A method for integrated treatment of high-salinity wastewater by nanofiltration desalination based on MBR

Abstract

The present application belongs to the technical field of wastewater treatment, and particularly relates to a high-salinity wastewater nanofiltration desalination integrated treatment method based on MBR. The present application aims to solve the problem of unstable effect of the existing MBR unit in treating high-salinity wastewater. The present application passes the high-salinity wastewater into a hydraulic homogenization unit, then into a chemical softening unit, and after treatment, into a biodegradation unit. The influent of the biodegradation unit includes the effluent of the chemical softening unit and the nanofiltration concentrated liquid from the substrate reuse unit; the effluent of the biodegradation unit is passed into a chemical flocculation unit, and then into a security filtration unit. The effluent of the security filtration unit enters the nanofiltration unit. The effluent of the nanofiltration unit includes desalinated water and nanofiltration raw liquid, the desalinated water is sent out of the treatment section, and the nanofiltration raw liquid is passed into the substrate reuse unit. The effluent of the substrate reuse unit includes nanofiltration concentrated liquid and nanofiltration waste liquid, the nanofiltration waste liquid is sent out of the treatment section, and the nanofiltration concentrated liquid is passed into the biodegradation unit.

Description

Technical Field

[0001] This invention belongs to the field of wastewater treatment technology, specifically relating to an integrated treatment method for high-salinity wastewater nanofiltration desalination based on MBR. Background Technology

[0002] High-salinity wastewater typically refers to wastewater with a total dissolved solids (TDS) content greater than 1% by mass. It originates from a wide range of sources, including chemical, dyeing, and pharmaceutical industries. The challenges in treating this wastewater lie in the strong inhibitory effect of high salinity on biological treatment and the high concentration of organic matter. Conventional treatment methods typically include physicochemical methods, biological methods, and membrane separation methods. Among these, biological methods have attracted considerable attention due to their lower cost, but their desalination capacity is limited.

[0003] Membrane bioreactors (MBRs) are a novel water treatment technology that combines activated sludge processes with membrane separation. In the pretreatment of high-salinity wastewater, MBRs are widely used for the efficient removal of organic pollutants (COD) and suspended solids (SS), producing relatively low effluent saturated water (SDI), theoretically suitable as influent for subsequent advanced treatment. However, MBRs themselves do not possess desalination capabilities. More importantly, in high-salinity environments, microbial activity is inhibited, leading to decreased biodegradation efficiency. Simultaneously, the effluent still contains large amounts of soluble microbial products (SMPs) and extracellular polymeric substances (EPS), which are major factors contributing to severe organic and biological fouling in subsequent membrane systems (such as nanofiltration or reverse osmosis). Furthermore, salinity fluctuations in high-salinity wastewater can easily impact the MBR biological system, resulting in unstable pretreatment effects. In turn, unstable MBR effluent quality can severely disrupt the stable operation of nanofiltration units, lacking a system-wide synergistic control mechanism. As a biological pretreatment method, MBRs cannot remove calcium and magnesium ions from the water. During the nanofiltration concentration process, the concentration of these scaling ions increases exponentially, making them highly susceptible to precipitation on the nanofiltration membrane surface, resulting in irreversible chemical scaling that severely affects the system's desalination efficiency and operating cycle.

[0004] To address the issue of unstable performance of existing MBR units in treating high-salinity wastewater, a nanofiltration desalination integrated treatment method for high-salinity wastewater based on MBR is proposed. Summary of the Invention

[0005] The purpose of this invention is to provide an integrated nanofiltration desalination treatment method for high-salinity wastewater based on MBR. This invention introduces high-salinity wastewater into a hydraulic homogenization unit, followed by a chemical softening unit. The effluent from the chemical softening unit is then introduced into a biodegradation unit. The influent to the biodegradation unit includes the effluent from the chemical softening unit and nanofiltration concentrate from a fractional filtration and reuse unit. The effluent from the biodegradation unit is then introduced into a chemical flocculation unit, and the effluent from the chemical flocculation unit enters a security filtration unit. The effluent from the security filtration unit enters the nanofiltration unit. The effluent from the nanofiltration unit includes demineralized water and nanofiltration concentrate. The demineralized water is discharged from the treatment stage, and the nanofiltration concentrate is introduced into the fractional filtration and reuse unit. The effluent from the fractional filtration and reuse unit includes nanofiltration concentrate and nanofiltration wastewater. The nanofiltration wastewater is discharged from the treatment stage, and the nanofiltration concentrate is reused and introduced into the biodegradation unit.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A nanofiltration desalination integrated treatment method for high-salinity wastewater based on MBR includes the following steps:

[0008] The high-salinity wastewater is fed into a hydraulic homogenization unit, and after treatment, it is fed into a chemical softening unit.

[0009] The effluent from the chemical softening unit is fed into the biodegradation unit.

[0010] The influent to the biodegradation unit includes the effluent from the chemical softening unit and the nanofiltration concentrate from the fractional recycling unit.

[0011] The effluent from the biodegradation unit is fed into the pollutant removal section.

[0012] The pollutant removal section includes a chemical flocculation unit and a security filtration unit. Effluent from the biodegradation unit first enters the chemical flocculation unit, and the effluent from the chemical flocculation unit enters the security filtration unit. The effluent from the security filtration unit then enters the nanofiltration unit.

[0013] The effluent from the nanofiltration unit includes demineralized water and nanofiltration feed solution. The demineralized water is sent out of the treatment section, while the nanofiltration feed solution is fed into the fractional filtration and reuse unit. The effluent from the fractional filtration and reuse unit includes nanofiltration concentrate and nanofiltration waste liquid. The nanofiltration waste liquid is sent out of the treatment section, while the nanofiltration concentrate is fed into the biodegradation unit.

[0014] Specifically, the operation of the hydraulic homogenization unit is as follows: the hydraulic retention time (HRT) is 8-10 hours; after the high-salinity wastewater is introduced into the hydraulic homogenization unit, it is aerated and stirred to maintain the dissolved oxygen (DO) > 0.5 mg / L. Then, the treated high-salinity wastewater is output at a constant flow rate and introduced into the chemical softening unit. This constant flow rate depends on the influent flow rate. Essentially, it transforms the uneven influent of high-salinity wastewater into a uniform wastewater flow. The constant output flow rate of the hydraulic homogenization unit of this invention is controlled at 150-200 m³ / h. 3 / h.

[0015] The specific operation of the chemical softening unit is as follows: The effluent from the hydraulic homogenization unit is treated by adding lime slurry and soda ash to control the pH value within the range of 10.5-11.0, with a HRT of 5 minutes for the pH adjustment stage. The pH-adjusted wastewater then enters the flocculation reactor, where the hydraulic retention time is 20-40 minutes, and 0.1 mg / L of anionic polyacrylamide is added to control the suspended solids (SS) concentration in the effluent to less than 30 mg / L. The supernatant from the flocculation reactor is sent to the pH adjustment tank, where the pH is adjusted to 7.0-8.0 before the effluent is sent to the biodegradation unit.

[0016] The specific operation of the biodegradation unit is as follows: The biodegradation unit includes an anoxic tank, an aerobic membrane tank, and a return system. The effluent from the chemical softening unit and the nanofiltration concentrate from the fractional reuse unit are mixed and sent to the anoxic tank. By adjusting the ratio of the two influents, the total salinity of the influent is maintained between 2.0-3.5%. The HRT (Heat Retention Time) of the anoxic tank is 3-5 hours, and the DO (Dissolved Oxygen) is controlled to be <0.5 mg / L. The effluent from the anoxic tank flows to the aerobic membrane tank, where the HRT is 8-12 hours, the DO is controlled within the range of 2.0-4.0 mg / L, the sludge concentration (MLSS) is maintained at 12000-15000 mg / L, and the sludge retention time (SRT) is 60-80 days. The membrane module has a pore size of 0.2 μm, is a PVDF hollow fiber membrane, and is designed with a membrane flux of 15 L / m³. 2 •h. The reflux system pumps the mixed liquor from the aerobic membrane tank back to the anoxic tank at a reflux ratio of 2.0-4.0 to control operating parameters within the target range. The effluent from the aerobic membrane tank is sent to the chemical flocculation unit.

[0017] The specific operation of the chemical flocculation unit is as follows: add 5-10 mg / L of polydimethyldiallyl ammonium chloride to the effluent of the aerobic membrane tank, mix for 30 seconds, then let it stand for 30 minutes, and then send the effluent to the security filtration unit.

[0018] The specific operation of the security filtration unit is as follows: the effluent from the chemical flocculation unit is sent into the filter area with a filtration precision of 5.0μm, the working pressure is maintained at 0.2-0.4MPa, the effluent turbidity is maintained at <0.2NTU, and the sludge density index is ≤3.0 after 15 minutes, and the effluent is sent into the nanofiltration unit.

[0019] The specific operation of the nanofiltration unit is as follows: the effluent from the security filtration unit is pressurized to 0.8-1.5 MPa, and the designed membrane flux is 25 L / m³. 2Monovalent inorganic salt ions permeate through the nanofiltration membrane module and are discharged as demineralized water; other inorganic salt ions are retained by the nanofiltration membrane module and collected as nanofiltration feed solution, which is then transported to the fractionation and reuse unit. The nanofiltration membrane module has a spacing thickness of 0.76 mm, is made of glass fiber, and has an effective membrane area of ​​33.8 m² / h per module. 2 The product model is Veolia DK8040F30.

[0020] The specific operation of the fractional-filtration reuse unit is as follows: Nanofiltration feed solution from the nanofiltration unit is introduced into the fractional-filtration reuse unit, and its salinity is monitored. The effluent salinity is controlled between 40,000 and 60,000 mg / L. The reuse ratio of the nanofiltration feed solution is adjusted according to the real-time salinity in the biodegradation unit, where real-time salinity refers to the real-time salinity in the aerobic membrane tank. Specifically, the adjustment method is as follows: when the real-time salinity in the biodegradation unit is ≥35,000 mg / L, the reuse ratio is adjusted to the minimum value of 10%; when the real-time salinity in the biodegradation unit is ≤20,000 mg / L, the reuse ratio is adjusted to the maximum value of 30%; when the real-time salinity is between 26,000 and 35,000 mg / L, the reuse ratio is linearly adjusted between 10% and 20%, with a salinity of 26,000 mg / L corresponding to a 20% reuse ratio. The recycling ratio is as follows: 10% for a salinity of 35,000 mg / L; when the real-time salinity is between 20,000 and 24,000 mg / L, the recycling ratio is linearly adjusted between 20% and 30%, with 20% for a salinity of 24,000 mg / L and 30% for a salinity of 20,000 mg / L; when the real-time salinity is between 24,000 and 26,000 mg / L, the recycling ratio remains unchanged at 20%.

[0021] The reused nanofiltration solution is sent to the anoxic tank of the biodegradation unit as nanofiltration concentrate, and the remaining part is discharged as nanofiltration waste liquid to the treatment section.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] High-salinity wastewater is first fed into a hydraulic homogenization unit, and the subsequent chemical softening unit is placed before the biodegradation unit. This design first removes the high concentration of hardness ions such as calcium and magnesium in the high-salinity wastewater in advance, preventing inorganic salt scaling in the subsequent nanofiltration unit. At the same time, the low-hardness wastewater environment provides a more hydraulically and load-balanced influent for the subsequent biodegradation unit, effectively protecting the MBR system.

[0024] The biodegradation unit employs an A / O-MBR process and operates under specific conditions. This design allows for the stable enrichment of highly efficient, salt-tolerant bacterial communities. Unlike traditional activated sludge processes, the biodegradation unit utilizes completely decoupled SRT and HRT regulation functions to ensure stable removal of COD and ammonia nitrogen even in high-salt environments. This stage not only provides high-quality influent with significantly reduced organic load to the subsequent desalination stage but also ensures the biodegradation unit's own good adaptability and stability to influent fluctuations.

[0025] A contaminant removal section was added after the biodegradation unit and before the nanofiltration unit. The effluent from the biodegradation unit contains a large amount of soluble microbial products and extracellular polymers, which would cause rapid fouling of the nanofiltration membrane if directly introduced into the nanofiltration unit. The contaminant removal section integrates a chemical flocculation unit and a security filtration unit. In addition to specifically removing colloidal contaminants that the MBR membrane cannot retain, it precisely adjusts the influent quality to the nanofiltration unit, achieving a match between the effluent characteristics of the biodegradation unit and the influent requirements of the nanofiltration unit, thus ensuring the reliability of the nanofiltration unit's operation.

[0026] The nanofiltration concentrate output from the nanofiltration unit is sent to the fractional recovery unit, where it is returned to the anoxic section of the biodegradation unit in a specific ratio dependent on the real-time salinity of the biodegradation unit. This design stabilizes the salinity environment of the biodegradation unit, maintaining its high salinity by utilizing the high salinity of the concentrate, ensuring the removal rate of recalcitrant COD, and allowing undegraded organic matter in the concentrate to undergo further degradation treatment. Simultaneously, the amount of nanofiltration wastewater requiring final disposal is significantly reduced, improving the overall environmental friendliness of the process. Attached Figure Description

[0027] Figure 1 This is a process flow diagram of an integrated treatment method for high-salinity wastewater nanofiltration desalination based on MBR according to the present invention. Detailed Implementation

[0028] The technical solution of the present invention will be clearly and completely described below through some embodiments and experimental examples. 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.

[0029] Reference Figure 1 The process flow diagram shown illustrates that this invention provides an integrated nanofiltration desalination treatment method for high-salinity wastewater based on MBR. The technical solution is as follows:

[0030] Example 1

[0031] The specific parameters of the high-salinity wastewater treated in this embodiment are as follows: salinity is 27400 mg / L, COD is 3680 mg / L, chloride ion content is 11240 mg / L, total hardness (calculated as CaCO3) is 2100 mg / L, and ammonia nitrogen is 220 mg / L.

[0032] After the high-salinity wastewater is fed into the hydraulic homogenizing unit, it is aerated and stirred continuously. The HRT (High-Temperature Retention Time) is 8-10 hours, and the dissolved oxygen (DO) is maintained above 0.5 mg / L. Then, the treated high-salinity wastewater is discharged at a constant flow rate and fed into the chemical softening unit. This constant flow rate depends on the influent flow rate. Essentially, it transforms the uneven influent of the high-salinity wastewater into a uniform wastewater flow. In this embodiment, the constant output flow rate of the hydraulic homogenizing unit is controlled at 150 m³ / h. 3 / h.

[0033] The effluent from the hydraulic homogenization unit is treated by adding lime slurry and soda ash to control the pH value within the range of 10.5-11.0, with a HRT of 5 minutes for the pH adjustment section. The pH-adjusted wastewater then enters the flocculation reactor, where the hydraulic retention time is 20-40 minutes, and 0.1 mg / L of anionic polyacrylamide is added to control the effluent SS concentration to be less than 30 mg / L. The supernatant from the flocculation reactor is sent to the pH adjustment tank to adjust the pH to 7.0-8.0 before being sent to the biodegradation unit.

[0034] The biodegradation unit comprises an anoxic tank, an aerobic membrane tank, and a return system. Effluent from the chemical softening unit and nanofiltration concentrate from the fractional-reuse unit are mixed and fed into the anoxic tank. The total salinity of the influent is maintained between 2.0-3.5% by adjusting the ratio of the two influents. The HRT (Heat Retention Time) in the anoxic tank is 3-5 hours, and DO (Dissolved Oxygen) is controlled to <0.5 mg / L. The effluent from the anoxic tank flows to the aerobic membrane tank, where the HRT is 8-12 hours, DO is controlled within the range of 2.0-4.0 mg / L, the sludge concentration (MLSS) is maintained at 12000-15000 mg / L, and the sludge retention time (SRT) is 60-80 days. The membrane module has a pore size of 0.2 μm and a designed membrane flux of 15 L / m³. 2 •h. The reflux system pumps the mixed liquor from the aerobic membrane tank back to the anoxic tank at a reflux ratio of 2.0-4.0 to control operating parameters within the target range. The effluent from the aerobic membrane tank is sent to the chemical flocculation unit.

[0035] Add 5 mg / L of polydimethyldiallyl ammonium chloride to the effluent from the aerobic membrane tank, mix for 30 seconds, then let it stand for 30 minutes before sending the effluent to the security filter unit.

[0036] The effluent from the chemical flocculation unit is fed into a filter area with a filtration precision of 5.0 μm, maintaining an operating pressure of 0.2-0.4 MPa, keeping the effluent turbidity <0.2 NTU, and the sludge density index ≤3.0 after 15 minutes before being fed into the nanofiltration unit.

[0037] The effluent from the security filtration unit is pressurized to 0.8 MPa, and the designed membrane flux is 25 L / m³. 2 •h, monovalent inorganic salt ions pass through the nanofiltration membrane module and are discharged as demineralized water; other inorganic salt ions are retained by the nanofiltration membrane module and collected as nanofiltration raw solution, which is then transported to the fractionation and reuse unit.

[0038] The nanofiltration solution from the nanofiltration unit is fed into the fractional salinity recycling unit, and its salinity is monitored to control the effluent salinity between 40,000 and 60,000 mg / L. The recycling ratio of the nanofiltration solution is adjusted based on the real-time salinity in the biodegradation unit, which refers to the real-time salinity in the aerobic membrane tank. Specifically, the adjustment method is as follows: when the real-time salinity in the biodegradation unit is ≥35,000 mg / L, the recycling ratio is adjusted to the minimum value of 10%; when the real-time salinity in the biodegradation unit is ≤20,000 mg / L, the recycling ratio is adjusted to the maximum value of 30%; when the real-time salinity is between 26,000 and 35,000 mg / L, the recycling ratio is linearly adjusted between 10% and 20%, with a salinity of 26,000 mg / L corresponding to a 20% recycling rate. The recycling ratio is as follows: 10% for a salinity of 35,000 mg / L; when the real-time salinity is between 20,000 and 24,000 mg / L, the recycling ratio is linearly adjusted between 20% and 30%, with 20% for a salinity of 24,000 mg / L and 30% for a salinity of 20,000 mg / L; when the real-time salinity is between 24,000 and 26,000 mg / L, the recycling ratio remains unchanged at 20%.

[0039] The reused nanofiltration solution is sent to the anoxic tank of the biodegradation unit as nanofiltration concentrate, and the remaining part is discharged as nanofiltration waste liquid to the treatment section.

[0040] Example 2

[0041] Unlike Example 1, the output flow rate of the hydraulic homogenizing unit is controlled at 200 m³ / s. 3 / h, the amount of polydimethyldiallyl ammonium chloride added in the chemical flocculation unit is 10mg / L, and the nanofiltration unit pressurizes the effluent from the security filtration unit to 1.5MPa.

[0042] Example 3

[0043] Unlike Example 1, the output flow rate of the hydraulic homogenizing unit is controlled at 170 m³ / s. 3 / h, the amount of polydimethyldiallyl ammonium chloride added in the chemical flocculation unit is 8mg / L, and the nanofiltration unit pressurizes the effluent from the security filtration unit to 1.0MPa.

[0044] Example 4

[0045] Unlike Example 1, the output flow rate of the hydraulic homogenizing unit is controlled at 185 m³ / s. 3 / h, the amount of polydimethyldiallyl ammonium chloride added in the chemical flocculation unit is 7mg / L, and the nanofiltration unit pressurizes the effluent from the security filtration unit to 0.8MPa.

[0046] Example 5

[0047] Unlike Example 1, the output flow rate of the hydraulic homogenizing unit is controlled at 175 m³ / s. 3 / h, the amount of polydimethyldiallyl ammonium chloride added in the chemical flocculation unit is 6mg / L, and the nanofiltration unit pressurizes the effluent from the security filtration unit to 1.3MPa.

[0048] The operating parameters of Examples 6-10 are the same as those of Examples 1-5, but the specific parameters of the high-salt wastewater used are: salinity of 23600 mg / L, COD of 3410 mg / L, chloride ion content of 9250 mg / L, total hardness (calculated as CaCO3) of 1950 mg / L, and ammonia nitrogen of 210 mg / L.

[0049] The operating parameters of Examples 11-15 are the same as those of Examples 1-5, but the specific parameters of the high-salt wastewater used are: salinity of 31500 mg / L, COD of 3740 mg / L, chloride ion content of 12150 mg / L, total hardness (calculated as CaCO3) of 2050 mg / L, and ammonia nitrogen of 205 mg / L.

[0050] The operating parameters of Examples 16-20 are the same as those of Examples 1-5, but the specific parameters of the high-salt wastewater used are: salinity of 29800 mg / L, COD of 2980 mg / L, chloride ion content of 9960 mg / L, total hardness (calculated as CaCO3) of 2350 mg / L, and ammonia nitrogen of 190 mg / L.

[0051] Comparative Example 1

[0052] Unlike Example 1, the chemical softening unit was removed, while all other process parameters remained the same.

[0053] Comparative Example 2

[0054] Unlike Example 1, the hydraulic homogenization unit was removed, while all other process parameters remained the same.

[0055] Comparative Example 3

[0056] Unlike Example 6, the biodegradation unit only accepts effluent from the chemical softening unit and does not accept nanofiltration concentrate from the fractional recycling unit; all other process parameters are the same.

[0057] Comparative Example 4

[0058] Unlike Example 6, the SRT in the biodegradation unit was adjusted to 20 days, while all other process parameters remained the same.

[0059] Comparative Example 5

[0060] Unlike Example 11, the chemical flocculation unit was removed, while all other process parameters remained the same.

[0061] Comparative Example 6

[0062] Unlike Example 11, the order of the security filtration unit and the chemical flocculation unit is changed, while all other process parameters remain the same.

[0063] Comparative Example 7

[0064] Unlike Example 16, a constant 20% recycling ratio was maintained, and all other process parameters remained the same.

[0065] Comparative Example 8

[0066] Unlike Example 16, the fractionation and recycling unit is removed, while all other process parameters remain the same.

[0067] Experimental Example 1

[0068] The pressure difference (MPa) between the nanofiltration unit influent and nanofiltration feed solution area and the ammonia nitrogen concentration (mg / L) in the biodegradation unit effluent were tested in the process flow of Examples 1-5 and Comparative Examples 1-2 after 8 hours of continuous operation. The relevant results are summarized in Table 1.

[0069] Table 1 Comparison of relevant intermediate process parameters between Examples 1-5 and Comparative Examples 1-2

[0070]

[0071] As shown in Table 1, the pressure difference of the nanofiltration unit and the ammonia nitrogen concentration in the effluent of the biodegradation unit in Examples 1-5 remained at a stable and low level. In contrast, the pressure difference in Comparative Example 1 increased significantly, while the ammonia nitrogen concentration in the effluent of Comparative Example 2 was extremely high, indicating that the design of Examples 1-5 has a clear advantage in protecting the nanofiltration membrane and the biological system.

[0072] Comparative Example 1 removed the chemical softening unit, resulting in high-salt wastewater with a high concentration of total hardness directly entering the subsequent membrane system. This caused severe inorganic salt scaling during the operation of the nanofiltration unit, significantly increasing the pressure differential of the nanofiltration unit, thus demonstrating the necessity of a pre-treatment chemical softening unit to prevent scaling in the subsequent nanofiltration unit. Comparative Example 2 removed the hydraulic homogenization unit, subjecting the subsequent chemical softening and biodegradation units to uneven wastewater impacts and load fluctuations. This severely disrupted the stable operating environment of the salt-tolerant nitrifying bacteria in the biodegradation unit, causing its denitrification function to fail and resulting in extremely high ammonia nitrogen concentrations in the effluent. Simultaneously, the unstable influent also affected the operational stability of the chemical softening unit, leading to a slight increase in the pressure differential of the nanofiltration unit.

[0073] In summary, this invention prioritizes the hydraulic homogenization unit and places the chemical softening unit before the biodegradation unit, resulting in a significant synergistic effect. The hydraulic homogenization unit, through its long hydraulic retention time and aeration, provides a more balanced feedwater in terms of hydraulics and load for all subsequent stages. This stable feedwater is a prerequisite for the chemical softening unit to achieve stable pH control and efficient hardness removal, and is also the foundation for the stable operation of the biodegradation unit. The pre-positioned chemical softening unit removes high concentrations of calcium and magnesium ions in advance, eliminating the risk of inorganic salt scaling in the nanofiltration unit at its source. This combined design synergistically ensures that the biodegradation unit is protected from load shocks while simultaneously protecting the nanofiltration unit from scaling threats, achieving stable and efficient operation of the entire integrated process.

[0074] Experimental Example 2

[0075] The COD removal rate and ammonia nitrogen removal rate in the final desalinated water of the entire process in Examples 6-10 and Comparative Examples 3-4 were tested, and the relevant results are summarized in Table 2.

[0076] Table 2 Final COD removal rate and ammonia nitrogen removal rate of Examples 6-10 and Comparative Examples 3-4

[0077]

[0078] As shown in Table 2, Examples 6-10 maintained a high and stable level of COD removal rate and ammonia nitrogen removal rate throughout the entire process. In contrast, Comparative Example 3 had a higher ammonia nitrogen removal rate, but a significantly lower COD removal rate; while Comparative Example 4 had extremely low COD and ammonia nitrogen removal rates, indicating severe system malfunction.

[0079] Comparative Example 4 significantly shortened the sludge retention time (SRT) of the biodegradation unit from 60-80 days to 20 days, which directly violated the specific operating conditions set by this invention for enriching salt-tolerant bacteria. In high-salinity environments, highly efficient salt-tolerant bacteria, especially slow-growing nitrifying bacteria, have generation cycles much longer than 20 days. The extremely short SRT prevents these functional bacteria from being enriched in the reactor and causes them to be completely washed out, resulting in the complete collapse of nitrification and a severe deterioration in COD degradation efficiency. Comparative Example 3 maintained the correct SRT and MLSS operating conditions but eliminated the recirculation of nanofiltration concentrate. Therefore, the salt-tolerant bacteria inside its biodegradation unit were retained and could still efficiently remove ammonia nitrogen; however, the secondary degradation step of recalcitrant COD in the nanofiltration concentrate was missing, causing this portion of COD to be directly discharged, resulting in a significantly lower final COD removal rate than the examples. The final COD concentration in the nanofiltration wastewater of Examples 6-10 was all below 100 mg / L, meeting the secondary standard for wastewater discharge.

[0080] In summary, this invention employs an A / O-MBR process in the biodegradation unit, forcing it to operate under specific conditions: a high MLSS of 12,000-15,000 mg / L and an ultra-long SRT of 60-80 days. This design fully utilizes the complete decoupling capability of MBR from SRT and HRT, providing the necessary conditions for the stable enrichment of slow-growing, highly efficient salt-tolerant bacteria and salt-tolerant nitrifying bacteria. This specific operating condition, in synergy with the A / O process, ensures that the system can still achieve efficient and stable removal of COD and ammonia nitrogen even at high salinity.

[0081] Experimental Example 3

[0082] The actual membrane flux of the nanofiltration membranes in Examples 11-15 and Comparative Examples 5-6 after 8 hours of operation, and the sludge density index 15 minutes after the nanofiltration unit was fed with water, were tested. The relevant results are summarized in Table 3.

[0083] It should be noted that Comparative Example 6 could not run for 8 hours, and the nanofiltration unit would become clogged shortly after the device started running. It was used as a reference control in this experimental example.

[0084] Table 3 Comparison of relevant intermediate process parameters between Examples 11-15 and Comparative Examples 5-6

[0085]

[0086] As shown in Table 3, the actual membrane flux of the nanofiltration membranes in Examples 11-15 remained stable at a high level close to the design value, and the sludge density index of the influent to the nanofiltration unit remained at a low acceptable level for 15 minutes. In contrast, the actual membrane flux of Comparative Example 5 decreased significantly, and the sludge density index also deteriorated significantly. Comparative Example 6 experienced nanofiltration unit clogging within a short period of time and could not operate normally.

[0087] Comparative Example 5 removed the chemical flocculation unit from the contaminant removal section, causing effluent from the biodegradation unit, rich in soluble microbial products and extracellular polymers, to directly enter the security filtration unit without effective conversion. These colloidal contaminants are much smaller than the filtration precision of the security filtration unit, thus penetrating into the nanofiltration unit in large quantities, causing rapid organic fouling of the nanofiltration membrane, manifested as an increased sludge density index and a significant decrease in actual membrane flux. Comparative Example 6 incorrectly placed the security filtration unit before the chemical flocculation unit. This allowed colloidal contaminants to penetrate the security filtration unit first, and then be successfully converted into floc particles in the chemical flocculation unit. However, there was no subsequent filtration unit to retain these newly generated particles, causing the flocs to be directly pumped into the nanofiltration unit, leading to severe physical clogging and demonstrating the necessity of the process sequence in this invention.

[0088] In summary, this invention adds a contaminant removal section after the biodegradation unit and before the nanofiltration unit, designing it as a specific series sequence of a chemical flocculation unit and a security filtration unit. The chemical flocculation unit first converts colloidal contaminants in the MBR effluent into easily filterable micro-flocculations, which are then completely retained by the subsequent security filtration unit. This design of the two units creates a crucial synergistic effect, enabling precise regulation of the nanofiltration membrane feed water quality and targeted removal of colloidal contaminants. This design successfully matches the effluent characteristics of the biodegradation unit with the feed water requirements of the nanofiltration unit, ensuring that the nanofiltration unit is protected from organic fouling and particulate clogging, allowing for long-term stable operation at the designed flux.

[0089] Experiment Example 4

[0090] The highest and lowest real-time salinity values ​​within the biodegradation unit, as well as the COD concentration of the discharged nanofiltration waste liquid, were measured during the 8-hour operation of Test Examples 16-20 and Comparative Examples 7-8. The relevant results are summarized in Table 4.

[0091] Table 4 Comparison of relevant intermediate process parameters between Examples 16-20 and Comparative Examples 7-8

[0092]

[0093] As shown in Table 4, the real-time salinity within the biodegradation units of Examples 16-20 was consistently controlled within the target operating range, and the COD concentration of the discharged nanofiltration wastewater remained at the lowest level. In contrast, the real-time salinity of Comparative Examples 7 and 8 both experienced severe runaway, with both their highest and lowest values ​​exceeding the target boundaries of the process design. Furthermore, the COD concentration of the discharged nanofiltration wastewater was significantly higher than that of the Examples.

[0094] Comparative Example 8 removed the fractional salinity reuse unit, resulting in complete loss of salinity control in the biodegradation unit and extremely large real-time salinity fluctuations. Simultaneously, undegraded organic matter in the nanofiltration concentrate could not be returned to the biological system for secondary treatment, leading to the highest COD concentration in the discharged nanofiltration wastewater. Although Comparative Example 7 included a reuse unit, it employed a constant 20% reuse ratio, lacking dynamic adjustment capability. Therefore, when influent salinity fluctuated, the system could not compensate, causing the real-time salinity to also exceed the process limits; while its COD removal effect was better than Comparative Example 8, it was still inferior to the examples.

[0095] In summary, this invention incorporates a fractional-reuse unit and dynamically controls the reuse ratio of nanofiltration concentrate in a closed-loop manner with the real-time salinity within the biodegradation unit. This design firstly, by adjusting the reuse ratio in real-time and proactively intervening when influent salinity fluctuates, utilizes the high-salinity nanofiltration concentrate as a regulator, successfully maintaining the salinity environment of the biodegradation unit at a specific high-salinity condition of 20,000-35,000 mg / L. Secondly, this reuse pathway allows the recalcitrant COD retained in the nanofiltration concentrate to be returned to the anoxic tank for secondary degradation treatment. This synergistic effect of salinity stabilization and enhanced pollutant degradation not only ensures the stable operation of the biodegradation unit but also significantly reduces the COD concentration of the final discharged nanofiltration wastewater.

[0096] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A nanofiltration desalination integrated treatment method for high-salinity wastewater based on MBR, characterized in that: The processing method is as follows: High-salinity wastewater is fed into a hydraulic homogenization unit, treated, and then fed into a chemical softening unit; the constant output flow rate of the hydraulic homogenization unit is controlled at 150-200 m³ / h. 3 / h; The effluent from the chemical softening unit is fed into the biodegradation unit; The influent to the biodegradation unit includes the effluent from the chemical softening unit and the nanofiltration concentrate from the fractional reuse unit; the effluent from the biodegradation unit is fed into the pollutant removal section; the biodegradation unit includes an anoxic tank, an aerobic membrane tank, and a reflux system; the effluent from the chemical softening unit and the nanofiltration concentrate from the fractional reuse unit are mixed and then fed into the anoxic tank, adjusting the total salinity of the influent to maintain between 2.0-3.5%; the reflux system pumps the mixed solution from the aerobic membrane tank back to the anoxic tank, with a reflux ratio of 2.0-4.0; the effluent from the aerobic membrane tank is sent to the chemical flocculation unit; the hydraulic retention time of the aerobic membrane tank is 8-12 hours, controlling the dissolved oxygen content within the range of 2.0-4.0 mg / L, maintaining the sludge concentration at 12000-15000 mg / L, and the sludge retention time at 60-80 days; The pollutant removal section includes a chemical flocculation unit and a security filtration unit. The effluent from the biodegradation unit first enters the chemical flocculation unit, and the effluent from the chemical flocculation unit enters the security filtration unit. The effluent from the security filtration unit enters the nanofiltration unit. The effluent from the nanofiltration unit includes demineralized water and nanofiltration concentrate. The specific operation of the chemical flocculation unit is as follows: 5-10 mg / L of polydimethyldiallyl ammonium chloride is added to the effluent from the aerobic membrane tank, the mixing HRT is 30 seconds, followed by a 30-minute retention period, and the effluent is then sent to the security filtration unit. The specific operation of the security filtration unit is as follows: the effluent from the chemical flocculation unit is sent into a filter area with a 5.0 μm filtration precision, maintaining an operating pressure of 0.2-0.4 MPa, maintaining an effluent turbidity <0.2 NTU, and a sludge density index ≤3.0 after 15 minutes, before the effluent is sent to the nanofiltration unit. The demineralized water is sent out of the treatment section, and the nanofiltration raw solution is fed into the fractional recycling unit; the effluent from the fractional recycling unit includes the nanofiltration concentrate and nanofiltration waste liquid, the nanofiltration waste liquid is sent out of the treatment section, and the nanofiltration concentrate is fed into the biodegradation unit.

2. The integrated treatment method for high-salinity wastewater nanofiltration desalination based on MBR according to claim 1, characterized in that: The nanofiltration unit pressurizes the effluent from the security filtration unit to 0.8-1.5 MPa.

3. The integrated treatment method for high-salinity wastewater nanofiltration desalination based on MBR according to claim 1, characterized in that: The fractional recycling unit adjusts the recycling ratio of the nanofiltration stock solution according to the real-time salinity in the biodegradation unit; the recycling ratio is adjusted to 10-30%; the recycled portion of the nanofiltration stock solution is used as the nanofiltration concentrate and is introduced into the biodegradation unit.

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