Method for treating nanofiltration concentrated water of waterworks by ultra-low pressure reverse osmosis technology

Abstract

The invention discloses a method for treating nanofiltration concentrated water of a waterworks by an ultra-low-pressure reverse osmosis technology, which comprises the following steps: (1) enabling the nanofiltration concentrated water of the waterworks to enter a reverse osmosis membrane assembly through a reverse osmosis high-pressure pump to obtain reverse osmosis produced water and reverse osmosis concentrated water; and (2) the reverse osmosis produced water obtained in the step (1) and the produced water of the nanofiltration system enter a clean water tank together. The water supply plant nanofiltration concentrated water reduction technology is achieved, meanwhile, the quality of reverse osmosis produced water is considered, the water recovery rate of the reverse osmosis membrane assembly reaches 33%-55% under long-term ultra-low-pressure operation, the desalination rate reaches 98%-99%, a more economical and sustainable water saving technical path is provided for a water supply plant using a nanofiltration system, and the water supply plant nanofiltration concentrated water reduction technology is suitable for large-scale popularization and application. And the problem of environmental pollution caused by nanofiltration concentrated water discharge is solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of water treatment, and particularly relates to a method for treating nanofiltration concentrate of a waterworks by ultra-low pressure reverse osmosis technology. Background Art

[0002] In recent years, more and more waterworks have adopted nanofiltration technology for advanced treatment to improve the quality of drinking water. Although the effluent quality can be improved, the problem of disposing of nanofiltration concentrate is also faced. If directly discharged, it will cause ecological pollution of the discharged water body and also lead to waste of water resources. At present, most of the research on the disposal of nanofiltration concentrate focuses on the field of treating sewage by nanofiltration technology. For example, patent document CN202223284739 uses the method of "primary filtration + photothermal evaporation" to treat nanofiltration concentrate. Although this method greatly reduces the thermal energy consumption compared with the traditional evaporation crystallization treatment method, it is not easy to be applied on a large scale. Patent document CN202211074348 provides a flocculation precipitation process for treating nanofiltration concentrate by a combined agent complex treatment. Although this process can greatly reduce the conductivity and total dissolved solids content of the treated nanofiltration concentrate, the treatment process is complex, and the addition of a settling agent and a flocculant is likely to have an impact on the environment. At the same time, the quality of nanofiltration concentrate produced by treating sewage by nanofiltration technology is very different from that of nanofiltration concentrate produced by treating drinking water by nanofiltration technology. Therefore, there is an urgent need to invent a method for treating nanofiltration concentrate suitable for the quality of nanofiltration concentrate in a waterworks, which can not only effectively remove ions in the nanofiltration concentrate, but also recover water resources and improve the overall water recovery rate of the nanofiltration process. Summary of the Invention

[0003] Object of the Invention: The present invention aims to provide a method for treating nanofiltration concentrate of a waterworks by ultra-low pressure reverse osmosis technology, which can effectively remove ions and organic substances in the nanofiltration concentrate and improve the water recovery rate of the nanofiltration technology in the waterworks.

[0004] Technical Solution: The method for treating nanofiltration concentrate of a waterworks by the ultra-low pressure reverse osmosis technology of the present invention includes the following steps:

[0005] (1) The nanofiltration concentrate of the waterworks is fed into a reverse osmosis membrane module by a reverse osmosis high-pressure pump to obtain reverse osmosis product water and reverse osmosis concentrate water;

[0006] (2) The reverse osmosis product water obtained in step (1) and the water produced by the nanofiltration system enter a clear water tank together.

[0007] The nanofiltration concentrate of the waterworks can intercept salt ions and organic substances in the water through the reverse osmosis membrane module, reducing the conductivity and TOC content.

[0008] Preferably, the pressure range of the osmotic high-pressure pump is 0.85 MPa to 0.95 MPa. The resource recovery of the nanofiltration concentrate water at a super-low pressure below 0.95 MPa is much lower than the operating pressure of the conventional reverse osmosis technology, greatly reducing the operating energy consumption of the reverse osmosis high-pressure pump.

[0009] Preferably, the temperature range of the nanofiltration concentrate water is 15°C to 30°C. More preferably, the temperature range of the nanofiltration concentrate water is 23°C to 27°C. Under the same operating conditions, the higher the temperature, the better the treatment effect of the reverse osmosis membrane module on the nanofiltration concentrate water. However, considering the energy consumption of the water treatment plant, without adding a temperature controller to regulate the water temperature, the room temperature of 23 - 27°C should be used for treatment.

[0010] Preferably, the pH value of the nanofiltration concentrate water is 3 to 12. The reverse osmosis membrane module cannot be used to treat strong acids and strong alkalis.

[0011] Preferably, the reverse osmosis membrane in the reverse osmosis membrane module is a polyamide composite membrane.

[0012] Preferably, the reverse osmosis water production recovery rate is 33% to 55%.

[0013] Preferably, the desalination rate of the reverse osmosis membrane module for treating the nanofiltration concentrate water is 98% to 99.5%.

[0014] Preferably, the TOC removal rate of the reverse osmosis membrane module for treating the nanofiltration concentrate water is 60% to 98%. More preferably, the TOC removal rate of the reverse osmosis membrane module for treating the nanofiltration concentrate water is 70% to 97%.

[0015] Beneficial effects: Compared with the prior art, the present invention has the following remarkable advantages: (1) The present invention adds a reverse osmosis unit at the end of the nanofiltration system, effectively intercepts salt ions and organic substances in the nanofiltration concentrate water by using the reverse osmosis membrane, realizes the effective treatment of the nanofiltration concentrate water, effectively recovers and utilizes the water resources in the nanofiltration concentrate water, and improves the total water recovery rate of the nanofiltration system; (2) The present invention realizes the resource recovery of the nanofiltration concentrate water at a super-low pressure below 0.95 MPa, which is much lower than the operating pressure of the conventional reverse osmosis technology, greatly reducing the operating energy consumption of the reverse osmosis high-pressure pump; (3) The desalination rate of the reverse osmosis membrane module for treating the nanofiltration concentrate water reaches 98% to 99.5%, and the TOC removal rate reaches 60% to 80%; (4) Compared with other conventional processes for treating the nanofiltration concentrate water, the operation method of the present invention is simpler and more convenient, and the equipment can operate independently, thus saving labor costs. Description of the Drawings

[0016] Figure 1 It is a schematic diagram of a laboratory bench-scale reverse osmosis device;

[0017] Figure 2 It is a practical picture of a laboratory bench-scale reverse osmosis device;

[0018] Figure 3 Separation performance diagram of the actual nanofiltration concentrate water experiment by reverse osmosis treatment in Example 1 under different temperature and pressure combinations;

[0019] Figure 4 Diagram showing the change of membrane separation performance of the simulated nanofiltration concentrate water treated by reverse osmosis in Example 2 over time;

[0020] Figure 5 SEM diagram of the reverse osmosis fouled membrane for treating the simulated nanofiltration concentrate water in Example 2;

[0021] Figure 6 EDS-mapping diagram of the reverse osmosis fouled membrane for treating the simulated nanofiltration concentrate water in Example 2;

[0022] Figure 7 Diagram showing the change of membrane separation performance of the actual nanofiltration concentrate water treated by reverse osmosis in Example 3 over time;

[0023] Figure 8 Diagram showing the change of TOC removal rate of the actual nanofiltration concentrate water treated by reverse osmosis in Example 3 over time;

[0024] Figure 9 SEM diagram of the reverse osmosis fouled membrane for treating the actual nanofiltration concentrate water in Example 3;

[0025] Figure 10 EDS-mapping diagram of the reverse osmosis fouled membrane for treating the actual nanofiltration concentrate water in Example 3. Detailed implementation mode

[0026] The technical solution of the present invention will be further described below in conjunction with the embodiments.

[0027] As Figure 1 、 2 shown are respectively the schematic diagram and the actual diagram of the laboratory bench-scale reverse osmosis device. The high-pressure pump provides pressure to push the concentrate water into the reverse osmosis module from the small water tank for separation and concentration. The treated concentrate water and clean water are discharged from the concentrate water pipeline and the clean water pipeline respectively.

[0028] The technical solution of the present invention will be further described below in conjunction with the embodiments.

[0029] Measured parameters: the reverse osmosis product water flow rate obtained by filtering through the reverse osmosis device, with the unit of mL / s; the reverse osmosis concentrate water flow rate, with the unit of mL / s; the conductivity of the reverse osmosis product water, with the unit of μS / cm; the conductivity of the reverse osmosis influent water, with the unit of μS / cm.

[0030] Reverse osmosis performance test

[0031] There are three important parameters for evaluating the reverse osmosis performance of the present invention: the feed membrane flux, the water recovery rate of the product water, and the salt rejection rate. The formula for defining the feed membrane flux is as follows:

[0032]

[0033] Among them, J w is the membrane flux, with the unit of L / (m 2 ·h); V is the volume of the product water, with the unit of L; t is the time, with the unit of s; S is the effective membrane area, which is 0.49 m 2 .

[0034] The formula for defining the water recovery rate of the product water is as follows:

[0035]

[0036] Among them, N is the water recovery rate, with the unit of %; Q 1 is the reverse osmosis product water flow rate, with the unit of mL / s; Q 2 is the reverse osmosis concentrate water flow rate, with the unit of mL / s.

[0037] The formula for defining the salt rejection rate is as follows:

[0038]

[0039] Among them, R is the desalination rate, with the unit of %; C R is the reverse osmosis feed water conductivity, with the unit of μS / cm; C P is the reverse osmosis product water conductivity, with the unit of μS / cm.

[0040] Example 1

[0041] In this example, the water sample introduced into the laboratory-scale reverse osmosis device is the concentrated water of the third-stage nanofiltration of a certain water plant, and its conductivity is 1162 μS / cm. The reverse osmosis device is operated, and the temperature of the nanofiltration concentrated water is stabilized at 15 °C, 20 °C, 25 °C, and 30 °C respectively by adjusting the temperature of the constant-temperature cold water bath, and the operating pressure is adjusted to 0.65 MPa, 0.75 MPa, and 0.85 MPa by adjusting the needle valve on the concentrated water pipeline under each temperature condition. The clean water flux, concentrated water flux, clean water conductivity, and feed water conductivity under different operating conditions are measured, and the reverse osmosis membrane flux, water recovery rate, and desalination rate are calculated. The test results are as Figure 3 shown.

[0042] From Figure 3It can be seen that both the membrane flux and the water recovery rate increase with the increase of the operating pressure and the temperature of the NF concentrated water; the salt rejection rate remains above 98% under all operating conditions. The test results show that in the actual application of reverse osmosis treatment of NF concentrated water, economic benefits should be considered, and higher operating pressure and operating temperature should be adopted as much as possible. Considering the economic cost of setting a constant temperature device in the water plant, the operating temperature should be set at room temperature of 25 °C, and the operating pressure is 0.85 MPa. Compared with the lower operating pressures of 0.65 MPa and 0.75 MPa, the water recovery rate at 0.85 MPa is 33.35%, and the salt rejection rate is 98.8%, and the reverse osmosis performance is better. At the operating temperature of 25 °C, the reverse osmosis performance is better than that at 15 °C and 20 °C, and the operating cost is lower than that at 30 °C.

[0043] Example 2

[0044] In this example, the water sample introduced into the laboratory-scale reverse osmosis device is a mixed solution of calcium sulfate and sodium chloride with humic acid as the background, which is configured with simulated NF concentrated water. Among them, the concentrations of humic acid, calcium sulfate, and sodium chloride are 6.5 mg / L, 354.4 mg / L, and 250 mg / L respectively, and the conductivity of the prepared water is 1100 - 1300 μS / cm. Among them, the operating pressure of the reverse osmosis process is 0.85 MPa, and the operating temperature is 25 °C. Prepare the simulated pollutant water sample and stir it fully to dissolve. Introduce the prepared mixed solution of calcium sulfate and sodium chloride with humic acid as the background into the reverse osmosis device. Set the operating temperature at 25 °C and the operating pressure at 0.95 MPa. Measure the reverse osmosis membrane flux, water recovery rate, and salt rejection rate every 10 h. It is specified that when the membrane flux drops to 30% of the initial membrane flux after long-term operation, it is considered that the reverse osmosis membrane is polluted. After the pollution, stop the test and take the polluted membrane for SEM electron microscopy scanning to observe the microscopic morphology of the pollutants, and combine EDS-MAP to analyze the pollution process of the target pollutants on the reverse osmosis membrane. The test results are as Figures 4 - 6 shown.

[0045] From Figure 4 (a) and (b), it can be seen that the initial reverse osmosis membrane flux is 61.71 L / (m 2 ·h). As the running time increases, with the accumulation of pollutants on the membrane surface, the membrane flux generally shows a downward trend. After running for 160 h, the membrane flux drops to 42.61 L / (m 2 ·h), which is 30.95% of the initial flux, indicating that the reverse osmosis membrane introduced with this simulated pollutant water sample is polluted at 160 h. From Figure 4 (c), it can be seen that as the running time increases, the water recovery rate generally shows a downward trend. After the device runs for 120 h, the water recovery rate drops below 50%. After running for 160 h, the water recovery rate drops to 39.19%, with an overall decrease of 40.4%. From Figure 4(d) It can be seen that compared with the membrane flux and water recovery rate, the salt rejection rate hardly changes and basically remains stable at 99% - 99.5%. The SEM electron microscope scans and EDS-MAP results of the fouled membrane are shown in Figure 5 and 6 respectively. By comparing Figure 5 the microscopic morphologies of the base membrane and the fouled membrane, it can be seen that the porous structure of the base membrane has been filled with pollutants to form a thin film of pollutants, and flaky pollutants of different sizes and a small number of larger rosette-shaped composite crystals are covered on the thin film; combined with Figure 6 the results, it can be known that the components of the pollutant thin film are a large number of humic acid molecules and calcium sulfate, the crystals are calcium sulfate hemihydrate crystals, and sodium chloride hardly participates in membrane fouling.

[0046] Example 3

[0047] The water sample to be treated in this example is the same as that in Example 1. The reverse osmosis device is operated by introducing the actual nanofiltration concentrated water sample. The operating temperature is set at 25 °C and the operating pressure is 0.85 MPa. The reverse osmosis membrane flux, water recovery rate, and salt rejection rate are measured every 10 h, and the TOC concentration of the produced water is measured every 20 h. It is stipulated that when the membrane flux drops to 30% of the initial membrane flux after long-term operation, it is considered that the reverse osmosis membrane is fouled. After fouling, the test is stopped and the fouled membrane is taken for SEM electron microscope scanning to observe the microscopic morphology of the pollutants, and the fouling process of the target pollutants on the reverse osmosis membrane is analyzed by combining EDS-MAP. The test results are as shown in Figures 7 - 10 .

[0048] From Figure 7 (a), it can be seen that the initial reverse osmosis membrane flux is 40.63 L / (m 2 ·h). As the operation time increases and pollutants accumulate on the membrane surface, the membrane flux generally shows a downward trend. After operating for 140 h, the membrane flux decreases to 27.82 L / (m 2 ·h), which is 31.28% of the initial flux, indicating that the reverse osmosis membrane fouled when the simulated pollutant water sample was introduced at 140 h; from Figure 7 (b), it can be known that the initial reverse osmosis water recovery rate is 45.29%. As the operation time increases, it can be seen that the water recovery rate generally shows a downward trend. After the membrane element is fouled at 140 h, the water recovery rate drops to 37.15%, and the average water recovery rate is 43.26%. From Figure 7 (c), it can be known that the salt rejection rate fluctuates between 98% and 99%. From Figure 8 , it can be seen that the TOC removal rate is 70.52% - 96.41%. The SEM electron microscope scans and EDS-MAP results of the fouled membrane are shown in Figure 9 and 10 respectively. By comparing Figure 9 the microscopic morphologies of the base membrane and the fouled membrane, it can be seen that the porous structure of the base membrane has been covered by the fouling layer, and there are pollutants of different sizes on the surface of the fouling layer; combined withFigure 10 The results show that the particulate pollutants on the contaminated membrane may be macromolecular complexes formed by organic pollutants and calcium ions, and sodium chloride hardly participates in membrane fouling.

Claims

1. A method for treating nanofiltration concentrated water from a water plant using ultra-low pressure reverse osmosis technology, characterized in that: The following steps are involved: (1) The nanofiltration concentrated water from the water plant enters the reverse osmosis membrane module through a reverse osmosis high-pressure pump to obtain reverse osmosis produced water and reverse osmosis concentrated water; (2) The reverse osmosis water obtained in step (1) enters the clean water tank together with the water produced by the nanofiltration system.

2. The method for treating nanofiltration concentrated water from a water plant using ultra-low pressure reverse osmosis technology according to claim 1, characterized in that: The pressure range of the osmotic high-pressure pump is 0.65MPa to 0.95MPa.

3. The method for treating nanofiltration concentrated water from a water plant using ultra-low pressure reverse osmosis technology according to claim 1, characterized in that: The temperature range of the nanofiltration concentrated water is 15°C to 30°C.

4. The method for treating nanofiltration concentrated water from a water plant using ultra-low pressure reverse osmosis technology according to claim 3, characterized in that: The temperature range of the nanofiltration concentrated water is 23°C to 27°C.

5. The method for treating nanofiltration concentrated water from a water plant using ultra-low pressure reverse osmosis technology according to claim 1, characterized in that: The pH value of the nanofiltration concentrated water is 3-12.

6. The method for treating nanofiltration concentrated water from a water plant using ultra-low pressure reverse osmosis technology according to claim 1, characterized in that: The reverse osmosis membrane in the reverse osmosis membrane assembly is a polyamide composite membrane.

7. The method for treating nanofiltration concentrated water from a water plant using ultra-low pressure reverse osmosis technology according to claim 1, characterized in that: The reverse osmosis water recovery rate is 33% to 55%.

8. The method for treating nanofiltration concentrated water from a water plant using ultra-low pressure reverse osmosis technology according to claim 1, characterized in that: The desalination rate of the reverse osmosis membrane assembly in treating nanofiltration concentrated water is 98% to 99.5%.

9. The method for treating nanofiltration concentrated water from a water plant using ultra-low pressure reverse osmosis technology according to claim 1, characterized in that: The TOC removal rate of the reverse osmosis membrane assembly in treating nanofiltration concentrated water is 60% to 98%.

10. The method for treating nanofiltration concentrated water from a water plant using ultra-low pressure reverse osmosis technology according to claim 9, characterized in that: The TOC removal rate of the reverse osmosis membrane assembly in treating nanofiltration concentrated water is 70% to 97%.

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