocculants: The surface of magnetic flocculants presents obvious colloidal cross-linked structure and has obvious wrinkles. The particles are stably connected to the surface of flocculants. The grafting rate of flocculants is maintained at 37% to 62%. The Zeta potential of the synthesized magnetic flocculants is 16.4 to 21.9 mV at a concentration of 600 mg / L, and 28.84 to -10.06 mV at a pH of 2.00-12.00. pzc =11.23, and the saturation magnetization is 35.88emu / g.

[0015] The application method of the magnetic flocculant for purifying combined sewer overflow pollution of the present invention can be carried out according to the following steps:

[0016] ① The magnetic flocculant prepared as above is suitable for on-site treatment of combined sewer overflow pollution or for enhanced primary treatment of sewage treatment plants during the rainy season.

[0017] ② When the magnetic flocculant prepared as above is used for on-site treatment of combined sewer overflow pollution, it is characterized in that, under the coagulation process conditions of fast stirring speed 450rpm / min, fast stirring time 1min, slow stirring speed 70rpm / min, slow stirring time 20min, magnetic sedimentation 2min, magnetic field strength 0.1-0.2T (1000-2000G), when the dosage of magnetic flocculant is selected as 100-600mg / L, the removal rate of suspended matter in combined sewer overflow is 95.8%-98.5%, the removal efficiency of COD is as high as 37.5%, the floc particle size after coagulation is about 187.627μm, and the sedimentation efficiency is 2.2455min -1 ;

[0018] ③ When the magnetic flocculant prepared as above is applied to the water plant with combined sewer overflow pollution to strengthen the primary treatment of multiple pollutants, it is characterized in that it is compounded with inorganic flocculant polyferric chloride according to different water quality backgrounds. When the influent turbidity is high and the suspended solids concentration is large, the magnetic flocculant (600 mg / L) should be added first, and then PFC should be added. When the PFC dosage is 120 mg / L, the removal rate of SS in the combined sewer overflow pollution can reach up to 96.1%, the removal rate of COD can reach up to 44.7%, and the removal rate of TP can reach up to 93.3%; when the influent suspended particles are fine and the turbidity is relatively low, PFC should be added first and then the magnetic flocculant (600 mg / L). When the PFC dosage is 80 mg / L, the removal rate of SS in the combined sewer overflow pollution can reach up to 87.4%, the removal rate of COD can reach up to 37.04%, and the removal rate of TP can reach up to 93.3%. The highest removal rate of TP is 72.6%; the step-by-step coagulation process conditions are: fast stirring speed of 450rpm / min, fast stirring time of 0.5min for dosing 1, fast stirring speed of 450rpm / min, fast stirring time of 0.5min for dosing 2, slow stirring speed of 70rpm / min, slow stirring time of 20min, magnetic sedimentation for 2min, and magnetic field intensity range of 0.1-0.2T (1000-2000G); when the water temperature is low in winter, magnetic flocculant (600mg / L) and inorganic flocculant need to be added at the same time to ensure the removal effect. When the dosage of inorganic flocculant is 100mg / L, the removal rate of SS in combined sewer overflow pollution can be 88.75%, the removal rate of COD is 28.24%, and the removal rate of TP is 84.12%. The coagulation process conditions added at the same time are consistent with the on-site treatment process of combined sewer overflow pollution.

[0019] ④ After the magnetic flocculant prepared above purifies the overflow pollution of the combined system, the recovery efficiency is as high as 80% to 100%, and the saturation magnetization intensity is still maintained at 32.03 to 34.58 emu / g.

[0020] The superior effect of the present invention lies in that: the present invention proposes a preparation method of a magnetic flocculant, a magnetic core is grafted into the flocculant skeleton structure, compared with the traditional magnetic flocculant, the addition method is simplified, the selection of cellulose monomer is cheaper and easier to obtain, and acrylamide monomer and cationic monomer are further grafted on cellulose as a matrix to form the flocculant skeleton structure. In addition, the synergistic effect of the cationic monomer, acrylamide and magnetic species enhances the electrical neutralization effect in the flocculation process, enhances the branch chain length of the flocculant, improves the ability of flocs to precipitate quickly, ensures the reuse efficiency of the flocculant, and explores the process parameters and removal efficiency of the on-site treatment process for overflow pollution and the enhanced primary short process of the water plant, thereby realizing efficient and rapid removal of combined sewer overflow sewage in different modes. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 (a) Infrared spectrum of magnetic flocculant, (b) X-ray diffraction pattern, (c) magnetic intensity pattern of vibrating sample, (d) scanning electron microscopy and element distribution pattern

[0022] Figure 2 Comparison of (a) removal efficiency and (b) sedimentation rate between magnetic flocculants and conventional flocculants

[0023] Figure 3 (a) Repeated removal efficiency and recovery efficiency of magnetic flocculants in removing combined sewer overflow pollution, (b) Vibrating sample magnetic intensity map before and after repeated use of magnetic flocculants

[0024] Figure 4 Removal efficiency of (a) SS, (b) COD and (c) TP in different combinations of magnetic flocculants and conventional polyferric chloride DETAILED DESCRIPTION

[0025] The present invention is further described in detail with reference to specific embodiments, but the present invention is not limited to the following embodiments.

[0026] Example 1 Preparation of magnetic flocculant

[0027] ① Preparation method of double-bond loaded Fe3O4 nanoparticles: weigh 4g of Fe3O4 nanoparticles, add them to a mixed solution of ethanol and water, wherein the ratio of ethanol to water is 5:1, add 12mL of ammonia water to ensure that the reaction is carried out under alkaline conditions, and then ultrasonically treat the mixed suspension for 30min to make the Fe3O4 nanoparticles evenly dispersed in the alkaline solution. After the ultrasonic treatment, add 9mL of vinyl triethoxysilane, a silane coupling agent containing double bonds, to the reaction container, place the reaction container in a water bath at a temperature of 50°C, and react at a stirring speed of 250-300rpm / min for 6h. The entire reaction needs to be carried out in a nitrogen atmosphere. After the reaction, separate the product with a magnet, and wash it repeatedly with ethanol and deionized water until the supernatant is transparent and the pH is maintained at neutral.

[0028] ② Dissolution of cellulose: weigh 7g of sodium hydroxide (NaOH) and 12.0g of urea (Urea), add them to 81mL of deionized water, stir at 800rpm / min to completely dissolve them to obtain an alkaline solution, put it in a 4℃ refrigerator to cool for 30min after dissolution, measure 30mL of the cooled alkaline solution, add 1.5g of microcrystalline cellulose, stir at 800rpm / min to dissolve it into a milky white mixed solution, and then put it in a -12℃ refrigerator for precooling for 2h to completely dissolve it into a transparent and uniform cellulose dispersion;

[0029] ③ Grafting of magnetic flocculant: Weigh 4g of the loaded double-bond Fe3O4 nanoparticles obtained in step ①, add them to 100mL of deionized water, then add 30mL of the cellulose solution obtained in step ② and mix, ultrasonicate for 30min to make it uniformly dispersed, after the ultrasonicate is finished, gradually add 4g of acrylamide monomer and 11g of dimethyldiallylammonium chloride cationic monomer to the reaction container, stir at a speed of 500rpm / min for 15min to make it fully mixed, then introduce nitrogen for 30min to remove oxygen in the reaction solution, slowly add 30mL of 0.184mol / L potassium persulfate solution at 45℃, ensure that the whole reaction is carried out in a nitrogen atmosphere for 8h, after the reaction is completed, separate the product with a magnet, use ethanol and deionized water to repeatedly wash the product until the supernatant of the solution is transparent and the pH is kept neutral, dry the product after washing in a vacuum oven at 60℃, grind the dried product powder, and control its particle size to be less than 150μm, that is, complete the grafting of the magnetic flocculant.

[0030] ④ The surface of the synthesized magnetic flocculant exhibits an obvious colloidal cross-linked structure with obvious wrinkles. The particles are stably connected to the surface of the flocculant. The grafting rate of the flocculant is maintained at 37% to 62%. The Zeta potential of the synthesized magnetic flocculant is 16.4 to 21.9 mV at a concentration of 600 mg / L, 28.84 to -10.06 mV at a pH of 2.00-12.00, and pzc =11.23, and the saturation magnetization is 35.88emu / g.

[0031] Depend on Figure 1 (a) It can be seen that the synthesized magnetic flocculant has a peak at 1635 cm -1 and 1426cm -1 The stretching vibration of the C=O double bond and -N + The absorption peak of (CH3)3, 1385cm -1 The Si-O-Fe absorption peak and 569cm -1 The absorption peaks of Fe-O bonds all prove that the magnetic species, cationic monomers and acrylamide monomers have been successfully grafted into the skeleton structure of the cellulose flocculant. Figure 1 The XRD pattern of (b) shows that the flocculant is mainly in the form of Fe3O4 crystals, indicating that the magnetic seeds have been successfully grafted. Figure 1 (c) shows that the saturation magnetization of the grafted flocculant is maintained at 35.88emu / g, and it exhibits paramagnetism, which can avoid magnetic loss in the subsequent reaction process; Figure 1(d) It can be seen that the surface of the synthesized flocculant presents an obvious colloidal cross-linked structure with obvious wrinkles. The particles are stably connected to the surface of the flocculant. The elements in each monomer of the flocculant are all present, proving that the flocculant has been successfully prepared.

[0032] Example 2: Application of magnetic flocculants in purifying combined sewer overflow pollution in situ

[0033] The magnetic flocculant prepared as above is applicable to the on-site treatment process of combined sewer overflow pollution. Specifically, the combined sewer overflow pollution is collected through a pipeline and enters the nearest combined sewer overflow pollution on-site treatment device. The combined sewer overflow sewage is separately doped with the magnetic flocculant through a dosing pump and then completes the coagulation process in the mixing reaction zone. The combined sewer overflow sewage then flows through the magnetic separation area for sedimentation and separation of the flocs. The treated supernatant is directly discharged into the water body, and the sludge is collected and recovered for the next step of recycling.

[0034] The coagulation process in the mixed reaction zone is achieved through the following steps: 1) the flocculant dosage is 100-600 mg / L; 2) the fast stirring speed is 450 rpm / min, the fast stirring time is 1 min, the slow stirring speed is 70 rpm / min, the slow stirring time is 20 min, the magnetic sedimentation time is 2 min, and the magnetic field strength is 0.1-0.2 T (1000-2000 G).

[0035] Depend on Figure 2 It can be seen that compared with conventional flocculants, the magnetic flocculant prepared above has a sedimentation efficiency of 2.2455 min when the dosage is 100-600 mg / L. -1 The removal rate of suspended solids in the combined sewer overflow is greater than 95%, and the highest COD removal efficiency is 8.9%. Its performance and sedimentation efficiency are better than those of conventional flocculation processes. The fastest sedimentation efficiency can achieve rapid purification of pollutants in 0.5 minutes.

[0036] from Figure 3 (a) It can be seen that after the magnetic flocculant prepared above repeatedly purified the combined sewage overflow pollution, the recovery efficiency was as high as 80% to 100% after 10 times, the removal efficiency was >80%, and the highest COD removal rate could reach 37.5%. Figure 3 (b) It can be seen that after being reused 10 times, the saturation magnetization intensity of the flocculant still remains at 32.03-34.58emu / g.

[0037] Example 3 Application of magnetic flocculants in purifying combined sewer overflow pollution in a water plant primary enhanced treatment process

[0038] The magnetic flocculant prepared as above is applied to the primary enhanced treatment process of combined sewer overflow pollution in water plants, specifically, the combined sewer overflow sewage is collected through pipes and then enters the regulating tank and then enters the enhanced primary treatment short process. When the influent turbidity is high and the suspended solids concentration is high, the combined sewer overflow sewage is sequentially doped with magnetic flocculants and inorganic flocculants polyaluminium chloride through a dosing pump, and the coagulation process is completed in the mixed reaction zone, and the high electrical neutralization ability of the magnetic flocculant is used to preferentially remove pollutants; when the influent suspended particles are fine and the turbidity is relatively low, the combined sewer overflow sewage is sequentially doped with inorganic flocculants polyaluminium chloride and magnetic flocculants through a dosing pump, and the inorganic flocculants are used to wrap the finer flocs, and then further captured and swept by the magnetic flocculants; when the water temperature is low in winter, magnetic flocculants and inorganic flocculants need to be added at the same time to ensure the removal effect. Combined sewer overflow pollution depends on the different background of influent water quality. The supernatant after the above treatment can be directly discharged into the water body when the rainy season is heavy. When the rainfall is small, the combined sewer overflow sewage needs to continue to flow into the sewage treatment plant's biochemical treatment tank for subsequent reactions within the allowable load range of the sewage treatment plant after completing the coagulation process.

[0039] The coagulation process in the mixed reaction zone is realized by the following steps: 1) the dosage of flocculant is 600 mg / L, and the dosage range of inorganic flocculant is 20-120 mg / L; 2) the process conditions of step-by-step addition are: after adding drug 1, the fast stirring speed is 450 rpm / min, the fast stirring time is 0.5 min, after adding drug 2, the fast stirring speed is 350 rpm / min, the fast stirring time is 0.5 min, the slow stirring speed is 70 rpm / min, the slow stirring time is 20 min, magnetic sedimentation is 2 min, and the magnetic field strength is 0.1-0.2 T (1000-2000 G); 3) the process conditions of synchronous addition are: fast stirring speed is 450 rpm / min, fast stirring time is 1 min, slow stirring speed is 70 rpm / min, slow stirring time is 20 min, magnetic sedimentation time is 2 min, and magnetic field strength is 0.1-0.2 T (1000-2000 G).

[0040] from Figure 4It can be seen that when the magnetic flocculant (600 mg / L) is added first and then the PFC is added, when the PFC dosage is 120 mg / L, the removal rate of SS from the combined sewer overflow pollution can reach a maximum of 96.1%, the removal rate of COD is up to 44.7%, and the removal rate of TP is up to 93.3%; when the PFC is added first and then the magnetic flocculant (600 mg / L), when the PFC dosage is 80 mg / L, the removal rate of SS from the combined sewer overflow pollution can reach a maximum of 96.1%, the removal rate of COD is up to 44.7%, and the removal rate of TP is up to 93.3%. The maximum removal rate of SS can reach 96.1%, the maximum removal rate of COD can reach 37.04%, and the maximum removal rate of TP can reach 72.6%; when magnetic flocculant (600 mg / L) and inorganic flocculant are added at the same time, when the dosage of inorganic flocculant is 100 mg / L, the removal rate of SS in combined sewer overflow pollution can reach 88.75%, the maximum removal rate of COD can reach 28.24%, and the maximum removal rate of TP can reach 84.12%.

[0041] The specific embodiments described above are only preferred implementations of the present invention, but other aspects and implementations will be obvious to those skilled in the art. Several modifications and improvements may be made without departing from the principles of the present invention, all of which fall within the scope of protection of this application.

Claims

1. A method for preparing a magnetic flocculant and using the same to purify combined sewer overflow pollution, characterized in that: Cellulose is used as a matrix to graft acrylamide monomers and cationic monomers to form a flocculant skeleton structure, and magnetic cores are grafted into the flocculant skeleton structure. The synergistic effect of cationic monomers, acrylamide and magnetic species enhances the electrical neutralization flocculation effect, enhances the branch chain length of the flocculant, improves the ability of flocs to quickly precipitate, ensures the reuse efficiency of the flocculant, and realizes the efficient removal of combined sewer overflow pollution under different treatment modes. The preparation and application methods of the magnetic flocculant are as follows: (1) Preparation method of magnetic flocculant: ① Preparation method of double-bond loaded Fe3O4 nanoparticles: weigh 2-4g Fe3O4 nanoparticles, add them to a mixed solution of ethanol and water, wherein the ratio of ethanol to water is 4-5:1, add 9-12mL ammonia water thereto, ensure that the reaction is carried out under alkaline conditions, and then ultrasonically treat the mixed suspension for 30min to make the Fe3O4 nanoparticles uniformly dispersed in the alkaline solution. After the ultrasonic treatment, add 8-10mL of vinyl triethoxysilane, a silane coupling agent containing double bonds, to the reaction container, place the reaction container in a water bath at a temperature of 50°C, and react at a stirring speed of 250-300rpm / min for 6h. The entire reaction needs to be carried out in a nitrogen atmosphere. After the reaction is completed, the product is separated by a magnet, and repeatedly washed with ethanol and deionized water until the supernatant is transparent and the pH is maintained at a neutral level. ② Dissolution of cellulose: weigh 7g of sodium hydroxide (NaOH) and 12.0g of urea (Urea), add them to 81mL of deionized water, stir at 800rpm / min to completely dissolve them to obtain an alkaline solution, put it in a 4°C refrigerator to cool for 30min after dissolution, measure 30mL of the cooled alkaline solution, add 1.5g of microcrystalline cellulose, stir at 800rpm / min to dissolve it into a milky white mixed solution, and then put it in a -12°C refrigerator for precooling for 1.5 to 2h to completely dissolve it into a transparent and uniform cellulose dispersion; ③ Grafting of magnetic flocculant: Weigh 4g of the double-bond Fe3O4 nanoparticles obtained in step ①, add them to 80-100mL of deionized water, then add 30mL of cellulose solution obtained in step ② and mix, ultrasonicate for 30min to make it uniformly dispersed, after the ultrasonicate is finished, gradually add 2-4g of acrylamide monomer and 9-11g of dimethyldiallylammonium chloride cationic monomer to the reaction container, stir at a speed of 500rpm / min for 15min to make it fully mixed, and then introduce nitrogen for 30mi n, to exclude oxygen from the reaction solution, slowly drop a proper amount of potassium persulfate solution with a concentration of 0.166-0.184 mol / L at 45°C, and ensure that the entire reaction is carried out in a nitrogen atmosphere for 6-8 hours. After the reaction is completed, the product is separated by a magnet, and the product is repeatedly washed with ethanol and deionized water until the supernatant of the solution is transparent and the pH is kept neutral. The product after washing is dried in a vacuum oven at 60°C, and the dried product powder is ground to control the particle size to be less than 150 microns, thus completing the grafting of the magnetic flocculant; ④ Surface characteristics of magnetic flocculants: The surface of magnetic flocculants presents obvious colloidal cross-linked structure and has obvious wrinkles. The particles are stably connected to the surface of flocculants. The grafting rate of flocculants is maintained at 37% to 62%. The Zeta potential of the synthesized magnetic flocculants is 16.4 to 21.9 mV at a concentration of 600 mg / L, and 28.84 to -10.06 mV at a pH of 2.00-12.

00. pzc =11.23, saturation magnetization is 35.88emu / g; (2) Application of magnetic flocculants to purify combined sewer overflow pollution: ① The magnetic flocculant prepared above is suitable for the on-site treatment process of combined sewer overflow pollution or for the enhanced primary treatment process of sewage treatment plants during the rainy season; ② When the magnetic flocculant prepared as above is used for on-site treatment of combined sewer overflow pollution, it is characterized in that, under the coagulation process conditions of fast stirring speed 450rpm / min, fast stirring time 1min, slow stirring speed 70rpm / min, slow stirring time 20min, magnetic sedimentation 2min, magnetic field strength 0.1-0.2T (1000-2000G), when the dosage of magnetic flocculant is selected as 100-600mg / L, the removal rate of suspended matter in combined sewer overflow is 95.8%-98.5%, the removal efficiency of COD is as high as 37.5%, the floc particle size after coagulation is about 187.627μm, and the sedimentation efficiency k can reach 2.2455min -1 ; ③ When the magnetic flocculant prepared as above is applied to the enhanced primary treatment of multiple pollutants in a water plant with combined sewer overflow pollution, it is characterized in that it is compounded with an inorganic flocculant polyferric chloride according to different water quality backgrounds. When the influent turbidity is high and the suspended matter concentration is large, the magnetic flocculant (600 mg / L) is added first, and then the PFC is added. When the PFC dosage is 120 mg / L, the removal rate of SS in the combined sewer overflow pollution can reach up to 96.1%, the removal rate of COD can reach up to 44.7%, and the removal rate of TP can reach up to 93.3%; when the influent suspended particles are fine and the turbidity is relatively low, PFC must be added first and then the magnetic flocculant (600 mg / L). When the PFC dosage is 80 mg / L, the removal rate of SS in the combined sewer overflow pollution can reach up to 87.4%, the removal rate of COD can reach up to 37.04%, and the removal rate of TP can reach up to 93.3%. The highest removal rate of TP is 72.6%; the step-by-step coagulation process conditions are: dosing 1, fast stirring speed 450rpm / min, fast stirring time 0.5min, dosing 2, fast stirring speed 450rpm / min, fast stirring time 0.5min, slow stirring speed 70rpm / min, slow stirring time 20min, magnetic sedimentation 2min, magnetic field intensity range 0.1-0.2T (1000-2000G); when the water temperature is low in winter, it is necessary to add magnetic flocculant (600mg / L) and inorganic flocculant at the same time to ensure the removal effect. When the dosage of inorganic flocculant is 100mg / L, the removal rate of SS in combined sewer overflow pollution can be 88.75%, the removal rate of COD is 28.24%, and the removal rate of TP is 84.12%. The coagulation process conditions added at the same time are consistent with the on-site treatment process of combined sewer overflow pollution. ④ After the magnetic flocculant prepared above purifies the overflow pollution of the combined system, the recovery efficiency is as high as 80% to 100%, and the saturation magnetization intensity is still maintained at 32.03 to 34.58 emu / g.

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