A gradient acid precipitation coupled fenton treatment process and system for refractory organic wastewater

Abstract

The present application belongs to the technical field of wastewater treatment, and relates to a gradient acid precipitation coupled Fenton treatment process and system for refractory organic wastewater, wherein waste titanium white acid liquor is used as the only acidification, seed and catalytic raw material, and the process relies on the Fe 2+ , Ti 4+ and other components of the waste titanium white acid liquor to sequentially carry out two-stage gradient acid precipitation, integrated flocculation, synergistic wide-pH Fenton oxidation and sludge closed-loop reuse four-unit treatment; by adding the waste titanium white acid liquor in stages, the fractional precipitation of macromolecular refractory organic matter is realized, the Fe 2+ / Ti 4+ bimetallic catalytic system is relied on to widen the effective Fenton reaction pH to 3.0-6.0, online pH monitoring and UV 254 monitoring are combined with AI closed-loop dosing of hydrogen peroxide to construct a closed-loop process. The present application does not need to add concentrated sulfuric acid and iron-based catalysts, and the process eliminates the repeated acid-base adjustment process between process sections, realizes the resource utilization of waste acid for waste treatment, and has the advantages of low operation cost, strong resistance to water quality impact and high industrialization adaptability.

Description

Technical Field

[0001] This invention belongs to the field of wastewater treatment technology, specifically relating to a gradient acid precipitation coupled with Fenton oxidation process and system for treating recalcitrant organic wastewater. In particular, it relates to a process that uses waste titanium dioxide acid liquid as acidification, seed crystal and catalytic raw materials, and treats recalcitrant organic wastewater through gradient acid precipitation coupled with Fenton oxidation. It is applicable to the treatment of recalcitrant organic wastewater generated by industries such as papermaking, chemical industry, and printing and dyeing. Background Technology

[0002] Recalcitrant organic wastewater is characterized by high content of recalcitrant organic matter and deep color. Conventional treatment processes are difficult to achieve discharge standards. Among them, acid precipitation flocculation + Fenton oxidation is one of the most widely used combined processes. However, the existing process has many drawbacks, which limit its industrial application and treatment effect.

[0003] First, traditional acid precipitation processes often use a single-stage, one-time pH adjustment method, using concentrated sulfuric acid as the acidifying agent. This not only results in high acid consumption and local over-acidification leading to acid waste, but also causes the instantaneous and disordered precipitation of organic matter, resulting in fine flocs that are difficult to settle and are prone to organic matter re-dissolution, increasing the load on subsequent treatments. At the same time, traditional acid precipitation requires the addition of additional seed crystals to induce precipitation, increasing the cost of reagents.

[0004] Secondly, the sulfuric acid process for titanium dioxide production generates a large amount of waste titanium dioxide acid liquor, the main components of which include free sulfuric acid and Fe. 2+ Ti 4+ Currently, most waste titanium dioxide acid solutions, including those containing sulfates, are neutralized before discharge, which not only wastes resources but also generates large amounts of sludge, causing secondary pollution. While there are reports of using waste titanium dioxide acid solutions for wastewater acid precipitation or Fenton oxidation, these are all single-stage additions and simple superpositions, failing to achieve deep coupling between waste titanium dioxide acid solutions and gradient acid precipitation or wide-pH Fenton oxidation, thus failing to fully utilize the Fe in the waste titanium dioxide acid solutions. 2+ Ti 4+ Resources are limited, and the problems of cumbersome acid-base adjustment, waste of chemicals, and large amount of sludge in traditional processes have not been solved.

[0005] Furthermore, the traditional Fenton oxidation process is greatly limited by pH, and can only react efficiently within a narrow pH range of 2 to 4. Frequent pH adjustments of the wastewater are required, which not only consumes a large amount of acid and alkali reagents, but also exacerbates equipment corrosion. At the same time, traditional Fenton processes often use a fixed H2O2 addition ratio to add hydrogen peroxide (H2O2), resulting in a high rate of ineffective self-decomposition of H2O2 and low utilization rate. In addition, FeSO4 is required as an external catalyst, generating a large amount of ferrous hazardous sludge, which leads to high treatment costs and significant environmental pressure.

[0006] In addition, the existing acid precipitation flocculation and Fenton oxidation processes have poor integration, and the pH of the effluent after acid precipitation does not match the optimal pH of Fenton oxidation. This requires repeated acid and alkali adjustments, resulting in a cumbersome process, unstable operation, and a lack of intelligent control methods, making it difficult to adapt to fluctuations in influent water quality and further affecting the treatment effect. Summary of the Invention

[0007] In view of this, the purpose of this invention is to overcome the above-mentioned shortcomings and provide a gradient acid precipitation coupled with Fenton treatment process and system for recalcitrant organic wastewater, so as to overcome the problems of high acid consumption, need for additional seed crystals, narrow pH range of Fenton reaction (only 2-4), low reagent utilization, need for external iron source to generate a large amount of hazardous sludge, cumbersome inter-stage acid-base adjustment and weak shock resistance due to lack of intelligent control in the existing acid precipitation flocculation + Fenton oxidation process.

[0008] To achieve the above objectives, the present invention provides the following technical solution: On one hand, the present invention provides a gradient acid precipitation coupled with Fenton treatment process for recalcitrant organic wastewater, comprising the following steps: (1) Gradient acid precipitation: The recalcitrant organic wastewater is introduced into the acid precipitation tank. The waste titanium dioxide acid solution is added to the acid precipitation tank in two stages. After the first stage is added, the pH of the wastewater is adjusted to 5.0-6.0 and reacted. After the second stage is added, the pH of the wastewater is adjusted to 3.0-5.0 and reacted, so that the ferrous ions and titanium ions in the waste titanium dioxide acid solution are released in situ and act as seed crystals to induce the directional crystallization of organic matter. (2) Integrated flocculation: The acid effluent obtained in step (1) is directly introduced into the flocculation tank without adjusting the pH, and flocculant is added to the flocculation tank and stirred. The effluent after the flocculation reaction is directly introduced into the Fenton reaction tank. (3) Synergistic wide-pH Fenton oxidation: The flocculated effluent obtained in step (2) is introduced into the Fenton reactor. The ferrous and titanium ions brought in by the waste titanium dioxide acid solution are used to construct a bimetallic synergistic catalytic system. No external catalyst is required. The pH of the Fenton reaction is controlled within the range of 3.0 to 6.0. Online pH monitoring and UV are used. 254 Monitoring and coordinating the adjustment of hydrogen peroxide dosage; (4) Sludge reuse and advanced treatment: The effluent after the Fenton reaction is introduced into the neutralization and degassing tank to adjust the pH to 6.0-7.5, and the effluent after neutralization and degassing is introduced into the sedimentation tank to obtain Fenton sludge through sedimentation and separation. Part of the Fenton sludge is returned to the flocculation tank in step (2) and / or the acid precipitation tank in step (1) as auxiliary seed crystals and coagulant. The remaining Fenton sludge is discharged after dewatering treatment. The supernatant after neutralization and sedimentation is discharged in compliance with the standards, or part of it is returned to the front end of the acid precipitation tank to dilute the high concentration of recalcitrant organic wastewater.

[0009] Further, in step (1), 80% to 90% of the total volume of the waste titanium dioxide acid solution is added in the first stage, and the reaction is carried out for 10 to 15 minutes; the remaining 10% to 20% is added in the second stage, and the reaction is carried out for 15 to 20 minutes.

[0010] Further, in step (2), the flocculant is one of modified PAC, modified PFS or modified PFC, and the flocculant is loaded with zinc ions and / or manganese ions, with a loading amount of 3% to 10% of the mass of the flocculant.

[0011] Furthermore, the waste titanium dioxide acid solution is an acidic waste liquid produced by the sulfuric acid process for titanium dioxide production, containing sulfuric acid, ferrous ions, and titanium ions; In step (3), in the bimetallic synergistic catalytic system, titanium ions promote the reduction of ferric ions to ferrous ions, thus extending the catalytic activity cycle.

[0012] Furthermore, in step (3), the linkage adjustment is executed by the AI ​​closed-loop control system, which performs feedforward pre-dosing based on the online influent COD, performs real-time correction based on the online pH of the reaction tank, and performs adjustments based on the effluent UV. 254 Perform closed-loop fine-tuning.

[0013] Further, in step (3), when the effluent UV... 254 When the average rise rate of the characteristic peak absorbance over 5 minutes is ≥0.01 A / min, hydrogen peroxide is automatically added at a ratio of 2% to 5%.

[0014] Furthermore, in step (4), 50% to 80% of the Fenton sludge is returned to the flocculation tank in step (2), and the aged iron oxide in the returned sludge is used as an auxiliary seed crystal and coagulant.

[0015] On the other hand, the present invention also provides a gradient acid precipitation coupled Fenton treatment system for recalcitrant organic wastewater, applied to the aforementioned gradient acid precipitation coupled Fenton treatment process for recalcitrant organic wastewater, comprising, in sequence along the water flow direction: The acid precipitation tank is equipped with a wastewater inlet, a waste titanium dioxide acid solution staged dosing port, a pH monitoring device, and a stirring device. The flocculation tank is connected to the outlet of the acid precipitation tank and is equipped with a flocculant dosing port, a stirring device, an online COD monitor and a Fenton sludge return port. The Fenton reaction tank, connected to the outlet of the flocculation tank, is equipped with a hydrogen peroxide dosing port, an online pH monitoring device, and a UV... 254 Online monitoring device and mixing aeration device; The neutralization and degassing tank is connected to the outlet of the Fenton reaction tank and is equipped with an alkali addition port, a PAM addition port (for adding PAM, i.e., polyacrylamide, as a coagulant aid) and a pH monitoring device. The sedimentation tank is connected to the outlet of the neutralization and degassing tank. Its bottom sludge outlet is connected to the Fenton sludge return port of the flocculation tank via a sludge return pipe, and the other path is connected to the sludge dewatering device. The clear water tank is connected to the top clear liquid outlet of the sedimentation tank, and the clear water tank is equipped with a qualified discharge pipe or a return pipe.

[0016] The beneficial effects of this invention are as follows: 1. Precise Coupling of Waste Titanium Dioxide Acid Liquor: Using waste titanium dioxide acid liquor, a byproduct of the sulfuric acid process, as a dedicated acid source for gradient acid precipitation not only achieves resource utilization of waste acid but also leverages its inherent Fe... 2+ Ti 4+ It simultaneously achieves the triple functions of "acidification + seed crystal + Fenton catalyst", eliminating the need for additional addition of acid, iron-based catalyst, seed crystal and iron source, saving the process of repeated acid-base adjustment between process stages, realizing the resource utilization of titanium dioxide waste acid, and has the advantages of low operating cost, strong resistance to water quality impact and high industrial adaptability.

[0017] 2. Synergistic effect of gradient acid precipitation and waste titanium dioxide acid solution: Waste titanium dioxide acid solution is added in two stages to adapt to the precipitation characteristics of different organic compounds, achieving graded and directional precipitation, while Fe... 2+ Ti 4+ Using in-situ seed crystals solves the problems of fine flocs, back dissolution, and high acid consumption in traditional acid precipitation. Furthermore, the pH of the effluent after acid precipitation directly connects to subsequent processes, improving coupling.

[0018] 3. Wide pH Fenton catalytic system: Construction of Fe 2+ / Ti 4+ The bimetallic synergistic catalytic system, combined with a weak buffer system, broadens the pH range of the Fenton reaction to 3.0–6.0, eliminating the need for repeated acid-base adjustments; combined with intelligent H2O2 dosing, it improves oxidation efficiency and H2O2 utilization, reducing sludge production.

[0019] 4. Intelligent Coupling and Closed Loop: The influent COD, reaction pH and effluent UV254 are integrated into a closed-loop intelligent dosing system to achieve adaptive adjustment of reagent dosing; Fenton sludge return and supernatant return form a closed loop to improve system stability and shock resistance, while achieving sludge reduction and maximum resource utilization.

[0020] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description

[0021] To make the objectives, technical solutions, and advantages of the present invention clearer, the preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein: Figure 1 This is a flowchart of a gradient acid precipitation coupled with Fenton treatment process for recalcitrant organic wastewater, as described in the embodiments.

[0022] Figure labels: 1-First-stage waste titanium dioxide acid inlet; 2-First-stage acid precipitation tank pH monitoring device; 3-Second-stage waste titanium dioxide acid inlet; 4-Second-stage acid precipitation tank pH monitoring device; 5-Flocculant inlet; 6-Online COD monitor; 7-Hydrogen peroxide inlet; 8-Fenton reactor pH monitoring device; 9-UV 254 Online monitoring device; 10-Alkali solution inlet; 11-pH monitoring device for neutralization and degassing tank; 12-PAM inlet. Detailed Implementation

[0023] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of the present invention. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0024] The accompanying drawings are for illustrative purposes only and are schematic diagrams, not actual images. They should not be construed as limiting the invention. To better illustrate the embodiments of the invention, some parts in the drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0025] In the accompanying drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components. In the description of the present invention, it should be understood that if terms such as "upper," "lower," "left," "right," "front," and "rear" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the terms used to describe positional relationships in the drawings are only for illustrative purposes and should not be construed as limiting the present invention. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0026] Example 1 This case study is used to treat the biochemical effluent of a paper mill wastewater, with an influent COD of 350 mg / L and a pH of 7.8.

[0027] (1) Gradient acid precipitation: using waste titanium dioxide acid solution (sulfuric acid concentration 20%, Fe) 2+ Concentration 8 g / L, Ti 4+ A 1.5 g / L concentration was used as the acidifying agent. In the first stage, 85% of the total volume of the waste titanium dioxide acid solution was added, the wastewater pH was adjusted to 5.3, and the reaction was allowed to proceed for 10 minutes, promoting the precipitation of large-molecule lignin and other colloidal and oily organic matter. In the second stage, the remaining 15% of the waste titanium dioxide acid solution was added, the wastewater pH was adjusted to 4.5, and the reaction was allowed to proceed for 15 minutes, further promoting the precipitation of small-molecule, recalcitrant organic matter. The Fe in the waste titanium dioxide acid solution... 2+ Ti 4+ It is released in situ and acts as a seed crystal, inducing the directional crystallization of organic matter to form dense and easily settled flocs.

[0028] (2) Integrated flocculation: The pH of the acid-precipitated effluent in step (1) is about 4.5. No pH adjustment is required. It is directly introduced into the flocculation tank, and 250 mg / L of modified PFC flocculant is added. After aeration and stirring for 15 min, dense flocs are formed. At this time, the pH of the effluent is about 4.3, and the COD of the effluent drops to 140 mg / L. The effluent after the flocculation reaction does not undergo sedimentation and directly enters the Fenton reaction tank.

[0029] (3) Synergistic wide pH Fenton oxidation: The flocculated effluent is introduced into the Fenton reactor, utilizing the Fe carried in by the waste titanium dioxide acid solution. 2+ With Ti 4+ A bimetallic synergistic catalytic system was constructed without the need for an external catalyst. The pH of the Fenton reaction was controlled at 4.3, and 300 mg / L of hydrogen peroxide (27.5%) was added. An AI closed-loop control system was adopted, linking online pH monitoring and UV254 monitoring, forming a process of "online influent COD feedforward pre-dosing → variable frequency H2O2 pre-dosing → online real-time pH correction in the reaction tank → effluent UV..." 254 The intelligent dosing process with "closed-loop fine-tuning" resulted in a COD of 40 mg / L in the Fenton post-effects effluent after 60 minutes of reaction.

[0030] (4) Sludge reuse and advanced treatment: The effluent after the Fenton reaction is introduced into the neutralization and degassing tank, alkali solution and PAM coagulant are added, the pH is adjusted to 6.0-7.5, and then introduced into the sedimentation tank for sedimentation and separation to obtain Fenton sludge; 60% of the Fenton sludge is returned to the acid precipitation tank of step (1) as auxiliary seed crystals and coagulant, and the remaining sludge is discharged after dewatering treatment, reducing the amount of hazardous sludge by 52%; the supernatant after neutralization and sedimentation meets the discharge standards.

[0031] After treatment in this embodiment, the final wastewater COD was 40 mg / L, the color was 10 times higher, and the discharge met the standards stably.

[0032] Example 2 This implementation example illustrates the method of the present invention in a wide pH operating range and sludge return mode in the flocculation tank. The treatment object is the same as the above-mentioned biochemical effluent from a certain papermaking wastewater (influent COD=350 mg / L, pH=7.8), and the composition of the waste titanium dioxide acid solution used is the same as in Example 1 (sulfuric acid concentration 20%, Fe...). 2+ Concentration 8 g / L, Ti 4+ Concentration 1.5 g / L).

[0033] (1) Gradient acid precipitation: In the first stage, 90% of the total volume of waste titanium dioxide acid solution is added, the pH of the wastewater is adjusted to 6.0 and reacted for 15 min; in the second stage, the remaining 10% of waste titanium dioxide acid solution is added, the pH of the wastewater is adjusted to 5.6 and reacted for 20 min, so as to achieve the graded and directional precipitation of macromolecules and small molecule organic matter.

[0034] (2) Integrated flocculation: Acid precipitation water is directly introduced into the flocculation tank, and modified PAC flocculant (loaded with zinc ions, the loading amount is 10% of the flocculant mass) is added. The agent is dispersed by mechanical stirring, and the effluent after flocculation reaction is directly introduced into the Fenton reaction tank.

[0035] (3) Synergistic wide pH Fenton oxidation: Flocculated effluent is introduced into the Fenton reactor, utilizing the Fe carried in by the waste titanium dioxide acid solution. 2+ With Ti 4+ Constructing a bimetallic synergistic catalytic system (titanium ion promoting Fe) 3+ To Fe 2+ (Reduction, extending the catalytic activity cycle), no external catalyst is required, and the pH of the Fenton reaction is controlled at 5.5 (located at the upper limit of the wide pH range of 3.0 to 6.0 in this invention). The hydrogen peroxide dosage is adjusted by an AI closed-loop control system: feedforward pre-dosing based on online influent COD, real-time correction based on online pH of the reaction tank, and closed-loop fine-tuning based on effluent UV254; when the average rise rate of the characteristic peak absorbance of effluent UV254 is ≥0.01A / min over 5 minutes, hydrogen peroxide is automatically added at a ratio of 2% to 5%; after 60 minutes of reaction, the COD of the effluent after Fenton reaction drops to 35 mg / L. (4) Sludge reuse and advanced treatment: The effluent after the Fenton reaction is introduced into the neutralization and degassing tank, and alkali and PAM are added to adjust the pH to 6.0-7.5. The effluent is then introduced into the sedimentation tank for sedimentation and separation to obtain Fenton sludge. 80% of the Fenton sludge is returned to the flocculation tank in step (2) as an auxiliary seed crystal and coagulant (the aged iron oxide in the returned sludge plays the role of auxiliary seed crystal and coagulant). The remaining Fenton sludge is discharged after dewatering. The supernatant after neutralization and sedimentation meets the discharge standards and is partially returned to the front end of the acid precipitation tank to dilute the high concentration of recalcitrant organic wastewater.

[0036] After treatment by the process in this embodiment, the effluent stably meets the discharge standards, with a COD removal rate of over 90% and a color removal rate of over 95%, verifying the treatment effect and operational stability of the present invention in a wide pH range (this embodiment operates at pH 5.5) and under the sludge return mode of the flocculation tank.

[0037] Example 3 This embodiment, based on Embodiment 2, provides a gradient acid precipitation coupled with Fenton treatment system for recalcitrant organic wastewater, applied to the aforementioned gradient acid precipitation coupled with Fenton treatment process for recalcitrant organic wastewater, comprising, along the water flow direction, the following: The acid precipitation tank is equipped with a wastewater inlet, a waste titanium dioxide acid solution staged dosing port, a pH monitoring device, and a stirring device (specifically, a stirring and aeration device arranged at the bottom of the acid precipitation tank). Specifically, the waste titanium dioxide acid solution segmented dosing ports include a first-stage waste titanium dioxide acid solution dosing port 1 and a second-stage waste titanium dioxide acid solution dosing port 3. The pH monitoring device of the acid precipitation tank includes a first-stage acid precipitation tank pH monitoring device 2 and a second-stage acid precipitation tank pH monitoring device 4, which are respectively used to detect the pH of the first-stage acid precipitation tank at the first-stage waste titanium dioxide acid solution dosing port 1 and the second-stage waste titanium dioxide acid solution dosing port 3. The flocculation tank is connected to the outlet of the acid precipitation tank and is equipped with a flocculant dosing port 5, a stirring device, an online COD monitor 6, and a Fenton sludge return port; specifically, the stirring device is a mechanical stirring device.

[0038] The Fenton reactor, connected to the outlet of the flocculation tank, is equipped with a hydrogen peroxide dosing port 7, an online pH monitoring device (i.e., the Fenton reactor pH monitoring device 8), and a UV... 254 Online monitoring device 9 and stirring and aeration device (arranged at the bottom of the Fenton reaction tank); The neutralization and degassing tank is connected to the outlet of the Fenton reaction tank and is equipped with an alkali inlet 10, a PAM inlet 12 and a pH monitoring device (i.e., the neutralization and degassing tank pH monitoring device). The alkali inlet 10 is used to add liquid alkali or lime slurry.

[0039] The sedimentation tank is connected to the outlet of the neutralization and degassing tank. Its bottom sludge outlet is connected to the Fenton sludge return port of the flocculation tank via a sludge return pipe, and the other path is connected to the sludge dewatering device. The clear water tank (not shown in the figure) is connected to the top clear liquid outlet of the sedimentation tank. The clear water tank is equipped with a compliant discharge pipe or a return pipe. The supernatant is discharged in compliance with standards through the compliant discharge pipe, or partially returned to the front-end equalization tank of the acid precipitation tank through the return pipe to dilute the high-concentration influent, improve the system's load resistance and shock resistance, and form a closed-loop process.

[0040] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A gradient acid precipitation coupled with Fenton treatment process for recalcitrant organic wastewater, characterized in that, Includes the following steps: (1) Gradient acid precipitation: The recalcitrant organic wastewater is introduced into the acid precipitation tank. The waste titanium dioxide acid solution is added to the acid precipitation tank in two stages. After the first stage is added, the pH of the wastewater is adjusted to 5.0-6.0 and reacted. After the second stage is added, the pH of the wastewater is adjusted to 3.0-5.0 and reacted, so that the ferrous ions and titanium ions in the waste titanium dioxide acid solution are released in situ and act as seed crystals to induce the directional crystallization of organic matter. (2) Integrated flocculation: The acid effluent obtained in step (1) is directly introduced into the flocculation tank without adjusting the pH. Flocculant is added to the flocculation tank and stirred. The effluent after flocculation reaction is directly introduced into the Fenton reaction tank. (3) Synergistic wide-pH Fenton oxidation: The flocculated effluent obtained in step (2) is introduced into the Fenton reactor. The ferrous and titanium ions brought in by the waste titanium dioxide acid solution are used to construct a bimetallic synergistic catalytic system. No external catalyst is required. The pH of the Fenton reaction is controlled within the range of 3.0 to 6.

0. Online pH monitoring and UV are used. 254 Monitor and coordinate the adjustment of hydrogen peroxide dosage; (4) Sludge reuse and advanced treatment: The effluent after the Fenton reaction is introduced into the neutralization and degassing tank to adjust the pH to 6.0-7.5, and the effluent after neutralization and degassing is introduced into the sedimentation tank to obtain Fenton sludge through sedimentation and separation. Part of the Fenton sludge is returned to the flocculation tank in step (2) and / or the acid precipitation tank in step (1) as auxiliary seed crystals and coagulant. The remaining Fenton sludge is discharged after dewatering treatment. The supernatant after neutralization and sedimentation is discharged in compliance with the standards, or part of it is returned to the front end of the acid precipitation tank to dilute the high concentration of recalcitrant organic wastewater.

2. The gradient acid precipitation coupled Fenton treatment process for recalcitrant organic wastewater according to claim 1, characterized in that, In step (1), 80% to 90% of the total volume of the waste titanium dioxide acid solution is added in the first stage, and the reaction is carried out for 10 to 15 minutes; the remaining 10% to 20% is added in the second stage, and the reaction is carried out for 15 to 20 minutes.

3. The gradient acid precipitation coupled Fenton treatment process for recalcitrant organic wastewater according to claim 1, characterized in that, In step (2), the flocculant is one of modified PAC, modified PFS or modified PFC, and the flocculant is loaded with zinc ions and / or manganese ions, with the loading amount being 3% to 10% of the mass of the flocculant.

4. The gradient acid precipitation coupled Fenton treatment process for recalcitrant organic wastewater according to claim 1, characterized in that, The waste titanium dioxide acid solution is an acidic waste liquid produced by the sulfuric acid process for titanium dioxide production, containing sulfuric acid, ferrous ions and titanium ions; In step (3), in the bimetallic synergistic catalytic system, titanium ions promote the reduction of ferric ions to ferrous ions, thus extending the catalytic activity cycle.

5. The gradient acid precipitation coupled Fenton treatment process for recalcitrant organic wastewater according to claim 1, characterized in that, In step (3), the linkage adjustment is executed by the AI ​​closed-loop control system, which performs feedforward pre-dosing based on the online influent COD, real-time correction based on the online pH of the reaction tank, and adjustment based on the effluent UV. 254 Perform closed-loop fine-tuning.

6. The gradient acid precipitation coupled Fenton treatment process for recalcitrant organic wastewater according to claim 5, characterized in that, In step (3), when the effluent UV... 254 When the average rise rate of the characteristic peak absorbance over 5 minutes is ≥0.01 A / min, hydrogen peroxide is automatically added at a ratio of 2% to 5%.

7. The gradient acid precipitation coupled Fenton treatment process for recalcitrant organic wastewater according to claim 1, characterized in that, In step (4), 50% to 80% of the Fenton sludge is returned to the flocculation tank in step (2), and the aged iron oxide in the returned sludge is used as an auxiliary seed crystal and coagulant.

8. A gradient acid precipitation coupled Fenton treatment system for recalcitrant organic wastewater, characterized in that, The gradient acid precipitation coupled Fenton treatment process for recalcitrant organic wastewater as described in any one of claims 1 to 7, comprising, in sequence along the water flow direction: The acid precipitation tank is equipped with a wastewater inlet, a waste titanium dioxide acid solution staged dosing port, a pH monitoring device, and a stirring device. The flocculation tank is connected to the outlet of the acid precipitation tank and is equipped with a flocculant dosing port, a stirring device, an online COD monitor and a Fenton sludge return port. The Fenton reaction tank, connected to the outlet of the flocculation tank, is equipped with a hydrogen peroxide dosing port, an online pH monitoring device, and a UV... 254 Online monitoring device and mixing aeration device; The neutralization and degassing tank is connected to the outlet of the Fenton reaction tank and is equipped with an alkali addition port, a PAM addition port and a pH monitoring device. The sedimentation tank is connected to the outlet of the neutralization and degassing tank. Its bottom sludge outlet is connected to the Fenton sludge return port of the flocculation tank via a sludge return pipe, and the other path is connected to the sludge dewatering device. The clear water tank is connected to the top clear liquid outlet of the sedimentation tank, and the clear water tank is equipped with a qualified discharge pipe or a return pipe.

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