JP6642082B2 - Membrane separation device - Google Patents

Description

  The present invention relates to a membrane separation device.

  In the semiconductor manufacturing process, cleaning of electronic parts, cleaning of medical instruments, and the like, high-purity pure water containing no impurities is used. This type of pure water is generally produced by treating raw water such as groundwater or tap water with a reverse osmosis membrane (hereinafter also referred to as “RO membrane”).

  As an example of a water treatment system using an RO membrane, there has been proposed a system for performing flow rate feedback control for maintaining a constant flow rate of permeated water in an RO membrane irrespective of the temperature of supply water (see Patent Document 1). In the flow rate feedback control, the operating frequency of the pressurizing pump that supplies water to the RO membrane is controlled by the inverter so that the flow rate of the permeated water produced by the RO membrane becomes a target value.

  The system of Patent Literature 1 performs further additional control in addition to the flow rate feedback control (for example, “temperature feed forward control”). The temperature feedforward control aims at suppressing the precipitation of silica-based scale in the RO membrane module while maximizing the recovery rate of permeated water. Specifically, by associating the silica concentration of the feedwater, the silica solubility determined from the temperature of the feedwater, and the target flow rate of the permeate, the concentration at which the recovery rate is maximized within the allowable concentration ratio is not exceeded. The target discharge rate of water is determined.

JP-A-2015-127056

  Since the recovery rate of the RO membrane greatly affects the running cost of the water treatment system using the RO membrane, it is required to maintain the recovery rate as high as possible. In addition to the control such as the temperature feedforward control disclosed in Patent Document 1, there is a demand for the development of a technique capable of maintaining the highest possible recovery rate while suppressing the deposition of scale.

  Therefore, an object of the present invention is to solve the above-mentioned problems, and to provide a membrane separation device capable of realizing a higher recovery rate while suppressing precipitation of scale.

  In order to achieve the above object, a membrane separation device according to one embodiment of the present invention includes a reverse osmosis membrane module that separates feed water into permeated water and concentrated water, and is driven at a rotation speed according to an input drive frequency. A pressure pump that sucks in the supply water and discharges the water toward the reverse osmosis membrane module, an inverter that outputs a drive frequency corresponding to the input operation value signal to the pressure pump, and a flow rate of the permeated water. A first calculating unit that calculates a driving frequency of the pressurizing pump using a physical quantity in the system so as to have a preset target flow rate value, and outputs a calculated value signal corresponding to the calculated value of the driving frequency to the inverter; A control unit, a temperature detection unit for detecting a temperature of the supply water, the permeated water or the concentrated water, and a drain valve capable of adjusting a drain flow rate of the concentrated water discharged to the outside of the apparatus, and the first control unit includes: (I) Supply water obtained in advance Calculating the allowable concentration ratio of silica in the concentrated water based on the concentration of silica and the solubility of silica determined from the temperature detected by the temperature detecting means and the pH value of the supply water obtained in advance; The drainage flow rate is calculated from the calculated value of the magnification and the target flow rate value of the permeated water, and (iii) the drain valve is controlled such that the actual drainage flow rate of the concentrated water becomes the calculated value of the drainage flow rate.

  In the membrane separation device, the first control unit may include: (i) calcium carbonate solubility determined from a previously obtained hardness of the supply water, a temperature value detected by the temperature detection unit, and a pH value of the supply water previously obtained. (Ii) when the allowable concentration ratio of calcium carbonate is smaller than the allowable concentration ratio of silica, instead of the allowable concentration ratio of silica, The drainage flow rate is calculated from the calculated value of the allowable concentration rate of calcium and the target flow rate value of the permeated water, and (iii) the drain valve is controlled such that the actual drainage flow rate of the concentrated water becomes the calculated value of the drainage flow rate. May be.

  In the membrane separation device, a pH adjusting agent adding device for adding a pH adjusting agent for adjusting the pH to the supply water, and a pH adjusting agent for controlling an addition amount of the pH adjusting agent supplied from the pH adjusting agent adding device. A second control unit, wherein the second control unit (i) calculates a target pH value based on the detected temperature and a target pH calculation table indicating a relationship between the temperature of the supply water, the pH value, and the recovery rate. (Ii) controlling the addition amount of the pH adjuster so as to maintain the pH value of the supply water at the target pH value, wherein the first control unit sets the pH value of the supply water obtained in advance as: The target pH value may be used.

  In the membrane separation device, the second control unit calculates a pH value at which the recovery rate is maximum at the detected temperature as a target pH value based on the target pH calculation table, and sets the pH value of the supply water to the target pH value. The addition amount of the pH adjuster may be controlled so as to maintain the value.

  The membrane separation device may further include a water softening device for reducing and softening a hardness component of the supply water, and the pH adjusting agent adding device may add an alkaline agent for increasing the pH as a agent.

  According to the membrane separation device of the present invention, it is possible to realize a higher recovery rate while suppressing precipitation of scale.

Overall configuration diagram of the water treatment system 1 according to the first embodiment The flowchart which shows the processing procedure when the control part 8 of 1st Embodiment performs flow volume feedback water volume control. The flowchart which shows the processing procedure when the control part 8 of 1st Embodiment performs temperature / pH feedforward collection | recovery rate control. The flowchart which shows the processing procedure when the control part 8 of 2nd Embodiment performs temperature / pH feedforward collection | recovery rate control. Overall configuration diagram of the water treatment system 1 according to the third embodiment The flowchart which shows the processing procedure when the control part 8 of 3rd Embodiment performs pH adjustment control. The figure which shows the typical target pH calculation table

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the present invention is not limited by these embodiments.

(1st Embodiment)
A water treatment system 1 according to a first embodiment of the present invention will be described with reference to the drawings. FIG. 1 is an overall configuration diagram of a water treatment system 1 according to the first embodiment. The water treatment system 1 is applied to, for example, a pure water production system that produces pure water from fresh water.

  As shown in FIG. 1, the water treatment system 1 according to the first embodiment includes a pressurizing pump 2, a temperature sensor 3 as temperature detecting means, a pH sensor 4 as pH detecting means, and an inverter 5. . Further, the water treatment system 1 includes an RO membrane module 6 as a reverse osmosis membrane module, a flow rate sensor 7 as flow rate detection means, a control unit 8, and first to third drain valves 9 to 11. . Among them, the pressurizing pump 2, the RO membrane module 6, the flow rate sensor 7, and the control unit 8 constitute a membrane separation device in the present embodiment. In FIG. 1, the path of the electrical connection is indicated by a broken line.

  The water treatment system 1 includes a raw water line L1, a permeated water line L2, and a concentrated water line L3. The “line” in this specification is a general term for lines such as a flow path, a path, and a pipe through which a fluid can flow.

  The upstream end of the raw water line L1 is connected to a supply source (not shown) of the raw water W1. On the other hand, the downstream end of the raw water line L1 is connected to the RO membrane module 6.

  The pressurizing pump 2 is provided in the raw water line L1. The pressurizing pump 2 pumps raw water W1 such as tap water or groundwater supplied from a supply source toward the RO membrane module 6. The pressurizing pump 2 is electrically connected to an inverter 5 (described later). The driving power whose frequency has been converted is supplied from the inverter 5 to the pressurizing pump 2. The pressurizing pump 2 is driven at a rotation speed according to the input driving frequency.

  The temperature sensor 3 is a device that detects the temperature of the raw water W1 flowing through the raw water line L1. The temperature sensor 3 is connected to the raw water line L1 at the connection J1. Further, the temperature sensor 3 is electrically connected to the control unit 8. The temperature of the raw water W1 detected by the temperature sensor 3 (hereinafter, also referred to as “detected temperature value”) is transmitted to the control unit 8 as a detection signal.

  The pH sensor 4 is a device that detects the pH of the raw water W1 flowing through the raw water line L1. The pH sensor 4 is connected to the raw water line L1 at a connection J2. Further, the pH sensor 4 is electrically connected to the control unit 8. The pH of the raw water W1 detected by the pH sensor 4 (hereinafter, also referred to as “detected pH value”) is transmitted to the control unit 8 as a detection signal.

  The inverter 5 is an electric circuit that supplies driving power whose frequency has been converted to the pressurizing pump 2. The inverter 5 is electrically connected to the control unit 8. The inverter 5 receives a current value signal from the control unit 8. Inverter 5 outputs a driving frequency corresponding to the input current value signal to pressurizing pump 2.

  The RO membrane module 6 is a facility that performs a membrane separation process on the raw water W1 into a permeated water W2 from which dissolved salts have been removed and a concentrated water W3 from which dissolved salts have been concentrated. The RO membrane module 6 includes a single or a plurality of RO membrane elements (not shown). The RO membrane module 6 performs a membrane separation process on the raw water W1 using these RO membrane elements to produce a permeated water W2 and a concentrated water W3. The downstream end of the raw water line L1 is connected to the primary inlet port of the RO membrane module 6.

  The upstream end of the permeated water line L2 is connected to the secondary port of the RO membrane module 6. The permeated water W2 obtained by the RO membrane module 6 is sent to a demand location or the like via a permeated water line L2. The upstream end of the concentrated water line L3 is connected to the primary outlet port of the RO membrane module 6. The concentrated water W3 obtained in the RO membrane module 6 is discharged out of the RO membrane module 6 via the concentrated water line L3. A first drain valve 9, a second drain valve 10, and a third drain valve 11 (described later) are connected to the downstream side of the concentrated water line L3.

  A part of the concentrated water W3 discharged from the concentrated water line L3 may be returned to the raw water line L1 on the upstream side of the pressurizing pump 2. By refluxing a part of the concentrated water W3 to the raw water line L1, the water flow velocity on the surface of the RO membrane module 6 can be maintained in a predetermined range.

  The RO membrane module 6 in this embodiment has a reverse osmosis membrane (not shown) having a negatively-charged skin layer made of crosslinked wholly aromatic polyamide formed on the membrane surface. This reverse osmosis membrane has a water permeability coefficient of 1.5 × 10 − when an aqueous sodium chloride solution having a concentration of 500 mg / L, a pH of 7.0 and a temperature of 25 ° C. is supplied at an operating pressure of 0.7 MPa and a recovery of 15%. 11m3.m-2.s-1.Pa-1 or more, and has a performance of a salt removal rate of 99% or more.

  The operating pressure is an average operating pressure defined in JIS K3802-1995 “Membrane term”. The operation pressure indicates an average value of the inlet pressure on the primary side and the outlet pressure on the primary side of the RO membrane module 6. The recovery refers to the ratio of the flow rate Qp of the permeated water to the flow rate Qf of the supply water (here, an aqueous solution of sodium chloride) to the RO membrane module 6 (that is, Qp / Qf × 100).

  The water permeability coefficient is a value obtained by dividing the flow rate of permeated water [m3 / s] by the membrane area [m2] and the effective pressure [Pa]. The water permeability coefficient is an index indicating the water permeability of the reverse osmosis membrane. That is, the water permeability coefficient means the amount of water that permeates a unit area of the membrane in a unit time when a unit effective pressure is applied. The effective pressure is defined in JIS K3802-1995 “Membrane term”. The effective pressure is a pressure obtained by subtracting the osmotic pressure difference and the secondary pressure (back pressure) from the operating pressure (average operating pressure).

  A reverse osmosis membrane satisfying the condition of the water permeability coefficient of the present embodiment is commercially available as a reverse osmosis membrane element. As the reverse osmosis membrane element, for example, Toray Co., Ltd .: Model name "TMG20-400", Unjin Chemical Co., Ltd .: Model name "RE8040-BLF", Nitto Denko Corporation: Model name "ESPA1", etc. it can.

  The flow sensor 7 is a device that detects the flow rate of the permeated water W2 flowing through the permeated water line L2. The flow sensor 7 is connected to the permeated water line L2 at the connection part J3. The flow sensor 7 is electrically connected to the control unit 8. The flow rate of the permeated water W2 detected by the flow rate sensor 7 (hereinafter, also referred to as “detected flow rate value”) is transmitted to the control unit 8 as a detection signal.

  The first drain valve 9 to the third drain valve 11 are valves for adjusting the drain flow rate of the concentrated water W3 discharged from the concentrated water line L3. The downstream side of the above-mentioned concentrated water line L3 is branched into a first drainage line L4, a second drainage line L5, and a third drainage line L6 at branch portions J4 and J5.

  A first drain valve 9 is provided in the first drain line L4. A second drain valve 10 is provided in the second drain line L5. A third drain valve 11 is provided in the third drain line L6.

  The first drain valve 9 can open and close the first drain line L4. The second drain valve 10 can open and close the second drain line L5. The third drain valve 11 can open and close the third drain line L6.

  Each of the first drain valve 9 to the third drain valve 11 includes a constant flow valve mechanism (not shown). In the constant flow valve mechanism, the first drain valve 9 to the third drain valve 11 are set to different flow values, respectively. For example, in the first drain valve 9, the drain flow rate is set such that the recovery rate of the RO membrane module 6 is 95% in the open state. The drainage flow rate of the second drainage valve 10 is set such that the recovery rate of the RO membrane module 6 is 90% in the open state. The drain flow rate of the third drain valve 11 is set such that the recovery rate of the RO membrane module 6 is 80% in the open state.

  The drainage flow rate of the concentrated water W3 discharged from the concentrated water line L3 can be adjusted stepwise by selectively opening and closing the first drain valve 9 to the third drain valve 11. For example, only the second drain valve 10 is opened, and the first drain valve 9 and the third drain valve 11 are closed. In this case, the recovery rate of the RO membrane module 6 can be 90%. Further, the first drain valve 9 and the second drain valve 10 are opened, and only the third drain valve 11 is closed. In this case, the recovery rate of the RO membrane module 6 can be set to 85%. Therefore, in the present embodiment, the drainage flow rate of the concentrated water W3 is 5% between 65% and 95% by selectively opening and closing the first drain valve 9 to the third drain valve 11. It can be adjusted step by step.

  The first drain valve 9 to the third drain valve 11 are each electrically connected to the control unit 8. Opening and closing of the valve elements in the first drain valve 9 to the third drain valve 11 is controlled by a drive signal from the control unit 8.

  The control unit 8 is configured by a microprocessor (not shown) including a CPU and a memory. Further, the control unit 8 calculates the drive frequency of the pressurizing pump 2 using the detected flow rate value of the permeated water W2 as a feedback value so that the flow rate of the permeated water W2 becomes a preset target flow value. Then, the control unit 8 outputs a current value signal corresponding to the calculated value of the calculated drive frequency to the inverter 5 (hereinafter, also referred to as “flow rate feedback water amount control”). This flow rate feedback control is a control mode executed when the flow rate of the permeated water W2 is normally detected by the flow rate sensor 7.

  Here, the flow rate feedback water amount control by the control unit 8 will be described. FIG. 2 is a flowchart illustrating a processing procedure when the control unit 8 executes the flow rate feedback water amount control. The process of the flowchart shown in FIG. 2 is repeatedly executed during the operation of the water treatment system 1.

  In step ST101 shown in FIG. 2, the control unit 8 acquires a target flow value Qp 'of the permeated water W2. The target flow rate value Qp 'is, for example, a set value input to the memory by a system administrator via a user interface (not shown).

  In step ST102, the control unit 8 determines whether or not the time t measured by the internal timer (not shown) has reached the control cycle of 100 ms. In this step ST102, when the control unit 8 determines that the time measured by the timer has reached 100 ms (YES), the process proceeds to step ST103. If the control unit 8 determines in step ST102 that the time measured by the timer has not reached 100 ms (NO), the process returns to step ST102.

  In step ST103 (step ST102: YES determination), the control unit 8 acquires a detected flow value Qp of the permeated water W2 detected by the flow sensor 7.

  In step ST104, the control unit 8 operates using the speed digital PID algorithm so that the deviation between the detected flow value (feedback value) Qp obtained in step ST103 and the target flow value Qp 'obtained in step ST101 becomes zero. Calculate the quantity U. In the speed type digital PID algorithm, a change amount of the operation amount is calculated for each control cycle (100 ms), and this is added to the previous operation amount to determine the current operation amount.

  In step ST105, the control unit 8 calculates the driving frequency F of the pressurizing pump 2 based on the operation amount U, the target flow rate Qp ', and the maximum driving frequency of the pressurizing pump 2 (set value of 50 Hz or 60 Hz). .

  In step ST106, the control unit 8 converts the calculated value of the drive frequency F into a corresponding current value signal (4 to 20 mA).

  In step ST107, the control unit 8 outputs the converted current value signal to the inverter 5. Thus, the process of this flowchart ends (return to step ST101).

  The control unit 8 further controls the recovery rate of the permeated water W2 based on the temperature and the pH value of the raw water W1 (hereinafter, also referred to as “temperature / pH feedforward recovery rate control”). In the first embodiment, in the RO apparatus that performs flow rate feedback water volume control, in order to realize a high recovery rate in a range where silica scale is not precipitated, the recovery rate is determined based on the temperature and pH value of the raw water W1 that is feed water. To control. This temperature / pH feedforward recovery rate control is executed in parallel with the flow rate feedback water flow control described above.

  Next, control of the temperature / pH feedforward recovery rate by the control unit 8 will be described. FIG. 3 is a flowchart illustrating a processing procedure when the control unit 8 executes the temperature / pH feedforward recovery rate control. The process of the flowchart shown in FIG. 3 is repeatedly executed during the operation of the water treatment system 1.

  In step ST201 shown in FIG. 3, the control unit 8 acquires a target flow rate Qp ′ of the permeated water W2. The target flow rate value Qp 'is, for example, a set value input to the memory by a system administrator via a user interface (not shown).

  In step ST202, the control unit 8 acquires the silica (SiO2) concentration Cs of the raw water W1. The silica concentration Cs is, for example, a set value input to the memory by a system administrator via a user interface (not shown). The silica concentration of the raw water W1 can be obtained by analyzing the water quality of the raw water W1 in advance. In the raw water line L1, the silica concentration of the raw water W1 may be measured by a sensor (not shown).

  In step ST203, the control unit 8 acquires the detected temperature value T of the raw water W1 from the temperature sensor 3.

  In step ST204, the control unit 8 acquires the detected pH value “Ph” of the raw water W1 from the pH sensor 4. The pH value of the raw water W1 can also be obtained by analyzing the water quality of the raw water W1 in advance.

  In step ST205, the control unit 8 determines the solubility Ss of silica in water based on the acquired detected temperature value T and detected pH value “Ph”. Here, even if the pH of the feed water is the same, the solubility of the silica increases as the temperature of the feed water increases. Further, even when the temperature of the feed water is the same, the solubility of silica increases as the pH of the feed water falls or rises from a reference value (neutral). Based on such a tendency, the relationship between the pH and temperature of the supply water and the corresponding silica solubility is measured in advance by experiments or the like, and stored in the control unit 8 as data. Based on the data, the silica solubility Ss is determined from both the measured feed water temperature and pH value.

In step ST206, the control unit 8 calculates an allowable concentration ratio Ns of silica in the concentrated water W3 based on the silica concentration Cs and the silica solubility Ss obtained or determined in the previous step. The allowable concentration ratio Ns of silica can be determined by the following equation (1).
Ns = Ss / Cs (1)

  For example, if the silica concentration Cs is 20 mg SiO2 / L, the silica solubility Ss is 100 mgSiO2 / L at a temperature of 25 ° C. and a pH (specific values are added), the allowable concentration ratio Ns is “5”.

In step ST207, the control unit 8 calculates the drain flow rate (target drain flow rate Qd ') at which the recovery rate becomes maximum based on the target flow rate value Qp' obtained or calculated in the previous step and the allowable concentration ratio Ns. . The target drainage flow rate Qd 'can be obtained by the following equation (2).
Qd '= Qp' / (Ns-1) (2)

  In step ST208, the control unit 8 controls the opening and closing of the first drain valve 9 to the third drain valve 11 so that the actual drain flow Qd of the concentrated water W3 becomes the target drain flow Qd 'calculated in step ST207. Thereby, the processing of this flowchart ends (return to step ST201).

  According to the water treatment system 1 according to the first embodiment described above, for example, the following effects can be obtained.

  According to the water treatment system 1 according to the first embodiment, the control unit 8 executes the temperature / pH feedforward recovery rate control in parallel with the flow rate feedback water volume control. By performing such temperature / pH feedforward recovery rate control, it is possible to realize a higher recovery rate for the permeated water W2 while suppressing the precipitation of the silica-based scale in the RO membrane module 6. In particular, as described in ST205 and ST206, the recovery rate control is performed by calculating the allowable concentration ratio Ns of silica from both the temperature and the pH value of the supply water. By considering not only the temperature of the feed water but also the pH value, the actual state of the feed water can be grasped more accurately than in the case where only the temperature is considered, and the allowable concentration ratio Ns of silica can be reduced. It can be calculated with high accuracy. Therefore, it is possible to realize a higher recovery rate of the permeated water W2 while accurately suppressing the precipitation of the silica-based scale.

  According to the recovery rate control of the first embodiment described above, (i) the silica concentration Cs of the supply water obtained in advance, the silica solubility Ss determined from the detected temperature value T and the pH value of the supply water obtained in advance, , The allowable concentration ratio Ns of silica in the concentrated water W3 is calculated. Further, (ii) the drainage flow rate Qd 'is calculated from the calculated value of the permissible concentration ratio Ns and the target flow rate Qp' of the permeated water W2. Lastly, (iii) the first drain valve 9 to the third drain valve 11 are controlled such that the actual drain flow rate of the concentrated water W3 becomes the calculated value of the drain flow rate Qd '. By such control, the recovery of the permeated water W2 can be improved as much as possible within a range where the silica-based scale does not precipitate, and a higher recovery of the permeated water W2 can be realized.

(2nd Embodiment)
Although the first embodiment focuses on the silica-based scale, other scale components may be focused on. In particular, in fresh water, not only silica but also calcium carbonate is likely to appear as a scale. Therefore, in the second embodiment, not only silica but also calcium carbonate is subjected to the same processing as in ST201 to ST206, and the allowable concentration ratio is calculated. The specific flow is as shown in FIG. Since the configuration of the water treatment system 1 is the same as that of the first embodiment (FIG. 1), description and illustration are omitted.

  As shown in FIG. 4, while performing ST201 to ST206 (silica) similar to FIG. 3, new ST209 to ST211 (calcium carbonate) are performed in parallel. ST209 is performed in parallel with ST202, and ST210 and ST211 are performed in parallel with ST205 and ST206. The contents of ST201 to ST206 are the same as the contents described with reference to FIG.

  In step ST209, the control unit 8 acquires the hardness Hd of the raw water W1. The hardness is calcium hardness or simple hardness (calcium hardness + magnesium hardness), and is used as an index indicating the concentration of calcium carbonate in the second embodiment. The hardness Hd is, for example, a set value input to the memory by a system administrator via a user interface (not shown). The hardness Hd of the raw water W1 can also be obtained by analyzing the water quality of the raw water W1 in advance. In the raw water line L1, the hardness of the raw water W1 may be measured by a sensor (not shown).

  In step ST210, the control unit 8 determines the calcium carbonate solubility Sc in water based on the detected temperature value T obtained in ST203 and the detected pH value “Ph” obtained in ST204. The present inventors have found that, similarly to silica, calcium carbonate has a correlation between the temperature and pH value of feed water and scale precipitation (solubility of calcium carbonate). Specifically, as the temperature of the feed water increases, the solubility of calcium carbonate decreases, and as the pH of the feed water increases, the solubility of calcium carbonate decreases. In addition, we found that scale control by calcium carbonate concentration is important. Based on such a tendency, the relationship between the pH and temperature of the supply water and the solubility of calcium carbonate corresponding thereto is grasped in advance, and stored in the control unit 8 as data. Based on the data, the calcium carbonate solubility Sc is determined from both the temperature and the pH value of the feed water.

In step ST211, the control unit 8 calculates the allowable concentration ratio Nc of calcium carbonate in the concentrated water W3 based on the hardness Hd obtained in ST209 and the solubility Sc of calcium carbonate determined in ST210. The allowable concentration ratio Nc of calcium carbonate can be determined by the following equation (3).
Nc = Sc / Hd (3)

  In step ST212, the control unit 8 determines, based on the target flow rate value Qp ', the allowable concentration rate Ns, and the allowable concentration rate Nc acquired or calculated in the previous step, the drainage flow rate at which the recovery rate becomes maximum (the target wastewater flow rate Qd). ') Is calculated. Specifically, a smaller one of the allowable concentration ratio Ns of silica and the allowable concentration ratio Nc of calcium carbonate is selected. Next, the target drainage flow rate Qd 'is calculated as follows using the selected allowable concentration ratio.

When the allowable concentration ratio Ns of silica is selected as the smaller allowable concentration ratio, the target drainage flow rate Qd 'can be obtained by the following equation (4).
Qd '= Qp' / (Ns-1) (4)
On the other hand, when the allowable concentration ratio Nc of calcium carbonate is selected as the smaller allowable concentration ratio, the target drainage flow rate Qd 'can be obtained by the following equation (5).
Qd '= Qp' / (Nc-1) (5)

  In step ST213, the control unit 8 controls the opening and closing of the first drain valve 9 to the third drain valve 11 so that the actual drain flow Qd of the concentrated water W3 becomes the target drain flow Qd 'calculated in step ST212. Thereby, the processing of this flowchart ends (return to step ST201).

  According to the recovery rate control of the second embodiment described above, the control unit 8 executes ST209 to ST211 in addition to the same recovery rate control as in the first embodiment. That is, (i) the allowable amount of calcium carbonate in the concentrated water W3 based on the hardness Hd of the supply water obtained in advance and the calcium carbonate solubility Sc determined from the detected temperature value T and the pH value of the supply water obtained in advance. The concentration ratio Nc is calculated. (Ii) When the allowable concentration ratio Nc of calcium carbonate is smaller than the allowable concentration ratio Ns of silica, the calculated value of the allowable concentration ratio Nc of calcium carbonate and the permeated water are used instead of the allowable concentration ratio Ns of silica. A drain flow Qd 'is calculated from the target flow Qp' of W2. Lastly, (iii) the first drain valve 9 to the third drain valve 11 are controlled such that the actual drain flow rate of the concentrated water W3 becomes the calculated value of the drain flow rate Qd '.

  As described above, the recovery rate control in the second embodiment determines the discharge flow rate and the recovery rate by selecting the smaller allowable concentration rate of the allowable concentration rate Ns of silica and the allowable concentration rate Nc of calcium carbonate. Things. Thereby, a new effect of achieving a higher recovery rate while preventing both silica and calcium carbonate from being precipitated is exhibited.

  In the second embodiment, in particular, as described in ST210 and ST211, the allowable concentration ratio Nc of calcium carbonate is calculated from both the temperature and the pH value of the supply water, and the allowable concentration ratio Nc is set to the allowable concentration ratio of silica. The recovery rate control is performed in comparison with the magnification Ns. Thus, by considering not only the temperature of the feed water but also the pH value, the actual state of the feed water can be grasped more accurately than in the case where only the temperature is considered, and the allowable concentration ratio Nc of calcium carbonate can be understood. Can be calculated with high accuracy. As described above, by operating at a recovery rate as high as possible within a range in which calcium carbonate scale does not precipitate, a higher recovery rate of the permeated water W2 is realized while suppressing precipitation of calcium carbonate scale in addition to silica scale. be able to.

(Third embodiment)
Next, a water treatment system 1A according to a third embodiment of the present invention will be described with reference to FIGS. FIG. 5 is an overall configuration diagram of a water treatment system 1A according to the third embodiment. FIG. 6 shows a control flow newly performed in addition to the control flows performed in the first and second embodiments.

  In the third embodiment, differences from the first and second embodiments will be mainly described. In the third embodiment, the same or equivalent components as those in the first and second embodiments are denoted by the same reference numerals and described. Further, in the third embodiment, description overlapping with the first and second embodiments will be appropriately omitted.

  As shown in FIG. 5, the water treatment system 1A according to the third embodiment has a new configuration in which a raw water pump 12, a hard water softening device 13, a salt water tank 14 as a regenerating liquid supply unit, and a pH adjusting agent addition. Device 15. In the third embodiment, a higher recovery rate is achieved by performing the softening treatment of the supply water supplied to the RO membrane module 6 using the water softening device 13 and performing the pH adjustment using the pH adjusting agent adding device 15. It will be realized.

  The upstream end of the raw water line L7 is connected to a supply source (not shown) of the raw water W1. On the other hand, the downstream end of the raw water line L7 is connected to the water softening device 13.

  The raw water pump 12 is provided on the raw water line L7. The raw water pump 12 pumps raw water W1 such as tap water or groundwater supplied from a supply source toward the hard water softening device 13. The raw water pump 12 is electrically connected to the control unit 8. The operation (drive and stop) of the raw water pump 12 is controlled by the control unit 8.

  The raw water line L7 and the raw water pump 12 supply the raw water W1 having an electric conductivity of 150 mS / m or less and a total hardness of 500 mg CaCO3 / L or less to the water softening device 13 in the softening process.

  The water softening device 13 is a facility for producing soft water W5 by replacing hardness components (calcium ions and magnesium ions) contained in raw water W1 with sodium ions (or potassium ions) in a cation exchange resin bed (not shown). is there.

  The salt water tank 14 stores the salt water W4 for regenerating the cation exchange resin bed. The upstream end of the salt water line L9 is connected to the salt water tank 14. The downstream end of the salt water line L9 communicates with a process control valve (not shown) and is connected to various lines constituting the process control valve.

  The salt water line L9 is provided with a salt water valve (not shown). The salt water valve opens and closes the salt water line L9. The salt water valve is built into the process control valve. In the salt water valve, the drive unit of the valve body is electrically connected to the control unit 8 via a signal line (not shown). The opening and closing of the salt water valve is controlled by the control unit 8. The salt water tank 14 sends out a salt water W4 for regenerating the cation exchange resin bed to a pressure tank (not shown) in the regeneration process.

  The pH adjusting agent adding device 15 is a device for adding a pH adjusting agent for adjusting the pH of the soft water W5 produced by the water softening device 13. The pH adjusting agent adding device 15 is connected to a soft water line L8 at a connection portion J7. By adding a pH adjuster to the soft water W5, the soft water W6 whose pH has been adjusted is produced. In the second embodiment, an alkaline agent (such as sodium hydroxide) that increases the pH value is used as a pH adjuster.

  The RO membrane module 6 is a facility for performing a membrane separation process on the pH-adjusted soft water W6 into permeated water W7 from which dissolved salts have been removed and concentrated water W8 from which dissolved salts have been concentrated. The primary inlet port of the RO membrane module 6 is connected to the downstream side of the water softening device 13 via a water softening line L8.

  As the RO membrane module 6 in the third embodiment, the same RO membrane module 6 as in the first embodiment is used. The reverse osmosis membrane of the RO membrane module 6 has a salt removal rate (i.e., (feed water EC-permeation) when evaluated in electrical conductivity (EC) as the hardness of the feed water is lower in the desalination treatment of fresh water. (Water EC) / supply water EC × 100). Therefore, as an example, the high-purity soft water W5 (0.8 mg CaCO3 or less as an ability value) produced by the water softening device 13 for performing split flow regeneration is constantly supplied, thereby increasing the salt removal rate (usually 98%). 0.5% or more).

  The salt removal ratio is a value calculated from the concentration of a specific salt before and after permeating the membrane (here, the concentration of sodium chloride). The salt removal rate is an index indicating the solute blocking performance of the reverse osmosis membrane. The salt removal rate is obtained from the concentration Cf of the supply water and the concentration Cp of the permeated water by (1−Cp / Cf) × 100.

  The reverse osmosis membrane satisfying the conditions of the water permeability coefficient and the salt removal rate of the present embodiment is commercially available as the above-described reverse osmosis membrane element.

  The control unit 8 controls the operation of the process control valve of the water softening device 13 based on a detection signal or the like input from a not-shown soft water flow sensor or salt water flow sensor. A control program for operating the water softening device 13 is stored in the memory of the control unit 8 in advance.

  The control unit 8 performs the flow rate feedback water amount control (FIG. 2) and the temperature / pH feedback recovery rate control (FIG. 3), as in the first and second embodiments. Perform pH adjustment control. The processing of the flowchart shown in FIG. 6 is repeatedly executed during the operation of the water treatment system 1A.

  In step ST301 shown in FIG. 6, the control unit 8 prepares a “target pH calculation table” indicating the relationship between the temperature of the water (soft water W6) supplied to the RO membrane module 6, the pH value, and the recovery rate. This table defines the recovery rate of the limit value at which silica is deposited on a scale according to the temperature and the pH value of the feed water. The target pH calculation table can be prepared in advance for each raw water.

  In step ST302, based on the detected temperature of the supply water detected by the temperature sensor 3, the control unit 8 calculates, from the target pH calculation table, the pH value at which the recovery rate becomes maximum at the detected temperature as the target pH value. . An example of a specific method will be described with reference to FIG.

  FIG. 7 shows a schematic target pH calculation table. As shown in FIG. 7, with respect to three kinds of temperatures (15 ° C., 20 ° C., and 25 ° C.) and three kinds of pHs (7.5, 8.0, and 8.5), the limit values at which silica scale is precipitated are shown. The recovery is determined in advance. These data are stored in the control unit 8 as a target pH calculation table.

  In the example shown in FIG. 7, when the measured temperature of the supply water is 15 ° C., the pH at which the recovery rate is highest is 8.5. Therefore, the pH value 8.5 may be calculated as the target pH value. On the other hand, when the measured temperature of the feed water is 20 ° C., the pH at which the recovery rate is highest is 8.0. Therefore, the pH value of 8.0 may be calculated as the target pH value. Further, when the measured temperature of the feed water is 25 ° C., the pH at which the recovery rate is highest is 7.5. Therefore, the pH value 7.5 may be calculated as the target pH value.

  In step ST303, the control unit 8 determines the amount of the pH adjuster to be added to the target pH value, and controls the pH adjuster adding device 15 to add the added amount.

  In step ST304, the control unit 8 acquires the calculated target pH value as the pH value of the supply water. The control unit 8 uses the acquired pH value of the supply water as the detected pH value “Ph” in ST204 in FIGS.

  According to the control according to the third embodiment described above, (i) the target pH value is calculated based on the detected temperature and the target pH calculation table indicating the relationship between the temperature of the supply water (soft water W6), the pH value, and the recovery rate. It is calculated (ST301-ST302). Further, (ii) the addition amount of the pH adjuster is controlled so as to maintain the ph value of the supply water at the target pH value (ST303). Further, the target pH value is used as the pH value of the previously obtained supply water (ST304).

  The above control is based on the following principle. That is, by providing the water softening device 13 and performing the softening treatment on the raw water W1, the hardness component containing calcium carbonate in the raw water W1 can be reduced. Thereby, since the hardness Hd obtained in ST207 (FIG. 4) described above decreases, the allowable concentration ratio Nc of calcium carbonate increases (ST209). Therefore, if it can be adjusted so as to increase the allowable concentration ratio Ns of the other silica, a higher recovery rate can be realized in a range where neither silica nor calcium carbonate is precipitated. As a means therefor, a pH adjuster adding device 15 is provided, and a pH adjuster is introduced into the soft water W5 to adjust it to an ideal pH value. Specifically, the pH value of the soft water W5 is increased by adding an alkaline agent to the soft water W5 (to become the soft water W6). The present inventors have noticed that when the pH value of the solution is increased from a value near neutrality, the solubility of silica is significantly increased. Therefore, by adding an alkaline agent to the soft water W5, the solubility of silica is greatly increased, and the allowable concentration ratio Ns of silica can be greatly increased. Increasing the pH value decreases the solubility of calcium carbonate, but has little effect since it has already been reduced by the softening treatment by the water softening device 13. As described above, in the third embodiment, the allowable concentration ratio Nc of calcium is increased by adjusting the pH of the soft water W5 while the allowable concentration ratio Nc of calcium carbonate is increased by the soft water treatment. As described above, by increasing the respective allowable concentration ratios of calcium carbonate and silica and controlling the recovery rate, it is possible to realize a higher recovery rate while suppressing both silica and calcium carbonate scale precipitation. it can.

  As described above, the present invention has been described with reference to the above-described first to third embodiments, but the present invention is not limited to the above-described first to third embodiments. For example, in the third embodiment, the case has been described in which the control unit 8 performs both the temperature / pH feedforward control and the pH adjustment control, but the control unit 8 may perform the control using separate control units (that is, the first control). Unit and the second control unit).

  In the third embodiment, the case where the water softening treatment by the water softening device 13 and the pH adjustment treatment by the pH adjusting agent adding device 15 are performed is described, but only one of the treatments may be performed.

  Further, in the third embodiment, the case where the pH is adjusted (increases the pH) on the alkali side using an alkaline agent as the pH adjuster has been described. However, when the soft water treatment is not particularly performed, the pH adjustment is performed on the acid side. (Reducing the pH).

  Note that by appropriately combining any of the various embodiments described above, the effects of the respective embodiments can be achieved.

DESCRIPTION OF SYMBOLS 1 Water treatment system 2 Pressure pump 3 Temperature sensor (temperature detection means)
4 pH sensor (pH detection means)
5 Inverter 6 RO membrane module 7 Flow rate sensor (flow rate detecting means)
8 control part 9 1st drain valve (drain valve)
10 Second drain valve (drain valve)
11 3rd drain valve (drain valve)
12 Raw water pump 13 Hard water softening device 14 Salt water tank 15 pH adjuster addition device L1 Raw water line L2 Permeated water line L3 Feed water line L4 First drain line L5 Second drain line L6 Third drain line L7 Raw water line L8 Soft water line L9 Salt water line

Claims (3)

A reverse osmosis membrane module for separating feed water into permeate and concentrate water,
A pressurized pump driven at a rotation speed according to the input drive frequency, sucking supply water and discharging toward the reverse osmosis membrane module,
An inverter that outputs a drive frequency corresponding to the input operation value signal to the pressure pump,
The driving frequency of the pressurizing pump is calculated using a physical quantity in the system so that the flow rate of the permeated water becomes a preset target flow value, and the calculated value signal corresponding to the calculated value of the driving frequency is calculated by the inverter. A first control unit that outputs the
Temperature detection means for detecting the temperature of feed water, permeate water or concentrated water,
A drain valve capable of adjusting the drain flow rate of the concentrated water discharged outside the apparatus,
The first controller is configured to: (i) concentrate the silica concentration based on the silica concentration obtained in advance and the silica solubility determined from the temperature value detected by the temperature detecting unit and the pH value of the supply water obtained in advance. Calculating a permissible concentration ratio of silica in the water; (ii) calculating a drainage flow rate from the calculated value of the permissible concentration ratio and the target flow rate value of the permeated water; Controlling the drain valve so that the calculated value of
a pH adjuster addition device for adding a pH adjuster for adjusting the pH to the feed water,
A second control unit for controlling the amount of the pH adjuster supplied from the pH adjuster addition device,
The second controller calculates (i) a target pH value based on the detected temperature and a target pH calculation table indicating a relationship between the temperature, the pH value, and the recovery rate of the supply water; Controlling the addition amount of the pH adjusting agent so as to maintain the pH value of the target pH value,
The said 1st control part uses the said target pH value as the pH value of the said previously acquired supply water , The membrane separation apparatus. The second control unit calculates a pH value at which the recovery rate is maximum at the detected temperature as a target pH value based on the target pH calculation table, and maintains the pH value of the supply water at the target pH value. controlling the amount of pH adjusting agent, film separation apparatus according to claim 1. Further provided is a water softening device for reducing and softening the hardness component of the feed water,
The pH adjusting agent addition device is adding an alkaline agent to increase the pH as a drug, membrane separation device according to claim 1 or 2.

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