High-salinity wastewater zero discharge and salt separation recycling integrated treatment system and method

Abstract

The invention relates to the technical field of high-salinity wastewater zero discharge treatment, in particular to a high-salinity wastewater zero discharge and salt separation recycling integrated treatment system and method.The system comprises a pretreatment unit used for intercepting suspended matter and macromolecular organic matter of low-salinity wastewater; the primary reverse osmosis unit is used for primarily concentrating the pretreated produced water, and the recovery rate is controlled to be 70-80%; the nanofiltration salt separation unit is used for separating monovalent ions and multivalent ions / organic matters in the primary RO concentrated solution; the super-efficient reverse osmosis unit is used for performing two-stage high-power concentration on the nanofiltration produced water; and the MVR evaporative crystallization unit is used for treating the super-effect RO concentrated water and recovering sodium chloride crystals. According to the invention, graded concentration-salt separation synergistic interaction is realized, the treatment cost is reduced, super-effect reverse osmosis energy circulation is realized, the energy consumption is greatly reduced, the whole-flow resource closed loop is realized, zero emission is realized, the stability and adaptability of the system are improved, high-efficiency, low-cost and environment-friendly treatment of the high-salinity wastewater is realized, and the method has remarkable economic and social benefits.

Description

Technical Field

[0001] The present invention relates to the technical field of zero-discharge treatment of high-salt wastewater, and specifically to an integrated treatment system and method for zero-discharge and salt separation and resource utilization of high-salt wastewater. Background Art

[0002] With the increasingly strict industrial wastewater discharge standards and the increasing demand for resource recycling, the "zero-discharge" treatment of high-salt wastewater (TDS≥10,000mg / L) has become an important challenge in the environmental protection field. Traditional high-salt wastewater treatment technologies mainly rely on evaporation crystallization or membrane concentration coupled with crystallization processes, but there are still the following technical bottlenecks:

[0003] (1) High energy consumption: Conventional evaporation processes (such as multi-effect evaporation) require a large amount of steam and electricity, and the treatment cost accounts for more than 60% of the total system operation cost; although reverse osmosis (RO) technology can achieve preliminary concentration, its concentration ratio is limited, resulting in a large load on the subsequent evaporation unit and still relatively high overall energy consumption.

[0004] (2) Low salt separation efficiency: High-salt wastewater usually contains mixed ions such as Na + , Cl - , SO4 2- etc. Traditional processes are difficult to effectively separate them, resulting in high impurity content in the crystalline salt products (such as the coexistence of sodium sulfate and sodium chloride), and the purity is generally less than 90%, which cannot meet the industrial reuse standards (such as the first-class product requirements of GB / T 5462-2015 "Industrial Salt" for Cl - ≥98.5%), limiting its resource utilization value.

[0005] (3) Risk of secondary pollution: If the high-salt concentrated water is directly solidified and landfilled without effective salt separation, it is easy to cause soil salinization; if incineration treatment is used without a coexisting salt separation process, toxic gases such as dioxins may be generated, causing secondary pollution to the environment.

[0006] In recent years, nanofiltration (NF) salt separation technology has been applied to a certain extent in high-salt wastewater treatment due to its selective separation ability for monovalent ions (such as Na + , Cl - ) and divalent ions (such as SO4 2- ). However, this technology still has the following problems:

[0007] (1) Insufficient pretreatment: Residual hardness ions (such as Ca 2+ , Mg 2+ ) and organic matter in the wastewater are not effectively removed, which easily leads to nanofiltration membrane fouling and affects the long-term operation stability of the system.

[0008] (2) Salt separation - concentration decoupling: The NF concentrated water is usually not co - disposed with industrial waste residues, resulting in the system not forming a true closed - loop and a low resource recovery rate.

[0009] (3) Low salt resource utilization rate: The purity of the crystalline salt is limited by the salt separation accuracy of the front - end NF, making it difficult to stably meet the requirements of industrial - grade salt products, and its economic value is limited.

[0010] For the related technical problems, no solutions have been proposed yet. Summary of the Invention

[0011] In view of the problems in the related art, the present invention proposes a zero - discharge and salt separation resource utilization integrated treatment system and method for high - salt wastewater to overcome the above - mentioned technical problems existing in the existing related technologies. The purpose of the present invention is to achieve efficient reduction of low - salt wastewater, precise separation of salts, and high - value recovery through hierarchical treatment and energy cycle optimization.

[0012] To achieve the above object, the present invention provides the following technical solutions: A zero - discharge and salt separation resource utilization integrated treatment system for high - salt wastewater, comprising:

[0013] A pretreatment unit for intercepting suspended solids and macromolecular organic matters in low - salt wastewater;

[0014] A primary reverse osmosis unit for preliminarily concentrating the pretreated water and controlling the recovery rate at 70 - 80%;

[0015] A nanofiltration salt separation unit for separating monovalent ions from multivalent ions / organic matters in the primary RO concentrated solution;

[0016] A super - efficient reverse osmosis unit for performing two - stage high - fold concentration on the NF produced water;

[0017] An MVR evaporation crystallization unit for treating the super - efficient RO concentrated water and recovering sodium chloride crystals.

[0018] Preferably, the pretreatment unit uses ultrafiltration or microfiltration membranes to treat low - salt wastewater with TDS ≤ 5,000 mg / L.

[0019] Preferably, the TDS of the water produced by the primary reverse osmosis unit is ≤ 500 mg / L, and the TDS of the concentrated solution is ≥ 20,000 mg / L.

[0020] Preferably, the operating parameters of the nanofiltration salt separation unit include: operating pressure 0.8 - 1.5 MPa, pH value 6.5 - 7.5, and the proportion of Cl - in the NF produced water ≥ 95%.

[0021] Preferably, the super - efficient reverse osmosis unit includes:

[0022] (1) The first-stage RO: uses a seawater desalination membrane, with an operating pressure of 5 - 7 MPa and a recovery rate of 60 - 70%;

[0023] (2) The second-stage RO: integrates a pressure-exchange type energy recovery device, and the total system recovery rate ≥ 90%;

[0024] (3) The TDS of the final product water ≤ 200 mg / L, and the TDS of the concentrated water ≥ 100,000 mg / L.

[0025] Preferably, the MVR evaporation crystallization unit adopts gradient heating and seed-induced crystallization to produce industrial-grade sodium chloride crystals with a purity ≥ 98.5%, and the heating temperature is 60°C - 90°C.

[0026] The present invention also provides the following technical solution: an integrated treatment method for zero discharge and salt separation and resource utilization of high-salt wastewater, including the following steps:

[0027] S1. Perform membrane pretreatment on low-salt wastewater;

[0028] S2. Achieve wastewater volume reduction through primary RO;

[0029] S3. Use nanofiltration to separate monovalent salts from polyvalent salts / organic matters;

[0030] S4. Perform two-stage RO high-fold concentration on the nanofiltration product water;

[0031] S5. Evaporate and crystallize the high-salt concentrated water to obtain sodium chloride products.

[0032] Preferably, in step S4, a pressure-exchange type energy recovery device is used to drive the feed water booster pump by using the residual pressure of the concentrated water.

[0033] Preferably, in step S5, the evaporated condensate water is recycled to the front-end process of the system to form a closed-loop water-salt cycle for the whole system.

[0034] Compared with the prior art, the beneficial effects of the present invention are:

[0035] (1) The present invention is an integrated treatment system and method for zero discharge and salt separation and resource utilization of high-salt wastewater, with hierarchical concentration, salt separation and synergistic efficiency improvement, reducing the treatment cost. Through ultrafiltration and reverse osmosis (pre-concentration), the wastewater volume is effectively reduced, and the treatment volume in the subsequent evaporation section is reduced by more than 30%, significantly reducing the overall operating energy consumption. The nanofiltration (NF) technology is used to selectively separate Cl - and SO4 2- , breaking through the limitation that traditional RO cannot separate salts, improving the purity of the crystalline salt from the source, and ensuring that the final salt product meets the industrial reuse standard;

[0036] (2) The present invention relates to an integrated treatment system and method for zero discharge of high-salt wastewater and salt separation and resource utilization. Through an energy cycle of hyperfiltration reverse osmosis, the energy consumption is significantly reduced. By adopting a two-stage RO series connection + pressure energy recovery technology, the residual pressure of the concentrated water is fully utilized to drive the operation of the system. Compared with the conventional RO process, the system energy consumption is reduced by 40 - 50%, and the power consumption for treating one ton of water is ≤ 3.5 kWh / m 3 , significantly improving the economy;

[0037] (3) The present invention relates to an integrated treatment system and method for zero discharge of high-salt wastewater and salt separation and resource utilization. A full-process resource closed-loop is achieved to realize zero discharge. The water produced by hyperfiltration RO can be directly reused in production. The overall water recovery rate of the system is ≥ 95%, significantly reducing the consumption of fresh water. The purity of the crystallized salt is ≥ 98.5%, meeting the first-class standard of industrial salt, and can replace the purchased salt for industries such as chlor-alkali and metallurgy, creating additional economic benefits. The NF concentrated water is completely harmless through co-firing or slag mixing, avoiding secondary pollution and meeting environmental protection requirements;

[0038] (4) The present invention relates to an integrated treatment system and method for zero discharge of high-salt wastewater and salt separation and resource utilization. By optimizing the pretreatment and online cleaning technologies, the system stability and adaptability are improved, the risk of membrane fouling is effectively reduced, the service life of the membrane is extended, and the long-term operation stability of the system is enhanced. It is applicable to high-salt wastewater with different salt compositions, has strong industrial applicability, and realizes the efficient, low-cost and environmentally friendly treatment of high-salt wastewater, with significant economic and social benefits. Detailed implementation manners

[0039] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the embodiments of the present invention.

[0040] Embodiment

[0041] The present invention proposes a technical solution for an integrated treatment system and method for zero discharge of high-salt wastewater and salt separation and resource utilization: An integrated treatment system for zero discharge of high-salt wastewater and salt separation and resource utilization, comprising:

[0042] A pretreatment unit for intercepting suspended solids and macromolecular organic matter in low-salt wastewater;

[0043] A primary reverse osmosis unit for preliminarily concentrating the water produced by pretreatment, controlling the recovery rate at 70 - 80%;

[0044] A nanofiltration salt separation unit for separating monovalent ions from polyvalent ions / organic matter in the primary RO concentrated solution;

[0045] A hyperfiltration reverse osmosis unit for performing two-stage high-fold concentration on the water produced by nanofiltration;

[0046] An MVR evaporation crystallization unit for treating the hyperfiltration RO concentrated water and recovering sodium chloride crystals.

[0047] Further, the pretreatment unit uses ultrafiltration or microfiltration membranes to treat low-salt wastewater with TDS ≤ 5,000 mg / L.

[0048] Further, the water produced by the primary reverse osmosis unit has TDS ≤ 500 mg / L, and the concentrate has TDS ≥ 20,000 mg / L.

[0049] Further, the operating parameters of the nanofiltration salt separation unit include: operating pressure 0.8 - 1.5 MPa, pH value 6.5 - 7.5, and the proportion of Cl in the nanofiltration-produced water - ≥ 95%.

[0050] Further, the hyper-efficient reverse osmosis unit includes:

[0051] (4) The first-stage RO: uses seawater desalination membranes, with an operating pressure of 5 - 7 MPa and a recovery rate of 60 - 70%;

[0052] (5) The second-stage RO: integrates a pressure-exchange type energy recovery device, and the total system recovery rate ≥ 90%;

[0053] (6) The final produced water has TDS ≤ 200 mg / L, and the concentrated water has TDS ≥ 100,000 mg / L.

[0054] Further, the MVR evaporation crystallization unit uses gradient heating and seed-induced crystallization to produce industrial-grade sodium chloride crystals with a purity ≥ 98.5%, and the heating temperature is 60°C - 90°C.

[0055] The present invention also provides the following technical solution: an integrated treatment method for zero discharge and salt separation and resource utilization of high-salt wastewater, including the following steps:

[0056] S1. Perform membrane pretreatment on low-salt wastewater;

[0057] S2. Achieve wastewater volume reduction through primary RO;

[0058] S3. Use nanofiltration to separate monovalent salts from polyvalent salts / organic matters;

[0059] S4. Perform two-stage RO high-fold concentration on the nanofiltration-produced water;

[0060] S5. Evaporate and crystallize the high-salt concentrated water to obtain sodium chloride products.

[0061] Further, in step S4, the pressure-exchange type energy recovery device is used to drive the feed water booster pump by using the residual pressure of the concentrated water.

[0062] Further, in step S5, the evaporated condensate is recycled to the front-end process of the system to form a closed-loop water-salt cycle for the whole system.

[0063] Taking the treatment of high-salt wastewater from coal chemical industry as an example:

[0064] 1. Wastewater characteristics

[0065] Source: High-salt wastewater from coal chemical industry

[0066] Water quality parameters: TDS 52,000mg / L, Cl - 32,000mg / L, SO4 2- 12,000mg / L, COD 800mg / L, total hardness (calculated as CaCO3) 2,500mg / L

[0067] 2. Treatment process

[0068] Step 1: Energy-saving pretreatment

[0069] Microfiltration (MF): Using a 0.1μm ceramic membrane to remove suspended solids (SS≤10mg / L);

[0070] Chemical softening: Adding NaOH (adjusting pH to 10.5) and Na2CO3 (dosage 1.2g / L) to precipitate and remove Ca 2+ , Mg 2 + , reducing the hardness to below 50mg / L;

[0071] Step 2: Nanofiltration for salt separation

[0072] Nanofiltration system: Selecting NF270-400 membrane modules, operating pressure 1.2MPa, pH 7.0;

[0073] Salt separation effect: The proportion of Cl in the produced water - is 96.5% (interception rate > 95%), and the concentration of SO4 in the concentrated water 2- increases to 28,000mg / L.

[0074] Step 3: Ultra-efficient reverse osmosis

[0075] First-stage RO: Seawater desalination membrane SW30HRLE-400, pressure 6.5MPa, recovery rate 65%;

[0076] Stage-level RO: Through energy recovery and pressurization, the recovery rate is increased to 92%, and the TDS of the concentrated water reaches 112,000mg / L;

[0077] Produced water reuse: Freshwater TDS 180mg / L, directly used for the circulating cooling water system.

[0078] Step 4: Resource utilization of concentrated water

[0079] MVR evaporation crystallization: Using a forced circulation evaporator, steam compressor power 75kW, crystallization temperature 85℃;

[0080] Salt product: The purity of the precipitated sodium chloride crystals is 99.3%, meeting the first-class standard of industrial salt in "GB / T 5462 - 2015", with an annual output of 1,095 tons;

[0081] Condensate reuse: The TDS of the evaporation condensate is ≤ 50 mg / L, and it is reused in the pretreatment unit.

[0082] Step 5: Treatment of nanofiltration concentrate

[0083] Co - firing treatment: The concentrate is mixed with pulverized coal in a mass ratio of 1:8 and sent to the gasification furnace for incineration (furnace temperature 1,050 °C), and SO4 2- decomposes into SO2 (meeting the discharge standard after subsequent desulfurization treatment).

[0084] 3. Technical effects

[0085] Energy consumption: The comprehensive energy consumption of the system is 13.2 kWh / t of water, a 68% reduction compared to traditional multi - effect evaporation;

[0086] Resource recovery rate: The recovery rate of sodium chloride is > 90%, reducing the cost of raw salt procurement by 1.2 million yuan / year;

[0087] Environmental protection: Zero wastewater discharge, and the SO2 concentration in the co - firing tail gas is < 50 mg / Nm 3 (ultra - low emission standard).

[0088] Taking the treatment of desulfurization wastewater in the power industry as an example:

[0089] 1. Wastewater characteristics

[0090] Source: A mixed solution of the regenerated wastewater from the make - up water of coal - fired power plant boilers and the concentrated water from reverse osmosis of concentrated water + desulfurization wastewater;

[0091] Water quality parameters:

[0092] TDS: 24,000 mg / L, Cl - : 6,500 mg / L, SO4 2- : 9,000 mg / L,,, suspended solids (SS): 200 mg / L, COD: 150 mg / L, total hardness (calculated as CaCO3): 1,800 mg / L

[0093] Step 1: Heavy metal removal and softening pretreatment

[0094] Chemical precipitation:

[0095] Sodium sulfide (Na2S) (0.08 g / L) and polyferric sulfate (PFS) (0.2 g / L) are added, and the reaction lasts for 30 minutes to precipitate and remove Hg 2+ 、Pb 2+ (effluent concentration < 0.01 mg / L);

[0096] Synchronously add lime (Ca(OH)2) to adjust the pH to 9.5, then add sodium carbonate to generate CaCO3 and CaSO4 precipitates, reducing the hardness to below 80 mg / L.

[0097] Multi-media filtration: Quartz sand + activated carbon filter to remove residual suspended solids (SS ≤ 20 mg / L) and organic matter (COD ≤ 30 mg / L).

[0098] Step 2: Nanofiltration for salt separation (Cl - / SO4 2- separation)

[0099] Membrane system configuration:

[0100] Select a fouling-resistant nanofiltration membrane NF1000, with an operating pressure of 1.0 MPa and the pH adjusted to 7.2;

[0101] Set the circulation flow ratio to 4:1 to improve the separation efficiency.

[0102] Salt separation effect:

[0103] Nanofiltration product water: The proportion of Cl - increases to 85% (interception rate > 90%), and the concentration of SO4 2- decreases to 1,200 mg / L;

[0104] Nanofiltration concentrate: The concentration of SO4 2- is concentrated to 32,000 mg / L, and the proportion of Cl - decreases to 15%.

[0105] Step 3: High-efficiency reverse osmosis for high-fold concentration

[0106] First-stage RO concentration:

[0107] Seawater desalination reverse osmosis membrane SW30XFR-400 / 34, with an operating pressure of 3.5 MPa, a recovery rate of 75%, and the product water TDS of 800 mg / L (reused for the boiler make-up water system);

[0108] The concentrate TDS increases to 48,000 mg / L.

[0109] Stage-level RO re-concentration zero-discharge membrane element XC70:

[0110] Through energy recovery and pressurization, with an operating pressure of 6.0 MPa and the total system recovery rate ≥ 90%;

[0111] The final concentrate TDS reaches 105,000 mg / L (Cl - purity ≥ 97%).

[0112] Step 4: Separate crystallization and resource utilization

[0113] Sodium chloride crystallization:

[0114] The ultra-efficient RO concentrated water enters the evaporator, and the evaporation temperature is controlled at 70 - 80 °C, and the purity of the crystalline salt is ≥98.8% (meeting the first-class standard of industrial salt);

[0115] About 500 tons of sodium chloride are recycled annually, replacing the electrolysis of water to produce acid and alkali and snow melting agent in power plants.

[0116] Sodium sulfate recovery:

[0117] The nanofiltration concentrated water is concentrated and reduced by the thermal method and mixed with pulverized coal in a mass ratio of 1:16, and then sent to the gasification furnace for incineration (furnace temperature 1,050 °C), and SO4 2- is decomposed into SO2 (subsequent desulfurization treatment meets the discharge standard).

[0118] Through the integrated design of energy-saving pretreatment - nanofiltration salt separation - ultra-efficient reverse osmosis - collaborative disposal, the present invention realizes:

[0119] (1) Precise salt separation: The nanofiltration membrane selectively separates Cl - and SO4 2- , combined with the secondary purification of ultra-efficient RO, to ensure that the purity of the crystalline salt is ≥98.5%;

[0120] (2) Optimal energy consumption: Pretreatment waste heat recovery + RO energy recycling device, the system energy consumption is reduced by 30% - 40% compared with traditional evaporation;

[0121] (3) Harmless treatment of waste residue: Incineration / mixing of nanofiltration concentrated water with slag to realize thermal decomposition or solidification and sealing of salts, avoiding secondary pollution.

[0122] The present invention breaks through the bottleneck of the existing technology from three aspects of "salt separation efficiency - energy cost - terminal disposal", and forms a complete chain solution for high-salt wastewater treatment and salt resource recovery.

[0123] Although the embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. An integrated treatment system for zero discharge of high-salt wastewater and salt separation and resource utilization, characterized in that, Including: A pretreatment unit for intercepting suspended solids and macromolecular organic matters in low-salt wastewater; A primary reverse osmosis unit for preliminarily concentrating the pretreated water production and controlling the recovery rate at 70 - 80%; A nanofiltration salt separation unit for separating monovalent ions from polyvalent ions / organic matters in the primary RO concentrate; An ultra-efficient reverse osmosis unit for performing two-stage high-fold concentration on the nanofiltration water production; An MVR evaporation crystallization unit for treating the ultra-efficient RO concentrated water and recovering sodium chloride crystals.

2. The system according to claim 1, wherein: The pretreatment unit adopts ultrafiltration or microfiltration membranes to treat low-salt wastewater with TDS ≤ 5,000 mg / L.

3. The system according to claim 1, wherein: The water production TDS of the primary reverse osmosis unit is ≤ 500 mg / L, and the concentrate TDS is ≥ 20,000 mg / L.

4. The system according to claim 1, characterized in that: The operating parameters of the nanofiltration salt separation unit include: an operating pressure of 0.8 - 1.5 MPa, a pH value of 6.5 - 7.5, and the proportion of Cl- in the nanofiltration water production ≥ 95%.

5. The system according to claim 1, wherein: The ultra-efficient reverse osmosis unit includes: (1) The first-stage RO: Adopting seawater desalination membranes, with an operating pressure of 5 - 7 MPa and a recovery rate of 60 - 70%; (2) The second-stage RO: Integrated with a pressure exchange type energy recovery device, and the total system recovery rate ≥ 90%; (3) The final water production TDS ≤ 200 mg / L, and the concentrated water TDS ≥ 100,000 mg / L.

6. The system according to claim 1, wherein: The MVR evaporation crystallization unit adopts gradient heating and seed-induced crystallization to produce industrial-grade sodium chloride crystals with a purity ≥ 98.5%, and the heating temperature is 60°C - 90°C.

7. A method for treating high-salt wastewater based on any one of the systems of claims 1-6, characterized in that, Including the following steps: S1. Conduct membrane pretreatment on low-salt wastewater; S2. Achieve wastewater volume reduction through primary RO; S3. Adopt nanofiltration to separate monovalent salts from polyvalent salts / organic matters; S4. Perform two-stage RO high-fold concentration on the nanofiltration water production; S5. Evaporate and crystallize the high-salt concentrated water to obtain sodium chloride products.

8. The method according to claim 7, wherein: In step S4, the pressure exchange type energy recovery device is used to drive the feed water booster pump by using the residual pressure of the concentrated water.

9. The method according to claim 7, wherein: In step S5, the evaporated condensate is recycled to the front-end process of the system to form a closed loop of water-salt circulation for the whole system.