Shale gas field gas well sewage pretreatment technology

Abstract

The invention discloses a shale gas field gas well sewage pretreatment technology, and relates to the field of industrial wastewater treatment. In the technology, the shale gas field gas well sewage is subjected to electrochemical oxidation through a PMS or three-electrode system to selectively remove bromine, and the recovered bromine and other high-value resources can be reused; then pre-oxidation is carried out through H2O2, Mn (VII) or Fe (VI), the precursor structure of the halogenated organic matter is changed, and the residual oxidizing agent is also beneficial to strengthening the treatment capacity of subsequent advanced oxidation; and finally, through AO or O3 advanced oxidation treatment, organic pollutants in the wastewater are efficiently removed, halogen-containing compounds are reduced or even not generated, and the method can be used as a pretreatment process for desalination advanced treatment such as membrane separation, evaporative crystallization and the like. The pretreatment technology for the gas well sewage of the shale gas field has the advantages of high efficiency, environment friendliness and resource recycling, and the operability of subsequent deep treatment can be greatly improved.

Description

Technical Field

[0001] The invention belongs to the field of industrial sewage treatment, and in particular relates to a method for pretreating sewage from a shale gas field gas well. Background Art

[0002] The large amount of wastewater generated during shale gas production may cause water and soil pollution, which has become a limiting factor for its sustainable development. Fracturing fluid is pumped into the formation at high pressure to fracture the target rock formation, and proppant is deposited and embedded in the cracks to form porous channels. Then a large amount of water-gas mixture returns to the surface and is separated and recycled. The gas well wastewater formed is a typical high-salt, difficult-to-degrade gas mine wastewater, which mainly includes organic matter such as fracturing fluid additives, geological organic matter and its transformation products, and inorganic matter such as halides, sulfides, sulfates, alkali (earth) metals, and natural radioactive substances. If not handled properly, it will cause serious environmental and health risks.

[0003] The treatment technology of gas well wastewater in shale gas fields is generally divided into primary, secondary and tertiary treatment according to the salinity of the wastewater. Among them, primary treatment can remove most solids, residual oil and grease, iron, fracturing fluid additives and bacteria, etc. The related processes include static gravity-assisted phase separation technology, dynamic separation methods, oxidation technology, coagulation, flotation and clarification, etc., which have the characteristics of strong treatment capacity and wide application. Among them, advanced oxidation processes, such as ozone oxidation, can use strong oxidizing substances (such as hydroxyl radicals (·OH)) generated in situ to quickly and effectively degrade organic pollutants in wastewater, but ·OH can also react with high concentrations of Cl in gas well wastewater. - Br - ,I - Active halogen species are formed, and active halogen species react with organic matter to form a large number of persistent, toxic, and bioaccumulative halogenated organic matter through free radical substitution, nucleophilic substitution, or electrophilic addition, which may also be further oxidized into highly toxic inorganic halogen compounds such as bromate, chlorate, and perchlorate. Secondary (including lime softening, ion exchange, nanofiltration, and electrically driven charge separation, etc.) and tertiary (including evaporation, distillation, crystallization, and (reverse, forward, or mixed) osmosis, etc.) deep treatment technologies are relatively less used due to the influence of water quality and technical economy.

[0004] At present, some research teams have adopted different oxidation technologies to treat shale gas field gas well wastewater and have achieved relatively satisfactory organic matter removal effects. For example, patents CN114314959A, CN108658319A, and CN112897787A respectively disclose a "primary oxidation / coagulation + advanced oxidation + membrane salt separation-desalting" process for treating shale gas fracturing flowback fluid, "pretreatment (ozone / electroflocculation) + step chemical precipitation + evaporation crystallization", and "coagulation + chemical precipitation + oxidation + evaporation crystallization" process. However, none of them paid attention to the generation of halogen-containing compounds with high potential environmental and health risks during the reaction process, nor did they involve relevant preventive measures. Summary of the invention

[0005] The present invention provides a combined pretreatment process for shale gas field gas well wastewater, aiming to overcome the deficiencies in the prior art, achieve efficient removal of organic matter in gas well wastewater, effectively control the generation of highly toxic halogen-containing compounds during the treatment process, and reduce the potential environmental and health risks of treated effluent.

[0006] The present invention adopts a "selective oxidation (PMS / electrochemical oxidation) + pre-oxidation (H2O2 / Mn (VII) / Fe (VI)) + advanced oxidation (AO / O3) combined process" to treat shale gas field gas well sewage. The specific technical scheme is: shale gas field gas well sewage is first electrochemically oxidized by PMS or a three-electrode system to selectively remove Br - (Based on Br - The standard oxidation potential is lower than that of Cl - ); then, it is oxidized by H2O2, Mn (VII) or Fe (VI) to change the structure of the precursor of the halogenated organic matter (electron-rich compounds, such as aromatic hydrocarbons and phenolic compounds); finally, it is treated with O3 to efficiently remove the organic pollutants and reduce or even eliminate the production of halogenated compounds. The operating conditions of each process are as follows: (1) In the combined process described in the present invention, PMS / electrochemical oxidation uses unactivated, cheap and stable PMS and a three-electrode system with precisely controlled oxidation potential to oxidize high concentrations of Br in gas well wastewater. - Selective oxidation is performed, and the stripped product is recovered using an absorbent. Specifically, in PMS oxidation, the initial pH of the water sample is adjusted to 1-5, and the PMS:Br - PMS is added at a molar ratio of 0.5-3; the three-electrode system electrochemical oxidation uses carbon felt or graphite electrode as the working electrode, saturated calomel or Ag / AgCl as the reference electrode, and metal titanium or platinum as the counter electrode, and the anode potential is 1.0-2.0 V (SHE). During the reaction, air is introduced into the system at a flow rate of 0.5-2 m / s to blow off the formed product. After the reaction, Br -The removal rate can reach nearly 100%, and Cl - There is no significant change in concentration. In addition, the high-value Br2 obtained in the process can be used as a raw material for industrial production.

[0007] (2) H2O2 / Mn (VII) / Fe (VI) pre-oxidation: H2O2 / Mn (VII) / Fe (VI) oxidant is used to change the structure of halogenated organic precursors in the selectively oxidized gas well wastewater. The initial pH and dosage conditions of H2O2, Mn (VII), and Fe (VI) oxidation are 4-10, 6-10, and 5-9, respectively, and the (mass ratio of oxidant to TOC of wastewater) is 0.5-5, 0.1-2, and 0.5-8. UV 254 (can indicate unsaturated hydrocarbon organic compounds with double bonds or benzene rings) value characterizes the removal effect of halogenated organic precursor substances, and the UV 254 It can effectively reduce. Moreover, the residual oxidant can enhance the treatment effect of the subsequent advanced oxidation system.

[0008] (3) The AO / O3 advanced oxidation process is used to treat the pre-oxidized gas well wastewater. The specific method is as follows: in the AO, under constant current mode, PbO2 or BDD is used as the anode, stainless steel or titanium is used as the cathode, the plate spacing is 0.5~5 cm, and the current density is 0.5~20 mA / cm 2 During ozonation, the initial pH of the water sample was adjusted to 6-13 and the ozone concentration was 10-100 g / m 3 , the flow rate is 0.2~2m / s. The organic matter removal rate of the treated water can reach more than 80%, and compared with the gas well sewage without selective oxidation and pre-oxidation treatment, the acute toxicity and cytotoxicity of the luminescent bacteria are greatly reduced.

[0009] The combined process of selective oxidation + pre-oxidation + advanced oxidation provided by the present invention as a shale gas field gas well wastewater pretreatment technology has the following beneficial effects: Before using advanced oxidation technology to treat shale gas field gas well wastewater, selective oxidation technology is first used to efficiently remove Br in gas well wastewater. -, and at the same time, the pre-oxidation technology is used to change the precursor structure of halogenated organic matter, so as to achieve efficient removal of organic matter in gas well wastewater while effectively reducing the formation of highly toxic halides during the treatment process. In addition, high-value resources such as bromine recovered in the pretreatment can be reused, and the residual oxidant is also beneficial to enhance the treatment capacity of subsequent advanced oxidation. Therefore, the use of this combined process as a pretreatment technology for gas well wastewater in shale gas fields has the advantages of high efficiency, environmental friendliness and resource recycling. Moreover, the flexibility of the combined process enables it to be adjusted according to the water quality characteristics of gas well wastewater in different shale gas fields, thereby achieving a wider range of applications. In addition, as a pretreatment for deep treatments such as desalination, this combined process effectively reduces harmful inorganic and organic impurities, thereby improving the purity and safety of recovered salts and increasing the feasibility of salt reuse. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 The present invention is a process flow chart for pretreatment of wastewater from shale gas fields and gas wells. DETAILED DESCRIPTION

[0011] The present invention will be further described in detail below in conjunction with specific embodiments, but the embodiments of the present invention are not limited thereto.

[0012] The gas well wastewater treated in the embodiment comes from a shale gas field in the Sichuan Basin. Its pH, TOC, COD, Br - , Cl - The concentrations are 6~8, 50~300 mg / L, 100~1000 mg / L, 50~95 mg / L, and 10~20 g / L respectively. Example

[0013] The pH of the gas well wastewater is adjusted to 2 in the pH adjustment tank (1), and then pumped into the reaction tank (1). - PMS was added at a molar ratio of 1.5, and air was introduced into the system at a flow rate of 1 m / s to blow off the formed Br2. After selective oxidation by PMS, Br - When the concentration drops below 1 mg / L, Cl - The concentration did not change significantly, and the Br2 recovery rate using the absorption liquid reached more than 90%, and then it flowed into the reaction tank (2). After adjusting the pH to neutral, H2O2 was added at a mass ratio of H2O2:TOC of 3. UV 254 The removal rate is nearly 50%, and the TOC removal rate is less than 5%. After that, it flows into the pH adjustment tank (2). After adjusting the pH to 12, it is pumped into the ozone reactor. The O3 concentration during the reaction is 40 g / m 3, the flow rate was 1 m / s, and the TOC of the effluent after ozone advanced oxidation treatment dropped to below 10-50 mg / L. Compared with direct ozone advanced oxidation of gas well wastewater under the same conditions, not only the TOC removal rate was increased, but bromate was not detected in the effluent, the adsorbable organic halogen was reduced by nearly 50%, and the acute toxicity and cytotoxicity were significantly reduced. Example

[0014] The gas well sewage is pumped into the electrochemical reaction device (1). The device (1) uses a graphite electrode as a working electrode, a saturated Ag / AgCl as a reference electrode, and a metal platinum as a counter electrode. The anode potential is controlled at 1.0 V. Air is introduced into the system at a flow rate of 1 m / s to blow off the formed Br2. After selective oxidation by the three-electrode system, Br2 in the effluent is - When the concentration drops below 0.5 mg / L, Cl - The concentration did not change significantly, and the Br2 recovery rate using the absorption liquid reached more than 90%, and then flowed into the reaction tank. After adjusting the pH to 8, Fe(VI) was added at a mass ratio of Fe(VI):TOC of 6. UV 254 When the removal rate exceeds 50% and the TOC removal rate is less than 10%, it is then pumped into the electrochemical reaction device (2). Device (2) uses BDD as the anode and stainless steel as the cathode. The distance between the plates is 0.5 cm and the current density is 5 mA / cm 2 The TOC of the effluent after AO treatment dropped to below 5-40 mg / L. Compared with direct AO treatment of gas well wastewater under the same conditions, no bromate was detected in the effluent and the acute toxicity was significantly reduced. Example

[0015] The pH of the gas well wastewater is adjusted to 2 in the pH adjustment tank (1) and then pumped into the reaction tank (1) to add PMS:Br - PMS was added at a molar ratio of 1.5, and air was introduced into the system at a flow rate of 1 m / s to blow off the formed Br2. After selective oxidation by PMS, Br - When the concentration drops below 1 mg / L, Cl - The concentration did not change significantly, and the Br2 recovery rate using the absorption liquid reached more than 90%, and then flowed into the reaction tank (2). The pH was adjusted to 8, and Mn(VII) was added at a mass ratio of Mn(VII):TOC of 0.5. UV 254 The removal rate is more than 70%, and the TOC removal rate is less than 10%. Then it flows into the pH adjustment tank (2). After adjusting the pH to 12, it is pumped into the ozone reactor. The O3 concentration during the reaction is 40 g / m 3, the flow rate was 1 m / s, and the TOC of the effluent after ozone advanced oxidation treatment dropped to below 15-55 mg / L. Compared with direct ozone advanced oxidation of gas well wastewater under the same conditions, not only the TOC removal rate was increased, but no bromate was detected in the effluent, the adsorbable organic halogen was reduced by more than 50%, and the acute toxicity and cytotoxicity were significantly reduced.

[0016] It can be seen from the above examples that the process of the present invention which combines different selective oxidation, pre-oxidation and advanced oxidation technologies can not only achieve efficient removal of organic matter in shale gas field gas well wastewater, but also greatly reduce the generation of halogen-containing compounds during the treatment process, thereby effectively reducing the toxicity of the treated water.

Claims

1. A shale gas field gas well wastewater pretreatment technology, characterized in that: The technology consists of selective oxidation, pre-oxidation and advanced oxidation processes. The specific process flow is as follows: shale gas field gas well wastewater is first electrochemically oxidized by monosulfate (PMS) or a three-electrode system to selectively remove bromine (Br - ); and then undergo pre-oxidation with hydrogen peroxide (H2O2), permanganate (Mn (VII)) or ferrate (Fe (VI)) to change the precursor structure of the halogenated organic matter; finally, undergo advanced oxidation treatment with anodic oxidation (AO) or ozone (O3) to efficiently remove organic pollutants and reduce or even eliminate the production of halogenated compounds, which can be used as a pretreatment for deep treatments such as desalination.

2. The method according to claim 1, characterized in that The initial pH of the PMS selective oxidation, PMS:Br - The molar ratios are 1~5 and 0.5~3 respectively.

3. The method according to claim 1, characterized in that The three-electrode system electrochemical selective oxidation uses one of carbon felt or graphite electrode as a working electrode, one of saturated calomel or Ag / AgCl as a reference electrode, and one of metal titanium or platinum as a counter electrode, and the anode potential is 1.0-2.0 V.

4. The method according to claim 1, characterized in that: In the selective oxidation reaction, air is introduced into the system at a flow rate of 0.5-2 m / s to blow off the formed product.

5. The method according to claim 1, characterized in that The initial pH and H2O2:TOC mass ratio of the H2O2 pre-oxidation are 4-10 and 0.5-5 respectively.

6. The method according to claim 1, characterized in that The initial pH and Mn (VII):TOC mass ratio of the Mn (VII) pre-oxidation are 6-10 and 0.1-2 respectively.

7. The method according to claim 1, characterized in that The initial pH and Fe(VI):TOC mass ratio of the Fe(VI) pre-oxidation are 5-9 and 0.5-8 respectively.

8. The method according to claim 1, characterized in that The AO advanced oxidation uses PbO2 or diamond (BDD) as the anode, stainless steel or titanium as the cathode, the plate spacing is 0.5-5 cm, and the current density is 0.5-20 mA / cm 2 .

9. The method according to claim 1, characterized in that: The initial pH, O3 concentration and flow rate of the O3 advanced oxidation are 6-13 and 10-100 g / m 3 , 0.2~2 m / s.

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