DESCRIPTION

TITLE OF THE INVENTION:
REVERSE OSMOSIS MEMBRANE SEPARATOR, START-UP METHOD
THEREFOR, AND METHOD FOR PRODUCING PERMEATE

TECHNICAL FIELD

[0001]
The present invention relates to a reverse osmosis membrane separation apparatus for separating components mixed or dissolved in a fluid using a reverse osmosis membrane module.

BACKGROUND ART

[0002]
As shown in Fig. 1, a reverse osmosis membrane separation apparatus used for seawater desalination etc. are basically configured in such a manner that liquid to be treated is fed to a reverse osmosis membrane module unit 2 after the pressure thereof is increased to a predetermined value through a high-pressure pump 1 and components dissolved in the liquid to be treated are separated by the reverse osmosis function of the reverse osmosis membrane module unit 2, thereby obtaining a permeate.

[0003]
Incidentally, as shown in Fig. 2, the reverse osmosis membrane module unit 2 is a module unit having one or plural membrane modules 20 (unit structures) in each of which separation membrane elements 22 (e.g., spiral separation membrane elements) each having a reverse osmosis membrane (separation membrane) are provided in a cylindrical vessel 21. When a pressurized liquid to be treated such as seawater is fed to an inlet 23 located at one end of the membrane module 20, solute components are separated by the separation membrane in the separation membrane element 22 of each stage and a low-pressure permeate after the separation passes through central pipes 24 and is discharged from a permeate drain 25. A high-pressure concentrate is discharged from a concentrate drain 26 which is located at the other end, that is, the end that is different from the end where the inlet 23 exists.

[0004]
In such a reverse osmosis membrane separation apparatus, the necessary motive power of the high-pressure pump may be reduced utilizing the energy of a high-pressure concentrate that is discharged from the apparatus. For example, a reverse pump, a turbo charger, or a Pelton turbine is used for recovering the energy of a concentrate. Recently, pressure exchange energy recovery devices which are high in energy recovery efficiency have come to be employed increasingly.

[0005]
Fig. 3 shows a general configuration of a reverse osmosis membrane separation apparatus which is equipped with a pressure exchange energy recovery device. A liquid to be treated is distributed to a high-pressure pump 1 and a pressure exchange energy recovery device 3. The liquid to be treated that is fed to the high-pressure pump 1 is increased in pressure thereof to a predetermined value and is fed to a reverse osmosis membrane module unit 2. A high-pressure concentrate that is discharged from the reverse osmosis membrane module unit 2 is also fed to the pressure exchange energy recovery device 3. In the pressure exchange energy recovery device 3, the liquid to be treated is increased in pressure by exchanging energy with the high-pressure concentrate, thereby discharging from the pressure exchange energy recovery device 3. On the other hand, the high-pressure concentrate whose energy has been given to the liquid to be treated is lowered in pressure and discharged as a low-pressure concentrate. The high-pressure liquid to be treated that is discharged from the pressure exchange energy recovery device 3 is fed to a booster pump 4, increased in pressure to the same value as of the liquid to be treated whose pressure has been increased by the high-pressure pump 1, merged with the liquid to be treated that is discharged from the high-pressure pump 1, and then fed to the reverse osmosis membrane module unit 2.

[0006]
A common operation (activation) procedure of the above-described reverse osmosis membrane separation apparatus which is equipped with the pressure exchange energy recovery device 3 is as follows. First, a liquid to be treated is fed only to the pressure exchange energy recovery device 3. The flow rate of the liquid to be treated is controlled by a flow rate control valve 5 disposed on a drain line of a concentrate discharged from the pressure exchange energy recovery device 3 so as to be approximately equal to a concentrate flow rate at the time of a steady-state operation. Then, the booster pump 4 is activated so that the liquid to be treated flows through the pressure exchange energy recovery device 3, the booster pump 4, the reverse osmosis membrane module unit 2, and the pressure exchange energy recovery device 3 in this order and is then discharged. In this state, it is common to control the motor rotation speed of the booster pump 4 with a variable frequency drive (inverter) so that the flow rate of the liquid to be treated that is discharged from the pressure exchange energy recovery device 3 in this state is also made approximately equal to the concentrate flow rate at the time of a steady-state operation. At this stage, the pressure of the liquid to be treated is low and separation of solute components is not performed by the reverse osmosis membranes.

[0007]
Then, the high-pressure pump 1 is activated. A liquid to be treated that flows out of the activated high-pressure pump 1 is merged with the liquid to be treated that is flowing through the booster pump 4, and fed to the reverse osmosis membrane module unit 2. The flow rate of the liquid to be treated that is flowing through the booster pump 4 is always controlled by the flow rate control valve 5 and the motor rotation speed of the booster pump 4 so as to be approximately equal to the concentrate flow rate at the time of a steady-state operation. Therefore, a liquid to be treated corresponding to the flow rate of the liquid to be treated that is discharged from the high-pressure pump 1, that is, only part, corresponding to an excess flow rate over the concentrate flow rate at the time of a steady-state operation, of the liquid to be treated that is fed to the reverse osmosis membrane module unit 2, passes through the reverse osmosis membranes and is discharged to the outside as a solute-components-separated permeate.

[0008]
In the above operation, it is common to cause the high-pressure pump 1 to discharge the liquid to be treated at a low rate at the beginning and then to increase the flow rate gradually. This control is made by disposing, on the discharge side of the high-pressure pump 1, a flow rate control value 6 for adjusting the flow rate of the liquid to be treated that is fed to the reverse osmosis membrane module unit 2 (see Fig. 4) or controlling the motor rotation speed of the high-pressure pump with a variable frequency drive (inverter) 7 (see Fig. 5).

[0009]
Incidentally, to obtain a permeate through the reverse osmosis membranes, it is necessary that the inlet pressure of the reverse osmosis membrane module unit 2 be higher than or equal to the osmotic pressure of the liquid to be treated. In the case where the liquid to be treated is seawater, for example, the osmotic pressure of the reverse osmosis membrane module unit 2 needs to be as high as about 3 MPa.

[0010]
At the beginning of an operation of the reverse osmosis membrane separation apparatus equipped with the pressure exchange energy recovery device 3, until activation of the high-pressure pump 1, the inlet pressure of the reverse osmosis membrane module unit 2 is approximately equal to the liquid-to be treated push-in pressure and is as low as about 0.3 MPa, for example. Once the high-pressure pump 1 is activated, the inlet pressure of the reverse osmosis membrane module unit 2 is increased to the osmotic pressure of the liquid to be treated (about 3 MPa) even if the flow rate control valve 6 is opened slightly to allow the liquid to be treated to flow only slightly, because as described above the liquid to be treated has no drain destination. Thus, the pressure of the reverse osmosis membrane module unit 2 is increased rapidly.

[0011]
If a high-pressure liquid to be treated (seawater or the like) is applied to the reverse osmosis membrane module unit 2 abruptly, the physical properties of the reverse osmosis membranes may deteriorate due to resulting pressure impact. The deteriorations of the physical properties of the reverse osmosis membranes are a factor in, for example, lowering the salt rejection in the reverse osmosis membrane module unit 2 and may lower its reverse osmosis processing ability.

[0012]
In this connection, Patent document 1 proposes a method for increasing the inlet pressure gradually by disposing, on the high-pressure pump discharge side, a bypass flow passage which bypasses a reverse osmosis membrane module unit and a bypass flow rate control valve. However, in the case of reverse osmosis membrane separation apparatus equipped with a pressure exchange energy recovery device, the flow rate of a concentrate is controlled by a flow rate control valve that is disposed on a drain line of a concentrate that is discharged from the pressure exchange energy recovery device. And the flow rate of a liquid to be treated that is discharged from the pressure exchange energy recovery device is controlled so as to be approximately equal to a concentrate flow rate at the time of a steady-state operation by controlling the motor rotation speed of a booster pump using a variable frequency drive (inverter). Therefore, the only way to prevent a rapid rise of the inlet pressure is to discharge, through the bypass flow rate control valve, a liquid to be treated that is discharged from the high-pressure pump at the time of its activation.

[0013]
In the case where the high-pressure pump is controlled by a variable frequency drive, it can be activated by this method. However, in many cases, a large-scale reverse osmosis membrane separation apparatus is not equipped with a variable frequency drive because the motor capacity of the booster pump is large and hence a very expensive variable frequency drive needs to be used. In such a case, even if it is attempted to activate the high-pressure pump by fully opening the bypass flow rate control valve, a trip occurs in the high-pressure pump due to overloading and this device itself cannot be activated. The high-pressure pump can be activated if it is attempted to activate it with the opening degree of the bypass flow rate control valve fixed so that a minimum flow rate of the high-pressure pump is secured to prevent a trip in the high-pressure pump. However, at this time, pressure that is higher than at the time of a steady-state operation is abruptly exerted on the reverse osmosis membrane module unit, and its inlet pressure cannot be increased gradually.

BACKGROUND ART DOCUMENT

PATENT DOCUMENT

[0014]

SUMMARY OF THE INVENTION

PROBLEMS THAT THE INVENTION IS TO SOLVE

[0015]
An object of the present invention is to provide a simplified reverse osmosis membrane separation apparatus which is equipped with an energy recovery device and can prevent an event that the pressure acting on a reverse osmosis membrane module varies rapidly at a start of operation, thereby preventing deteriorations of physical properties of reverse osmosis membranes effectively.

MEANS FOR SOLVING THE PROBLEMS

[0016]

To attain the above object, the invention is characterized by one of the following items (1) to (4).

[0017]

(1) A reverse osmosis membrane separation apparatus including:

a pump A for feeding a part of liquid to be treated to a reverse osmosis membrane module after increasing a pressure thereof to a predetermined value;

an energy recovery device for increasing a pressure of a remaining part of the liquid to be treated utilizing a pressure of a concentrate that is discharged from the reverse osmosis membrane module;

a pump B for feeding the liquid to be treated whose pressure has been increased by the energy recovery device to the reverse osmosis membrane module after further increasing the pressure thereof to a predetermined value;

a flow rate control valve A for adjusting a flow rate of the liquid to be treated that is discharged from the pump A;

a bypass flow passage which bypasses the reverse osmosis membrane module from the flow rate control valve A; and

a flow rate control valve B which is disposed in the bypass flow passage and adjusts a bypass rate of the liquid to be treated.

[0018]

(2) The reverse osmosis membrane separation apparatus according to (1), further including a pump C for feeding the liquid to be treated to the pump A and the energy recovery device, and a frequency converter for controlling a rotation speed of the pumpC.

[0019]
(3) An activation method of the reverse osmosis membrane separation apparatus
according to (1), the method including:

before activation of the pump A, making adjustments so that a liquid to be treated flows through the energy recovery device, a booster pump, the reverse osmosis membrane module and the energy recovery device in this order and is discharged thereafter;
then activating the pump A while setting opening degrees of the flow rate control valve A and the flow rate control valve B to predetermined values; and

then controlling the flow rate control valve A and the flow rate control valve B step by step so that the flow rate control valve A operates in an opening direction and the flow rate control valve B operates in a closing direction, until an inlet pressure of the reverse osmosis membrane module is increased to a predetermined value.

[0020]
(4) A method for producing a permeate, including activating the reverse osmosis membrane separation apparatus by the activation method of a reverse osmosis membrane separation apparatus according to (3), followed by feeding liquid to be treated to the reverse osmosis membrane module to obtain a permeate.

ADVANTAGE OF THE INVENTION

[0021]
In a reverse osmosis membrane separation apparatus which is equipped with an energy recovery device, the invention makes it possible to gradually increase the pressure of liquid to be treated that is fed to a reverse osmosis membrane module at a start of activation of a high-pressure pump by adjusting the bypass flow rate of a bypass passage using a flow rate control valve disposed in the bypass passage to thereby prevent deteriorations of physical properties of the reverse osmosis membrane module effectively in a simple manner.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]

[Fig. 1] Fig. 1 is a schematic view showing a reverse osmosis membrane separation apparatus.

[Fig. 2] Fig. 2 is a view showing a general structure of a reverse osmosis membrane module unit.

[Fig. 3] Fig. 3 is a schematic view showing a reverse osmosis membrane separation apparatus which is equipped with a pressure exchange energy recovery device.

[Fig. 4] Fig. 4 is a schematic view showing that a flow rate control valve is disposed on the discharge side of a high-pressure pump shown in Fig. 3.

[Fig. 5] Fig. 5 is a schematic view showing that the high-pressure pump shown in Fig. 3 is equipped with a variable frequency drive.

[Fig. 6] Fig. 6 is a schematic view showing a reverse osmosis membrane separation apparatus according to an embodiment of the present invention.

[Fig. 7] Fig. 7 is a schematic view showing a reverse osmosis membrane separation apparatus according to another embodiment of the invention.

MODE FOR CARRYING OUT THE INVENTION

[0023]
A reverse osmosis membrane separation apparatus according to the present invention will be described with reference to Fig. 6.

[0024]
Fig. 6 shows a schematic view of a reverse osmosis membrane separation apparatus for separating components mixed or dissolved in a fluid. A liquid to be treated such as seawater is distributed to a high-pressure pump 1 and a pressure exchange energy recovery device 3. The liquid to be treated that is fed to the high-pressure pump 1 is increased in pressure to a predetermined value (e.g., about 6.0 MPa) and fed to a reverse osmosis membrane module unit 2. The pressure exchange energy recovery device 3 is fed with the distributed liquid to be treated and a high-pressure concentrate that is discharged from the reverse osmosis membrane module unit 2. In the pressure exchange energy recovery device 3, the liquid to be treated is increased in pressure by exchanging energy with the high-pressure concentrate. A resulting high-pressure liquid to be treated is discharged from the pressure exchange energy recovery device 3. On the other hand, the high-pressure concentrate whose energy has been given to the liquid to be treated is lowered in pressure and discharged as a low-pressure concentrate. The high-pressure liquid to be treated that is discharged from the pressure exchange energy recovery device 3 is fed to a booster pump 4, increased in pressure to the same value as of the liquid to be treated whose pressure has been increased by the high-pressure pump 1, merged with the liquid to be treated that is discharged from the high-pressure pump 1, and fed to the reverse osmosis membrane module unit 2. Receiving the liquid to be treated whose pressure has been increased to the predetermined value, the reverse osmosis membrane module unit 2 produces a solute-components-separated permeate by reverse osmosis function thereof and also produces a concentrate.

[0025]
A flow rate control valve 6 for controlling the flow rate of the liquid to be treated that is discharged from the high-pressure pump 1 is disposed on the discharge side of the high-pressure pump 1. As described above, the flow rate of the liquid to be treated that is discharge from the high-pressure pump 1 is approximately equal to the permeate flow rate. Therefore, it is a common practice that in order to control the permeate flow rate, the flow rate control valve 6 is flow-rate-controlled by a permeate flowmeter 9 which is disposed on a permeate pipe and a permeate flow rate control unit 10 therefor.

[0026]
A bypass flow passage which bypasses the reverse osmosis membrane module unit 2 is provided on the output side of the flow rate control valve 6 and a flow rate control valve 8 for controlling the bypass rate is disposed on the bypass flow passage. The flow rate control valve 8 is flow-rate-controlled by a flowmeter 11 disposed on the feed side of the high-pressure pump 1 and a high-pressure pump minimum flow rate control unit 12 in order to secure a minimum flow rate of the high-pressure pump 1 when the reverse osmosis membrane separation apparatus is activated.

[0027]
An operation manipulation procedure of this reverse osmosis membrane separation apparatus is as follows. First, liquid to be treated is fed only to the pressure exchange energy recovery device 3. The flow rate of the liquid to be treated is controlled by a flow rate control valve 5 disposed on a drain line of a concentrate discharged from the pressure exchange energy recovery device 3 so as to be approximately equal to a concentrate flow rate at the time of a steady-state operation. Then, the booster pump 4 is activated so that the liquid to be treated flows through the pressure exchange energy recovery device 3, the booster pump 4, the reverse osmosis membrane module unit 2, and the pressure exchange energy recovery device 3 in this order and is then discharged. In this state, it !s common to control the motor rotation speed of the booster pump 4 with a variable frequency drive (inverter) so that the flow rate of the liquid to be treated that is discharged from the pressure exchange energy recovery device 3 in this state is also made approximately equal to the concentrate flow rate at the time of a steady-state operation. At this stage, the pressure of the liquid to be treated is low and separation of solute components is not performed by the reverse osmosis membranes.

[0028]
Then, the high-pressure pump 1 is activated. To prevent the high-pressure pump 1 from being damaged due to excessive vibration or heating, it is necessary to increase the flow rate of the high-pressure pump 1 to its minimum flow rate immediately after the activation. To this end, the high-pressure pump 1 is activated with presetting the opening degrees of the flow rate control valves 6 and 8 to predetermined values so that the minimum flow rate is secured. More specifically, the initial opening degree of the flow rate control valve 6 is determined taking into consideration a pressure loss of a secondary-side pipe of the flow rate control valve 6 and a pressure loss of the flow rate control valve 8 in the case where the flow rate control valve 8 is fully opened. If the high-pressure pump 1 is activated in this state, the input side pressure of the flow rate control valve 6 becomes higher than a rated pressure and has a value 7.0 MPa, for example, although it depends on the flow rate characteristic of the high-pressure pump 1. On the other hand, the inlet pressure of the reverse osmosis membrane module unit 2 exhibits almost no increase and has a value of about 0.5 MPa because the flow rate control valve 8 is fully opened and hence the liquid to be treated that is discharged from the high-pressure pump 1 is discharged to the bypass side through the flow rate control valve 8.

[0029]
Then, in order to obtain a permeate, the flow rate control valve 6 is opened gradually according to an instruction from the permeate flow rate control unit 10. As a result, the discharge rate of the high-pressure pump 1 is increased instantaneously. However, since the discharge rate of the high-pressure pump 1 is controlled so as to be kept equal to its minimum flow rate by the flow rate control valve 8 with the function of the high-pressure pump minimum flow rate control unit 12, the flow rate control valve 8 operates in the closing direction and the discharge rate of the high-pressure pump 1 is kept at its minimum flow rate. The flow rate control valves 6 and 8 operate in the opening direction and the closing direction, respectively, whereby the inlet pressure of the reverse osmosis membrane module unit 2 is increased. As these two kinds of controls are performed simultaneously step by step, the reverse osmosis membrane module unit inlet pressure is increased gradually. A permeate starts to be discharged when the inlet pressure of the reverse osmosis membrane module unit 2 has reached 3.0 MPa, for example.

[0030]
Even after the start of discharging of a permeate, the high-pressure pump minimum flow rate control unit 12 continues to control the flow rate of the high-pressure pump 1 to its minimum flow rate. Therefore, the flow rate of the liquid to be treated that is discharged to the bypass side from the flow rate control valve 8 is decreased gradually, and a permeate corresponding to the above decreased flow rate is discharged from the reverse osmosis membrane module unit 2. Finally, the flow rate control valve 8 is fully closed.

[0031]
From this time onward, the discharge rate of the high-pressure pump 1 is equal to the permeation rate of the reverse osmosis membrane module unit 2. The permeate flow rate control unit 10 thereafter continues the control of opening the flow rate control valve 6 gradually until the permeate flowmeter 9 shows a rated flow rate. The activating operation of this apparatus is finished when the permeate flow rate has reached the rated flow rate.

[0032]
More specifically, it is best that about 300 sec or more be taken from the activation of the high-pressure pump 1 to a time point when the permeation rate reaches the rated flow rate. To this end, it is appropriate to perform a program control for gradually increasing the flow rate setting value of the permeate flow rate control unit 10 to the rate flow rate in 300 sec and to restrict the variation rate of the flow rate control valve 6 (manipulation end) to prevent an extremely fast pressure or flow rate variation.

[0033]
In the above-described apparatus configuration, each of the high-pressure pump 1 and the booster pump 4 includes a centrifugal pump or a plunger pump and each of the flow rate control valves 6 and 8 includes a globe valve, a cage valve, or a needle valve. A bypassed liquid to be treated may be either discharged from the system or returned to a tank or the like for storing a liquid to be treated so as to be used again as a liquid to be treated.

[0034]
In the above-described method, the inlet pressure of the reverse osmosis membrane module unit 2 is increased in a certain preset time by the permeate flow rate control unit 10 and the high-pressure pump minimum flow rate control unit 12. Alternatively, as shown in Fig. 7, the opening degree of the flow rate control valve 6 may be controlled by performing a cascade control on the permeate flow rate control unit 10 by an inlet pressure control unit 14 on the basis of a pressure value of liquid to be treated, obtained by a pressure transmitter 13 which is disposed on the input side of the reverse osmosis membrane module unit 2.

[0035]
The necessary input pressure of the reverse osmosis membrane module unit 2 varies depending on the quality and the temperature of liquid to be treated. In an ordinary operation, adaptation to a variation (increase or decrease) of the necessary input pressure of the reverse osmosis membrane module unit 2 is made in the following manner. When the necessary input pressure of the reverse osmosis membrane module unit 2 is increased, the opening degree of the flow rate control valve 6 is increased according to an instruction from the permeate flow rate control unit 10. Conversely, when the necessary input pressure of the reverse osmosis membrane module unit 2 is decreased, the opening degree of the flow rate control valve 6 is decreased.

[0036]
From the viewpoint of the dynamic energy consumption, no useless dynamic energy consumption occurs if the opening degree of the flow rate control valve 6 is close to full-opening. On the other hand, the opening degree of the flow rate control valve 6 being lower than the full-opening means that the flow rate control valve 6 is consuming energy, that is, useless dynamic energy consumption is occurring in the high-pressure pump 1. This dynamic energy corresponds to electric power for operating a device such as the high-pressure pump 1 ((dynamic energy) = (electric energy)). In seawater desalinating apparatus, the reduction of the electric energy consumption is an important factor. Therefore, in order to reduce such a useless electric energy consumption, it is preferable to equip a feed pump 15 of liquid to be treated with a variable frequency drive (inverter) 7 (see Fig. 7).

[0037]
If small pipe pressure losses are not taken into consideration, a necessary inlet pressure of the reverse osmosis membrane module unit 2 can be calculated by adding a discharge pressure of the high-pressure pump 1 to a discharge pressure of the feed pump 15 and subtracting therefrom a pressure corresponding to a dynamic energy loss of the flow rate control valve 6. Now assume that the necessary inlet pressure of the reverse osmosis membrane module unit 2 is 7.5 MPa when, for example, the rated discharge pressure of the feed pump 15 is 1.0 MPa and the rated discharge pressure of the high-pressure pump 1 is 7.0 MPa. In this case, dynamic energy corresponding to 0.5 MPa should be lost in the flow rate control valve 6. If the feed pump 15 is equipped with a variable frequency drive (inverter) 7, useless electric energy consumption can be avoided by decreasing the discharge pressure of the feed pump 15 from the rated value 1.0 MPa to 0.5 MPa by adjusting the output rotation speed of the feed pump 15 by converting the frequency of the power that is fed to the feed pump 15. This method makes it unnecessary to consume dynamic energy uselessly in the flow rate control valve 6. As exemplified above, there may occur a case that the feed pump 15 of liquid to be treated is equipped with a variable frequency drive (inverter) 7 to prevent useless dynamic energy consumption. Also in such a case, the above-described activation method and operation method which use the flow rate control valve 6 and the flow rate control valve 5 which is disposed on the bypass line are effective.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

[0038]

1: High-pressure pump (pump A)

2: Reverse osmosis membrane module unit

3: Pressure exchange energy recovery device

4: Booster pump (pump B)

5: Flow rate control valve

6: Flow rate control valve (flow rate control valve A)

7: Variable frequency drive (inverter)

8: Flow rate control valve (flow rate control valve B)

9: Permeate flowmeter

10: Permeate flow rate control unit

11: Flowmeter

12: High-pressure pump minimum flow rate control unit

13: Pressure transmitter

14: Inlet pressure control unit

15: Feed pump (pump C)

20: Membrane module

21: Cylindrical vessel

22: Separation membrane element

23: Inlet

24: Central pipe

25: Permeate drain

26: Concentrate drain

CLAIMS [Claim 1]

A reverse osmosis membrane separation apparatus comprising:

a pump A for feeding a part of liquid to be treated to a reverse osmosis membrane module after increasing a pressure thereof to a predetermined value;

an energy recovery device for increasing a pressure of a remaining part of the liquid to be treated utilizing a pressure of a concentrate that is discharged from the reverse osmosis membrane module;

a pump B for feeding the liquid to be treated whose pressure has been increased by the energy recovery device to the reverse osmosis membrane module after further increasing the pressure thereof to a predetermined value;

a flow rate control valve A for adjusting a flow rate of the liquid to be treated that is discharged from the pump A;

a bypass flow passage which bypasses the reverse osmosis membrane module from the flow rate control valve A; and

a flow rate control valve B which is disposed in the bypass flow passage and adjusts a bypass rate of the liquid to be treated.

[Claim 2]
The reverse osmosis membrane separation apparatus according to claim I, further comprising a pump C for feeding the liquid to be treated to the pump A and the energy recovery device, and a frequency converter for controlling a rotation speed of the pumpC.

[Claim 3]
An activation method of the reverse osmosis membrane separation apparatus according to claim 1, the method comprising:

before activation of the pump A, making adjustments so that a liquid to be treated flows through the energy recovery device, a booster pump, the reverse osmosis membrane module and the energy recovery device in this order and is discharged thereafter;
then activating the pump A while setting opening degrees of the flow rate control valve A and the flow rate control valve B to predetermined values; and

then controlling the flow rate control valve A and the flow rate control valve B step by step so that the flow rate control valve A operates in an opening direction and the flow rate control valve B operates in a closing direction, until an inlet pressure of the reverse osmosis membrane module is increased to a predetermined value.

[Claim 4]
A method for producing a permeate, comprising activating the reverse osmosis membrane separation apparatus by the activation method of a reverse osmosis membrane separation apparatus according to claim 3, followed by feeding liquid to be treated to the reverse osmosis membrane module to obtain a permeate.