A coagulation-coupled advanced oxidation pretreatment system and method for landfill leachate

Abstract

This invention relates to a coagulation-coupled advanced oxidation pretreatment system and method for landfill leachate, comprising: a pre-acidification unit connected to a landfill leachate source; an acidic coagulation reaction unit located downstream of the pre-acidification unit; a primary sedimentation unit located downstream of the acidic coagulation reaction unit; an advanced oxidation reaction unit located downstream of the primary sedimentation unit; a secondary sedimentation unit located downstream of the advanced oxidation reaction unit; and a sludge treatment unit located downstream of the secondary sedimentation unit. By pre-acidifying the initial pH of the landfill leachate, pollutants can be efficiently removed through coagulation, while the acidic environment stimulates the in-situ generation of a large number of Fe(II) ions. Only the addition of an oxidant is needed to initiate the advanced oxidation reaction, deeply removing remaining recalcitrant pollutants and improving biodegradability.

Description

Technical Field

[0001] This invention relates to the field of landfill leachate treatment technology, specifically to a landfill leachate coagulation coupled with advanced oxidation pretreatment system and method. Background Technology

[0002] When household waste, industrial waste, or other types of waste are transported to landfills and accumulated, they undergo a series of physical, chemical, and biological processes. During these processes, the liquid components of the waste interact with external water sources such as rainwater to form a complex liquid mixture known as landfill leachate. This leachate is often dark gray or blackish-brown and has a foul odor. It possesses a complex and variable chemical composition, typically rich in organic matter, inorganic salts, heavy metals, and other harmful substances. In particular, the organic components, such as volatile fatty acids, phenolic compounds, and other recalcitrant substances, pose a serious threat to the environment. These organic pollutants not only increase the chemical oxygen demand (COD) and biological oxygen demand (BOD) of the leachate but can also damage surrounding ecosystems through groundwater infiltration and diffusion into surface water bodies. Furthermore, as landfill leachate accumulates over time, the organic pollutants it contains gradually shift towards becoming more recalcitrant and less biodegradable, posing even greater challenges to subsequent treatment.

[0003] Currently, various landfills in China employ numerous technological methods to treat leachate, including evaporation, dual-membrane methods, chemical oxidation, and biological methods. Evaporation and dual-membrane methods utilize the principles of liquid evaporation and semi-permeable membrane separation, respectively, to physically separate leachate into a concentrated solution with higher pollutant concentration and clean permeate. Due to their good effluent quality, they are commonly used in emergency leachate treatment. However, besides their high investment and operating costs, their most significant drawback is their inability to truly remove pollutants, resulting in large quantities of more difficult-to-treat concentrated solution. While chemical oxidation can relatively effectively degrade pollutants using various reaction mechanisms, its high reagent costs and reduced pollutant removal efficiency limit its practical application in leachate treatment. In contrast, most landfills in China are equipped with biochemical facilities, and utilizing microorganisms to degrade various pollutants remains the primary method for leachate treatment. However, limited by the increasing volume and pollution load of leachate, as well as its decreasing biodegradability, standalone biochemical units are no longer sufficient to meet current treatment demands. Therefore, how to develop a biochemical pretreatment system that can significantly reduce organic load, improve biodegradability, and maintain stable low cost is an urgent problem that needs to be solved. Summary of the Invention

[0004] To address the technical problems existing in the prior art, the first objective of this invention is to provide a coagulation-coupled advanced oxidation pretreatment system for landfill leachate, comprising a pre-acidification unit, an acidic coagulation reaction unit, a primary sedimentation unit, an advanced oxidation reaction unit, a secondary sedimentation unit, and a sludge treatment unit. By pre-acidifying and adjusting the initial pH value of the landfill leachate, pollutants can be efficiently removed through coagulation, while a large amount of Fe(II) ions can be generated in situ under acidic conditions. Only the addition of an oxidant is needed to initiate the advanced oxidation reaction, deeply removing the remaining recalcitrant pollutants, reducing the pollution load, and improving the biodegradability of the treated leachate, thereby improving the effect of subsequent biochemical processes.

[0005] The second objective of this invention is to provide a method for pretreatment of landfill leachate by coagulation coupled with advanced oxidation.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A landfill leachate coagulation coupled with advanced oxidation pretreatment system is disclosed. The system includes: a pre-acidification unit connected to a landfill leachate source, wherein the pre-acidification unit mixes the landfill leachate with an acidifying agent to adjust the pH to a weakly acidic level; an acidic coagulation reaction unit located downstream of the pre-acidification unit, wherein the acidic coagulation reaction unit mixes the landfill leachate from the pre-acidification unit with an iron-based coagulant to induce hydrolysis coagulation and reduction conversion; and a primary sedimentation unit located downstream of the acidic coagulation reaction unit, wherein the primary sedimentation unit mixes the landfill leachate from the acidic coagulation reaction unit with a coagulant aid. The system comprises: a primary sedimentation unit for initial sedimentation separation; an advanced oxidation reaction unit, located downstream of the primary sedimentation unit, for mixing the supernatant from the primary sedimentation unit with an oxidant for advanced oxidation; a secondary sedimentation unit, also located downstream of the advanced oxidation reaction unit, for mixing the solution from the advanced oxidation reaction unit with a regulator and a coagulant aid, respectively, for secondary sedimentation separation; and a sludge treatment unit for dewatering the sludge from the upstream primary and secondary sedimentation units and conveying the filtrate to the pre-acidification unit.

[0008] Furthermore, the pre-acidification unit includes a first tank, the top of which is provided with a first dosing port and a second dosing port, and the interior of the first tank is axially provided with a first stirrer.

[0009] Furthermore, the acidic coagulation reaction unit includes a second tank, the top of which is provided with a third dosing port, and the interior of the second tank is axially equipped with a second stirrer.

[0010] Furthermore, the primary sedimentation unit sequentially includes a first reaction tank and a first sedimentation tank. The first reaction tank has a first inlet for receiving landfill leachate on its side, a fourth dosing port on its top, and a third agitator axially arranged inside the first reaction tank. The first sedimentation tank has a first outlet for discharging landfill leachate on its side and a first sludge outlet for discharging sludge on its bottom.

[0011] Furthermore, the advanced oxidation reaction unit includes a third tank, the side of which is provided with a second inlet for receiving landfill leachate and a second outlet for discharging landfill leachate, the top of which is provided with a fifth dosing port, and the interior of the third tank is provided with a plurality of fourth agitators arranged side by side.

[0012] Furthermore, the secondary sedimentation unit sequentially includes a second reaction tank and a second sedimentation tank. The second reaction tank is provided with a third inlet for receiving landfill leachate on its side, a sixth dosing port and a seventh dosing port on its top, a fifth agitator axially arranged inside the second reaction tank, a third drain port for discharging pretreated landfill leachate on its side, and a second sludge outlet for discharging sludge at the bottom of the second sedimentation tank.

[0013] Furthermore, the sludge treatment unit is a plate and frame filter press.

[0014] Furthermore, the iron-based coagulant is one or more of ferric chloride, ferric sulfate, or polyferric sulfate; the acidifying agent is sodium hydroxide and sulfuric acid; the oxidizing agent is hydrogen peroxide or persulfate; the regulating agent is sodium hydroxide; and the coagulant aid is cationic polyacrylamide.

[0015] The above-mentioned landfill leachate coagulation coupled with advanced oxidation pretreatment method includes the following steps:

[0016] The leachate source is supplied to the pre-acidification unit, where an acidifying agent is added and mixed to adjust the pH to a slightly acidic level. The acidified leachate is then introduced into an acidic coagulation reaction unit, where an iron-based coagulant is added and mixed to induce hydrolysis coagulation and reduction conversion. Finally, the coagulated leachate is introduced into a primary sedimentation unit, where a coagulant aid is added and mixed to initiate the initial sedimentation process. The process involves sedimentation, where gravity separates the supernatant and sludge. The supernatant is then introduced into an advanced oxidation reaction unit, where an oxidant is added to the primary sedimentation unit. The supernatant and oxidant are mixed and stirred, and advanced oxidation is performed. The oxidized solution is then introduced into a secondary sedimentation unit, where a regulator and coagulant are added. The solution is mixed with the regulator and coagulant, and secondary sedimentation is performed, where gravity separates the pretreated water and sludge. The sludge from both the primary and secondary sedimentation units is then introduced into a sludge treatment unit for dewatering. The dewatered filtrate is then returned to the pre-acidification unit.

[0017] Furthermore, the supernatant in the primary precipitation unit contains ferrous ions, the pH in the pre-acidification unit is 4.5-5.5, and the pH in the secondary precipitation unit is 6-9.

[0018] The present invention has the following advantages:

[0019] This invention discloses a landfill leachate coagulation coupled with advanced oxidation pretreatment system. Through initial pH pre-acidification, it simultaneously promotes iron coagulation and generates Fe(II) ions in situ within the acidic coagulation reaction unit, which can be used to initiate advanced oxidation reactions, eliminating the need for additional catalysts and saving reagent costs. By adding an oxidant in the advanced oxidation reaction unit to form a coupled advanced oxidation reaction, pollutants that cannot be removed by coagulation are oxidized and degraded, achieving secondary coagulation removal and improving the pollution removal efficiency of the single coagulation process. Ultimately, it achieves a significant reduction in the organic pollution load in landfill leachate, reaching a removal efficiency of over 80%, while also improving the B / C ratio and enhancing biodegradability, which is of great significance for improving subsequent biological treatment processes. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the process of a landfill leachate coagulation coupled with advanced oxidation pretreatment system according to the present invention.

[0021] Figure 2 This is a graph showing the COD removal rate of the supernatant after coagulation of landfill leachate under different initial pH conditions in Example 1 of the present invention.

[0022] Figure 3 This is a graph showing the Fe(II) ion production of the supernatant after coagulation of landfill leachate under different initial pH conditions in Example 1 of the present invention.

[0023] Figure 4 This is a graph showing the COD removal rate of the coupled advanced oxidation process under different hydrogen peroxide addition conditions in Example 2 of the present invention.

[0024] Figure 5 This is a graph showing the COD removal rate of the coupled advanced oxidation process under different sodium persulfate addition conditions in Example 3 of the present invention.

[0025] Wherein, 1 is the pre-acidification unit, 101 is the first tank, 102 is the first dosing port, 103 is the second dosing port, and 104 is the first stirrer; 2 is the acidic coagulation reaction unit, 201 is the second tank, 202 is the third dosing port, and 203 is the second stirrer; 3 is the primary sedimentation unit, 301 is the first reaction tank, 301a is the first liquid inlet, 301b is the fourth dosing port, 301c is the third stirrer, 302 is the first sedimentation tank, 302a is the first discharge port, and 302b is the second primary sedimentation unit. 4 is the sludge outlet, 4 is the advanced oxidation reaction unit, 401 is the third tank, 402 is the second liquid inlet, 403 is the second liquid outlet, 404 is the fifth dosing port, 405 is the fourth agitator, 5 is the secondary sedimentation unit, 501 is the second reaction tank, 501a is the third liquid inlet, 501b is the sixth dosing port, 501c is the seventh dosing port, 501d is the fifth agitator, 502 is the second sedimentation tank, 502a is the third liquid outlet, 502b is the second sludge outlet, and 6 is the sludge treatment unit. Detailed Implementation

[0026] The following description is merely illustrative in nature and is in no way intended to limit the invention, its application, or use. It will be further understood that the terms “comprising” and / or “including” as used herein specify the presence of the mentioned features, integrals, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components, and / or groups thereof. As used herein, the term “and / or” includes any and all combinations of one or more of the associated listed items. It will be understood that when an element, component, and / or portion is referred to as “connected to another element, component, and / or portion,” it may be directly connected to another element, component, and / or portion, or there may be intermediate elements. It will be understood that although the terms “first,” “second,” etc., may be used herein to describe various elements, components, and / or portions, these elements, components, and / or portions should not be limited by these terms. These terms are used only to distinguish one element, component, or portion from another element, component, or portion. Therefore, the first element, component, or part discussed below may be referred to as the second element, component, or part without departing from the teachings of the invention. Unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. It will be further understood that terms such as those defined in commonly used dictionaries should be interpreted as having meanings consistent with their meanings in the relevant field and / or the context of this specification, and will not be interpreted in an idealized or overly formal sense unless expressly defined herein.

[0027] It should be understood that, for clarity, the accompanying drawings are not drawn to scale, and the same or similar reference numerals indicate the same or similar parts or components. Furthermore, it should be understood that any embodiments described in this application and the technical features they include can be combined with each other.

[0028] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0029] like Figure 1 As shown, a landfill leachate coagulation coupled with advanced oxidation pretreatment system includes a pre-acidification unit 1, an acid coagulation reaction unit 2, a primary sedimentation unit 3, an advanced oxidation reaction unit 4, a secondary sedimentation unit 5, and a sludge treatment unit 6 arranged in series.

[0030] like Figure 1As shown, a pre-acidification unit 1 is connected to the landfill leachate source. Pre-acidification unit 1 is used to mix the landfill leachate with an acidifying agent to adjust the pH to a weakly acidic level. Pre-acidification unit 1 includes a first tank 101, with a first dosing port 102 and a second dosing port 103 at the top. A first stirrer 104 is axially arranged inside the first tank 101. An online pH monitor is also installed at the top of the first tank 101 to monitor the pH after acidification. The acidifying agent is sodium hydroxide and sulfuric acid. The first dosing port 102 is connected to an external sodium hydroxide dosing device via a pipeline, and the second dosing port 103 is connected to an external sulfuric acid dosing device via a pipeline. A transfer pump is installed on the input pipeline of the landfill leachate source.

[0031] like Figure 1 As shown, the acidic coagulation reaction unit 2 is located downstream of the pre-acidification unit 1. It is used to mix the leachate from the pre-acidification unit 1 with an iron-based coagulant to induce hydrolysis coagulation and reduction conversion. The acidic coagulation reaction unit 2 includes a second tank 201, with a third dosing port 202 at its top. A second agitator 203 is axially mounted inside the second tank 201. The second tank 201 is made of an acid-resistant and corrosion-resistant material. The iron-based coagulant is one or more of ferric chloride, ferric sulfate, or polyferric sulfate. The third dosing port 202 is connected to an external iron-based coagulant dosing device via a pipeline. A transfer pump is installed on the pipeline between the acidic coagulation reaction unit 2 and the pre-acidification unit 1.

[0032] like Figure 1 As shown, the primary sedimentation unit 3 is located downstream of the acidic coagulation reaction unit 2. It is used to mix the landfill leachate from the acidic coagulation reaction unit 2 with a coagulant aid for initial sedimentation and separation. The primary sedimentation unit 3 sequentially includes a first reaction tank 301 and a first sedimentation tank 302. The first reaction tank 301 has a first inlet 301a on its side for receiving landfill leachate, a fourth dosing port 301b on its top, and a third agitator 301c axially arranged inside the first reaction tank 301. The first sedimentation tank 302 has a first drain 302a on its side for discharging landfill leachate and a first sludge outlet at its bottom for discharging sludge. The sludge discharged from the first sludge outlet is pumped to the sludge treatment unit 6. The coagulant aid is cationic polyacrylamide, and the fourth dosing port 301b is connected to an external coagulant aid dosing device via a pipeline. A delivery pump is installed on the pipeline between the primary sedimentation unit 3 and the acidic coagulation reaction unit 2.

[0033] like Figure 1As shown, the advanced oxidation reaction unit 4 is located downstream of the primary sedimentation unit 3. It is used to mix the supernatant from the primary sedimentation unit 3 with an oxidant for advanced oxidation. The advanced oxidation reaction unit 4 includes a third tank 401. The side of the third tank 401 is equipped with a second inlet 402 for receiving landfill leachate and a second outlet 403 for discharging landfill leachate. The top of the third tank 401 is equipped with a fifth dosing port 404, located near the second inlet 402 to ensure rapid mixing of the oxidant and supernatant, improving oxidation efficiency. Multiple fourth agitators 405 are arranged side-by-side inside the third tank 401. The oxidant is hydrogen peroxide or persulfate. The fifth dosing port 404 is connected to an external coagulant dosing device via a pipeline.

[0034] like Figure 1 As shown, the secondary sedimentation unit 5 is located downstream of the advanced oxidation reaction unit 4. It is used to mix the solution from the advanced oxidation reaction unit 4 with a regulator and a coagulant aid for secondary sedimentation separation. The secondary sedimentation unit 5 sequentially includes a second reaction tank 501 and a second sedimentation tank 502. The second reaction tank 501 has a third inlet 501a on its side for receiving landfill leachate, and a sixth dosing port 501b and a seventh dosing port 501c on its top. A fifth agitator 501d is axially installed inside the second reaction tank 501. The second sedimentation tank 502 has a third drain port 502a on its side for discharging pretreated landfill leachate, and a second sludge outlet 502b at its bottom for discharging sludge. The sludge discharged from the second sludge outlet 502b is pumped to the sludge treatment unit 6. An online pH monitor is also installed at the top of the second reaction tank 501 to monitor the adjusted pH. The regulator is sodium hydroxide, and the coagulant is cationic polyacrylamide.

[0035] like Figure 1 As shown, sludge treatment unit 6 is used to dewater the sludge from the upstream primary sedimentation unit 3 and secondary sedimentation unit 5, and to transport the filtrate to the pre-acidification unit 1. Sludge treatment unit 6 is a plate and frame filter press. After receiving the sludge, the plate and frame filter press performs mechanical dewatering. The filtered sludge is utilized as a resource, while the resulting filtrate is pumped to the pre-acidification unit 1 for treatment.

[0036] The working mechanism of the landfill leachate coagulation coupled with advanced oxidation pretreatment system is as follows: by adjusting the initial pH value of the landfill leachate to a weakly acidic condition of 4.5-5.5 in the pre-acidification unit 1, anions such as carbonate / bicarbonate in the water are removed to prevent them from affecting the formation of insoluble solid impurities such as iron carbonate by iron-based coagulants and reducing the coagulation effect. At the same time, the acidic conditions are conducive to the coagulation and removal effect of humic organic matter. In acidic coagulation reaction unit 2, an iron-based coagulant is added. The iron-based coagulant is mainly composed of ferric iron, such as ferric chloride, ferric sulfate, or polyferric sulfate. The pH value is further reduced to about 3. At this time, the ferric ions in the iron-based coagulant undergo two processes: hydrolysis coagulation and reduction transformation. Some ferric ions hydrolyze to form iron flocs, which undergo double-layer compression, adsorption neutralization, adsorption bridging, and precipitate trapping, resulting in the removal of organic matter by precipitation. Other ferric ions remain in an ionic state under acidic conditions and are excited and reduced in situ to ferrous ions by organic matter with reducing ability, dissolving in the coagulation supernatant. The leachate undergoes a change in properties after a period of anaerobic landfilling, distinguishing itself from the aerobic oxidation state and entering a reducing state, thereby acquiring reducing ability. Sludge-water separation is performed in the primary sedimentation unit 3. The supernatant after sludge-water separation is introduced into the advanced oxidation reaction unit 4, where an oxidant (hydrogen peroxide or persulfate, etc.) is added. Under the catalysis of ferrous ions, a Fenton or Fenton-like advanced oxidation reaction occurs, generating a large number of highly oxidizing free radicals (·OH, ·SO4). - (etc.) Deeply oxidizes and decomposes recalcitrant and difficult-to-biochemically remove pollutants into CO2, H2O, and small molecule organic matter. Finally, the pH is adjusted to neutral through the secondary sedimentation unit 5, forming new iron flocs, achieving secondary coagulation and removal of the remaining organic matter that could not be removed by coagulation, and obtaining pretreated landfill leachate.

[0037] The above-mentioned landfill leachate coagulation coupled with advanced oxidation pretreatment method includes the following steps:

[0038] The leachate source is supplied to the pre-acidification unit 1, and the acidifying agent is added to the pre-acidification unit 1. The leachate source and the acidifying agent are mixed and stirred to adjust the pH to a weakly acidic level. Specifically, the leachate to be treated enters the first tank 101, and sodium hydroxide and sulfuric acid are added to the leachate through the dosing device. The mixture is stirred evenly and the pH is adjusted to a weakly acidic range of 4.5-5.5.

[0039] The acidified landfill leachate is introduced into the acidic coagulation reaction unit 2, and the iron-based coagulant is added into the acidic coagulation reaction unit 2. The acidified landfill leachate and the iron-based coagulant are mixed and stirred to undergo hydrolysis coagulation and reduction conversion. Specifically, the acidified landfill leachate enters the second tank 201, and a certain amount of iron-based coagulant is added to the landfill leachate through the dosing device to make it fully stirred and uniform, so that a coagulation reaction occurs.

[0040] The coagulated landfill leachate is introduced into the primary sedimentation unit 3. The coagulant aid is added to the primary sedimentation unit 3, and the coagulated landfill leachate and coagulant aid are mixed and stirred. The initial sedimentation is carried out, and the supernatant and sludge are separated by gravity. Specifically, the uniformly coagulated solution enters the first reaction tank 301. The coagulant aid is added to the coagulation solution through the dosing device. After thorough stirring, the sludge particles become larger and then enter the first sedimentation tank 302. The sludge naturally settles to the bottom by gravity, and the supernatant overflows and is discharged.

[0041] The supernatant is introduced into the advanced oxidation reaction unit 4, and the oxidant is added into the primary precipitation unit 3. The supernatant and oxidant are mixed and stirred, and advanced oxidation is carried out. Specifically, the supernatant enters the third tank 401, and the oxidant is added to the supernatant through the dosing device. The mixture is fully mixed and reacted through multi-stage stirring.

[0042] The solution after advanced oxidation is introduced into the secondary sedimentation unit 5. A regulator and coagulant aid are added to the secondary sedimentation unit 5, and the solution is mixed with the regulator and coagulant aid. Secondary sedimentation is then performed, and pretreated water and sludge are separated by gravity. Specifically, the solution after advanced oxidation enters the second reaction tank 501. A regulator is added to the solution through a dosing device to adjust the pH value to the range of 6-9, forming a large amount of iron flocs. Then, a coagulant aid is added to the solution, and after thorough stirring, the iron flocs clump together and enter the second sedimentation tank 502. The sludge naturally settles to the bottom using gravity, and the supernatant is the permeable water from the pretreatment system, which overflows and is discharged.

[0043] The sludge from the primary sedimentation unit 3 and the secondary sedimentation unit 5 is introduced into the sludge treatment unit 6 for dewatering. The dewatered filtrate is then returned to the pre-acidification unit 1. Specifically, when the sludge settled at the bottom of the first sedimentation tank 302 and the second sedimentation tank 502 reaches a certain amount, it is pumped to a plate filter press for mechanical dewatering. The resulting filtrate is returned to the first tank 101, while the dewatered sludge is subsequently utilized for resource recovery.

[0044] The supernatant in the primary precipitation unit 3 contains ferrous ions, the pH in the pre-acidification unit 1 is 4.5-5.5, and the pH in the secondary precipitation unit 5 is 6-9.

[0045] By initially adjusting the leachate to a suitable weakly acidic pH value, an iron-based coagulant is added to carry out an acidic coagulation process. While deodorizing, decolorizing, and removing most of the humic organic matter, the coagulated supernatant can generate a large amount of Fe(II) ions in situ. Without adjusting the pH, a certain amount of oxidant can be directly added to achieve advanced oxidation reactions such as Fenton / Fenton-like reactions. This process deeply removes the remaining organic pollutants in the supernatant that cannot be flocculated, are difficult to degrade, and have poor biodegradability. This achieves a physicochemical pretreatment process that couples coagulation with advanced oxidation, solving the problems of high organic load, low biodegradability, and poor biochemical effect in the existing biochemical treatment process of landfills.

[0046] Example 1

[0047] Figure 2 and Figure 3 The effects of different initial pH values ​​on the COD removal rate and Fe(II) ion generation of the supernatant after landfill leachate coagulation are shown.

[0048] The leachate to be treated enters the pre-acidification unit 1. The initial pH value is adjusted by controlling the dosage of sodium hydroxide and sulfuric acid according to the pH online monitoring instrument. The pH values ​​are 7.5, 7.0, 6.5, 6.0, 5.5, 5.0, 4.5, 4.0, 3.5, and 3.0. After adjusting the initial pH value, the leachate enters the acidic coagulation reaction unit 2, where 1% (by volume) of commercially available polyferric sulfate (total iron mass fraction of 11.8%) is added. After thorough stirring, the mixture enters the primary sedimentation unit 3. Cationic polyacrylamide coagulant is added to the first reaction tank 301 to increase the size of the coagulated sludge particles before they enter the first sedimentation tank 302 for sedimentation. After sludge-water separation, the coagulated supernatant is obtained. The COD value and Fe(II) ion concentration of the coagulated supernatant are tested.

[0049] The COD value test method is based on the dichromate method (GB 11914-89); the Fe(II) ion concentration test method is based on the o-phenanthroline spectrophotometric method (HJ / T 345-2007).

[0050] like Figure 2 As shown, under different initial pH conditions, adding 1% polyferric sulfate (by volume) can remove COD from landfill leachate, especially achieving a better removal effect of over 65% in the pH range of 4.5-5.5. However, the COD removal effect is not ideal in higher or lower pH ranges.

[0051] like Figure 3As shown, by detecting the Fe(II) ion concentration in the supernatant, divalent iron ions could be detected in the supernatant, indicating that through the coagulation process, the trivalent iron (derived from polyferric sulfate) dissolved in the landfill leachate can be effectively converted into the divalent state, and under the condition of an initial pH of 4.5-5.5, a high Fe(II) ion yield of more than 850 mg / L can be obtained.

[0052] Example 2

[0053] Figure 4 The effect of adding different proportions of hydrogen peroxide on the advanced oxidation of landfill leachate for COD removal is shown based on the supernatant of the first sedimentation tank 302 in Example 1.

[0054] The initial pH value was set to 5, and the dosage of the coagulant, polyferric sulfate, was 1% by volume. The supernatant produced in the first sedimentation tank 302 entered the advanced oxidation reaction unit 4, where commercially available 30% hydrogen peroxide was directly added. The addition ratio was based on the molar ratio of Fe(II) ions produced in the supernatant, which were 1:0, 1:1, 1:2, 1:3, 1:4, and 1:5, respectively. After thorough stirring and reaction, the COD was tested to obtain the organic matter mineralization rate. Subsequently, the solution entered the secondary sedimentation unit 5, where the pH value was adjusted to the range of 6-9 in the second reaction tank 501 to form a large amount of iron flocs. Then, a coagulant aid was added to the solution, and after thorough stirring, the iron flocs agglomerated and entered the second sedimentation tank 502. The sludge naturally settled to the bottom by gravity, and the COD value of the supernatant was tested to obtain the total COD removal rate of the advanced oxidation process.

[0055] The COD value test method is based on the dichromate method (GB 11914-89).

[0056] like Figure 4 As shown, without hydrogen peroxide, advanced oxidation reactions (AORs) do not occur, and mineralization and total removal are not significant. After adding hydrogen peroxide, Fenton's AOR occurs in the presence of Fe(II), further mineralizing and removing organic matter. The mineralization rate gradually increases with increasing hydrogen peroxide dosage. After the reaction, pH adjustment forms secondary iron flocs, further removing organic matter. This demonstrates that the AOR process can partially alter organic matter, allowing it to be removed through coagulation. Overall, the Fenton AOR process, achieved through hydrogen peroxide addition, can deeply remove organic matter from the coagulated supernatant, with an overall removal rate exceeding 25% and a maximum removal rate of 40%. Through AOR and pH adjustment coagulation, the B / C ratio of the landfill leachate increases from 0.05 to over 2.5, significantly improving its biodegradability and benefiting subsequent biological treatment.

[0057] Example 3

[0058] Figure 5 The effect of adding different proportions of hydrogen peroxide to the supernatant of the first sedimentation tank 302 in Example 1 on the advanced oxidation deep removal of COD from landfill leachate is shown.

[0059] The initial pH value was set to 5, and the dosage of the coagulant polyferric sulfate was 1% by volume. The supernatant produced in the first sedimentation tank 302 entered the advanced oxidation reaction unit 4, where a commercially available 1M sodium persulfate solution was directly added. The addition ratio was based on the molar ratio of Fe(II) ions produced in the supernatant, which were 1:0, 1:1, 1:2, 1:3, 1:4, and 1:5, respectively. After thorough stirring and reaction, the COD was tested to obtain the organic matter mineralization rate. Subsequently, the solution entered the secondary sedimentation unit 5, where the pH value was adjusted to the range of 6-9 in the second reaction tank 501 to form a large amount of iron flocs. Then, a coagulant aid was added to the solution, and after thorough stirring, the iron flocs agglomerated and entered the second sedimentation tank 502. The sludge naturally settled to the bottom by gravity, and the COD value of the supernatant was tested to obtain the total COD removal rate of the advanced oxidation process.

[0060] The COD value test method is based on the dichromate method (GB 11914-89).

[0061] like Figure 5 As shown, without hydrogen peroxide, advanced oxidation reactions do not occur, and mineralization and total removal are not significant. However, with the addition of sodium persulfate, a Fenton-like advanced oxidation reaction occurs in the presence of Fe(II), further mineralizing and removing organic matter. The mineralization rate gradually increases with increasing sodium persulfate dosage. After the reaction, pH adjustment forms secondary iron flocs, further removing organic matter, demonstrating that the advanced oxidation process can partially alter organic matter, allowing it to be removed through coagulation. Overall, the Fenton advanced oxidation process formed by adding hydrogen peroxide can achieve deep removal of coagulated supernatant, with an overall removal rate exceeding 12% and a maximum removal rate of 30%. Through advanced oxidation and pH adjustment coagulation, the B / C ratio of landfill leachate increases from 0.05 to over 2.0, significantly improving biodegradability and benefiting subsequent biological treatment.

[0062] Comparative Example 1

[0063] To compare with traditional advanced oxidation pretreatment, the initial pH of the landfill leachate was adjusted to 3 before entering the advanced oxidation reaction unit 4, where ferrous sulfate and hydrogen peroxide were directly added. The Fe(II) ion concentration in the ferrous sulfate solution was 850 mg / L, and the molar ratio of Fe(II):hydrogen peroxide was 1:4. After thorough stirring and reaction, the leachate entered the sedimentation tank reaction zone, where the pH was adjusted to 6-9. After thorough stirring, the leachate entered the second sedimentation tank 502 for precipitation. The COD value of the supernatant was tested to obtain the COD removal rate.

[0064] The COD value test method is based on the dichromate method (GB 11914-89).

[0065] Tests revealed that traditional Fenton advanced oxidation has a total COD removal rate of only 10% for landfill leachate, which is far lower than the coagulation-coupled advanced oxidation pretreatment system of this invention. At the same time, its cost is slightly higher than that of this invention, and the B / C ratio is not significantly improved.

[0066] In summary, the landfill leachate coagulation coupled with advanced oxidation pretreatment system of this invention, through initial pH pre-acidification adjustment, simultaneously promotes iron coagulation and generates Fe(II) ions in situ within the acidic coagulation reaction unit, which can be used to initiate advanced oxidation reactions, without the need for additional catalysts, thus saving reagent costs. By adding an oxidant in the advanced oxidation reaction unit to form a coupled advanced oxidation reaction, pollutants that cannot be removed by coagulation are oxidized and degraded, achieving secondary coagulation removal and improving the pollution removal efficiency of the single coagulation process. Ultimately, it achieves a significant reduction in the organic pollution load in landfill leachate, reaching a removal efficiency of over 80%, while also improving the B / C ratio and enhancing biodegradability, which is of great significance for improving subsequent biological treatment processes.

[0067] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A landfill leachate coagulation coupled with advanced oxidation pretreatment system, characterized in that, The system includes: A pre-acidification unit connected to a landfill leachate source, the pre-acidification unit being used to mix the landfill leachate with an acidifying agent to adjust the pH to a weakly acidic level; An acidic coagulation reaction unit, located downstream of the pre-acidification unit, is used to mix landfill leachate from the pre-acidification unit with an iron-based coagulant. The acidic coagulation reaction unit utilizes the reducing organic matter contained in the landfill leachate to cause hydrolysis, coagulation, and reduction conversion of ferric ions in the iron-based coagulant. A primary sedimentation unit, located downstream of the acidic coagulation reaction unit, is used to mix the landfill leachate from the acidic coagulation reaction unit with a coagulant aid for initial sedimentation and separation. An advanced oxidation reaction unit, located downstream of the primary precipitation unit, is used to mix the supernatant from the primary precipitation unit with an oxidant to generate Fe(II) ions in situ in the acidic coagulation reaction unit to initiate the advanced oxidation reaction without the need for additional catalyst. A secondary sedimentation unit, located downstream of the advanced oxidation reaction unit, is used to mix the solution from the advanced oxidation reaction unit with a regulator and a coagulant, respectively, for secondary sedimentation separation. The sludge treatment unit is used to dewater the sludge from the upstream primary sedimentation unit and secondary sedimentation unit, and to transport the filtrate to the pre-acidification unit. The supernatant in the primary precipitation unit contains ferrous ions, the pH in the pre-acidification unit is 4.5-5.5, and the pH in the secondary precipitation unit is 6-9.

2. The landfill leachate coagulation coupled with advanced oxidation pretreatment system according to claim 1, characterized in that, The pre-acidification unit includes a first tank, the top of which is provided with a first dosing port and a second dosing port, and the interior of the first tank is axially provided with a first stirrer.

3. The landfill leachate coagulation coupled with advanced oxidation pretreatment system according to claim 1, characterized in that, The acidic coagulation reaction unit includes a second tank, a third dosing port is provided on the top of the second tank, and a second stirrer is axially arranged inside the second tank.

4. The landfill leachate coagulation coupled with advanced oxidation pretreatment system according to claim 1, characterized in that, The primary sedimentation unit includes a first reaction tank and a first sedimentation tank in sequence. The first reaction tank has a first inlet for receiving landfill leachate on its side and a fourth dosing port on its top. A third agitator is axially arranged inside the first reaction tank. The first sedimentation tank has a first outlet for discharging landfill leachate on its side and a first sludge outlet for discharging sludge on its bottom.

5. The landfill leachate coagulation coupled with advanced oxidation pretreatment system according to claim 1, characterized in that, The advanced oxidation reaction unit includes a third tank, the side of which is provided with a second inlet for receiving landfill leachate and a second outlet for discharging landfill leachate, the top of which is provided with a fifth dosing port, and the interior of the third tank is provided with multiple fourth agitators arranged side by side.

6. The landfill leachate coagulation coupled with advanced oxidation pretreatment system according to claim 1, characterized in that, The secondary sedimentation unit includes a second reaction tank and a second sedimentation tank in sequence. The second reaction tank is provided with a third inlet for receiving landfill leachate on its side, a sixth dosing port and a seventh dosing port on its top, a fifth agitator axially arranged inside the second reaction tank, a third drain port for discharging pretreated landfill leachate on its side, and a second sludge outlet for discharging sludge at the bottom of the second sedimentation tank.

7. The landfill leachate coagulation coupled with advanced oxidation pretreatment system according to claim 1, characterized in that, The sludge treatment unit is a plate and frame filter press.

8. The landfill leachate coagulation coupled with advanced oxidation pretreatment system according to claim 1, characterized in that, The iron-based coagulant is one or more of ferric chloride, ferric sulfate, or polyferric sulfate; the acidifying agent is sodium hydroxide and sulfuric acid; the oxidizing agent is hydrogen peroxide or persulfate; the regulating agent is sodium hydroxide; and the coagulant aid is cationic polyacrylamide.

9. A method for pretreatment of landfill leachate using coagulation coupled with advanced oxidation, characterized in that, The landfill leachate coagulation coupled with advanced oxidation pretreatment system according to any one of claims 1 to 8, the method comprising the following steps: The landfill leachate source is supplied to the pre-acidification unit, and the acidifying agent is added to the pre-acidification unit. The landfill leachate source and the acidifying agent are mixed and stirred to adjust the pH to a weakly acidic state. The acidified landfill leachate is introduced into the acid coagulation reaction unit, and the iron-based coagulant is added into the acid coagulation reaction unit. The acidified landfill leachate and the iron-based coagulant are mixed and stirred to cause hydrolysis coagulation and reduction conversion. The coagulated landfill leachate is introduced into the primary sedimentation unit, and the coagulant aid is added into the primary sedimentation unit. The coagulated landfill leachate and the coagulant aid are mixed and stirred, and the initial sedimentation is carried out. The supernatant and sludge are separated by gravity. The supernatant is introduced into the advanced oxidation reaction unit, the oxidant is added into the primary precipitation unit, the supernatant and the oxidant are mixed and stirred, and advanced oxidation is carried out. The advanced oxidized solution is introduced into the secondary sedimentation unit, and the regulator and coagulant are added to the secondary sedimentation unit. The solution is mixed with the regulator and the coagulant, and secondary sedimentation is carried out. The pretreated water and sludge are separated by gravity. The sludge from the primary and secondary sedimentation units is introduced into the sludge treatment unit for dewatering, and the dewatered filtrate is returned to the pre-acidification unit.

10. The method for pretreatment of landfill leachate by coagulation coupled with advanced oxidation according to claim 9, characterized in that, The supernatant in the primary precipitation unit contains ferrous ions, the pH in the pre-acidification unit is 4.5-5.5, and the pH in the secondary precipitation unit is 6-9.

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