A method for purifying and desalinizing high-salinity water

Abstract

This invention belongs to the field of industrial wastewater treatment technology, specifically relating to a method for purifying and desalinizing high-salinity water. This invention employs a synergistic combination process of "electrodialysis pretreatment + nanofiltration / reverse osmosis deep desalination." Electrodialysis first achieves preliminary desalination of the high-salinity water, reducing the operating pressure and pollution risk of the subsequent nanofiltration / reverse osmosis system. Ultimately, it achieves efficient purification and desalination of the high-salinity water, ensuring stable system operation and reducing energy consumption. While achieving efficient water resource recovery, it creates more favorable conditions for the subsequent resource utilization of salts. This not only creates direct economic benefits for enterprises but also represents a technological path that aligns with the concepts of green and circular development, demonstrating significant environmental and social benefits and more thorough resource recovery. The process route is simple, operation and maintenance are convenient, and parameters can be flexibly adjusted according to different treatment scales, adapting to the high-salinity water treatment needs of various titanium dioxide production enterprises, and is easy to promote industrially.

Description

Technical Field

[0001] This invention belongs to the field of industrial wastewater treatment technology, specifically relating to a method for purifying and desalinating high-salinity water, which is particularly suitable for the deep treatment and resource utilization of high-salinity water containing sodium sulfate generated after the third washing process in titanium dioxide production. Background Technology

[0002] The sulfuric acid process is currently the mainstream technology for titanium dioxide production in China. In its post-processing stage, to ensure the pigment performance of the titanium dioxide product (such as tinting strength and hiding power), the coated titanium dioxide slurry must undergo rigorous water washing to remove residual soluble salt impurities. The third washing process is a crucial step in removing residual sulfate impurities from the finished titanium dioxide product; however, this process generates a large amount of high-salt water containing sodium sulfate, typically ranging from 8000 to 12000 mg / L, as well as small amounts of titanium dioxide dust, calcium and magnesium ions, and other impurities. To achieve water resource recycling or meet discharge standards, this high-salt water needs to be purified and desalinated. Whether for achieving in-plant water resource recycling, reducing fresh water consumption, or meeting increasingly stringent environmental emission standards, efficient and economical purification and desalination of the high-salt water from the third washing process for titanium dioxide has become an urgent need and a technological bottleneck for the sustainable development of the entire industry.

[0003] Currently, the treatment of high-salinity wastewater from the third washing of titanium dioxide typically employs a single desalination process: sedimentation removal followed by nanofiltration / reverse osmosis. This involves first removing solid particles through sedimentation before directly introducing the wastewater into a nanofiltration or reverse osmosis system for desalination. However, this process has significant drawbacks: First, the high salt content of the third washing wastewater creates a high osmotic pressure environment for the membrane elements when directly introduced into the nanofiltration or reverse osmosis system, leading to high system operating pressure and energy consumption. Second, concentration polarization is highly likely to occur in high-salinity environments, accelerating membrane surface fouling. If pretreatment (such as sedimentation) is incomplete, residual ultrafine titanium dioxide particles will directly clog the membrane pores. This results in rapid decline in membrane flux and desalination rate, requiring frequent chemical cleaning, increasing maintenance costs and significantly shortening the lifespan of the membrane elements. Third, it has weak anti-fouling capabilities; if sedimentation removal is incomplete, residual trace solid particles will further exacerbate membrane fouling, affecting treatment efficiency and the stability of the effluent quality.

[0004] In existing processes, electrodialysis is mostly used as a final concentration unit to concentrate RO concentrate to higher salinity (e.g., 12-15% or even higher) to reduce evaporation load. Electrodialysis is a mature desalination technology characterized by stable operation at high salt concentrations, low energy consumption, and strong resistance to fouling. However, using electrodialysis alone is insufficient for achieving deep desalination. Currently, there are no known technical solutions combining electrodialysis as a pretreatment unit with nanofiltration / reverse osmosis for the purification and desalination of high-salinity water in the third washing stage of titanium dioxide production.

[0005] In summary, existing technologies for treating high-salinity wastewater from titanium dioxide washing processes face multiple challenges, including complex and expensive procedures, unstable and easily contaminated core membrane systems, and difficulties in deep resource utilization. Therefore, the industry urgently needs to develop a novel purification and desalination process that combines high technical efficiency, economical operation, and system stability. Summary of the Invention

[0006] The technical problem to be solved by this invention is to provide a method for purifying and desalinating high-salinity water. This method employs a synergistic combination of "electrodialysis pretreatment + nanofiltration / reverse osmosis deep desalination." Electrodialysis first achieves preliminary desalination of the high-salinity water, reducing the operating pressure and contamination risk of the subsequent nanofiltration / reverse osmosis system. Ultimately, it achieves efficient purification and desalination of the high-salinity water, ensuring stable system operation and reducing energy consumption. While achieving efficient water resource recovery, it also creates more favorable conditions for the subsequent resource utilization of salts, providing a better solution for the green upgrading and zero wastewater discharge of the titanium dioxide industry.

[0007] The technical solution adopted is as follows: A method for purifying and desalinizing high-salinity water includes the following steps: (1) Pretreatment and impurity removal: The high-salt water is introduced into a precision filter for filtration and impurity removal to obtain pretreated saline water that meets the requirements; (2) Electrodialysis desalination: The pretreated brine is pumped into the electrodialysis unit, and the operating parameters of the electrodialysis unit are controlled to perform preliminary desalination of the brine to obtain intermediate water and concentrated brine after desalination; the concentrated brine is collected and recovered. (3) Nanofiltration / reverse osmosis deep desalination: According to the effluent water quality requirements, the desalinated intermediate water obtained in step (2) is introduced into a nanofiltration device or a reverse osmosis device for deep desalination.

[0008] Preferably, the treated high-salt water has a sodium sulfate content of 8000–12000 mg / L and a solid content of 50–100 mg / L; the precision filter uses a polypropylene pleated filter membrane with a pore size of 5–10 μm to remove solid particles from the water, and controls the solid content of the high-salt water after filtration to be ≤5 mg / L.

[0009] Preferably, the filtration pressure of the precision filter is controlled at 0.1 to 0.3 MPa, and the filtration velocity is 1 to 2 m / s, ensuring that the solid particulate matter removal rate is ≥95%.

[0010] Preferably, the ion exchange membranes of the electrodialysis device are homogeneous cation exchange membranes and homogeneous anion exchange membranes, with the cation exchange membranes and homogeneous anion exchange membranes arranged alternately. The membrane surface resistance is ≤5Ω·cm², and the selected permeability is ≥95%, which is suitable for the desalination requirements of high saline water containing sodium sulfate.

[0011] Preferably, the operating parameters of the electrodialysis device are set as follows: operating voltage 20-40V, current density 20-30mA / cm², dilute chamber flow rate 15-25m³ / h, and concentrated chamber flow rate 8-12m³ / h.

[0012] Preferably, the desalination rate of the electrodialysis device in the initial desalination is controlled at 50-60%, and the sodium sulfate content of the intermediate water after desalination is 3200-5800 mg / L, and the sodium sulfate content of the concentrated brine is 18000-22000 mg / L.

[0013] Preferably, in step (3), the nanofiltration / reverse osmosis device is filtered by a security filter before the water enters, and the pore size of the filter membrane is 1 to 3 μm.

[0014] Preferably, in step (3), if a nanofiltration device is used, the operating pressure is controlled at 1.5–2.5 MPa, the temperature at 25–35 °C, and the recovery rate at 60–70%. The nanofiltration membrane is a polyamide composite nanofiltration membrane, which is suitable for Na+. + Al 3+ and Zr 4+ ZrO 2+ The retention rate is ≥98%, and the sodium sulfate content in the treated effluent is ≤500mg / L.

[0015] Preferably, in step (3), if a reverse osmosis device is used, the operating pressure is controlled at 3.0-4.0 MPa, the temperature at 25-35℃, the recovery rate at 70-80%, the reverse osmosis membrane is a polyamide composite reverse osmosis membrane, the total desalination rate is ≥99%, and the conductivity of the treated effluent is ≤50μS / cm.

[0016] Preferably, the ion exchange membrane of the electrodialysis device is cleaned every 30 to 45 days by circulating a 2 to 3% hydrochloric acid solution for 30 to 60 minutes; the filter membrane used in the nanofiltration / reverse osmosis device is cleaned online with an alkaline or acidic cleaning agent to ensure long-term stable operation of the system; wherein, the alkaline cleaning agent is a 0.5 to 1.0% sodium hydroxide solution and the acidic cleaning agent is a 1.0 to 2.0% citric acid solution.

[0017] This invention is applicable to the deep treatment and resource utilization of high-sodium sulfate brine generated after the third washing process in titanium dioxide production.

[0018] Compared with the prior art, the beneficial effects of the method of the present invention are as follows: (1) Significantly improved operational stability: This invention first desalinates high saline water through electrodialysis (desalination rate 50-60%), which greatly reduces the salt content of the feed water of the subsequent nanofiltration / reverse osmosis system, avoids the impact of high osmotic pressure on membrane elements, reduces the risk of concentration polarization and membrane fouling, and the system can operate continuously and stably for ≥180 days, which is more than twice as good as the traditional direct nanofiltration / reverse osmosis process.

[0019] (2) Energy consumption is significantly reduced: high saline water is first desalinated by electrodialysis, which significantly reduces the salt load entering the subsequent nanofiltration (NF) / reverse osmosis (RO) system. The operating pressure of the subsequent nanofiltration / reverse osmosis system can be reduced by 30-40%, and the energy consumption for desalination per unit of water is reduced to 1.2-1.8 kWh / m³, which is 25-35% lower than the traditional process. At the same time, electrodialysis has significant energy consumption advantages in high saline environment, which further reduces the overall energy cost of the process.

[0020] (3) Strong anti-fouling ability: The electrodialysis process can effectively intercept impurities such as fine titanium dioxide particles, acting as a "protective shield" for the core membrane system of NF / RO. This greatly reduces the main risk substances that cause membrane fouling, clogging and concentration polarization from the source. The dual impurity removal and desalination design of pretreatment + electrodialysis not only removes solid particles, but also reduces the ion concentration in the water, reduces the adsorption and deposition of pollutants on the membrane surface, extends the membrane cleaning frequency from 15-20 days / time in the traditional process to 30-45 days / time, and extends the chemical cleaning cycle by 2 to 3 times, reducing reagent consumption and downtime, extending the service life of membrane elements by 1.5 to 2.0 times, and reducing replacement costs.

[0021] (4) Strong adaptability and controllable effluent quality: The subsequent deep desalination unit (nanofiltration or reverse osmosis) can be flexibly selected according to the actual effluent demand. The nanofiltration effluent can meet the general industrial water reuse requirements, and the reverse osmosis effluent can meet the high-quality water reuse or discharge standards. At the same time, the concentrated brine produced by electrodialysis can further recover sodium sulfate and realize resource recycling.

[0022] (5) Simple process and easy to promote industrialization: Through this gradient treatment, the overall desalination rate and water recovery rate of the system are improved simultaneously, and higher quality recycled water can be produced stably. The equipment used (precision filter, electrodialysis device, nanofiltration / reverse osmosis device) are all conventional industrial equipment. The process route is simple, the operation and maintenance are convenient, and the parameters can be flexibly adjusted according to different treatment scales. It is suitable for the high salinity treatment needs of various titanium dioxide production enterprises and is easy to promote industrialization.

[0023] (6) Promoting green recycling and zero emissions: This process simultaneously and efficiently realizes the dual resource utilization of "water" and "salt". A large amount of produced water is recycled back to the main production line, saving fresh water intake; sodium sulfate or conversion products are sold or reused as products. This not only creates direct economic benefits for enterprises, but also is a technical path that conforms to the concept of green and circular development, with outstanding environmental and social benefits and more thorough resource recycling. Attached Figure Description

[0024] Figure 1 This is a flowchart of the method of the present invention. Detailed Implementation

[0025] The accompanying drawings are for illustrative purposes only; to enable those skilled in the art to better understand the technical solutions of the present invention, the preferred embodiments of the present invention are described below in conjunction with specific examples, but should not be construed as limiting the present patent.

[0026] Unless otherwise specified, the test methods or experimental methods described in the following examples are conventional methods or obtained from conventional commercial sources.

[0027] Example 1 A method for purifying and desalinizing high-salt water, using high-salt water containing sodium sulfate after three washings of titanium dioxide as the treatment target, wherein the high-salt water has a sodium sulfate content of 8500 mg / L, a solid content of 60 mg / L, a pH value of 6.5, and a temperature of 25℃, and the process steps are as follows: (1) Pretreatment and impurity removal: The high-salt water containing titanium dioxide after three washes is introduced into a precision filter and filtered using a polypropylene pleated filter membrane with a pore size of 5 μm. The pressure is controlled at 0.1 MPa and the filtration speed is 1 m / s to remove solid particles from the water, ensuring that the solid particle removal rate is ≥95%. The solid content of the high-salt water after filtration is controlled to be ≤5 mg / L to obtain qualified pretreated saline water.

[0028] (2) Preliminary desalination by electrodialysis: The pretreated qualified brine obtained in step (1) is pumped into the electrodialysis device, and the operating parameters of the electrodialysis device are controlled as follows: operating voltage 20V, current density 20mA / cm², dilute chamber flow rate 15m³ / h, concentrated chamber flow rate 10m³ / h; the high brine is preliminarily desalinated by the electrodialysis device, and the desalination rate is controlled at 55%, to obtain intermediate water (sodium sulfate content 3800mg / L) and concentrated brine (sodium sulfate content 20500mg / L) after desalination; the concentrated brine can be collected and used for subsequent salt recovery processes.

[0029] (3) Nanofiltration Deep Desalination: According to the effluent quality requirements, the desalinated intermediate water obtained in step (2) is introduced into the nanofiltration unit for deep desalination. The operating pressure is controlled at 1.5 MPa, the temperature at 25℃, and the recovery rate at 62%. A polyamide composite nanofiltration membrane is selected for the nanofiltration. + Al 3+ and Zr4+ ZrO 2+ The retention rate is ≥98%, and the sodium sulfate content in the treated effluent is ≤500mg / L.

[0030] System operation and maintenance optimization: The ion exchange membranes of the electrodialysis unit are cleaned regularly (once every 40 days) using a 2% hydrochloric acid solution for 40 minutes to restore membrane performance. For the nanofiltration unit, depending on the membrane fouling condition, if colloids, sludge, or other contaminants adhere to the membrane, alkaline washing is used with a 1.0% sodium hydroxide solution for online cleaning. If the membrane is fouled by aluminum ions, specifically aluminum hydroxide scale, acid washing is used to remove the aluminum with a 2.0% citric acid solution for online cleaning, ensuring long-term stable system operation.

[0031] Example 2 A method for purifying and desalinizing high-salt water, using high-salt water containing sodium sulfate after three washings of titanium dioxide as the treatment target, wherein the high-salt water has a sodium sulfate content of 11000 mg / L, a solid content of 85 mg / L, a pH value of 7.5, and a temperature of 25℃, and the process steps are as follows: (1) Pretreatment and impurity removal: The high-salt water containing titanium dioxide after three washes is introduced into a precision filter and filtered using a polypropylene pleated filter membrane with a pore size of 8μm. The pressure is controlled at 0.3MPa and the filtration speed is 2m / s to remove solid particles from the water, ensuring that the solid particle removal rate is ≥95%. The solid content of the high-salt water after filtration is controlled to be ≤5mg / L to obtain qualified pretreated saline water.

[0032] (2) Preliminary desalination by electrodialysis: The pretreated qualified brine obtained in step (1) is pumped into the electrodialysis device, and the operating parameters of the electrodialysis device are controlled as follows: operating voltage 40V, current density 30mA / cm², dilute chamber flow rate 20m³ / h, concentrated chamber flow rate 12m³ / h; the high brine is preliminarily desalinated by the electrodialysis device, and the desalination rate is controlled at 50-60%, to obtain intermediate water (sodium sulfate content 4000mg / L) and concentrated brine (sodium sulfate content 19500mg / L) after desalination; the concentrated brine can be collected and used for subsequent salt recovery processes. (3) Deep desalination by reverse osmosis: According to the requirements of the effluent quality, the desalinated intermediate water obtained in step (2) is introduced into the reverse osmosis unit for deep desalination. The operating pressure is controlled at 3.0 MPa, the temperature at 30℃, and the recovery rate at 75%. The reverse osmosis membrane is a polyamide composite reverse osmosis membrane with a total desalination rate of ≥99%. The conductivity of the treated effluent is ≤50 μS / cm, which meets the requirements for industrial reuse or discharge standards.

[0033] System operation and maintenance optimization: The ion exchange membranes of the electrodialysis unit are cleaned regularly (every 35 days) using a 3% hydrochloric acid solution for 40 minutes to restore membrane performance. For the reverse osmosis unit, depending on the membrane fouling condition, if colloids, sludge, or other contaminants are attached to the membrane, alkaline washing is used with a 0.8% sodium hydroxide solution for online cleaning. If the membrane is fouled by aluminum ions, specifically aluminum hydroxide scale, acid washing is used to remove the aluminum with a 1.6% citric acid solution for online cleaning to ensure long-term stable system operation.

[0034] Application Example 1 Taking the high-salt water treatment of the third wash of a 100,000-ton-per-year sulfuric acid process titanium dioxide production line as an example, the specific parameters and process of this invention are as follows: (1) Raw material water quality parameters: titanium dioxide three-wash high saline: sodium sulfate content 10000mg / L, solid content 80mg / L, pH value 6.5-7.5, temperature 25-30℃, treatment water volume 50m³ / h, and the final effluent conductivity is required to be ≤50μS / cm (for industrial circulating water makeup).

[0035] (2) Pretreatment to remove impurities Two parallel precision filters (model JL-50) were used, with 5μm polypropylene pleated membranes, a filtration pressure of 0.2MPa, and a filtration velocity of 1.5m / s. After filtration, the solids content of the high-salinity water was reduced to 3mg / L, meeting the feed water requirements of the subsequent electrodialysis unit.

[0036] (3) Preliminary desalination by electrodialysis An electrodialysis unit (model EDR-50) was selected, equipped with homogeneous cation exchange membranes and homogeneous anion exchange membranes (membrane surface resistance 4 Ω·cm², selective permeability 96%). Operating parameters were controlled as follows: operating voltage 30V, current density 25mA / cm², dilute chamber flow rate 20m³ / h, and concentrated chamber flow rate 10m³ / h. After treatment, the dilute chamber effluent (intermediate water after desalination) contained 4200 mg / L of sodium sulfate, achieving a desalination rate of 58%; the concentrated chamber effluent (concentrated brine) contained 20500 mg / L of sodium sulfate, which was collected and sent to a sodium sulfate recovery unit.

[0037] (4) Reverse osmosis deep desalination After desalination, the intermediate water is first filtered again through a security filter (1μm membrane) and then introduced into a reverse osmosis unit (model RO-50). A polyamide composite reverse osmosis membrane is selected, and the operating parameters are controlled as follows: operating pressure 3.5MPa, temperature 30℃, and recovery rate 75%. The treated effluent has a conductivity of 38μS / cm and a sodium sulfate content ≤20mg / L, which fully meets the requirements for industrial circulating water makeup.

[0038] (5) System operation and maintenance optimization The electrodialysis unit was cleaned every 40 days with a 2.5% hydrochloric acid solution for 45 minutes, after which the membrane resistivity recovered to more than 95% of its initial value. The reverse osmosis unit was cleaned online every 45 days with a 0.8% sodium hydroxide solution for 60 minutes, after which the membrane flux recovered to more than 90% of its initial value. The system operated continuously for 190 days with stable operation and no serious membrane fouling problems occurred.

[0039] (6) Implementation results In this embodiment, the overall process energy consumption per unit of water desalination is 1.5 kWh / m³, which is 31.8% lower than that of the traditional direct reverse osmosis process (energy consumption per unit of 2.2 kWh / m³). The system can operate continuously and stably for 190 days, the membrane cleaning frequency is reduced by 50%, and the membrane element lifespan is expected to be extended by 1.8 times. It can save about RMB 547,200 in electricity costs per year (calculated based on an electricity price of RMB 0.75 / kWh and 8,000 hours of operation per year). At the same time, the recovery of sodium sulfate from concentrated brine can generate additional economic benefits, resulting in significant environmental and economic benefits.

[0040] The results of the core parameters of Application Example 1 of the present invention are compared with those of the traditional process, and the results are shown in Table 1.

[0041] Table 1. Comparison of core parameter results of application example 1 of the present invention with traditional processes. The process in Application Example 1 of the present invention was compared with the conventional direct deep desalination process, and the results are shown in Table 2.

[0042] Table 2 Comparison of parameters and treatment results between Application Example 1 of the present invention and the traditional direct deep desalination process. In summary, this invention constructs a synergistic process of "precision filtration-electrodialysis-nanofiltration / reverse osmosis" to reduce post-treatment salt load and membrane fouling, thereby overcoming the shortcomings of existing technologies such as high energy consumption, severe membrane fouling, and unstable operation. While achieving efficient water resource recovery, it provides a better solution for the green upgrading of the titanium dioxide industry and zero wastewater discharge, and brings additional economic benefits.

[0043] Of course, the above description is not intended to limit the present invention, and the present invention is not limited to the examples given above. Any changes, modifications, additions or substitutions made by those skilled in the art within the scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A method for purifying and desalinizing high-salinity water, characterized in that, Includes the following steps: (1) Pretreatment and impurity removal: The high-salt water is introduced into a precision filter for filtration and impurity removal to obtain pretreated saline water that meets the requirements; (2) Electrodialysis desalination: The pretreated brine is pumped into the electrodialysis unit, and the operating parameters of the electrodialysis unit are controlled to perform preliminary desalination of the brine to obtain intermediate water and concentrated brine after desalination; the concentrated brine is collected and recovered. (3) Nanofiltration / reverse osmosis deep desalination: According to the effluent water quality requirements, the desalinated intermediate water obtained in step (2) is introduced into a nanofiltration device or a reverse osmosis device for deep desalination.

2. The method for purifying and desalinizing high-salinity water according to claim 1, characterized in that, The treated high-salt water has a sodium sulfate content of 8000–12000 mg / L and a solids content of 50–100 mg / L. The precision filter uses a polypropylene pleated filter membrane with a pore size of 5–10 μm to remove solid particles from the water, and controls the solids content of the high-salt water after filtration to be ≤5 mg / L.

3. The method for purifying and desalinizing high-salinity water according to claim 1, characterized in that, The filtration pressure of the precision filter is controlled at 0.1 to 0.3 MPa, and the filtration velocity is 1 to 2 m / s, ensuring a solid particulate matter removal rate of ≥95%.

4. The method for purifying and desalinizing high-salinity water according to claim 1, characterized in that, The ion exchange membranes of the electrodialysis device are selected from homogeneous cation exchange membranes and homogeneous anion exchange membranes, which are arranged alternately. The membrane surface resistance is ≤5Ω·cm², and the permeability is ≥95%, which is suitable for the desalination needs of high saline water containing sodium sulfate.

5. The method for purifying and desalinizing high-salinity water according to claim 1, characterized in that, The operating parameters of the electrodialysis unit are set as follows: operating voltage 20-40V, current density 20-30mA / cm², dilute chamber flow rate 15-25m³ / h, and concentrated chamber flow rate 8-12m³ / h.

6. The method for purifying and desalinizing high-salinity water according to claim 5, characterized in that, The initial desalination rate of the electrodialysis unit is controlled at 50-60%, and the sodium sulfate content of the intermediate water after desalination is 3200-5800 mg / L, while the sodium sulfate content of the concentrated brine is 18000-22000 mg / L.

7. The method for purifying and desalinizing high-salinity water according to claim 1, characterized in that, In step (3), the nanofiltration / reverse osmosis device is filtered by a security filter before the water enters the device. The pore size of the filter membrane is 1 to 3 μm.

8. The method for purifying and desalinizing high-salinity water according to claim 1, characterized in that, In step (3), if a nanofiltration device is used, the operating pressure is controlled at 1.5–2.5 MPa, the temperature at 25–35 °C, and the recovery rate at 60–70%. A polyamide composite nanofiltration membrane is selected for the nanofiltration process. + Al 3+ and Zr 4+ ZrO 2+ The retention rate is ≥98%, and the sodium sulfate content in the treated effluent is ≤500mg / L.

9. The method for purifying and desalinizing high-salinity water according to claim 1, characterized in that, In step (3), if a reverse osmosis device is used: control the operating pressure to be 3.0-4.0 MPa, the temperature to be 25-35℃, the recovery rate to be 70-80%, the reverse osmosis membrane to be a polyamide composite reverse osmosis membrane, the total desalination rate to be ≥99%, and the conductivity of the treated effluent to be ≤50μS / cm.

10. The method for purifying and desalinizing high-salinity water according to claim 1, characterized in that, The ion exchange membrane of the electrodialysis device is cleaned every 30 to 45 days by circulating a 2 to 3% hydrochloric acid solution for 30 to 60 minutes. The filter membrane used in the nanofiltration / reverse osmosis device is cleaned online using an alkaline or acidic cleaning agent. The alkaline cleaning agent is a 0.5 to 1.0% sodium hydroxide solution, and the acidic cleaning agent is a 1.0 to 2.0% citric acid solution.

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