A device and method for desalination pretreatment of shale gas produced water using slow filtration

Abstract

The present application relates to unconventional oil and gas field produced wastewater treatment and resource utilization technical field. Specifically relates to a kind of device and method using slow filtration as the pretreatment of shale gas produced water desalination, the device includes the water inlet adjustment system, slow filtration pretreatment system and desalination treatment system communicated in turn, the slow filtration pretreatment system is set before desalination treatment system, for removing suspended particles, colloidal material, organic pollutants and scale precursors in shale gas produced water.The present application is by constructing low hydraulic load, long residence time slow filtration process, without high energy consumption or chemical reagent conditions significantly improve water quality, effectively reduce the pollution and wetting risk of subsequent desalination membrane process, improve system stability and operating life.The present application simple structure, low operating energy consumption, maintenance cost is small, suitable for high salinity shale gas produced water engineering pretreatment.

Description

Technical Field

[0001] This invention relates to the field of unconventional oil and gas field produced water treatment and resource utilization technology, specifically to a device and method for using slow filtration as a pre-treatment method for desalination of shale gas produced water. Background Technology

[0002] Shale gas is an important unconventional natural gas resource, and its commercial development heavily relies on hydraulic fracturing technology. After fracturing operations, a large amount of fracturing fluid injected into the formation is returned to the surface as flowback fluid and produced water, collectively referred to as shale gas produced water. This type of wastewater is extremely complex, characterized by high salinity, high suspended particulate content, high concentration of colloidal substances, and a wide variety of organic pollutants. Direct discharge or reinjection without effective treatment will cause serious pollution to groundwater, surface water, and soil, and also represent a significant waste of water resources. Therefore, developing efficient desalination technologies suitable for shale gas produced water is of significant engineering and environmental value for promoting green shale gas extraction and realizing the resource utilization of wastewater.

[0003] Membrane distillation is a thermal membrane separation technology that uses a hydrophobic porous membrane as the medium and a vapor pressure difference generated by a temperature difference as the driving force. Water vapor evaporates on the feed side (hot side) of the membrane, passes through the membrane pores, and condenses into liquid pure water on the cold side. Dissolved salts, organic matter, and other non-volatile solutes are completely retained by the hydrophobic membrane. It can handle high-salt or saturated brine that pressure-driven membrane technologies such as reverse osmosis cannot handle, and is therefore considered a highly promising technical route for deep desalination and resource utilization of shale gas produced water.

[0004] However, membrane distillation faces significant operational challenges in treating shale gas produced water, hindering its widespread engineering application. These challenges are mainly reflected in the following aspects: First, membrane fouling is a prominent issue. Suspended particles, colloidal organic matter, and natural high-molecular-weight organic matter commonly found in shale gas produced water easily form a fouling layer on the surface of hydrophobic membranes, leading to membrane pore blockage, a significant increase in mass transfer resistance, a rapid decline in membrane flux, and a substantial decrease in system treatment efficiency.

[0005] Secondly, the risk of membrane wetting is high. High concentrations of polyvalent ions such as calcium, magnesium, barium, and strontium, under the high-temperature operation conditions of membrane distillation, readily form insoluble inorganic scale such as calcium carbonate, barium sulfate, and strontium sulfate on the membrane surface and within the membrane pores. These scale deposits not only exacerbate membrane fouling but also damage the membrane's hydrophobicity, leading to membrane wetting, salt leakage, and a sharp deterioration or even complete failure of the desalination effect. Furthermore, surfactants and low-molecular-weight organic acids in the produced water can also reduce the membrane contact angle, further increasing the risk of wetting.

[0006] Third, existing pretreatment schemes have significant shortcomings. To alleviate membrane fouling and wetting problems, current technologies typically incorporate pretreatment processes such as coagulation sedimentation, flotation, rapid sand filtration, or precision filtration before membrane distillation. Some schemes also require the addition of chemical agents such as scale inhibitors and dispersants. However, these pretreatment schemes share the following common problems: First, they consume large amounts of chemicals, resulting in high operating costs; second, they generate large amounts of saline sludge, causing secondary pollution; and third, they are inconsistent in their effectiveness in removing colloidal substances and organic matter in high-salt systems. Furthermore, their operation and management are complex, making them difficult to adapt to the large fluctuations in water quality at shale gas sites, and they struggle to maintain stable pretreatment results under long-term continuous operation.

[0007] Slow filtration, as a filtration technology with low hydraulic loading and long retention time, can synergistically remove pollutants from water through multiple mechanisms such as physical interception, biodegradation, and interfacial adsorption. Compared with traditional rapid filtration, slow filtration has a more stable removal capacity for colloids, organic matter, and scaling precursors, and it hardly relies on chemical agents during operation, offering advantages such as low energy consumption and stable operation. However, currently, slow filtration technology is mainly used in drinking water and low-salinity wastewater treatment, and its systematic research and engineering application as a pretreatment unit for high-salinity shale gas produced water membrane distillation are still relatively lacking. Therefore, developing a shale gas produced water desalination technology that organically combines slow filtration and membrane distillation is of great significance for improving the operational stability of membrane distillation systems and reducing the risk of membrane fouling. Summary of the Invention

[0008] The purpose of this invention is to address the technical problems in existing shale gas produced water membrane distillation desalination pretreatment processes, such as complex processes, high reagent consumption, and susceptibility to membrane fouling and wetting. This invention proposes a device and method for using slow filtration as a pretreatment method for shale gas produced water desalination. The aim is to significantly improve the quality of the influent to membrane distillation by constructing a low-energy, reagent-free slow filtration pretreatment unit, effectively reducing the risk of membrane fouling and wetting, and improving the operational stability and service life of the membrane distillation system.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: On one hand, the present invention provides an apparatus for pre-treatment of shale gas produced water using slow filtration, comprising an influent regulating unit, a slow filtration pretreatment unit, a membrane distillation desalination unit, and a product water collection unit arranged in sequence. The influent regulating unit receives the shale gas produced water to be treated and regulates its flow rate and stabilizes its water level. The slow filtration pretreatment unit is connected to and follows the influent regulating unit and includes a slow filtration tank, the inside of which is arranged from bottom to top as a support layer, a slow filtration media layer, and a biological filter layer. The membrane distillation desalination unit is connected to and follows the slow filtration pretreatment unit and includes a membrane distillation influent tank, a membrane distillation assembly, a heating device, and a condensation device arranged in sequence. It utilizes temperature difference to drive water vapor to permeate through a hydrophobic membrane to achieve deep desalination of high-salinity produced water.

[0010] Preferably, the water inlet regulating unit includes an inlet water tank and an outlet valve. The inlet end of the inlet water tank is connected to the shale gas wastewater source through a pipeline, and the outlet end is equipped with an outlet valve that is connected to the slow filtration pretreatment unit.

[0011] Preferably, the support layer is laid at the bottom of the slow filter tank to support the filter layer above and prevent the filter media from being lost. It is made of a support plate with a pore size of 2 to 5 mm or pebbles with a particle size of 2 to 5 mm. The thickness of the support layer is 5 to 10 cm.

[0012] Preferably, the slow filtration medium comprises zeolite or biochar with a particle size range of 0.1–2 mm, serving as the main functional layer for physical interception and adsorption, with a layer thickness of 80–100 cm. The filtration rate is controlled within the range of 0.05–0.5 m / h by the outlet valve of the inlet water regulating unit. Zeolite has ion exchange and adsorption characteristics, effectively removing ammonia nitrogen and some polyvalent scaling ions. Biochar has a large specific surface area and abundant surface functional groups, exhibiting good adsorption and removal capabilities for dissolved organic matter, colloidal substances, and heavy metal ions. The synergistic effect of the two significantly reduces the pollutant load in the produced water.

[0013] Preferably, the biofilter layer is a biofilm layer that is naturally formed or artificially inoculated on the surface of the slow filter medium. As the main functional layer for biodegradation, the biofilm layer has a thickness of 1 to 3 mm. This biofilm layer is rich in microbial communities and can remove dissolved organic matter, ammonia nitrogen and microbial metabolic byproducts from the extracted water through biodegradation, thereby further reducing the risk of organic matter contaminating the membrane surface of subsequent membrane distillation.

[0014] Preferably, the membrane distillation assembly is a hydrophobic membrane assembly, and the membrane material is selected from polyvinylidene fluoride, polytetrafluoroethylene or polypropylene, and the membrane assembly is in the form of a flat sheet membrane.

[0015] Preferably, the membrane distillation desalination unit adopts a direct contact membrane distillation operation mode; the heating device is used to control the temperature of the membrane distillation feed water side at 50-80℃, and the condensing device is used to control the temperature of the condensing side at 10-30℃.

[0016] At the device level, the slow filtration pretreatment unit works synergistically with the membrane distillation desalination unit. The slow filtration unit significantly reduces the pollutant loading on the membrane surface of the membrane distillation unit by removing suspended solids, colloids, organic pollutants and reducing the activity of scaling ions, thereby reducing the probability of inorganic scaling-induced wetting and achieving long-term stable desalination treatment of shale gas produced water. It also significantly reduces the fouling rate and wetting probability during the membrane distillation process.

[0017] Preferably, the product water collection unit includes a product water collection tank and an outlet valve four, used to collect high-quality product water after membrane distillation and desalination treatment.

[0018] On the other hand, the present invention also provides a method for desalination pretreatment of shale gas produced water using the aforementioned apparatus, comprising the following steps: S1. The shale gas produced water is transported to the water inlet regulating unit for water volume and level regulation. Specifically, the shale gas produced water is transported to the water inlet tank of the water inlet regulating unit for water volume buffering (buffering time not less than 30 minutes). The water flow rate is controlled by the water outlet valve to ensure that the water flow rate entering the slow filter pretreatment unit is uniform and stable. S2. The regulated produced water enters the slow filtration pretreatment unit, where suspended solids, organic matter, and inorganic pollutants are removed through a low-rate slow filtration process. Specifically, the regulated produced water enters evenly from the top of the slow filter tank, and the water flows from top to bottom through the biological filter layer, the slow filter media layer, and the support layer under the action of gravity. Filtration is carried out at a low filtration rate of 0.05–0.5 m / h under normal temperature and pressure conditions, without the addition of any chemical agents. The biological filter layer removes dissolved organic matter and ammonia nitrogen through biodegradation, while the slow filter media layer removes suspended solids, colloids, and scaling precursors through physical interception and adsorption. The pretreated water is discharged from the bottom outlet of the slow filter tank and enters the membrane distillation inlet tank. S3. The slow-filtered effluent is transported to the membrane distillation desalination unit. Driven by temperature difference, selective water vapor transport is achieved, resulting in low-salinity permeate. Specifically, the pretreated effluent in the membrane distillation feed tank is heated to 50–80°C by a heater and then pumped into the feed side of the membrane distillation module by a gear pump. Driven by the vapor pressure difference generated by the transmembrane temperature difference, water molecules preferentially permeate the hydrophobic membrane pores as vapor, entering the condenser side and being condensed into liquid permeate by the condenser. Multivalent ions, residual organic matter, and most of the salt are retained by the hydrophobic membrane, forming concentrated water which is then returned to the membrane distillation feed tank for recycling. The low-salinity permeate is collected in the permeate collection tank via the permeate pipeline, completing the entire desalination process. It is worth noting that the conductivity of the permeate after membrane distillation desalination is less than 500 μS / cm, and the salt removal rate is greater than 99%, meeting the requirements of the "Design Code for Urban Wastewater Reuse Engineering" (GB). According to the industrial water quality requirements in 50335-2016, it can be directly reused for shale gas fracturing water or other industrial uses.

[0019] Preferably, the slow filtration process in S2 operates under normal temperature and pressure conditions, and the slow filtration pretreatment unit can operate continuously and stably for more than 60 days without the addition of additional chemical agents during operation.

[0020] Preferably, the shale gas produced water is flowback fluid, produced water, or a mixture thereof, and is suitable for shale gas wastewater with high salt and high organic matter content.

[0021] Compared with the prior art, the present invention has the following outstanding technical advantages: 1. Significant pretreatment effect, green and chemical-free: The slow filtration pretreatment unit utilizes multiple synergistic mechanisms of biodegradation, physical interception and adsorption to effectively remove key precursors that cause membrane fouling and wetting under normal temperature and pressure conditions and without the addition of chemical reagents, thus controlling the risk of membrane distillation failure from the source.

[0022] 2. Significantly extended membrane module lifespan: After slow filtration pretreatment, the membrane flux of the membrane distillation system remains stable at a normal level during continuous operation, greatly improving system stability and extending the expected lifespan of the membrane modules, thus significantly reducing operating and replacement costs.

[0023] 3. Excellent desalination performance: Membrane distillation produces water with low conductivity, high salt removal rate, and excellent water quality that meets industrial reuse standards.

[0024] 4. The device has a simple structure and is suitable for modular engineering applications: Each functional unit is connected by standard pipelines. The structure is simple and compact, which facilitates modular deployment and rapid installation at shale gas well sites and adapts to working conditions with limited on-site operation and maintenance capabilities.

[0025] 5. Low operating energy consumption and good economic efficiency: The slow filtration process is gravity driven and does not require additional pumping power, resulting in extremely low operating energy consumption. Combined with the characteristic of membrane distillation that it can utilize low-grade waste heat (such as the heat energy associated with shale gas extraction), the overall operating cost of the system can be significantly reduced, combining good economic efficiency and sustainability.

[0026] The technical solution of the present invention will be further described in detail below through embodiments. Attached Figure Description

[0027] Figure 1 A schematic diagram of the structure of the device for pretreatment of shale gas produced water by slow filtration as a membrane distillation desalination pretreatment method provided by the present invention. Figure 2 This is a comparison chart showing the effectiveness of ammonia nitrogen removal. Figure 3 Comparison chart showing the effect of organic matter removal; Figure 4 This is a comparison chart of membrane flux stability; Figure 5 This is a comparison chart of the conductivity of produced water.

[0028] Figure label: 1. Inlet Water Regulation Unit; 1.1 Inlet Water Tank; 1.2 Outlet Valve 1; 2. Slow Filtration Pretreatment Unit; 2.1 Slow Filtration Tank; 2.2 Support Layer; 2.3 Slow Filtration Media Layer; 2.4 Biological Filter Layer; 2.5 Outlet Valve 2; 3. Membrane Distillation Desalination Unit; 3.1 Membrane Distillation Inlet Water Tank; 3.2 Inlet Valve 1; 3.3 Inlet Gear Pump; 3.4 Heater; 3.5 Membrane Distillation Membrane Module; 3.6 Outlet Gear Pump; 3.7 Outlet Valve 3; 3.8 Condensation Device; 3.9 Membrane Distillation Outlet Water Tank; 4. Product Water Collection Unit; 4.1 Outlet Valve 4; 4.2 Product Water Collection Tank. Detailed Implementation

[0029] To enable those skilled in the art to better understand the present application, the technical solutions in specific embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Unless otherwise defined, the technical or scientific terms used in this invention should have the ordinary meaning understood by those skilled in the art.

[0030] The terms "first," "second," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects.

[0031] First, the structural part of the invention, see below. Figure 1 The present invention provides an apparatus for desalination pretreatment of shale gas produced water using slow filtration, comprising an inlet regulating unit 1, a slow filtration pretreatment unit 2, a membrane distillation desalination unit 3, and a product water collection unit 4 arranged in sequence. Each unit is sealed and connected by a pipeline system to ensure stable water flow.

[0032] The inlet water regulating unit 1 includes an inlet water tank 1.1 and an outlet valve 1.2. The inlet water tank 1.1 is connected to the shale gas wastewater source via an inlet pipeline, and is used to receive raw water from the produced water storage facility, buffering the water flow and ensuring a uniform and stable inlet flow to subsequent treatment units. The outlet valve 1.2 is installed on the connecting pipeline from the inlet water tank 1.1 to the slow filter 2.1, and is used to control the water volume and flow rate entering the slow filter unit. A flow meter is installed on the inlet pipeline to monitor the inlet flow rate in real time, ensuring that the filtration rate is controlled within the set range.

[0033] The core equipment of the slow filtration pretreatment unit 2 is the slow filter tank 2.1. The slow filter tank 2.1 is a sealed container, and inside, from bottom to top, are laid a support layer 2.2, a slow filtration media layer 2.3, and a biological filter layer 2.4. The three layers together constitute the slow filtration system.

[0034] The support layer 2.2 is located at the bottom of the slow filter 2.1, immediately above the bottom outlet. It is constructed using a perforated support plate with a pore size of 2–5 mm, or by laying clean pebbles with a particle size of 2–5 mm, with a thickness of 5–10 cm. The main function of the support layer 2.2 is to provide uniform mechanical support for the upper slow filter media layer 2.3, prevent fine filter media from being lost with the effluent, and ensure that the effluent is evenly distributed at the bottom to avoid short-circuiting.

[0035] The slow-filtration media layer 2.3 is located above the support layer 2.2 and consists of zeolite or biochar with a particle size of 0.5–1 mm, or a mixture of both, with a thickness of 80–100 cm. Zeolite is a natural aluminosilicate mineral with a regular crystalline pore structure and high ion exchange capacity, effectively adsorbing ammonia nitrogen and some polyvalent scaling ions such as calcium and magnesium, reducing the hardness and scaling tendency of the produced water. Biochar is prepared by thermal decomposition of biomass under anaerobic or oxygen-deficient conditions and has a highly developed pore structure (specific surface area can reach 200–800 m²). 2 The filter media contains a large number of oxygen-containing functional groups (such as hydroxyl, carboxyl, and ketone groups), exhibiting excellent adsorption and removal capabilities for dissolved organic matter, colloidal substances, and heavy metal ions. The synergistic combination of these two components gives the slow-filter media layer the functions of ion exchange, physical retention, and multi-mechanism adsorption, enabling efficient removal of various types of pollutants. The filtration rate is controlled at 0.05–0.5 m / h via the outlet valve (1.2) to ensure sufficient contact time between the water flow and the filter media, achieving a deep purification effect.

[0036] The biofilter layer 2.4 is located on the top surface of the slow filter media layer 2.3. It is a biofilm layer formed on the surface of the slow filter media particles through natural domestication or artificial inoculation, with a thickness of 1–3 mm. This biofilm layer is composed of various microorganisms such as bacteria, protozoa, and algae, as well as their extracellular polymeric substances (EPS). Under aerobic conditions, it can convert dissolved organic matter (such as low molecular weight organic acids, alcohols, and aldehydes), ammonia nitrogen, etc. in the extracted water into harmless substances such as CO2, H2O, and N2 through aerobic biodegradation. It also removes colloidal substances and microbial metabolic byproducts through biological predation and adsorption.

[0037] The bottom outlet of the slow filter 2.1 is located below the support layer 2.2 and is connected to the outlet pipeline through outlet valve 2.5 to transport the effluent after slow filtration to the membrane distillation inlet tank 3.1 of the membrane distillation desalination unit 3.

[0038] The membrane distillation desalination unit 3 includes a membrane distillation inlet tank 3.1, an inlet valve 1 3.2, an inlet gear pump 3.3, a heater 3.4, a membrane distillation membrane module 3.5, an outlet gear pump 3.6, an outlet valve 3.7, a condenser 3.8, and a membrane distillation outlet tank 3.9.

[0039] The effluent from the slow filtration pretreatment enters the membrane distillation feed water tank 3.1 for temporary storage via the effluent pipeline. A heater 3.4 is placed in the membrane distillation feed water tank 3.1 to heat the pretreated effluent to 50–80°C and maintain a constant temperature. The heated feed water is then pumped at a stable flow rate into the feed water side (hot side) of the membrane distillation membrane module 3.5 by the feed water gear pump 3.3 after the inlet valve 3.2 is opened.

[0040] The membrane distillation module 3.5 uses a hydrophobic flat sheet membrane, specifically made of one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and polypropylene (PP). In this embodiment, polyvinylidene fluoride (PVDF) is selected. The membrane pore size is 0.45 μm, and the effective membrane area is 10 cm². 2 (Can be scaled up proportionally according to the project scale); The membrane module consists of two parallel operating units. Each unit has a rectangular flow channel with a length of 5 cm, a width of 2 cm, and a depth of 0.5 cm. Hot water flows in the flow channel on the inlet side (above the membrane), and condensate flows in the flow channel on the permeate side (below the membrane), forming a direct contact membrane distillation (DCMD) operating mode. Driven by the transmembrane vapor pressure difference generated by the transmembrane temperature difference (50-80℃ on the feed side, 10-30℃ on the condenser side, temperature difference ΔT = 30-60℃), water molecules evaporate at the hot side interface, pass through the hydrophobic membrane pores in the form of vapor, and condense into pure liquid water at the cold side interface. Dissolved salts, polyvalent ions, non-volatile organic compounds, etc. are completely retained by the hydrophobic membrane, forming high-salt concentrated water, which is returned to the membrane distillation feed water tank 3.1 through the reflux pipeline via the outlet gear pump 3.6 and outlet valve 3.7. After mixing with fresh pretreated effluent, it is circulated into the membrane module for treatment to achieve concentration and volume reduction. The low-salinity permeate water that permeates through the membrane enters the membrane distillation outlet water tank 3.9, is condensed by the condenser 3.8, and then enters the permeate collection unit 4.

[0041] The product water collection unit 4 includes an outlet valve 4.1 and a product water collection tank 4.2. The product water from the membrane distillation outlet tank 3.9 is controlled by the outlet valve 4.1 and transported to the product water collection tank 4.2 through the product water pipeline to complete the collection and storage of high-quality product water.

[0042] Example 1 Taking the backflow wastewater from a shale gas well in the Sichuan Basin as the treatment target, the raw water quality parameters are as follows: turbidity 796 NTU, UV... 254 It is 0.253 cm. -1 The TOC was 20.45 mg / L, TDS was 21,900 mg / L, ammonia nitrogen concentration was 86.7 mg / L, and pH was 7.2–7.8. In this embodiment, the filtration rate of the slow filtration media layer 2.3 was controlled at 0.1 m / h, the influent temperature of the membrane distillation was controlled at 70℃, and the condensate temperature was controlled at 20℃. The system's slow filtration pretreatment ran continuously for 60 days, and the membrane distillation was run for 12 hours per cycle. The specific operating steps are as follows: S1. Water Inlet Adjustment: Inject shale gas backflow wastewater into inlet tank 1.1 through the inlet pipe for a 30-minute water volume buffer. Open outlet valve 1.2 and adjust the opening to match the outlet flow rate with the design filtration rate of the slow filter (0.1 m / h). The extracted water enters the top of the slow filter tank 2.1 evenly through the inlet pipe.

[0043] S2. Slow Filtration Pretreatment: The treated water enters from the top of the slow filter 2.1 and flows sequentially from top to bottom under gravity through the biological filter layer 2.4 (approximately 1-3 mm thick in this embodiment), the slow filter media layer 2.3 (a mixture of zeolite and biochar in this embodiment, with a particle size of 0.5-1 mm and a layer thickness of 80 cm), and the support layer 2.2 (a support plate with a pore size of 3 mm and a thickness of 8 cm in this embodiment). The filtration rate is controlled at 0.1 m / h, and the entire process is carried out under normal temperature and pressure conditions without the addition of any chemical agents. The biological filter layer converts dissolved organic matter (TOC removal >60%) and ammonia nitrogen (removal >98%) into harmless substances through the biodegradation of aerobic microorganisms. The slow filter media layer removes ammonia nitrogen and some scale-forming polyvalent ions through ion exchange adsorption of zeolite (calcium and magnesium ion content reduced by >60%), removes colloidal organic matter and residual organic pollutants through adsorption of biochar, and removes suspended particles through physical interception (turbidity removal >95%). After treatment, the effluent is discharged from the bottom outlet of the slow filter 2.1 through the outlet valve 2.5 into the membrane distillation feed tank 3.1.

[0044] S3, Membrane Distillation Desalination: The pretreated effluent in the membrane distillation feed tank 3.1 is heated to 70°C by the heater 3.4 and pumped into the membrane distillation membrane module 3.5 at a constant flow rate by the feed gear pump 3.3; the condensate on the condensation side is maintained at 20°C by the outlet gear pump 3.6; the temperature difference between the feed side and the condensation side is 50°C, and the resulting transmembrane vapor pressure difference drives water vapor to pass through the hydrophobic PVDF membrane with a pore size of 0.45 μm, and condenses into permeate on the condensation side; salt and contaminants are retained by the membrane as concentrate, which is returned to the feed tank for circulation through the return pipeline; the permeate enters the membrane distillation outlet tank 3.9 and is collected in the permeate collection tank 4.2 through the outlet valve 4.1.

[0045] The final test results (average indicators after 60 days of continuous operation) are as follows: After treatment, the turbidity of the wastewater decreased to 38 NTU, achieving a removal rate of 95.2%; TOC decreased from 20.45 mg / L to 7.8 mg / L, achieving a removal rate of 61.9%; ammonia nitrogen concentration decreased from 86.7 mg / L to 1.21 mg / L, achieving a removal rate of 98.6%; suspended solids content decreased to below 5 mg / L; and the activity of scaling ions (calcium and magnesium ions) decreased by more than 60%, effectively removing key precursors of membrane fouling and wetting; UV... 254 From 0.253 cm -1 Decreased to 0.089 cm -1The removal rate reached 64.8%, indicating that a large amount of macromolecular organic matter was removed, significantly reducing the risk of organic pollution on the membrane surface.

[0046] The membrane distillation permeate has a turbidity of ≤1 NTU, conductivity of ≤8 μS / cm, TDS of ≤35 mg / L, ammonia nitrogen concentration of ≤0.5 mg / L, and salt removal rate of >99%. The permeate quality meets the industrial water quality requirements in the "Design Code for Urban Wastewater Reuse Engineering" (GB 50335-2016) and can be directly reused for shale gas fracturing.

[0047] Furthermore, with the synergistic effect of slow filtration pretreatment, the membrane flux of the membrane distillation module remained stable at 8–10 L / (m³) during continuous 12 hours of operation. 2 ·h), no significant flux decline was observed, and the stability was significantly improved compared with the control group without slow filtration pretreatment (flux decline >40%). After 60 days of continuous supply of effluent from slow filtration pretreatment, the surface fouling layer quality and inorganic scale deposition of the membrane module were reduced by about 55% and 62%, respectively, and the membrane lifespan was predicted to be extended by 1.5 to 2 times, which greatly reduced the operation, maintenance and replacement costs of the membrane module.

[0048] Specific reference Figure 2-5 As shown in the four charts, by comparing experimental data, these charts intuitively demonstrate the significant advantages of slow filtration pretreatment (SSF-S) over untreated shale gas produced water (SGPW) in terms of water quality improvement and membrane distillation operational stability.

[0049] Figure 2 and Figure 3 This shows how the slow filtration pretreatment effect changes over time. Figure 4 and Figure 5 This demonstrates how the performance of the membrane distillation process changes with water recovery rate.

[0050] Specifically, Figure 2 To compare ammonia nitrogen removal efficiency, the vertical axis represents ammonia nitrogen concentration, and the horizontal axis represents slow filter operation time.

[0051] SGPW (gray squares) represents raw water, i.e., shale gas produced water. Its ammonia nitrogen concentration fluctuates wildly between 60 and 120 mg / L, remaining at a high level.

[0052] SSF-S (pink diamond) represents the effluent after slow filtration pretreatment.

[0053] By reading the charts, it is clear that the ammonia nitrogen concentration in the slow-filtered effluent gradually decreases during the first 10 to 15 days of operation. After about 15 days, the ammonia nitrogen concentration in the effluent drops to almost 0 mg / L and remains stable, no longer fluctuating with the raw water concentration.

[0054] This proves that the biofilter layer in the slow filtration system is mature. After the biofilm is formed, ammonia nitrogen is efficiently removed through the biodegradation of nitrifying bacteria, verifying the biodegradation effect mentioned in the instructions.

[0055] Figure 3 This chart compares the effectiveness of organic matter removal. The vertical axis represents the concentration of recalcitrant organic matter such as humic substances and aromatic compounds in the water. The horizontal axis represents the slow filtration time.

[0056] SGPW (gray square) represents raw water, its UV... 254 The value is between 0.15 and 0.30 cm. -1 Fluctuations between.

[0057] SSF-S (pink diamond) represents slow-filtered water, with its UV... 254 The value was consistently significantly lower than that of the raw water, with minimal fluctuations.

[0058] This indicates that the slow filtration media layer (zeolite / biochar) and the biofilter layer effectively retained and mineralized dissolved organic matter through the synergistic effect of physical adsorption and biodegradation. It also reduced UV radiation. 254 These hydrophobic organic compounds are the main precursors leading to subsequent membrane fouling and wetting in membrane distillation.

[0059] Figure 4 To compare membrane flux stability and demonstrate antifouling performance, the vertical axis represents membrane flux, i.e., standardized flux, with 1.0 representing the initial flux and a decrease in value indicating membrane fouling. The horizontal axis represents water recovery rate (%), representing the proportion of water produced during the treatment process.

[0060] SGPW (gray squares) indicate that when directly treating raw water, the membrane flux rapidly declines as the recovery rate increases. When the recovery rate reaches 75%, the flux is only about 20% of the initial value.

[0061] SSF-S (green circle) indicates that after slow filtration pretreatment, the membrane flux remains extremely stable throughout the process, almost maintaining around 1.0 with no significant decline.

[0062] This reflects that direct treatment of raw water can lead to severe membrane fouling, which may be caused by suspended solids deposition or organic matter adsorption, as mentioned in the instructions.

[0063] The slow filtration device of the present invention effectively removes fouling factors, enabling the membrane distillation system to maintain high efficiency even with high recovery rates, thus solving the technical problem of rapid membrane flux decay.

[0064] Figure 5 The comparison of permeate conductivity demonstrates anti-wetting performance. The vertical axis represents conductivity, which corresponds to the salinity of the permeate. Lower values ​​indicate better water quality. The horizontal axis represents water recovery rate (%).

[0065] SGPW (gray squares) indicates that when directly treating raw water, the conductivity of the product water increases exponentially and sharply after the recovery rate exceeds 50%.

[0066] SSF-S (green circle) indicates that after slow filtration pretreatment, the conductivity of the produced water remains at an extremely low level throughout the process.

[0067] A surge in SGPW conductivity indicates membrane wetting, meaning brine has penetrated the hydrophobic membrane pores into the product water side, leading to desalination failure. This is because surfactants in the feed water reduce the membrane's hydrophobicity.

[0068] The stability curve of SSF-S, representing the process of this invention, proves that slow filtration pretreatment effectively removes organic matter and colloids that cause membrane wetting, ensuring the hydrophobic stability of the membrane distillation process and high-quality water production.

[0069] In summary, Figures 2-5 The four tables in the paper jointly demonstrate the core logic of this technical solution: slow filtration pretreatment removes ammonia nitrogen and organic matter through biological and physical processes, thereby eliminating the risks of membrane fouling and membrane wetting in the subsequent membrane distillation process and achieving long-term stable and efficient desalination.

[0070] Finally, it should be noted that the described embodiments are merely some, not all, of the embodiments of the present invention. Those skilled in the art will understand that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the claims and their equivalents; that is, all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A device for pre-treatment of shale gas produced water using slow filtration, characterized in that, It includes an influent regulating unit, a slow filtration pretreatment unit, a membrane distillation desalination unit, and a product water collection unit, which are connected in sequence. The water inlet regulating unit receives the shale gas produced water to be processed and regulates its water volume and controls its water level stability. The slow filtration pretreatment unit is connected to and communicates with the influent regulating unit, and includes a slow filtration tank. The slow filtration tank is provided with a support layer, a slow filtration medium layer and a biological filter layer from bottom to top. The membrane distillation desalination unit is connected to and communicates with the slow filtration pretreatment unit, and includes a membrane distillation inlet tank, a membrane distillation assembly, a heating device, and a condensation device connected in sequence.

2. The apparatus according to claim 1, characterized in that, The water inlet regulating unit includes an inlet water tank and an outlet valve. The inlet end of the inlet water tank is connected to the shale gas wastewater source through a pipeline, and the outlet end is equipped with an outlet valve that is connected to the slow filtration pretreatment unit.

3. The apparatus according to claim 1, characterized in that, The support layer is laid at the bottom of the slow filter tank, using a support plate with a pore size of 2-5mm or pebbles with a particle size of 2-5mm, and the thickness of the support layer is 5-10cm.

4. The apparatus according to claim 1, characterized in that, The slow filtration medium is composed of zeolite or biochar with a particle size range of 0.1 to 2 mm, and the thickness of each slow filtration medium layer is 80 to 100 cm. It is configured to control the filtration rate at 0.05 to 0.5 m / h.

5. The apparatus according to claim 1, characterized in that, The biofilter layer is a biofilm layer that is naturally formed or artificially inoculated on the surface of the slow filter medium, with a thickness of 1 to 3 mm.

6. The apparatus according to claim 1, characterized in that, The membrane distillation unit is a hydrophobic membrane unit, and the membrane material is selected from polyvinylidene fluoride, polytetrafluoroethylene or polypropylene, and the membrane form is a flat sheet membrane.

7. The apparatus according to claim 1, characterized in that, The membrane distillation desalination unit adopts a direct contact membrane distillation operation mode; the heating device is used to control the temperature of the membrane distillation feed water side at 50-80℃, and the condensing device is used to control the temperature of the condensing side at 10-30℃.

8. A method for pre-treatment of shale gas produced water desalination using the apparatus described in any one of claims 1-7, characterized in that, Includes the following steps: S1. Transport the shale gas produced water to the water intake regulating unit for water volume and level regulation; S2. The adjusted produced water enters the slow filtration pretreatment unit, where suspended solids, organic matter and inorganic pollutants are removed through a low-speed filtration process. S3. The slow-filtered water is transported to the membrane distillation desalination unit, where selective water vapor transport is achieved under temperature difference drive, thereby obtaining low-salinity product water.

9. The method according to claim 8, characterized in that, The slow filtration process in S2 operates under normal temperature and pressure conditions, and no additional chemical agents are added during the operation.

10. The method according to claim 8, characterized in that, The shale gas produced water is flowback fluid, produced water, or a mixture thereof.

Images