CN116395871B - Treatment method of high-oil high-salt natural gas fracturing flowback fluid - Google Patents

Treatment method of high-oil high-salt natural gas fracturing flowback fluid Download PDF

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CN116395871B
CN116395871B CN202310071617.1A CN202310071617A CN116395871B CN 116395871 B CN116395871 B CN 116395871B CN 202310071617 A CN202310071617 A CN 202310071617A CN 116395871 B CN116395871 B CN 116395871B
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杨红梅
肖德龙
何天智
曾文懿
刘同远
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Sichuan Entech Environment Technology Co ltd
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Abstract

The application discloses a treatment method of a high-oil high-salt natural gas fracturing flowback fluid, and belongs to the technical field of wastewater treatment. The method comprises the following steps: oil removal pretreatment, primary flocculation precipitation, air floatation, iron carbon micro electrolysis, primary advanced oxidation, secondary flocculation precipitation, evaporation desalination, a biochemical treatment system, tertiary flocculation precipitation, secondary advanced oxidation and a sludge dewatering machine system; the application adopts the coupling effect of the persulfate advanced oxidation and the biochemical method and the synergistic effect of the iron-carbon micro-electrolysis and the persulfate advanced oxidation to treat the high-concentration organic matters, adopts the evaporative crystallization to treat the high-salt pollutants, effectively solves the problem of removing the high-oil high-salt pollutants of the natural gas fracturing flowback fluid, has lower sludge quantity and energy consumption than the existing treatment method, is convenient to operate, has strong impact load resistance, has obvious economic value and environmental value, and has great significance in promoting the development of new natural gas energy, protecting the local ecological environment, strengthening the reutilization of water resources, reducing pollution and reducing carbon and the like.

Description

Treatment method of high-oil high-salt natural gas fracturing flowback fluid
Technical Field
The invention belongs to the technical field of wastewater treatment, and particularly relates to a treatment method of a high-oil high-salt natural gas fracturing flowback fluid.
Background
Hydraulic fracturing is the primary form of natural gas production and requires hydraulic fracturing with large amounts of chemically incorporated water poured into the shale formation to release the natural gas. A large amount of fracturing flowback fluid can be generated in the production process, and the wastewater contains a large amount of pollutants such as organic matters, salts, chloride ions, heavy metals, ammonia nitrogen, suspended matters and the like, has extremely high pollution degree, and has large water quality change at different exploitation sites and different exploitation periods. The pollutants are derived from additives of fracturing fluid, dissolved matters of underground rock stratum and pollutants in stratum water, and have biotoxicity, and if the pollutants are directly discharged, the pollutants can cause great harm to the ecological environment.
At present, the treatment methods of natural gas fracturing flowback fluid mainly comprise 3 types: reinjecting the groundwater layer after pretreatment, recycling the prepared fracturing flowback fluid after pretreatment, and discharging the fracturing flowback fluid reaching the standard after advanced treatment. The specific manner of treatment is related to the local environmental regulations and whether the mining site belongs to an environmentally sensitive area. With the attention of people on environmental protection and the research on whether the pollution risk of reinjection to underground water exists, the approval quantity of the reinjection wells by local environmental protection departments is gradually reduced. The recycling of the flowback fluid is mainly used for preparing the fracturing fluid, but the preparation water quantity of the fracturing fluid is limited, the reinjection water quantity is limited, and a large amount of residual waste water is inevitably discharged.
The existing discharging mode mainly adopts a transport vehicle to transport the wastewater to a sewage treatment plant which meets the environmental protection requirement for centralized treatment and then discharging the wastewater, but the transportation cost generated by the method is huge. Therefore, the construction of a sewage centralized treatment device at a mining site for directly discharging sewage after reaching the standard or recycling the sewage at the local site is the most economical mode and is also the development direction in the future.
The fracturing flowback fluid contains not only added chemical substances, but also a large amount of petroleum hydrocarbon organic matters, calcium, magnesium, barium, strontium and sodium in stratum, dissolved salts and the like, so that the flowback fluid is waste water with high organic matters (with concentration of 8000-20000 mg/L), high salt (with concentration of 40000-80000 mg/L), high hardness (with concentration of 5000-10000 mg/L) and high suspended matters (with concentration of 2000-3000 mg/L), has aromatic flavor (oil-water has obvious layering, COD concentration is up to 20000 mg/L), is mainly chloride salt, and the treatment difficulty is very high. GC-MS (see FIG. 2) and three-dimensional fluorescence (see FIG. 3) were performed on raw water, EEM fluorescence spectrum was roughly divided into 5 regions, I (Ex/Em=220-250/280-330 nm) and II (Ex/Em 220-250/330-380 nm) were aromatic proteins, III (Ex/Em=220-250/380-480 nm) were fulvic acid substances, IV (Ex/Em=250-440/280-330 nm) were soluble microbial products or petroleum hydrocarbons, and V (Ex/Em=250-400/380-540 nm) were humic acid. The organic matters in the wastewater are mainly humic acid and petroleum hydrocarbon, and the specific components are mainly a mixture of alkane (the content is 94.59%), aromatic hydrocarbon (the content is 3.92%) and a small amount of other organic matters (the content is 1.49%), such as sulfide, nitride, naphthenic acid and the like. The wastewater also contains a small amount of ammonia nitrogen (the concentration is 40-80 mg/L), total nitrogen (the concentration is 50-100 mg/L), sulfate ions (the concentration is 50-400 mg/L), bicarbonate (the concentration is 150-1000 mg/L), heavy metal ions As (the concentration is 0.1-0.6 mg/L), fe (the concentration is 1-5 mg/L) and the like.
According to different local requirements, the standard of the discharge after the treatment of the natural gas fracturing flowback fluid mainly comprises the first-level standard (COD is less than or equal to 60mg/L, BOD) of the integrated wastewater discharge Standard GB16297-1996 5 Less than or equal to 20mg/L, SS less than or equal to 70mg/L, petroleum less than or equal to 5mg/L, ammonia nitrogen less than or equal to 30 mg/L), and III standard (COD less than or equal to 20mg/L, BOD) of surface water environment quality standard GB3838-2002 5 Less than or equal to 4mg/L, less than or equal to 250mg/L of chloride, less than or equal to 0.05mg/L of arsenic, less than or equal to 1mg/L of ammonia nitrogen, less than or equal to 0.3mg/L of Fe), and the like (maximum value) in the standard of farmland irrigation GB 5084-2021COD≤200mg/L,BOD 5 150mg/L or less, 200mg/L or less of SS or less, 2000mg/L or less of TDS or less, 350mg/L or less of chloride or less, 0.05mg/L or less of arsenic or less, and 30mg/L or less of Kjeldahl nitrogen or less. The requirements of COD, TDS, chloride ions, ammonia nitrogen, petroleum, arsenic, suspended substances and the like are met in the standards, so that the removal measures of the pollutants need to be considered.
The TDS of the fracturing flowback fluid is up to 40000-80000 mg/L, the TDS of the yielding water is less than or equal to 2000mg/L, and the chloride is less than or equal to 250mg/L, so that desalting measures are needed. At present, the desalting measures mainly comprise MVR evaporation and RO membrane filtration, but high-concentration COD can cause the MVR evaporation concentrate to be sticky, influence the heat transfer performance and the crystallization separation of salt, and easily pollute and block the RO membrane, so the COD needs to be reduced before desalting.
The COD concentration of the fracturing flowback fluid is up to 20000mg/L, and is mainly contributed by the included petroleum hydrocarbon organic matters, so that measures for oil separation and air floatation are very necessary. The residual COD after oil separation is still 7000-8000 mg/L, the color is milky white, the smell is large, the colloidal suspended substances are more, and therefore, flocculation precipitation measures are very necessary; the solution becomes transparent after flocculation and precipitation, but the COD concentration is still 2000-3000 mg/L, which indicates that the solubility COD is still very high. The prior soluble COD treatment method mainly comprises an advanced oxidation method, a biochemical method, an adsorption method and the like. However, the fracturing flowback fluid contains high-concentration salt and macromolecular alkane organic matters with poor biodegradability, is not suitable for direct biochemistry, has high COD concentration and is not suitable for direct adsorption, so that the advanced oxidation method has great advantages in the treatment of the wastewater.
The current common advanced oxidation methods include Fenton oxidation, ozone catalytic oxidation, electrocatalytic oxidation, wet oxidation and the like. However, the quenching effect of Fenton ferrous oxide on hydroxyl free radicals ensures that the COD removal rate is only about 50%, the efficiency is low, acid and alkali are required to be repeatedly added, a large amount of reagent is added, the salt content of wastewater is increased, and a large amount of sludge is generated (the volume ratio of the sludge is 40% -50%); the sludge yield of ozone catalytic oxidation and electrocatalytic is low, but the equipment investment is large and the power consumption is large; wet oxidation requires reaction under high temperature and high pressure, and has high operation condition and high power consumption. Therefore, it is very necessary to find an advanced oxidation method which is convenient to operate, high in efficiency, low in power consumption and low in solid waste yield to remove high-concentration COD in the wastewater.
In addition, the wastewater contains a small amount of arsenic, ammonia nitrogen and volatile organic compounds, so that the measures of removing arsenic from the wastewater and removing COD and ammonia nitrogen from condensate are also considered; the quality of the natural gas fracturing flowback fluid has great change, the salt content and the concentration of pollutants such as organic matters of the fracturing flowback fluid at different times in different places are different, and measures for ensuring the stability and the standard of the yielding water for different qualities of water are also required to be considered.
Therefore, in view of the above technical problems, there is a need for a treatment method of high-oil high-salt natural gas fracturing flowback fluid, which can effectively solve the problem of pollutant discharge of the fracturing flowback fluid, reduce the amount of solid waste, reduce energy consumption and simplify the operation conditions.
Disclosure of Invention
Aiming at the high-oil high-salt natural gas fracturing flowback fluid, the invention aims to provide the treatment method of the high-oil high-salt natural gas fracturing flowback fluid, which effectively solves the problem of pollutant discharge of the waste water, reduces the generation amount of solid waste such as sludge, crystalline salt and the like, reduces the energy consumption, simplifies the operation condition, adapts to different variable water quality and ensures that the high-oil high-salt natural gas fracturing flowback fluid stably meets the standard for discharge.
In order to achieve the above purpose, the technical scheme adopted by the invention is as follows:
(1) Oil removal pretreatment: oil-water separation is carried out on the high-oil high-salt natural gas fracturing flowback fluid to obtain petroleum and wastewater;
(2) Primary flocculation precipitation: regulating the pH value of the wastewater in the step (1) to 8-9, and then adding PAC and PAM for flocculation precipitation to obtain supernatant and precipitated sludge;
(3) Air floatation: removing floating oil in the supernatant fluid in the step (2) to obtain high-oil emulsion and wastewater;
(4) Iron-carbon micro-electrolysis: adding hydrochloric acid into the wastewater obtained in the step (3) to adjust the pH value to 3, performing iron-carbon micro-electrolysis reaction, and judging whether COD (chemical oxygen demand) after the iron-carbon micro-electrolysis reaction is more than 2000mg/L;
COD is less than or equal to 2000mg/LAdopting an air pipe for aeration and stirring, and automatically adjusting the pH value of the wastewater to 8-9 to obtain the Fe-containing wastewater 3+ Is a waste water of (2); when COD is more than 2000mg/L, stirring by adopting an internal reflux pump to obtain the Fe-containing material 2+ Is a waste water of (2);
(5) High-grade oxidation: if the step (4) adopts an internal reflux pump for stirring, the Fe is contained 2+ Adding persulfate into the wastewater to perform oxidation reaction to obtain Fe-containing wastewater 3+ Is a waste water of (2); if the step (4) adopts air aeration stirring, the step (5) is not added with a medicament and is only used as an overcurrent unit;
(6) Secondary flocculation precipitation: fe is contained in the step (4) or the step (5) 3+ Adding sodium carbonate, PAC and PAM into the wastewater to perform flocculation precipitation to obtain supernatant and precipitated sludge;
(7) Evaporating to remove salt: judging whether the concentrated solution needs to be reinjected into the gas well according to the actual engineering requirement of the gas well, and concentrating the supernatant if the concentrated solution needs to be reinjected into the gas well to obtain concentrated solution and condensate; if reinjection is not needed, concentrating and crystallizing to obtain crystallized salt and condensate;
(8) AO biochemical treatment: carrying out AO biochemical treatment on the condensate in the step (7) to obtain condensate and activated sludge after biochemical treatment;
(9) Three-stage flocculation precipitation: adding PAC and PAM into the condensate after biochemical treatment in the step (8) for flocculation and precipitation to obtain supernatant and precipitated sludge;
(10) Secondary advanced oxidation: judging whether the COD concentration of the supernatant fluid in the step (9) reaches the following emission standard according to the requirements of different areas: the COD is less than or equal to 60mg/L according to the first-level standard of the integrated wastewater discharge standard GB 16297-1996; III standard of surface water environment quality standard GB3838-2002, COD is less than or equal to 20mg/L; "farm irrigation Standard" GB 5084-2021, maximum COD is less than or equal to 200mg/L;
when COD reaches the standard, discharging; when the COD is not up to standard, adding activated carbon particles and persulfate into the supernatant to perform adsorption and oxidation reaction, and discharging after the COD is stable and reaches the standard.
Further: the steps also include (11) sludge dewatering: and (3) discharging the sludge obtained in the step (2), the step (6), the step (8) and the step (9) into a dehydration system for dehydration to obtain filtrate and a filter cake, and reinjecting the filtrate into a primary flocculation precipitation unit for continuous treatment.
Further: in the step (1), oil and water are separated in an oil separation tank unit through an oil separator, the obtained petroleum is recovered, and the volume ratio content of the petroleum in the high-oil high-salt natural gas fracturing flowback fluid is 8 percent at most.
Further: in the step (2), sodium hydroxide is added into the wastewater in a primary flocculation precipitation unit, the pH value of the wastewater is regulated to 8-9, the addition amount of PAC is 1000-1500 mg/L, the addition amount of PAM is 2-5 mg/L, and calcium hydroxide generated in the wastewater is used as a coagulant aid, so that the flocculation effect is improved.
After the pH value is adjusted to 8-9, HCO in the wastewater 3- With OH - Neutralization to produce CO 3 2- ,CO 3 2- And then with Ca in the wastewater 2+ CaCO production 3 Precipitation of at the same time OH - Ca in wastewater 2+ Production of Ca (OH) 2 And the produced calcium hydroxide precipitate can be used as a coagulant aid while removing precipitable ions such as calcium, magnesium and the like in the wastewater, so that the precipitation effect is quickened, and the PAC and PAM precipitation effect is better in an alkaline environment.
Further: in the step (3), in the air floatation unit, residual floating oil and suspended matters in the supernatant fluid are carried on the water surface by adopting an air dissolution air floatation method and removed, so as to obtain high-oil emulsion and wastewater, and the high-oil emulsion is recovered.
Further: in the step (4), in an iron-carbon micro-electrolysis unit, the reaction pH value of the iron-carbon micro-electrolysis is 3, the reaction time is 2-4 hours, the filler of the iron-carbon micro-electrolysis reactor is iron-carbon, and is formed by melting ferric oxide, graphite powder and a catalyst, the iron content is 75%, the carbon content is 10-15%, the catalyst content is 10-15%, and the bulk density is 1.2-1.4 kg/m 3 The porosity is 50-70%, and the filler strength is more than or equal to 600 kg f/cm.
Further: in the step (4), when COD is less than or equal to 2000mg/L after the iron-carbon micro-electrolysis reaction in the iron-carbon micro-electrolysis unit, air aeration stirring is adopted during the iron-carbon micro-electrolysis reaction, and the air-water ratio is 3:1-51, introducing oxygen while stirring, automatically raising the pH value to 8-9 after the reaction, and Fe 2+ Is oxidized to Fe 3+ Ferric hydroxide precipitate is generated.
When the iron-carbon micro-electrolysis reaction is carried out, the innumerable primary batteries formed in the iron-carbon micro-electrolyzer are utilized to carry out electrochemical treatment on organic matters in the wastewater, so that partial COD is degraded, and the biodegradability is improved.
Further: in the step (4), when COD is more than 2000mg/L after the iron-carbon micro-electrolysis reaction in the iron-carbon micro-electrolysis unit, an internal reflux pump is adopted for hydraulic stirring during the iron-carbon micro-electrolysis reaction, so that the dissolved iron is Fe 2+ To obtain Fe-containing alloy 2+ Is the waste water of Fe 2+ As an activator of the primary advanced oxidation reaction in step (5).
Further: in the step (5), if the Fe is contained in the primary advanced oxidation unit obtained in the step (4) 3+ The wastewater is directly subjected to the next step without treatment; if the Fe is contained in the mixture obtained in the step (4) 2+ The concentration ratio of the persulfate to COD is 0.5:1-3:1, and the reaction time is 2-6 h, wherein Fe is added into the wastewater 2+ As an activator; the persulfate is sodium persulfate.
After adding persulfate, fe in the wastewater is utilized 2+ As an activator, persulfate is activated to generate sulfate radical and hydroxyl radical, so that COD in the wastewater is degraded, and biodegradability is improved.
Further: in the step (5), in order to enhance the oxidation effect, hydrogen peroxide is added, and the molar ratio of the added hydrogen peroxide to persulfate is 1:10.
To enhance the oxidation effect, after adding hydrogen peroxide, fe 2+ As a catalyst, the hydrogen peroxide is catalyzed to generate hydroxyl free radicals to oxidize organic matters, and excessive hydrogen peroxide can cause quenching of the hydroxyl free radicals and also can cause the sludge in a subsequent flocculation tank to float upwards, so that the hydrogen peroxide is only added when the oxidation effect needs to be enhanced, and the excessive hydrogen peroxide is not suitable to be added.
Further: in the step (6), if Fe is contained in the secondary flocculation precipitation unit 3+ The waste water obtained in the step (4) is directly introduced into the reaction kettleAdding sodium carbonate, PAC and PAM to perform flocculation precipitation; if Fe is contained 3+ And (2) adding sodium hydroxide into the wastewater obtained in the step (5) to adjust the pH value to 8-9, and then adding sodium carbonate, PAC and PAM to perform flocculation precipitation.
Due to the Fe content obtained in the step (4) 3+ In the step (4), air aeration stirring is adopted, the pH value of the wastewater is automatically increased to 8-9, and Fe 2+ Is oxidized to Fe 3+ Generating Fe (OH) 3 And (3) precipitation, wherein persulfate is not added in the step (5), and the step (5) is only used as an overflow unit, so that sodium carbonate, PAC and PAM can be directly added in the step (6) for flocculation precipitation.
Due to the Fe content obtained in the step (5) 3+ In the step (4), an internal reflux pump is adopted for hydraulic stirring, the pH value after the reaction is about 5.2, the pH value can be automatically reduced to below 2.9 after sodium persulfate is added, a large amount of sulfate radicals are generated by activating sodium persulfate under an acidic condition to oxidize organic matters, the pH value after the reaction is 2.4, therefore, sodium hydroxide is added to adjust the pH value to 8-9, iron ions are precipitated and removed under an alkaline condition, arsenic ions and iron ions generate ferric arsenate precipitates, and the arsenic ions are removed.
Further: in the step (6), after sodium carbonate is added into the secondary flocculation precipitation unit, the concentration of calcium ions in supernatant is less than or equal to 50 mg/L, the adding amount of PAC is 150-250 mg/L, and the adding amount of PAM is 2-5 mg/L.
Further: in the step (7), in the evaporation desalting unit, when the actual situation of the gas well is judged and the concentrated solution needs to be refilled, the concentrated solution is concentrated by adopting an evaporator, and the concentration multiple is 1-3 times, so as to obtain the concentrated solution with the volume percent concentration of 25% -50%; when the concentrated solution does not need to be refilled, an evaporator is adopted for crystallization, and the obtained crystallized salt is used as industrial salt to be recovered.
Further: in the step (7), the evaporator adopts an MVR evaporator, the operating temperature is 90 ℃, the operating pressure is-55 kPa to-65 kPa, the heat source is steam, and the steam pressure is more than or equal to 0.3Mpa.
Further: in the step (8), if the pH value of the condensate liquid is acidic in an AO biochemical system unit, adding alkali to adjust the pH value to 8-9 before entering the unit, and performing biochemical treatment after the condensate liquid enters a biochemical system to obtain the condensate liquid and activated sludge after the biochemical treatment, wherein the biochemical system adopts an A/O process to remove COD and ammonia nitrogen in the condensate liquid, and the treatment time is 15-20 h; the biochemical system comprises a secondary sedimentation tank and a mixed liquor reflux system, and mixed liquor sludge with the concentration of 4000-5000 mg/L can be obtained.
After the volatile macromolecular organic matters such as aromatic hydrocarbon, naphthene and the like are degraded in the step (4) and the step (5), part of the organic matters in the condensate are changed into small molecular organic matters such as alcohol, ketone, aldehyde, acid and the like, so that the biodegradability of the condensate is improved, and COD and ammonia nitrogen are further degraded in an AO biochemical treatment system.
Further: in the step (9), in the three-stage flocculation precipitation unit, the addition amount of PAC is 150-250 mg/L, and the addition amount of PAM is 1-2 mg/L.
The condensate after AO biochemical treatment still contains partial organic suspended matters and colloid, so PAC and PAM are added again for flocculation precipitation.
Further: in the step (10), in a secondary advanced oxidation unit, the diameter of the activated carbon particles is 4mm, the length is 10mm, the mass ratio of the added activated carbon particles to sodium persulfate is 30:1-50:1, and the concentration ratio of the added persulfate to COD is 0.5:1-3:1; the treatment time of the secondary advanced oxidation unit is 4-6 hours; at least two secondary oxidation reactors are arranged in the secondary advanced oxidation unit, one secondary oxidation reactor is ensured to run, and the rest secondary oxidation reactors are regenerated for standby.
Adding active carbon and persulfate, and further degrading COD by utilizing the adsorption effect of the active carbon and activating the persulfate to generate sulfate radical and hydroxyl radical; at least two secondary oxidation reactors are arranged, one of the secondary oxidation reactors operates, the rest of the secondary oxidation reactors are regenerated for standby, and the standby secondary oxidation reactors are used for back flushing and regenerating the activated carbon periodically.
Compared with the prior art, the technical scheme of the invention has the following beneficial effects:
1) The invention adopts the coupling effect of persulfate advanced oxidation and biochemical method, the synergistic effect of iron-carbon micro-electrolysis and persulfate advanced oxidation, and the synergistic effect of activated carbon adsorption and persulfate advanced oxidation to treat high-concentration organic matters, adopts evaporative crystallization to treat high-concentration salt, effectively solves the problems of removing high-salt and high-organic pollutant in natural gas fracturing flowback fluid, and simultaneously takes into account the effective removal of pollutants such as ammonia nitrogen, arsenic and the like;
2) The invention adopts a small amount of ferrous iron and granular activated carbon dissolved by iron carbon micro-electrolysis as a catalyst to activate persulfate, generates hydroxyl free radicals and sulfate radical to oxidize organic matters, greatly reduces the sludge content and salt content of the Fenton treatment method in the prior art, and reduces the discharge amount of solid waste; the equipment is simple, the electricity consumption is low, and the equipment configuration and the electricity consumption of the existing ozone catalytic oxidation and wet oxidation methods are greatly reduced;
3) According to the invention, the adding mode and the dosage of the sodium persulfate are determined according to the condition of the water quality of the inlet water, the operation is flexible and convenient, the water quality of the outlet water is ensured to reach the standard stably, and the problem of large water quality change of the natural gas fracturing flowback fluid is solved;
4) The invention has remarkable economic and environmental values, and has great significance in promoting the development of new natural gas energy, protecting local ecological environment, enhancing the reutilization of water resources, reducing pollution, reducing carbon and the like.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings that are needed in the embodiments will be briefly described below, it being understood that the following drawings only illustrate some examples of the present invention and therefore should not be considered as limiting the scope, and other related drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
FIG. 1 is a flow chart of the processing of each unit of the processing method of the high-oil high-salt natural gas fracturing flow-back fluid.
Fig. 2 is a GC-MS detection diagram of raw water of a method for treating a high-oil high-salt natural gas fracturing flow-back fluid provided by the present invention.
Fig. 3 is a three-dimensional fluorescence detection diagram of raw water of the treatment method of the high-oil high-salt natural gas fracturing flowback fluid.
Fig. 4 is a three-dimensional fluorescence detection chart of wastewater after iron-carbon reaction in the treatment method of the high-oil high-salt natural gas fracturing flowback fluid provided by the embodiment 1 of the invention.
Fig. 5 is a three-dimensional fluorescence detection chart of condensate after evaporation and desalting for the treatment method of the high-oil high-salt natural gas fracturing flowback fluid provided by the embodiment 1 of the invention.
Fig. 6 is a three-dimensional fluorescence detection chart of condensate after biochemical treatment for 20 hours in the treatment method of the high-oil high-salt natural gas fracturing flow-back fluid provided by the embodiment 1 of the invention.
Fig. 7 is a three-dimensional fluorescence detection chart of the supernatant fluid after activated carbon adsorption in the treatment method of the high-oil high-salt natural gas fracturing flowback fluid provided in the embodiment 1 of the invention.
Fig. 8 is a three-dimensional fluorescence detection chart of the supernatant fluid after the high-level reaction of activated carbon activated sodium persulfate in the treatment method of the high-oil high-salt natural gas fracturing flow-back fluid provided by the embodiment 1 of the invention.
Fig. 9 is a three-dimensional fluorescence detection diagram of wastewater after sodium persulfate is added into a primary oxidation unit and oxidized in the treatment method of the high-oil high-salt natural gas fracturing flowback fluid provided in the embodiment 2 of the invention.
Fig. 10 is a three-dimensional fluorescence detection diagram of condensate after sodium persulfate is added into a first-stage oxidation unit and is subjected to evaporation and desalination in the treatment method of the high-oil high-salt natural gas fracturing flowback fluid provided in the embodiment 2 of the invention.
FIG. 11 is a GC-MS chromatogram of condensate after evaporation and desalting of sodium persulfate not added to a primary advanced oxidation unit in the treatment method of the high-oil high-salt natural gas fracturing flow-back fluid provided by the embodiment 1 of the invention.
Fig. 12 is a GC-MS chromatogram of condensate obtained by adding sodium persulfate to a first-stage oxidation unit and desalting by evaporation in the treatment method of a high-oil high-salt natural gas fracturing flow-back fluid provided in embodiment 2 of the invention.
Detailed Description
To make the objects, technical solutions and advantages of the present invention more apparent, the technical solutions in the embodiments of the present invention will be clearly and completely described below, and it is apparent that the described embodiments are some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the present invention without making any inventive effort, are intended to fall within the scope of the present invention. Accordingly, the detailed description of the embodiments of the invention provided below is not intended to limit the scope of the invention as claimed, but is merely representative of selected embodiments of the invention.
Example 1:
the embodiment provides a treatment method of a high-oil high-salt natural gas fracturing flowback fluid, which is suitable for the treatment of standard discharge of high-oil-content and high-organic-content natural gas fracturing flowback fluid wastewater, wherein the wastewater contains a large amount of organic matters, salts, chloride ion suspended matters, calcium ions and magnesium ions, and a small amount of pollutants such as heavy metals, ammonia nitrogen and the like.
The parameters of various pollutants in the initial high-oil high-salt natural gas fracturing flowback fluid of the embodiment are shown in table 1:
TABLE 1
The standard to be achieved by the water output of the embodiment is the standard of dry land crops (COD is less than or equal to 200mg/L, BOD) of GB 5084-2021 (Standard for irrigation of farmlands) 5 Less than or equal to 100mg/L, less than or equal to 100mg/L of SS, less than or equal to 1000mg/L of TDS (non-saline-alkaline land), less than or equal to 350mg/L of chloride, less than or equal to 0.05mg/L of arsenic, and less than or equal to 30mg/L of Kjeldahl nitrogen.
As shown in fig. 1, the initial flowback fluid is treated by the steps of:
(1) Oil removal pretreatment:
the initial flowback fluid contains an obvious separable oil layer, oil-water in the flowback fluid is separated in an oil separation tank unit through an oil separator to obtain petroleum and wastewater with the volume ratio of 8%, the petroleum is recovered, and the wastewater enters a next-stage treatment unit.
After oil removal pretreatment, the pH value of the wastewater is 5.77, the COD concentration is reduced from 20000 mg/L to 7822 mg/L, and the concentrations of ammonia nitrogen, TDS, chloride ions and SS are unchanged.
(2) Primary flocculation precipitation:
in a primary flocculation precipitation unit, the wastewater subjected to oil removal in the step (1) is yellow-white emulsion (COD concentration is 7822 mg/L), the pH value is 5.77, sodium hydroxide is added into the wastewater, the pH value is adjusted to 9, and 1500 PAC/L and 5 PAM/L are added under alkaline conditions to perform flocculation precipitation, so that HCO in the wastewater is caused 3 - With OH - Neutralization to produce CO 3 2- ,CO 3 2- And then with Ca in the wastewater 2+ CaCO production 3 Precipitation of at the same time OH - Ca in wastewater 2+ Production of Ca (OH) 2 The calcium hydroxide precipitate generated when the precipitable ions such as calcium, magnesium and the like are removed is used as a coagulant aid to accelerate the precipitation effect; and (3) obtaining supernatant and precipitated sludge after primary flocculation precipitation, discharging the precipitated sludge into a sludge dewatering system for dewatering, and enabling the supernatant to enter a next-stage treatment unit.
After primary flocculation precipitation, the pH value of the supernatant is 9, the COD concentration is reduced from 7822 mg/L to 2692.28 mg/L, the ammonia nitrogen concentration is unchanged, and due to the addition of the medicament, the TDS concentration is increased from 34150 mg/L to 37531.86 mg/L, the chloride ion concentration is increased from 29200mg/L to 30620mg/L, and the SS concentration is reduced from 2770 mg/L to 210 mg/L.
(3) Air floatation:
and (3) introducing the supernatant (COD concentration is 2692.28 (m/L)) obtained in the step (2) into an air floatation unit, removing emulsified oil and suspended matters again by adopting a dissolved air floatation method, introducing gas into the supernatant, introducing residual floating oil in the supernatant into the water surface by bubbles, obtaining high-oil emulsion and wastewater with pH value of 8, recovering the high-oil emulsion, and introducing the wastewater into a next-stage treatment unit.
After dissolved air floatation, the pH value of the wastewater is 8, the COD concentration of the supernatant is reduced from 2692.28 mg/L to 2623.05 mg/L, the ammonia nitrogen concentration is unchanged, the TD and chloride ion concentrations are unchanged, and the SS concentration is reduced from 210 mg/L to 180 mg/L.
(4) Iron-carbon micro-electrolysis:
the wastewater (COD concentration is 2623.05 (m/L)) in the step (3) enters an iron-carbon micro-electrolysis unit, hydrochloric acid is added to adjust the pH value to 3, and in the iron-carbon micro-electrolysis reactor, the organic matters in the wastewater are subjected to electrochemical treatment by utilizing innumerable primary batteries formed in the iron-carbon micro-electrolysis, so that partial COD is degraded, and the biodegradability is improved; wherein, the filler of the iron-carbon micro-electrolysis reactor is iron-carbon, which is formed by melting ferric oxide, graphite powder and a catalyst, the iron content is 75%, the carbon content is 10-15%, the catalyst content is 10-15%, and the bulk density is 1.2-1.4 kg/m 3 The porosity is 50-70%, and the filler strength is more than or equal to 600 kg f/cm;
if COD is less than or equal to 2000mg/L after the iron-carbon micro-electrolysis reaction (under normal conditions), air aeration is adopted to stir the wastewater, the air-water ratio is 3:1-5:1, oxygen is introduced while stirring, the pH value is automatically increased to 9, and Fe is added 2+ Is oxidized to Fe 3+ Ferric hydroxide sediment is generated under alkaline condition, partial organic matters are net-caught by flocculent sediment, COD is removed again, the micro-electrolysis reaction time of iron and carbon is 2-4 h, and finally Fe is obtained 3+ Is a waste water of (2); if COD is more than 2000mg/L after the iron-carbon micro-electrolysis reaction, an internal reflux pump is adopted for stirring to obtain the Fe-containing material 2+ Is a waste water of (a) and (b).
In the step, after the iron-carbon micro-electrolysis reaction, the pH value of the wastewater is 9, the COD concentration is reduced from 2623.05 mg/L to 1990.37 mg/L, the COD is less than or equal to 2000mg/L, air aeration stirring is adopted, the air-water ratio is 3:1, the ammonia nitrogen concentration is reduced from 45 mg/L to 39 mg/L, the pH value is regulated by adding hydrochloric acid, the TDS concentration is increased from 37531.86 mg/L to 37753.83 mg/L, the chloride ion concentration is increased from 30620mg/L to 30841.94mg/L, and the SS concentration is increased from 180 mg/L to 2000mg/L due to the generation of ferric hydroxide precipitation. The three-dimensional fluorescence detection diagram of the wastewater is shown in fig. 4.
(5) High-grade oxidation:
fe-containing in step (4) 3+ Is a mg waste water (COD concentration is 1990.37)I) enters a first-stage oxidation unit, and COD after reaction is less than 2000mg/L, so the first-stage oxidation unit is used as an overflow unit, sodium persulfate is not added, and Fe is contained 3+ The wastewater of the (a) directly enters the next stage of treatment unit.
(6) Secondary flocculation precipitation:
the step (5) contains Fe 3+ The waste water (COD concentration is 1990.37 per liter, SS concentration is 2000 per liter) enters a secondary flocculation precipitation unit, the pH value of the waste water is 9, the calcium ion concentration in the waste water is controlled within 50 mg per liter after sodium carbonate is added, PAC of 150 mg per liter and PAM of 2 mg per liter are added for flocculation precipitation, iron ions, magnesium ions and calcium ions in the waste water are precipitated, supernatant fluid and precipitated sludge are obtained, the precipitated sludge is discharged into a sludge dewatering system, and the supernatant fluid enters a next-stage treatment unit;
Due to the inclusion of Fe 3+ In the step (4), air aeration stirring is adopted, fe 2+ Is oxidized to Fe 3+ The pH value of the wastewater automatically rises to 9, the wastewater is alkaline, and Fe (OH) is generated 3 And (3) precipitation, wherein persulfate is not added in the step (5), and the step (5) is only used as an overflow unit, so that sodium carbonate, PAC and PAM can be directly added in the step (6) for flocculation precipitation.
After the secondary flocculation precipitation, the pH value of the wastewater is 9, the concentrations of COD and ammonia nitrogen are basically unchanged, and due to the addition of the medicament, the TDS concentration is increased from 37753.83 mg/L to 41653.93 mg/L, the chloride ion concentration is increased from 30841.94mg/L to 31031.31mg/L, and the SS concentration is reduced from 2000mg/L to 100 mg/L.
(7) Evaporating to remove salt:
the supernatant fluid (COD concentration is 1990.37 (m/L)) obtained in the step (6) enters an evaporation desalting unit, and when the concentrated solution needs to be refilled according to the actual situation of a gas well, the supernatant fluid is concentrated through an MVR evaporator to obtain concentrated solution and condensate with the concentration multiple of 1-3 times, namely, the volume percent concentration is 25-50%, and the concentrated solution is discharged outside and refilled into the gas well; when the concentrated solution does not need to be refilled, the MVR evaporator crystallizes the supernatant to obtain crystallized salt and condensate, the crystallized salt is recycled as industrial salt, and the condensate enters the next-stage treatment unit; the operation temperature of the MVR evaporator is 90 ℃, the pressure is-55 kPa to-65 kPa, the heat source is steam, and the steam pressure is more than or equal to 0.3MPa.
After evaporation for desalting, the pH value of the condensate liquid is 8.5, the COD concentration is reduced from 1990.37 mg/L to 1393.26 mg/L, the ammonia nitrogen concentration is reduced from 39 mg/L to 31.1 mg/L, the TDS concentration is reduced from 41653.93 mg/L to 416.54 mg/L, the chloride ion concentration is reduced from 31031.31mg/L to 31.03mg/L, and the SS concentration is reduced from 100mg/L to 0mg/L. The three-dimensional fluorescence detection diagram of the condensate is shown in FIG. 5, and the GC-MS chromatogram of the condensate is shown in FIG. 11.
(8) AO biochemical treatment:
the condensate liquid (COD concentration is 1393.26/L, pH value is 8.5) in the step (7) enters an AO biochemical system unit, biochemical treatment is carried out through a biochemical system, and the biochemical system adopts an A/O technology to remove COD and ammonia nitrogen in the condensate liquid; as the condensate liquid contains volatile macromolecules such as organic aromatic hydrocarbon, cycloparaffin and the like, BOD/COD=0.31 and has poor biodegradability, COD and ammonia nitrogen in the condensate liquid are degraded through an AO biochemical system, and the treatment time is 20 hours;
the biochemical system comprises a secondary sedimentation tank and a mixed liquor reflux system, the sludge concentration of the mixed liquor is 4000-5000 mg/L, condensate after biochemical treatment enters the secondary sedimentation tank for solid-liquid separation, precipitated sludge is discharged into a sludge dewatering system, and condensate after biochemical treatment enters a next-stage treatment unit.
After AO biochemical treatment, the pH value of condensate liquid is 8.0, COD concentration is reduced from 1393.26 mg/L to 348.31 mg/L, ammonia nitrogen concentration is reduced from 31.1 mg/L to 7.42 mg/L, and the concentrations of TDS and chloride ions are unchanged, and because the condensate liquid is added with activated sludge, SS concentration is increased from 0mg/L to 80mg/L. The three-dimensional fluorescence detection diagram of the condensate after biochemical treatment is shown in figure 6.
(9) Three-stage flocculation precipitation:
and (3) feeding the condensate (COD concentration is 348.31 (m/L)) after biochemical treatment in the step (8) into a three-stage flocculation precipitation unit, and adding 150mg/L PAC and 1mg/L PAM for flocculation precipitation because part of organic suspended matters and colloid are also contained in the condensate, further removing the organic suspended matters and the colloid to obtain supernatant and precipitated sludge, discharging the precipitated sludge into a sludge dewatering system, and feeding the supernatant into a next-stage treatment unit.
After three-stage flocculation precipitation, the pH value of the supernatant is 7.8, the concentration of the dissolved COD and ammonia nitrogen is basically unchanged, and as the medicament is added, the TDS concentration is increased from 416.54 mg/L to 605.93 mg/L, the chloride ion concentration is increased from 31.03mg/L to 220.39mg/L, and the SS concentration is reduced from 80mg/L to 50mg/L.
(10) Secondary advanced oxidation:
the supernatant fluid (COD concentration is 348.31/L) in the step (9) enters a secondary oxidation unit, and because COD is larger than the effluent standard (COD is less than or equal to 200 mg/L), the supernatant fluid enters a persulfate secondary oxidation reactor, granular activated carbon and persulfate are added, the activated carbon is utilized for adsorption, the persulfate is activated to generate sulfate radical and hydroxyl radical to further degrade COD, the mass ratio of the added activated carbon particles to sodium persulfate is 30:1-50:1, the concentration ratio of the added persulfate to COD is 0.5:1-3:1, and operators can reasonably select according to actual conditions; the three-dimensional fluorescence detection chart of the supernatant after the adsorption of the activated carbon is shown in figure 7;
The addition amount in this example is: sodium persulfate (mg/L): COD (mg/L) =3: 1, granular activated carbon: sodium persulfate=50: 1, residence time 6h.
After secondary advanced oxidation, the pH value of the supernatant is 7.4, the COD concentration is reduced from 348.31 mg/L to 139.33 mg/L, the ammonia nitrogen concentration is unchanged from 7.42 mg/L, the TDS concentration is increased from 605.93 mg/L to 784.95 mg/L, the chloride ion concentration is unchanged, and the short-term SS concentration is increased from 50mg/L to 70mg/L due to the small amount of activated carbon powder, so that the later-stage SS change is not great. The three-dimensional fluorescence graph of the supernatant after activated by the activated carbon and secondary advanced oxidation is shown in fig. 8.
The pollutant concentration in the supernatant is degraded to the standard of dry land crops (COD is less than or equal to 200mg/L, BOD) of GB 5084-2021 Standard for field irrigation 5 100mg/L or less, 100mg/L or less of SS, 1000mg/L or less of TDS (non-saline-alkaline land), 350mg/L or less of chloride, 0.05mg/L or less of arsenic and 30mg/L or less of Kjeldahl nitrogen), and directly discharging, and further performing membrane filtration treatment and then discharging if the water reaches the surface III water quality or reclaimed water is required to be recycled.
(11) And (3) sludge dewatering:
and (3) discharging the sludge obtained in the step (2), the step (6), the step (8) and the step (9) into a sludge dewatering system, periodically pumping the sludge into a sludge dewatering machine for dewatering, reducing the water content of the sludge to below 65%, obtaining filtrate and a filter cake, reinjecting the filtrate into a primary flocculation precipitation unit for continuous treatment, and treating the filter cake as solid waste.
In this example, the degradation effect of the treatment unit on COD and ammonia nitrogen in each step is shown in table 2:
TABLE 2
According to the table, after the treatment method, through the coupling effect of the persulfate advanced oxidation (step 5 and step 10) and the biochemical method (step 8), the synergistic effect of the iron-carbon micro electrolysis (step 4) and the persulfate advanced oxidation (step 5 and step 10) and the synergistic effect of the activated carbon adsorption and the persulfate advanced oxidation (step 10), the COD removal rate of the flowback fluid is 99.30 percent and the ammonia nitrogen removal rate is 83.51 percent, compared with the existing independent Fenton oxidation method (COD removal rate 50 percent and ozone oxidation (COD removal rate 50-60 percent), the invention has excellent removal effect on COD and ammonia nitrogen in the natural gas fracturing flowback fluid with high oil and high salt, and the final effluent accords with the dry land crop standard (maximum COD 200mg/L, BOD) of 'farm irrigation standard' GB 5084-2021 5 150mg/L or less, 200mg/L or less of SS or less, 2000mg/L or less of TDS or less, 350mg/L or less of chloride or less, 0.05mg/L or less of arsenic or less, and 30mg/L or less of Kjeldahl nitrogen or less.
The three-dimensional fluorescence detection diagrams of fig. 4 to 8 can clearly show the removal process and effect of the organic pollutants in the high-oil high-salt natural gas fracturing flowback fluid by adopting the treatment method, thereby proving the effectiveness of the treatment method.
Example 2:
the various contaminant parameters in the initial high-oil high-salt natural gas fracturing flowback fluid in this example are shown in table 3:
TABLE 3 Table 3
Contaminants (S) Content (% L) Contaminants (S) Content (% L)
COD 20000 Ca 2+ 1980
Ammonia nitrogen 45 Mg 2+ 185
Total nitrogen 46.9 HCO 3 152
TDS 34150 Cl - 29200
SS 3000 SO 4 2- 345
The standard to be achieved by the effluent of the embodiment is the first-level standard (COD is less than or equal to 60mg/L, BOD) of integrated wastewater discharge Standard GB16297-1996 5 Less than or equal to 20mg/L, SS less than or equal to 70mg/L, petroleum less than or equal to 5mg/L, ammonia nitrogen less than or equal to 30mg/L
As shown in fig. 1, the initial flowback fluid is treated by the steps of:
(1) Oil removal pretreatment:
the initial flowback fluid contains an obvious separable oil layer, oil-water in the flowback fluid is separated in an oil separation tank unit through an oil separator to obtain petroleum and wastewater with the volume ratio of 8%, the petroleum is recovered, and the wastewater enters a next-stage treatment unit.
After oil removal pretreatment, the pH value of the wastewater is 5.77, the COD concentration is reduced from 20000 mg/L to 8320 mg/L, and the concentrations of ammonia nitrogen, TDS, chloride ions and SS are unchanged.
(2) Primary flocculation precipitation:
in a primary flocculation precipitation unit, the wastewater subjected to oil removal in the step (1) is yellow-white emulsion (COD concentration is 8320 mg/L), the pH value is 5.77, sodium hydroxide is added into the wastewater, the pH value is regulated to 9, and 1000/L PAC and 2/L PAM are added under alkaline conditions to perform flocculation precipitation, so that HCO in the wastewater is caused 3 - With OH - Neutralization to produce CO 3 2- ,CO 3 2- And then with Ca in the wastewater 2+ CaCO production 3 Precipitation of at the same time OH - Ca in wastewater 2+ Production of Ca (OH) 2 The calcium hydroxide precipitate generated when the precipitable ions such as calcium, magnesium and the like are removed is used as a coagulant aid to accelerate the precipitation effect; and (3) obtaining supernatant and precipitated sludge after primary flocculation precipitation, discharging the precipitated sludge into a sludge dewatering system for dewatering, and enabling the supernatant to enter a next-stage treatment unit.
After the primary flocculation precipitation, the pH value of the wastewater is 9, the COD concentration is reduced from 8320 mg/L to 3185 mg/L, the ammonia nitrogen concentration is unchanged, and the TDS concentration is increased from 34150 mg/L to 37531.86 mg/L, the chloride ion concentration is increased from 29200mg/L to 30620mg/L, and the SS concentration is reduced from 3000 mg/L to 240 mg/L due to the addition of the medicament.
(3) Air floatation:
and (3) introducing the supernatant (COD concentration is 3185/L) obtained in the step (2) into an air floatation unit, removing emulsified oil and suspended matters again by adopting a dissolved air floatation method, introducing gas into the supernatant, introducing residual floating oil in the supernatant into the water surface by bubbles, obtaining high-oil emulsion and wastewater with pH value of 8, recovering the high-oil emulsion, and introducing the wastewater into a next-stage treatment unit.
After dissolved air flotation, the pH value of the wastewater is 8, the COD concentration is reduced from 3185 mg/L to 3118 mg/L, the ammonia nitrogen concentration is unchanged, the TD and chloride ion concentrations are unchanged, and the SS concentration is reduced from 240 mg/L to 200 mg/L.
(4) Iron-carbon micro-electrolysis:
the wastewater (COD concentration is 3118 mg/L) in the step (3) enters an iron-carbon micro-electrolysis unit, hydrochloric acid is added to adjust the pH value to 3, and in the iron-carbon micro-electrolysis reactor, the organic matters in the wastewater are subjected to electrochemical treatment by utilizing countless galvanic cells formed in the iron-carbon micro-electrolysis to degrade part of COD, so that the biodegradability is improved; wherein, the filler of the iron-carbon micro-electrolysis reactor is iron-carbon, which is formed by melting ferric oxide, graphite powder and a catalyst, the iron content is 75%, the carbon content is 10-15%, the catalyst content is 10-15%, and the bulk density is 1.2-1.4 kg/m 3 The porosity is 50-70%, and the filler strength is more than or equal to 600 kg f/cm;
if COD is less than or equal to 2000mg/L after the iron-carbon micro-electrolysis reaction (under normal conditions), stirring the wastewater by adopting air aeration, wherein the air-water ratio is 3:1-5:1, and introducing oxygen while stirring to automatically raise the pH value to 9, and the Fe 2+ Is oxidized to Fe 3+ Ferric hydroxide sediment is generated under alkaline condition, partial organic matters are net-caught by flocculent sediment, COD is removed again, the micro-electrolysis reaction time of iron and carbon is 2-4 h, and finally Fe is obtained 3+ Is a waste water of (2); if COD is more than 2000mg/L after the iron-carbon micro-electrolysis reaction, an internal reflux pump is adopted for stirring to obtain the Fe-containing material 2+ Is a waste water of (a) and (b).
In the step, after the iron-carbon micro-electrolysis reaction, the pH value of the wastewater is 5.4, the COD concentration is reduced from 3118 mg/L to 2359 mg/L, and the COD is more than 2000mg/L, so that the wastewater is stirred by an internal reflux pump, the ammonia nitrogen concentration is reduced from 45 mg/L to 39 mg/L, the pH value is adjusted by adding acid, the TDS concentration is increased from 37531.86 mg/L to 37753.83 mg/L, the chloride ion is increased from 30620mg/L to 30841.94mg/L, and Fe is generated due to iron dissolution 2+ The SS concentration increased from 200 mg/L to 1000 mg/L.
(5) High-grade oxidation:
fe-containing in step (4) 2+ The waste water (COD concentration is 2359 mg/L) enters a primary high-grade oxidation unit, and the COD is more than 2000mg/L, so that sodium persulfate is added into the primary high-grade oxidation unit, and the adding amount of sodium persulfate is as follows: cod=1: 1 (the operator can select the adding amount in 0.5:1-3:1 according to the actual situation), and the reaction time is 2-6 h.
After the primary advanced oxidation, the macromolecular organic matters are oxidized into micromolecular organic matters, the organic nitrogen is oxidized into ammonia nitrogen, the pH value of the wastewater is reduced to 2.4, the COD concentration is increased from 2359 mg/L to 2433 mg/L, the ammonia nitrogen concentration is increased from 39 mg/L to 70.2 mg/L, and the TDS concentration is increased from 37753.83 mg/L to 38946.92 mg/L and the chloride ion concentration is unchanged due to the addition of the medicament. Although Fe is 2+ Is oxidized to Fe 3+ However, under acidic conditions, no ferric hydroxide precipitate was generated, and the SS concentration was unchanged. The three-dimensional fluorescence detection diagram of the wastewater is shown in fig. 9.
(6) Secondary flocculation precipitation:
the step (5) contains Fe 3+ Feeding the waste water (COD concentration is 2433 mg/L, SS is 1000 mg/L) into a secondary flocculation precipitation unit, wherein the pH value of the waste water is 2.4, sodium hydroxide is required to be added to adjust the pH value to 9, sodium carbonate is added to control the calcium ion concentration in the waste water to be within 50 mg/L, PAC with the concentration of 250 mg/L and PAM with the concentration of 5 mg/L are added to perform flocculation precipitation, iron ions, magnesium ions and calcium ions in the waste water are precipitated to obtain supernatant and precipitated sludge, the precipitated sludge is discharged into a sludge dewatering system, and the supernatant enters a next-stage treatment unit;
due to the inclusion of Fe 2+ In the step (4), the wastewater is stirred by an internal reflux pump, feIs oxidized to Fe 2+ The pH value of the wastewater is automatically raised to 5.4, the wastewater is acidic, and persulfate is added in the step (5) to add Fe 2+ Oxidation to Fe 3+ The pH value is reduced to 2.4, so that in the step (6), sodium hydroxide is firstly added to adjust the pH value to 9, and Fe is added 3+ Generating ferric hydroxide precipitate, and then adding sodium carbonate, PAC and PAM to perform flocculation precipitation.
After secondary flocculation precipitation, the pH value of the wastewater is 8.1, and the concentration of COD and ammonia nitrogen is basically unchanged. The TDS concentration is increased from 38946.92 mg/L to 43144.71 mg/L, the chloride ion concentration is increased from 30841.94mg/L to 31031.31mg/L, and the SS concentration is decreased from 1000mg/L to 90 mg/L.
(7) Evaporating to remove salt:
the supernatant fluid (COD concentration is 2433 (m/L)) obtained in the step (6) enters an evaporation desalting unit, and when the concentrated solution needs to be reinjected according to the actual condition of a gas well, the supernatant fluid is concentrated through an MVR evaporator to obtain concentrated solution and condensate with the concentration multiple of 1-3 times, namely, the volume percentage concentration of 25-50%, and the concentrated solution is discharged outside and reinjected to the gas well; when the gas well does not need reinjection, the MVR evaporator crystallizes the supernatant to obtain crystallized salt and condensate, the crystallized salt is recovered as industrial salt, and the condensate enters the next-stage treatment unit; the operation temperature of the MVR evaporator is 90 ℃, the pressure is-55 kPa to-65 kPa, the heat source is steam, and the steam pressure is more than or equal to 0.3MPa.
After evaporation for desalting, the pH value of the condensate is 2.2, the COD concentration is reduced from 2433 mg/L to 1089 mg/L, the ammonia nitrogen concentration is reduced from 70.2 mg/L to 28.5 mg/L, the TDS concentration is reduced from 43144.71 mg/L to 419.52 mg/L, the chloride ion concentration is reduced from 31031.31mg/L to 31.03mg/L, and the SS concentration is reduced to 0mg/L. The three-dimensional fluorescence detection diagram of the condensate is shown in fig. 10, and the GC-MS chromatogram of the condensate is shown in fig. 12.
(8) AO biochemical treatment:
the condensate liquid (COD concentration is 1089/L) in the step (7) is added with sodium hydroxide to adjust the pH value to 8 because the pH value of the condensate liquid is 2.2, and then enters an AO biochemical system unit to carry out biochemical treatment through a biochemical system, and the biochemical system adopts an A/O technology to remove COD and ammonia nitrogen in the condensate liquid; the step (5) oxidizes macromolecular organic matters in the wastewater into micromolecular organic matters, so that volatile micromolecular organic matters such as alcohol, ketone, aldehyde, acid and the like (shown in fig. 9 and 12) are contained in condensate, BOD/COD=0.45, biodegradability is good, COD and ammonia nitrogen in the condensate are degraded through an AO biochemical system, and the treatment time is 15 hours;
The biochemical system comprises a secondary sedimentation tank and a mixed liquor reflux system, the sludge concentration of the mixed liquor is 4000-5000 mg/L, condensate after biochemical treatment enters the secondary sedimentation tank for solid-liquid separation, precipitated sludge is discharged into a sludge dewatering system, and condensate after biochemical treatment enters a next-stage treatment unit.
After AO biochemical treatment, the pH value of condensate liquid is 8.0, COD concentration is reduced from 1089 mg/L to 105 mg/L, ammonia nitrogen concentration is reduced from 28.5 mg/L to 5.53 mg/L, and due to the addition of sodium hydroxide, the TDS concentration is increased from 419.52 mg/L to 586.23 mg/L, the chloride ion concentration is unchanged, and due to the addition of activated sludge into condensate liquid, the SS concentration is increased from 0mg/L to 80mg/L.
(9) Three-stage flocculation precipitation:
feeding the condensate (COD concentration is 105 (m/L)) after biochemical treatment in the step (8) into a three-stage flocculation precipitation unit, and adding 250mg/L PAC and 2mg/L PAM for flocculation precipitation because part of organic suspended matters and colloid are also contained in the condensate, further removing the organic suspended matters and the colloid to obtain supernatant and precipitated sludge, discharging the precipitated sludge into a sludge dewatering system, and feeding the supernatant into a next-stage treatment unit;
after three-stage flocculation precipitation, the pH value of the supernatant is 7.8, the concentration of the dissolved COD and ammonia nitrogen is basically unchanged, and as the medicament is added, the TDS concentration is increased from 586.23 mg/L to 775.62 mg/L, the chloride ion concentration is increased from 31.03mg/L to 220.39mg/L, and the SS concentration is reduced from 80mg/L to 50mg/L.
(10) Secondary advanced oxidation:
the supernatant fluid (COD concentration is 105 mg/L) in the step (9) enters a secondary oxidation unit, and because COD is larger than the effluent standard (COD is less than or equal to 60 mg/L), the supernatant fluid enters a persulfate secondary oxidation reactor, granular activated carbon and persulfate are added, the activated carbon is utilized for adsorption, sulfate radical and hydroxyl radical are generated by activating the persulfate to further degrade COD, the mass ratio of the added activated carbon particles to sodium persulfate is 30:1-50:1, the concentration ratio of the added persulfate to COD is 0.5:1-3:1, and operators can reasonably select according to actual conditions;
the addition amount in this example is: sodium persulfate (mg/L): COD (% L) =0.5: 1, granular activated carbon: sodium persulfate=30: 1, residence time 4h.
After secondary advanced oxidation, the pH value of the supernatant is 7.4, the COD concentration is reduced from 105 mg/L to 45 mg/L, the ammonia nitrogen concentration is unchanged, the TDS concentration is increased from 775.62 mg/L to 835.62 mg/L, the chloride ion concentration is unchanged, the short-term SS concentration is increased from 50mg/L to 60mg/L due to the small amount of activated carbon powder, and the later-stage SS concentration is not changed greatly.
The pollutant concentration in the supernatant is degraded to the first level standard (COD is less than or equal to 60mg/L, BOD) of the integrated wastewater discharge Standard GB16297-1996 5 Less than or equal to 20mg/L, SS less than or equal to 70mg/L, petroleum less than or equal to 5mg/L, ammonia nitrogen less than or equal to 30 mg/L), and if the water quality or reclaimed water on the surface III needs to be recycled, the water is further discharged after membrane filtration treatment.
(11) And (3) sludge dewatering:
and (3) discharging the sludge obtained in the steps (2), 6, 8 and 9) into a sludge dewatering system, periodically pumping the sludge into a sludge dewatering machine for dewatering, reducing the water content of the sludge to below 65%, obtaining filtrate and a filter cake, reinjecting the filtrate into a primary flocculation precipitation unit for continuous treatment, and treating the filter cake as solid waste.
In this example, the degradation effect of the treatment unit on COD and ammonia nitrogen in each step is shown in table 4:
TABLE 4 Table 4
From the above table, through the above pointsAfter the treatment method, the COD removal rate of the flowback fluid is 99.78% and the ammonia nitrogen removal rate is 87.71% through the coupling effect of the persulfate advanced oxidation (step 5 and step 10) and the biochemical method (step 8), the synergistic effect of the iron-carbon micro-electrolysis (step 4) and the persulfate advanced oxidation (step 5 and step 10) and the synergistic effect of the activated carbon adsorption and the persulfate advanced oxidation (step 10), so that compared with the existing Fenton oxidation method (the COD removal rate is only 50% and the ozone oxidation (the COD removal rate is 50-60%), the invention has excellent removal effect on COD and ammonia nitrogen in the natural gas fracturing flowback fluid with high oil and high salt, and the final effluent accords with the first level standard (COD is less than or equal to 60mg/L and BOD) of the integrated wastewater discharge standard GB16297-1996 5 Less than or equal to 20mg/L, less than or equal to 70mg/L of SS, less than or equal to 5mg/L of petroleum, less than or equal to 30mg/L of ammonia nitrogen).
According to the three-dimensional fluorescence detection diagrams of fig. 9 and 10, after the iron-carbon reaction, the sodium persulfate is added for oxidation and fluorescent organic matters in the evaporated condensate after oxidation are obviously reduced compared with the wastewater and the evaporated condensate without persulfate in fig. 4 and 5; as can be seen from the GC-MS spectra of fig. 11 and 12, the condensate evaporated after adding sodium persulfate is basically small molecular organic matters such as alcohol, ketone, aldehyde, acid, etc., while the evaporating condensate without adding sodium persulfate is still large molecular aromatic hydrocarbon, etc., which indicates that adding sodium persulfate can oxidize large molecular organic matters into small molecular organic matters, improving the biodegradability of the condensate.
According to example 1 and example 2, the effluent quality of the treated flowback wastewater was compared as shown in table 5:
TABLE 5
Therefore, a small amount of ferrous iron and granular activated carbon dissolved by iron-carbon micro-electrolysis is used as a catalyst to activate persulfate, so that hydroxyl free radicals and sulfate radical oxidation organic matters are generated, the sludge content and the salt content of the Fenton treatment method in the prior art are greatly reduced, and the discharge amount of solid waste is reduced; the equipment is simple, the electricity consumption is low, and the equipment configuration and the electricity consumption of the existing ozone catalytic oxidation and wet oxidation methods are greatly reduced; the method and the device have the advantages that the addition mode and the consumption of the sodium persulfate are determined according to the condition of the water inflow quality, the operation is flexible and convenient, the water outflow quality is ensured to be stable and reach the standard, the problem of large water quality change of the natural gas fracturing flowback fluid is solved, the high-oil and high-salt pollutants of the natural gas fracturing flowback fluid are effectively solved, the sludge quantity and the energy consumption are lower than those of the existing treatment method, the operation is convenient, the impact load resistance is high, the economic value and the environmental value are obvious, and the method and the device have great significance in the aspects of promoting the development of new natural gas energy, protecting the local ecological environment, reinforcing the reutilization of water resources, reducing pollution and reducing carbon and the like.
The foregoing is merely a preferred embodiment of the present invention, and it should be noted that the above-mentioned preferred embodiment should not be construed as limiting the invention, and the scope of the invention should be defined by the appended claims. It will be apparent to those skilled in the art that various modifications and adaptations can be made without departing from the spirit and scope of the invention, and such modifications and adaptations are intended to be comprehended within the scope of the invention.

Claims (10)

1. The treatment method of the high-oil high-salt natural gas fracturing flowback fluid is characterized by comprising the following steps of:
(1) Oil removal pretreatment: oil-water separation is carried out on the high-oil high-salt natural gas fracturing flowback fluid to obtain petroleum and wastewater;
(2) Primary flocculation precipitation: regulating the pH value of the wastewater in the step (1) to 8-9, and then adding PAC and PAM for flocculation precipitation to obtain supernatant and precipitated sludge;
(3) Air floatation: removing floating oil in the supernatant fluid in the step (2) to obtain high-oil emulsion and wastewater;
(4) Iron-carbon micro-electrolysis: adding hydrochloric acid into the wastewater obtained in the step (3) to adjust the pH value to 3, performing iron-carbon micro-electrolysis reaction, and judging whether COD (chemical oxygen demand) after the iron-carbon micro-electrolysis reaction is more than 2000mg/L;
When COD is less than or equal to 2000mg/L, the aeration and stirring are carried out by adopting an air pipe, and the pH value of the wastewater is automatically regulated to 8-9, thus obtaining the wastewater containing Fe 3+ Is a waste water of (2); when COD is more than 2000mg/L, stirring by adopting an internal reflux pump to obtain the Fe-containing material 2+ Is a waste water of (2);
(5) High-grade oxidation: if the step (4) adopts an internal reflux pump for stirring, the Fe is contained 2+ Adding persulfate into the wastewater to perform oxidation reaction to obtain Fe-containing wastewater 3+ Is a waste water of (2); if the step (4) adopts air pipe aeration stirring, the step (5) is not added with a medicament and is only used as an overcurrent unit;
(6) Secondary flocculation precipitation: fe is contained in the step (4) or the step (5) 3+ Adding sodium carbonate, PAC and PAM into the wastewater to perform flocculation precipitation to obtain supernatant and precipitated sludge;
(7) Evaporating to remove salt: judging whether the concentrated solution needs to be reinjected into the gas well according to the actual engineering requirement of the gas well, and concentrating the supernatant if the concentrated solution needs to be reinjected into the gas well to obtain concentrated solution and condensate; if reinjection is not needed, concentrating and crystallizing to obtain crystallized salt and condensate;
(8) AO biochemical treatment: carrying out AO biochemical treatment on the condensate in the step (7) to obtain condensate and activated sludge after biochemical treatment;
(9) Three-stage flocculation precipitation: adding PAC and PAM into the condensate after biochemical treatment in the step (8) for flocculation and precipitation to obtain supernatant and precipitated sludge;
(10) Secondary advanced oxidation: judging whether the COD concentration of the supernatant fluid in the step (9) reaches the emission standard according to the requirements of different areas; when COD reaches the standard, discharging; when the COD is not up to standard, adding activated carbon particles and persulfate into the supernatant to perform adsorption and oxidation reaction, and discharging after the COD is stable and reaches the standard.
2. The method for treating a high-oil high-salt natural gas fracturing flow-back fluid according to claim 1, wherein said steps further comprise (11) sludge dewatering: and (3) discharging the sludge obtained in the step (2), the step (6), the step (8) and the step (9) into a dehydration system for dehydration to obtain filtrate and a filter cake, and reinjecting the filtrate into a primary flocculation precipitation unit for continuous treatment.
3. The method for treating the high-oil high-salt natural gas fracturing flowback fluid according to claim 1, wherein in the step (1), oil and water are separated in an oil separation tank unit through an oil separator, the obtained petroleum is recovered, and the volume ratio content of the petroleum in the high-oil high-salt natural gas fracturing flowback fluid is 8% at most; in the step (2), sodium hydroxide is added into the wastewater in a primary flocculation precipitation unit, the pH value of the wastewater is regulated to 8-9, the addition amount of PAC is 1000-1500 mg/L, the addition amount of PAM is 2-5 mg/L, and calcium hydroxide generated in the wastewater is used as a coagulant aid, so that flocculation effect is improved; in the step (3), in the air floatation unit, residual floating oil and suspended matters in the supernatant fluid are carried on the water surface by adopting an air dissolution air floatation method and removed, so as to obtain high-oil emulsion and wastewater, and the high-oil emulsion is recovered.
4. The method for treating a high-oil high-salt natural gas fracturing flow-back fluid according to claim 1, wherein in the step (4), in an iron-carbon micro-electrolysis unit, the reaction pH value of iron-carbon micro-electrolysis is 3, the reaction time is 2-4 hours, the filler of the iron-carbon micro-electrolysis reactor is iron-carbon, and is formed by melting iron oxide, graphite powder and a catalyst, wherein the iron content is 75%, the carbon content is 10-15%, the catalyst content is 10-15%, the bulk density is 1.2-1.4 kg/m, the porosity is 50-70%, and the filler strength is more than or equal to 600kgf/cm 2
5. The method for treating a high-oil and high-salt natural gas fracturing flow-back fluid according to claim 1, wherein in the step (5), if the primary advanced oxidation unit is obtained in the step (4), fe is contained 3+ The wastewater is directly subjected to the next step without treatment; if the Fe is contained in the mixture obtained in the step (4) 2+ The amount of persulfate is added into the wastewater of (1)The concentration ratio of COD is 0.5:1-3:1, the reaction time is 2-6 h, wherein Fe 2+ As an activator; the persulfate is sodium persulfate; in order to enhance the oxidation effect, hydrogen peroxide is added, and the mol ratio of the added hydrogen peroxide to persulfate is 1:10.
6. The method for treating a high-oil and high-salt natural gas fracturing flow-back fluid according to claim 1, wherein in the step (6), if Fe is contained in the secondary flocculation precipitation unit 3+ The wastewater obtained in the step (4) is directly added with sodium carbonate, PAC and PAM for flocculation precipitation; if Fe is contained 3+ And (2) adding sodium hydroxide into the wastewater obtained in the step (5) to adjust the pH value to 8-9, and then adding sodium carbonate, PAC and PAM to perform flocculation precipitation.
7. The method for treating the high-oil high-salt natural gas fracturing flow-back fluid according to claim 1, wherein in the step (6), the concentration of calcium ions in the supernatant fluid is less than or equal to 50 mg/L after sodium carbonate is added into the secondary flocculation precipitation unit, the adding amount of PAC is 150-250 mg/L, and the adding amount of PAM is 2-5 mg/L.
8. The method for treating the natural gas fracturing flow-back fluid with high oil and high salt according to claim 1, wherein in the step (7), in the evaporation desalting unit, when the concentrated solution is required to be refilled according to the actual situation of a gas well, the concentrated solution is concentrated by adopting an evaporator, and the concentration multiple is 1-3 times, so that the concentrated solution with the volume percent concentration of 25% -50% is obtained; when the concentrated solution does not need to be refilled, an evaporator is adopted for crystallization, and the obtained crystallized salt is used as industrial salt for recycling; the evaporator adopts an MVR evaporator, the operating temperature is 90 ℃, the operating pressure is-55 kPa to-65 kPa, the heat source is steam, and the steam pressure is more than or equal to 0.3Mpa.
9. The method for treating the natural gas fracturing flow-back fluid with high oil and high salt as claimed in claim 1, wherein in the step (8), in the AO biochemical system unit, if the pH value of condensate is acidic, alkali is added to adjust the pH value to 8-9 before the condensate enters the unit, the condensate enters a biochemical system for biochemical treatment, and the biochemical system adopts an A/O technology to remove COD and ammonia nitrogen in the condensate, and the treatment time is 15-20 h; the biochemical system comprises a secondary sedimentation tank and a mixed liquor reflux system, and mixed liquor sludge with the concentration of 4000-5000 mg/L can be obtained; in the step (9), in the three-stage flocculation precipitation unit, the addition amount of PAC is 150-250 mg/L, and the addition amount of PAM is 1-2 mg/L.
10. The method for treating the high-oil high-salt natural gas fracturing flow-back fluid according to claim 1, wherein in the step (10), in a secondary advanced oxidation unit, the diameter of the activated carbon particles is 4 mm, the length is 10 mm, the mass ratio of the added activated carbon particles to sodium persulfate is 30:1-50:1, and the concentration ratio of the added persulfate to COD is 0.5:1-3:1; the treatment time of the secondary advanced oxidation unit is 4-6 hours; at least two secondary oxidation reactors are arranged in the secondary advanced oxidation unit, one secondary oxidation reactor is ensured to run, and the rest secondary oxidation reactors are regenerated for standby.
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