Method for producing water

Abstract

Provided is a technical means for suppressing the development of dirt on a semipermeable membrane when fresh water is obtained by desalting seawater, salt water, or the like using a membrane, and recycled water, or the like, is obtained by purifying wastewater treatment water, industrial wastewater, or the like. A method for producing water according to the present invention comprises a membrane treatment step in which semipermeable membrane supply water is treated by a semipermeable membrane and separated into permeated water and concentrated water, and at least one of (A) a pretreatment step in which water to be treated is pretreated to obtain semipermeable membrane supply water, (B) a cleaning step in which the semipermeable membrane is cleaned, and (C) a bactericide addition step in which a bactericide is added to the semipermeable membrane. Or when the biopolymer concentration of the concentrated water exceeds 75 * 1 / (1-R) [mu] gC / L when the recovery rate of the permeate water in the membrane treatment step is set as R, the operating conditions of at least one of the pretreatment step, the cleaning step and the bactericide addition step are added or enhanced.

Description

Technical Field

[0001] The present invention relates to a method for producing fresh water by desalinating seawater or salt water using a membrane, or purifying sewage treatment water or industrial wastewater to obtain reuse water. Background Art

[0002] Freshwater systems using semipermeable membranes are used in many industries and water treatment fields, including desalination of seawater, and have proven to be superior to other separation methods in terms of separation performance and energy efficiency. On the other hand, in freshwater systems, due to the proliferation of microorganisms on the membrane surface or the attachment of biofilms (biofilms) to the membrane surface, i.e. biofouling, there are problems such as a sharp increase in membrane pressure difference and a decrease in membrane permeability and separation performance.

[0003] When the membrane pressure difference increases due to biological fouling, or the membrane permeability and separation properties decrease, the membrane is usually cleaned. As a cleaning method, there can be so-called flushing cleaning in which filtration is temporarily stopped and raw water or filtered water is supplied to the membrane surface for cleaning, or pharmaceutical (chemical) cleaning in which a cleaning agent is used for cleaning. However, once fouling develops, the membrane pressure difference, permeability, and separation properties will not be fully restored even after cleaning, and the frequency of cleaning will gradually increase, and eventually the membrane will not be able to operate and need to be replaced. Therefore, it is important to clean the membrane at an appropriate stage before the fouling develops to inhibit the development of fouling.

[0004] In addition, as a means of inhibiting the development of scale, many technologies have been proposed as effective methods to add agents that inhibit the proliferation of biofilms (hereinafter referred to as "fungicides") to the treated water. If the concentration, frequency, time, etc. of these fungicides are added too little, the development of scale cannot be inhibited. On the other hand, if too much, although the development of scale can be inhibited, it will lead to an increase in drug costs. Therefore, it is important to master the appropriate concentration, frequency, time, etc. of the fungicides added to inhibit the development of scale.

[0005] In addition, if the treated water contains turbidity and other components, if it is directly filtered with a semipermeable membrane, the components attached to the membrane surface will increase, the pressure difference will rise sharply, and it will not be able to operate. In order to avoid this, the raw water will be pre-treated before being supplied to the semipermeable membrane, but since the operating conditions of the pre-treatment process will also affect the development of fouling of the semipermeable membrane, in order to suppress the development of fouling of the semipermeable membrane, it is necessary to appropriately control the operating conditions of the pre-treatment process.

[0006] In view of the above, in order to suppress the progress of fouling of the semipermeable membrane, it is important to appropriately evaluate the biofouling potential of the water to be treated and to appropriately control the conditions of the cleaning, bactericide addition, and pretreatment steps.

[0007] Patent Documents 1 and 2 focus on biopolymers contained as part of organic matter in treated water, and disclose a method for membrane separation in which the biopolymers in treated water are adjusted to below a predetermined threshold value, wherein the predetermined threshold value is any value in the range of 9 μg / L to 12 μg / L, or 9 μg / L to 17 μg / L.

[0008] Prior Art Literature

[0009] Patent Document 1: Japanese Patent No. 6561082

[0010] Patent Document 2: Japanese Patent No. 6630689 Summary of the invention

[0011] Problems to be solved by the invention

[0012] However, in the technologies of Patent Documents 1 and 2, microfiltration membranes or ultrafiltration membranes are envisioned as membrane separation, but they are difficult to apply directly because of the different fouling mechanisms from semipermeable membranes. In addition, the predetermined threshold values ​​disclosed in these documents are very low concentrations, which are difficult to achieve in general seawater, sewage treatment water and other treated water.

[0013] Therefore, an object of the present invention is to provide a technical means for suppressing the development of fouling of a semipermeable membrane when desalting seawater or salt water to obtain fresh water or purifying sewage treatment water or industrial wastewater to obtain reuse water.

[0014] Means for solving problems

[0015] The present inventors conducted intensive research and found that the above-mentioned problems can be solved by appropriately evaluating the biofouling potential of the water to be treated and appropriately controlling the conditions of the semipermeable membrane cleaning, bactericide addition and pretreatment steps, thereby completing the present invention.

[0016] In order to solve the above-mentioned problems, the water production method of the present invention is constituted by any one of the following technical means.

[0017] (1) A method for producing fresh water, comprising a membrane treatment step of treating semipermeable membrane feed water with a semipermeable membrane to separate it into permeated water and concentrated water, and at least one of the following steps (A) to (C),

[0018] (A) a pretreatment step of pretreating the water to be treated to obtain the semipermeable membrane feed water,

[0019] (B) a cleaning step of cleaning the semipermeable membrane,

[0020] (C) a bactericide adding step of adding a bactericide to the semipermeable membrane,

[0021] When the biopolymer concentration of the semipermeable membrane supply water exceeds 75 μgC / L, or when the biopolymer concentration of the concentrated water exceeds 75×1 / (1-R) ​​μgC / L when the recovery rate of the permeated water in the membrane treatment process is set to R, the operating conditions of at least one of the pretreatment process, the cleaning process and the bactericide addition process are added or strengthened.

[0022] (2) The fresh water production method according to (1), wherein in the pretreatment step, a coagulant is added to supplement or strengthen the operating conditions of the pretreatment step.

[0023] (3) The method for producing fresh water according to (2), wherein when the biopolymer concentration of the semipermeable membrane feed water exceeds 75 μgC / L, ferric chloride is added as the coagulant at a concentration of (P-75)×0.1[mg-Fe / L] or more and (P-75)×0.33[mg-Fe / L] or less, with the concentration being defined as P[μgC / L].

[0024] (4) The fresh water production method according to any one of (1) to (3), wherein the pretreatment step includes a microfiltration membrane method or an ultrafiltration membrane method.

[0025] (5) The method for producing fresh water according to (4), wherein the ultrafiltration membrane used in the ultrafiltration membrane method has a removal rate of 55% to 99% for dextran having a weight average molecular weight of 200,000 Da.

[0026] (6) The method for producing fresh water according to (4) or (5), wherein the ultrafiltration membrane used in the ultrafiltration membrane method has a removal rate of 60% to 95% for dextran having a weight average molecular weight of 40,000 Da.

[0027] (7) The method for producing fresh water according to (5) or (6), wherein the number of pores on the surface of the ultrafiltration membrane is 200 / μm 2 Above and 2000 / μm 2 the following.

[0028] (8) The method for generating fresh water according to any one of (5) to (7), wherein the average value of the surface pore diameter of the ultrafiltration membrane is 5.0 nm or more and 12.0 nm or less.

[0029] (9) The method for producing fresh water according to any one of (5) to (8), wherein the value obtained by dividing the number of pores on the surface of the ultrafiltration membrane by the average pore size on the surface of the ultrafiltration membrane is 30 pores / μm 2 / nm or more and 100 / μm 2 / nm or less.

[0030] (10) A method for producing fresh water, comprising a membrane treatment step of treating semipermeable membrane feed water with a semipermeable membrane to separate it into permeated water and concentrated water, and at least one of the following steps (A) to (C),

[0031] (A) a pretreatment step of pretreating the water to be treated to obtain the semipermeable membrane feed water,

[0032] (B) a cleaning step of cleaning the semipermeable membrane,

[0033] (C) a bactericide adding step of adding a bactericide to the semipermeable membrane,

[0034] When the increase in filtration resistance (δA) when filtering the semipermeable membrane supply water using an ultrafiltration membrane having a removal rate of more than 60% and less than 95% for dextran with a weight-average molecular weight of 40,000 Da exceeds a predetermined threshold, the operating conditions of at least one of the pretreatment process, the cleaning process and the disinfectant addition process are added or strengthened.

[0035] (11) According to the water production method described in (10), the pretreatment step includes pretreating the treated water using an ultrafiltration membrane having a removal rate of 60% or more and 95% or less for dextran with a weight-average molecular weight of 40,000 Da, and when the increase in filtration resistance (δA) when the semipermeable membrane supplies water to the ultrafiltration membrane and flows back to the ultrafiltration membrane exceeds a predetermined threshold value, the operating conditions of at least any one of the pretreatment step, the cleaning step and the bactericide addition step are added or strengthened.

[0036] (12) The fresh water production method according to (10) or (11), wherein the threshold value of the filtration resistance increase (δA) is 2.5×10 -12 / m 2 .

[0037] Effects of the Invention

[0038] According to the present invention, the conditions of the cleaning, bactericide addition and pretreatment steps of the semipermeable membrane can be appropriately controlled to suppress the development of fouling of the semipermeable membrane. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is a schematic diagram showing an example of the water generation method of the present invention.

[0040] Figure 2 This is a schematic diagram showing a treatment method of the seawater desalination facility A.

[0041] Figure 3 This is a schematic diagram showing another treatment method of the seawater desalination facility A.

[0042] Figure 4This is a schematic diagram showing the treatment method of the sewage reuse facility B.

[0043] Figure 5 This is a schematic diagram showing another example of the water generation method of the present invention. DETAILED DESCRIPTION

[0044] Below, using Figure 1 The present invention will be described in detail, but the contents of the present invention are not limited to the embodiments of the drawings.

[0045] In addition, in this specification, "mass" and "weight" are synonymous.

[0046] The method for producing fresh water of the present invention is carried out in a fresh water producing system in which water to be treated 1 is treated through a semipermeable membrane 2 and separated into permeated water 3 and concentrated water 4 .

[0047] As the water to be treated, for example, seawater, salt water, river water, lake water, groundwater, sewage, sewage secondary treatment water, etc. In the case where the water to be treated contains solid components such as turbidity, if it is directly filtered with a semipermeable membrane, the solid components attached to the membrane surface will increase, the pressure difference will rise sharply, and it will not be able to operate. Therefore, in this case, the water to be treated is treated in the pretreatment unit 5 in advance and then supplied to the semipermeable membrane.

[0048] A commonly used pretreatment method is a coagulant sand filtration method in which a coagulant is added to the raw water to flocculate the solid components and filter the solid components with sand or anthracite. On the other hand, in this method, since the treated water quality is unstable due to the influence of the fluctuation of the raw water, a membrane pretreatment using a microfiltration membrane or an ultrafiltration membrane may also be used. In addition, in the case where the raw water is organic wastewater such as sewage, in order to reduce the organic matter contained in the wastewater, a pretreatment for solid-liquid separation may be performed after the activated sludge treatment to separate the activated sludge. The solid-liquid separation method may be the sedimentation separation using a sedimentation tank as used in the past. In order to improve the treated water quality, a solid-liquid separation method using a separation membrane such as a microfiltration membrane or an ultrafiltration membrane may also be used.

[0049] The treated water that has been pretreated, i.e., the semipermeable membrane supply water 6, is sent to the high-pressure pump 8 by the water delivery pump 7, and is pressurized by the high-pressure pump 8, thereby being supplied to the semipermeable membrane 2 at the pressure required for filtration, and separated into permeated water 3 and concentrated water 4. In the middle of the supply piping, a bactericide 9 is sometimes added to inhibit the development of biological fouling on the semipermeable membrane. Regarding the device for adding the bactericide, in order to control the addition conditions of the bactericide, it is preferably equipped with a control mechanism having a valve or pump that can control the addition amount, addition time, addition frequency, etc.

[0050] In addition, a pipeline for introducing a cleaning agent 10 is provided upstream of the semipermeable membrane 2 for chemical cleaning. There is no particular limitation on the location of introducing the cleaning agent, but depending on the type of cleaning agent, it may corrode the high-pressure pump 8, so it is preferably downstream. In addition, the cleaning agent is usually introduced from the middle of the pipe of the concentrated water 4 and circulated.

[0051] As a semipermeable membrane, any raw material can be used as long as the salt concentration can be reduced so that the treated water can be used for drinking water, industrial water, urban water, etc. For example, a membrane composed of cellulose acetate and polyamide raw materials can be cited. Among them, a membrane composed of polyamide raw materials is particularly effective in the method of the present invention. Polyamide membranes have low resistance to chlorine, which is most commonly used to prevent the proliferation of biological films. Even very low concentrations of chlorine can significantly cause membrane deterioration, so it is difficult to prevent biofouling. Therefore, the effect brought about by the implementation of the present invention is significantly manifested.

[0052] The method for producing fresh water of the present invention is characterized by comprising, in the fresh water producing system, a membrane treatment step of treating semipermeable membrane feed water with a semipermeable membrane to separate it into permeated water and concentrated water, and at least one of the following steps (A) to (C).

[0053] (A) a pretreatment step of pretreating the water to be treated to obtain the semipermeable membrane feed water,

[0054] (B) a cleaning step of cleaning the semipermeable membrane,

[0055] (C) a bactericide adding step of adding a bactericide to the semipermeable membrane,

[0056] When the biopolymer concentration of the semipermeable membrane supply water exceeds 75 μgC / L, or when the biopolymer concentration of the concentrated water exceeds 75×1 / (1-R) ​​μgC / L when the recovery rate of the permeated water in the membrane treatment process is set to R, the operating conditions of at least one of the pretreatment process, the cleaning process and the bactericide addition process are added or strengthened.

[0057] Biopolymers are hydrophilic high molecular weight organic substances (polysaccharides, proteins, etc.) with a molecular weight of about 10 to 20 kDa or more in organic matter. As for the definition and determination method of biopolymers, for example, as described in Huber, SA, Balz, A., Abert, M., Pronk, W., 2011. Characterization of aquatic humic and non-humic matter with size-exclusion chromatography e organic carbon detection e organic carbon detection e organic nitrogen detection (LC-OCD-OND). Water Research 45 (2), 879-885, it can be measured by organic carbon detection type exclusion chromatography (LC-OCD). Here, the LC-OCD method is an analytical method that divides the total organic carbon (TOC) component in the sample according to molecular weight, and represents it as a chromatogram. On the chromatogram, organic matter with larger molecular weight and hydrophilicity tends to have a shorter retention time. As the measurement conditions of the LC-OCD method, a 250 mm × 20 mm TSKHW50S is used as a column, the flow rate is set to 1.1 mL / min, the sample injection amount is set to 1 mL, the UV wavelength is set to 254 nm, the acid injection amount to the wet total organic carbon meter (OCD meter) is set to 0.2 mL / min, a phosphate buffer with a pH value of 6.85 is used as an eluent, and as an acidified solution, a solution obtained by adding 4 mL of O-phosphoric acid (85%) and 0.5 g of potassium peroxodisulfate to 1 L of ultrapure water is used. As a measuring device used in the LC-OCD method, for example, an LC-OCD device (manufactured by DOC-Labar) in which an OCD meter is connected to a high performance liquid chromatograph (HPLC) can be used.

[0058] In the present invention, when the biopolymer concentration of the semipermeable membrane supply water measured by the above method exceeds 75μgC / L, the cleaning conditions and / or bactericide addition conditions of the semipermeable membrane are supplemented or strengthened. Here, as cleaning conditions, in the case of flushing cleaning, the time, frequency, and method can be cited, and in the case of drug cleaning, the type, concentration, injection method, time, and method of the cleaning agent can be cited. For example, in flushing cleaning, the high-pressure pump is temporarily stopped, the filtration is stopped, and only the water supply pump is operated, and the biofilm attached to the membrane surface is peeled off and cleaned and removed by using the semipermeable membrane supply water. As a method for supplementing or strengthening cleaning, increasing the cleaning time and frequency (including cleaning that is not usually performed), increasing the flow rate, repeatedly increasing and decreasing, and allowing air to flow in together to improve the cleaning effect can be cited.

[0059] In chemical cleaning, there is no particular limitation on the type of cleaning agent, but in the case of biological fouling, alkali cleaning is generally used. As a cleaning agent, for example, 0.1% sodium hydroxide solution can be cited. The cleaning agent is usually placed in a cleaning tank, etc., and is introduced into the RO piping from the downstream of the high-pressure pump by a pump, and is led out from the middle of the concentrated water piping for circulation. As a cleaning method, for example, after circulating cleaning for about 1 hour (repeat 2 to 3 times depending on the situation), depending on the degree of fouling, immersion for 2 to 24 hours, and finally rinsing to complete the cleaning. As a method of additional or enhanced cleaning, increasing the cleaning time and frequency (including cleaning that is not usually performed), increasing the flow rate, repeated increase and decrease, and allowing air to flow in to improve the cleaning effect can be cited.

[0060] In addition, as the conditions for adding the bactericide, the type, concentration, injection method, time, etc. of the bactericide can be cited. As the type of bactericide, for example, bactericides with the above-mentioned 2-methyl-4-isothiazoline-3-one or 5-chloro-2-methyl-4-isothiazoline-3-one or their salts and their mixtures as active ingredients, or 2,2-dibromo-3-nitrilopropionamide (DBNPA), sulfuric acid, etc. can be cited. As the concentration, for example, in the case of DBNPA, the semipermeable membrane is supplied with water in a final concentration of 10ppm, or in the case of sulfuric acid, the semipermeable membrane is supplied with water in a pH value of 3. As an injection method, it can be added continuously, or it can be added intermittently, for example, once a day for 1 hour, and intermittent addition can usually reduce the cost of the bactericide. As a method of adding or strengthening the conditions for adding the bactericide, it can be cited to increase the concentration, increase the time, increase the frequency (also including the addition that is not usually implemented), etc.

[0061] In addition, the target water for measuring the biopolymer concentration may not be the semipermeable membrane supply water but the concentrated water. In this case, the biopolymer is concentrated by the semipermeable membrane, so when the recovery rate of the permeated water is set to R, when the biopolymer concentration of the concentrated water exceeds 75×1 / (1-R)μgC / L, the cleaning conditions of the semipermeable membrane and / or the disinfectant injection conditions are added or strengthened.

[0062] In addition, the operating conditions of the pretreatment unit can be added or strengthened when the biopolymer concentration of the semipermeable membrane supply water exceeds 75μgC / L. As a method of adding or strengthening the operating conditions, adding a coagulant and reducing the biopolymer are the most easily applicable methods in operation management. Specifically, it is effective to add a coagulant according to the concentration exceeded by the biopolymer concentration of the semipermeable membrane supply water. When the biopolymer concentration of the semipermeable membrane supply water is set to P[μgC / L], adding ferric chloride as a coagulant at a concentration of (P-75)×0.1[mg-Fe / L] or more and (P-75)×0.33[mg-Fe / L] or less is effective and preferred in terms of effect and cost. (P-75) represents an amount exceeding 75μgC / L, which means that it is added at a concentration of 0.1 times or more and 0.33 times or less of the excess amount. In addition, it is more effective and preferred to add at a concentration of 0.1 times or more and 0.2 times or less of the excess amount of the biopolymer concentration.

[0063] In addition, as a pretreatment method, there is no particular limitation as long as it is a method such as the above-mentioned coagulation sand filtration method, membrane pretreatment method, etc. that can remove solid components such as turbidity, and membrane pretreatment method is more preferably used. In the case where the pretreatment method is the coagulation sand filtration method, the concentration of coagulant addition is sometimes optimized in order to form flocs of turbidity components in the treated water. By increasing the concentration of coagulant addition in order to adjust the concentration of biopolymers in the semipermeable membrane supply water, the removal performance of turbidity components that should be removed is sometimes affected. On the other hand, in the case of membrane pretreatment methods such as microfiltration membrane method or ultrafiltration membrane method, the removal of turbidity components is achieved by membrane pretreatment itself, so the turbidity removal rate is always stable. In addition, there is no need to add coagulants for turbidity components, so only when the biopolymer concentration exceeds 75μgC / L, coagulants can be added to reduce the biopolymer concentration, so it is efficient.

[0064] In addition, as a means of improving the biopolymer removal rate in membrane pretreatment without adding a coagulant, increasing the specific filtration resistance of a microfiltration membrane or an ultrafiltration membrane can also be cited. This is a method of improving the biopolymer removal rate by clogging the microfiltration membrane or the ultrafiltration membrane. The specific filtration resistance is represented by the following formula (1).

[0065] R m =(TMP-TMP 0 ) / (μ×J)···Formula (1)

[0066] Among them, R m : Specific filtration resistance [1 / m], TMP: Pressure difference between membranes during operation [Pa], TMP 0 : initial membrane pressure difference [Pa], μ: water viscosity coefficient [Pa·s], J: membrane filtration flux [m / s].

[0067] The operation of a microfiltration membrane or an ultrafiltration membrane is generally carried out as follows. First, the water to be treated is supplied and filtered at a predetermined membrane filtration flux for a certain period of time (20 to 30 minutes). Then, backflow cleaning using filtered water or air cleaning using a blower (physical cleaning) is carried out for a certain period of time (30 to 60 seconds), and filtration is started again, and the cycle is repeated. When performing backflow cleaning, in order to improve the cleaning effect, a liquid medicine such as acid, alkali, sodium hypochlorite, etc. is sometimes injected (liquid backflow cleaning). As a method of clogging a microfiltration membrane or an ultrafiltration membrane to increase the specific filtration resistance, there are citing methods such as increasing the membrane filtration flux, extending the filtration time (reducing the frequency of physical cleaning), reducing the flow rate or time of each physical cleaning, reducing the frequency of liquid backflow cleaning, and reducing the flow rate or time of each liquid backflow cleaning. By improving the removal rate of biopolymers by such a method, since there is no need to add a coagulant, the cost of drugs can be reduced.

[0068] As an ultrafiltration membrane, it is preferred that the removal rate of dextran with a weight average molecular weight of 200,000 Da is 55% or more and 99% or less. In addition, it is more preferred that the removal rate of dextran with a weight average molecular weight of 40,000 Da is 60% or more and 95% or less. The removal rate of dextran with a weight average molecular weight of 40,000 Da is more preferably 68% or more and 90% or less, and particularly preferably 70% or more and 90% or less. By using such a membrane, the biopolymer removal rate of the membrane itself becomes higher, so the number of times the biopolymer concentration of the semipermeable membrane supply water exceeds 75μgC / L is reduced, or the concentration when it exceeds will not become so high, so not only the operation control becomes easier, but also the amount of coagulant added can be reduced.

[0069] The dextran removal rate can be calculated from the following formula (2) by using a commercially available dextran aqueous solution prepared to 1000 ppm at 25°C and filtering it through a porous membrane at a cross flow linear velocity of 1.0 m / sec and a transmembrane pressure difference of 10 kPa.

[0070] Dextran removal rate T (%) = {(refractive index of stock solution) - (refractive index of permeate)} / (refractive index of stock solution) × 100 Formula (2)

[0071] The cross flow linear velocity is the value obtained by dividing the flow rate in the direction perpendicular to the filtration direction of the filtrate by the cross-sectional area of ​​the flow path. In addition, the transmembrane pressure difference refers to the difference between the pressure on the filtrate side and the pressure on the permeate side across the porous membrane.

[0072] In addition, in ultrafiltration membranes, there is a trade-off between high biopolymer removal rate and filtration flux and its maintenance rate. In order to make the ultrafiltration membrane show high biopolymer removal rate and maintain high filtration flux, the surface pore number of the ultrafiltration membrane is preferably 200 / μm. 2 Above and 2000 / μm2 Therefore, as the filtration proceeds, even if the dirt components in the filtration stock solution block part of the surface pores of the ultrafiltration membrane, the number of flow paths for the filtration stock solution to pass through the ultrafiltration membrane can be fully ensured, so the ultrafiltration membrane is easy to show excellent stain resistance, and is therefore preferred. The number of surface pores of the ultrafiltration membrane is further preferably 290 / μm 2 Above and 1500 / μm 2 Below, particularly preferably 350 pieces / μm 2 More than 1000 pieces / μm 2 the following.

[0073] Regarding the number of pores on the surface of the ultrafiltration membrane, the free software "ImageJ" was used to binarize the image obtained by observing the SEM of the ultrafiltration membrane surface. When binarizing, set it to 1 pixel in Subtract Background to CreateBackground, and then select the condition: RenyiEntropy in Threshold (binarization threshold). It can be obtained by analyzing it through Analyze Particles based on the obtained binary image. The number of surface pores is divided by the area of ​​the analyzed image, which is used as the number of pores per unit area. Similar to the pore size, it is calculated by analyzing an image containing more than one thousand pores.

[0074] In addition, the average value of the surface pore size of the ultrafiltration membrane is 5.0 nm or more and 12.0 nm or less, which can prevent the coarse pollutants (turbidity) in the filtration stock solution and the biopolymers as the removal object from invading the ultrafiltration membrane, and it is easy to show high fouling resistance, so it is preferred. The average value of the surface pore size [nm] is more preferably 5.0 nm or more and 9.0 nm or less, and particularly preferably 5.0 nm or more and 8.0 nm or less.

[0075] The surface pore size refers to the diameter of the holes located within the surface when observing the surface of the ultrafiltration membrane. When calculating the surface pore size of the ultrafiltration membrane, use the free software "ImageJ" to binarize the image obtained by observing the SEM of the ultrafiltration membrane surface. When binarizing, set 1 pixel in Subtract Background to Create Background, and then select the condition: RenyiEntropy in Threshold (binarization threshold). In the obtained binary image, select Area in Analyze Particles to calculate the area of ​​each hole, and use the diameter calculated assuming that each hole is a circle as the surface pore size. When calculating the average value of the surface pore size, the pore size of more than one thousand holes is averaged and calculated.

[0076] The value X obtained by dividing the number of pores on the surface of the ultrafiltration membrane by the average value of the surface pore diameter is 30 pores / μm. 2 / nm or more and 100 / μm 2 / nm or less, it is possible to prevent the coarse pollutants (turbidity) in the filtered stock solution or the biopolymers as the removal object from invading the ultrafiltration membrane, and to fully ensure the number of flow paths for the filtered stock solution to pass through the ultrafiltration membrane, so it is easy to show excellent stain resistance, so it is preferred. A large X means more small pores. Generally, when the pore size is small, the number of pores tends to decrease, and there is a trade-off relationship between the pore size and the number of pores. The inventors and others have repeatedly conducted in-depth research and found that by having small pores, it is possible to prevent the coarse pollutants (turbidity) in the filtered stock solution or the biopolymers as the removal object from invading the ultrafiltration membrane, and by having many pores, it is possible to fully ensure the number of flow paths for the filtered stock solution to pass through the ultrafiltration membrane, and to disperse the dirt components, so it is easy to show excellent stain resistance. There is a correlation between the number of pores and the pore size, both of which contribute to stain resistance, so it is preferred to use the X value that takes both into account as an indicator of stain resistance. The ultrafiltration membrane used in the present invention has an X of 30 / μm. 2 / nm or more and 100 / μm 2 / nm or less, showing excellent stain resistance, so it is preferred, and more preferably 32 pieces / μm 2 / nm or more and 80 / μm 2 / nm or less, particularly preferably 50 / μm 2 / nm or more and 70 / μm 2 / nm or less.

[0077] Examples of polymers used as raw materials for ultrafiltration membranes include polysulfone resins, polyethersulfone resins, polyvinylidene fluoride resins, nylon, cellulose esters such as cellulose acetate or cellulose acetate propionate, fatty acid vinyl esters, polyvinyl alcohol, polyvinyl acetate, polyvinyl pyrrolidone, polymers of acrylic acid esters or methacrylic acid esters such as ethylene oxide, propylene oxide or polymethyl methacrylate, and copolymers thereof.

[0078] In particular, in order to use the ultrafiltration membrane for filtration for a long time, it is preferred to regularly perform chemical cleaning on the accumulated dirt components, and it is particularly preferred to contain a polyvinylidene fluoride resin with excellent chemical resistance. Polyvinylidene fluoride resin refers to a homopolymer of vinylidene fluoride or a copolymer of vinylidene fluoride. Here, a copolymer of vinylidene fluoride refers to a polymer having a vinylidene fluoride residue structure. A polymer having a vinylidene fluoride residue structure is typically a copolymer of a vinylidene fluoride monomer and a fluorine-based monomer other than the vinylidene fluoride monomer. As such a fluorine-based monomer, for example, vinyl fluoride, tetrafluoroethylene, hexafluoropropylene or chlorotrifluoroethylene can be cited. In the above-mentioned copolymer of vinylidene fluoride, ethylene other than the above-mentioned fluorine-based monomer can also be copolymerized to the extent that the effect of the present invention is not impaired.

[0079] The polymer used as the raw material of the ultrafiltration membrane may be used in combination of two or more. When the weight of the ultrafiltration membrane is 100%, the polyvinylidene fluoride resin is more preferably contained in an amount of 50% by weight or more, and particularly preferably in an amount of 60% by weight or more.

[0080] When the surface of an ultrafiltration membrane containing a polyvinylidene fluoride resin is measured by the ATR method (total reflection measurement method), the α-type structure crystals (H α ) and β-type structure crystal (H β ) ratio (H α / H β ) is preferably greater than 0 and less than 0.50. Here, as the crystal structure of polyvinylidene fluoride resin, there are three known structures: α-type, β-type, and a very small amount of γ-type. Among them, the β-type structure crystal is a planar sawtooth structure of TTTT, the dipole moments are arranged in the same direction, and the electron supply component on the surface of the film is large and polar. Therefore, it interacts strongly with water molecules that also have polarity, and organic matter is difficult to adsorb on the contrary. Through H α / H β The α-type structure crystal (H α ) and β-type structure crystal (H β ) ratio (H α / H β ), can be obtained by ATR method in the IR spectrum, according to 763cm -1 The peak height (H α ) and at 840cm -1 The peak height (H β ), and the ratio of α-type structure crystals to β-type structure crystals was calculated using the following formula (3). α / H β It is more preferably 0.25 or more and 0.40 or less, and still more preferably 0.30 or more and 0.40 or less.

[0081] The ratio of α-type / β-type structure crystals = H α / H β Formula (3)

[0082] The permeability of ultrafiltration membrane through pure water is 0.25m 3 / m 2 / h / 50kPa or above and 1.2m 3 / m 2 / h / 50kPa or less, even for easily contaminated filtration stock solution, it is easy to filter at a lower pressure, and the pollutants will not be pressed on the membrane surface due to high pressure, so it can inhibit the adsorption and clogging of the ultrafiltration membrane, and it is easy to obtain sufficient permeate even in the filtration of filtration stock solution with many pollutants. The pure water permeability is more preferably 0.30m 3 / m 2 / h / 50kPa or above and 1.2m 3 / m 2 / h / 50kPa or less, more preferably 0.40m 3 / m 2 / h / 50kPa or above and 1.2m 3 / m 2 / h / below 50kPa.

[0083] Since the ultrafiltration membrane is hollow fiber-shaped and the surface elastic modulus of the outer side of the hollow fiber membrane is more than 200 MPa, it is easy to suppress the friction of the hollow fiber membrane and reduce the reduction of the removal rate. As a result, it is easy to stably maintain a high removal rate, and the reduction of the removal rate due to friction will not be caused. The surface elastic modulus of the outer surface of the hollow fiber membrane is an indicator of the restoring force of the membrane relative to the pressure. The greater the restoring force, the less likely it is to undergo irreversible deformation. If irreversible deformation, i.e., plastic deformation, occurs, the shape of the pores of the hollow fiber membrane changes, and the components that should be removed by the hollow fiber membrane are likely to leak into the permeate. It was found that by using a hollow fiber membrane with a high surface elastic modulus, even when turbid matter contacts the hollow fiber membrane and is about to rub against the hollow fiber membrane, it is easy to suppress rubbing through the strong restoring force. In order to exert sufficient restoring force when turbidity is squeezed, the surface elastic modulus is preferably above 230 MPa, more preferably above 250 MPa, and particularly preferably above 300 MPa. In addition, in order to make the hollow fiber membrane tough and inhibit rupture during turbidity collision, the surface elastic modulus is preferably below 450 MPa, more preferably below 400 MPa, and particularly preferably below 350 MPa.

[0084] The surface elastic modulus can be tested and calculated using a commercially available nanoindenter using a method based on ISO14577. In the case of measuring a hollow fiber membrane, a dry hollow fiber membrane is fixed on a metal or glass plate, the maximum load is set to 0.1 mN, the application time is set to 15 seconds, the maximum load holding time is set to 30 seconds, and the measurement is performed at room temperature. The Poisson's ratio of the main component constituting the outer surface of the hollow fiber membrane is used to calculate the range of the indentation depth of 0.4 μm or more and 0.8 μm or less. The Poisson's ratio can be used in the case of polyvinylidene fluoride resins. A value of 0.35 can be used, a value of 0.37 can be used in the case of polysulfones, a value of 0.40 can be used in the case of polyethersulfones, and a value of 0.30 can be used in the case of cellulose acetate. Here, among the components constituting the hollow fiber membrane, the component with a weight fraction of 50% or more is taken as the main component. The main component can be obtained using a general composition analysis instrument by known techniques. For example, IR, NMR, ICP, etc. The surface elastic modulus was determined by measuring the surface elastic modulus five times using the same hollow fiber membrane while changing the measurement site, and then measuring three different hollow fiber membranes, and calculating the average of all the measured values.

[0085] The ultrafiltration membrane is preferably in the form of a hollow fiber, with a crystalline polymer resin containing polyvinylidene fluoride as the main component, and within 50 μm from the outer surface of the hollow fiber membrane, the crystallinity of the polymer resin constituting the hollow fiber membrane is 30% or more. By making the crystallinity 30% or more, when the turbidity collides with the hollow fiber membrane, it is easy to suppress the deformation of the pores, and the turbidity is easy to rebound from the membrane surface, so it is preferred. In order to better suppress the deformation of the pores and rebound the turbidity, the crystallinity is more preferably 35% or more, and further preferably 40% or more. However, if the crystallinity exceeds 80%, the softness of the hollow fiber membrane is lost, and it is easy to be damaged during operation, such as cross-flow operation, so it is preferably less than 80%.

[0086] When calculating the crystallinity of the hollow fiber membrane, it can be calculated based on the measurement results of a differential scanning calorimeter (hereinafter referred to as DSC). The slices within 50 μm from the outer surface of the hollow fiber membrane used in the measurement of the crystallinity are collected using a commercially available cryo-microtome. In the microtome, the hollow fiber membrane can be cut by bringing the blade into contact with the hollow fiber membrane after the hollow fiber membrane is moved at a certain moving distance. The blade is set in a direction parallel to the surface of the hollow fiber membrane. First, the moving distance is approached to the blade at intervals of 5 μm, and the hollow fiber membrane is cut once. Then, the moving distance is set to 40 μm, and it is cut once more, so that a surface portion with a thickness of 40 to 45 μm can be collected from the surface.

[0087] In addition, biopolymer concentration is generally measured by the above-mentioned LC-OCD method, such as Figure 5As shown, the filtration resistance increase (δA) when filtering the semipermeable membrane supply water 6 with an ultrafiltration membrane 72 having a dextran removal rate of 60% or more and 95% or less with a weight average molecular weight of 40,000 Da can also be used as a substitute for the biopolymer concentration. By using the filtration resistance increase (δA) of the ultrafiltration membrane as an indicator, it is easy to perform online measurements and operation control based on the measurement results, so it is preferred. The ultrafiltration membrane 72 is a membrane with a high biopolymer removal rate, and on the other hand, it allows low molecular weight components to pass through. In addition, the semipermeable membrane supply water is water that has been pretreated in advance, so it does not contain solid components such as turbidity. Therefore, the filtration resistance increase (δA) when filtering the semipermeable membrane supply water 6 with the ultrafiltration membrane 72 comes from the biopolymer, so δA becomes a substitute indicator for the biopolymer concentration.

[0088] δA is obtained, for example, as follows: When semipermeable membrane feed water is supplied to the ultrafiltration membrane at a predetermined filtration pressure P1 and filtration is performed, and the time until a predetermined amount W1 of filtered water is obtained is defined as t1, the filtration resistance R1 [1 / m] is obtained as shown in the following formula (4).

[0089] (Filtration resistance R1) = (Filtration pressure P1) × (Filtration time t1) × (Membrane area A) / {(Raw water viscosity μ) × (Predetermined amount W1)} ... Formula (4)

[0090] By continuously filtering, the filtration resistance R1 is continuously obtained, the total filtration water volume per unit membrane area is plotted on the horizontal axis, and R1 is plotted on the vertical axis, and the rate of change (slope) of the filtration resistance relative to the total filtration water volume per unit membrane area is taken as the filtration resistance rise (δA). Alternatively, the filtration flow rate q1 can be measured using a flow meter, etc., and the filtration resistance R1 can be calculated using the following formula (5).

[0091] (Filtration resistance R1) = (Filtration pressure P1) × (Membrane area A) / {(Raw water viscosity μ) × (Filtration flow rate q1)} ···Formula (5)

[0092] In addition, when an ultrafiltration membrane having a weight average molecular weight of 40,000 Da and a glucan removal rate of 60% or more and 95% or less is used in the pretreatment section 5, the increase in filtration resistance (δA) when the semipermeable membrane feed water 6 is made to flow back to the ultrafiltration membrane can be used as a substitute indicator for the biopolymer concentration. Figure 5 As shown, when an ultrafiltration membrane is used in the pretreatment section 5, in order to clean and remove the dirt components attached to the ultrafiltration membrane by filtration, it usually has the function of backflow cleaning using the filtered water of the ultrafiltration membrane (semipermeable membrane supply water), so there is no need to add a new device for measuring δA.

[0093] δA can be obtained using the filtration resistance R1 calculated by the above formula (5) based on the pressure difference between the membranes (filtration pressure) during backflow cleaning. However, in the operation of a normal ultrafiltration membrane, after 20 to 30 minutes of filtration, backflow cleaning is performed for 30 to 60 seconds. Since the pressure difference between the membranes during the backflow cleaning period includes the resistance from the dirt components remaining in the membrane, it is sometimes difficult to accurately measure δA. Therefore, when measuring δA, it is preferable to extend the backflow cleaning period longer than usual, and after the removal of the dirt remaining in the membrane is completed, δA is obtained based on the data during the period when the pressure difference between the membranes is observed to rise. In addition, in the operation of the ultrafiltration membrane, a process of injecting a chemical solution 15 (sodium hypochlorite, etc.) for backflow cleaning is often set about 1 to 2 times a day, so that the ultrafiltration membrane is immersed in the chemical solution for about 20 minutes to remove the dirt in the membrane. After the backflow cleaning process of the chemical solution is completed, backflow cleaning can be performed to obtain δA.

[0094] In the present invention, when the δA obtained by the above method exceeds a predetermined threshold, the conditions of at least one of the above pretreatment step, the above cleaning step, and the above bactericide addition step are added or strengthened. The threshold of δA is preferably 2.5×10 -12 / m 2 Thereby, the development of fouling of the semipermeable membrane can be effectively suppressed.

[0095] Example

[0096] Hereinafter, the present invention will be described in detail, but the present invention is not limited to the embodiments.

[0097] (Reference Example 1)

[0098] A hollow fiber porous membrane A including a support membrane was obtained by the following production method.

[0099] 38% by mass of PVDF (KUREHA; KF1300, weight average molecular weight 350,000 Da) and 62% by mass of γ-butyrolactone were mixed and dissolved at 160°C to prepare a support membrane stock solution. While an 85% by mass γ-butyrolactone aqueous solution was allowed to accompany the support membrane stock solution as a hollow portion forming liquid, it was expelled from a double nozzle die. The expelled support membrane stock solution was solidified in a cooling bath set at 30 mm below the die at a temperature of 20°C and added with an 85% by mass γ-butyrolactone aqueous solution, to produce a hollow fiber support membrane with a spherical structure.

[0100] 12 mass% of PVDF (Kynar (registered trademark) 710 manufactured by ARKEMA, with a weight average molecular weight of 180,000 Da), 4.0 mass% of cellulose diacetate (CA-398-3 manufactured by EASTMAN), 4.0 mass% of cellulose triacetate (CA-436-80S manufactured by EASTMAN), and 80.0 mass% of N-methyl-2-pyrrolidone were mixed and stirred at 120°C for 4 hours to prepare a polymer solution.

[0101] Next, the outer surface of the hollow fiber-shaped support membrane was uniformly coated with a polymer solution (thickness 50 μm) at 10 m / min. The support membrane coated with the polymer solution was pulled at 10 m / min, and after 1 second of coating, it was immersed in a coagulation bath of distilled water at 25°C for 10 seconds to coagulate, forming a porous membrane A with a three-dimensional network structure. The obtained porous membrane A was evaluated, and the results showed that the removal rate of dextran with a weight average molecular weight of 200,000 Da was 94%, the removal rate of dextran with a weight average molecular weight of 40,000 Da was 55%, and the number of surface pores was 196 / μm 2 The average surface pore size is 15 nm, and the value X obtained by dividing the number of surface pores by the average surface pore size is 13 / μm. 2 / nm. In addition, the ratio of α-type / β-type structure crystal = H α / H β is 1.0, and the permeability of pure water is 0.41m 3 / m 2 / h / 50kPa, the surface elastic modulus is 150MPa, and the crystallinity is 11%.

[0102] Here, the removal rate of dextran with a weight average molecular weight of 200,000 Da is calculated according to the following steps. Dextran (manufactured by Aldrich; weight average molecular weight 200,000 Da) is mixed with 1000 ppm distilled water to prepare a dextran aqueous solution. The prepared dextran aqueous solution is supplied to a porous membrane at 25°C so that the membrane pressure difference becomes 10 kPa, and cross-flow filtration is performed at a cross flow linear velocity of 1.0 m / sec, and the permeate is sampled. Here, the cross flow linear velocity is the value obtained by dividing the flow rate in the direction perpendicular to the filtration direction of the filtered stock solution by the cross-sectional area of ​​the flow path of the flow. In addition, the membrane pressure difference refers to the difference between the pressure on the filtered stock solution side across the porous membrane and the pressure on the permeate side. At the time of sampling the permeate, the dextran aqueous solution (stock solution) supplied to the porous membrane is sampled. The refractive indices of the permeate and the stock solution are measured, and the removal rate is calculated based on the following formula (2): T.

[0103] Dextran removal rate T (%) = {(refractive index of stock solution) - (refractive index of permeate)} / (refractive index of stock solution) × 100 Formula (2)

[0104] The removal rate of dextran with a weight average molecular weight of 40,000 Da was determined by the same test method as that of dextran with a weight average molecular weight of 200,000 Da. The dextran used was manufactured by Aldrich and had a weight average molecular weight of 40,000 Da.

[0105] After the porous membrane was vacuum dried at 25°C for one night, it was observed using a SEM (manufactured by Hitachi High-Technologies Co., Ltd.; S-5500) at a magnification of 30,000 to 100,000 times. The image obtained by observing the SEM of the porous membrane surface was binarized using the free software "ImageJ". When binarizing, after setting Subtract Background to 1 pixel and performing CreateBackground, select the condition "RenyiEntropy" in Threshold (binarization threshold). In the resulting binary image, the area of ​​each hole was determined by selecting Area in Analyze Particles, and the diameter calculated assuming that each hole was a circle was used as the surface pore diameter. When calculating the average value of the surface pore diameter, the pore diameters of more than one thousand holes were averaged and calculated. The number of holes was divided by the area of ​​the observed area to obtain the number of holes per unit area.

[0106] The IRTracer-100 manufactured by Shimadzu Corporation was used to measure the total reflection at a resolution of 8 cm -1 The IR spectrum of the ultrafiltration membrane surface was measured. In the obtained spectrum, according to the wavelength at 763 cm -1 The peak height (H α ) and at 840cm -1 The peak height (H β ), and the ratio of α-type structure crystals to β-type structure crystals was calculated using the following formula (3).

[0107] The ratio of α-type / β-type structure crystals = H α / H β Formula (3)

[0108] The slices within 50 μm from the outer surface of the hollow fiber ultrafiltration membrane used in the determination of crystallinity are collected using a commercially available cryostat (manufactured by Leica; Jung CM3000). The hollow fiber membrane immersed in distilled water is frozen at -20°C using a cryostat (manufactured by Leica; Jung CM3000), and a blade is set in a direction parallel to the surface of the hollow fiber membrane. First, the moving distance is approached to the blade at intervals of 5 μm, and the hollow fiber membrane is cut once. Then, the moving distance is set to 40 μm, and it is cut once more, thereby collecting hollow fiber membrane slices with a thickness of 40 to 45 μm from the surface. The slices are set on the DSC (manufactured by SEIKO: DSC6200), and when the temperature rises from room temperature to 300°C at a rate of 5°C / min, the endothermic peak of each polymer resin is regarded as the heat of fusion, and the heat is divided by the heat of complete crystallization fusion of the polymer resin to calculate as a percentage. For example, for polyvinylidene fluoride resin, the endothermic peak observed in the range of 100 to 190° C. is regarded as the heat of fusion of the polyvinylidene fluoride resin, and this heat is divided by the heat of complete crystal fusion of the polyvinylidene fluoride resin, 104.6 J / g, to calculate the percentage.

[0109] The surface elastic modulus of the hollow fiber ultrafiltration membrane is measured by installing a diamond Berkovich indenter SA2 manufactured by KLM, and applying and releasing the compression load in the room temperature atmosphere using the method based on ISO14577. The dried hollow fiber membrane is cut into pieces of about 1 cm in length and fixed on a 1 cm square metal plate with double-sided tape for measurement. The maximum load is set to 0.1 mN, the application time is set to 15 seconds, and the maximum load holding time is set to 30 seconds. The measurement is carried out at room temperature in the atmosphere. In the case of measuring the hollow fiber membrane with polyvinylidene fluoride resin as the main component, the surface elastic modulus is calculated with a Poisson's ratio of 0.35. With N=5, different positions are pressed in respectively, and the surface elastic modulus is calculated within the range of 0.4μm to 0.8μm of the pressing depth. The same measurement is carried out using 3 different hollow fiber membranes, and the average value of all the measured values ​​is taken as the measurement result of the surface elastic modulus.

[0110] The pure water permeability of the hollow fiber ultrafiltration membrane was determined by the following method: a small module of about 10 cm in length was prepared, consisting of about 1 to 10 hollow fiber membranes, and distilled water was supplied from the surface A for full filtration under the conditions of a temperature of 25°C and a filtration pressure difference of 18.6 kPa. The permeable water volume (m 3 ) and the values ​​obtained are expressed in terms of unit time (hr), unit effective membrane area (m 2 ), convert every 50kPa.

[0111] (Reference Example 2)

[0112] Based on the membrane preparation of Reference Example 1, the composition ratio of the polymer solution was changed to 12 mass% of PVDF (manufactured by ARKEMA; Kynar (registered trademark) 710, weight average molecular weight 180,000 Da), 4.8 mass% of diacetyl cellulose (manufactured by EASTMAN; CA-398-3), 2.4 mass% of triacetyl cellulose (manufactured by EASTMAN; CA-436-80S), 68.7 mass% of N-methyl-2-pyrrolidone, and 12.1 mass% of 2-pyrrolidone. The membrane preparation was carried out in the same manner as above to obtain a porous membrane B.

[0113] The porous membrane B obtained was evaluated in the same manner as in Reference Example 1. The removal rate of dextran with a weight average molecular weight of 200,000 Da was 96%, the removal rate of dextran with a weight average molecular weight of 40,000 Da was 71%, and the number of surface pores was 444 / μm. 2 The average surface pore size is 7.2 nm, and the value X obtained by dividing the number of surface pores by the average surface pore size is 62 / μm. 2 / nm. In addition, the ratio of α-type / β-type structure crystal H α / H β The permeability of pure water is 0.43m 3 / m 2 / h / 50kPa, the surface elastic modulus is 310MPa, and the crystallinity is 49%.

[0114] (Reference Example 3)

[0115] The hollow fiber support membrane obtained in the membrane formation of Reference Example 1 was designated as a porous membrane C.

[0116] The porous membrane C obtained was evaluated in the same manner as in Reference Example 1. The removal rate of dextran with a weight average molecular weight of 200,000 Da was 48%, and the number of pores on the surface was 0.1 / μm. 2 The average surface pore size is 1230 nm, and the value X obtained by dividing the number of surface pores by the average surface pore size is 8×10 -5 Pieces / μm 2 / nm. In addition, the water permeability of pure water is 1.3m 3 / m 2 / h / 50kPa, and the surface elastic modulus is 130MPa.

[0117] (Example 1)

[0118] In seawater desalination equipment A, according to Figure 2The treatment method shown in the figure is to treat seawater. First, seawater 21 is taken and stored in a seawater storage tank 22. Then, the seawater is fed to a sand filter tank 24 (filtration area: 17 m 2 , filter layer height: 1.5m, filter material: sand (average pore size 0.6mm)) is supplied and filtered to pre-treat the seawater. The filtration speed is set to 10m / h, and back pressure cleaning and air cleaning are performed once a day. In addition, a coagulant 25 (ferric chloride 3mg-Fe / L) is added to the front section of the sand filter tank. The seawater pre-treated by the coagulation sand filtration method is temporarily stored in the semipermeable membrane supply water storage tank 26, and then transported to the high-pressure pump 28 by the water delivery pump 27. The seawater is pressurized by the high-pressure pump and filtered by the semipermeable membrane 29 to obtain permeated water 30 and concentrated water 31.

[0119] The semipermeable membrane is made of polyamide, with a desalination rate of 99.8% and a membrane area of ​​37m 2 Spiral reverse osmosis membrane. When in operation, the membrane filtration flux is 14L / m 2 / hr, the recovery rate is 37%. In addition, the recovery rate is calculated by the flow rate of permeated water / (flow rate of permeated water + flow rate of concentrated water) × 100. In addition, during the operation of the semipermeable membrane, the pressure difference between the semipermeable membrane supply water and the concentrated water (hereinafter referred to as the operating pressure difference) is monitored to observe the changes in the operating performance. In addition, a bactericide 32 can be added between the water supply pump and the high-pressure pump. In addition, a pipeline for introducing cleaning chemicals 33 can be set between the high-pressure pump and the semipermeable membrane for chemical cleaning. In addition, the cleaning chemicals can be exported from the middle of the concentrated water piping for circulation cleaning.

[0120] During the operation of equipment A, the biopolymer concentration in the semipermeable membrane supply water is regularly measured by the LC-OCD method. In addition, the period when the biopolymer concentration exceeds 75μgC / L is regarded as a period with high potential for biofouling, and as an additional condition for the cleaning process, flushing and cleaning using the semipermeable membrane supply water is regularly performed. Specifically, the high-pressure pump is temporarily stopped, the filtration is stopped, and only the water supply pump is operated, and cleaning is performed for 30 to 60 minutes. After cleaning, the high-pressure pump is operated again and the operation is restarted. The period when the biopolymer concentration exceeds 75μgC / L is about 3 months in total, and flushing and cleaning are performed once a week during this period. By performing such operation, in equipment A, the operating pressure difference for 1 year rises to within 10% of the initial pressure difference, and it can be operated without chemical cleaning.

[0121] (Example 2)

[0122] In another series of equipment A, during the period when the biopolymer concentration in the semipermeable membrane feed water exceeded 75 μgC / L, a bactericide was added as an additional condition of the bactericide addition process instead of flushing. Specifically, DBNPA was added once every two days for one hour to a final concentration of 10 ppm. By performing such operation, in this series, the operating differential pressure rose to within 10% of the initial differential pressure over one year, and it was possible to operate without chemical cleaning.

[0123] (Example 3)

[0124] In another series of apparatus A, the biopolymer concentration in the concentrated water was measured regularly, and during the period when the biopolymer concentration exceeded 75×1 / (1-R)μgC / L (R is the recovery rate, and the recovery rate in apparatus A is 37%, so R=0.37), the same method as in Example 2 was used to add a bactericide as an additional condition of the bactericide addition step. By performing such operation, in this series, the operating differential pressure over one year increased to within 10% of the initial differential pressure, and the operation was possible without chemical cleaning.

[0125] (Example 4)

[0126] In another series of equipment A, the biopolymer concentration in the semipermeable membrane feed water was measured regularly. At the beginning of operation, the biopolymer concentration fluctuated around 65 μgC / L, but rose to about 90 μgC / L during a certain period. Therefore, as a reinforcement of the conditions of the pretreatment process, the concentration of ferric chloride added as a coagulant 25 was increased in the raw water before entering the sand filter tank 24.

[0127] First, the concentration of ferric chloride added was increased by 1 mg-Fe / L (0.07 times the amount exceeding 75 μgC / L) (i.e., the added concentration was 4 mg-Fe / L), but the biopolymer concentration in the semipermeable membrane feed water did not fall below 75 μgC / L. Then, 2 mgFe / L (0.13 times the amount exceeding 75 μgC / L) was increased (i.e., the added concentration was 5 mg-Fe / L), and although the removal rate of turbidity in sand filtration slightly decreased, the biopolymer concentration in the semipermeable membrane feed water fell below 75 μgC / L, so the operation was continued under this condition. After that, since the biopolymer concentration in the semipermeable membrane feed water fell to 40 μgC / L, the added concentration of the coagulant was restored to the original 3 mg-Fe / L, and the biopolymer concentration in the semipermeable membrane feed water became about 60 μgC / L, so the operation was continued in this state. When the biopolymer concentration in the semipermeable membrane feed water exceeded 75 μgC / L again, ferric chloride was added in an amount of 0.1 to 0.33 times the amount exceeding 75 μgC / L. In this series, the operating differential pressure in one year increased to within 15% of the initial differential pressure, and it was possible to operate without chemical cleaning. In addition, when the conditions of the pretreatment process were not strengthened, the average removal rate of biopolymers in the pretreatment was about 50%.

[0128] (Example 5)

[0129] In another series of device A, using Figure 3 The treatment method shown is used to treat seawater. First, seawater 41 is taken and stored in a seawater storage tank 42. Then, a supply pump 43 is used to supply the seawater to a microfiltration membrane 44 for filtration, thereby pre-treating the seawater. The microfiltration membrane is the porous membrane C shown in Reference Example 3. The membrane filtration flux is 1.0 m / d, and backflow cleaning and air cleaning (physical cleaning) are performed once every 20 minutes. In addition, once a day, sodium hypochlorite 45 is injected into the backflow cleaning water 46 to make its final concentration 300 mg / L, and liquid backflow cleaning is performed. After the pre-treated seawater is temporarily stored in a semipermeable membrane supply water storage tank 47, it is sent to a high-pressure pump 49 by a water supply pump 48, and is pressurized by a high-pressure pump and filtered by a semipermeable membrane 50 to obtain permeated water 51 and concentrated water 52.

[0130] The semipermeable membrane is made of polyamide, with a desalination rate of 99.8% and a membrane area of ​​37m 2 Spiral reverse osmosis membrane. When in operation, the membrane filtration flux is 14L / m 2 / hr, the recovery rate was 37%. During the operation of the semipermeable membrane, the operating pressure difference was monitored to observe the changes in the operating performance.

[0131] In the operation of this series of equipment A, as in the case of Example 4, when the biopolymer concentration in the semipermeable membrane feed water exceeds 75μgC / L, as an additional condition of the pretreatment process, operation is performed by adding ferric chloride in an amount of 0.1 times or more and 0.33 times or less than 75μgC / L. However, due to the membrane pretreatment, ferric chloride is not added when the biopolymer concentration in the semipermeable membrane feed water does not exceed 75μgC / L. As a result, in this series, the operating pressure difference for 1 year rose to within 10% of the initial pressure difference, and it was possible to operate without chemical cleaning. In addition, due to the membrane pretreatment, the turbidity removal rate in the pretreatment was always stable. In addition, during the period when the conditions of the pretreatment process were not added, the average removal rate of biopolymers in the pretreatment was about 50%.

[0132] (Example 6)

[0133] In another series of equipment A, the microfiltration membrane used in the pretreatment was changed to the porous membrane A shown in Reference Example 1, that is, the ultrafiltration membrane, and the pretreatment was performed. As in the case of Example 5, when the biopolymer concentration in the semipermeable membrane feed water exceeded 75μgC / L, as an additional condition of the pretreatment process, an operation was performed by adding ferric chloride in an amount of 0.1 times or more and 0.33 times or less exceeding 75μgC / L. As a result, in this series, the operating pressure difference in one year increased to within 10% of the initial pressure difference, and it was possible to operate without chemical cleaning. In addition, during the period when the pretreatment process was not added, the average removal rate of biopolymers in the pretreatment was about 70%, and the removal rate was improved compared with the case of Example 5, so the amount of coagulant added could be reduced to 1 / 4 in one year.

[0134] (Example 7)

[0135] In another series of equipment A, the microfiltration membrane used in the pretreatment was changed to the porous membrane B shown in Reference Example 2, that is, the ultrafiltration membrane, and the pretreatment was performed. As in the case of Example 5, when the biopolymer concentration in the semipermeable membrane feed water exceeded 75μgC / L, as an additional condition of the pretreatment process, an operation was performed by adding ferric chloride in an amount of 0.1 times or more and 0.33 times or less exceeding 75μgC / L. As a result, in this series, the operating pressure difference in one year increased to within 10% of the initial pressure difference, and it was possible to operate without chemical cleaning. In addition, during the period when the pretreatment process was not added, the average removal rate of biopolymers in the pretreatment was about 90%, and the removal rate was further improved compared with the case of Example 5, so the amount of coagulant added could be reduced to 1 / 10 in one year.

[0136] (Example 8)

[0137] In sewage reuse equipment B, Figure 4 The treatment method shown is used to treat the sewage secondary treatment water. First, the sewage secondary treatment water 61 is collected and stored in the sewage secondary treatment water storage tank 62. Then, the supply pump 43 is used to supply to the ultrafiltration membrane 44 for filtration, thereby pre-treating the sewage secondary treatment water. The ultrafiltration membrane is the porous membrane B shown in Reference Example 2. The membrane filtration flux is 1.0m / d, and backflow cleaning and air cleaning (physical cleaning) are performed once every 20 minutes. In addition, once a day, sodium hypochlorite 45 is injected into the backflow cleaning water 46 to make its final concentration 300mg / L, and the liquid backflow cleaning is implemented. After the pre-treated sewage secondary treatment water is temporarily stored in the semipermeable membrane supply water storage tank 47, it is sent to the high-pressure pump 49 by the water supply pump 48, and is pressurized by the high-pressure pump and filtered by the semipermeable membrane 50 to obtain permeated water 51 and concentrated water 52.

[0138] The semipermeable membrane is made of polyamide, with a desalination rate of 99.8% and a membrane area of ​​37m 2 Spiral reverse osmosis membrane. When in operation, the membrane filtration flux is 14L / m 2 / hr, the recovery rate is 75%. During the operation of the semipermeable membrane, the operating pressure difference is monitored to observe the changes in the operating performance. In addition, a bactericide 32 can be added between the water supply pump and the high-pressure pump.

[0139] During the operation of equipment B, the biopolymer concentration in the semipermeable membrane feed water was regularly measured by LC-OCD method. In addition, the period when the biopolymer concentration exceeded 75 μgC / L was regarded as a period with high potential for biofouling, and a bactericide was added as an additional condition for the bactericide addition process. Specifically, DBNPA was added once every 2 days and 1 hour to a final concentration of 10 ppm. By performing such operation, in this series, the operating pressure difference in 1 year rose to within 10% of the initial pressure difference, and it was possible to operate without chemical cleaning.

[0140] (Example 9)

[0141] During the operation of the device A (pretreatment process is the same as that of Example 5), instead of measuring the biopolymer concentration in the semipermeable membrane feed water, the filtration resistance increase (δA) when the semipermeable membrane feed water is filtered using the porous membrane B, i.e., the ultrafiltration membrane shown in Reference Example 2, is measured. Specifically, the semipermeable membrane feed water is pumped at a pressure of 60 kPa to the ultrafiltration membrane (membrane area of ​​0.18 m 2) is supplied, the filtration flux is measured by a flow meter, and the filtration resistance R1 is calculated by the above formula (5). By continuously filtering, R1 is continuously obtained, the total filtration water volume per unit membrane area is plotted on the horizontal axis, and R1 is plotted on the vertical axis. The rate of change (slope) of filtration resistance relative to the total filtration water volume per unit membrane area is calculated as the filtration resistance rise δA. After obtaining δA, in order to clean the ultrafiltration membrane, the RO permeate water of device A is used for backflow cleaning, and then filtration is started again to obtain δA. By repeating this operation cycle, δA is continuously measured over time. In addition, when δA exceeds 2.5×10 -12 / m 2 During the period, as an additional condition of the bactericide addition process, bactericide was added. Specifically, DBNPA was added once every 2 days and 1 hour to a final concentration of 10 ppm. By performing such operation, in this series, the operating pressure difference in 1 year increased to within 10% of the initial pressure difference, and it was possible to operate without chemical cleaning.

[0142] (Example 10)

[0143] During the operation of device A (pretreatment process is the same as that in Example 7), instead of measuring the biopolymer concentration in the semipermeable membrane feed water, the filtration resistance increase (δA) when the semipermeable membrane feed water is made to flow back to the ultrafiltration membrane of the pretreatment process is measured. Specifically, after the physical cleaning process (backflow cleaning and air cleaning) performed once every 20 minutes is completed, only one membrane unit is further backflow cleaned. After the pressure difference between the membranes turns to increase, the time data of the filtration resistance R1 is obtained by the above formula (5), and the total filtered water volume per unit membrane area is plotted on the horizontal axis, and R1 is plotted on the vertical axis. The rate of change (slope) of the filtration resistance relative to the total filtered water volume per unit membrane area is calculated as the filtration resistance increase δA. After obtaining δA, filtration of the membrane unit is started again. By repeating this operation, δA is continuously measured over time. When δA is greater than 2.5×10 -12 / m 2 During the period, as an additional condition of the bactericide addition process, bactericide was added. Specifically, DBNPA was added once every 2 days and 1 hour to a final concentration of 10 ppm. By performing such operation, in this series, the operating pressure difference in 1 year increased to within 10% of the initial pressure difference, and it was possible to operate without chemical cleaning.

[0144] (Comparative Example 1)

[0145] During the operation of device A, the biopolymer concentration in the semipermeable membrane supply water is not measured, but the operating differential pressure is monitored, and chemical cleaning is performed when the operating differential pressure exceeds the reference value. As a method of chemical cleaning, a 0.1% sodium hydroxide solution is used as a cleaning agent 10, which is placed in a cleaning tank, introduced into the RO piping from the downstream of the high-pressure pump 8 by a pump, and then discharged from the middle of the piping of the concentrated water 4 and circulated for 1 hour of cleaning. Then, it is immersed overnight and finally rinsed to complete the cleaning. As a result of the operation, the operating differential pressure increased sharply due to biological fouling 3 months after the start of operation. Although chemical cleaning was performed, the operating differential pressure did not fully recover, and the operating differential pressure increased again 1 month later, so the membrane had to be replaced.

[0146] (Comparative Example 2)

[0147] During another operation of the device A, the biopolymer concentration in the semipermeable membrane feed water was regularly measured by LC-OCD method. From the beginning of the operation, the biopolymer concentration in the semipermeable membrane feed water exceeded 75μgC / L and fluctuated between 80 and 85μgC / L. Despite this, no additional or enhanced pretreatment process, cleaning process, or bactericide addition process was implemented. Three months after the start of operation, the operating pressure difference increased sharply due to biofouling. Although chemical cleaning was performed, the operating pressure difference did not fully recover, and the operating pressure difference increased again one month later, which necessitated membrane replacement.

[0148] (Comparative Example 3)

[0149] During another operation period of the device A, the biopolymer concentration in the semipermeable membrane feed water was regularly measured by LC-OCD method, and the biopolymer concentration was below 75μgC / L for about 1 year. At this time, the operation was carried out without adding or strengthening the pretreatment process, cleaning process, and disinfectant addition process. The operating pressure difference during 1 year increased to within 10% of the initial pressure difference, and the operation was able to be carried out without chemical cleaning.

[0150] The present invention is described in detail using specific embodiments, but various changes and modifications can be made by those skilled in the art without departing from the intent and scope of the present invention. In addition, this application is based on the Japanese patent application (Japanese Patent Application No. 2022-152225) filed on September 26, 2022, and is cited herein in its entirety.

[0151] Industrial Availability

[0152] The present invention can be applied to obtaining fresh water by desalinating seawater or salt water using a membrane, or obtaining reuse water by purifying sewage treatment water or industrial wastewater.

[0153] Description of Reference Numerals

[0154] 1 Treated water

[0155] 2 Semipermeable membrane

[0156] 3. Through water

[0157] 4 Concentrated water

[0158] 5 Pre-processing unit

[0159] 6 Semipermeable membrane water supply

[0160] 7 Water pump

[0161] 8 High pressure pump

[0162] 9. Fungicide

[0163] 10. Cleaning agent

[0164] 11 Supply pump

[0165] 12 Coagulants

[0166] 13 Backflow cleaning pump

[0167] 14 Backflow cleaning water

[0168] 15. Liquid

[0169] 16 Backflow cleaning and drainage

[0170] 21 Seawater

[0171] 22 Seawater storage tank

[0172] 23 Supply pump

[0173] 24 Sand filter tank

[0174] 25 Coagulant

[0175] 26 Semipermeable membrane water supply storage tank

[0176] 27 Water pump

[0177] 28 High pressure pump

[0178] 29 Semipermeable membrane

[0179] 30 Through water

[0180] 31 Concentrated water

[0181] 32 Fungicide

[0182] 33 Cleaning chemicals

[0183] 41 Seawater

[0184] 42 Seawater storage tank

[0185] 43 Supply Pump

[0186] 44 Microfiltration membrane or ultrafiltration membrane

[0187] 45 Sodium hypochlorite

[0188] 46 Backflow cleaning water

[0189] 47 Semipermeable membrane water supply storage tank

[0190] 48 Water pump

[0191] 49 High pressure pump

[0192] 50 Semipermeable membrane

[0193] 51 Filtered Water

[0194] 52 Concentrated water

[0195] 53 Backflow cleaning pump

[0196] 61 Secondary sewage treatment water

[0197] 62 Sewage secondary treatment water storage tank

[0198] 71 Supply pump

[0199] 72 Ultrafiltration membrane

Claims

1. A method for producing fresh water, comprising a membrane treatment step of treating semipermeable membrane feed water with a semipermeable membrane to separate it into permeated water and concentrated water, and at least one of the following steps A to C, A: a pretreatment step of pretreating the water to be treated to obtain the semipermeable membrane feed water, B: a cleaning step of cleaning the semipermeable membrane, C: a bactericide adding step of adding a bactericide to the semipermeable membrane, When the biopolymer concentration of the semipermeable membrane supply water exceeds 75 μgC / L, or when the biopolymer concentration of the concentrated water exceeds 75×1 / (1-R) ​​μgC / L when the recovery rate of the permeated water in the membrane treatment process is set to R, the operating conditions of at least one of the pretreatment process, the cleaning process and the bactericide addition process are added or strengthened.

2. The method for producing fresh water according to claim 1, In the pretreatment step, the operating conditions of the pretreatment step are supplemented or strengthened by adding a coagulant.

3. The method for producing water according to claim 2, When the biopolymer concentration of the semipermeable membrane feed water exceeds 75 μgC / L, ferric chloride is added as the coagulant at a concentration of (P-75)×0.1 mg-Fe / L or more and (P-75)×0.33 mg-Fe / L or less, with the concentration being P and the unit being μgC / L.

4. The method for producing fresh water according to any one of claims 1 to 3, The pretreatment process includes a microfiltration membrane method or an ultrafiltration membrane method.

5. The method for producing fresh water according to claim 4, The ultrafiltration membrane used in the ultrafiltration membrane method has a removal rate of 55% or more and 99% or less for dextran having a weight average molecular weight of 200,000 Da.

6. The method for producing fresh water according to claim 4 or 5, The ultrafiltration membrane used in the ultrafiltration membrane method has a removal rate of 60% or more and 95% or less for dextran having a weight average molecular weight of 40,000 Da.

7. The method for producing fresh water according to claim 5 or 6, The surface pore number of the ultrafiltration membrane is 200 / μm 2 Above and 2000 / μm 2 the following.

8. The method for producing fresh water according to any one of claims 5 to 7, The average value of the surface pore diameter of the ultrafiltration membrane is 5.0 nm or more and 12.0 nm or less.

9. The method for producing fresh water according to any one of claims 5 to 8, The value obtained by dividing the number of pores on the surface of the ultrafiltration membrane by the average value of the surface pore size of the ultrafiltration membrane is 30 pores / μm 2 / nm or more and 100 / μm 2 / nm or less.

10. A method for producing fresh water, comprising a membrane treatment step of treating semipermeable membrane feed water with a semipermeable membrane to separate it into permeated water and concentrated water, and at least one of the following steps A to C: A: a pretreatment step of pretreating the water to be treated to obtain the semipermeable membrane feed water, B: a cleaning step of cleaning the semipermeable membrane, C: a bactericide adding step of adding a bactericide to the semipermeable membrane, When the filtration resistance increase δA when filtering the semipermeable membrane supply water using an ultrafiltration membrane having a removal rate of more than 60% and less than 95% for dextran with a weight-average molecular weight of 40,000 Da exceeds a predetermined threshold, the operating conditions of at least one of the pretreatment process, the cleaning process and the disinfectant addition process are added or strengthened.

11. The method for producing fresh water according to claim 10, The pretreatment process includes pretreating the treated water using an ultrafiltration membrane with a removal rate of 60% or more and 95% or less for dextran with a weight-average molecular weight of 40,000 Da. When the increase in filtration resistance δA when the semipermeable membrane supplies water to the ultrafiltration membrane and flows back to the ultrafiltration membrane exceeds a predetermined threshold, the operating conditions of at least one of the pretreatment process, the cleaning process and the disinfectant addition process are added or strengthened.

12. The method for producing fresh water according to claim 10 or 11, The threshold value of the filtration resistance increase δA is 2.5×10 -12 / m 2 .

Images