JP2002085941A - Fresh water making process and fresh water maker - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fresh water making process which involves subjecting feed water to membrane treatment to obtain permeate water and enables attainment of a higher recovery ratio of the permeate water (i.e., fresh water), and also to provide a fresh water maker for the process. SOLUTION: This process involves: supplying feed water to pretreatment membrane to obtain pretreated water; and thereafter, supplying the pretreated water to reverse osmosis membrane to obtain permeate water and concentrated water; wherein a portion of the feed water is supplied to the reverse osmosis membrane while bypassing the pretreatment membrane.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fresh water producing method and a fresh water producing apparatus which can be preferably used for treating raw water such as seawater using a pretreatment membrane and a reverse osmosis membrane to obtain permeated water.

[0002]

2. Description of the Related Art With respect to separation of a mixture, there are various techniques for removing substances (eg, salts) dissolved in a solvent (eg, water). In recent years, however, membrane separation has been used as a process for saving energy and resources. The law is being used. Above all, the reverse osmosis method, which does not have a phase change such as evaporation and is advantageous in energy and is easy to operate and manage, has become widespread, but according to this method, seawater and low-concentration salt water (brine) are used. It is possible to provide industrial, agricultural or domestic water by removing impurities such as salt from raw water. Further, the reverse osmosis method is used not only for desalinating seawater and brackish water to obtain drinking water, but also for treating harmful water and wastewater, collecting valuable resources, and manufacturing industrial ultrapure water. be able to.

[0003] In implementing this reverse osmosis method, the focus of technology is how to produce water at low cost. Therefore, efforts are made to increase the recovery rate. When doing so, the concentration of solute components on the concentrated water side of the reverse osmosis membrane increases, and scale components with low solubility in water precipitate out,
This has caused problems such as a reduction in membrane life and permeated water quality.

Therefore, a method has been developed in which raw water to be supplied to the reverse osmosis membrane is pretreated with a membrane in advance to remove scale components to a certain extent to increase the recovery rate.
In order to increase the recovery rate of the entire apparatus, the recovery rate in the pretreatment process must be increased.
As a result, there was a problem that scale components were precipitated in the pretreatment step, and the total recovery rate could not be increased.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned conventional problems and to provide a fresh water producing method and a fresh water producing method capable of achieving a higher recovery rate in membrane treatment of raw water to obtain permeated water. It is to provide a device.

[0006]

SUMMARY OF THE INVENTION The present invention for achieving the above object is directed to a method for supplying treated water obtained by supplying raw water to a pretreatment membrane and supplying the treated water to a reverse osmosis membrane to obtain permeated water and concentrated water. The method is characterized in that a part of raw water is supplied to the reverse osmosis membrane by bypassing the pretreatment membrane.

Here, the supply pressure of the treated water supplied to the reverse osmosis membrane, the pressure difference between the supply pressure of the treated water supplied to the reverse osmosis membrane and the pressure of the concentrated water obtained from the reverse osmosis membrane, The quality of the permeated water to be measured, and at least one selected from the group consisting of the flow rate of the permeated water obtained from the reverse osmosis membrane is measured, and when the measured value exceeds a predetermined range, the measured value is measured. The desalination method described above, in which the ratio of raw water that bypasses the pretreatment membrane is controlled based on the value, is also preferable.

When the ratio of raw water that bypasses the pretreatment membrane is X, the recovery rate in the pretreatment membrane is Y, and the recovery rate in the reverse osmosis membrane is Z, X, Y and Z are expressed by the following equations (1). The water freshening method described above, in which the ratio of raw water that bypasses the pretreatment film is controlled so as to simultaneously satisfy the requirements (3) to (3), is also preferable.

0.6 ≦ Y <1 (1) 0.6 <Z ≦ 0.8 (2) 0.6 ≦ (X + (1−X) Y) Z (3) Further, the present invention provides a method for treating raw water A pretreatment membrane device having a pretreatment membrane for processing, a bypass flow path for bypassing raw water to the pretreatment membrane device, a flow control means provided in the bypass flow passage, A reverse osmosis membrane device having a reverse osmosis membrane, which separates the obtained treated water into permeated water and concentrated water, pressure measurement means for measuring the pressure of water supplied to the reverse osmosis membrane device, and water supplied to the reverse osmosis membrane device Differential pressure measuring means for measuring the pressure difference between the pressure and the concentrated water pressure, water quality measuring means for measuring the quality of the permeated water obtained from the reverse osmosis membrane device, and measuring the flow rate of the permeated water obtained from the reverse osmosis membrane device At least one measuring means selected from the group consisting of And a controller for controlling the flow rate control means based on the value measured by the means.

[0010] Further, water obtained by the above-described fresh water producing method or fresh water producing apparatus is also preferable.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A desalination apparatus and desalination method according to one embodiment of the present invention will be described with reference to FIG. In FIG. 1, a fresh water generator 1 includes an intake pump 21, a water pump 22, and an intake pump 21 in order from the upstream side in the flow direction of raw water such as seawater.
A pretreatment membrane module (pretreatment membrane device) 41, a high-pressure pump 23, and a reverse osmosis membrane module (reverse osmosis membrane treatment device) 42 are connected. In addition, the water intake pump 21 and the water supply pump 2
A bypass flow passage 32 is provided from the raw water flow passage between the raw water flow passage 2 and the treatment water flow passage 61 while bypassing the pretreatment membrane module. The bypass flow passage 32 has a valve (flow rate control) for controlling a bypass flow rate. Means) 81 are provided. Further, a turbine 70 for recovering pressure energy is connected between the high-pressure pump 23 and the concentrated water channel 52. A pressure gauge (pressure measuring means) 91 between the reverse osmosis membrane module 42 and the high-pressure pump 23; a differential pressure gauge (differential pressure measuring means) 92 between the reverse osmosis membrane module 42 and the concentrated water channel 52; A water quality meter (water quality measurement means) 93 and a flow meter (flow rate measurement means) 94 are provided in the permeated water flow path 62, and both are electrically connected to the control device 100. The control device 100 is also electrically connected to the valve 81.

Raw water such as seawater is taken in by a water intake pump 21 through a raw water flow path 11, a part of the raw water is bypassed through a pre-treatment membrane module through a bypass flow path 32, and the remaining raw water is supplied to the pre-treatment membrane module by a water supply pump 22. 41. Of the water supplied to the pretreatment membrane module 41, water that has not passed through the pretreatment membrane is discharged through the non-permeate water channel 51, while treated water that has passed through the pretreatment membrane is passed through the treatment water channel 61. The raw water flowing through the bypass passage 32 is mixed with the raw water and supplied to the reverse osmosis membrane module 42 by the high-pressure pump 23 as supply water. The supplied water is separated into permeated water and concentrated water through the reverse osmosis membrane module 42 and is taken out. Among them, the concentrated water is discharged after the pressure energy is recovered by the turbine 70 and used as power for the high-pressure pump 23.
When the above fresh water is produced, the pressure supplied to the reverse osmosis membrane module 42, the reverse osmosis membrane differential pressure, the permeated water quality, the permeated water quality are measured by the pressure gauge 91, the differential pressure gauge 92, the water quality meter 93, and the flow meter 94. The water flow rate is measured, the measured values are collected in the control device 100, the valve opening of the valve 81 is controlled based on the measured value, and the bypass flow rate is controlled.

In the case where fresh water is to be obtained from the seawater using a membrane separation method, seawater from which specific ions (polyvalent ions such as sulfate ions, calcium ions, and magnesium ions) that cause scale generation have been removed in advance. If the is separated by reverse osmosis membrane, the recovery rate of fresh water can be increased without generating scale. Furthermore, removing sodium ions and chlorine ions with the pretreatment membrane leads to a reduction in the concentration of seawater, that is, a reduction in osmotic pressure, which allows operation at a lower pressure. This not only reduces the size of the high-pressure pump used to supply the treated water to the reverse osmosis membrane and reduces power costs by saving labor, but also prevents the reverse osmosis membrane from becoming more compact. This means that the service life can be extended, that is, the element replacement ratio can be reduced. However, if all removal of polyvalent ions supplied to the reverse osmosis membrane is to be performed by pretreatment membrane treatment, for example, if the removal rate of both cations such as calcium ions and magnesium ions and anions such as sulfate ions is high, Not only does the life of the pretreatment membrane element itself decrease due to the generation of scale inside the pretreatment membrane, but also to prevent scale,
Eventually, a low recovery rate operation results in an increase in cost such as an increase in the number of pretreatment membrane elements, which is not a preferable situation. Therefore, as a means to solve this problem, by using a method of passing only a part of the pumped seawater through the pretreatment membrane, there was a margin for scale generation by using a small amount of pretreatment membrane module. The recovery rate can be set, and the required amount of water supplied to the reverse osmosis membrane can be easily secured by mixing the permeated water of the pretreatment membrane with the remaining seawater that does not pass through the pretreatment membrane. At this time, the division ratio of the pumped seawater, that is, the ratio of the seawater that bypasses the pretreatment membrane, depends on the supply pressure of the treatment water supplied to the reverse osmosis membrane, the supply pressure of the treatment water supplied to the reverse osmosis membrane, and the reverse osmosis membrane. It is preferable to measure and determine the pressure difference from the pressure of the concentrated water obtained from the above, the quality of the permeated water obtained from the reverse osmosis membrane, the flow rate of the permeated water obtained from the reverse osmosis membrane, and the like.

For example, the supply pressure of the treated water to be supplied to the reverse osmosis membrane is in a predetermined range.
When the pressure exceeds 10 MPa ± 20%, for example, the solute concentration in the treated water supplied to the reverse osmosis membrane is controlled to be reduced by decreasing the bypass amount and increasing the throughput in the pretreatment membrane.

The pressure difference between the supply pressure of the treated water supplied to the reverse osmosis membrane and the pressure of the concentrated water obtained from the reverse osmosis membrane is within a predetermined range.
When the pressure exceeds the range defined by the differential pressure at the initial stage of operation × 1.5, control is performed to decrease the bypass amount in the same manner as described above.

Further, also in the case where the permeated water quality of the permeated water obtained from the reverse osmosis membrane, for example, when the total solute concentration becomes 500 mg / l or more, the control for reducing the bypass amount may be performed in the same manner as described above. . The above-mentioned total solute concentration can be obtained, for example, by measuring electric conductivity.

Further, when the flow rate of the permeated water obtained from the reverse osmosis membrane falls below a predetermined range, for example, the flow rate at the initial operation × 0.8 to the flow rate at the initial operation, for example, Control is performed to increase the bypass amount and decrease the processing amount in the pretreatment film.

Although the bypass amount may be controlled based on the measured value in one state as described above, it is needless to say that the bypass amount may be controlled based on the measured values in two or more states. For example, the quality of the permeated water obtained from the reverse osmosis membrane deteriorates, that is, the total solute concentration exceeds a certain range,
In addition, when the amount of permeated water obtained from the reverse osmosis membrane also increases beyond a certain range, supply to the reverse osmosis membrane by reducing the bypass amount and increasing the throughput in the pretreatment membrane It is preferable to control so as to lower the solute concentration of the treated water.

Further, when the supply pressure to the reverse osmosis membrane is increased, the differential pressure is increased, the quality of the permeated water is deteriorated, and the amount of the permeated water is decreased, the control for reducing the bypass amount is similarly performed. Good. However, in such a case, since the reverse osmosis membrane may be deteriorated, it may be preferable to wash the reverse osmosis membrane element or replace the element with a new one.

In any case, by controlling the bypass amount as described above, the operating conditions of the fresh water generator can be optimized, and the permeated water can be obtained at a high recovery rate.

In the present invention, when the ratio of raw water that bypasses the pretreatment membrane is X, the recovery rate in the pretreatment membrane is Y, and the recovery rate in the reverse osmosis membrane is Z,
It is preferable to control the ratio of raw water that bypasses the pretreatment film so that X, Y and Z simultaneously satisfy the following expressions (1) to (3).

0.6 ≦ Y <1 (1) 0.6 <Z ≦ 0.8 (2) 0.6 ≦ (X + (1-X) Y) Z (3) where X = (pretreatment film) Volume of raw water that bypasses
(Volume of all raw water) Y = (permeate water volume of pretreatment membrane) / (supply water volume of pretreatment membrane) Z = (permeate water volume of reverse osmosis membrane) / (supply water volume of reverse osmosis membrane) Can be

Since X, Y and Z simultaneously satisfy the above expressions (1) to (3), the load ratio between the pretreatment membrane and the reverse osmosis membrane is kept within an appropriate range, and The amount of permeated water can be increased while maintaining the quality of permeated water, that is, the recovery rate can be increased.

In the above fresh water generator, various methods such as surface water intake, permeate water intake, and deep water intake can be used as the seawater intake method. Above all, when using the osmotic water intake method that removes seawater that has penetrated the sand layer on the seabed, which can remove suspended substances in seawater to some extent in advance,
It is preferable because fouling hardly occurs due to a small amount of turbid components.

It is preferable that the pretreatment membrane device includes a pretreatment membrane module having a pretreatment membrane and containing a plurality of pretreatment membrane elements in a pressure vessel.

As the pretreatment membrane, an ultrafiltration membrane (MF membrane)
Or a microfiltration membrane (UF membrane), a nanofiltration membrane (NF membrane), or the like. Among them, it is preferable to use a film capable of removing an ion component, and it is particularly preferable to use an NF film because polyvalent ions that are scale generation sources can be selectively removed. Further, as a material constituting the pretreatment film, for example, cellulose acetate, aromatic polyamide, aliphatic polyamide, polyvinyl alcohol, polyether sulfone, or the like can be used. Also, from the viewpoint of the film form,
A membrane having a hollow fiber membrane form or a flat membrane form can be used, and further, any form of an asymmetric membrane or a composite membrane can be used.

When a flat membrane type reverse osmosis membrane is used as the pretreatment membrane element, a spiral type element, a tubular type element, or a plate and frame type element is used as in the case of a reverse osmosis membrane element to be described later. Can be used. When a reverse osmosis membrane in the form of a hollow fiber membrane is used, an element bundled in a U-shape or an I-shape and housed in a housing may be used.

As for the pretreatment membrane module, one or a plurality of the above-mentioned elements are housed in a pressure vessel and used as in the case of a reverse osmosis membrane module described later. Of course, it is also possible to use this module as a pretreatment membrane module unit in which a plurality of modules are arranged in parallel.

As a high-pressure pump for supplying treated water to the reverse osmosis membrane device, various types of pumps such as a spiral pump, a turbine pump, and a plunger pump can be used.

The reverse osmosis membrane device preferably includes a reverse osmosis membrane module having a plurality of reverse osmosis membrane elements having a reverse osmosis membrane and housed in a pressure vessel.

As the reverse osmosis membrane, those using a cellulose acetate polymer or polyamide are preferable, those having an asymmetric structure, those having a composite structure, and those having a hollow fiber membrane form. And those having a flat film form can be used. Above all, a reverse osmosis membrane using a cellulose acetate polymer having an asymmetric structure, or a reverse osmosis membrane using a polyamide having a composite structure has a large effect, and in particular, an aromatic polyamide composite membrane is very effective. Great effect.

The element is an element in which the above-described reverse osmosis membrane is housed in a housing for easy use. When a reverse osmosis membrane in the form of a flat membrane is used, the element is constituted together with a permeated water flow path material and a raw water flow path material. A spiral element in which a membrane unit is wound around a water collecting pipe, a tubular element, a plate and frame element, or the like can be used. When a hollow fiber reverse osmosis membrane is used, U
It can be in a form of being bundled in a letter shape or an I shape and housed in a housing.

The reverse osmosis membrane module contains one or more of these elements in a pressure vessel.
Preferably, one containing 4 to 8 elements is used. Further, a module in which a plurality of such modules are arranged in parallel is referred to as a reverse osmosis membrane module unit, which can also be suitably used as a reverse osmosis membrane device.

In the present invention, in order to produce water more economically and efficiently, especially when fresh water is obtained from seawater, the above reverse osmosis membrane modules and reverse osmosis membrane module units are arranged in multiple stages to reverse water. It is preferable to configure a permeable membrane device.
For example, in the case of arranging two stages in the front and rear stages, the concentrated water obtained in the front stage is pressurized to supply water to the subsequent stage, and the concentrated water two-stage method in which permeated water (fresh water) is obtained from each of the front and rear stages, A permeate water two-stage method for obtaining high-quality permeate water (fresh water) by further using permeate water as a supply water to the subsequent stage can be applied.
Of course, these are not limited to the two-stage arrangement, and a multi-stage configuration of three or more stages can be used if necessary.

In the case of using the above-mentioned concentrated water two-stage method or multi-stage method, since the concentrated water obtained from the subsequent stage has high pressure energy, it is preferable to collect and use this as energy for increasing the pressure. . This can be realized using, for example, a recovery turbine.

[0036]

EXAMPLES In the following examples and comparative examples, a nanofiltration membrane (NF membrane) was used as a pretreatment membrane, and a polyamide-based composite membrane was used as a reverse osmosis membrane. (Example 1) Fresh water was produced using the fresh water producing apparatus shown in FIG.

First, raw water (seawater having a salt concentration of 3.5% by weight)
Is increased to 2.0 MPa by a water supply pump 22 and the pretreatment membrane
Supply to module 41 and process at recovery rate Y = 0.7
Was. The treatment water of the pretreatment membrane module 41 was bypassed.
Raw water (the ratio of bypass amount X = 0.5 at the start of operation)
Evaporation residue concentration of mixed feed water is about 25,000 m
g / l. In addition, as the pretreatment membrane module 41
Operates a 500 mg / l NaCl solution at 0.5 MPa.
Desalination rate 85% when turned, membrane water production 1.20m ^Three/
(M^Two-Spiral using NF membrane with d) performance
Six mold elements are connected in series and assembled into a pressure vessel
Was used.

Next, the supply water obtained by mixing the treated water of the pretreatment membrane module 41 and the raw water that has been bypassed is pressurized to 9.0 MPa by a high-pressure pump to increase the reverse osmosis membrane module 4
2 and operated continuously for 2,000 hours at a recovery rate Z = 0.75 to produce fresh water. As the reverse osmosis membrane module 42, seawater having a salt concentration of 3.5% by weight was converted to 5.5.
Six spiral reverse osmosis membrane elements using a reverse osmosis membrane having a desalination rate of 99.85% and a water production of 0.75 m ³ / (m ² · d) when operated at MPa. One connected and housed in a pressure vessel was used.

As a result, the recovery rate of the whole fresh water generator was 0.6.
4 and the quality of the permeated water is 148 mg / l
And was of high quality. During operation, the pretreatment membrane module
Calcium sulfate in concentrated water of reverse osmosis membrane module
No kale deposition was observed and the transmembrane pressure in the module
It is within a certain range and there is no need to change the bypass amount
Was. (Example 2) Fresh water was produced using the fresh water generator shown in FIG.
Was. In FIG. 2, the fresh water generator 2 is a reverse osmosis membrane device.
Reverse osmosis membrane modules 42 and 43 are connected in two stages in series,
The concentrated water obtained from the stage is used as feed water to the subsequent stage.
The pressure energy of the concentrated water obtained from the latter stage is
It was configured to be collected by a charger. And, set in the previous stage
The reverse osmosis membrane module 42 has a salt concentration of 3.5% by weight.
Desalination rate is 99.8 when the seawater is operated at 5.5 MPa.
5%, membrane water production 0.7m^Three/ (M^TwoA reverse osmosis membrane which is d)
The reverse osmosis membrane module 43 provided at the subsequent stage
When running 3.5 wt% seawater at 5.5 MPa
Desalination rate 99.85%, membrane water production 0.75m ^Three/ (M^Two・
A reverse osmosis membrane having the performance of d) was used. About other
Was the same as the fresh water generator shown in FIG.

First, raw water (seawater having a salt concentration of 3.5% by weight)
Is increased to 1.8 MPa by a water pump, and is supplied to a pretreatment membrane module 41 in which six spiral elements using an NF membrane are connected in series and incorporated into a pressure vessel, and a recovery rate Y = 0.67. Processed. The evaporation residue concentration of the supply water obtained by mixing the treated water of the pretreatment membrane module 41 with the bypassed raw water (the ratio of the bypass amount at the start of operation X = 0.7) was about 27,000 mg / l. Was. Next, the pressure of the supplied water was increased to 5.0 MPa by a high-pressure pump, and supplied to the reverse osmosis membrane module 42 in the preceding stage. The recovery rate in the former stage was set to 0.55. Next, the reverse osmosis membrane module 42 of the preceding stage is
Was supplied at a pressure of 8.5 MPa, and was continuously operated for 2,000 hours at a recovery rate of 0.45 to produce fresh water. Recovery rate Z of reverse osmosis membrane module as a whole
Was 0.75.

As a result of desalination, the recovery rate of the entire apparatus was 0.67.
, And the quality of the permeated water was high quality with a total solute concentration of 250 mg / l. During operation, precipitation of calcium sulfate scale was not observed in any of the modules. (Comparative Example 1) Raw water (sea water with a salt concentration of 3.5% by weight) was not passed through the pretreatment membrane module, but was 9.0MP with a high-pressure pump.
a, and the reverse osmosis membrane module 4 similar to that of Example 1
2, and operated for 2,000 hours with Z = 0.65.

As a result, 100 hours after the start of the operation, calcium sulfate was precipitated in the concentrated water at the outlet of the reverse osmosis membrane module 42. Further, 500 hours after the start of operation, the permeated water quality deteriorated to about 400 mg / l. Therefore, the permeated water quality of each element in the reverse osmosis membrane module was measured by passing a pipe through the center pipe of the reverse osmosis membrane element in the reverse osmosis membrane module. Inspection by the central pipe method for judging whether or not the element was damaged revealed that the element disposed on the inlet side of the reverse osmosis membrane module 42 was damaged. For this reason, the reverse osmosis membrane element on the inlet side was replaced with a new one, and the operating pressure was 8.0 MPa and Z = 0.55.
And resumed operation. (Comparative Example 2) Except that the entire amount of raw water (seawater with a salt concentration of 3.5% by weight) was supplied to the pretreatment membrane module 41 without bypassing (X = 0) and the operation was performed at Z = 0.9. Is
The operation was performed in the same manner as in Example 1. However, the differential pressure gradually increased immediately after the start of the operation, and the differential pressure became twice as large as that immediately after the start of the operation 200 hours after the start of the operation, so the operation was stopped. As a result of disassembling the NF membrane element in the pretreatment membrane module and analyzing attached matter on the membrane surface, precipitation of calcium sulfate was confirmed.

[0043]

According to the present invention, raw water bypassing the pretreatment membrane and treated water treated by the pretreatment membrane are mixed and supplied to the reverse osmosis membrane. Raw water can be separated without imposing excessive burden,
The permeated water can be obtained at a high recovery rate while keeping the quality of the permeated water high.

The supply pressure of the treated water supplied to the reverse osmosis membrane, the pressure difference between the supply pressure of the treated water supplied to the reverse osmosis membrane and the pressure of the concentrated water obtained from the reverse osmosis membrane, and the pressure obtained from the reverse osmosis membrane The quality of the permeated water, and at least one state selected from the group consisting of the flow rate of the permeated water obtained from the reverse osmosis membrane is measured, and when the measured value exceeds a predetermined range, the measured value is measured. By controlling the ratio of raw water that bypasses the pretreatment membrane based on the value, it is possible to set operating conditions that match the properties of the raw water and operating conditions that match the state of deterioration of the membrane, thus further improving the recovery rate. Can be.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a fresh water generator according to one embodiment of the present invention.

FIG. 2 is a schematic diagram showing a fresh water generator used in an example.

[Explanation of symbols]

 1: fresh water generator 2: fresh water generator 11: raw water flow path 21: water intake pump 22: water supply pump 23: high pressure pump 32: bypass flow path 41: pretreatment membrane module (pretreatment membrane device) 42: reverse osmosis membrane module (Reverse osmosis membrane device) 43: Reverse osmosis membrane module (reverse osmosis membrane device) 51: Non-permeated water channel 52: Concentrated water channel 61: Treated water channel 62: Permeated water channel 70: Recovery turbine 71: Turbocharger 81: Valve (Flow control means) 91: Pressure gauge (pressure measuring means) 92: Differential pressure gauge (Differential pressure measuring means) 93: Water quality meter (Water quality measuring means) 94: Flow meter (Flow rate measuring means) 100: Control device

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4D006 GA03 GA07 HA01 HA41 KA52 KA53 KA56 KA57 KE01P KE01Q KE01R KE06P KE06Q KE30P MA01 MA03 MA06 MB07 MC18 MC33 MC54 MC63 PB03

Claims (5)

[Claims] When the treated water obtained by supplying raw water to a pretreatment membrane is supplied to a reverse osmosis membrane to obtain permeated water and concentrated water, a part of the raw water is passed through the pretreatment membrane to perform reverse osmosis. A fresh water producing method characterized by supplying to a membrane. 2. The supply pressure of the treated water supplied to the reverse osmosis membrane, the pressure difference between the supply pressure of the treated water supplied to the reverse osmosis membrane and the pressure of the concentrated water obtained from the reverse osmosis membrane, and the pressure obtained from the reverse osmosis membrane. The quality of the permeated water, and at least one selected from the group consisting of the flow rate of the permeated water obtained from the reverse osmosis membrane is measured, and when the measured value exceeds a predetermined range, the measured value The fresh water producing method according to claim 1, wherein a ratio of raw water for bypassing the pretreatment membrane is controlled based on the condition. 3. The ratio of raw water that bypasses the pretreatment membrane is X
When the recovery rate in the pretreatment membrane is Y and the recovery rate in the reverse osmosis membrane is Z, the pretreatment membrane is bypassed so that X, Y and Z simultaneously satisfy the following equations (1) to (3). The fresh water producing method according to claim 1, wherein a ratio of raw water to be produced is controlled. 0.6 ≦ Y <1 (1) 0.6 <Z ≦ 0.8 (2) 0.6 ≦ (X + (1-X) Y) Z (3) 4. A pretreatment membrane device having a pretreatment membrane for treating raw water, a bypass flow passage for bypassing raw water to the pretreatment membrane device, and a flow control means provided in the bypass flow passage. A reverse osmosis membrane device having a reverse osmosis membrane, which separates treated water obtained from the pretreatment membrane device into permeated water and concentrated water, and a pressure measuring means for measuring a pressure of water supplied to the reverse osmosis membrane device; Differential pressure measuring means for measuring the pressure difference between the supply water pressure and the concentrated water pressure to the osmosis membrane device, water quality measurement means for measuring the quality of the permeated water obtained from the reverse osmosis membrane device, and obtained from the reverse osmosis membrane device At least one measuring means selected from the group consisting of flow rate measuring means for measuring the flow rate of permeated water to be supplied, and a control device for controlling the flow rate controlling means based on a value measured by the measuring means. Features of fresh water . 5. The water obtained by the method for producing fresh water according to claim 1 or the water producing apparatus according to claim 4.