Method for removing pollutants in nanofiltration membrane concentrated solution through cooperation of coagulating sedimentation, Fenton oxidation and electrochemical oxidation

Abstract

The invention discloses a method for removing pollutants in a nanofiltration membrane concentrated solution through cooperation of coagulating sedimentation, Fenton oxidation and electrochemical oxidation, and belongs to the technical field of water pollution control. Aiming at the kitchen waste leachate nanofiltration membrane concentrated solution, the invention discloses a'coagulating sedimentation-Fenton oxidation-electrochemical oxidation 'cooperative treatment method, and provides an optimal parameter combination adapted to the method at the same time, the coagulating sedimentation process under the parameter combination can effectively remove organic matters (COD and TOC), nitrogen and phosphorus nutritive salts (TN and TP) and chromaticity pollutants in the nanofiltration membrane concentrated solution, and the effluent quality of the nanofiltration membrane concentrated solution is improved. The Fenton oxidation process can balance the synergistic effect of free radical oxidation and flocculation, so that efficient degradation of organic matters can be ensured, and stable operation of the system can be maintained; the chemical oxygen demand (COD), the biochemical oxygen demand (BOD5), the total nitrogen (TN), the total phosphorus (TN) and the chromaticity of the water sample treated by the method can meet the emission standards, and an industrial-grade solution for reference is provided for harmlessness of the nanofiltration membrane concentrated solution.

Description

Technical Field

[0001] This invention belongs to the field of water pollution control technology, specifically relating to a method for the synergistic removal of pollutants from nanofiltration membrane concentrate by coagulation sedimentation-Fenton oxidation-electrochemical oxidation. Background Technology

[0002] For the treatment of leachate from food waste, physical, chemical, and biological methods, as well as combinations thereof, are currently the main approaches. In practical engineering applications, a three-stage treatment process of "pretreatment + biological treatment + advanced treatment" is commonly used. The pretreatment stage often employs dissolved air flotation (DAF) technology to remove suspended solids and grease; the biological treatment stage typically involves a two-stage AO process to degrade organic matter and remove nitrogen; and the advanced treatment stage uses ultrafiltration + nanofiltration to ensure effluent quality. It is worth noting that this process system generates nanofiltration membrane concentrate during operation, which is characterized by high salt content, rich in recalcitrant organic matter, and potentially contains trace amounts of heavy metals and other toxic substances. The treatment and disposal of this concentrate has become a major technical bottleneck restricting the large-scale application of membrane technology in leachate treatment.

[0003] The main methods for treating nanofiltration membrane concentrate include evaporation, reinjection, advanced oxidation, and coagulation sedimentation. Evaporation separates water and salt through heating, making it particularly suitable for treating high-salinity wastewater. While it offers good treatment results and is simple to operate, it consumes a lot of energy and requires significant equipment investment. Reinjection involves reinjecting leachate into the landfill layer, where soil purification and microbial action allow for the adsorption, degradation, and filtration of organic matter through physical and biological processes. This method is simple to operate, requires low investment, and has low operating costs. However, it carries the risk of groundwater pollution and can only be used as pretreatment; further treatment is necessary to achieve complete leachate neutralization.

[0004] Advanced oxidation technology is particularly suitable for treating recalcitrant and highly toxic wastewater. It effectively decomposes large organic molecules in the concentrate by generating highly oxidizing hydroxyl radicals (·OH). It features strong oxidizing power (redox potential up to 2.8V) and rapid reaction speed (half-life of only 10). -9 While it has significant advantages such as high processing costs and high professional requirements for operators, it also has limitations such as high processing costs and high requirements for operators.

[0005] In contrast, coagulation sedimentation, through the addition of chemical agents such as PAC and PAM, promotes the sedimentation of colloids and suspended particles in the water through charge neutralization, adsorption bridging, and other processes, effectively removing 30%-50% of organic pollutants. This technology has advantages such as simple operation, high efficiency, low operating costs, and good treatment results, and is particularly suitable for the treatment of high-turbidity wastewater, heavy metal wastewater, and phosphorus-containing wastewater. It has already been applied on a large scale in fields such as food industry wastewater treatment.

[0006] Coagulation and sedimentation are frequently used as pretreatment in advanced oxidation processes, but their effectiveness is influenced by many factors, including water temperature, initial pH, coagulant dosage, and coagulant aid dosage. The regulation and optimization of these factors are crucial for achieving optimal coagulation and sedimentation results. Commonly used coagulants include polyaluminum chloride (PAC), aluminum sulfate, ferric chloride, polyferric sulfate, ferrous sulfate, and alum. In 2017, Wen Chen et al. disclosed the use of PAC and polyacrylamide to treat wastewater. Under conditions of pH 8.0-9.0, PAC dosage of 1.5 g / L, and polyacrylamide dosage of 1.5 mg / L, the removal rates of chemical oxygen demand (COD) and color in the wastewater were 81.7% and 56.3%, respectively. In 2025, Li Qiuyu et al. provided a study on the treatment of dissolved organic pollutants in coal mine water using PAC. When the PAC dosage was 350 mg / L, the removal rate of colloidal dissolved organic matter in the coal mine water was 97.78%. Syaichurrozi I et al. published a study in 2024 on the treatment of palm oil factory wastewater in Indonesia using alum. When the alum dosage was 6 g / L, the chemical oxygen demand (COD) removal rate was 70.02%. These studies demonstrate that coagulation and sedimentation processes are highly effective in treating COD, color, and organic pollutants in wastewater.

[0007] Among advanced oxidation processes, Fenton oxidation technology has advantages such as high degradation efficiency, fast reaction, and ease of operation. Furthermore, iron is a harmless metal, and hydrogen peroxide is easily treated, resulting in relatively low environmental impact when using Fenton oxidation to treat wastewater. The principle of Fenton oxidation is primarily based on ferrous ions (Fe²⁺). 2+ The chain reaction between Fe and hydrogen peroxide (H2O2), under acidic conditions, occurs with Fe 2+ As a catalyst, H₂O₂ generates highly oxidizing hydroxyl radicals (·OH). These hydroxyl radicals exhibit strong addition reaction characteristics, reacting with recalcitrant organic compounds, disrupting their organic structure, and attacking them, thus causing their degradation. Simultaneously, Fe... 2+ The coagulation effect produced when Fe(OH)3 is precipitated can further improve the removal rate of pollutants. Therefore, Fenton oxidation can effectively treat wastewater with high content of recalcitrant organic matter. Since Fenton oxidation is mainly dominated by ·OH, Fe... 2+ Factors such as the dosage of H2O2 and the pH value of the solution directly and significantly affect the generation of ·OH and the reaction efficiency, while the oxidation time directly affects the degree of reaction. The regulation and optimization of these factors are crucial to the treatment effect of Fenton oxidation.

[0008] Electrochemical oxidation is also an advanced oxidation water treatment technology. The operating mechanism of electrochemical oxidation involves placing organic wastewater into a reaction device, inserting a direct current to oxidize the organic matter, or first oxidizing low-valence metal ions into high-valence metal ions, which then oxidize the organic matter in the solution. The wastewater flows between the energized electrodes, and the following oxidation reaction occurs on the anode surface: pollutants are directly oxidized by electron transfer, or indirectly degraded by strong oxidizing free radicals (such as ·OH) or active chlorine generated at the anode until they are mineralized. Therefore, electrochemical oxidation technology can indirectly oxidize organic matter such as ammonia nitrogen and total nitrogen in wastewater, and is particularly suitable for treating wastewater containing high salinity and recalcitrant organic matter (such as nanofiltration membrane concentrate). It efficiently removes COD, color, and toxic substances without requiring the addition of large amounts of chemical reagents, and has advantages such as good treatment effect, ease of automation, no secondary pollution, and small footprint. However, the high investment and operating costs of electrochemical oxidation equipment, the susceptibility of electrodes to contamination, and short lifespan also limit the application of this technology in practical engineering.

[0009] In summary, coagulation sedimentation, Fenton oxidation, and electrochemical oxidation technologies each have their own advantages in treating recalcitrant wastewater. Therefore, when treating nanofiltration membrane concentrate from kitchen waste leachate, a synergistic treatment method of coagulation sedimentation-Fenton oxidation-electrochemical oxidation can be designed to cover different pollutants and is suitable for complex water samples. At the same time, the operating parameters and reagent dosage of each unit can be optimized to achieve the best treatment effect and economic benefits. Summary of the Invention

[0010] To address the problems existing in the prior art, this invention provides a method for the synergistic removal of pollutants from nanofiltration membrane concentrate by coagulation sedimentation-Fenton oxidation-electrochemical oxidation. The water sample treated by this method can meet the emission standards for chemical oxygen demand (COD), biochemical oxygen demand (BOD5), total nitrogen (TN), ammonia nitrogen, total phosphorus (TP), and color, providing a reference industrial-grade solution for the harmless treatment of nanofiltration membrane concentrate.

[0011] The technical solution of the present invention is as follows:

[0012] The purpose of this invention is to provide a method for the synergistic removal of contaminants from nanofiltration membrane concentrate by coagulation-precipitation-Fenton oxidation-electrochemical oxidation, comprising the following steps:

[0013] S1. The effluent from the nanofiltration membrane concentrate after coagulation and sedimentation is adjusted to an initial pH of 3.0 using 1.0 mol / L H2SO4 or 2.0 mol / L NaOH solution. Then, FeSO4·7H2O and 30% H2O2 solution are added to the effluent in the designed proportion.

[0014] S2. Stir to start the oxidation reaction. After the reaction is terminated, adjust the pH of the effluent to 9.0, wait for it to settle, and discharge the supernatant, which is the Fenton oxidation effluent.

[0015] S3. Immerse the positive and negative electrodes in the Fenton oxidation water for electrochemical oxidation, and obtain the treated water sample after continuous electrolysis.

[0016] The pH value of the nanofiltration membrane concentrate is 6.0-8.5;

[0017] Chemical oxygen demand (COD) is 300-600 mg / L;

[0018] Biochemical oxygen demand (BOD5) is 30-80 mg / L;

[0019] Total organic carbon (TOC) is 200-300 mg / L;

[0020] Total nitrogen (TN) was 40-68 mg / L;

[0021] Total phosphorus (TP) was 0.029 mg / L;

[0022] Ammonia nitrogen content is 0-30 mg / L;

[0023] H2O2 / Fe in S1 2+ The molar ratio is 7, and the relationship between the H2O2 dosage and the volume of water effluent from the nanofiltration membrane concentrate coagulation and sedimentation is 20 mL / L.

[0024] The oxidation reaction time in S2 is 60 min.

[0025] Furthermore, the coagulation and sedimentation process in S1 is as follows: the initial pH of the nanofiltration membrane concentrate derived from the anaerobic fermentation biogas slurry of kitchen waste is adjusted and stirred, and then polyferric sulfate (PFS) is used as a coagulant and wet-added to the nanofiltration membrane concentrate. After rapid stirring, cationic polyacrylamide (PAM) is wet-added, and stirring is stopped at low speed. The flocs or sludge are separated from the coagulation and sedimentation effluent by sedimentation.

[0026] Furthermore, the coagulant is a 10% polyferric sulfate (PFS) solution, and the coagulant aid is a 0.1% cationic polyacrylamide (PAM) solution.

[0027] Furthermore, when polyferric sulfate (PFS) is selected as a coagulant, the initial pH is adjusted to 7.0.

[0028] Further, the polyferric sulfate (PFS) is added to a concentration of 1.0 g / L.

[0029] Further, the coagulant, cationic polyacrylamide (PAM), is added to a concentration of 4.0 mg / L.

[0030] Furthermore, after adding the coagulant, stir rapidly at 200 rpm for 5 minutes.

[0031] Furthermore, after adding the coagulant, the stirring speed was reduced to 100 r / min, and stirring was stopped after 20 minutes.

[0032] Furthermore, in S3, the cathode electrode material is a titanium plate, and the anode electrode material is a lead dioxide plate.

[0033] Furthermore, the continuous electrolysis time in S3 is 2 hours.

[0034] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0035] 1. This invention discloses for the first time a method for removing pollutants from nanofiltration membrane concentrate of kitchen waste leachate, and innovates a three-stage pollutant removal mechanism tailored to the water quality characteristics of this nanofiltration membrane concentrate. Coagulation and sedimentation serve as pretreatment, specifically removing colloids, suspended solids, and phosphorus, significantly reducing the subsequent oxidation load. Its optimal parameter combination (PFS 1g / L, pH 7.0, PAM 4mg / L) can efficiently remove color and some organic TOC, while avoiding "secondary pollution" caused by excessive PAM addition. The Fenton oxidation step technology builds upon this, utilizing the low turbidity characteristics of the coagulated effluent, and at the optimal parameter combination (pH 3.0, H2O2 / Fe... 2+ Under conditions of a molar ratio of 7 and H2O2 / nanofiltration membrane concentrate coagulation and sedimentation effluent of 20 mL / L, deep degradation is achieved through the synergistic effect of oxidation and flocculation. ·OH free radicals mineralize macromolecular organic matter, while Fe 3+ The Fe(OH)3 colloids generated by hydrolysis simultaneously adsorb and co-precipitate nitrogen-containing pollutants, overcoming the limitations of single oxidation processes. The electrochemical oxidation step directly utilizes the high salinity of the concentrate as a natural electrolyte, thoroughly mineralizing small-molecule organic matter remaining after Fenton oxidation under the action of lead dioxide anodes, avoiding the need for additional electrolyte addition and significantly reducing energy consumption. This three-stage cascade design forms a progressive treatment procedure of "colloid / phosphorus removal → macromolecular oxidation → small-molecule mineralization," breaking through the current bottleneck of single processes in treating complex pollutants and providing a new paradigm for the treatment of high-salt, recalcitrant organic wastewater.

[0036] 2. This invention also discloses the optimal parameter combinations for each step in the synergistic removal method. For the coagulation and sedimentation stage, based on the significant differences in the effects of various coagulation and sedimentation parameters on the treatment of TOC, TN, TP, and color, an optimal parameter combination was designed (polyferric sulfate dosage 1 g / L, initial pH 7.0, polyacrylamide dosage 4 mg / L). Under this combination, the coagulation and sedimentation process can effectively remove organic matter (COD, TOC), nitrogen and phosphorus nutrients (TN, TP), and color pollutants from the nanofiltration membrane concentrate, significantly reducing the organic matter content in the wastewater and creating favorable conditions for subsequent advanced treatment processes. For the Fenton oxidation stage, based on the drastically different changes in the removal of TOC and TN by various parameters and the differences in their removal mechanisms in the Fenton system, an optimal parameter combination was designed (initial pH 3.0, H2O2 / Fe...). 2+ With a molar ratio of 7, H2O2 / nanofiltration membrane concentrate coagulation and sedimentation effluent of 20 mL / L, and an oxidation time of 60 min, the effluent treated by Fenton oxidation under this parameter combination can achieve complete mineralization of small molecule organic matter through continuous electrochemical oxidation. The synergistic removal method described in this invention, through innovative design, in-depth parameter optimization, and engineering adaptability, achieves efficient, economical, and stable treatment of nanofiltration membrane concentrate. The final effluent has organic carbon (TOC), total nitrogen (TN), total phosphorus (TN), and color values ​​far below the limits of the "Pollution Control Standard for Municipal Solid Waste Landfills" (GB 16889-2008).

[0037] 3. The method for synergistic removal of pollutants from nanofiltration membrane concentrate by coagulation sedimentation-Fenton oxidation-electrochemical oxidation described in this invention has the following three advantages: 1) Deep purification of pollutants, with the final effluent being colorless and transparent, color completely eliminated, and all indicators comprehensively superior to the national standard (GB16889-2008), with extremely low TP concentration and a removal rate close to 100%; TN and COD... Cr Both the BOD5 data are better than the national standard limits, solving the removal dilemma of recalcitrant organic matter, nitrogen and phosphorus nutrients, and color pollutants in nanofiltration membrane concentrate, and the process is stable; 2) The cost and efficiency are balanced. The use of low-dose PFS and PAM in the coagulation and sedimentation step reduces the cost of reagents; the precise control of H2O2 addition and reaction time in the Fenton oxidation step avoids ineffective consumption; the electrochemical oxidation step directly utilizes high salinity to save electrolyte costs, and the overall operating cost is significantly lower than that of single advanced oxidation; 3) The process has strong compatibility. The coagulation and sedimentation step effectively protects the electrodes of the subsequent oxidation unit from colloidal contamination, and the neutral pH effluent after the Fenton oxidation step is directly adapted to the electrochemical input, avoiding the complex operation of secondary pH adjustment. In addition, the entire process can be automated, providing a reliable path for the industrial-grade treatment of kitchen waste leachate membrane concentrate. Attached Figure Description

[0038] Figure 1 This is a graph showing the effect of various coagulation and sedimentation parameters on TOC removal efficiency during the performance test of this invention.

[0039] Figure 2 This is a graph showing the effect of various coagulation and sedimentation parameters on TN removal efficiency during the performance test of this invention.

[0040] Figure 3 This is a graph showing the effect of various coagulation and sedimentation parameters on TP removal efficiency during the performance test of this invention.

[0041] Figure 4 This is a graph showing the effect of various coagulation and sedimentation parameters on CN removal efficiency during the performance test of this invention.

[0042] Figure 5 The changes in spectral characteristics of the nanofiltration membrane concentrate before and after treatment under optimal coagulation and sedimentation conditions are shown in the performance test of this invention. Figure 5 a represents the ultraviolet-visible absorption spectrum. Figure 5 b represents the infrared spectrum;

[0043] Figure 6 The effect of different initial pH values ​​on the removal of TOC and TN by Fenton oxidation in the performance test of this invention is shown in the graph.

[0044] Figure 7 In the performance test of this invention, different H2O2 / Fe ratios 2+ Graph showing the effect of molar ratio on the removal of TOC and TN by Fenton oxidation;

[0045] Figure 8 The effect of different H2O2 dosages on the removal of TOC and TN by Fenton oxidation in the performance test of this invention is shown in the graph.

[0046] Figure 9 The graph shows the effect of different oxidation times on the removal of TOC and TN by Fenton oxidation in the performance test of this invention. Detailed Implementation

[0047] The present invention will be further described below with reference to preferred embodiments. The endpoints and any values ​​of the ranges disclosed in the present invention are not limited to the precise ranges or values. These ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be regarded as specifically disclosed herein.

[0048] Unless otherwise specified, the experimental methods in the following embodiments are conventional methods, performed in accordance with the techniques or conditions described in the literature in this field or in accordance with the product instructions.

[0049] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0050] The main experimental reagents used in the following examples are shown in Table 1:

[0051] Table 1. Main experimental reagents and related information

[0052]

[0053]

[0054] The main experimental instruments and equipment used in the following embodiments are shown in Table 2:

[0055] Table 2 Main experimental instruments and equipment

[0056]

[0057] Table 3 shows the specific water quality indicators and corresponding discharge standards of the nanofiltration membrane concentrate used in the following examples:

[0058] Table 3. Water quality indicators of the concentrate

[0059]

[0060] The methods for determining various indicators of coagulation and sedimentation in the following examples are shown in Table 4:

[0061] Table 4. Methods and Standards for Determination of Water Quality Indicators

[0062]

[0063]

[0064] Colorimetry was measured using a UV-Vis spectrophotometer with a scanning wavelength range of 200-800 nm and a scanning interval of 0.5 nm. The absorbance was measured using a direct detection detector unit.

[0065] The formula for calculating chromaticity is as follows:

[0066]

[0067] In the formula: A 436 Represents the absorbance measured at a wavelength of 436 nm; A 525 Represents the absorbance measured at a wavelength of 525 nm; A 620 This represents the absorbance measured at a wavelength of 620 nm.

[0068] The scanning wavenumber range of the infrared spectrum is 400-4000 cm⁻¹.-1 The scan count was 128, the resolution was 4, and the final format was % transmittance.

[0069] Example 1

[0070] This embodiment provides a method for the synergistic removal of contaminants from nanofiltration membrane concentrate by coagulation precipitation-Fenton oxidation-electrochemical oxidation, including the following steps:

[0071] S1. Stir the nanofiltration membrane concentrate at 200 r / min for 30 s, then adjust the initial pH of the water sample to 7.0, add 10% polyferric sulfate (PFS) solution by wet method to make the concentration in the solution 1.0 g / L, and stir rapidly at 200 r / min for 5 min.

[0072] S2. Wet addition of a 0.1% polyacrylamide (PAM) solution to achieve a concentration of 4.0 mg / L in the solution. Reduce the stirring speed to 100 r / min, stop stirring after 20 min, and let stand to separate the coagulated sediment effluent from the flocs or sludge.

[0073] S3. The effluent from the nanofiltration membrane concentrate, after coagulation and sedimentation treatment, is adjusted to an initial pH of 3.0 using 1.0 mol / L H2SO4 or 2.0 mol / L NaOH solution. Then, FeSO4·7H2O and 30% H2O2 solution are added to the effluent at the designed ratio to achieve a H2O2 / Fe... 2+ The molar ratio is 7, and the relationship between the H2O2 dosage and the volume of water effluent from the nanofiltration membrane concentrate coagulation and sedimentation is 20 mL / L.

[0074] S4. Start the oxidation reaction by stirring. Stop the reaction after 60 minutes. Adjust the pH of the effluent to 9.0. Let it stand and settle. Discharge the supernatant, which is the Fenton oxidation effluent.

[0075] S5. Electrochemical oxidation was carried out by immersing the cathode and anode electrodes in the Fenton oxidation effluent. The cathode electrode material was a titanium plate, and the anode electrode material was a lead dioxide plate. After continuous electrolysis for 2 hours, the treated water sample was obtained.

[0076] Performance testing

[0077] 1. Coagulation and sedimentation treatment effect test

[0078] A verification experiment was conducted under the optimal parameter combination determined in the above experiment (polyferric sulfate dosage 1 g / L, initial pH value 7.0, polyacrylamide dosage 4 mg / L). The results of the water quality index analysis after treatment are shown in Table 5.

[0079] Table 5 Treatment effect of optimal coagulation and sedimentation conditions

[0080]

[0081] As shown in the table, the coagulation and sedimentation process exhibits varying removal efficiencies for different types of pollutants. The removal efficiency for total nitrogen (TP) is the most outstanding, reaching 99.6% with an effluent concentration of only 0.1 mg / L. The removal rates for total nitrogen (TN) and ammonia nitrogen are 47.8% and 64.8%, respectively, with effluent concentrations of 32.0 mg / L and 7.40 mg / L. Color removal is also significantly improved, decreasing from 50 times to 6 times. All these indicators meet the emission requirements of the "Standard for Pollution Control of Municipal Solid Waste Landfills" (GB16889-2008).

[0082] It is important to note that although the coagulation and sedimentation process also showed some removal effect on organic pollutants such as COD, BOD5, and TOC (removal rates of 61.1%, 26.5%, and 46.7%, respectively), the high concentration of organic matter in the raw water (COD 578 mg / L, BOD5 71.4 mg / L, TOC 242.0 mg / L) meant that complete compliance with standards could not be achieved solely through coagulation. This result is consistent with expectations, as coagulation and sedimentation, as a physicochemical treatment method, primarily removes colloidal and large molecular organic matter, while its ability to remove dissolved small molecular organic matter is limited.

[0083] Furthermore, the experiment revealed that the total dissolved solids (TDS) content increased from 4400 mg / L to 6592 mg / L after treatment. This was mainly due to the introduction of new ionic components by the addition of coagulants and pH adjusters. This phenomenon suggests that in practical engineering applications, TDS control needs to be comprehensively considered based on the intended use of the effluent.

[0084] Figure 5 This study demonstrates the changes in spectral characteristics of nanofiltration membrane concentrate before and after treatment under optimal coagulation and sedimentation conditions (1 g / L polyferric sulfate, initial pH 7.0, and 4 mg / L polyacrylamide). Comparative analysis of UV-Vis absorption and infrared spectroscopy provides a deeper understanding of the removal mechanism of organic pollutants from the concentrate by the coagulation and sedimentation process.

[0085] Ultraviolet-Visible Absorption Spectroscopy Figure 5 a) Analysis showed that the absorbance of the treated concentrate in the 200-400 nm ultraviolet region was significantly reduced, indicating that the coagulation and sedimentation process effectively removed aromatic compounds and organic matter containing conjugated double bonds from the concentrate. This result is corroborated by the significant improvement in color index (from 50 times to 6 times), confirming the good removal effect of the coagulation process on colorimetric substances.

[0086] Infrared spectroscopy Figure 5(b) This further revealed the removal characteristics of coagulation and sedimentation for organic compounds with different functional groups. A comparison of the spectra of samples before and after treatment showed that: in the 3000-3600 cm⁻¹ range... -1 The broad absorption peaks in the region are significantly weakened, corresponding to a reduction in the stretching vibrations of OH and NH, indicating that organic compounds containing hydroxyl and amino groups are effectively removed; in the 1600-1700 cm⁻¹ region... -1 A decrease in absorption peaks within the specified range reflects the removal of carbonyl compounds (such as aldehydes, ketones, and carboxylic acids); while the fingerprint region (500-1500 cm⁻¹) shows a decrease in absorption peaks. -1 The disappearance or weakening of some characteristic peaks indicates that coagulation and sedimentation also have a good removal effect on organic pollutants containing benzene rings and other complex structures. These changes in spectral characteristics are in good agreement with the decrease in TOC in the water quality index (from 242.0 mg / L to 128.9 mg / L).

[0087] These spectral analysis results corroborate each other, confirming that coagulation and sedimentation can effectively remove various organic pollutants from the concentrate. This method not only reduces the color of the concentrate, but more importantly, it can specifically remove organic pollutants containing specific functional groups (such as hydroxyl, amino, and carbonyl groups), which is of great significance for improving the treatment quality of the concentrate. The results of this study provide a reliable theoretical basis for optimizing coagulation and sedimentation process parameters and also offer important references for selecting concentrate treatment schemes in practical engineering applications.

[0088] 2. Test on the influence of various coagulation and sedimentation parameters on removal efficiency

[0089] The effects of polyferric sulfate dosage, initial coagulation pH, and polyacrylamide dosage on the removal efficiency of TOC, TN, TP, and CN were investigated using an orthogonal experimental system. Experimental data and analysis results are detailed in Tables 6 and 7.

[0090] Table 6. Orthogonal Experiment Table for Concrete Coagulation

[0091]

[0092] Table 7. Intuitive Analysis Table

[0093]

[0094]

[0095] (1) Impact on TOC removal efficiency

[0096] Experimental results showed that the optimal TOC removal efficiency was achieved when the dosage of polyferric sulfate was 3 g / L, the initial pH was 7.0, and the dosage of polyacrylamide was 4 mg / L. Range analysis revealed that the influence of each factor on TOC removal efficiency was in the following order: polyferric sulfate dosage (range 22.0) > polyacrylamide dosage (range 19.1) > initial pH (range 9.0), indicating that the dosage of polyferric sulfate was the most critical factor affecting TOC removal.

[0097] The effect of coagulation and sedimentation on TOC removal is as follows: Figure 1 As shown, the effects of various coagulation parameters on the treatment effect differed significantly. Regarding the dosage of polyferric sulfate, the TOC removal rate increased rapidly when the dosage increased from 0.5 g / L to 3 g / L, with the best TOC removal effect at level 4 (dosage 3 g / L), reaching 59.2%. This phenomenon is mainly attributed to the polynuclear hydroxyl complexes generated during the hydrolysis of polyferric sulfate. These complexes effectively reduced the Zeta potential on the surface of colloidal particles through charge neutralization, thereby promoting colloid destabilization and accelerating floc formation and sedimentation.

[0098] The effect of pH on TOC removal rate initially increases and then decreases, reaching a peak (51.1%) at pH 7.0. This phenomenon stems from the morphological changes of the hydrolysis products of polyferric sulfate at different pH values. Under neutral conditions (pH≈7.0), the polynuclear hydroxy complex [Fe2(OH)2(H2O)8] generated by the hydrolysis of polyferric sulfate... 4+ It possesses high charge density and a three-dimensional network structure, which can effectively neutralize the surface charge of colloids and promote flocculation through adsorption bridging; in acidic environments (pH < 5.0), it is mainly composed of [Fe(H2O)6]. 3+ It mainly consists of mononuclear hydrolysis products, with insufficient charge density and molecular size, and readily reacts with SO4. 2- The effectiveness is reduced by combining with other substances; under alkaline conditions (pH>8.0), the [Fe(OH)4]- generated will electrostatically repel the pollutant colloids, leading to a significant reduction in flocculation efficiency. This pH dependence of hydrolysis chemistry directly determines the actual effect of coagulation treatment.

[0099] It is worth noting that there is a negative correlation between polyacrylamide dosage and TOC removal rate, with the optimal dosage of 4 mg / L (level 1) achieving a removal rate of 57.5%. Excessive polyacrylamide dosage not only weakens its bridging effect due to steric hindrance, but more importantly, polyacrylamide primarily improves solid-liquid separation by accelerating floc settling, offering limited improvement in actual pollutant removal capacity. Furthermore, as an organic polymer, polyacrylamide residues can lead to an increase in the system's TOC value; this "secondary pollution" effect requires special attention in practical engineering applications.

[0100] (2) Impact on TN removal efficiency

[0101] Experimental results show that the removal efficiency of TN by coagulation and sedimentation is affected by multiple factors. Orthogonal experimental data analysis revealed that the optimal equilibrium state for TN removal was achieved when the dosage of polyferric sulfate was 1 g / L, the initial pH was 7.0, and the dosage of polyacrylamide was 4 mg / L, at which point the removal rate was 45.7%. Range analysis showed that the influence of each factor on the TN removal efficiency was in the following order: polyferric sulfate dosage > initial pH > polyacrylamide dosage, with ranges of 17.3, 9.8, and 5.6, respectively.

[0102] The effect of coagulation and sedimentation on TN removal efficiency is as follows: Figure 2 As shown, the effects of various coagulation parameters on removal efficiency differed significantly. The effect of polyferric sulfate (PFLS) dosage on TN removal rate showed a trend of first increasing and then decreasing. When the PFLS dosage increased from 0.5 g / L (level 1) to 2 g / L (level 3), the TN removal rate increased from 29.3% to 46.6%. This is mainly attributed to [Fe2(OH)2] generated by PFS hydrolysis. 4+ Polynuclear complexes effectively remove nitrogen-containing organic matter through charge neutralization and adsorption bridging. However, when the dosage was further increased to 3 g / L (level 4), the removal rate decreased to 35.8%. This was due to excess Fe. 3+ It will react with protonated amino groups (-NH3) + This generates electrostatic repulsion, which in turn causes the zeta potential of the flocs to reverse, triggering the colloid restoring phenomenon.

[0103] The effect of pH also showed a pattern of first increasing and then decreasing, with the highest TN removal rate (44.2%) at pH 7.0. This was mainly due to three synergistic mechanisms: First, the neutral environment brought nitrogen-containing organic matter such as proteins close to their isoelectric point, balancing the surface charge and enhancing hydrophobicity, thus promoting their spontaneous aggregation; second, the abundant hydroxyl groups on the surface of Fe(OH)3 colloids formed a stable hydrogen bond network with nitrogen-containing groups; and third, humic acids formed insoluble precipitates with iron ions, enhancing the removal effect through co-precipitation. In contrast, under acidic conditions (pH < 5), Fe... 3+ Incomplete hydrolysis resulted in the formation of [Fe(H2O)6]. 3+ Inefficient monomers are present in alkaline environments (pH>8), while the formation of [Fe(OH)4]- anions and enhanced ionization of organic matter in alkaline environments lead to electrostatic repulsion, both resulting in decreased treatment efficiency. This pH-dependent variation provides an important basis for optimizing process parameters in practical engineering.

[0104] The dosage of polyacrylamide was negatively correlated with the TN removal rate. The highest TN removal rate (42.5%) was achieved at a polyacrylamide dosage of 4 mg / L, where polyacrylamide effectively strengthened the floc structure through bridging. However, when the dosage increased to 10 mg / L, the removal rate decreased to 36.9%. This is because the high molecular weight of excessive polyacrylamide can mask the active sites of iron hydrolysis products, competing with nitrogen-containing organic matter for adsorption sites. Simultaneously, the dissolution of its own nitrogen-containing segments leads to an increase in the TN concentration in the effluent. This negative effect is particularly pronounced when treating membrane concentrates rich in medium-molecular-weight organic nitrogen.

[0105] (3) Impact on TP removal efficiency

[0106] As shown in Tables 6 and 7, coagulation and sedimentation (FFS) exhibited excellent removal efficiency for total phosphorus (TP). The dosage of polyferric sulfate (PFS), initial pH, and polyacrylamide (PAM) had little impact on TP removal efficiency, with all experimental groups maintaining a TP removal rate above 95%. Range analysis revealed that the influence of each factor on TP removal efficiency was in the following order: PFS dosage (range 1.3) > PAM dosage (range 0.5) > initial pH (range 0.3). Considering both treatment effectiveness and economic efficiency, the optimal conditions were determined to be: PFS dosage 1 g / L, initial pH 9.0, and PAM dosage 4 mg / L.

[0107] from Figure 3 It can be seen that the parameters have a relatively small impact on the TP removal rate, which is mainly attributed to the following mechanisms: In terms of chemical precipitation, the Fe produced by the hydrolysis of polyferric sulfate 3+ It can react directly with phosphate ions to form insoluble FePO4 precipitate, a process that is key to achieving efficient TP removal. In terms of adsorption and coprecipitation, polyacrylamide can effectively remove ferric phosphate precipitate particles and large organic phosphorus molecules (such as phospholipids and nucleic acids) through the entrapment properties of its long-chain molecules. In terms of pH regulation, alkaline conditions (pH=9) are more conducive to the formation of phosphate precipitates and can also promote the hydrolysis and transformation of organic phosphorus.

[0108] It is worth noting that although pH changes affect the form of phosphorus and the hydrolysis products of flocculants, the high efficiency of chemical precipitation makes the TP removal rate relatively insensitive to pH changes. This characteristic provides greater operational flexibility for adjusting process parameters in practical engineering applications.

[0109] (4) Impact on color removal effect

[0110] Range analysis revealed that the order of influence on CN removal efficiency is: polyferric sulfate dosage > polyacrylamide dosage > initial pH value. This ranking helps us better understand the impact of each parameter on the removal efficiency, thereby optimizing the treatment effect of coagulation and sedimentation.

[0111] The effects of polyferric sulfate dosage, pH value, and polyacrylamide (PAM) dosage on color removal efficiency were studied experimentally. Figure 4 As shown, with the increase in polyferric sulfate dosage from 0.5 g / L (level 1) to 3 g / L (level 3), the removal rate of CN increased rapidly, reaching an optimal removal rate of 78.3% at the highest dosage. Fe produced by the hydrolysis of polyferric sulfate... 3 + ions can reduce the stability of colloidal particles through charge neutralization, while their hydrolysis products can promote floc formation through adsorption bridging. Furthermore, the generated Fe(OH)3 precipitate can trap chromatic substances in the water and cause them to settle together. However, excessive PFS dosage not only wastes reagents but may also lead to the restabilization of colloidal particles, thus reducing the treatment effect.

[0112] The effect of pH on CN removal shows a trend of initially promoting and then inhibiting it. When the pH increases from 3.0 to 9.0, the CN removal rate increases to 72.7%. This is because moderate pH conditions are favorable for Fe... 3+ Hydrolysis produces polynuclear hydroxyl complexes with strong adsorption capacity. However, when the pH continues to rise into the alkaline range, Fe... 3+ It will form Fe(OH)3 colloid, and its charge neutralization ability will decrease.

[0113] The effect of PAM dosage on color removal showed a decreasing trend. When the polyacrylamide dosage increased from 4 mg / L (level 1) to 10 mg / L (level 4), the CN removal rate decreased, reaching its optimal level at the lowest dosage (level 1, 4 mg / L), where the CN removal rate was 75.9%. Polyacrylamide connects Fe through molecular chain bridging. 3+Hydrolysis products and chromogenic substances simultaneously capture fine chromogenic particles through the network effect of long-chain molecules and improve the surface charge distribution of flocs through charge-compensating effects, thereby enhancing the chromogenic removal effect. However, with increasing dosage, the removal efficiency decreases, mainly due to the potential for new color-developing effects from PAM as an organic polymer. Although a slight rebound in removal rate was observed at the highest dosage, this may be due to experimental errors or the mechanical retention effect of the three-dimensional network formed by PAM on chromogenic substances. Based on multi-factor influence analysis and cost-effectiveness assessment, the optimal conditions for coagulation and sedimentation are: 1 g / L polyferric sulfate dosage, a system pH of 7.0, and 4 mg / L polyacrylamide dosage. These conditions not only effectively remove TOC, TN, and CN but also avoid potential problems caused by excessive dosage.

[0114] 3. Test on the influence of various parameters of Fenton oxidation on the removal effect

[0115] The Fenton oxidation process was used to treat the coagulation and sedimentation effluent from nanofiltration membrane concentrate. Single-factor analysis was used to systematically investigate the initial pH value and H₂O₂ / Fe ratio of the solution. 2+ The effects of four parameters—molar ratio, H2O2 dosage, and reaction time—on the treatment effect.

[0116] All experiments were conducted at room temperature (25±3℃). 100 mL of the concentrated solution coagulated effluent was placed in a 250 mL beaker, and the initial pH was adjusted to the set value using 1.0 mol / L H₂SO₄ or 2.0 mol / L NaOH solution. FeSO₄·7H₂O and 30% H₂O₂ solution were added according to the designed ratio, and the reaction was started with stirring at 200 rpm. After terminating the reaction, the pH was adjusted to 9.0 to ensure sufficient precipitation of iron ions. The solution was filtered through a 0.45 μm microporous membrane, and the TOC and TN concentrations of the filtrate were measured.

[0117] (1) Different initial pH values

[0118] The effect of pH values ​​within the range of 2.0, 3.0, 4.0, 5.0, and 6.0 was investigated. The dosage of H₂O₂ was fixed at 2 mL, and the dosage of FeSO₄·7H₂O was 1.0891 g. The H₂O₂ / Fe 2+ The molar ratio was 5, and the reaction time was 90 min. (Calculation basis: 2 mL × 1.11 g / mL × 30% ÷ 34 g / mol × 0.2 × 278 g / mol) to achieve the designed ratio. The effect of the initial pH of the solution on the Fenton oxidation effect is as follows. Figure 6 As shown.

[0119] The TOC removal rate peaked at pH 3.0 (67.8%), a phenomenon that can be explained by the reaction mechanism. Within the strongly acidic pH range of 2.0-3.0, on the one hand, Fe... 2+It can maintain a completely dissolved state, avoiding precipitation at higher pH levels; on the other hand, H + The concentration is moderate, neither too high nor too low, which would lead to the ineffective decomposition of H2O2, nor too low a concentration, which would allow H2O2 to react with Fe. 2+ The reaction generates hydroxyl radicals (·OH) at the optimal rate (too low a pH will accelerate H2O2 consumption, while too high a pH will reduce reaction activity). It is noteworthy that the TN removal rate shows an abnormal decrease at pH 2.0 (with a large error bar range), which may be due to the following reasons: ① Strongly acidic conditions (pH≤2) promote the protonation of NH3 to NH4+. 4+ (pKa = 9.25), making it more difficult to oxidize; ②Fe 3+ / Fe 2+ With the cycle disrupted, iron species may form [Fe(H2O)6]. 3+ Inert complexes can affect oxidation efficiency. Based on the overall experimental results, pH 3.0 is the optimal condition that balances TOC removal efficiency and TN removal rate, ensuring both efficient degradation of organic matter and stable system operation.

[0120] (2) Different H2O2 / Fe 2+ Moor ratio

[0121] Design H2O2 / Fe under the condition of initial pH 3.0. 2+ Molar ratios of 1, 3, 5, 7, and 9 were used, with a fixed H₂O₂ dosage of 2 mL. Different molar ratios were achieved by adjusting the FeSO₄·7H₂O dosage (0.2178, 0.6535, 1.0891, 1.5247, and 1.9604 g), with a reaction time of 90 min. Different H₂O₂ / Fe₂O₄ ratios were observed. 2+ The effect of molar ratio on Fenton oxidation effect is as follows: Figure 7 As shown.

[0122] Experimental results show that the removal rates of TOC and TN exhibit distinctly different patterns, reflecting the differences in their removal mechanisms within the Fenton system. The TOC removal rate initially increases and then decreases with increasing molar ratio, reaching a peak (59.4%) at a molar ratio of 7. This phenomenon can be explained by the free radical chain reaction mechanism of the Fenton reaction. Appropriately increasing H2O2 can facilitate the removal of TOC by Fe... 2+ Catalytic generation of highly active ·OH(Fe) 2+ +H₂O₂→Fe 3+ +·OH+OH - It promotes the mineralization of organic matter. However, excessive H2O2 will undergo a quenching reaction with ·OH (H2O2 + ·OH → HO2· + H2O), reducing the oxidation efficiency.

[0123] The TN removal rate decreased continuously with increasing molar ratio, indicating that TN removal mainly depends on Fe. 3+The flocculation effect is not an oxidation effect; Fe 3+ Hydrolysis forms Fe(OH)3 colloid (Fe 3+ +3H₂O→Fe(OH)₃+3H + Nitrogen compounds are removed through surface adsorption and entrapment; an increased H2O2 ratio consumes more Fe. 2+ The presence of Fe(OH)3 in the free radical reaction reduces the amount of Fe(OH)3 generated, leading to a decrease in TN removal rate. Control experiments further verified this mechanism. When H2O2 was added alone, the TN removal rate was -3.4%, and the TOC removal rate was only 4.8%, indicating that Fe(OH)3 flocculation is the main pathway for TN removal, while TOC removal depends on Fe... 2+ Catalytic oxidation reaction.

[0124] Based on the actual situation that the effluent TN concentration already meets the discharge standard (≤40mg / L), but TOC still requires further treatment, H2O2 / Fe was ultimately selected. 2+ A molar ratio of 7 is optimal. This ratio balances the synergistic effects of free radical oxidation and flocculation, conforming to the design principle of the Fenton process: "synergistic effect of oxidation and flocculation."

[0125] (3) H2O2 dosage

[0126] The effect of H2O2 dosage (0.3, 0.5, 1.0, 1.5, 2.0, 3.0 mL) was investigated at an initial pH of 3.0. The H2O2 / Fe ratio was maintained. 2+ With a molar ratio of 7, 0.1167, 0.1945, 0.3890, 0.5834, 0.7779, and 1.1669 g of FeSO4·7H2O were added respectively, with a reaction time of 90 min. The effect of different H2O2 dosages on the Fenton oxidation effect is as follows: Figure 8 As shown.

[0127] With increasing H2O2 dosage, the TOC removal rate significantly improved. When the H2O2 dosage increased from 3 mL / L to 20 mL / L, the TOC removal rate rapidly increased from 51.2% to 65.6%. This indicates that within this dosage range, the addition of H2O2 effectively promoted the generation of hydroxyl radicals (·OH), thereby enhancing the oxidation efficiency of organic matter. However, when the H2O2 dosage exceeded 20 mL / L, the increase in TOC removal rate tended to level off, and even slightly decreased. This may be because excess H2O2 can undergo a side reaction with ·OH (H2O2 + ·OH → HO2· + H2O), leading to free radical quenching and thus a slower increase in TOC removal rate.

[0128] In contrast, the TN removal rate remained below 5% with no significant improvement. This result indicates that Fenton oxidation is less effective at degrading nitrogen-containing organics than ordinary organics, possibly due to the following reasons: some nitrogen-containing organics (such as nitro compounds and heterocyclic nitrogen compounds) have more stable structures and require stronger oxidation conditions for complete degradation; ·OH may preferentially attack non-nitrogen-containing organics, resulting in relatively low oxidation efficiency for nitrogen-containing compounds.

[0129] Considering both treatment effectiveness and economy, 20 mL / L is the optimal H2O2 dosage. At this level, the TOC removal rate is 65.6% and the reagent utilization efficiency is optimal. However, the low TN removal rate indicates that it is necessary to combine it with other denitrification processes for enhanced treatment.

[0130] (4) Different reaction times

[0131] At an initial pH of 3.0, H2O2 / Fe 2+ Under optimal conditions (molar ratio 7, H₂O₂ dosage 2 mL, FeSO₄·7H₂O dosage 0.7779 g), the effect of reaction time (15, 30, 45, 60, 90, 120 min) on the treatment effect was investigated. The effect of different reaction times on the Fenton oxidation effect is shown below. Figure 9 As shown.

[0132] Experimental results show that, with the extension of oxidation time, the removal efficiency of Fenton oxidation for pollutants exhibits a trend of rapid initial increase followed by a gradual plateau. In the initial stage of the reaction (0-60 min), the removal rates of TOC (total organic carbon) and TN (total nitrogen) increase significantly, indicating that H2O2 and Fe2O3 react at this time. 2+ The hydroxyl radicals (·OH) generated in the reaction can effectively degrade organic matter and nitrogen-containing pollutants. The pollutant removal effect reaches its optimal level when the oxidation time reaches 60 minutes, at which point the system reaction is complete and the utilization rate of free radicals is highest.

[0133] However, when the oxidation time exceeds 60 minutes, the removal rates of TOC and TN slow down or even stabilize. This may be because as the reaction proceeds, H2O2 is gradually consumed, reducing the amount of free radicals generated; simultaneously, the intermediate products accumulated in the reaction system may inhibit the activity of free radicals, leading to a decrease in oxidation efficiency. Furthermore, excessively long reaction times may cause some oxidized organic matter to be re-released or side reactions to occur, thus affecting the overall treatment effect.

[0134] In conclusion, the optimal oxidation time should be controlled at 60 minutes. This ensures high pollutant removal efficiency while avoiding resource waste or decreased effectiveness caused by excessively prolonged reaction time.

[0135] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention specification, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method for synergistic removal of contaminants from nanofiltration membrane concentrate by coagulation-precipitation-Fenton oxidation-electrochemical oxidation, characterized in that, Includes the following steps: S1. The effluent from the nanofiltration membrane concentrate after coagulation and sedimentation is adjusted to an initial pH of 3.0 using 1.0 mol / L H2SO4 or 2.0 mol / L NaOH solution. Then, FeSO4·7H2O and 30% H2O2 solution are added to the effluent in the designed proportion. S2. Stir to start the oxidation reaction. After the reaction is terminated, adjust the pH of the effluent to 9.0, wait for it to settle, and discharge the supernatant, which is the Fenton oxidation effluent. S3. Immerse the positive and negative electrodes in the Fenton oxidation water for electrochemical oxidation, and obtain the treated water sample after continuous electrolysis. The pH value of the nanofiltration membrane concentrate is 6.0-8.5; Chemical oxygen demand (COD) is 300-600 mg / L; Biochemical oxygen demand (BOD5) is 30-80 mg / L; Total organic carbon (TOC) is 200-300 mg / L; Total nitrogen (TN) was 40-68 mg / L; Total phosphorus (TP) was 0-29 mg / L; Ammonia nitrogen content is 0-30 mg / L; H2O2 / Fe in S1 2+ The molar ratio is 7, and the relationship between the H2O2 dosage and the volume of water effluent from the nanofiltration membrane concentrate coagulation and sedimentation is 20 mL / L. The oxidation reaction time in S2 is 60 min.

2. The method for synergistic removal of contaminants from nanofiltration membrane concentrate by coagulation-precipitation-Fenton oxidation-electrochemical oxidation according to claim 1, characterized in that, The coagulation and sedimentation process in S1 is as follows: the initial pH of the nanofiltration membrane concentrate derived from the anaerobic fermentation biogas slurry of kitchen waste is adjusted and stirred. Then, polyferric sulfate (PFS) is used as a coagulant and wet-added to the nanofiltration membrane concentrate. After rapid stirring, cationic polyacrylamide (PAM) is wet-added. After low-speed stirring, the stirring is stopped, and the flocs or sludge are separated from the coagulation and sedimentation effluent by sedimentation.

3. The method for synergistic removal of contaminants from nanofiltration membrane concentrate by coagulation-precipitation-Fenton oxidation-electrochemical oxidation according to claim 2, characterized in that, The coagulant is a 10% polyferric sulfate (PFS) solution, and the coagulant aid is a 0.1% cationic polyacrylamide (PAM) solution.

4. The method for synergistic removal of contaminants from nanofiltration membrane concentrate by coagulation sedimentation-Fenton oxidation-electrochemical oxidation according to claim 3, characterized in that, When using polyferric sulfate (PFS) as a coagulant, adjust the initial pH to 7.

0.

5. The method for synergistic removal of contaminants from nanofiltration membrane concentrate by coagulation-precipitation-Fenton oxidation-electrochemical oxidation according to claim 2, characterized in that, The polyferric sulfate (PFS) was added until its concentration was 1.0 g / L.

6. The method for synergistic removal of contaminants from nanofiltration membrane concentrate by coagulation-precipitation-Fenton oxidation-electrochemical oxidation according to claim 2, characterized in that, The coagulant, cationic polyacrylamide (PAM), was added to a concentration of 4.0 mg / L.

7. The method for synergistic removal of contaminants from nanofiltration membrane concentrate by coagulation-precipitation-Fenton oxidation-electrochemical oxidation according to claim 2, characterized in that, After adding the coagulant, stir rapidly at 200 rpm for 5 minutes.

8. The method for synergistic removal of contaminants from nanofiltration membrane concentrate by coagulation sedimentation-Fenton oxidation-electrochemical oxidation according to claim 2, characterized in that, After adding the coagulant, reduce the stirring speed to 100 r / min and stop stirring after 20 min.

9. The method for synergistic removal of contaminants from nanofiltration membrane concentrate by coagulation-precipitation-Fenton oxidation-electrochemical oxidation according to claim 1, characterized in that, In S3, the cathode electrode material is a titanium plate, and the anode electrode material is a lead dioxide plate.

10. The method for synergistic removal of contaminants from nanofiltration membrane concentrate by coagulation-precipitation-Fenton oxidation-electrochemical oxidation according to claim 1, characterized in that, The continuous electrolysis time in S3 is 2 hours.

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