Water treatment method and water treatment device

Abstract

This water treatment method includes: a step for supplying water being treated to a reverse osmosis membrane to separate the water being treated into percolated water and concentrated water; and a step for intermittently adding a bactericide to the water being treated that is supplied to the reverse osmosis membrane, with a bromine-based oxide, a stabilized hypobromous acid composition including bromine and a sulfamic acid compound, an iodine-based oxide, or 2,2-dibromo-3-nitropropionamide (DBNPA) being added as the bactericide. The step for intermittently adding the bactericide includes: a step for evaluating the extent of organic-matter contamination of the reverse osmosis membrane; and a step for adjusting the amount of bactericide added to the water being treated per prescribed time within a range in which the redox potential and / or the total chlorine concentration of the water being treated to which the bactericide is added does not exceed a preset prescribed value, on the basis of the evaluated extent of organic-matter contamination.

Description

[Technical Field]

[0001] The present invention relates to a water treatment method and a water treatment device. [Background technology]

[0002] Water treatment equipment using reverse osmosis (RO) membranes is known as a water treatment equipment for removing impurities from water to be treated. In this equipment, the water to be treated (raw water) is supplied to the RO membrane at a predetermined supply pressure, and the RO membrane separates the water into permeated water and concentrated water. This allows the production of treated water (permeated water) from which impurities have been removed.

[0003] Water treatment equipment with RO membranes is required to operate stably and continuously. To achieve this, it is important to suppress biofouling, which occurs when organisms in raw water adhere to the membrane surface of the RO membrane. A conventional countermeasure against biofouling involves adding a disinfectant to the raw water to inhibit the growth of organisms. Typical disinfectants include oxidizing agents such as hypochlorous acid, hypobromous acid, and stabilized compositions thereof (see, for example, Patent Document 1). Meanwhile, in recent years, with demands for cost reduction and growing environmental awareness, there has been a demand for effective suppression of biofouling while minimizing the amount of disinfectant used. For example, Patent Document 2 proposes a method for adjusting the amount of disinfectant added depending on the level of biofouling. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6401491 [Patent Document 2] International Publication No. 2020 / 158645 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the method described in Patent Document 2 does not take into consideration the effect of the disinfectant on the RO membrane, and in particular, does not take into consideration the effect of an oxidizing agent such as that described in Patent Document 1 on the RO membrane.

[0006] Therefore, an object of the present invention is to provide a water treatment method and a water treatment device that suppresses clogging of a reverse osmosis membrane caused by biofouling and exhibits stable water treatment performance. [Means for solving the problem]

[0007] In order to achieve the above-mentioned object, the water treatment method of the present invention includes a step of supplying water to be treated to a reverse osmosis membrane and separating the water into permeate and concentrate, and a step of intermittently adding a bactericide to the water to be treated that is supplied to the reverse osmosis membrane, wherein the bactericide is selected from the group consisting of a bromine-based oxidizing agent, a stabilized hypobromous acid composition containing bromine and a sulfamic acid compound, an iodine-based oxidizing agent, and 2,2-dibromo-3-nitrobenzoate. R and adding lopropionamide (DBNPA), wherein the step of intermittently adding the fungicide comprises: a step of detecting a current water temperature of any of the water to be treated supplied to the reverse osmosis membrane, the permeated water flowing out from the reverse osmosis membrane, and the concentrated water flowing out from the osmosis membrane; a step of detecting a current supply pressure of the water to be treated supplied to the reverse osmosis membrane; and a step of detecting a current water temperature based on an initial value of any of the water temperatures detected in advance when the reverse osmosis membrane is started to be used, an initial value of the supply pressure detected in advance when the reverse osmosis membrane is started to be used, the detected current water temperature, and the detected current supply pressure. The degree of biological contamination of reverse osmosis membranes Calculate the level of contamination and calculation did Pollution level and adjusting the amount of disinfectant added to the water to be treated per predetermined time based on the above, so that at least one of the oxidation-reduction potential and the total chlorine concentration of the water to be treated to which the disinfectant has been added does not exceed a predetermined value. A process including changing the addition time of the disinfectant per predetermined time period in accordance with the calculated contamination level while maintaining the concentration of the disinfectant in the water to be treated constant. and includes. In one aspect, the step of calculating the contamination level includes converting the initial value of the supply pressure to a value at the detected current water temperature based on the detected current water temperature and an initial value of any water temperature, and calculating the difference between the detected current supply pressure and the converted initial value of the supply pressure as the contamination level. In another aspect, the step of calculating the contamination level includes converting the detected current supply pressure and the initial value of the supply pressure to values ​​at a standard temperature based on the detected current water temperature and an initial value of any water temperature, and calculating the difference between the converted current supply pressure and the converted initial value of the supply pressure as the contamination level.

[0008] The water treatment device of the present invention separates the water to be treated into permeate and concentrated water. Has a reverse osmosis membrane A reverse osmosis membrane device and a disinfectant adding device that adds a disinfectant to the water to be treated that is supplied to the reverse osmosis membrane device, wherein the disinfectant is selected from the group consisting of a bromine-based oxidizing agent, a stabilized hypobromous acid composition containing bromine and a sulfamic acid compound, an iodine-based oxidizing agent, and 2,2-dibromo-3-nitrobenzoate. R a fungicide adding device for adding lopropionamide (DBNPA); a temperature sensor for detecting the temperature of the water to be treated supplied to the reverse osmosis membrane device, the permeated water flowing out from the reverse osmosis membrane device, or the concentrated water flowing out from the reverse osmosis membrane device; and a pressure sensor for detecting the supply pressure of the water to be treated supplied to the reverse osmosis membrane device. Intermittent disinfectant addition using a disinfectant addition device vinegar R together , Based on an initial value of any water temperature detected in advance by a temperature sensor when the reverse osmosis membrane is started to be used, an initial value of a supply pressure detected in advance by a pressure sensor when the reverse osmosis membrane is started to be used, any current water temperature detected by the temperature sensor, and the current supply pressure detected by the pressure sensor, reverse osmosis membrane The extent of biological contamination Calculate the level of contamination death, calculation did Pollution level Based on the above, the amount of disinfectant added to the water to be treated per predetermined time is adjusted so that at least one of the oxidation-reduction potential and the total chlorine concentration of the water to be treated to which the disinfectant has been added does not exceed a predetermined value. a control device; . In one aspect, the control device converts the initial supply pressure to a value at one of the detected current water temperatures based on one of the detected current water temperatures and an initial value of one of the water temperatures, calculates the difference between the detected current supply pressure and the converted initial supply pressure as a contamination level, and adjusts the amount of disinfectant added by changing the disinfectant addition time per predetermined time period in accordance with the calculated contamination level while maintaining the disinfectant concentration in the water to be treated constant. In another aspect, the control device converts the detected current supply pressure and the initial supply pressure to values ​​at a standard temperature based on one of the detected current water temperatures and an initial value of one of the water temperatures, calculates the difference between the converted current supply pressure and the converted initial supply pressure as a contamination level, and adjusts the disinfectant addition time per predetermined time period in accordance with the calculated contamination level while maintaining the disinfectant concentration in the water to be treated constant.

[0009] According to this water treatment method and water treatment device, the bactericide can be added to the water to be treated (raw water) in an amount appropriate for the level of biological contamination (biofouling), as long as the oxidizing power of the bactericide does not adversely affect the reverse osmosis membrane. [Effects of the Invention]

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

[0011] [Figure 1] 1 is a schematic diagram showing the configuration of a water treatment device according to one embodiment of the present invention. [Figure 2] 1 is a graph showing the change over time in raw water pressure and the addition time of a disinfectant per 24 hours in Example 1. [Figure 3] 1 is a graph showing the change over time in raw water pressure and the concentration of a disinfectant in raw water in Comparative Example 1. [Figure 4] 10 is a graph showing the change over time in raw water pressure in Comparative Example 2. [Figure 5] 1 is a graph showing the change in the addition time of the fungicide per 12 hours over time in Examples 2 and 3. DETAILED DESCRIPTION OF THE INVENTION

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

[0013] The water treatment device 10 of this embodiment includes a raw water tank 11 and a reverse osmosis membrane (RO membrane) device 12. The RO membrane device 12 treats raw water (water to be treated) stored in the raw water tank 11 to remove impurities from the raw water and produce treated water. The RO membrane device 12 separates the raw water supplied from the raw water tank 11 into concentrated water containing impurities and permeated water from which the impurities have been removed, and includes an RO membrane. The RO membrane device 12 is connected to a water supply line L1 that supplies raw water from the raw water tank 11 to the RO membrane device 12, a permeated water line L2 that supplies permeated water flowing out from the RO membrane device 12 to a treated water tank or a point-of-use, and a wastewater line L3 that discharges concentrated water flowing out from the RO membrane device 12 to the outside. The raw water tank 11 is connected to a raw water line L4 that supplies raw water that has been pretreated, such as by clarification and dechlorination, in a pretreatment system (not shown) to the raw water tank 11.

[0014] The water treatment device 10 also includes a booster 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 booster pump 13 has its rotation speed controlled by an inverter (not shown) and functions to adjust the supply pressure (raw water pressure) of raw water supplied to the RO membrane device 12 through the water supply line L1. The raw water pressure sensor 14 functions to detect the raw water pressure. The temperature sensor 15 functions to detect the water temperature (raw water temperature) of the raw water supplied to the RO membrane device 12. The temperature sensor 15 may be configured to detect the water temperature of either the permeate or concentrated water flowing out of the RO membrane device 12; that is, it may be provided in the permeate line L2 or the drain line L3. The concentrated water pressure sensor 16, together with the raw water pressure sensor 14, has the function of detecting the water flow differential pressure across the RO membrane (the pressure difference between the supply pressure of raw water supplied to the RO membrane and the outflow pressure of concentrated water flowing out of the RO membrane). The manual valve V1 functions as a flow rate adjusting means for adjusting the flow rate of 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 operation of the water treatment device 10, raw water stored in the raw water tank 11 is supplied to the RO membrane device 12 by operation of the pressure pump 13, where it is treated and separated into permeate and concentrated water. The permeate is supplied to a treated water tank or a point-of-use via a permeate line L2, and the concentrated water is discharged to the outside via a drainage line L3. Raw water that has been pretreated, such as for turbidity removal and dechlorination, in a pretreatment system (not shown) is continuously supplied to the raw water tank 11 via a raw water line L4, depending on the flow rate of the raw water supplied to the RO membrane device 12. Note that the raw water tank 11 is not necessarily required from the viewpoint of the functionality of the water treatment device 10, but is preferably provided from the viewpoint of adding a disinfectant to the raw water, as described below.

[0016] The water treatment device 10 also has a disinfectant addition device 20 for adding a disinfectant to the raw water supplied to the RO filtration device 12 to suppress biofouling of the RO membrane, and a control device 30 for controlling the operation of the water treatment device 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 for storing disinfectant and a chemical injection pump 22 connected to the raw water tank 11 via a disinfectant supply line L5 and for injecting the disinfectant stored in the disinfectant tank 21 into the raw water tank 11. The disinfectant addition by the disinfectant addition device 20 is preferably performed intermittently, as described below, from the viewpoints of running costs and environmental impact. However, in this case, there is a concern that biofouling may progress during the time when the disinfectant is not being added. Therefore, the disinfectant to be added is preferably one with a higher disinfecting power, i.e., one with a higher oxidation-reduction potential (ORP), which is one indicator of disinfectant power. Specifically, a disinfectant with an ORP exceeding 500 mV when the aqueous solution is adjusted to a total chlorine concentration of 10 mg / L and a pH of 7.3 is preferred. Examples of such disinfectants include bromine-based oxidizing agents, stabilized hypobromous acid compositions containing bromine and sulfamic acid compounds, iodine-based oxidizing agents, and 2,2-dibromo-3-nitrobenzoates. R Examples of suitable disinfectants include lopropionamide (DBNPA). Using a chlorine-based oxidizing agent (such as sodium hypochlorite) as a disinfectant is undesirable because it may deteriorate the polyamide-based RO membrane. The disinfectant injection point does not have to be the raw water tank 11, and may be, for example, on the water supply line L1 between the pressure pump 13 and the raw water pressure sensor 14. However, in this case, the pressure at the injection point will be higher than when the disinfectant is injected into the raw water tank 11, requiring a chemical injection pump 22 with a large capacity, which is undesirable from a cost perspective. Therefore, the disinfectant injection point, i.e., the connection point of the disinfectant supply line L5, is preferably the raw water tank 11 as shown in the figure.

[0018] During operation of the water treatment device 10, the controller 30 controls the booster pump 13 to maintain a constant flow rate of the permeate through the permeate line L2 (a predetermined set flow rate). For example, when the water temperature changes, the viscosity of the water changes, which in turn changes the flow rate of the permeate separated by the RO membrane. In response to this change, the controller 30 controls the rotation speed of the booster pump 13 via an inverter. That is, when the water temperature decreases, the viscosity of the water increases, resulting in a decrease in the flow rate of the permeate separated by the RO membrane. Therefore, the controller 30 increases the rotation speed of the booster pump 13 to compensate for this decrease, thereby increasing the raw water pressure. On the other hand, when the water temperature increases, the viscosity of the water decreases, resulting in an increase in the flow rate of the permeate separated by the RO membrane. Therefore, the controller 30 reduces the rotation speed of the booster pump 13 to offset this increase, thereby reducing the raw water pressure. In this way, the rotation speed of the booster pump 13, i.e., the raw water pressure, is adjusted, thereby adjusting the flow rate of the permeate through the permeate line L2 to the set flow rate.

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

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

[0021] In the disinfectant addition process, first, prior to the addition of the disinfectant, the level of biofouling (biological contamination) of the RO membrane at that time is evaluated. Specifically, a contamination level indicating the level of biofouling is calculated based on the detection values ​​of the sensors 14, 15, and 16. The method for calculating the contamination level will be described later. Once the contamination level of the RO membrane is calculated, the amount of disinfectant to be added to the raw water per disinfectant addition process is determined based on the calculated contamination level. Specifically, a new addition amount is determined by adding a predetermined minimum addition amount to an addition amount corresponding to (proportional to) the calculated contamination level. The disinfectant addition process is then executed by controlling the chemical feed pump 22 based on the determined addition amount, and raw water is supplied to the raw water tank 11 at a predetermined flow rate corresponding to the flow rate of raw water supplied to the RO membrane device 12. That is, during the disinfectant addition process, raw water containing a predetermined concentration of disinfectant is supplied to the RO membrane device 12. In this way, it becomes possible to accurately grasp the level of biofouling and then add the minimum necessary amount of disinfectant to the raw water accordingly, which ultimately makes it possible to reduce running costs and environmental impact.

[0022] In order to reduce the impact of disinfectants, especially oxidants, on membranes, it is believed that keeping the CT value (the product of the disinfectant concentration and the contact time of the disinfectant with the membrane) low is effective. Conversely, even if the disinfectant concentration or contact time is changed, if the CT value is the same, it is believed that there will be little difference in the impact of the disinfectant on the membrane. Therefore, if the total amount of disinfectant added to raw water during the disinfectant addition process is the same, the impact of the disinfectant on the RO membrane should be the same regardless of the disinfectant concentration in the raw water or the disinfectant addition time (the execution time of the disinfectant addition process). In other words, whether the disinfectant concentration in the raw water is changed without changing the disinfectant concentration in the raw water or the disinfectant concentration in the raw water without changing the disinfectant addition time to adjust (change) the disinfectant addition amount per disinfectant addition process, there should be little difference in the impact of the disinfectant on the RO membrane.

[0023] However, in practice, the inventors have confirmed that, under certain conditions, when the disinfectant concentration in the raw water is changed without changing the disinfectant addition time, the disinfectant may indirectly adversely affect the RO membrane. Specifically, as shown in the examples described below, it has been confirmed that when the disinfectant concentration is increased until the ORP of the raw water after disinfectant addition exceeds a certain upper limit, the increase in raw water pressure, which should be suppressed, cannot be suppressed. Based on the results of analysis of deposits on RO membranes where an increase in raw water pressure was observed, it is inferred that this is because the use of a disinfectant with a high ORP causes viscous substances to be released from organisms attached to the RO membrane surface, and these viscous substances clog the RO membrane.

[0024] Therefore, in this embodiment, to prevent such an increase in raw water pressure, the disinfectant concentration in the raw water is not changed from the initial setting. However, as the level of biofouling changes, the execution time of the disinfectant addition process, i.e., the disinfectant addition time per predetermined time period corresponding to the execution cycle, is changed accordingly. Specifically, when the contamination level of the RO membrane is calculated, a new addition time is set by adding a time corresponding to (proportional to) the calculated contamination level to a preset minimum addition time (a value obtained by dividing a preset minimum addition amount by the disinfectant set concentration). The disinfectant set concentration is a concentration within a range in which the ORP of the raw water after disinfectant addition does not exceed the aforementioned upper limit. Preferably, this upper limit is determined by experimentally verifying the range in which the RO membrane is not clogged by viscous substances of biological origin, as shown in the examples described below. The disinfectant concentration in the raw water can be easily determined from the respective flow rates of the raw water and the disinfectant. However, it is preferably determined by manually sampling the raw water flowing through the water supply line L1 and measuring the total chlorine concentration in the raw water using the DPD method with a portable residual chlorine meter.

[0025] Thus, according to this embodiment, the amount of disinfectant added to raw water per disinfectant addition step is adjusted so that the ORP of the raw water after disinfectant addition does not exceed a preset upper limit (predetermined value). Specifically, the disinfectant addition amount is adjusted by changing the disinfectant addition time depending on the level of biofouling while maintaining the disinfectant concentration in the raw water at a constant concentration such that the ORP of the raw water after disinfectant addition does not exceed the upper limit. This allows the disinfectant to be added to raw water in just the right amount depending on the level of biofouling, within a range in which the oxidizing power of the disinfectant does not adversely affect the RO membrane.

[0026] The set disinfectant concentration is not particularly limited as long as the ORP of the raw water after disinfectant addition does not exceed a predetermined upper limit. However, if the concentration is too low, sufficient disinfection effect may not be achieved, and the desired results may not be obtained. Therefore, the set disinfectant concentration is preferably set within a range in which the ORP of the raw water after disinfectant addition does not fall below a predetermined lower limit so as to exert the minimum disinfecting effect. Note that total chlorine concentration may be used instead of or in addition to ORP as an indicator for determining the set disinfectant concentration. Furthermore, the disinfectant addition time increases as biofouling progresses, but if it becomes too long, it is undesirable from the perspective of running costs and environmental impact. Therefore, it is preferable that the disinfectant addition time not exceed a predetermined maximum addition time. In other words, if the addition time calculated by the above method exceeds the predetermined maximum addition time, it is preferable to set the predetermined maximum addition time as the new addition time, rather than the calculated addition time. Note that, if suppressing RO membrane clogging is prioritized over reducing running costs and environmental impact, continuous disinfectant addition may be temporarily performed until the next disinfectant addition step.

[0027] Here, we will explain three methods for calculating the contamination level, which indicates the degree of biofouling of RO membranes.

[0028] (First calculation method) Biofouling in an RO membrane clogs the raw water flow path, increasing pressure loss. Therefore, as described above, when flow control is performed to maintain a constant flow rate of permeate through the permeate line L2, its effect manifests as a change (increase) in raw water pressure. Therefore, calculating this increase can accurately determine the level of biofouling. However, raw water pressure not only varies depending on the level of biofouling, but also, as described above, varies with water temperature. Therefore, to accurately calculate the increase in raw water pressure due to biofouling, the current raw water pressure should not be directly compared with the raw water pressure at the start of RO membrane use (initial raw water pressure), but should be compared with a value corrected for the initial raw water pressure, taking into account the effects of fluctuations in water temperature. This means that the initial raw water pressure should be converted to a pressure at the current water temperature.

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

[0030] First, the raw water pressure sensor 14 detects the current raw water pressure, and simultaneously, the temperature sensor 15 detects the current raw water temperature. In practice, a moving average of the detected values ​​of each sensor 14, 15 is calculated, and these are acquired (detected) as the current raw water pressure and raw water temperature. Then, using temperature correction coefficient information (e.g., a table or a function) pre-stored in an internal storage device or an external server, a temperature correction coefficient for the detected current raw water temperature and a temperature correction coefficient for 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 an RO membrane measured at an arbitrary temperature to a value at a standard temperature (e.g., 25°C). Manufacturers provide temperature correction coefficients for each temperature for each model of RO membrane. The temperature correction coefficient for the initial raw water temperature may be acquired in advance when the RO membrane is first used and stored in the control device 30. Once the respective temperature correction coefficients are acquired, the initial raw water pressure is converted to a value for the current raw water temperature based on the acquired temperature correction coefficients. Specifically, if the initial raw water pressure is P0, the converted initial pressure P is calculated by converting the initial raw water pressure to the value at the current raw water temperature. R0 is given by the following equation (1): P R0 =P0×(K i / K0) (1) where 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] The converted initial pressure calculated by the above formula (1) is then compared with the detected current raw water pressure, and if the current raw water pressure is higher than the converted initial pressure, it is determined that biofouling is occurring, and the difference is calculated as the contamination level.On the other hand, if the current raw water pressure is equal to or lower than the converted initial pressure, it is determined that biofouling is not occurring, and the contamination level is calculated as zero.

[0032] (Second calculation method) In the second calculation method, in order to offset the effect of water temperature fluctuations on the raw water pressure, rather than converting the initial raw water pressure to pressure at the current water temperature as in the first calculation method, the initial raw water pressure and the current raw water pressure are each converted to pressure at a standard temperature (e.g., 25°C) and the level of RO membrane contamination is calculated based on the results of comparing the converted pressures. As with the first calculation method, this allows for accurate calculation of the increase in raw water pressure over time that is due to biofouling, i.e., the level of RO membrane contamination. Note that the second calculation method also uses the detection values ​​of the sensors 14 and 15 to calculate the level of RO membrane contamination, as described below. However, as with the first calculation method, it is preferable to actually use the moving average values ​​of the sensors.

[0033] In the second calculation method, first, a converted initial pressure obtained by converting the initial raw water pressure into a value at standard temperature is stored in the control device 30. In this case, the converted initial pressure P R0 ' is given by the following equation (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) Like the initial raw water pressure and the initial raw water temperature in the first calculation method, this converted initial pressure is newly acquired and stored in the control device 30 and updated every time the RO membrane is replaced with a new one.

[0034] Then, when the raw water pressure sensor 14 and the temperature sensor 15 detect the current raw water pressure and raw water temperature, respectively, a temperature correction coefficient for the detected current raw water temperature is acquired, and the current raw water pressure is converted to a value at standard temperature based on the acquired temperature correction coefficient. Specifically, the current raw water pressure is converted to P i and the temperature correction coefficient K i Then, the converted raw water pressure P is calculated by converting the current raw water pressure to the value at standard temperature. Ri is given by the following equation (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 equation (2) above) stored in advance in the control device 30, and as in the first calculation method, if the converted raw water pressure is higher than the converted initial pressure, the difference is calculated as the pollution level. On the other hand, as in the first calculation method, if the converted raw water pressure is equal to or lower than the converted initial pressure, the pollution level is calculated to be zero.

[0036] (Third calculation method) Both of the above-described two calculation methods calculate the contamination level of the RO membrane using the raw water pressure detected by the raw water pressure sensor 14. These methods are effective when the outlet pressure of the concentrated water flowing out of the RO membrane device 12 cannot be obtained due to reasons such as the inability to install a pressure sensor in the discharge line L3. However, this does not apply when the concentrated water pressure sensor 16 is installed in the discharge line L3, as shown in the figure. Specifically, as described above, biofouling of the RO membrane clogs the raw water flow path, increasing pressure loss and manifesting as an increase in the water flow differential pressure (the difference between the raw water supply pressure and the concentrated water outlet pressure). Therefore, if the RO membrane water flow differential pressure can be detected using the raw water pressure sensor 14 and the concentrated water pressure sensor 16, the detected value may be used to calculate the contamination level of the RO membrane. This third calculation method has the advantage that the RO membrane water flow differential pressure is hardly affected by fluctuations in water temperature, and therefore does not need to be considered in calculating the contamination level of the RO membrane.

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

[0038] (Fourth calculation method) As mentioned above, the third calculation method is advantageous in that the RO membrane water flow differential pressure is hardly affected by fluctuations in water temperature. However, when using this method, the following points must be taken into consideration. Specifically, the RO membrane water flow differential pressure is considered to be proportional to the nth power (1≦n≦2) of the raw water flow rate (primary flow rate) passing through the primary side of the RO membrane, based on the relationship between the flow rate of a fluid flowing through a circular pipe and pressure loss (e.g., Fanning's equation or Hagen-Poiseuille's equation). Therefore, the RO membrane water flow differential pressure changes not only when biofouling occurs, but also when the flow rate of the concentrated water flowing through the drainage line L3 changes, resulting in a change in the primary flow rate of the raw water. Such changes in the concentrated water flow rate can occur, for example, when the set flow rate of the concentrated water is changed even when the above-mentioned flow rate control of the concentrated water is being performed, or even if the set flow rate is not changed, due to infrequent flow rate adjustment using the manual valve V1. Therefore, if the third calculation method is simply used, it may not be possible to accurately estimate the increase in the water flow differential pressure across the RO membrane caused by biofouling, and the degree of contamination of the RO membrane may not be calculated accurately.

[0039] Therefore, in the fourth calculation method, the contamination level of the RO membrane is calculated based on a value obtained by correcting the current RO membrane water flow differential pressure, taking into account changes in the concentrate flow rate, rather than on the current RO membrane water flow differential pressure itself. Specifically, as in the third calculation method, when the current RO membrane water flow differential pressure is detected, a flow sensor (not shown) installed in the discharge line L3 simultaneously detects the current concentrate flow rate, and preferably, a moving average of the detected values ​​is calculated. Then, based on the detected current concentrate flow rate and the concentrate flow rate at the start of use of the RO membrane (initial concentrate flow rate) stored in the control device 30, the current RO membrane water flow differential pressure is converted to a value at the initial concentrate flow rate. The converted RO membrane water flow differential pressure is compared with the initial RO membrane water flow differential pressure, and the contamination level of the RO membrane is calculated as in the third calculation method. That is, if the converted RO membrane water flow differential pressure is higher than the initial RO membrane water flow differential pressure, the difference is calculated as the contamination level. If the converted RO membrane water flow differential pressure is equal to or lower than the initial RO membrane water flow differential pressure, the contamination level is calculated as zero. This calculation method is expected to prevent overestimation of the increase in water flow differential pressure across the RO membrane caused by biofouling, further reducing the amount of disinfectant used.

[0040] As a conversion formula for the water flow differential pressure, it is preferable to use Fanning's equation (corresponding to the case where the above-mentioned exponent n is 2) assuming that the raw water flow passing through the upstream side of the RO membrane is turbulent, but this is not limited to this. For example, the actual flow state of the raw water (i.e., the value of the above-mentioned exponent n) may be verified, and the water flow differential pressure may be converted using a relational expression obtained from the verification results. Ideally, the water flow differential pressure would be corrected based on the average flow rate of the raw water on the upstream side rather than the flow rate of the concentrated water flowing through the discharge line L3. However, it is practically impossible to measure the average flow rate of the raw water on the upstream side with high accuracy. Furthermore, the flow rate of the raw water flowing through the feed line L1 may be used as the flow rate used to correct the water flow differential pressure. However, in this case, the influence of changes in the flow rate of the permeated water flowing through the permeated water line L2 must also be taken into consideration. Therefore, in practice, it is preferable to correct the water flow differential pressure based on the flow rate of the concentrated water flowing through the discharge line L2, as described above.

[0041] The method for calculating the contamination level of an RO membrane, i.e., the method for evaluating the degree of biofouling, is not limited to the method of calculating the increase in raw water pressure or water flow differential pressure due to biofouling as described above. For example, the degree of biofouling may be evaluated by measuring the total organic carbon (TOC) concentrations in the raw water and the concentrated water and continuously monitoring the difference between them, or by measuring the viable bacteria counts in the raw water and the concentrated water and continuously monitoring the difference between them.

[0042] Next, the effects of the present invention will be described with reference to specific examples.

[0043] Example 1 In this example, a test device simulating the water treatment device shown in Figure 1 was used, and continuous operation was performed with a disinfectant addition process performed once every 24 hours. The raw water pressure (the supply pressure of raw water supplied to the RO membrane device) was measured over time. The raw water used was well water that had undergone predetermined pretreatment (disinfection with sodium hypochlorite, solid-liquid separation using a turbidity membrane, and dechlorination using activated carbon), to which acetic acid, a nutrient for microorganisms, was added at a concentration of 2.5 mg / L. The raw water temperature was 17 to 22°C, and the pH was 6.7 to 7.0 throughout the operation. Nitto Denko Corporation's RO membrane element (product number: ESPA2-4040) was used as the RO membrane. The permeate and concentrate flow rates were controlled throughout the operation, with the permeate flow rate set to 120 L / h and the concentrate flow rate set to 480 L / h.

[0044] The disinfectant used was Organo Corporation's "Orpersion" (product number: E266), a stabilized hypobromous acid composition containing bromine and sulfamic acid compounds. The disinfectant concentration in the raw water during each disinfectant addition step was fixed at a total chlorine concentration of 1.0 mg / L (corresponding to an ORP of 545 mV). Furthermore, during the first disinfectant addition step immediately after starting operation, the execution time (minimum addition time) was set to 1 hour so that the disinfectant addition amount (minimum addition amount) would be 1 h·mg / L. From the second time onward, the RO membrane fouling level was calculated using the first calculation method, and the execution time of the disinfectant addition step, i.e., the disinfectant addition time per 24 hours, was changed based on the calculation results to adjust the disinfectant addition amount per step.

[0045] (Comparative Example 1) Measurements were performed under the same conditions as in Example 1, except that the execution time of each disinfectant addition step was fixed at 3 hours, and the disinfectant concentration in the raw water was changed according to the degree of RO membrane fouling to adjust the amount of disinfectant added per disinfectant addition step. Note that, because this comparative example was performed at a different time than Example 1, in the first disinfectant addition step, the disinfectant concentration in the raw water was set to a total chlorine concentration of 0.6 mg / L (equivalent to an ORP of 530 mV) so that the minimum disinfectant addition amount was 1.8 h mg / L.

[0046] (Comparative Example 2) Measurements were carried out under the same conditions as in Comparative Example 1, except that the disinfectant concentration in the raw water in each disinfectant addition process was fixed at 1.0 mg / L in total chlorine concentration, and the execution time was fixed at 3 hours, without adjusting the amount of disinfectant added according to the degree of contamination of the RO membrane.

[0047] Figures 2, 3, and 4 are graphs showing the measurement results in Example 1, Comparative Example 1, and Comparative Example 2, respectively. For reference, Figures 2 and 3 also show the changes in parameters that vary over time (the disinfectant addition time per 24 hours and the disinfectant concentration in the raw water), and Figure 2 also shows the changes in the temperature of the raw water during the operation period.

[0048] In Example 1, as shown in Figure 2, the raw water pressure was maintained almost constant throughout the operation period. It was confirmed that adjusting the amount of disinfectant added depending on the level of RO membrane contamination effectively suppressed RO membrane clogging due to biofouling. In contrast, in Comparative Examples 1 and 2, as shown in Figures 3 and 4, it was ultimately impossible to maintain a constant raw water pressure. In particular, in Comparative Example 1, once the disinfectant concentration in the raw water exceeded 2.0 mg / L (equivalent to an ORP of 570 mV) in total chlorine concentration, the rise in raw water pressure could no longer be suppressed, even though the disinfectant was added at an amount that appropriately reflected the effects of biofouling, as in Example 1. Based on the analysis of deposits on the RO membrane where a rise in raw water pressure was observed, it is inferred that this is because, when the ORP of the disinfectant-containing raw water exceeds a certain level, organisms attached to the RO membrane surface release viscous substances due to cell wall destruction or excessive stress, and these viscous substances clog the RO membrane.

[0049] Example 2 In this example, the water treatment device shown in FIG. 1 was continuously operated while performing a disinfectant addition process once every 12 hours. The change in the disinfectant addition amount per process was investigated when the RO membrane fouling level was calculated using a method different from that used in Example 1. The raw water used was industrial water that had undergone predetermined pretreatment (disinfection with sodium hypochlorite, solid-liquid separation by sand filtration, and dechlorination with sodium bisulfite). The raw water temperature was 20-25°C and the pH was 6.8-7.2 throughout the operation. An RO membrane module equipped with 40 RO membrane elements (product number: ESPA1) manufactured by Nitto Denko Corporation was used as the RO membrane device, and the flow rates of the permeate and concentrate were controlled throughout the operation. The set flow rates of the permeate and concentrate were 35 L / h and 15 L / h, respectively.

[0050] The disinfectant used was the same as in Example 1, and the disinfectant addition amount per disinfectant addition step was adjusted using the same procedure as in Example 1, except that the RO membrane fouling level was calculated using the third calculation method. The disinfectant concentration in the raw water in each disinfectant addition step was fixed at a total chlorine concentration of 1.0 mg / L, as in Example 1. The execution time (minimum addition time) of the first disinfectant addition step immediately after starting operation was set to 15 minutes so that the disinfectant addition amount (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 fouling level of the RO membrane was calculated using the fourth calculation method.

[0052] 5 is a graph showing the change in the addition time of the fungicide per 12 hours over time in Examples 2 and 3. The vertical axis of the graph shows values ​​normalized by the minimum addition time.

[0053] As is clear from Figure 5, compared to Example 2, in which the RO membrane fouling level was calculated without considering changes in the flow rate of the concentrated water, Example 3, in which the RO membrane fouling level was calculated taking this into consideration, showed a smaller increase in the disinfectant addition time overall. On the other hand, although not shown in detail here, neither Examples 2 nor 3 showed a significant increase in the water flow differential pressure (the pressure difference between the supply pressure of raw water supplied to the RO membrane device and the outflow pressure of concentrated water flowing out of the RO membrane device) throughout the operating period, allowing stable operation to be continued. From this, it can be considered that both Examples 2 and 3 are good in terms of preventing RO membrane clogging, but Example 3 is better in terms of reducing the amount of disinfectant used. [Explanation of symbols]

[0054] 10 Water Treatment Equipment 11 Raw water tank 12 Reverse osmosis membrane (RO membrane) equipment 13 Pressure pump 14 Raw water pressure sensor 15 Temperature Sensor 16 Concentrated water pressure sensor 20 Disinfectant Addition Device 21 Fungicide tank 22 Chemical injection pump 30 Control device L1 water supply line L2 permeate line L3 drain line L4 raw water line L5 Fungicide supply line

Claims

1. A step of supplying the water to be treated to a reverse osmosis membrane and separating the water into permeate and concentrate; a step of intermittently adding a disinfectant to the water to be treated that is supplied to the reverse osmosis membrane, wherein the disinfectant 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); The step of intermittently adding the fungicide comprises: Detecting the current water temperature of any of the water to be treated supplied to the reverse osmosis membrane, the permeated water flowing out from the reverse osmosis membrane, and the concentrated water flowing out from the reverse osmosis membrane; detecting a current supply pressure of the water to be treated supplied to the reverse osmosis membrane; calculating a contamination level indicating the degree of biological contamination of the reverse osmosis membrane based on an initial value of any one of the water temperatures detected in advance at the start of use of the reverse osmosis membrane, an initial value of the supply pressure detected in advance at the start of use of the reverse osmosis membrane, the detected current water temperature, and the detected current supply pressure; and a step of adjusting the amount of the disinfectant added to the water to be treated per predetermined time based on the calculated contamination level, so that at least one of the oxidation-reduction potential and the total chlorine concentration of the water to be treated to which the disinfectant has been added does not exceed a predetermined value, the step including changing the addition time of the disinfectant per predetermined time in accordance with the calculated contamination level while maintaining the concentration of the disinfectant in the water to be treated constant; a water treatment method, wherein the step of calculating the degree of contamination includes converting an initial value of the supply pressure to a value at the detected current water temperature based on the detected current water temperature and an initial value of any one of the water temperatures, and calculating the difference between the detected current supply pressure and the converted initial value of the supply pressure as the degree of contamination.

2. A process of supplying the water to be treated to a reverse osmosis membrane and separating the water into permeate and concentrate; a step of intermittently adding a disinfectant to the water to be treated that is supplied to the reverse osmosis membrane, wherein the disinfectant 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); The step of intermittently adding the fungicide comprises: Detecting the current water temperature of any of the water to be treated supplied to the reverse osmosis membrane, the permeated water flowing out from the reverse osmosis membrane, and the concentrated water flowing out from the reverse osmosis membrane; detecting a current supply pressure of the water to be treated supplied to the reverse osmosis membrane; calculating a contamination level indicating the degree of biological contamination of the reverse osmosis membrane based on an initial value of any one of the water temperatures detected in advance at the start of use of the reverse osmosis membrane, an initial value of the supply pressure detected in advance at the start of use of the reverse osmosis membrane, the detected current water temperature, and the detected current supply pressure; and a step of adjusting the amount of the disinfectant added to the water to be treated per predetermined time based on the calculated contamination level, so that at least one of the oxidation-reduction potential and the total chlorine concentration of the water to be treated to which the disinfectant has been added does not exceed a predetermined value, the step including changing the addition time of the disinfectant per predetermined time in accordance with the calculated contamination level while maintaining the concentration of the disinfectant in the water to be treated constant; a water treatment method, wherein the step of calculating the degree of contamination includes converting the detected current supply pressure and the initial value of the supply pressure to values ​​at a standard temperature based on the detected current water temperature and an initial value of any of the water temperatures, and calculating the difference between the converted current supply pressure and the converted initial value of the supply pressure as the degree of contamination.

3. 3. The water treatment method according to claim 1, wherein changing the addition time includes setting a new addition time to a value obtained by adding a time corresponding to the degree of contamination to the addition time that was preset when the reverse osmosis membrane was first used.

4. 3. The water treatment method according to claim 1, 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.

5. a reverse osmosis membrane device having a reverse osmosis membrane that separates the water to be treated into permeate and concentrated water; a disinfectant adding device that adds a disinfectant to the water to be treated that is supplied to the reverse osmosis membrane device, the disinfectant being 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); a temperature sensor for detecting the temperature of any of the water to be treated supplied to the reverse osmosis membrane device, the permeated water flowing out from the reverse osmosis membrane device, and the concentrated water flowing out from the reverse osmosis membrane device; a pressure sensor for detecting a supply pressure of the water to be treated supplied to the reverse osmosis membrane device; a control device that intermittently adds the disinfectant by the disinfectant adding device, calculates a contamination level indicating the degree of biological contamination of the reverse osmosis membrane based on an initial value of any of the water temperatures previously detected by the temperature sensor when use of the reverse osmosis membrane is started, an initial value of the supply pressure previously detected by the pressure sensor when use of the reverse osmosis membrane is started, any of the water temperatures currently detected by the temperature sensor, and the current supply pressure detected by the pressure sensor, and adjusts the amount of disinfectant added to the water to be treated per predetermined time based on the calculated contamination level, within a range where at least one of the oxidation-reduction potential and the total chlorine concentration of the water to which the disinfectant has been added does not exceed a predetermined value; The control device converts the initial value of the supply pressure to a value at one of the detected current water temperatures based on one of the detected current water temperatures and the initial value of one of the water temperatures, calculates the difference between the detected current supply pressure and the converted initial value of the supply pressure as the degree of contamination, and adjusts the amount of addition of the bactericide by changing the addition time of the bactericide per specified time according to the calculated degree of contamination while maintaining the concentration of the bactericide in the treated water constant.

6. A reverse osmosis membrane device having a reverse osmosis membrane that separates the water to be treated into permeate and concentrated water; a disinfectant adding device that adds a disinfectant to the water to be treated that is supplied to the reverse osmosis membrane device, the disinfectant being 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); a temperature sensor for detecting the temperature of any of the water to be treated supplied to the reverse osmosis membrane device, the permeated water flowing out from the reverse osmosis membrane device, and the concentrated water flowing out from the reverse osmosis membrane device; a pressure sensor for detecting a supply pressure of the water to be treated supplied to the reverse osmosis membrane device; a control device that intermittently adds the disinfectant by the disinfectant adding device, calculates a contamination level indicating the degree of biological contamination of the reverse osmosis membrane based on an initial value of any of the water temperatures previously detected by the temperature sensor when use of the reverse osmosis membrane is started, an initial value of the supply pressure previously detected by the pressure sensor when use of the reverse osmosis membrane is started, any of the water temperatures currently detected by the temperature sensor, and the current supply pressure detected by the pressure sensor, and adjusts the amount of disinfectant added to the water to be treated per predetermined time based on the calculated contamination level, within a range where at least one of the oxidation-reduction potential and the total chlorine concentration of the water to which the disinfectant has been added does not exceed a predetermined value; The control device converts the detected current supply pressure and the initial value of the supply pressure to values ​​at standard temperature based on one of the detected current water temperatures and the initial value of one of the water temperatures, calculates the difference between the converted current supply pressure and the converted initial value of the supply pressure as the degree of contamination, and adjusts the amount of disinfectant added by changing the addition time of the disinfectant per specified time period according to the calculated degree of contamination while maintaining the concentration of the disinfectant in the treated water constant.

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