JP7106283B2 - Membrane filtration device - Google Patents

Description

The present invention relates to membrane filtration devices having reverse osmosis membranes or nanofiltration membranes.

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

In many cases, in a membrane filtration device having an RO membrane or an NF membrane, from the viewpoint of effective use of water (water saving), part of the concentrated water containing impurities is discharged to the outside as concentrated waste water, and the rest is discharged as concentrated reflux water. A configuration is adopted in which the gas is returned to the upstream side of the RO membrane or the NF membrane. As a result, the recovery rate (the ratio of the permeate flow rate to the sum of the permeate flow rate and the concentrated wastewater flow rate) can be improved compared to the case where all the concentrated water is discharged as concentrated waste water, resulting in water savings. can be realized. At the same time, in the membrane filtration device, in order to respond to changes in the flow rate of permeate due to changes in water temperature (i.e., changes in water viscosity), the rotation speed of the pressure pump is controlled to allow the RO or NF membrane to Flow rate control is also performed to maintain a constant flow rate of permeate by adjusting the supply pressure of the raw water.

In permeate flow rate control, when the feed pressure of raw water is adjusted so that the flow rate of permeate water is constant, the flow rate of concentrated water separated by the RO membrane or NF membrane also changes accordingly. Such changes in the flow rate of the concentrate lead to clogging of the membrane due to fouling and scaling, and damage to the membrane due to increased pressure loss. Control is required.

Patent Documents 1 to 3 propose specific methods for controlling the flow rate of concentrated water. In the methods described in Patent Documents 1 and 2, in order to achieve stable flow rate control, a constant flow rate valve is provided in the concentrated water line through which the concentrated water flows, so that the flow rate of the concentrated water is always kept constant. . On the other hand, in the method described in Patent Document 3, the flow rate of the concentrated water flowing through the concentrated water line is maintained at the set flow rate by adjusting the flow rate of the concentrated reflux water in accordance with the flow rate adjustment of the concentrated waste water.

Japanese Patent No. 6161384 JP 2016-203084 A JP 2008-000658 A

The flow rate of concentrated water flowing through the concentrated water line must be set in consideration of not only the minimum flow rate of concentrated water required for the filtration means (RO membrane or NF membrane), but also the recovery rate adjustment range. . That is, the lower limit of the adjustable recovery rate is defined by the minimum flow rate of the concentrated water required for the filtration means, but considering the risk of fouling and scaling, operation at a recovery rate lower than the lower limit is recommended. is sometimes requested. In that case, in order to lower the lower limit of the adjustable recovery rate, for example, in the methods described in Patent Documents 1 and 2, a constant flow valve with a specified flow rate larger than the minimum flow rate of concentrated water required for the filtration means must be selected. In such a situation, in particular, the filtration means is composed of multiple RO membranes or NF membranes, and their primary sides (raw water and concentrated water distribution sides) are connected in series and finally connected to the concentrated water line. , the secondary side (permeate distribution side) is connected in parallel and finally connected to the permeate line.

However, it is not always necessary to operate at a low recovery rate, and if the risk of fouling and scaling is low due to conditions such as water quality and water temperature, it is possible to operate at a higher recovery rate, thereby saving water. It may be possible. However, in the methods described in Patent Documents 1 and 2, since a constant flow valve is provided in the concentrated water line, even if the flow rate of concentrated wastewater is reduced in order to increase the recovery rate and save water, cannot be used to reduce the flow rate of concentrated water. In addition, although the constant flow valve is not used, the method described in Patent Document 3 is substantially the same as the methods described in Patent Documents 1 and 2 in that the set flow rate of the concentrated water flowing through the concentrated water line is maintained. Therefore, the flow rate of the concentrated water cannot be reduced only by increasing or decreasing the flow rate of the concentrated reflux water in accordance with the increase or decrease of the flow rate of the concentrated waste water. Therefore, the methods described in Patent Documents 1 to 3 cannot adjust the discharge flow rate of the pressure pump according to the adjustment of the recovery rate, and waste energy is consumed, which is not preferable from the viewpoint of energy saving. .

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a membrane filtration device that realizes stable flow rate control and is excellent in energy saving.

In order to achieve the above object, a membrane filtration device according to one aspect of the present invention includes filtration means having a reverse osmosis membrane or nanofiltration membrane that separates water to be treated into permeated water and concentrated water, and is connected to the filtration means. A supply line for supplying water to be treated to the filtering means, a permeated water line connected to the filtering means for circulating permeated water from the filtering means, and a permeated water line connected to the filtering means for circulating concentrated water from the filtering means A concentrated water line, a drain line that branches from the concentrated water line and discharges part of the concentrated water flowing through the concentrated water line to the outside, and a concentrated water line that branches from the concentrated water line and is connected to the supply line and flows through the concentrated water line. A reflux water line for returning the remaining water to the supply 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, and a flow rate of the concentrated water flowing through the drain line is adjusted to the set flow rate. and second flow rate control means for adjusting the flow rate of the concentrated water flowing through the reflux line to a set flow rate. The second flow rate control means includes a first flow rate adjustment valve provided in the drainage line, a first flow rate detection means for detecting the flow rate of the concentrated water flowing through the drainage line, and a second flow rate detection means provided in the recirculated water line. a flow rate control valve, a second flow rate detection means for detecting the flow rate of the concentrated water flowing through the recirculated water line, and a first flow rate control valve based on the flow rate of the concentrated water detected by the first flow rate detection means and a controller that adjusts the opening of the second flow control valve based on the flow rate of the concentrated water detected by the second flow rate detecting means. The control unit of the second flow rate control means has a predetermined recovery rate, which is the ratio of the flow rate of the permeate flowing through the permeate line to the sum of the flow rate of the permeate flowing through the permeate line and the flow rate of the concentrated water flowing through the drain line. Determine the set flow rate of the concentrated water flowing through the drainage line so that the value of If the determined set flow rate is smaller than the predetermined minimum flow rate, the set flow rate of the concentrated water flowing through the reflux line is changed from the predetermined minimum flow rate to the set flow rate determined from the predetermined minimum flow rate. If the reduced flow rate is determined and is equal to or greater than the predetermined minimum flow rate, then the set flow rate of the concentrate flowing through the reflux line is determined to be zero.

In addition, a membrane filtration device according to another aspect of the present invention is a plurality of filtration means connected in series, each of which comprises a reverse osmosis membrane or a nanofiltration membrane for separating the water to be treated into permeated water and concentrated water. a plurality of filtering means, a supply line connected to the most upstream filtering means among the plurality of filtering means and supplying the water to be treated to the most upstream filtering means, and the most downstream of the plurality of filtering means A permeated water line connected to the filtering means and circulating the permeated water from the most downstream filtering means, and an intermediate permeating water line connected to the most upstream filtering means and circulating the permeated water from the most upstream filtering means line, a concentrated water line that is connected to the most upstream filtering means and circulates the concentrated water from the most upstream filtering means, and a part of the concentrated water that branches from the concentrated water line and flows through the concentrated water line is sent to the outside. a waste water line that discharges to , a reflux line that is branched from the concentrated water line and connected to the supply line to return the rest of the concentrated water flowing through the concentrated water line to the supply line, and a flow rate of the permeated water flowing through the permeated water line A first flow rate control means for adjusting the flow rate to a set flow rate, and a second flow rate control means for adjusting the flow rate of the concentrated water flowing through the drainage line to the set flow rate and adjusting the flow rate of the concentrated water flowing through the reflux line to the set flow rate. and have The second flow rate control means includes a first flow rate adjustment valve provided in the drainage line, a first flow rate detection means for detecting the flow rate of the concentrated water flowing through the drainage line, and a second flow rate detection means provided in the recirculated water line. a flow rate control valve, a second flow rate detection means for detecting the flow rate of the concentrated water flowing through the recirculated water line, and a first flow rate control valve based on the flow rate of the concentrated water detected by the first flow rate detection means and a controller that adjusts the opening of the second flow control valve based on the flow rate of the concentrated water detected by the second flow rate detecting means. The control unit of the second flow rate control means has a predetermined recovery rate, which is the ratio of the flow rate of the permeate flowing through the permeate line to the sum of the flow rate of the permeate flowing through the permeate line and the flow rate of the concentrated water flowing through the drain line. The set flow rate of the concentrated water flowing through the drainage line is determined so that the value of Compare with the minimum flow rate of the concentrated water that should flow through the line, and if the determined set flow rate is smaller than the predetermined minimum flow rate, reduce the set flow rate of the concentrated water flowing through the reflux line from the predetermined minimum flow rate. A value obtained by subtracting the determined set flow rate is determined, and if it is equal to or greater than the predetermined minimum flow rate, the set flow rate of the concentrated water flowing through the reflux line is determined to be zero.

According to such a membrane filtration device, the flow rate of concentrated water flowing through the concentrated water line can be adjusted to an optimum flow rate based on the recovery rate and the minimum flow rate of concentrated water required for the filtration means. Therefore, even when a pressurizing pump is used to adjust the supply pressure of the water to be treated (raw water) to the filtering means, the discharge flow rate of the pressurizing pump can be optimally adjusted, which saves unnecessary energy consumption. can be reduced and energy consumption can be minimized.

As described above, according to the present invention, it is possible to provide a membrane filtration device that realizes stable flow rate control and is excellent in energy saving.

BRIEF DESCRIPTION OF THE DRAWINGS It is the schematic which shows the structure of the membrane filtration apparatus which concerns on the 1st Embodiment of this invention. It is a schematic diagram showing the configuration of a membrane filtration device according to a second embodiment of the present invention.

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

(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.

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. It has filtering means 11 for separating from water. The filtering means 11 has a reverse osmosis membrane (RO membrane) or a nanofiltration membrane (NF membrane).

In addition, the membrane filtration device 10 includes a plurality of lines respectively connected to the filtration means 11, that is, a supply line 1 that supplies raw water to the filtration means 11, and a permeated water line 2 that distributes the permeated water from the filtration means 11. , and a concentrated water line 3 for circulating the concentrated water from the filtering means 11 . In addition, the membrane filtration device 10 has two lines branched from the concentrated water line 3, that is, a drain 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 the rest of the concentrated water. It has a reflux water line 5 for refluxing to 1. After being branched from the concentrated water line 3, the reflux water line 5 is connected to the supply line 1 on the upstream side of the pressure pump 21, which will be described later. Note that the return water line 5 may be connected to a raw water tank (not shown) provided on the supply line 1 instead of being directly connected to the supply line 1 .

Furthermore, the membrane filtration device 10 has a permeate flow rate control mechanism (first flow rate control means) 20 that adjusts the flow rate of permeate flowing through the permeate line 2 to a set flow rate.

The permeate flow rate control mechanism 20 is provided in the supply line 1 and includes a pressure pump (pressure adjustment means) 21 for adjusting the pressure of the raw water flowing through the supply line 1 (supply pressure of the raw water to the filtration means 11), and the permeate A permeate flow meter (flow rate detection means) 22 provided in the line 2 for detecting the flow rate of the permeate flowing through the permeate line 2, and based on the flow rate of the permeate detected by the permeate flow meter 22, pressurization and a permeate water flow control unit 23 for controlling the pump 21 .

The permeate flow rate control unit 23 includes an inverter (not shown) that controls the rotation speed of the pressure pump 21, and controls the pressure pump so that the permeate flow rate detected by the permeate flow meter 22 is constant. 21 rotation speed is controlled. For example, when the water temperature changes, the flow rate of the permeate separated by the RO or NF membrane also changes due to the change in the viscosity of the water. The permeate water flow control unit 23 controls the rotation speed of the pressure pump 21 according to this change. That is, the lower the water temperature, the higher the viscosity of the water, resulting in a decrease in the flow rate of the permeate separated by the RO or NF membrane. Therefore, the permeate water flow control unit 23 increases the supply pressure of the raw water by increasing the rotational speed of the pressure pump 21 so as to compensate for this decrease. Also, as the water temperature increases, the viscosity of the water decreases, resulting in an increase in the flow rate of the permeate separated by the RO or NF membrane. Therefore, the permeate water flow rate control unit 23 reduces the supply pressure of the raw water by reducing the rotational speed of the pressure pump 21 so as to cancel out this increase.

Thus, 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. The pressure of the concentrated water separated by the RO membrane or NF membrane also changes according to the change in the supply pressure of . A pressure reducing valve 12 is provided in the concentrated water line 3 in order to suppress such a change in the pressure of the concentrated water itself. The pressure reducing valve 12 not only reduces the pressure of the concentrated water flowing through the concentrated water line 3 (that is, the pressure on the secondary side can be lower than the pressure on the primary side), but also keeps the pressure on the secondary side constant. It also has the function of holding As a result, even when the rotation speed of the pressurizing pump 21 is changed by the permeated water flow rate control unit 23 and the supply pressure of the raw water to the filtering means 11 is changed, the pressure of the concentrated water can be kept constant. .

In this way, by providing the pressure reducing valve 12 in the concentrated water line 3, the pressure of the concentrated water separated by the filtration means 11 is always kept constant, and the flow rate control of the permeated water (pressure fluctuation on the primary side) is controlled by the drainage. The pressure of the concentrated water flowing through the line 4 and the reflux water line 5 (the pressure on the secondary side) is not affected. As a result, no matter what kind of flow rate control is performed in the drainage line 4 or the return water line 5, it will not interfere with the flow rate control of the permeated water, so hunting can be avoided. In addition, since the pressure reducing valve 12 does not keep the flow rate of the concentrated water flowing through the concentrated water line 3 constant, it is possible to adjust the flow rate of the concentrated water flowing through the concentrated water line 3 to an arbitrary flow rate, as described later. can be done. As a result, the discharge flow rate of the pressurizing pump 21 can be adjusted in accordance with the flow rate adjustment of the concentrated water, and wasteful consumption of energy can be suppressed. Furthermore, the provision of the pressure reducing valve 12 does not require that much pressure resistance performance to the peripheral members (piping etc.) downstream of it, so it is not only advantageous in terms of safety, but also has a low pressure resistance that is not so high. It is also advantageous in terms of cost because general-purpose products can be used.

It should be noted that the type of the pressure reducing valve 12 should be selected such that the pressure on the secondary side of the pressure reducing valve 12 is greater than the water flow differential pressure of the drainage line 4 and the recirculating water line 5 . Also, the flow rate range of the pressure reducing valve 12 may be such that clogging of the membrane due to fouling or scaling does not occur, and that the membrane is not damaged due to increased pressure loss. However, enlarging the flow rate range of the pressure reducing valve 12 more than necessary is not preferable in that the cost is wasted. Therefore, the flow rate range of the pressure reducing valve 12 is set in consideration of the permeation flux of the filtration means 11 and the minimum flow rate of concentrated water required for the filtration means 11, and the recovery rate (flow rate of permeated water and concentration The ratio of the flow rate of permeated water to the sum of the flow rate of wastewater) is also considered and set. For example, when an RO membrane having a diameter of about 20.32 cm (8 inches) is used as the filtering means 11, the flow rate range of the pressure reducing valve 12 is in the range of 1 to 15 m ³ /h. The minimum flow rate of the concentrated water required for the filtering means 11 means the minimum flow rate of the concentrated water to be flowed through the concentrated water line 3 so as not to cause clogging of the membrane due to fouling or scaling. Hereinafter, this is also simply referred to as "minimum concentrated water amount".

As described above, by installing the pressure reducing valve 12, the flow rate control of the permeated water (pressure fluctuation on the primary side) no longer affects the pressure of the concentrated water (pressure on the secondary side). and the flow rate control of the concentrated water flowing through the reflux water line 5 can be easily executed. Therefore, the membrane filtration device 10 of the present embodiment adjusts the flow rate of the concentrated water flowing through the drainage line 4 (hereinafter referred to as "concentrated drainage") to a set flow rate, and the concentrated water flowing through the reflux water line 5 (hereinafter referred to as " It has a concentrated water flow rate control mechanism (second flow rate control means) 30 for adjusting the flow rate of the "concentrated reflux water") to a set flow rate. The concentrated water flow rate control mechanism 30 can adjust the flow rate of the concentrated water flowing through the concentrated water line 3 to an arbitrary flow rate as a result of adjusting the respective flow rates of the concentrated waste water and the concentrated reflux water. The flow rate control of the concentrated waste water and the concentrated reflux water by the concentrated water 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 concentrated water flow rate control mechanism 30 includes a waste water flow rate control valve (first flow rate control valve) 31 provided in the waste water line 4, and a waste water flow meter (first flow rate detection means) 32 for detecting the flow rate of the concentrated waste water. , a reflux water flow rate adjustment valve (second flow rate adjustment valve) 33 provided in the reflux water line 5, a reflux water flow meter (second flow rate detection means) 34 for detecting the flow rate of the concentrated reflux water, and a waste water flow rate. Based on the flow rate of the concentrated waste water detected by the meter 32, the opening degree of the waste water flow rate adjustment valve 31 is adjusted, and based on the flow rate of the concentrated reflux water detected by the reflux water flow meter 34, the reflux water flow rate adjustment valve and a concentrated water flow control unit 35 that adjusts the opening degree of 33 .

The flow rate control of the concentrated waste water and the flow rate control of the concentrated reflux water by the concentrated water flow rate control mechanism 30 will be described in detail below.

(Flow rate control of concentrated wastewater)
The concentrated water flow rate control unit 35 determines the set flow rate of the concentrated waste water 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 waste water. The opening degree of the drainage flow control valve 31 is adjusted so that the value 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 the concentrated waste water is as small as possible. However, as will be described later, the flow rate of concentrated water flowing through the concentrated water line 3 is adjusted so as not to fall below the minimum amount of concentrated water. flow rate increases. As a result, when the concentration of impurities in the raw water increases, scaling, in which impurities (especially silica or calcium) are deposited on the membrane surface of the RO membrane or NF membrane of the filtering means 11, tends to occur. Therefore, the flow rate of the concentrated wastewater is set so that the recovery rate is maximized in a range where the concentration of impurities in the concentrated water does not exceed the solubility, that is, the recovery rate is maximized in a range where silica or calcium as impurities does not precipitate. is set to

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

Specifically, first, the theoretical recovery rate at which silica precipitates at the detected water temperature (hereinafter referred to as "silica precipitation recovery rate") and the theoretical recovery rate at which calcium (calcium carbonate) precipitates at the detected water temperature The recovery rate of (hereinafter referred to as "calcium deposition recovery rate") is calculated. Methods for calculating the silica precipitation recovery rate and the calcium precipitation recovery rate will be described later. Next, the silica deposition recovery rate and the calcium deposition recovery rate are compared, and the smaller deposition recovery rate is set as the target recovery rate. Then, based on this target recovery rate and the flow rate of the permeated water detected by the permeated water flow meter 22, the target flow rate of the concentrated waste water is calculated and set by the following equation (1).
(Target flow rate of concentrated wastewater) =
(Detected flow rate of permeated water/Target recovery rate) - (Detected flow rate of permeated water) (1)

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

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

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

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

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

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

The recovery rate, which is the ratio of the permeate flow rate to the sum of the permeate flow rate and the concentrated wastewater flow rate, is the ratio of the concentrated water flow rate to the sum of the permeate flow rate and the concentrated wastewater flow rate. It can be expressed as a magnification. That is, the recovery rate Y can be expressed by the following formula (4), where N is the allowable concentration factor.
Y=(N−1)/N (4)

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 permeated water) / (Allowable concentration ratio - 1) (1')
N _S =C _S /F _S (2′)
N _C =C _C /F _C (3′)
Here, NS is the permissible concentration ratio corresponding to the precipitation recovery rate of _calcium , and _NC is the permissible concentration ratio corresponding to the precipitation recovery rate of silica.

The method of calculating the precipitation recovery rate of silica and calcium and the method of calculating the set flow rate of concentrated wastewater may be used when there are restrictions on the recovery rate and flow rate in advance due to restrictions on device design such as the capacity of the pressurizing pump and the flow rate of raw water. is not limited to the above. In addition, a preset target flow rate of permeate can be used to calculate the set flow rate of concentrated wastewater. It is not preferable because the recovery rate of may deviate from the target recovery rate. That is, if the actual permeate flow rate is greater than the target flow rate, scaling will occur if the actual recovery exceeds the target recovery rate, or if the actual permeate flow rate is less than the target flow rate. If the actual recovery rate falls below the target recovery rate, it becomes impossible to save water.

Therefore, as described above, it is preferable to use the detected flow rate of the permeated water by the permeated water flow meter 12 for calculating the set flow rate of the concentrated waste water. As a result, even if the flow rate control of the permeated water is not appropriately performed, it is possible to prevent the actual recovery rate from deviating from the target recovery rate. In actual calculation, it is preferable to use an average flow rate for a predetermined detection time or a predetermined number of times of detection in order to minimize the influence of variations in the detected flow rate of permeated water.

However, if the flow rate of the permeate is not stable and the variation in the detected flow rate is very large, such as when starting up the device or restarting operation, the permeate Using the target flow rate of, the set flow rate of the concentrated waste water may be calculated. 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 and the actual flow rate of the permeated water. That is, if the difference is within a predetermined range, the target flow rate may be used for calculation, and if the difference is outside the predetermined range, the actual flow rate may be used for calculation.

When performing recovery rate control as described above, it is preferable to use an electric proportional control valve as the waste water flow rate control valve 31 . As a result, the degree of opening can be finely adjusted according to the resolution of the electric proportional control valve, and the recovery rate can be adjusted more smoothly than when adjusting the degree of opening in a stepwise manner using a combination of solenoid valves. For example, in a staged formula that can only control the recovery rate in the range of 50 to 70% in 5 stages (50%, 55%, 60%, 65%, 70%), when the target recovery rate is set to 64%, The recovery rate can only be adjusted to 60%, resulting in useless concentrated waste water. Therefore, the use of an electric proportional control valve as the waste water flow rate adjustment valve 31 can reduce waste of such concentrated waste water, which is advantageous from the viewpoint of water saving.

However, when an electric proportional control valve is used as the waste water flow rate adjustment 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 waste water by the concentrated water flow control unit 35. be. For example, if the two speeds are significantly different, hunting may occur if the set flow rate of the concentrated waste water is changed before the electric proportional control valve is completely opened and closed and the flow rate of the concentrated waste water is stabilized. . In addition, since the set flow rate of the concentrated waste water is determined based on the flow rate of the permeated water detected by the permeated water flow meter 22, the flow rate control of the concentrated waste water also depends on the response speed of the inverter that controls the rotation speed of the pressure pump 21. may be affected. Therefore, it is preferable to consider the opening/closing speed of the electric proportional control valve and the response speed of the inverter when determining the calculation speed of the set flow rate of the concentrated waste water by the concentrated water flow control unit 35 . That is, it is preferable to slow the response speed of the inverter when the opening/closing speed of the electric proportional control valve is slow, and to increase the response speed of the inverter when the opening/closing speed of the electric proportional control valve is fast. In this embodiment, as described above, the pressure reducing valve 12 is installed so that the flow rate control of the permeated water and the flow rate control of the concentrated water are performed independently. can be done. As a result, the occurrence of hunting as described above can be suppressed as much as possible, and deviation of the actual recovery rate from the target recovery rate can be suppressed. Also from this point, it is preferable that the concentrated water line 3 is provided with the pressure reducing valve 12 .

(Flow rate control of concentrated reflux water)
By the way, as described above, the RO membrane or NF membrane of the filtration means 11 has a minimum amount of concentrated water (minimum flow rate of concentrated water to be flowed through the concentrated water line 3 to prevent clogging of the membrane due to fouling or scaling). stipulated. Therefore, the concentrated water flow rate control unit 35 controls the flow rate of the concentrated reflux water so that the flow rate of the concentrated water flowing through the concentrated water line 3 does not fall below this minimum amount of concentrated water. Specifically, the concentrated water flow rate control unit 35 compares the set flow rate of the concentrated waste water determined in consideration of the recovery rate with the minimum amount of concentrated water, and adjusts the set flow rate of the concentrated reflux water based on the comparison result. Then, the opening degree of the recirculated water flow control valve 33 is adjusted so that the value detected by the recirculated water flow meter 34 becomes the set flow rate. As the recirculated water flow rate adjustment valve 33, it is preferable to use an electric proportional control valve, like the waste water flow rate adjustment valve 31, in that the degree of opening can be finely adjusted.

Specifically, the set flow rate of the concentrated reflux water is determined as follows. That is, the set flow rate of concentrated waste water and the minimum amount of concentrated water are compared, and if the set flow rate of concentrated waste water is smaller than the minimum amount of concentrated water, the set flow rate of concentrated reflux water is determined by the following equation (5). is equal to or greater than the minimum concentrated water flow rate, the following formula (6) determines the set flow rate of the concentrated reflux water.
(Set flow rate of concentrated reflux water) = (Minimum amount of concentrated water) - (Set flow rate of concentrated waste water) (5)
(Set flow rate of concentrated reflux water) = 0 (6)

In this way, when the set flow rate of the concentrated water is smaller than the minimum concentrated water amount, the set flow rate of the concentrated water is equal to or greater than the minimum concentrated water amount so that the flow rate of the concentrated water flowing through the concentrated water line 3 is the minimum concentrated water amount. , the flow rate of the concentrated reflux water is controlled so that the flow rate of the concentrated water flowing through the concentrated water line 3 becomes the set flow rate of the concentrated waste water. Therefore, the flow rate of the concentrated water flowing through the concentrated water line 3 can be adjusted to an optimum flow rate based on the recovery rate and the minimum amount of concentrated water, and the discharge flow rate of the pressurizing pump 21 can be adjusted optimally. As a result, wasteful energy consumption can be suppressed and energy consumption can be minimized.

In this embodiment, the target value of the recovery rate is set higher, and in order to further save water, a scale inhibitor is added to the raw water for the purpose of increasing the precipitation recovery rate described above. can be In this case, the flow rate range of the pressure reducing valve 12 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 scale inhibitor can be done by a dosing pump.

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

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

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

As described above, in the present embodiment, one flow control unit (concentrated water flow control unit 35) adjusts the flow rates of concentrated waste water and concentrated reflux water, but these are performed by separate flow control units. It can be like this. In addition, instead of the permeated water flow rate control unit 23 and the concentrated water flow rate control unit 35 separately adjusting the flow rates of the permeated water and the concentrated water (concentrated waste water and concentrated reflux water), all flow rate adjustments are performed as one flow rate control. It may be performed by the department.

(Second embodiment)
FIG. 2 is a schematic diagram showing the configuration of a membrane filtration device according to a second embodiment of the present invention. In the following, configurations similar to those of the first embodiment are denoted by the same reference numerals in the drawings, description thereof is omitted, and only configurations different from those of the first embodiment are described.

In this embodiment, in addition to the filtering means (first filtering means) 11 of the first embodiment, another filtering means (second filtering means) 13 is provided downstream thereof. The second filtering means 13 is connected in series with the first filtering means 11 and treats permeated water separated by the first filtering means 11 as water to be treated. That is, a first permeate line (intermediate permeate line) 6 for circulating the permeated water from the first filter means 11 is connected to the upstream side of the second filtration means 13, and a second A second permeated water line 2 for passing the permeated water from the filtering means 13 of No. 2 is connected. Thereby, the membrane filtration device 10 of the present embodiment can generate treated water of better quality than the first embodiment.

A second concentrated water line 7 for circulating the concentrated water from the second filtering means 13 is connected to the second filtering means 13 . In the second filtering means 13, the permeated water from the first filtering means 11 is further separated into permeated water and concentrated water. does not need to be discharged to Therefore, from the viewpoint of saving water, the second concentrated water line 7 is arranged upstream of the pressurizing pump 21 to return all of the concentrated water separated by the second filtering means 13 to the supply line 1. 1. Alternatively, the second concentrated water line 7 may be connected to a raw water tank (not shown) provided on the supply line 1 instead of being directly connected to the supply line 1 . In the second concentrated water line 7, part or all of the concentrated water from the second filtering means 13 is discharged to the outside when cleaning the RO membrane or NF membrane of the second filtering means 13. A drain line may be connected.

The second concentrated water line 7 is provided with a manual valve 14 and a concentrated water flow meter 15 for adjusting the flow rate of the concentrated water flowing through the second concentrated water line 7 . As a result, the recovery rate of the second filtering means 13 (the sum of the flow rate of the permeated water from the second filtering means 13 and the flow rate of the concentrated water from the second filtering means 13, from the second filtering means 13 permeate flow rate) can be arbitrarily adjusted. In addition, in order to eliminate the complexity of manual adjustment of the recovery rate, instead of the manual valve 14, a proportional control valve whose opening can be adjusted based on the flow rate of the concentrated water detected by the concentrated water flow meter 15 is provided. may have been Alternatively, a constant flow valve may be provided in place of the manual valve 14 and the concentrate flow meter 15 in order to keep the recovery rate within a certain range. In this case, depending on the conditions, the pressure difference between the primary side and the secondary side of the constant flow valve may exceed the operating differential pressure range (the allowable range of the pressure difference for normal operation of the constant flow valve). However, in order to avoid this, a pressure reducing valve may be provided upstream of the constant flow valve.

In this embodiment, the permeate flow meter 22 of the permeate flow control mechanism 20 is used to supply a constant flow rate of treated water to, for example, an electrodeionization water production device connected downstream of the membrane filtration device 10. is provided in the second permeate line 2 . Therefore, the concentrated water flow rate control unit 35 needs to separately know the flow rate of the permeated water flowing through the first permeated water line 6 when calculating the set flow rate of the concentrated waste water based on the target value of the recovery rate. In this embodiment, the flow rate can be detected indirectly. That is, the concentrated water flow control unit 35 controls the measured value by the permeated water flow meter 22 (the flow rate of the permeated water from the second filtering means 13) and the measured value by the concentrated water flow meter 15 (from the second filtering means 13 The flow rate of the permeate flowing through the first permeate line 6 can be calculated from the sum of the flow rate of the concentrated water of . Also, as described above, when a constant flow valve is provided instead of the manual valve 14 and the concentrated water flow meter 15, the specified flow rate of the constant flow valve is used instead of the measured value by the concentrated water flow meter 15. , the permeate flow rate through the first permeate line 6 can be calculated. Alternatively, a flow meter (not shown) may be provided in the first permeated water line 6 to directly detect the flow rate of the permeated water from the first filtering means 11 .

In this embodiment, since it is necessary to supply raw water to the two filtering means 11 and 13 with one pressurizing pump 21, the supply pressure of the raw water to the first filtering means 11 by the pressurizing pump 21 is It becomes larger than in the first embodiment. Therefore, it is necessary to select a type of the pressure reducing valve 12 having higher pressure resistance. For example, when RO membranes having a diameter of about 20.32 cm (8 inches) are used as the two filtration means 11 and 13, respectively, the application temperature range of the first filtration means 11 is 5 to 35 ° C., and the pressure reducing valve 12 A pressure reducing valve manufactured by Yoshitake Co., Ltd. (product number: GD-200H) can be used as the valve.

In the above-described embodiment, two filtering means are connected in series, but the number of filtering means is not limited to this, and even if three or more filtering means are connected in series, good. Even in that case, the pressure reducing valve is provided in the concentrated water line connected to the most upstream filtering means among the three or more filtering means, and the permeated water separated by the most downstream filtering means is the set flow rate ( preset target flow rate). However, it is needless to say that the flow rate distribution between the permeated water and the concentrated water needs to be appropriately set and adjusted by any flow rate adjusting means in all the filtering means except for the most upstream filtering 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 is used instead of the permeate separated by the most downstream filtration means. Note that used Here, "connected in series" means that the water to be treated is sequentially treated by a plurality of filtration means, and in two adjacent filtration means, separated by the upstream filtration means It means that the permeated water is supplied to the filtering means on the downstream side as the water to be treated. Moreover, each filtering 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 (distribution side of raw water and concentrated water) and finally connected to the concentrated water line, and the secondary side (distribution side of permeated water) is It will be connected in parallel and finally connected to the permeate line.

1 feed line 2 permeate line (second permeate line)
3 Concentrated water line (first concentrated water line)
4 drainage line 5 reflux water line 6 first permeated water line 7 second concentrated water line 10 membrane filtration device 11 filtration means (first filtration means)
12 Pressure reducing valve 13 Second filtering means 14 Manual valve 15 Concentrated water flow meter 20 Permeated water flow control mechanism 21 Pressurizing pump 22 Permeated water flow meter 23 Permeated water flow control unit 30 Drainage flow control mechanism 31 Drainage flow rate adjustment valve 32 Drainage Flow meter 33 Reflux water flow control valve 34 Reflux water flow meter 35 Concentrated water flow controller

Claims (6)

Filtration means having a reverse osmosis membrane or nanofiltration membrane for separating the water to be treated into permeated water and concentrated water;
a supply line connected to the filtering means for supplying water to be treated to the filtering means;
a permeated water line connected to the filtering means for circulating the permeated water from the filtering means;
a concentrated water line connected to the filtering means and for circulating the concentrated water from the filtering means;
a drain line branching from the concentrated water line and discharging a part of the concentrated water flowing through the concentrated water line to the outside;
a reflux water line that is branched from the concentrated water line and connected to the supply line to return the rest of the concentrated water flowing through the concentrated water line to the supply line;
a first flow rate control means for adjusting the flow rate of the permeate flowing through the permeate line to a set flow rate;
a second flow control means for adjusting the flow rate of the concentrated water flowing through the drainage line to a set flow rate and adjusting the flow rate of the concentrated water flowing through the return water line to a set flow rate;
The second flow rate control means includes a first flow rate adjustment valve provided in the drainage line, a first flow rate detection means for detecting a flow rate of concentrated water flowing through the drainage line, and a return water line provided in the return water line. based on the flow rate of the concentrated water detected by the second flow control valve, the second flow rate detecting means for detecting the flow rate of the concentrated water flowing through the recirculated water line, and the first flow rate detecting means; , control to adjust the opening degree of the first flow rate adjustment valve and adjust the opening degree of the second flow rate adjustment valve based on the flow rate of the concentrated water detected by the second flow rate detection means; and
The control unit of the second flow control means controls the ratio of the flow rate of the permeate flowing through the permeate line to the sum of the flow rate of the permeate flowing through the permeate line and the flow rate of the concentrated water flowing through the drain line. The set flow rate of the concentrated water flowing through the drainage line is determined so that a certain recovery rate becomes a predetermined value, and the determined set flow rate and the clogging of the reverse osmosis membrane or nanofiltration membrane of the filtration means compared with the minimum flow rate of concentrated water that should flow through the concentrated water line so as not to occur, and if the determined set flow rate is smaller than the predetermined minimum flow rate, the A set flow rate is determined to be a value obtained by subtracting the determined set flow rate from the predetermined minimum flow rate, and if the set flow rate is equal to or greater than the predetermined minimum flow rate, the concentrated water flowing through the reflux water line to zero the set flow rate of the membrane filtration device. a plurality of filtering means connected in series, each filtering means having a reverse osmosis membrane or a nanofiltration membrane for separating the water to be treated into permeated water and concentrated water;
a supply line connected to the most upstream filtering means among the plurality of filtering means and supplying water to be treated to the most upstream filtering means;
a permeated water line connected to the most downstream filtering means among the plurality of filtering means and for circulating the permeated water from the most downstream filtering means;
an intermediate permeate line connected to the most upstream filtering means and for circulating the permeated water from the most upstream filtering means;
a concentrated water line connected to the most upstream filtering means and for circulating the concentrated water from the most upstream filtering means;
a drainage line branching from the concentrated water line and discharging a part of the concentrated water flowing through the concentrated water line to the outside;
a reflux water line that is branched from the concentrated water line and connected to the supply line to return the rest of the concentrated water flowing through the concentrated water line to the supply line;
a first flow rate control means for adjusting the flow rate of the permeate flowing through the permeate line to a set flow rate;
a second flow control means for adjusting the flow rate of the concentrated water flowing through the drainage line to a set flow rate and adjusting the flow rate of the concentrated water flowing through the return water line to a set flow rate;
The second flow rate control means comprises a first flow rate control valve provided in the drainage line, a second flow rate adjustment valve provided in the recirculated water line, and a flow rate of the concentrated water flowing through the drainage line. a first flow rate detecting means for detecting; a second flow rate detecting means for detecting a flow rate of the concentrated water flowing through the recirculated water line; and based on the flow rate of the concentrated water detected by the first flow rate detecting means. , control to adjust the opening degree of the first flow rate adjustment valve and adjust the opening degree of the second flow rate adjustment valve based on the flow rate of the concentrated water detected by the second flow rate detection means; and
The control unit of the second flow control means controls the flow rate of the permeate flowing through the intermediate permeate line with respect to the sum of the flow rate of the permeate flowing through the intermediate permeate line and the flow rate of the concentrated water flowing through the drain line. The set flow rate of the concentrated water flowing through the drainage line is determined so that the recovery rate, which is a ratio, has a predetermined value, and the determined set flow rate and the reverse osmosis membrane of the most upstream filtration means or comparing with the minimum flow rate of the concentrated water that should flow through the concentrated water line to prevent clogging of the nanofiltration membrane, and if the determined set flow rate is smaller than the predetermined minimum flow rate, the reflux water line; determining the set flow rate of the concentrated water flowing through to a value obtained by subtracting the determined set flow rate from the predetermined minimum flow rate, and if equal to or greater than the predetermined minimum flow rate, the return A membrane filtration device, wherein the set flow rate of the concentrated water flowing through the running water line is determined to be zero. water temperature detection 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 rate control means detects the recovery rate based on the water temperature detected by the water temperature detection means, silica or silica on the membrane surface of the reverse osmosis membrane or nanofiltration membrane of the filtration means. The membrane filtration device according to claim 1, wherein said set flow rate of concentrated water flowing through said drainage line is determined so as to achieve a maximum recovery rate at which calcium is not precipitated. Water temperature detection means for detecting the water temperature of any one of the water to be treated supplied to the most upstream filtering means, the permeated water from the most upstream filtering means, and the concentrated water from the most upstream filtering means. has
The control unit of the second flow rate control means determines that the recovery rate is the reverse osmosis membrane or nanofiltration membrane of the most upstream filtration means based on the water temperature detected by the water temperature detection means. 3. The membrane filtration device according to claim 2, wherein said set flow rate of concentrated water flowing through said drainage line is determined so as to achieve a maximum recovery rate at which silica or calcium is not precipitated on the surface. 5. The pressure reducing valve according to any one of claims 1 to 4, which is provided in said concentrated water line and has a pressure reducing valve that reduces the pressure of the concentrated water flowing through said concentrated water line and keeps the pressure on the secondary side constant. Membrane filtration device. The first flow rate control means is provided in the supply line, pressure adjustment means for adjusting the pressure of the water flowing through the supply line, and flow rate detection means for detecting the flow rate of the permeated water flowing through the permeate line. 6. The membrane filtration device according to any one of claims 1 to 5, further comprising: a controller for controlling said pressure adjusting means based on the flow rate of said permeated water detected by said flow rate detecting means.

Images