Method for evaluating the degree of clogging and fouling of ultrafiltration membranes

Abstract

The present invention provides an evaluation method for the degree of clogging and contamination of ultrafiltration membranes, which belongs to the field of membrane water treatment technology. The evaluation method uses the UF normalized membrane permeability reduction ratio (R i ) as an assessment indicator of membrane clogging and fouling, categorized into three levels of fouling, this method eliminates the influence of factors such as temperature and sludge concentration on membrane permeability changes, objectively and directly assessing membrane permeability and, consequently, determining the degree of membrane clogging and fouling. This provides a basis for developing and optimizing UF membrane cleaning and maintenance programs, facilitating the scientific and rational determination of membrane cleaning and maintenance cycles, improving membrane operating efficiency, and extending membrane service life. Furthermore, this method establishes a rapid increase in the percentage decrease in UF normalized membrane permeability as a characteristic indicator of severe membrane clogging and irreversible fouling, enabling an effective, accurate, and rapid assessment of the membrane's current condition.

Description

Technical Field

[0001] The invention belongs to the technical field of membrane water treatment and relates to a method for evaluating the blockage and contamination degree of an ultrafiltration membrane. Background Art

[0002] Ultrafiltration (UF) membranes typically have pore sizes between 1 and 100 nm. Under certain operating pressures, they can retain colloids, macromolecular organic matter, bacteria, and more. UF membrane types include hollow fiber membranes, plate membranes, and tubular membranes. Hollow fiber membranes offer advantages such as high processing loads, high mechanical strength, and excellent pollution resistance. Pressure-type hollow fiber ultrafiltration membranes are widely used in seawater desalination projects and tap water deep treatment projects.

[0003] According to the "2021 National Seawater Utilization Report" released by the Ministry of Natural Resources, as of the end of 2021, there were 144 desalination projects in China, with a total capacity of 1,856,433 cubic meters per day. Of these, 126 projects utilized reverse osmosis technology, with a capacity of 1,228,803 cubic meters per day, accounting for 66.19% of the total project capacity. Ultrafiltration (UF) is a key pretreatment process for reverse osmosis (RO), directly impacting the production capacity and water quality of desalination projects. UF can intercept suspended matter, colloids, and microorganisms in raw seawater, reducing turbidity and the silt contamination index (SDI), ensuring stable RO influent quality, improving RO membrane flux, mitigating membrane fouling, and extending RO cleaning cycles.

[0004] According to the "2021 Urban and Rural Construction Statistical Yearbook" published by the Ministry of Housing and Urban-Rural Development, my country's urban water supply reached 317.3767 million cubic meters per day by the end of 2021. The new version of the "Standard for Drinking Water Quality" (GB 5749-2022), which will come into effect on April 1, 2023, imposes more targeted and higher standards on drinking water quality and water purification processes within the domestic water supply industry. UF, a third-generation water purification process, effectively removes microorganisms and colloids from water. It is increasingly being used in new construction and upgrading of water plants, playing a vital role in ensuring the safety of urban water supply quality.

[0005] However, UF applications present a series of challenges, including long-term membrane clogging and contamination, resulting in reduced water production rates and high cleaning and maintenance costs. Membrane cleaning and maintenance are crucial aspects of UF process operation, encompassing physical cleaning, maintenance, and restorative chemical cleaning. Currently, UF membrane cleaning and maintenance plans are primarily determined by process technicians based on changes in parameters such as membrane flux and transmembrane pressure differential. These factors, in addition to the degree of membrane clogging and contamination, also include inlet water temperature and turbidity. For example, within a certain temperature range, membrane flux increases with increasing temperature, while transmembrane pressure differential decreases accordingly. Similarly, as turbidity increases, membrane flux decreases and transmembrane pressure differential increases.

[0006] Due to the lack of an objective assessment method for membrane clogging and fouling, it is impossible to accurately formulate and optimize cleaning and maintenance plans based on the degree of membrane clogging and fouling, resulting in excessive or inadequate membrane cleaning frequency and poor cleaning results. Therefore, establishing a UF membrane clogging and fouling assessment method to predict and evaluate the degree of membrane clogging and fouling, and thus accurately guide membrane cleaning and maintenance, is of great significance for improving UF treatment efficiency and ensuring the quality and quantity of produced water. Summary of the Invention

[0007] The present invention provides a method for evaluating the degree of clogging and contamination of ultrafiltration membranes. The method uses the UF normalized membrane permeability reduction ratio R i As an evaluation indicator of membrane blockage and pollution degree, it can objectively and directly evaluate the water permeability of the membrane, and then judge the degree of membrane blockage and pollution, which is conducive to scientifically and reasonably determining the membrane cleaning and maintenance cycle, improving membrane operating efficiency, and extending membrane service life.

[0008] In order to achieve the above object, the present invention provides a method for evaluating the degree of clogging and contamination of an ultrafiltration membrane, comprising the following steps:

[0009] Calculate the temperature normalization coefficient A of UF membrane permeability and the turbidity normalization coefficient B;

[0010] Calculate the actual permeability L of the current UF membrane i , using the temperature normalization coefficient A and turbidity normalization coefficient B to calculate the current UF membrane normalized membrane permeability L in =L i ×[1+(T i -20)×A] -1 ×[1+(C i -7)×B] -1 ;

[0011] Calculate the normalized membrane permeability L of the current UF membrane in Compared with the normalized permeability L of the standard UF membrane within a certain period 0a The decrease ratio R i ;

[0012] By R i The range of values ​​evaluates the degree of UF membrane clogging and fouling.

[0013] Preferably, the temperature normalization coefficient A of the UF membrane water permeability is calculated by the following method:

[0014] The turbidity of UF inlet water was controlled at 7NTU, and 5-8 temperature conditions were averaged in the temperature range of 5-30℃ to test the membrane flux J of UF membrane at different temperatures. i and transmembrane pressure difference ΔP i , calculate the actual membrane permeability L i =Ji / ΔP i ;

[0015] L i is the dependent variable, (T i -20) as the independent variable to perform linear fitting and obtain the approximate linear fitting formula L i =L0+L0×(T i -20)×A, and then calculate the temperature normalization coefficient A of the UF membrane permeability, the unit is ℃ -1 ; Among them, T i represents the temperature value collected by the online instrument, in °C; L0 is the UF normalized membrane permeability, in L·m -2 ·h -1 KPa -1 .

[0016] Preferably, the UF membrane water permeability turbidity normalization coefficient B is calculated by the following method:

[0017] The UF inlet water temperature was controlled at 20°C, and 5-8 turbidity conditions were averaged in the turbidity range of 1-15 NTU to test the flux J of the UF membrane under different turbidities. i and transmembrane pressure difference ΔP i , calculate the actual membrane permeability L i ;

[0018] L i is the dependent variable, (C i -7) is the independent variable for linear fitting, and the approximate linear fitting formula L is obtained. i =L0+L0×(C i -7)×B, and then calculate the UF membrane permeability turbidity normalization coefficient B, the unit is NTU -1 ;

[0019] Among them, C i represents the turbidity value collected by the online instrument, in NTU; L0 is the UF normalized membrane permeability, in L·m -2 ·h -1 KPa -1 .

[0020] As a preference, the standard permeability of the standard UF membrane under normalized conditions within a certain period is L 0a Specifically, it is the average value of the UF normalized membrane permeability L0 after each restorative chemical cleaning in a certain period.

[0021] Preferably, the UF normalized membrane permeability L0 is calculated by the following method:

[0022] Taking the UF inlet water temperature of 20℃ and turbidity of 7NTU as normalization conditions, the UF normalized membrane permeability was measured as L0=J0 / ΔP0;

[0023] Among them, J0 and ΔP0 represent the membrane flux and transmembrane pressure difference measured after each restorative chemical cleaning, respectively, and only the average value of the normalized membrane permeability after each cleaning can represent the performance of the UF membrane before pollution and clogging.

[0024] Preferably, the certain period is 2-3 months.

[0025] As a preferred option, the current UF membrane normalized membrane permeability reduction ratio R i The calculation formula is as follows:

[0026] Reduction ratio R i =(L 0a -L in ) / L 0a ×100%.

[0027] As a preference, by R i The range of values ​​for evaluating the degree of UF membrane clogging and fouling is as follows:

[0028] When R i ≤10%, indicating that the UF membrane is slightly blocked and contaminated;

[0029] When 10% <R i ≤20%, indicating moderate blockage and contamination of the UF membrane;

[0030] When R i >20% indicates that the UF membrane is severely blocked and contaminated.

[0031] As a preference, in the i Before evaluating the degree of UF membrane clogging and contamination, it is also necessary to determine whether the membrane has been broken or damaged.

[0032] Preferably, the step of determining whether the membrane has been broken or damaged is as follows:

[0033] When the UF membrane flux J i >J max Or transmembrane pressure difference ΔP i <ΔP min When the membrane is broken or damaged.

[0034] As a preference, when the UF membrane is broken or damaged, the membrane fibers should be repaired in time; when the UF membrane is slightly blocked and contaminated, it should be backwashed and air-washed regularly; when the UF membrane is slightly blocked and contaminated, it should be maintained by chemical cleaning; when the UF membrane is severely blocked and contaminated, it should be restored by chemical cleaning; when the ratio of the decrease in the normalized membrane permeability increases rapidly over a period of time (R i / t value increases sharply), the membrane may be seriously blocked and irreversibly contaminated, and the cleaning plan (including cleaning time, type of reagent, concentration, etc.) should be adjusted and optimized.

[0035] Of course, as a further optimization, the normalized membrane permeability reduction ratio of the UF membrane can be increased rapidly R i / t is a characteristic indicator of serious membrane blockage and irreversible pollution. Specifically, by comparing the Ri / t of the UF membrane in the current period with the Ri / t of the UF membrane in historical accumulated data, if the current Ri / t increases sharply, it means that the UF membrane after cleaning is quickly polluted and blocked, and irreversible pollution has occurred.

[0036] Compared with the prior art, the advantages and positive effects of the present invention are:

[0037] 1. The evaluation method provided by the present invention adopts the UF normalized membrane permeability reduction ratio (R i ) is used as an evaluation indicator for the degree of membrane clogging and contamination. Setting boundary conditions to divide it into three levels of contamination can eliminate the influence of factors such as temperature and sludge concentration on the change of membrane permeability, objectively and directly evaluate the membrane permeability, and then judge the degree of membrane clogging and contamination, providing a basis for the formulation and optimization of UF membrane cleaning and maintenance plans, which is conducive to scientifically and reasonably determining the membrane cleaning and maintenance cycle, improving membrane operating efficiency, and extending membrane service life.

[0038] 2. The evaluation method provided by the present invention sets the rapid increase in the ratio of decrease in UF normalized membrane permeability as a characteristic indicator of severe blockage and irreversible pollution of the membrane, thereby achieving an effective, accurate and rapid evaluation of the current status of the membrane. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is a scanning electron microscope photograph of a cross section of a UF membrane fiber provided in Example 1 of the present invention, magnified 1000 times. DETAILED DESCRIPTION

[0040] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0041] Example 1

[0042] A membrane desalination plant has a production capacity of 100,000 cubic meters per day. The UF membrane is made of polyvinylidene fluoride (PVDF) with an average pore size of 0.02 μm. It is an internal pressure hollow fiber ultrafiltration membrane with a designed transmembrane pressure difference of 20-80 kPa and a designed membrane flux of 85 L·m -2 ·h -1 .

[0043] In the past 3 months, at 20℃ and turbidity 7NTU, the membrane flux (J0) and transmembrane pressure difference (ΔP0) of the UF membrane after chemically enhanced cleaning were tested. The UF membrane permeability L0=J0 / ΔP0, the UF membrane standard permeability L 0a is the average value of L0 in the past three months, L 0a =1.8922L·m -2 ·h -1 KPa -1 .

[0044] Calculation of UF membrane permeability temperature normalization coefficient A

[0045] Control the turbidity of UF inlet water to be consistent (select 7NTU), take an average of 5-8 temperature conditions (8℃, 12℃, 16℃, 20℃, 24℃) in the temperature range of 5-30℃, and test the membrane flux J of UF membrane at different temperatures i and transmembrane pressure difference ΔP i , calculate the actual membrane permeability L i =J i / ΔP i ;

[0046] L i is the dependent variable, (T i -20) as the independent variable to perform linear fitting and obtain the approximate linear fitting formula L i =1.8922+1.8922×(T i -20)×0.0469, and then the UF membrane permeability temperature normalization coefficient A=0.0469℃ -1 .

[0047] Calculation of UF membrane permeability turbidity normalization coefficient B

[0048] The UF inlet water temperature was controlled to be consistent (20°C), and 5-8 turbidity conditions (2NTU, 5NTU, 8NTU, 11NTU, 14NTU) were averaged in the turbidity range of 1-15NTU to test the flux J of the UF membrane under different turbidities. i and transmembrane pressure difference ΔP i , calculate the actual membrane permeability L i =Ji / ΔP i ;

[0049] L i is the dependent variable, (C i -7) is the independent variable for linear fitting, and the approximate linear fitting formula L is obtained. i =1.8922+1.8922×(C i -7)×(-0.0115), and then the UF membrane permeability turbidity normalization coefficient B = -0.0115NTU -1 .

[0050] Online collection of UF operating parameters

[0051] The actual inlet water temperature of the UF is collected through online monitoring instruments (T i ), turbidity (C i ) and membrane flux (J i ), transmembrane pressure difference (ΔP i ) and other operating data. Current UF membrane inlet water temperature T i =23.9℃, turbidity C i =6.2NTU, membrane flux J i =78L·m -2 ·h -1 , transmembrane pressure difference ΔP i =36KPa.

[0052] A quick membrane integrity test can be performed first to determine whether there is a break or damage. The judgment standard is the UF membrane flux J i >J max Or transmembrane pressure difference ΔP i <ΔP min Current UF membrane flux J i <85L·m -2 ·h -1且 Transmembrane pressure difference ΔP i When the pressure is >20KPa, the membrane integrity is currently good and the degree of clogging and fouling can be assessed.

[0053] Calculate the normalized membrane permeability L of the current UF membrane in

[0054] L in =L i ×[1+(T i -20)×A] -1 ×[1+(C i -7)×B] -1 =(78 / 36)×[1+(23.9-20)

[0055] ×0.0469] -1×[1+(6.2-7)×(-0.0115)] -1 =1.814L·m -2 ·h -1 KPa -1 .

[0056] Calculate the current UF normalized membrane permeability decrease ratio

[0057] R i =(L 0a -L in ) / L 0a ×100%=(1.8922-1.814) / 1.8922×100%=4.1%.

[0058] Combined with the UF membrane blockage and pollution degree evaluation criteria (the criteria are as follows: when R i ≤10%, it means the UF membrane is slightly blocked and polluted; when 10% <R i ≤20%, it means the UF membrane is moderately blocked and contaminated; when R i >20%, it means that the UF membrane is severely blocked and polluted; when the UF normalized membrane permeability decreases, the ratio increases rapidly (R i / t value increases sharply), it is severely blocked and polluted;) It can be seen that the UF membrane provided in Example 1 is currently slightly polluted and should be backwashed and air-washed regularly. Figure 1 From the scanning electron microscope photo of the UF membrane cross section shown, it can be seen that the membrane surface is relatively clean with only a small amount of pollutants attached.

[0059] As can be seen from the above, the UF membrane blockage and contamination degree assessment method and corresponding evaluation criteria provided in this application can objectively and directly evaluate the water permeability of the membrane, accurately determine the current blockage and contamination degree of the UF membrane, and help to scientifically and reasonably determine the membrane cleaning and maintenance cycle, improve membrane operating efficiency, and extend the membrane service life. Figure 1 Corresponding evidence is also provided for this, which shows that this method is indeed objective, accurate and efficient.

Claims

1. A method for evaluating the degree of clogging and contamination of an ultrafiltration membrane, characterized in that: The following steps are involved: Calculate the temperature normalization coefficient A of UF membrane permeability and the turbidity normalization coefficient B; Calculate the actual membrane permeability L of the current UF membrane i , using the UF membrane permeability temperature normalization coefficient A and turbidity normalization coefficient B to calculate the current UF membrane normalized membrane permeability L in =L i ×[1+(T i -20) × A] -1 ×[1+(C i -7) × B] -1 ; Calculate the standard permeability L of the UF membrane under normalized conditions within a certain period 0a ; Calculate the normalized membrane permeability L of the current UF membrane in Compared with the standard permeability of UF membrane within a certain period, L 0a The decrease ratio R i ; By R i The range of values ​​is used to evaluate the degree of UF membrane clogging and contamination; the UF membrane permeability temperature normalization coefficient A is calculated by the following method: The turbidity of UF inlet water was controlled at 7 NTU, and 5-8 temperature conditions were averaged in the temperature range of 5-30℃ to test the membrane flux J of UF membrane at different temperatures. i and transmembrane pressure difference ΔP i , calculate the actual membrane permeability L i =J i / ΔP i ; L i is the dependent variable, (T i -20) is used as the independent variable for linear fitting, and the approximate linear fitting formula L is obtained. i =L0+L0×(T i -20) × A, and then calculate the temperature normalization coefficient A of the UF membrane permeability, unit is ℃ -1 ; Among them, T i represents the temperature value collected by the online instrument, in °C; L0 is the UF normalized membrane permeability, in L·m -2 ·h -1 KPa -1 The UF membrane permeability turbidity normalization coefficient B is calculated by the following method: The UF inlet water temperature was controlled at 20°C, and 5-8 turbidity conditions were averaged in the turbidity range of 1-15 NTU to test the flux J of the UF membrane under different turbidities. i and transmembrane pressure difference ΔP i , calculate the actual membrane permeability L i ; L i is the dependent variable, (C i -7) Perform linear fitting for the independent variable and obtain the approximate linear fitting formula L i =L0+L0×(C i -7) × B, and then calculate the UF membrane permeability turbidity normalization coefficient B, the unit is NTU -1 ; Among them, C i Represents the turbidity value collected by the online instrument, the unit is NTU, L0 is the UF normalized membrane permeability, the unit is L·m -2 ·h -1 KPa -1 .

2. The evaluation method according to claim 1, wherein: UF membrane standard permeability L under normalized conditions within a certain period 0a Specifically, it is the average value of the UF normalized membrane permeability L0 after each restorative chemical cleaning within a certain period.

3. The evaluation method according to claim 2, wherein: The UF normalized membrane permeability L0 is calculated by the following method: Taking the UF inlet water temperature of 20℃ and turbidity of 7 NTU as the normalized conditions, the UF normalized membrane permeability was measured as L0=J0 / ΔP0.

4. The evaluation method according to claim 2 or 3, characterized in that: The certain period is 2-3 months.

5. The evaluation method according to claim 1, wherein: Current UF membrane normalized membrane permeability reduction ratio R i The calculation formula is as follows: Reduction ratio R i =(L 0a -L in ) / L 0a ×100%.

6. The evaluation method according to claim 5, characterized in that By R i The range of values ​​for evaluating the degree of UF membrane clogging and fouling is as follows: When R i ≤10%, indicating slight blockage and contamination of the UF membrane; When 10% <R i ≤20%, indicating moderate blockage and contamination of the UF membrane; When R i >20% indicates that the UF membrane is severely blocked and contaminated.

7. The evaluation method according to claim 6, characterized in that In R i Before evaluating the degree of UF membrane clogging and contamination, it is also necessary to determine whether the membrane has been broken or damaged.

8. The evaluation method according to claim 7, characterized in that The specific steps to determine whether the membrane has been broken or damaged are: When the UF membrane flux J i >J max Or transmembrane pressure difference ΔP i <ΔP min When the membrane is broken or damaged.

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