A reverse nanofiltration membrane filtration system and a method for operating control thereof

Abstract

The application provides a kind of turnover nanofiltration membrane filtration system and its operation control method, for solving the problem of uneven membrane pollution distribution and system operation efficiency decline in existing multi-stage series nanofiltration membrane system, belongs to membrane separation water treatment technical field.The application sets up pressure monitoring unit, valve control unit and PLC control unit in multi-stage nanofiltration membrane filtration system, real-time acquisition of inlet and outlet pressure and calculation of operating pressure difference and trend, combined with historical operation data to establish adaptive threshold decision mechanism, which can dynamically evaluate the membrane pollution state;When the system reaches the turnover condition, automatically execute membrane cleaning and water direction switching, and adjust the valve opening and water pump operating parameters during the reversing process to achieve smooth transition, the whole process is automatically completed pollution assessment, membrane cleaning and flow direction turnover, without frequent manual intervention, effectively improve the system operation stability, water treatment efficiency and water recovery rate.This is suitable for nanofiltration membrane water treatment and intelligent operation control of membrane system.

Description

Technical Field

[0001] This invention relates to the field of membrane separation water treatment technology, and in particular to a multi-stage nanofiltration membrane flow direction control system based on differential pressure feedback and its operation method. Background Technology

[0002] Nanofiltration membranes are pressure-driven membrane separation technologies with high retention capacity for divalent and multivalent ions, small organic molecules, and some microorganisms in water. They are widely used in drinking water treatment, industrial wastewater treatment, and resource recovery. Compared to traditional water treatment processes, nanofiltration membrane separation technology offers advantages such as high treatment efficiency, small footprint, and ease of automation, thus showing promising application prospects in the water treatment field. In practical engineering applications, to improve system recovery rates or treatment efficiency, nanofiltration membrane systems typically employ a multi-stage series operation mode. This involves connecting multiple nanofiltration membrane modules sequentially, using the concentrate produced by the previous stage as feed water for further treatment by the next stage, thereby achieving staged treatment of raw water and improving the overall water recovery rate and treatment efficiency of the system.

[0003] However, during long-term operation of nanofiltration membranes, pollutants in the water tend to gradually deposit on the membrane surface and form a fouling layer, leading to a decrease in membrane flux and an increase in system operating pressure differential, thus affecting the stable operation of the nanofiltration system. For multi-stage series nanofiltration membrane systems, as the influent water quality is concentrated at each stage in the system, pollutants tend to accumulate gradually, causing the downstream membrane modules to bear a high fouling load for a long time. This results in a significant uneven distribution of membrane fouling within the system, exacerbating irreversible fouling of some membrane modules and further affecting the overall operating efficiency of the system.

[0004] In existing technologies, membrane fouling is typically mitigated through periodic chemical cleaning or adjustments to operating conditions. However, these common solutions are often costly, difficult to apply continuously in drinking water treatment, and may affect the stable operation of the system. Furthermore, they are ineffective in addressing the uneven distribution of fouling levels among membrane modules in multi-stage nanofiltration systems. Therefore, there is an urgent need to propose a new nanofiltration membrane operation control method that ensures uniform distribution of membrane fouling among membrane modules, slows down the progression of membrane fouling, improves the overall operating efficiency of the system, and reduces system operation and maintenance costs. Summary of the Invention

[0005] This invention provides a flip-type nanofiltration membrane system and its operation control method to solve the problems of uneven membrane fouling distribution and reduced system operating efficiency in existing multi-stage series nanofiltration membrane systems. By optimizing the operation mode of the nanofiltration membrane system, membrane fouling is more evenly distributed among the membrane modules, slowing down the rate of membrane fouling development and improving the overall stability of the multi-stage nanofiltration membrane system.

[0006] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a flip-type nanofiltration membrane filtration system, the system comprising a multi-stage nanofiltration membrane module, a membrane flushing water tank, a valve control unit, a pressure monitoring unit, and a control unit; The multi-stage nanofiltration membrane module is used to filter the raw water to be treated in stages. The membrane flushing tank is used to store the membrane flushing solution; The valve control unit is used to control the water flow path of the multi-stage nanofiltration membrane module and to switch the water inlet direction; The pressure monitoring unit includes pressure detection components respectively installed at the inlet and outlet of the multi-stage nanofiltration membrane module, used to collect inlet and outlet pressures; The control unit is electrically connected to the valve control unit and the pressure monitoring unit respectively, and is configured as follows: The control unit receives inlet and outlet pressure data collected by the pressure monitoring unit and calculates the system operating pressure difference and its changing trend to dynamically assess the membrane fouling status. Based on historical system operating data, it analyzes the operating pressure difference variation pattern and establishes an adaptive threshold determination mechanism. When the system operating conditions reach the reversal condition, the control valve control unit switches the inlet direction of the multi-stage nanofiltration membrane modules, ensuring uniform distribution of membrane fouling among the modules. During the inlet direction switching process, the system flow rate and pressure are gradually changed by adjusting the valve opening or feed pump operating parameters to achieve a smooth transition, thereby avoiding pressure shocks and sudden flow changes and improving system operational stability.

[0007] Furthermore, the multi-stage nanofiltration membrane assembly of the present invention includes a first nanofiltration membrane assembly, a second nanofiltration membrane assembly, a third nanofiltration membrane assembly, a fourth nanofiltration membrane assembly, and a fifth nanofiltration membrane assembly arranged in series, for performing step-by-step filtration treatment on raw water, with the concentrate produced by the previous stage nanofiltration membrane assembly serving as the feed water for the next stage nanofiltration membrane assembly.

[0008] Furthermore, the pressure monitoring unit of the present invention includes a first pressure gauge disposed at the inlet end of the multi-stage nanofiltration membrane module and a second pressure gauge disposed at the outlet end, for monitoring and acquiring system operating pressure data.

[0009] Furthermore, the system described in this invention also includes a raw water tank and a water supply pump; The raw water tank is used to store raw water to be treated; The water supply pump is located between the raw water tank and the multi-stage nanofiltration membrane assembly, and is used to drive the raw water into the filtration system.

[0010] Furthermore, the system of the present invention also includes an operation terminal, which includes a human-machine interface for users to manually set and adjust the preset operating parameters, thresholds and flip cycles of the control unit.

[0011] Furthermore, the control unit described in this invention is a PLC control system, which uses a PID intelligent control algorithm to perform closed-loop control of the system operating parameters.

[0012] Furthermore, the system of the present invention also includes a rotor flow meter, which is used to monitor the system's outlet water flow rate in real time.

[0013] Furthermore, the operating differential pressure setting threshold described in this invention adopts an adaptive threshold mechanism, which is dynamically adjusted based on historical system operating data and real-time differential pressure changes, specifically as follows: During system operation, the control unit obtains the transmembrane pressure difference ΔP in real time based on the inlet and outlet water pressure data, and collects the operating flux through the rotor flow meter; calculates the rate of pressure difference change per unit time according to formula (1) and calculates the flux decay rate FDR according to formula (2);

[0014]

[0015] in, This represents the initial flux of the system. The control unit uses a PID-fuzzy composite algorithm to calculate the comprehensive pollution index F according to equation (3):

[0016] Where α, β, and γ are weighting coefficients. and These represent the initial pressure difference and initial growth rate of the system, respectively. The adaptive setpoint for the transmembrane pressure difference is calculated according to equation (4). And based on this, determine the timing of the reversal:

[0017] When satisfied ≥ When any of the following conditions are met, such as FDR≥0.75, the control unit immediately performs membrane flushing and flow direction reversal to form an adaptive closed-loop control.

[0018] Furthermore, the control unit of the present invention is also configured to: automatically control the valve control unit to perform a periodic reversal operation of the water inlet direction based on at least one of a preset time period, a membrane module pressure difference change value, or a percentage decrease in permeate flux as a trigger condition, so that each nanofiltration membrane module in the multi-stage nanofiltration membrane module alternately bears a high pollution load.

[0019] Based on the same inventive concept, the present invention also provides an operation control method for a multi-stage nanofiltration membrane system. This method is based on the aforementioned flip-type nanofiltration membrane filtration system and includes the following steps: S1. Start the water supply pump to deliver the raw water in the raw water tank to the multi-stage nanofiltration membrane module for filtration treatment. The raw water passes through the first nanofiltration membrane module, the second nanofiltration membrane module, the third nanofiltration membrane module, the fourth nanofiltration membrane module and the fifth nanofiltration membrane module in sequence to complete the filtration process. During this process, the system is in normal operation mode and the operating parameters are monitored in real time. S2. The pressure monitoring unit monitors the inlet and outlet pressures of the multi-stage nanofiltration membrane system in real time and calculates the system operating pressure difference. At the same time, based on the operating pressure difference and its changing trend, the membrane fouling status is dynamically assessed, and the operating pressure difference change pattern is analyzed in combination with the system's historical operating data to establish an adaptive threshold judgment basis and enter the flip-over judgment process. S3. When the operating pressure difference reaches the adaptive threshold or meets the flip-over judgment condition, the control unit determines that the flip-over condition is met, the control system stops the filtration operation, and starts the flushing liquid in the membrane flushing water tank to flush the multi-stage nanofiltration membrane module to alleviate membrane fouling. S4. After the membrane flushing is completed, the system pipeline connection method is changed by controlling the valve control unit, so that the filtration direction of the raw water in the multi-stage nanofiltration membrane module is changed. During the reversal process, the control unit adjusts the valve opening and the operating parameters of the feed water pump to gradually change the system flow and pressure, so as to achieve a smooth transition and avoid pressure shock and sudden flow change. After the new flow direction is established, the system enters the stable operation process after the reversal and starts the next round of filtration operation.

[0020] The beneficial effects of this invention are as follows: 1. This invention, by setting up a pressure monitoring unit, valve control unit, and PLC control unit in a multi-stage nanofiltration membrane filtration system, collects inlet and outlet water pressures in real time and calculates the operating pressure difference and its changing trend. Combined with historical system operating data, an adaptive threshold judgment mechanism is established to dynamically assess the membrane fouling status. When the system reaches the flip condition, it automatically performs membrane flushing and inlet water direction switching, and adjusts the valve opening and feed water pump operating parameters during the switching process to achieve a smooth transition. The entire process of fouling assessment, membrane flushing, and flow direction flipping is completed automatically without frequent manual intervention, effectively improving the system's operational stability, water treatment efficiency, and water recovery rate.

[0021] 2. This invention monitors operating parameters in real time and flushes membrane modules promptly, alternating the inlet water direction of multi-stage nanofiltration membrane modules to ensure uniform distribution of membrane fouling among each stage, slowing down the rate of fouling development and avoiding excessive fouling of upstream membrane modules and low utilization of downstream membrane modules. Compared to traditional chemical cleaning, which easily damages membranes and causes secondary pollution, and physical cleaning, which is costly and has short-lasting effects, this invention can significantly reduce cleaning frequency, lower operating and maintenance costs, extend the service life of nanofiltration membrane modules, and ensure long-term, efficient, and stable operation of the system.

[0022] This invention is applicable to nanofiltration membrane water treatment and intelligent operation control of membrane systems. Attached Figure Description

[0023] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0024] Figure 1 This is a schematic diagram of the structure of a flip-type nanofiltration membrane filtration system according to the present invention; Figure 2 The curves showing the normalized membrane flux variation under the flow direction reversal operation and unidirectional filtration conditions described in this invention; Figure 3 The reversible fouling resistance of nanofiltration membrane modules under different operating modes; Figure 4 The irreversible fouling resistance of nanofiltration membrane modules under different operating modes; Figure 5 TOC content (mg / cm³) on the surface of the nanofiltration membrane module under different operating modes 2 ); Figure 6 The ATP content (μg / cm³) on the surface of the nanofiltration membrane under different operating modes. 2 ).

[0025] Among them, 1-raw water tank, 2-first valve, 3-second valve, 4-membrane flushing water tank, 5-feed water pump, 6-third valve, 7-fourth valve, 8-fifth valve, 9-first pressure gauge, 10-PLC control unit, 11-operation terminal, 12-first nanofiltration membrane group, 13-second nanofiltration membrane group, 14-third nanofiltration membrane group, 15-fourth nanofiltration membrane group, 16-fifth nanofiltration membrane group, 17-second pressure gauge, 18-rotameter, 19-product water storage tank, 20-sixth valve, 21-concentrate tank. Detailed Implementation

[0026] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings. The following examples will help those skilled in the art to further understand the present invention, but do not limit the present invention in any way. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention.

[0027] Example 1: This example provides a flip-type nanofiltration membrane filtration system, the specific structure of which includes a multi-stage nanofiltration membrane module, a membrane flushing water tank, a valve control unit, a pressure monitoring unit, and a control unit. The specific arrangement, connection relationship, and working principle of each component are as follows: 1. Multi-stage nanofiltration membrane module: In this embodiment, the multi-stage nanofiltration membrane module uses multiple nanofiltration membrane modules connected in series, namely a first nanofiltration membrane module, a second nanofiltration membrane module, a third nanofiltration membrane module, a fourth nanofiltration membrane module, and a fifth nanofiltration membrane module. Each nanofiltration membrane unit is connected sequentially through pipelines, ensuring that the concentrate produced by the previous nanofiltration membrane unit can be directly used as the feed water for the next nanofiltration membrane unit, realizing progressive deep filtration of the raw water, improving the filtration effect, and increasing the system water recovery rate. The filtration area and retention precision of each nanofiltration membrane unit can be adapted and selected according to the water quality of the raw water to be treated (such as hardness and organic matter content), ensuring effective retention of divalent and polyvalent ions and small molecule organic matter in the raw water.

[0028] 2. Membrane flushing tank: The volume of the membrane flushing tank can be designed according to the total filtration area of ​​the multi-stage nanofiltration membrane modules to ensure that a single flush can completely cover the membrane surface of each stage of the nanofiltration membrane modules. The membrane flushing tank is equipped with an outlet pipeline, which is connected to the flushing interface of the multi-stage nanofiltration membrane modules to supply flushing fluid to each stage of the nanofiltration membrane modules, thereby alleviating membrane fouling and restoring membrane flux.

[0029] 3. Valve Control Unit: The valve control unit includes multiple control valves, which are respectively installed on the inlet main pipe, outlet main pipe, connecting pipes of each stage of nanofiltration membrane modules, and the outlet pipe of the membrane flushing tank. The valves on the inlet and outlet main pipes control the inflow of raw water and the outflow of filtered product water. The valves on the connecting pipes at each stage are used to switch the water flow path, thereby reversing the inflow direction of the multi-stage nanofiltration membrane modules. The valve on the outlet pipe of the membrane flushing tank controls the supply and stop of the flushing solution. All valves are electrically connected to the control unit, which uniformly controls their opening and closing states to ensure a smooth and precise switching process and avoid pressure surges that could damage the membrane modules.

[0030] 4. Pressure Monitoring Unit: The pressure monitoring unit includes two pressure sensors, a first pressure sensor and a second pressure sensor. The first pressure sensor is fixedly installed on the inlet manifold of the multi-stage nanofiltration membrane module to collect the system's inlet water pressure in real time. The second pressure sensor is fixedly installed on the outlet manifold of the multi-stage nanofiltration membrane module to collect the system's outlet water pressure in real time. Both pressure sensors are high-precision pressure transmitters, and their signal output terminals are electrically connected to the signal input terminal of the control unit, enabling the real-time transmission of the collected pressure data to the control unit, providing accurate data support for calculating the system's operating pressure differential.

[0031] 5. Control Unit: The control unit adopts a PLC controller, which is electrically connected to two pressure sensors of all control valve pressure monitoring units of the valve control unit. It has the functions of data reception, calculation, and command output, and its specific work includes at least: (1) Differential pressure calculation: The control unit receives the inlet pressure data transmitted by the first pressure sensor and the outlet pressure data transmitted by the second pressure sensor in real time, calculates the system operating differential pressure through the built-in algorithm, and stores the pressure data and the calculated differential pressure data in real time, which is convenient for subsequent operation status analysis and parameter adjustment.

[0032] (2) Inlet water direction switching control: The control unit dynamically assesses the membrane fouling status based on the system's preset operating parameters and the real-time monitored system operating pressure difference, and analyzes the trend of operating condition changes in conjunction with historical data to establish an adaptive threshold judgment mechanism. When the system operating pressure difference reaches the adaptive threshold or meets the flip judgment condition, the control unit sends a control command to adjust the opening and closing of the corresponding valves to achieve the switching of the inlet water direction of the multi-stage nanofiltration membrane modules. During the inlet water direction switching process, the control unit adjusts the valve opening and the operating parameters of the feed water pump to gradually change the system flow rate and pressure to achieve a smooth transition. After the inlet water direction switching is completed, the raw water enters from the inlet end of the original last-stage nanofiltration membrane module and passes through each stage of the nanofiltration membrane module in sequence to achieve the flip of the inlet water direction. Each stage of the nanofiltration membrane module alternately bears the fouling load, thereby making the membrane fouling evenly distributed among each stage of the nanofiltration membrane module and avoiding the accelerated deterioration of the downstream membrane module due to long-term exposure to high concentrations of pollutants.

[0033] (3) Membrane flushing control: The control unit presets a membrane flushing differential pressure threshold. When the real-time calculated system operating differential pressure reaches the set threshold, it is determined that the membrane module fouling has reached the level that requires flushing. At this time, the control unit sends a command to close the raw water inlet valve and open the valve on the outlet pipeline of the membrane flushing water tank. The flushing liquid in the membrane flushing water tank is then introduced into the multi-stage nanofiltration membrane module under its own gravity or the action of an auxiliary pump (which can be added to increase the flushing pressure) to flush the membrane surface of each stage of the membrane module. After flushing for a certain period of time, the flushing valve is closed and the raw water inlet valve is reopened. The system resumes normal filtration operation. The membrane flux is restored through flushing, the system operating differential pressure is reduced, and the system is ensured to operate stably.

[0034] The flip-type nanofiltration membrane filtration system in this embodiment achieves step-by-step filtration of raw water through the coordinated work of its components. It also solves the problem of uneven distribution of membrane fouling in multi-stage series nanofiltration membrane systems by controlling the flip-type water inlet direction through the control unit. At the same time, the membrane flushing function further alleviates membrane fouling, extends the service life of the membrane module, and improves the overall operating efficiency of the system. Moreover, it has a simple structure, is easy to implement, and is suitable for various application scenarios such as drinking water treatment and industrial wastewater treatment.

[0035] Example 2, in conjunction with the following Figures 1 to 6 The specific implementation of the flip-type nanofiltration membrane filtration system described in this invention is explained in detail. See Figure 1 As shown, the system includes a raw water tank 1, a membrane flushing water tank 4, a feed water pump 5, a valve control unit, a multi-stage nanofiltration membrane assembly, a pressure monitoring unit, a PLC control unit 10, and a rotor flow meter 18, which are used to realize the adaptive control of the multi-stage nanofiltration membrane system.

[0036] In this embodiment, the raw water tank 1 is used to store the raw water to be treated and is connected to the water supply pump 5 through a pipeline, so that the raw water can be transported to the nanofiltration membrane system for filtration under the action of the water supply pump 5.

[0037] The membrane flushing tank 4 is used to store membrane flushing fluid. When the operating pressure difference of the nanofiltration membrane system reaches a set threshold, flushing fluid is supplied to the multi-stage nanofiltration membrane modules through the system pipeline to flush the nanofiltration membrane modules, thereby restoring membrane flux and reducing the operating pressure difference of the system.

[0038] The first nanofiltration membrane group 12, the second nanofiltration membrane group 13, the third nanofiltration membrane group 14, the fourth nanofiltration membrane group 15, and the fifth nanofiltration membrane group 16 are connected in series to form a multi-stage nanofiltration membrane module, which is used to filter raw water step by step. The concentrate produced by the previous stage nanofiltration membrane module is used as the feed water for the next stage nanofiltration membrane module for further filtration.

[0039] In this embodiment, the pressure monitoring unit includes a first pressure gauge 9 and a second pressure gauge 17. The first pressure gauge 9 is located at the inlet end of the multi-stage nanofiltration membrane module and is used to monitor the system inlet pressure. The second pressure gauge 17 is located at the outlet end of the multi-stage nanofiltration membrane module and is used to monitor the system outlet pressure. The PLC control unit 10 is electrically connected to the first pressure gauge 9 and the second pressure gauge 17. By acquiring the system inlet pressure and outlet pressure, it calculates the system operating pressure difference, thereby realizing real-time monitoring of the operating status of the nanofiltration membrane system.

[0040] In this embodiment, the valve control unit includes a first valve 2, a second valve 3, a third valve 6, a fourth valve 7, a fifth valve 8, and a sixth valve 20, which are used to control the on / off of raw water inlet, concentrated water return, backwashing, and system sewage discharge path; In addition, the PLC control unit 10 is also electrically connected to the first valve 2, the second valve 3, the third valve 6, the fourth valve 7, the fifth valve 8 and the sixth valve 20, and is used to control the opening or closing of each valve, thereby realizing the automatic adjustment of the system operation mode.

[0041] In this embodiment, the rotor flowmeter 18 is used to monitor the system's effluent flow rate in real time. The PLC control unit 10 is electrically connected to the rotor flowmeter 18 and calculates the system's operating flux by acquiring the effluent flow rate, thereby realizing real-time monitoring of the nanofiltration membrane system's water production efficiency.

[0042] In this embodiment, an operation terminal 11 is also included, which is used to operate and set parameters of the PLC control unit 10.

[0043] Under normal system operation, open valves 2 (first), 7 (fourth), and 20 (sixth), and close valves 3 (second), 6 (third), and 8 (fifth). At this time, raw water enters the multi-stage nanofiltration membrane module under the action of the feed pump 5, sequentially passing through the first nanofiltration membrane group 12, the second nanofiltration membrane group 13, the third nanofiltration membrane group 14, the fourth nanofiltration membrane group 15, and the fifth nanofiltration membrane group 16 to complete the filtration process. The permeate produced during filtration enters the product water storage tank 19, and the concentrate enters the concentrate tank 21.

[0044] During system operation, the PLC control unit 10 acquires the system operating pressure in real time through the first pressure gauge 9 and the second pressure gauge 17, and calculates the system operating pressure difference in real time. The set threshold for the system operating pressure difference is dynamically determined by the control unit based on an adaptive control strategy. The control unit uses a PID control algorithm, taking the system operating pressure difference (ΔP), the rate of change of pressure difference (dΔP / dt), and the specific flux (J / J0) as control input parameters, to comprehensively judge the degree of membrane fouling, and adjusts the system operating conditions in real time according to parameter changes. When the system meets the flushing conditions, the PLC control unit 10 controls the system to stop filtration operation. The membrane flushing program is started, and flushing liquid is supplied to the multi-stage nanofiltration membrane module through the membrane flushing water tank 4 to flush the nanofiltration membrane module.

[0045] Specifically, the closed-loop control of the system operating parameters using the PID intelligent control algorithm is as follows: During system operation, the control unit obtains the transmembrane pressure difference ΔP in real time based on the inlet and outlet water pressure data, and collects the operating flux through the rotor flow meter; calculates the rate of pressure difference change per unit time per Δt (preferably 30 s) according to formula (1), and calculates the flux decay rate FDR according to formula (2);

[0046]

[0047] in, This represents the initial flux of the system. The control unit uses a PID-fuzzy composite algorithm to calculate the comprehensive pollution index F according to equation (3):

[0048] Wherein, α, β, and γ are weighting coefficients, preferably α:β:γ = 0.4:0.3:0.3, and α+β+γ = 1; and These represent the initial pressure difference and initial growth rate of the system, respectively. The adaptive setpoint for the transmembrane pressure difference is calculated according to equation (4). And based on this, determine the timing of the reversal:

[0049] When satisfied ≥ When any of the following conditions are met, such as FDR≥0.75, the control unit immediately performs membrane flushing and flow direction reversal to form an adaptive closed-loop control.

[0050] After membrane flushing is completed, the PLC control unit 10 controls the relevant valves to switch the system pipelines, thereby changing the flow direction of the raw water in the multi-stage nanofiltration membrane modules. For example, in the next filtration cycle, the first valve 2, the third valve 6, and the fifth valve 8 are opened, while the second valve 3, the fourth valve 7, and the sixth valve 20 are closed. This causes the raw water to pass through the fifth nanofiltration membrane module 16, the fourth nanofiltration membrane module 15, the third nanofiltration membrane module 14, the second nanofiltration membrane module 13, and the first nanofiltration membrane module 12 in the opposite direction to the previous cycle.

[0051] During the flow direction switching process, the PLC control unit 10 employs a buffer control strategy to regulate the system. Before the switching begins, the operating parameters of the feed water pump 5 are adjusted to gradually reduce the system's inlet flow rate and operating pressure to a set transition state. When the system is in a stable transition state, the control valve control unit adjusts the relevant valves in the current operating path, gradually closing the original flow path. Subsequently, the valves in the reverse operating path are gradually opened, allowing raw water to enter the multi-stage nanofiltration membrane module in the opposite direction. After the new flow path is established, the operating status of the feed water pump 5 is further controlled to ensure that the system flow rate and pressure smoothly return to the set operating conditions. Through the above continuous adjustment process, a smooth transition of system flow rate and pressure is achieved, effectively avoiding pressure shocks and sudden changes in flow rate caused by abrupt changes in flow direction, and improving the operational stability and safety of the multi-stage nanofiltration membrane system during the flow direction switching process.

[0052] This embodiment also provides an operation control method for a multi-stage nanofiltration membrane system. The method is based on the above-mentioned multi-stage nanofiltration membrane filtration system and includes the following steps: S1. Start the water supply pump to deliver the raw water in the raw water tank to the multi-stage nanofiltration membrane module for filtration treatment. The raw water passes through the first nanofiltration membrane module, the second nanofiltration membrane module, the third nanofiltration membrane module, the fourth nanofiltration membrane module and the fifth nanofiltration membrane module in sequence to complete the filtration process. During this process, the system is in normal operation mode and the operating parameters are monitored in real time. S2. The pressure monitoring unit monitors the inlet and outlet pressures of the multi-stage nanofiltration membrane system in real time and calculates the system operating pressure difference. At the same time, based on the operating pressure difference and its changing trend, the membrane fouling status is dynamically assessed, and the operating pressure difference change pattern is analyzed in combination with the system's historical operating data to establish an adaptive threshold judgment basis and enter the flip-over judgment process. S3. When the operating pressure difference reaches the adaptive threshold or meets the flip-over judgment condition, the control unit determines that the flip-over condition is met, the control system stops the filtration operation, and starts the flushing liquid in the membrane flushing water tank to flush the multi-stage nanofiltration membrane module to alleviate membrane fouling. S4. After the membrane flushing is completed, the system pipeline connection method is changed by controlling the valve control unit, so that the filtration direction of the raw water in the multi-stage nanofiltration membrane module is changed. During the reversal process, the control unit adjusts the valve opening and the operating parameters of the feed water pump to gradually change the system flow and pressure, so as to achieve a smooth transition and avoid pressure shock and sudden flow change. After the new flow direction is established, the system enters the stable operation process after the reversal and starts the next round of filtration operation.

[0053] Through the above operating mode, the multi-stage nanofiltration membrane system alternately changes the filtration direction in different operating cycles, so that membrane fouling is more evenly distributed among the nanofiltration membrane modules, and the rate of membrane fouling development is slowed down, thereby improving the overall operating efficiency of the system.

[0054] Figure 2 The curves showing the normalized membrane flux changes under flow-direction reversal operation and unidirectional filtration conditions demonstrate that, within the same operating time, the rate of decrease in normalized membrane flux under unidirectional filtration is significantly faster than that under flow-direction reversal operation. Specifically, in unidirectional filtration, as the operating time increases, the membrane flux of the downstream membrane units of the multi-stage nanofiltration membrane module rapidly declines due to the continuous exposure to high concentrations of pollutants. After a period of operation, the normalized membrane flux drops to a low level, requiring frequent flushing to maintain basic filtration efficiency. However, when using the flow-direction reversal operation mode of this invention, because each stage of the nanofiltration membrane unit alternately bears a high fouling load, membrane fouling is evenly distributed among each stage of the module, effectively slowing down the rate of membrane flux decline. Within the same operating cycle, the normalized membrane flux remains at a high level with smaller fluctuations. This curve result fully verifies the technical effect of this invention in effectively slowing down the membrane fouling process and maintaining stable system filtration efficiency by controlling the influent direction reversal, further proving the practicality and advancement of the technical solution of this invention.

[0055] Figure 3 – Figure 6 The distribution characteristics of reversible fouling resistance, irreversible fouling resistance, and TOC and ATP content on the membrane surface of a multi-stage nanofiltration membrane module are presented under both flow-reversing operation and unidirectional filtration conditions. It can be seen that in the unidirectional filtration mode, the fouling distribution of each stage of the nanofiltration membrane module exhibits a clear gradient accumulation characteristic, with the downstream membrane module bearing a higher fouling load. This is manifested by a gradual increase in both reversible and irreversible fouling resistance, and a significant increase in TOC and ATP content on the membrane surface. In contrast, under the flow-reversing operation mode, the fouling load distribution of each stage of the nanofiltration membrane module tends to be more uniform, the difference in reversible and irreversible fouling resistance between different membrane segments is significantly reduced, and the TOC and ATP content on the membrane surface remains at a relatively stable level. The overall membrane fouling degree is also significantly alleviated.

[0056] These results demonstrate that flow-direction reversing operation effectively alters the distribution of pollutants within the membrane module, allowing each membrane unit to alternately bear the fouling load. This prevents excessive accumulation of pollutants in the final membrane module, significantly inhibits the formation of irreversible fouling, and slows down the deposition of organic and biological pollutants on the membrane surface. In summary, flow-direction reversing operation achieves uniform pollutant distribution while effectively mitigating membrane fouling trends, thereby improving the overall system operating efficiency and lifespan.

[0057] In summary, the tilting nanofiltration membrane filtration system and its operation control method described in this embodiment, by setting up a pressure monitoring unit, a valve control unit, and a PLC control system, monitors the operating pressure difference of the multi-stage nanofiltration membrane system in real time, and automatically controls the opening or closing of valves according to changes in the system operating pressure difference and real-time specific flux, thereby realizing the automatic switching of the filtration direction of the nanofiltration membrane system. When the system operating pressure difference reaches a set threshold, the system automatically stops filtration and starts the membrane flushing program. After flushing, the filtration direction is changed and the next round of filtration begins, making the membrane fouling more evenly distributed among the nanofiltration membrane modules, thereby slowing down the development of membrane fouling and improving the system's operational stability and treatment efficiency. At the same time, the multi-stage nanofiltration membrane modules adopt a series operation mode, so that the concentrate produced by the previous stage nanofiltration membrane is used as the feed water for the next stage nanofiltration membrane for further treatment. While ensuring stable system operation, it improves the overall water recovery rate of the system. Therefore, the multi-stage nanofiltration membrane filtration system described in this embodiment has the advantages of high automation, stable operation, high treatment efficiency, and low operation and maintenance costs.

[0058] The above description of the technical solution provided by the present invention through several specific embodiments is intended to highlight the advantages and benefits of the technical solution provided by the present invention. However, the above-described specific embodiments are not intended to limit the present invention. Any reasonable modifications and improvements to the present invention, reasonable combinations of implementation methods and equivalent substitutions based on the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A flip-type nanofiltration membrane filtration system, characterized in that, The system includes a multi-stage nanofiltration membrane module, a membrane flushing water tank, a valve control unit, a pressure monitoring unit, and a control unit; The multi-stage nanofiltration membrane module is used to filter the raw water to be treated in stages. The membrane flushing tank is used to store the membrane flushing solution; The valve control unit is used to control the water flow path of the multi-stage nanofiltration membrane module and to switch the water inlet direction; The pressure monitoring unit includes pressure detection components respectively installed at the inlet and outlet of the multi-stage nanofiltration membrane module, used to collect inlet and outlet pressures; The control unit is electrically connected to the valve control unit and the pressure monitoring unit respectively, and is configured as follows: The control unit receives inlet and outlet water pressure data collected by the pressure monitoring unit, calculates the system operating pressure difference and its changing trend, thereby dynamically assessing the membrane fouling status; it analyzes the changing pattern of operating pressure difference based on the system's historical operating data and establishes an adaptive threshold determination mechanism. When the system operating conditions reach the reversal condition, the control valve control unit switches the water inlet direction of the multi-stage nanofiltration membrane module, so that the membrane fouling is evenly distributed among the nanofiltration membrane modules. During the water inlet direction switching process, the system flow rate and pressure are gradually changed by adjusting the valve opening or the operating parameters of the feed water pump to achieve a smooth transition.

2. The flip-type nanofiltration membrane filtration system according to claim 1, characterized in that, The multi-stage nanofiltration membrane assembly includes a first nanofiltration membrane assembly (12), a second nanofiltration membrane assembly (13), a third nanofiltration membrane assembly (14), a fourth nanofiltration membrane assembly (15), and a fifth nanofiltration membrane assembly (16) arranged in series, for filtering raw water step by step, with the concentrated water produced by the previous stage nanofiltration membrane assembly serving as the feed water for the next stage nanofiltration membrane assembly.

3. The flip-type nanofiltration membrane filtration system according to claim 1, characterized in that, The pressure monitoring unit includes a first pressure gauge (9) installed at the inlet end of the multi-stage nanofiltration membrane module and a second pressure gauge (17) installed at the outlet end, used to monitor and acquire system operating pressure data.

4. The flip-type nanofiltration membrane filtration system according to claim 1, characterized in that, The system also includes a raw water tank (1) and a water supply pump (5); The raw water tank (1) is used to store raw water to be treated; The water pump (5) is located between the raw water tank (1) and the multi-stage nanofiltration membrane assembly, and is used to drive the raw water into the filtration system.

5. The flip-type nanofiltration membrane filtration system according to claim 1, characterized in that, The system also includes an operation terminal (11), which includes a human-machine interface for users to manually set and adjust the preset operating parameters, thresholds and flip cycles of the control unit.

6. The flip-type nanofiltration membrane filtration system according to claim 1, characterized in that, The control unit is a PLC control system, which uses a PID intelligent control algorithm to perform closed-loop control of the system operating parameters.

7. The flip-type nanofiltration membrane filtration system according to claim 1, characterized in that, The system also includes a rotor flow meter (18) for real-time monitoring of the system's effluent flow rate.

8. The flip-type nanofiltration membrane filtration system according to claim 7, characterized in that, The set threshold for operating differential pressure adopts an adaptive threshold mechanism, which is dynamically adjusted based on historical system operating data and real-time differential pressure changes. Specifically: During system operation, the control unit obtains the transmembrane pressure difference ΔP in real time based on the inlet and outlet water pressure data, and collects the operating flux through the rotor flow meter; calculates the rate of pressure difference change per unit time according to formula (1) and calculates the flux decay rate FDR according to formula (2); in, This represents the initial flux of the system. The control unit uses a PID-fuzzy composite algorithm to calculate the comprehensive pollution index F according to equation (3): Where α, β, and γ are weighting coefficients. and These represent the initial pressure difference and initial growth rate of the system, respectively. The adaptive setpoint for the transmembrane pressure difference is calculated according to equation (4). And based on this, determine the timing of the reversal: When satisfied ≥ When any of the following conditions are met, such as FDR≥0.75, the control unit immediately performs membrane flushing and flow direction reversal to form an adaptive closed-loop control.

9. The flip-type nanofiltration membrane filtration system according to claim 1, characterized in that, The control unit is also configured to automatically control the valve control unit to perform a periodic reversal operation of the inlet direction based on at least one of a preset time period, a membrane module pressure difference change value, or a decrease in permeate flux as a trigger condition, so that each nanofiltration membrane module in the multi-stage nanofiltration membrane module alternately bears a high pollution load.

10. A method for operating and controlling a multi-stage nanofiltration membrane system, the method being implemented based on the flip-type nanofiltration membrane filtration system according to any one of claims 1-9, characterized in that, Includes the following steps: S1. Start the water supply pump to deliver the raw water in the raw water tank to the multi-stage nanofiltration membrane module for filtration treatment. The raw water passes through the first nanofiltration membrane module, the second nanofiltration membrane module, the third nanofiltration membrane module, the fourth nanofiltration membrane module and the fifth nanofiltration membrane module in sequence to complete the filtration process. During this process, the system is in normal operation mode and the operating parameters are monitored in real time. S2. The pressure monitoring unit monitors the inlet and outlet pressures of the multi-stage nanofiltration membrane system in real time and calculates the system operating pressure difference. At the same time, based on the operating pressure difference and its changing trend, the membrane fouling status is dynamically assessed, and the operating pressure difference change pattern is analyzed in combination with the system's historical operating data to establish an adaptive threshold judgment basis and enter the flip-over judgment process. S3. When the operating pressure difference reaches the adaptive threshold or meets the flip-over judgment condition, the control unit determines that the flip-over condition is met, the control system stops the filtration operation, and starts the flushing liquid in the membrane flushing water tank to flush the multi-stage nanofiltration membrane module to alleviate membrane fouling. S4. After the membrane flushing is completed, the system pipeline connection method is changed by controlling the valve control unit, so that the filtration direction of the raw water in the multi-stage nanofiltration membrane module is changed. During the reversal process, the control unit adjusts the valve opening and the operating parameters of the feed water pump to gradually change the system flow and pressure, so as to achieve a smooth transition and avoid pressure shock and sudden flow change. After the new flow direction is established, the system enters the stable operation process after the reversal and starts the next round of filtration operation.

Images