A method for synergistic oxidation treatment of oilfield wastewater

Abstract

The present application relates to the oilfield wastewater treatment and resource recycling technical field, disclose a kind of oilfield wastewater collaborative oxidation treatment method, comprising the following steps: the oilfield wastewater to be treated is introduced into flocculation reaction tank, and polyaluminium chloride is added to the oilfield wastewater to be treated, and after wastewater pH is adjusted, stirring is carried out, so that suspended oil, emulsified oil, colloidal particles and polyaluminium chloride hydrolysis product in wastewater collide, destabilize, coagulate, and form floc.The present application is coupled by micro-nano bubble enhanced air floatation and electroflocculation-ozone-hydrogen peroxide depth, so that the oilfield wastewater to be treated is sequentially subjected to flocculation, air-type micro-nano bubble air floatation, electroflocculation-ozone-hydrogen peroxide collaborative deep oxidation and standing precipitation treatment, which can eliminate the need for separate oxidation unit, simplify the process flow and improve the integration of the treatment process.

Description

Technical Field

[0001] This invention relates to the field of oilfield wastewater treatment and resource recovery technology, and in particular to a method for synergistic oxidation treatment of oilfield wastewater. Background Technology

[0002] Oilfield wastewater typically contains suspended oil, emulsified oil, dissolved organic matter, polymers, and surfactants. It is characterized by strong emulsification stability, high chemical oxygen demand concentration, poor biodegradability, and large fluctuations in water quality. If not treated adequately, it is difficult to meet the requirements for oilfield reinjection or compliant discharge.

[0003] Currently, oilfield wastewater treatment processes mainly include gravity sedimentation, conventional air flotation, chemical flocculation, biological treatment, electrocoagulation, and advanced ozone oxidation. Some technologies also employ coupled air flotation-oxidation or electrocoagulation-oxidation systems to treat oily wastewater. However, conventional air flotation produces millimeter-sized bubbles with small specific surface areas and fast rising speeds, limiting its ability to capture fine oil droplets and stable emulsions. Chemical flocculation requires large dosages of chemicals, easily generating significant amounts of chemical sludge. Biological treatment is easily inhibited by oil and high-salinity environments, resulting in low removal efficiency for recalcitrant organic matter. Single ozone oxidation systems suffer from low ozone solubility, poor mass transfer efficiency, and insufficient free radical utilization, making it difficult to achieve deep mineralization of high-concentration organic matter. Traditional electrocoagulation processes have shortcomings such as electrode passivation, insufficient floc stability, and limited deep oxidation capabilities.

[0004] Micro- and nanobubbles possess characteristics such as high specific surface area, low buoyancy rate, high interfacial zeta potential, and the generation of reactive oxygen species upon bubble collapse, which can enhance gas-liquid mass transfer and the oxidative degradation of pollutants. However, most existing technologies only apply micro- and nanobubbles to single flotation or single oxidation processes, lacking deep synergistic coupling with electrocoagulation and ozone-hydrogen peroxide advanced oxidation, making it difficult to simultaneously achieve efficient separation of suspended oil and deep mineralization of recalcitrant organic matter. Therefore, it is necessary to provide a synergistic oxidation treatment method for oilfield wastewater to synergistically enhance oil removal, mass transfer, and advanced oxidation processes. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing technologies by proposing a method for the synergistic oxidation treatment of oilfield wastewater.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for co-oxidation treatment of oilfield wastewater includes the following steps: S1. The oilfield wastewater to be treated is introduced into the flocculation reaction tank. Polyaluminum chloride is added to the oilfield wastewater, and the pH of the wastewater is adjusted and stirred so that the suspended oil, emulsified oil, colloidal particles in the wastewater collide, destabilize, and coagulate with the hydrolysis products of polyaluminum chloride to form flocs. S2. The floc-containing wastewater treated in step S1 is transported to the micro-nano bubble flotation unit, and air-type micro-nano bubbles are introduced to allow the air-type micro-nano bubbles to adhere to and encapsulate the flocs and suspended oil and emulsified oil in the wastewater, forming bubble-floc composite scum that floats to the water surface. The scum is then removed to obtain the flotation effluent. S3. The air flotation effluent obtained in step S2 is directly fed into the electrocoagulation-ozone-hydrogen peroxide coupled reactor, and hydrogen peroxide is added to the electrocoagulation-ozone-hydrogen peroxide coupled reactor. Simultaneously, ozone micro-nano bubbles are prepared and introduced into the electrocoagulation-ozone-hydrogen peroxide coupled reactor. The electrocoagulation-ozone-hydrogen peroxide coupled reactor is equipped with an iron anode and a stainless steel cathode, which are connected to a DC power supply. The air flotation effluent undergoes deep oxidation treatment under the synergistic effect of electrocoagulation, ozone micro-nano bubbles and hydrogen peroxide to obtain oxidized effluent. S4. The oxidized effluent obtained in step S3 is allowed to settle in a sedimentation tank. The supernatant from the sedimentation tank is taken as the treated wastewater, and the sludge at the bottom of the sedimentation tank is discharged.

[0007] Preferably, in step S1, the initial pH of the oilfield wastewater to be treated is 8.0~9.0, the suspended oil content is 120~180 mg / L, and the chemical oxygen demand (COD) concentration is 1500~2000 mg / L.

[0008] Preferably, in step S1, the dosage of polyaluminum chloride is 110~130 mg / L, the pH of the wastewater is adjusted to 6.5~7.5 by an acid-base adjuster, and the mixture is stirred at a stirring rate of 150~250 r / min for 8~12 min.

[0009] Preferably, in step S2, the proportion of micron bubbles smaller than 50 μm in the air-type micro-nano bubbles is not less than 93%, the proportion of nano bubbles smaller than 1 μm is not less than 60%, and the Zeta potential of the air-type micro-nano bubbles is maintained at -25 to -35 mV.

[0010] Preferably, in step S2, the air inlet flow rate of the air-type micro / nano bubbles is 180~220 mL / min, and the total air flotation time is 20~30 min.

[0011] Preferably, in step S3, the air flotation effluent enters the electrocoagulation-ozone-hydrogen peroxide coupled reactor directly without intermediate buffering or separate oxidation treatment. In step S3, the concentration of hydrogen peroxide in the electrocoagulation-ozone-hydrogen peroxide coupled reactor is 3.5~4.5 mmol / L, the ozone concentration of the ozone micro-nano bubbles is 8.0~9.0 mg / L, and the bubble flow rate of the ozone micro-nano bubbles is 190~210 mL / min.

[0012] Preferably, in step S3, the electrode spacing between the iron anode and the stainless steel cathode is 0.8~1.2 cm, and the current density of the deep oxidation treatment is 32~40 mA / cm². 2 The pH of the wastewater in the electrocoagulation-ozone-hydrogen peroxide coupled reactor is 8.5~9.5, and the reaction time of the deep oxidation treatment is 25~35 min.

[0013] Preferably, in step S4, the settling time is 25-35 minutes.

[0014] The beneficial effects of this invention are as follows: This invention deeply couples micro-nano bubble enhanced flotation with electrocoagulation-ozone-hydrogen peroxide, allowing the oilfield wastewater to be treated to undergo flocculation, air-type micro-nano bubble flotation, electrocoagulation-ozone-hydrogen peroxide synergistic deep oxidation, and static sedimentation treatment in sequence. This eliminates the need for a separate oxidation unit, simplifies the process, and improves the integration of the treatment process.

[0015] This invention employs air-type micro-nano bubbles for flotation treatment. Compared to conventional millimeter-level ordinary bubble flotation, it enhances the adhesion, entrainment, and flotation separation of suspended oil, emulsified oil, and flocs, increasing the suspended oil removal efficiency by 24.6%, thereby significantly reducing the organic load in subsequent deep oxidation treatments. Through micro-nano bubble-enhanced flotation treatment, this invention can efficiently remove suspended oil, achieving a removal rate of over 97%.

[0016] In step S3 of this invention, ozone micro-nano bubbles are used to participate in deep oxidation treatment. Compared with ozone oxidation alone, ozone micro-nano bubbles can improve the mass transfer efficiency and liquid phase residence time of ozone, promote the generation of active oxygen species, increase the free radical production in the ozone micro-nano system, and significantly improve the chemical oxygen demand removal rate.

[0017] This invention introduces electrocoagulation into an ozone micro-nano bubble-hydrogen peroxide system, through the dissolution of Fe by an iron anode. 2+ Furthermore, in synergy with ozone micro-nano bubbles and hydrogen peroxide, it can improve the oxidative degradation and mineralization of recalcitrant organic matter. Compared with the ozone-hydrogen peroxide system without electrocoagulation, the chemical oxygen demand removal rate increased from 87.05% to 95.95% after coupling electrocoagulation, indicating that there is a synergistic effect among electrocoagulation, ozone micro-nano bubbles, and hydrogen peroxide.

[0018] Under optimized operating parameters, the entire process of this invention produces effluent with suspended oil content below 5 mg / L, chemical oxygen demand (COD) below 50 mg / L, and a total pollutant removal rate exceeding 96%, directly meeting the water quality requirements for oilfield reinjection and external discharge. Compared with traditional segmented treatment processes, this invention reduces reagent usage, sludge generation, and energy consumption, while achieving efficient, stable, low-consumption, and low-pollution oilfield wastewater treatment. Attached Figure Description

[0019] Figure 1 Figure 1 shows the residual amount and removal rate of pollutants after micro-nano bubble flotation. Figure 2 The graph shows the residual amount and removal rate of pollutants after electrocoagulation-ozone micro-nano bubbles-hydrogen peroxide treatment. Figure 3 This is a graph showing the effect of ozone concentration on the organic matter removal rate. Figure 4 This is a graph showing the effect of electrode spacing on the organic matter removal rate. Figure 5 This is a graph showing the effect of current density on the organic matter removal rate. Figure 6 This is a graph showing the effect of wastewater pH on the organic matter removal rate. Figure 7 The graph shows the effect of hydrogen peroxide concentration on the organic matter removal rate. Figure 8 This is a graph showing the effect of ozone micro / nanobubble flow rate on organic matter removal rate. Detailed Implementation

[0020] To provide a clearer understanding of the technical features, objectives, and beneficial effects of this invention, the technical solution of this invention is described in detail below, but this should not be construed as limiting the scope of implementation of this invention. Unless otherwise specified, the methods used in this invention are conventional methods in this technical field. In this invention, materials, reagents, or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0021] In one embodiment, the present invention provides a method for the synergistic oxidation treatment of oilfield wastewater, comprising the following steps: S1. The oilfield wastewater to be treated is introduced into the flocculation reaction tank. Polyaluminum chloride is added to the oilfield wastewater, and the pH of the wastewater is adjusted and stirred so that the suspended oil, emulsified oil, colloidal particles in the wastewater collide, destabilize, and coagulate with the hydrolysis products of polyaluminum chloride to form flocs. S2. The floc-containing wastewater treated in step S1 is transported to the micro-nano bubble flotation unit, and air-type micro-nano bubbles are introduced to allow the air-type micro-nano bubbles to adhere to and encapsulate the flocs and suspended oil and emulsified oil in the wastewater, forming bubble-floc composite scum that floats to the water surface. The scum is then removed to obtain the flotation effluent. S3. The air flotation effluent obtained in step S2 is directly fed into the electrocoagulation-ozone-hydrogen peroxide coupled reactor. Hydrogen peroxide is added to the electrocoagulation-ozone-hydrogen peroxide coupled reactor. The ozone generator and micro-nano bubble generator are turned on to prepare ozone micro-nano bubbles using ozone as the gas source. The prepared ozone micro-nano bubbles are then introduced into the electrocoagulation-ozone-hydrogen peroxide coupled reactor. The electrocoagulation-ozone-hydrogen peroxide coupled reactor is equipped with an iron anode and a stainless steel cathode, which are connected to a DC power supply. The air flotation effluent undergoes deep oxidation treatment under the synergistic effect of electrocoagulation, ozone micro-nano bubbles, and hydrogen peroxide to obtain oxidized effluent. S4. The oxidized effluent obtained in step S3 is allowed to settle in a sedimentation tank. The supernatant from the sedimentation tank is taken as the treated wastewater, and the sludge at the bottom of the sedimentation tank is discharged.

[0022] In a preferred embodiment of the present invention, in step S1, the initial pH of the oilfield wastewater to be treated is 8.0~9.0, the suspended oil content is 120~180 mg / L, and the COD concentration is 1500~2000 mg / L.

[0023] In a preferred embodiment of the present invention, in step S1, the dosage of polyaluminum chloride is 110~130 mg / L, the pH of the wastewater is adjusted to 6.5~7.5 by an acid-base adjuster, and the mixture is stirred at a stirring rate of 150~250 r / min for 8~12 min. In step S1, the dosage of polyaluminum chloride is 120 mg / L, the pH of the wastewater is adjusted to 7.0, and the stirring time is 10 min.

[0024] In a preferred embodiment of the present invention, in step S2, the air-type micro-nano bubbles are prepared by dissolved gas release method, wherein the proportion of micron bubbles smaller than 50 μm is not less than 93%, the proportion of nano bubbles smaller than 1 μm is not less than 60%, and the Zeta potential of the air-type micro-nano bubbles is maintained at -25~-35 mV.

[0025] In a preferred embodiment of the present invention, in step S2, the air intake flow rate of the air-type micro-nano bubbles is 180~220 mL / min, and the total air flotation time is 20~30 min; preferably, the air intake flow rate of the air-type micro-nano bubbles is 200 mL / min, and the total air flotation time is 25 min.

[0026] In a preferred embodiment of the present invention, in step S3, the effluent from the air flotation enters directly into the electrocoagulation-ozone-hydrogen peroxide coupled reactor without intermediate buffering or separate oxidation treatment. In step S3, the concentration of hydrogen peroxide in the electrocoagulation-ozone-hydrogen peroxide coupled reactor is 3.5~4.5 mmol / L, the ozone concentration of the ozone micro-nano bubbles is 8.0~9.0 mg / L, and the bubble flow rate of the ozone micro-nano bubbles is 190~210 mL / min. Preferably, the concentration of hydrogen peroxide is 3.96 mmol / L, the ozone concentration of the ozone micro-nano bubbles is 8.51 mg / L, and the bubble flow rate of the ozone micro-nano bubbles is 198.11 mL / min.

[0027] In a preferred embodiment of the present invention, in step S3, the electrode spacing between the iron anode and the stainless steel cathode is 0.8~1.2 cm, and the current density of the deep oxidation treatment is 32~40 mA / cm². 2 The pH of the wastewater in the electrocoagulation-ozone-hydrogen peroxide coupled reactor is 8.5-9.5, and the reaction time for the deep oxidation treatment is 25-35 min; preferably, the electrode spacing is 1 cm, and the current density for the deep oxidation treatment is 36.02 mA / cm². 2 The wastewater has a pH of 9.0, and the reaction time for the deep oxidation treatment is 30 min.

[0028] In a preferred embodiment of the present invention, in step S4, the settling time is 25-35 min; preferably, the settling time is 30 min, and the suspended oil content of the treated wastewater is less than 5 mg / L and the COD concentration is less than 50 mg / L.

[0029] In one embodiment, the present invention proposes an integrated processing device for implementing the above method, comprising a primary micro-nano bubble flotation chamber, a secondary electrocoagulation coupling reaction chamber, a micro-nano bubble generator, an ozone generator, a DC electrocoagulation power supply, a reagent quantitative dosing component, an online water quality monitoring component, and an exhaust gas collection component; The primary micro-nano bubble flotation chamber is used to receive oilfield raw water and to remove suspended oil by capturing oil flocs through micro-nano bubbles after the addition of polyaluminum chloride. The secondary electrocoagulation coupling reaction chamber is connected to the primary micro-nano bubble flotation chamber and is used to receive the effluent from the primary micro-nano bubble flotation chamber. An iron electrode plate is installed inside the secondary electrocoagulation coupling reaction chamber. The iron electrode plate is connected to the DC electrocoagulation power supply for dissolving Fe by energizing it. 2+This process aims to form an in-situ Fenton-like system and achieve deep mineralization of pollutants by adsorbing organic intermediates through electrolytic flocs.

[0030] In a preferred embodiment of the present invention, the integrated processing device further includes an automatic slag scraping component, which is used to remove the slag formed in the primary micro-nano bubble flotation chamber.

[0031] In a preferred embodiment of the present invention, the integrated treatment device further includes an ozone exhaust gas adsorption component.

[0032] As a preferred embodiment of the present invention, the integrated processing device further includes a pH, ozone and current linkage automatic control system.

[0033] In a preferred embodiment of the present invention, the primary micro-nano bubble flotation chamber and the secondary electrocoagulation coupling reaction chamber are constructed as two self-flowing interconnected structures.

[0034] In a preferred embodiment of the present invention, the integrated processing device is an integrated sealed design.

[0035] The specific implementation of the present invention will be described in detail below with reference to specific embodiments.

[0036] Example 1: This example includes the following steps.

[0037] S1. The oilfield wastewater to be treated is introduced into a flocculation reaction tank. The initial pH of the oilfield wastewater is 8.0, the suspended oil content is 120 mg / L, and the chemical oxygen demand concentration is 1500 mg / L. Polyaluminum chloride is added to the oilfield wastewater at a dosage of 110 mg / L. The pH of the wastewater is adjusted to 6.5 using an acid-base adjuster, and the mixture is stirred at a stirring rate of 150 r / min for 8 min to allow the suspended oil, emulsified oil, colloidal particles, and polyaluminum chloride hydrolysis products in the wastewater to fully collide, destabilize, and coagulate, forming dense flocs with good settling properties, thus providing a basis for subsequent air flotation separation.

[0038] S2. The floc-containing wastewater treated in step S1 is transported to the micro / nano bubble flotation unit. The micro / nano bubble generator is turned on, and air-type micro / nano bubbles are introduced. The air-type micro / nano bubbles are prepared by dissolved air release method, wherein the proportion of micron bubbles smaller than 50 μm is not less than 93%, and the proportion of nano bubbles smaller than 1 μm is not less than 60%. The Zeta potential of the air-type micro / nano bubbles is maintained at -25 mV. The air flow rate of the air-type micro / nano bubbles is controlled at 180 mL / min, and the total flotation time is 20 min. Due to their high specific surface area, low buoyancy rate, and strong interfacial adsorption characteristics, the air-type micro / nano bubbles adhere to and encapsulate the flocs and suspended oil and emulsified oil in the wastewater, forming a stable bubble-floc composite scum that floats to the water surface. After the flotation is completed, the scum is removed to obtain the flotation effluent.

[0039] S3. The air flotation effluent obtained in step S2 is directly fed into the electrocoagulation-ozone-hydrogen peroxide coupled reactor without intermediate buffering or separate oxidation treatment. Hydrogen peroxide is added to the electrocoagulation-ozone-hydrogen peroxide coupled reactor to a concentration of 3.5 mmol / L. The ozone generator and micro / nano bubble generator are turned on to prepare ozone micro / nano bubbles, which are then introduced into the electrocoagulation-ozone-hydrogen peroxide coupled reactor. The ozone concentration of the ozone micro / nano bubbles is 8.0 mg / L, and the bubble flow rate is 190 mL / min. The electrocoagulation-ozone-hydrogen peroxide coupled reactor is equipped with an iron anode and a stainless steel cathode, with a plate spacing of 0.8 cm between them. The iron anode and the stainless steel cathode are connected to a DC power supply, and the current density for the deep oxidation treatment is 32 mA / cm². 2 The pH of the wastewater in the electrocoagulation-ozone-hydrogen peroxide coupled reactor is controlled to 8.5 by a pH adjustment device, and the reaction time of the deep oxidation treatment is 25 min. The air flotation effluent undergoes deep oxidation treatment under the synergistic effect of electrocoagulation, ozone micro-nano bubbles and hydrogen peroxide to obtain oxidized effluent.

[0040] In step S3, the iron anode dissolves Fe under the action of an electric field. 2+ Fe 2+ It participates in the catalytic generation of hydroxyl radicals; the ozone micro-nano bubbles improve ozone mass transfer efficiency, prolong bubble residence time and enhance interfacial reaction. At the same time, the bubble collapse generates local high temperature and active oxygen, which, together with the hydroxyl radicals, deeply oxidize and mineralize refractory organic matter, polymers and surface pollutants, thereby achieving the degradation of chemical oxygen demand.

[0041] S4. The oxidized effluent obtained in step S3 is introduced into a sedimentation tank for settling. The settling time is 25 minutes to allow the flocculants, oxidation byproducts, and residual solids generated in the reaction to settle. The supernatant from the sedimentation tank is taken as treated wastewater, and the sludge at the bottom of the sedimentation tank is discharged. The suspended oil content of the treated wastewater is less than 5 mg / L, and the chemical oxygen demand concentration is less than 50 mg / L, which can be used for oilfield reinjection or discharged in compliance with standards.

[0042] Example 2: This example includes the following steps.

[0043] S1. The oilfield wastewater to be treated is introduced into a flocculation reaction tank. The initial pH of the oilfield wastewater to be treated is 8.0~9.0, the suspended oil content is 120~180 mg / L, and the chemical oxygen demand concentration is 1500~2000 mg / L. Polyaluminum chloride is added to the oilfield wastewater to be treated at a dosage of 120 mg / L. The pH of the wastewater is adjusted to 7.0 using an acid-base adjuster, and the mixture is stirred at a stirring rate of 150~250 r / min for 10 min to allow the suspended oil, emulsified oil, colloidal particles and polyaluminum chloride hydrolysis products in the wastewater to fully collide, destabilize and coagulate, forming a dense floc with good settling performance, providing a basis for subsequent air flotation separation.

[0044] S2. The floc-containing wastewater treated in step S1 is transported to the micro / nano bubble flotation unit. The micro / nano bubble generator is turned on, and air-type micro / nano bubbles are introduced. The air-type micro / nano bubbles are prepared by dissolved air release method, wherein the proportion of micron bubbles smaller than 50 μm is not less than 93%, and the proportion of nano bubbles smaller than 1 μm is not less than 60%. The zeta potential of the air-type micro / nano bubbles is maintained at -25~-35 mV. The air flow rate of the air-type micro / nano bubbles is controlled at 200 mL / min, and the total flotation time is 25 min. Due to their high specific surface area, low buoyancy rate, and strong interfacial adsorption characteristics, the air-type micro / nano bubbles adhere to and encapsulate the flocs and suspended oil and emulsified oil in the wastewater, forming a stable bubble-floc composite scum that floats to the water surface. After the flotation is completed, the scum is removed to obtain the flotation effluent.

[0045] S3. The air flotation effluent obtained in step S2 is directly fed into the electrocoagulation-ozone-hydrogen peroxide coupled reactor without intermediate buffering or separate oxidation treatment. Hydrogen peroxide is added to the electrocoagulation-ozone-hydrogen peroxide coupled reactor to achieve a concentration of 3.96 mmol / L. The ozone generator and micro / nano bubble generator are turned on to prepare ozone micro / nano bubbles, which are then introduced into the electrocoagulation-ozone-hydrogen peroxide coupled reactor. The ozone concentration of the ozone micro / nano bubbles is 8.51 mg / L, and the bubble flow rate is 198.11 mL / min. The electrocoagulation-ozone-hydrogen peroxide coupled reactor is equipped with an iron anode and a stainless steel cathode, with a 1 cm distance between the plates. The iron anode and the stainless steel cathode are connected to a DC power supply, and the current density for the deep oxidation treatment is 36.02 mA / cm². 2 The pH of the wastewater in the electrocoagulation-ozone-hydrogen peroxide coupled reactor is controlled to be 9.0 by a pH adjustment device, and the reaction time of the deep oxidation treatment is 30 min. The air flotation effluent undergoes deep oxidation treatment under the synergistic effect of electrocoagulation, ozone micro-nano bubbles and hydrogen peroxide to obtain oxidized effluent.

[0046] In step S3, the iron anode dissolves Fe under the action of an electric field. 2+ Fe 2+ It participates in the catalytic generation of hydroxyl radicals; the ozone micro-nano bubbles improve ozone mass transfer efficiency, prolong bubble residence time and enhance interfacial reaction. At the same time, the bubble collapse generates local high temperature and active oxygen, which, together with the hydroxyl radicals, deeply oxidize and mineralize refractory organic matter, polymers and surface pollutants, thereby achieving the degradation of chemical oxygen demand.

[0047] S4. The oxidized effluent obtained in step S3 is introduced into a sedimentation tank for settling. The settling time is 30 minutes to allow the flocculants, oxidation byproducts, and residual solids generated in the reaction to settle. The supernatant from the sedimentation tank is taken as treated wastewater, and the sludge at the bottom of the sedimentation tank is discharged. The suspended oil content of the treated wastewater is less than 5 mg / L, and the chemical oxygen demand concentration is less than 50 mg / L, which can be used for oilfield reinjection or discharged in compliance with standards.

[0048] Example 3: This example includes the following steps.

[0049] S1. The oilfield wastewater to be treated is introduced into a flocculation reaction tank. The initial pH of the oilfield wastewater is 9.0, the suspended oil content is 180 mg / L, and the chemical oxygen demand concentration is 2000 mg / L. Polyaluminum chloride is added to the oilfield wastewater at a dosage of 130 mg / L. The pH of the wastewater is adjusted to 7.5 using an acid-base adjuster, and the mixture is stirred at a stirring rate of 250 r / min for 12 min to allow the suspended oil, emulsified oil, colloidal particles, and polyaluminum chloride hydrolysis products in the wastewater to fully collide, destabilize, and coagulate, forming dense flocs with good settling properties, thus providing a foundation for subsequent air flotation separation.

[0050] S2. The floc-containing wastewater treated in step S1 is transported to the micro / nano bubble flotation unit. The micro / nano bubble generator is turned on, and air-type micro / nano bubbles are introduced. The air-type micro / nano bubbles are prepared by dissolved air release method, wherein the proportion of micron bubbles smaller than 50 μm is not less than 93%, and the proportion of nano bubbles smaller than 1 μm is not less than 60%. The Zeta potential of the air-type micro / nano bubbles is maintained at -35 mV. The air flow rate of the air-type micro / nano bubbles is controlled at 220 mL / min, and the total flotation time is 30 min. Due to their high specific surface area, low buoyancy rate, and strong interfacial adsorption characteristics, the air-type micro / nano bubbles adhere to and encapsulate the flocs and suspended oil and emulsified oil in the wastewater, forming a stable bubble-floc composite scum that floats to the water surface. After the flotation is completed, the scum is removed to obtain the flotation effluent.

[0051] S3. The air flotation effluent obtained in step S2 is directly fed into the electrocoagulation-ozone-hydrogen peroxide coupled reactor without intermediate buffering or separate oxidation treatment. Hydrogen peroxide is added to the electrocoagulation-ozone-hydrogen peroxide coupled reactor to a concentration of 4.5 mmol / L. The ozone generator and micro / nano bubble generator are turned on to prepare ozone micro / nano bubbles, which are then introduced into the electrocoagulation-ozone-hydrogen peroxide coupled reactor. The ozone concentration of the ozone micro / nano bubbles is 9.0 mg / L, and the bubble flow rate is 210 mL / min. The electrocoagulation-ozone-hydrogen peroxide coupled reactor is equipped with an iron anode and a stainless steel cathode, with a plate spacing of 1.2 cm between them. The iron anode and the stainless steel cathode are connected to a DC power supply, and the current density for the deep oxidation treatment is 40 mA / cm². 2The pH of the wastewater in the electrocoagulation-ozone-hydrogen peroxide coupled reactor is controlled to be 9.5 by a pH adjustment device, and the reaction time of the deep oxidation treatment is 35 min. The air flotation effluent undergoes deep oxidation treatment under the synergistic effect of electrocoagulation, ozone micro-nano bubbles and hydrogen peroxide to obtain oxidized effluent.

[0052] In step S3, the iron anode dissolves Fe under the action of an electric field. 2+ Fe 2+ It participates in the catalytic generation of hydroxyl radicals; the ozone micro-nano bubbles improve ozone mass transfer efficiency, prolong bubble residence time and enhance interfacial reaction. At the same time, the bubble collapse generates local high temperature and active oxygen, which, together with the hydroxyl radicals, deeply oxidize and mineralize refractory organic matter, polymers and surface pollutants, thereby achieving the degradation of chemical oxygen demand.

[0053] S4. The oxidized effluent obtained in step S3 is introduced into a sedimentation tank for settling. The settling time is 35 minutes, allowing the flocculants, oxidation byproducts, and residual solids generated in the reaction to settle. The supernatant from the sedimentation tank is taken as treated wastewater, and the sludge at the bottom of the sedimentation tank is discharged. The suspended oil content of the treated wastewater is less than 5 mg / L, and the chemical oxygen demand concentration is less than 50 mg / L, which can be used for oilfield reinjection or discharged in compliance with standards.

[0054] To verify the oil removal advantages of air-type micro-nano bubbles compared to conventional millimeter-sized ordinary bubbles, the synergistic effect of electrocoagulation and the ozone micro-nano bubble-hydrogen peroxide system, and the mass transfer enhancement and oxidation enhancement effect of ozone micro-nano bubbles compared to ozone alone, the following comparative experiments were conducted based on the above embodiments.

[0055] Comparative Experiment Example 1: Under the same experimental conditions, the oil removal efficiency of conventional millimeter-level ordinary bubble flotation treatment and air-type micro / nano bubble flotation treatment were compared. The dosage of polyaluminum chloride was 120 mg / L, the wastewater pH was 7.0, the temperature was 25℃, the total flotation time was 25 min, the air inlet flow rate was 200 mL / min, and a process of flocculation followed by flotation was adopted. The wastewater from an oilfield in Jilin Province was used as the treatment target. The initial suspended oil content of the wastewater was 150 mg / L, and the chemical oxygen demand (COD) concentration was 1814 mg / L.

[0056] Under the above conditions, after air flotation treatment using conventional millimeter-level ordinary bubbles, the suspended oil content in the effluent was 73.2 mg / L, with a suspended oil removal rate of 51.20%, a dissolved oil removal rate of 13.60%, and a chemical oxygen demand (COD) removal rate of 31.64%. After air flotation treatment using air-type micro / nano bubbles, the suspended oil content in the effluent was 4.28 mg / L, with a suspended oil removal rate of 97.10%, representing an overall oil removal efficiency improvement of 24.6%. The removal effects of dissolved oil and COD were also simultaneously improved. The above results indicate that air-type micro-nano bubbles have smaller particle size, larger specific surface area, and longer residence time in water, and their Zeta potential is -25 to -35 mV. This enhances the collision, adhesion, and capture effects between oil droplets and flocs, forming stable bubble-floc composite scum. In contrast, ordinary bubbles have a fast rising speed and short gas-liquid contact time, making it difficult to capture fine emulsified oil. Their ability to separate suspended oil and organic pollutants is weaker than that of air-type micro-nano bubble flotation, and it increases the reagent and energy consumption for subsequent ozone and electrocoagulation treatments.

[0057] Comparative Experiment Example 2: Under the same process conditions of ozone micro-nano bubble activated hydrogen peroxide system, the treatment effects of the ozone micro-nano bubble-hydrogen peroxide process without electrocoagulation and the system with iron electrode electrocoagulation coupling were compared. The ozone concentration was 9.85 mg / L, the hydrogen peroxide concentration was 2.87 mmol / L, the pH was 7.7, the temperature was 33.88℃, and the treated wastewater was oilfield wastewater with a chemical oxygen demand (COD) concentration of 847 mg / L after flotation.

[0058] Under the above conditions, without the introduction of electrocoagulation, the chemical oxygen demand (COD) removal rate using only the ozone micro-nano bubble-hydrogen peroxide process was 87.05%. After introducing the iron electrode electrocoagulation coupling system, the COD removal rate was 87.05% at an ozone concentration of 8.51 mg / L and a current density of 36.02 mA / cm². 2 Treatment under conditions of 3.96 mmol / L hydrogen peroxide concentration and 198.11 mL / min ozone micro-nanobubble flow rate achieved a chemical oxygen demand (COD) removal rate of 95.95%, significantly improving the pollutant mineralization capacity. This is attributed to the dissolution of Fe from the iron anode during the electrocoagulation process. 2+ This system can construct a Fenton-like microenvironment in situ and synergistically enhance oxidative degradation with hydroxyl radicals generated by the collapse of ozone micro-nano bubbles. Simultaneously, the iron hydroxide flocs generated by electrolysis can adsorb recalcitrant organic intermediates in the water and are separated by flotation with the assistance of electrolytic microbubbles. In contrast, using only the ozone-hydrogen peroxide system lacks the synergistic effect of electrochemical catalysis and flocculation adsorption, resulting in limited free radical generation, incomplete removal of persistent organic pollutants, and higher chemical oxygen demand in the effluent, making it difficult to stably meet the requirements for oilfield reinjection discharge.

[0059] Comparative Experiment Example 3: Under consistent reaction conditions, the oxidation treatment effects of a standalone ozone system and the ozone micro / nano bubble process were compared. The hydrogen peroxide concentration was 2.87 mmol / L, pH was 7.7, temperature was 33.88℃, reaction time was 30 min, and oilfield wastewater with a post-flotation chemical oxygen demand (COD) concentration of 847 mg / L was used as the treatment target.

[0060] Under the aforementioned conditions, when using a standalone ozone system for treatment, due to ozone's poor water solubility, small gas-liquid contact area, and low mass transfer efficiency, the treatment mainly relies on direct ozone oxidation, resulting in a low generation of hydroxyl radicals and a chemical oxygen demand (COD) removal rate of 56.2%. However, when using ozone micro-nanobubble technology, the ultrafine bubbles increase the gas-liquid specific surface area, prolonging the ozone residence time in the liquid phase. Bubble collapse creates a localized high-temperature, high-pressure environment, promoting ozone decomposition to generate hydroxyl radicals, which, in conjunction with hydrogen peroxide, form a chain oxidation reaction, increasing the COD removal rate to 70.6%. Therefore, compared to ozone oxidation alone, ozone micro-nanobubble technology can enhance oxidative degradation efficiency, promote the decomposition of recalcitrant aromatic organic pollutants in wastewater, and reduce the load on subsequent treatment processes.

[0061] To further investigate the effects of each process parameter in step S3 on the organic matter removal efficiency of the electrocoagulation-ozone micro / nanobubble-hydrogen peroxide system, and to provide a basis for the selection of process parameters such as ozone concentration, electrode spacing, current density, wastewater pH, hydrogen peroxide concentration, and ozone micro / nanobubble flow rate, the following single-factor influence experiment was conducted while keeping other conditions relatively fixed.

[0062] Single-factor influence experiment example 1: Under the conditions of fixed reaction temperature, initial pH, hydrogen peroxide dosage, gas inlet flow rate, electrode spacing and current density, the ozone concentration was changed to investigate the effect of ozone concentration on the removal efficiency of organic matter.

[0063] Ozone concentration is a core process parameter affecting the organic matter removal efficiency of the electrocoagulation-ozone micro / nanobubble-hydrogen peroxide system, and it plays a regulatory role in the generation rate of reactive oxygen species and the oxidative degradation efficiency of pollutants in the system. To investigate the influence of this factor, the following parameters were used: reaction temperature 25℃, initial pH 7.0, hydrogen peroxide dosage 2 mmol / L, gas inlet flow rate 250 mL / min, electrode spacing 1.5 cm, and current density 30 mA / cm². 2 Under fixed conditions, the ozone was treated with micro-nano bubbles for 30 min, and samples were taken every 5 min for analysis. The ozone concentration was varied by adjusting the output power of the ozone generator to obtain ozone concentrations of 1 mg / L, 3 mg / L, 5 mg / L, 7 mg / L, and 10 mg / L, and the effect of different ozone concentrations on the removal efficiency of organic matter was investigated.

[0064] Experimental results show that... Figure 3 As shown, in the electrocoagulation-ozone micro / nanobubble activated hydrogen peroxide system, ozone concentration significantly affects the chemical oxygen demand (COD) removal efficiency. With the gradual increase of ozone concentration, the COD removal rate shows an increasing trend throughout the aeration process. For example, after 5 minutes of aeration, the COD removal rate increased from 11.7% at an ozone concentration of 1 mg / L to 29.4% at an ozone concentration of 10 mg / L, indicating that a higher ozone concentration can increase the oxidation rate of organic matter in the initial stage of the system. Electrocoagulation provides a fundamental support for COD removal in the system. It adsorbs some organic matter in the water by generating hydroxide precipitates from iron ions, creating reaction conditions for the ozone-activated hydrogen peroxide oxidation process, enabling the organic matter in the system to be oxidized by free radicals.

[0065] With prolonged aeration time, the effect of ozone concentration on the chemical oxygen demand (COD) removal rate varied further. After 15 minutes of aeration, the COD removal rate increased from 30.9% to 68.2% with increasing ozone concentration. This indicates that higher ozone concentrations not only promote the activation of hydrogen peroxide and the generation of hydroxyl radicals, but also, in conjunction with the anolyte and suspended particles generated by electrocoagulation, improve the contact efficiency and oxidative degradation efficiency of organic matter. In the later stages of the reaction, although the removal rate still increased with increasing ozone concentration under various ozone concentration conditions, the rate of increase gradually slowed down. For example, after 30 minutes of aeration, the COD removal rate increased from 53.4% ​​at an ozone concentration of 1 mg / L to 95.1% at an ozone concentration of 10 mg / L, reflecting that the enhancing effect of higher ozone concentrations on the system tended to stabilize in the later stages, possibly limited by the decrease in the concentration of oxidizable organic matter and the increase in free radical consumption.

[0066] Single-factor influence experiment example 2: Under the conditions of fixed reaction temperature, initial pH, hydrogen peroxide dosage, gas intake, ozone concentration and current density, the electrode spacing was changed to investigate the effect of electrode spacing on the removal efficiency of organic matter.

[0067] The electrode spacing is a key process parameter affecting the organic matter removal efficiency of the electrocoagulation-ozone micro / nanobubble-hydrogen peroxide system. By changing the electric field distribution and mass transfer conditions between the electrodes, it directly affects the floc formation characteristics and the synergistic effect of active species, thereby regulating the oxidative degradation efficiency of organic matter. To investigate the influence of this factor, the reaction was conducted at a reaction temperature of 25℃, an initial pH of 7.0, a hydrogen peroxide dosage of 2 mmol / L, a gas inlet flow rate of 250 mL / min, an ozone concentration of 10 mg / L, and a current density of 30 mA / cm². 2Under fixed conditions, the samples were treated with micro-nano bubbles for 30 min, and samples were taken for testing every 5 min. The electrode installation position was adjusted to change the electrode spacing, with electrode spacings of 0.5 cm, 1.0 cm, 1.5 cm, 2.0 cm and 2.5 cm, to investigate the effect of different electrode spacings on the removal efficiency of organic matter.

[0068] Experimental results show that... Figure 4 As shown, the chemical oxygen demand (COD) removal rate gradually increased with the extension of aeration time, and the differences under different electrode spacings gradually became more prominent as the reaction progressed. When the electrode spacing was 1 cm, the system exhibited the best COD removal performance, with a removal rate of 97.83% after 30 min, which was higher than other spacing groups; the 0.5 cm spacing group was the second best, with a removal rate of 96.41% after 30 min. As the electrode spacing increased to 1.5 cm, 2 cm, and 2.5 cm, the COD removal rate decreased stepwise, dropping to 95.11%, 89.33%, and 83.99% respectively after 30 min. Under the condition of small spacing, the electric field strength was higher, and the metal ions dissolved from the anode could quickly diffuse into the liquid phase, synergistically generating hydroxyl radicals with hydrogen peroxide, ozone, and micro / nano bubbles, while providing suitable space for floc growth, achieving a synergistic effect of oxidative degradation and flocculation sedimentation; while excessively large electrode spacing would weaken the electric field strength, prolong the diffusion path of metal ions, reduce the efficiency of active species generation, lead to the attenuation of electrocoagulation contribution, and ultimately weaken the overall removal capacity of the system for organic matter.

[0069] Single-factor influence experiment example 3: Investigating the effect of current density on the removal efficiency of organic matter. Current density is a core process parameter affecting the removal efficiency of organic matter in the electrocoagulation-ozone micro / nanobubble-hydrogen peroxide system. By controlling the electrode reaction rate, it directly affects the floc formation characteristics, the efficiency of active species generation, and their synergistic oxidation, thereby regulating the degradation pathway and removal efficiency of organic matter.

[0070] In this single-factor influence experiment (Example 3), under fixed conditions of reaction temperature 25℃, initial pH 7.0, hydrogen peroxide dosage 2 mmol / L, gas inlet flow rate 250 mL / min, ozone concentration 10 mg / L, and electrode spacing 1.5 cm, micro / nanobubbles were used for treatment for 30 min, with samples taken every 5 min for analysis. The current density was varied by adjusting the power supply output, with current densities of 10 mA / cm². 2 20 mA / cm 2 30 mA / cm 2 40 mA / cm 2 and 50 mA / cm 2 The effect of different current densities on the removal efficiency of organic matter was investigated.

[0071] Experimental results are as follows Figure 5 As shown, the chemical oxygen demand (COD) removal rate continuously increases with prolonged aeration time, and exhibits a trend of first increasing and then slowly decreasing with increasing current density. When the current density is 30 mA / cm²... 2 At that time, the system exhibited the best chemical oxygen demand (COD) removal performance, reaching a removal rate of 98.33% after 30 min; 20 mA / cm 2 With 40 mA / cm 2 The next group showed removal rates of 91.22% and 97.83% at 30 min, respectively; while at 10 mA / cm 2 The group exhibited the lowest removal efficiency, at only 73.99% after 30 min, at 50 mA / cm². 2 The group was 95.11%, slightly below 30 mA / cm². 2 Group.

[0072] The above phenomena can be attributed to the following: under lower current density conditions, the anolyte metal ion dissolution rate is insufficient, the electrocoagulation effect is weak, and the electron transfer efficiency is limited, resulting in insufficient generation of strong oxidizing species such as hydroxyl radicals, making it difficult to efficiently degrade organic matter; as the current density increases to 30 mA / cm², the degradation of organic matter becomes more efficient. 2 The metal ion dissolution rate and electron transfer efficiency reach a relatively optimal balance, providing metal ion carriers for floc growth and enabling efficient synergy with hydrogen peroxide, ozone, and micro / nano bubbles to enhance free radical chain reactions, achieving synergistic effects of oxidative degradation and flocculation precipitation; when the current density is further increased to 40~50 mA / cm 2 Excessive current can easily trigger aggravated side reactions, including oxygen evolution reaction and hydrogen evolution reaction, leading to decreased current efficiency and increased energy consumption. At the same time, excessively rapid dissolution of metal ions may cause the floc structure to become loose and the sedimentation performance to deteriorate, thereby weakening the overall removal efficiency of the system.

[0073] Single-factor influence experiment example 4: Investigating the effect of wastewater pH on the removal efficiency of organic matter. Wastewater pH is a key process parameter affecting the removal efficiency of organic matter in the electrocoagulation-ozone micro-nanobubble-hydrogen peroxide system. By changing the pH of the solution... + / OH - The balance directly affects the speciation and distribution of reactive oxygen species, the formation characteristics of flocs, and their synergistic oxidation effects, thereby regulating the degradation pathway and removal efficiency of organic matter.

[0074] In this single-factor influence experiment example 4, the reaction temperature was 25℃, the ozone concentration was 10 mg / L, the hydrogen peroxide dosage was 2 mmol / L, the gas inlet flow rate was 250 mL / min, the electrode spacing was 1.5 cm, and the current density was 30 mA / cm². 2Under fixed conditions, the solution was treated with micro-nano bubbles for 30 min, with samples taken every 5 min for analysis. The effect of different pH values ​​on the removal efficiency of organic matter was investigated by adjusting the initial pH value of the solution to 3, 5, 7, 9, and 11.

[0075] Experimental results are as follows Figure 6 As shown, the effect of different pH values ​​on the chemical oxygen demand (COD) removal rate exhibits a trend of first increasing and then decreasing, with the effect becoming increasingly pronounced with prolonged aeration time. When pH is 9, the system demonstrates the best COD removal performance, achieving a removal rate of 93.44% after 30 minutes; pH 7 is the second best, with a removal rate of 88.66% after 30 minutes; however, the removal efficiency decreases under acidic and strongly alkaline conditions, with removal rates of 66.4%, 78.33%, and 83.2% after 30 minutes at pH 3, 5, and 11, respectively.

[0076] The above pattern can be attributed to the following: Under weakly alkaline conditions, metal ions dissolved at the anolyte readily hydrolyze to form highly active hydroxyl complexes and hydroxide flocs. These provide an interface for the catalytic activation of hydrogen peroxide, ozone, and micro / nano bubbles, enhancing the generation and chain reaction of strong oxidizing species such as hydroxyl radicals. Furthermore, they can capture degradation intermediates and organic colloids through adsorption bridging and entrapment, achieving a synergistic effect of oxidative degradation and flocculation precipitation. Under acidic conditions, metal ions mostly exist in a free state, resulting in weaker flocculation. Simultaneously, H+... + It will quench some active free radicals, thus limiting the oxidation efficiency; while under strongly alkaline conditions, excess OH- will quench some of the active free radicals, thus limiting the oxidation efficiency; - It can easily lead to the precipitation and aggregation of metal hydroxides, resulting in a decrease in the specific surface area of ​​the flocs and the covering of catalytic active sites. At the same time, ozone self-decomposition is intensified and the free radical pathway is shifted, ultimately weakening the system's overall ability to remove organic matter.

[0077] Single-factor influence experiment example 5: Investigating the effect of hydrogen peroxide concentration on the removal efficiency of organic matter. Hydrogen peroxide concentration is a core process parameter affecting the organic matter removal efficiency of the electrocoagulation-ozone micro-nanobubble-hydrogen peroxide system. By regulating the generation rate and oxidation capacity of reactive oxygen species such as hydroxyl radicals in the system, it directly affects the degradation efficiency and mineralization degree of organic matter.

[0078] In this single-factor influence experiment example 5, the reaction temperature was 25℃, the initial pH was 7.0, the ozone concentration was 10 mg / L, the gas inlet flow rate was 250 mL / min, the electrode spacing was 1.5 cm, and the current density was 30 mA / cm². 2Under fixed conditions, the system was treated with micro / nanobubbles for 30 min, with samples taken every 5 min for analysis. The concentration of hydrogen peroxide in the system was varied by adjusting the amount of hydrogen peroxide added, with concentrations of 1 mmol / L, 2 mmol / L, 3 mmol / L, 4 mmol / L, and 5 mmol / L, to investigate the effect of different hydrogen peroxide concentrations on the removal efficiency of organic matter.

[0079] Experimental results are as follows Figure 7 As shown, the effect of different hydrogen peroxide concentrations on the chemical oxygen demand (COD) removal efficiency showed a trend of first increasing and then gradually decreasing, and the difference gradually increased with the extension of aeration time. When the hydrogen peroxide concentration was 3 mmol / L, the system exhibited the best COD removal performance, with a removal rate of 92.7% after 30 min; the 2 mmol / L and 4 mmol / L groups were next, with removal rates of 86.4% and 90.5% after 30 min, respectively; while the 1 mmol / L group had the lowest removal efficiency, only 76.22% after 30 min, and the 5 mmol / L group was 88.1%, slightly lower than the 3 mmol / L group.

[0080] The above pattern may be due to the following reasons: Under low concentrations of hydrogen peroxide, there are insufficient free radical precursors in the system, making it difficult to fully utilize the catalytic activation effect of electrocoagulation-ozone micro / nanobubbles, resulting in insufficient generation of hydroxyl radicals and thus limiting the degradation rate of organic matter; as the concentration of hydrogen peroxide increases to 3 mmol / L, the matching degree between substrate supply and catalytic interface reaches a relatively optimal state; when the concentration of hydrogen peroxide further increases to 4-5 mmol / L, excess hydrogen peroxide will undergo side reactions with hydroxyl radicals, leading to a decrease in the effective free radical concentration, and may also cause a shift in the oxidation pathway within the system, ultimately reducing the overall removal capacity of the system for organic matter.

[0081] Single-factor influence experiment example 6: Investigating the effect of ozone micro-nanobubble flow rate on organic matter removal efficiency. The ozone micro-nanobubble flow rate is a key process parameter affecting the organic matter removal efficiency of the electrocoagulation-ozone micro-nanobubble-hydrogen peroxide system. By controlling the number of micro-nanobubbles, residence time, and gas-liquid mass transfer efficiency, the ozone dissolution rate, the generation of reactive oxygen species such as hydroxyl radicals, and their contact opportunities with pollutants are directly affected, thereby regulating the oxidative degradation efficiency of organic matter.

[0082] In this single-factor influence experiment example 6, the reaction temperature was 25℃, the initial pH was 7.0, the ozone concentration was 10 mg / L, the hydrogen peroxide dosage was 2 mmol / L, the electrode spacing was 1.5 cm, and the current density was 30 mA / cm. 2Under fixed conditions, ozone micro-nanobubbles were used for treatment for 30 min, with samples taken every 5 min for analysis. The flow rate of the ozone micro-nanobubbles was varied by adjusting the gas flow meter to 50 mL / min, 100 mL / min, 150 mL / min, 200 mL / min, and 250 mL / min, to investigate the effect of different ozone micro-nanobubble flow rates on the removal efficiency of organic matter.

[0083] Experimental results are as follows Figure 8 As shown, the effect of different ozone micro-nanobubble flow rates on the chemical oxygen demand (COD) removal efficiency gradually increases and then stabilizes, with the difference increasing with prolonged aeration time. When the ozone micro-nanobubble flow rate is 200 mL / min, the system exhibits the best COD removal performance, reaching a removal rate of 93.3% after 30 min; the 150 mL / min and 250 mL / min groups are second best, with removal rates of 91.8% and 92.5% respectively after 30 min; while the removal efficiency of the low bubble flow rate groups is significantly lower, with removal rates of 73.66% and 84.9% after 30 min under conditions of 50 mL / min and 100 mL / min, respectively.

[0084] The above pattern may be due to the following reasons: at low bubble flow rates, the gas-liquid mass transfer interface is insufficient, limiting the ozone dissolution and diffusion rates. Simultaneously, the interfacial catalytic effect of micro- and nano-bubbles is difficult to fully realize, resulting in insufficient generation of reactive species such as hydroxyl radicals and low organic matter degradation efficiency. As the bubble flow rate of ozone micro- and nano-bubbles increases to 200 mL / min, the high specific surface area and interfacial characteristics of the micro- and nano-bubbles are fully realized, enhancing ozone dissolution and mass transfer, providing reaction sites for the catalytic activation of hydrogen peroxide, and further promoting free radical generation through the local high-energy effect of bubble rupture, achieving a synergistic effect of enhanced mass transfer, catalytic activation, and flocculation. When the bubble flow rate of ozone micro- and nano-bubbles further increases to 250 mL / min, excessive bubbles may cause intensified liquid-phase turbulence and floc shearing and breakage. At the same time, excessive competition at the gas-liquid interface leads to saturation of catalytic efficiency, thus slowing the increase in removal rate and failing to show significant gains.

[0085] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for co-oxidation treatment of oilfield wastewater, characterized in that, Includes the following steps: S1. The oilfield wastewater to be treated is introduced into the flocculation reaction tank. Polyaluminum chloride is added to the oilfield wastewater, and the pH of the wastewater is adjusted and stirred so that the suspended oil, emulsified oil, colloidal particles in the wastewater collide, destabilize, and coagulate with the hydrolysis products of polyaluminum chloride to form flocs. S2. The floc-containing wastewater treated in step S1 is transported to the micro-nano bubble flotation unit, and air-type micro-nano bubbles are introduced to allow the air-type micro-nano bubbles to adhere to and encapsulate the flocs and suspended oil and emulsified oil in the wastewater, forming bubble-floc composite scum that floats to the water surface. The scum is then removed to obtain the flotation effluent. S3. The air flotation effluent obtained in step S2 is directly fed into the electrocoagulation-ozone-hydrogen peroxide coupled reactor, and hydrogen peroxide is added to the electrocoagulation-ozone-hydrogen peroxide coupled reactor. Simultaneously, ozone micro-nano bubbles are prepared and introduced into the electrocoagulation-ozone-hydrogen peroxide coupled reactor. The electrocoagulation-ozone-hydrogen peroxide coupled reactor is equipped with an iron anode and a stainless steel cathode, which are connected to a DC power supply. The air flotation effluent undergoes deep oxidation treatment under the synergistic effect of electrocoagulation, ozone micro-nano bubbles and hydrogen peroxide to obtain oxidized effluent. S4. The oxidized effluent obtained in step S3 is allowed to settle in a sedimentation tank. The supernatant from the sedimentation tank is taken as the treated wastewater, and the sludge at the bottom of the sedimentation tank is discharged.

2. The method for co-oxidation treatment of oilfield wastewater according to claim 1, characterized in that, In step S1, the initial pH of the oilfield wastewater to be treated is 8.0~9.0, the suspended oil content is 120~180 mg / L, and the chemical oxygen demand concentration is 1500~2000 mg / L.

3. The method for co-oxidation treatment of oilfield wastewater according to claim 1, characterized in that, In step S1, the dosage of polyaluminum chloride is 110~130 mg / L, the pH of the wastewater is adjusted to 6.5~7.5 by acid-base adjuster, and the mixture is stirred at a stirring rate of 150~250 r / min for 8~12 min.

4. The method for co-oxidation treatment of oilfield wastewater according to claim 1, characterized in that, In step S2, the proportion of micron bubbles smaller than 50 μm in the air-type micro-nano bubbles is not less than 93%, the proportion of nano bubbles smaller than 1 μm is not less than 60%, and the Zeta potential of the air-type micro-nano bubbles is maintained at -25~-35 mV.

5. The method for co-oxidation treatment of oilfield wastewater according to claim 1, characterized in that, In step S2, the air inlet flow rate of the air-type micro-nano bubbles is 180~220 mL / min, and the total air flotation time is 20~30 min.

6. The method for co-oxidation treatment of oilfield wastewater according to claim 1, characterized in that, In step S3, the concentration of hydrogen peroxide in the electrocoagulation-ozone-hydrogen peroxide coupled reactor is 3.5~4.5 mmol / L, the ozone concentration of the ozone micro-nano bubbles is 8.0~9.0 mg / L, and the bubble flow rate of the ozone micro-nano bubbles is 190~210 mL / min.

7. The method for co-oxidation treatment of oilfield wastewater according to claim 1, characterized in that, In step S3, the distance between the iron anode and the stainless steel cathode is 0.8~1.2 cm, and the current density for the deep oxidation treatment is 32~40 mA / cm². 2 The pH of the wastewater in the electrocoagulation-ozone-hydrogen peroxide coupled reactor is 8.5~9.5, and the reaction time of the deep oxidation treatment is 25~35 min.

8. The method for co-oxidation treatment of oilfield wastewater according to claim 1, characterized in that, In step S4, the settling time is 25-35 minutes.

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