Water treatment method and water treatment device

Abstract

To deliver a stable water treatment performance by suppressing blockage of reverse osmosis membranes caused by biofouling.SOLUTION: A water treatment method includes a step for intermittently adding a disinfectant to water to be treated supplied to a reverse osmosis membrane, in which the step includes the steps of: correcting a current detected value of a water flow differential pressure of the reverse osmosis membrane based on a current detected value of a flow rate of concentrated water from the reverse osmosis membrane and an initial value of the flow rate of the concentrated water from the reverse osmosis membrane, and calculating a difference between the corrected detected value and an initial value of the water flow differential pressure of the reverse osmosis membrane as a degree of contamination indicating the degree of biological contamination of the reverse osmosis membrane; and adjusting the amount of the disinfectant added to the water to be treated for a predetermined time based on the calculated degree of contamination to the extent that at least one of redox potential and the total chlorine concentration of the water to be treated to which the disinfectant is added does not exceed a predetermined value, and while maintaining the concentration of the disinfectant in the treated water at a constant level, changing the time of addition of the disinfectant per predetermined amount of time according to the calculated contamination level.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a water treatment method and a water treatment apparatus.

Background Art

[0002] As a water treatment apparatus for removing impurities contained in water to be treated, one having a reverse osmosis membrane (RO membrane) is known. In this apparatus, the water to be treated (raw water) supplied to the RO membrane at a predetermined supply pressure is separated by the RO membrane into permeated water and concentrated water. Thereby, treated water (permeated water) from which impurities have been removed can be obtained.

[0003] In a water treatment apparatus having an RO membrane, it is required to continuously operate stably. For this purpose, it is important to suppress biofouling in which organisms in the raw water adhere to the membrane surface of the RO membrane. As a countermeasure against such biofouling, conventionally, a method of adding a bactericide that suppresses the growth of organisms to the raw water has been used. As typical bactericides, oxidizing agents such as hypochlorous acid, hypobromous acid, and their stabilized compositions are known (see, for example, Patent Document 1). On the other hand, in recent years, with the demand for cost reduction and the increasing environmental awareness, it has been required to effectively suppress biofouling while minimizing the use amount of bactericides. For example, Patent Document 2 proposes a method of adjusting the addition amount of a bactericide according to the degree of biofouling.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, in the method described in Patent Document 2, no consideration is given to the impact of the bactericide on the RO membrane. In particular, when using an oxidizing agent as described in Patent Document 1, no consideration is given to the impact it has on the RO membrane.

[0006] Therefore, an object of the present invention is to provide a water treatment method and a water treatment apparatus that suppress the clogging of a reverse osmosis membrane caused by biofouling and exhibit stable water treatment performance.

Means for Solving the Problems

[0007] To achieve the above-described object, the water treatment method of the present invention includes a step of supplying raw water to a reverse osmosis membrane and separating it into permeated water and concentrated water, and a step of intermittently adding a bactericide to the raw water supplied to the reverse osmosis membrane. As the bactericide, a bromine-based oxidizing agent, a stabilized hypobromous acid composition containing bromine and a sulfamic acid compound, an iodine-based oxidizing agent, or 2,2-dibromo-3-nitrilopropionamide (DBNPA) is added. The step of intermittently adding the bactericide includes a step of detecting the current water passage differential pressure, which is the difference between the current supply pressure of the raw water supplied to the reverse osmosis membrane and the current outflow pressure of the concentrated water flowing out from the reverse osmosis membrane; a step of detecting the current flow rate of the concentrated water flowing out from the reverse osmosis membrane; a step of calculating the contamination degree indicating the degree of biofouling of the reverse osmosis membrane based on the initial value of the water passage differential pressure detected in advance at the start of use of the reverse osmosis membrane, the initial value of the flow rate detected in advance at the start of use of the reverse osmosis membrane, the detected current water passage differential pressure, and the detected current flow rate; and a step of adjusting the addition amount of the bactericide per predetermined time to the raw water within a range where at least one of the oxidation-reduction potential and the total chlorine concentration of the raw water to which the bactericide is added does not exceed a predetermined value set in advance based on the calculated contamination degree, including changing the addition time of the bactericide per predetermined time according to the calculated contamination degree while maintaining the concentration of the bactericide in the raw water constant. The step of calculating the contamination degree includes correcting the detected current water passage differential pressure based on the detected current flow rate and the initial value of the flow rate, and calculating the difference between the corrected current water passage differential pressure and the initial value of the water passage differential pressure as the contamination degree.

[0008] In addition, the water treatment apparatus of the present invention includes a reverse osmosis membrane device having a reverse osmosis membrane that separates raw water to be treated into permeated water and concentrated water, and a bactericide addition device that adds a bactericide to the raw water supplied to the reverse osmosis membrane device. As the bactericide, a bromine-based oxidant, a stabilized hypobromous acid composition containing bromine and a sulfamic acid compound, an iodine-based oxidant, or a bactericide addition device that adds 2,2-dibromo-3-nitrilopropionamide (DBNPA); a pressure sensor that detects the water passing differential pressure, which is the difference between the supply pressure of the raw water supplied to the reverse osmosis membrane device and the outflow pressure of the concentrated water flowing out from the reverse osmosis membrane device; a flow rate sensor that detects the flow rate of the concentrated water flowing out from the reverse osmosis membrane device; the bactericide addition device intermittently adds the bactericide, and based on the initial value of the water passing differential pressure detected in advance by the pressure sensor at the start of use of the reverse osmosis membrane, the initial value of the flow rate detected in advance by the flow rate sensor at the start of use of the reverse osmosis membrane, the current water passing differential pressure detected by the pressure sensor, and the current flow rate detected by the flow rate sensor, calculates the degree of fouling indicating the degree of biological fouling of the reverse osmosis membrane device, and based on the calculated degree of fouling, adjusts the addition amount of the bactericide per predetermined time to the raw water within a range where at least one of the oxidation-reduction potential and the total chlorine concentration of the raw water to which the bactericide is added does not exceed a predetermined value set in advance. The control device corrects the detected current water passing differential pressure based on the detected current flow rate and the initial value of the flow rate, calculates the difference between the corrected current water passing differential pressure and the initial value of the water passing differential pressure as the degree of fouling, and adjusts the addition amount by changing the addition time of the bactericide per predetermined time according to the calculated degree of fouling while maintaining the concentration of the bactericide in the raw water constant.

[0009] According to such a water treatment method and water treatment apparatus, the bactericide can be added to the raw water (raw water) appropriately according to the degree of biological fouling (biofouling) within a range where the oxidizing power of the bactericide does not adversely affect the reverse osmosis membrane.

Advantages of the Invention

[0010] As described above, according to the present invention, it is possible to suppress the blockage of the reverse osmosis membrane caused by biofouling and exhibit stable water treatment performance.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Modes for Carrying Out the Invention

[0012] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0013] The water treatment apparatus 10 of the present embodiment has a raw water tank 11 and a reverse osmosis membrane (RO membrane) apparatus 12, and by treating the raw water (water to be treated) stored in the raw water tank 11 with the RO membrane apparatus 12, impurities contained in the raw water are removed to generate treated water. The RO membrane apparatus 12 separates the raw water supplied from the raw water tank 11 into concentrated water containing impurities and permeated water from which impurities have been removed, and has an RO membrane. Connected to the RO membrane apparatus 12 are a water supply line L1 for supplying raw water from the raw water tank 11 to the RO membrane apparatus 12, a permeated water line L2 for supplying the permeated water flowing out from the RO membrane apparatus 12 to a treated water tank or a use point, and a drainage line L3 for discharging the concentrated water flowing out from the RO membrane apparatus 12 to the outside. Connected to the raw water tank 11 is a raw water line L4 for supplying raw water that has been subjected to pretreatment such as turbidity removal and dechlorination in a pretreatment system (not shown) to the raw water tank 11.

[0014] The water treatment device 10 also includes a pressure pump 13 provided in the water supply line L1, a raw water pressure sensor 14 and a temperature sensor 15 also provided in the water supply line L1, and a concentrated water pressure sensor 16 and a manual valve V1 provided in the drain line L3. The pressure pump 13 is configured such that its rotation speed is controlled by an inverter (not shown), and has a function of adjusting the supply pressure (raw water pressure) of the raw water supplied to the RO membrane device 12 through the water supply line L1. The raw water pressure sensor 14 has a function of detecting the raw water pressure. The temperature sensor 15 has a function of detecting the water temperature (raw water temperature) of the raw water supplied to the RO membrane device 12. Note that the temperature sensor 15 may be configured to detect the water temperature of either the permeated water or the concentrated water flowing out from the RO membrane device 12, that is, it may be provided in the permeated water line L2 or the drain line L3. The concentrated water pressure sensor 16, together with the raw water pressure sensor 14, has a function of detecting the differential pressure of water flow through the RO membrane (the differential pressure between the supply pressure of the raw water supplied to the RO membrane and the outflow pressure of the concentrated water flowing out from the RO membrane). The manual valve V1 functions as a flow rate adjusting means for adjusting the flow rate of the concentrated water flowing through the drain line L3. Note that, as will be described later, when the concentrated water pressure sensor 16 is provided, the temperature sensor 15 may be omitted.

[0015] During the operation of the water treatment device 10, the raw water stored in the raw water tank 11 is supplied to the RO membrane device 12 by the operation of the pressure pump 13, where it is treated and separated into permeated water and concentrated water. The permeated water is supplied to a treated water tank or a use point through the permeated water line L2, and the concentrated water is discharged to the outside through the drain line L3. Then, the raw water tank 11 is continuously supplied with raw water that has been pretreated, such as turbidity removal and dechlorination, in a pretreatment system (not shown) through the raw water line L4 according to the flow rate of the raw water supplied to the RO membrane device 12. Note that the raw water tank 11 is not necessarily provided in terms of the function of the water treatment device 10, but it is preferably provided from the viewpoint of adding a bactericide to the raw water, as will be described later.

[0016] The water treatment apparatus 10 also includes a disinfectant addition device 20 for adding a disinfectant that suppresses biofouling of the RO membrane to the raw water supplied to the RO membrane device 12, and a control device 30 that controls the operation of the water treatment apparatus 10 described above, including the addition of the disinfectant by the disinfectant addition device 20.

[0017] The disinfectant addition device 20 includes a disinfectant tank 21 that stores the disinfectant, and a chemical injection pump 22 that is connected to the raw water tank 11 via a disinfectant supply line L5 and injects the disinfectant stored in the disinfectant tank 21 into the raw water tank 11. The addition of the disinfectant by the disinfectant addition device 20 is preferably performed intermittently as described later from the viewpoints of running cost and environmental load. However, in that case, there is a concern that biofouling may progress while the addition is not being performed. Therefore, as the disinfectant to be added, those having a higher bactericidal power, that is, those having a higher oxidation-reduction potential (ORP) which is a measure of bactericidal power are preferred. Specifically, a disinfectant having an ORP exceeding 500 mV when an aqueous solution is adjusted so that the total chlorine concentration is 10 mg / L and the pH is 7.3 is preferred. Examples of such disinfectants include bromine-based oxidants, stabilized hypobromous acid compositions containing bromine and sulfamic acid compounds, iodine-based oxidants, or 2,2-dibromo-3-nitrilopropionamide (DBNPA). It should be noted that using a chlorine-based oxidant (for example, sodium hypochlorite, etc.) as the disinfectant is not preferable in that it may deteriorate the polyamide-based RO membrane. The injection position of the disinfectant may not be the raw water tank 11, and may be, for example, on the water supply line L1 between the pressurizing pump 13 and the raw water pressure sensor 14. However, in that case, since the pressure at the injection point becomes higher compared to the case of injecting into the raw water tank 11, a large-capacity chemical injection pump 22 is required, which is not preferable in terms of cost. Therefore, the injection position of the disinfectant, that is, the connection position of the disinfectant supply line L5 is preferably the raw water tank 11 as shown in the figure.

[0018] During the operation of the water treatment device 10 described above, the control device 30 executes flow rate control to control the pressure pump 13 so that the flow rate of the permeate flowing through the permeate line L2 becomes constant (a preset set flow rate). For example, when the water temperature changes, the viscosity of the water changes, and as a result, the flow rate of the permeate separated by the RO membrane also changes. In response to this change, the control device 30 controls the rotation speed of the pressure pump 13 through an inverter. That is, when the water temperature decreases, the viscosity of the water increases, and as a result, the flow rate of the permeate separated by the RO membrane decreases. Therefore, the control device 30 increases the rotation speed of the pressure pump 13 to increase the raw water pressure so as to compensate for this decrease. Also, when the water temperature increases, the viscosity of the water decreases, and as a result, the flow rate of the permeate separated by the RO membrane increases. Therefore, the control device 30 decreases the rotation speed of the pressure pump 13 to decrease the raw water pressure so as to cancel out this increase. In this way, by adjusting the rotation speed of the pressure pump 13, that is, the raw water pressure, the flow rate of the permeate flowing through the permeate line L2 is adjusted to the set flow rate.

[0019] During the operation of the water treatment device 10, in addition to the above-described flow rate control of the permeate, it is preferable to also adjust the flow rate of the concentrated water flowing through the drain line L3 in order to suppress scaling in which impurities (particularly, silica or calcium) precipitate on the membrane surface of the RO membrane. Specifically, from the impurity concentration of the raw water measured in advance, a target recovery rate (the ratio of the flow rate of the permeate to the sum of the flow rate of the permeate and the flow rate of the concentrated drain) is set so that the impurity concentration of the concentrated water does not exceed the solubility at the measured water temperature in advance, and it is preferable that the flow rate of the concentrated water is adjusted so as to reach the set target recovery rate. The flow rate adjustment at this time is performed by a manual valve V1 provided in the drain line L3, and the set flow rate is determined based on the target recovery rate and the set flow rate of the permeate.

[0020] Also, the control device 30 controls the disinfectant addition device 20 during the operation of the water treatment device 10, and intermittently, preferably periodically (for example, once every 24 hours), executes a disinfectant addition step of adding a disinfectant to the raw water supplied to the RO membrane device 12.

[0021] In the bactericide addition step, first, prior to the addition of the bactericide, the degree of biofouling (biological contamination) of the RO membrane at that time is evaluated. Specifically, based on the detection values of each of the sensors 14, 15, and 16, a contamination degree indicating the degree of biofouling is calculated. The method for calculating this contamination degree will be described later. When the contamination degree of the RO membrane is calculated, based on the calculated contamination degree, the amount of bactericide added to the raw water per bactericide addition step is determined. Specifically, a value obtained by adding an addition amount corresponding to (proportional to) the calculated contamination degree to a preset minimum addition amount is determined as the new addition amount. Then, by controlling the chemical injection pump 22 based on the determined addition amount, the bactericide addition step is executed, and raw water is supplied to the raw water tank 11 at a predetermined flow rate corresponding to the flow rate of the raw water supplied to the RO membrane device 12. That is, during the bactericide addition step, raw water containing a bactericide at a predetermined concentration is supplied to the RO membrane device 12. In this way, it becomes possible to accurately grasp the degree of biofouling and add the bactericide to the raw water in the minimum necessary amount corresponding thereto, and as a result, it also becomes possible to reduce the running cost and environmental load.

[0022] Incidentally, in order to reduce the influence of bactericides, particularly oxidants, on the membrane, it is considered effective to keep the CT value (the product of the concentration of the bactericide and the time the bactericide contacts the membrane) low. Conversely, even if the concentration or contact time of the bactericide is changed, if the CT value is the same, it is considered that there is almost no difference in the influence of the bactericide on the membrane. According to this, if the total amount of bactericide added to the raw water during the bactericide addition step is the same, regardless of the bactericide concentration in the raw water or the bactericide addition time (the execution time of the bactericide addition step), the influence of the bactericide on the RO membrane should be the same. That is, in order to adjust (change) the amount of bactericide added per bactericide addition step, even if the bactericide addition time is changed without changing the bactericide concentration in the raw water, or the bactericide concentration in the raw water is changed without changing the bactericide addition time, there should be almost no difference in the influence of the bactericide on the RO membrane.

[0023] However, in reality, through the verification by the inventors, when the concentration of the bactericide in the raw water is changed without changing the addition time of the bactericide, it has been confirmed that under certain conditions, the bactericide may indirectly have an adverse effect on the RO membrane. Specifically, as shown in the examples described later, when the concentration of the bactericide is increased until the ORP of the raw water after the addition of the bactericide exceeds a certain upper limit value, it has been confirmed that the increase in the raw water pressure that should originally be suppressed cannot be suppressed. From the analysis results of the deposits on the RO membrane where an increase in the raw water pressure was observed, it is presumed that this is because when using a bactericide with a high ORP, a viscous substance is released from the organisms attached to the membrane surface of the RO membrane, and this viscous substance blocks the RO membrane.

[0024] Therefore, in the present embodiment, in order to suppress such an increase in the raw water pressure, the concentration of the bactericide in the raw water is not changed from the initial set concentration. When the degree of biofouling changes, accordingly, the execution time of the bactericide addition step, that is, the addition time of the bactericide per predetermined time corresponding to its execution cycle is changed. Specifically, when the degree of contamination of the RO membrane is calculated, a value obtained by adding a time corresponding to (proportional to) the calculated degree of contamination to the preset minimum addition time (the value obtained by dividing the preset minimum addition amount by the set concentration of the bactericide) is set as the new addition time. The set concentration of the bactericide is a concentration within the range where the ORP of the raw water after the addition of the bactericide does not exceed the above-mentioned upper limit value. As shown in the examples described later, it is preferable that the upper limit value is determined by experimentally verifying in advance the range in which the RO membrane is not blocked by the viscous substance derived from organisms. Also, the concentration of the bactericide in the raw water can be simply obtained from the flow rates of the raw water and the bactericide respectively. Preferably, the raw water flowing through the water supply line L1 is manually sampled, and the total chlorine concentration in the raw water is measured by the DPD method using a portable residual chlorine meter.

[0025] Thus, according to this embodiment, the adjustment of the amount of the bactericide added to the raw water per bactericide addition step is performed within a range where the ORP of the raw water after the bactericide addition does not exceed a preset upper limit value (predetermined value). Specifically, while maintaining the concentration of the bactericide in the raw water at a constant concentration such that the ORP of the raw water after the bactericide addition does not exceed the upper limit value, the addition time of the bactericide is changed according to the degree of biofouling, thereby adjusting the amount of the bactericide added. Thereby, within a range where the oxidizing power of the bactericide does not adversely affect the RO membrane, the bactericide can be added to the raw water in an appropriate amount according to the degree of biofouling.

[0026] The set concentration of the bactericide is not particularly limited as long as the ORP of the raw water after the bactericide addition does not exceed the preset upper limit value. However, if it is too low, a sufficient bactericidal effect cannot be obtained, and the desired result may not be achieved. Therefore, the set concentration of the bactericide is preferably a concentration within a range where the ORP of the raw water after the bactericide addition does not fall below a predetermined lower limit value so that the minimum bactericidal power is exerted. In addition, as an index for determining the set concentration of the bactericide, the total chlorine concentration may be used instead of or in addition to the ORP. Also, the addition time of the bactericide becomes longer as the biofouling progresses, but if it becomes too long, it is not preferable from the viewpoints of running cost and environmental load. Therefore, the addition time of the bactericide is preferably adjusted so as not to exceed a preset maximum addition time. That is, when the addition time calculated by the above calculation method exceeds the preset maximum addition time, it is preferable that the preset maximum addition time, rather than the calculated addition time, is set as the new addition time. At this time, when suppressing the clogging of the RO membrane is prioritized over reducing the running cost and environmental load, continuous addition of the bactericide may be temporarily performed until the next bactericide addition step.

[0027] Here, four calculation methods for the degree of contamination indicating the degree of biofouling of the RO membrane will be described.

[0028] (First calculation method) The occurrence of biofouling in the RO membrane blocks the flow path of the raw water and increases the pressure loss. Therefore, as described above, when flow control is performed to keep the flow rate of the permeate flowing through the permeate line L2 constant, the influence appears as a change (increase) in the raw water pressure. Therefore, by calculating the increase amount, the degree of biofouling can be accurately grasped. However, the raw water pressure not only changes depending on the degree of biofouling but also changes depending on the water temperature as described above. Therefore, in order to accurately calculate the increase amount of the raw water pressure caused by biofouling, instead of directly comparing the current raw water pressure with the raw water pressure (initial raw water pressure) at the start of use of the RO membrane, the initial raw water pressure should be corrected considering the influence of the water temperature fluctuation, that is, it is necessary to compare it with the value obtained by converting the initial raw water pressure to the pressure at the current water temperature.

[0029] Therefore, in the first calculation method, the contamination degree of the RO membrane is calculated as follows. As a premise, the control device 30 stores the initial value of the raw water pressure (initial raw water pressure) detected in advance by the raw water pressure sensor 14 and the initial value of the raw water temperature (initial raw water temperature) detected in advance by the temperature sensor 15 at the start of use of the RO membrane. The initial raw water pressure and the initial raw water temperature may be those immediately after the start of use of the RO membrane, but it is preferably those obtained after a certain period of time has passed since the start of use and the performance has stabilized, and may be the moving average value thereof. Also, the initial raw water pressure and the initial raw water temperature are newly obtained each time the RO membrane is replaced with a new one, stored in the control device 30, and updated.

[0030] First, the current raw water pressure is detected by the raw water pressure sensor 14, and at the same time, the current raw water temperature is detected by the temperature sensor 15. Actually, the moving average of the detection values of each sensor 14, 15 is calculated, and they are acquired (detected) as the current raw water pressure and raw water temperature. Then, using the information (such as tables or functions) of the temperature correction coefficient pre-stored in the internal storage device or external server, etc., the temperature correction coefficient at the detected current raw water temperature and the temperature correction coefficient at the initial raw water temperature pre-stored in the control device 30 are acquired. The temperature correction coefficient is a coefficient for correcting the permeation flux of the RO membrane measured at an arbitrary temperature to a value at a standard temperature (for example, 25°C), and the temperature correction coefficient for each temperature is provided by the manufacturer for each type of RO membrane. Note that the temperature correction coefficient at the initial raw water temperature may be acquired in advance at the start of use of the RO membrane and stored in the control device 30. When each temperature correction coefficient is acquired, based on the acquired temperature correction coefficient, the initial raw water pressure is converted to a value at the current raw water temperature. Specifically, assuming the initial raw water pressure is P0, the converted initial pressure P R0 is given by the following formula (1). P R0 =P0×(K i / K0) (1) Here, K i is the temperature correction coefficient at the current raw water temperature, and K0 is the temperature correction coefficient at the initial raw water temperature.

[0031] Then, the converted initial pressure calculated by the above formula (1) and the detected current raw water pressure are compared. When the current raw water pressure is higher than the converted initial pressure, it is determined that biofouling has occurred, and the difference is calculated as the contamination degree. On the other hand, when the current raw water pressure is equal to or lower than the converted initial pressure, it is considered that no biofouling has occurred, and the contamination degree is calculated as zero.

[0032] (Second calculation method) The second calculation method is a method of calculating the degree of contamination of the RO membrane based on the result of comparing, instead of converting the initial raw water pressure to the pressure at the current water temperature as in the first calculation method, each of the initial raw water pressure and the current raw water pressure to the pressure at a standard temperature (for example, 25°C) in order to cancel out the influence of the water temperature fluctuation on the raw water pressure. Thereby, similar to the first calculation method, it is possible to accurately calculate the increase due to biofouling in the change over time of the raw water pressure, that is, the degree of contamination of the RO membrane. In the second calculation method as well, as shown below, the detection values of the respective sensors 14 and 15 are used to calculate the degree of contamination of the RO membrane, but in practice, it is preferable to use the respective moving average values, which is the same as in the first calculation method.

[0033] In the second calculation method, first, as a premise, the control device 30 stores a converted initial pressure obtained by converting the initial raw water pressure to a value at the standard temperature. In this case, the converted initial pressure P R0 ’ is given by the following formula (2), where the initial raw water pressure is P0 and the temperature correction coefficient at the initial raw water temperature is K0. P R0 ’ = P0 / K0 (2) Note that this converted initial pressure is newly obtained and stored in the control device 30 and updated every time the RO membrane is replaced with a new one, similar to the initial raw water pressure and the initial raw water temperature in the first calculation method.

[0034] Then, when the current raw water pressure and the raw water temperature are respectively detected by the raw water pressure sensor 14 and the temperature sensor 15, the temperature correction coefficient at the detected current raw water temperature is acquired, and based on the acquired temperature correction coefficient, the current raw water pressure is converted to a value at the standard temperature. Specifically, when the current raw water pressure is P i and the temperature correction coefficient K i at the current raw water temperature is taken, the converted raw water pressure P Ri obtained by converting the current raw water pressure to a value at the standard temperature is given by the following formula (3). P Ri = P i / K i (3)

[0035] The converted raw water pressure calculated in this way is compared with the converted initial pressure (see the above formula (2)) previously stored in the control device 30. Similar to the case of the first calculation method, when the converted raw water pressure is higher than the converted initial pressure, the difference is calculated as the contamination degree. On the other hand, when the converted raw water pressure is equal to or lower than the converted initial pressure, the contamination degree is calculated as zero, which is the same as the case of the first calculation method.

[0036] (The third calculation method) Both of the above two calculation methods are methods for calculating the contamination degree of the RO membrane using the detected value of the raw water pressure by the raw water pressure sensor 14. These methods are effective when the outflow pressure of the concentrated water flowing out from the RO membrane device 12 cannot be obtained due to reasons such as the inability to install a pressure sensor in the drain line L3 in the device configuration. However, as shown in the figure, this is not the case when the concentrated water pressure sensor 16 is provided in the drain line L3. That is, as described above, the occurrence of biofouling of the RO membrane also appears as an increase in the water passing differential pressure (the differential pressure between the supply pressure of the raw water and the outflow pressure of the concentrated water) because it blocks the flow path of the raw water and increases the pressure loss. Therefore, when the water passing differential pressure of the RO membrane can be detected by the raw water pressure sensor 14 and the concentrated water pressure sensor 16, the contamination degree of the RO membrane may be calculated using the detected value. Such a third calculation method is advantageous in that the water passing differential pressure of the RO membrane is hardly affected by the fluctuation of the water temperature, so the influence does not need to be considered in the calculation of the contamination degree of the RO membrane.

[0037] Therefore, in the third calculation method, when the differential pressure of the current RO membrane is detected from the difference between the detected value of the raw water pressure sensor 14 (preferably, its moving average value) and the detected value of the concentrated water pressure sensor 16 (preferably, its moving average value), the detected value is directly compared with the differential pressure of the RO membrane at the start of use (initial differential pressure) stored in the control device 30. When the current differential pressure is higher than the initial differential pressure, the difference is calculated as the degree of contamination. When it is equal to or lower than the initial differential pressure, the degree of contamination is calculated as zero. Note that the initial differential pressure is preferably obtained after a certain period of time has elapsed since the start of use of the RO membrane and the performance has stabilized, and more preferably, it is its moving average value. When implementing the third calculation method, the temperature sensor 15 may be omitted, and instead of the two pressure sensors 14 and 16, one differential pressure sensor may be provided.

[0038] (Fourth calculation method) As described above, the third calculation method is an advantageous method in that the differential pressure of the RO membrane is hardly affected by fluctuations in the water temperature. However, when using this method, attention needs to be paid to the following points. That is, the differential pressure of the RO membrane is considered to be proportional to the nth power (1 ≤ n ≤ 2) of the flow rate of the raw water passing through the primary side of the RO membrane (primary side flow rate) by analogy from the relational expression between the flow rate and pressure loss of the fluid flowing in the circular pipe (such as the Fanning equation or the Hagen - Poiseuille equation). Therefore, the differential pressure of the RO membrane will change not only when biofouling occurs but also when the flow rate of the concentrated water flowing through the drain line L3 changes, thereby changing the primary side flow rate of the raw water. Such a change in the flow rate of the concentrated water may occur, for example, when the above - mentioned flow rate control of the concentrated water is being executed, whether the set flow rate is changed or not, or when the frequency of flow rate adjustment by the manual valve V1 is low. Therefore, if only the third calculation method is used as it is, it may not be possible to accurately estimate the increase in the differential pressure of the RO membrane due to biofouling, and there is a risk that the degree of contamination of the RO membrane cannot be accurately calculated.

[0039] Therefore, in the fourth calculation method, the degree of contamination of the RO membrane is calculated based on a value obtained by correcting the current water permeation differential pressure of the RO membrane itself in consideration of the change in the flow rate of the concentrated water. Specifically, when the current water permeation differential pressure of the RO membrane is detected in the same manner as in the third calculation method, at the same time, the flow rate of the current concentrated water is detected by a flow sensor (not shown) provided in the drain line L3, and preferably, the moving average of the detected value is calculated. Then, based on the detected current flow rate of the concentrated water and the flow rate of the concentrated water at the start of use of the RO membrane (initial concentrated water flow rate) stored in the control device 30, the current water permeation differential pressure is converted into a value at the initial concentrated water flow rate. The water permeation differential pressure thus converted is compared with the initial water permeation differential pressure, and the degree of contamination of the RO membrane is calculated in the same manner as in the third calculation method. That is, when the converted water permeation differential pressure is higher than the initial water permeation differential pressure, the difference is calculated as the degree of contamination, and when it is equal to or lower than that, the degree of contamination is calculated as zero. According to such a calculation method, it is possible to suppress an overestimation of the increase in the water permeation differential pressure of the RO membrane due to biofouling, and a further reduction in the amount of biocide used is expected.

[0040] As a conversion formula for the water permeation differential pressure, it is preferable to assume that the flow of the raw water passing through the primary side of the RO membrane is turbulent flow and use the Fanning's formula (corresponding to the case where the above power index n is 2), but it is not limited thereto. For example, the actual state of the raw water flow (that is, what value the above power index n takes) may be verified, and the water permeation differential pressure may be converted using the relational expression obtained from the verification result. Ideally, it is preferable to correct the water permeation differential pressure based on the average value of the primary side flow rate of the raw water instead of the flow rate of the concentrated water flowing through the drain line L3, but it is substantially impossible to measure the average value of the primary side flow rate of the raw water with high accuracy. Also, as the flow rate used for correcting the water permeation differential pressure, the flow rate of the raw water flowing through the feed water line L1 may be used, but in that case, it is necessary to consider the influence of the change in the flow rate of the permeated water flowing through the permeate line L2. Therefore, in reality, as described above, it is preferable to correct the water permeation differential pressure based on the flow rate of the concentrated water flowing through the drain line L3.

[0041] The method for calculating the degree of contamination of the RO membrane, that is, the method for evaluating the degree of biofouling, is not limited to the method of calculating the increase in the raw water pressure or the flow-through differential pressure caused by biofouling as described above. For example, the total organic carbon (TOC) concentrations in the raw water and the concentrated water are measured, and the difference is continuously monitored, or the viable cell count in the raw water and the concentrated water is measured, and the difference is continuously monitored to evaluate the degree of biofouling.

[0042] Next, specific examples will be given to explain the effects of the present invention.

[0043] (Example 1) In this example, using a test apparatus that simulated the water treatment apparatus shown in FIG. 1, continuous operation was performed while executing a bactericide addition step at a frequency of once every 24 hours, and the change over time of the raw water pressure (the supply pressure of the raw water supplied to the RO membrane apparatus) was measured. As the raw water, well water that had been subjected to predetermined pretreatment (sterilization treatment with sodium hypochlorite, solid-liquid separation treatment with a turbidity removal membrane, and dechlorination treatment with activated carbon) and to which acetic acid serving as a nutrient for microorganisms was added at a concentration of 2.5 mg / L was used. During the operation period, the temperature of the raw water was 17 to 22°C, and the pH was 6.7 to 7.0. Also, as the RO membrane, an RO membrane element (product number: ESPA2-4040) manufactured by Nitto Denko Corporation was used, and the flow rates of the permeated water and the concentrate were controlled during the operation period, and the flow rate of the permeated water was adjusted to 120 L / h and the flow rate of the concentrate was adjusted to 480 L / h, respectively.

[0044] As a bactericide, “Orpajon” (product number: E266) manufactured by Orga Co., Ltd., which is a stabilized hypobromous acid composition containing bromine and a sulfamic acid compound, was used, and the bactericide concentration in the raw water in each bactericide addition step was fixed at 1.0 mg / L in terms of total chlorine concentration (equivalent to 545 mV in ORP). Also, in the first bactericide addition step immediately after the start of operation, the execution time (minimum addition time) was set to 1 hour so that the addition amount (minimum addition amount) of the bactericide would be 1 h·mg / L. Then, after the second time, the degree of contamination of the RO membrane was calculated using the first calculation method, and based on the calculation result, the execution time of the bactericide addition step, that is, the addition time of the bactericide per 24 hours, was changed to adjust the addition amount of the bactericide per one execution of this step.

[0045] (Comparative Example 1) The measurement was carried out under the same conditions as in Example 1, except that the execution time of each bactericide addition step was fixed at 3 hours, and the addition amount of the bactericide per one execution of the bactericide addition step was adjusted by changing the bactericide concentration in the raw water according to the degree of contamination of the RO membrane. In this comparative example, since the implementation time was different from that of Example 1, in the first bactericide addition step, the bactericide concentration in the raw water was set at 0.6 mg / L in terms of total chlorine concentration (equivalent to 530 mV in ORP) so that the minimum addition amount of the bactericide would be 1.8 h·mg / L.

[0046] (Comparative Example 2) The measurement was carried out under the same conditions as in Comparative Example 1, except that the adjustment of the addition amount of the bactericide according to the degree of contamination of the RO membrane was not performed, the bactericide concentration in the raw water in each bactericide addition step was fixed at 1.0 mg / L in terms of total chlorine concentration, and the execution time was fixed at 3 hours.

[0047] Figures 2, 3, and 4 are graphs showing the measurement results in Example 1, Comparative Example 1, and Comparative Example 2, respectively. In Figures 2 and 3, for reference, the transitions of the parameters (the addition time of the bactericide per 24 hours and the bactericide concentration in the raw water) that vary with time are also shown, and further, in Figure 2, the transition of the water temperature of the raw water during the operation period is also shown.

[0048] In Example 1, as shown in FIG. 2, the raw water pressure was maintained almost constant throughout the operation period, and by adjusting the dosage of the bactericide according to the degree of fouling of the RO membrane, it was confirmed that the blockage of the RO membrane caused by biofouling was well suppressed. On the other hand, in Comparative Examples 1 and 2, as shown in FIGS. 3 and 4, in both cases, the raw water pressure could not be finally maintained constant. In particular, in Comparative Example 1, after the bactericide concentration in the raw water exceeded 2.0 mg / L in terms of total chlorine concentration (corresponding to 570 mV in ORP), although the bactericide was added at a dosage that appropriately reflected the influence of biofouling as in Example 1, it was confirmed that the increase in the raw water pressure could not be suppressed. From the analysis results of the deposits on the RO membrane where an increase in the raw water pressure was observed, it is presumed that when the ORP of the raw water containing the bactericide becomes higher than a certain level, viscous substances are released from the organisms attached to the membrane surface of the RO membrane due to the destruction of the cell wall or excessive stress, and the viscous substances blocked the RO membrane.

[0049] (Example 2) In this example, using the water treatment apparatus shown in FIG. 1, continuous operation was performed while executing the bactericide addition step once every 12 hours, and the change in the dosage of the bactericide per execution of the step was examined when the degree of fouling of the RO membrane was calculated by a method different from that in Example 1. As the raw water, industrial water subjected to predetermined pretreatment (sterilization treatment with sodium hypochlorite, solid-liquid separation treatment by sand filtration, dechlorination treatment with sodium bisulfite) was used. During the operation period, the water temperature of the raw water was 20 to 25°C, and the pH was 6.8 to 7.2. Also, as the RO membrane device, an RO membrane module equipped with 40 RO membrane elements (product number: ESPA1) manufactured by Nitto Denko Corporation was used, and the flow rates of the permeate and concentrate were controlled throughout the operation period. The set flow rates of the permeate and concentrate water at that time were 35 L / h and 15 L / h, respectively.

[0050] As a bactericide, the same one as in Example 1 was used, and the amount of bactericide added per bactericide addition step was adjusted in the same procedure as in Example 1, except that the degree of fouling of the RO membrane was calculated using the third calculation method. Note that the concentration of the bactericide in the raw water in each bactericide addition step was fixed at 1.0 mg / L in terms of the total chlorine concentration, the same as in Example 1. Also, the execution time (minimum addition time) of the first bactericide addition step immediately after the start of operation was set to 15 minutes so that the amount of bactericide added (minimum addition amount) would be 0.25 h·mg / L.

[0051] (Example 3) Continuous operation was carried out under the same conditions as in Example 2, except that the degree of fouling of the RO membrane was calculated using the fourth calculation method.

[0052] Figure 5 is a graph showing the change over time in the addition time of the bactericide per 12 hours in Examples 2 and 3. Note that the vertical axis of the graph shows the value normalized by the minimum addition time.

[0053] As is clear from Figure 5, in Example 3 where the degree of fouling of the RO membrane was calculated taking into account the change in the flow rate of the concentrated water, compared with Example 2 where the degree of fouling of the RO membrane was calculated without considering the change in the flow rate of the concentrated water, the increase in the addition time of the bactericide is generally smaller. On the other hand, although not shown in detail here, in both Examples 2 and 3, a significant increase in the flow-through differential pressure (the differential pressure between the supply pressure of the raw water supplied to the RO membrane device and the outflow pressure of the concentrated water flowing out of the RO membrane device) was not observed throughout the operation period, and the operation could be continued stably. From this, it is considered that both Examples 2 and 3 are good in terms of suppressing the blockage of the RO membrane, but Example 3 is better in terms of reducing the usage amount of the bactericide.

Explanation of symbols

[0054] 10 Water treatment device 11 Raw water tank 12 Reverse osmosis membrane (RO membrane) device 13 Pressure pump 14 Raw water pressure sensor 15 Temperature sensor 16 Concentrated water pressure sensor 20 Disinfectant Adding Device 21 Disinfectant Tank 22 Chemical Injection Pump 30 Control Device L1 Feed Water Line L2 Permeate Water Line L3 Drainage Line L4 Raw Water Line L5 Disinfectant Supply Line

Claims

1. A step of supplying treated water to a reverse osmosis membrane and separating it into permeate water and concentrated water; A step of intermittently adding a bactericide to the treated water supplied to the reverse osmosis membrane, wherein the bactericide is a bromine-based oxidizing agent, a stabilized hypobromous acid composition containing bromine and a sulfamic acid compound, an iodine-based oxidizing agent, or 2,2-dibromo-3-nitrilopropionamide (DBNPA), and adding the step; The step of intermittently adding the bactericide; A step of detecting a current water passage differential pressure, which is a difference between a current supply pressure of the treated water supplied to the reverse osmosis membrane and a current outflow pressure of the concentrated water flowing out from the reverse osmosis membrane; A step of detecting a current flow rate of the concentrated water flowing out from the reverse osmosis membrane; Based on the initial value of the water passage differential pressure detected in advance at the start of use of the reverse osmosis membrane, the initial value of the flow rate detected in advance at the start of use of the reverse osmosis membrane, the detected current water passage differential pressure, and the detected current flow rate, calculating a contamination degree indicating the degree of biofouling of the reverse osmosis membrane; Based on the calculated contamination degree, in a range where at least one of the oxidation-reduction potential and the total chlorine concentration of the treated water to which the bactericide is added does not exceed a preset predetermined value, adjusting the addition amount of the bactericide to the treated water per predetermined time, including changing the addition time per predetermined time of the bactericide according to the calculated contamination degree while maintaining the concentration of the bactericide in the treated water constant; The step of calculating the contamination degree includes correcting the detected current water passage differential pressure based on the detected current flow rate and the initial value of the flow rate, and calculating the difference between the corrected current water passage differential pressure and the initial value of the water passage differential pressure as the contamination degree. A water treatment method.

2. The water treatment method according to claim 1, wherein changing the addition time includes setting, as a new addition time, a value obtained by adding a time corresponding to the contamination degree to an addition time preset at the start of use of the reverse osmosis membrane.

3. The water treatment method according to claim 1 or 2, wherein the predetermined value for the oxidation-reduction potential is 570 mV, and the predetermined value for the total chlorine concentration is 2.0 mg / L.

4. A reverse osmosis membrane device having a reverse osmosis membrane for separating treated water into permeate water and concentrated water; A biocide addition device for adding a biocide to the water to be treated supplied to the reverse osmosis membrane device, wherein the biocide is a bromine-based oxidant, a stabilized hypobromous acid composition containing bromine and a sulfamic acid compound, an iodine-based oxidant, or 2,2-dibromo-3-nitrilopropionamide (DBNPA), and a biocide addition device for adding the same. A pressure sensor for detecting a water passage differential pressure, which is the difference between the supply pressure of the water to be treated supplied to the reverse osmosis membrane device and the outflow pressure of the concentrated water flowing out from the reverse osmosis membrane device. A flow rate sensor for detecting the flow rate of the concentrated water flowing out from the reverse osmosis membrane device. The biocide addition device intermittently adds the biocide, and based on the initial value of the water passage differential pressure previously detected by the pressure sensor at the start of use of the reverse osmosis membrane, the initial value of the flow rate previously detected by the flow rate sensor at the start of use of the reverse osmosis membrane, the currently detected water passage differential pressure by the pressure sensor, and the currently detected flow rate by the flow rate sensor, calculates a contamination degree indicating the degree of biological fouling of the reverse osmosis membrane device, and based on the calculated contamination degree, within a range where at least one of the oxidation-reduction potential and the total chlorine concentration of the water to be treated to which the biocide is added does not exceed a preset predetermined value, and a control device for adjusting the addition amount of the biocide to the water to be treated per predetermined time. The control device corrects the currently detected water passage differential pressure based on the currently detected flow rate and the initial value of the flow rate, calculates the difference between the corrected currently detected water passage differential pressure and the initial value of the water passage differential pressure as the contamination degree, and while maintaining the concentration of the biocide in the water to be treated constant, adjusts the addition amount by changing the addition time of the biocide per predetermined time according to the calculated contamination degree. A water treatment device.

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