Operational method of membrane filtration system

Abstract

To provide an operational method of a membrane filtration system that can shorten time to reach an objective flow rate and is easy of control until reaching to the objective flow rate even if changes occur in water qualities caused by changes in water temperature, etc.SOLUTION: Provided is an operational method of a membrane filtration system for controlling a flow rate of a membrane filtration water by a rotation frequency of a pump, the operational method of a membrane filtration system including: an initial step of execution of operation where a rotation frequency of the pump is controlled by an output frequency (fixed output frequency) of the inverter with a fixed numeric value; an intermediate step of switching to the operation where the rotation frequency of the pump is controlled by elevating or lowering the output frequency gradually and conducting judgement of transition to a proportional integral derivative (PID)-controlled operation by comparing the present value of the flow rate of the membrane filtration water with the objective flow rate; and a final step of execution of the operation with a rotation frequency of the pump is PID-controlled.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for operating a membrane filtration system.

Background Art

[0002] In the conventional flow rate control method of a membrane filtration device, a target value (Set Variable: SV; in one aspect, the target flow rate) is set, and control using P (proportional), I (integral), and D (derivative) is performed based on the difference from the current value (Process Variable: PV), and the manipulated variable (Manupilative Variable: MV) is determined to reach the target value. This is common.

[0003] At the initial stage of startup when the difference between SV and PV is large, an overshoot that greatly deviates from SV occurs, so various control methods have been proposed in the stage before shifting to PID (proportional integral derivative) control.

[0004] For example, in Patent Document 1, after performing an initial operation in which the rotational speed of a feed water pump is controlled according to the output frequency of an inverter controlled by P control so that the treated water flow rate becomes the target flow rate, before the treated water flow rate reaches the target flow rate, a method for operating a membrane filtration system that switches to PID control so that the treated water flow rate becomes the target flow rate is disclosed. In the method described in Patent Document 1, overshoot can be prevented by using P control that results in a gentle manipulated variable initially.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] In the method described in Patent Document 1, while it is possible to surely control without causing overshoot, as a characteristic of P control, the operation amount decreases as it approaches the target flow rate. Therefore, there is a problem that it takes time to reach the target flow rate including the time after the transition from P control to PID control. In addition, since the quality of water changes due to changes in the water temperature in the membrane filtration system, control that can easily cope with these changes has been desired.

[0007] An object of the present invention is to provide an operation method for a membrane filtration system that shortens the time until the target flow rate is reached and enables easy control until the target flow rate is reached even if the water quality changes due to a change in water temperature or the like.

Means for Solving the Problems

[0008] Aspects of the present invention are as follows.

[0009] <<Aspect 1>> An operation method for a membrane filtration system that controls the flow rate of membrane-filtered water by the rotation speed of a pump, An initial step of performing an operation in which the rotation speed of the pump is controlled at a fixed output frequency (fixed output frequency) of an inverter with a fixed value, A middle step of switching to an operation in which the rotation speed of the pump is controlled by gradually increasing or decreasing the output frequency, and making a determination to shift to an operation controlled by PID by comparing the current value of the flow rate of the membrane-filtered water with the target flow rate, and An operation method for a membrane filtration system including a final step of performing an operation in which the rotation speed of the pump is controlled by PID.

[0010] <<Aspect 2>> The operation method for a membrane filtration system according to Aspect 1, wherein when the difference between the current value of the flow rate of the membrane-filtered water and the target flow rate is within ±100%, a shift is made to an operation controlled by PID.

[0011] <<Aspect 3>> The operation method of the membrane filtration system according to Embodiment 2, wherein when the difference between the current value of the flow rate of the membrane-filtered water and the target flow rate is within ±0% to ±20%, a shift is made to a PID-controlled operation.

[0012] <<Embodiment 4>> In the final stage of the operation of the membrane filtration system, during the PID-controlled operation, by storing the output frequency of the inverter every 0.1 to 600 seconds, In the initial stage of the next operation of the membrane filtration system, using the output frequency stored in the final stage of the previous operation of the membrane filtration system as the fixed output frequency, an operation in which the rotational speed of the pump is controlled is performed. The operation method of the membrane filtration system according to any one of Embodiments 1 to 3.

Advantages of the Invention

[0013] According to the present invention, the time until the target flow rate of the membrane filtration system is reached is shortened. Further, according to one aspect of the present invention, even if the water quality changes due to a change in water temperature or the like, the control until the target flow rate is reached is easy.

Brief Description of the Drawings

[0014]

Figure 1

Modes for Carrying Out the Invention

[0015] Hereinafter, a mode for carrying out the present invention (hereinafter also referred to as the present embodiment) will be described in detail. Note that the present embodiment is not limited to the embodiments described below, and various modifications can be made and used within the scope shown.

[0016] The present embodiment will be described below with reference to the drawings. FIG. 1 is a diagram showing an example of the operation method of the membrane filtration system according to the present embodiment. Note that parts unnecessary for the description are omitted and only the illustrated parts are shown.

[0017] The membrane filtration system of this embodiment filters raw water. In FIG. 1, the membrane filtration system includes a water supply line 11, and a hollow fiber membrane module 4 is provided in this water supply line 11. In the water supply line 11 on the upstream side of the hollow fiber membrane module 4, a raw water tank 1, a flow meter 3, a controller 5, a pump 8, and an inverter 9 are provided. Further, a flow meter 6 is provided in the reflux line 7 on the downstream side of the hollow fiber membrane module 4.

[0018] The hollow fiber membrane module 4 filters the raw water. This hollow fiber membrane module 4 is formed using a filtration membrane (not shown). Specifically, it is formed using a microfiltration membrane (MF membrane), an ultrafiltration membrane (UF membrane), a reverse osmosis membrane (RO membrane), a nanofiltration membrane (NF membrane), etc. The reverse osmosis membrane and the nanofiltration membrane are synthetic polymer membranes such as polyamide-based and polyether-based membranes. And the reverse osmosis membrane is a liquid separation membrane that can block the permeation of substances with a molecular weight of about several tens. Also, the nanofiltration membrane is a liquid separation membrane that can block the permeation of particles and polymers smaller than about 2 nm (substances with a maximum molecular weight of about several hundreds). The nanofiltration membrane has a function that is positioned between the reverse osmosis membrane and the ultrafiltration membrane (UF membrane) that can separate substances with a molecular weight of about 1,000 to 300,000 in terms of the filtration function. The filtration membrane is selected so that the raw water becomes the desired water quality (in one aspect, the water quality that can be used for pharmaceutical applications).

[0019] In the hollow fiber membrane module 4, the raw water flowing in from the raw water tank 1 is filtered by the filtration membrane and becomes treated water, which flows out to the downstream side. As a result, the treated water (in one aspect, membrane-filtered water) that has become the desired water quality flows out to a treated water line (not shown). On the other hand, the treated water that has not become the desired water quality flows out to the reflux line 7 connected to the hollow fiber membrane module 4. This reflux line 7 is connected to the water supply line 11 on the upstream side of the pump 8. And the treated water that has not become the desired water quality and has flowed out to the reflux line 7 flows back to the water supply line 11 on the upstream side of the hollow fiber membrane module 4 via the reflux line 7.

[0020] The pump 8 is controlled such that its rotational speed varies according to the output frequency of the inverter 9 so that the treated water flow rate reaches the target flow rate. Specifically, when the controller 5 receives the flow rate detection signal 10 from the flow meter 6, it outputs this as a frequency instruction signal to the inverter 9. The inverter 9 then converts this frequency instruction signal into the output frequency (in one aspect, the operation output 2) to control the rotational speed of the pump 8. Here, the control function such as the PID control of the controller 5 is a function that compares the flow rate detection signal 10 from the flow meter 6 as a feedback value with the target flow rate and operates to make the deviation zero.

[0021] The flow meter 6 provided on the downstream side of the hollow fiber membrane module 4 measures the current value (PV), which is the actual value of the treated water flow rate flowing out from the hollow fiber membrane module 4. The PV measured by the flow meter 6 provided on the downstream side of the hollow fiber membrane module 4 is converted into the flow rate detection signal 10 and input to the controller 5 equipped with a CPU. When the controller 5 receives the flow rate detection signal 10 from the flow meter 6, it calculates the operation amount (MV) of the treated water flow rate based on the deviation between the SV and the PV, and outputs this as a frequency instruction signal (for example, a current value of 4 to 20 mA or a voltage value of 1 to 5 V) to the inverter 9. Then, according to the operation output 2 of the inverter 9 that has received the frequency instruction signal, the treated water flow rate from the hollow fiber membrane module 4 is controlled by the rotational speed of the controlled pump 8 so as to reach the target flow rate. The flow meter 3 provided on the upstream side of the hollow fiber membrane module 4 measures the current value (PV), which is the actual value of the raw water flow rate flowing into the hollow fiber membrane module 4.

[0022] This embodiment is an operation method of a membrane filtration system that controls the flow rate of the membrane-filtered water by the rotational speed of a pump. The operation method of the membrane filtration system of this embodiment (hereinafter referred to as the operation method of this embodiment) includes the following steps as the membrane filtration process: An initial step of performing an operation in which the rotational speed of the pump is controlled at a fixed output frequency (fixed output frequency) of the inverter with fixed numerical values. By gradually increasing or decreasing the output frequency, the operation is switched to an operation in which the rotational speed of the pump is controlled, and a medium-term process of determining to shift to a PID-controlled operation by comparing the current value of the membrane filtration water flow rate with the target flow rate, and A final process in which the rotational speed of the pump performs an operation controlled by PID control is included.

[0023] In one aspect, the operation method of the present embodiment repeats a membrane filtration process including an initial process, a medium-term process, and a final process as one cycle. The operation method of the present embodiment is advantageous as an operation method of a membrane filtration system that performs such a repeated cycle.

[0024] ≪Initial process≫ The initial process is a process of performing an operation in which the rotational speed of the pump is controlled at a fixed output frequency of the inverter (fixed output frequency). In one aspect, in this process, the inverter 9 is fixed at a certain numerical value of the output frequency (hereinafter referred to as the fixed output frequency), the rotational speed of the pump 8 is controlled, and the controller 5 controls the frequency instruction signal to the inverter 9 so that the treated water flow rate becomes the target flow rate. In one aspect, fixing the output frequency of the inverter 9 to a certain numerical value means setting the output frequency to a certain numerical value. In one aspect, a certain numerical value may be appropriately set according to the embodiment within the range of 0.0 to 120.0 Hz of the output frequency of the inverter 9, preferably within the range of 10.0 to 60.0 Hz. In one aspect, the operation time of this process is 0 to 999 seconds, preferably 0 to 60 seconds.

[0025] After the previous operation of the membrane filtration system, in the initial process of the next operation of the membrane filtration system, an operation in which the rotational speed of the pump is controlled may be performed using the output frequency stored in the final process described later of the previous operation of the membrane filtration system as the fixed output frequency. In the operation method of the present embodiment, even if the treated water flow rate control of the membrane filtration system is performed at the time of switching each process, since the set value of the treated water flow rate does not start from 0, the time until the target flow rate is reached is shortened.

[0026] In the present disclosure, a large water temperature change refers to a case where the water temperature during the first operation and the next operation of the membrane filtration device is measured, and the difference in water temperature between the first operation and the next operation is ±3°C or more. For example, if the water temperature drops by -3°C or more from the first operation to the next operation, the water quality changes and the flow rate of the membrane-filtered water during the next operation decreases compared to the flow rate of the membrane-filtered water during the first operation. According to the operation method of the present embodiment, even if the water quality changes due to a large water temperature change, it is easy to cope with these changes.

[0027] ≪Middle-stage process≫ The middle-stage process is a process of switching to an operation in which the rotation speed of the pump is controlled by gradually increasing or decreasing the output frequency, and making a determination to shift to a PID-controlled operation by comparing the current value of the flow rate of the membrane-filtered water with the target flow rate. In one aspect, this process is a process of switching from an initial process in which the rotation speed of the pump is controlled at the fixed output frequency of the inverter 9 to an operation in which the rotation speed of the pump is controlled by gradually increasing or decreasing the output frequency. In one aspect, this process is a process of measuring the current value of the treated water flow rate with the flow meter 6, comparing the current value with the target flow rate, and obtaining the difference therebetween, thereby making a determination to shift to a PID-controlled operation. The transition from the initial process to the middle-stage process is carried out after the elapse of the operation time set in the initial process (in one aspect, 0 to 999 seconds, preferably 0 to 60 seconds).

[0028] In order to effectively implement the PID control to be carried out in the final stage process described later, it is necessary to control the treated water flow rate in the middle-stage process before shifting to the PID control to reduce overshoot. When starting the operation of the pump 8, after performing the initial process, start the middle-stage process and switch to the final stage process before the treated water flow rate reaches the target flow rate. Specifically, when the difference between the current value of the flow rate of the membrane-filtered water and the target flow rate as the treated water flow rate reaches within ±100%, the process shifts from the middle stage to the final stage. For example, when the difference between the current value of the flow rate of the membrane-filtered water and the target flow rate reaches within ±100%, it means that the flow rate of the membrane-filtered water is 0 to 200% of the target flow rate. In a preferred embodiment, when the difference between the current value of the flow rate of the membrane-filtered water and the target flow rate is within ±0% to ±20%, the operation shifts to a PID-controlled operation. For example, when the difference between the current value of the flow rate of the membrane-filtered water and the target flow rate reaches within ±20%, it means that the flow rate of the membrane-filtered water is 80 to 120% of the target flow rate.

[0029] Also, in one aspect, controlling the rotation speed of the pump by the inverter 9 step by step means that the frequency of the inverter 9 may be increased or decreased every second until the difference between the target flow rate and the current value reaches within the predetermined range, or may be increased or decreased step by step so as to maintain the set frequency at a certain value (for example, 1.0 Hz) for 5 seconds. The frequency of the inverter 9 in the middle stage is preferably 0.1 to 2.0 Hz per second.

[0030] ≪Final Stage≫ The final stage is a process of performing an operation in which the rotation speed of the pump is PID-controlled. In one aspect, the final stage is a process in which the rotation speed of the pump 8 is controlled according to the output frequency of the inverter 9 controlled by PID control so that the treated water flow rate becomes the target flow rate.

[0031] According to the operation method of the present embodiment, at the start of the operation of the pump 8, an initial process is performed in which the rotation speed of the pump is controlled by the fixed output frequency of the inverter 9, so that the treated water flow rate approaches the target flow rate in a shorter time. Further, when the difference between the current value of the flow rate of the membrane-filtered water and the target flow rate reaches within the predetermined range, by switching to the final stage in which the frequency of the inverter 9 is controlled by PID control, the overshoot amount of the treated water flow rate with respect to the target flow rate is suppressed.

[0032] In a preferred embodiment, in the final stage of the operation method of this embodiment, during the operation under PID control, the output frequency of the inverter 9 is memorized every 0.1 to 600 seconds, preferably every 0.1 to 10 seconds. Specifically, during the operation under PID control, the output frequency of the inverter 9 that controls the treated water flow rate (MV) every 0.1 to 600 seconds, preferably every 0.1 to 10 seconds, is memorized. In a more preferred embodiment, in the operation method of the membrane filtration system of this embodiment, from the stop of the operation of the membrane filtration system to the initial stage of the next operation, using the output frequency memorized in the previous operation as the fixed output frequency, an operation in which the rotation speed of the pump is controlled can be carried out. Thereby, in the initial stage of the next operation of the membrane filtration system, since the set value of the treated water flow rate does not start from 0, the time until the target flow rate is reached becomes shorter.

[0033] <Circulation step> In one embodiment, in the operation method of this embodiment, in addition to the membrane filtration step, a circulation step and the like carried out after the membrane filtration step may be included in a repeated cycle. The circulation step of this embodiment is carried out between the membrane filtration step of the previous cycle and the membrane filtration step of the next cycle. In one embodiment, the circulation step of this embodiment is carried out when the raw water does not flow into the membrane filtration system or the timing of the raw water inflow is unknown, such as when a problem occurs in the membrane filtration step, and it is a step to maintain the water quality of the water treated in the membrane filtration step. In one embodiment, the circulation step of this embodiment is a step of carrying out an operation in which the rotation speed of the pump is controlled at the output frequency of the inverter with a fixed value (fixed output frequency). In one embodiment, the value of the fixed output frequency in the circulation step may be appropriately set according to the embodiment within the range of 10 to 30 Hz of the output frequency of the inverter 9.

Example

[0034] Hereinafter, preferred embodiments of the present invention will be described in detail by way of example, but the present invention is not limited thereby.

[0035] [Example 1] As the membrane filtration device, an ultrafiltration (UF) membrane module (molecular weight cut-off 6000) was used. For the PID control, a controller with the product name KV-7500, manufactured by Keyence Corporation, was used. The target flow rate of the membrane-filtered water (treated water) was 0.5 m 3 / h.

[0036] [Initial operation] In the initial process, the output frequency of the inverter (product name FRN2.2G1S, manufactured by Fuji Electric Co., Ltd.) was fixed at 25 Hz, and after operating the pump for 10 seconds, the process shifted to the middle process. In the middle process, the output frequency was increased step by step from 25 Hz in the initial process by 0.5 Hz every second, and the output frequency was set to 29 Hz. Also, when the current value of the flow rate of the membrane-filtered water was measured with a flow meter (product name FD-H20, manufactured by Keyence Corporation), it was 0.42 m 3 / h. By comparing the current value of the flow rate of this membrane-filtered water with the target flow rate, since the difference was within ±20%, the operation shifted to the PID-controlled operation. The time from the middle process to the final process was 8 seconds. In the final process, the PID control operation was carried out. When the current value of the flow rate of the membrane-filtered water was measured with a flow meter, the target flow rate was reached 12 seconds after shifting from the middle process to the final process. Also, in the final process, the output frequency of the inverter was memorized during the PID control operation. The memorized output frequency every 5 seconds was 30.8 Hz.

[0037] [Next operation] Using the output frequency 30.8 Hz memorized in the initial operation as the output frequency at the start of the pump in the next operation, the process immediately shifted from the initial process to the middle process. In the middle process, the frequency was set to 31.3 Hz by increasing or decreasing step by step from 30.8 Hz in the initial process by 0.5 Hz every second. Also, when the current value of the flow rate of the membrane-filtered water was measured with a flow meter, it was 0.51 m 3 / h. When the current value of the flow rate of the membrane-filtered water was compared with the target flow rate, the difference was within ±20%, so the process shifted to the final stage. The time from the intermediate stage to the final stage was 1 second. In the final stage, PID control operation was performed. When the current value of the flow rate of the membrane-filtered water was measured with a flow meter, the target flow rate was reached 2 seconds after the start of the PID control operation. In addition, when the water temperature during the first operation and the next operation of the membrane filtration device was measured, the water temperature change was +1°C between the first operation and the next operation.

[0038] [Example 2] [First operation] In the initial stage, the output frequency of the inverter (product name FRN2.2G1S, manufactured by Fuji Electric Co., Ltd.) was fixed at 10 Hz, and after operating the pump for 10 seconds, the process shifted to the intermediate stage. In the intermediate stage, the output frequency was increased step by step from 10 Hz in the initial stage by 0.5 Hz every second, and the output frequency was set to 12 Hz. Also, when the current value of the flow rate of the membrane-filtered water was measured with a flow meter (product name FD-H20, manufactured by Keyence Corporation), 0.11 m 3 / h. When the current value of the flow rate of this membrane-filtered water was compared with the target flow rate, the difference was within ±80%, so the operation shifted to the PID-controlled operation. The time from the intermediate stage to the final stage was 4 seconds. In the final stage, PID control operation was performed. When the current value of the flow rate of the membrane-filtered water was measured with a flow meter, the target flow rate was reached 50 seconds after the start of the PID control operation. Also, in the final stage, the output frequency of the inverter was memorized during the PID control operation. The memorized output frequency every 5 seconds was 30.8 Hz.

[0039] [Next operation] Using the output frequency 30.8 Hz memorized in the first operation as the output frequency at the start of the pump in the next operation, the process shifted directly from the initial stage to the intermediate stage. In the intermediate process, the frequency was set to 31.3 Hz by gradually increasing or decreasing it by 0.5 Hz per second from the output frequency of 30.8 Hz in the initial process. Also, when the current value of the flow rate of the membrane-filtered water was measured with a flow meter, it was 0.5 m 3 / h. By comparing the current value of the flow rate of this membrane-filtered water with the target flow rate, and since the difference was within ±20%, the process shifted to the final process. The time from the intermediate process to the shift to the final process was 1 second. In the final process, PID control operation was performed. When the current value of the flow rate of the membrane-filtered water was measured with a flow meter, the target flow rate was reached 2 seconds after the start of the PID control operation. In addition, when the water temperature during the first operation and the next operation of the membrane filtration device was measured, the water temperature change was +1°C between the first operation and the next operation.

[0040] [Example 3] [First operation] In the initial process, the output frequency of the inverter (product name FRN2.2G1S, manufactured by Fuji Electric Co., Ltd.) was fixed at 25 Hz, and after operating the pump for 10 seconds, the process shifted to the intermediate process. In the intermediate process, the output frequency was set to 29 Hz by gradually increasing it by 0.5 Hz per second from the output frequency of 25 Hz in the initial process. Also, when the current value of the flow rate of the membrane-filtered water was measured with a flow meter (product name FD-H20, manufactured by Keyence Corporation), it was 0.41 m 3 / h. By comparing the current value of the flow rate of this membrane-filtered water with the target flow rate, and since the difference was within ±20%, the operation shifted to the PID-controlled operation. The time from the intermediate process to the shift to the final process was 8 seconds. In the final process, PID control operation was performed. When the current value of the flow rate of the membrane-filtered water was measured with a flow meter, the target flow rate was reached 12 seconds after the start of the PID control operation. Also, in the final process, the output frequency of the inverter was memorized during the PID control operation. The output frequency every 5 seconds that was memorized was 30.8 Hz.

[0041] [Next operation] The output number frequency of 30.8 Hz memorized during the first operation was set as the output number frequency at the start of the pump during the next operation, and the process immediately shifted from the initial process to the intermediate process. During the intermediate process, the frequency was set to 31.3 Hz by gradually increasing or decreasing it by 0.5 Hz every second from the output frequency of 30.8 Hz in the initial process. Also, when the current value of the flow rate of the membrane-filtered water was measured with a flow meter, it was 0.43 m 3 / h. By comparing the current value of the flow rate of this membrane-filtered water with the target flow rate, and since the difference was within ±20%, the process shifted to the final process. The time from the intermediate process to the shift to the final process was 1 second. During the final process, PID control operation was carried out. When the current value of the flow rate of the membrane-filtered water was measured with a flow meter, the target flow rate was reached 10 seconds after starting the PID control operation. In addition, when the water temperature during the first operation and the next operation of the membrane filtration device was measured, the water temperature change between the first operation and the next operation was -4°C.

[0042] [Comparative Example 1] During the first operation, the pump was operated while maintaining the output frequency of the inverter at 30.7 - 31.2 Hz where it stably reached the target flow rate. It took 500 seconds after the start of operation to reach the target flow rate during the first operation. During the next operation, the pump was operated in the same manner as in the first operation. It took 510 seconds after the start of operation to reach the target flow rate during the next operation. In addition, when the water temperature during the first operation and the next operation of the membrane filtration device was measured, the water temperature change between the first operation and the next operation was +0.5°C. Here, the output frequency of the inverter that stably reaches the target flow rate was defined as the frequency when the amount of operation change after the membrane-filtered water flow rate reached the target flow rate was less than the amount of operation change until the target flow rate was reached. The output frequency of the inverter that stably reaches the target flow rate was determined by appropriately setting the water temperature, etc. in advance and operating the membrane filtration device multiple times.

[0043] [Comparative Example 2] In the initial operation, the output frequency of the inverter (product name: FRN2.2G1S, manufactured by Fuji Electric Co., Ltd.) was fixed at 25 Hz, and after operating the pump for 10 seconds, PID control operation was carried out. The target flow rate was reached 80 seconds after the start of PID control operation. It took 90 seconds from the start of operation to reach the target flow rate in the initial operation. Also, the output frequency of the inverter was not memorized during the PID control operation. In the next operation, the output frequency of the inverter was fixed at 31 Hz, and after operating the pump for 10 seconds, PID control operation was carried out. The target flow rate was reached 80 seconds after the start of PID control operation. It took 90 seconds from the start of operation to reach the target flow rate in the next operation. When the water temperature of the membrane filtration device was measured during the initial operation and the next operation, the change in water temperature was +0.2 °C from the initial operation to the next operation.

[0044] [Comparative Example 3] In the initial operation, the output frequency of the inverter (product name: FRN2.2G1S, manufactured by Fuji Electric Co., Ltd.) was fixed at 25 Hz, and after operating the pump for 10 seconds, PID control operation was carried out. The target flow rate was reached 80 seconds after the start of PID control operation. It took 90 seconds from the start of operation to reach the target flow rate in the initial operation. Also, the output frequency of the inverter was not memorized during the PID control operation. In the next operation, the output frequency of the inverter was fixed at 34 Hz, and after operating the pump for 10 seconds, PID control operation was carried out. The target flow rate was reached 120 seconds after the start of PID control operation. It took 130 seconds from the start of operation to reach the target flow rate in the next operation. When the water temperature of the membrane filtration device was measured during the initial operation and the next operation, the change in water temperature was -4 °C from the initial operation to the next operation.

[0045] In Comparative Examples 1 to 3, since overshoot occurred, the time taken for the flow rate of the membrane-filtered water to reach the target flow rate in the initial operation was 500 seconds, 90 seconds, and 90 seconds respectively from the start of pump operation. In Comparative Examples 1 to 3, since the output frequency was not memorized during the first operation, the times for the flow rate of the membrane-filtered water to reach the target flow rate during the next operation were 510 seconds, 90 seconds, and 130 seconds respectively from the start of the pump operation, and the time until the target flow rate was reached could not be shortened compared to the first operation. Particularly in Comparative Example 3, since the water temperature changed greatly by -4°C between the first operation and the next operation, it took significantly more time to reach the target flow rate in the next operation compared to the time taken to reach the target flow rate in the first operation.

[0046] In Examples 1 to 3, when the difference between the current value and the target flow rate of the membrane-filtered water reached within a predetermined range, by switching to the final stage process, overshoot did not occur. In Examples 1 to 3, the times for the flow rate of the membrane-filtered water to reach the target flow rate during the first operation were 30 seconds, 64 seconds, and 30 seconds respectively from the start of the pump operation. In addition, in Examples 1 to 3, since the output frequency was memorized during the first operation, the times for the flow rate of the membrane-filtered water to reach the target flow rate during the next operation were 3 seconds, 3 seconds, and 11 seconds respectively from the start of the pump operation, and the time until the target flow rate was reached could be significantly shortened compared to the first operation. Particularly in Example 3, although the water temperature decreased by -4°C between the first operation and the next operation and the water quality changed resulting in a decrease in the flow rate of the membrane-filtered water in the next operation, the time to reach the target flow rate in the next operation could be shortened compared to the first operation.

[0047] The operation method of the membrane filtration system according to the present embodiment can control the overshoot of the flow rate of the membrane-filtered water to be extremely small. Further, according to the operation method of the membrane filtration system according to the present embodiment, the time until the target flow rate is reached can be shortened, and even if the water quality changes due to a change in water temperature or the like, the control until the target flow rate is reached becomes easy.

Explanation of Reference Numerals

[0048] 1 Raw water tank 2 Operation output 3 Flow meter 4 Hollow fiber membrane module 5 Controller 6 Flow meter 7 reflux line 8 pump 9 inverter 10 flow detection signal 11 water supply line

Claims

1. An operation method for a membrane filtration system that controls the flow rate of membrane-filtered water by the rotational speed of a pump, comprising: An initial step of performing an operation in which the rotational speed of the pump is controlled at a fixed output frequency (fixed output frequency) of an inverter with fixed numerical values; A middle step of switching to an operation in which the rotational speed of the pump is controlled by gradually increasing or decreasing the output frequency, and comparing the current value of the flow rate of the membrane-filtered water with the target flow rate to determine whether to shift to a PID-controlled operation; and A final step of performing an operation in which the rotational speed of the pump is PID-controlled, the operation method of the membrane filtration system.

2. The operation method of the membrane filtration system according to claim 1, wherein when the difference between the current value of the flow rate of the membrane-filtered water and the target flow rate is within ±100%, a shift is made to a PID-controlled operation.

3. The operation method of the membrane filtration system according to claim 2, wherein when the difference between the current value of the flow rate of the membrane-filtered water and the target flow rate is within ±0% to ±20%, a shift is made to a PID-controlled operation.

4. In the final step of the operation of the membrane filtration system, during the PID-controlled operation, by storing the output frequency of the inverter every 0.1 to 600 seconds, In the initial step of the next operation of the membrane filtration system, using the output frequency stored in the final step of the previous operation of the membrane filtration system as the fixed output frequency, an operation in which the rotational speed of the pump is controlled is performed, the operation method of the membrane filtration system according to claim 2 or 3.

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