Water purification method and raw water evaluation method

Abstract

To provide a water purification method capable of suppressing increase in a transmembrane pressure difference in a water purification method where a processing object is clear raw water.SOLUTION: A water purification method 1 is a water purification method 1 that can obtain filtration water by membrane filtration of raw water W having been injected with a flocculant F and includes a particle number measurement step S12, a flocculant injection ratio determination step S13, and a flocculant injection step S14. In the particle number measurement step S12, a number of particles in a prescribed diameter range among particles in raw water W is measured as a measurement particle number. In the flocculant injection ratio determination step S13, an injection ratio of the flocculant F is determined as a determined injection ratio based on the number of the particle. In the flocculant injection step S14, the flocculant F is injected based on the determined injection ratio. The prescribed diameter range is a diameter range of a particle in the vicinity of the membrane pore size.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a water purification method and a raw water evaluation method. [Background technology]

[0002] Conventionally, in water treatment processes such as water purification, one operation cycle consists of a membrane filtration operation in which raw water is filtered through a membrane using a membrane filtration device, and a backwashing operation in which the membrane filtration operation is paused and backwashing is performed. This operation cycle is repeated. During membrane filtration operation, clogging substances (turbidity, organic matter, etc.) that clog the surface of the separation membrane or the pores of the separation membrane (membrane pores) (membrane clogging) are removed by the backwashing operation. This suppresses the increase in transmembrane pressure at the start of membrane filtration operation in each operation cycle. However, if the clogging substances are not completely removed by the backwashing operation and remain inside the pores of the separation membrane and accumulate, the transmembrane pressure at the start of membrane filtration operation will increase over the long-term repetition of the operation cycle.

[0003] In contrast, a coagulant is injected into the raw water in a mixing basin prior to membrane filtration to coagulate turbid substances in the raw water, producing flocs that are larger than the pores of the separation membrane. This prevents clogging substances from entering and accumulating in the pores of the separation membrane. The injection rate of the coagulant into the raw water is adjusted (controlled) so that the turbidity, color, etc. of the raw water are used as indicators. This, combined with repeated membrane filtration and backwash operations, prevents clogging substances from accumulating in the pores of the separation membrane, and suppresses an increase in transmembrane pressure at the start of membrane filtration operation. An example of such a water purification method is the water purification method described in Patent Document 1.

[0004] When using clear raw water with low turbidity and color, the water purification method described in Patent Document 1 makes it difficult to accurately measure turbidity and color because the low levels make it difficult to properly control the coagulant injection rate. In other words, when using clear raw water with the water purification method described in Patent Document 1, if the coagulant injection rate is not properly controlled, fine clogging substances may accumulate in the pores of the separation membrane of the membrane filtration device, resulting in an increase in transmembrane pressure at the start of membrane filtration operation. An increase in transmembrane pressure increases the power costs required for membrane filtration operation, the frequency of backwashing operations, and the cost of chemical cleaning to clear membrane blockages using chemical solutions, or increases the coagulant costs due to the injection of excess coagulant to suppress transmembrane pressure. Furthermore, with conventional water purification methods, even when raw water is so clear that it is deemed unnecessary to inject a coagulant, repeated operation cycles without injecting a coagulant can cause problems with the rise in transmembrane pressure at the start of membrane filtration operation. Therefore, a new water purification method that can appropriately adjust and control the coagulant injection rate even for clear raw water is desired. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 3830085 Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made in view of the above-mentioned problems, and aims to provide a water purification method for treating clear raw water, which can suppress an increase in transmembrane pressure difference. [Means for solving the problem]

[0007] According to one aspect of the present invention, a water purification method is a water purification method in which raw water into which a coagulant has been injected is subjected to membrane filtration to obtain filtrate, a particle number measuring step of measuring the number of particles in the raw water within a predetermined particle size range as a measured particle number; a flocculant injection rate determination step of determining the flocculant injection rate as a determined injection rate based on the measured particle number; a flocculant injection step in which the flocculant is injected based on the determined injection rate; Equipped with The predetermined particle size range is a particle size range of the particles in the vicinity of the average membrane pore size of the separation membrane used in the membrane filtration.

[0008] According to this, in the water purification method, the injection rate of the coagulant is determined using the number of particles in the raw water whose particle size range is close to the average membrane pore size of the separation membrane as an indicator. Particles close to the average membrane pore size contribute greatly to clogging of the separation membrane pores. Therefore, in the water purification method, the injection rate of the coagulant can be adjusted so that the number of particles that contribute greatly to clogging of the separation membrane pores is, for example, equal to or less than a desired particle number.

[0009] The water purification method according to the second invention further includes a coagulation condition setting step in which the relationship between the particle number and the required coagulant injection rate is set as a coagulant injection condition, the required injection rate is the injection rate of the flocculant required to make the number of particles in the specified particle size range in the raw water equal to or less than a predetermined threshold value, In the coagulant injection rate determination step, the determined injection rate is determined based on the coagulant injection conditions and the number of measured particles.

[0010] According to this, in the water purification method, the relationship between the number of particles within a predetermined particle size range and the required injection rate of the coagulant can be set in advance as a coagulant injection condition. Then, a determined injection rate is determined based on the coagulant injection condition and the measured particle count. By injecting the coagulant at the determined injection rate, the number of particles near the average membrane pore size becomes equal to or less than a predetermined threshold. Particles near the average membrane pore size contribute significantly to clogging of the pores in the separation membrane. Therefore, in the water purification method, the coagulant injection rate can be controlled so that the number of particles that contribute significantly to clogging of the pores in the separation membrane becomes equal to or less than a predetermined threshold.

[0011] In the water purification method according to the third invention, the particle number measuring step includes: A particle volume, which is the volume of the particles, is measured as a measured particle volume instead of the number of particles. Alternatively, instead of the number of particles, a particle weight, which is the weight of the particles, is measured as the measured particle weight instead of the number of particles measured.

[0012] According to this, the particle number measuring step of the water purification method can be configured to measure particle volume or particle weight instead of particle number.

[0013] In the water purification method according to the fourth invention, the predetermined particle size range is any range that is 0.5 times or more the average membrane pore size and 10 times or less the average membrane pore size.

[0014] According to this, in the water purification method, by setting the predetermined particle size range to the above range, the predetermined particle size range becomes a range close to the average membrane pore size, and particles within the predetermined particle size range contribute greatly to clogging of the separation membrane pores. Therefore, in the water purification method, the injection rate of the coagulant can be adjusted and controlled so that the number of particles that contribute greatly to clogging of the separation membrane pores is, for example, equal to or less than a desired particle number.

[0015] In the water purification method according to the fifth invention, the predetermined particle size range is any range that is 0.8 times or more the average membrane pore size and 5 times or less the average membrane pore size.

[0016] According to this, in the water purification method, by setting the predetermined particle size range to the above range, the predetermined particle size range becomes a range close to the average membrane pore size, and particles within the predetermined particle size range contribute more to clogging of the separation membrane pores. Therefore, in the water purification method, the injection rate of the coagulant can be adjusted and controlled so that the number of particles that contribute more to clogging of the separation membrane pores is, for example, equal to or less than a desired particle number.

[0017] In the water purification method according to the sixth invention, the average membrane pore size is 2 μm or less.

[0018] According to this, the water purification method can be configured as a water purification method for treating clear raw water by using a separation membrane having an average membrane pore size within the above range.

[0019] The water purification method according to the seventh invention is a water purification method in which raw water into which a coagulant has been injected is subjected to membrane filtration to obtain filtrate, The concentration of dissolved organic matter in the raw water is less than 1 mg / L, a dissolved organic matter concentration measuring step of measuring the dissolved organic matter concentration in the raw water as a measured dissolved organic matter concentration; a flocculant injection rate determination step of determining the flocculant injection rate as a determined injection rate based on the measured dissolved organic matter concentration; a flocculant injection step in which the flocculant is injected based on the determined injection rate; It is equipped with the following.

[0020] According to this, the water purification method can be configured as a water purification method for treating clear raw water, as long as the concentration of dissolved organic matter in the raw water is less than 1 mg / L. Furthermore, in the water purification method, the injection rate of the coagulant is determined using the concentration of dissolved organic matter in the raw water as an index. The concentration of dissolved organic matter contributes greatly to clogging of the pores of the separation membrane in a membrane filtration device that handles clear raw water. Therefore, in the water purification method, the injection rate of the coagulant can be adjusted so that the concentration of dissolved organic matter, which contributes greatly to clogging of the pores of the separation membrane, is, for example, a desired concentration or less.

[0021] The water purification method according to the eighth invention further comprises a coagulation condition setting step in which the relationship between the concentration of dissolved organic matter and the required injection rate of the coagulant is set as a coagulant injection condition, the required injection rate is the injection rate of the coagulant required to make the concentration of dissolved organic matter in the raw water equal to or lower than a predetermined threshold value, In the coagulant injection rate determination step, the determined injection rate is determined based on the coagulant injection conditions and the measured dissolved organic matter concentration.

[0022] According to this, in the water purification method, the relationship between the concentration of dissolved organic matter and the required injection rate of the coagulant can be set in advance as a coagulant injection condition. Then, a determined injection rate is determined based on the coagulant injection condition and the measured concentration of dissolved organic matter. By injecting the coagulant at the determined injection rate, the concentration of dissolved organic matter becomes equal to or less than a predetermined threshold. The concentration of dissolved organic matter contributes greatly to clogging of the pores of the separation membrane in a membrane filtration device that handles clear raw water. Therefore, in the water purification method, the injection rate of the coagulant can be controlled so that the concentration of dissolved organic matter, which contributes greatly to clogging of the pores of the separation membrane, becomes equal to or less than a predetermined threshold.

[0023] In the water purification method according to the ninth invention, in the coagulation condition setting step, the coagulation agent injection conditions are set based on the results of a coagulation condition setting test in which the coagulation properties of the raw water are tested for a number of different coagulant injection rates to evaluate the coagulation properties of the raw water.

[0024] According to this, in the water purification method, the coagulation condition setting step is tested and evaluated by a coagulation condition setting test, and the coagulant injection conditions are experimentally set. As a result, the water purification method can set the coagulant injection conditions that have been experimentally verified in advance at a stage before the water purification facility is put into operation.

[0025] The water purification method according to the tenth aspect of the present invention further comprises an oxidant injection step of injecting an oxidant into the raw water before injecting the coagulant, based on the measured dissolved organic matter concentration.

[0026] According to this method, the growth of microorganisms in dissolved organic matter is suppressed by injecting an oxidizing agent. Therefore, in the water purification method, the injection rate of the coagulant is appropriately adjusted and controlled so that the concentration of dissolved organic matter, which contributes greatly to clogging of the pores of the separation membrane, is kept below a desired concentration, for example, and the growth of microorganisms is further suppressed, thereby maintaining that state.

[0027] A raw water evaluation method according to the eleventh invention is a method for evaluating raw water supplied to a water purification treatment device that obtains filtrate by membrane filtration of raw water into which a coagulant has been injected, the method comprising: a particle number measuring step for measuring the number of particles within a predetermined particle size range in the raw water as a measured particle number; The predetermined particle size range is a particle size range of the particles in the vicinity of the average membrane pore size of the separation membrane used in the membrane filtration, The raw water is evaluated based on the number of particles measured.

[0028] According to this, the raw water evaluation method can evaluate raw water using the number of particles in the raw water that are close to the average membrane pore size as an index. Particles close to the average membrane pore size contribute greatly to clogging of the separation membrane pores. Therefore, in the raw water evaluation method, the injection rate of the coagulant can be adjusted so that the number of particles that contribute greatly to clogging of the separation membrane pores is, for example, equal to or less than a desired particle number.

[0029] In the raw water evaluation method according to the twelfth aspect of the present invention, the particle number measuring step comprises: A particle volume, which is the volume of the particles, is measured as a measured particle volume instead of the number of particles. Alternatively, instead of the number of particles, a particle weight, which is the weight of the particles, is measured as the measured particle weight instead of the number of particles measured.

[0030] According to this, the particle number measurement step of the raw water evaluation method can be configured to measure particle volume or particle weight instead of particle number.

[0031] A raw water evaluation method according to the thirteenth invention is a method for evaluating raw water supplied to a water purification treatment device that obtains filtered water by membrane filtration of raw water into which a coagulant has been injected, comprising: The concentration of dissolved organic matter in the raw water is less than 1 mg / L, a dissolved organic matter concentration measuring step of measuring the dissolved organic matter concentration in the raw water as a measured dissolved organic matter concentration; The raw water is evaluated based on the measured dissolved organic matter concentration.

[0032] According to this, the raw water evaluation method can be configured as a water purification method for treating clear raw water when the dissolved organic matter concentration in the raw water is less than 1 mg / L. Furthermore, the raw water evaluation method can evaluate the raw water using the dissolved organic matter concentration in the raw water as an index. The dissolved organic matter concentration in clear raw water contributes greatly to clogging of the pores in the separation membrane. Therefore, in the raw water evaluation method, the injection rate of the coagulant can be controlled so that the dissolved organic matter concentration, which contributes greatly to clogging of the pores in the separation membrane, is, for example, below a desired concentration. [Effects of the Invention]

[0033] According to the present invention, in a water purification method in which clear raw water is treated, an increase in transmembrane pressure difference due to membrane clogging can be suppressed. [Brief explanation of the drawings]

[0034] [Figure 1] 1 is a diagram showing a water purification treatment facility in which a water purification treatment method according to a first embodiment of the present invention is used. [Figure 2] FIG. 2 is a vertical cross-sectional view of a water purification treatment device in the water purification treatment facility. [Figure 3] FIG. 2 is a perspective view of a membrane element provided in the water purification treatment device. [Figure 4] FIG. 1 is a flow diagram of a water purification method according to a first embodiment of the present invention. [Figure 5] FIG. 2 is a diagram showing an example of a flocculant injection condition set in the flocculation condition setting step in the water purification method. [Figure 6]3 is a flowchart showing the process of the water purification method. [Figure 7] FIG. 10 is a diagram showing the results of measuring the number of particles in a predetermined particle size range in raw water flocculated under a plurality of different flocculation conditions. [Figure 8] FIG. 10 is a diagram showing the measurement results of the transmembrane pressure difference when raw water flocculated under a plurality of different flocculation conditions is subjected to membrane filtration. [Figure 9] FIG. 10 is a diagram showing an example of the results of measuring the concentration of dissolved organic matter and the number of particles within a predetermined particle size range. [Figure 10] FIG. 1 is a diagram showing the relationship between the concentration of dissolved organic matter and the number of particles within a predetermined particle size range. [Figure 11] FIG. 4 is a flow diagram of a water purification method according to a second embodiment of the present invention. [Figure 12] FIG. 2 is a diagram showing an example of a flocculant injection condition set in the flocculation condition setting step in the water purification method. [Figure 13] 3 is a flowchart showing the process of the water purification method. [Figure 14] 10A and 10B are diagrams showing examples of processing results of the water purification method. DETAILED DESCRIPTION OF THE INVENTION

[0035] A water purification method 1 according to a first embodiment of the present invention will be described below with reference to the drawings. In the drawings, identical or corresponding parts are designated by the same reference symbols, and descriptions thereof will not be repeated. In the following description, terms indicating positions or directions, such as "upper," "lower," "horizontal," and "vertical," may also be used. These terms are used for convenience to facilitate understanding of the embodiment, and are not limited to positions or directions when actually implemented.

[0036] <Embodiment 1> [Water purification facility 4] A water purification treatment facility 4 in which a water purification treatment method 1 according to a first embodiment of the present invention is used will be described with reference to Fig. 1. Fig. 1 is a diagram showing a water purification treatment facility 4 in which a water purification treatment method 1 according to a first embodiment of the present invention is used.

[0037] As shown in FIG. 1, the water purification treatment facility 4 includes a raw water storage section 5 that stores raw water W taken from a water source, and a membrane filtration device 10 (an example of a water purification treatment device) that performs membrane filtration of the supplied raw water W. The raw water storage section 5 includes a receiving well 6 and a mixing basin 7. The mixing basin 7 includes a coagulant injection section 7b and an agitator 7c. The coagulant injection section 7b injects coagulant F into the raw water W. The agitator 7c agitates the raw water W to which coagulant F has been injected. In the mixing basin 7, the raw water W to which coagulant F has been injected is agitated, thereby generating a large number of flocs (not shown) in the raw water W. A raw water supply pipeline 8 is connected between the mixing basin 7 and the membrane filtration device 10, and is equipped with a membrane filtration pump 8p and a first valve 8v. A backwash wastewater feed pipeline 9 branches off from the raw water supply pipeline 8. A second valve 9v is provided in the backwash drainage feed pipe 9. If the water source from which water is taken is not clear, such as river water or lake water, impurity removal means for removing large amounts of solids from the raw water W may be provided upstream of the raw water storage section 5.

[0038] [Membrane filtration equipment 10] Next, the configuration of the membrane filtration device 10 will be described with reference to Figures 2 and 3. Figure 2 is a longitudinal cross-sectional view of the membrane filtration device 10. Figure 3 is a perspective view of a membrane element 20 provided in the membrane filtration device 10. The membrane filtration device 10 is a casing-housed membrane filtration device in which the membrane element 20 is housed in a closed space 12 formed in a casing 11. The membrane filtration device 10 performs membrane filtration on the entire amount of raw water W supplied thereto.

[0039] The membrane element 20 is composed of a plurality of porous ceramic formed bodies 21 each having a substantially rectangular parallelepiped shape, and the porous ceramic formed bodies 21 are bonded to one another via a bonding material layer 22. A plurality of through-flow passages 23 are formed in the membrane element 20, penetrating between both end faces 24, 25. A separation membrane tank is formed on the inner surface of the through-flow passages 23. Raw water W is supplied into the through-flow passages 23, so that the inner surface of the through-flow passages 23 forms the primary side. Treated water (purified water) is filtered through the outer surface 27 of the membrane element 20, so that the outer surface 27 of the membrane element 20 forms the secondary side. A plurality of slits 26 are formed in the membrane element 20. The slits 26 extend in the axial direction of the through-flow passages 23 and are groove-shaped and open to the outer surface 27 of the membrane element 20, with both ends of the slits 26 closing near the end faces 24, 25 of the membrane element 20.

[0040] The casing 11 has an upper primary-side chamber 13 formed in its upper part, and a lower primary-side chamber 14 formed in its lower part. An upper end 24 of the membrane element 20 is exposed in the upper primary-side chamber 13, and a lower end 25 of the membrane element 20 is exposed in the lower primary-side chamber 14. A through-flow passage 23 connects the upper primary-side chamber 13 and the lower primary-side chamber 14. The lower primary-side chamber 14 has a raw water supply port 14b, and the upper primary-side chamber 13 has a compressed air inlet 13b. The casing 11 also has a purified water inlet and outlet 11b, which communicates with a space 12 surrounding the membrane element 20.

[0041] The water purification process using the water purification treatment facility 4 as described above will be described below. The water purification process includes a membrane filtration operation and a backwashing operation. In the membrane filtration operation, raw water W is subjected to membrane filtration using the membrane filtration device 10. In the backwashing operation, backwashing is performed while the membrane filtration operation is paused. The membrane filtration operation and the backwashing operation constitute one operation cycle, and this operation is repeated.

[0042] In membrane filtration operation, raw water W is subjected to membrane filtration in the membrane filtration device 10 to obtain filtered water. That is, in FIG. 1, the first valve 8v is opened, and the raw water W in the mixing basin 7 is supplied from the raw water W supply pipe to the raw water supply port 14b of the membrane filtration device 10. The supplied raw water W is membrane filtered from the through-flow passage 23 of the membrane element 20 of the membrane filtration device 10 from the primary side to the secondary side, thereby obtaining purified water. The obtained purified water is discharged from the purified water inlet / outlet 11b to outside the water purification facility (for example, to a purified water reservoir, etc.).

[0043] The backwash operation is a step in which, after membrane filtration operation has been performed for a predetermined time, purified water is used to backwash the membrane filtration device 10. Specifically, the first valve 8v is closed and the second valve 9v is opened. At this time, a portion of the purified water is passed through a pressure tank (not shown) in a pressurized state and supplied to the purified water outlet / inlet 11b of the membrane filtration device 10 through the purified water pipe 15. Pressurized air pressurized in the pressure tank also flows in through the pressurized air inlet 13b. The supplied purified water is pressurized by the pressurized air and flows back through the through-flow passage 23 of the membrane element 20 from the secondary side to the primary side, thereby backwashing the membrane element 20. The purified water that has flowed back through the membrane element 20 is discharged from the raw water supply port 14b as backwash wastewater and passed through the backwash wastewater feed pipe 9 to be discharged outside the facility (for example, to a wastewater tank).

[0044] In the backwashing operation, membrane filtration is performed to remove clogging substances (such as turbidity and organic matter) that clog the surface of the separation membrane and the pores of the separation membrane (membrane clogging). This suppresses the increase in transmembrane pressure at the start of membrane filtration operation in each operation cycle due to membrane clogging. However, some of the clogging substances that were not completely removed during the backwashing operation may continue to remain inside the pores of the separation membrane. In this case, repeated water purification processes over a long period of time may cause the clogging substances to accumulate inside the pores of the separation membrane, resulting in an increase in transmembrane pressure. The water purification method 1 of the present invention reduces the clogging substances that continue to remain inside the pores of the separation membrane and suppresses the increase in transmembrane pressure when the water purification processes are repeated over a long period of time.

[0045] [Water purification method 1] Next, the treatment steps of the water purification method 1 according to the first embodiment of the present invention will be described with reference to Fig. 4. Fig. 4 is a flow diagram of the water purification method 1. As shown in Fig. 4, the water purification method 1 includes a raw water treatment step S1 and a water purification step S2. The raw water treatment step S1 includes a coagulation condition setting step S11, a particle number measurement step S12, a coagulant injection rate determination step S13, and a coagulant injection step S14. The water purification step S2 includes a membrane filtration step S15 and a backwashing step S16.

[0046] In the water purification method 1, raw water W into which a coagulant F has been injected is subjected to membrane filtration to obtain filtered water. In the particle number measurement step S12, the number of particles [particles / mL] of particles within a predetermined particle size range per unit volume in the raw water W downstream (M in FIG. 1) of the raw water storage section 5 is measured as the measured particle number. Note that in the particle number measurement step S12, instead of the measured particle number, the particle volume, which is the volume of particles within the predetermined particle size range among the particles in the raw water W, may be measured as the measured particle volume. Alternatively, in the particle number measurement step S12, instead of the measured particle number, the particle weight, which is the weight of particles within the predetermined particle size range among the particles in the raw water W, may be measured as the measured particle weight.

[0047] The predetermined particle size range is the particle size range of particles close to the average membrane pore size of the separation membrane. Specifically, the predetermined particle size range is preferably 0.5 times or more the average membrane pore size and 10 times or less the average membrane pore size. When the predetermined particle size range is within the above range, particles within the predetermined particle size range contribute more to clogging of the separation membrane pores. It is more preferable that the predetermined particle size range is 0.8 times or more the average membrane pore size and 5 times or less the average membrane pore size. When the predetermined particle size range is within the above range, particles within the predetermined particle size range contribute even more to clogging of the separation membrane pores. In this embodiment, the predetermined particle size range is, for example, 0.1 μm or more and 0.45 μm or less. In this embodiment, the average membrane pore size is, for example, 0.1 μm.

[0048] Particle counts are measured using methods such as light scattering and electrophoresis. Light scattering estimates particle counts by measuring the intensity of light scattered by particles. Electrophoresis estimates particle counts by applying an electric field to particles and measuring their migration speed. Particle volume is estimated by first calculating the volume per particle, assuming the particle diameter is the average value within a predetermined particle size range and the particle shape is spherical. The calculated volume per particle is then multiplied by the measured number of particles to estimate the total volume of particles per unit volume in the raw water W. Particle weight is estimated by first estimating particle density, for example, by analyzing the particle components. The particle weight is then estimated by multiplying this density by the particle volume to estimate the total weight of particles per unit volume in the raw water W.

[0049] In the coagulation condition setting step S11, coagulation conditions are set. The coagulation conditions include, for example, the coagulant injection rate, coagulation pH, stirring strength, and coagulation time. Setting the coagulation conditions is an important step for achieving optimal floc formation and sedimentation of the formed flocs. The coagulant injection rate affects, for example, the rate of floc formation and the size of the flocs. The coagulation pH affects, for example, the charge state of the coagulant F and the charge state of the particle surface. The coagulation time affects, for example, the measurement of floc formation and the sedimentation of the flocs. The coagulation time affects, for example, the rate of floc formation.

[0050] These flocculation conditions are set, for example, by a jar test (an example of a flocculation condition setting test). Specifically, a jar test is performed for each flocculation condition at a plurality of different values ​​(for example, a plurality of flocculant injection rates, a plurality of flocculation pHs, etc.), and the value at which the number of particles in a predetermined particle size range after flocculation is the smallest (an example of flocculation property) is set as the optimum value.

[0051] Among the aggregation conditions, the flocculant injection rate is set according to the number of particles within a predetermined particle size range before aggregation (hereinafter referred to as "number of particles within a predetermined particle size range"). Specifically, in the aggregation condition setting step S11, the relationship between the number of particles within a predetermined particle size range N (horizontal axis) and the required injection rate R of flocculant F (vertical axis) is set as the flocculant injection condition, as shown in Fig. 5. Fig. 5 is a diagram showing an example of the flocculant injection condition set in the aggregation condition setting step S11.

[0052] The required injection rate R is the injection rate of the coagulant required to make the number N of particles in a predetermined particle size range in the raw water W after the coagulant injection equal to or less than a predetermined threshold value Nt. In this embodiment, the threshold value Nt is, for example, 10 × 10 6 The coagulant injection conditions are determined by conducting jar tests for combinations of different coagulant injection rates and different particle counts within a specified particle size range. As shown in Figure 5, the coagulant injection conditions are shown by lines 31, 32, and 33, which indicate the relationship between the number of particles in a specified particle size range, N, and the required injection rate, R.

[0053] In the coagulant injection rate determination step S13, the injection rate of coagulant F is determined as a determined injection rate based on the number of measured particles. As will be described in detail later, in the coagulant injection rate determination step S13, the determined injection rate is determined based on the coagulant injection conditions and the number of measured particles described above. In the coagulant injection step S14, coagulant F is injected into the raw water W based on the determined injection rate. In the membrane filtration step S15 in the water purification process S2, the membrane filtration operation described above is performed. In the backwashing step S16, the backwashing operation described above is performed.

[0054] In the water purification method 1, the raw water treatment step S1 and the water purification step S2 are configured as separate cycles, and the raw water treatment step S1 and the water purification step S2 may or may not be performed for the same number of cycles. The water purification method 1 may be configured, for example, so that one cycle of the raw water treatment step S1 is performed followed by ten cycles of the water purification step S2. In the water purification method 1, the raw water treatment step S1 and the water purification step S2 are repeated as appropriate. As shown in Figure 4, in the water purification method 1, when the raw water treatment step S1 is repeated, the coagulation conditions set initially are used, so that the coagulation condition setting step S11 is not performed from the second time onwards.

[0055] Next, the processing of the raw water treatment step S1 in the water purification method 1 will be described with reference to Figures 5 and 6. Figure 6 is a flowchart showing the processing of the water purification method 1.

[0056] As shown in FIG. 6, first, in step S21, the aggregation conditions described above are set. Next, in step S22, the number of particles within a predetermined particle size range is measured as the measured particle number. Then, it is determined whether the measured particle number is less than the lower limit particle number NL. If the measured particle number is less than the lower limit particle number NL (Yes in step S23), the lower limit injection rate RL is determined as the determined injection rate (line 31 shown in FIG. 5). Then, the flocculant F is injected at the lower limit injection rate RL (step S24), and the process ends (END). If the measured particle number is equal to or greater than the lower limit particle number NL (No in step S23), it is determined whether the measured particle number is equal to or greater than the upper limit particle number NU (step S25). If the measured particle number is equal to or greater than the upper limit particle number NU (Yes in step S25), the upper limit injection rate RU is determined as the determined injection rate (line 33 shown in FIG. 5). Then, the flocculant F is injected at the upper limit injection rate RU (step S26), and the process ends (END). When the number of measured particles is less than the upper limit particle number NU (No in step S25), the final injection rate is determined by the coagulant injection rate determination function [R = f(N)] (line 32 shown in Figure 5). For example, when the number of measured particles is N1, the final injection rate is determined as R1 (i.e., f(N1)). Then, coagulant F is injected at the final injection rate R1 (step S28), and the process ends (END). By injecting coagulant F into the raw water W at each of the final injection rates RL, RU, and R1 described above, the number of particles in the specified particle size range in the raw water W becomes equal to or less than the threshold value Nt shown in Figure 5.

[0057] [Relationship between the number of particles in a specified size range and the increase in transmembrane pressure] Next, the relationship between the number of particles in a predetermined particle size range and the increase in transmembrane pressure will be described with reference to Figures 7 and 8. Figure 7 shows the measurement results of the number of particles in a predetermined particle size range for raw water W aggregated under a plurality of different aggregation conditions. Figure 8 shows the measurement results of the transmembrane pressure when raw water W aggregated under a plurality of different aggregation conditions is subjected to membrane filtration. Here, the predetermined particle size range is 0.1 μm or more and 0.45 μm or less, as explained above.

[0058] 7, the number of particles in the specified particle size range (Na) under the coagulation conditions (condition A: no adjustment of coagulation pH), the number of particles in the specified particle size range (Nb) under the condition (condition B: 6.8) and the number of particles in the specified particle size range (Nc) under the condition (coagulation pH: high basicity) increase in this order. Note that the mixing intensity and mixing time for conditions A, B and C are 1 minute of rapid mixing and 10 minutes of slow mixing, and the coagulant injection rate is also constant.

[0059] Figure 8 shows the transmembrane pressure difference when the coagulation conditions are Condition A, Condition B, and Condition C in Figure 7. In Figure 8, the horizontal axis is time, and the vertical axis is transmembrane pressure difference. In Figure 8, the period indicated by reference numeral 36 is the period during which membrane filtration operation is performed, the backwashing operation is performed during the period indicated by reference numeral 37, and air bleeding is performed during the period indicated by reference numeral 38. As shown in Figure 8, five cycles of membrane filtration operation (period 36), backwashing operation (period 37), and air bleeding (period 38) are repeated.

[0060] As shown in Figure 8, the increase in transmembrane pressure during a single membrane filtration operation (period 36) during five repeated cycles of operation increases in the order of condition A, condition B, and condition C. That is, the increase in transmembrane pressure increases as the number of particles in the specified particle size range increases. In other words, there is a correlation between the number of particles in the specified particle size range and the increase in transmembrane pressure during a single membrane filtration operation (period 36). Furthermore, the greater the increase in transmembrane pressure during a single membrane filtration operation (period 36), the more clogging substances remain in the pores of the separation membrane. Therefore, the increase in transmembrane pressure at the start of membrane filtration operation due to repeated long-term water purification processes also increases in the order of condition A, condition B, and condition C. Therefore, in order to suppress the increase in transmembrane pressure at the start of membrane filtration operation during each operation cycle, it is effective to suppress the number of particles in the specified particle size range using the number of particles in the specified particle size range as an indicator.

[0061] [About action and effects] The operation and effect of the water purification method 1 with the above configuration and process will be described below. As described above, in the water purification method 1, the injection rate of the coagulant F is determined as the determined injection rate using the number of particles in a predetermined particle size range in the raw water W as an index. Particles close to the average membrane pore size contribute greatly to clogging of the pores in the separation membrane. Therefore, in the water purification method 1, the injection rate of the coagulant F can be adjusted so that the number of particles that contribute greatly to clogging of the pores in the separation membrane is, for example, equal to or less than a desired particle number.

[0062] In the water purification method 1 according to this embodiment, the relationship between the number of measured particles and the required injection rate of flocculant F can be set in advance as a flocculant injection condition. Then, a determined injection rate is determined based on the flocculant injection condition and the number of measured particles. By injecting flocculant F at the determined injection rate, the number of particles near the average membrane pore size becomes equal to or less than a predetermined threshold. Therefore, in the water purification method 1, the injection rate of flocculant F can be controlled so that the number of particles that contribute greatly to clogging of the pores of the separation membrane becomes equal to or less than a predetermined threshold.

[0063] As explained above, there is a correlation between the increase in transmembrane pressure at the start of long-term membrane filtration operation and the number of particles in a predetermined particle size range. Water purification method 1 can suppress particles in a predetermined particle size range to a predetermined threshold or less, thereby suppressing an increase in transmembrane pressure due to membrane blockage. As explained above as an example of this embodiment, it is preferable that the average membrane pore size of membrane element 20 is 2 μm or less. This allows water purification method 1 to be configured to treat clear raw water.

[0064] <Embodiment 2> Next, a water purification method 2 according to a second embodiment of the present invention will be described. In the water purification method 1 according to the first embodiment, the injection rate of the coagulant F was controlled using the number of particles in a predetermined particle size range as an indicator. In the water purification method 2 according to the second embodiment, the injection rate of the coagulant F is controlled using the dissolved organic matter concentration [mg / L] as an indicator instead of the number of particles in a predetermined particle size range. The dissolved organic matter concentration is the concentration of organic matter dissolved in water. Specifically, the total amount of organic matter passing through a 0.45 μm filter is measured as dissolved organic carbon (DOC). In this embodiment, the dissolved organic matter concentration in the raw water W is less than 1 mg / L.

[0065] One method for measuring the concentration of dissolved organic matter is, for example, LC-OCD (Liquid Chromatography-Organic Carbon Detection). LC-OCD is a device used to analyze organic matter in water. Unlike TOC meters (Total Organic Carbon Analyzers), which are conventional indicators of organic matter, LC-OCD separates and measures organic matter by molecular weight, allowing for more detailed analysis.

[0066] [Relationship between dissolved organic matter concentration and number of particles in a specified size range] First, the relationship between the concentration of dissolved organic matter and the number of particles in a predetermined particle size range will be described with reference to Figures 9 and 10. Figure 9 is a diagram showing an example of the results of measuring the concentration of dissolved organic matter and the number of particles in a predetermined particle size range, and Figure 10 is a diagram showing the relationship between the measured concentration of dissolved organic matter and the number of particles in a predetermined particle size range.

[0067] In Figure 9, the dissolved organic matter concentrations 46b, 47b, 48b, and 49b and the number of particles in a specified size range 46c, 47c, 48c, and 49c in raw water W are measured on different observation days as four observation data 46, 47, 48, and 49. In Figure 10, the four observation data 46, 47, 48, and 49 in Figure 9 are plotted with the number of particles in a specified size range on the horizontal axis and the dissolved organic matter concentration on the vertical axis. Figure 10 shows that there is a high correlation between the dissolved organic matter concentration and the number of particles in a specified size range (correlation coefficient: 0.99).

[0068] From this, it is estimated that the particles within the predetermined particle size range in the raw water W observed in Figure 9 are mainly composed of dissolved organic matter. Water purification method 2 is intended to treat such raw water W, which is composed mainly of dissolved organic matter and has particles within the predetermined particle size range.

[0069] Next, the treatment steps of water purification method 2 will be described with reference to Fig. 11. Fig. 11 is a flow diagram of water purification method 2. As shown in Fig. 11, water purification method 2 includes a raw water treatment step S3 and a water purification step S2. The raw water treatment step S3 includes a coagulation condition setting step S31, a dissolved organic matter concentration measurement step S32, a coagulant injection rate determination step S33, and a coagulant injection step S35. The water purification step S2 is the same as in the first embodiment described above, and therefore will not be described again.

[0070] In the dissolved organic matter concentration measurement step S32, the dissolved organic matter concentration [mg / L] in the raw water W in the downstream area (M in FIG. 1) of the raw water storage section 5 is measured as the measured dissolved organic matter concentration.

[0071] In the coagulation condition setting step S31, coagulation conditions are set. Among the coagulation conditions, the coagulant injection condition is set according to the dissolved organic matter concentration before coagulation. Specifically, in the coagulation condition setting step S31, the relationship between the dissolved organic matter concentration C (horizontal axis) and the required injection rate R of coagulant F (vertical axis) is set as the coagulant injection condition, as shown in FIG. 12. FIG. 12 is a diagram showing an example of the coagulant injection condition set in the coagulation condition setting step S31. As shown by the dotted line in FIG. 12, the coagulant injection condition of water purification method 2 may be set so that sodium hypochlorite (NaClO: an example of an oxidant) is added to the raw water W when the dissolved organic matter concentration reaches or exceeds a predetermined value (required concentration for adding oxidant) Cp (arrow 44 shown in FIG. 12).

[0072] The required injection rate R is the coagulant injection rate required to maintain the dissolved organic matter concentration C in the raw water W after coagulant injection at or below a predetermined threshold Ct. In this embodiment, the threshold Ct is, for example, 0.01 mg / L. The coagulant injection conditions are determined by conducting jar tests for combinations of multiple different coagulant injection conditions and multiple different dissolved organic matter concentrations. As shown in Figure 12, the coagulant injection conditions are represented by lines 41, 42, and 43, which indicate the relationship between the dissolved organic matter concentration C and the required injection rate R.

[0073] As will be described in detail later, in the flocculant injection rate determination step S33, the injection rate of flocculant F is determined as a determined injection rate based on the flocculant injection conditions and the measured dissolved organic matter concentration described above. In the flocculant injection step S35, flocculant F is injected into the raw water W based on the determined injection rate. As shown in FIG. 11, the water purification method 2 may be configured to include an oxidant injection step S34. In the oxidant injection step S34, sodium hypochlorite (NaClO: an example of an oxidant) is injected into the raw water W before the flocculant F is injected.

[0074] As in the case of embodiment 1, in the water purification method 2, the raw water treatment step S3 and the water purification step S2 are configured as separate cycles, and the raw water treatment step S3 and the water purification step S2 may or may not be performed for the same number of cycles. Also, as in the case of embodiment 1, in the water purification method 2, when the raw water treatment step S3 is repeated, the coagulation conditions set initially may be used, so that the coagulation condition setting step S31 is not performed from the second time onwards.

[0075] Next, the processing of the raw water treatment step S3 in the water purification method 2 will be described with reference to Figures 12 and 13. Figure 13 is a flowchart of the processing of the water purification method 2.

[0076] As shown in FIG. 13, first, in step S41, the flocculation conditions described above are set. Next, in step S42, the dissolved organic matter concentration is measured as the measured dissolved organic matter concentration. Then, it is determined whether the measured dissolved organic matter concentration is less than the lower limit concentration CL (step S43). If the measured dissolved organic matter concentration is less than the lower limit concentration CL (Yes in step S43), the lower limit injection rate RL is determined as the determined injection rate (line 41 shown in FIG. 12). Then, flocculant F is injected at the lower limit injection rate RL (step S44), and the process ends (END). If the measured dissolved organic matter concentration is equal to or greater than the lower limit concentration CL (No in step S43), it is determined whether the measured dissolved organic matter concentration is equal to or greater than the upper limit concentration CU (step S45). If the measured dissolved organic matter concentration is equal to or greater than the upper limit concentration CU (Yes in step S45), the upper limit injection rate RU is determined as the determined injection rate (line 43 shown in FIG. 12). Then, the flocculant F is injected at the upper limit injection rate RU (step S46), and the process ends (END). When the measured dissolved organic matter concentration is less than the upper limit concentration CU (No in step S45), it is determined whether the measured dissolved organic matter concentration is less than the oxide addition required concentration Cp (step S47).

[0077] When the measured dissolved organic matter concentration is less than the oxide addition required concentration Cp (Yes in step S47), the determined injection rate is determined by the coagulant injection rate determination function [R = f(C)] (line 42 shown in Figure 12). For example, when the measured dissolved organic matter concentration is C1 (see Figure 12), the determined injection rate is determined as R1 (i.e., f(C1)) (step S48). Then, coagulant F is injected into the raw water W at the determined injection rate R1 (step S49), and the process ends (END). When the measured dissolved organic matter concentration is greater than or equal to the oxide addition required concentration Cp (No in step S47), for example, when the measured dissolved organic matter concentration is C2 (see Figure 12), sodium hypochlorite (NaClO) is first added to the raw water W (step S50). Then, the determined injection rate is determined as R2 (i.e., f(C2)) (step S51). Then, the coagulant F is injected into the raw water W at the determined injection rate R2 (step S52), and the process ends (END). By injecting the coagulant F into the raw water W at each of the determined injection rates RL, RU, R1, and R2 described above, the concentration of dissolved organic matter in the raw water W becomes equal to or less than the threshold value Ct shown in FIG.

[0078] [About action and effects] The operation and effect of water purification method 2 with the above configuration and processing will be described. As described above, water purification method 2 can be configured as a water purification method for treating clear raw water by ensuring that the dissolved organic matter concentration in raw water W is less than 1 mg / L. Furthermore, water purification method 2 determines the injection rate of coagulant F using the dissolved organic matter concentration in raw water W as an index. The dissolved organic matter concentration is correlated with the number of particles within a predetermined particle size range described in embodiment 1 above. The number of particles within a predetermined particle size range contributes greatly to clogging of separation membrane pores. In other words, the dissolved organic matter concentration contributes greatly to clogging of separation membrane pores. In other words, water purification method 2 can appropriately control the injection rate of coagulant F so that the dissolved organic matter concentration, which contributes greatly to clogging of separation membrane pores, is, for example, at or below a desired concentration.

[0079] Furthermore, in water purification method 2, the relationship between the measured dissolved organic matter concentration and the required injection rate of coagulant F can be set in advance as a coagulant injection condition. Then, a final injection rate is determined based on the coagulant injection condition and the measured dissolved organic matter concentration. By injecting coagulant F at the final injection rate, the dissolved organic matter concentration becomes equal to or less than a predetermined threshold. Therefore, in water purification method 2, the injection rate of coagulant F can be controlled so that the dissolved organic matter concentration, which contributes greatly to clogging of the pores in the separation membrane, becomes equal to or less than a predetermined threshold. As a result, water purification method 2 can suppress an increase in transmembrane pressure due to membrane clogging.

[0080] In addition, in water purification method 2, when the dissolved organic matter concentration reaches or exceeds a predetermined value (the concentration required for the addition of oxides, Cp), sodium hypochlorite is added in addition to coagulant F. Sodium hypochlorite has a bactericidal effect and disinfects and sterilizes bacteria, viruses, and other contaminants in raw water W. Dissolved organic matter serves as food and an energy source for microorganisms, so a high concentration of dissolved organic matter facilitates the proliferation of microorganisms. The proliferated microorganisms may form biofilms, causing membrane blockage. Adding sodium hypochlorite decomposes the dissolved organic matter and inhibits microbial proliferation. In other words, when the dissolved organic matter concentration reaches or exceeds the concentration required for the addition of oxides, water purification method 2 further suppresses the increase in transmembrane pressure at the start of membrane filtration operation in each operating cycle by adding sodium hypochlorite in addition to controlling the injection rate of coagulant F based on the dissolved organic matter concentration.

[0081] The water purification treatment facility 4 using the water purification treatment method 1 of embodiment 1 and the water purification treatment method 2 of embodiment 2 may be configured to have a control unit and a memory unit. In this case, the control unit includes a processor such as a CPU (Central Processing Unit). The memory unit includes a storage device and stores data and computer programs. Also, for example, some of the steps of the water purification treatment method 1 and the water purification treatment method 2 may be configured to be stored as computer programs in the storage device of the storage unit. In this case, the processor of the control unit may be configured to execute the computer program stored in the storage device of the storage unit to control the water purification treatment facility 4.

[0082] Next, an example of the treatment results of water purification method 2 will be described with reference to Fig. 14. Fig. 14 is a diagram showing an example of the treatment results of water purification method 2. Fig. 14 plots the average transmembrane pressure in each operation cycle of the membrane filtration device 10 when water purification method 2 is performed over a long period of time in the water purification facility 4. In the water purification step S2 of water purification method 2, membrane filtration operation and backwashing operation are repeated as one operation cycle, and one plot represents the performance of one operation cycle.

[0083] During the period indicated by reference numeral 51 in FIG. 14, the increase in the average transmembrane pressure is stable over a long period (several months). The transmembrane pressure during period 51 remains below the upper transmembrane pressure limit (not shown) at which maintenance is required. Therefore, no membrane maintenance is required during period 51. During the subsequent period indicated by reference numeral 52, the average transmembrane pressure increases more rapidly than during period 51. Although not shown, the air temperature and the water temperature of the raw water W tend to increase during period 52 compared to period 51. The increase in the average transmembrane pressure is presumably due to the proliferation of microorganisms in the raw water W due to the rise in water temperature, which in turn leads to membrane blockage. Subsequently, at the time indicated by reference numeral 53, the measured dissolved organic matter concentration in the raw water W exceeds the oxide addition required concentration Cp described above (No in step S47 of FIG. 13). Therefore, in water purification method 2, sodium hypochlorite was added to the raw water W in addition to the coagulant F at the time indicated by reference numeral 53 (step S50 of the same figure). In period 54 after sodium hypochlorite is added, the increase in average transmembrane pressure is suppressed more than in periods 51 and 52 before sodium hypochlorite is added. This is presumably because, as explained above, the growth of microorganisms is suppressed by the bactericidal action of sodium hypochlorite in addition to the appropriate control of the injection rate of coagulant F. As described above, water purification method 2 according to the present invention makes it possible to suppress the increase in transmembrane pressure over a long period of time.

[0084] The water purification method 1 according to the first embodiment described above does not include the oxidant injection step S34. However, the water purification method 1 according to the first embodiment may be configured to include the oxidant injection step S34. In this case, the oxidant injection step S34 may be configured to add sodium hypochlorite (NaClO) in addition to the coagulant F when the number of particles in a predetermined particle size range reaches or exceeds a predetermined value (e.g., the number of particles required for adding an oxide, Np).

[0085] Furthermore, in the above-described first and second embodiments, the membrane element 20 is made of a porous ceramic body, but the membrane element 20 may be made of other materials, such as a hollow fiber membrane, a tubular membrane, or a flat membrane.

[0086] Furthermore, the particle number measurement process S12 of the water purification treatment method 1 in embodiment 1 described above and the dissolved organic matter concentration measurement process S32 of the water purification treatment method 2 in embodiment 2 may be configured as a raw water evaluation method for evaluating the raw water W.

[0087] The embodiments of the present invention have been described above with reference to the drawings. However, the present invention is not limited to the above embodiments and can be embodied in various forms without departing from the spirit and scope of the present invention. The drawings mainly show each component in a schematic manner for ease of understanding, and the thickness, length, number, spacing, etc. of each component shown in the drawings may differ from the actual ones due to the convenience of creating the drawings. Furthermore, the materials, shapes, dimensions, etc. of each component shown in the above embodiments are merely examples and are not particularly limited, and various modifications are possible within a scope that does not substantially deviate from the configuration of the present invention. [Explanation of symbols]

[0088] 1. Water purification method 5. Water purification facilities 6 Raw Water Storage Section 7 Water landing well 8 Mixing pond 10. Membrane filtration equipment 20 Membrane element S1, S3 raw water treatment process S2 Water purification process S11,S31 Agglomeration condition setting process S12 Particle number measurement process S13, S33 Coagulant injection rate determination process S14, S35 Coagulant injection process S32 Dissolved organic matter concentration measurement process S34 Oxidant injection process

Claims

1. A water purification method in which raw water into which a coagulant has been injected is subjected to membrane filtration to obtain filtered water, a particle number measuring step of measuring the number of particles in the raw water within a predetermined particle size range as a measured particle number; a flocculant injection rate determination step of determining the flocculant injection rate as a determined injection rate based on the measured particle number; a flocculant injection step in which the flocculant is injected based on the determined injection rate; Equipped with The water purification method, wherein the predetermined particle size range is a particle size range of the particles in the vicinity of an average membrane pore size of a separation membrane used in the membrane filtration.

2. The method further includes a flocculation condition setting step in which a relationship between the number of particles and a required injection rate of the flocculant is set as a flocculant injection condition, the required injection rate is the injection rate of the flocculant required to make the number of particles in the specified particle size range in the raw water equal to or less than a predetermined threshold value, The water purification method according to claim 1 , wherein in the coagulant injection rate determination step, the determined injection rate is determined based on the coagulant injection conditions and the measured particle count.

3. In the particle number measuring step, A particle volume, which is the volume of the particles, is measured as a measured particle volume instead of the number of particles. The water purification method according to claim 1 or 2, wherein a particle weight, which is the weight of the particles, is measured instead of the particle number as the measured particle weight.

4. 3. The water purification method according to claim 1, wherein the predetermined particle size range is any range between 0.5 times the average membrane pore size and 10 times the average membrane pore size.

5. 3. The water purification method according to claim 1, wherein the predetermined particle size range is any range between 0.8 times the average membrane pore size and 5 times the average membrane pore size.

6. The water purification method according to claim 1 or 2, wherein the average membrane pore size is 2 μm or less.

7. A water purification method in which raw water into which a coagulant has been injected is subjected to membrane filtration to obtain filtered water, The concentration of dissolved organic matter in the raw water is less than 1 mg / L, a dissolved organic matter concentration measuring step of measuring the dissolved organic matter concentration in the raw water as a measured dissolved organic matter concentration; a flocculant injection rate determination step of determining the flocculant injection rate as a determined injection rate based on the measured dissolved organic matter concentration; a flocculant injection step in which the flocculant is injected based on the determined injection rate; A water purification method comprising:

8. a coagulation condition setting step for setting a relationship between the concentration of dissolved organic matter and a required injection rate of the coagulant as a coagulant injection condition; the required injection rate is the injection rate of the coagulant required to make the concentration of dissolved organic matter in the raw water equal to or lower than a predetermined threshold value, The water purification method according to claim 7 , wherein in the coagulant injection rate determination step, the determined injection rate is determined based on the coagulant injection conditions and the measured dissolved organic matter concentration.

9. The water purification method of claim 2 or claim 8, wherein in the coagulation condition setting step, the coagulation agent injection conditions are set based on the results of a coagulation condition setting test that tests the coagulation properties of the raw water for the injection rates of a plurality of different coagulants and evaluates the coagulation properties of the raw water.

10. The water purification method according to claim 7 or 8, further comprising an oxidant injection step of injecting an oxidant into the raw water before injecting the coagulant based on the measured dissolved organic matter concentration.

11. A method for evaluating raw water supplied to a water purification treatment device in which raw water into which a coagulant has been injected is subjected to membrane filtration to obtain filtered water, comprising: a particle number measuring step for measuring the number of particles within a predetermined particle size range in the raw water as a measured particle number; The predetermined particle size range is a particle size range of the particles in the vicinity of the average membrane pore size of the separation membrane used in the membrane filtration, A raw water evaluation method for evaluating the raw water based on the measured particle count.

12. In the particle number measuring step, A particle volume, which is the volume of the particles, is measured as a measured particle volume instead of the number of particles. The raw water evaluation method according to claim 11 , wherein a particle weight, which is the weight of the particles, is measured instead of the particle number as the measured particle weight.

13. A method for evaluating raw water supplied to a water purification treatment device in which raw water into which a coagulant has been injected is subjected to membrane filtration to obtain filtered water, comprising: The concentration of dissolved organic matter in the raw water is less than 1 mg / L, a dissolved organic matter concentration measuring step of measuring the dissolved organic matter concentration in the raw water as a measured dissolved organic matter concentration; A raw water evaluation method for evaluating the raw water based on the measured dissolved organic matter concentration.

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