JP2010120015A - Method of membrane filtration - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To achieve a method of membrane filtration which can maintain the amount and quality of permeable water. <P>SOLUTION: In the method of membrane filtration for separating the permeable water and condensed water, and discharging the condensed water by flowing feed water containing dissolved salts with a pump 7 into a filtering membrane module 4 having a reverse osmosis membrane or a nanofiltration membrane, the rotation number of the pump 7 is controlled so that an amount of desirable water may be obtained, and the recovery ratio is adjusted based on the electric conductivity of the feed water and/or the permeable water. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a membrane filtration method in which feed water containing dissolved salts is introduced into a filtration membrane module having a reverse osmosis membrane or a nanofiltration membrane by a pump, separated into permeated water and concentrated water, and the concentrated water is discharged.

  There exists a membrane filtration system which has a filtration membrane part which filters impurities contained in feed water, for example, dissolved salts, as a water treatment system for feed water to equipment (for example, refer to patent documents 1). In this membrane filtration system, feed water flows from one side of the filtration membrane section, and impurities contained in the feed water are captured. And the permeated water which flowed out from the other side of the said filtration membrane part is supplied to the said apparatus.

  By the way, from the other side of the filtration membrane part, concentrated water flows out in addition to the permeated water. In the membrane filtration system, there is a configuration (cross flow filtration) in which only a part of the concentrated water is drained and the remainder is returned to the upstream side of the filtration membrane unit in order to effectively use water.

Japanese Patent Laid-Open No. 5-220480

  In general, in the membrane filtration system method configured as described above, the quality of the permeated water is affected by the concentration of impurities contained in the water supply. Further, if impurities are excessively concentrated in the vicinity of the surface of the filtration membrane, the filtration membrane is clogged due to phenomena such as fouling and scaling. Here, fouling refers to a phenomenon in which suspended substances, colloids, organic substances, etc. in water are deposited or adsorbed on the membrane surface. Scaling is the concentration of dissolved salts dissolved in water beyond the solubility. This means a phenomenon that deposits and deposits on the film surface. Furthermore, when the filtration membrane is used for a long time, the polymer forming the filtration membrane is gradually oxidized by an oxidizing substance contained in the water supply (for example, sodium hypochlorite which is a bactericide). In some cases, the filter membrane may deteriorate over time due to decomposition. In the membrane filtration system adapted to perform cross-flow filtration, the concentration of the oxidizing substance gradually increases in the vicinity of the surface of the filtration membrane, and deterioration of the filtration membrane is likely to proceed. This deterioration usually acts on the side of enlarging the pores of the filtration membrane, making it difficult to prevent impurities.

When the concentration of impurities contained in the water supply to the filtration membrane increases and the concentration of impurities increases near the surface of the filtration membrane, the amount of impurities leaking into the permeated water increases, and the quality of the permeated water deteriorates. Arise. Therefore, in the membrane filtration method, the drainage amount of concentrated water is set to an amount that does not cause excessive concentration near the surface of the filtration membrane, thereby maintaining the removal rate of impurities within a predetermined range, It is necessary to suppress deterioration of water quality. In addition, when the filtration membrane is clogged in the filtration membrane portion, the permeate flux of water is lowered, so that the amount of permeate is lowered. Therefore, in the membrane filtration system, it is necessary to prevent fouling and scaling and suppress a decrease in the amount of permeated water by setting the drainage amount of concentrated water to an amount that does not cause excessive concentration near the surface of the filtration membrane. There is. Further, when the filtration membrane deteriorates in the filtration membrane portion, it becomes difficult to prevent impurities. Therefore, when the concentration of impurities on the water supply side increases, the amount of impurities leaking into the permeate increases and the quality of the permeate deteriorates. Arise. Therefore, in the membrane filtration method, the drainage amount of concentrated water is set to an amount that does not cause excessive concentration near the surface of the filtration membrane, thereby maintaining the removal rate of impurities within a predetermined range, It is necessary to suppress deterioration of water quality.

  However, the quality and temperature of the water supplied to the filtration membrane, especially the water supplied from the groundwater, varies depending on local factors, seasonal factors, factors of the water treatment condition of the water supplied to the filtration membrane. . Further, the solubility of dissolved salts contained in the water supply, such as silica and hardness components, varies depending on the water quality and the water temperature. Therefore, in the conventional membrane filtration system, the permeated water is maintained at a predetermined water quality regardless of the quality of the feed water, fouling and scaling are prevented, and further, the impurity removal rate is maintained within a predetermined range. The amount of concentrated water drainage was set assuming the worst conditions of water temperature. For this reason, there existed a problem that the amount of drainage increased and it was difficult to aim at effective utilization of water.

  The problem to be solved by the present invention is to realize a membrane filtration method capable of maintaining the amount of permeate and the quality of the permeate.

  The present invention has been made to solve the above-mentioned problems, and the invention according to claim 1 is characterized in that a feedwater containing dissolved salts is caused to flow into a filtration membrane module having a reverse osmosis membrane or a nanofiltration membrane by a pump. A membrane filtration method that separates water into concentrated water and discharges the concentrated water, and controls the number of rotations of the pump so as to obtain a target amount of permeated water, and also conducts electric conduction of water supply and / or permeated water. It is characterized by adjusting the recovery rate based on the degree.

  In the first aspect of the present invention, the target permeated water amount is obtained by controlling the number of revolutions of the pump, and the recovery rate is adjusted based on the electric conductivity of the feed water and / or the permeated water, and the permeated water amount and The quality of the permeated water is maintained.

  When the electrical conductivity of the feed water or the permeated water is increased in the first aspect of the invention, the drainage amount of the concentrated water is increased while the electrical conductivity of the feed water or the permeated water is decreased. It is characterized by reducing the amount of concentrated water discharged.

  In the second aspect of the present invention, the quality of the permeated water is maintained by adjusting the recovery rate by increasing or decreasing the drainage amount of the concentrated water based on the increase or decrease of the electrical conductivity of the feed water or the permeated water.

  Furthermore, the invention according to claim 3 is the invention according to claim 1, wherein the residual rate of dissolved salt of the permeated water is obtained from the electrical conductivity of the feed water and the permeated water, and the amount of concentrated water discharged is increased according to the residual rate of the dissolved salt. Or it is characterized by decreasing.

  In the invention according to claim 3, the residual rate of the dissolved salt in the permeated water is obtained from the electric conductivity of the feed water and the permeated water, and the amount of concentrated water discharged is increased or decreased according to the residual rate of the dissolved salt, and the recovery rate is increased. Adjust to maintain the quality of the permeate.

  According to this invention, the amount of permeated water and the quality of permeated water can be maintained.

It is a schematic explanatory drawing which shows the membrane filtration system for implementing 1st Example of this invention. It is a schematic explanatory drawing which shows the membrane filtration system for implementing 2nd Example of this invention. It is a flowchart which shows an example of the process of the control part in 2nd Example. It is a flowchart which shows an example of the process of the control part in 2nd Example. It is a flowchart which shows an example of the process of the control part in 2nd Example.

  Hereinafter, the best mode for carrying out the operation method of the membrane filtration system according to the present invention will be described.

(First embodiment)
First, the first embodiment will be described. The operation method of the membrane filtration system according to the first embodiment includes a filtration membrane section provided on a water supply line to equipment, a drain line for draining concentrated water from the filtration membrane section, and an upstream of the filtration membrane section. It is implemented in a membrane filtration system having a circulating water line connecting the water supply line on the side.

  The said filtration membrane part filters the impurity contained in feed water with a filtration membrane. Examples of the filtration membrane include a reverse osmosis membrane (RO membrane) and a nanofiltration membrane (NF membrane). The reverse osmosis membrane is a liquid separation membrane capable of filtering out ions having a molecular weight of about several tens. The nanofiltration membrane is a liquid separation membrane that can prevent permeation of particles and polymers (substances having a maximum molecular weight of about several hundreds) smaller than about 2 nm. In terms of filtration function, the nanofiltration membrane is an ultrafiltration membrane. It has a function located between the reverse osmosis membrane (a membrane capable of filtering out a substance having a molecular weight of about 1,000 to 300,000) and the reverse osmosis membrane.

  In the membrane filtration system, a part of the concentrated water from the filtration membrane part is drained from the drainage line, and the remaining part is returned to the upstream side of the filtration membrane part through the circulating water line. That is, the membrane filtration system performs cross flow filtration.

  In the membrane filtration system, the water temperature of any one of the water supply to the filtration membrane unit, the permeated water from the filtration membrane unit and the concentrated water from the filtration membrane unit, or the quality of the water supply to the filtration membrane unit Based on this, the amount of concentrated water discharged from the drain line is adjusted. For example, when the temperature of any one of feed water, permeated water and concentrated water changes, the drainage amount of the concentrated water is increased or decreased within a range where concentration higher than the solubility of various impurities at the water temperature does not occur, and the filtration membrane is clogged. By preventing this, while ensuring a predetermined amount of permeated water, the quality of the permeated water is maintained within a predetermined range by reducing the impurity concentration in the vicinity of the surface of the filtration membrane. Further, for example, when the quality of the feed water is deteriorated, the drainage amount of concentrated water is increased so as not to decrease the impurity removal rate, and the filter membrane is prevented from being clogged, thereby ensuring a predetermined amount of permeated water, and By reducing the impurity concentration in the vicinity of the surface of the filtration membrane, the quality of the permeated water is maintained within a predetermined range. Conversely, when the quality of the permeated water is improved, the removal rate of impurities is unlikely to decrease, so the amount of concentrated water discharged is reduced so that water can be used effectively. Here, the water quality of the feed water can include hardness, silica, suspended solids, and the like, which are likely to cause clogging of the filtration membrane, and electrical conductivity capable of comprehensively grasping the water quality. it can.

  As described above, according to the operation method of the membrane filtration system of the first embodiment, it is possible to prevent a decrease in the amount of permeated water and a deterioration in the quality of the permeated water, and to prevent the drainage of concentrated water more than necessary. Can do. As a result, it is possible to save water while stably using the membrane filtration system for a long period of time.

(Second embodiment)
Next, a second embodiment will be described. The operation method of the membrane filtration system of the second embodiment is the same as the first embodiment, the filtration membrane part provided on the water supply line to the equipment, and the drainage for draining the concentrated water from this filtration membrane part It implements in the membrane filtration system which has a circulating water line which connects a line and the said water supply line of the upstream of the said filtration membrane part. About the structure of this membrane filtration system, the description of said 1st embodiment is used and the description is abbreviate | omitted.

  In the membrane filtration system according to the second embodiment, the drainage amount of the concentrated water from the drainage line is adjusted based on the clogging state of the filtration membrane in the filtration membrane unit or the degradation state of the filtration membrane. Here, the clogged state or the deteriorated state of the filtration membrane is, for example, comparing the permeate flux in the initial stage of operation (hereinafter referred to as “permeate flux”) and the permeate flux after a predetermined time of operation. Judge with. In addition, the clogged state or the deteriorated state of the filtration membrane may be, for example, the pressure difference between the water supply side and the permeate side of the filtration membrane portion in the initial operation, and the water supply side and the permeate side of the filtration membrane portion after a predetermined time operation. Judgment is made by comparing the pressure difference. When it is determined that clogging or deterioration of the filtration membrane has occurred, the drainage amount of concentrated water is increased, and the progress of clogging or deterioration of the filtration membrane is suppressed, thereby ensuring a predetermined amount of permeated water and permeation. Maintain the water quality within the specified range.

  Further, the deterioration state of the filtration membrane can be judged from, for example, the amount of impurities remaining by monitoring the quality of the permeated water. Further, the deterioration state of the filtration membrane can be determined by, for example, comparing the quality of the feed water with the quality of the permeated water and determining the residual rate of impurities. When it is determined that the filtration membrane has deteriorated, the amount of concentrated water discharged is increased so as not to reduce the impurity removal rate, and the concentration of impurities near the surface of the filtration membrane and the oxidation that degrades the filtration membrane are increased. By reducing the concentration of the active substance, the quality of the permeated water is maintained within a predetermined range, and the progress of deterioration of the filtration membrane is suppressed. Here, the water quality of the feed water and the permeated water can be exemplified by electric conductivity capable of comprehensively grasping the water quality, and the concentration of residual chlorine (oxidizing substance concentration) that easily causes deterioration of the filtration membrane. .

  As described above, according to the operation method of the membrane filtration system of the second embodiment, the amount of permeated water and the quality of the permeated water can be maintained. As a result, the membrane filtration system can be used stably for a long period of time.

(First Example)
Hereinafter, specific embodiments of the present invention will be described in detail with reference to the drawings. First, a first embodiment of the present invention will be described. FIG. 1 is a schematic explanatory view showing a membrane filtration system for carrying out the first embodiment of the present invention.

  In FIG. 1, a membrane filtration system 1 performs water treatment of raw water supplied from a raw water tank (not shown) in which tap water, industrial water, groundwater, etc. are stored, and supplies this water to the boiler 2 as feed water. Is. The membrane filtration system 1 includes a water supply line 3 to the boiler 2, and further includes a filtration membrane unit 4 and a deaeration membrane unit 5 connected to the water supply line 3 in this order from the upstream side. The membrane filtration system 1 includes a water supply tank 6 that stores water supply, and a pump 7 that is provided on the upstream side of the filtration membrane unit 4 and supplies the water supply to the filtration membrane unit 4.

  Here, the said filtration membrane part 4 is provided with the nanofiltration membrane. This nanofiltration membrane is a polyamide-based, polyether-based, etc. synthetic polymer membrane, and is a liquid separation membrane that can prevent the passage of particles and polymers (substances having a maximum molecular weight of several hundreds) smaller than about 2 nm. It is. The nanofiltration membrane is usually configured as a filtration membrane module. Examples of the form of the filtration membrane module include a spiral module, a hollow fiber module, and a flat membrane module.

  Further, the membrane filtration system 1 includes temperature sensors 8, 8 ′, 8 ″ connected to any position of the balloons A, A ′, A ″ and a water quality sensor 9 connected to the position of the balloon B. And have. The water quality sensor 9 is a measuring device for obtaining water quality information of the water supplied to the filtration membrane unit 4, specifically, an electrical conductivity sensor for measuring the electrical conductivity of the water supply, and the hardness included in the water supply. One or more types of sensors selected from a hardness sensor that measures the concentration of water, a silica sensor that measures the concentration of silica contained in the water supply, and a turbidity sensor that measures the concentration of suspended solids contained in the water supply. Further, the membrane filtration system 1 has a control unit 10 that opens and closes each drain valve described later based on the detected values of the temperature sensors 8, 8 ′, 8 ″ and the water quality sensor 9.

  Here, for the hardness sensor or the silica sensor, for example, a colorimetric sensor that detects the hardness component or the concentration of silica by color development when a reagent containing a dye is added is used. In this colorimetric sensor, a reagent is added to a transparent container containing a predetermined amount of sample water, and the hue change of the sample water due to the reaction of the dye is measured from the absorbance when irradiated with light of a specific wavelength. Based on the absorbance, the hardness in the sample water and the concentration of silica are determined. Further, as the turbidity sensor, for example, a transmitted light sensor that determines the turbidity of the sample water from the transmitted light intensity or a scattered light sensor that determines from the scattered light intensity is used.

  Now, the feed water sent out from the pump 7 flows into one side of the filtration membrane part 4. The feed water that has flowed into the filtration membrane 4 is adapted to capture corrosion-promoting components and permeate corrosion-inhibiting components by the nanofiltration membrane.

  Here, the corrosion promoting component and the corrosion inhibiting component will be described. First, when the boiler 2 is a once-through boiler, the corrosion promoting component is a portion where corrosion of a plurality of heat transfer tubes (not shown) made of non-passivated metal in the once-through boiler is likely to occur, in particular, moisture ( Here, boiler water is in contact with it and acts on the inner surface of the heat transfer tube heated from the outside to promote corrosion, and usually contains sulfate ions, chloride ions and other components. . Incidentally, both sulfate ions and chloride ions are important as corrosion promoting components. By the way, JIS B8223: 1999 prescribes | regulates the various management items and recommended standard regarding the water quality of boiler water in these boilers from a viewpoint of suppressing the corrosion of the special circulation boiler containing the said once-through boiler. This regulation provides a reference value for the chloride ion concentration of boiler water. On the other hand, although the sulfuric acid ion concentration in the boiler water is not mentioned, the applicant of the present application has confirmed that the sulfuric acid ions contained in the boiler water act on the heat transfer tube as a corrosion promoting component. .

  Next, the corrosion-inhibiting component is a portion where the corrosion of the heat transfer tube of the boiler 2 is likely to occur, in particular, the inner surface of the heat transfer tube that is in contact with moisture (here, boiler water) and is heated from the outside. It usually has silica (i.e., silicon dioxide). By the way, the silica contained in the water supply is generally recognized as a scale generating component in the heat transfer tube, and it is considered preferable to suppress its concentration as much as possible. However, the applicant of the present application has confirmed that silica contained in boiler water acts on the heat transfer tube and the like as a corrosion inhibiting component. Here, silica is a component usually contained in tap water, industrial water, groundwater, and the like used as water supply.

From the other side of the filtration membrane part 4, permeated water containing a large amount of corrosion inhibiting components and concentrated water containing a large amount of corrosion promoting components are separated and flow out. The permeated water flows through the water supply line 3 and is stored in the water supply tank 6. On the other hand, a part of the concentrated water is drained from the drainage line 11, and the remaining part flows through the circulating water line 12 connecting the drainage line 11 and the water supply line 3 upstream of the pump 7. 7 is returned to the upstream side.

  The drainage line 11 is branched to a first drainage line 13, a second drainage line 14, and a third drainage line 15 on the downstream side of the connection location of the circulating water line 12. Each drain line 13, 14, 15 is provided with a first drain valve 16, a second drain valve 17, and a third drain valve 18, respectively.

  Here, each of the drain valves 16, 17, 18 includes a constant flow valve mechanism (not shown). This constant flow valve mechanism is set to a different flow rate value in each of the drain valves 16, 17, and 18. The amount of drainage from each of the drain valves 16, 17, 18 is set as follows, for example. That is, the drainage amount when only the first drain valve 16 is opened is set so that the recovery rate is 95%, and the drainage amount when only the second drain valve 17 is opened is the recovery rate. The rate is set to 90%, and the drainage amount when only the third drain valve 18 is opened is set so that the recovery rate is 80%. Here, the recovery rate refers to the ratio of the amount of permeated water to the sum of the amount of permeated water from the filtration membrane unit 4 and the amount of drainage to the outside of the system. In the following, the amount of drainage is described as the ratio of the amount of drainage to the sum of the amount of permeate and the amount of drainage. For example, the drainage amount when only the first drain valve 16 is opened, that is, the drainage amount when the recovery rate is 95% is referred to as 5% drainage, and when only the second drain valve 17 is opened. The amount of drainage, that is, the amount of drainage when the recovery rate is 90% is called 10% drainage, and the amount of drainage when only the third drain valve 18 is opened, that is, the amount of drainage when the recovery rate is 80%, is drained by 20%. It is said.

  The degassing membrane unit 5 includes a gas permeable membrane module (not shown) having a large number of gas permeable membranes, and a water-sealed type that vacuums the dissolved gas in the feed water, specifically, dissolved oxygen through the gas permeable membrane module. And a vacuum pump (not shown).

  The control unit 10 is connected to the pump 7 and the drain valves 16, 17, 18 through signal lines 19, 19,..., And is connected to the control unit 10 through signal lines or communication lines (not shown). Temperature sensors 8, 8 ′, 8 ″ and the water quality sensor 9 are connected. The control unit 10 is based on detection values of the temperature sensors 8, 8 ′, 8 ″ and the electrical conductivity sensor 9. The drain valves 16, 17, and 18 are controlled to open and close, and the rotation speed of the pump 7 is controlled based on the detection value of the temperature sensor 8 so that the target permeated water amount is always obtained.

  Next, an operation method of the membrane filtration system 1 will be described. In the membrane filtration system 1, water is supplied to the filtration membrane unit 4 by the pump 7, and corrosion promoting components are filtered by the nanofiltration membrane (not shown). The permeated water that has been filtered through the nanofiltration membrane contains a corrosion inhibiting component. Next, the dissolved oxygen in the permeated water containing the corrosion inhibiting component is degassed by the degassing membrane unit 5, and this water is stored in the water supply tank 6 as water supplied to the boiler 2.

A part of the concentrated water from the filtration membrane part 4 is drained from the drain line 11, and the remaining part is returned to the upstream side of the pump 7 through the circulating water line 12. The controller 10 controls the opening and closing of the drain valves 16, 17, 18 based on the detected values of the temperature sensors 8, 8 ', 8 "and the water quality sensor 9 to adjust the drainage amount of the concentrated water. Specifically, when the detected value of the temperature sensor 8, 8 ', 8 "changes, concentration over the solubility of various impurities (for example, calcium carbonate and silica) in the water supply at that water temperature does not occur. By increasing / decreasing the drainage amount of the concentrated water within a range and preventing clogging of the nanofiltration membrane, the predetermined permeated water amount is ensured, and by reducing the impurity concentration in the vicinity of the surface of the nanofiltration membrane, Maintain water quality within a specified range. Further, when the water quality sensor 9 detects an increase in concentration of, for example, hardness, silica, suspended matter, etc., the drainage amount of the concentrated water is increased so as not to reduce the removal rate of impurities, and the nanofiltration membrane By preventing clogging, a predetermined amount of permeated water is secured, and by reducing the impurity concentration in the vicinity of the surface of the nanofiltration membrane, the quality of the permeated water is maintained within a predetermined range. Conversely, when the water quality sensor 9 detects a decrease in the concentration of, for example, hardness, silica, suspended matter, etc., the removal rate of impurities is unlikely to decrease. Try to improve efficiency.

  The drainage amount of the concentrated water can be adjusted in stages by opening and closing the drain valves 16, 17, and 18, respectively. For example, by setting only the second drain valve 17 to an open state and closing the first drain valve 16 and the third drain valve 18, 10% drainage can be achieved (that is, a recovery rate of 90%). ). Further, for example, the first drain valve 16 and the second drain valve 17 are opened, and only the third drain valve 18 is closed, so that 15% drainage can be achieved (that is, the recovery rate is 85). %). Accordingly, the drainage amount of the concentrated water can be adjusted in steps of 5% from 5% drainage to 35% drainage by the combination of the drain valves 16, 17, and 18, in other words, the recovery rate. Can be adjusted in steps of 5% from 65% to 95%.

(Second embodiment)
Next, a second embodiment of the present invention will be described. FIG. 2 is a schematic explanatory view showing a membrane filtration system for carrying out the second embodiment of the present invention.

  The basic configuration of the membrane filtration system 20 in the second embodiment shown in FIG. 2 is the same as the membrane filtration system 1 in the first embodiment, and the same components are denoted by the same reference numerals and the description thereof is omitted. Omitted.

  The filtration membrane system 20 is connected to the positions of the temperature sensors 8, 8 ', 8 "and balloons B, B', B" connected to any position of the balloons A, A ', A ". A plurality of water quality sensors 9, 9 ', 9 ", a flow rate sensor 21 connected to the position of the balloon C, and a plurality of pressure sensors 22, connected to the positions of the balloons D, D', D". 22 ', 22 ". The temperature sensors (8, 8 ′, 8 ″), the water quality sensors (9, 9, 9 ″), the flow rate sensor 21 and the pressure sensors (22, 22 ′, 22 ″) are signal lines and communication The membrane filtration system 20 is connected to the nanofiltration membrane (not shown) in the filtration membrane section 4 when clogged or deteriorated. It has a reporting means, for example, a notification lamp or buzzer (not shown).

  The operation method of the membrane filtration system 20 is different from the operation method of the membrane filtration system 1 of the first embodiment in that the amount of concentrated water discharged is adjusted based on the clogged or degraded state of the nanofiltration membrane. It is a point. This will be specifically described below.

The control unit 10 determines whether the nanofiltration membrane is clogged or deteriorated, and performs processing from step S1 to step S3 shown in FIG. 3 in order to adjust the drainage amount of concentrated water. That is, in step S1, the control unit 10 is based on the detected values of the temperature sensors 8, 8 ', 8 ", the detected values of the flow sensor 21, and the detected values of the pressure sensors 22, 22', 22". Then, the calculation for obtaining the permeation flux for the permeated water in operation is performed, and the process proceeds to step S2. In step S2, the control unit 10 compares the permeation flux obtained in step S1 with the permeation flux calculated in advance and stored, and whether the nanofiltration membrane is clogged or deteriorated. Judge whether or not. When an opening exceeding an allowable value occurs with respect to the permeation flux in the initial stage of operation, it is determined that the nanofiltration membrane is clogged or deteriorated (YES in step S2), and the process proceeds to step S3. In step S3, the control unit 10 controls the opening and closing of the drain valves 16, 17, and 18 to increase the drainage amount of concentrated water, thereby suppressing the progress of clogging or deterioration of the nanofiltration membrane. The amount of permeated water is ensured and the quality of the permeated water is maintained within a predetermined range. At this time, the control unit 10 may report that the nanofiltration membrane is clogged or deteriorated via the reporting unit. On the other hand, in step S2, when there is no opening exceeding the permissible value with respect to the permeation flux in the initial stage of operation, the control unit 10 determines that the nanofiltration membrane is not clogged or deteriorated (in step S2). NO), and the process of step S1 is executed again.

  Further, the control unit 10 determines whether the nanofiltration membrane is clogged or deteriorated and adjusts the drainage amount of the concentrated water by performing the processes of steps S4 to S6 shown in FIG. 4 instead of the process shown in FIG. Also good. That is, in step S4, the control unit 10 determines the filtration membrane based on the detection values of the pressure sensor 22 and the pressure sensor 22 ′ or based on the detection values of the pressure sensor 22 and the pressure sensor 22 ″. The calculation for obtaining the pressure difference of the unit 4 is performed, and the process proceeds to step S5, where the control unit 10 calculates the pressure difference obtained in step S4 and the pressure difference in the initial stage of operation calculated and stored in advance. In order to determine whether the nanofiltration membrane is clogged or deteriorated, and when the opening exceeds the allowable value with respect to the pressure difference in the initial operation, the nanofiltration membrane is clogged or deteriorated. It is determined that it has occurred (YES in step S5), and the process proceeds to step S6, in which the control unit 10 opens and closes the drain valves 16, 17, and 18. Then, the drainage amount of the concentrated water is increased and the progress of clogging and deterioration of the nanofiltration membrane is suppressed to ensure a predetermined permeate amount and maintain the permeate water quality within a predetermined range. The control unit 10 may report that the nanofiltration membrane is clogged or deteriorated through the reporting means, whereas in step S5, an opening exceeding the allowable value with respect to the pressure difference at the initial stage of operation. If not, the control unit 10 determines that the nanofiltration membrane is not clogged or deteriorated (NO in step S5), and executes the process of step S4 again.

  Further, the control unit 10 determines the deterioration of the nanofiltration membrane and adjusts the drainage amount of the concentrated water, so that the processing from step S7 to step S9 shown in FIG. 5 is performed instead of the processing shown in FIG. 3 and FIG. You may go. That is, in step S7, the control unit 10 is based on the detection value of the water quality sensor 9 and the water quality sensor 9 ′ (or the water quality sensor 9 ″) or based on the detection value of the water quality sensor 9 ′ alone. The calculation for obtaining the residual rate or amount of impurities in the permeated water is performed, and the process proceeds to step 8. In step S8, the control unit 10 calculates in advance the residual rate or residual amount of impurities obtained in step S7. The residual rate or amount of impurities at the initial stage of operation stored in the memory is compared with each other to determine whether the nanofiltration membrane has deteriorated or not. It is determined that an opening exceeding the value has occurred (YES in step S8), and the process proceeds to step S9, where the control unit 10 determines that each drain valve 16, 17, 8 is controlled to open and close to increase the drainage amount of the concentrated water and suppress the progress of the deterioration of the nanofiltration membrane, thereby ensuring a predetermined amount of permeated water and maintaining the quality of the permeated water within a predetermined range. The control unit 10 may report that the nanofiltration membrane has deteriorated through the reporting unit, whereas the opening of the remaining amount or remaining amount of impurities at the initial stage of operation exceeds an allowable value. If not, the control unit 10 determines in step S8 that the nanofiltration membrane is not clogged or deteriorated (NO in step S8), and executes the process of step S7 again.

  As mentioned above, although this invention was demonstrated by the 1st Example and the 2nd Example, this invention is not restricted to the aspect implemented with each said membrane filtration system 1 and 20, of course. For example, in each of the membrane filtration systems 1 and 20, an oxidant removing unit for removing an oxidant such as a chlorine agent derived from sodium hypochlorite in the feed water with an adsorbent such as activated carbon, and a hardness in the feed water A water-softening treatment section that removes the portion with an ion exchange resin or the like may be provided on the upstream side of the filtration membrane section 4 in this order.

1,20 Membrane filtration system 4 Filtration membrane section 7 Pump 9, 9 ', 9 "Water quality sensor

Claims (3)

A membrane filtration method in which feed water containing dissolved salts is introduced into a filtration membrane module having a reverse osmosis membrane or a nanofiltration membrane by a pump, separated into permeated water and concentrated water, and drained concentrated water,
While controlling the number of revolutions of the pump so that the target permeated water amount can be obtained,
A membrane filtration method characterized by adjusting a recovery rate based on electric conductivity of water supply and / or permeated water.   The concentrated water discharge amount is increased when the electrical conductivity of the feed water or the permeated water is increased, while the drainage amount of the concentrated water is decreased when the electrical conductivity of the feed water or the permeated water is decreased. 2. The membrane filtration method according to 1. Obtain the residual rate of dissolved salts of the permeated water from the electrical conductivity of the feed water and permeated water,
The membrane filtration method according to claim 1, wherein the drainage amount of the concentrated water is increased or decreased in accordance with the residual rate of dissolved salts.

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