A pretreatment system and process for phenolic-containing wastewater

Abstract

This invention belongs to the field of wastewater treatment technology in the petrochemical industry, specifically relating to a pretreatment system and process for phenol-containing wastewater. The pretreatment system for phenol-containing wastewater includes an inlet pipeline, a coagulation zone, a flocculation zone, a sedimentation zone, and an effluent tank, sequentially connected along the wastewater flow direction. The effluent tank is connected to the coagulation zone via a return pipeline. A first online colorimeter for monitoring the inlet color value is installed on the inlet pipeline, and a second online colorimeter for monitoring the effluent color value is installed on the effluent tank. Both the first and second online colorimeters are connected to a DCS remote control system, which adjusts the return ratio of the return pipeline based on the monitored inlet and effluent color values. The treatment system and process of this invention have good treatment effects on phenol-containing wastewater from the petrochemical industry, achieving a removal rate of over 95% for suspended solids and asphaltenes, and a COD removal rate of over 10%.

Description

Technical Field

[0001] This invention belongs to the field of wastewater treatment technology in the petrochemical industry, specifically relating to a pretreatment system and process for phenol-containing wastewater. Background Technology

[0002] Wastewater from the petrochemical industry has a complex composition, including large amounts of phenolic substances such as phenol, cresol, and xylenol, as well as large amounts of organic matter such as asphaltene, benzene, toluene, and xylene. It is also accompanied by inorganic salts and heavy metal ions. This type of phenol-containing wastewater has a wide range of pollution and is highly harmful, posing serious threats to human health, water bodies, fish, and crops.

[0003] Steam dephenolization is an early and simple method for removing phenols, suitable for treating wastewater containing mainly volatile phenols. The essence of this method lies in the azeotropic mixture formed by phenol and steam. The phenol in the water is transferred into the steam, purifying the wastewater. The phenol-containing steam is then washed with alkaline solution to recover the phenol. The dephenolization rate is approximately 80%. Some factories in the United States have used this method to treat wastewater from tar extraction and p-isopropylphenol production, achieving a dephenolization efficiency of 97%. This method does not use organic solvents, recovers high-quality phenol, can treat large volumes of water, and is relatively simple to operate. Currently, the conventional process involves the phenol-containing wastewater first entering a storage tank for settling and slag removal. The upper and middle layers of liquid then enter an oil-water separator for further separation. The separated light components enter the steam dephenolization unit, and the process flow includes multiple stages of filtration for slag removal.

[0004] The current problems with this process are as follows: ① During the treatment process, the filtration effects of static sedimentation and slag removal in the storage tank, oil-water separator, basket filter, and membrane filter are poor, leaving some insoluble matter and asphalt residue. This causes the heat exchanger, steam phenol removal tower, and other devices to be easily clogged, requiring frequent maintenance and seriously affecting the purification of phenol-containing wastewater. ② The influent ammonia nitrogen and COD content of the steam phenol removal unit are too high (ammonia nitrogen around 10,000 mg / L, COD around 100,000 mg / L, oil content around 1,500 mg / L, and color around 10,000), resulting in poor removal efficiency. Consequently, even after subsequent combined processes of deacidification and deammoniation, extraction and oil removal, and recovery of residual extractant in the water, the effluent COD is still around 10,000 mg / L, leading to high COD levels in the subsequent biological wastewater treatment and high treatment costs.

[0005] In view of this, the present invention is hereby proposed. Summary of the Invention

[0006] The purpose of this invention is to provide a pretreatment system and process for phenol-containing wastewater, in order to solve the problems of easy clogging of equipment, high COD index of effluent, poor treatment effect and high cost in existing treatment processes.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] A pretreatment system for phenol-containing wastewater includes an inlet pipeline, a coagulation zone, a flocculation zone, a sludge sedimentation zone, and an effluent tank, all connected sequentially along the wastewater flow direction. The effluent tank is connected to the coagulation zone via a return pipeline. A first online colorimeter for monitoring the inlet color value is installed on the inlet pipeline, and a second online colorimeter for monitoring the effluent color value is installed on the effluent tank. Both the first and second online colorimeters are connected to a DCS remote control system, which adjusts the return ratio of the return pipeline based on the monitored inlet and effluent color values.

[0009] Furthermore, the coagulation zone includes a coagulation zone A and a coagulation zone B that are interconnected; the coagulation zone A is connected to the inlet pipeline and the return pipeline respectively, and the coagulation zone B is connected to the flocculation zone; the coagulation zone A is equipped with a demulsifier dosing pipeline, and the coagulation zone B is equipped with a coagulant dosing pipeline.

[0010] Furthermore, a first flow meter for monitoring the inlet flow rate is installed on the inlet pipeline, and a second flow meter for monitoring the return flow rate is installed on the return pipeline; both the first flow meter and the second flow meter are connected to the DCS remote control system, and the DCS remote control system adjusts the inlet flow rate and the return flow rate through the first flow meter and the second flow meter.

[0011] In addition, the present invention also provides a pretreatment process for phenol-containing wastewater, which employs the pretreatment system described in any one of claims 1-3, and includes the following steps:

[0012] S1. Perform online color detection on the influent containing phenol;

[0013] S2. Phenolic wastewater is treated sequentially through a coagulation zone, a flocculation zone, and a sludge sedimentation zone. Part of the effluent is then returned to the coagulation zone to dilute the influent. At the same time, the effluent is subjected to online color detection to ensure that the effluent color value is <4000.

[0014] S3. Substitute the influent color value and effluent color value into formula (1) to calculate the K value; when the effluent color value is ≥4000, adjust the reflux ratio to be greater than the K value, and after the effluent meets the standard, it will enter the subsequent treatment.

[0015] K = (influent color value - 6000) ÷ (6000 - effluent color value) Equation (1).

[0016] Furthermore, after adjusting the reflux ratio to be greater than the K value, the effluent color value is ≤4000.

[0017] Furthermore, a portion of the effluent is returned to the coagulation zone to dilute the influent. The color of the diluted wastewater is calculated as: influent color ÷ dilution factor.

[0018] Furthermore, adjust the reflux ratio to <5.

[0019] Furthermore, the coagulation treatment process includes: adding a demulsifier to coagulation zone A to demulsify the wastewater and reduce the viscosity of the oily substance; then sending it to coagulation zone B, adding a coagulant, and coagulating the wastewater.

[0020] Furthermore, in coagulation zone A, the demulsification time is 6–10 min; in coagulation zone B, the coagulation time is 6–10 min.

[0021] Furthermore, the flocculation time is 10–15 min.

[0022] Compared with the closest existing technology, the technical solution provided by the present invention has the following superior effects:

[0023] (1) The pretreatment system and process for phenol-containing wastewater of the present invention have good treatment effect on phenol-containing wastewater in the petrochemical industry. The removal rate of heavy components such as suspended solids and asphalt in the wastewater can reach more than 95%, which can avoid the blockage of subsequent towers, heat exchangers and other equipment. The COD removal rate can reach more than 10%, which removes some of the difficult-to-degrade COD and is beneficial to subsequent biochemical treatment.

[0024] (2) The pretreatment process of phenol-containing wastewater of the present invention automatically controls the appropriate influent dilution ratio (recirculation ratio self-control) through DCS remote control system, which achieves better treatment effect. Moreover, some of the reagents are still present in the effluent and are recycled back to the coagulation zone, which can reduce the amount of reagents added and reduce the operating cost.

[0025] (3) The pretreatment system for phenol-containing wastewater of the present invention is simple to operate, has a short treatment time, a large treatment volume, a small footprint, low construction cost, and a high degree of automation, thus achieving the purpose of saving costs and extending the service life of the device. Attached Figure Description

[0026] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. Wherein:

[0027] Figure 1 This is a flowchart of the pretreatment system for phenol-containing wastewater according to the present invention;

[0028] Explanation of reference numerals in the attached diagram: 1. Inlet water pipeline; 2. Coagulation zone; 21. Coagulation zone A; 22. Coagulation zone B; 3. Flocculation zone; 4. Sludge sedimentation zone; 5. Outlet water tank; 6. Return pipeline; 11. First online colorimeter; 51. Second online colorimeter; 110. First flow meter; 610. Second flow meter.

[0029] Figure 2A linear graph showing the direct proportional relationship between the color of phenol-containing wastewater and the concentration of heavy substances in phenol-containing wastewater;

[0030] Figure 3 This is a comparison diagram of the phenol-containing wastewater before and after treatment in Example 1. Detailed Implementation

[0031] The present invention will now be described in detail with reference to the accompanying drawings and embodiments. It should be noted that, unless otherwise specified, the embodiments and features described herein can be combined with each other.

[0032] In the description of this invention, the terms "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," and "bottom," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and do not require the invention to be constructed and operated in a specific orientation; therefore, they should not be construed as limitations on the invention. The terms "connected" and "linked" used in this invention should be interpreted broadly. For example, they can refer to a fixed connection or a detachable connection; they can refer to a direct connection or an indirect connection through intermediate components. Those skilled in the art can understand the specific meaning of the above terms according to the specific circumstances.

[0033] According to a first aspect of the invention, such as Figure 1 As shown, a pretreatment system for phenol-containing wastewater is provided, including an inlet pipeline 1, a coagulation zone 2, a flocculation zone 3, a sludge sedimentation zone 4, and an outlet tank 5, which are connected sequentially along the wastewater flow direction. The outlet tank 5 is connected to the coagulation zone 2 via a return pipeline 6. A first online colorimeter 11 for monitoring the inlet color value is installed on the inlet pipeline 1, and a second online colorimeter 51 for monitoring the outlet color value is installed on the outlet tank 5. Both the first online colorimeter 11 and the second online colorimeter 51 are connected to a DCS remote control system, which adjusts the return ratio of the return pipeline according to the monitored inlet and outlet color values.

[0034] Direct coagulation and sedimentation of phenol-containing wastewater leads to a high sludge content, difficulty in sedimentation, poor liquid flowability, and easy clogging. Furthermore, because the sedimentation tank is not a sealed device, the raw water is highly volatile and has a strong odor and toxicity, making routine maintenance difficult. Therefore, this method uses a mixture of influent and a portion of effluent for recirculation. The influent is diluted to a certain ratio before coagulation and sedimentation. A first flow meter 110 is installed on the influent pipeline 1 to monitor the influent flow rate, and a second flow meter 610 is installed on the recirculation pipeline 6 to monitor the recirculation flow rate. Both the first and second flow meters 110 are connected to a DCS remote control system, which adjusts the influent and recirculation flow rates using these meters. The influent and recirculation flow rates are controlled proportionally, i.e., the recirculation ratio is automatically controlled. This automatic recirculation ratio control is based on logic settings for influent and effluent color. The entire process is automated through the DCS remote control system, improving treatment efficiency and reducing costs.

[0035] In the aforementioned pretreatment system for phenol-containing wastewater, as a preferred embodiment, the coagulation zone 2 specifically includes coagulation zone A 21 and coagulation zone B 22, which are interconnected. Coagulation zone A 21 is connected to the influent pipeline 1 and the return pipeline 6, respectively, while coagulation zone B 22 is connected to the flocculation zone 3. The influent and return liquid are mixed in coagulation zone A 21, which is equipped with a demulsifier dosing pipeline. The demulsifier dosage is automatically controlled according to the influent flow rate and is used to break the internal equilibrium of the phenol-containing wastewater, which helps with subsequent sedimentation. When the system is first started, since there is no demulsifier in the circulating water, the demulsifier dosage is relatively large during startup. After stable operation, some of the demulsifier settles, while some remains in the effluent and is recycled back to the flocculation zone for reuse, which can reduce the dosage and save operating costs.

[0036] The demulsifier used in this invention is a conventional demulsifier, i.e., a surfactant that can destroy emulsions. Demulsifiers primarily disrupt emulsions by partially replacing the stabilizing film, and can also be used to reduce the viscosity of oily substances, making them less prone to clogging. Preferably, the demulsifier used in this invention is a conventional reverse demulsifier, whose molecular structure has a hydrophilic-to-hydrophobic ratio that is biased towards hydrophobicity. The molecules are relatively stable in the oil phase, but can adsorb onto the surface of oil droplets in the aqueous phase, forming a structure where "hydrophilic groups face the oil droplet, and hydrophobic groups face outwards." This disrupts the interaction between the oil droplet and the oil phase, causing the oil droplet to detach from the oil phase. Its main function is to reduce the tension at the oil-water interface, making the originally stable emulsion system unstable, promoting the aggregation, merging, and separation of oil droplets from the aqueous phase.

[0037] A coagulant dosing pipeline is installed in coagulation zone B22. The dosage is automatically controlled according to the influent flow. In practice, it was found that iron salts would combine with organic matter in the sewage and become ineffective. Therefore, aluminum salts are selected as the coagulant in this method to make the insoluble matter and asphaltenes in the raw water clump together.

[0038] Wastewater that has undergone coagulation treatment enters flocculation tank 3. A guide tube is arranged in flocculation tank 3, and the flocculant dosing pipe extends to the bottom of the guide tube. Wastewater that has undergone flocculation reaction flows by gravity from the inclined wall push flow tank into sludge sedimentation zone 4. The sedimentation zone is an inclined tube sedimentation zone. The flocs settle down under the action of the inclined tube and are collected by the sludge collection hopper in the center of the sedimentation zone.

[0039] According to a second aspect of the present invention, in order to further understand the pretreatment system for phenol-containing wastewater of the present invention, the present invention also provides a pretreatment process for phenol-containing wastewater, comprising the following steps:

[0040] Step 1: Perform online colorimetry detection on the influent containing phenol wastewater using the first online colorimetry detector 11;

[0041] Step 2: The phenol-containing wastewater is treated sequentially through coagulation zone 2, flocculation zone 3 and sludge sedimentation zone 4. The effluent is then returned to coagulation zone 2 via return pipeline 6 to dilute the influent. At the same time, the effluent is subjected to online color detection by the second colorimeter 51 to ensure that the effluent color value is <4000.

[0042] Step 3: Substitute the real-time influent color value and effluent color value monitored by the first online colorimeter 11 and the first online colorimeter 51 into formula (1) to calculate the K value; when the effluent color value is ≥4000, adjust the reflux ratio to be greater than the K value, and after the effluent meets the standard, it will enter the subsequent treatment.

[0043] K = (influent color value - 6000) ÷ (6000 - effluent color value) Equation (1).

[0044] The pretreatment process for phenol-containing wastewater of the present invention achieves better treatment results by automatically controlling the appropriate influent dilution ratio (i.e., automatic control of the reflux ratio). Moreover, some of the reagents are still present in the effluent and are recycled back to the coagulation zone, which can reduce the amount of reagents added and lower operating costs.

[0045] The reflux ratio is mainly affected by two factors: ① Treatment difficulty of phenol-containing wastewater: Since the treated wastewater is often in a fluctuating state, when there are more heavy substances in the wastewater, the treatment difficulty is high and the required dilution ratio is high, and vice versa. The content of heavy substances (asphaltite and some insoluble substances) can be characterized by the color of the influent; ② Treatment effect under current operating conditions: It is affected by the current dosage, reagent quality and actual treatment effect under the environment. Good treatment effect means high removal rate of heavy substances, and vice versa. The actual treatment effect can be characterized by the color of the effluent.

[0046] For phenol-containing wastewater in petrochemical processes, when the raw water color is 10,000, the raw water is diluted and its color is measured. The results show that the color of the diluted wastewater = raw water color ÷ dilution factor. This means the color of the phenol-containing wastewater is directly proportional to the concentration of heavy substances in the wastewater. The linear relationship between the two is shown in the attached figure. Figure 2 As shown in Table 1 below, assuming the original water concentration is 1:

[0047] Table 1

[0048] Dilution factor concentration Detecting colorimetry 10 0.1 993 5 0.2 2022 4 0.25 2561 3 0.33 3297 2 0.5 5031 1 1.0 10000

[0049] The physicochemical properties of the treated effluent are similar to those of the influent. The mixing of the influent and effluent colors is a mixing of concentrations, and this is also reflected in the actual color measurement after mixing. That is, the color after mixing = (influent flow rate × influent color + reflux flow rate × effluent color) ÷ (influent flow rate + reflux flow rate);

[0050] Reflux ratio = Reflux flow rate ÷ Inlet flow rate;

[0051] Therefore, the color after mixing = (inlet color + reflux ratio × outlet color) ÷ (reflux ratio + 1).

[0052] Extensive practical experience has shown that the demulsification effect is better when the color of the influent and return water mixture is below 6000. When the color is greater than 6000, the demulsifier is ineffective, and the water treatment becomes more difficult. Therefore, the operating requirement is set as follows: the color of the influent and return water mixture is <6000, which means: return ratio > (influent color value - 6000) ÷ (6000 - effluent color value).

[0053] Considering experience and cost, the reflux ratio should not be too large, otherwise it will exceed the water volume design load. Adjust the reflux ratio to <5, that is: (inlet water color - 6000) ÷ (6000 - outlet water color) < reflux ratio < 5.

[0054] If the calculated (influent color - 6000) ÷ (6000 - effluent color) > 5, it is considered an abnormal situation. The automatic control of the reflux ratio needs to be stopped, and circulating water needs to be added again to dilute the influent. After the system stabilizes, the automatic control of the reflux ratio can be restarted.

[0055] When the system is first started, there is no outflow water, so there is no return water. Therefore, circulating water is used to dilute the inflow water. The color of the circulating water can be considered to be close to 0, so the mixed dilution can basically meet the requirement of an inflow water color of 6000. Since the inflow water color is around 10,000, a lower return ratio can be used when diluting with circulating water. Empirically, the initial set return ratio is 3. Under this condition, the outflow water color is very low, between 1000 and 2000, and there is no need to use automatic return ratio control. When the outflow tank reaches a certain level, the outflow water can be used as the return diluent. In the initial stage, the circulating water still accounts for a large proportion, and a return ratio of 3 can still be used. The system meets the requirement of effluent color < 4000. As the circulating water is gradually discharged, when the effluent color reaches 4000, automatic control can be activated. At this time, the reflux ratio will be gradually increased to maintain the effluent color at 4000. When the effluent color is controlled within 4000, impurities (asphalt and some insoluble substances) are basically removed. After the influence of the circulating water is completely eliminated, the system reaches a steady state, and the reflux ratio reaches a stable value. During stable operation, the reflux ratio is adjusted according to the fluctuation of the influent water quality. That is, when the water quality is good, the reflux ratio is low to save operating costs, and when the water quality is poor, the reflux ratio is high. Through automatic control of the reflux ratio, the treatment requirements are met.

[0056] In the above pretreatment process, as a preferred embodiment, the coagulation treatment process includes: adding a demulsifier to coagulation zone A 21 to demulsify the wastewater and reduce the viscosity of the oily substance; then sending it to coagulation zone B 22, adding a coagulant, and coagulating the wastewater.

[0057] In the above pretreatment process, as a preferred embodiment, the demulsification time in coagulation zone A 21 is 6 to 10 minutes (e.g., 6 minutes, 7 minutes, 8 minutes, 9 minutes, 10 minutes); and the coagulation time in coagulation zone B 22 is 6 to 10 minutes (e.g., 6 minutes, 7 minutes, 8 minutes, 9 minutes, 10 minutes).

[0058] Optionally, the flocculation time is 10 to 15 minutes (e.g., 10 minutes, 11 minutes, 12 minutes, 13 minutes, 14 minutes, 15 minutes).

[0059] The present invention will now be described in detail with reference to embodiments thereof. These examples are provided by way of explanation and not by way of limitation. In fact, those skilled in the art will recognize that modifications and variations can be made to the present invention without departing from its scope or spirit. For example, a feature shown or described as part of one embodiment may be used in another embodiment to produce yet another embodiment. Therefore, it is desirable that the present invention encompass such modifications and variations that fall within the scope of the appended claims and their equivalents.

[0060] In the embodiments of the present invention, unless otherwise specified, the experimental methods used are conventional methods, and the materials and reagents used are commercially available unless otherwise specified.

[0061] Example 1

[0062] This embodiment provides a pretreatment process for phenol-containing wastewater. The experimental subject is a phenol-containing wastewater from a company in Shaanxi Province. The influent COD is about 100,000 mg / L and the color is about 11,000. The influent water is blackish-brown and very turbid. In the initial stage of operation, it is diluted by reflux at a ratio of 5, that is, the reflux ratio is 4. In the initial stage of operation, due to the low color of the circulating water, the color control dilution ratio (automatic reflux ratio control) is not put into use.

[0063] In the first part of the coagulation zone, the demulsifier dosage is 50 mg / L, and the mixture is stirred rapidly for 8 minutes. In the second part of the coagulation zone, the coagulant PAC dosage is 100 mg / L, and the mixture is stirred rapidly for 8 minutes. In the flocculation zone, the PAM dosage is 3 mg / L, and the mixture is stirred slowly for 12 minutes. In the sedimentation zone, sludge and water are rapidly separated. The clear water flows upward, is separated through an inclined tube, enters the effluent tank through the collection trough, and the sludge sinks downward. The sludge is concentrated by a thickening scraper and then discharged.

[0064] When the water level in the outlet pipe rises to 30%, the circulating water line is shut off, and the outlet water replaces the circulating water as a diluent. When the water level rises to 50%, the outlet water is discharged, and the water level is controlled between 30% and 50%. As the outlet water gradually flows back, the color of the outlet water gradually increases. When it rises to 4000, the color reflux ratio is automatically controlled. As the influence of the circulating water decreases, in order to maintain the color of the outlet water at 4000, the reflux ratio gradually increases. After stabilization, the reflux ratio is 4.3.

[0065] After stable operation, some demulsifier will precipitate, while some demulsifier will still exist in the effluent. It will be recycled back to the flocculation zone for reuse. The dosage of demulsifier will be adjusted to 30 mg / L.

[0066] The final effluent COD was 89,300 mg / L, with a COD removal rate of 10.7%. Along with the removal of difficult-to-treat heavy substances, after subsequent treatment by a combined process of deacidification and deammoniation, extraction and oil removal, and recovery of residual extractant in the water, the effluent COD was about 4,000 mg / L, meeting the influent standards for subsequent biological wastewater treatment. Figure 3 The changes in appearance of phenol-containing wastewater before and after treatment are shown. The previous process required a shutdown for maintenance every two months; after switching to this method, the equipment has operated stably for six months without any blockages.

[0067] Comparative Example 1

[0068] The conventional process flow was used, and the experimental subjects were the same as in Example 1. A demulsifier was added, i.e., the process involved sedimentation in a storage tank → oil-water separator → multi-stage filter. A reaction tank was set up before the storage tank, and the demulsifier was added and stirred to react. During operation, the treatment effect was no different from the old process, but frequent maintenance and cleaning were required.

[0069] Comparative Example 2

[0070] This comparative example provides a pretreatment process for phenol-containing wastewater. All other steps and parameters are the same as in Example 1, except that no demulsifier is added. During operation, the COD removal rate is low, requiring frequent replenishment of circulating water for dilution, resulting in high operating costs.

[0071] Comparative Example 3

[0072] This comparative example provides a pretreatment process for phenol-containing wastewater. All other steps and parameters are the same as in Example 1, except that the effluent is not recirculated and the influent is not diluted. During operation, the odor is strong, making it difficult for personnel to control. Furthermore, due to the poor fluidity of the liquid, severe coking occurs in the agitators of the coagulation and flocculation zones, resulting in poor sedimentation and rendering the process unusable.

[0073] Comparative Example 4

[0074] This comparative example provides a pretreatment process for phenol-containing wastewater. All other steps and parameters are the same as in Example 1, except that: instead of automatic control of the color reflux ratio, the reflux ratio is fixed at 4. During operation, the effluent color fluctuates continuously. When the water quality is good, the effluent color is low; when the water quality is poor, it cannot be adjusted in time, easily leading to back-end blockage.

[0075] Table 2 shows the relevant indicators and operating conditions of the effluent water quality of Example 1 and Comparative Examples 1-4:

[0076] Table 2

[0077]

[0078] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A pretreatment system for phenol-containing wastewater, characterized in that, The system includes an inlet pipeline, a coagulation zone, a flocculation zone, a sludge sedimentation zone, and an outlet tank, which are connected sequentially along the wastewater flow direction. The outlet tank is connected to the coagulation zone via a return pipeline. A first online colorimeter for monitoring the inlet color value is installed on the inlet pipeline, and a second online colorimeter for monitoring the outlet color value is installed on the outlet tank. Both the first and second online colorimeters are connected to a DCS remote control system. The DCS remote control system adjusts the return ratio of the return pipeline based on the monitored inlet and outlet color values, including: substituting the inlet and outlet color values ​​into formula (1) to calculate the K value; when the outlet color value is ≥4000, adjusting the return ratio to be greater than the K value, and the outlet water enters the subsequent treatment after meeting the standard; K = (inlet color value - 6000) ÷ (6000 - outlet color value) formula (1).

2. The preprocessing system according to claim 1, characterized in that, The coagulation zone includes a coagulation zone A and a coagulation zone B that are interconnected; the coagulation zone A is connected to the inlet pipeline and the return pipeline respectively, and the coagulation zone B is connected to the flocculation zone; the coagulation zone A is equipped with a demulsifier dosing pipeline, and the coagulant dosing pipeline is equipped with a coagulant dosing pipeline.

3. The preprocessing system according to claim 1, characterized in that, A first flow meter for monitoring the inlet flow rate is installed on the inlet pipeline, and a second flow meter for monitoring the return flow rate is installed on the return pipeline; both the first and second flow meters are connected to the DCS remote control system, and the DCS remote control system adjusts the inlet and return flow rates through the first and second flow meters.

4. A pretreatment process for phenol-containing wastewater, characterized in that, It employs the preprocessing system described in any one of claims 1-3, comprising the following steps: S1. Perform online color detection on the influent containing phenol; S2. Phenolic wastewater is treated sequentially through a coagulation zone, a flocculation zone, and a sludge sedimentation zone. Part of the effluent is then returned to the coagulation zone to dilute the influent. At the same time, the effluent is subjected to online color detection to ensure that the effluent color value is <4000. S3. Substitute the influent color value and effluent color value into formula (1) to calculate the K value. When the effluent color value is ≥4000, adjust the reflux ratio to be greater than the K value. After the effluent meets the standard, it will enter the subsequent treatment. K = (Inlet color value - 6000) ÷ (6000 - Outlet color value) Equation (1).

5. The pretreatment process according to claim 4, characterized in that, After adjusting the reflux ratio to be greater than the K value, the effluent color value is <4000.

6. The pretreatment process according to claim 4, characterized in that, The effluent is partially returned to the coagulation zone to dilute the influent. The color of the diluted wastewater is equal to the color of the influent divided by the dilution factor.

7. The pretreatment process according to claim 4, characterized in that, Adjust the reflux ratio to <5.

8. The pretreatment process according to claim 4, characterized in that, The coagulation treatment process includes: adding a demulsifier in coagulation zone A to demulsify the wastewater and reduce the viscosity of the oily substances; then sending it to coagulation zone B, adding a coagulant, and coagulating the wastewater.

9. The pretreatment process according to claim 8, characterized in that, In zone A of coagulation, the demulsification time is 6-10 minutes; in zone B of coagulation, the coagulation time is 6-10 minutes.

10. The pretreatment process according to claim 4, characterized in that, The flocculation time is 10-15 minutes.

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