JP7307665B2 - MEMBRANE FILTRATION DEVICE AND METHOD OF OPERATION THEREOF - Google Patents

Description

The present invention relates to a membrane filtration device and its operating method.

Membrane filtration devices having reverse osmosis membranes (RO membranes) or nanofiltration membranes (NF membranes) are known as water treatment devices for removing impurities contained in water to be treated. In this apparatus, the water to be treated (raw water) supplied to the RO membrane or NF membrane at a predetermined supply pressure is separated into permeated water and concentrated water by the RO membrane or NF membrane. As a result, treated water (permeated water) from which impurities have been removed can be obtained.

In many cases, in a membrane filtration device having an RO membrane or an NF membrane, from the viewpoint of effective use of water (water saving), part of the concentrated water containing impurities is discharged to the outside as concentrated waste water, and the rest is discharged as concentrated reflux water. A configuration is adopted in which the gas is returned to the upstream side of the RO membrane or the NF membrane. As a result, the recovery rate (ratio of the permeate flow rate to the sum of the permeate flow rate and the concentrated waste water flow rate) can be improved compared to the case where all the concentrated water is discharged as concentrated waste water, resulting in water savings. can be realized. At the same time, in such a membrane filtration device, in order to respond to changes in the flow rate of permeated water due to changes in water temperature (that is, changes in water viscosity), by controlling the rotation speed of the pressure pump, the RO membrane or NF Flow rate control is performed to maintain a constant flow rate of permeate by adjusting the supply pressure of raw water to the membrane. In the flow rate control of permeated water, when the supply pressure of raw water is adjusted so that the flow rate of permeated water is constant, the flow rate of concentrated water also changes accordingly. Such changes in the flow rate of the concentrated water lead to clogging of the membrane due to fouling and scaling, and damage to the membrane due to increased pressure loss. It is also desirable to control the flow rate of running water or concentrated waste water). For example, Patent Document 1 describes performing flow rate control to adjust the flow rate of concentrated wastewater to a set flow rate. It is described to calculate

On the other hand, in membrane filtration devices having RO membranes or NF membranes, raw water is also sequentially treated with a plurality of RO membranes or NF membranes for the purpose of improving the quality of treated water. In Patent Document 1, another RO membrane or NF membrane is provided downstream of the RO membrane or NF membrane where the flow rate control described above is performed, and the permeated water separated by the upstream RO membrane or NF membrane is sent downstream. Further treatment with side RO or NF membranes is described.

JP 2018-167146 A

Membrane filtration equipment with multiple filtration means (RO membrane or NF membrane) treats while maintaining a high recovery rate of the entire system (the ratio of the flow rate of treated water obtained in the system to the flow rate of raw water supplied to the system) It is required to perform flow rate control to improve water quality. However, although it is described in Patent Document 1 that it is possible to generate treated water with better water quality, two filtration means (RO membrane or NF membrane) are used, especially in relation to the recovery rate of the entire system. It is not explicitly stated what kind of flow rate control should be performed specifically for .

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a membrane filtration apparatus and an operation method thereof that can improve the quality of treated water while taking into consideration the recovery rate of the entire system.

In order to achieve the above object, the membrane filtration device of the present invention comprises first and second filtration means connected in series, each of which separates water to be treated into permeated water and concentrated water. First and second filtering means having an osmotic membrane or a nanofiltration membrane, the second filtering means being located downstream of the first filtering means, and supplying the water to be treated to the first filtering means a supply line for circulating the permeated water from the first filtration means and supplying it to the second filtration means; and a first concentrated water for circulating the concentrated water from the first filtration means. line, a drain line branching from the first concentrated water line and discharging a part of the concentrated water flowing through the first concentrated water line to the outside, and a first concentrated water line branching from the first concentrated water line, the first concentrated water A reflux water line for returning the rest of the concentrated water flowing through the line to the supply line, a second permeated water line for circulating the permeated water from the second filtering means, and a concentrated water from the second filtering means for circulating. A second concentrated water line that returns to the supply line, a pressure adjusting means that adjusts the pressure of the water to be treated flowing through the supply line, a first flow rate adjusting means that adjusts the flow rate of the concentrated water flowing through the drain line, Second flow rate adjusting means for adjusting the flow rate of concentrated water flowing through the second concentrated water line, first flow rate detecting means for detecting the flow rate of permeated water flowing through the second permeated water line, and second permeated water The second recovery rate, which is the ratio of the permeate flow rate through the second permeate line to the sum of the permeate flow rate through the water line and the concentrate flow rate through the second concentrate line, is the first so as to fall below a first recovery rate, which is the ratio of the permeate flow rate through the first permeate line to the sum of the permeate flow rate through the permeate line and the concentrate flow rate through the waste line. a control unit that adjusts the flow rate of the permeate flowing through the second permeate line, the flow rate of the concentrated water flowing through the drain line, and the flow rate of the concentrated water flowing through the second concentrated water line ; A first flow rate control for controlling the pressure adjusting means so that the flow rate of the permeated water flowing through the permeated water line of 2 becomes a set flow rate, and a concentrated water flowing through the drainage line from the flow rate of the permeated water flowing through the first permeated water line. A second flow rate control that controls the first flow rate adjusting means so that the flow rate of the concentrated water flowing through the drainage line becomes the target flow rate, and the flow rate of the permeated water flowing through the second permeate line to calculate the target flow rate of the concentrated water flowing through the second concentrated water line, and control the second flow rate adjusting means so that the flow rate of the concentrated water flowing through the second concentrated water line becomes the target flow rate. While executing control and in parallel, the target value of the second recovery rate is set to a value lower than the target value of the first recovery rate, and the control unit, in the third flow rate control, the second A target flow rate of the concentrated water flowing through the second concentrated water line is calculated based on the target recovery rate value and the value detected by the first flow rate detection means.

In addition, the operating method of the membrane filtration apparatus of the present invention is the first and second filtration means connected in series, each of which separates the water to be treated into permeated water and concentrated water. First and second filtration means having filtration membranes, the second filtration means being located downstream of the first filtration means, and a supply line for supplying water to be treated to the first filtration means , a first permeated water line that circulates permeated water from the first filtering means and supplies it to the second filtering means, a first concentrated water line that circulates the concentrated water from the first filtering means, and a first A drain line branching from one concentrated water line and discharging a part of the concentrated water flowing through the first concentrated water line to the outside, and a concentration branching from the first concentrated water line and flowing through the first concentrated water line A reflux line for returning the rest of the water to the supply line, a second permeated water line for circulating the permeated water from the second filtering means, and a concentrated water from the second filtering means for circulating and returning to the supply line. A method of operating a membrane filtration device having a second concentrated water line to flow, wherein the sum of the flow rate of permeated water flowing through the second permeated water line and the flow rate of concentrated water flowing through the second concentrated water line is the ratio of the permeate flow rate through the second permeate line to the sum of the permeate flow rate through the first permeate line and the concentrate flow rate through the drain line to the The pressure of the water to be treated flowing through the supply line and the flow rate of the concentrate flowing through the drain line and the second concentrate line so as to be below the first recovery rate, which is the ratio of the flow rate of the permeate flowing through the first permeate line. and adjusting the flow rate of the concentrated water flowing through the
The adjusting step includes a first flow rate control for adjusting the pressure of the water to be treated flowing through the supply line so that the flow rate of the permeate flowing through the second permeate line becomes the set flow rate, and the first permeate line. A second flow rate control that calculates the target flow rate of the concentrated water flowing through the drainage line from the flow rate of the flowing permeated water, and adjusts the flow rate of the concentrated water flowing through the drainage line so that the flow rate of the concentrated water flowing through the drainage line reaches the target flow rate. Then, the target flow rate of the concentrated water flowing through the second concentrated water line is calculated from the flow rate of the permeated water flowing through the second permeated water line, and the flow rate of the concentrated water flowing through the second concentrated water line becomes the target flow rate. A third flow rate control that adjusts the flow rate of the concentrated water flowing through the second concentrated water line, and while executing in parallel, the target value of the second recovery rate and the target value of the first recovery rate In a third flow control, the permeate flow rate through the second permeate line is sensed, and based on the sensed flow rate and the second recovery target value , the target flow rate of the concentrated water flowing through the second concentrated water line is calculated .

According to such a membrane filtration device and its operation method, by relatively increasing the recovery rate of the first filtration means, the recovery rate of the entire system can be increased, and water can be saved. At the same time, by making the recovery rate of the second filtration means relatively small, the water quality of the permeate from the second filtration means can be improved, and in addition, the raw water ( water to be treated) can be increased. Since the concentrated water from the second filtering means is clearer than the raw water, the increased flow rate of the concentrated water from the second filtering means can improve the quality of the water supplied to the first filtering means. Also by this, the water quality of the permeated water from the second filtering means can be improved.

As described above, according to the present invention, it is possible to improve the quality of treated water in consideration of the recovery rate of the entire system.

BRIEF DESCRIPTION OF THE DRAWINGS It is the schematic which shows the structure of the membrane filtration apparatus which concerns on one Embodiment of this invention.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a schematic diagram showing the configuration of a membrane filtration device according to one embodiment of the present invention.

The membrane filtration device 10 is a device that removes impurities contained in water to be treated (raw water) to generate treated water, and has two filtration means 11 and 12 connected in series. Each of the filtration means 11 and 12 has a reverse osmosis membrane (RO membrane) or a nanofiltration membrane (NF membrane) that separates the water to be treated into concentrated water containing impurities and permeated water from which impurities have been removed. there is Here, "connected in series" means that the water to be treated is sequentially treated by a plurality of filtration means, and therefore, in the present embodiment, the first filtration means 11 on the upstream side It means that the separated permeated water is supplied to the second filtering means 12 on the downstream side as the water to be treated. Hereinafter, the permeated water and concentrated water separated by the first filtering means 11 are also referred to as "primary permeated water" and "primary concentrated water", respectively, and the permeated water and concentrated water separated by the second filtering means 12 are respectively Also referred to as "secondary permeated water" and "secondary concentrated water".

The membrane filtration device 10 includes a plurality of lines respectively connected to the first filtration means 11, that is, a supply line L1 for supplying raw water to the first filtration means 11, and the primary permeate from the first filtration means 11. A primary permeated water line (first permeated water line) L2 that circulates and supplies it to the second filtration means 12, and a primary concentrated water line that circulates the primary concentrated water from the first filtration means 11 (first concentrated water line) L3. In addition, the membrane filtration device 10 has two lines branched from the primary concentrated water line L3, that is, a drainage line L4 that discharges a part of the primary concentrated water flowing through the primary concentrated water line L3 to the outside, and the rest. and a reflux water line L5 for refluxing to the line L1. After being branched from the primary concentrated water line L3, the reflux water line L5 is connected to the supply line L1 on the upstream side of the pressure pump 13, which will be described later. Note that the return water line L5 may be connected to a raw water tank (not shown) provided on the supply line L1 instead of being directly connected to the supply line L1.

In addition, the membrane filtration device 10 includes a plurality of lines respectively connected to the second filtration means 12, that is, secondary permeate lines (second permeate water line) L6 and a secondary concentrated water line (second concentrated water line) L7 through which the secondary concentrated water from the second filtering means 12 is distributed. In the second filtering means 12, the primary permeated water from the first filtering means 11 is further separated into secondary permeated water and secondary concentrated water. No need to eject. Therefore, from the viewpoint of saving water, the secondary concentrated water line L7 is connected to the supply line L1 in order to return all of the secondary concentrated water to the supply line L1. The secondary concentrated water line L7 may be connected to the raw water tank instead of being directly connected to the supply line L1, similarly to the recirculated water line L5.

Furthermore, the membrane filtration device 10 includes a pressure pump 13 provided in the supply line L1, a constant flow valve 14 provided in the primary concentrated water line L3, a flow rate adjustment valve CV1 provided in the drainage line L4, and a drainage flow rate A meter 15, a manual valve MV1 provided in the reflux water line L5, a permeate flow meter 16 provided in the secondary permeate line L6, a flow control valve CV2 provided in the secondary concentrated water line L7, and a concentration and a water flow meter 17 .

The pressurizing pump 13 has its rotation speed controlled by an inverter (not shown), and adjusts the pressure of the raw water flowing through the supply line L1 (supply pressure of the raw water to the first filtering means 11). It functions as a pressure regulating means. The constant flow rate valve 14 has a function of keeping the flow rate of the primary concentrated water flowing through the primary concentrated water line L3 constant, suppressing interference between two flow rate controls described later, and avoiding hunting. The flow control valve CV1 functions as flow control means for adjusting the flow rate of the primary concentrated water (hereinafter also referred to as "concentrated waste water") flowing through the waste water line L4, and the waste water flow meter 15 detects the flow rate of the concentrated waste water. It functions as a detection means. The manual valve MV1 functions as a pressure control valve that adjusts the pressure balance between the primary concentrated water flowing through the drain line L4 and the primary concentrated water flowing through the reflux line L5. The permeated water flow meter 16 functions as flow rate detection means for detecting the flow rate of the secondary permeated water flowing through the secondary permeated water line L6. The flow control valve CV2 functions as flow control means for adjusting the flow rate of the secondary concentrated water (hereinafter also referred to as "concentrated return water") flowing through the secondary concentrated water line L7, and the concentrated water flow meter 17 controls the concentration return. It functions as flow rate detection means for detecting the flow rate of running water.

In addition, the membrane filtration device 10 has a control section 20 that controls the operation of the membrane filtration device 10 . During normal operation (membrane filtration) of the membrane filtration device 10, the control unit 20 performs three flow rate controls, that is, the first flow rate control for secondary permeate water flow control and the second flow rate control for concentrated waste water. and the third flow control, which is the flow control of the concentrated return water, are executed in parallel. Specifically, in the first flow rate control, the pressure pump 13 is controlled so that the flow rate of the secondary permeated water flowing through the secondary permeated water line L6 becomes the set flow rate. In the second flow rate control, the target flow rate of the concentrated wastewater (primary concentrated water flowing through the wastewater line L4) is calculated from the flow rate of the primary permeated water flowing through the primary permeated water line (first permeated water line) L2. The opening of the flow control valve CV1 is controlled so that the flow reaches the target flow. In the third flow rate control, the target flow rate of the concentrated return water (secondary concentrated water flowing through the secondary concentrated water line L7) from the flow rate of the secondary permeated water flowing through the secondary permeated water line (second permeated water line) L6 is calculated, and the opening degree of the flow control valve CV2 is controlled so that the flow rate of the concentrated return water becomes the target flow rate. The details of these three flow rate controls are described below.

In the first flow rate control, the pressurizing pump 13 is controlled so that the detected flow rate (detected value) of the secondary permeated water by the permeated water flow meter 16 is constant (a preset target flow rate). For example, when the water temperature changes, the flow rate of the permeate separated by the first filtration means 11 changes due to the change in the viscosity of the water, and the flow rate of the permeate separated by the second filtration means 12 also changes. According to this change, the controller 20 controls the rotation speed of the pressure pump 13 through the inverter. That is, when the water temperature decreases, the viscosity of the water increases, and as a result, the flow rate of the secondary permeate from the second filtering means 12 decreases. Therefore, the control unit 20 increases the supply pressure of the raw water by increasing the rotational speed of the pressure pump 13 so as to compensate for this decrease. Moreover, when the water temperature rises, the viscosity of the water becomes low, and as a result, the flow rate of the secondary permeated water from the second filtering means 12 increases. Therefore, the control unit 20 reduces the supply pressure of the raw water by reducing the rotational speed of the pressure pump 13 so as to cancel out this increase. By adjusting the rotation speed of the pressure pump 13, that is, the supply pressure of the raw water, the flow rate of the secondary permeate flowing through the secondary permeate line L6 is kept constant.

Although the flow rate of the concentrated water separated by the first filtering means 11 also changes according to changes in the supply pressure of the raw water to the first filtering means 11 (changes in the rotation speed of the pressurizing pump 13), The primary concentrated water line L3 is provided with the constant flow valve 14 as described above. Therefore, by the first flow rate control, even when the rotation speed of the pressurizing pump 13 changes and the supply pressure of the raw water changes, the flow rate of the primary concentrated water flowing through the primary concentrated water line L3 can be kept constant. can. As a result, the first flow rate control does not affect the flow rate of the primary concentrated water flowing through the drainage line L4 and the reflux water line L5, and the second flow rate control, which will be described later, interferes with the first flow rate control. will be performed independently.

Here, the specified flow rate of the constant flow valve 14 should be such that clogging of the membrane due to fouling or scaling does not occur on the one hand, and that the membrane is not damaged due to an increase in pressure loss. However, if the specified flow rate of the constant flow valve 14 is increased more than necessary, the flow rate required of the pressure pump 13 will be increased more than necessary, and as a result the size of the pressure pump 13 will increase, resulting in energy consumption. It is not preferable in terms of Therefore, the specified flow rate of the constant flow valve 14 is set in consideration of the permeation flux of the first filtration means 11 and the minimum flow rate of concentrated water required for the first filtration means 11. For example, the first filtration When using an RO membrane with a diameter of about 20.32 cm (8 inches) as means 11, it is in the range of 1-15 m ³ /h. The minimum flow rate of concentrated water required for the first filtration means 11 means the minimum flow rate of concentrated water that should flow through the primary concentrated water line L3 to prevent clogging of the membrane due to fouling or scaling. On the other hand, in the present embodiment, since it is necessary to supply raw water to the two filtration means 11 and 12 with one pressure pump 13, the supply pressure of the raw water to the first filtration means 11 by the pressure pump 13 is become relatively large. Therefore, the prescribed flow rate of the constant flow rate valve 14 must be set in consideration of this point as well. For example, when RO membranes having a diameter of about 20.32 cm (8 inches) are used as the two filtration means 11 and 12, respectively, when the application temperature range of the first filtration means 11 is 5 to 35 ° C., for example, a constant flow rate As the valve 14, a constant flow valve manufactured by Keihin Corporation (product number: NSPW-25, set flow rate: 55 L/min) can be used.

By the way, the constant flow valve 14 has an operating differential pressure range (permissible range of pressure difference between the primary side and the secondary side of the constant flow valve) for operating the constant flow valve 14 normally. Therefore, depending on the conditions, for example, when a medium- and high-pressure RO membrane is used as the first filtration means 11, or when the water temperature drops extremely, the supply pressure of the raw water rises significantly and the pressure of the primary concentrated water increases. increases, and the pressure difference between the primary side and the secondary side of the constant flow valve 14 may exceed the operating differential pressure range. In that case, the flow rate of the primary concentrated water flowing through the primary concentrated water line L3 may not be kept constant.

Therefore, the pressure of the primary concentrated water flowing through the primary concentrated water line L3 is reduced in the primary concentrated water line L3 on the upstream side of the constant flow valve 14 (that is, the pressure on the secondary side is made lower than the pressure on the primary side. A pressure reducing valve may be provided. As a result, even when the supply pressure of the raw water to the first filtering means 11 rises significantly, the pressure difference between the primary side and the secondary side of the constant flow valve 14 is kept within the operating differential pressure range and the constant flow rate is maintained. The valve 14 can be operated normally, and the flow rate of the primary concentrated water flowing through the primary concentrated water line L3 can be kept constant. Also, if a pressure reducing valve is provided, the constant flow valve 14 will not operate normally and the flow rate of the primary concentrated water will not increase. When adjusted, the flow rate of the primary concentrated water flowing through the recirculated water line L5 does not increase, and the discharge flow rate of the pressure pump 13 does not increase. Therefore, there is no possibility that the necessary flow rate of the permeated water cannot be obtained due to the lowering of the pump head of the pressurizing pump 13 . Furthermore, providing a pressure reducing valve does not require a high degree of pressure resistance performance for peripheral members (piping, etc.) downstream of it, so it is not only advantageous in terms of safety, but it is also a low-cost type that does not have high pressure resistance performance. Being able to use general-purpose products is also advantageous in terms of cost. The type of pressure reducing valve is not particularly limited as long as it can reduce the pressure of the primary concentrated water to within the operating differential pressure range of the constant flow valve 14, but the specified flow rate of the constant flow valve 14 It is necessary to select one that allows the above flow rate and one that has a secondary side pressure greater than the sum of the water flow differential pressure of the drainage line L4 or the recirculated water line L5 and the back pressure on the drainage side.

In the second flow rate control, the target flow rate of the concentrated wastewater is calculated considering the recovery rate of the first filtration means 11 (the ratio of the flow rate of the primary permeated water to the sum of the flow rate of the primary permeated water and the flow rate of the concentrated wastewater). Then, the opening degree of the flow control valve CV1 is adjusted so that the flow rate (detection value) of the concentrated waste water detected by the waste water flow meter 15 becomes the target flow rate. The recovery rate at this time is preferably as high as possible from the viewpoint of effective use of water (water saving). That is, it is preferable that the flow rate of the concentrated waste water is as small as possible. However, since the flow rate of the primary concentrated water is kept constant by the constant flow valve 14, when the flow rate of the concentrated waste water decreases, naturally, the flow rate of the primary concentrated water that flows back from the reflux water line L5 to the supply line L1 increases. As a result, when the concentration of impurities in the raw water increases, scaling, in which impurities (especially silica or calcium) are deposited on the membrane surface of the RO membrane or NF membrane of the first filtration means 11, tends to occur. Therefore, the flow rate of the concentrated wastewater should be adjusted so that the recovery rate is maximized within a range in which the concentration of impurities in the primary concentrated water does not exceed the solubility level, that is, the recovery rate is maximized within a range in which impurities such as silica or calcium do not precipitate. is set to be

However, the solubility of impurities varies depending on the water temperature. For example, in the case of silica, its solubility increases proportionally with increasing temperature, and in the case of calcium (calcium carbonate), its solubility decreases with increasing temperature. Therefore, when the water temperature is low, the solubility of silica is relatively low, and silica tends to precipitate (silica scale is likely to occur). is likely to precipitate (calcium scale is likely to occur). Therefore, although not shown, the membrane filtration device 10 is provided with a temperature sensor (water temperature detection means) for detecting the water temperature of any one of the raw water, the primary permeated water, and the primary concentrated water. Based on the water temperature detected by this temperature sensor, the optimum target flow rate of concentrated waste water is calculated.

Specifically, first, the theoretical recovery rate at which silica precipitates at the detected water temperature (hereinafter referred to as "silica precipitation recovery rate") and the theoretical recovery rate at which calcium (calcium carbonate) precipitates at the detected water temperature The recovery rate of (hereinafter referred to as "calcium deposition recovery rate") is calculated. Methods for calculating the silica precipitation recovery rate and the calcium precipitation recovery rate will be described later. Next, the silica deposition recovery rate and the calcium deposition recovery rate are compared, and the smaller deposition recovery rate is set as the target recovery rate. Then, based on this target recovery rate and the flow rate of the primary permeate indirectly detected by the control unit 20, the target flow rate of the concentrated waste water is calculated and set by the following equation (1).
(Target flow rate of concentrated wastewater) =
(Detected flow rate of primary permeated water/Target recovery rate) - (Detected flow rate of primary permeated water) (1)

Indirect detection of the flow rate of the primary permeated water can be performed using the permeated water flow meter 16 and the concentrated water flow meter 17 . That is, the detected flow rate of the primary permeated water is calculated (acquired) as the sum of the flow rate of the secondary permeated water detected by the permeated water flow meter 16 and the flow rate of the secondary concentrated water detected by the concentrated water flow meter 17. can do. However, the primary permeate water line L2 may be provided with a flow meter (not shown) to directly detect the flow rate of the primary permeate water.

From the viewpoint of reliably suppressing the occurrence of scaling, a flow rate exceeding the target flow rate calculated by the above formula (1) can be set as the set flow rate of the concentrated wastewater. It is preferable to set the target flow rate as the set flow rate of the concentrated waste water. As the recovery rate (target recovery rate), a value expressed in percent is usually used, but in the above formula (1), it goes without saying that a value expressed in decimals is used.

Here, the methods for calculating the silica deposition recovery rate and the calcium deposition recovery rate will be described respectively.

(Method for calculating precipitation recovery rate of silica)
The silica precipitation recovery rate Y _S is expressed as follows, where _CS is the solubility of silica (mg/L) at the detected water temperature and _FS is the silica concentration (mg/L) of the raw water measured in advance. It is calculated from the formula (2).
Y _S =(C _S −F _S )/C _S (2)

As a method for calculating the solubility of silica, a method specified in ASTM (American Society for Testing and Materials) D4993-89 or the like can be used.

(Method for calculating precipitation recovery rate of calcium)
The precipitation recovery rate of calcium is calculated using a method for calculating the Langerier index of concentrated water. Here, the Langerier index (saturation index) is an index indicating the possibility of precipitation of calcium (calcium carbonate), and the actual pH of water and the theoretical pH (pHs: calcium carbonate in water neither dissolves nor precipitates It means the difference (pH - pHs) from the pH at equilibrium. That is, the larger the absolute value of the positive value of the Langerier index, the easier it is for calcium carbonate to precipitate, while the negative value does not precipitate calcium carbonate. Therefore, the precipitation recovery rate of calcium is calculated as the recovery rate when the Langelier index of the concentrated water becomes zero. In order to set a safer value, the recovery rate of calcium precipitation may be the recovery rate when the Langelier index of the concentrated water becomes a negative value.

The Langerier index of the concentrate is calculated from the pH of the concentrate, the concentration of impurities in the concentrate (calcium concentration, total alkalinity, and evaporation residue concentration), and the detected water temperature. As a method for calculating the Langelier index, for example, the method described in Japanese Patent Application Laid-Open No. 11-267687 (paragraphs [0025] to [0027]) can be used. In addition, the concentration of impurities in the concentrated water (calcium concentration, total alkalinity, and concentration of evaporation residue) is calculated from the previously measured impurity concentration of raw water (calcium concentration, total alkalinity, and concentration of evaporation residue) and the recovery rate. calculated from Therefore, the calcium deposition recovery rate Y _C is _the impurity concentration (mg / L) of the concentrated water when the Langerier index of the concentrated water becomes zero, and the impurity concentration (mg / L) of the raw water measured in advance. is represented by the relationship of the following _formula (3).
Y _C =(C _C −F _C )/C _C (3)

In addition, the method of calculating the precipitation recovery rate of silica and calcium and the method of calculating the set flow rate of concentrated wastewater are subject to restrictions on the recovery rate and flow rate in advance, such as the capacity of the pressurizing pump and the flow rate of raw water. In some cases, this is not the case. Further, the flow rate of the secondary permeate flowing through the secondary permeated water line L6 is adjusted to be constant by the first flow control, and the flow rate of the secondary concentrated water flowing through the secondary concentrated water line L7 is controlled by the third flow control described later. is adjusted to be constant, the primary permeate flowing through the primary permeate water line L2 is also adjusted to be substantially constant. Therefore, such a substantial target flow rate of the primary permeate can also be used for calculating the set flow rate of the concentrated waste water. However, this method is not preferable because the actual recovery rate may deviate from the target recovery rate when the actual target flow rate of the primary permeate does not match the actual flow rate. That is, if the actual flow rate of the primary permeate is greater than the target flow rate, scaling occurs due to the actual recovery exceeding the target recovery rate, or the actual flow rate of the primary permeate is less than the target flow rate. In some cases, the actual recovery rate falls below the target recovery rate, making it impossible to save water.

Therefore, for the calculation of the set flow rate of the concentrated waste water, as described above, the flow rate of the primary permeated water indirectly detected from the detected value by the permeated water flow meter 16 and the detected value by the concentrated water flow meter 17 is used. is preferred. As a result, even if the flow rate control of the secondary permeate is not appropriately performed in the first flow rate control, it is possible to prevent the actual recovery rate from deviating from the target recovery rate. In actual calculation, it is preferable to use an average flow rate for a predetermined detection time or a predetermined number of times of detection in order to minimize the influence of variations in the detected flow rate of the secondary permeated water and the secondary concentrated water.

However, if the flow rate of the secondary permeate or secondary concentrated water is not stable, such as when starting up the device or restarting operation, and the variation in the detected flow rate is very large, the flow rate of the secondary permeated water or secondary concentrated water The set flow rate of the concentrated waste water may be calculated using the substantial target flow rate of the primary permeate described above for a certain period of time until is stabilized. Further, the flow rate of the primary permeated water used for calculating the set flow rate of the concentrated waste water may be switched according to the difference between the substantial target flow rate and the actual flow rate of the primary permeated water. That is, if the difference is within a predetermined range, the target flow rate may be used for calculation, and if the difference is outside the predetermined range, the actual flow rate may be used for calculation.

When performing recovery rate control as described above, it is preferable to use an electric proportional control valve as the flow control valve CV1. As a result, the degree of opening can be finely adjusted according to the resolution of the electric proportional control valve, and the recovery rate can be adjusted more smoothly than when adjusting the degree of opening in a stepwise manner using a combination of solenoid valves. For example, in a staged formula that can only control the recovery rate in the range of 50 to 70% in 5 stages (50%, 55%, 60%, 65%, 70%), when the target recovery rate is set to 64%, The recovery rate can only be adjusted to 60%, resulting in useless concentrated wastewater. Therefore, the use of an electric proportional control valve as the flow rate control valve CV1 is advantageous from the viewpoint of water saving because it is possible to reduce waste of such concentrated waste water.

However, when an electric proportional control valve is used as the flow control valve CV1, it is necessary to pay attention to the relationship between the opening/closing speed and the calculation speed (calculation speed) of the set flow rate of the concentrated waste water. For example, if the two speeds are significantly different, hunting may occur if the set flow rate is changed before the electric proportional control valve is completely opened and closed and the concentrated waste water flow rate is stabilized. In addition, the set flow rate of the concentrated waste water is determined based on the detected flow rate of the primary permeated water (the sum of the detected flow rate of the secondary permeated water by the permeated water flow meter 16 and the detected flow rate of the secondary concentrated water by the concentrated water flow meter 17). Therefore, the control of the flow rate of the concentrated waste water may be affected by the response speed of the inverter that controls the rotation speed of the pressure pump 13 . Therefore, when determining the calculation speed of the set flow rate of the concentrated waste water, it is preferable to consider the opening/closing speed of the electric proportional control valve and the response speed of the inverter. That is, when the opening/closing speed of the electric proportional control valve is slow, it is preferable to slow down the response speed of the inverter, and when the opening/closing speed of the electric proportional control valve is fast, it is preferable to speed up the response speed of the inverter. In addition, as described above, the second flow control (flow control of the primary concentrated water) is performed independently of the first flow control (flow control of the secondary permeate) by installing the constant flow valve 14. , the mutual interference of the flow control is suppressed. As a result, the occurrence of hunting as described above can be suppressed as much as possible, and deviation of the actual recovery rate from the target recovery rate can be suppressed. Also from this point, it is preferable that the constant flow valve 14 is provided in the primary concentrated water line L3.

In this embodiment, the target value of the recovery rate is set higher, and in order to further save water, a scale inhibitor is added to the raw water for the purpose of increasing the precipitation recovery rate described above. can be In this case, the specified flow rate of the constant flow rate valve 14 can be reduced, and as a result, energy saving can be realized by using the pressure pump 13 with a smaller capacity. The addition of scale inhibitor can be done by a dosing pump.

The scale inhibitor is not particularly limited as long as it is a substance capable of suppressing precipitation of scale components such as silica and calcium. Examples thereof include phosphonic acids such as 1-hydroxyethylidene-1,1-diphosphonic acid, 2-phosphonobutane-1,2,4-tricarboxylic acid, ethylenediaminetetramethylenephosphonic acid, nitrilotrimethylphosphonic acid, and salts thereof. Phosphonic acid-based compounds; phosphoric acid-based compounds such as orthophosphates and polymerized phosphates; maleic acid-based compounds such as polymaleic acid and maleic acid copolymers; are copolymers such as poly(meth)acrylic acid, maleic acid/(meth)acrylic acid, (meth)acrylic acid/sulfonic acid, (meth)acrylic acid/nonionic group-containing monomers, and (meth)acrylic acid/sulfonic acid /nonionic group-containing monomer, (meth)acrylic acid/acrylamide-alkylsulfonic acid/substituted (meth)acrylamide, and (meth)acrylic acid/acrylamide-arylsulfonic acid/substituted (meth)acrylamide terpolymer. Examples of the (meth)acrylic acid constituting the terpolymer include methacrylic acid, acrylic acid, and (meth)acrylic acid salts such as sodium salts thereof. Examples of acrylamide-alkylsulfonic acids constituting the terpolymer include 2-acrylamido-2-methylpropanesulfonic acid and salts thereof. Examples of substituted (meth)acrylamides constituting the terpolymer include t-butylacrylamide, t-octylacrylamide and dimethylacrylamide.

Among these, it is preferable to use one containing at least one of a phosphonic acid-based compound and an acrylic acid-based polymer. In order to simultaneously suppress scale derived from calcium and silica, 2-phosphonobutane-1,2,4-tricarboxylic acid, acrylic acid and (meth)acrylic acid/2-acrylamido-2-methylpropanesulfonic acid It is particularly preferred to use a scale inhibitor consisting of a mixture of /substituted (meth)acrylamide with a terpolymer.

In addition, commercially available scale inhibitors for RO membranes include the "Orpersion" series manufactured by Organo Corporation, the "Flocon (registered trademark)" series manufactured by BWA Water Additives, and the "PermaTreat (registered trademark)" manufactured by Nalco. series, General Electric Company's "Hypersperse (registered trademark)" series, and Kurita Water Industries Ltd.'s "Kuriverter (registered trademark)" series.

As described above, in the present embodiment, since the flow rate of the primary concentrated water is kept constant by the constant flow valve 14, the flow rate of the primary concentrated water flowing through one of the drainage line L4 and the recirculated water line L5 can be simply defined. , the flow rate of the primary retentate flowing through the other can also be defined. Therefore, in the illustrated embodiment, the drainage line L4 is provided with a flow rate control valve CV1 and a drainage flow meter 15 as flow rate control means, and the recirculated water line L5 is provided with a manual valve MV1 for pressure balance adjustment. , and vice versa. That is, the return water line L5 may be provided with a flow rate control valve (proportional control valve) and a flow meter, and the drainage line L4 may be provided with a manual valve for pressure balance adjustment. Alternatively, both the drain line L4 and the return water line L5 may be provided with a flow control valve (proportional control valve) and a flow meter.

In the third flow rate control, the concentration return water ( The target flow rate of the secondary concentrated water flowing through the secondary concentrated water line L7 is calculated, and the flow rate adjustment valve CV2 is adjusted so that the detected flow rate (detection value) of the concentrated return water by the concentrated water flow meter 17 becomes the target flow rate. opening is adjusted. In this specification, the term "recovery rate" is also used for the second filtering means 12, but in the second filtering means 12, all of the secondary concentrated water is returned to the supply line L1. Therefore, it should be noted that the meaning is different from the case of the first filtering means 11 . Hereinafter, the recovery rate (first recovery rate) of the first filtering means 11 is also referred to as "primary recovery rate", and the recovery rate (second recovery rate) of the second filtering means 12 is referred to as "secondary recovery rate". Also called

The target flow rate (set flow rate) of the concentrated return water is preferably calculated based on the target value of the secondary recovery rate and the detected flow rate of the secondary permeate by the permeate flow meter 16 . As a result, even if the flow rate control of the secondary permeate is not properly performed in the first flow rate control, it is possible to suppress the deviation of the actual secondary recovery rate from the target secondary recovery rate. . In actual calculation, it is preferable to use an average flow rate over a predetermined detection time or a predetermined number of times of detection in order to minimize the influence of variation in the detected flow rate of the secondary permeated water.

However, if the flow rate of the secondary permeate is not stable and the variation in the detected flow rate is very large, such as when starting up the device or restarting operation, a certain period of time until the flow rate of the secondary permeate stabilizes is set in advance. The set flow rate of the concentrated return water may be calculated using the target flow rate of the secondary permeate. Further, the flow rate of the secondary permeate used for calculating the set flow rate of the concentrated return water may be switched according to the difference between the target flow rate and the actual flow rate of the secondary permeate. That is, if the difference is within a predetermined range, the target flow rate may be used for calculation, and if the difference is outside the predetermined range, the actual flow rate may be used for calculation.

When the secondary recovery control is performed as described above, it is preferable to use an electric proportional control valve as the flow control valve CV2, so that the opening degree can be finely adjusted according to the resolution of the electric proportional control valve. However, when an electric proportional control valve is used as the flow control valve CV2, it is necessary to pay attention to the relationship between its opening/closing speed and the calculation speed (calculation speed) of the set flow rate of the concentrated return water. For example, if the two speeds are significantly different, hunting may occur if the set flow rate is changed before the electric proportional control valve is completely opened and closed and the concentrated return water flow rate is stabilized. Further, when the set flow rate of the concentrated returned water is determined based on the flow rate of the secondary permeated water detected by the permeated water flow meter 16, the flow rate control of the concentrated returned water is performed by the inverter that controls the rotation speed of the pressure pump 13. Response speed may also be affected. Therefore, when determining the calculation speed of the set flow rate of the concentrated return water, it is preferable to consider the opening/closing speed of the electric proportional control valve and the response speed of the inverter. That is, when the opening/closing speed of the electric proportional control valve is slow, it is preferable to slow down the response speed of the inverter, and when the opening/closing speed of the electric proportional control valve is fast, it is preferable to speed up the response speed of the inverter. However, in this embodiment, since a constant flow valve is not installed in the secondary concentrated water line L7, the third flow control (flow control of concentrated return water) is the first flow control (flow rate of secondary permeate control). Therefore, it is preferable that the opening/closing speed of the electric proportional control valve is set to be slow in order to suppress mutual interference between the flow rate controls. As a result, the occurrence of hunting as described above can be suppressed as much as possible, and deviation of the actual secondary recovery rate from the target secondary recovery rate can be suppressed.

By the way, in this embodiment, since all of the secondary concentrated water is returned to the supply line L1, the recovery rate of the entire system (the ratio of the flow rate of the secondary permeate to the flow rate of the raw water supplied to the system) is It changes in conjunction with the recovery rate (primary yield) of the filtration means 11 of 1. That is, if the primary yield is high, the recovery rate of the entire system will also be high, and if the primary yield is low, the recovery rate of the entire system will also be low. Therefore, considering water saving, it is preferable that the primary yield is as high as possible. Moreover, generally, a decrease in the recovery rate leads to a decrease in the salt concentration of the concentrated water, which leads to an improvement in the water quality of the permeate. Therefore, from the viewpoint of improving the quality of the secondary permeate, it is preferable that the recovery rate (secondary recovery rate) of the second filtration means 12 is as low as possible. A value lower than the recovery rate is preferred.

In other words, in the present embodiment, the flow rate of the secondary permeate flowing through the secondary permeate line L6 and the flow rate of the concentrated wastewater flowing through the drainage line L4 and the secondary concentrated water so that the secondary recovery rate is lower than the primary yield It is preferable that the flow rate of the secondary concentrated water flowing through the line L7 is adjusted. Thus, by relatively increasing the primary yield, the recovery rate of the entire system can be increased, and water can be saved. At the same time, by making the secondary recovery relatively small, the water quality of the secondary permeate can be improved, and in addition, the second filtration means 12 that joins the raw water through the secondary concentrated water line L7. The flow rate of secondary concentrated water (concentrated return water) from can be increased. Since the concentrated return water is clearer than the raw water, an increase in the flow rate of the concentrated return water can improve the quality of the water supplied to the first filtering means 11, and this also improves the quality of the water supplied to the second filtering means 12. can improve the water quality of the secondary permeate.

Specific target values for primary yield and secondary recovery are not particularly limited as long as the target value for secondary recovery is lower than the target value for primary yield. A value can be set for each. However, since the maximum permeate flow rate is specified for the RO membrane or NF membrane of the first filtration means 11, the target value of the secondary recovery rate is set by the target flow rate of the concentrated return water and the secondary permeate water. It is preferable that the sum with the target flow rate (the flow rate of the primary permeated water required for the second filtering means 12) does not exceed the maximum permeated water flow rate. That is, in a state where the flow rate of the secondary permeate is kept constant by the first flow rate control, when the target value of the secondary recovery rate becomes low, the target flow rate of the concentrated return water increases accordingly, and the second flow rate control The flow rate of primary permeate required for the filtration means 12 may exceed the maximum permeate flow rate of the first filtration means 11 . Therefore, it is preferable that the target value of the secondary recovery rate is not too low. In addition, when the target value of the secondary recovery rate cannot be set to the desired value within the range where the flow rate of the primary permeate required for the second filtration means 12 does not exceed the maximum permeate flow rate of the first filtration means 11 It is also conceivable to change the target flow rate of the secondary permeate in the first flow rate control to a lower value. As a result, the flow rate of the primary permeate required for the second filtration means 12 is reduced, and the target value of the secondary recovery rate is set to the desired value within a range that does not exceed the maximum permeate flow rate of the first filtration means 11. It is also possible to set Alternatively, if such a change in the target flow rate of the secondary permeated water is not preferable, a plurality of RO membranes or NF membranes are used as the first filtration means 11, so that the maximum permeation of the entire first filtration means 11 The water flow rate may be increased in advance. On the other hand, the target value of the primary yield is preferably 75% or more from the viewpoint of saving water, and 55% or more from the viewpoint of improving the quality of treated water, as shown in the examples described later. preferable.

As described above, in the above-described embodiment, the case where one control unit executes three flow rate controls was exemplified, but each flow rate control may be executed by a separately provided control unit.

Next, the present invention will be described in more detail with reference to specific examples.

(Example 1)
In this example, the membrane filtration apparatus shown in FIG. 1 was continuously operated for 24 hours, and the water quality (conductivity and sodium concentration) of the secondary permeate after the operation was measured. Tap water was used as raw water, and two RO membranes (product number: XLE-4040) manufactured by Dow Chemical Co., Ltd. connected in parallel were used as the first and second filtration means. The target flow rate of the secondary permeated water flowing through the secondary permeated water line is set to 320 L / h, and the recovery rate of the first filtration means (primary yield) and the recovery rate of the second filtration means (secondary recovery rate) were set at 75% and 40%, respectively.

(Example 2)
The water quality of the secondary permeate was measured under the same conditions as in Example 1, except that the target value of the primary yield was set to 55%.

(Comparative example 1)
The water quality of the secondary permeate was measured under the same conditions as in Example 1, except that the target values of the primary yield and secondary recovery were set to 45% and 90%, respectively.

(Comparative example 2)
The water quality of the secondary permeate was measured under the same conditions as in Comparative Example 1, except that the target value of the secondary recovery rate was set to 75%.

Table 1 shows the measured values of the conductivity and sodium concentration of the secondary permeate in Examples 1 and 2 and Comparative Examples 1 and 2. For reference, Table 1 shows the target value of the primary yield, the target value of the secondary recovery rate, and the recovery rate of the entire system (the ratio of the flow rate of the secondary permeate to the flow rate of the raw water supplied to the system). percentage) are also shown.

It was confirmed that in Examples 1 and 2, as compared with Comparative Examples 1 and 2, good results were obtained with respect to both conductivity and sodium concentration. In addition to the effect of lowering the secondary recovery rate, the increased flow rate of the secondary concentrated water (concentrated return water), which is sufficiently clearer than the raw water, improved the quality of the water supplied to the first filtration means. This is considered to be the effect (the effect of diluting the raw water by the concentrated return water). On the other hand, when Example 1 and Example 2 are compared, Example 2 is better in terms of the water quality of the secondary permeate, but this is considered to be the effect of a decrease in primary yield.

10 Membrane Filtration Device 11 First Filtration Means 12 Second Filtration Means 13 Pressure Pump 14 Constant Flow Valve 15 Waste Water Flow Meter 16 Permeated Water Flow Meter 17 Concentrated Water Flow Meter 20 Control Unit L1 Supply Line L2 Primary Permeate Line ( first permeate line)
L3 primary concentrated water line (first concentrated water line)
L4 drainage line L5 reflux water line L6 secondary permeate line (second permeate line)
L7 secondary concentrated water line (second concentrated water line)
CV1, CV2 Flow control valve MV1 Manual valve

Claims (6)

First and second filtration means connected in series, each having a reverse osmosis membrane or a nanofiltration membrane for separating the water to be treated into permeated water and concentrated water, wherein the second filtration means first and second filtering means located downstream of the first filtering means;
a supply line for supplying water to be treated to the first filtering means;
a first permeated water line for circulating the permeated water from the first filtering means and supplying it to the second filtering means;
a first concentrated water line for circulating the concentrated water from the first filtering means;
A drain line branching from the first concentrated water line and discharging a part of the concentrated water flowing through the first concentrated water line to the outside;
a reflux water line that branches from the first concentrated water line and returns the rest of the concentrated water flowing through the first concentrated water line to the supply line;
a second permeate line for circulating permeate from the second filtering means;
a second concentrated water line that circulates the concentrated water from the second filtering means and returns it to the supply line;
pressure adjusting means for adjusting the pressure of the water to be treated flowing through the supply line;
a first flow rate adjusting means for adjusting the flow rate of concentrated water flowing through the drainage line;
a second flow rate adjusting means for adjusting the flow rate of the concentrated water flowing through the second concentrated water line;
a first flow rate detection means for detecting a flow rate of permeate flowing through the second permeate line;
A second ratio of the flow rate of the permeate flowing through the second permeate line to the sum of the flow rate of the permeate flowing through the second permeate line and the flow rate of the concentrated water flowing through the second concentrate line is the ratio of the permeate flow rate through the first permeate line to the sum of the permeate flow rate through the first permeate line and the concentrate flow rate through the drain line. a control unit that adjusts the pressure of the water to be treated flowing through the supply line, the flow rate of the concentrated water flowing through the drain line, and the flow rate of the concentrated water flowing through the second concentrated water line so as to fall below the recovery rate of 1; , has
The control unit includes first flow rate control for controlling the pressure adjusting means so that the flow rate of the permeated water flowing through the second permeated water line becomes a set flow rate, and the permeated water flowing through the first permeated water line. A second flow rate control that calculates a target flow rate of concentrated water flowing through the drainage line from the flow rate of and controls the first flow rate adjusting means so that the flow rate of concentrated water flowing through the drainage line becomes the target flow rate; , calculating a target flow rate of the concentrated water flowing through the second concentrated water line from the flow rate of the permeated water flowing through the second permeated water line, and calculating the target flow rate of the concentrated water flowing through the second concentrated water line; While executing in parallel a third flow rate control that controls the second flow rate adjustment means so that the target value of the second recovery rate is set to the target value of the first recovery rate set it to a value below
In the third flow rate control, the control unit controls the concentrated water flowing through the second concentrated water line based on the target value of the second recovery rate and the value detected by the first flow rate detection means. A membrane filtration device that calculates the target flow rate of In the second flow rate control, the control unit acquires the flow rate of permeate flowing through the first permeate line, and sets the target value of the first recovery rate and the acquired flow rate of the permeate. The membrane filtration device according to claim 1 , wherein said target flow rate of concentrated water flowing through said drainage line is calculated based on said target flow rate. The control unit calculates a value obtained by subtracting the obtained flow rate of the permeated water from a value obtained by dividing the obtained flow rate of the permeated water by the target value of the first recovery rate, as the concentrated water flowing through the drainage line. The membrane filtration device according to claim 2 , which is calculated as a target flow rate. water temperature detection means for detecting the water temperature of any one of the water to be treated supplied to the first filtering means, the permeated water from the first filtering means, and the concentrated water from the first filtering means; ,
The control unit calculates a maximum first recovery rate at which silica or calcium does not precipitate on the membrane surface of the reverse osmosis membrane or nanofiltration membrane of the first filtration means based on the value detected by the water temperature detection means. 4. The membrane filtration device according to any one of claims 1 to 3 , wherein the calculated value is set as the target value of the first recovery rate. Having a second flow rate detection means for detecting the flow rate of the concentrated water flowing through the second concentrated water line,
The control unit acquires the sum of the detection value by the first flow rate detection means and the detection value by the second flow rate detection means as the flow rate of the permeate flowing through the first permeate water line. 5. The membrane filtration device according to any one of 1 to 4 . First and second filtration means connected in series, each having a reverse osmosis membrane or a nanofiltration membrane for separating the water to be treated into permeated water and concentrated water, wherein the second filtration means , first and second filtering means located downstream of the first filtering means, a supply line for supplying water to be treated to the first filtering means, and permeation from the first filtering means A first permeated water line for circulating water and supplying it to the second filtering means, a first concentrated water line for circulating concentrated water from the first filtering means, and the first concentrated water line A drain line branching from and discharging a part of the concentrated water flowing through the first concentrated water line to the outside , and a concentrated water branching from the first concentrated water line and flowing through the first concentrated water line A reflux water line that circulates the remainder to the supply line, a second permeated water line that circulates permeated water from the second filtering means, and a concentrated water from the second filtering means that circulates the supply line. A method of operating a membrane filtration device having a second concentrated water line that returns to
A second ratio of the flow rate of the permeate flowing through the second permeate line to the sum of the flow rate of the permeate flowing through the second permeate line and the flow rate of the concentrated water flowing through the second concentrate line is the ratio of the permeate flow rate through the first permeate line to the sum of the permeate flow rate through the first permeate line and the concentrate flow rate through the drain line. and adjusting the pressure of the water to be treated flowing through the supply line, the flow rate of the concentrated water flowing through the drain line, and the flow rate of the concentrated water flowing through the second concentrated water line so as to fall below the recovery rate of 1. fruit,
The adjusting step includes a first flow rate control for adjusting the pressure of the water to be treated flowing through the supply line so that the flow rate of the permeated water flowing through the second permeated water line becomes a set flow rate; A target flow rate of the concentrated water flowing through the drainage line is calculated from the flow rate of the permeated water flowing through the permeate line, and the flow rate of the concentrated water flowing through the drainage line is adjusted so that the flow rate of the concentrated water flowing through the drainage line becomes the target flow rate. and a target flow rate of the concentrated water flowing through the second concentrated water line from the flow rate of the permeated water flowing through the second permeated water line, and the second concentrated water line While executing in parallel a third flow rate control that adjusts the flow rate of concentrated water flowing through the second concentrated water line so that the flow rate of the flowing concentrated water becomes the target flow rate, the second recovery rate The target value of is set to a value below the target value of the first recovery rate,
In the third flow rate control, the flow rate of the permeate flowing through the second permeate line is detected, and based on the detected flow rate and the target value of the second recovery rate, the second A method of operating a membrane filtration device , wherein the target flow rate of concentrated water flowing through a concentrated water line is calculated .

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