Membrane filtration device and membrane filtration method

Abstract

To provide a membrane filtration apparatus and membrane filtration method which can suppress adhesion of a substance to a separation membrane while suppressing a load on the separation membrane.SOLUTION: A membrane filtration apparatus includes: a separation part which has a separation membrane for separating a liquid to be treated into a permeated liquid and concentrated liquid; a pump which supplies the liquid to be treated to the separation part; and a flow rate control part which controls a flow rate of the liquid to be treated which is supplied by the pump. The flow rate control part suppresses adhesion of a substance to the separation membrane by reducing the flow rate Q of the liquid to be treated from a flow rate QS in a steady state and then increasing the flow rate Q of the liquid to be treated to return to the flow rate QS in the steady state.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a membrane filtration device and a membrane filtration method. [Background technology]

[0002] BACKGROUND ART Conventionally, membrane filtration devices are known that separate a liquid to be treated into a permeate liquid and a concentrate using a separation membrane (see, for example, Patent Documents 1 and 2). Membrane filtration devices can separate substances larger than the pore size of the separation membrane without heating, and are widely used for concentrating and sterilizing liquid foods, separating and purifying proteins and sugars, etc. While membrane filtration devices can easily perform concentration and separation operations, they can also cause membrane fouling, where substances adhere to the separation membrane over time.

[0003] The membrane filtration device described in Patent Document 1 is designed so that when the flow rate of the permeate falls below a certain value, an on-off valve provided in the bypass piping is opened to increase the flow rate of the concentrated liquid side above normal, and the pump is operated alternately between forward and reverse rotation to strip off substances adhering to the reverse osmosis membrane (separation membrane). The membrane filtration device described in Patent Document 2 has a cylindrical object installed inside the pipe that sends the liquid to be treated to the filter (separation membrane), which fluctuates the transmembrane pressure difference in accordance with the natural frequency of the separation membrane, thereby preventing impurities from adhering to and accumulating on the separation membrane. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 9-34086 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-268494 Summary of the Invention [Problem to be solved by the invention]

[0005] However, conventional membrane filtration devices such as those described in Patent Documents 1 and 2 have the problem of placing a strain on the separation membrane because they increase the flow rate of the concentrated liquid side above normal, alternately operate the pump in forward and reverse rotation, and vary the transmembrane pressure difference in accordance with the natural frequency of the separation membrane.

[0006] An object of the present invention is to provide a membrane filtration device and a membrane filtration method that can suppress adhesion of substances to a separation membrane while reducing the burden on the separation membrane. [Means for solving the problem]

[0007] The membrane filtration device of the present invention comprises a separation section having a separation membrane that separates a liquid to be treated into a permeate liquid and a concentrate, a pump that supplies the liquid to be treated to the separation section, and a flow rate control section that controls the flow rate of the liquid to be treated supplied by the pump, wherein the flow rate control section reduces the flow rate of the liquid to be treated from a steady state flow rate and then increases the flow rate of the liquid to be treated to return it to the steady state flow rate, thereby suppressing adhesion of substances to the separation membrane.

[0008] According to the present invention, the flow rate of the liquid to be treated is reduced from the steady-state flow rate, and then the flow rate is increased to return to the steady-state flow rate, thereby suppressing the adhesion of substances to the separation membrane, thereby suppressing the adhesion of substances to the separation membrane while reducing the burden on the separation membrane.

[0009] In the membrane filtration device of the present invention, it is preferable that the flow control unit increases the flow rate of the treated liquid at a velocity gradient that is steeper than the velocity gradient at which the flow rate is reduced from the steady state to return it to the steady state flow rate.

[0010] According to the present invention, the flow rate of the treated liquid is increased at a velocity gradient that is steeper than the velocity gradient at which the flow rate is reduced from the steady state flow rate, thereby returning the flow rate to the steady state flow rate. This allows the flow rate of the treated liquid to be quickly returned to the steady state flow rate, and shortens the time during which the flow rate of the treated liquid becomes less than the steady state flow rate.

[0011] In the membrane filtration device of the present invention, it is preferable that the flow control unit reduces the flow rate of the treated liquid from the steady-state flow rate by reducing the rotation speed of the pump, and then increases the rotation speed of the pump to increase the flow rate of the treated liquid and return it to the steady-state flow rate.

[0012] According to the present invention, the adhesion of substances to the separation membrane is suppressed by changing the rotation speed of the pump, so there is no need to add a new component, and the device configuration can be prevented from becoming complicated.

[0013] In the membrane filtration device of the present invention, the separation membrane is preferably a reverse osmosis membrane, a nanofiltration membrane, an ultrafiltration membrane, or a microfiltration membrane.

[0014] According to the present invention, a separation membrane suitable for the substance to be separated in the liquid to be treated can be selected.

[0015] In the membrane filtration device of the present invention, the separation section is a separation membrane module in which a plurality of the separation membranes are housed in a case, and the separation membrane module is preferably a ceramic membrane module, a spiral membrane module, a hollow fiber membrane module, or a flat membrane module.

[0016] According to the present invention, a separation membrane module suitable for the substance to be separated in the liquid to be treated can be selected.

[0017] The membrane filtration method of the present invention comprises a supply step of supplying the treated liquid by a pump to a separation section having a separation membrane that separates the treated liquid into a permeate liquid and a concentrate, and a flow rate control step of controlling the flow rate of the treated liquid supplied by the pump, wherein the flow rate control step is characterized in that the flow rate of the treated liquid is reduced from a steady state flow rate, and then the flow rate of the treated liquid is increased to return it to the steady state flow rate, thereby suppressing adhesion of substances to the separation membrane.

[0018] According to the present invention, the same effects as those of the above-mentioned membrane filtration device can be obtained. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a schematic diagram of a membrane filtration device according to one embodiment of the present invention. [Figure 2] FIG. 2 is an explanatory diagram of the operation of the membrane filtration device of FIG. 1. DETAILED DESCRIPTION OF THE INVENTION

[0020] An embodiment of the present invention will be described below with reference to FIGS. In Figure 1, the membrane filtration device 1 is a device that separates substances to be separated that have particles larger than the pore size of the separation membrane by passing the liquid to be treated through a separation membrane. The membrane filtration device 1 of this embodiment performs cross-flow filtration in which the liquid to be treated flows parallel to the membrane surface of the separation membrane, and performs separation processing on the liquid to be treated, which is a liquid food. Examples of the liquid to be treated include an aqueous sugar solution, an aqueous protein solution, milk, whey, and skim milk.

[0021] The membrane filtration device 1 includes a tank 2 , a separation section 3 , a pump 4 , a flow rate control section 5 , and a valve 6 .

[0022] The liquid to be treated is stored in the tank 2. The type, size, shape, etc. of the tank 2 are not particularly limited.

[0023] The separation unit 3 has a separation membrane that separates the liquid to be treated into a permeate and a concentrate, and is composed of, for example, a separation membrane module in which multiple separation membranes are housed in a case. The separation unit 3 of this embodiment is configured so that the liquid to be treated flows parallel to the membrane surface of the separation membrane, thereby performing cross-flow filtration. Types of separation membranes used in the separation unit 3 include, for example, reverse osmosis membranes, nanofiltration membranes, ultrafiltration membranes, microfiltration membranes, and electrodialysis membranes, and shapes of the separation membranes include flat membranes, spiral membranes, hollow fiber membranes, and porous membranes.

[0024] The pump 4 is connected to the tank 2 and the separation unit 3 via pipes 41 and 42, and supplies the liquid to be treated from the tank 2 to the separation unit 3. Examples of the pump 4 include, but are not limited to, a volumetric pump and a turbo pump. The pump 4 of this embodiment supplies the liquid to be treated by rotating a rotor or the like.

[0025] The flow rate control unit 5 includes a control device 51 and an inverter 52, and controls the flow rate Q of the liquid to be treated supplied by the pump 4. The control device 51 is composed of a signal generator, a computer, etc., and outputs a signal waveform for driving the pump 4 to the inverter 52 , and controls the rotation speed of the pump 4 via the inverter 52 .

[0026] The valve 6 is connected to the downstream side (concentrated water side) of the separation section 3 via a pipe 61, and adjusts the back pressure on the concentrated water side by adjusting the valve opening.

[0027] A method for separating the liquid to be treated into a permeated liquid and a concentrated liquid using the membrane filtration device 1 described above will be described. First, a signal waveform for driving the pump 4 is set in the control device 51 via an input device such as a keyboard or touch panel. This involves setting a signal waveform that maintains a steady-state flow rate QS (see FIG. 2 ), where the flow rate Q of the liquid being treated is constant, and setting a signal waveform for flow rate fluctuation control that repeatedly reduces the flow rate Q from the steady state and then returns it to the steady-state flow rate QS. The signal waveform for flow rate fluctuation control is set so that it is output each time the steady-state flow rate QS continues for a certain period of time, and so that the flow rate Q during flow rate fluctuation control does not exceed the steady-state flow rate QS. In this embodiment, as shown in FIG. 2 , the signal waveform for flow rate fluctuation control is a waveform that reduces the flow rate Q from the steady-state flow rate QS to a minimum flow rate QM at a constant velocity gradient θ1 and then returns the flow rate Q to the steady-state flow rate QS at a constant velocity gradient θ2 that is steeper than the velocity gradient θ1. Note that in FIG. 2 , the vertical axis represents the flow rate Q of the liquid being treated supplied from the pump 4, and the horizontal axis represents time t.

[0028] Here, the minimum flow rate QM during flow rate fluctuation control is preferably set to 1 / 10 to 1 / 2 of the steady-state flow rate QS. Furthermore, the period of the signal waveform for flow rate fluctuation control is preferably set to 2.5 to 50 seconds, and the ratio of the deceleration time (decrease time) TD of the flow rate Q from the steady-state flow rate QS to the minimum flow rate QM to the acceleration time (increase time) TA of the flow rate Q from the minimum flow rate QM to the steady-state flow rate QS is preferably set to 4:1, 3:2, 2:3, or 1:4 (the ratio of the deceleration to acceleration of the flow rate Q is 1:4, 2:3, 3:2, or 4:1).

[0029] When a signal to start operation is input to the control device 51 via the input means, the control device 51 outputs a steady-state signal waveform to the inverter 52, and drives the pump 4 in a steady state via the inverter 52. As a result, the pump 4 rotates at a predetermined rotation speed and supplies the treated liquid to the separation section 3 at a steady-state flow rate QS. In addition, the valve 6 adjusts its opening so that the pressure on the concentrate side in the separation section 3 and the flow rate of the permeate liquid each reach a predetermined value.

[0030] After that, when the steady-state flow rate QS continues for a certain period of time, the control device 51 outputs a signal waveform for flow rate fluctuation control to the inverter 52, and fluctuates the rotation speed of the pump 4 via the inverter 52. That is, the flow rate control unit 5 repeatedly reduces the rotation speed of the pump 4 to decrease the flow rate Q from the steady-state flow rate QS at a constant rate gradient θ1, and then increases the rotation speed of the pump 4 to increase the flow rate Q at a constant rate gradient θ2, thereby returning the flow rate QS to the steady-state flow rate. This causes turbulence in the flow of the treated liquid. Flow turbulence in the treated liquid often occurs when the flow rate Q is reduced from the steady-state flow rate QS, but can also occur when the flow rate Q changes from a decrease to an increase or from an increase to a steady state. This causes pulsation in the flow of the treated liquid, which prevents substances from adhering to the separation membrane.

[0031] Then, when the flow rate fluctuation control is completed, the control device 51 again outputs a steady-state signal waveform to the inverter 52, and drives the pump 4 in the steady state via the inverter 52. Thereafter, the flow rate control unit 5 continues to perform flow rate fluctuation control every time the steady-state flow rate QS continues for a certain period of time, and periodically fluctuates the flow rate Q of the liquid to be treated.

[0032] Using the above membrane filtration device 1, various treated liquids were separated into permeate and concentrate using Examples 1 to 6 in Table 1 below as examples.Compared to when flow rate fluctuation control was not performed (when the steady-state flow rate QS was continued), it was found that the permeate flow rate increased in all cases and that adhesion of substances to the separation membrane was suppressed. In Table 1, "minimum flow rate ratio relative to steady state" refers to the ratio of the minimum flow rate QM during flow rate fluctuation control to the steady-state flow rate QS, "deceleration time" refers to the deceleration time TD and acceleration time TA shown in Figure 2, and "permeation flow rate increase rate" refers to the increase rate of the permeation flow rate during flow rate fluctuation control relative to the permeation flow rate when flow rate fluctuation control is not performed. Furthermore, the WPI aqueous solution refers to an aqueous solution of WPI (Whey Protein Isolate).

[0033] [Table 1]

[0034] According to the above-described embodiment, the flow rate Q of the liquid to be treated is reduced from the steady-state flow rate QS, and then the flow rate Q is increased and returned to the steady-state flow rate QS, thereby suppressing the adhesion of substances to the separation membrane, thereby reducing the burden on the separation membrane and suppressing the adhesion of substances to the separation membrane. That is, the flow rate control unit 5 controls the flow rate Q of the liquid to be treated by controlling the flow rate fluctuation, rather than by varying the pressure of the liquid to be treated, and therefore the load on the separation membrane can be reduced.

[0035] Furthermore, the flow rate Q of the treated liquid is increased and returned to the steady-state flow rate QS at a velocity gradient θ2 that is steeper than the velocity gradient θ1 used to reduce the flow rate QS from the steady-state flow rate QS, so that the flow rate Q of the treated liquid can be quickly returned to the steady-state flow rate QS, and the time during which the flow rate Q of the treated liquid becomes less than the steady-state flow rate QS can be shortened.

[0036] Furthermore, by changing the rotation speed of the pump 4, adhesion of substances to the separation membrane is suppressed, so there is no need to add a new component, and the device configuration can be prevented from becoming complicated.

[0037] Furthermore, the flow rate control unit 5 only decreases or increases the flow rate Q through flow rate fluctuation control without causing the liquid to flow in the reverse direction, so that separation processing of the liquid to be treated can be carried out continuously even during flow rate fluctuation control.

[0038] As described above, the best configurations, methods, and the like for implementing the present invention have been disclosed in the above description, but the present invention is not limited thereto. That is, although the present invention has been particularly illustrated and described mainly with reference to specific embodiments, those skilled in the art can make various modifications to the above-described embodiments in terms of shape, material, quantity, and other detailed configurations without departing from the scope of the technical idea and purpose of the present invention. Furthermore, the above-disclosed descriptions limiting the shape, material, and the like are provided as examples to facilitate understanding of the present invention and are not intended to limit the present invention. Therefore, descriptions using names of components that are free from some or all of the limitations on shape, material, and the like are included in the present invention.

[0039] For example, the membrane filtration device 1 may be a device that performs full-volume filtration by flowing the liquid to be treated in a direction intersecting the membrane surface of the separation membrane and filtering the entire amount of the liquid to be treated, or it may perform separation processing of a liquid to be treated other than a liquid food.

[0040] The pump 4 may be of a variable displacement type, in which case the flow rate control unit 5 may control the flow rate Q of the liquid to be treated by changing the displacement of the pump 4. The pump 4 may supply the liquid to be treated by reciprocating a piston, plunger, etc., and in this case, the flow rate control unit 5 may control the flow rate Q of the liquid to be treated by changing the speed of the reciprocating movement.

[0041] The flow rate control unit 5 may be configured to perform flow rate fluctuation control at a predetermined time during the separation treatment of the liquid to be treated, or may be configured to perform flow rate fluctuation control at all times during the separation treatment. The flow control unit 5 may reduce the flow rate Q from the steady-state flow rate QS and then increase it back to the steady-state flow rate QS in one cycle or multiple cycles during flow rate fluctuation control, and the number of such cycles performed during flow rate fluctuation control is not particularly limited. The flow rate control unit 5 may set the velocity gradients θ1 and θ2 to be the same gradient, or may set the velocity gradient θ2 to be gentler than the velocity gradient θ1. The flow rate control unit 5 may perform flow rate fluctuation control using a triangular wave, a sine wave, or a rectangular wave as a signal waveform. [Explanation of symbols]

[0042] 1...membrane filtration device, 2...tank, 3...separation section, 4...pump, 5...flow rate control section, 6...valve, Q...flow rate of treated liquid, QS...steady-state flow rate.

Claims

1. a separation section having a separation membrane for separating the liquid to be treated into a permeate liquid and a concentrate; a pump for supplying the liquid to be treated to the separation section; a flow rate control unit for controlling the flow rate of the liquid to be treated supplied by the pump, The flow control unit reduces the flow rate of the treated liquid from a steady state flow rate, and then increases the flow rate of the treated liquid at a rate gradient steeper than the rate gradient at which the flow rate is reduced from the steady state flow rate, thereby returning the flow rate to the steady state flow rate, thereby suppressing adhesion of substances to the separation membrane.

2. The membrane filtration device described in claim 1, characterized in that the flow control unit reduces the flow rate of the treated liquid from the steady state flow rate by reducing the rotation speed of the pump, and then increases the rotation speed of the pump to increase the flow rate of the treated liquid and return it to the steady state flow rate.

3. 3. The membrane filtration device according to claim 1, wherein the separation membrane is a reverse osmosis membrane, a nanofiltration membrane, an ultrafiltration membrane, or a microfiltration membrane.

4. the separation unit is a separation membrane module in which a plurality of the separation membranes are housed in a case, 4. The membrane filtration device according to claim 1, wherein the separation membrane module is a ceramic membrane module, a spiral membrane module, a hollow fiber membrane module, or a flat membrane module.

5. a supply step of supplying the liquid to be treated by a pump to a separation section having a separation membrane that separates the liquid to be treated into a permeate liquid and a concentrate; a flow rate control step of controlling the flow rate of the liquid to be treated supplied by the pump, The flow rate control step reduces the flow rate of the treated liquid from a steady state flow rate, and then increases the flow rate of the treated liquid at a rate gradient steeper than the rate gradient at which the flow rate is reduced from the steady state flow rate to return it to the steady state flow rate, thereby suppressing adhesion of substances to the separation membrane.

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