Method of operating reverse osmosis membrane apparatus

Abstract

To enable energy-saving operation and stable supply of permeate water while water quality level of the permeate water is satisfied during operation, clarified seawater sw subjected to pretreatment and stored in to-be-treated water tank 12 is supplied to reverse osmosis membrane module 14 via clarified seawater supply passage 16. Permeate water pw obtained by an earlier stage element of the reverse osmosis membrane elements 36 of the reverse osmosis membrane module 14 is sent to a subsequent process from permeate water sending passage 46, and permeate water pw obtained by the reverse osmosis membrane element 36 is circulated into the to-be-treated water tank 12 via permeate water circulation passage 48. The flow rate of the clarified seawater sw supplied to the reverse osmosis membrane module 14 is controlled by variable flow rate high-pressure pump 22 provided in the clarified seawater supply passage 16, and opening degree of flow regulating valve 68 provided in the permeate water circulation passage 48 is controlled, whereby the circulation flow rate of the permeate water is controlled.

Claims

1. A method of operating a reverse osmosis membrane apparatus including a reverse osmosis membrane module having a plurality of reverse osmosis membrane elements arranged in series inside a high pressure vessel, the method comprising: a step of supplying to-be-treated water to the reverse osmosis membrane module from a to-be-treated water tank in which the to-be-treated water is stored; a step of treating the to-be-treated water by the plurality of reverse osmosis membrane elements sequentially to obtain: upstream permeate water separated by at least one upstream element including a first stage element among the plurality of reverse osmosis membrane elements; and downstream permeate water separated by at least one downstream element disposed downstream of the at least one upstream element; a step of circulating at least a part of the downstream permeate water to the to-be-treated water tank; a step of measuring a salinity of the upstream permeate water obtained by the at least one upstream element; and a step of controlling a circulation flow rate of the downstream permeate water based on measurement results of the salinity of the upstream permeate water.

2. The method of operating a reverse osmosis membrane apparatus according to claim 1, further comprising: a step of measuring a flow rate of the upstream permeate water obtained by the at least one upstream element; and a step of controlling a flow rate of the to-be-treated water supplied to the reverse osmosis membrane module to maintain the flow rate of the upstream permeate water at a target value, based on measurement results of the flow rate of the upstream permeate water.

3. The method of operating a reverse osmosis membrane apparatus according to claim 2, further comprising: a step of directing a part of the to-be-treated water supplied to a power recovery device disposed on a branched passage branching from a to-be-treated water supply passage, and then directing the part of the to-be-treated water to flow into the to-be-treated water supply passage after pressurizing the part of the to-be-treated water by the power recovery device in the branched passage, wherein, in the step of controlling the flow rate of the to-be-treated water, the flow rate of the upstream permeate water is maintained at the target value by controlling both of a flow rate of the to-be-treated water in the to-be-treated water supply passage and a flow rate of the part of the to-be-treated water in the branched passage.

4. The method of operating a reverse osmosis membrane apparatus according to claim 2, wherein when a temperature of the to-be-treated water is increased, the circulation flow rate of the downstream permeate water is increased, and the flow rate of the to-be-treated water supplied to the reverse osmosis membrane module is increased; and when the temperature of the to-be-treated water is decreased, the circulation flow rate of the downstream permeate water is decreased, and the flow rate of the to-be-treated water supplied to the reverse osmosis membrane module is decreased.

5. The method of operating a reverse osmosis membrane apparatus according to claim 2, wherein when a salinity of the to-be-treated water (sw) is increased, the circulation flow rate of the downstream permeate water is increased, and the flow rate of the to-be-treated water supplied to the reverse osmosis membrane module is increased; and when the salinity of the to-be-treated water is decreased, the circulation flow rate of the downstream permeate water is decreased, and the flow rate of the to-be-treated water supplied to the reverse osmosis membrane module is decreased.

6. The method of operating a reverse osmosis membrane apparatus according to claim 2, wherein when the reverse osmosis membrane of the reverse osmosis membrane elements is deteriorated, the circulation flow rate of the downstream permeate water is increased, and the flow rate of the to-be-treated water supplied to the reverse osmosis membrane module is increased.

7. The method of operating a reverse osmosis membrane apparatus according to claim 2, wherein the reverse osmosis membrane apparatus includes a variable flow rate pump having an inverter device capable of controlling a rotational speed of the pump, the pump being provided in a to-be-treatment water supply passage through which the to-be-treated water is supplied to the reverse osmosis membrane module from the to-be-treated water tank, and wherein in the step of controlling flow rate of the to-be-treated water, the rotational speed of the variable flow rate high-pressure pump is controlled to control the flow rate of the to-be-treated water flowing into the reverse osmosis membrane module.

8. The method of operating a reverse osmosis membrane apparatus according to claim 2, wherein the reverse osmosis membrane apparatus includes a flow regulating valve provided in a to-be-treated water supply passage through which the to-be-treated water is supplied to the reverse osmosis membrane module from the to-be-treated water tank, and wherein in the permeate water flow rate control step, the opening degree is controlled to control the flow rate of the to-be-treated water flowing into the reverse osmosis membrane module.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a flow diagram of a reverse osmosis membrane apparatus according to an embodiment of the present embodiment.

(2) FIG. 2 is a block diagram illustrating a control system according to an embodiment.

(3) FIG. 3 is a chart showing a relationship among a seawater temperature, deterioration degree of a reverse osmosis membrane and a circulation flow rate of permeate water.

(4) FIG. 4 is a chart showing a relationship among a seawater salinity, deterioration degree of a reverse osmosis membrane and a circulation flow rate of permeate water.

(5) FIG. 5 is a flow diagram illustrating a modified example of an embodiment.

(6) FIG. 6 is a cross-sectional diagram of a conventional reverse osmosis membrane module.

DETAILED DESCRIPTION

(7) Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. It is intended, however, that unless particularly specified, dimensions, materials, shapes, relative positions and the like of components described in the embodiments shall be interpreted as illustrative only and not limitative of the scope of the present invention.

(8) An embodiment of the present invention will be described with reference to FIG. 1 to FIG. 4. FIG. 1 is a flow diagram of a reverse osmosis membrane apparatus 10 applied to a seawater desalination plant. In FIG. 1, in a to-be-treated water tank 12, stored is clarified seawater sw obtained by subjecting raw seawater to pretreatment of sterilization and removal of relatively large foreign substances such as dusts and microbes. To the to-be-treated water tank 12, a clarified seawater supply passage 16 for supplying clarified seawater sw to a reverse osmosis membrane module 14, is connected. The clarified seawater sw stored in the to-be-treated water tank 12 is flown into the clarified seawater supply passage 16 by a water supply pump 18 provided in the clarified seawater supply passage 16.

(9) Impurities in the clarified seawater sw flown into the clarified water supply passage 16 are removed by a safety filter device 20. On a downstream side of the safety filter device 20, a high-pressure pump 22 is provided. The high-pressure pump 22 having an inverter device 22a is a variable flow rate pump of which rotational speed is controllable, and the flow rate of the clarified seawater sw is controlled by the high-pressure pump 22. At an inlet of the high-pressure pump 22, a branched passage 24 is branched from the clarified seawater supply passage 16. In the branched passage 24, a power recovery device 26 is provided. The power recovery device 26 is a pressure-exchange-type power recovery device to increase a pressure of the clarified seawater sw flown from the branched passage 24 and to send it to a booster pump 54, utilizing a pressure of concentrated seawater cs flown from a concentrated seawater discharging passage 50 which will be described later. The power recovery device 26 has, for example, a known structure (for example, see JP 2011-56439 A).

(10) The branched passage 24 is connected to the inlet of the power recovery device 26, and a branched passage 28 is connected to the outlet of the power recovery device 26. The other end of the branched passage 28 is connected to the clarified seawater supply passage 16 on the upstream side of the reverse osmosis membrane module 14. On the clarified seawater supply passage 16, a pressure sensor 30 and a temperature sensor 32 are provided between the joint portion of the branched passage 28 and reverse osmosis membrane module 14.

(11) The reverse osmosis membrane module 14 has a high pressure vessel 34 and a plurality of reverse osmosis membrane elements 36 arranged in series inside the high pressure vessel 34. At an inlet end of the high pressure vessel 34, an inlet opening 34a to which the clarified seawater supply passage 16 is connected, is formed, and from the inlet opening 34a, the clarified seawater sw is flown into the high pressure vessel 34. The reverse osmosis membrane module 14 has a cylindrical shape and has the same configuration as the spiral-type reverse osmosis membrane module 100 as shown in FIG. 6. Along the central axis of the reverse osmosis membrane module 14, provided is a center pipe 38 to which permeate water is collected.

(12) The center pipe 38 of each of the reverse osmosis membrane elements 36 is connected with a connector 40, and permeate water from each of the reverse osmosis membrane elements 36 is joined together in the center pipes 38. On the other hand, in the intermediate portion is blocked by an end cap 42 so that permeate water from an earlier stage element of the reverse osmosis membrane elements 36 and permeate water from a later stage element of the reverse osmosis membrane elements 36 are not mixed together. The internal space of the high pressure vessel 34 is separated with a brine seal 44 provided on an outer peripheral surface of each of the reverse osmosis membrane elements 36.

(13) The clarified water sw flown from the inlet opening 34a into the high pressure vessel 34 is separated into permeate water pw and concentrated seawater cs with a reverse osmosis membrane provided in the first stage element of the reverse osmosis membrane elements 14. An inlet opening 34b is formed at an inlet end of the high pressure vessel 34, and a permeate water sending passage 46 is connected to the inlet opening 34b. The permeate water pw from the earlier stage element of the reverse osmosis membrane elements 36 is joined to the center pipe 38, and is flown out to the permeate water sending passage 46 on the earlier stage side via the connector 40 and the inlet opening 34b. The permeate water pw flown out to the permeate water sending passage 46 on the earlier stage side is sent to a downstream subsequent process.

(14) At the outlet end of the high pressure vessel, an outlet openings 34c and 34d are formed. A permeate water circulation passage 48 is connected to the outlet opening 34c, and a concentrated seawater discharging passage 50 is connected to the outlet opening 34d. The permeate water separated with the reverse osmosis membrane provided in the later stage element of the reverse osmosis membrane elements 36 is joined to the center pipe 38, and is flown out to the permeate water circulation passage 48 on the later stage side via the connector 40 and the outlet opening 34c.

(15) The concentrated seawater cs separated from the permeate water pw by the first stage element of the reverse osmosis membrane elements 36 is flown out from the outlet end of the first stage element of the reverse osmosis membrane elements 36, and flown into the second stage element of the reverse osmosis membrane elements 36 to be separated into permeate water pw and concentrated seawater cs. Concentrated seawater cs is thereby separated into permeate water pw and concentrated seawater cs by the reverse osmosis membrane elements 36 sequentially to be concentrated gradually. The concentrated seawater cs flown from the last stage element of the reverse osmosis membrane elements 36 is discharged from the outlet opening 34d to the concentrated seawater discharging passage 50. The concentrated seawater discharging passage 50 is connected to the power recovery device 26.

(16) The high-pressure concentrated seawater cs flown out to the concentrated seawater discharging passage 50 is flown into the power recovery device 26 to increase the pressure of the clarified seawater sw entered from the branched passage 24 and send out the seawater sw to the branched passage 28. As the pressure of the clarified seawater sw at the inlet of the high pressure vessel 34 is thereby increased, a part of the power for the high-pressure pump 22 can be provided by the power recovery device 26.

(17) To the outlet of the power recovery device 26, a concentrated seawater discharging passage 52 is provided, and the concentrated seawater cs having a low pressure and flown out from the power recovery device 26 is discharged from the concentrated seawater discharging passage 52. In the branched passage 28, a booster pump 54 is provided, and the pressure of the clarified seawater sw can be increased by the booster pump 54 to increase the flow rate of the clarified seawater. The booster pump 54 has provided an inverter device 54a capable of controlling the rotational speed of the pump. A flowmeter 56 is provided in the branched passage 24, and a flow regulating valve 58 is provided in the concentrated seawater discharging passage 52.

(18) In the permeate water sending passage 46, a flowmeter 60, a salinity meter 62 to detect an electric conductivity of the permeate water pw and to obtain the salinity from the detected value, and a flow regulating valve 64 are provided. In the permeate water circulation passage 48, a flowmeter 66 and a flow regulating valve 68 are provided.

(19) FIG. 2 is a block diagram illustrating a control system for the reverse osmosis membrane apparatus 10. In FIG. 2, detected values of the pressure sensor 30, the temperature sensor 32, flowmeters 56, 60 and 66, and the salinity meter 62 are input to the controller 70. On the basis of the detected values, the controller 70 controls the behavior of the water supply pump 18, the high pressure pump 22 and the booster pump 54, and controls the opening degree of the flow regulating valves 58, 64 and 68.

(20) According to the embodiment, the permeate water pw having a better water quality from the earlier stage element of the reverse osmosis membrane elements 36 is sent to the downstream subsequent process, and the permeate water pw having a worse water quality from the later stage element of the reverse osmosis membrane elements 36 is returned to the to-be-treated water tank 12. By returning the permeate water pw from the later stage element of the reverse osmosis membrane elements 36 to the to-be-treated water tank 12 to decrease the salinity of the clarified seawater sw to be supplied to the high pressure vessel 34, it is possible to decrease the salinity of the permeate water pw.

(21) In such a configuration, by controlling the rotational speed of the high pressure pump 22 to control the pressure of the clarified seawater sw at the inlet of the high pressure vessel 34, the flow rate of the permeate water from the earlier stage element of the reverse osmosis membrane elements 36 is controlled so as to be constant. In addition, while the detected value of the salinity meter 62 is monitored, the flow rate of the permeate water from the later stage element of the reverse osmosis membrane elements 36 flowing in the permeate water circulation passage 48 is controlled so that the salinity becomes the reference value.

(22) FIG. 3 is a chart where the horizontal axis represents the temperature of the clarified seawater sw, and the vertical axis represents the circulation flow rate of the permeate water from the later stage element of the reverse osmosis membrane elements 36. In FIG. 3, each of the three lines represents the circulation flow rate of the permeate water required to satisfy the required value of the water quality of the permeate water: the line A is for a case where the deterioration degree of a reverse osmosis membrane of the reverse osmosis membrane elements 36 is large, the line B is for a case where the deterioration degree of the reverse osmosis membrane is small, and the line C is for a case where the reverse osmosis membrane is not deteriorated. In a case where temperature of the clarified seawater is increased, as the water quality of the permeate water pw is declined, the circulation flow rate of the permeate water is increased so that the water quality of the permeate water pw satisfies the required value. In a case where the reverse osmosis membrane is deteriorated, as the water quality of the permeate water pw is declined, the circulation flow rate of the permeate water is increased so that the water quality of the permeate water pw satisfies the required value.

(23) FIG. 4 is a chart where the horizontal axis represents the salinity of the clarified seawater sw, and the vertical axis represents the circulation flow rate of the permeate water from the later stage element of the reverse osmosis membrane elements 36. In a case where the salinity of the clarified seawater sw is increased, the circulation flow rate of the permeate water is increased so that the water quality of the permeate water pw satisfies the required value. In a case where the reverse osmosis membrane is deteriorated, as the water quality of the permeate water pw is declined, the circulation flow rate of the permeate water is increased so that the water quality of the permeate water pw satisfies the required value.

(24) According to the embodiment, during the operation of the reverse osmosis membrane apparatus 10A, by controlling the flow rate of the clarified seawater sw supplied to the reverse osmosis membrane module 14 and the circulation flow rate of the permeate water returned to the to-be-treated water tank 12 depending upon the temperature or salinity of the clarified seawater sw or the deterioration degree of the reverse osmosis membrane module 14, it is possible to satisfy the required value of the permeate water pw and to supply the permeate water pw stably. Further, as the required value of the permeate water pw is less likely to be excessively satisfied, energy-saving operation becomes possible. In addition, the control as described above can be automated by using the controller 70.

(25) Since the permeate water pw from the later stage element of the reverse osmosis membrane elements 14 is returned to the to-be-treated water tank 12, there is no need to consider the pressure balance between the flow passage for returning the permeate water pw and the flow passage for receiving the permeate water pw. Thus, a device to adjust pressure therebetween becomes unnecessary, and it is thereby possible to reduce cost and to permit change in flow rate of the permeate water to be returned without limitation.

(26) By increasing the pressure of the clarified seawater sw flown into the branched passage 24 by the power recovery device 26, it is possible to increase the pressure of the clarified seawater sw flown into the inlet opening 34a of the high pressure vessel 34. It is thereby possible to reduce the power for the high-pressure pump 22, and energy-saving operation becomes possible. In addition to the above control, by controlling the rotational speed of the booster pump 54 to control the flow rate of the clarified seawater sw flown into the branched passage 24, it is possible to control the water quality and the supply amount of the permeate water pw more accurately.

(27) According to the embodiment, the total amount of the permeate water pw from the later stage element of the reverse osmosis membrane elements 14 is returned to the to-be-treated water tank 12. However, by using a branched passage branched from the permeate water circulation passage 48 and connected to the permeate water sending passage 46, only a part of the permeate water pw may be circulated to the to-be-treated water tank 12. It is thereby possible to facilitate the control of the circulation flow rate of the permeate water.

(28) Now, a modified example of the embodiment will be described with reference to FIG. 5. In this modified example, as the means for controlling the flow rate of the clarified seawater sw flown into the high pressure vessel 34, a constant flow rate high-pressure pump 72, which is not of a variable flow rate type, is provided in the clarified seawater supply passage 16, and a flow regulating valve 74 is provided on the outlet side of the high-pressure pump 72. In addition, a booster pump 76, which is not of a variable flow rate type, is provided in the branched passage 28, and a flow regulating valve 78 is provided on the outlet side of the booster pump 76. The opening degrees of the flow regulating valves 74 and 78 are controlled by the controller 70. Except for what is specified in the above description, this example has the same configuration as the above embodiment.

(29) In this modified example, operation is carried out in the same manner as in the above embodiment. That is, while the detected values of by the pressure sensor 30, the temperature sensor 32, the salinity meter 62 and the flowmeters 56, 60 and 66 are monitored by the controller 70, the flow rate of the clarified seawater sw flowing in the clarified seawater supply passage 16 and the flow rate of the permeate water pw flowing in the permeate water circulation passage 48 are controlled. According to this modified example, there is an advantage that it is possible to simplify the flow rate control of the clarified seawater supply passage 16 and the branched passage 28 and to reduce cost thereof.

(30) In the above embodiment and the above modified example, the controller 70 is provided and control of the high-pressure pump 22 and the booster pump 54, and regulation of the flow regulating valves 58, 64, 68, 74 and 78 are automated. However, alternatively, such control may be carried out manually by an operator without providing a controller 70.

(31) In the above embodiment and the above modified example, a spiral-type reverse osmosis membrane element 14 is used as the reverse osmosis membrane module. However, instead of the spiral-type reverse osmosis membrane element 14, a flat membrane-type reverse osmosis membrane element may be employed. The present invention may also be applied to, for example, pure water production apparatuses.

INDUSTRIAL APPLICABILITY

(32) According to the present invention, a reverse osmosis membrane apparatus which enables energy-saving operation and stable supply of permeate water while satisfying a required value of the permeate water during operation, is provided.

REFERENCE SIGNS LIST

(33) 10 Reverse osmosis membrane apparatus 12 To-be-treated water tank 14, 100 Reverse osmosis membrane module 16 Clarified seawater supply passage 18 Water supply pump 20 Safety filter device 22, 72 High pressure pump 22a Inverter device 24, 28 Branched passage 26 Power recovery device 30 Pressure sensor 32 Temperature sensor 34, 102 High pressure vessel 34a, 34b, 102a Inlet opening 34c, 34d, 102b, 102c Outlet opening 36, 104 Reverse osmosis membrane element 38, 106 Center pipe 40, 110 Connector 42, 108 End cap 44, 112 Brine seal 46 Permeate water sending passage on earlier stage side 48 Permeate water circulation passage on later stage side 50, 52 Concentrated seawater discharging passage 54, 76 Booster pump 54a Inverter device 56, 60, 66 Flowmeter 58, 64, 68, 74, 78 Flow regulating valve 62 Salinity meter 70 Controller 114 To-be-treated water supply passage 116 Permeate water discharging passage 118 Concentrated water discharging passage Cs Concentrated seawater Cw Concentrated water Sw Clarified seawater Pw Permeate water Tw To-be-treated water