A nanofiltration membrane integrated process for leachate treatment at a waste transfer station and a flocculant for its pretreatment stage.

Abstract

This invention relates to the field of water treatment technology, specifically to a nanofiltration membrane integrated process for treating leachate from landfill transfer stations and a flocculant for its pretreatment stage. This flocculant utilizes ionic liquid-grafted diatomaceous earth and sulfonate-modified branched polyamide crosslinking in synergy, combined with in-situ aluminum and iron salts to generate a highly efficient composite flocculant. This achieves rapid removal of multiple pollutants and enhanced floc stability, significantly reducing the risk of subsequent membrane fouling. The process employs an "innovative pretreatment-stage nanofiltration-deep purification" model. The membrane types for each nanofiltration stage are optimized and screened according to pollutant characteristics, achieving molecular-level stepwise filtration. The final stage combines photo-Fenton oxidation, dense nanofiltration, or reverse osmosis in a flexible combination, balancing efficient COD and salinity reduction with energy consumption control. This process produces stable effluent quality, low membrane fouling, significantly reduced reagent and operating costs, and is adaptable to different discharge standards, demonstrating significant engineering application value.

Description

Technical Field

[0001] This invention relates to the field of water treatment technology, and in particular to a nanofiltration membrane integrated process for treating leachate from a landfill transfer station and a flocculant for its pretreatment stage. Background Technology

[0002] Landfill leachate from landfill transfer stations has a complex composition and high pollutant concentration, containing large amounts of organic matter (such as humic acid and protein), colloidal suspended solids, oils, and heavy metal ions. Its typical characteristics include high COD (usually exceeding 20,000 mg / L), high turbidity (2000-3000 NTU), strong acidity, and high color. This type of wastewater has poor biodegradability, and conventional treatment processes (such as biological methods) have limited effectiveness. Existing technologies mostly employ a combination of "pretreatment + membrane separation," but this approach has significant drawbacks.

[0003] Traditional flocculants (such as inorganic aluminum and iron salts) have low capture efficiency for dissolved organic matter and colloids, leading to severe clogging of subsequent membrane systems. Physicochemical methods (such as activated carbon adsorption) are costly and difficult to regenerate. Furthermore, colloids and organic matter in the leachate easily form an irreversible fouling layer on the membrane surface, requiring frequent chemical cleaning, reducing membrane flux and lifespan. Ordinary ultrafiltration (UF) membranes have an irreversible fouling index as high as 48% after filtration, significantly increasing operating costs. Moreover, existing nanofiltration membrane selection is limited (e.g., focusing only on molecular weight cutoff) and does not consider the interaction between pollutant characteristics and membrane surface charge. For example, hydrophobic membranes easily adsorb oily substances, while ordinary nanofiltration membranes have insufficient rejection rates for negatively charged organic matter. In addition, end-stage photo-Fenton oxidation consumes a large amount of reagents and produces iron sludge, while reverse osmosis (RO), although highly efficient, is energy-intensive. Unoptimized membrane combination processes (such as "UF+RO") still result in effluent COD as high as 1,900 mg / L, making it difficult to meet discharge standards.

[0004] Therefore, there is an urgent need to develop an integrated membrane process that is low-pollution, highly adaptable, and energy-controllable, in order to reduce the risk of membrane fouling at the source and improve overall treatment efficiency. Summary of the Invention

[0005] In view of this, the purpose of this invention is to propose a nanofiltration membrane integrated process for treating leachate from landfill transfer stations and a flocculant for the pretreatment stage, so as to provide a landfill leachate nanofiltration membrane integrated process with low membrane fouling and high treatment efficiency and a flocculant for the pretreatment stage, and solve the problems of incomplete pretreatment and easy clogging of membrane systems.

[0006] To achieve the above objectives, this invention provides a flocculant for the pretreatment stage of a nanofiltration membrane integrated process for leachate treatment in waste transfer stations. The flocculant uses ionic liquid-grafted diatomaceous earth and sulfonate-modified branched polyamide crosslinking as carriers, and is obtained by in-situ generation of polyaluminum ferric chloride from aluminum and iron salts. The weight ratio of the ionic liquid-grafted diatomaceous earth, sulfonate-modified branched polyamide crosslinking, aluminum salt, and iron salt is 1-3:2-10:10-30:5-15.

[0007] Preferably, the aluminum salt is aluminum chloride hexahydrate.

[0008] Preferably, the iron salt is ferric sulfate.

[0009] Furthermore, the ionic liquid-grafted diatomaceous earth is obtained by modifying diatomaceous earth with a silane-type ionic liquid prepared from 3-chloropropyltrimethoxysilane and N-methylimidazole; the weight ratio of 3-chloropropyltrimethoxysilane and N-methylimidazole is 1:0.35-0.45; the weight ratio of the silane-type ionic liquid to diatomaceous earth is 2:0.3-0.8.

[0010] Preferably, the diatomaceous earth is ground through a 200-mesh sieve and dried before modification.

[0011] Preferably, the preparation steps of the ionic liquid-grafted diatomaceous earth are as follows:

[0012] (1) Mix 3-chloropropyltrimethoxysilane and N-methylimidazolium, stir and react for 24 h under nitrogen atmosphere, wash with diethyl ether, and rotary evaporate to obtain silane-type ionic liquid;

[0013] (2) Add diatomaceous earth and silane-type ionic liquid to toluene, heat to 108-112℃, reflux for 10-15h, centrifuge, wash with ethanol, and vacuum dry to obtain ionic liquid-grafted diatomaceous earth.

[0014] Furthermore, the sulfonate-modified branched polyamide crosslinker is obtained by polymerization of sodium isophthalate-5-sulfonate, polyetheramine D400 and tris(2-aminoethyl)amine; the weight ratio of sodium isophthalate-5-sulfonate, polyetheramine D400 and tris(2-aminoethyl)amine is 2.68:3.2:0.3.

[0015] Preferably, the preparation steps of the sulfonate-modified branched polyamide crosslinker are as follows: sodium isophthalate-5-sulfonate, polyetheramine D400 and tris(2-aminoethyl)amine are added to N-methylpyrrolidone and stirred for 30 min. Then, pyridine, triphenyl phosphite, calcium chloride and lithium chloride are added. The mixture is heated to 110°C under a nitrogen atmosphere and stirred for 24 h. After the reaction is completed, the mixture is poured into acetone and allowed to stand to precipitate. The mixture is then filtered, and the filter residue is washed with acetone and dried under vacuum to obtain the sulfonate-modified branched polyamide.

[0016] Furthermore, the preparation steps of the flocculant for the pretreatment section of the nanofiltration membrane integrated process for treating leachate from a landfill transfer station are as follows: Ionic liquid-grafted diatomaceous earth and sulfonate-modified branched polyamide are added to deionized water, heated to 38-42℃, and stirred for 20-40 minutes to form a crosslinked compound. Then, aluminum chloride hexahydrate and ferric sulfate are added, and stirring is continued for 0.5-1.5 hours. The pH is then adjusted to 6.3-6.6 using sodium hydroxide solution, cooled to 25℃, and stirred for 10-15 hours to generate polyaluminum ferric chloride in situ on the crosslinked compound. The mixture is then vacuum dried, ground, and passed through a 50-mesh sieve to obtain the flocculant for the leachate treatment process of the landfill transfer station.

[0017] Preferably, the concentration of the sodium hydroxide solution is 25wt%-35wt%.

[0018] Furthermore, the present invention also provides a nanofiltration membrane integrated process for leachate treatment in waste transfer stations, comprising the following steps:

[0019] S1: The leachate from the waste transfer station enters the pretreatment section from the collection tank. First, Ca(OH)2 is added to adjust the pH to alkaline, and then flocculant is added for sedimentation.

[0020] S2: Take the supernatant after sedimentation in step S1, add NaOH until pH 12-13, react for 10 minutes, then add flocculant and let it settle.

[0021] S3: Take the supernatant after sedimentation in step S2, add Na2CO3 and react for 10 min, then add flocculant and let it settle through CaCO3;

[0022] S4: Filter the solution after sedimentation in step S3, then add H2SO4 to adjust the pH to weakly acidic to obtain pretreated wastewater;

[0023] S5: Pretreated wastewater directly enters the loose nanofiltration section, where COD is reduced by 29%-32% through the membrane's retention effect;

[0024] S6: Loose nanofiltration permeate re-enters the nanofiltration section, where the COD is reduced to 4000-5500 mg / L and desalination is achieved by 40%-60% through the membrane's retention effect.

[0025] S7: Nanofiltration permeate enters the deep purification unit, where COD is further reduced through powerful free radical oxidation by photo-Fenton and efficient retention by dense nanofiltration or reverse osmosis membranes.

[0026] Preferably, the loose nanofiltration section requires the nanofiltration membrane to have a molecular weight cutoff of approximately 500-2000D, and the membrane material to be polyamide. This section can also be replaced by ceramic membranes or organic dense ultrafiltration membranes, but the membrane needs to have high antifouling properties. The nanofiltration section requires the nanofiltration membrane to have a molecular weight cutoff of 150-400D, and the membrane material to be polyamide, with a divalent salt rejection rate of ≥98%.

[0027] Preferably, the initial pH of the photo-Fenton stage is 2-3, the reaction time is 2-4 hours, the oxidant dosage is H2O2:COD(m / m) = 1-2.5:1, the catalyst dosage is H2O2:Fe(m / m) = 20-80:1, and the ultraviolet light source power is ≥100mW / cm². 2 .

[0028] Preferably, the dense nanofiltration section requires the dense nanofiltration membrane to have a molecular weight cutoff of ≤150D, the membrane material to be polyamide, and a monovalent salt rejection rate of ≥80%.

[0029] Preferably, the reverse osmosis section requires the reverse osmosis membrane to retain a molecular weight cutoff of ≤100D, and the reverse osmosis section requires the reverse osmosis membrane to retain monovalent salts with a rejection rate of ≥90%.

[0030] The beneficial effects of this invention are:

[0031] (I) Preprocessing mechanism for deep collaboration

[0032] Achieving a triple synergistic effect through innovative flocculant design:

[0033] Enhanced electrostatic adsorption: Silane-type ionic liquid grafted onto the surface of diatomaceous earth with quaternary ammonium cations efficiently neutralizes the negative charge of the colloidal material, and combines with hydrophobic groups to target and adsorb oils and humic acids, significantly compressing the double layer.

[0034] Spatial network construction: The anionic groups of sulfonate-modified branched polyamide form an ionic cross-linking network with the cation sites of diatomaceous earth, which serves as an efficient crystal nucleus template for hydroxyl-modified aluminum-iron, inducing the formation of a dense porous flocculent structure.

[0035] Shear stabilization: The flexible molecular bridges of the polyamide branched chains enhance the mechanical strength of the flocs, and simultaneously trap soluble COD and colloidal particles through a net-like sweeping action.

[0036] This composite flocculation system can reduce oil concentration to below 3 mg / L and turbidity to below 1 NTU, providing low-load feed water for subsequent membrane systems.

[0037] (II) Membrane Fouling Control and Process Adaptability

[0038] Gradient membrane screening mechanism:

[0039] Loose nanofiltration section: Using a low molecular weight cutoff (500Da) and low charge density membrane (NF5), it preferentially removes large molecular colloids, avoids high pressure differential clogging, and reduces the contamination index to below 5%.

[0040] Fine nanofiltration stage: Select medium-to-high molecular weight cutoff membrane (NF2) to focus on medium molecular weight organic matter, balancing flux and pollutant removal rate.

[0041] Deep purification stage: Flexible matching of photo-Fenton oxidation (for treating recalcitrant organic matter), dense nanofiltration (NF1), or reverse osmosis (RO) according to water quality requirements, wherein the COD of RO effluent can be stably below 350mg / L.

[0042] Synergistic pollution suppression: The porous layer formed by the pretreated flocs creates a dynamic protective barrier on the membrane surface, reducing direct adhesion of colloids. Simultaneously, the ionic liquid graft layer regulates the hydrophilic / hydrophobic balance at the membrane-pollutant interface, reducing the risk of oil adsorption.

[0043] (III) Optimization of Operating Costs and Stability

[0044] Reduced reagent consumption: The amount of flocculant added is reduced by more than 50%, and the amount of H2O2 used in the deep section is only 30% of that in the traditional process.

[0045] Extended membrane life: The irreversible fouling index of each membrane segment is ≤10%, and the chemical cleaning cycle is extended by more than 2 times.

[0046] High process flexibility: By adjusting the deep purification section modules (photo-Fenton / NF1 / RO), it can adapt to different emission standards, balancing economy and treatment efficiency.

[0047] This integrated process achieves low membrane fouling throughout the leachate treatment process, reduces overall treatment costs by 40%, and ensures stable and compliant effluent quality. Detailed Implementation

[0048] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0049] In the specific embodiment of the present invention, the diatomite was purchased from Yixing Junlian Diatomite Co., Ltd., and the model was diatomite granular adsorbent 3#1-3mm.

[0050] Example 1:

[0051] (1) Mix 1g of 3-chloropropyltrimethoxysilane and 0.4g of N-methylimidazolium and stir the mixture under a nitrogen atmosphere for 24h. Wash the mixture three times with diethyl ether to remove unreacted excess reagents. Remove the diethyl ether by rotary evaporation to obtain a silane-type ionic liquid.

[0052] (2) 2g of diatomaceous earth (ground through a 200-mesh sieve) was treated at 100℃ for 3h, then 80g of toluene and 0.3g of silane-type ionic liquid were added, the temperature was raised to 108℃, and the reaction was refluxed for 10h. After centrifugation, the mixture was washed with ethanol and dried under vacuum to obtain ionic liquid-grafted diatomaceous earth.

[0053] (3) 2.68g sodium isophthalic acid-5-sulfonate, 3.2g polyetheramine D400 and 0.3g tris(2-aminoethyl)amine were added to 15g N-methylpyrrolidone and stirred for 30min. Then 2g pyridine, 6.2g triphenyl phosphite, 0.6g calcium chloride and 0.2g lithium chloride were added. The mixture was heated to 110℃ under nitrogen atmosphere and stirred for 24h. After the reaction was completed, the mixture was poured into 250g acetone and allowed to stand to precipitate. The mixture was filtered, and the filter residue was washed with acetone and dried under vacuum to obtain sulfonate modified branched polyamide.

[0054] (4) Add 1g of ionic liquid-grafted diatomaceous earth and 2g of sulfonate-modified branched polyamide to 50g of deionized water, heat to 38℃, stir for 20-40min, then add 10g of aluminum chloride hexahydrate and 5g of ferric sulfate, continue stirring for 0.5h, then adjust the pH to 6.3 with a 30wt% sodium hydroxide solution, cool to 25℃, stir for 10h, vacuum dry, grind through a 50-mesh sieve to obtain the flocculant for the pretreatment section of the nanofiltration membrane integrated process for leachate treatment in the landfill transfer station.

[0055] Example 2:

[0056] (1) Mix 1g of 3-chloropropyltrimethoxysilane and 0.4g of N-methylimidazolium and stir the mixture under a nitrogen atmosphere for 24h. Wash the mixture three times with diethyl ether to remove unreacted excess reagents. Remove the diethyl ether by rotary evaporation to obtain a silane-type ionic liquid.

[0057] (2) 2g of diatomaceous earth (ground through a 200-mesh sieve) was treated at 100℃ for 3h, then 100g of toluene and 0.5g of silane-type ionic liquid were added, the temperature was raised to 110℃, and the reaction was refluxed for 12h. After centrifugation, the mixture was washed with ethanol and dried under vacuum to obtain ionic liquid-grafted diatomaceous earth.

[0058] (3) 2.68g sodium isophthalic acid-5-sulfonate, 3.2g polyetheramine D400 and 0.3g tris(2-aminoethyl)amine were added to 15g N-methylpyrrolidone and stirred for 30min. Then 2g pyridine, 6.2g triphenyl phosphite, 0.6g calcium chloride and 0.2g lithium chloride were added. The mixture was heated to 110℃ under nitrogen atmosphere and stirred for 24h. After the reaction was completed, the mixture was poured into 250g acetone and allowed to stand to precipitate. The mixture was filtered, and the filter residue was washed with acetone and dried under vacuum to obtain sulfonate modified branched polyamide.

[0059] (4) Add 2g of ionic liquid-grafted diatomaceous earth and 5g of sulfonate-modified branched polyamide to 100g of deionized water, heat to 40℃, stir for 30min, then add 20g of aluminum chloride hexahydrate and 10g of ferric sulfate, continue stirring for 1h, then adjust the pH to 6.5 with a 30wt% sodium hydroxide solution, cool to 25℃, stir for 12h, vacuum dry, grind through a 50-mesh sieve to obtain the flocculant for the pretreatment section of the nanofiltration membrane integrated process for leachate treatment in the landfill transfer station.

[0060] Example 3:

[0061] (1) Mix 1g of 3-chloropropyltrimethoxysilane and 0.4g of N-methylimidazolium and stir the mixture under a nitrogen atmosphere for 24h. Wash the mixture three times with diethyl ether to remove unreacted excess reagents. Remove the diethyl ether by rotary evaporation to obtain a silane-type ionic liquid.

[0062] (2) 2g of diatomaceous earth (ground through a 200-mesh sieve) was treated at 100℃ for 3h, then 120g of toluene and 0.8g of silane-type ionic liquid were added, the temperature was raised to 112℃, and the reaction was refluxed for 15h. After centrifugation, the mixture was washed with ethanol and dried under vacuum to obtain ionic liquid-grafted diatomaceous earth.

[0063] (3) 2.68g sodium isophthalic acid-5-sulfonate, 3.2g polyetheramine D400 and 0.3g tris(2-aminoethyl)amine were added to 15g N-methylpyrrolidone and stirred for 30min. Then 2g pyridine, 6.2g triphenyl phosphite, 0.6g calcium chloride and 0.2g lithium chloride were added. The mixture was heated to 110℃ under nitrogen atmosphere and stirred for 24h. After the reaction was completed, the mixture was poured into 250g acetone and allowed to stand to precipitate. The mixture was filtered, and the filter residue was washed with acetone and dried under vacuum to obtain sulfonate modified branched polyamide.

[0064] (4) Add 3g of ionic liquid-grafted diatomaceous earth and 10g of sulfonate-modified branched polyamide to 200g of deionized water, heat to 42℃, stir for 40min, then add 30g of aluminum chloride hexahydrate and 15g of ferric sulfate, continue stirring for 1.5h, then adjust the pH to 6.6 with a 30wt% sodium hydroxide solution, cool to 25℃, stir for 15h, vacuum dry, grind through a 50-mesh sieve to obtain the flocculant for the pretreatment section of the nanofiltration membrane integrated process for leachate treatment in the landfill transfer station.

[0065] Comparative Example 1:

[0066] The difference between Comparative Example 1 and Example 2 is that the ionic liquid grafted diatomaceous earth in step (4) is replaced with diatomaceous earth;

[0067] Comparative Example 2:

[0068] The difference between Comparative Example 2 and Example 2 is that the 2g of ionic liquid-grafted diatomaceous earth in step (4) is replaced with a mixture of 1.6g of diatomaceous earth and 0.4g of 1-methylimidazolium chloride.

[0069] Comparative Example 3:

[0070] The difference between Comparative Example 3 and Example 2 is that no sulfonate-modified branched polyamide was added in step (4);

[0071] Comparative Example 4:

[0072] The difference between Comparative Example 4 and Example 2 is that sodium isophthalic acid-5-sulfonate in step (3) is replaced with an equimolar amount of isophthalic acid;

[0073] Comparative Example 5:

[0074] The difference between Comparative Example 5 and Example 2 is that the tris(2-aminoethyl)amine in step (3) is replaced with an equimolar amount of polyetheramine D400;

[0075] The following process validation used leachate from a waste transfer station in Anhui Province as a sample. The water quality parameters of this sample are shown in Table 1 below, and the parameters of the loose nanofiltration membrane, nanofiltration membrane, dense nanofiltration membrane, reverse osmosis membrane, and other control membranes used are shown in Table 2 below.

[0076] Table 1. Water quality parameters of leachate from a certain waste transfer station.

[0077] index numerical values pH 3.6-3.8 Appearance Yellow-brown emulsion Turbidity (NTU) 2000-3000 COD (mg / L) 19800-21000 <![CDATA[NH3-N(mg / L)]]> 100-120 Oils (mg / L) 61.7

[0078] Table 2 lists the specific parameters of various membranes used.

[0079]

[0080] Example 4:

[0081] (1) Take 500 mL of leachate from the landfill transfer station, add 2.5 g of Ca(OH)2, stir for 10 min, the pH of the reaction solution is 11.5, continue to add 300 mg / L of the flocculant prepared in Example 2, stir for 5 min, and let stand for 15 min;

[0082] (2) Next, take 400 mL of the supernatant after standing, add 0.80 g NaOH, stir for 10 min, the pH of the reaction solution is 12.2, then add 1 mg / L of the flocculant prepared in Example 2, stir for 10 min, and let stand for 15 min;

[0083] (3) Take 300 mL of the supernatant after standing, add 1.5 g Na2CO3, stir for 10 min, then add 1 mg / L of the flocculant prepared in Example 2, stir for 10 min, let stand for 5 min, and filter the supernatant with neutral filter paper.

[0084] (4) Then add H2SO4 to adjust the pH to 6.0, stir for 5 min to obtain the pretreated leachate, and measure the COD, turbidity and oil content of the product water;

[0085] (5) The pretreated water enters the loose nanofiltration section and is filtered using NF5. The irreversible pollution index and COD of the water after the concentration experiment are measured to comprehensively judge the performance.

[0086] (6) The permeate from the loose nanofiltration section enters the nanofiltration section and is filtered using NF2. The irreversible pollution index and COD of the permeate are measured after the concentration experiment to comprehensively judge the performance.

[0087] (7) The nanofiltration stage permeate is further purified using photo-Fenton technology. The initial pH of the wastewater is adjusted to 2.5, the H2O2 dosage is H2O2:COD(m / m) = 2:1, the FeSO4 dosage is H2O2:Fe(m / m) = 20:1, and the ultraviolet light source power is 200mW / cm². 2 The reaction time was 3 hours. After the reaction was completed, NaOH was added to adjust the pH to neutral. The mixture was then filtered through neutral filter paper, and the final COD of the produced water was measured.

[0088] Example 5:

[0089] (1) Take 500 mL of leachate from the landfill transfer station, add 2.86 g of Ca(OH)2, stir for 10 min, the pH of the reaction solution is 11.8, continue to add 300 mg / L of the flocculant prepared in Example 2, stir for 5 min, and let stand for 15 min;

[0090] (2) Next, take 400 mL of the supernatant after standing, add 0.85 g NaOH, stir for 10 min, the pH of the reaction solution is 12.4, then add 1 mg / L of the flocculant prepared in Example 2, stir for 10 min, and let stand for 15 min;

[0091] (3) Take 300 mL of the supernatant after standing, add 1.5 g Na2CO3, stir for 10 min, then add 1 mg / L of the flocculant prepared in Example 2, stir for 10 min, let stand for 5 min, and filter the supernatant with neutral filter paper.

[0092] (4) Then add H2SO4 to adjust the pH to 6.0, stir for 5 min to obtain the pretreated leachate, and measure the COD, turbidity and oil content of the product water;

[0093] (5) The pretreated water enters the loose nanofiltration section and is filtered using NF5. The irreversible pollution index and COD of the water after the concentration experiment are measured to comprehensively judge the performance.

[0094] (6) The permeate from the loose nanofiltration section enters the nanofiltration section and is filtered using NF2. The irreversible pollution index and COD of the permeate are measured after the concentration experiment to comprehensively judge the performance.

[0095] (7) The nanofiltration stage permeate is further purified using photo-Fenton technology. The initial pH of the wastewater is adjusted to 2.5, the H2O2 dosage is H2O2:COD(m / m) = 2:1, the FeSO4 dosage is H2O2:Fe(m / m) = 20:1, and the ultraviolet light source power is 200mW / cm². 2 The reaction time was 3 hours. After the reaction was completed, NaOH was added to adjust the pH to neutral. The mixture was then filtered through neutral filter paper, and the final COD of the produced water was measured.

[0096] Example 6:

[0097] (1) Take 500 mL of leachate from the landfill transfer station, add 2.86 g of Ca(OH)2, stir for 10 min, the pH of the reaction solution is 11.8, continue to add 300 mg / L of the flocculant prepared in Example 2, stir for 5 min, and let stand for 15 min;

[0098] (2) Next, take 400 mL of the supernatant after standing, add 0.85 g NaOH, stir for 10 min, the pH of the reaction solution is 12.4, then add 1 mg / L of the flocculant prepared in Example 2, stir for 10 min, and let stand for 15 min;

[0099] (3) Take 300 mL of the supernatant after standing, add 1.5 g Na2CO3, stir for 10 min, then add 1 mg / L of the flocculant prepared in Example 2, stir for 10 min, let stand for 5 min, and filter the supernatant with neutral filter paper.

[0100] (4) Then add H2SO4 to adjust the pH to 6.0, stir for 5 min to obtain the pretreated leachate, and measure the COD, turbidity and oil content of the product water;

[0101] (5) The pretreated water enters the loose nanofiltration section and is filtered using NF5. The irreversible pollution index and COD of the water after the concentration experiment are measured to comprehensively judge the performance.

[0102] (6) The permeate from the loose nanofiltration section enters the nanofiltration section and is filtered using NF2. The irreversible pollution index and COD of the permeate are measured after the concentration experiment to comprehensively judge the performance.

[0103] (7) The water produced by nanofiltration enters the deep purification section and is further treated with NF1 to determine the COD of the final water.

[0104] Example 7:

[0105] (1) Take 500 mL of leachate from the landfill transfer station, add 2.86 g of Ca(OH)2, stir for 10 min, the pH of the reaction solution is 11.8, continue to add 300 mg / L of the flocculant prepared in Example 2, stir for 5 min, and let stand for 15 min;

[0106] (2) Next, take 400 mL of the supernatant after standing, add 0.85 g NaOH, stir for 10 min, the pH of the reaction solution is 12.4, then add 1 mg / L of the flocculant prepared in Example 2, stir for 10 min, and let stand for 15 min;

[0107] (3) Take 300 mL of the supernatant after standing, add 1.5 g Na2CO3, stir for 10 min, then add 1 mg / L of the flocculant prepared in Example 2, stir for 10 min, let stand for 5 min, and filter the supernatant with neutral filter paper.

[0108] (4) Then add H2SO4 to adjust the pH to 6.0, stir for 5 min to obtain the pretreated leachate, and measure the COD, turbidity and oil content of the product water;

[0109] (5) The pretreated water enters the loose nanofiltration section and is filtered using NF5. The irreversible pollution index and COD of the water after the concentration experiment are measured to comprehensively judge the performance.

[0110] (6) The permeate from the loose nanofiltration section enters the nanofiltration section and is filtered using NF2. The irreversible pollution index and COD of the permeate are measured after the concentration experiment to comprehensively judge the performance.

[0111] (7) The water produced by nanofiltration enters the deep purification section and is further treated by RO. The COD of the final water is then measured.

[0112] Comparative Example 6:

[0113] The difference between Comparative Example 6 and Example 5 is that the loose nanofiltration section uses CRUF for concentration filtration.

[0114] Comparative Example 7:

[0115] The difference between Comparative Example 7 and Example 5 is that the loose nanofiltration section uses UF for concentration filtration.

[0116] Performance testing:

[0117] Preparation of simulated landfill leachate: Referring to the typical water quality indicators in the "Technical Specification for Leachate Treatment Engineering of Municipal Solid Waste Landfills (Trial)" (HJ 564-2010), 1L of distilled water was mixed with 3.0g glucose, 0.5g peptone, 0.3g ammonium chloride, 0.5g potassium dihydrogen phosphate, 0.5g kaolin, and 0.1g humic acid. After stirring for 30 minutes, the mixture was allowed to stand for 24 hours to mature, resulting in a simulated leachate with a COD of 3025mg / L, SS of 520mg / L, turbidity of 208NTU, and color of 410 times.

[0118] Flocculation experiment: Take 6 1000mL beakers and fill each with 500mL of simulated landfill leachate. Add 100mg / L (based on leachate volume) of the flocculant samples prepared in Examples 1-3 and Comparative Examples 1-5 respectively. First, stir and disperse at 200rpm for 2min, then stir slowly at 50rpm for 10min to promote floc growth. After stopping stirring, let stand and settle for 30min. After settling, use a pipette to draw the supernatant 2cm below the liquid surface. COD determination was performed according to GB 11914-89 "Determination of Chemical Oxygen Demand in Water - Dichromate Method". The sample was added to potassium dichromate solution and digested at 165℃ for 2 hours under silver sulfate catalysis, followed by titration with ferrous ammonium sulfate. SS determination was performed according to GB 11901-89 "Determination of Suspended Solids in Water - Gravimetric Method". 100 mL of the supernatant was filtered through a 0.45 μm filter membrane, dried at 105℃ to constant weight, and weighed. Turbidity determination was performed according to GB 13200-91 "Determination of Turbidity in Water", using a turbidimeter for direct reading. Color determination was performed according to GB... The method of determining the color of water was followed according to 11903-89 "Method for Determination of Color in Water". The supernatant was diluted until it was visually colorless. The settling velocity was determined by recording the height of the interface between the liquid surface and the floc layer per minute during the settling process, plotting the settling curve and calculating the average settling velocity of the linear segment (unit: mm / min). The removal rate was calculated based on the blank sample without flocculant, according to the formula "Removal rate (%) = (C0-C) / C0×100%", where C0 is the blank measurement value and C is the measurement value of the supernatant after flocculation. The results are shown in Table 3.

[0119] Table 3 Performance Test Results

[0120]

[0121]

[0122] Data Analysis:

[0123] According to Table 3, stable high removal rates and sedimentation performance were observed in Examples 1-3, indicating that the flocculant of the present invention has universal effectiveness in complex leachate systems. It is speculated that silane-type ionic liquid-grafted diatomaceous earth enhances the electrostatic attraction of negatively charged colloidal pollutants through quaternary ammonium cations, while its surface organic groups adsorb humic acid-like organic matter through hydrophobic interactions. The sulfonic acid groups of sulfonate-modified branched polyamide ionize in the aqueous system to form multiple negative charge sites, generating a "cation-anion synergistic effect" with the ionic liquid-grafted diatomaceous earth, forming a three-dimensional network structure carrier. This carrier serves as a crystal nucleus template during the subsequent formation of polyaluminum ferric chloride, inducing the formation of denser Al-Fe polymers from hydroxyl-containing polyaluminum ferric chloride, resulting in multiple effects of charge neutralization, adsorption bridging, and network capture sweeping, achieving efficient capture of soluble COD, colloidal SS, and chromogenic substances.

[0124] According to Table 3, the removal rate and settling velocity of Comparative Example 1 were significantly worse than those of Example 2, proving that diatomaceous earth alone cannot replace ionic liquid grafted modified materials. It is speculated that the insufficient number of silanol polar sites on the surface of diatomaceous earth leads to a low charge density in the flocs formed with aluminum-iron salts, and the lack of steric hindrance from organic components results in loose and easily broken flocs. In contrast, the ionic liquid graft layer regulates the surface hydrophilic / hydrophobic balance through alkyl chains, enhancing the adsorption force on non-polar organic matter. Its rigid imidazole ring structure provides skeletal support for the flocs, giving the formed Al-Fe polymers a stable mesoporous structure, thereby increasing floc density and settling velocity.

[0125] According to Table 3, Comparative Example 2, which introduced the ionic liquid component through physical mixing, exhibited lower overall performance than the chemically grafted modification in Example 2. It is inferred that the physically adsorbed 1-methylimidazolium chloride is prone to desorption and release during flocculation, failing to form a stable complex with the sulfonate-modified branched polyamide. In contrast, the grafted ionic liquid is firmly anchored to the diatomaceous earth surface through chemical bonds. Its quaternary ammonium cations can form an ionic cross-linking network with the anionic groups of the sulfonate-modified branched polyamide, regulating the polymerization pathway of aluminum and iron hydrolysis products during metal salt hydrolysis, generating composite flocs with a zeolite-like structure, and enhancing the sieving and retention capacity for organic macromolecules.

[0126] According to Table 3, the absence of the sulfonate-modified branched polyamide component in Comparative Example 3 resulted in a significant performance decrease, indicating the crucial regulatory role of this component in the flocculation system. It is speculated that the branched structure of the polyamide molecular chain forms coordination complexes with aluminum and iron ions through amide bonds, thus slowing down the hydrolysis rate of the metal salt. The ionization of its sulfonate groups provides space charge, synergistically constructing a "micro-electric field" with the diatomaceous earth ionic liquid sites, causing the colloidal particles to migrate and aggregate in a directional manner. Simultaneously, the flexible long chains of polyamide form molecular bridges between flocs, enhancing the flocs' shear resistance, especially improving the removal efficiency of hydrophobic chromatic substances.

[0127] According to Table 3, the polyamide synthesized using isophthalic acid without sulfonic acid groups in Comparative Example 4 showed significantly reduced performance, confirming the unique role of the sulfonate group. It is inferred that the strong hydrophilicity of the sulfonate group significantly improves the dispersibility of the polyamide in the aqueous phase, and its high charge density effectively compresses the electric double layer of pollutant particles. During the hydrolysis of aluminum-iron salts, the sulfonate group forms a stable coordination structure with the Al-Fe polymer, regulating the generation of a ternary composite active center. This structure exhibits a specific adsorption effect on benzene ring-containing chromogenic substances (such as humic acid), significantly improving the color removal rate.

[0128] According to Table 3, the use of linear polyamide in Comparative Example 5 resulted in a decrease in sedimentation velocity, highlighting the key value of the branched structure. It is speculated that the branching sites provided by tris(2-aminoethyl)amine generate a dendritic topology that provides more binding sites during the floc growth stage, capturing fine particles through van der Waals forces and hydrogen bonds to form dense "grape bunch"-like flocs. In contrast, the flexible chains of linear polyetheramine D400 easily coil and encapsulate metal ions, limiting the floc expansion space, resulting in larger floc volume but lower density, thus slowing down the sedimentation process.

[0129] The process test results of Examples 4-7 and Comparative Examples 6-7 are shown in Table 4.

[0130] Table 4 Process Test Results

[0131]

[0132]

[0133] Data Analysis:

[0134] According to Table 4, the pretreatment stage can effectively reduce turbidity and oil content, with a COD removal rate of 22%-30%. Compared with the two comparative membranes (Comparative Example 6-CRUF and Comparative Example 7-UF), NF5 is more suitable for the loose nanofiltration stage, and the irreversible fouling of the membrane is maintained at a low level. The overall difference in the nanofiltration stage is not significant, and the irreversible fouling of the membrane can be controlled below 10% through process condition control. The deep purification stage uses photo-Fenton technology to achieve an effluent COD of below 2000 mg / L. If dense nanofiltration (Example 6) or reverse osmosis (Example 7) is used, the COD content can be reduced to below 1000 mg / L.

[0135] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A flocculant for a pretreatment section of a nanofiltration membrane integrated process for treating leachate of a waste transfer station, characterized by, The ion liquid grafted diatomite and sulfonate modified branched polyamide crosslinker is used as a carrier, and polyaluminum ferric chloride is generated in situ by aluminum salt and iron salt; the weight ratio of the ion liquid grafted diatomite, the sulfonate modified branched polyamide, the aluminum salt and the iron salt is 1-3:2-10:10-30:5-15; The preparation steps of the ion liquid grafted diatomite are as follows: (1) 3-chloropropyl trimethoxysilane and N-methyl imidazole are mixed, stirred and reacted for 24 hours under a nitrogen atmosphere, washed with diethyl ether, and rotary evaporated to obtain a silane type ionic liquid; (2) diatomite and the silane type ionic liquid are added into toluene, heated to 108-112℃, refluxed and reacted for 10-15 hours, centrifuged, washed with ethanol, and vacuum dried to obtain the ion liquid grafted diatomite; The preparation steps of the sulfonate modified branched polyamide are as follows: isophthalic acid-5-sodium sulfonate, polyetheramine D400 and tri(2-aminoethyl)amine are added into N-methyl pyrrolidone, stirred for 30 minutes, pyridine, triphenyl phosphite, calcium chloride and lithium chloride are added, heated to 110℃ under a nitrogen atmosphere, stirred and reacted for 24 hours, and then poured into acetone to precipitate, filtered, washed with acetone, and vacuum dried to obtain the sulfonate modified branched polyamide; The flocculant for the pretreatment section of the integrated nanofiltration membrane process for treating the landfill station leachate is prepared by the following steps: the ion liquid grafted diatomite and the sulfonate modified branched polyamide are added into deionized water, heated to 38-42℃, stirred for 20-40 minutes to form a crosslinker, aluminum chloride hexahydrate and ferric sulfate are added, continuously stirred for 0.5-1.5 hours, then sodium hydroxide solution is added to adjust the pH to 6.3-6.6, cooled to 25℃, stirred for 10-15 hours, polyaluminum ferric chloride is generated in situ on the crosslinker, vacuum dried, ground through a 50 mesh sieve, and the flocculant for the integrated process of the landfill station leachate is obtained.

2. The flocculant used in the pretreatment stage of the nanofiltration membrane integrated process for leachate treatment at waste transfer stations according to claim 1, characterized in that, The weight ratio of the 3-chloropropyl trimethoxysilane and the N-methyl imidazole is 1:0.35-0.

45.

3. The flocculant for the pretreatment stage of the integrated nanofiltration membrane process of landfill leachate treatment of the waste transfer station according to claim 1, characterized by, The weight ratio of the silane type ionic liquid and the diatomite is 2:0.3-0.

8.

4. The flocculant used in the pretreatment stage of the nanofiltration membrane integrated process for leachate treatment at waste transfer stations according to claim 1, characterized in that, The weight ratio of the isophthalic acid-5-sodium sulfonate, the polyetheramine D400 and the tri(2-aminoethyl)amine is 2.68:3.2:0.

3.

5. A nanofiltration membrane integrated process for leachate treatment in a waste transfer station using the flocculant according to any one of claims 1 to 4, characterized in that, The steps include: S1: landfill station leachate from the collection tank enters the pretreatment section, Ca(OH)2 is first added to adjust the pH to alkaline, and then the flocculant is added for sedimentation; S2: the supernatant after the sedimentation in step S1 is taken, NaOH is continuously added to pH 12-13, reacted for 10 minutes, and then the flocculant is added for sedimentation; S3: the supernatant after the sedimentation in step S2 is taken, Na2CO3 is added and reacted for 10 minutes, and then the flocculant is added to pass through CaCO3 for sedimentation; S4: the solution after the sedimentation in step S3 is filtered, H2SO4 is then added to adjust the pH to weakly acidic to obtain pretreated wastewater; S5: the pretreated wastewater directly enters the loose nanofiltration section, and the COD is reduced by 29%-32% through the retention effect of the membrane; S6: The loose nanofiltration product water reenters the nanofiltration section, and through the retention effect of the membrane, COD is reduced to 4000-5500 mg / L, and desalination is 40%-60%; S7: The nanofiltration product water enters a deep purification unit, and through the strong oxidation of free radicals of photo-Fenton, the high-efficiency retention of dense nanofiltration or reverse osmosis membrane, further reduction of COD is realized.

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