Heavy medium coupled oil and gas field wastewater pretreatment process and treatment system thereof

Abstract

The application relates to the technical field of oilfield wastewater treatment, and discloses a heavy medium coupled oil and gas field wastewater pretreatment process and a treatment system thereof, which comprises the following steps: S1, pre-oxidation adjustment: sending oil and gas field wastewater into a pre-oxidation pool; S2, coagulation destabilization: pre-oxidation pool effluent flows into a coagulation reaction pool from top to bottom; S3, heavy medium composite reaction: adding heavy medium with a density greater than 4 g / cm3 into the pool; S4, flocculation and sedimentation: heavy medium reaction pool effluent flows into a flocculation reaction pool from top to bottom, and a flocculating agent is added; S5, sludge circulation and recovery: part of sludge containing heavy medium at the bottom of a sedimentation pool is returned to the heavy medium reaction pool through a reflux branch; S6, deep ozone oxidation: supernatant of the sedimentation pool is pumped to a deep oxidation pool for deep oxidation treatment; and S7, residual ozone stripping and degassing: the deep oxidation pool effluent is sent into a stripping tower to remove residual ozone. The application has the characteristics of small occupied area, low cost and high treatment efficiency.

Description

Technical Field

[0001] This application relates to the technical field of oil and gas field wastewater treatment, and in particular to a heavy media coupled oil and gas field wastewater pretreatment process and treatment system. Background Technology

[0002] Oil and gas field development generates a large amount of produced water, fracturing flowback fluid, drilling wastewater, well washing wastewater, and other oil and gas field wastewater. This type of wastewater has complex water quality and high pollutant concentration. It generally contains a large amount of reducing iron-containing polymers, sulfides, colloidal suspended solids, and small molecule organic pollutants, and has the characteristics of strong reducing properties, easy fouling, and strong corrosiveness.

[0003] Currently, the industry commonly uses traditional coagulation and sedimentation processes for the pretreatment of oil and gas field wastewater. This involves adding coagulants and flocculants to achieve the coagulation and sedimentation of suspended solids and colloids, providing feed water for downstream RO membrane treatment. However, this traditional process has many unavoidable technical defects in practical applications, severely restricting wastewater treatment efficiency and system operational stability, as follows: (i) Secondary turbidity in the effluent indicates an extremely high risk of membrane fouling: Oil and gas field wastewater is a reducing complex system, with iron-containing polymers in a stable reduced state. Traditional coagulation and sedimentation can only achieve superficial clarification and cannot completely oxidize the reduced iron-containing polymers. After the treated effluent is left to stand for several tens of minutes, the residual reduced iron-containing polymers react with oxygen in the air and are oxidized to ferric ions. These ferric ions further hydrolyze to form ferric hydroxide colloids, causing secondary turbidity in the water. When this water enters the RO membrane equipment, it will continue to generate ferric hydroxide colloids, which adhere to the membrane surface, causing membrane pore blockage, membrane flux reduction, significantly increasing the risk of membrane fouling, and shortening the RO membrane's lifespan.

[0004] (ii) Low sludge settling efficiency, large equipment footprint, and cumbersome operation and maintenance: Traditional coagulation and sedimentation processes produce flocs with low density and slow settling rates. To ensure effective sludge-water separation, the hydraulic retention time in the sedimentation tank must be extended, thereby increasing the tank volume and overall equipment footprint, resulting in low equipment integration and poor site adaptability. Furthermore, traditional equipment often employs manual sludge removal, making precise timing of removal difficult and prone to delays, sludge accumulation, and ultimately, excessive suspended solids in the effluent and significant water quality fluctuations.

[0005] (III) High sulfide levels cause equipment corrosion, safety hazards, and water quality exceeding standards: Oil and gas field wastewater contains high levels of sulfides, which traditional processes cannot effectively remove. On the one hand, sulfides cause continuous electrochemical corrosion to steel processing equipment and pipelines, significantly increasing equipment maintenance costs, and the release of hydrogen sulfide gas poses on-site safety hazards. On the other hand, sulfides can penetrate RO membranes and enter the permeate water, causing odor and excessive COD levels. Furthermore, residual sulfides are easily oxidized to form elemental sulfur, which adheres to the RO membrane surface, forming stubborn fouling and severely affecting the stability of the membrane system.

[0006] (iv) Insufficient removal capacity of small molecule organic matter, making it difficult to meet COD standards in the produced water: Oil and gas field wastewater contains a large number of small-molecule, highly permeable organic pollutants. These substances cannot be removed by traditional coagulation and sedimentation processes and can directly penetrate RO membranes, ultimately leading to continuous COD exceeding the standard in RO membrane permeate, which fails to meet the standards for oil and gas field wastewater discharge and reuse.

[0007] (v) The process is complicated and the operating cost is high: To ensure the quality of the RO membrane feed water, existing pretreatment processes require pretreatment equipment such as filtration and ultrafiltration after coagulation and sedimentation. This results in a lengthy process, a large number of devices, high energy consumption, and complex operation and maintenance, significantly increasing the construction and operating costs of oil and gas field wastewater treatment.

[0008] In summary, existing traditional oil and gas field wastewater pretreatment processes suffer from core problems such as poor effluent stability, severe membrane fouling, large equipment footprint, cumbersome operation and maintenance, incomplete pollutant removal, and complex process flow. These issues make it difficult to meet the current industry demand for efficient pretreatment, low operation and maintenance costs, and stable compatibility with RO membranes for deep treatment of oil and gas field wastewater. Summary of the Invention

[0009] Firstly, this application provides a heavy media coupled oil and gas field wastewater pretreatment process. The core process flow is as follows: pre-oxidation conditioning → coagulation and destabilization → heavy media composite flocculation → high-density sedimentation → sludge recycling → deep ozone oxidation → residual ozone stripping. Through multi-stage synergistic reactions, the gradient removal of pollutants is achieved.

[0010] The technical solution adopted in this application is as follows: A pretreatment process for oil and gas field wastewater coupled with heavy media includes the following steps: S1. Pre-oxidation conditioning: Oil and gas field wastewater is sent into the pre-oxidation tank, where it is pre-oxidized and conditioned by bottom aeration and micro-nano gas-liquid mixing to degrade the reducing iron-containing polymers, sulfides and small molecule organic matter in the wastewater. S2, Coagulation and Destabilization: The effluent from the pre-oxidation tank flows into the coagulation reaction tank from top to bottom, and coagulant is added to generate fine flocs; S3, Heavy Media Composite Reaction: The effluent from the coagulation reaction tank flows into the heavy media reaction tank from bottom to top. Heavy media with a density greater than 4 g / cm³ is added to the tank to fully combine with the fine flocs and form composite flocs that coat the heavy media. S4. Flocculation and sedimentation: The effluent from the heavy medium reaction tank flows into the flocculation reaction tank from top to bottom. Flocculant is added to cause the composite flocs to agglomerate and grow into large flocs. Then, they enter the sedimentation tank from the bottom to settle and obtain the clarified supernatant. The sludge is deposited at the bottom of the sedimentation tank. S5. Sludge circulation and recycling: Part of the sludge containing heavy media at the bottom of the sedimentation tank is returned to the heavy media reaction tank through the return branch, and the other part is separated from the heavy media through the recycling branch and then sent back to the heavy media reaction tank. S6. Deep ozone oxidation: The supernatant from the sedimentation tank is pumped to the deep oxidation tank for deep oxidation treatment; S7. Residual ozone stripping and degassing: The effluent from the deep oxidation tank is sent to the stripping tower to remove residual ozone and then sent to the downstream RO membrane equipment.

[0011] By adopting the above technical solution, the air oxidation reaction is first enhanced in the pre-oxidation tank to completely oxidize the unstable reduced iron-containing polymers in the wastewater in advance, thus avoiding the problem of ferric hydroxide colloid formation after the effluent has been left to stand. The effluent remains clear and turbid for a long time. By combining high-density heavy media and flocs, the sludge settling rate is increased by 5 times compared with traditional processes, and the hydraulic retention time in the sedimentation tank is shortened by more than 80%. It has the characteristics of high settling efficiency and small equipment footprint. Deep ozone thoroughly degrades sulfides in the wastewater, avoids corrosion of equipment and pipelines by sulfides, and eliminates the safety hazard of hydrogen sulfide emission.

[0012] Optionally, in step S1, the oxidative degradation rate of the reducing iron-containing polymer, sulfide, and small molecule organic matter is ≥85%.

[0013] By adopting the above technical solution, the degradation rate can be controlled. If the degradation rate is lower than this value, the residual reduced iron-containing polymer will still be oxidized again during the subsequent standing process to generate ferric hydroxide colloid.

[0014] Optionally, in step S2, the coagulant is selected from one or more of polyferric sulfate, ferric chloride, polyaluminum chloride, and polyaluminum sulfate.

[0015] By adopting the above technical solution, the coagulant can generate high-valence metal cations (Al³⁺ or Fe³⁺) after dissolving in water, which destabilize and coagulate negatively charged colloidal particles through charge neutralization.

[0016] Optionally, in step S3, the heavy medium is selected from high-density inorganic inert media.

[0017] By adopting the above technical solution, inorganic inert heavy media can be repeatedly recovered and recycled in the system without being consumed or deteriorated.

[0018] Optionally, in step S3, the flocculant is selected from organic flocculants.

[0019] By adopting the above technical solution, organic flocculants (such as polyacrylamide) have long molecular chains containing a large number of active groups, which can simultaneously adsorb multiple fine floc particles. Through the "adsorption bridging" effect, the particles are connected into huge flocs, which, in combination with heavy media, greatly improve the sedimentation efficiency.

[0020] Optionally, in step S5, the proportion of heavy media sludge recirculation is 10%-15%.

[0021] By adopting the above technical solution, the returned sludge contains three useful substances: (1) residual coagulant and flocculant molecules, which have not yet fully reacted and can continue to participate in the reaction after reuse; (2) high-density heavy media particles, which serve as "seeds" for floc growth; and (3) the already formed sludge floc skeleton, on which newly generated flocs can attach and grow rapidly. A return ratio of 10%-15% is the equilibrium point optimized by engineering: below 10%, the return flow is too small and cannot achieve effective "inoculation" and reagent reuse; above 15%, the return flow is too large, which will lead to excessive sludge concentration in the heavy media reaction tank, increased stirring energy consumption, and aggravated equipment wear. Moreover, excessive return will reduce treatment efficiency due to excessive competition among flocs.

[0022] Optionally, in step S7, the residual ozone removal rate is ≥99%.

[0023] By adopting the above technical solution, the residual ozone concentration after stripping is reduced to below the safe threshold, ensuring that the RO membrane in subsequent processes is not oxidized and damaged.

[0024] Secondly, this application provides a heavy media coupling oil and gas field wastewater pretreatment system.

[0025] A heavy media coupling oil and gas field wastewater pretreatment system for implementing the above-mentioned pretreatment process includes the following components connected in sequence: The pre-oxidation tank is equipped with an aeration system at the bottom and a micro-nano gas-liquid mixer. The coagulation reaction tank is connected to the pre-oxidation tank, and the liquid from the pre-oxidation tank overflows into the coagulation reaction tank. A heavy medium reaction tank is connected to the coagulation reaction tank, and the liquid from the coagulation reaction tank enters the heavy medium reaction tank from below. A flocculation reaction tank is connected to the heavy medium reaction tank, and the liquid overflows from above into the flocculation tank; A sedimentation tank is connected to the flocculation reaction tank. The bottom of the sedimentation tank is provided with a sludge output pipeline. The sludge output pipeline is provided with a return branch and a recovery branch. The return branch is connected to the heavy media reaction tank. The deep oxidation tank is connected to the supernatant outlet of the sedimentation tank. The deep oxidation tank is equipped with an ozone micro-nano gas-liquid mixer and connected to an external ozone generator. The stripping tower is connected to the deep oxidation tank, its tail gas outlet is connected to the ozone destroyer, and its outlet is used to connect to the downstream RO membrane equipment.

[0026] By adopting the above technical solutions and the above system, the system structure required for the above process is realized, which is the "hardware carrier" corresponding to the process rights, so as to achieve the above technical effects.

[0027] Optionally, a separation device is provided on the recycling branch to separate the sludge and heavy media, so as to recover the heavy media and send it into the heavy media reaction tank, and send the sludge into the sludge tank.

[0028] In summary, this application includes at least one of the following beneficial effects: 1. Completely solves secondary turbidity in effluent: The unstable reduced iron-containing polymers are completely oxidized in advance through pre-oxidation, which avoids the formation of ferric hydroxide colloids from the source and eliminates colloidal fouling of the RO membrane.

[0029] 2. High settling efficiency and small footprint: The high-density heavy medium coating of flocs increases the settling rate by 5 times and shortens the hydraulic retention time of the sedimentation tank by more than 80%, significantly reducing the equipment footprint.

[0030] 3. Highly efficient removal of sulfides: Two-stage oxidation process of pre-oxidation and deep ozone thoroughly degrades sulfides, eliminates corrosion and safety hazards, and prevents odor and excessive COD in the produced water.

[0031] 4. Precise degradation of small molecule organic matter: The two-stage oxidation process specifically degrades small molecule organic matter that easily penetrates the RO membrane, ensuring that the COD of the produced water meets the standards.

[0032] 5. Streamlined process flow: The integrated structure eliminates the need for filtration and ultrafiltration pre-treatment equipment, shortening the process and reducing investment and operating costs.

[0033] 6. Harmless operation throughout the process: The stripping tower + ozone destroyer completely remove residual ozone and protect the RO membrane from oxidative damage. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the processing system according to an embodiment of this application.

[0035] Explanation of reference numerals in the attached diagram: 1. Pre-oxidation tank; 2. Aeration system; 3. Micro-nano gas-liquid mixer; 5. Coagulation reaction tank; 6. Heavy media reaction tank; 7. Flocculation reaction tank; 8. Sedimentation tank; 9. Deep oxidation tank; 10. Stripping tower; 11. Return branch; 12. Recovery branch; 13. Separation equipment; 14. Sludge discharge pipeline. Detailed Implementation

[0036] The present application will be further described in detail below with reference to the accompanying drawings.

[0037] This application discloses a heavy media coupling oil and gas field wastewater pretreatment system. (Refer to...) Figure 1 The treatment system is a long, trough-shaped frame structure, separated by overflow plates. From the inlet to the outlet, it sequentially forms a pre-oxidation tank 1, a coagulation reaction tank 5, a heavy media reaction tank 6, a flocculation reaction tank 7, a sedimentation tank 8, a deep oxidation tank 9, and a stripping tower 10. Each of the pre-oxidation tank 1, coagulation reaction tank 5, heavy media reaction tank 6, and flocculation reaction tank 7 is equipped with a stirring device to agitate the wastewater being treated.

[0038] An aeration system 2, preferably an aeration disc, is evenly distributed at the bottom of the pre-oxidation tank 1. This system generates numerous microbubbles in the water, increasing the contact area between air and wastewater and allowing oxygen to dissolve efficiently. A micro / nano gas-liquid mixer 3 is vertically installed inside the tank. Wastewater and air enter the mixer, where they mix to generate micro / nano-sized bubbles, thus promoting the pre-oxidation treatment effect. The overflow outlets of the pre-oxidation tank 1 and the coagulation reaction tank 5 are located at the top of the overflow plate, allowing water from the pre-oxidation tank 1 to flow into the coagulation reaction tank 5 from above.

[0039] Coagulant is added to coagulation reaction tank 5, and under the action of stirring, the colloids and suspended solids in the water are destabilized and initially coagulated to form fine flocs.

[0040] The connection between the coagulation reaction tank 5 and the heavy media reaction tank 6 is located at the lower end of the overflow plate, allowing the fine flocs in the coagulation reaction tank 5 to flow from bottom to top into the heavy media reaction tank 6. After adding heavy media to the heavy media reaction tank 6, the flocs can aggregate and grow, forming sludge flocs that coat the heavy media.

[0041] The sludge flocs in the heavy media reaction tank 6 can enter the flocculation reaction tank 7 from top to bottom. Flocculant is added to the flocculation reaction tank 7, and under the action of stirring, the sludge flocs can form large flocs. Then, they enter the sedimentation tank 8 from the bottom and can settle quickly. The upper part of the sedimentation tank 8 is clear water, and the sludge is deposited at the bottom of the sedimentation tank 8.

[0042] The clean water in the sedimentation tank 8 is sent to the deep oxidation tank 9 by means of a booster pump. The deep oxidation tank 9 is also equipped with a micro-nano gas-liquid mixer 3. The water in the sedimentation tank 8 enters the gas-liquid mixer, and ozone is introduced into the micro-nano gas-liquid mixer 3 to fully mix ozone and water, further oxidizing and degrading residual sulfides and small molecule organic matter, thus preventing the COD and sulfide of the RO membrane permeate from exceeding the standard in subsequent processes.

[0043] The stripping tower 10 is located at the end of the system. The stripping tower 10 can be selected from a packed tower structure. The lower part of the tower body has an inlet pipe and a circulating water outlet, which is pumped to the top spray device for circulating spraying. The bottom of the tower body has a blower inlet, and the top of the tower body has an exhaust gas outlet connected to an ozone depletor. A level controller is installed on the side wall of the tower body, and the outlet is connected to an outlet pump. The outlet pump is electrically connected to the level controller and automatically starts and stops according to the liquid level inside the tower. The stripping tower 10 removes ozone from the water, preventing ozone from affecting the subsequent RO membrane. The specific structure of the stripping tower 10 is existing technology and will not be described in detail here.

[0044] This application also discloses a heavy media coupling oil and gas field wastewater pretreatment process, which is implemented based on the above-mentioned treatment system and specifically includes the following steps; S1. Pre-oxidation conditioning: Oil and gas field wastewater is sent into pre-oxidation tank 1. Through the bottom aeration system 2 and the micro-nano gas-liquid mixer 3, highly soluble micro-nano air bubbles are prepared to fully oxidize the reducing iron-containing polymers, sulfides and small molecule organic matter that can easily penetrate the RO membrane in the wastewater, so as to achieve the initial degradation of pollutants. The oxidation degradation rate of reducing iron-containing polymers, sulfides and small molecule organic matter is ≥85%, and the water quality is conditioned at the same time. S2, Coagulation and Destabilization: The effluent from the pre-oxidation tank 1 flows from top to bottom into the coagulation reaction tank 5. Coagulant is added. The coagulant is one or more of polyferric sulfate, ferric chloride, polyaluminum chloride, and aluminum sulfate, which destabilizes the colloids and suspended solids in the water and initially coagulates to form fine flocs. S3. Heavy Media Composite Reaction: The effluent from coagulation reaction tank 5 flows from bottom to top into heavy media reaction tank 6. Heavy media with a density greater than 4 g / cm³ is added to the tank. The heavy media is selected from high-density inorganic inert media, such as magnetite powder, garnet, ceramic sand and other components. The heavy media and fine flocs are fully combined to form composite fine sludge flocs coated with heavy media. By adding heavy media, the sludge settling speed is 5 times faster, which greatly reduces the overall equipment footprint. S4. Flocculation and sedimentation: The effluent from the heavy medium reaction tank 6 flows from top to bottom into the flocculation reaction tank 7. Flocculants are added to cause the composite fine flocs to agglomerate and grow, forming high-density large-particle flocs. The flocculants are preferably organic flocculants, such as polyacrylamide. Then, the effluent enters the sedimentation tank 8 from the bottom for rapid sedimentation to achieve mud-water separation and obtain a clear supernatant. The sludge is deposited at the bottom of the sedimentation tank 8. S5. Sludge Recycling and Recovery: The heavy media sludge at the bottom of sedimentation tank 8 is divided into two streams. 10%-15% of the heavy media sludge is returned to the heavy media reaction tank 6 to increase the sludge concentration in the tank and reduce the amount of reagents added. The remaining part is sent to the recovery branch 12, where it is separated by high-precision separation equipment, such as shearing machine crushing and centrifuge separation. The separated heavy media is returned to the heavy media reaction tank 6 for recycling. The sludge without media residue is sent to the sludge tank for disposal. S6. Deep ozone oxidation: The supernatant of sedimentation tank 8 is pumped to deep oxidation tank 9, and ozone micro-nano bubbles are introduced through micro-nano gas-liquid mixer 3 to further oxidize and degrade residual sulfides and small molecule organic matter, thus eliminating the problem of excessive COD and sulfides in RO membrane permeate. S7. Residual Ozone Stripping and Degassing: The effluent from the deep oxidation tank 9 is sent to the stripping tower 10, where it is aerated by a blower and circulated by a circulating pump pipeline to remove residual ozone from the water. The stripped ozone is then harmlessly decomposed by an ozone destroyer. The residual ozone removal rate is ≥99%, and the qualified effluent is directly sent to the RO membrane equipment for subsequent treatment.

[0045] Application Example 1: Pretreatment of low-concentration produced water from conventional oil and gas fields (small-scale treatment) This application example treats conventional low-concentration produced water from oil and gas fields. The influent water quality parameters are: turbidity 80-120 NTU, iron polymer concentration 15-25 mg / L, sulfide concentration 30-50 mg / L, and wastewater treatment capacity 5 m³ / h.

[0046] The above process is as follows: Wastewater first enters the pre-oxidation tank 1, and the bottom aeration system 2 and the micro-nano gas-liquid mixer 3 are turned on. The air micro-nano bubbles fully contact the wastewater, and the hydraulic retention time is 25 minutes, completing the initial oxidation of reduced iron-containing polymers and sulfides. Then, the upper part of the wastewater overflows into the coagulation reaction tank 5, where polyaluminum chloride coagulant is added at a dosage of 300 mg / L, and the mixture is stirred for 3 minutes to achieve colloid destabilization. The lower part of the wastewater enters the heavy medium reaction tank 6, where 4.2 g / cm³ high-density inorganic heavy medium (magnetite powder) is added at a dosage of 80 mg / L, and the mixture reacts for 2 minutes to form composite flocs. Then, it enters the flocculation reaction tank 7, where polyacrylamide flocculant is added at a dosage of 3 mg / L, and the mixture quickly agglomerates to form high-density large flocs. The flocs are carried by the water flow into the sedimentation tank 8 for settling, with a hydraulic retention time of 5 minutes.

[0047] 10% of the sludge at the bottom of sedimentation tank 8 is recycled to heavy media reaction tank 6, increasing the sludge concentration in the tank and reducing the amount of reagents added by 30%; the remaining 90% of the sludge is broken up by a high-speed shear machine and separated by a centrifuge, with a heavy media recovery rate of ≥98%, and is recycled for reuse. The supernatant from sedimentation tank 8 is pumped into deep oxidation tank 9, where ozone micro-nano bubbles are introduced at a dosage of 20 mg / L and a residence time of 15 min, to deeply degrade residual small molecule organic matter and sulfides; finally, the wastewater enters stripping tower 10 and is circulated and stripped for 10 min, where residual ozone is completely removed and decomposed harmlessly, and the effluent directly enters the RO membrane equipment.

[0048] Treatment results: The turbidity of the pretreated effluent is ≤5 NTU, with no suspended solids and no secondary turbidity; the removal rate of iron-containing polymers is 99.2%, and the removal rate of sulfides is 99.5%; the RO membrane operates without significant clogging, the COD of the produced water is stable at ≤60 mg / L, with no odor, fully meeting the discharge requirements; the equipment occupies 45% less space than traditional processes, requires no manual sludge removal, and is easy to operate and maintain.

[0049] Application Example 2: Pretreatment of medium-concentration well-washing wastewater from oil and gas fields (medium-scale treatment) This embodiment treats oil and gas field fracturing flowback fluid wastewater, which has large fluctuations in water quality. The influent water quality parameters are: turbidity 200-300 NTU, iron polymer concentration 30-45 mg / L, sulfide concentration 60-80 mg / L, and wastewater treatment capacity 20 m³ / h.

[0050] The above process is used as follows: the hydraulic retention time in pre-oxidation tank 1 is extended to 35 minutes to enhance the aeration and micro-nano gas-liquid mixing effect, improve the initial oxidation efficiency of high-concentration pollutants, and thoroughly oxidize unstable reduced pollutants; the dosage of polyaluminum chloride in coagulation reaction tank 5 is adjusted to 350 mg / L, and the reaction is stirred for 5 minutes to enhance the destabilization of colloids in high-turbidity water; the dosage of heavy medium (magnetite powder) in heavy medium reaction tank 6 is increased to 120 mg / L to ensure the coating and modification effect of high-concentration flocs, and the reaction is carried out for 8 minutes; the dosage of polyacrylamide in flocculation reaction tank 7 is 4.5 mg / L to fully aggregate large flocs.

[0051] Sedimentation tank 8 has a hydraulic retention time of 35 minutes, and the sludge return ratio is adjusted to 15%, further increasing the sludge concentration in the reaction tank to meet the needs of high-concentration wastewater treatment. The overall dosage of reagents is reduced by 35% compared to traditional processes. The heavy media recovery system operates continuously, with a stable recovery rate of ≥97.5%. Deep oxidation tank 9 has an ozone dosage of 30 mg / L and a retention time of 20 minutes, thoroughly degrading residual difficult-to-remove small molecule organic matter and sulfides. Stripping tower 10 continuously circulates and strips, with automated liquid level control ensuring stable effluent.

[0052] Treatment results: The turbidity of the pretreated effluent is ≤5 NTU, and there is no back turbidity or sedimentation after standing for 24 hours; the removal rate of iron polymers is 98.8%, and the removal rate of sulfides is 99.1%; the corrosion and safety hazards of sulfides are completely eliminated, the RO membrane is free from sulfur and ferric hydroxide fouling, and the quality of the produced water is stable and meets the standards; the equipment tank volume is reduced by 40% compared with the traditional process, and the whole process is automated without human intervention.

[0053] Application Example 3: Pretreatment of high-concentration reducing oil and gas field wastewater (large-scale treatment) This embodiment treats wastewater produced from highly reducing oil and gas fields. The influent water quality is poor, with the following parameters: turbidity 350-450 NTU, iron polymer concentration 50-70 mg / L, sulfide concentration 1000-150 mg / L, and wastewater treatment capacity 50 m³ / h.

[0054] The above process is used as follows: the pre-oxidation tank 1 is fully loaded with the whole-area aeration system 2 and the dual-group micro-nano gas-liquid mixer 3, with a hydraulic retention time of 60 min, to maximize the air oxidation efficiency and thoroughly decompose high-concentration reduced iron-containing polymers and sulfides; the coagulation reaction tank 5 adopts a gradient stirring mode, with a polyaluminum chloride dosage of 500 mg / L, to destabilize high-concentration colloidal suspended solids in stages; the heavy medium reaction tank 6 uses 4.5 g / cm³ ultra-high density heavy medium, with a dosage of 180 mg / L, to enhance the formation of high-density flocs, and the reaction time is 15 min; the flocculation reaction tank 7 precisely adds 6 mg / L of polyacrylamide to achieve rapid agglomeration and densification of flocs.

[0055] Sedimentation tank 8 has a hydraulic retention time of 40 minutes and a sludge return rate of 15%, minimizing reagent consumption. The heavy media recovery system operates at high frequency, accurately separating sludge and media with a recovery rate of ≥98%. Deep oxidation tank 9 uses high-concentration ozone micro-nano aeration with an ozone dosage of 45 mg / L and a retention time of 25 minutes to specifically degrade stubborn small-molecule organic pollutants. The stripping tower 10 increases the blower air volume and extends the circulation stripping time to 15 minutes to ensure complete removal of residual ozone and thoroughly protect the RO membrane equipment.

[0056] Treatment results: Under high concentration and poor water quality, the turbidity of the pretreated water is stable at ≤5 NTU, the water quality is clear and stable, and there is no secondary turbidity; the removal rate of iron-containing polymers is 98.5%, and the removal rate of sulfides is 98.9%; the RO membrane has a stable flux during long-term operation, without stubborn fouling, and the COD and odor indicators of the produced water fully meet the standards; the equipment integration has significant advantages, occupying only 55% of the space of traditional processes, and is suitable for large-scale wastewater treatment scenarios in oil fields.

[0057] Comparative example: Traditional coagulation and sedimentation process (current mainstream process) This comparative example adopts the industry's traditional oil and gas field wastewater pretreatment coagulation and sedimentation process, without pre-oxidation, heavy media coupling, deep ozone oxidation and degassing units. It only uses the conventional process of coagulation + flocculation + sedimentation to treat the wastewater. The same medium-concentration oil and gas field fracturing flowback fluid wastewater as in Application Example 2 is selected. The influent water quality is: turbidity 200-300 NTU, iron polymer concentration 30-45 mg / L, sulfide concentration 60-80 mg / L, and treatment scale 20 m³ / h.

[0058] Process operation mode: Wastewater directly enters coagulation sedimentation tank 8, and equal amounts of polyaluminum chloride 350mg / L and polyacrylamide 4.5mg / L are added. After thorough stirring and reaction, the water settles naturally. The hydraulic retention time in sedimentation tank 8 is 35min. Sludge is manually removed periodically. The effluent is then connected to the RO membrane equipment after simple filtration.

[0059] Treatment results: The initial effluent turbidity was 20 NTU, and the water appeared clear. However, after standing for 30-60 minutes, the water became noticeably turbid again, producing a large amount of yellowish-brown iron hydroxide colloidal suspended matter. The removal rate of iron-containing polymers was only about 65%, and the removal rate of sulfides was less than 40%. The wastewater contained a large amount of residual sulfides and small molecule organic matter. The RO membrane showed obvious fouling after 7 days of operation, with a 40% decrease in membrane flux. The permeate had a noticeable odor and exceeded the COD standard. The sludge in sedimentation tank 8 settled slowly, occupied a large area, and untimely manual sludge removal easily caused fluctuations in the effluent. The overall operational stability was extremely poor, and it could not meet the long-term stable pretreatment requirements at all.

[0060] The above are all preferred embodiments of this application, and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the structure, shape and principle of this application should be covered within the scope of protection of this application.

Claims

1. A heavy media coupling pretreatment process for oil and gas field wastewater, characterized in that: Includes the following steps: S1. Pre-oxidation conditioning: The oil and gas field wastewater is sent into the pre-oxidation tank (1), and pre-oxidation conditioning is carried out through bottom aeration and micro-nano gas-liquid mixing to degrade the reducing iron-containing polymers, sulfides and small molecule organic matter in the wastewater. S2, Coagulation and Destabilization: The effluent from the pre-oxidation tank (1) flows from top to bottom into the coagulation reaction tank (5), where coagulant is added to coagulate and generate fine flocs; S3, Heavy medium composite reaction: The effluent from the coagulation reaction tank (5) flows from bottom to top into the heavy medium reaction tank (6), and heavy medium with a density greater than 4g / cm³ is added into the tank so that the heavy medium and fine flocs are fully combined to form a composite flocs that cover the heavy medium. S4, Flocculation and Sedimentation: The effluent from the heavy medium reaction tank (6) flows from top to bottom into the flocculation reaction tank (7). Flocculant is added to make the composite flocs agglomerate and grow, forming large flocs. Then, they enter the sedimentation tank (8) from the bottom to settle, and the clarified supernatant is obtained. The sludge is deposited at the bottom of the sedimentation tank (8). S5. Sludge recycling and recovery: Part of the sludge containing heavy media at the bottom of the sedimentation tank (8) is returned to the heavy media reaction tank (6) through the return branch (11), and the other part is separated from the heavy media through the recovery branch (12) and returned to the heavy media reaction tank (6). S6, Deep ozone oxidation: The supernatant of the sedimentation tank (8) is pumped to the deep oxidation tank (9) for deep oxidation treatment; S7. Residual ozone stripping and degassing: The effluent from the deep oxidation tank (9) is sent to the stripping tower (10) to remove residual ozone and then sent to the downstream RO membrane equipment.

2. The heavy media coupling oil and gas field wastewater pretreatment process according to claim 1, characterized in that: In step S1, the oxidative degradation rate of the reducing iron-containing polymer, sulfide, and small molecule organic matter is ≥85%.

3. The heavy media coupling oil and gas field wastewater pretreatment process according to claim 1, characterized in that: In step S2, the coagulant is selected from one or more of polyferric sulfate, ferric chloride, polyaluminum chloride, and polyaluminum sulfate.

4. The heavy media coupling oil and gas field wastewater pretreatment process according to claim 1, characterized in that: In step S3, the heavy medium is selected from high-density inorganic inert media.

5. The heavy media coupling oil and gas field wastewater pretreatment process according to claim 1, characterized in that: In step S3, the flocculant is selected from organic flocculants.

6. The heavy media coupling oil and gas field wastewater pretreatment process according to claim 5, characterized in that: In step S5, the proportion of heavy media sludge recirculation is 10%-15%.

7. The heavy media coupling oil and gas field wastewater pretreatment process according to claim 6, characterized in that: In step S7, the residual ozone removal rate is ≥99%.

8. A heavy media coupled oil and gas field wastewater pretreatment system according to claim 6, used to implement the pretreatment process according to any one of claims 1-7, characterized in that: Including those connected in sequence: The pre-oxidation tank (1) is equipped with an aeration system (2) at the bottom and a micro-nano gas-liquid mixer (3). The coagulation reaction tank (5) is connected to the pre-oxidation tank (1), and the liquid in the pre-oxidation tank (1) overflows from the top into the coagulation reaction tank (5). The heavy medium reaction tank (6) is connected to the coagulation reaction tank (5), and the liquid in the coagulation reaction tank (5) enters the heavy medium reaction tank (6) from below. The flocculation reaction tank (7) is connected to the heavy medium reaction tank (6), and the liquid overflows into the flocculation tank from above; The sedimentation tank (8) is connected to the flocculation reaction tank (7). The bottom of the sedimentation tank (8) is provided with a sludge output pipeline (14). The sludge output pipeline (14) is provided with a return branch (11) and a recovery branch (12). The return branch (11) is connected to the heavy medium reaction tank (6). The deep oxidation tank (9) is connected to the supernatant outlet of the sedimentation tank (8). The deep oxidation tank (9) is equipped with an ozone micro-nano gas-liquid mixer (3) and connected to an external ozone generator. The stripping tower (10) is connected to the deep oxidation tank (9), its tail gas outlet is connected to the ozone destroyer, and its outlet is used to connect to the back-end RO membrane equipment.

9. The heavy media coupled oil and gas field wastewater pretreatment system according to claim 8, characterized in that: The recycling branch (12) is equipped with a separation device (13) for separating sludge and heavy media, so as to recover the heavy media and send it into the heavy media reaction tank (6), and send the sludge into the sludge field.

Images