Method and apparatus for evaluating membrane fouling index

Abstract

The present invention relates to the technical field of water treatment, specifically to a method and device for evaluating membrane fouling. The existing methods for evaluating membrane fouling have insufficient mechanisms and multiple technical problems. The method and device for evaluating membrane fouling disclosed in the present invention can accurately evaluate the membrane fouling condition. Further, the method of the present invention can be used for different membrane materials and structures, and is also feasible under different test conditions, including different pressures, temperatures and flow rates. The present invention also includes corrections for various parameters such as temperature, viscosity and membrane area, thereby improving the accuracy and reliability of the evaluation. The method and related device of the present invention solve multiple technical problems in the prior art and improve the accuracy and reliability of membrane fouling evaluation. This invention is expected to have wide and beneficial applications in the fields of water resource management, wastewater treatment and environmental protection.

Description

Technical Field

[0001] This invention relates to the field of water treatment technology, specifically to a method and equipment for evaluating membrane fouling index. Background Technology

[0002] Membrane separation technology has broad application prospects in seawater desalination, pure water production, high-quality reclaimed water production, and industrial wastewater treatment due to its efficient removal of various impurities in water (including suspended particulate matter, colloids, and dissolved substances). However, membrane separation technology faces the problem of membrane fouling, which directly affects the economics and sustainability of the technology. Therefore, rapid and accurate measurement of the fouling potential of the feed membrane is crucial for effective control of membrane fouling.

[0003] The American Society for Testing and Materials (ASTM) developed the Sludge Density Index (SDI), one of the earliest and most commonly used fouling indices. The primary purpose of the SDI is to predict the membrane fouling potential of particulate matter in the feed water to membrane devices. While the SDI provides useful information for membrane separation processes to some extent, it has several significant limitations. First, the SDI is based on empirical values ​​rather than specific filtration mechanisms. Second, the SDI focuses primarily on particulate matter measurement, with limited effectiveness in detecting and quantifying colloids and dissolved substances. Third, the SDI does not adjust for changes in key factors such as pressure, temperature, viscosity, and membrane area, which can lead to inaccurate membrane fouling assessments. Finally, the SDI is not sensitive enough to feed water with high fouling potential, and therefore cannot accurately predict the fouling nature of the feed water to membrane devices.

[0004] Furthermore, to improve the fouling index, the ASTM D8002-15 standard proposed a fouling index model based on microfiltration (MFI) membrane sheets (0.45 μm pore size, 47 mm diameter), such as MFI-0.45. This model relies on the cake filtration mechanism and makes corrections for temperature, viscosity, and membrane area. Although MFI-0.45 represents an improvement over SDI in some aspects, it still has several issues. First, MFI-0.45 still employs dead-end filtration, while in reality, the number, size, and shape of particles in the influent sample can vary greatly, meaning the cake filtration mechanism is not necessarily the sole dominant filtration mechanism. In many cases, membrane pore clogging and / or cake compression mechanisms may coexist, causing interference to the MFI-0.45 measurement.

[0005] Existing methods or equipment for evaluating membrane fouling have the following problems. Both SDI and MFI-0.45 employ dead-end filtration measurement methods rather than being based on a specific filtration mechanism. This may be inaccurate in practical applications because different water samples may exhibit different filtration mechanisms. SDI and MFI-0.45 primarily focus on particulate matter measurement, limiting their effectiveness in detecting and quantifying colloids and dissolved substances. In real-world water samples, various types of contaminants may be present, necessitating a more comprehensive fouling index to account for different contaminant types. Furthermore, SDI and MFI-0.45 do not correct for variations in pressure, temperature, viscosity, and membrane area, potentially leading to errors under different conditions.

[0006] Furthermore, SDI and MFI-0.45 lack sufficient sensitivity when dealing with feed water with high fouling potential, and cannot accurately assess the fouling status of the membrane unit's feed water. Additionally, the SDI and MFI-0.45 evaluation methods are designed for water containing air bubbles, making them highly susceptible to permeate flux deviations caused by bubble accumulation on the membrane surface. This is primarily because slightly larger bubbles cannot pass through hydrophilic membrane sheets or modules, and their accumulation on the membrane surface increases filtration resistance and reduces the contact area between the test water and the membrane, resulting in significant fluctuations in the final evaluation value and failing to reflect the true membrane fouling index.

[0007] In view of the above problems, it is necessary to propose a more advanced method and equipment to more accurately and comprehensively assess the membrane fouling potential of the feed water of membrane devices, so as to meet the actual needs of membrane device processes in different application fields, especially when treating highly difficult wastewater, improve their performance and reliability, reduce operating costs, and achieve sustainable water resource management. Summary of the Invention

[0008] To address the shortcomings of existing technologies, this invention provides a method and apparatus for evaluating membrane fouling index, thereby further promoting and applying membrane technology. The method of this invention is not bound by existing theoretical frameworks, allowing for accurate reflection of membrane fouling conditions, unaffected by issues such as membrane pore blockage and / or filter cake compression and / or the presence of microbubbles in the solution.

[0009] The CMFI mentioned in this invention refers to the Cross Mambrance Fouling Index (CMFI) of cross-flow filtration.

[0010] The CMFI described in this invention adopts a cross-flow filtration mode. The filtrate permeating the membrane is metered online by a metering device, and the data is transmitted to a data recording device. By continuously recording the volume V of the filtered filtrate, or the volume V converted from filtrate mass or flow rate, and the corresponding filtration time t, a t / V - V curve is plotted and linearly fitted to determine whether it conforms to the cake filter model. The minimum value of this slope tanα is the uncorrected CMFI.

[0011] Furthermore, to achieve the goal of accurately evaluating membrane fouling, the present invention provides the following technical solution: In a first aspect, a method for evaluating the membrane fouling index (CMFI) is provided, comprising the following steps: S1. Fill the membrane module with the liquid to be tested, ensuring that there are no air bubbles between the liquid to be tested and the membrane module; S2. Start the data recording and storage device and enter one or more of the following recording parameters: recording time interval, filtered liquid volume, flow rate, mass, filtration time, test temperature or pressure; S3. Control the liquid to be measured to maintain the transmembrane pressure difference within the range of 0.05 - 0.6 MPa and / or the membrane surface flow velocity within the range of 0.05 - 1 m / s; S4. Record the curve of the flow rate or mass of the liquid passing through the membrane over time; S5. Collect the filtrate passing through the membrane, plot the time-volume ratio and volume graph, perform linear fitting to obtain curve L, and calculate the slope (tanα) of each point on the curve; S6. Create a graph of slope (tanα) versus time, and determine the minimum value of slope (tanα). This minimum value is the value of the membrane fouling index (CMFI). S7. Based on different test conditions, apply the following formula to correct the obtained CMFI value:

[0012] In the formula, This indicates the corrected membrane fouling index; The numerical value representing the slope is taken as the minimum value described in S6; This indicates the viscosity of the liquid being tested at 20°C. This indicates the viscosity of the liquid being tested at the test temperature. This represents the transmembrane pressure difference under test conditions; This represents the transmembrane pressure difference under specified conditions. Indicates the effective membrane area of ​​the test membrane; This indicates the membrane area to be used; This indicates the pure water flux of the test membrane under the test conditions; This indicates the pure water flux of the membrane under specified conditions; Used to correct the results obtained under test membrane conditions to the membrane fouling index value based on the set membrane.

[0013] Furthermore, the recording parameters include a recording time interval between 0.01-2 seconds, 2-10 seconds, 10-120 seconds, or 60-300 seconds.

[0014] Furthermore, the transmembrane pressure tested is in the range of 0.05 - 0.6 MPa.

[0015] Furthermore, the membrane can be one or more of ceramic membrane, organic membrane, metal membrane, inorganic membrane or composite membrane of multiple materials, and the membrane structure can be one or more of tubular single-channel, tubular multi-channel, hollow fiber, flat plate, etc.

[0016] Furthermore, the membrane pore size can be between 1 and 50 nm, or between 20 and 50 nm, or between 50 and 2000 nm.

[0017] Furthermore, the effective test membrane area of ​​the membrane module is between 0.005 and 0.05 m². 2 0.01 - 0.25 m 2 or 0.25 - 30 m 2 between.

[0018] A second aspect of the present invention provides an apparatus for evaluating membrane fouling index (CMFI), employing the above-described method for evaluating membrane fouling index (CMFI), wherein the apparatus for evaluating membrane fouling index (CMFI) comprises: a.) The first pump assembly is capable of providing a test pressure of at least 0.6 MPa for liquid supply or cleaning; b.) Membrane module, composed of membranes, including inlet, recirculation outlet and product water outlet; c.) Metering devices that can accurately measure the mass or flow rate of fluids and continuously record and transmit data; d.) Data recording and storage device for real-time recording and storage of the recorded parameters of the evaluation method.

[0019] Furthermore, the equipment for evaluating CMFI membrane fouling also includes: e.) A second pump assembly capable of providing a liquid pressure of at least 0.6 MPa for cleaning or supplying liquid; f.) Circulation side pressure gauge, used to detect the pressure of the circulating water; h.) An electric ball valve is used to switch between the first pump assembly and the second pump assembly, thereby enabling the cleaning and liquid supply process. i.) Temperature sensor, used to measure the temperature of liquids; j.) Pressure reducing valve, used to regulate a constant pressure when liquid enters the membrane module to ensure the stability of the test; k.) Exhaust valve, used to remove excess gas from the system to ensure the normal operation of the system; l.) A pressure gauge before the membrane is used to detect the pressure of the liquid entering the membrane; m.) The first water tank is used to store the water sample to be tested or the cleaning solution; n.) Waste liquid tank, used as a storage container for waste liquid; o .) The second water tank is used to store the water sample to be tested or the cleaning solution. Beneficial effects

[0020] This invention introduces a novel method for evaluating membrane fouling index. Compared with existing methods, the method described in this invention adopts membrane operation under actual operating conditions, thus reflecting membrane fouling more realistically and accurately. This improves the reliability and performance of membrane fouling assessment.

[0021] The evaluation method described in this invention solves several technical problems existing in current methods, such as interference from membrane pore blockage and filter cake compression. This provides a solution for the further promotion and application of membrane technology, and is expected to improve membrane performance and service life.

[0022] Furthermore, this invention fully considers the special case of liquids containing micro / nano bubbles. In this situation, the presence of bubbles may lead to adsorption and aggregation on the membrane surface, thereby changing the effective filtration area of ​​the membrane. This invention employs a cross-flow filtration evaluation method, where the liquid has a flow velocity on the membrane surface, preventing the aggregation and adsorption of hydrophobic bubbles on the membrane surface. This allows for accurate assessment of membrane fouling regardless of the presence of micro / nano bubbles in the liquid, improving the accuracy of the assessment.

[0023] The method and related equipment described in this invention provide a more accurate and reliable method for evaluating membrane fouling in the field of water treatment, which is expected to improve the performance of membrane technology and the service life of membranes, and has broad application prospects. Attached Figure Description

[0024] Figure 1 The membrane fouling index evaluation method of the present invention shows a graph of tanα calculated from the time-volume (t / V) versus volume (V) curve; Where: Region A represents the stage in the membrane fouling process where membrane pore blockage is the dominant behavior; Region B represents the stage in the membrane fouling process where the filter cake layer's filtration behavior is dominant. Region C represents the stage in the membrane fouling process where filter cake compression is the dominant behavior. Figure 2 This is a tanα curve of the membrane fouling index evaluation method of the present invention; Figure 3 This is a schematic diagram of the structure of a device for evaluating membrane fouling index (CMFI) containing two sets of pumps according to the present invention. Figure 4 This is a schematic diagram of the structure of a device for evaluating membrane fouling index (CMFI) containing a set of pumps according to the present invention. in Figure 3 , Figure 4 middle: a.) indicates the first pump assembly; b.) indicates a membrane module; c.) indicates a measuring device; d.) indicates a data recording and storage device; e.) indicates the second pump assembly; f.) indicates the pressure gauge on the circulating side; h.) indicates an electric ball valve; i.) indicates a temperature sensor; j.) indicates a pressure reducing valve; k.) indicates the exhaust valve; l.) indicates the pressure gauge before the membrane; m.) indicates the first water tank; n.) represents a waste liquid container; o .) indicates the second water tank. Detailed Implementation

[0025] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0026] Please see Figure 1-2 The present invention provides a method for evaluating membrane fouling index, comprising the following steps: S1. Fill the membrane module with the liquid to be tested, ensuring that there are no air bubbles between the liquid to be tested and the membrane module; S2. Start the data recording and storage device and enter one or more of the following recording parameters: recording time interval, filtered liquid volume, flow rate, mass, filtration time, test temperature or pressure; S3. Control the liquid to be measured to maintain the transmembrane pressure difference within the range of 0.05 - 0.6 MPa and / or the membrane surface flow velocity within the range of 0.05 - 1 m / s; S4. Record the curve of the flow rate or mass of the liquid passing through the membrane over time; S5. Collect the filtrate passing through the membrane, plot the time-volume ratio and volume graph, perform linear fitting to obtain curve L, and calculate the slope (tanα) of each point on the curve; S6. Create a graph of slope (tanα) versus time, and determine the minimum value of slope (tanα). This minimum value is the value of the membrane fouling index (CMFI). S7. Based on different test conditions, apply the following formula to correct the obtained CMFI value:

[0027] This indicates the corrected membrane fouling index; The numerical value representing the slope is taken as the minimum value described in S6; This indicates the viscosity of the liquid being tested at 20°C. This indicates the viscosity of the liquid being tested at the test temperature. This represents the transmembrane pressure difference under test conditions; This represents the transmembrane pressure difference under specified conditions. Indicates the effective membrane area of ​​the test membrane; This indicates the membrane area to be used; This indicates the pure water flux of the test membrane under the test conditions; This indicates the pure water flux of the membrane under specified conditions; Used to correct the results obtained under test membrane conditions to the membrane fouling index value based on the set membrane.

[0028] Furthermore, the recording parameters include a recording time interval between 0.01-2 seconds, 2-10 seconds, 10-120 seconds, or 60-300 seconds.

[0029] Furthermore, the transmembrane pressure tested is in the range of 0.05 - 0.6 MPa.

[0030] Furthermore, the membrane can be one or more of ceramic membrane, organic membrane, metal membrane, inorganic membrane or composite membrane of multiple materials, and the membrane structure can be one or more of tubular single-channel, tubular multi-channel, hollow fiber, flat plate, etc.

[0031] Furthermore, the membrane pore size can be between 1 and 50 nm, or between 20 and 50 nm, or between 50 and 2000 nm.

[0032] Furthermore, the effective test membrane area of ​​the membrane module is between 0.005 and 0.05 m². 2 0.01 - 0.25 m 2 or 0.25 - 30 m 2 between.

[0033] Example 1: According to the above-mentioned method for evaluating CMFI membrane fouling, the primary reverse osmosis concentrate of coking wastewater was taken as the test sample. The membrane module used was a hollow fiber membrane with a pore size of 20 nm and a membrane area of ​​0.03 m². 2 The membrane has a pure water flux of 13.2 mL / s; The pure water flux of the membrane module was tested with pure water and found to be 13.2 mL / s, confirming that the membrane module was clean. The liquid to be tested was placed in the water tank, the inlet was placed in the water tank, and the concentrate outlet, the desalination outlet and the exhaust port were placed in the waste liquid tank. Turn on the water supply pump, and the liquid to be tested will fill the pipeline and membrane module. Open the vent valve, and after venting, close the vent valve and turn off the water supply pump. Turn on the computer, input the parameters required for data recording, with a time interval of 2 seconds. The computer automatically records the volume and mass of the filtered filtrate, the filtration time, the solution temperature of 28℃, and the test pressure of 0.1 MPa. The viscosity of the solution at 28℃ was measured to be 1.80 × 10⁻⁶. -3 N·s / m 2 The viscosity of the solution at 20℃ is 1.94 × 10⁻⁶. -3 N·s / m 2 .

[0034] Turn on the water supply pump, adjust the pressure reducing valve and regulating valve to control the transmembrane pressure difference at 0.1 MPa, and at the same time control the membrane surface flow velocity at 0.16 m / s; Once the membrane permeate flow rate reaches 4.0 g / s and the curve derivative is 0, stop recording and turn off the water supply pump. Process the collected data: Convert the mass of the collected filtrate into volume V; Plot a t / VV graph based on time and volume, calculate tanα at each point, plot tanα against time, and determine tanα. min = 33.2 s / L 2 ; Example

[0035] The coking wastewater was used as permeate from the MBR membrane. Membrane module B used a hollow fiber membrane with a pore size of 50 nm and a membrane area of ​​0.03 m². 2 The membrane has a pure water flux of 15.6 mL / s; The pure water flux of the membrane module was tested with pure water and found to be 15.6 mL / s, confirming that the membrane module was clean. Place the liquid to be tested into the water tank, place the inlet into the water tank, and place the concentrate outlet, the desalination outlet, and the vent into the waste liquid container. Turn on the water supply pump, and the liquid to be tested will fill the pipeline and membrane module. Open the vent valve, and after venting, close the vent valve and turn off the water supply pump. Turn on the computer, input the parameters required for data recording, with a time interval of 2 seconds. The computer automatically records the volume and mass of the filtered filtrate, the filtration time, the solution temperature of 25℃, and the test pressure of 0.1 MPa. The measured viscosity of the solution at 25℃ is 1.86 × 10⁻⁶. -3 N·s / m 2 The viscosity of the solution at 20℃ is 1.90 × 10⁻⁶. -3 N·s / m 2 ; Turn on the water supply pump, adjust the pressure reducing valve and regulating valve to control the transmembrane pressure difference at 0.1 MPa, and at the same time control the membrane surface flow velocity at 0.22 m / s; When the membrane permeate flow rate reaches 5.2 g / s, the curve derivative is 0. Stop recording and turn off the water supply pump. Process the collected data: Convert the mass of the collected filtrate into volume V; Plot a t / VV graph based on time and volume, calculate tanα at each point, plot tanα against time, and determine tanα. min = 56 s / L 2 ; The membrane module uses a hollow fiber membrane with a pore size of 20 nm and a membrane area of ​​0.03 m². 2 The membrane has a pure water flux of 13.2 mL / s; Correction was applied to the CMFI measured on membrane B:

[0036] That is, using membrane A as the reference membrane, the solution to be tested... 68.1 s / L 2 .

[0037] Based on the same inventive concept, the present invention also provides a device for evaluating CMFI membrane fouling, which employs the above-mentioned method for evaluating CMFI membrane fouling. The device for evaluating CMFI membrane fouling includes: a.) The first pump assembly is capable of providing a test pressure of at least 0.6 MPa for liquid supply; b.) Membrane module, composed of membranes, including inlet, recirculation outlet and product water outlet; c.) Metering devices that can accurately measure the mass or flow rate of fluids and continuously record and transmit data; d.) Data recording and storage device for real-time recording and storage of the recorded parameters of the evaluation method.

[0038] Furthermore, the equipment for evaluating CMFI membrane fouling also includes: e.) A second pump assembly capable of providing a liquid pressure of at least 0.6 MPa for cleaning; f.) Circulation side pressure gauge, used to detect the pressure of the circulating water; h.) An electric ball valve is used to switch between the first pump assembly and the second pump assembly, thereby enabling the cleaning and liquid supply process. i.) Temperature sensor, used to measure the temperature of liquids; j.) Pressure reducing valve, used to regulate a constant pressure when liquid enters the membrane module to ensure the stability of the test; k.) Exhaust valve, used to remove excess gas from the system to ensure the normal operation of the system; l.) A pressure gauge before the membrane is used to detect the pressure of the liquid entering the membrane; m.) The first water tank is used to store the water sample or cleaning solution to be tested; n.) Waste liquid tank, used as a storage container for waste liquid; o .) The second water tank is used to store the water sample to be tested or the cleaning solution.

[0039] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0040] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for evaluating membrane fouling index, characterized in that, The cross-flow filtering method includes the following steps: S1. Fill the membrane module with the liquid to be tested, ensuring that there are no air bubbles between the liquid to be tested and the membrane module; S2. Start the data logging device and input one or more of the following recording parameters: recording time interval, filtered liquid volume, flow rate, mass, filtration time, test temperature or pressure; S3. Control the liquid to be measured to maintain the transmembrane pressure difference within the range of 0.05 - 0.6 MPa and the membrane surface flow velocity within the range of 0.05 - 1 m / s; S4. Record the curve of the flow rate or mass of the liquid passing through the membrane over time; S5. Collect the filtrate passing through the membrane, plot a time-volume ratio versus volume graph to obtain curve L, and calculate the slope at each point on the curve. ; S6. Create slope Determine the slope using a time-based graph. The minimum value of this value is the membrane fouling index. The value; S7. Based on different test conditions, apply the following formula to the obtained... The value is corrected: In the formula, This indicates the corrected membrane fouling index; The numerical value representing the slope is taken as the minimum value described in S6; This indicates the viscosity of the liquid being tested at 20°C. This indicates the viscosity of the liquid being tested at the test temperature. This represents the transmembrane pressure difference under test conditions; This represents the transmembrane pressure difference under specified conditions. Indicates the effective membrane area of ​​the test membrane; This indicates the membrane area to be used; This indicates the pure water flux of the test membrane under the test conditions; This indicates the pure water flux of the membrane under specified conditions; Used to correct the results obtained under test membrane conditions to the membrane fouling index value based on the set membrane; The membrane pore size is between 1 - 50 nm, 20 - 50 nm, or 50 - 2000 nm.

2. The method for evaluating membrane fouling index according to claim 1, characterized in that, The recording parameters include a recording time interval between 0.01-2 seconds, 2-10 seconds, 10-120 seconds, or 60-300 seconds.

3. The method for evaluating membrane fouling index according to claim 1, characterized in that, The membrane can be an organic or inorganic membrane, and its structure can be one or more of the following: tubular single-channel, tubular multi-channel, hollow fiber, or flat sheet.

4. The method for evaluating membrane fouling index according to claim 1, characterized in that, The effective test membrane area of ​​the membrane module is 0.005 - 0.05 m². 2 0.01 - 0.25 m 2 or 0.25 - 30 m 2 between.

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