Water treatment method and water treatment device

Abstract

The water treatment method of the present invention includes the following steps: supplying the water to be treated to a reverse osmosis membrane to separate it into permeate and concentrate; and intermittently adding a bactericide to the water supplied to the reverse osmosis membrane, wherein the bactericide is a bromine-based oxidant, a stabilized hypobromic acid composition containing bromine and amine sulfonic acid compounds, an iodine-based oxidant, or 2,2-dibromo-3-nitropropamide (DBNPA); the step of intermittently adding the bactericide includes the following steps: assessing the degree of biofouling of the reverse osmosis membrane; and adjusting the amount of bactericide added to the water at predetermined intervals, based on the assessed degree of biofouling, within a range where at least one of the oxidation-reduction potential and total chlorine concentration of the water to which the bactericide has been added does not exceed a predetermined value.

Description

Technical Field

[0001] This invention relates to a water treatment method and a water treatment apparatus. Prior Technology

[0002] Regarding water treatment devices for removing impurities from water to be treated, devices equipped with reverse osmosis membranes (RO membranes) are known. In this device, the water to be treated (raw water) supplied to the RO membrane at a predetermined supply pressure is separated into permeate and concentrate by the RO membrane. This yields treated water (permeate) with impurities removed.

[0003] Water treatment systems equipped with RO membranes require stable and continuous operation; therefore, suppressing biofouling caused by biological deposits in the raw water adhering to the RO membrane surface is crucial. Conventionally, a method to combat such biofouling has been to add bactericides that inhibit biological growth to the raw water. Representative bactericides include oxidants such as hypochlorous acid, hypobromic acid, and their stabilized components (see, for example, Patent Document 1). However, in recent years, with increasing demands for cost reduction and environmental awareness, there is a need to minimize the amount of bactericide used while effectively suppressing biofouling. For example, Patent Document 2 proposes a method for adjusting the amount of bactericide added according to the degree of biofouling. [Previous Technical Documents] [Patent Literature]

[0004] [Patent Document 1] Japanese Patent No. 6401491 [Patent Document 2] International Publication No. 2020 / 158645 Summary of the Invention

[0005] [The problem that the invention aims to solve]

[0006] However, the method described in Patent Document 2 does not take into account the effect of the bactericide on the RO membrane at all, especially when using an oxidant as described in Patent Document 1, it does not take into account its effect on the RO membrane at all.

[0007] Therefore, the object of the present invention is to provide a water treatment method and water treatment apparatus that inhibits the clogging of reverse osmosis membranes caused by biofouling and achieves stable water treatment performance. [Methods for solving problems]

[0008] To achieve the above objectives, the water treatment method of the present invention includes the following steps: supplying the water to be treated to a reverse osmosis membrane to separate it into permeate and concentrate; and intermittently adding a bactericide to the water supplied to the reverse osmosis membrane, wherein the bactericide is a bromine-based oxidant, a stabilized hypobromic acid composition containing bromine and amine sulfonic acid compounds, an iodine-based oxidant, or 2,2-dibromo-3-nitropropamide (DBNPA); the step of intermittently adding the bactericide includes the following steps: assessing the degree of biofouling of the reverse osmosis membrane; and adjusting the amount of bactericide added to the water at predetermined intervals, based on the assessed degree of biofouling, within a range where at least one of the oxidation-reduction potential and total chlorine concentration of the water to which the bactericide has been added does not exceed a predetermined value.

[0009] Furthermore, the water treatment apparatus of the present invention comprises: a reverse osmosis membrane device for separating the water to be treated into permeate and concentrate; a bactericide addition device for adding a bactericide to the water supplied to the reverse osmosis membrane device, wherein a bromine-based oxidant, a stabilized hypobromic acid composition containing bromine and amine sulfonic acid compounds, an iodine-based oxidant, or 2,2-dibromo-3-nitropropionic acid (DBNPA) is added as a bactericide; and a control device for intermittently performing the addition of bactericide by the bactericide addition device; and the control device assesses the degree of biofouling of the reverse osmosis membrane device and, based on the assessed degree of biofouling, adjusts the amount of bactericide added to the water at predetermined intervals within a range where at least one of the oxidation-reduction potential and total chlorine concentration of the water to which the bactericide has been added does not exceed a predetermined value.

[0010] According to such water treatment methods and devices, the bactericide can be added to the treated water (raw water) in appropriate amounts, depending on the degree of biological fouling (biofouling), within a range where the oxidizing power of the bactericide will not have an adverse effect on the reverse osmosis membrane. [Effects of the Invention]

[0011] According to the present invention, the blockage of reverse osmosis membranes caused by biological fouling can be suppressed and stable water treatment performance can be achieved. Simple Explanation of the Diagram

[0012] [Figure 1] is a schematic diagram showing the configuration of a water treatment apparatus according to one embodiment of the present invention. [Figure 2] is a graph showing the changes over time in raw water pressure and the time of addition of bactericide every 24 hours in Example 1. [Figure 3] is a graph showing the changes over time in the raw water pressure and the concentration of bactericide in the raw water in Comparative Example 1. [Figure 4] is a graph showing the change of raw water pressure over time in Comparative Example 2. [Figure 5] is a graph showing the change over time in the addition time of the bactericide every 12 hours in Examples 2 and 3. Implementation

[0013] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0014] The water treatment apparatus 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 the raw water (treated water) stored in the raw water tank 11, thereby removing impurities contained in the raw water and producing treated water. The RO membrane device 12 separates the raw water supplied from the raw water tank 11 into concentrated water containing impurities and permeate water with impurities removed. It has an RO membrane. A water supply line L1 for supplying raw water from the raw water tank 11 to the RO membrane device 12, a permeate water line L2 for supplying permeate water flowing out of the RO membrane device 12 to the treated water tank or the user end, and a drain line L3 for discharging the concentrated water flowing out of the RO membrane device 12 to the outside. A raw water line L4 for supplying raw water that has undergone pretreatment such as turbidity removal and dechlorination in a pretreatment system (not shown) to the raw water tank 11 is connected to the raw water tank 11.

[0015] Furthermore, the water treatment device 10 includes a booster pump 13 installed on the water supply line L1, a raw water pressure sensor 14 and a temperature sensor 15 also installed on the water supply line L1, and a concentrate pressure sensor 16 and a manual valve V1 installed on the drain line L3. The booster pump 13 is configured to control its speed via an inverter (not shown) and has the 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 the function of detecting the raw water pressure. The temperature sensor 15 has the function of detecting the temperature (raw water temperature) of the raw water supplied to the RO membrane device 12. In addition, the temperature sensor 15 can also be configured to detect the temperature of either the permeate or the concentrate flowing out of the RO membrane device 12, that is, it can also be installed on the permeate line L2 or the drain line L3. The concentrate pressure sensor 16, together with the raw water pressure sensor 14, functions to detect the pressure difference across the RO membrane (the pressure difference between the supply pressure of the raw water to the RO membrane and the outflow pressure of the concentrate flowing out of the RO membrane). The manual valve V1 functions as a flow adjustment means to adjust the flow rate of the concentrate flowing in the drain line L3. Furthermore, as described later, when the concentrate pressure sensor 16 is provided, the temperature sensor 15 can be omitted.

[0016] When the water treatment device 10 is operating, the raw water stored in the raw water tank 11 is supplied to the RO membrane device 12 by the action of the booster pump 13, where it is treated and separated into permeate and concentrate. The permeate is supplied to the treated water tank or the user end through the permeate pipeline L2, and the concentrate is discharged to the outside through the drain pipeline L3. Moreover, depending on the flow rate of the raw water supplied to the RO membrane device 12, the raw water that has undergone pretreatment such as turbidity removal and dechlorination in the pretreatment system (not shown) is continuously supplied to the raw water tank 11 through the raw water pipeline L4. In addition, from the perspective of the function of the water treatment device 10, it is not necessary to set up a raw water tank 11, but as will be described later, it is advisable to set it up from the viewpoint of adding bactericide to the raw water.

[0017] Furthermore, the water treatment device 10 includes a bactericide addition device 20 and a control device 30. The bactericide addition device 20 is used to add a bactericide that inhibits biofouling of the RO membrane to the raw water supplied to the RO membrane device 12. The control device 30 controls the operation of the water treatment device 10, including the addition of the bactericide by the bactericide addition device 20.

[0018] The bactericide addition device 20 includes a bactericide tank 21 for storing bactericide and a chemical injection pump 22 connected to the raw water tank 11 via a bactericide supply line L5, which injects the bactericide stored in the bactericide tank 21 into the raw water tank 11. From the viewpoint of operating cost and environmental impact, the addition of bactericide by the bactericide addition device 20 should preferably be performed intermittently, as described later. However, in this case, biodegradation may progress during periods when addition is not performed. Therefore, the bactericide added should preferably be one with high bactericidal power, that is, one with a high oxidation-reduction potential (ORP), which is a standard for bactericidal power. Specifically, it should preferably be a bactericide whose ORP exceeds 500 mV when the aqueous solution is adjusted to a total chlorine concentration of 10 mg / L and a pH of 7.3. Examples of suitable bactericides include bromine-based oxidants, stabilized hypobromic acid compositions containing bromine and amine sulfonic acid compounds, iodine-based oxidants, and 2,2-dibromo-3-nitropropionic acid (DBNPA). Furthermore, the use of chlorine-based oxidants (such as sodium hypochlorite) as bactericides may lead to the degradation of polyamide-based RO membranes, which is less desirable. The bactericide injection point may not be the raw water tank 11; for example, it could be on the water supply line L1 between the booster pump 13 and the raw water pressure sensor 14. However, in this case, the pressure at the injection point would be higher than when injected into the raw water tank 11, requiring a large-capacity chemical injection pump 22, which is less cost-effective. Therefore, the bactericide injection point, i.e., the connection point of the bactericide supply line L5, should preferably be the raw water tank 11, as shown in the diagram.

[0019] When the water treatment device 10 is operating, the control device 30 controls the flow rate of the booster pump 13 to keep the flow rate of the permeate flowing in the permeate pipeline L2 constant (a pre-determined set flow rate). For example, if the water temperature changes, the viscosity of the water will change, and thus the flow rate of the permeate separated by the RO membrane will also change. In response to this change, the control device 30 controls the rotation speed of the booster pump 13 via an inverter. That is, if 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, to compensate for this decrease, the control device 30 increases the rotation speed of the booster pump 13 to increase the raw water pressure. Conversely, if 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, to eliminate this increase, the control device 30 decreases the rotation speed of the booster pump 13 to decrease the raw water pressure. Thus, by adjusting the speed of the booster pump 13, that is, adjusting the raw water pressure, the flow rate of the permeate flowing in the permeate pipeline L2 is adjusted to the set flow rate.

[0020] Furthermore, during the operation of the water treatment unit 10, in order to suppress scaling caused by the precipitation of impurities (especially silicon dioxide or calcium) on the RO membrane surface, it is advisable to adjust the flow rate of the concentrate flowing in the drainage line L3, in addition to controlling the flow rate of the permeate water as described above. Specifically, it is advisable 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 of the concentrate does not reach the solubility at a pre-measured water temperature, and adjust the flow rate of the concentrate in a manner that achieves the set target recovery rate. The flow rate adjustment is performed by a manual valve V1 installed in the drainage line L3, and this set flow rate is determined based on the target recovery rate and the set flow rate of the permeate water.

[0021] Furthermore, during the operation of the water treatment device 10, the control device 30 controls the bactericide addition device 20 to perform the bactericide addition step for the raw water supplied to the RO membrane device 12 intermittently, preferably periodically (e.g., once every 24 hours).

[0022] In the disinfectant addition step, firstly, before adding the disinfectant, the degree of biofouling (bioaccumulation) on the RO membrane at that point in time is assessed. Specifically, the fouling level, indicating the degree of biofouling, is calculated based on the detection values ​​of each sensor 14, 15, and 16. The calculation method for this fouling level is described later. Once the fouling level of the RO membrane is calculated, the amount of disinfectant added to the raw water for each disinfectant addition step is determined based on the calculated fouling level. Specifically, the new addition amount is the value obtained by adding a pre-set minimum addition amount to the addition amount corresponding to (proportional to) the calculated fouling level. Then, based on the determined addition amount, the reagent injection pump 22 is controlled to perform the disinfectant addition step, and the 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 unit 12. That is, during the disinfectant addition step, raw water containing a predetermined concentration of disinfectant is supplied to the RO membrane unit 12. Thus, once the degree of biofouling is correctly determined, the bactericide can be added to the raw water at the minimum required amount, which in turn reduces operating costs and environmental impact.

[0023] However, to reduce the impact of disinfectants, especially oxidants, on the membrane, it is believed that setting a lower CT value (the product of the disinfectant concentration and the time the disinfectant contacts the membrane) is effective. In other words, even if the concentration of the disinfectant and the contact time change, it is believed that as long as the CT value remains the same, the impact of the disinfectant on the membrane will be almost the same. According to this, as long as the total amount of disinfectant added to the raw water in the disinfectant addition step is the same, regardless of the disinfectant concentration in the raw water or the addition time (execution time of the disinfectant addition step), the impact of the disinfectant on the RO membrane should be the same. That is, to adjust (change) the amount of disinfectant added in each disinfectant addition step, whether the disinfectant concentration in the raw water is kept the same but the addition time is changed, or the disinfectant concentration in the raw water is kept the same but the addition time is changed, the impact of the disinfectant on the RO membrane should be almost the same.

[0024] However, in practice, according to the verification by the inventors of this case, it has been confirmed that when the concentration of the bactericide in the raw water is changed without changing the time of addition of the bactericide, under certain conditions, the bactericide may indirectly have an adverse effect on the RO membrane. Specifically, as shown in the examples described later, it has been confirmed that if the concentration of the bactericide is increased to the point that the ORP of the raw water after the addition of the bactericide exceeds a certain upper limit, the rise in raw water pressure, which should have been suppressed, becomes uncontrollable. Based on the analysis results of the deposits on the RO membrane observed to have increased raw water pressure, it is speculated that this is because the use of a bactericide with a high ORP causes the release of sticky substances from the organisms attached to the membrane surface of the RO membrane, which clogs the RO membrane.

[0025] Therefore, in this embodiment, in order to suppress such an increase in raw water pressure, the concentration of the bactericide in the raw water is not changed from the initial set concentration. If the degree of biofouling changes, the execution time of the bactericide addition step is changed accordingly, that is, the bactericide addition time is changed for each predetermined time corresponding to its execution cycle. Specifically, if the fouling degree of the RO membrane is calculated, the new addition time is obtained by adding the preset minimum addition time (the value obtained by dividing the preset minimum addition amount by the set concentration of the bactericide) to the time corresponding to (proportional to) the calculated fouling degree. In addition, the set concentration of the bactericide is the concentration within the range where the ORP of the raw water after the bactericide addition does not exceed the above-mentioned upper limit value. However, this upper limit value should be determined by experimentally verifying in advance that the RO membrane will not be blocked by sticky substances from organisms, as shown in the embodiments described later. Furthermore, the concentration of disinfectant in the raw water can be easily determined from the flow rates of the raw water and the disinfectant, but it is preferable to manually sample the raw water flowing in the water supply line L1 and determine the total chlorine concentration in the raw water using the DPD method with a portable residual chlorine meter.

[0026] Thus, according to this embodiment, the amount of bactericide added to the raw water is adjusted for each bactericide addition step, ensuring that the ORP of the raw water after adding the bactericide does not exceed a preset upper limit (predetermined value). Specifically, while maintaining a fixed concentration of bactericide in the raw water that ensures the ORP of the raw water after adding the bactericide does not exceed the aforementioned upper limit, the bactericide addition time is changed according to the degree of biofouling, thereby adjusting the amount of bactericide added. In this way, the bactericide can be added to the raw water in an appropriate amount according to the degree of biofouling, within a range where the oxidizing power of the bactericide will not adversely affect the RO membrane.

[0027] The set concentration of the disinfectant is not particularly limited as long as the ORP of the raw water after adding the disinfectant does not exceed the preset 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 concentration of the disinfectant should be within the range where the ORP of the raw water after adding the disinfectant is not lower than the predetermined lower limit, so as to exert a minimum disinfection effect. In addition, as an indicator for determining the set concentration of the disinfectant, the total chlorine concentration can be used instead of ORP, or the total chlorine concentration can be used in addition to ORP. Furthermore, the disinfectant addition time will increase with the progress of biofouling, but if it is too long, it is not ideal from the perspective of operating costs and environmental impact. Therefore, the disinfectant addition time should be adjusted so as not to exceed the preset maximum addition time. That is, if the addition time obtained by the above calculation method exceeds the preset maximum addition time, the preset maximum addition time should be set instead of the calculated addition time as the new addition time. Alternatively, if reducing operating costs and environmental impact is a greater priority than preventing RO membrane blockage, then the bactericide can be added continuously until the next bactericide addition step.

[0028] Here, we will explain four methods for calculating the degree of fouling that indicates the extent of biofouling on RO membranes.

[0029] (Calculation Method 1) Biofouling on the RO membrane can clog the feedwater flow path, increasing pressure loss. Therefore, when flow control is implemented as described above to maintain a constant flow rate of permeate in the L2 line, the effect will manifest as a change (increase) in feedwater pressure. Thus, the degree of biofouling can be accurately assessed by calculating this increase. However, feedwater pressure not only varies with the degree of biofouling but also, as mentioned above, with water temperature. Therefore, to accurately calculate the increase in feedwater pressure due to biofouling, it is not necessary to directly compare the current feedwater pressure with the initial feedwater pressure when the RO membrane was first used (initial feedwater pressure). Instead, it is necessary to compare the current feedwater pressure with the value obtained after adjusting for the effects of water temperature variations, i.e., with the value obtained by converting the initial feedwater pressure to the pressure at the current water temperature.

[0030] Therefore, the first calculation method calculates the fouling degree of the RO membrane as follows. As a prerequisite, the control device 30 stores the initial value of the raw water pressure (initial raw water pressure) detected by the raw water pressure sensor 14 and the initial value of the raw water temperature (initial raw water temperature) detected by the temperature sensor 15 when the RO membrane is first used. Furthermore, the initial raw water pressure and initial raw water temperature can be measured immediately after the RO membrane is first used, but it is preferable to obtain them after a certain period of time from the start of use and after the performance has stabilized; they can also be moving averages. Moreover, whenever the RO membrane is replaced with a new one, the initial raw water pressure and initial raw water temperature are obtained again and stored and updated in the control device 30.

[0031] First, the current raw water pressure is detected by the raw water pressure sensor 14, and simultaneously, the current raw water temperature is detected by the temperature sensor 15. In practice, the moving average of the detected values ​​from each sensor 14 and 15 is calculated to obtain the current raw water pressure and temperature. Then, using information (tables, functions, etc.) of temperature correction coefficients pre-stored in an internal memory device or external server, the temperature correction coefficient at the detected current raw water temperature and the temperature correction coefficient pre-stored in the control device 30 at the initial raw water temperature are obtained. The temperature correction coefficient refers to the coefficient used to correct the permeate flow of the RO membrane measured at any temperature to a value at a standard temperature (e.g., 25°C). The manufacturer provides temperature correction coefficients for each temperature for each model of the RO membrane. Alternatively, the temperature correction coefficient at the initial raw water temperature can be obtained in advance when the RO membrane is first used and stored in the control device 30. If the respective temperature correction coefficients are obtained, the initial raw water pressure is converted to a value at the current raw water temperature based on the obtained temperature correction coefficients. Specifically, when the initial raw water pressure is P0, the converted initial pressure P R0 obtained by converting the initial raw water pressure to the value at the current raw water temperature can be provided by the following formula (1). PR = P0 × (Ki / K0) (1) Here, Ki is the temperature correction factor at the current raw water temperature, and K0 is the temperature correction factor at the initial raw water temperature.

[0032] Then, the converted initial pressure calculated by the above formula (1) is compared with the current raw water pressure that has been detected. If the current raw water pressure is higher than the converted initial pressure, it is determined that biodegradation has occurred, and the difference is calculated as the degree of contamination. On the other hand, if the current raw water pressure is equal to or lower than the converted initial pressure, it is determined that no biodegradation has occurred, and the degree of contamination is calculated to be zero.

[0033] (Second Calculation Method) The second calculation method, designed to offset the impact of water temperature variations on raw water pressure, does not convert the initial raw water pressure to the pressure at the current water temperature as in the first method. Instead, it converts both the initial and current raw water pressures to pressures at a standard temperature (e.g., 25°C) and calculates the RO membrane fouling level based on the comparison results. In this way, similar to the first method, it accurately calculates the increase in raw water pressure due to biofouling over time, i.e., the RO membrane fouling level. Furthermore, while the second calculation method uses the detection values ​​of sensors 14 and 15 to calculate the RO membrane fouling level, as shown below, it is actually preferable to use their respective moving averages, just as in the first method.

[0034] In the second calculation method, firstly, the control device 30 stores the converted initial pressure obtained by converting the initial raw water pressure to the value at the standard temperature. At this time, if the initial raw water pressure is P0 and the temperature correction coefficient at the initial raw water temperature is K0, then the converted initial pressure PR0' can be provided by the following formula (2). P R0'=P 0 / K 0(2) In addition, similar to the initial raw water pressure and initial raw water temperature in the first calculation method, this converted initial pressure is obtained again whenever the RO membrane is replaced with a new one, and it is memorized and updated in the control device 30.

[0035] Then, by using the raw water pressure sensor 14 and the temperature sensor 15 to detect the current raw water pressure and temperature respectively, a temperature correction coefficient at the detected current raw water temperature will be obtained. Based on the obtained temperature correction coefficient, the current raw water pressure will be converted to a value at the standard temperature. Specifically, if the current raw water pressure is Pi and the temperature correction coefficient at the current raw water temperature is Ki, the converted raw water pressure Pra obtained by converting the current raw water pressure to a value at the standard temperature can be provided by the following formula (3). PRi = Pi / Ki(3)

[0036] The converted raw water pressure calculated in this way is compared with the converted initial pressure pre-memorized in the control device 30 (refer to the above formula (2)). As with the first calculation method, if the converted raw water pressure is higher than the converted initial pressure, the difference is calculated as the pollution degree. On the other hand, if the converted raw water pressure is equal to or lower than the converted initial pressure, the pollution degree is calculated to be zero, which is also the same as the first calculation method.

[0037] (Third calculation method) Both of the above calculation methods use the raw water pressure measured by the raw water pressure sensor 14 to calculate the fouling degree of the RO membrane. These methods are effective when the outflow pressure of the concentrate flowing from the RO membrane unit 12 cannot be obtained due to the inability to install a pressure sensor or similar device on the drain line L3. However, when a concentrate pressure sensor 16 is installed on the drain line L3 as shown in the diagram, these methods are not limited to this approach. That is, as described above, the formation of biofouling on the RO membrane can clog the raw water flow path, increasing pressure loss, and thus manifesting as an increase in the flow pressure difference (the pressure difference between the raw water supply pressure and the concentrate outflow pressure). Therefore, when the flow pressure difference of the RO membrane can be detected by the raw water pressure sensor 14 and the concentrate pressure sensor 16, the fouling degree of the RO membrane can also be calculated using their measured values. This third calculation method is more advantageous because the pressure difference of the RO membrane is almost unaffected by water temperature changes, and therefore the fouling degree of the RO membrane can be disregarded when calculating it.

[0038] Therefore, in the third calculation method, if the current RO membrane pressure difference is detected by the difference between the detection value of the raw water pressure sensor 14 (preferably its moving average) and the detection value of the concentrated water pressure sensor 16 (preferably its moving average), then the detected value is directly compared with the initial pressure difference (when the RO membrane was first used) stored in the control device 30. Furthermore, if the current pressure difference is higher than the initial pressure difference, the difference is calculated as the fouling degree; if it is equal to or lower than the initial pressure difference, the fouling degree is calculated to be zero. In addition, the initial pressure difference should preferably be obtained after a certain period of time since the RO membrane was first used and its performance has stabilized, and preferably as its moving average. Also, when implementing the third calculation method, the temperature sensor 15 can be omitted, and one differential pressure sensor can be used instead of two pressure sensors 14 and 16.

[0039] (Calculation Method 4) The third calculation method, as mentioned above, is advantageous because it considers that the pressure differential across the RO membrane is almost unaffected by water temperature fluctuations. However, the following points should be noted when using this method. Specifically, the pressure differential across the RO membrane, derived by analogy from the relationship between flow rate and pressure loss in a circular pipe (Fanning's equation, Hagen-Poiseuille's equation, etc.), is believed to be proportional to the nth power (1≦n≦2) of the primary flow rate of the raw water passing through the RO membrane (primary flow rate). Therefore, the pressure differential across the RO membrane changes not only when biofouling occurs but also when the flow rate of the concentrate flowing in the drain line L3 changes, thus causing changes in the primary flow rate of the raw water. Such changes in the flow rate of concentrated water, for example, may occur even when the flow rate control described above is implemented, due to changes in the set flow rate, or even if the set flow rate is not changed, due to the infrequent flow rate adjustments made using the manual valve V1. Therefore, if only the third calculation method is used directly, there are concerns that the increase in the pressure difference of the RO membrane caused by biofouling may not be accurately estimated, and the fouling degree of the RO membrane may not be accurately calculated.

[0040] Therefore, in the fourth calculation method, the fouling degree of the RO membrane is calculated not based on the current RO membrane pressure difference itself, but on the value obtained after correcting the pressure difference by considering the change in the concentrate flow rate. Specifically, if the current RO membrane pressure difference is detected in the same way as in the third calculation method, the current concentrate flow rate is detected simultaneously using a flow sensor (not shown) installed on the drain line L3, preferably by calculating a moving average of the detected value. Then, based on the detected current concentrate flow rate and the concentrate flow rate stored in the control device 30 when the RO membrane is first used (initial concentrate flow rate), the current pressure difference is converted to a value under the initial concentrate flow rate. The pressure difference after this conversion is compared with the initial pressure difference, and the fouling degree of the RO membrane is calculated in the same way as in the third calculation method. In other words, if the converted pressure difference is higher than the initial pressure difference, the difference is calculated as the fouling degree; if it is equal to or lower than the initial pressure difference, the fouling degree is calculated as zero. Following this calculation method, the overestimation of the increase in pressure difference of the RO membrane due to biofouling can be prevented, potentially leading to a further reduction in the amount of disinfectant used.

[0041] Regarding the conversion formula for differential pressure, it is advisable to assume that the flow of raw water through the primary side of the RO membrane is turbulent and to use the Van der Waals equation (equivalent to the case where the exponent n is 2), but this is not the only possibility. For example, it is also possible to practically verify the state of the raw water flow (i.e., to what extent the exponent n will be), and use the relationship obtained from this verification result to perform the conversion of differential pressure. Furthermore, ideally, it is advisable to correct the differential pressure based on the average flow rate of the primary side of the raw water rather than the flow rate of the concentrate flowing in the drainage line L3, but in reality, it is not possible to measure the average flow rate of the primary side of the raw water with high precision. Also, the flow rate of the raw water flowing in the supply line L1 can be used as the flow rate for the differential pressure correction, but in this case, the influence caused by the change in the flow rate of the permeate flowing in the permeate line L2 must also be considered. Therefore, in practice, it is advisable to correct the differential pressure based on the flow rate of the concentrate flowing in the drainage line L2, as described above.

[0042] The methods for calculating the fouling degree of RO membranes, that is, the methods for assessing the degree of biofouling, are not limited to the methods described above that calculate the increase in raw water pressure and inlet pressure difference caused by biofouling. For example, the degree of biofouling can also be assessed by measuring the total organic carbon (TOC) concentration in the raw water and concentrate and continuously monitoring the difference, or by measuring the number of bacteria in the raw water and concentrate and continuously monitoring the difference.

[0043] Secondly, specific embodiments will be used to illustrate the effects of the present invention.

[0044] (Example 1) In this embodiment, a test apparatus simulating the water treatment device shown in Figure 1 was used. The apparatus was continuously operated while performing a disinfectant addition step every 24 hours, and the change in raw water pressure (the supply pressure of the raw water to the RO membrane device) over time was measured. The raw water was obtained by adding acetic acid at a concentration of 2.5 mg / L as a nutrient for microorganisms to well water that had undergone predetermined pretreatment (disinfection using sodium hypochlorite, solid-liquid separation using a turbidity removal membrane, and dechlorination using activated carbon). Furthermore, throughout the operation, the raw water temperature was 17–22°C, and the pH was 6.7–7.0. The RO membrane was a Nitto Denko Corporation RO membrane element (model: ESPA2-4040), and the flow rates of permeate and concentrate were controlled throughout the operation, with the permeate flow rate adjusted to 120 L / h and the concentrate flow rate adjusted to 480 L / h, respectively.

[0045] Regarding the disinfectant, a stabilized hypobromous acid composition containing bromine and sulfamic acid compounds, namely "Orpersion" (model: E266) manufactured by Orpersion Co., Ltd., is used. In each disinfectant addition step, the disinfectant concentration in the raw water is fixed at 1.0 mg / L (equivalent to 545 mV in ORP). Furthermore, in the first disinfectant addition step immediately following startup, the execution time (minimum addition time) is set to 1 hour, so that the disinfectant addition amount (minimum addition amount) is 1 h·mg / L. Then, thereafter, the fouling degree of the RO membrane is calculated using the first calculation method, and the execution time of the disinfectant addition step is changed based on the calculation results, i.e., the disinfectant addition time is changed every 24 hours, thereby adjusting the disinfectant addition amount for each step.

[0046] (Comparative Example 1) The execution time for each disinfectant addition step was fixed at 3 hours, and the concentration of disinfectant in the raw water was adjusted according to the fouling level of the RO membrane to adjust the amount of disinfectant added in each step. Otherwise, the measurements were performed under the same conditions as in Example 1. In addition, since the implementation period of this comparative example is different from that of Example 1, the concentration of disinfectant in the raw water in the first disinfectant addition step was set to 0.6 mg / L (equivalent to 530 mV in terms of ORP) to make the minimum addition amount of disinfectant 1.8 mg / L.

[0047] (Comparative Example 2) The amount of bactericide added was not adjusted according to the fouling level of the RO membrane. In each bactericide addition step, the concentration of bactericide in the raw water was fixed at 1.0 mg / L based on the total chlorine concentration, and the execution time was fixed at 3 hours. Otherwise, the measurements were performed under the same conditions as in Comparative Example 1.

[0048] Figures 2, 3, and 4 are graphs showing the measurement results of Example 1, Comparative Example 1, and Comparative Example 2, respectively. In addition, Figures 2 and 3 also show the changes in parameters (the time of addition of the bactericide every 24 hours and the concentration of the bactericide in the raw water) over time for reference, and Figure 2 also shows the changes in the temperature of the raw water during operation.

[0049] In Example 1, as shown in Figure 2, the raw water pressure remained relatively constant throughout the operation, confirming that adjusting the amount of bactericide added according to the fouling level of the RO membrane effectively suppressed RO membrane clogging caused by biofouling. In contrast, Comparative Examples 1 and 2, as shown in Figures 3 and 4, ultimately failed to maintain a constant raw water pressure. Specifically, in Comparative Example 1, it was confirmed that when the bactericide concentration in the raw water exceeded 2.0 mg / L (equivalent to 570 mV in terms of ORP), even with the addition of bactericide at an appropriate amount reflecting the effect of biofouling, as in Example 1, the rise in raw water pressure could not be suppressed. Based on the analysis of the deposits on the RO membrane observed during the rise in raw water pressure, it is speculated that when the ORP of the raw water containing the bactericide rises above a certain value, the organisms attached to the RO membrane surface release viscous substances due to cell wall damage and excessive pressure load, leading to RO membrane clogging.

[0050] (Example 2) In this embodiment, the water treatment apparatus shown in Figure 1 was used, and the process was continuously operated while performing a disinfectant addition step every 12 hours. The study investigated how the amount of disinfectant added in each step would change when the RO membrane fouling level was calculated using a different method than in Example 1. The raw water used was industrial water that had undergone predetermined pretreatment (disinfection using sodium hypochlorite, solid-liquid separation using sand filtration, and dechlorination using sodium bisulfite). Furthermore, the raw water temperature was maintained at 20-25°C and the pH at 6.8-7.2 throughout the operation. The RO membrane unit used was an RO membrane module equipped with 40 RO membrane elements (model: ESPA1) manufactured by Nitto Denko Corporation, and the flow rates of permeate and concentrate were controlled throughout the operation. The set flow rates for permeate and concentrate were 35 L / h and 15 L / h, respectively.

[0051] Regarding the bactericide, the same one as in Example 1 was used. Except for calculating the RO membrane fouling level using the third calculation method, the bactericide addition amount for each addition step was adjusted according to the same procedure as in Example 1. Furthermore, the bactericide concentration in the raw water during each addition step was fixed at 1.0 mg / L, the same as in Example 1, based on the total chlorine concentration. Also, the execution time (minimum addition time) for the first bactericide addition step immediately following the start of operation was set to 15 minutes, so that the bactericide addition amount (minimum addition amount) was 0.25 mg / L.

[0052] (Example 3) The fouling degree of the RO membrane was calculated using the fourth calculation method. Otherwise, continuous operation was performed under the same conditions as in Example 2.

[0053] Figure 5 is a graph showing the change in the addition time of the fungicide every 12 hours in Examples 2 and 3. In addition, the vertical axis of the graph shows the value obtained by standardization with the minimum addition time.

[0054] As can be clearly seen from Figure 5, compared to Example 2, which calculated the RO membrane fouling degree without considering the change in concentrate flow rate, Example 3, which calculated the RO membrane fouling degree considering the change in concentrate flow rate, showed a smaller overall increase in the amount of disinfectant added over time. On the other hand, although not shown in detail here, no significant increase in the flow pressure difference (the pressure difference between the supply pressure of the raw water to the RO membrane unit and the outflow pressure of the concentrate flowing out of the RO membrane unit) was observed in Examples 2 and 3 throughout the entire operation, allowing for stable and continuous operation. Therefore, from the viewpoint of suppressing RO membrane clogging, Examples 2 and 3 are both good, but from the viewpoint of reducing the amount of disinfectant used, Example 3 is better.

[0055] 10: Water treatment equipment 11: Original sink 12: Reverse osmosis membrane (RO membrane) device 13: Booster Pump 14: Raw water pressure sensor 15: Temperature sensor 16: Concentrated water pressure sensor 20: Bactericide Addition Device 21: Disinfectant Tank 22: Drug Injection Pump 30: Control device L1: Water supply pipeline L2: Permeable water pipeline L3: Drainage pipe L4: Raw water pipeline L5: Bactericide supply pipeline V1: Manual valve

Claims

1. A water treatment method, comprising the following steps: supplying water to be treated to a reverse osmosis membrane to separate it into permeate and concentrate; and intermittently adding a bactericide to the water supplied to the reverse osmosis membrane, wherein, The bactericide is prepared by adding a bromine-based oxidant, a stabilized hypobromic acid composition containing bromine and amine sulfonic acid compounds, an iodine-based oxidant, or 2,2-dibromo-3-nitropropionic acid (DBNPA). The step of intermittently adding the bactericide includes the following steps: assessing the degree of biofouling of the reverse osmosis membrane; and, based on the assessed degree of biofouling, adjusting the amount of the bactericide added to the treated water at predetermined intervals, provided that at least one of the oxidation-reduction potential and total chlorine concentration in the treated water does not exceed a predetermined value. The step of adjusting the dosage of the bactericide includes: maintaining a fixed concentration of the bactericide in the treated water, and changing the addition time of the bactericide at predetermined intervals according to the assessed degree of biofouling; The step of assessing the degree of biofouling of the reverse osmosis membrane includes the following steps: detecting the pressure difference between the current supply pressure of the treated water supplied to the reverse osmosis membrane and the current outflow pressure of the concentrate flowing out of the reverse osmosis membrane; and calculating the fouling degree, which indicates the degree of biofouling of the reverse osmosis membrane, based on the detected current pressure difference and the initial value of the pressure difference detected when the reverse osmosis membrane was first used; The step of assessing the degree of biofouling of the reverse osmosis membrane further includes the following steps: detecting the current flow rate of the concentrate flowing out of the reverse osmosis membrane; and calculating the fouling degree includes: The detected current pressure difference is corrected based on the current flow rate and the initial value of the flow rate detected when the reverse osmosis membrane was first used, and the difference between the corrected current pressure difference and the initial value of the pressure difference is calculated as the fouling degree.

2. As in request item 1, the water treatment method, wherein, The steps for assessing the degree of biofouling of the reverse osmosis membrane include the following steps: detecting the current temperature of any one of the treated water supplied to the reverse osmosis membrane, the permeate flowing out of the reverse osmosis membrane, and the concentrate flowing out of the reverse osmosis membrane; detecting the current supply pressure of the treated water supplied to the reverse osmosis membrane; and calculating the degree of biofouling of the reverse osmosis membrane based on the initial values ​​of the water temperatures detected in advance when the reverse osmosis membrane is first used, the initial values ​​of the supply pressure detected in advance when the reverse osmosis membrane is first used, the detected current water temperature, and the detected current supply pressure.

3. As in request item 2, the water treatment method, wherein, The steps for calculating the contamination level include: converting the initial value of the supply pressure to the value at the current detected water temperature based on the detected current water temperature and the initial value of either 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.

4. The water treatment method as described in request item 2, wherein, The steps for calculating the contamination level include: converting the detected current water temperature and the initial value of either water temperature into values ​​at standard temperatures based on the detected current supply pressure and the initial value of the supply pressure, and calculating the difference between the converted current supply pressure and the converted initial value of the supply pressure as the contamination level.

5. The water treatment method as described in Request 1, wherein, The steps for calculating the contamination level include: calculating the difference between the detected current pressure difference and the initial value of the pressure difference as the contamination level.

6. The water treatment method described in any of requests 1 to 5, wherein, Changing the addition time includes: setting the new addition time to be the value obtained by adding the time corresponding to the fouling level to the addition time preset when the reverse osmosis membrane is first used.

7. The water treatment method described in any of requests 1 to 5, wherein, The predetermined value for the redox potential is 570 mV, and the predetermined value for the total chlorine concentration is 2.0 mg / L.

8. A water treatment apparatus comprising: a reverse osmosis membrane device for separating water to be treated into permeate and concentrate; and a bactericide addition device for adding a bactericide to the water to be treated supplied to the reverse osmosis membrane device, wherein... The method involves adding a bromine-based oxidant, a stabilized hypobromic acid composition containing bromine and amine sulfonic acid compounds, an iodine-based oxidant, or 2,2-dibromo-3-nitropropionic acid (DBNPA) as a bactericide; and a control device that intermittently performs the addition of the bactericide by the bactericide addition device; the control device assesses the degree of biofouling of the reverse osmosis membrane unit, and, based on the assessed degree of biofouling, adjusts the amount of the bactericide added to the treated water at predetermined intervals, provided that at least one of the oxidation-reduction potential and total chlorine concentration of the treated water containing the added bactericide does not exceed a predetermined value; when adjusting the amount of the bactericide added, the control device performs the following process: while maintaining a fixed concentration of the bactericide in the treated water, the addition time of the bactericide at predetermined intervals is changed according to the assessed degree of biofouling; when assessing the degree of biofouling of the reverse osmosis membrane unit, the control device performs the following process: The control device detects the pressure difference between the current supply pressure of the treated water supplied to the reverse osmosis membrane unit and the current outflow pressure of the concentrate flowing out of the reverse osmosis membrane unit; and calculates a fouling degree, indicating the degree of biofouling of the reverse osmosis membrane unit, based on the detected current pressure difference and an initial value of the pressure difference detected when the reverse osmosis membrane unit was first used; when assessing the degree of biofouling of the reverse osmosis membrane unit, the control device further performs the following processing: detects the current flow rate of the concentrate flowing out of the reverse osmosis membrane unit; and when calculating the fouling degree, the control device performs the following processing: corrects the detected current pressure difference based on the detected current flow rate and an initial value of the flow rate detected when the reverse osmosis membrane unit was first used, and calculates the difference between the corrected current pressure difference and the initial value of the pressure difference as the fouling degree.

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