JP7017365B2 - Membrane filtration device - Google Patents

Description

The present invention relates to a membrane filtration apparatus 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 permeated water and concentrated water by the RO membrane or the NF membrane. As a result, treated water (permeated water) from which impurities have been removed is obtained.

In membrane filtration devices with RO membranes or NF membranes, in many cases, from the viewpoint of effective use of water (water saving), part of the concentrated water containing impurities is discharged to the outside as concentrated wastewater, and the rest is used as concentrated reflux water. A configuration is adopted in which the water is refluxed to the upstream side of the RO membrane or the NF membrane. As a result, the recovery rate (the ratio of the permeated water flow rate to the sum of the permeated water flow rate and the concentrated drainage flow rate) can be improved as compared with the case where all the concentrated water is discharged as concentrated wastewater, and water saving can be achieved. It can be realized. At the same time, in the membrane filtration device, in order to respond to the change in the flow rate of the permeated water due to the change in the water temperature (that is, the change in the viscosity of the water), the rotation speed of the pressurizing pump is controlled to change to the RO membrane or the NF membrane. The flow rate is also controlled to keep the flow rate of the permeated water constant by adjusting the supply pressure of the raw water.

In the flow rate control of the permeated water, when the supply 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 changes accordingly. Such a change in the flow rate of concentrated water leads to clogging of the membrane due to fouling and scaling, and damage to the membrane due to an increase in pressure loss. It is required to control the flow rate of.

Patent Documents 1 and 2 describe that, in parallel with the flow rate control of the permeated water, the flow rate control of the concentrated water that adjusts the flow rate of the concentrated water discharged to the outside to the set flow rate is performed. As a method for determining the set flow rate of concentrated water, Patent Document 1 describes that a calculated value of an allowable concentration ratio of silica in concentrated water and a preset target flow rate value of permeated water are used. In 2, it is described that the calculated value of the permissible concentration ratio of calcium carbonate in the concentrated water and the preset target flow rate value of the permeated water are used. That is, in the flow rate control of the concentrated water described in Patent Documents 1 and 2, the set flow rate of the concentrated water discharged to the outside is determined based on the target flow rate value of the permeated water set in advance.

Japanese Patent No. 5768615 Japanese Patent No. 5904299

However, in reality, as shown below, a situation may occur in which the flow rate of the permeated water cannot be adjusted to the target flow rate in the flow rate control of the permeated water. 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 there is a risk of scale generation. There is a possibility that the water will be high and wasteful drainage will occur.

For example, as the water temperature increases, the viscosity of the water decreases, and as a result, the flow rate of the permeated water separated by the RO membrane or the NF membrane increases. In this case, the raw water supply pressure required to keep the permeated water flow rate constant decreases, so the raw water supply pressure is adjusted by reducing the rotation speed of the pressurizing pump to keep the permeated water flow rate constant. Try to maintain. However, since the minimum rotation speed is specified for the pressurizing pump, when the water temperature rises significantly, even if the rotation speed of the pressurizing pump is reduced to the minimum rotation speed, the supply pressure of raw water is set as the target pressure. It may not be lowered to. Therefore, the flow rate of the permeated water cannot 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, as the water temperature decreases, the viscosity of the water increases, and as a result, the flow rate of the permeated water separated by the RO membrane or the NF membrane decreases. Therefore, the supply pressure of raw water required to keep the flow rate of permeated water constant increases, but when the water temperature drops significantly, the supply pressure of raw water is increased even if the rotation speed of the pressurizing pump is maximized. It may not be possible to increase the target pressure. Therefore, the flow rate of the permeated water cannot be increased to the target flow rate, and as a result, the actual recovery rate may be lower than the target recovery rate, and wastewater may be discharged.

Therefore, an object of the present invention is to provide a membrane filtration device that suppresses deviation of the actual recovery rate from a preset target recovery rate.

In order to achieve the above-mentioned object, the membrane filtering apparatus of the present invention is connected to a filtering means having a back-penetrating membrane or a nano-filtering membrane that separates the water to be treated into permeated water and concentrated water, and filters. A supply line that supplies water to be treated to the means, a permeated water line that is connected to the filtering means and flows the permeated water from the filtering means, and a concentrated water line that is connected to the filtering means and flows the concentrated water from the filtering means. A drainage line that branches off from the concentrated water line and discharges a part of the concentrated water flowing through the concentrated water line to the outside, a first flow rate detecting means for detecting the flow rate of the permeated water flowing through the permeated water line, and a drainage line. A second flow rate detecting means for detecting the flow rate of the concentrated water flowing through the water, a 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, and a flow rate of the concentrated water flowing through the drainage line are set. A second flow control means for adjusting the flow rate, a flow control valve provided in the drain line, and a control unit for adjusting the opening degree of the flow control valve based on the value detected by the second flow detection means. The control unit of the second flow control means has a second flow control means having the Concentrated water flowing through the drainage line, which is determined from the target value of the 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 flowing concentrated water, and the value detected by the first flow rate detecting means. The opening degree of the flow rate adjusting valve is adjusted so as to reach the set flow rate of.

According to such a membrane filtration device, the set flow rate of the concentrated water flowing through the drainage line is determined based on the flow rate (detection value) of the permeated water actually flowing through the permeated water line. Therefore, even if a situation occurs in which the flow rate control of the permeated water is not properly implemented, the flow rate of the concentrated water flowing through the drainage line should be adjusted to the set flow rate determined based on the actual flow rate of the permeated water (detection value). Therefore, it is possible to prevent the actual recovery rate from deviating from the target recovery rate.

As described above, according to the present invention, it is possible to provide a membrane filtration device that suppresses an actual recovery rate deviation from a preset target recovery rate.

It is a schematic diagram 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 a configuration of a membrane filtration device according to an embodiment of the present invention.

The membrane filtration device 10 of the present embodiment is a device that removes impurities contained in raw water (water to be treated) to generate treated water, and the raw water is permeated with concentrated water containing impurities and permeation from which impurities have been removed. It has a filtration means 11 that separates from water. The filtration means 11 has a reverse osmosis membrane (RO membrane) or a nanofiltration membrane (NF membrane).

Further, the membrane filtration device 10 includes a plurality of lines connected to the filtration means 11, that is, a supply line 1 for supplying raw water to the filtration means 11, and a permeation water line 2 for circulating the permeated water from the filtration means 11. It also has a concentrated water line 3 for circulating concentrated water from the filtration means 11. In addition, the membrane filtration device 10 has two lines branched from the concentrated water line 3, that is, a drainage line 4 for discharging a part of the concentrated water flowing through the concentrated water line 3 to the outside, and a supply line for supplying the rest of the concentrated water. It has a reflux water line 5 for refluxing to 1. The recirculated water line 5 is connected to the supply line 1 on the upstream side of the pressurizing pump 21, which will be described later, after branching 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 being directly connected to the supply line 1.

Further, the membrane filtration device 10 includes a permeated water flow meter (first flow rate detecting means) 12 that detects the flow rate of the permeated water flowing through the permeated water line 2, and a permeated water flow rate control mechanism that adjusts the flow rate to a set flow rate. It has a first flow rate control means) 20.

The permeated water flow rate control mechanism 20 is provided in the supply line 1 and has a pressurizing pump (pressure adjusting means) 21 for adjusting the pressure of the raw water flowing through the supply line 1 (the pressure of supplying the raw water to the filtering means 11), and the permeated water. 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) that controls the rotation speed of the pressurizing pump 21, so that the permeated water flow rate detected by the permeated water flow meter 12 becomes constant. It controls the number of rotations. For example, when the water temperature changes, the viscosity of the water changes, so that the flow rate of the permeated water 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 rotation speed of the pressurizing pump 21. That is, as the water temperature decreases, the viscosity of the water increases, and as a result, the flow rate of the permeated water separated by the RO membrane or the NF membrane decreases. Therefore, the permeated water flow rate control unit 22 increases the supply pressure of raw water by increasing the rotation speed of the pressurizing pump 21 so as to compensate for this decrease. Further, as the water temperature increases, the viscosity of the water decreases, and as a result, the flow rate of the permeated water separated by the RO membrane or the NF membrane increases. Therefore, the permeated water flow rate control unit 22 lowers the supply pressure of the raw water by lowering the rotation speed of the pressurizing pump 21 so as to cancel this increase. The rotation speed of the pressurizing pump 21 is controlled by the permeated water flow rate control unit 22 so as not to exceed a preset upper limit value or a preset lower limit value. Therefore, even if the rotation speed of the pressurizing pump 21 is controlled to be the lower limit value, the flow rate of the permeated water may exceed the set flow rate. A manual valve or a proportional control valve for adjusting the supply pressure of raw water may be provided between the pump 21 and the filtration means 11.

As described above, in the present embodiment, the flow rate of the permeated water is maintained constant (preset target flow rate) by adjusting the rotation speed of the pressurizing pump 21, that is, the supply pressure of the raw water, but the raw water is maintained. The flow rate of the concentrated water separated by the RO membrane or the NF membrane also changes according to the change in the supply pressure of. In order to suppress such a change in the flow rate of the concentrated water itself, the concentrated water line 3 is provided with a constant flow rate valve 13 that keeps the flow rate of the concentrated water flowing through the concentrated water line 3 constant. As a result, 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 means 11 is changed, the flow rate of the concentrated water can be kept constant. ..

Here, the specified flow rate of the constant flow rate valve 13 may be such that, on the one hand, the film is not clogged due to fouling or scaling, and on the other hand, the film is not damaged due to an increase in pressure loss. However, if the specified flow rate of the constant flow valve 13 is increased more than necessary, the flow rate required for the pressurizing pump 21 becomes larger than necessary, and as a result, the size of the pressurizing pump 21 becomes large, so that energy consumption is consumed. It is not preferable in terms of. Therefore, the specified 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 using a (8 inch) RO membrane, the range is 1 to 15 m ³ / h.

By the way, the constant flow rate valve 13 defines an operating differential pressure range (allowable range of pressure difference between the primary side and the secondary side of the constant flow rate valve) for operating the constant flow rate valve 13 normally. Therefore, depending on the conditions, for example, when an RO membrane for medium and high pressure is used as the filtration means 11 or when the water temperature is extremely lowered, the supply pressure of the raw water rises remarkably and the pressure of the concentrated water rises. The pressure difference between the primary side and the secondary side of the constant flow rate 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 to 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. As a result, even when the supply pressure of raw water to the filtration means 11 rises remarkably, the pressure difference between the primary side and the secondary side of the constant flow rate valve 13 is kept within the operating differential pressure range, and the constant flow rate valve 13 is provided. It can be operated normally, 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 (pipes, etc.) on the downstream side of the valve are not required to have such pressure resistance. Therefore, the installation of the pressure reducing valve is not only advantageous in terms of safety, but also advantageous in terms of cost because an inexpensive general-purpose product whose pressure resistance performance is not so high can be used. The type of the pressure reducing valve is not particularly limited as long as the pressure of the concentrated water can be reduced within the operating differential pressure range of the constant flow rate valve 13, but the flow rate is equal to or higher than the specified flow rate of the constant flow rate valve 13. It is necessary to select one in which the flow rate of the water flows and the pressure on the secondary side is larger than the water flow differential pressure of the drainage line 4 and the recirculation water line 5.

As described above, by installing the constant flow rate valve 13, the flow rate control of the permeated water does not affect 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 recirculation water line 5 is controlled. It becomes easy to execute. Therefore, the membrane filtration device 10 of the present embodiment has a drainage flow meter (second flow rate detecting means) 14 for detecting the flow rate of concentrated water (hereinafter referred to as "concentrated drainage") flowing through the drainage line 4, and the flow rate thereof. It has a drainage flow rate control mechanism (second flow rate control means) 30 that adjusts to a set flow rate. The flow rate control of the concentrated drainage by the drainage flow rate control mechanism 30 is performed independently of the flow rate control of the permeated water by the permeated water flow rate control mechanism 20.

The drainage flow rate control mechanism 30 adjusts the opening degree of the flow rate adjusting valve 31 based on the flow rate adjusting valve 31 provided in the drainage line 4 and the detected flow rate (detected value) of the concentrated drainage by the drainage flow meter 14. It has a control unit 32.

The drainage flow rate control unit 32 determines the set flow rate of the concentrated drainage in consideration of the recovery rate, which is 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 drainage, and the detection value by the drainage flow meter 14. Is adjusted so that the opening degree of the flow rate adjusting valve 31 becomes the set flow rate. The recovery rate at this time is preferably as high as possible from the viewpoint of effective use of water (water saving). That is, it is preferable that the flow rate of concentrated waste water is as small as possible. However, since the flow rate of the concentrated water is kept constant by the constant flow rate valve 13, when the flow rate of the concentrated drainage decreases, the flow rate of the concentrated water returning from the reflux water line 5 to the supply line 1 naturally increases. do. As a result, when the concentration of impurities in the raw water increases, scaling in which impurities (particularly silica or calcium) are deposited on the film surface of the RO film or NF film of the filtration means 11 tends to occur. Therefore, the flow rate of the concentrated waste water is such that the recovery rate is maximized in the range where the impurity concentration of the concentrated water does not exceed the solubility, that is, the recovery rate is maximized in the range where the impurities silica or calcium do not precipitate. Is set to.

However, the solubility of impurities changes depending on the water temperature. For example, in the case of silica, its solubility increases in proportion to temperature, and in the case of calcium (calcium carbonate), its solubility decreases as the temperature rises. Therefore, when the water temperature is low, the solubility of silica is relatively low and silica is likely to precipitate (silica scale is likely to occur), but when the water temperature is high, the solubility of calcium is relatively low and therefore calcium. Is likely to precipitate (calcium scale is likely to occur). Therefore, although not shown in the present embodiment, a temperature sensor (water temperature detecting means) for detecting the water temperature of any of raw water, permeated water, and concentrated water is provided, and the water temperature detected by this temperature sensor is provided. Based on, the optimum set flow rate of concentrated wastewater is calculated.

Specifically, first, the theoretical recovery rate at which silica precipitates at the detected water temperature (hereinafter referred to as "silica precipitation recovery rate") and the theoretical recovery rate at which calcium (calcium carbonate) precipitates at the detected water temperature. Recovery rate (hereinafter referred to as "calcium precipitation recovery rate") is calculated. The methods for calculating the silica precipitation recovery rate and the calcium precipitation recovery rate will be described 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 this 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 wastewater is calculated and set by the following formula (1).
(Target flow rate of concentrated wastewater) =
(Detected flow rate of permeated water / target recovery rate)-(Detected flow rate of permeated water) (1)

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

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

(Calculation method of silica precipitation recovery rate)
The silica precipitation recovery rate YS is as follows, where the solubility (mg / _L ) of silica at the detected water temperature is CS and the silica concentration (mg / _L ) of the raw water measured in advance is _FS . Calculated from equation (2).
Y _S = ( _CS - _FS ) / _CS (2)

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

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

The Langeria index of concentrated water is calculated from the pH of the concentrated water, the impurity concentration (calcium concentration, total alkalinity, and evaporation residue concentration) of the concentrated water, and the detected water temperature. As a method for calculating the Langeria index, for example, the method described in JP-A No. 11-267687 (paragraphs [0025] to [0027]) can be used. In addition, the impurity concentration (calcium concentration, total alkalinity, and evaporation residue concentration) of the concentrated water is the impurity concentration (calcium concentration, total alkalinity, and evaporation residue concentration) of the raw water measured in advance, and the recovery rate. It is calculated from. Therefore, for the calcium precipitation recovery rate YC, the impurity concentration (mg / L) of the concentrated water when the _Langeria index of the concentrated water becomes zero is defined as CC, and the impurity concentration (mg / _L ) of the raw water measured in advance is used. When is _FC , it is expressed by the relation of the following equation (3).
Y _C = ( _CC - _FC ) / _CC (3)

The method of calculating the precipitation recovery rate of silica and calcium and the method of calculating the set flow rate of concentrated wastewater are limited in advance due to device design restrictions such as the capacity of the pressurizing pump and the flow rate of raw water. In some cases, this is not the case as described above. In addition, a preset target flow rate of permeated water can be used to calculate the set flow rate of concentrated wastewater, but this method actually performs when the target flow rate of permeated water and the actual flow rate do not match. It is not preferable because the recovery rate 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 because the actual recovery rate exceeds the target recovery rate, or when the actual flow rate of the permeated water is smaller than the target flow rate. If the actual recovery rate is lower than the target recovery rate, it may not be possible to save water.

Therefore, as described above, it is preferable to use the permeated water detected flow rate by the permeated water flow meter 12 for calculating the set flow rate of the concentrated wastewater. As a result, even if a situation occurs in which the flow rate control of the permeated water is not properly performed, it is possible to prevent the actual recovery rate from deviating from the target recovery rate. In the actual calculation, it is preferable to use the average flow rate at the predetermined detection time and the predetermined number of detections in order to minimize the influence of the variation in the detected flow rate of the permeated water.

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

When controlling the recovery rate as described above, it is preferable to use an electric proportional control valve as the flow rate adjusting valve 31. As a result, the opening degree can be finely adjusted according to the resolution of the electric proportional control valve, and the recovery rate can be smoothly adjusted as compared with the stepwise opening degree adjustment by a combination of solenoid valves or the like. For example, in a stepped system where the recovery rate in the range of 50 to 70% can be controlled only in 5 steps (50%, 55%, 60%, 65%, 70%), when the target recovery rate is set to 64%, The recovery rate can only be adjusted to 60%, resulting in wasteful concentrated wastewater. Therefore, using the electric proportional control valve as the flow rate adjusting valve 31 is advantageous from the viewpoint of water saving because it is possible to reduce the waste of such concentrated wastewater.

However, when an electric proportional control valve is used as the flow rate adjusting valve 31, it is necessary to pay attention to the relationship between the opening / closing speed and the calculation speed (calculation speed) of the set flow rate of the concentrated drainage by the drainage flow rate control unit 32. For example, if the two speeds are significantly different, hunting may occur if the set flow rate of the concentrated drainage is changed before the opening and closing of the electric proportional control valve is completed and the flow rate of the concentrated drainage is stabilized. .. Further, since the set flow rate of the concentrated drainage is determined based on the detected flow rate of the permeated water by the permeated water flow meter 12, the flow rate control of the concentrated drainage also affects the response speed of the inverter that controls the rotation speed of the pressurizing pump 21. May be affected. Therefore, when determining the calculation speed of the set flow rate of the 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, the flow rate control of the permeated water and the flow rate control of the concentrated water are independently performed by installing the constant flow rate valve 13, so that the mutual flow rate control is suppressed from interfering with each other. 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 suppressed from deviating from the target recovery rate. From this point as well, it is preferable that the concentrated water line 3 is provided with the constant flow rate valve 13.

In order to further save water, it is necessary to set a higher target value for the recovery rate, but in the present embodiment, an antiscale agent is added to the raw water for the purpose of increasing the above-mentioned precipitation recovery rate. You may be able to do it. In this case, the specified 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 having a smaller capacity. The addition of the anti-scale agent can be done by a medicated pump.

The anti-scale agent is not limited to a specific substance as long as it is a substance capable of suppressing the precipitation of scale components such as silica and calcium. Examples thereof include phosphonic acids such as 1-hydroxyethylidene-1,1-diphosphonic acid, 2-phosphonobutane-1,2,4-tricarboxylic acid, ethylenediaminetetramethylenephosphonic acid and nitrilotrimethylphosphonic acid and salts thereof. Phosphoric acid-based compounds; Phosphoric acid-based compounds such as orthophosphates and polymerized phosphates; Maleic acid-based compounds such as polymaleic acid and maleic acid copolymers; Is a copolymer of poly (meth) acrylic acid, maleic acid / (meth) acrylic acid, (meth) acrylic acid / sulfonic acid, (meth) acrylic acid / nonionic group-containing monomer, and (meth) acrylic acid / sulfonic acid. / Nonion group-containing monomer, (meth) acrylic acid / acrylamide-alkylsulfonic acid / substituted (meth) acrylamide, (meth) acrylic acid / acrylamide-arylsulfonic acid / substituted (meth) acrylamide tarpolymer and the like can be mentioned. Examples of the (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 acrylamide-alkylsulfonic acid constituting the terpolymer include 2-acrylamide-2-methylpropanesulfonic acid and a salt thereof. Examples of the substituted (meth) acrylamide constituting the terpolymer include t-butyl acrylamide, t-octyl acrylamide, and dimethyl acrylamide.

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

Commercially available scale inhibitors for RO membranes include the "Olpage" series manufactured by Organo Corporation, the "Flocon (registered trademark)" series manufactured by BWA Water Adaptives, and the "PermaTreat (registered trademark)" manufactured by Nalco. Series, "Hypersperse (registered trademark)" series manufactured by General Electric Co., Ltd., "Cliberter (registered trademark)" series manufactured by Kurita Water Industries, Ltd., and the like.

As described above, in the present embodiment, since the flow rate of the concentrated water is kept constant by the constant flow rate valve 13, only the flow rate of the concentrated water flowing through one of the drainage line 4 and the recirculation water line 5 is specified, and the other is specified. The flow rate of concentrated water flowing through the water can also be specified. Therefore, in the illustrated embodiment, the drainage line 4 is provided with the drainage flow meter 14 and the flow rate control means (flow rate adjusting valve 31), and the return water line 5 is the concentrated water flowing through the drainage line 4 and the return water line 5. A manual valve (pressure adjusting valve) 15 for adjusting the pressure balance is provided, but vice versa. That is, the recirculation water line 5 may be provided with a flow meter and a flow rate adjusting valve (proportional control valve) as a flow rate 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 recirculation water line 5 may be provided with a flow meter and a flow rate adjusting valve (proportional control valve) as a flow rate control means. Further, in the above-described embodiment, the permeated water flow rate control unit and the drainage flow rate control unit are separately provided, but the flow rate adjustment of the permeated water and the flow rate adjustment of the concentrated drainage are performed by one flow rate control unit. It may be like this.

Further, the number of filtration means is not limited to one, and two or more filtration means may be provided by being connected in series. Even in that case, the constant flow rate valve is provided in the concentrated water line connected to the most upstream filtration means among the two or more filtration means, and the permeated water separated by the most downstream filtration means is the set flow rate. It will be adjusted to (preset target flow rate). However, it goes without saying that in all the filtration means except the most upstream filtration means, it is necessary to appropriately set and adjust the flow rate distribution of the permeated water and the concentrated water by any flow rate adjusting means. Furthermore, in calculating the set flow rate of concentrated wastewater from the most upstream filtration means, the flow rate of the permeated water separated by the most upstream filtration means (not the permeated water separated by the most downstream filtration means). Note that the detected flow rate) is used. In addition, "connected in series" here means that the water to be treated is sequentially treated by a plurality of filtering means, and is separated by the filtering means on the upstream side in the two adjacent filtering means. It means that the permeated water is supplied to the filtration means on the downstream side as water to be treated. Further, each filtration means may be composed of a plurality of RO membranes or NF membranes. In this case, in the plurality of RO membranes or NF membranes, the primary side (raw water and concentrated water distribution side) is connected in series and finally connected to the concentrated water line, and the secondary side (permeated water distribution side) is connected. It will be connected in parallel and eventually connected to the permeated water line.

1 Supply line 2 Permeated water line 3 Concentrated water line 4 Drainage line 5 Reflux water line 10 Film filtration device 11 Filtering means 12 Permeated water flow meter 13 Constant flow valve 14 Drainage flow meter 15 Manual valve 20 Permeated water flow control mechanism 21 Pressurization Pump 22 Permeated water 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 that separates the water to be treated into permeated water and concentrated water,
A supply line connected to the filtration means and supplying water to be treated to the filtration means,
A permeated water line connected to the filtering means and circulating the permeated water from the filtering means,
A concentrated water line connected to the filtering means and circulating concentrated water from the filtering means,
A drainage line that branches off from the concentrated water line and discharges a part of the concentrated water flowing through the concentrated water line to the outside.
A first flow rate detecting means for detecting the flow rate of the permeated water flowing through the permeated water line,
A second flow rate detecting means for detecting the flow rate of concentrated water flowing through the drainage line,
A 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 rate control means for adjusting the flow rate of concentrated water flowing through the drainage line to a set flow rate, based on a flow rate adjusting valve provided in the drainage line and a value detected by the second flow rate detection means. A second flow rate control means having a control unit for adjusting the opening degree of the flow rate adjusting valve, and a second flow rate control means.
In the control unit of the second flow rate control means, the value detected by the second flow rate detecting means is 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 drainage line. The set flow rate of concentrated water flowing through the drainage line is determined from the target value of the recovery rate, which is the ratio of the flow rate of the permeated water flowing through the permeated water line, and the value detected by the first flow rate detecting means. In addition, a membrane filtration device that adjusts the opening degree of the flow rate adjusting valve . The control unit of the second flow rate control means is a value obtained by subtracting the detection value by the first flow rate detection means from the value obtained by dividing the detection value by the first flow rate detection means by the target value of the recovery rate. The membrane filtration device according to claim 1, wherein is determined as the set flow rate of concentrated water flowing through the drainage line. It has a water temperature detecting means for detecting the water temperature of any one of the water to be treated supplied to the filtering means, the permeated water from the filtering means, and the concentrated water from the filtering means.
The control unit of the second flow control means theoretically deposits silica and calcium on the membrane surface of the reverse osmosis membrane or the nanofiltration membrane of the filtration means at the water temperature detected by the water temperature detecting means. The membrane filtration apparatus according to claim 1 or 2, wherein the recovery rates of the above are calculated, the calculated values are compared, and the smaller 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 is based on a pressure adjusting means for adjusting the pressure of water to be treated flowing through the supply line and a value detected by the first flow rate detecting means. The membrane filtration device according to any one of claims 1 to 3, further comprising a control unit for controlling the adjusting means.

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