JP2019076838A - Membrane filtration device - Google Patents

Abstract

To suppress a deviation in an actual recovery rate from a target recovery rate set in advance.SOLUTION: A membrane filtration device 10 includes: filtration means 11; a supply line 1 for supplying treated water to the filtration means 11; a permeated water line 2 for circulating permeated water from the filtration means 11; a concentrated water line 3 for circulating concentrated water from the filtration means 11; a drainage line 4 for discharging a part of the concentrated water to outside; first flow rate detecting means 12 for detecting a flow rate of the permeated water flowing through the permeated water line 2; second flow rate detection means 14 for detecting a flow rate of the concentrated water (concentrated drainage) flowing through the drainage line 4; first flow control means 20 for adjusting a flow rate of the permeated water to a set flow rate; and a second flow control means 30 for adjusting a flow rate of a concentrated drainage to a set flow rate. The second flow control means 30 determines the set flow rate of the concentrated drainage based on a recovery target value, which is a ratio of the permeated water flow rate to the sum of the permeated water flow rate and the concentrated drainage flow rate, and a detection value by the first flow rate detecting means 12.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a membrane filtration device having a reverse osmosis membrane or a nanofiltration membrane.

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

  In a membrane filtration apparatus having an RO membrane or an NF membrane, in most cases, from the viewpoint of effective use of water (water saving), a part of concentrated water containing impurities is discharged to the outside as concentrated drainage, and the remaining is used as concentrated reflux water A configuration is adopted in which the reflux is performed upstream of the RO membrane or the NF membrane. As a result, the recovery rate (the ratio of the flow rate of the permeated water to the sum of the flow rate of the permeated water and the flow rate of the concentrated drained water) can be improved as compared to the case where all the concentrated water is discharged as concentrated waste water. It can be realized. At the same time, in the membrane filtration apparatus, in order to cope with the change in the flow rate of permeated water due to the change in water temperature (that is, the change in viscosity of water), the RO membrane or NF membrane is controlled by controlling the rotation speed of the pressure pump. The flow control of maintaining the flow rate of the permeate water constant is also performed by adjusting the supply pressure of the raw water.

  In the flow rate control of the permeated water, when the feed pressure of the raw water is adjusted so that the flow rate of the permeated water becomes constant, the flow rate of the concentrated water separated by the RO membrane or the NF membrane also changes accordingly. Since such a change in flow rate of concentrated water leads to clogging of the membrane due to fouling and scaling and breakage of the membrane due to an increase in pressure loss, the same applies to concentrated water as in the flow rate control of permeated water It is required to control the flow rate of

  Patent Literatures 1 and 2 describe performing flow control of concentrated water for adjusting the flow rate of concentrated water discharged to the outside to a set flow rate in parallel with flow control of permeated water. As a method of determining the set flow rate of the concentrated water, Patent Document 1 describes using the calculated value of the allowable concentration ratio of silica in the concentrated water and the target flow rate value of the permeated water set in advance. In No. 2, it is described that the calculated value of the allowable concentration ratio of calcium carbonate in the concentrated water and the target flow rate value of the permeated water set in advance are used. That is, in the flow rate control of concentrated water described in Patent Documents 1 and 2, the set flow rate of concentrated water discharged to the outside is determined based on a target flow rate value of permeated water set in advance.

Patent No. 5768615 gazette Patent No. 5904299

  However, in practice, as described below, there may occur a situation where the flow rate of the permeate can not be adjusted to the target flow rate in controlling the flow rate of the permeate. In that case, even if the flow rate of the concentrated water discharged to the outside is adjusted to the set flow rate based on the target flow rate value of the permeated water, the actual recovery rate deviates from the target recovery rate, and the risk of scale generation And wasteful drainage may occur.

  For example, as the water temperature increases, the viscosity of the water decreases, as a result, the flow rate of permeate separated by the RO membrane or the NF membrane increases. In this case, since the supply pressure of the raw water necessary to maintain the flow rate of the permeate water is reduced, the feed pressure of the raw water is adjusted by lowering the rotational speed of the pressure pump to make the flow rate of the permeate water constant. Try to maintain. However, since the minimum number of revolutions is specified for the pressure pump, if the water temperature rises significantly, even if the number of revolutions of the pressure pump is reduced to the minimum number of revolutions, the supply pressure of the raw water May not be reduced. Therefore, the flow rate of the permeate can not be reduced to the target flow rate, and as a result, the actual recovery rate may exceed the target recovery rate, and the risk of scale generation may increase. On the other hand, when the water temperature is lowered, the viscosity of the water is increased, and as a result, the flow rate of the permeate separated by the RO membrane or the NF membrane is decreased. Therefore, although the supply pressure of the raw water required to maintain the flow rate of the permeate water constant increases, if the water temperature drops significantly, the supply pressure of the raw water can be increased even if the number of rotations of the pressure pump is maximized. There is a possibility that the target pressure can not be increased. Therefore, the flow rate of the permeated water can not be increased to the target flow rate, and as a result, the actual recovery rate falls below the target recovery rate, and unnecessary drainage may occur.

  Therefore, an object of the present invention is to provide a membrane filtration apparatus that suppresses deviation of an actual recovery rate from a target recovery rate set in advance.

  In order to achieve the above-mentioned object, the membrane filtration apparatus of the present invention is connected to filtration means having a reverse osmosis membrane or a nanofiltration membrane for separating treated water into permeate water and concentrated water, and filtration means, A feed water line for supplying treated water to the means, and a permeate water line connected to the filter means for circulating permeate water from the filter means, and a concentrated water line connected to the filter means for circulating the concentrated water from the filter means And a drainage line branching from the concentrated water line and discharging part of the concentrated water flowing in the concentrated water line to the outside, a first flow rate detecting means for detecting the flow rate of permeated water flowing in the permeate water line, and a drainage line Second flow rate detection means for detecting the flow rate of the concentrated water flowing through the first flow control means for adjusting the flow rate of the permeate water flowing through the permeate water line to the set flow rate, and setting the flow rate of the concentrated water flowing through the drainage line Adjust to flow rate A second flow control unit having a flow control valve provided in the drainage line, and a control unit for adjusting an opening degree of the flow control valve based on a detection value by the second flow detection unit; And the control unit of the second flow rate control means controls the permeated water flow through the permeated water line to the sum of the permeated water flow rate flowing through the permeated water line and the concentrated water flow rate flowing through the drainage line The set flow rate of the concentrated water flowing through the drainage line is determined based on the target value of the recovery rate, which is the ratio of the flow rate of 1 and the detection value of the first flow rate detection means.

  According to such a membrane filtration apparatus, the set flow rate of the concentrated water flowing through the drainage line is determined based on the flow rate (detected value) of the permeated water actually flowing through the permeated water line. Therefore, even if the flow control of permeated water is not properly performed, the flow rate of concentrated water flowing in the drainage line should be adjusted to the set flow rate determined based on the actual flow rate of permeated water (detection value). Thus, it is possible to suppress that the actual recovery rate deviates from the target recovery rate.

  As described above, according to the present invention, it is possible to provide a membrane filtration apparatus that suppresses deviation of the actual recovery rate from the target recovery rate set in advance.

It is the schematic which shows the structure of the membrane filtration apparatus which concerns on one Embodiment of this invention.

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

  FIG. 1 is a schematic view showing the configuration of a membrane filtration apparatus according to an embodiment of the present invention.

  The membrane filtration apparatus 10 according to the present embodiment is an apparatus for removing impurities contained in raw water (to-be-treated water) to generate treated water, and the raw water is a permeated water from which concentrated water containing impurities and impurities are removed. It has filtration means 11 which separates into water. The filtration means 11 has a reverse osmosis membrane (RO membrane) or a nanofiltration membrane (NF membrane).

  In addition, the membrane filtration apparatus 10 includes a plurality of lines respectively connected to the filtration means 11, that is, a supply line 1 for supplying raw water to the filtration means 11, and a permeate water line 2 for circulating permeate water from the filtration means 11. , And a concentrated water line 3 for circulating the concentrated water from the filtration means 11. In addition, the membrane filtration apparatus 10 includes two lines branched from the concentrated water line 3, ie, a drainage line 4 for discharging a part of the concentrated water flowing in the concentrated water line 3 to the outside, and a line for supplying the remaining concentrated water. And a reflux water line 5 for refluxing to 1. The return water line 5 is connected to the supply line 1 on the upstream side of a pressure pump 21 described later, after being branched from the concentrated water line 3. The reflux water line 5 may be connected to a raw water tank (not shown) provided in the supply line 1 instead of directly connected to the supply line 1.

  Furthermore, the membrane filtration apparatus 10 is a permeated water flow meter (first flow rate detecting means) 12 for detecting the flow rate of permeated water flowing through the permeated water line 2, and a permeated water flow rate control mechanism ( First flow rate control means) 20 is provided.

  A permeated water flow rate control mechanism 20 is provided in the supply line 1, and a pressure pump (pressure adjustment means) 21 for adjusting the pressure of the raw water flowing through the supply line 1 (the supply pressure of the raw water to the filtration means 11) It has a permeated water flow rate control unit 22 that controls the pressurizing pump 21 based on the detected flow rate (detected value) of the permeated water by the flow meter 12.

  The permeated water flow rate control unit 22 includes an inverter (not shown) for controlling the number of rotations of the pressurizing pump 21, and the flow rate of the pressurized pump 21 is controlled so that the detected flow rate of permeated water by the permeated water flowmeter 12 becomes constant. It controls the number of revolutions. For example, when the water temperature changes, the viscosity of the water changes, and the flow rate of the permeate separated by the RO membrane or the NF membrane also changes. In response to this change, the permeated water flow rate control unit 22 controls the number of rotations of the pressure pump 21. That is, as the water temperature decreases, the viscosity of the water increases, and as a result, the flow rate of the permeate separated by the RO membrane or the NF membrane decreases. Therefore, the permeated water flow rate control unit 22 increases the supply pressure of the raw water by increasing the rotational speed of the pressure pump 21 so as to compensate for the decrease. Also, as the water temperature increases, the viscosity of the water decreases, as a result, the flow rate of the permeate separated by the RO membrane or the NF membrane increases. Therefore, the permeated water flow rate control unit 22 reduces the supply pressure of the raw water by reducing the rotational speed of the pressure pump 21 so as to cancel out the increase. The rotation speed of the pressure pump 21 is controlled by the permeated water flow rate control unit 22 so as not to exceed the preset upper limit value or fall below the preset lower limit value. Therefore, even when the rotation speed of the pressure pump 21 is controlled to be the lower limit value, there is a possibility that the flow rate of permeated water may exceed the set flow rate. A manual valve or a proportional control valve may be provided between the pump 21 and the filtration means 11 to adjust the supply pressure of the raw water.

  As described above, in the present embodiment, the flow rate of the permeated water is maintained constant (predetermined target flow rate) by adjusting the rotational speed of the pressure pump 21, that is, the supply pressure of the raw water, but the raw water In accordance with the change in the supply pressure, the flow rate of the concentrated water separated by the RO membrane or the NF membrane also changes. In order to suppress such a flow rate change of the concentrated water itself, the concentrated water line 3 is provided with a constant flow valve 13 which holds the flow rate of the concentrated water flowing through the concentrated water line 3 constant. Thereby, even when the rotation speed of the pressurizing pump 21 is changed by the permeated water flow rate control unit 22 and the supply pressure of the raw water to the filtration unit 11 is changed, the flow rate of the concentrated water can be kept constant. .

Here, the prescribed flow rate of the constant flow valve 13 may be on the one hand not to cause clogging of the membrane due to fouling or scaling, and on the other hand on the other hand as long as the membrane is not broken due to an increase in pressure loss. However, increasing the prescribed flow rate of the constant flow rate valve 13 more than necessary means that the flow rate required for the pressure pump 21 becomes larger than necessary, and as a result, the size of the pressure pump 21 becomes larger. Unfavorable in terms of Therefore, the prescribed flow rate of the constant flow rate valve 13 is set in consideration of the permeation flux of the filtration means 11 and the minimum flow rate of the concentrated water required for the filtration means 11; for example, the diameter of the filtration means 11 is about 20.32 cm. When an (8 inch) RO membrane is used, it is in the range of 1 to 15 m ³ / h.

  By the way, in the constant flow valve 13, an operation differential pressure range (a tolerance range of pressure difference between the primary side and the secondary side of the constant flow valve) for operating the constant flow valve 13 normally is defined. Therefore, for example, when using a RO membrane for medium to high pressure as the filtration means 11 or when the water temperature drops extremely, depending on the conditions, the supply pressure of the raw water significantly increases and the pressure of the concentrated water rises. The pressure difference between the primary side and the secondary side of the constant flow valve 13 may exceed the operating differential pressure range. In that case, the flow rate of the concentrated water flowing through the concentrated water line 3 may not be kept constant.

  Therefore, the pressure of the concentrated water flowing through the concentrated water line 3 is reduced in the concentrated water line 3 on the upstream side of the constant flow valve 13 (that is, the pressure on the secondary side can be made lower than the pressure on the primary side) A pressure reducing valve may be provided. Thereby, even if the supply pressure of the raw water to the filtration means 11 rises remarkably, the pressure difference between the primary side and the secondary side of the constant flow valve 13 is contained within the differential pressure range, and the constant flow valve 13 is The normal operation can be performed, and the flow rate of the concentrated water flowing through the concentrated water line 3 can be kept constant. Further, by providing the pressure reducing valve, the peripheral members (such as piping) located downstream of the pressure reducing valve are not required to have much pressure resistance. Therefore, the installation of the pressure reducing valve is advantageous not only in terms of safety, but also in terms of cost because an inexpensive general-purpose product whose pressure resistance performance is not very high can be used. The type of pressure reducing valve is not particularly limited as long as the pressure of concentrated water can be reduced to the differential pressure range of operation of constant flow valve 13, but the specified flow rate of constant flow valve 13 or more It is necessary to select one having a flow rate of 1 or a pressure at which the pressure on the secondary side becomes larger than the differential pressure of water flow through the drainage line 4 and the reflux water line 5.

  As described above, the installation of the constant flow valve 13 prevents the flow rate control of the permeate from affecting the flow rate of the concentrated water, and as a result, the flow rate control of the concentrated water flowing through the drainage line 4 or the reflux water line 5 It becomes easily executable. Therefore, the membrane filtration apparatus 10 according to the present embodiment includes a drainage flow meter (second flow rate detection means) 14 for detecting the flow rate of concentrated water (hereinafter referred to as "concentrated drainage") flowing through the drainage line 4 and its flow rate. And a drainage flow control mechanism (second flow control means) 30 for adjusting to a set flow rate. The flow rate control of concentrated drainage by the drainage flow rate control mechanism 30 is performed independently of the flow rate control of permeate water by the permeate water flow rate control mechanism 20.

  The drainage flow control mechanism 30 adjusts the opening degree of the flow adjustment valve 31 based on the flow adjustment valve 31 provided in the drainage line 4 and the detection flow (detection value) of concentrated drainage by the drainage flow meter 14 And a control unit 32.

  The drainage flow rate control unit 32 determines the set flow rate of concentrated drainage in consideration of the recovery rate, which is the ratio of the flow rate of permeated water to the sum of the flow rate of permeated water and the flow rate of concentrated drainage, The opening degree of the flow rate adjusting valve 31 is adjusted so that the set flow rate is obtained. 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 concentrated drainage be as small as possible. However, since the flow rate of concentrated water is kept constant by the constant flow valve 13, when the flow rate of concentrated drainage decreases, it goes without saying that the flow rate of concentrated water returned from the reflux water line 5 to the supply line 1 increases. Do. As a result, when the impurity concentration of the raw water increases, scaling in which an impurity (in particular, silica or calcium) is precipitated on the film surface of the RO film or the NF film of the filtration means 11 easily occurs. Therefore, the flow rate of concentrated drainage is such that the recovery rate is maximized in the range where the concentration of impurities in the concentrated water does not become higher than the solubility, that is, the recovery is maximized in the range where the impurities silica or calcium do not precipitate Is set as

  However, the solubility of the impurities varies with the water temperature. For example, in the case of silica, its solubility increases in proportion to the temperature, and in the case of calcium (calcium carbonate), its solubility decreases as the temperature increases. Therefore, when the water temperature is low, the solubility of silica is relatively low and silica is likely to precipitate (silica scale tends to occur), but when the water temperature is high, the solubility of calcium becomes relatively low, so calcium Is likely to precipitate (calcium scale is likely to occur). So, in this embodiment, although not shown in figure, the temperature sensor (water temperature detection means) which detects the water temperature of either raw water, permeate water, or concentrated water is provided, and the water temperature detected by this temperature sensor The optimal set flow rate of concentrated drainage is calculated based on

Specifically, first, a theoretical recovery rate at which silica precipitates at the detected water temperature (hereinafter referred to as "silica precipitation recovery rate"), and theoretically, calcium (calcium carbonate) precipitates at the detected water temperature Recovery rate (hereinafter referred to as "calcium precipitation recovery rate") is calculated. In addition, each calculation method of the precipitation recovery rate of a silica, and the precipitation recovery rate of calcium is mentioned later. Next, the precipitation recovery rate of silica and the precipitation recovery rate of calcium are compared, and the smaller precipitation recovery rate is set as the target recovery rate. Then, based on the target recovery rate and the detected flow rate of the permeated water by the permeated water flow meter 12, the target flow rate of the concentrated drainage is calculated and set by the following equation (1).
(Target flow rate for concentrated drainage) =
(Detected flow rate of permeated water / target recovery rate)-(detected flow rate of permeated water) (1)

  Although the flow rate exceeding the target flow rate calculated by the above equation (1) can be set as the set flow rate of concentrated drainage from the viewpoint of reliably suppressing the occurrence of scaling, it is calculated from the viewpoint of water conservation It is preferable to set the target flow rate as the set flow rate of concentrated drainage. As the recovery rate (target recovery rate), a value expressed in percentage is usually used, but in the above formula (1), it is needless to say that a value expressed in decimal is used.

  Here, methods of calculating the precipitation recovery rate of silica and the precipitation recovery rate of calcium will be respectively described.

(Calculation method of precipitation recovery rate of silica)
Assuming that the solubility (mg / L) of silica at the detected water temperature is C _S and the silica concentration (mg / L) of the raw water measured in advance is F _S , the precipitation recovery rate Y _S of silica is as follows: It is calculated from equation (2).
Y _S = (C _S −F _S ) / C _S (2)

  In addition, as a calculation method of the solubility of a silica, the method prescribed | regulated to ASTM (American Society for Testing and Materials) D4993-89 etc. can be used.

(Calculation method of precipitation recovery rate of calcium)
The precipitation recovery rate of calcium is calculated using the method of calculating the Langeria index of the concentrated water. Here, the Langeria index (saturation index) is an index showing the possibility of precipitation of calcium (calcium carbonate), and the actual pH of water and the theoretical pH (pHs: calcium carbonate in water does not dissolve or precipitate) It means the difference (pH-pHs) from the pH at equilibrium. That is, calcium carbonate is more likely to precipitate as the Langeria index is a positive value and the absolute value is larger, and calcium carbonate is not precipitated at a negative value. Therefore, the precipitation recovery rate of calcium is calculated as the recovery rate when the Langeria index of the concentrated water becomes zero. In addition, in order to set as a value on the more safe side, the precipitation recovery rate of calcium may be a recovery rate when the Langeria index of the concentrated water becomes a negative value.

The Langeria index of the concentrated water is calculated from the pH of the concentrated water, the concentration of impurities in the concentrated water (calcium concentration, total alkalinity, and evaporation residue concentration), and the detected water temperature. As a calculation method of the Langeria index, for example, the method described in JP-A-11-267687 (paragraphs [0025] to [0027]) can be used. In addition, the impurity concentration (calcium concentration, total alkalinity and evaporation residue concentration) of concentrated water is the impurity concentration (calcium concentration, total alkalinity and evaporation residue concentration) of raw water measured in advance, and the recovery rate Calculated from Therefore, the precipitation recovery rate Y _C of calcium is defined as the impurity concentration of the concentrated water (mg / L) when the Langeria index of the concentrated water is zero, as C _C, and the impurity concentration of the raw water measured in advance (mg / L) Let F be _C , it is expressed by the following equation (3).
Y _C = (C _C −F _C ) / C _C (3)

  In addition, the calculation method of precipitation recovery rate of silica and calcium and the calculation method of setting flow rate of concentrated drainage are restricted in recovery rate and flow rate beforehand by restrictions on the device design such as capacity of pressurized pump and flow rate of raw water. In some cases, it is not limited to the above. Also, although it is possible to use the target flow rate of permeated water set in advance for calculation of the set flow rate of concentrated drainage, this method is actually used when the target flow rate of permeated water and the actual flow rate do not match. It is not preferable because the recovery rate of H may deviate from the target recovery rate. That is, when the actual flow rate of the permeated water is larger than the target flow rate, scaling occurs when the actual recovery rate exceeds the target recovery rate, or the actual flow rate of the permeated water is smaller than the target flow rate. If the actual recovery rate falls below the target recovery rate, water conservation can not be achieved.

  Therefore, as described above, it is preferable to use the detection flow rate of the permeated water by the permeated water flow meter 12 for calculation of the set flow rate of the concentrated drainage. Thereby, even if the situation where the flow rate control of the permeated water is not properly performed, it is possible to suppress the actual recovery rate from deviating from the target recovery rate. In addition, it is preferable to use the average flow volume in predetermined | prescribed detection time or the predetermined | prescribed detection frequency in order to minimize the influence by the dispersion | variation in the detection flow volume of permeated water, etc. for actual calculation.

  However, when the flow rate of permeated water is not stable, such as at the time of device start-up or resumption of operation, and the variation in detected flow rate is very large, the permeated water set in advance for a fixed period until the flow rate of permeated water stabilizes. The set flow rate of the concentrated drainage may be calculated using the target flow rate of Further, the flow rate of the permeated water used to calculate the set flow rate of the concentrated drainage may be switched according to the difference between the target flow rate of the permeated water and the actual flow rate. That is, when the difference is within the predetermined range, calculation may be performed using the target flow rate, and when the difference is outside the predetermined range, calculation may be performed using the actual flow rate.

  When the recovery rate control is performed as described above, it is preferable to use an electric proportional control valve as the flow rate adjustment valve 31. Thus, the degree of opening adjustment can be finely performed in accordance with the resolution of the electric proportional control valve, and the recovery rate can be smoothly adjusted as compared with the degree of opening adjustment in a stepwise manner by a combination of solenoid valves. For example, if the target recovery rate is set to 64% in a stage system in which the recovery rate in the range of 50 to 70% can be controlled only to 5 stages (50%, 55%, 60%, 65%, 70%) The recovery rate can only be adjusted to 60% and wasteful concentrated drainage will occur. Therefore, using the electric proportional control valve as the flow rate adjustment valve 31 is advantageous also from the viewpoint of water saving because such waste of concentrated drainage can be reduced.

  However, when using an electric proportional control valve as the flow rate adjustment valve 31, it is necessary to pay attention to the relationship between the opening / closing speed thereof and the calculation speed (calculation speed) of the set flow rate of concentrated drainage by the drainage flow control unit 32. For example, if the two speeds are significantly different, hunting may occur if the set flow rate of concentrated drainage is changed before the opening and closing of the motorized proportional control valve is completed and the flow rate of concentrated drainage is stabilized. . Further, since the set flow rate of concentrated drainage is determined based on the detection flow rate of permeated water by the permeated water flow meter 12, the flow control of concentrated drainage also corresponds to the response speed of the inverter controlling the number of rotations of the pressurizing pump 21 May be affected. Therefore, when determining the calculation speed of the set flow rate of concentrated drainage by the drainage flow rate control unit 32, it is preferable to consider the opening / closing speed of the electric proportional control valve and the response speed of the inverter. In the present embodiment, as described above, since the flow control of the permeate and the flow control of the concentrated water are performed independently by the installation of the constant flow valve 13, interference of the flow control with each other is suppressed. be able to. As a result, the occurrence of hunting as described above can be suppressed as much as possible, and the actual recovery rate can be prevented from deviating from the target recovery rate. Also from this point of view, it is preferable that the concentrated water line 3 be provided with the constant flow valve 13.

  In order to achieve further water saving, it is necessary to set a higher target recovery rate, but in the present embodiment, a scale inhibitor is added to the raw water for the purpose of increasing the above-mentioned precipitation recovery rate. You may come to In this case, the prescribed flow rate of the constant flow rate valve 13 can be reduced, and as a result, energy saving can be realized by using the pressurizing pump 21 with a smaller capacity. The addition of the scale inhibitor can be done by means of a dosing pump.

  The scale inhibitor is not limited to a specific one as long as it is a substance that can suppress the precipitation of scale components such as silica and calcium. As the type, for example, 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 compounds; Phosphoric acid compounds such as orthophosphates and polymerized phosphates; Maleic acid compounds such as polymaleic acid and maleic acid copolymers; Acrylic acid polymers and the like, and acrylic acid polymers And copolymers such as poly (meth) acrylic acid, maleic acid / (meth) acrylic acid, (meth) acrylic acid / sulfonic acid, (meth) acrylic acid / nonion group-containing monomers, or (meth) acrylic acid / sulfonic acid / Nonion group containing monomer, (meth) acrylic acid / acrylamide-alkyl Sulfonic acid / substituted (meth) acrylamide, (meth) acrylic acid / acrylamide - like arylsulfonic acid / substituted (meth) acrylamide terpolymer. Examples of (meth) acrylic acid constituting the terpolymer include methacrylic acid and acrylic acid, and (meth) acrylic acid salts such as sodium salts thereof. Examples of the acrylamido-alkylsulfonic acid constituting the terpolymer include 2-acrylamido-2-methylpropanesulfonic acid and salts thereof. Moreover, as a substituted (meth) acrylamide which comprises terpolymer, t-butyl acrylamide, t-octyl acrylamide, dimethyl acrylamide etc. are mentioned, for example.

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

  In addition, as a commercially available scale inhibitor for RO membranes, "Orpartion" series manufactured by Organo Corporation, "Flocon (registered trademark)" series manufactured by BWA Water Additives, and "PermaTreat (registered trademark)" manufactured by Nalco Series, “Hypersperse (registered trademark)” series manufactured by General Electric Co., Ltd., “Krivator (registered trademark)” series manufactured by Kurita Kogyo Co., Ltd., and the like.

  As described above, in the present embodiment, since the flow rate of the concentrated water is maintained constant by the constant flow rate valve 13, only by defining the flow rate of the concentrated water flowing through one of the drainage line 4 and the reflux water line 5, The flow rate of the concentrated water flowing through can also be defined. Therefore, in the illustrated embodiment, the drainage line 4 is provided with the drainage flow meter 14 and the flow rate control means (flow control valve 31), and the reflux water line 5 contains concentrated water flowing through the drainage line 4 and the reflux water line 5. A manual valve (pressure regulating valve) 15 is provided to adjust the pressure balance, but the reverse may be possible. That is, the reflux water line 5 may be provided with a flow meter and a flow control valve (proportional control valve) as flow control means, and the drainage line 4 may be provided with a manual valve for pressure balance adjustment. Alternatively, both the drainage line 4 and the reflux water line 5 may be provided with a flow meter and a flow control valve (proportional control valve) as flow control means. Further, in the embodiment described above, the permeated water flow rate control unit and the drainage flow rate control unit are separately provided, but one flow rate control unit performs adjustment of the flow rate of permeated water and the flow rate adjustment of concentrated drainage. It may be like that.

  Further, the number of filtration means is not limited to one, and two or more filtration means may be connected in series. Also in this case, the constant flow rate valve is provided in the concentrated water line connected to the most upstream filter means among the two or more filter means, and the permeated water separated by the most downstream filter means is the set flow rate It will be adjusted to (predetermined target flow rate). However, it is needless to say that the flow distribution of the permeated water and the concentrated water needs to be appropriately set and adjusted by any flow rate adjustment means in all the filtration means excluding the filtration means on the most upstream side. Furthermore, in order to calculate the set flow rate of concentrated drainage from the most upstream filtration means, the flow rate of permeated water separated by the most upstream filtration means, not the permeate water separated by the most downstream filtration means It should be noted that the detected flow rate is used. Here, “connected in series” means that the water to be treated is sequentially treated by a plurality of filtration means, and is separated by the filtration means on the upstream side in two adjacent filtration means It means that the permeated water is supplied to the downstream filtration means as treated water. Each filtration means may be composed of a plurality of RO membranes or NF membranes. In this case, a plurality of RO membranes or NF membranes are connected in series on the primary side (flowing side of raw water and concentrated water) and finally connected to the concentrated water line, and the secondary side (flowing side of permeated water) is It will be connected in parallel and will eventually be connected to the permeate water line.

Reference Signs List 1 supply line 2 permeate water line 3 concentrated water line 4 drainage line 5 reflux water line 10 membrane filtration apparatus 11 filtration means 12 permeate water flow meter 13 constant flow valve 14 drainage flow meter 15 manual valve 20 permeate water flow control mechanism 21 pressurization Pump 22 Permeate flow control unit 30 Drainage flow control mechanism 31 Flow control valve 32 Drainage flow control unit

Claims (4)

A filtration means having a reverse osmosis membrane or a nanofiltration membrane, which separates the water to be treated into permeate and concentrated water;
A supply line connected to the filtration means for supplying treated water to the filtration means;
A permeate water line connected to the filtration means for circulating permeate water from the filtration means;
A concentrated water line connected to the filtration means and circulating the concentrated water from the filtration means;
A drainage line branched from the concentrated water line and discharging a part of the concentrated water flowing in the concentrated water line to the outside;
First flow rate detection means for detecting the flow rate of the permeated water flowing through the permeated water line;
Second flow rate detection means for detecting the flow rate of the concentrated water flowing through the drainage line;
First flow rate control means for adjusting the flow rate of the permeated water flowing through the permeated water line to a set flow rate;
A second flow control means for adjusting the flow rate of the concentrated water flowing in the drainage line to a set flow rate, which is based on a flow rate adjusting valve provided in the drainage line and a detected value by the second flow rate detection means A second flow control unit having a control unit that adjusts the opening degree of the flow control valve;
The control unit of the second flow rate control means is a ratio of the flow rate of permeated water flowing through the permeated water line to the sum of the flow rate of permeated water flowing through the permeated water line and the flow rate of concentrated water flowing through the drainage line The membrane filtration apparatus which determines the said setting flow rate of the concentrated water which flows through the said drainage line based on the target value of a certain recovery rate, and the detected value by the said 1st flow rate detection means.   The control unit of the second flow control means is a value obtained by subtracting the detection value by the first flow detection means from the value obtained by dividing the detection value by the first flow detection means by the target value of the recovery rate. The membrane filtration apparatus according to claim 1, wherein the set flow rate of the concentrated water flowing through the drainage line is determined. It has water temperature detection means for detecting the water temperature of any of the water to be treated supplied to the filtration means, the permeated water from the filtration means, and the concentrated water from the filtration means,
The control unit of the second flow rate control means is a maximum at which silica or calcium does not precipitate on the membrane surface of the reverse osmosis membrane or nanofiltration membrane of the filtration means based on the water temperature detected by the water temperature detection means The membrane filtration apparatus according to claim 1 or 2, wherein the recovery rate of is calculated and the calculated value is set as the target value of the recovery rate.   The first flow rate control means is provided in the supply line, and the pressure adjustment means for adjusting the pressure of the water to be treated flowing through the supply line; and the pressure based on the detection value by the first flow rate detection means. The membrane filtration apparatus according to any one of claims 1 to 3, further comprising: a control unit that controls the adjusting unit.

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