Membrane filtration equipment

Abstract

To provide a stable and continuous operation of a membrane filtration apparatus.SOLUTION: A membrane filtration apparatus 10 includes: filtration means 12 having a reverse osmosis membrane or nanofiltration membrane that separates treated water into permeate water and concentrated water; a tank 11 that stores treated water to be treated by the filtration means 12; a treated water line L1 that supplies treated water from the tank 11 to the filtration means 12; a treated water line L2 that distributes the permeate water from the filtration means 12; a concentrated water line L3 that distributes the concentrated water from the filtration means 12; a drainage line L4 that branches off from the concentration line 3 and discharges some of the concentrated water flowing through the concentrated water line L3 outside; a water supply line L6 that supplies treated water to the tank 11; and a control unit 20 that adjusts a flow rate of the treated water flowing through the feed water line L6 so that the flow rate of the water supply line 6 becomes the sum of the flow rate of the permeate water flowing through the treated water line L2 and the flow rate of the concentrate water flowing through the drain line L4.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a membrane filtration device. [Background technology]

[0002] Membrane filtration devices using reverse osmosis (RO) or nanofiltration (NF) membranes are known as water treatment devices that remove impurities from water to be treated. In these devices, the water to be treated (raw water) is supplied to the RO or NF membrane at a predetermined supply pressure, and the RO or NF membrane separates the water into permeate and concentrated water. This allows the production of treated water (permeate) from which impurities have been removed.

[0003] In order to effectively utilize water (water conservation), membrane filtration systems using RO or NF membranes often employ a configuration in which a portion of the concentrated water containing impurities is discharged externally as concentrated wastewater and the remaining portion is returned upstream of the RO or NF membrane as concentrated reflux water. This configuration improves the recovery rate (the ratio of the permeate flow rate to the sum of the permeate flow rate and the concentrated wastewater flow rate) compared to discharging all of the concentrated water as concentrated wastewater, thereby achieving water conservation. At the same time, in these membrane filtration systems, to accommodate changes in the permeate flow rate due to changes in water temperature (i.e., changes in water viscosity), flow control is performed by adjusting the raw water supply pressure to the RO or NF membrane by controlling the rotation speed of the pressure pump to maintain a constant permeate flow rate. When controlling the permeate flow rate, adjusting the raw water supply pressure to maintain a constant permeate flow rate also changes the concentrated water flow rate accordingly. Because such changes in the concentrated water flow rate can lead to membrane clogging due to fouling or scaling, or membrane damage due to increased pressure loss, it is desirable to also control the concentrated water flow rate (concentrated reflux water or concentrated wastewater) when controlling the permeate flow rate. For example, Patent Document 1 describes that in addition to controlling the flow rate of the permeate, flow rate control is performed to adjust the flow rate of the concentrated wastewater to a set flow rate, and that the set flow rate of the concentrated wastewater is calculated based on a target value for recovery rate that is set according to the water temperature.

[0004] Incidentally, some of the above-mentioned membrane filtration devices are equipped with a raw water tank that stores raw water to be supplied to the RO membrane or NF membrane, as described in Patent Document 1. In such a configuration, the water level in the raw water tank drops depending on the wastewater flow rate (the flow rate of permeate and the flow rate of concentrated wastewater) from the membrane filtration device to the outside, so it is necessary to perform water supply control to supply raw water to the raw water tank as needed. Normally, raw water is supplied from the water supply line to the raw water tank by opening and closing an automatic on-off valve installed in the water supply line based on the detection results of a water level sensor installed in the raw water tank. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2018-176033 Summary of the Invention [Problem to be solved by the invention]

[0006] In the above-described water supply control, the flow rate of raw water flowing through the water supply line when the automatic on-off valve is open is preferably adjusted to as high a flow rate as possible so that the water level in the raw water tank does not drop and stop the operation of the membrane filtration device. Specifically, the flow rate is preferably adjusted to a value higher than the flow rate at which the discharge flow rate from the membrane filtration device to the outside is maximized so that the water level in the raw water tank always rises even when the recovery rate (target recovery rate) changes depending on the water temperature. However, in this case, the water level in the raw water tank frequently reaches a predetermined upper water level, which causes the automatic on-off valve to open and close more than necessary, shortening the life of the automatic on-off valve and, as a result, making it difficult to operate the membrane filtration device stably and continuously.

[0007] Therefore, an object of the present invention is to provide a membrane filtration device that realizes stable and continuous operation. [Means for solving the problem]

[0008] In order to achieve the above-mentioned object, a membrane filtration device according to one aspect of the present invention comprises a filtration means having a reverse osmosis membrane or a nanofiltration membrane that separates water to be treated into permeate and concentrated water, a tank that stores the water to be treated by the filtration means, a water to be treated line that supplies the water to be treated from the tank to the filtration means, a treated water line that circulates the permeate from the filtration means, a concentrated water line that circulates the concentrated water from the filtration means, and a drainage line that branches off from the concentrated water line and discharges a portion of the concentrated water flowing through the concentrated water line to the outside. a return water line that branches off from the concentrated water line and returns the remaining concentrated water flowing through the concentrated water line to the tank; a water supply line for supplying the water to be treated to the tank; a pressure adjusting means for adjusting the pressure of the water to be treated flowing through the water to be treated line, a first flow rate adjusting means for adjusting the flow rate of the concentrated water flowing through the drainage line, and a second flow rate adjusting means for adjusting the flow rate of the water to be treated flowing through the water supply line; a control unit that adjusts the flow rate of the water to be treated flowing through the water supply line so that the flow rate of the water to be treated flowing through the water supply line is the sum of the flow rate of the permeated water flowing through the treated water line and the flow rate of the concentrated water flowing through the drainage line. The control unit concurrently performs a first flow rate control that controls the pressure adjustment means so that the flow rate of the permeated water flowing through the treated water line becomes a predetermined set flow rate; a second flow rate control that calculates a target wastewater flow rate, which is a target flow rate of the concentrated water flowing through the drainage line, from the flow rate of the permeated water flowing through the treated water line, and controls the first flow rate adjustment means so that the flow rate of the concentrated water flowing through the drainage line becomes the target wastewater flow rate; and a third flow rate control that sets a target feedwater flow rate, which is a target flow rate of the water to be treated flowing through the water supply line, from the flow rates of the permeated water flowing through the treated water line and the concentrated water flowing through the drainage line, and controls the second flow rate adjustment means so that the water to be treated flowing through the water supply line becomes the target feedwater flow rate. .

[0009] In addition, a membrane filtration device according to another aspect of the present invention includes a plurality of filtration means connected in series, including a first filtration means that is the most upstream of the plurality of filtration means and a second filtration means that is the most downstream of the plurality of filtration means, each of which has a reverse osmosis membrane or a nanofiltration membrane that separates the water to be treated into permeate and concentrated water; a tank that stores the water to be treated by the plurality of filtration means; a water to be treated line that supplies the water to be treated from the tank to the first filtration means; and a treated water line that circulates the permeate from the second filtration means. a permeate line through which permeate from the first filtration means flows; a concentrated water line through which the concentrated water from the first filtering means flows; and a drain line branching from the concentrated water line and discharging a portion of the concentrated water flowing through the concentrated water line to the outside; a return water line that branches off from the concentrated water line and returns the remaining concentrated water flowing through the concentrated water line to the tank; a water supply line for supplying the water to be treated to the tank; a pressure adjusting means for adjusting the pressure of the water to be treated flowing through the water to be treated line, a first flow rate adjusting means for adjusting the flow rate of the concentrated water flowing through the drainage line, and a second flow rate adjusting means for adjusting the flow rate of the water to be treated flowing through the water supply line; a control unit that adjusts the flow rate of the water to be treated flowing through the water supply line so that the flow rate of the water to be treated flowing through the water supply line is the sum of the flow rate of the permeated water flowing through the treated water line and the flow rate of the concentrated water flowing through the drainage line. The control unit concurrently performs a first flow rate control that controls the pressure adjustment means so that the flow rate of the permeated water flowing through the treated water line becomes a predetermined set flow rate; a second flow rate control that calculates a target wastewater flow rate, which is a target flow rate of the concentrated water flowing through the drainage line, from the flow rate of the permeated water flowing through the permeated water line, and controls the first flow rate adjustment means so that the flow rate of the concentrated water flowing through the drainage line becomes the target wastewater flow rate; and a third flow rate control that sets a target feedwater flow rate, which is a target flow rate of the water to be treated flowing through the water supply line, from the flow rates of the permeated water flowing through the treated water line and the concentrated water flowing through the drainage line, and controls the second flow rate adjustment means so that the water to be treated flowing through the water supply line becomes the target feedwater flow rate. .

[0010] With this membrane filtration device, raw water can be supplied to the tank (raw water tank) at a flow rate that corresponds to the flow rate of permeate flowing through the treated water line and the flow rate of concentrated water (concentrated wastewater) flowing through the wastewater line. Therefore, even if the flow rate of concentrated wastewater changes due to a change in water temperature, for example, fluctuations in the water level in the raw water tank can be minimized, and the membrane filtration device can continue to operate stably. [Effects of the Invention]

[0011] As described above, according to the present invention, stable and continuous operation of a membrane filtration device can be achieved. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a schematic diagram showing the configuration of a membrane filtration device according to a first embodiment of the present invention. [Figure 2] FIG. 4 is a schematic diagram showing the configuration of a membrane filtration device according to a second embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

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

[0014] (First embodiment) FIG. 1 is a schematic diagram showing the configuration of a membrane filtration device according to a first embodiment of the present invention.

[0015] The membrane filtration device 10 has a raw water tank 11 and a filtration means 12, and is an apparatus that removes impurities contained in the raw water and produces treated water by treating the raw water (water to be treated) stored in the raw water tank 11 with the filtration means 12. The filtration means 12 separates the raw water supplied from the raw water tank 11 into concentrated water containing impurities and permeated water from which the impurities have been removed, and has a reverse osmosis membrane (RO membrane) or a nanofiltration membrane (NF membrane).

[0016] The filtration means 12 is connected to a raw water line (water to be treated) L1 that supplies raw water from a raw water tank 11 to the filtration means 12, a treated water line L2 that circulates permeated water from the filtration means 12, and a concentrated water line L3 that circulates concentrated water from the filtration means 12. The concentrated water line L3 branches into two lines: a drainage line L4 that discharges a portion of the concentrated water to the outside, and a return water line L5 that returns the remainder to the raw water tank 11. That is, the return water line L5 branches off from the concentrated water line L3 and is connected to the raw water tank 11. A water supply line L6 is connected to the raw water tank 11, and raw water that has been subjected to pretreatment such as turbidity removal and dechlorination in a pretreatment system (not shown) is supplied through the water supply line L6.

[0017] The membrane filtration device 10 also has a pressure pump 13 provided in the raw water line L1, a constant flow valve 14 provided in the concentrated water line L3, a treated water flow meter 15 provided in the treated water line L2, a flow control valve CV1 and a wastewater flow meter 16 provided in the wastewater line L4, a manual valve MV1 provided in the return water line L5, and a flow control valve CV2 and a feedwater flow meter 17 provided in the feedwater line L6.

[0018] The pressure pump 13, whose rotation speed is controlled by an inverter (not shown), functions as a pressure regulator that adjusts the pressure of the raw water flowing through the raw water line L1 (the supply pressure of the raw water to the filtration means 11). The constant flow valve 14 maintains the flow rate of the concentrated water flowing through the concentrated water line L3 at a constant level and has the function of suppressing interference between two flow rate controls (described below) to avoid hunting. The treated water flow meter 15 functions as a flow rate detector that detects the flow rate of the permeated water flowing through the treated water line L2. The flow control valve CV1 functions as a flow rate regulator that adjusts the flow rate of the concentrated water (hereinafter also referred to as "concentrated wastewater") flowing through the drainage line L4, and the drainage flow meter 16 functions as a flow rate detector that detects the flow rate of the concentrated wastewater. The manual valve MV1 functions as a pressure regulator that adjusts the pressure balance between the concentrated water flowing through the drainage line L4 and the concentrated water (hereinafter also referred to as "concentrated reflux water") flowing through the reflux water line L5. The flow control valve CV2 functions as a flow control means for adjusting the flow rate of raw water (hereinafter also referred to as "supply water") flowing through the water supply line L6, and the water supply flow meter 17 functions as a flow detection means for detecting the flow rate of the supply water.

[0019] Furthermore, the membrane filtration apparatus 10 includes a control unit 20 that controls the operation of the membrane filtration apparatus 10. During normal operation (membrane separation process) of the membrane filtration apparatus 10, the control unit 20 executes three flow rate controls in parallel: a first flow rate control for the permeate flow rate, a second flow rate control for the concentrated wastewater flow rate, and a third flow rate control for the feedwater flow rate. Specifically, in the first flow rate control, the booster pump 13 is controlled so that the flow rate of the permeate flowing through the treated water line L2 becomes a set flow rate. In the second flow rate control, a target flow rate of the concentrated wastewater (concentrated water flowing through the wastewater line L4) is calculated from the flow rate of the permeate flowing through the treated water line L2, and the aperture of the flow control valve CV1 is controlled so that the flow rate of the concentrated wastewater becomes the target flow rate. In the third flow rate control, a target flow rate of the feedwater (raw water flowing through the water supply line L6) is calculated from the flow rates of the permeate and the concentrated wastewater, and the aperture of the flow control valve CV2 is controlled so that the flow rate of the feedwater becomes the target flow rate. These three flow rate controls will be described in detail below.

[0020] In the first flow rate control, the booster pump 13 is controlled so that the flow rate (detected value) of the permeated water detected by the treated water flow meter 15 remains constant (a predetermined set flow rate). For example, when the water temperature changes, the viscosity of the water changes, and the flow rate of the permeated water separated by the filtration means 12 also changes. In response to this change, the control unit 20 controls the rotation speed of the booster pump 13 via an inverter. That is, when the water temperature decreases, the viscosity of the water increases, resulting in a decrease in the flow rate of the permeated water from the filtration means 12. Therefore, to compensate for this decrease, the control unit 20 increases the rotation speed of the booster pump 13, thereby increasing the supply pressure of the raw water. On the other hand, when the water temperature increases, the viscosity of the water decreases, resulting in an increase in the flow rate of the permeated water from the filtration means 12. Therefore, to offset this increase, the control unit 20 reduces the rotation speed of the booster pump 13, thereby reducing the supply pressure of the raw water. In this way, the rotation speed of the booster pump 13, i.e., the supply pressure of the raw water, is adjusted, thereby maintaining a constant flow rate of the permeated water flowing through the treated water line L2.

[0021] The flow rate of the concentrated water separated by the filtration means 12 changes in accordance with changes in the supply pressure of the raw water to the filtration means 12 (changes in the rotation speed of the pressure pump 13), but the concentrated water line L3 is provided with the constant flow valve 14 as described above. Therefore, the first flow rate control can maintain a constant flow rate of the concentrated water flowing through the concentrated water line L3 even when the rotation speed of the pressure pump 13 changes and the supply pressure of the raw water changes. As a result, the first flow rate control does not affect the flow rate of the concentrated water flowing through the drain line L4 or the reflux water line L5, and the second flow rate control, which will be described later, is performed independently of the first flow rate control without interfering with it.

[0022] 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, and such that the membrane is not damaged due to an increase in pressure loss. However, setting the specified flow rate of the constant flow valve 14 higher than necessary is undesirable in terms of energy consumption, as the flow rate required for the booster pump 13 becomes larger than necessary, resulting in a larger size of the booster pump 13. Therefore, the specified flow rate of the constant flow valve 14 is set taking into consideration the permeation flux of the filtration means 12 and the minimum flow rate of the concentrate required for the filtration means 12. For example, when an RO membrane with a diameter of approximately 20.32 cm (8 inches) is used as the filtration means 12, the specified flow rate should be set to 1 to 15 m. 3 / h. The minimum flow rate of concentrated water required for the filtering means 12 means the minimum flow rate of concentrated water that should be passed through the concentrated water line L3 to prevent clogging of the membrane due to fouling or scaling.

[0023] Incidentally, the constant flow valve 14 has a specified operating differential pressure range (the allowable range of the pressure difference between the primary and secondary sides of the constant flow valve) for normal operation of the constant flow valve 14. Therefore, depending on the conditions, for example, when a medium- to high-pressure RO membrane is used as the filtration means 12 or when the water temperature drops extremely, the supply pressure of the raw water may rise significantly, causing the pressure of the concentrated water to rise, and the pressure difference between the primary and secondary sides of the constant flow valve 14 may exceed the operating differential pressure range. In such cases, the flow rate of the concentrated water flowing through the concentrated water line L3 may not be maintained constant.

[0024] Therefore, a pressure reducing valve may be provided in the concentrated water line L3 upstream of the constant flow valve 14 to reduce the pressure of the concentrated water flowing through the concentrated water line L3 (i.e., to lower the pressure on the secondary side below the pressure on the primary side). This allows the constant flow valve 14 to operate normally, even if the supply pressure of raw water to the filtration means 12 significantly increases, by keeping the pressure difference between the primary and secondary sides of the constant flow valve 14 within the operating differential pressure range. This allows the constant flow valve 14 to operate normally and maintain a constant flow rate of the concentrated water flowing through the concentrated water line L3. Furthermore, the provision of a pressure reducing valve prevents the constant flow valve 14 from operating normally and increasing the flow rate of the concentrated water. Therefore, when the flow rate of the concentrated wastewater is adjusted to the target flow rate by the second flow rate control (described later), the flow rate of the concentrated water flowing through the reflux water line L5 does not increase, and the discharge flow rate of the pressure pump 13 does not increase. This eliminates the risk of the required permeate flow rate not being obtained due to a decrease in the head of the pressure pump 13. Furthermore, the provision of a pressure reducing valve not only has safety advantages, since peripheral components downstream (such as piping) are not required to have as high a pressure resistance, but also has cost advantages, since it allows the use of inexpensive, general-purpose products that do not have as high a pressure resistance. The type of pressure reducing valve is not particularly limited as long as it can reduce the pressure of the concentrated water to within the operating differential pressure range of constant flow valve 14, but it is necessary to select one that allows a flow rate greater than the specified flow rate of constant flow valve 14, or one whose secondary pressure is greater than the sum of the water flow differential pressure in drain line L4 and return water line L5 and the back pressure on the drain side.

[0025] In the second flow rate control, a target flow rate of the concentrated wastewater is calculated taking into account the recovery rate of the filtration means 12 (the ratio of the permeate flow rate to the sum of the permeate flow rate and the concentrated wastewater flow rate), and the aperture of the flow control valve CV1 is adjusted so that the detected flow rate (detected value) of the concentrated wastewater by the wastewater flow meter 16 is equal to the target flow rate. In this case, the recovery rate is preferably as high as possible from the viewpoint of effective water utilization (water conservation). That is, the flow rate of the concentrated wastewater is preferably as low as possible. However, because the flow rate of the concentrated water is kept constant by the constant flow valve 14, a decrease in the flow rate of the concentrated wastewater naturally increases the flow rate of the concentrated water returned from the return water line L5 to the raw water tank 11. This increases the impurity concentration in the raw water, which increases the likelihood of scaling, in which impurities (particularly silica or calcium) precipitate on the surface of the RO membrane or NF membrane of the filtration means 12. Therefore, the flow rate of the concentrated wastewater is set so that the recovery rate is maximized within a range in which the impurity concentration in the concentrated water does not exceed its solubility, i.e., within a range in which the impurities silica or calcium do not precipitate.

[0026] However, the solubility of impurities varies depending on the water temperature. For example, the solubility of silica increases in proportion to the temperature, while the solubility of calcium (calcium carbonate) decreases as the temperature increases. Therefore, when the water temperature is low, the solubility of silica is relatively low, making it more likely to precipitate (silica scale is more likely to form). However, when the water temperature is high, the solubility of calcium becomes relatively low, making it more likely to precipitate (calcium scale is more likely to form). Therefore, although not shown, the membrane filtration device 10 is provided with a temperature sensor (water temperature detection means) that detects the temperature of any of the raw water, permeate, or concentrated water. The optimal target flow rate of the concentrated wastewater is calculated based on the water temperature detected by this temperature sensor.

[0027] Specifically, first, the theoretical recovery rate at which silica precipitates at the detected water temperature (hereinafter referred to as the "silica precipitation recovery rate") and the theoretical recovery rate at which calcium (calcium carbonate) precipitates at the detected water temperature (hereinafter referred to as the "calcium precipitation recovery rate") are calculated. Note that the methods for calculating the silica precipitation recovery rate and the calcium precipitation recovery rate will be described later. Next, the silica precipitation recovery rate and the calcium precipitation recovery rate are compared, and the smaller precipitation recovery rate is set as the target recovery rate. Then, based on this target recovery rate and the flow rate of the permeate detected by the treated water flow meter 15, the target flow rate of the concentrated wastewater is calculated and set using the following formula (1): (Target flow rate of concentrated wastewater) = (Detected flow rate of permeate water / target recovery rate) - (Detected flow rate of permeate water) (1)

[0028] 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, but from the viewpoint of water conservation, it is preferable to set the calculated target flow rate as the set flow rate of the concentrated wastewater. Note that, although a value expressed as a percentage is usually used as the recovery rate (target recovery rate), it goes without saying that a value expressed as a decimal is used in the above formula (1).

[0029] Here, the methods for calculating the precipitation recovery rate of silica and the precipitation recovery rate of calcium will be described.

[0030] (Calculation method for silica precipitation recovery rate) Silica precipitation recovery rate Y S is the solubility of silica (mg / L) at the detected water temperature, C S The silica concentration (mg / L) of the raw water measured in advance is taken as F S Then, it is calculated using the following formula (2). Y S =(C S -F S ) / C S (2)

[0031] The solubility of silica can be calculated using a method specified in ASTM (American Society for Testing and Materials) D4993-89 or the like.

[0032] (Calculation method for calcium precipitation recovery rate) The calcium precipitation recovery rate is calculated using a method for calculating the Langelier index of the concentrate. Here, the Langelier index (saturation index) is an index indicating the possibility of calcium (calcium carbonate) precipitation and refers to the difference (pH-pH) between the actual pH of the water and the theoretical pH (pH: the pH at which calcium carbonate in water is in an equilibrium state, neither dissolving nor precipitating). That is, the greater the positive absolute value of the Langelier index, the more likely calcium carbonate is to precipitate, whereas a negative value prevents calcium carbonate from precipitating. Therefore, the calcium precipitation recovery rate is calculated as the recovery rate when the Langelier index of the concentrate is zero. To set a more conservative value, the calcium precipitation recovery rate may also be the recovery rate when the Langelier index of the concentrate is negative.

[0033] The Langelier index of the concentrated water is calculated from the pH of the concentrated water, the impurity concentrations (calcium concentration, total alkalinity, and evaporation residue concentration) of the concentrated water, and the detected water temperature. The Langelier index can be calculated, for example, by the method described in JP-A-11-267687 (paragraphs

[0025] to

[0027] ). The impurity concentrations (calcium concentration, total alkalinity, and evaporation residue concentration) of the concentrated water are calculated from the impurity concentrations (calcium concentration, total alkalinity, and evaporation residue concentration) of the raw water measured in advance and the recovery rate. Therefore, the calcium precipitation recovery rate Y C is the impurity concentration (mg / L) of the concentrated water when the Langelier index of the concentrated water becomes zero. C The impurity concentration (mg / L) of the raw water measured in advance is F C Then, the relationship is expressed by the following equation (3). Y C =(C C -F C ) / CC (3)

[0034] The recovery rate, which is the ratio of the flow rate of the permeate to the sum of the flow rate of the permeate and the concentrated wastewater, can be expressed by the allowable concentration ratio, which is the ratio of the flow rate of the concentrated water to the sum of the flow rate of the permeate and the concentrated wastewater. That is, when the allowable concentration ratio is N, the recovery rate Y can be expressed by the following equation (4). Y=(N-1) / N (4)

[0035] Therefore, the above formulas (1) to (3) can be expressed as follows using the above formula (4): (Target flow rate of concentrated wastewater) = (Detected flow rate of permeate) / (Allowable concentration ratio - 1) (1') N S =C S / F S (2') N C =C C / F C (3') where N S is the allowable concentration factor corresponding to the precipitation recovery rate of calcium, and N C is the allowable concentration factor corresponding to the silica precipitation recovery rate.

[0036] The methods for calculating the silica and calcium precipitation recovery rates and the target flow rate of the concentrated wastewater are not limited to those described above if there are pre-defined limitations on the recovery rate and flow rate due to, for example, constraints on the equipment design, such as the capacity of the booster pump or the flow rate of the raw water. Furthermore, because the first flow rate control adjusts the flow rate of the permeate flowing through the treated water line L2 to a constant value, a predetermined set flow rate of the permeate can also be used to calculate the target flow rate of the concentrated wastewater. However, this method is not preferable because if the set flow rate of the permeate does not match the actual flow rate, the actual recovery rate may deviate from the target recovery rate. Specifically, if the actual flow rate of the permeate is greater than the set flow rate, the actual recovery rate may exceed the target recovery rate, resulting in scaling. Alternatively, if the actual flow rate of the permeate is smaller than the set flow rate, the actual recovery rate may fall below the target recovery rate, making it impossible to conserve water.

[0037] Therefore, as described above, it is preferable to use the detected flow rate of the permeate by the treated water flow meter 15 to calculate the target flow rate of the concentrated wastewater. This makes it possible to prevent the actual recovery rate from deviating from the target recovery rate even if the first flow rate control is not performed properly. Note that, in the actual calculation, it is preferable to use the average flow rate over a predetermined detection time or a predetermined number of detections to minimize the influence of variations in the detected flow rate of the permeate.

[0038] However, when the permeate flow rate is unstable and the detected flow rate varies significantly, such as when the apparatus is started up or restarted, the target flow rate of the concentrated wastewater may be calculated using a predetermined set flow rate of the permeate for a certain period until the permeate flow rate stabilizes. Furthermore, the permeate flow rate used to calculate the target flow rate of the concentrated wastewater may be switched depending on the difference between the set flow rate of the permeate and the actual flow rate. That is, when the difference is within a predetermined range, the set flow rate may be used for calculation, and when the difference is outside the predetermined range, the actual flow rate may be used for calculation.

[0039] When controlling the recovery rate as described above, it is preferable to use an electric proportional control valve as the flow control valve CV1. This allows for fine adjustment of the opening depending on the resolution of the electric proportional control valve, allowing for smoother adjustment of the recovery rate compared to step-wise adjustment of the opening using a combination of solenoid valves, etc. For example, in a step-wise system that can only control the recovery rate in five steps (50%, 55%, 60%, 65%, 70%) within the range of 50-70%, if the target recovery rate is set to 64%, the recovery rate can only be adjusted to 60%, resulting in wasted concentrated wastewater. Therefore, using an electric proportional control valve as the flow control valve CV1 is advantageous from the perspective of water conservation because it can reduce the waste of concentrated wastewater.

[0040] However, when using an electric proportional control valve as the flow control valve CV1, attention must be paid to the relationship between its opening / closing speed and the calculation speed (computation speed) of the target flow rate of the concentrated wastewater. For example, if the two speeds are significantly different, hunting may occur if the target flow rate is changed before the opening and closing of the electric proportional control valve is completed and the flow rate of the concentrated wastewater stabilizes. Furthermore, because the target flow rate of the concentrated wastewater is determined based on the flow rate of the permeated water detected by the treated water flow meter 15, the flow rate control of the concentrated wastewater may also 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 target flow rate of the concentrated wastewater, it is preferable to take into account the opening / closing speed of the electric proportional control valve and the response speed of the inverter. That is, if the opening / closing speed of the electric proportional control valve is slow, it is preferable to slow the response speed of the inverter, and if the opening / closing speed of the electric proportional control valve is fast, it is preferable to speed up the response speed of the inverter. As described above, the second flow rate control (flow rate control of the concentrated wastewater) is performed independently of the first flow rate control (flow rate control of the permeated water) due to the installation of the constant flow valve 14, and therefore interference between the two flow rate controls is suppressed. As a result, the occurrence of hunting as described above can be minimized, and deviation of the actual recovery rate from the target recovery rate can be suppressed. From this point of view, it is also preferable to provide a constant flow valve 14 in the concentrated water line L3.

[0041] In this embodiment, in order to set a higher target value for the recovery rate and achieve further water savings, a scale inhibitor may be added to the raw water in order to further increase the above-mentioned precipitation recovery rate. In this case, the specified flow rate of the constant flow valve 14 can be reduced, and as a result, energy savings can be achieved by using a pressure pump 13 with a smaller capacity. The scale inhibitor can be added by a chemical dosing pump.

[0042] The scale inhibitor is not limited to a specific one as long as it is a substance that can inhibit the deposition of scale components such as silica and calcium. Examples of such compounds include phosphonic acid compounds such as phosphonic acids and salts thereof, such as 1-hydroxyethylidene-1,1-diphosphonic acid, 2-phosphonobutane-1,2,4-tricarboxylic acid, ethylenediaminetetramethylenephosphonic acid, and nitrilotrimethylphosphonic acid; phosphoric acid compounds such as orthophosphates and polymerized phosphates; maleic acid compounds such as polymaleic acid and maleic acid copolymers; and acrylic acid polymers. Examples of the acrylic acid polymers include copolymers such as poly(meth)acrylic acid, maleic acid / (meth)acrylic acid, (meth)acrylic acid / sulfonic acid, and (meth)acrylic acid / nonionic group-containing monomer; (meth)acrylic acid / sulfonic acid / nonionic group-containing monomer; terpolymers of (meth)acrylic acid / acrylamide-alkylsulfonic acid / substituted (meth)acrylamide; and (meth)acrylic acid / acrylamide-arylsulfonic acid / substituted (meth)acrylamide. Examples of (meth)acrylic acids constituting the terpolymer include methacrylic acid, acrylic acid, and (meth)acrylate salts thereof, such as sodium salts. Examples of acrylamide-alkylsulfonic acids constituting the terpolymer include 2-acrylamido-2-methylpropanesulfonic acid and its salts. Examples of substituted (meth)acrylamides constituting the terpolymer include t-butylacrylamide, t-octylacrylamide, and dimethylacrylamide.

[0043] Among these, it is preferable to use one containing at least one of a phosphonic acid compound and an acrylic acid polymer. Furthermore, in order to simultaneously inhibit scale derived from calcium and silica, it is particularly preferable to use a scale inhibitor consisting of 2-phosphonobutane-1,2,4-tricarboxylic acid and a mixture of acrylic acid and a terpolymer of (meth)acrylic acid / 2-acrylamido-2-methylpropanesulfonic acid / substituted (meth)acrylamide.

[0044] Commercially available scale inhibitors for RO membranes include the "Orpersion" series manufactured by Organo Corporation, the "Flocon®" series manufactured by BWA Water Additives, the "PermaTreat®" series manufactured by Nalco, the "Hypersperse®" series manufactured by General Electric, and the "Kuriverter®" series manufactured by Kurita Water Industries Ltd.

[0045] As described above, in this embodiment, the constant flow rate of the concentrated water is maintained constant by the constant flow valve 14. Therefore, simply by determining the flow rate of the concentrated water flowing through one of the drain line L4 and the reflux water line L5, the flow rate of the concentrated water flowing through the other line can also be determined. Therefore, in the illustrated embodiment, the drain line L4 is provided with a flow control valve CV1 and a drain flow meter 16 as flow control means, and the reflux water line L5 is provided with a manual valve MV1 for adjusting the pressure balance. However, the reverse is also possible. That is, the reflux water line L5 may be provided with a flow control valve (proportional control valve) and a flow meter, and the drain line L4 may be provided with a manual valve for adjusting the pressure balance. Alternatively, both the drain line L4 and the reflux water line L5 may be provided with flow control valves (proportional control valves) and flow meters.

[0046] In the third flow rate control, a target flow rate of the feedwater (raw water flowing through the feedwater line L6) is first set based on a predetermined set flow rate of the permeated water and the target flow rate of the concentrated wastewater (concentrated water flowing through the wastewater line L4) calculated in the second flow rate control. Specifically, the sum of the set flow rate of the permeated water and the target flow rate of the concentrated wastewater is set as the target flow rate of the feedwater. The aperture of the flow control valve CV2 is then adjusted so that the detected flow rate of the feedwater by the feedwater flow meter 17 becomes equal to the target flow rate. This allows raw water to be supplied to the raw water tank 11 through the feedwater line L6 at a flow rate corresponding to the flow rate of the permeated water and the flow rate of the concentrated wastewater. Therefore, even if a change in water temperature changes the target value of the recovery rate, i.e., the target flow rate of the concentrated wastewater, and thereby changes the flow rate of the concentrated wastewater, fluctuations in the water level in the raw water tank 11 can be minimized, allowing the membrane filtration device 10 to continue operating stably.

[0047] The flow rate of the raw water supplied to the feedwater line L6 has a predetermined upper limit (hereinafter also referred to as the "maximum feedwater flow rate") depending on the device configuration of the pretreatment system (not shown) connected upstream. Therefore, for example, if the target recovery rate decreases due to a change in water temperature, causing the target flow rate of the feedwater set as described above to exceed the maximum feedwater flow rate, the water level in the raw water tank 11 will continue to drop even if the flow control valve CV2 is fully opened, and operation of the membrane filtration device 10 will have to be stopped. Therefore, in practice, the sum of the set flow rate of the permeate and the target flow rate of the concentrated wastewater is compared with the maximum feedwater flow rate, and if it does not exceed the maximum feedwater flow rate, the sum of the set flow rate of the permeate and the target flow rate of the concentrated wastewater is set as the target flow rate of the feedwater, as described above.

[0048] On the other hand, when the sum of the set flow rate of the permeate and the target flow rate of the concentrated wastewater exceeds the maximum feedwater flow rate, the maximum feedwater flow rate is set as the target flow rate of the feedwater, and the predetermined set flow rate of the permeate is changed to a lower value based on the set target flow rate of the feedwater. IN If the target recovery rate is Y, the set flow rate of permeate water Q p is given by the following equation (5). Q p =Q IN ×Y (5)

[0049] In this way, even when the maximum feedwater flow rate is set as the target flow rate of the feedwater, the target recovery rate calculated based on the water temperature can be maintained, thereby reducing the risk of scale formation. Note that if the set permeate flow rate is changed to a lower value, the permeate flow rate may fall below the minimum permeate flow rate (predetermined lower limit) required for a water treatment device (not shown), such as an electrodeionization water production device, connected to the treated water line L2. In such a case, operation of the membrane filtration device 10 will have to be stopped. Therefore, in practice, if the value calculated using the above formula (5) does not fall below such minimum permeate flow rate, the set permeate flow rate is changed to the value calculated using the above formula (5), as described above. Then, based on the changed set permeate flow rate and the target recovery rate, the target flow rate of the concentrated wastewater is also recalculated and reset using a calculation method similar to that of the above formula (1).

[0050] On the other hand, if the value calculated by the above formula (5) is below the minimum permeate flow rate, the set permeate flow rate is changed to that minimum permeate flow rate. In this case, the target flow rate of the feedwater becomes a fixed value (maximum feedwater flow rate), and the set permeate flow rate also becomes a fixed value (minimum permeate flow rate), so the recalculated target flow rate of the concentrated wastewater (the difference between the maximum feedwater flow rate and the minimum permeate flow rate) also becomes a fixed value. This is not advantageous from the perspective of suppressing scaling because the recovery rate also becomes a fixed value, but it is advantageous in that it allows the membrane filtration device 10 to operate continuously without stopping.

[0051] As described above, the target flow rate of the feedwater is set using a predetermined set flow rate of the permeated water. Therefore, for example, if the set flow rate of the permeated water does not match the actual flow rate, the flow rate balance is disrupted, and the water level in the raw water tank 11 may fluctuate, falling below a predetermined lower limit or exceeding a predetermined upper limit. Such significant water level fluctuations may also occur due to measurement errors in the feedwater flow meter 17. Therefore, to prevent the water level in the raw water tank 11 from falling below the predetermined lower limit due to an insufficient feedwater flow rate, the target flow rate of the feedwater may be set to a value obtained by adding a predetermined value to the sum of the set flow rate of the permeated water and the target flow rate of the concentrated wastewater, as long as the sum does not exceed the maximum feedwater flow rate. Furthermore, to prepare for the possibility that the water level in the raw water tank 11 may exceed the predetermined upper limit, water supply control using a water level sensor and an automatic on-off valve may be used in combination. That is, the automatic on-off valve provided in the feedwater line L6 may be opened or closed based on the detection result of the water level sensor provided in the raw water tank 11. Specifically, the automatic on-off valve may be open while the water level in raw water tank 11 is below a predetermined upper limit, and may be closed when the water level in raw water tank 11 reaches the predetermined upper limit. However, the target flow rate of the feed water may be set using the flow rate of the permeated water detected by treated water flow meter 15. Also, instead of flow control valve CV2, an inverter-controlled booster pump may be used as the means for adjusting the flow rate of the feed water.

[0052] In the above-described embodiment, three flow rate controls are performed by one control unit, but each flow rate control may be performed by a separate control unit.

[0053] (Second embodiment) 2 is a schematic diagram showing the configuration of a membrane filtration device according to a second embodiment of the present invention. Hereinafter, the same components as those in the first embodiment will be denoted by the same reference numerals in the drawing and their description will be omitted, and only the components different from those in the first embodiment will be described.

[0054] This embodiment differs from the first embodiment in that, in addition to the filtration means (first filtration means) 12 of the first embodiment, another filtration means (second filtration means) 18 is provided downstream thereof. The second filtration means 18 is connected in series to the first filtration means 12 so as to treat the permeated water separated by the first filtration means 12 as water to be treated. As a result, the membrane filtration device 10 of this embodiment can produce treated water of better quality than the first embodiment. Hereinafter, the permeated water and concentrated water separated by the first filtration means 12 will also be referred to as the "primary permeated water" and the "primary concentrated water," respectively, and the permeated water and concentrated water separated by the second filtration means 18 will also be referred to as the "secondary permeated water" and the "secondary concentrated water," respectively.

[0055] Accordingly, in this embodiment, some of the configurations of the first embodiment are changed and some new configurations are added. That is, in this embodiment, the treated water line L2 is connected to the second filtration means 18 and allows the secondary permeate from the second filtration means 18 to flow. A permeate line L7 is connected between the first filtration means 12 and the second filtration means 18, allowing the primary permeate from the first filtration means 12 to flow and supply it to the second filtration means 18. In addition, a secondary concentrate line L8 is connected to the second filtration means 18, allowing the secondary concentrate from the second filtration means 18 to flow. In the second filtration means 18, the primary permeate from the first filtration means 12 is further separated into secondary permeate and secondary concentrate. Therefore, from the viewpoint of water quality, it is not necessary to discharge the secondary concentrate to the outside. Therefore, the secondary concentrate line L8 is connected to the raw water tank 11 to return all of the secondary concentrate to the raw water tank 11 in order to save water. However, in some cases, part or all of the secondary concentrated water may be discharged to the outside, and a drain line for this purpose may be connected to the secondary concentrated water line L7.

[0056] The secondary concentrate line L8 is provided with a manual valve MV2 and a concentrate flow meter 19 for adjusting the flow rate of the secondary concentrate flowing through the secondary concentrate line L8. This allows the return rate of the second filtration means 18 (the ratio of the flow rate of the secondary concentrate to the sum of the flow rate of the secondary permeate and the flow rate of the secondary concentrate) to be adjusted as desired. To eliminate the complexity of manually adjusting the return rate, a proportional control valve whose opening can be adjusted based on the flow rate of the secondary concentrate detected by the concentrate flow meter 19 may be provided instead of the manual valve MV2. Alternatively, to maintain the return rate within a certain range, a constant flow valve may be provided instead of the manual valve MV2 and the concentrate flow meter 19. In this case, depending on the conditions, the pressure difference between the primary and secondary sides of the constant flow valve may exceed the operating differential pressure range (the allowable pressure difference range for the constant flow valve to operate normally). To avoid this, a pressure reducing valve may be provided upstream of the constant flow valve. As described above, the second filtration means 18 is supplied with the primary permeate from the first filtration means 12, which has a low impurity concentration, so from the viewpoint of water conservation, it is preferable to set the return rate of the second filtration means 18 low.

[0057] In this embodiment, as in the first embodiment, three flow rate controls are executed in parallel, but each flow rate control differs from the first embodiment in several respects, as will be described below.

[0058] In this embodiment, the treated water flow meter 15 is provided on the treated water line L2 because the secondary permeate flowing through the treated water line L2 is adjusted to a predetermined set flow rate in the first flow rate control. Therefore, when calculating the target flow rate of the concentrated wastewater based on the target value of the recovery rate in the second flow rate control, the flow rate of the primary permeate flowing through the permeate line L7 is indirectly detected using the permeate flow meter 15 and the concentrated water flow meter 19. That is, the detected flow rate of the primary permeate is calculated (acquired) as the sum of the flow rate of the secondary permeate detected by the permeate flow meter 15 and the flow rate of the secondary concentrated water detected by the concentrated water flow meter 19. However, a flow meter (not shown) may be provided on the permeate line L7, which may directly detect the flow rate of the primary permeate. Furthermore, as described above, if a constant flow valve is provided on the secondary concentrated water line L8 instead of the manual valve MV2 and the concentrated water flow meter 19, the flow rate of the primary permeate may be indirectly detected using the specified flow rate of the constant flow valve instead of the value detected by the concentrated water flow meter 19. Alternatively, if the flow rate of the secondary permeate is adjusted to a constant value by the first flow rate control and the flow rate of the secondary concentrated water is also adjusted to a constant value based on a predetermined target value for the return rate of the second filtration means 18, the primary permeate will also be adjusted to a substantially constant value, and the target flow rate of the concentrated wastewater may be calculated using such a substantial target flow rate of the primary permeate.

[0059] In the third flow rate control of this embodiment, the target flow rate of the feedwater is set based on the sum of a predetermined set flow rate of the secondary permeate and the target flow rate of the concentrated wastewater calculated in the second flow rate control, as in the first embodiment. That is, if the sum of the set flow rate of the secondary permeate and the target flow rate of the concentrated wastewater does not exceed the maximum feedwater flow rate, the sum is set as the target flow rate of the feedwater. If the sum exceeds the maximum feedwater flow rate, the maximum feedwater flow rate is set as the target flow rate of the feedwater. Also, like the first embodiment, the set flow rate of the secondary permeate is changed to a lower value at the same time as the maximum feedwater flow rate is set as the target flow rate of the feedwater. However, the calculation method differs from the first embodiment in that it is necessary to consider not only the recovery rate of the first filtration means 12 but also the return rate of the second filtration means 18. That is, the set flow rate Q of the secondary permeate2p is the target flow rate of the supply water, Q IN Assuming that the target recovery rate of the first filtering means 12 is Y1 and the return rate of the second filtering means 18 is Y2, the following formula (6) is given. Q 2p =Q IN ×(Y1×(1-Y2)) / (1-(Y1×Y2)) (6)

[0060] As in the first embodiment, the set flow rate of the secondary permeate is changed to the value calculated by the above formula (6) when the calculated value is not below the minimum permeate flow rate. When the set flow rate of the secondary permeate is changed, as described above, a substantial target flow rate of the primary permeate is calculated from the predetermined return rate of the second filtration means 18. Based on this calculated value and the target recovery rate of the first filtration means 12, the target flow rate of the concentrated wastewater is also recalculated and reset using a calculation method similar to that of the above formula (1). However, as described above, since the return rate of the second filtration means 18 is preferably set low, changing the set flow rate of the secondary permeate to a lower value would correspondingly further reduce the flow rate of the secondary concentrate, potentially resulting in the generation of bacteria in the piping constituting the secondary concentrate line L8. Therefore, the set flow rate of the secondary permeate may be changed to a value lower than the value calculated by the above formula (6) as long as it is within a range that does not fall below the minimum permeate flow rate, thereby maintaining the flow rate of the secondary concentrate at the flow rate before the set flow rate of the secondary permeate was changed.

[0061] Furthermore, when the calculated value by the above formula (6) is lower than the minimum permeate flow rate, the set flow rate of the secondary permeate is changed to the minimum permeate flow rate, as in the first embodiment. Then, since the target flow rate of the feedwater becomes a fixed value (maximum feedwater flow rate) and the set flow rate of the secondary permeate also becomes a fixed value (minimum permeate flow rate), the recalculated target flow rate of the concentrated wastewater (the difference between the maximum feedwater flow rate and the minimum permeate flow rate) also becomes a fixed value, as in the first embodiment.

[0062] In this embodiment, raw water must be supplied to the two filtration means 12, 18 using one pressure pump 13, and therefore the pressure at which raw water is supplied to the first filtration means 12 by the pressure pump 13 is greater than in the first embodiment. Therefore, the specified flow rate of the constant flow valve 14 must be set taking this into consideration. For example, if RO membranes with a diameter of approximately 20.32 cm (8 inches) are used as the two filtration means 12, 18, the applicable temperature range for the first filtration means 12 is 5 to 35°C, and the recovery rate control range is expected to be 50 to 85% based on the silica concentration and calcium concentration of the raw water, then, for example, a constant flow valve manufactured by Keihin Corporation (product number: NSPW-25, set flow rate: 55 L / min) can be used as the constant flow valve 14.

[0063] In the above-described embodiment, two filtration means are connected in series. However, the number of filtration means is not limited to this. Three or more filtration means may be connected in series. In this case, the most upstream filtration means among the three or more filtration means corresponds to the first filtration means of the present invention, and the most downstream filtration means corresponds to the second filtration means of the present invention. Note that, when calculating the set flow rate of concentrated wastewater from the most upstream filtration means, the flow rate of the permeate separated by the most upstream filtration means is used, not the permeate separated by the most downstream filtration means. Furthermore, "connected in series" as used herein means that the water to be treated is sequentially treated by multiple filtration means, and between two adjacent filtration means, the permeate separated by the upstream filtration means is supplied as the water to be treated to the downstream filtration means. Furthermore, each filtration means may be composed of multiple RO or NF membranes. In this case, the multiple RO or NF membranes are connected in series on their primary sides (the sides through which raw water and concentrated water flow) and are ultimately connected to the concentrated water line, and their secondary sides (the sides through which permeate water flows) are connected in parallel and are ultimately connected to the permeate line. [Explanation of symbols]

[0064] 10. Membrane filtration equipment 11 Raw water tank 12 Filtration means (first filtration means) 13 Pressure pump 14 Constant flow valve 15 Treated water flow meter 16 Drain flow meter 17 Water supply flow meter 18 Secondary filtration means 19 Concentrated water flow meter 20 Control Unit L1 raw water line L2 treated water line L3 Concentrated water line L4 drain line L5 Return water line L6 Water supply line L7 Secondary concentrate line CV1, CV2 flow control valve MV1, MV2 manual valves

Claims

1. A filtration means having a reverse osmosis membrane or a nanofiltration membrane that separates the water to be treated into permeate and concentrated water; a tank for storing the water to be treated by the filtering means; a water line for supplying water to be treated from the tank to the filtering means; a treated water line through which permeated water from the filtration means flows; a concentrated water line through which the concentrated water from the filtering means flows; a drain line branching from the concentrated water line and discharging a portion of the concentrated water flowing through the concentrated water line to the outside; a return water line branching from the concentrated water line and returning the remaining concentrated water flowing through the concentrated water line to the tank; a water supply line for supplying the water to be treated to the tank; a pressure adjusting means for adjusting the pressure of the water to be treated flowing through the water to be treated line; a first flow rate adjusting means for adjusting the flow rate of the concentrated water flowing through the drainage line; a second flow rate adjusting means for adjusting the flow rate of the water to be treated flowing through the water supply line; a control unit that adjusts the flow rate of the water to be treated flowing through the water supply line so that the flow rate of the water to be treated flowing through the water supply line is the sum of the flow rate of the permeated water flowing through the treated water line and the flow rate of the concentrated water flowing through the drainage line, The control unit concurrently performs the following operations in a membrane filtration device: a first flow rate control that controls the pressure adjustment means so that the flow rate of the permeated water flowing through the treated water line becomes a predetermined set flow rate; a second flow rate control that calculates a target wastewater flow rate, which is a target flow rate of the concentrated water flowing through the drainage line, from the flow rate of the permeated water flowing through the treated water line, and controls the first flow rate adjustment means so that the flow rate of the concentrated water flowing through the drainage line becomes the target wastewater flow rate; and a third flow rate control that sets a target water supply flow rate, which is a target flow rate of the water to be treated flowing through the water supply line, from the flow rate of the permeated water flowing through the treated water line and the flow rate of the concentrated water flowing through the drainage line, and controls the second flow rate adjustment means so that the water to be treated flowing through the water supply line becomes the target water supply flow rate.

2. a plurality of filtration means connected in series, each of which has a reverse osmosis membrane or a nanofiltration membrane that separates the water to be treated into permeate and concentrate; a tank for storing the water to be treated by the plurality of filtering means; a water line for supplying water to be treated from the tank to the first filtering means; a treated water line through which permeated water from the second filtration means flows; a permeate line through which permeate from the first filtration means flows; a concentrated water line through which the concentrated water from the first filtration means flows; a drain line branching from the concentrated water line and discharging a portion of the concentrated water flowing through the concentrated water line to the outside; a return water line branching from the concentrated water line and returning the remaining concentrated water flowing through the concentrated water line to the tank; a water supply line for supplying the water to be treated to the tank; a pressure adjusting means for adjusting the pressure of the water to be treated flowing through the water to be treated line; a first flow rate adjusting means for adjusting the flow rate of the concentrated water flowing through the drainage line; a second flow rate adjusting means for adjusting the flow rate of the water to be treated flowing through the water supply line; a control unit that adjusts the flow rate of the water to be treated flowing through the water supply line so that the flow rate of the water to be treated flowing through the water supply line is the sum of the flow rate of the permeated water flowing through the treated water line and the flow rate of the concentrated water flowing through the drainage line, The control unit concurrently performs the following operations in the membrane filtration device: a first flow rate control that controls the pressure adjustment means so that the flow rate of the permeated water flowing through the treated water line becomes a predetermined set flow rate; a second flow rate control that calculates a target wastewater flow rate, which is the target flow rate of the concentrated water flowing through the drainage line, from the flow rate of the permeated water flowing through the permeated water line, and controls the first flow rate adjustment means so that the flow rate of the concentrated water flowing through the drainage line becomes the target wastewater flow rate; and a third flow rate control that sets a target water supply flow rate, which is the target flow rate of the water to be treated flowing through the water supply line, from the flow rate of the permeated water flowing through the treated water line and the flow rate of the concentrated water flowing through the drainage line, and controls the second flow rate adjustment means so that the water to be treated flowing through the water supply line becomes the target water supply flow rate.

3. The control unit calculates the target wastewater flow rate in the second flow rate control based on a target value of a recovery rate, which is the ratio of the flow rate of the permeated water flowing through the treated water line to the sum of the flow rate of the permeated water flowing through the treated water line and the flow rate of the concentrated water flowing through the wastewater line, and sets the target water supply flow rate in the third flow rate control based on the predetermined set flow rate and the target wastewater flow rate.

4. The control unit compares the sum of the predetermined set flow rate and the target wastewater flow rate with a predetermined upper limit value of the flow rate of the treated water flowing through the water supply line, and if the sum does not exceed the upper limit value, sets the sum as the target water supply flow rate, and if the sum exceeds the upper limit value, sets the upper limit value as the target water supply flow rate.

5. The control unit, when setting the upper limit value as the target water supply flow rate, compares the value obtained by multiplying the target water supply flow rate by the target value of the recovery rate with a predetermined lower limit value of the flow rate of the permeate water flowing through the treated water line, and if the multiplied value is not below the lower limit value, changes the predetermined set flow rate to the multiplied value, and if the multiplied value is below the lower limit value, changes the predetermined set flow rate to the lower limit value.The membrane filtration device described in claim 4.

6. 3. The membrane filtration device according to claim 2, wherein the control unit calculates the target wastewater flow rate based on a target value of a recovery rate, which is the ratio of the flow rate of the permeated water flowing through the permeated water line to the sum of the flow rate of the permeated water flowing through the permeated water line and the flow rate of the concentrated water flowing through the wastewater line, in the second flow rate control, and sets the target water supply flow rate based on the predetermined set flow rate and the target wastewater flow rate, in the third flow rate control.

7. The control unit compares the sum of the predetermined set flow rate and the target wastewater flow rate with a predetermined upper limit value of the flow rate of the treated water flowing through the water supply line, and if the sum does not exceed the upper limit value, sets the sum as the target water supply flow rate, and if the sum exceeds the upper limit value, sets the upper limit value as the target water supply flow rate.

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