Shale gas fracturing flow-back wastewater treatment system and method

Abstract

This invention belongs to the field of oil and gas field wastewater treatment, and specifically relates to a shale gas fracturing flowback wastewater treatment system and method, including treatment units such as equalization and homogenization, dissolved air flotation, multi-stage chemical sedimentation, coagulation separation, electrochemical catalytic oxidation, membrane treatment, and magnetron evaporation crystallization. The dissolved air flotation unit generates micro-nano bubbles through a Venturi-cyclone coupling device, achieving efficient flotation in conjunction with PAC / PAM; the multi-stage sedimentation tank adds softening agents in stages and implements closed-loop control to remove soluble barium, boron, fluorides, and hardness; the electrochemical oxidation unit enhances mass transfer through ultrasound to degrade recalcitrant organic matter; the membrane treatment uses an integrated system of hollow fiber membrane bioreactor, spiral wound ultrafiltration membrane, and antifouling reverse osmosis membrane to achieve compliant wastewater discharge; the magnetron evaporation crystallizer utilizes the synergistic regulation of magnetic field gradient and temperature gradient to control the crystallization process, achieving membrane separation and fractional treatment of concentrated brine to produce high-purity salt products. This invention can effectively achieve the standard discharge treatment of shale gas fracturing backflow wastewater and the resource utilization of evaporated crystallized salt. It is also applicable to the treatment of shale gas production wastewater with similar water quality characteristics, and has good prospects for promotion and application.

Description

Technical Field

[0001] This invention relates to the field of oil and gas field wastewater treatment, and in particular to a shale gas fracturing backflow wastewater treatment system and method. Background Technology

[0002] Shale gas is a beneficial supplement to conventional natural gas. Sichuan and Chongqing are the main battlegrounds for the commercial development of shale gas in my country, and are focusing on building a national natural gas (shale gas) production base with a capacity of hundreds of billions of cubic meters in the Sichuan-Chongqing region to ensure national energy security. Large-scale hydraulic fracturing is currently the dominant technology for successful shale gas extraction both domestically and internationally. Hydraulic fracturing fluid is mainly composed of water and sand (approximately 98%), containing various organic treatment agents and inorganic salt additives, such as corrosion inhibitors, surfactants, drag reducers, acids, and flow improvers. Furthermore, different geological structures, different fracturing fluid systems, and different flowback processes lead to significant spatial and temporal variations in pollutants in the flowback fluid. Fracturing flowback wastewater is characterized by large volume (the flowback volume accounts for an average of 5%-30% of the fracturing fluid), high salinity, diverse pollutants, high concentration of organic pollutants, poor biodegradability, and high treatment difficulty. In addition, inconsistent discharge standards pose a significant challenge to achieving compliant discharge treatment of fracturing flowback wastewater. Most technologies for treating shale gas fracturing backflow wastewater to meet discharge standards are still in the laboratory research or pilot-scale stages. Currently, apart from a very few shale gas fields that have built compliant treatment systems for wastewater directly discharged into natural water bodies (treatment processes include "coagulation sedimentation + Fenton oxidation + electrolytic oxidation + multi-media filtration + ultrafiltration + reverse osmosis + MVR", "chemical coagulation + electrolytic oxidation + breakpoint chlorination + solid-liquid separation + four-effect evaporation", "air flotation + catalytic oxidation coupled with chemical softening and hardening removal + ultrafiltration + resin softening + electrodialysis + MBR + reverse osmosis + multi-effect evaporation crystallization", "homogenization and conditioning + flocculation air flotation + ozone catalytic oxidation + softening + tubular membrane filtration + nanofiltration + reverse osmosis + MVR", etc.), the effluent quality mainly complies with the "Integrated Wastewater Discharge Standard" (GB In addition to the Class I emission standard (8978-1996) with a chloride emission limit of ≤350mg / L, other shale gas field (block) fracturing backflow wastewater is mainly pre-treated and then transported by tanker truck to surrounding towns or industrial wastewater treatment plants for further treatment before being discharged. It mainly follows the wastewater treatment plant discharge standards, which has put some pressure on these wastewater treatment plants and ecological environment supervision.

[0003] To strengthen the management of shale gas fracturing runoff wastewater treatment, Sichuan and Chongqing have jointly formulated a unified mandatory standard for shale gas extraction wastewater pollutant discharge. The Sichuan provincial standard was released in December 2024, and the Chongqing municipal standard is scheduled for release in March 2025, with implementation commencing on July 1, 2025. This standard covers 21 indicators, including chemical oxygen demand (COD), fluoride, chloride, total dissolved solids (TDS), boron, and soluble barium, setting new requirements for the compliant treatment of shale gas fracturing runoff wastewater. Therefore, how to achieve large-scale, low-carbon, economical, and efficient treatment and compliant discharge of shale gas fracturing runoff wastewater has become a critical challenge that urgently needs to be addressed. Summary of the Invention

[0004] To address the aforementioned key technical issues, this invention provides a shale gas fracturing flowback wastewater treatment system and method. By coupling the process parameters of each treatment unit and coordinating the material / energy flow, it achieves large-scale, low-carbon, economical, and efficient treatment and discharge of shale gas fracturing flowback wastewater that meets emission standards.

[0005] The objective of this invention is achieved through the following technical solution:

[0006] A shale gas fracturing flowback wastewater treatment system and method, characterized in that:

[0007] The following treatment units are connected sequentially by pipelines along the wastewater treatment flow direction:

[0008] Equalization tank (S1): Equipped with stratified baffles, a dynamic equalization system at the bottom of the tank, and an online water quality monitoring system;

[0009] The dissolved air flotation unit (S2) equipped with a dissolved air release device is configured to generate bubbles with a particle size of 10-50μm through a micro-nano bubble generator, and add PAC and ionic PAM for flotation. The micro-nano bubble generator includes a combination structure of a Venturi jet generator and a cyclone cutter.

[0010] The multi-stage series chemical precipitation softening reaction tank (S3) is equipped with an online monitoring and control system for the reaction environment, with each stage equipped with a mechanical stirring device, pH sensor, conductivity meter and automatic dosing unit interlocked with the sensor. Sodium sulfate, sodium carbonate and calcium hydroxide are added in stages through a preset program to remove soluble barium, boron and fluoride and hardness.

[0011] The coagulation and separation composite device (S4) is configured as a gradient dosing system with three dosing ports, and a multi-stage solid-liquid separation structure including an inclined plate sedimentation zone and a tubular centrifuge;

[0012] The electrochemical catalytic oxidation reactor (S5) is configured as a plate electrode assembly with staggered titanium-based ruthenium-iridium coated anodes and graphite cathodes, and integrates an auxiliary mass transfer device with a fixed ultrasonic transducer.

[0013] The membrane treatment integrated module (S6) includes a submerged PVDF hollow fiber membrane bioreactor, a spiral wound ultrafiltration membrane module, and an antifouling reverse osmosis membrane stack with an antiscalant dosing device connected in sequence.

[0014] The magnetron evaporator crystallizer (S7) is configured with a spiral mass transfer channel with rare earth permanent magnets arranged in a Halbach array and a falling film vacuum evaporation system.

[0015] The system, through the coupling of process parameters of each unit and the coordination of material / energy flow, ensures that the effluent COD is ≤30mg / L, chloride ≤500mg / L, soluble barium ≤2mg / L, boron ≤2mg / L, fluoride ≤10mg / L, TDS ≤1000mg / L, total hardness ≤150mg / L, and benzene series compounds are not detected, meeting or exceeding the direct discharge limits in Table 1 of the "Sichuan Province Shale Gas Extraction Water Pollutant Discharge Standard" (DB51 / 3203-2024).

[0016] The equalization tank (S1) has a multi-layered horizontal guide plate. The distance between the upper guide plate and the bottom of the tank is 1 / 5-1 / 3 of the total height of the tank. The guide plate is tilted at an angle of 15-30°, and a zigzag flow channel is formed between adjacent guide plates. The perforated aeration pipe at the bottom of the dynamic equalization system has a hole diameter of Φ3-5mm and a hole spacing of 50-100mm. The aeration intensity is controlled by a variable frequency blower at 0.05-0.15m3 / (m2·min). The online monitoring module collects parameters such as pH, COD, chloride, total dissolved solids (TDS), and flow rate in real time, with a data collection frequency of no less than once every 30 minutes.

[0017] The Venturi jet nozzle diameter to inlet pipe diameter ratio of the dissolved air flotation unit (S2) is 1:3-1:5, the dissolved air-water reflux ratio (volume ratio) is 30%-50%, the working pressure is 0.35-0.45MPa, the PAC dosage is 20-40mg / L, the PAM dosage is 0.8-1.5mg / L, and the bubble surface zeta potential is -25mV to -35mV (controlled by adding anionic surfactants).

[0018] The mechanical agitator blade linear velocity of the chemical precipitation softening reaction tank (S3) is controlled at 0.8-1.5 m / s; sodium sulfate (concentration 5-15%) is added to the primary reaction tank, and the dosage is controlled by pH sensor interlock to maintain the pH in the range of 9.0-10.5, with a hydraulic retention time of 30-45 min; sodium carbonate (concentration 8-20%) is added to the secondary reaction tank, and the pH is controlled in the range of 10.5-11.5, with a hydraulic retention time of 20-35 min; calcium hydroxide (concentration 15-30%) is added to the tertiary reaction tank, and the pH is controlled in the range of 11.5-12.5, with a hydraulic retention time of 15-25 min. An online hardness monitor is installed between the secondary and tertiary reaction tanks (forming a closed-loop control with the dosing pump).

[0019] The coagulation-separation composite unit (S4) uses three dosing ports: the first port adds PAC (concentration 5-10wt%, dosage 50-200mg / L); the second port adds ionic PAM (concentration 0.1-0.5wt%, dosage 3-10mg / L); and the third port adds a pH adjuster (concentration 5-15wt%, pH value controlled at 7.5-9.5). The distance between each dosing port is 3-5 times the pipe diameter. A diaphragm pump is used to achieve an adjustable dosing rate of 0.5-2.0L / min. The mixing reaction zone uses a three-stage baffle mixer with baffle angles... The angle of inclination is 60-75°, the spacing between adjacent baffles is 100-150mm, and the hydraulic retention time is 2-5 minutes. The inclined plate is made of polypropylene composite material, with a plate length of 1.2-1.8m, an inclination angle of 55-60°, a plate spacing of 50-80mm, and a hydrophilic coating on the surface. The upward flow velocity is controlled at 0.8-1.2mm / s. The tubular centrifuge is connected to the bottom sludge discharge port of the sedimentation zone through a guide channel. The length-to-diameter ratio of the centrifuge drum is 15:1-20:1, the rotation speed is 8000-12000rpm, and it is equipped with an automatic slag discharge device with a slag discharge cycle of 15-30 minutes / time.

[0020] The plate electrode assembly of the electrochemical catalytic oxidation reactor (S5) has a single plate thickness of 2±0.1mm, an electrode spacing of 5-10mm, a current density of 15-25mA / cm2, and a fixed ultrasonic device with a working frequency of 28kHz±5% and a power density of 0.5-1.2W / cm3. The oxidation-reduction potential in the reactor is maintained in the range of 800-1200mV in real time by an ORP sensor.

[0021] The PVDF hollow fiber membrane in the membrane treatment integrated module (S6) has an average pore size of 0.1-0.2 μm, a porosity of 60-80%, an inner diameter of 0.8-1.2 mm, and an outer diameter of 1.5-2.0 mm, and adopts an external pressure filtration method. The reverse osmosis membrane is a seawater desalination-grade polyamide composite membrane with a rejection rate >99.5% and a permeate rate >80%. The effective area of ​​a single membrane element is ≥37 m², the operating pressure is 4.0-5.5 MPa, and the temperature tolerance range is 5-45℃. H tolerance range 2-11; set up a membrane fouling early warning system based on transmembrane pressure difference, permeate flow rate changes and data acquisition, with a data acquisition frequency of no less than once / minute. When the transmembrane pressure difference increases by 15% or the permeate flow rate decreases by 20%, trigger an alternating cleaning program of citric acid (concentration 2-5wt%) and sodium hypochlorite (effective chlorine concentration 0.1-0.5wt%). The number of alternating cleaning cycles is set to acid washing: alkaline washing = 2:1, and the cleaning flow rate is controlled at a membrane surface flow rate of 1.5-2.0 m / s.

[0022] The spiral mass transfer channel of the magnetically controlled evaporator crystallizer (S7) is made of 316L stainless steel with a pitch of 150-200mm, a channel diameter of Φ300-500mm, a wall thickness of 2.0±0.1mm, and an inner surface treated with electrolytic polishing (Ra≤0.8μm). A permanent magnet array with a magnetic field gradient of 0.5-0.7T / cm and a pole spacing of 20-30mm is installed inside the channel. The evaporation system maintains a temperature gradient of 70-85℃ (preheating section 70-75℃ (30%), evaporation section 78-82℃ (50%), crystallization section 83-85℃ (20%), with a temperature control accuracy of ±0.5℃). It is equipped with a hydrocyclone crystallizer with a centrifugal force ≥2000g, and the outlet is connected to a vibrating fluidized bed dryer. The sodium chloride purity in the crystallized product is ≥95%.

[0023] The beneficial effects of this invention are as follows:

[0024] This invention provides a shale gas fracturing backflow wastewater treatment system and method that combines regulation and homogenization, dissolved air flotation, multi-stage chemical precipitation, coagulation separation, electrochemical catalytic oxidation, membrane treatment, and magnetically controlled evaporation crystallization. The system first pre-treats the wastewater using regulation and homogenization, dissolved air flotation, multi-stage chemical precipitation, coagulation separation, and electrochemical catalytic oxidation. Then, it concentrates the wastewater using an integrated membrane treatment system (membrane bioreactor + ultrafiltration + reverse osmosis). Finally, the concentrated wastewater undergoes magnetically controlled evaporation crystallization. This system achieves coupling of process parameters across treatment units and synergy of material / energy flows, improving the synergistic removal of multiple pollutants and ensuring that the treated effluent meets the "Sichuan Province Shale Gas Extraction Water Pollutant Discharge Standard" (DB51 / 3203-20). 24) The direct emission limits in Table 1 are significantly reduced, and energy consumption and direct treatment costs are greatly reduced. The online water quality monitoring system of the equalization tank (S1), the online monitoring and control system of the reaction environment of the chemical precipitation softening reaction tank (S3), the real-time ORP control of the electrochemical catalytic oxidation reactor (S5), and the membrane fouling early warning system of the membrane treatment integrated module (S6) provided by this invention realize intelligent control of the entire wastewater treatment process and ensure the stability of the system. This invention provides a magnetically controlled evaporator crystallizer (S7) that can achieve zero discharge of reverse osmosis concentrated wastewater through complete crystallization and produce high-purity crystalline sodium chloride that can be utilized by resources. This invention is also applicable to the treatment of shale gas production wastewater with similar water quality characteristics. Attached Figure Description

[0025] Figure 1 This is a process flow diagram of a shale gas fracturing flowback wastewater treatment system and method. Detailed Implementation

[0026] The technical solution of this invention is: a shale gas fracturing flowback wastewater treatment system and method, comprising the following treatment units connected sequentially by pipelines along the wastewater treatment flow direction:

[0027] Equalization tank (S1): Equipped with stratified baffles, a dynamic equalization system at the bottom of the tank, and an online water quality monitoring system;

[0028] The dissolved air flotation unit (S2) equipped with a dissolved air release device is configured to generate bubbles with a particle size of 10-50μm through a micro-nano bubble generator, and add PAC and ionic PAM for flotation. The micro-nano bubble generator includes a combination structure of a Venturi jet generator and a cyclone cutter.

[0029] The multi-stage series chemical precipitation softening reaction tank (S3) is equipped with an online monitoring and control system for the reaction environment, with each stage equipped with a mechanical stirring device, pH sensor, conductivity meter and automatic dosing unit interlocked with the sensor. Sodium sulfate, sodium carbonate and calcium hydroxide are added in stages through a preset program to remove soluble barium, boron and fluoride and hardness.

[0030] The coagulation and separation composite device (S4) is configured as a gradient dosing system with three dosing ports, and a multi-stage solid-liquid separation structure including an inclined plate sedimentation zone and a tubular centrifuge;

[0031] The electrochemical catalytic oxidation reactor (S5) is configured as a plate electrode assembly with staggered titanium-based ruthenium-iridium coated anodes and graphite cathodes, and integrates an auxiliary mass transfer device with a fixed ultrasonic transducer.

[0032] The membrane treatment integrated module (S6) includes a submerged PVDF hollow fiber membrane bioreactor, a spiral wound ultrafiltration membrane module, and an antifouling reverse osmosis membrane stack with an antiscalant dosing device connected in sequence.

[0033] The magnetron evaporator crystallizer (S7) is configured with a spiral mass transfer channel with rare earth permanent magnets arranged in a Halbach array and a falling film vacuum evaporation system.

[0034] The system, through the coupling of process parameters of each unit and the coordination of material / energy flow, ensures that the effluent COD is ≤30mg / L, chloride ≤500mg / L, soluble barium ≤2mg / L, boron ≤2mg / L, fluoride ≤10mg / L, TDS ≤1000mg / L, total hardness ≤150mg / L, and benzene series compounds are not detected, meeting or exceeding the direct discharge limits in Table 1 of the "Sichuan Province Shale Gas Extraction Water Pollutant Discharge Standard" (DB51 / 3203-2024).

[0035] The equalization tank (S1) has a multi-layered horizontal guide plate. The distance between the upper guide plate and the bottom of the tank is 1 / 5-1 / 3 of the total height of the tank. The guide plate is tilted at an angle of 15-30°, and a zigzag flow channel is formed between adjacent guide plates. The perforated aeration pipe at the bottom of the dynamic equalization system has a hole diameter of Φ3-5mm and a hole spacing of 50-100mm. The aeration intensity is controlled by a variable frequency blower at 0.05-0.15m3 / (m2·min). The online monitoring module collects parameters such as pH, COD, chloride, total dissolved solids (TDS), and flow rate in real time, with a data collection frequency of no less than once every 30 minutes.

[0036] The Venturi jet nozzle diameter to inlet pipe diameter ratio of the dissolved air flotation unit (S2) is 1:3-1:5, the dissolved air-water reflux ratio (volume ratio) is 30%-50%, the working pressure is 0.35-0.45MPa, the PAC dosage is 20-40mg / L, the PAM dosage is 0.8-1.5mg / L, and the bubble surface zeta potential is -25mV to -35mV (controlled by adding anionic surfactants).

[0037] The mechanical agitator blade linear velocity of the chemical precipitation softening reaction tank (S3) is controlled at 0.8-1.5 m / s; sodium sulfate (concentration 5-15%) is added to the primary reaction tank, and the dosage is controlled by pH sensor interlock to maintain the pH in the range of 9.0-10.5, with a hydraulic retention time of 30-45 min; sodium carbonate (concentration 8-20%) is added to the secondary reaction tank, and the pH is controlled in the range of 10.5-11.5, with a hydraulic retention time of 20-35 min; calcium hydroxide (concentration 15-30%) is added to the tertiary reaction tank, and the pH is controlled in the range of 11.5-12.5, with a hydraulic retention time of 15-25 min. An online hardness monitor is installed between the secondary and tertiary reaction tanks (forming a closed-loop control with the dosing pump).

[0038] The coagulation-separation composite unit (S4) uses three dosing ports: the first port adds PAC (concentration 5-10wt%, dosage 50-200mg / L); the second port adds ionic PAM (concentration 0.1-0.5wt%, dosage 3-10mg / L); and the third port adds a pH adjuster (concentration 5-15wt%, pH value controlled at 7.5-9.5). The distance between each dosing port is 3-5 times the pipe diameter. A diaphragm pump is used to achieve an adjustable dosing rate of 0.5-2.0L / min. The mixing reaction zone uses a three-stage baffle mixer with baffle angles... The angle of inclination is 60-75°, the spacing between adjacent baffles is 100-150mm, and the hydraulic retention time is 2-5 minutes. The inclined plate is made of polypropylene composite material, with a plate length of 1.2-1.8m, an inclination angle of 55-60°, a plate spacing of 50-80mm, and a hydrophilic coating on the surface. The upward flow velocity is controlled at 0.8-1.2mm / s. The tubular centrifuge is connected to the bottom sludge discharge port of the sedimentation zone through a guide channel. The length-to-diameter ratio of the centrifuge drum is 15:1-20:1, the rotation speed is 8000-12000rpm, and it is equipped with an automatic slag discharge device with a slag discharge cycle of 15-30 minutes / time.

[0039] The plate electrode assembly of the electrochemical catalytic oxidation reactor (S5) has a single plate thickness of 2±0.1mm, an electrode spacing of 5-10mm, a current density of 15-25mA / cm2, and a fixed ultrasonic device with a working frequency of 28kHz±5% and a power density of 0.5-1.2W / cm3. The oxidation-reduction potential in the reactor is maintained in the range of 800-1200mV in real time by an ORP sensor.

[0040] The PVDF hollow fiber membrane in the membrane treatment integrated module (S6) has an average pore size of 0.1-0.2 μm, a porosity of 60-80%, an inner diameter of 0.8-1.2 mm, and an outer diameter of 1.5-2.0 mm, and adopts an external pressure filtration method. The reverse osmosis membrane is a seawater desalination-grade polyamide composite membrane with a rejection rate >99.5% and a permeate rate >80%. The effective area of ​​a single membrane element is ≥37 m², the operating pressure is 4.0-5.5 MPa, and the temperature tolerance range is 5-45℃. H tolerance range 2-11; set up a membrane fouling early warning system based on transmembrane pressure difference, permeate flow rate changes and data acquisition, with a data acquisition frequency of no less than once / minute. When the transmembrane pressure difference increases by 15% or the permeate flow rate decreases by 20%, trigger an alternating cleaning program of citric acid (concentration 2-5wt%) and sodium hypochlorite (effective chlorine concentration 0.1-0.5wt%). The number of alternating cleaning cycles is set to acid washing: alkaline washing = 2:1, and the cleaning flow rate is controlled at a membrane surface flow rate of 1.5-2.0 m / s.

[0041] The spiral mass transfer channel of the magnetically controlled evaporator crystallizer (S7) is made of 316L stainless steel with a pitch of 150-200mm, a channel diameter of Φ300-500mm, a wall thickness of 2.0±0.1mm, and an inner surface treated with electrolytic polishing (Ra≤0.8μm). A permanent magnet array with a magnetic field gradient of 0.5-0.7T / cm and a pole spacing of 20-30mm is installed inside the channel. The evaporation system maintains a temperature gradient of 70-85℃ (preheating section 70-75℃ (30%), evaporation section 78-82℃ (50%), crystallization section 83-85℃ (20%), with a temperature control accuracy of ±0.5℃). It is equipped with a hydrocyclone crystallizer with a centrifugal force ≥2000g, and the outlet is connected to a vibrating fluidized bed dryer. The sodium chloride purity in the crystallized product is ≥95%.

[0042] The present invention will be further described below with reference to the embodiments, but the present invention is not limited to these embodiments.

[0043] Example 1: This example describes the treatment of shale gas fracturing flowback wastewater, including the following treatment units connected sequentially by pipelines along the wastewater treatment flow direction:

[0044] (1) Equalization tank (S1): Equipped with layered guide plates, a dynamic equalization system at the bottom of the tank and an online water quality monitoring system; The layered guide plates are three-layered horizontal guide plates, with the distance between the upper guide plate and the bottom of the tank being 1 / 3 of the total height of the tank, and the guide plate tilt angle being 30° (based on computational fluid dynamics (CFD) simulation optimization, with turbulence intensity up to ≥0.15m² / s²), forming a broken flow channel between adjacent guide plates; The bottom perforated aeration pipe of the dynamic equalization system at the bottom of the tank has a hole diameter of Φ3mm, a hole spacing of 90mm, and a variable frequency blower aeration intensity of 0.06m³ / (m²·min) (when it is lower than 0.06m³ / (m²·min), the equalization efficiency decreases by 15%-20%); The online monitoring module collects parameters such as pH, COD, chloride, total dissolved solids (TDS), and flow rate in real time at a frequency of 1 time / 15min.

[0045] (2) A dissolved air flotation unit (S2) equipped with a dissolved air release device is configured to generate bubbles with a particle size of 10-50μm through a micro-nano bubble generator and add PAC and anionic PAM for flotation. The micro-nano bubble generator includes a combination structure of a Venturi jet injector and a cyclone cutter (the outlet of the Venturi jet injector and the inlet of the cyclone cutter are connected by a flange, the inner diameter of the cyclone cutter is twice the diameter of the jet injector throat, the blade inclination angle is 45°, and the cyclone angular velocity is ≥120rad / s); the ratio of the Venturi jet injector throat diameter to the inlet pipe diameter is 1:3, the dissolved air water reflux ratio is 30%, the working pressure is 0.35MPa, the PAC dosage is 35mg / L, the PAM dosage is 1.0mg / L, and the anionic surfactant is added to adjust the bubble surface ζ potential to -25mV.

[0046] (3) A multi-stage series chemical precipitation softening reaction tank (S3), each stage of the reaction tank is independently equipped with an online monitoring and control system for the reaction environment, including a mechanical stirring device, a pH sensor, a conductivity meter, and an automatic dosing unit interlocked with the sensor. Sodium sulfate, sodium carbonate, and calcium hydroxide are added in stages according to a preset program to remove soluble barium, boron, fluoride, and hardness. The blade linear velocity of the mechanical stirring device is controlled at 1.0 m / s. Sodium sulfate (concentration 8%) is added to the first-stage reaction tank, and the dosage is controlled by the pH sensor to maintain the pH at a certain level. The pH was 9.0±0.2, and the hydraulic retention time was 30 min. Sodium carbonate (concentration 12%) was added to the secondary reaction tank, and the pH was controlled at 10.5±0.2 with a hydraulic retention time of 25 min. Calcium hydroxide (concentration 15%) was added to the tertiary reaction tank, and the pH was controlled at 11.5±0.2 with a hydraulic retention time of 20 min (determined by optimization using a reaction kinetic model (quasi-secondary rate constant k=0.15L / (mol·min))). The hardness of the effluent was ensured to be ≤150 mg / L by an online hardness monitor.

[0047] (4) The coagulation and separation composite device (S4) is configured as a gradient dosing system with three dosing ports, and a multi-stage solid-liquid separation structure including an inclined plate sedimentation zone and a tubular centrifuge; the first dosing port adds PAC (concentration 8wt%, dosage 80mg / L), the second dosing port adds PAM (concentration 0.2wt%, dosage 2mg / L), and the third dosing port adds pH adjuster (concentration 10wt%, pH value controlled at 8.0±0.1). The distance between each dosing port is 5 times the pipe diameter, and the dosing rate is 1.0. L / min; the mixing reaction zone adopts a three-stage baffle mixer with a baffle angle of 65°, a spacing of 100mm between adjacent baffles, and a hydraulic retention time of 5min; the inclined plate is made of polypropylene composite material with an inclination angle of 55°, a plate spacing of 60mm, and a surface treated with a hydrophilic coating, and the upward flow velocity is controlled at 1.0mm / s; the tubular centrifuge is connected to the bottom sludge discharge port of the sedimentation zone through a guide channel, the centrifuge drum has a length-to-diameter ratio of 20:1, a rotation speed of 10000rpm, and is equipped with an automatic slag discharge device with a slag discharge cycle of 30min / time.

[0048] (5) Electrochemical catalytic oxidation reactor (S5) is configured as a plate electrode group with staggered titanium-based ruthenium-iridium coated anodes and graphite cathodes, and an auxiliary mass transfer device with a fixed ultrasonic transducer. The plate electrode group has a single plate thickness of 2.0 mm, an electrode spacing of 8 mm, a current density of 20 mA / cm2, and an integrated fixed ultrasonic transducer with a working frequency of 28 kHz and a power density of 1.0 W / cm3. The oxidation-reduction potential in the reactor is maintained at about 1000 ± 50 mV in real time by an ORP sensor.

[0049] (6) Membrane treatment integrated module (S6), comprising a submerged PVDF hollow fiber membrane bioreactor, a spiral wound ultrafiltration membrane module, and an antifouling reverse osmosis membrane stack with antiscalant dosing device connected in sequence; the PVDF hollow fiber membrane has an average pore size of 0.1 μm, a porosity of 70%, an inner diameter of 1.2 mm and an outer diameter of 2.0 mm, and adopts an external pressure filtration method; the reverse osmosis membrane is selected from Dow SW30HRLE-400 seawater desalination grade polyamide composite membrane (test conditions: 25℃, 32000 mg / L). NaCl solution (desalination rate ≥99.7%), effective area of ​​a single membrane element is 37 m2, operating pressure is 4.5 MPa, and permeate rate is 82%; a membrane fouling early warning system is set up. When the transmembrane pressure difference increases by 15% or the permeate flow rate decreases by 20%, an alternating cleaning program of citric acid (concentration 5 wt%) and sodium hypochlorite (effective chlorine concentration 0.5 wt%) is triggered. The number of alternating cleaning cycles is set to acid wash: alkali wash = 2:1, and the cleaning flow rate is controlled at a membrane surface velocity of 2.0 m / s.

[0050] (7) Magnetically controlled evaporator crystallizer (S7) is configured as a spiral mass transfer channel with rare earth permanent magnets arranged in a Halbach array and a falling film vacuum evaporation system; the spiral mass transfer channel is made of 316L stainless steel with a pitch of 200mm, a channel diameter of Φ500mm, a wall thickness of 2.0mm, and an inner surface treated by electrolytic polishing (Ra≤0.8μm); a permanent magnet array with a magnetic field gradient of 0.7T / cm is set in the channel, with a magnetic pole spacing of 30mm; the evaporation system maintains a temperature gradient of 70-85℃ (preheating section 70-75℃ (30%), evaporation section 78-82℃ (50%), crystallization section 83-85℃ (20%), with a temperature control accuracy of ±0.5℃), equipped with a cyclone crystallizer with a centrifugal force ≥2000g, and the outlet is connected to a vibrating fluidized bed dryer, with a sodium chloride purity ≥95% in the crystallized product.

[0051] Example 2: This example follows the steps of Example 1, with some parameters adjusted:

[0052] (1) Equalization tank (S1): The guide plate is tilted at an angle of 20° and the spacing is 1 / 4 of the tank height (CFD simulated turbulence intensity ≥ 0.18 m² / s²), the aeration intensity is 0.12 m³ / (m²·min), and the monitoring frequency is once every 20 min;

[0053] (2) Dissolved air flotation unit (S2): Venturi jet nozzle throat diameter to inlet pipe diameter ratio 1:4 (inlet pipe Φ100mm → throat Φ25mm), PAC 35mg / L, PAM 1.0mg / L, ζ potential -28mV (adjusted by adding sodium dodecyl sulfate 0.5mg / L).

[0054] (3) Chemical precipitation softening reaction tank (S3): First-stage reaction: 10% sodium sulfate, pH controlled at 10.0±0.2, hydraulic retention time 40min (blade linear velocity 1.2m / s); Second-stage reaction: 15% sodium carbonate, pH controlled at 11.0±0.2, hydraulic retention time 25min; Third-stage reaction: 25% calcium hydroxide, pH controlled at 12.2±0.2, hydraulic retention time 18min.

[0055] (4) Coagulation and separation composite device (S4): Gradual addition of PAC 130mg / L (8wt%) → PAM 4mg / L (0.3wt%) → NaOH to adjust pH to 8.8±0.1, upward flow velocity in inclined plate sedimentation zone 1.0mm / s, tubular centrifuge speed 10000rpm (length-to-diameter ratio 18:1).

[0056] (5) Electrochemical catalytic oxidation reactor (S5): Electrode spacing 6mm, current density 20mA / cm², ultrasonic power density 1.2W / cm³;

[0057] (6) Membrane treatment integrated module (S6): Reverse osmosis operating pressure 4.8MPa, scale inhibitor (aminophosphonic acid) dosage 4mg / L, after membrane fouling warning is triggered, use 3wt% citric acid (flow rate 1.8m / s) and 0.3wt% sodium hypochlorite for alternating cleaning;

[0058] (7) Magnetically controlled evaporator crystallizer (S7): magnetic field gradient 0.6T / cm, evaporation temperature gradient preheating 73℃ → evaporation 80℃ → crystallization 84℃, centrifugal force of cyclone crystallizer 2500g, crystallized salt purity 96.8%.

[0059] Example 3: This example follows the steps of Example 1, with some parameters adjusted:

[0060] (1) Dissolved air flotation unit (S2): The dissolved air water reflux ratio is increased to 45%, and the working pressure is 0.42MPa;

[0061] (2) Chemical precipitation softening reaction tank (S3): The HRT of the three-stage reaction tank is extended to 22 min;

[0062] (3) Coagulation and separation composite device (S4): The dosage of PAC and PAM is increased to 200 mg / L (8 wt%) and 7 mg / L (0.3 wt%), respectively.

[0063] (4) Membrane treatment integrated module (S6): The reverse osmosis adopts two-stage series connection, the operating pressure is 5.3MPa, and the scale inhibitor dosage is increased to 6mg / L;

[0064] (5) Magnetically controlled evaporator crystallizer (S7): The magnetic field gradient in the magnetically controlled evaporation section is enhanced to 0.7T / cm, and the purity of the crystallized salt is ≥96.5%.

[0065] Comparative Example 1: The magnetically controlled evaporator crystallizer (S7) was removed and replaced with a conventional multi-effect evaporator (without magnetic field enhancement). Other procedures were performed according to Example 1. The scaling cycle of the evaporator was shortened from 30 days in Example 1 to 9 days (data verified by 3 repeated experiments, CV=4.2%), the steam consumption per ton of water increased from 1.8 t / m³ to 2.25 t / m³, and the purity of the crystallized salt decreased from 95% to 88.5% (containing CaSO4 impurities >8%).

[0066] Comparative Example 2: The gradient dosing sequence in the chemical precipitation softening reaction tank (S3) was changed to calcium hydroxide → sodium sulfate → sodium carbonate, while other steps were performed as in Example 1. The barium ion removal rate decreased from 99.8% in Example 1 to 72% (requiring an additional extension of HRT to 35 min to recover to 90%), and the sodium sulfate dosage was increased from 10 kg / m³ to 17 kg / m³ (comparative experimental data SD = ±0.5 kg / m³).

[0067] Comparative Example 3: In the electrochemical catalytic oxidation reactor (S5), the ultrasonic device was removed, the electrode spacing was increased to 15 mm, and other procedures were performed according to Example 1. The COD removal efficiency decreased from 84% to 58% (under the same residence time), the electrode passivation rate was accelerated, and the cleaning cycle was shortened from 72 hours to 8 hours (comparative experimental data CV=6.1%).

[0068] In Examples 1-3, shale gas fracturing backflow wastewater with different water quality characteristics was treated by coupling the process parameters of each treatment unit and coordinating the material flow / energy flow. All 21 indicators, including COD, chloride, fluoride, and TDS, met or exceeded the direct discharge limits in Table 1 of the "Sichuan Province Shale Gas Extraction Water Pollutant Discharge Standard" (DB51 / 3203-2024) (see table below). Compared with Comparative Examples 1-3, Examples 1-3, through innovative practices such as precise parameter matching (e.g., the correlation between the inclination angle of the guide plate and the turbulence intensity in S1), reaction sequence optimization (e.g., the suppression of the encapsulation effect by gradient dosing in S3), and energy field coupling (S5 ultrasound + S7 magnetic field), have achieved multiple objectives, including improved treatment efficiency (e.g., a 5-15% increase in the removal rate of key indicators such as COD and TDS), reduced operating costs (e.g., a 29-44% reduction in energy consumption per ton of water and a 20-46% saving in reagent costs), and enhanced system stability (e.g., a 233% extension in equipment scaling cycle and a 50% increase in electrode life). They have fully leveraged the synergistic effect of multiple physical fields among the various treatment units, solving the low-carbon, economical, and efficient treatment challenges of high salt content, high TDS, recalcitrant organic matter, and complex ions in fracturing flowback wastewater. This provides a scalable and replicable solution for the compliant treatment and discharge of shale gas fracturing flowback wastewater.

[0069]

[0070] The above embodiments are merely illustrative of the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. 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 essence and scope of the present invention. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A shale gas fracturing flowback wastewater treatment system, characterized in that, The following treatment units are connected sequentially by pipelines along the wastewater treatment flow direction: Equalization tank: Equipped with stratified baffles, a dynamic equalization system at the bottom of the tank, and an online water quality monitoring system; A dissolved air flotation unit equipped with a dissolved air energy release device is configured to generate bubbles with a particle size of 10-50μm through a micro-nano bubble generator, and to add PAC and ionic PAM for flotation. The micro-nano bubble generator includes a combination structure of a Venturi jet generator and a cyclone cutter. The multi-stage series chemical precipitation softening reaction tanks are equipped with an online monitoring and control system for the reaction environment. Each stage of the reaction tank is independently equipped with a mechanical stirring device, pH sensor, conductivity meter and automatic dosing unit interlocked with the sensor. Sodium sulfate, sodium carbonate and calcium hydroxide are added in stages through a preset program to remove soluble barium, boron and fluoride and hardness. The coagulation and separation composite device is configured as a gradient dosing system with three dosing ports, and a multi-stage solid-liquid separation structure including an inclined plate sedimentation zone and a tubular centrifuge; An electrochemical catalytic oxidation reactor is configured with a plate electrode assembly having staggered titanium-based ruthenium-iridium coated anodes and graphite cathodes, and an auxiliary mass transfer device with a fixed ultrasonic transducer. The membrane treatment integrated module includes a submerged PVDF hollow fiber membrane bioreactor, a spiral wound ultrafiltration membrane module, and an antifouling reverse osmosis membrane stack with an antiscalant dosing device connected in sequence. The magnetron evaporator crystallizer is configured with a spiral mass transfer channel with rare earth permanent magnets arranged in a Halbach array and a falling film vacuum evaporation system. The shale gas fracturing backflow wastewater treatment system, through the coupling of process parameters of each unit and the synergy of material flow / energy flow, ensures that the effluent COD ≤ 30 mg / L, chloride ≤ 500 mg / L, soluble barium ≤ 2 mg / L, boron ≤ 2 mg / L, fluoride ≤ 10 mg / L, TDS ≤ 1000 mg / L, total hardness ≤ 150 mg / L, and benzene series compounds are not detected, meeting or exceeding the direct discharge limits in Table 1 of the "Sichuan Province Shale Gas Extraction Water Pollutant Discharge Standard" (DB51 / 3203-2024). The spiral mass transfer channel of the magnetically controlled evaporator crystallizer is made of 316L stainless steel with a pitch of 150-200mm, a channel diameter of Φ300-500mm, and a wall thickness of 2.0±0.1mm. The inner surface is electrolytically polished to achieve Ra≤0.8μm. A permanent magnet array with a magnetic field gradient of 0.5-0.7T / cm and a pole spacing of 20-30mm is set inside the channel. The evaporation system maintains a temperature gradient of 70-85℃, with the preheating section at 70-75℃ (30%), the evaporation section at 78-82℃ (50%), and the crystallization section at 83-85℃ (20%). The temperature control accuracy is ±0.5℃. It is equipped with a cyclone crystallizer with a centrifugal force ≥2000g, and the outlet is connected to a vibrating fluidized bed dryer. The sodium chloride purity in the crystallized product is ≥95%.

2. The shale gas fracturing flowback wastewater treatment system according to claim 1, characterized in that: The equalization tank features multi-layered horizontal guide plates. The distance between the upper guide plate and the tank bottom is 1 / 5 to 1 / 3 of the total tank height. The guide plates are tilted at an angle of 15-30°, forming a zigzag flow channel between adjacent guide plates. The perforated aeration pipes at the bottom of the dynamic equalization system have a diameter of Φ3-5mm and a spacing of 50-100mm. The aeration intensity is controlled at 0.05-0.15m by a variable frequency blower. 3 / (m 2 The online monitoring module collects pH, COD, chloride, total dissolved solids (TDS), and flow parameters in real time, with a data collection frequency of no less than once every 30 minutes.

3. The shale gas fracturing flowback wastewater treatment system according to claim 1, characterized in that: The ratio of the Venturi jet throat diameter to the inlet pipe diameter of the dissolved air flotation unit is 1:3-1:5, the dissolved air water reflux volume ratio is 30%-50%, the working pressure is 0.35-0.45MPa, the PAC dosage is 20-40mg / L, the PAM dosage is 0.8-1.5mg / L, and the surface zeta potential of the bubbles is adjusted to -25mV to -35mV by adding anionic surfactants.

4. The shale gas fracturing flowback wastewater treatment system according to claim 1, characterized in that: The mechanical agitator in the chemical precipitation softening reaction tank has a paddle linear velocity controlled at 0.8-1.5 m / s. Sodium sulfate at a concentration of 5-15% is added to the primary reaction tank, with the dosage controlled by a pH sensor to maintain the pH within the range of 9.0-10.5, and a hydraulic retention time of 30-45 min. Sodium carbonate at a concentration of 8-20% is added to the secondary reaction tank, maintaining the pH within the range of 10.5-11.5, and a hydraulic retention time of 20-35 min. Calcium hydroxide at a concentration of 15-30% is added to the tertiary reaction tank, maintaining the pH within the range of 11.5-12.5, and a hydraulic retention time of 15-25 min. An online hardness monitor and dosing pump are installed between the secondary and tertiary reaction tanks to form a closed-loop control system.

5. A shale gas fracturing flowback wastewater treatment system according to claim 1, characterized in that: The coagulation and separation composite device adds PAC at a concentration of 5-10 wt% and a dosage of 50-200 mg / L at the first dosing port; adds ionic PAM at a concentration of 0.1-0.5 wt% and a dosage of 3-10 mg / L at the second dosing port; and adds a pH adjuster at a concentration of 5-15 wt%, controlling the pH value at 7.5-9.5 at the third dosing port. The distance between each dosing port is 3-5 times the pipe diameter. A diaphragm pump is provided to achieve an adjustable dosing rate of 0.5-2.0 L / min. The mixing reaction zone uses a three-stage baffle mixer with baffle angles... The angle of inclination is 60-75°, the spacing between adjacent baffles is 100-150mm, and the hydraulic retention time is 2-5 minutes. The inclined plate is made of polypropylene composite material, with a plate length of 1.2-1.8m, an inclination angle of 55-60°, a plate spacing of 50-80mm, and a hydrophilic coating on the surface. The upward flow velocity is controlled at 0.8-1.2mm / s. The tubular centrifuge is connected to the bottom sludge discharge port of the sedimentation zone through a guide channel. The length-to-diameter ratio of the centrifuge drum is 15:1-20:1, the rotation speed is 8000-12000rpm, and it is equipped with an automatic slag discharge device with a slag discharge cycle of 15-30 minutes / time.

6. The shale gas fracturing flowback wastewater treatment system according to claim 1, characterized in that: The plate electrode assembly of the electrochemical catalytic oxidation reactor has a single plate thickness of 2±0.1 mm, an electrode spacing of 5-10 mm, and a current density of 15-25 mA / cm². 2 The fixed ultrasonic device operates at a frequency of 28kHz ± 5% and a power density of 0.5-1.2W / cm². 3 The redox potential inside the reactor is maintained in the range of 800-1200mV in real time by using an ORP sensor.

7. A shale gas fracturing flowback wastewater treatment system according to claim 1, characterized in that: The PVDF hollow fiber membrane in the membrane treatment integrated module has an average pore size of 0.1-0.2 μm, a porosity of 60-80%, an inner diameter of 0.8-1.2 mm, and an outer diameter of 1.5-2.0 mm, and adopts an external pressure filtration method. The reverse osmosis membrane is a seawater desalination-grade polyamide composite membrane with a rejection rate >99.5% and a water production rate >80%, and the effective area of ​​a single membrane element is ≥37 m². 2 Operating pressure: 4.0-5.5 MPa; temperature tolerance range: 5-45℃; pH tolerance range: 2-11; a membrane fouling early warning system based on transmembrane pressure difference, permeate flow rate changes, and data acquisition is set up, with a data acquisition frequency of no less than once per minute. When the transmembrane pressure difference increases by 15% or the permeate flow rate decreases by 20%, an alternating cleaning program of 2-5 wt% citric acid and 0.1-0.5 wt% sodium hypochlorite is triggered. The number of alternating cleaning cycles is set to acid washing:alkali washing = 2:1, and the cleaning flow rate is controlled at a membrane surface velocity of 1.5-2.0 m / s.

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