Shale gas drilling wastewater comprehensive treatment system and application thereof

Abstract

The application relates to the technical field of industrial wastewater treatment, in particular to a comprehensive treatment system for shale gas drilling and mining wastewater and application thereof, which comprises the following steps: collecting shale gas drilling and mining wastewater, and performing preliminary oil removal; sequentially performing water quality homogenization and fine pretreatment on the effluent after oil removal; sequentially performing anaerobic hydrolysis acidification and AO biochemical treatment; performing electro-catalytic oxidation treatment on the biochemical effluent; sequentially performing tubular membrane ultrafiltration treatment, nanofiltration membrane treatment and two-stage reverse osmosis membrane treatment, the produced clean water is introduced into an external discharge monitoring pool, the produced concentrated water is subjected to evaporation concentration crystallization and drying to obtain industrial salt, and the produced mother liquor is made into miscellaneous salt. Through the reasonably connected system process and the unit design with high synergy, the treatment ideas of efficient pretreatment, stable biochemical treatment, deep oxidation, long-term operation of the membrane system and salt quality crystallization recovery are utilized, the resource utilization of the salt in the shale gas wastewater and the safe disposal of the waste are successfully realized, and the output of the solid waste is greatly reduced.

Description

Technical Field

[0001] This application relates to the field of industrial wastewater treatment technology, specifically to a comprehensive treatment system for shale gas drilling wastewater and its application. Background Technology

[0002] Shale gas, a typical unconventional natural gas resource, is found in organic-rich shale reservoir systems. It is characterized by wide reservoir distribution, stable thickness, and continuous gas reservoir development, enabling long-term stable production from single wells. It exhibits development advantages such as huge resource reserves, long development cycles, and low production decline rates. Currently, shale gas is mainly extracted economically using engineering technologies such as horizontal well fracturing. However, the treatment of complex waste fluids generated during fracturing has become a key technical challenge restricting the sustainable development of the industry.

[0003] During shale gas drilling and extraction, functional fluids injected into the formation are returned to the surface after reservoir stimulation, forming a complex pollution system with significant environmental risks. This wastewater not only contains incompletely degraded thickeners (such as modified polysaccharide polymers), cross-linking agents, and various functional additives, but also carries minerals dissolved from the formation, sulfur-oxidizing bacteria, and heavy metals. Its fluid characteristics include high apparent viscosity (15–25 mPa·s), strong emulsification stability, and multiphase dispersion, making effective separation difficult with conventional water treatment technologies. More seriously, the drilling and extraction components dynamically change with geological conditions, containing oil phase substances, dissolved organic matter (COD values ​​generally exceeding 1500 mg / L), and total dissolved solids (TDS often reaching 5–15 g / L), posing multiple pollution threats to the environment.

[0004] Currently, the industry commonly employs various single or combined processes to treat shale gas drilling wastewater. Common pretreatment methods include simple oil separation and sedimentation, aimed at removing floating oil and some suspended solids. Subsequent treatment largely relies on biological processes (such as aerobic and anaerobic processes) to degrade organic matter. However, the toxic and harmful substances and high salinity in shale gas wastewater strongly inhibit microbial activity, leading to low efficiency or even failure of traditional biological treatment. Residual recalcitrant organic matter and ammonia nitrogen in the effluent are also difficult to completely remove using conventional methods. In terms of advanced treatment and zero discharge, membrane concentration (such as reverse osmosis) and evaporation crystallization (such as MVR) are commonly used key technologies, but existing technologies have significant shortcomings: First, the lack of systematic and refined pretreatment means that hardness, barium and strontium ions, and fine oil in the wastewater easily cause rapid fouling and scaling of the subsequent membrane system, resulting in decreased water production, frequent cleaning, and increased operating costs. Secondly, the traditional simple combination of "biochemical treatment + membrane + evaporation" fails to effectively address the toxicity and pollution of recalcitrant organic matter to microorganisms and membrane elements. Furthermore, it lacks sufficient quality control over crystalline salts, resulting in large quantities of mixed salts, high disposal costs, and ineffective resource recovery. In addition, existing treatment systems often struggle to consistently meet increasingly stringent emission standards, such as the limits for various pollutants in the "Shale Gas Extraction Wastewater Pollutant Discharge Standard" (DB50 / 1806-2025).

[0005] Therefore, a new comprehensive treatment system needs to be developed to solve the problem of wastewater treatment in shale gas drilling and production, and to promote the green and sustainable development of the shale gas industry. Summary of the Invention

[0006] The problem this application aims to solve is to provide a comprehensive treatment system for shale gas drilling wastewater and its application. Through a rationally integrated system process and a highly synergistic unit design, and by utilizing treatment approaches such as efficient pretreatment, stable biochemical treatment, deep oxidation, long-term operation of membrane systems, and salt crystallization recovery, the system successfully achieves the resource utilization of salts in shale gas wastewater and the safe disposal of waste, greatly reducing the production of solid waste.

[0007] To solve the above-mentioned technical problems, this application adopts the following technical solution: On the one hand, this application provides a comprehensive treatment system for shale gas drilling wastewater, including the following steps: S1. After the shale gas drilling wastewater is collected in the unloading pool, a high-efficiency oil-water separator is used for preliminary oil removal. S2. The effluent after S1 treatment is passed sequentially through a homogenization equalization tank and a multi-functional pretreatment tank for water quality homogenization and fine pretreatment. S3. The effluent after S2 treatment is subjected to anaerobic hydrolysis acidification and AO biological treatment in sequence; S4. Electrocatalytic oxidation treatment is performed on the biochemical effluent after S3 treatment; S5. The effluent after S4 treatment is sequentially subjected to tubular membrane ultrafiltration, nanofiltration, and two-stage reverse osmosis. After treatment, the resulting clean water enters the external discharge monitoring tank. S6. The concentrated water produced in S5 is sent to an MVR evaporator for evaporation, concentration, crystallization and drying to obtain industrial salt. The mother liquor produced is added to a mother liquor evaporator to produce mixed salt.

[0008] In the above technical solution, shale gas drilling wastewater is pretreated sequentially through an unloading tank, a high-efficiency oil-water separator, a homogenization equalization tank, and a multi-functional pretreatment tank. Then, it undergoes anaerobic hydrolysis acidification and AO biochemical treatment to effectively remove organic matter and ammonia nitrogen. Next, an electrocatalytic oxidation unit is introduced to further treat the biochemical effluent, effectively degrading recalcitrant organic matter and reducing the risk of membrane fouling. After further desalination and purification via a combination of tubular membrane ultrafiltration, nanofiltration, and two-stage reverse osmosis, the treated water is directly discharged. The concentrated water generated by the membrane system is crystallized and dried using an MVR evaporation crystallization system to obtain industrial salt, and the resulting mother liquor is further evaporated to form mixed salts.

[0009] Furthermore, in S1, the oil content in the effluent from the high-efficiency oil-water separator is <50 mg / L.

[0010] In the above technical solution, at this oil content, floating oil and dispersed oil with an oil droplet size of 10~100 μm in shale gas drilling wastewater have been basically removed.

[0011] Furthermore, in S2, the multifunctional pretreatment tank includes an air flotation unit, a chemical dosing and mixing unit, and a sedimentation unit.

[0012] In the above technical solution, the combined pretreatment method of air flotation unit, chemical dosing and mixing unit and sedimentation unit can effectively remove calcium, magnesium, barium and suspended solids in wastewater, and can also perform deep oil removal.

[0013] For example, the air flotation unit contains at least polyaluminum chloride and anionic polyacrylamide. The dosage of polyaluminum chloride is 20-150 mg / L and the dosage of anionic polyacrylamide is 0.5-5 mg / L based on the volume of influent. In this embodiment, polyaluminum chloride is used as a coagulant, and anionic polyacrylamide is used as a flocculant.

[0014] For example, the chemical dosing mixing unit contains at least sodium carbonate and sodium hydroxide for removing calcium, magnesium, and barium ions, and the hydraulic retention time of the chemical dosing mixing unit is 10-30 min.

[0015] For example, the hydraulic retention time of the sedimentation unit is 2 to 4 hours.

[0016] Furthermore, in S3, the anaerobic hydrolysis acidification is completed in an anaerobic hydrolysis acidification tank. The steps include: adding anaerobic granular sludge at 15%~35% (v / v) of the effective volume of the anaerobic hydrolysis acidification tank, controlling the temperature at 25~35℃, and the hydraulic retention time at 24~48 h, to perform anaerobic hydrolysis acidification on the effluent after S2 treatment.

[0017] In the above technical solution, the added anaerobic granular sludge can convert large molecular organic matter in wastewater into small molecular organic matter, thereby improving the biodegradability of wastewater.

[0018] Furthermore, in S3, during the AO biochemical treatment process, the dissolved oxygen in the anoxic zone is controlled below 0.2 mg / L, the dissolved oxygen in the aerobic zone is controlled at 2~4 mg / L, the hydraulic retention time in the anoxic zone is controlled at 2~4 h, the hydraulic retention time in the aerobic zone is controlled at 6~12 h, the temperature is controlled at 25~35℃, and every 24~48 h, 0.05%~0.2% (v / v) of anaerobic / aerobic microbial complex nutrients and 0.1%~0.3% (v / v) of corn steep liquor are added.

[0019] In the above technical solution, by controlling the temperature and time of the aerobic / anaerobic treatment process, as well as the amount of compound nutrients and corn steep liquor, the metabolic activity of microorganisms can be enhanced, and the efficient degradation of COD can be promoted.

[0020] Furthermore, in S4, the electrocatalytic oxidation treatment is performed using ruthenium-iridium-titanium electrode plates with a plate spacing of 10-20 mm, a current density of 10-30 mA / cm3, and a reaction time of 15-60 min.

[0021] In the above technical solution, by using specific electrode plates and electrocatalytic oxidation parameters, ammonia nitrogen and recalcitrant organic matter in wastewater can be effectively degraded.

[0022] Furthermore, in S5, the tubular membrane ultrafiltration process is carried out using a tubular membrane with a pore size of 0.05~0.2 μm at an operating pressure of 0.2~0.4 MPa.

[0023] Furthermore, in S5, the nanofiltration membrane treatment is carried out using a spiral wound membrane with a molecular weight cutoff of 150~300 Da at an operating pressure of 1.2~1.8 MPa.

[0024] In the above technical solution, sodium chloride in water can be separated and purified through nanofiltration membrane treatment, thereby improving the quality and yield of crystalline salt.

[0025] Furthermore, in S5, the first-stage operating pressure of the two-stage reverse osmosis membrane is 3.5~4.5 MPa, and the second-stage operating pressure is 5.0~6.0 MPa.

[0026] Furthermore, in S6, the temperature for evaporation, concentration, and crystallization is 70~90°C.

[0027] Furthermore, the drying method includes centrifugal drying.

[0028] On the other hand, this application provides the application of the above method in treating shale gas wastewater or in preparing industrial salt from shale gas wastewater.

[0029] This application has the following beneficial effects: (1) This application enhances the pretreatment effect through “high-efficiency oil-water separation + multi-functional pretreatment”; “anaerobic hydrolysis acidification + AO” improves the efficiency and stability of biochemical treatment; “electrocatalytic oxidation” serves as a bridge between biochemical and membrane methods, effectively removing membrane fouling substances, significantly reducing the fouling risk of subsequent nanofiltration and reverse osmosis membranes, and ensuring the long-term stable operation and high water production rate of the membrane system.

[0030] (2) This application introduces “electrocatalytic oxidation” as the core connecting unit. This unit not only deeply degrades the residual recalcitrant organic matter in the biochemical effluent and reduces the membrane burden, but also further improves the influent water quality of the membrane system through its micro-flocculation effect and bactericidal effect.

[0031] (3) This application uses "tubular membrane ultrafiltration" as the pretreatment for nanofiltration. Tubular membranes have strong anti-fouling ability and can effectively retain the tiny flocs and colloidal substances that may be formed after electrocatalytic oxidation, providing strong protection for nanofiltration membranes that are more sensitive to fouling.

[0032] (4) This application uses MVR to evaporate and crystallize the membrane concentrate, and recovers the crystallized solid as industrial salt, thus realizing the resource utilization of salt in wastewater; the high-concentration mother liquor is evaporated separately to generate mixed salt, thus realizing the final safe disposal of waste and effectively controlling the overall solid waste production.

[0033] (5) The entire system of this application is optimized and integrated, with each unit taking advantage of its strengths and avoiding its weaknesses, thereby reducing reagent consumption, membrane replacement frequency and energy consumption. The reverse osmosis has a high water production rate and fully recovers water resources; the MVR has high energy recycling efficiency and recovers industrial salt, generating additional economic value.

[0034] (6) In view of the characteristics of water quality fluctuation in shale gas wastewater, this application has set up homogenization regulation and multi-level barriers. The system has strong resistance to shock loads and stable effluent water quality, which can meet the indirect discharge standard of the "Shale Gas Extraction Water Pollutant Discharge Standard" (DB50 / 1806-2025). Attached Figure Description

[0035] Figure 1 This is a process flow diagram of the comprehensive treatment system for shale gas drilling wastewater in this application. Detailed Implementation

[0036] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings, but the scope of protection of the present invention is not limited to the following description.

[0037] This application takes the drilling wastewater from a shale gas field as an example for comprehensive treatment, and the influent water quality is shown in Table 1.

[0038] Table 1 Influent Water Quality Table The process for comprehensive treatment of shale gas field drilling wastewater in this application is as follows: Figure 1 As shown, the specific steps are as follows: S1. Collection and preliminary oil removal: The shale gas field drilling wastewater is collected in an unloading pool and then pumped into a high-efficiency oil-water separator using inclined plate coalescence technology. S2. Water Homogenization and Fine Pretreatment: The effluent after preliminary oil removal enters a homogenization and equalization tank. After the water quality and quantity are balanced, it enters a multi-functional pretreatment tank equipped with an air flotation unit, a chemical dosing and mixing unit, and a sedimentation unit. In the air flotation unit, 20-150 mg / L polyaluminum chloride (PAC) and 0.5-5 mg / L anionic polyacrylamide (PAM) are added to remove fine oil droplets and colloidal substances. In the chemical dosing and mixing unit, sodium carbonate (Na2CO3) and sodium hydroxide (NaOH) are added, and the reaction time is 10-30 min to effectively precipitate and remove calcium, magnesium, barium, and other ions. The hydraulic residence time in the sedimentation unit is 2-4 h. S3. Biological treatment: The pretreated effluent enters the anaerobic hydrolysis acidification tank, and anaerobic granular sludge is added at 15%~35% (v / v) of the effective volume. The temperature is controlled at 25~35℃, and the hydraulic retention time is 24~48 h. Then the effluent enters the anoxic zone (A zone) with dissolved oxygen (DO) controlled below 0.2 mg / L and aerobic zone (O zone) with DO controlled at 2~4 mg / L. The hydraulic retention time of the A zone is 2~4 h, and the hydraulic retention time of the O zone is 6~12 h. The temperature is maintained at 25~30℃. Every 24~48 h, 0.05% (v / v)~0.2% (v / v) of anaerobic and aerobic bacteria-specific compound nutrients and 0.5% (v / v)~3% (v / v) of corn steep liquor are added. S4. Electrocatalytic oxidation deep treatment: The biochemical effluent enters an electrocatalytic oxidation reactor with ruthenium-iridium-titanium (Ru-Ir / Ti-Ti) electrodes, with an electrode spacing of 10~20 mm, a current density of 10~30 mA / cm², and a reaction time of 15~60 min. S5. Membrane Deep Desalination and Purification: The effluent from electrocatalytic oxidation is treated by a tubular membrane ultrafiltration unit with a membrane pore size of 0.05~0.2 μm and an operating pressure of 0.2~0.4 MPa. The resulting concentrate is returned to the multifunctional pretreatment tank. The purified water enters a nanofiltration membrane system with a molecular weight cutoff of 150~500 Da and spiral wound membranes as the material, with an operating pressure of 1.2~1.8 MPa. The nanofiltration membrane permeate enters a two-stage reverse osmosis system, and the concentrate (rich in high-valent salts) enters the reverse osmosis unit. The first stage of the two-stage reverse osmosis system operates at a pressure of 3.5~4.5 MPa, and the second stage operates at a pressure of 5.0~6.0 MPa, thereby highly concentrating the nanofiltration permeate. S6. Concentrated Water Evaporation Crystallization and Resource Utilization: The concentrated water from the reverse osmosis unit and nanofiltration membrane system enters the MVR evaporator and undergoes forced circulation evaporation concentration at an evaporation temperature of 70~90℃. After centrifugation and drying, the crystallized slurry yields industrial-grade sodium chloride salt with a purity ≥98.5%. The evaporation condensate enters the discharge monitoring pool. The mother liquor produced by the MVR enters the mother liquor evaporator and is further evaporated at 90℃ using a forced circulation evaporation crystallization process to generate mixed salts whose main components are sodium sulfate and a small amount of heavy metal salts. This is used as solid waste for safe disposal. The distilled water produced by the mixed salt evaporator also enters the discharge monitoring pool. S7. Final discharge: The mixed clean water from the membrane system and the distilled water from the evaporation system in the discharge monitoring tank will be discharged to the park's wastewater treatment plant.

[0039] To verify the effectiveness of this solution in the comprehensive treatment of shale gas field drilling wastewater, the following exemplary embodiments are designed in this application.

[0040] Example 1 The shale gas field drilling wastewater was comprehensively treated using the following method: S1. Collection and preliminary oil removal: The shale gas field drilling wastewater is collected in an unloading pool and then pumped into a high-efficiency oil-water separator using inclined plate coalescence technology. After oil-water separation, the oil content of the effluent is reduced to below 50 mg / L, and floating oil and dispersed oil with a particle size in the range of 10~100 μm are effectively removed from the wastewater. S2. Water Homogenization and Fine Pretreatment: The effluent after preliminary oil removal enters a homogenization tank. After the water quality and quantity are balanced, it enters a multi-functional pretreatment tank equipped with an air flotation unit, a chemical dosing and mixing unit, and a sedimentation unit. In the air flotation unit, 80 mg / L polyaluminum chloride (PAC) and 1.5 mg / L anionic polyacrylamide (PAM) are added to remove fine oil droplets and colloidal substances. In the chemical dosing and mixing unit, 200 mg / L sodium carbonate (Na2CO3) and 80 mg / L sodium hydroxide (NaOH) are added, and the reaction time is 20 min to effectively precipitate and remove calcium, magnesium, barium, and other ions. The hydraulic retention time in the sedimentation unit is 3 h. After pretreatment, the SS in the effluent is reduced to below 35 mg / L, and the oil content in the effluent is further reduced to below 8 mg / L. S3. Biological Treatment: The pretreated effluent enters an anaerobic hydrolysis acidification tank, and anaerobic granular sludge with a VSS ≥ 40 g / L is added at 25% (v / v) of the effective volume. The temperature is controlled at 30℃, and the hydraulic retention time is 24 h, thereby converting large molecular organic matter into small molecular organic matter. At this time, the effluent BOD5 / COD ratio is 0.35. Then, the effluent enters an AO biological system with dissolved oxygen (DO) controlled at 0.2 mg / L in the anoxic section (Section A) and DO controlled at 3 mg / L in the aerobic section (Section O). The hydraulic retention time in Section A is 3 h, and the hydraulic retention time in Section O is 10 h. The temperature is maintained at 30℃. Every 24 h, 0.1% (v / v) of anaerobic and aerobic bacteria-specific compound nutrients and 0.2% (v / v) of corn steep liquor are added as biological activators. After the biological treatment is completed, the effluent COD is reduced to below 500 mg and NH3-N is reduced to below 50 mg / L. S4. Electrocatalytic oxidation deep treatment: The biochemical effluent enters the electrocatalytic oxidation reactor with ruthenium-iridium-titanium (Ru-Ir / Ti-Ti) electrodes and a spacing of 15 mm between the electrodes. The current density is 20 mA / cm², and the reaction time is 30 min. After the electrocatalytic treatment, the recalcitrant organic matter is effectively degraded, COD is further reduced to below 300 mg / L, and NH3-N is reduced to below 15 mg / L. At the same time, the fouling tendency of the subsequent membrane system is reduced. S5. Membrane Deep Desalination and Purification: The effluent from electrocatalytic oxidation is treated by a tubular membrane ultrafiltration unit with a membrane pore size of 0.1 μm at an operating pressure of 0.3 MPa. The resulting concentrate is returned to the multifunctional pretreatment tank, while the purified water enters a nanofiltration membrane system with a molecular weight cutoff of 300 Da and spiral wound membranes at an operating pressure of 1.8 MPa, thereby separating 90% of the sodium chloride. The nanofiltration membrane permeate enters a two-stage reverse osmosis system, while the concentrate (rich in high-valent salts) enters the reverse osmosis unit. The first stage of the two-stage reverse osmosis system operates at an operating pressure of 4.5 MPa, and the second stage operates at an operating pressure of 6.0 MPa, thereby highly concentrating the nanofiltration permeate to a TDS of 80,000 mg / L. The purified water from the reverse osmosis unit and the nanofiltration membrane system has a TDS of <500 mg / L and can be discharged into the external discharge monitoring tank. S6. Concentrated Water Evaporation Crystallization and Resource Utilization: The concentrated water from the reverse osmosis unit and nanofiltration membrane system enters the MVR evaporator and undergoes forced circulation evaporation concentration at an evaporation temperature of 75℃. After centrifugal drying, the crystallized slurry yields industrial-grade sodium chloride salt with a purity ≥98.5%. The evaporation condensate enters the discharge monitoring pool. The mother liquor produced by the MVR enters the mother liquor evaporator and undergoes forced circulation evaporation crystallization at 90℃ to further evaporate and generate mixed salts whose main components are sodium sulfate and a small amount of heavy metal salts. This is used as solid waste for safe disposal. The distilled water produced by the mixed salt evaporator also enters the discharge monitoring pool. S7. Final discharge: After the mixed clean water from the membrane system and the distilled water from the evaporation system are discharged into the monitoring tank, the water quality parameters are shown in Table 2. They meet the indirect discharge limits of the "Shale Gas Extraction Water Pollutant Discharge Standard" (DB50 / 1806-2025) and can be discharged to the park's wastewater treatment plant.

[0041] Table 2 Water quality parameters after discharge into the monitoring pond After the shale gas field drilling wastewater is treated by the system described in this application, the total removal rate of key pollutants is >85% for COD, >86% for NH3-N, >92% for oil, and >87% for TDS. Furthermore, the system operates stably, the membrane fouling cycle is extended to more than 90 days, and MVR evaporation and crystallization realize the resource utilization of salts. The industrial salt recovery rate can reach 85%, the miscellaneous salt yield is <10%, and the overall treatment cost is reduced by about 20% compared with traditional methods.

[0042] Example 2 The method for comprehensive treatment of shale gas field drilling wastewater in this embodiment is the same as in Embodiment 1; the only difference between Embodiment 2 and Embodiment 1 is that the current density of the electrocatalytic oxidation unit reactor in S4 is adjusted to 30 mA / cm².

[0043] Treatment results: The COD degradation efficiency is improved in the electrocatalytic oxidation stage, with the final effluent COD ≤ 70 mg / L and the total COD removal rate reaching 88%; the total NH3-N removal rate is 89%, and the total oil removal rate is 93%; the membrane fouling cycle is extended to 95 days, the industrial salt recovery rate is 86%, and the miscellaneous salt yield is 9%. The overall treatment cost is slightly higher than that of Example 1 by 3% (due to the increased operating cost caused by the increased current density), but it is still 17% lower than that of the traditional method, making it suitable for shale gas drilling wastewater treatment scenarios with high COD loads.

[0044] Example 3 The method for comprehensive treatment of shale gas field drilling wastewater in this embodiment refers to Embodiment 1; the only difference between Embodiment 3 and Embodiment 1 is that the hydraulic retention time of the anaerobic hydrolysis acidification process in S3 is shortened to 16 h, and the effluent BOD5 / COD ratio is 0.3.

[0045] Treatment effect: The final effluent quality can still meet the emission standards shown in Table 2 of Example 1. Only the system's resistance to shock load is slightly lower than that of Example 1. The membrane fouling cycle is about 75 days, which is suitable for the treatment of shale gas drilling wastewater with low COD load and short hydraulic retention time.

[0046] Comparative Example 1 The traditional shale gas drilling wastewater treatment process is adopted, and the specific process is as follows: oil collection and removal → homogenization and conditioning → chemical coagulation and sedimentation → conventional activated sludge biochemical treatment → ultrafiltration → single-stage reverse osmosis → evaporation and drying. It does not have the anaerobic hydrolysis acidification unit and MVR evaporation crystallization unit of this application, and the electrocatalytic oxidation is replaced by conventional Fenton oxidation (Fe²+ dosage 500mg / L, H2O2 dosage 1000mg / L).

[0047] Treatment results: The final effluent COD is ≤120mg / L, which does not meet the DB50 / 1806-2025 indirect discharge limit (100mg / L); the total removal rate of NH3-N is 75%, the total removal rate of oil is 82%, and the total removal rate of TDS is 78%; the membrane fouling cycle is only 30 days, requiring frequent cleaning and replacement of membrane elements; there is no salt resource recovery, the miscellaneous salt yield is 35%, the treatment cost is 20% higher than that of Example 1 of this application, and the operation stability is poor, which is easily affected by water quality fluctuations.

[0048] Comparative Example 2 The method for comprehensive treatment of shale gas field drilling wastewater in this comparative example is the same as that in Example 1; the only difference between Comparative Example 2 and Example 1 is that the electrocatalytic oxidation deep treatment in S4 is not performed.

[0049] Treatment results: The biological effluent directly enters the ultrafiltration unit, and the recalcitrant organic matter cannot be effectively removed. The final effluent COD is ≤110mg / L, close to the discharge standard limit; the total removal rate of NH3-N is 82%, the total removal rate of oil is 90%, and the total removal rate of TDS is 87%; the membrane fouling cycle is shortened to 45 days (due to the adhesion of recalcitrant organic matter), the industrial salt recovery rate is 83%, and the miscellaneous salt production rate is 12%. Although the treatment cost is reduced by 8% compared with Example 1, the treatment effect does not meet the standard and cannot meet the stable discharge requirements, highlighting the necessity of the electrocatalytic oxidation unit.

[0050] In summary, by adjusting the core parameters, Examples 1-3 of this application can be adapted to different water quality loads and treatment requirements, and can achieve compliant discharge and salt resource utilization. Compared with the traditional process (Comparative Example 1) and the simplified process (Comparative Example 2), this application has significant advantages in pollutant removal rate, operational stability, resource utilization effect and cost control, and the technical solution is more practical and advanced.

[0051] The above description is merely a preferred embodiment of the present invention. It should be understood that the present invention is not limited to the forms disclosed herein and should not be construed as excluding other embodiments. It can be used in various other combinations, modifications, and environments, and can be altered within the scope of the concept described herein through the above teachings or related technologies or knowledge. Modifications and variations made by those skilled in the art that do not depart from the spirit and scope of the present invention should be within the protection scope of the appended claims.

Claims

1. A comprehensive treatment system for shale gas drilling wastewater, characterized in that, Includes the following steps: S1. After the shale gas drilling wastewater is collected in the unloading pool, a high-efficiency oil-water separator is used for preliminary oil removal. S2. The effluent after S1 treatment is passed sequentially through a homogenization equalization tank and a multi-functional pretreatment tank for water quality homogenization and fine pretreatment. S3. The effluent after S2 treatment is subjected to anaerobic hydrolysis acidification and AO biological treatment in sequence; S4. Electrocatalytic oxidation treatment is performed on the biochemical effluent after S3 treatment; S5. The effluent after S4 treatment is sequentially subjected to tubular membrane ultrafiltration, nanofiltration, and two-stage reverse osmosis. After treatment, the resulting clean water enters the external discharge monitoring tank. S6. The concentrated water produced in S5 is sent to an MVR evaporator for evaporation, concentration, crystallization and drying to obtain industrial salt. The mother liquor produced is added to a mother liquor evaporator to produce mixed salt.

2. The integrated processing system according to claim 1, characterized in that, In S1, the oil content in the effluent from the high-efficiency oil-water separator is <50 mg / L.

3. The integrated processing system according to claim 1, characterized in that, In S2, the multifunctional pretreatment tank includes an air flotation unit, a chemical dosing and mixing unit, and a sedimentation unit.

4. The integrated processing system according to claim 3, characterized in that, The air flotation unit contains at least polyaluminum chloride and anionic polyacrylamide. Based on the volume of influent, the dosage of polyaluminum chloride is 20-150 mg / L, and the dosage of anionic polyacrylamide is 0.5-5 mg / L. And / or, at least sodium carbonate and sodium hydroxide are added to the chemical dosing mixing unit, and the hydraulic residence time of the chemical dosing mixing unit is 10 to 30 minutes; And / or, the hydraulic retention time of the sedimentation unit is 2 to 4 hours.

5. The integrated processing system according to claim 1, characterized in that, In S3, the anaerobic hydrolysis acidification is completed in an anaerobic hydrolysis acidification tank. The steps include: adding anaerobic granular sludge at 15%~35% (v / v) of the effective volume of the anaerobic hydrolysis acidification tank, controlling the temperature at 25~35℃, and the hydraulic retention time at 24~48 h to perform anaerobic hydrolysis acidification on the effluent after S2 treatment.

6. The integrated processing system according to claim 1, characterized in that, In S3, during the AO biochemical treatment process, the dissolved oxygen in the anoxic zone is controlled below 0.2 mg / L, the dissolved oxygen in the aerobic zone is controlled at 2~4 mg / L, the hydraulic retention time in the anoxic zone is controlled at 2~4 h, the hydraulic retention time in the aerobic zone is controlled at 6~12 h, and the temperature is controlled at 25~35℃. During this period, 0.05%~0.2% (v / v) of anaerobic / aerobic microbial complex nutrients and 0.5%~3% (v / v) of corn steep liquor are added every 24~48 h.

7. The integrated processing system according to claim 1, characterized in that, In S4, the electrocatalytic oxidation treatment is performed using ruthenium-iridium titanium-titanium electrode plates with a plate spacing of 10–20 mm and a current density of 10–30 mA / cm². 3 The reaction time is 15-60 minutes.

8. The integrated processing system according to claim 1, characterized in that, In S5, the tubular membrane ultrafiltration process is carried out using a tubular membrane with a pore size of 0.05~0.2 μm at an operating pressure of 0.2~0.4 MPa; And / or, in S5, the nanofiltration membrane treatment is carried out using a spiral wound membrane with a molecular weight cutoff of 150-300 Da at an operating pressure of 1.2-1.8 MPa; And / or, the first-stage operating pressure of the two-stage reverse osmosis membrane is 3.5~4.5 MPa, and the second-stage operating pressure is 5.0~6.0 MPa.

9. The integrated processing system according to claim 1, characterized in that, In S6, the temperature for evaporation, concentration, and crystallization is 70~90℃; And / or, in S6, the drying method includes centrifugal drying.

10. The application of the method according to any one of claims 1 to 9 in the treatment of shale gas wastewater or the preparation of industrial salt from shale gas wastewater.

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