Method and system for treating salt-containing wastewater

Abstract

The application discloses a treatment method and a treatment system for salt-containing wastewater, and realizes reduced treatment and resource utilization of miscellaneous salt. The treatment method comprises the following steps: pretreating the salt-containing wastewater to reduce the contents of calcium and magnesium ions, silicon dioxide, suspended solids, iron ions, manganese ions and part of organic matters in the high-content wastewater, and obtaining pretreated wastewater; performing electrodialysis crystallization on the pretreated wastewater to obtain crystalline salt, concentrated brine and dilute brine; and performing bipolar membrane treatment on the dilute brine to obtain acidic products, alkaline products and desalted water. The treatment method can treat high-concentration brine or saturated concentrated brine by coupling pretreatment with membrane concentration, electrodialysis crystallization and bipolar membrane treatment, greatly improves the purity and recovery rate of the crystalline salt, produces acid and alkali with higher added value, reduces the production amount of miscellaneous salt, reduces the treatment cost of miscellaneous salt, and realizes the resource utilization of miscellaneous salt.

Description

Technical Field

[0001] This invention relates to the field of water treatment, specifically to a method and system for treating saline wastewater, achieving the reduction and resource utilization of miscellaneous salts. Background Technology

[0002] Currently, ultra-low emission and zero-discharge wastewater technologies for flue gas from domestic coal-fired power plants, coal chemical industry, metallurgy, steel, and petrochemical industries are becoming increasingly mature. However, many water treatment processes in these sectors generate large amounts of industrial wastewater, much of which contains high concentrations of salt components. Saline wastewater typically refers to wastewater with a total dissolved solids content greater than 3.5%, and also contains ionic components related to its production processes. Therefore, industrial wastewater treatment often faces the challenge of treating high-salinity wastewater. Industrial saline wastewater is more difficult to treat than other types of wastewater, and it suffers from low resource recovery rates, high treatment costs, and the easy generation of waste byproducts.

[0003] Zero-discharge treatment of industrial wastewater generally involves five steps: pretreatment, pre-concentration, deep concentration, evaporation, and crystallization. To achieve true zero discharge of industrial wastewater, the reduction and resource utilization of miscellaneous salts are crucial. The majority of salts in industrial wastewater are sodium chloride and sodium sulfate, primarily originating from three sources: ① salts introduced during demineralized water and circulating water production processes; ② chemicals added during wastewater treatment and reuse; ③ salts introduced into the fresh water required for plant production. Concentrated brine treatment is the final step in achieving zero discharge of industrial wastewater. Concentrated brine has a COD exceeding 1000 mg / L and a total dissolved solids (TDS) of 30,000-260,000 mg / L. It contains a large amount of recalcitrant organic matter, various salts, and heavy metals. Currently, there are two main process routes for fractional crystallization: The first route involves the wastewater being deeply concentrated and then directly fed into an evaporator crystallizer. Based on the different solubilities of each solute at corresponding temperatures in the solution, phase diagram theory is used to separate the salts and obtain different salt products. The quality of the separated salts is poor and the recovery rate is low. The mother liquor from the crystallizer enters the mixed salt crystallizer, ultimately producing a large amount of mixed salts. The second route utilizes the special pore size range and charge effect of nanofiltration membranes to separate sodium chloride and sodium sulfate in the wastewater, followed by evaporation crystallization or freeze crystallization to achieve salt recovery. This process yields crystallized salts with higher purity, but ultimately, a portion of the mother liquor from the crystallizer also enters the mixed salt crystallizer, producing a portion of mixed salts.

[0004] However, existing fractional crystallization technologies achieve a salt recovery rate of only 45%-80%. After producing industrial salt, approximately 20%-55% of crystalline miscellaneous salts are still generated, which are difficult to reuse. Besides sodium and potassium salts and sulfhydryl chlorides, these miscellaneous salts are also enriched with complex organic compounds such as benzene, lipids, quinoline, and pyridine, and even small amounts of heavy metals. Therefore, they cannot be directly transported to slag yards for simple landfilling with gasification ash or boiler ash; they must be disposed of separately as hazardous solid waste. As can be seen, current methods can only further reduce the volume of miscellaneous salts but cannot completely eliminate them. There are no successful precedents for recovering miscellaneous salts. Therefore, reducing and recycling miscellaneous salts is of great significance. Summary of the Invention

[0005] The purpose of this invention is to overcome the problems of low salt recovery rate and low resource utilization rate in existing technologies for saline wastewater, and to provide a method and system for treating saline wastewater. This invention improves the purity and recovery rate of crystalline salt, reduces the production of hazardous waste from mixed salts, and reduces the treatment costs of hazardous waste from mixed salts, thereby realizing the resource utilization of mixed salts.

[0006] To achieve the above objectives, a first aspect of the present invention provides a method for treating saline wastewater, comprising:

[0007] Pre-treatment of saline wastewater reduces the content of calcium and magnesium ions, silica, suspended solids, iron ions, manganese ions, and some organic matter in the wastewater, resulting in pre-treated wastewater.

[0008] The pretreated wastewater was subjected to electrodialysis crystallization to obtain crystalline salt, concentrated brine and dilute brine.

[0009] The brine was subjected to bipolar membrane treatment to obtain acidic products, alkaline products, and desalinated water.

[0010] A second aspect of the present invention provides a treatment system for saline wastewater, the treatment system comprising a pretreatment device, an optional first membrane concentration device, an electrodialysis crystallization device, and a bipolar membrane device connected in sequence; wherein,

[0011] The pretreatment device includes a softening coagulation sedimentation unit, an ultrafiltration-resin unit, and an advanced oxidation unit connected in sequence.

[0012] The first membrane concentration device includes: a membrane concentration membrane element, a membrane housing, and a matching water pump, a high-pressure pump, and a pipeline system;

[0013] The electrodialysis crystallization device includes: a freshwater tank, a concentrated water tank, an electrode water tank, an electrodialysis membrane stack, a DC power supply, a control cabinet, and pipelines, pumps, and testing instruments between the water tanks and the membrane stack;

[0014] The bipolar membrane device is a three-compartment bipolar membrane electrodialysis membrane stack, which is composed of multiple membrane pairs. Each membrane pair consists of an anion membrane, an cation membrane and a bipolar membrane. A water distribution baffle is provided between two adjacent membranes, and electrodes and end plates are located at both ends, which are fastened and pressed together with multiple bolts.

[0015] Through the above technical solution, the method for treating saline wastewater proposed in this invention couples electrodialysis crystallization technology with bipolar membrane technology. First, electrodialysis crystallization is performed to obtain dilute brine, which is then treated with a bipolar membrane. Then, it is combined with membrane concentration technology as needed to improve the purity and recovery rate of the crystallized salt. At the same time, it can convert the salt in the aqueous solution into the corresponding acid and alkali without introducing new components. The acid and alkali can be reused in the wastewater treatment plant to adjust the pH value or regenerate the resin, thus realizing the resource utilization of miscellaneous salts.

[0016] The treatment method of this invention can treat not only low-concentration brine but also high-concentration brine or saturated concentrated brine. The purity and recovery rate of the obtained crystalline salt are significantly improved compared to existing processes, ultimately reducing the generation of impurity salts, lowering the cost of hazardous waste treatment, and realizing the resource utilization of impurity salts. Existing fractional crystallization processes for impurity salts achieve a maximum recovery rate of 45%-80%. After producing industrial salt, approximately 20%-55% of unusable crystalline impurity salts are still generated, which must be disposed of as hazardous solid waste, wasting resources and incurring high treatment costs (3500-5000 RMB / ton). Using the treatment method of this invention, the salt recovery rate is greater than 90%, and the purity of the crystalline salt reaches 99% or higher. The prepared acids and alkalis can be reused in wastewater treatment plants to adjust pH or for resin regeneration, significantly increasing the added value compared to crystalline salts. Attached Figure Description

[0017] Figure 1 This is a process flow diagram of a method for treating saline wastewater with a salt content ≥12wt% according to one embodiment of the present invention.

[0018] Figure 2 This is a process flow diagram of a method for treating saline wastewater with a salt content of <12wt% proposed in another embodiment of the present invention. Detailed Implementation

[0019] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0020] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0021] In this invention, saline wastewater includes any one or more combinations of NaCl, Na2SO4, and other soluble inorganic salts, organic matter, calcium ions, magnesium ions, and heavy metal ions.

[0022] In this invention, salinity refers to the percentage by mass of total salt content in wastewater relative to the total volume of wastewater. Salt in wastewater, also known as total dissolved solids (TDS), is the collective term for inorganic salts and organic matter dissolved in water. Its main components include calcium, magnesium, sodium, and potassium ions, as well as carbonate, bicarbonate, chloride, sulfate, and nitrate ions.

[0023] In this invention, equivalent concentration refers to the concentration of a solution expressed as the number of gram equivalents of solute contained in 1 liter of solution, denoted by the symbol N.

[0024] Because saline wastewater (especially high-salinity wastewater) contains a large amount of recalcitrant organic matter, various salts, and heavy metals, the salt recovery rate is low, the purity of the resulting crystalline impurities is low, and subsequent disposal is difficult, making it hard to achieve resource utilization. This invention integrates the process and system, first obtaining dilute brine through electrodialysis crystallization, and then treating it with a bipolar membrane to improve the purity and recovery rate of the crystalline salt. At the same time, it can convert the salt in the aqueous solution into corresponding acids and bases without introducing new components, which can be reused in the plant's wastewater treatment plant to adjust the pH value or regenerate the resin, thereby achieving resource utilization of impurities.

[0025] The first aspect of this invention provides a method for treating saline wastewater, comprising the following steps:

[0026] (1) Pre-treat saline wastewater to reduce the content of calcium and magnesium ions, silica, suspended solids, iron ions, manganese ions and some organic matter in high saline wastewater to obtain pre-treated wastewater;

[0027] (2) The pretreated wastewater described in step (1) is subjected to electrodialysis crystallization to obtain crystalline salt, concentrated brine and dilute brine;

[0028] (3) The brine obtained in step (2) is subjected to bipolar membrane treatment to obtain acidic product, alkaline product and desalinated water.

[0029] In step (1) of the present invention, the pretreated wastewater meets the following conditions: calcium ions < 1 mg / L, magnesium ions < 1 mg / L, iron ions < 0.1 mg / L, manganese ions < 0.1 mg / L, silicon < 1 mg / L, fluoride ions < 15 mg / L, suspended solids < 0.1 mg / L, and COD < 25 mg / L.

[0030] To achieve the above processing results, the pretreatment sequentially includes precipitation, oxidation, and ultrafiltration-resin.

[0031] Specifically, the sedimentation is primary coagulation sedimentation, secondary coagulation sedimentation, or tertiary coagulation sedimentation. The appropriate sedimentation method can be selected according to the actual situation to remove hardness, silicon, fluoride ions, and some organic matter from the water.

[0032] It should be noted that in this invention, primary coagulation sedimentation refers to the first coagulation sedimentation, secondary coagulation sedimentation refers to the sequential occurrence of the first and second coagulation sedimentation, and tertiary coagulation sedimentation refers to the sequential occurrence of the first, second, and third coagulation sedimentation.

[0033] The oxidation process includes, but is not limited to, one or more of the following: ozone oxidation, ozone catalytic oxidation, Fenton oxidation, electrocatalytic oxidation, ozone-co-Fenton oxidation, and ozone-co-UV oxidation, in order to remove organic matter from wastewater.

[0034] The ultrafiltration membrane is selected from at least one of hollow fiber ultrafiltration membrane, inorganic ceramic ultrafiltration membrane, tubular ultrafiltration membrane and flat sheet ultrafiltration membrane; it is used to remove suspended solids or particulate matter from water.

[0035] The resin is selected from at least one of chelating resin, strong acid cation exchange resin and weak acid cation exchange resin, preferably chelating resin, to remove residual hardness in water.

[0036] In some preferred embodiments, the pretreatment sequentially includes: three-stage coagulation sedimentation, ozone catalytic oxidation, hollow fiber ultrafiltration membrane filtration, and chelating resin; wherein the three-stage coagulation sedimentation is preferably a three-stage softening coagulation sedimentation.

[0037] The specific control conditions are as follows: the effluent from the three-stage coagulation sedimentation meets the following requirements: calcium ions < 10 mg / L, magnesium ions < 2 mg / L, iron ions < 0.1 mg / L, manganese ions < 0.1 mg / L, silicon < 1 mg / L, fluoride ions < 15 mg / L, and suspended solids < 100 mg / L.

[0038] The ozone catalytic oxidation effluent meets the following requirement: COD < 25 mg / L;

[0039] The hollow fiber ultrafiltration membrane filtration and chelating resin effluent meet the following requirements: calcium ions <1 mg / L, magnesium ions <1 mg / L, iron ions <0.1 mg / L, manganese ions <0.1 mg / L, silicon <1 mg / L, fluoride ions <15 mg / L, and suspended solids <0.1 mg / L.

[0040] This invention achieves the required influent water for subsequent processes through pretreatment methods such as reducing hardness and removing silica, removing suspended solids, removing iron and manganese ions, and removing some organic matter.

[0041] It should be noted that when using the treatment method of this invention to treat high-salinity wastewater, since the incoming water is high-concentration brine or saturated brine, the composition of the wastewater becomes more complex as the salt concentration increases, requiring overcoming the impact of increased ionic strength on the treatment effect. For example: in the softening coagulation sedimentation unit, it is necessary to adjust the use of multi-stage reactions, as well as the specific reaction time and dosage; in the resin unit, it is necessary to select a chelating resin suitable for high-salinity wastewater, and the chelating resin also needs to be selected before the experiment; in the advanced oxidation unit, general advanced oxidation is not very effective at removing organic matter from high-salinity wastewater, so it is necessary to compare advanced oxidation technologies and optimize the existing process before the experiment.

[0042] Those skilled in the art should understand that the values ​​listed above are average water quality indicators for saline wastewater. In actual operation, these values ​​may vary significantly, but the final effluent quality will still meet design requirements, demonstrating the method's strong resilience. Therefore, limiting the various indicators in saline wastewater is not the actual inventive point that needs protection in this invention. Different chemical plants produce saline wastewater of varying quality, and those skilled in the art need to adjust the operating parameters according to different water qualities.

[0043] According to the present invention, in step (2), the electrodialysis crystallization process includes: using the pretreated wastewater as the fresh water inlet for electrodialysis crystallization, using the saturated sodium chloride solution as the concentrated water inlet for electrodialysis crystallization, and performing electrodialysis crystallization under the action of a DC electric field to obtain crystalline salt, concentrated brine and dilute brine respectively.

[0044] Furthermore, the conditions for the electrodialysis crystallization include: a current density of 200-500 A / m³. 2 Preferably 300-400A / m 2 The DC voltage applied to each electrodialysis membrane is 0.1-1V, preferably 0.3-0.7V.

[0045] Furthermore, the flow rate ratio of the concentrated water inlet to the fresh water inlet is 1:0.3-5, preferably 1:1-2.5.

[0046] The inventors discovered through research that the mass percentage of sodium chloride in the brine used for electrodialysis crystallization affects the purity of the salt in the electrodialysis crystallization. When the mass percentage of sodium chloride in the brine used for electrodialysis crystallization is less than 3 wt%, the purity of the salt in the electrodialysis crystallization will be significantly reduced.

[0047] To improve the salt purity of electrodialysis crystallization, in some embodiments, the sodium chloride content of the brine is ≥3 wt%, preferably 6-9 wt%.

[0048] Furthermore, a portion of the concentrated brine is mixed with the pretreated wastewater and used as the freshwater inlet for the electrodialysis crystallization; another portion of the concentrated brine is directly used as the concentrated water inlet for the electrodialysis crystallization and circulated in the electrodialysis crystallization process.

[0049] To improve the efficiency of electrodialysis crystallization and salt recovery rate, this invention ensures that the salinity of the freshwater influent is ≥10wt%, preferably ≥12wt%. Therefore, it is necessary to control the salinity of the pretreated wastewater to ≥10wt%, preferably ≥12wt%. If the salinity of the pretreated wastewater is <12wt%, it is necessary to concentrate the pretreated wastewater using a first membrane concentration method to make the salinity of the pretreated wastewater ≥10wt%, preferably ≥12wt%. The first membrane concentration method can concentrate the salinity of the pretreated wastewater from 3-5% to 15%, and reuse the product water. Therefore, in step (1) of this invention, whether to perform the first membrane concentration method is determined based on the salinity of the saline wastewater.

[0050] In some embodiments, the salinity of the saline wastewater is ≥12wt%, and the salinity of the pretreated wastewater is about 12wt%. When the salinity of the pretreated wastewater is ≥10wt%, preferably ≥12wt%, there is no need to perform a first membrane concentration, and the pretreated wastewater can be directly subjected to electrodialysis crystallization.

[0051] In other embodiments, the salinity of the saline wastewater is <12wt%, and the treatment method further includes: before electrodialysis crystallization of the pretreated wastewater, the pretreated wastewater is first concentrated by a first membrane to make the salinity of the pretreated wastewater ≥10wt%, preferably ≥12wt%.

[0052] Furthermore, the first membrane concentration includes at least one of electrodialysis, membrane distillation, forward osmosis, and disc tube reverse osmosis.

[0053] In this invention, the recovery rate of the first membrane concentration is 30%-90%, preferably 50%-80%; the processing temperature is 20-50℃.

[0054] This invention uses pretreated wastewater with a salt content ≥12wt% as the freshwater inlet for electrodialysis crystallization, and a saturated sodium chloride solution as the concentrated water inlet for electrodialysis crystallization. Under the action of a DC electric field, salt ions migrate to a supersaturated state. The freshwater effluent from electrodialysis crystallization is used as brine, and the concentrated water effluent from electrodialysis crystallization is subjected to solid-liquid separation treatment to obtain sodium chloride crystal salt and concentrated brine.

[0055] According to the present invention, in step (3), the bipolar membrane treatment process includes: using the brine as the feed water for the bipolar membrane treatment, separating the anions and cations in the brine under the action of a DC electric field, wherein the anions react with hydrogen ions to form an acid, and the cations react with hydroxide ions to form an alkali, thereby obtaining desalinated water.

[0056] In some embodiments, the bipolar membrane treatment employs a three-compartment bipolar membrane electrodialysis stack, wherein the number of membrane pairs in the three-compartment bipolar membrane electrodialysis stack is ≤200, and the operating current density is 100-2000 A / m. 2 The membrane surface flow rate is 1-15 cm / s, the electrode liquid is a sodium hydroxide solution with a mass concentration of 1-5%, and the thickness of the water distribution baffle is ≤5 mm.

[0057] The inventors discovered through research that when dilute brine is treated with a bipolar membrane, a salt content of less than 8.5% in the dilute brine obtained by electrodialysis crystallization will affect the current efficiency of the bipolar membrane treatment.

[0058] To improve the salt purity of electrodialysis crystallization, in some embodiments, the salt content of the dilute brine is ≥8.5wt%, preferably 8.5-18wt%, for example, 9wt%, 10wt%, 11wt%, 12wt%, 13wt%, 14wt%, 16wt%, 17wt%. Different salt contents are selected according to different salt compositions to improve the salt purity of electrodialysis crystallization.

[0059] Considering both the salt purity from electrodialysis crystallization and the current efficiency of bipolar membrane treatment, this invention further controls the salinity of the influent to the bipolar membrane to improve overall system efficiency and salt recovery rate. In some embodiments, when the salinity of the dilute brine from electrodialysis crystallization is 8.5-18 wt% and the sodium chloride content is 6-9 wt%, it is discharged for subsequent bipolar membrane treatment.

[0060] In some preferred embodiments, the sodium salt in the brine is pure NaCl, and the NaCl content is 8.5-15 wt%.

[0061] In some other preferred embodiments, the sodium salt in the brine is pure Na2SO4, and the content of Na2SO4 is 10.7-18 wt%.

[0062] In some other preferred embodiments, the sodium salt in the brine is a mixture of NaCl and Na2SO4, with a combined percentage content of 9-16 wt%.

[0063] To ensure both the salt purity of electrodialysis crystallization and the current efficiency of bipolar membrane treatment, it is preferable that the sodium salt in the brine is pure NaCl or a mixture of NaCl and Na2SO4.

[0064] Furthermore, when the salinity of the brine is >18 wt%, the brine is further subjected to electrodialysis crystallization until the salinity of the brine is 8.5-18 wt%.

[0065] In this invention, the equivalent concentration of the alkaline and acidic products obtained by the bipolar membrane treatment is 0.5-4N, preferably 1.5-2.5N.

[0066] In this invention, the salt content of the desalinated water is 4-7 wt%, preferably 5-6 wt%.

[0067] A bipolar membrane is an ion exchange membrane with special functions. Under the influence of a direct current electric field, the water in its intermediate interface layer dissociates, resulting in hydrogen ions and hydroxide ions on both sides of the membrane. Utilizing this characteristic, a bipolar membrane electrodialysis system, combining bipolar membranes with other anion and cation exchange membranes, can convert salts in aqueous solutions into their corresponding acids and bases without introducing new components. The desalination solution from the electrodialysis unit enters the bipolar membrane unit, where it is prepared into acids and bases, which are then reused in wastewater treatment plants to adjust pH or regenerate ion exchange resins.

[0068] To achieve resource recovery from saline wastewater, the desalinated water treated by bipolar membranes is returned for further treatment. This invention proposes two different recycling processes based on the salinity of the wastewater.

[0069] In some embodiments, when the salinity of the saline wastewater is ≥12wt%, since the first membrane concentration is not required, a second membrane concentration step is added after the bipolar membrane treatment. The treatment method further includes: subjecting the demineralized water to a second membrane concentration to obtain concentrated demineralized water, and mixing the concentrated demineralized water with the dilute brine for further bipolar membrane treatment;

[0070] Furthermore, the salt content of the concentrated desalinated water is 8.5-18 wt%.

[0071] In other embodiments, when the salinity of the saline wastewater is <12wt%, since a first membrane concentration is required, only the existing first membrane concentration is needed for concentration, and a second membrane concentration is not required. The treatment method further includes: mixing the desalinated water with the pretreated wastewater, performing a first membrane concentration to obtain a first membrane concentrate permeate and a first membrane concentrate concentrate, ensuring that the salinity of the first membrane concentrate concentrate is ≥10wt%, preferably ≥12wt%. The first membrane concentrate concentrate is used as the feed water for electrodialysis crystallization, and a saturated sodium chloride solution is used as the feed water for electrodialysis crystallization. Electrodialysis crystallization is then performed under the action of a direct current electric field.

[0072] To improve overall system efficiency and salt recovery rate, a first membrane concentration method is used to concentrate pretreated wastewater with a salinity of <12wt%, followed by electrodialysis crystallization, and the permeate is reused; alternatively, a second membrane concentration method is used to concentrate desalinated water treated by a bipolar membrane, followed by bipolar membrane treatment, and the permeate is reused. The first or second membrane concentration process of this invention can concentrate brine from a salinity of 3-5% to 15%.

[0073] This invention couples bipolar membrane treatment with pretreatment, membrane concentration, and electrodialysis crystallization. Pretreatment ensures that the product water quality meets the influent requirements of subsequent units. Membrane concentration concentrates low-concentration brine, and electrodialysis crystallizes saturated brine. By controlling process conditions, while ensuring the purity of the electrodialysis crystallized salt, the desalinated water from electrodialysis crystallization precisely meets the influent salt concentration requirements of the bipolar membrane, achieving optimal operating results. This improves the economic efficiency of treating mixed brine wastewater, reduces the economic cost of treating mixed brine wastewater, and realizes the resource-based treatment of mixed brine wastewater.

[0074] A second aspect of the present invention provides a treatment system for saline wastewater, the treatment system comprising a pretreatment device, an optional first membrane concentration device, an electrodialysis crystallization device, and a bipolar membrane device connected in sequence; wherein the pretreatment device comprises a softening coagulation sedimentation unit, an ultrafiltration-resin unit, and an advanced oxidation unit connected in sequence.

[0075] The first membrane concentration device includes: a membrane concentration membrane element, a membrane housing, and a matching water pump, a high-pressure pump, and a pipeline system;

[0076] The electrodialysis crystallization device includes: a freshwater tank, a concentrated water tank, an electrode water tank, an electrodialysis membrane stack, a DC power supply, a control cabinet, and pipelines, pumps, and testing instruments between the water tanks and the membrane stack;

[0077] The bipolar membrane device is a three-compartment bipolar membrane electrodialysis membrane stack, which is composed of multiple membrane pairs. Each membrane pair consists of an anion membrane, an cation membrane and a bipolar membrane. A water distribution baffle is provided between two adjacent membranes, and electrodes and end plates are located at both ends, which are fastened and pressed together with multiple bolts.

[0078] In this invention, the softening coagulation and sedimentation unit is a primary coagulation and sedimentation treatment unit, a secondary coagulation and sedimentation treatment unit, or a tertiary coagulation and sedimentation sub-treatment unit, to remove hardness, silicon, fluoride ions, and some organic matter from the water; a suitable coagulation and sedimentation treatment unit can be selected according to the actual situation.

[0079] The ultrafiltration-resin unit includes an ultrafiltration unit and a resin unit, wherein the ultrafiltration unit is selected from at least one of hollow fiber ultrafiltration membranes, inorganic ceramic ultrafiltration membranes, tubular ultrafiltration membranes, and flat sheet ultrafiltration membranes to remove suspended solids or particulate matter from water; and the resin unit includes chelating resin to remove residual hardness from water.

[0080] The advanced oxidation unit is selected from at least one of ozone oxidation unit, ozone catalytic oxidation unit, Fenton oxidation unit, electrocatalytic oxidation unit, ozone-co-Fenton oxidation unit, and ozone-co-UV oxidation unit to remove organic matter from wastewater.

[0081] The main purpose of the pretreatment device of the present invention is to reduce the hardness of saline wastewater and reduce the content of silicon and some organic matter, based on the influent water quality of subsequent electrodialysis crystallization.

[0082] In this invention, the specific assembly method of the three-compartment bipolar membrane electrodialysis membrane stack is as follows: a group is formed by assembling one anion membrane, one cation membrane and one bipolar membrane, starting from the anode from left to right, with the anion membrane, bipolar membrane and cation membrane arranged in sequence until the right end is the last of the anion membrane, bipolar membrane, cation membrane and cathode.

[0083] The anion membrane and cation membrane can be homogeneous membranes or heterogeneous membranes, and both anion membranes and cation membranes are preferably homogeneous membranes.

[0084] The cathode plate of the bipolar membrane electrodialysis device is made of stainless steel, titanium coated with ruthenium, or nickel, and the anode plate is made of titanium coated with ruthenium, titanium coated with platinum, or platinum. The cathode plate is connected to the cathode of the power supply, and the anode plate is connected to the anode of the power supply. The power supply can be a regulated power supply or a regulated current power supply.

[0085] Furthermore, the number of membrane pairs in the bipolar membrane electrodialysis membrane stack is ≤200 pairs, and the thickness of the water distribution baffle is ≤5mm.

[0086] In this invention, the solution between the bipolar membrane and the anion membrane is acidic, the solution between the bipolar membrane and the cation membrane is alkaline, and the solution between the cation and anion membranes is brine. Similar to an electrodialysis membrane stack, many membrane pairs can be installed in a single stack, with multiple feeds entering the corresponding compartments in parallel, forming three pathways through the stack: acidic, alkaline, and brine solutions. Therefore, under the influence of a DC electric field, the H2O between the anion and cation membrane composite layers in the bipolar membrane device dissociates into H2O. + and OH - And through the anion and cation membranes respectively, as H + and OH -Ion source.

[0087] In a preferred embodiment, the processing system includes a pretreatment device, an electrodialysis crystallization device, a bipolar membrane device, and a second membrane concentration device connected in sequence; the second membrane concentration device includes: a membrane concentration membrane element, a membrane shell, and a matching water pump, a high-pressure pump, and a pipeline system.

[0088] The treatment system of this invention targets high-salinity wastewater and mainly includes a pretreatment unit, an electrodialysis crystallization unit, and a bipolar membrane unit. First, the pretreatment unit removes hardness, silica, and some organic matter from the water to ensure the stability of subsequent processes. Then, the pretreated saline wastewater enters the electrodialysis crystallization unit, where, under the action of a DC electric field, highly soluble salts are concentrated to saturation, and some crystallized salts precipitate. Finally, the desalinated solution from the electrodialysis crystallization unit enters the bipolar membrane unit, where inorganic salts are converted into inorganic acids and bases, which can be reused in wastewater treatment plants to adjust pH levels, ultimately realizing the resource recovery of impurities.

[0089] The saline wastewater treatment system proposed in this invention can further utilize high-salinity wastewater (such as crystallizer mother liquor) by converting a portion of impurities into high-purity crystalline salts, preparing another portion of the impurities into acids and alkalis, reducing the amount of impurities, and thus realizing the resource utilization of impurities. This treatment system can replace the evaporator or crystallizer in existing zero-emission processes, and the purity and recovery rate of the obtained crystalline salts are significantly improved compared to existing processes.

[0090] This invention proposes a method and system for reducing and recycling miscellaneous salts in the terminal high-concentration brine of zero-emission processes. This method can process high-concentration brine or saturated brine, and the purity and recovery rate of the obtained crystalline salt are significantly improved compared with existing processes, ultimately reducing the amount of miscellaneous salts generated and realizing the recycling of miscellaneous salts.

[0091] The advantages of this invention are mainly: ① It replaces or partially replaces the evaporation and crystallization units in existing zero-emission processes, significantly improving the purity and recovery rate of the obtained crystalline salt compared to existing processes. ② It can further utilize the crystallizer mother liquor (saturated salt solution), converting some impurities into higher-purity crystalline salts and preparing some impurities into acids and alkalis, thus reducing the amount of impurities and achieving resource utilization. The purpose of this invention is to improve the purity and recovery rate of crystalline salts in existing salt separation and crystallization processes, reduce the production of hazardous waste from impurities, and reduce the treatment costs of hazardous waste from impurities.

[0092] The present invention can flexibly select the above treatment methods according to the salinity of the wastewater to realize the resource utilization of saline wastewater.

[0093] According to a particularly preferred embodiment of the present invention, when the salinity of the saline wastewater is ≥12wt%, the method for treating the saline wastewater is as follows: Figure 1 As shown, the specific steps include:

[0094] S11. Pretreatment Steps: Using a pretreatment device, saline wastewater with a salt content ≥12wt% is first subjected to softening, coagulation, and sedimentation to remove hardness, silicon, fluoride ions, and some organic matter from the water; then, it undergoes advanced oxidation to remove residual organic matter; and finally, it undergoes ultrafiltration-resin to remove residual suspended solids, solid particles, and hardness from the water; so that calcium ions <1mg / l, magnesium ions <1mg / l, iron ions <0.1mg / l, manganese ions <0.1mg / l, silicon <1mg / l, fluoride ions <15mg / l, suspended solids <0.1mg / l, and COD <25mg / l, to obtain pretreated wastewater;

[0095] S12. Electrodialysis crystallization step: An electrodialysis crystallization device is used, with the pretreated wastewater as the fresh water inlet for electrodialysis crystallization and the saturated sodium chloride solution as the concentrated water inlet for electrodialysis crystallization. Electrodialysis crystallization is carried out under the action of a DC electric field to obtain sodium chloride crystals, concentrated brine, and dilute brine, respectively. During the electrodialysis crystallization process, a portion of the concentrated brine is returned and mixed with the pretreated wastewater as the fresh water inlet for electrodialysis crystallization; the other portion of the concentrated brine is directly used as the concentrated water inlet for electrodialysis crystallization and circulated in the electrodialysis crystallization process.

[0096] The salinity of the brine is controlled to be 8.5-18 wt%. When the salinity of the brine is >18 wt%, the brine is further subjected to electrodialysis crystallization until the salinity of the brine is 8.5-18 wt%, while ensuring that the sodium chloride content of the brine is ≥3 wt%.

[0097] S13. Bipolar membrane treatment steps: Using a bipolar membrane treatment device, the dilute brine is subjected to bipolar membrane treatment to obtain acidic products, alkaline products, and desalinated water; the desalinated water is concentrated by membrane treatment to obtain concentrated desalinated water, and the concentrated desalinated water is returned and mixed with the dilute brine for bipolar membrane treatment, and the cycle is repeated.

[0098] According to another particularly preferred embodiment of the present invention, when the salinity of the saline wastewater is <12wt%, the method for treating the saline wastewater is as follows: Figure 2 As shown, the specific steps include:

[0099] S21. Pretreatment Steps: Using a pretreatment device, saline wastewater with a salt content <12wt% is first subjected to softening, coagulation, and sedimentation to remove hardness, silicon, fluoride ions, and some organic matter from the water; then, it undergoes advanced oxidation to remove residual organic matter; and then, it undergoes ultrafiltration-resin to remove residual suspended solids, solid particles, and hardness from the water; ensuring that calcium ions <1mg / L, magnesium ions <1mg / L, iron ions <0.1mg / L, manganese ions <0.1mg / L, silicon <1mg / L, fluoride ions <15mg / L, suspended solids <0.1mg / L, and COD <25mg / L, thus obtaining pretreated wastewater; the pretreated wastewater is then subjected to membrane concentration to obtain membrane concentrate permeate and membrane concentrate concentrate, and the salt content of the membrane concentrate concentrate is ≥12wt%.

[0100] S22. Electrodialysis crystallization step: Using an electrodialysis crystallization apparatus, the concentrated water from the membrane is used as the feed water for electrodialysis crystallization, and a saturated sodium chloride solution is used as the feed water for concentrated water in electrodialysis crystallization. Electrodialysis crystallization is carried out under the action of a DC electric field to obtain sodium chloride crystals, concentrated brine, and dilute brine, respectively. During the electrodialysis crystallization process, a portion of the concentrated brine is returned and mixed with the first membrane concentrated water as the feed water for electrodialysis crystallization; another portion of the concentrated brine is directly used as the feed water for concentrated water in electrodialysis crystallization and circulated in the electrodialysis crystallization process.

[0101] The salinity of the brine is controlled to be 8.5-18 wt%. When the salinity of the brine is >18 wt%, the brine is further subjected to electrodialysis crystallization until the salinity of the brine is 8.5-18 wt%, while ensuring that the sodium chloride content of the brine is ≥3 wt%.

[0102] S23. Bipolar membrane treatment step: The dilute brine is treated with a bipolar membrane device to obtain acidic product, alkaline product and desalinated water.

[0103] The desalinated water is returned and mixed with the pretreated wastewater for membrane concentration, so that the salt content of the first membrane concentrate is ≥12wt%. Subsequent steps S12 and S13 are then performed in a cyclical operation.

[0104] The saline wastewater treatment system comprises a pretreatment unit, an electrodialysis crystallization unit, and a bipolar membrane unit connected in sequence. The pretreated wastewater (with impurities removed and a salt content ≥10 wt%) enters the electrodialysis crystallization unit, where salt ions migrate and concentrate during electrodialysis. Upon saturation, the ions are directly converted into crystalline salts, which precipitate out in solid form, thus reducing the volume of impurities. The brine from the electrodialysis crystallization unit enters the bipolar membrane unit, where, without introducing other media, inorganic salts are converted into inorganic acids and bases. These can be reused at wastewater treatment plants for pH adjustment or resin regeneration, ultimately achieving the resource recovery of impurities.

[0105] To further understand the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0106] Unless otherwise specified, the reagents and devices involved in the embodiments of this invention are all commercially available products and can be purchased through commercial channels. The invention will be described in detail below through embodiments.

[0107] According to GB / T 30902-2014, inductively coupled plasma optical emission spectrometry (ICP-OES) was used to determine the content of elements such as calcium, magnesium, and silicon in samples. The instrument model was Spectro Arcos, and the test conditions were 1400W power and 0.8 mL / min nebulizer flow rate. -1 Plasma gas flow rate: 1.0 mL / min -1 .

[0108] According to GB / T 14642-2009, the content of anions such as fluoride and sulfate in samples was determined by ion chromatography. The instrument model was Integrion HPIC, and the test conditions were: eluent 30 mM and flow rate 1.0 mL / min. -1 Run time 15 min, injection volume 25 μL.

[0109] The suspended solids content was determined according to the gravimetric method for the determination of suspended solids in water, GB11901-1989.

[0110] The COD content was determined according to the dichromate method (HJ 828—2017) for the determination of chemical oxygen demand in water.

[0111] The water quality test results of a certain industrial mixed saline wastewater 1 are shown in Table 1.

[0112] Table 1. Water quality test results of a certain industrial mixed saline wastewater.

[0113] project Concentration (mg / L) <![CDATA[Ca 2+ ]]> 650 <![CDATA[Mg 2+ ]]> 415 <![CDATA[Fe 3+ ]]> 14 Mn 3 <![CDATA[Na 3+ ]]> 98253 <![CDATA[SO4 2- ]]> 39123 <![CDATA[Cl - ]]> 125000 <![CDATA[SiO2]]> 110 <![CDATA[F - ]]> 125 TDS 263441 COD 190 suspended matter 120

[0114] The water quality test results of a certain industrial mixed saline wastewater 2 are shown in Table 2.

[0115] Table 2. Water quality test results of a certain industrial mixed saline wastewater.

[0116]

[0117]

[0118] Example 1

[0119] Adopting such Figure 1 The process route shown for treating a certain industrial mixed brine wastewater 1 includes the following steps:

[0120] (1) Pretreatment: The industrial mixed salt wastewater 1 was treated sequentially by “three-stage softening coagulation sedimentation + ozone catalytic oxidation + hollow fiber ultrafiltration + chelating resin” to obtain pretreated liquid 1. The water quality test results of pretreated liquid 1 are shown in Table 3.

[0121] Table 3. Water quality test results of pretreated solution 1

[0122] project Concentration (mg / L) <![CDATA[Ca 2+ ]]> 0.4 <![CDATA[Mg 2+ ]]> 0.2 <![CDATA[Fe 3+ ]]> 0.06 Mn 0.02 <![CDATA[Na 3+ ]]> 102053 <![CDATA[SO4 2- ]]> 39125 <![CDATA[Cl - ]]> 128500 <![CDATA[SiO2]]> 0.6 <![CDATA[F - ]]> 15 TDS 269676 COD 20 suspended matter 0.1

[0123] (2) Electrodialysis crystallization: Pretreatment solution 1 is used as the freshwater feed for the electrodialysis crystallization unit, and a saturated sodium chloride solution is used as the concentrated water feed for the same unit. The electrodialysis system uses a domestically produced homogeneous electrodialysis membrane with a membrane surface flow rate of 3 cm / s and an operating current density of 350 A / m³. 2 Each electrodialysis membrane is subjected to a DC voltage of 0.7V. The flow rate ratio of the electrodialysis concentrate feed water to the electrodialysis desalination feed water is 1:1.5. Under the action of the DC electric field, salt ions in the desalination chamber migrate to the concentrate chamber, causing the concentrate chamber to reach a supersaturated state. The electrodialysis system produces electrodialysis crystallization desalinated water 1 and concentrate. When the salt content in the electrodialysis crystallization desalinated water reaches 11.61wt% (of which the sodium chloride mass percentage reaches 9.1wt%), it is discharged for further treatment.

[0124] The resulting concentrated water was centrifuged to obtain sodium chloride product and electrodialysis crystallization concentrated brine. The sodium chloride product had a purity of 99.1%, meeting the requirements of Grade I industrial salt in the GB / T 5462-2015 standard for industrial salt. The current efficiency of electrodialysis crystallization reached 85%. The increased water volume of the electrodialysis crystallization concentrated brine was refluxed and mixed with pretreatment solution 1 as the freshwater feed water for the electrodialysis crystallization system. The water quality test results of electrodialysis crystallization brine 1 are shown in Table 4.

[0125] Table 4. Water quality test results of electrodialysis crystallization saline solution 1

[0126] project Concentration (mg / L) <![CDATA[Ca 2+ ]]> 0.3 <![CDATA[Mg 2+ ]]> 0.1 <![CDATA[Fe 3+ ]]> 0.02 Mn 0.01 <![CDATA[Na 3+ ]]> 43938 <![CDATA[SO4 2- ]]> 16823 <![CDATA[Cl - ]]> 55255 <![CDATA[SiO2]]> 0.3 <![CDATA[F - ]]> 12 TDS 116162 COD 10 suspended matter 0.1

[0127] (3) Bipolar membrane treatment: The electrodialysis crystallization brine 1 was introduced into the salt chamber of the bipolar membrane electrodialysis (BPED) device for bipolar membrane treatment. The BPED operating current density was 1000 A / m 2The membrane surface flow rate is 4.5 cm / s, the electrode solution is a 4% sodium hydroxide solution, and the thickness of the water distribution baffle is 3 mm. When the BPED is energized and the salt content of the feed solution in the salt chamber drops to 6 wt%, the bipolar membrane treated demineralized water 1 is transported to the disc tube high-pressure reverse osmosis (DTRO) unit.

[0128] A disc tube high-pressure reverse osmosis (DTRO) device was used to concentrate the desalinated water treated by the bipolar membrane. The system operating pressure was 11 MPa, the system recovery rate was 70%, and the treatment temperature was 35℃. After membrane concentration, the TDS of the membrane concentrate permeate was 800 mg / L, and the TDS of the membrane concentrate concentrate was 116162 mg / L. The membrane concentrate concentrate was then recycled to the salt chamber of the bipolar membrane electrodialysis device.

[0129] During the BPED process, deionized water is added to both the acid and alkali chambers. Sulfuric acid produced in the acid chamber and sodium hydroxide produced in the alkali chamber are recovered, ultimately yielding a 2.5N sodium hydroxide product and a 2.0N mixed acid product. The current efficiency of BPED reaches 85%.

[0130] The overall salt recovery rate in this embodiment reaches 95%.

[0131] Example 2

[0132] Adopting such Figure 2 The process route shown for treating industrial mixed brine wastewater 2 includes the following steps:

[0133] (1) Pretreatment: The industrial mixed salt wastewater 2 was treated sequentially by “three-stage softening coagulation sedimentation + ozone catalytic oxidation + hollow fiber ultrafiltration + chelating resin” to obtain pretreated liquid 2. The water quality test results of pretreated liquid 2 are shown in Table 5.

[0134] Table 5. Water quality test results of pretreated solution 2

[0135] project Concentration (mg / L) <![CDATA[Ca 2+ ]]> 0.3 <![CDATA[Mg 2+ ]]> 0.1 <![CDATA[Fe 3+ ]]> 0.05 Mn 0.01 <![CDATA[Na 3+ ]]> 21444 <![CDATA[SO4 2- ]]> 6099 <![CDATA[Cl - ]]> 28541 <![CDATA[SiO2]]> 0.5 <![CDATA[F - ]]> 14 TDS 56098 COD 24 suspended matter 0.1

[0136] (2) Membrane concentration treatment: The pretreatment liquid 2 was concentrated by a disc tube high pressure reverse osmosis (DTRO) device. The system operating pressure was 11 MPa, the system recovery rate was 70%, and the treatment temperature was 35℃. After membrane concentration, the TDS of the membrane concentrate permeate was 800 mg / L, and the TDS of the membrane concentrate concentrate was 149784 mg / L. The specific water quality is shown in Table 6.

[0137] Table 6. Water quality test results of membrane concentrate.

[0138]

[0139]

[0140] (3) Electrodialysis crystallization: The concentrated water from the membrane is used as the feed water for electrodialysis crystallization, and a saturated sodium chloride solution is used as the feed water for the concentrated water in the electrodialysis crystallization process. The electrodialysis system uses a domestically produced homogeneous electrodialysis membrane with a membrane surface flow rate of 3 cm / s and an operating current density of 350 A / m³. 2 Each electrodialysis membrane is subjected to a DC voltage of 0.7V. The flow rate ratio of the electrodialysis concentrate feed water to the electrodialysis desalination feed water is 1:1.5. Under the action of the DC electric field, salt ions in the desalination chamber migrate to the concentrate chamber, causing the concentrate chamber to reach a supersaturated state. The electrodialysis system produces electrodialysis desalinated brine 2 and concentrate. When the salt content in the electrodialysis crystallized desalinated brine reaches 8.68wt% (of which the sodium chloride mass percentage reaches 7.28wt%), it is discharged for further treatment.

[0141] The resulting concentrated water was centrifuged to obtain sodium chloride product and electrodialysis crystallization concentrated brine. The sodium chloride product had a purity of 99%, meeting the requirements of Grade I industrial salt in the GB / T 5462-2015 standard for industrial salt. The current efficiency of electrodialysis crystallization reached 75%. The increased water volume of the electrodialysis crystallization concentrated brine was refluxed and mixed with the membrane concentration concentrate as the feed water for electrodialysis crystallization. The water quality test results of electrodialysis crystallization concentrated brine 2 are shown in Table 7.

[0142] Table 7. Water quality test results of electrodialysis crystallization saline solution 2

[0143]

[0144]

[0145] (4) Bipolar membrane treatment: The electrodialysis crystallization brine 2 is introduced into the salt chamber of the bipolar membrane electrodialysis (BPED) device for bipolar membrane treatment. The BPED operating current density is 1000 A / m 2 The membrane surface flow rate is 4 cm / s, the electrode solution is a 4% sodium hydroxide solution, and the thickness of the water distribution baffle is 3 mm. When the BPED is energized and the salt content of the feed solution in the salt chamber drops to 4 wt%, the deionized water treated by the bipolar membrane is returned to the disc tube high-pressure reverse osmosis (DTRO) unit to mix with the DTRO feed water for re-concentration.

[0146] During the BPED process, deionized water is added to both the acid and alkali chambers. Sulfuric acid produced in the acid chamber and sodium hydroxide produced in the alkali chamber are recovered, ultimately yielding a 1.8N sodium hydroxide product and a 1.5N mixed acid product. The current efficiency of BPED reaches 75%.

[0147] The overall salt recovery rate in this embodiment reaches 95%.

[0148] Comparative Example 1

[0149] The only difference from Example 1 is that the industrial mixed salt wastewater 1 entered the electrodialysis crystallization device directly without pretreatment. Due to inorganic impurities (including high-valence ions such as calcium, magnesium, iron, manganese, and silica, which easily cause membrane scaling) and organic impurities (organic matter is generally negatively charged and easily adsorbs on the surface of the anion exchange membrane to form a double electric layer structure, increasing the resistance of anions to pass through the anion exchange membrane, or organic matter containing benzene rings that easily cause membrane swelling or damage), the membrane fouling or damage was easily caused. The electrodialysis crystallization desalination rate was significantly reduced, and the current efficiency of electrodialysis crystallization was only 30%. After running for 1 hour, the conductivity of the electrodialysis crystallization desalination chamber no longer decreased, and the device was forced to shut down.

[0150] Comparative Example 2

[0151] The only difference from Example 1 is that the industrial mixed saline wastewater 1 did not undergo the electrodialysis crystallization step; instead, it directly entered the bipolar membrane treatment step after the pretreatment step. The TDS of this industrial wastewater was 269,676 mg / L. After entering the bipolar membrane electrodialysis (BPED) device, the osmotic pressure difference between the salt chamber and the acid / alkali chambers (initially deionized water) was large, allowing water from the acid / alkali chambers to easily permeate into the salt chamber. Simultaneously, due to the high salt content in the salt chamber, the selectivity of the ion-exchange membrane deteriorated, resulting in poor acid and alkali quality. Furthermore, this led to a decrease in the current efficiency of the BPED device, with the current efficiency reaching only 20%.

[0152] As can be seen from the above, the current efficiency of electrodialysis crystallization in Example 1 reached 85%, the current efficiency of BPED reached 85%, and the overall salt recovery rate reached 95%. In contrast, the current efficiency of electrodialysis crystallization in Comparative Example 1 was only 30%, forcing the device to shut down. The current efficiency of BPED in Comparative Example 2 was only 20%. Therefore, pretreatment, electrodialysis crystallization, and bipolar membrane treatment form an organic whole; the absence of any one step will prevent the achievement of resource utilization. After adopting the treatment method of the present invention, the purity of the crystallized salt is as high as 99% or above, the salt recovery rate is greater than 90%, and the treatment cost is also reduced.

[0153] The processes described in Examples 1 and 2 are used to treat mixed salt wastewater for resource recovery. The produced acid and alkali products can be used for pH adjustment at the front end of wastewater treatment and for other acid and alkali needs in the plant area. Simultaneously, the obtained sodium chloride product has high purity and can be sold as industrial salt. This reduces the plant's acid and alkali consumption, improves the purity of the industrial by-product (sodium chloride), avoids the generation of mixed salts, and significantly reduces reagent costs and solid waste treatment costs. The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as part of the content disclosed in the present invention and fall within the protection scope of the present invention.

Claims

1. A method for treating salt-containing wastewater, characterized by, The treatment method comprises: The salt-containing wastewater is pretreated to reduce the contents of calcium and magnesium ions, silicon dioxide, suspended solids, iron ions, manganese ions and part of organic matters in the high-content wastewater, to obtain pretreated wastewater; the pretreatment comprises, in sequence, three-stage coagulation sedimentation, ozone catalytic oxidation, hollow fiber ultrafiltration membrane filtration and chelating resin; the pretreated wastewater meets the following conditions: calcium ion < 1 mg / l, magnesium ion < 1 mg / l, iron ion < 0.1 mg / l, manganese ion < 0.1 mg / l, silicon < 1 mg / l, fluorine ion < 15 mg / l, suspended solids < 0.1 mg / l, and COD < 25 mg / l; The pretreated wastewater is subjected to electrodialysis crystallization to obtain crystalline salt, concentrated brine and dilute brine; the conditions of the electrodialysis crystallization include: current density is 300-400 A / m 2 , and a direct current voltage of 0.3-0.7 V is applied to each electrodialysis membrane. The pretreated wastewater is used as fresh water for electrodialysis crystallization, saturated sodium chloride solution is used as concentrated water for electrodialysis crystallization, and under the action of a direct current electric field, electrodialysis crystallization is carried out to obtain crystalline salt, concentrated brine and dilute brine; the flow rate ratio of the concentrated water to the fresh water is 1:1-2.5; the salt content of the dilute brine is 8.5-18 wt%; The dilute brine is treated by bipolar membrane to obtain an acid product, an alkali product and desalted water; the bipolar membrane treatment adopts a three-compartment bipolar membrane electrodialysis membrane stack composed of multiple groups of membrane pairs, each group of membrane pairs is composed of one negative membrane, one positive membrane and one bipolar membrane, and a water distribution partition plate is arranged between two adjacent membranes; the number of membrane pairs of the three-compartment bipolar membrane electrodialysis membrane stack is ≤200 pairs, the operating current density is 100-2000 A / m 2 , the membrane surface flow rate is 1-15 cm / s, the electrolyte is a sodium hydroxide solution with a mass concentration of 1-5%, and the thickness of the water distribution partition plate is ≤5 mm; the equivalent concentration of the alkali product and the acid product obtained by the bipolar membrane treatment is 1.5-2.5 N, and the salt content of the desalted water is 4-7 wt%.

2. The treatment method of claim 1, wherein, The pretreatment comprises, in sequence, sedimentation, oxidation and ultrafiltration-resin, wherein the sedimentation is one-stage coagulation sedimentation, two-stage coagulation sedimentation or three-stage coagulation sedimentation; the oxidation is at least one selected from ozone oxidation, ozone catalytic oxidation, Fenton oxidation, electro-catalytic oxidation, ozone synergistic Fenton oxidation and ozone synergistic ultraviolet oxidation; the ultrafiltration is at least one selected from hollow fiber ultrafiltration membrane, inorganic ceramic ultrafiltration membrane, tubular ultrafiltration membrane and flat plate ultrafiltration membrane; and the resin is at least one selected from chelating resin, strong acid cation exchange resin and weak acid cation exchange resin.

3. The treatment method of claim 2, wherein, The resin is selected from chelating resin; And / or, the specific control conditions of the pretreatment are: The effluent of the three-stage coagulation sedimentation meets the following conditions: calcium ion < 10 mg / l, magnesium ion < 2 mg / l, iron ion < 0.1 mg / l, manganese ion < 0.1 mg / l, silicon < 1 mg / l, fluorine ion < 15 mg / l, and suspended solids < 100 mg / l; The effluent of the ozone catalytic oxidation meets the following condition: COD < 25 mg / l; The effluent of the hollow fiber ultrafiltration membrane filtration and chelating resin meets the following conditions: calcium ion < 1 mg / l, magnesium ion < 1 mg / l, iron ion < 0.1 mg / l, manganese ion < 0.1 mg / l, silicon < 1 mg / l, fluorine ion < 15 mg / l, and suspended solids < 0.1 mg / l.

4. The treatment method of claim 1, wherein, The mass percentage content of sodium chloride in the dilute brine is ≥ 3 wt%.

5. The treatment method of claim 1, wherein, Part of the concentrated brine is mixed with the pretreated wastewater as the fresh water for electrodialysis crystallization; and the other part of the concentrated brine is directly used as the concentrated water for electrodialysis crystallization to circulate in the electrodialysis crystallization.

6. The treatment method according to claim 1 or 2, wherein, The process of the bipolar membrane treatment comprises: using the dilute brine from the electrodialysis crystallization as the water for the bipolar membrane treatment, separating anions and cations in the dilute brine under the action of a direct current electric field, generating acid from the anions and hydrogen ions, generating base from the cations and hydroxyl ions, and obtaining desalinated water.

7. The treatment method of claim 6, wherein, The sodium salt in the dilute brine is pure NaCl, and the content of NaCl is 8.5-15 wt%.

8. The treatment method of claim 6, wherein, The sodium salt in the dilute brine is pure Na2SO4, and the content of Na2SO4 is 10.7-18wt%.

9. The treatment method of claim 6, wherein, The sodium salt in the dilute brine is a mixture of NaCl and Na2SO4, and the sum of the percentage contents of the two is 9-16wt%.

10. The treatment method of claim 6, wherein, When the salinity of the dilute brine is >18wt%, the dilute brine is continuously subjected to electrodialysis crystallization until the salinity of the dilute brine is 8.5-18wt%.

11. The treatment method of claim 6, wherein, The salinity of the desalinated water is 5-6wt%.

12. The treatment method of claim 1 or 2, wherein, The salinity of the saline wastewater is ≥12wt%, and the salinity of the pretreated wastewater is ≥10wt%.

13. The treatment method of claim 12, wherein, The salinity of the pretreated wastewater is ≥12wt%.

14. The treatment method of claim 12, wherein, The treatment method further comprises: subjecting the desalinated water to second membrane concentration to obtain concentrated desalinated water, and mixing the concentrated desalinated water with the dilute brine to continue the bipolar membrane treatment.

15. The treatment method of claim 14, wherein, The salinity of the concentrated desalinated water is 8.5-18wt%.

16. The treatment method of claim 1 or 2, wherein, The salinity of the saline wastewater is <12wt%, and the treatment method further comprises: before subjecting the pretreated wastewater to electrodialysis crystallization, subjecting the pretreated wastewater to first membrane concentration to make the salinity of the pretreated wastewater ≥10wt%.

17. The treatment method of claim 16, wherein, The salinity of the pretreated wastewater is ≥12wt%.

18. The treatment method of claim 16, wherein, The first membrane concentration comprises at least one of electrodialysis, membrane distillation, forward osmosis and disc-tube reverse osmosis.

19. The treatment method of claim 16, wherein, The recovery rate of the first membrane concentration is 30%-90%, and the treatment temperature is 20-50℃.

20. The treatment method of claim 19, wherein, The recovery rate of the first membrane concentration is 50%-80%.

21. The treatment method of claim 16, wherein, The treatment method further comprises: mixing the desalinated water with the pretreated wastewater to perform first membrane concentration to obtain first membrane concentration water and first membrane concentration concentrated water, and making the salinity of the first membrane concentration concentrated water ≥10wt%, taking the first membrane concentration concentrated water as dilute water feed for electrodialysis crystallization, taking saturated sodium chloride solution as concentrated water feed for electrodialysis crystallization, and performing electrodialysis crystallization under the action of a direct current electric field.

22. The treatment method of claim 21, wherein, And making the salinity of the first membrane concentration concentrated water ≥12wt%.

23. A system for treating salt-laden wastewater for use in the treatment method of any one of claims 1-22, characterized in that, The treatment system comprises, which are connected in sequence, a pretreatment device, an optional first membrane concentration device, an electrodialysis crystallization device and a bipolar membrane device; wherein the pretreatment device comprises, which are connected in sequence, a softening coagulation sedimentation unit, an ultrafiltration-resin unit and an advanced oxidation unit; the pretreated wastewater satisfies: calcium ion <1mg / l, magnesium ion <1mg / l, iron ion <0.1mg / l, manganese ion <0.1mg / l, silicon <1mg / l, fluoride ion <15mg / l, suspended solids <0.1mg / l, and COD <25mg / l; The first membrane concentration device comprises: a membrane concentration membrane element, a membrane shell and a supporting water delivery pump, a high-pressure pump and a pipeline system; The electrodialysis crystallization device comprises a fresh water tank, a concentrated water tank, a polar water tank, an electrodialysis membrane stack, a direct current power supply, a control cabinet and pipelines, pumps and detecting instruments between the water tanks and the membrane stack; the operation conditions of the electrodialysis membrane stack comprise that the current density is 300-400 A / m 2 and the direct current voltage applied to each electrodialysis membrane is 0.3-0.7 V. The bipolar membrane device is a three-compartment bipolar membrane electrodialysis membrane stack, which is composed of multiple groups of membrane pairs, each group of membrane pairs is composed of one negative membrane, one positive membrane and one bipolar membrane, water distribution partitions are arranged between two adjacent membranes, electrodes and end plates are arranged at both ends, and multiple bolts are used for fastening and pressing; the number of membrane pairs of the bipolar membrane electrodialysis membrane stack is ≤200 pairs, the thickness of the water distribution partition is ≤5mm, and the operating current density is 100-2000A / m 2 .

24. The processing system of claim 23, wherein, The softening coagulation sedimentation unit is a primary coagulation sedimentation treatment unit, a secondary coagulation sedimentation treatment unit or a tertiary coagulation sedimentation treatment unit to remove hardness, silicon, fluoride ions and part of organic matter in water; The ultrafiltration-resin unit comprises an ultrafiltration unit and a resin unit, wherein the ultrafiltration unit is selected from at least one of hollow fiber ultrafiltration membrane, inorganic ceramic ultrafiltration membrane, tubular ultrafiltration membrane and flat plate ultrafiltration membrane to remove suspended solids or particulate matters in water; the resin unit comprises a chelating resin to remove residual hardness in water; The advanced oxidation unit is selected from at least one of an ozone oxidation unit, an ozone catalytic oxidation unit, a Fenton oxidation unit, an electro-catalytic oxidation unit, an ozone synergistic Fenton oxidation unit and an ozone synergistic ultraviolet oxidation unit to remove organic matters in wastewater.

25. The processing system of claim 23 or 24, wherein, The treatment system comprises a pretreatment device, an electrodialysis crystallization device, a bipolar membrane device and a second membrane concentration device connected in sequence. The second membrane concentration device comprises a membrane concentration membrane element, a membrane shell and a supporting water delivery pump, a high-pressure pump and a pipeline system.

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