JP4923427B2 - Membrane separation method and membrane separation apparatus - Google Patents

Description

  The present invention relates to a microfiltration membrane module or an ultrafiltration membrane module, and a membrane separation method and membrane separation apparatus using a reverse osmosis membrane module. More specifically, a membrane separation method in which at least a portion of the concentrated water obtained by the second-stage reverse osmosis membrane module constituting the membrane separation apparatus is used as washing water for a microfiltration membrane or an ultrafiltration membrane, and such The present invention relates to a membrane separation apparatus having various means.

  For example, a membrane filtration device equipped with a reverse osmosis membrane module is used for the production of fresh water from seawater and brine, and the production of clean water from rivers and lakes. As shown in FIG. 1, this type of membrane filtration device basically uses raw water (seawater, etc.) subjected to pretreatment such as sterilization and removal of turbid components to a predetermined pressure (for example, 6.0 MPa) by a high-pressure pump 2. The reverse osmosis membrane module 3 is supplied to the reverse osmosis membrane module 3 to obtain permeated water and concentrated water permeated by reverse osmosis. The obtained permeate has a water quality that almost satisfies the WHO water quality guideline value, but the problem remains only for boron.

  Boron exists as boric acid in seawater and is contained at about 4 to 5 mg / L. Boric acid has a dissociation constant pKa of 9, and is almost non-dissociated in seawater. However, a composite reverse osmosis membrane having a cross-linked wholly aromatic polyamide, which is a typical reverse osmosis membrane, in the separation functional layer has unreacted carboxyl groups and amino groups as terminal groups in the separation functional layer. The reverse osmosis membrane module for seawater desalination has a characteristic of well eliminating ionic substances, and the boric acid rejection rate may not be sufficiently satisfied depending on seawater desalination conditions. For this reason, it may be difficult to make the boron concentration below the provisional value (0.5 mg / L) defined in the WHO Water Quality Guidelines.

  Therefore, Patent Document 1 discloses a second reverse method for removing boron in the permeated water at the subsequent stage of the reverse osmosis membrane module for the purpose of removing boron remaining in the permeated water obtained by the reverse osmosis membrane module. A water treatment method for providing an osmotic membrane module is disclosed.

On the other hand, as a method for treating the feed water of such a reverse osmosis membrane separation device, for example, sand filtration, activated carbon filtration, membrane filtration and the like are mentioned in Patent Document 1, and membrane filtration is particularly preferably used. It is disclosed. However, when the membrane filtration is continuously operated, organic pollutants and the like adhere to the membrane surface, the pressure loss due to the membrane module 7 increases, and the amount of filtered water obtained decreases. Therefore, it is common to add an oxidizing agent such as sodium hypochlorite to the treated water and back flow into the membrane module 7 to clean the membrane surface (back washing) by a chemical cleaning action by the oxidizing agent in addition to the physical action. Is. The higher the concentration of the oxidizing agent used for chemical cleaning, the better the cleaning effect, but in the case of seawater treatment, the pH is partially reduced by backwashing with high concentration sodium hypochlorite. The scale component in seawater may be precipitated because When backwashing with such backwashing water, scales may adhere to the inner surface of the membrane and clogging may occur, and there is a problem that the concentration of sodium hypochlorite cannot be increased.
Japanese Patent Laid-Open No. 11-253761 (paragraphs [0020], [0022], [0056], etc.)

  The present invention solves the above-mentioned conventional problems, and efficiently cleans and removes organic pollutants adhered and deposited on the film surface without generating scale even when a high concentration oxidizing agent is injected. An object is to provide a separation device and a membrane separation device.

The present invention for solving the above-described problems is characterized by the following configurations (1) to (4) .

(1) Treated water obtained by treating raw water with a microfiltration membrane module or ultrafiltration membrane module is subjected to reverse osmosis treatment with a first reverse osmosis membrane module, and further reversely treated with the first reverse osmosis membrane module. in membrane separation process for reverse osmosis treatment penetration treated permeate in the second reverse osmosis membrane module, at least a portion only of the concentrated water obtained by the second reverse osmosis membrane module, wherein the microfiltration membrane module A membrane separation method comprising a washing step of washing water for the microfiltration membrane used in the ultrafiltration membrane or the ultrafiltration membrane used in the ultrafiltration membrane module.

(2) When using at least a part of the concentrated water obtained by the second reverse osmosis membrane module as washing water for the microfiltration membrane or the ultrafiltration membrane, an oxidizing agent is added to the washing water. (2) The membrane separation method according to (1).

(3) A first reverse osmosis membrane module comprising a microfiltration membrane module or an ultrafiltration membrane module for treating raw water, and performing reverse osmosis treatment on treated water obtained by the microfiltration membrane module or the ultrafiltration membrane module And having a second reverse osmosis membrane module for treating the permeated water obtained by the first reverse osmosis membrane module with a reverse osmosis membrane, obtained by the second reverse osmosis membrane module film only at least part of the concentrated water, characterized in that it comprises means for returning a cleaning water ultrafiltration membrane used in the microfiltration membrane or the ultrafiltration membrane module used in the microfiltration membrane module Separation device.

(4) An oxidant tank for oxidant storage is further provided, and an oxidant addition port from the oxidant tank is provided in the middle of the means for returning at least a part of the concentrated water as washing water. (3) The membrane separator according to (3).

  According to the present invention, the microfiltration membrane or ultrafiltration is not generated even if a high-concentration oxidizing agent is injected into the microfiltration membrane or ultrafiltration membrane while continuing the process of separating the raw water into the membrane, without generating scale. It is possible to efficiently wash and remove the organic pollutant adhered and deposited on the film surface of the film. Therefore, long-term stable operation of a membrane separation apparatus using a reverse osmosis membrane is possible.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 2 is a schematic flow diagram of a membrane separation apparatus showing an embodiment of the present invention. In FIG. 2, 1 is a pre-treated treated water tank, 2 is a high-pressure pump capable of increasing the pressure to 6 to 10 MPa, 3 is a first reverse osmosis membrane module, and 4 is a first reverse osmosis membrane module 3. A low-pressure pump capable of boosting the permeated water and feeding it to the second reverse osmosis membrane module 5 up to 0.5 MPa, 5 is a second reverse osmosis membrane module, and 6 is a microfiltration membrane module or ultrafiltration membrane for raw water Supply pump for sending water to the module, 7 is a microfiltration membrane module or ultrafiltration membrane module, 9 is an oxidant tank, 10 is a chemical injection pump for injecting oxidant, and 11 is a second reverse osmosis membrane module 5 concentrated water tank to accumulate a concentrated water of 12 backwash pump for water in microfiltration membrane module or ultrafiltration membrane module concentrate as backwash _{water, V 1, V 2, V} 4 valve der . Moreover, in FIG. 2, the piping at the time of filtration is shown with the continuous line for convenience of explanation, and the piping at the time of backwashing is shown with the broken line.

  As the pretreatment, it is only necessary to remove suspended substances contained in the raw water. For example, sand filtration, activated carbon filtration, and membrane filtration can be adopted. However, in the present invention, the microfiltration membrane module is used because it can be separated compactly and efficiently. Or it is essential to perform membrane filtration using an ultrafiltration membrane module. Such filtration membranes include hollow fiber membranes and flat membranes, and spiral membranes are preferably used as membrane modules using flat membranes, and cylindrical types are preferably used as membrane modules using hollow fiber membranes. In the present invention, since the raw water is directly filtered by the filtration membrane module, a cylindrical membrane module using a hollow fiber membrane suitable for separation of a liquid containing a high concentration of turbidity is preferably used.

  In the present invention, the microfiltration membrane module or the ultrafiltration membrane module has, for example, a plurality of hollow fiber membranes accommodated in a cylindrical case, and the hollow fiber membrane is particularly limited as long as it is porous. Instead of polyethylene, polypropylene, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polysulfone, polyethersulfone (PES), polyether-etherketone (PEEK), polyphenylene sulfide sulfone (PPSS), polyphenylenesulfone ( PPSO), polyvinyl alcohol, cellulose acetate, polyacrylonitrile, polyamide, organic materials such as polyimide, inorganic materials such as ceramic and metal, and other materials can be selected. In particular, a polyvinylidene fluoride (PVDF) film having excellent chemical resistance is preferable.

  Among such membranes, a membrane having an average pore size of 0.001 to 10 μm is preferable, and a membrane having an average pore size of 0.05 to 1 μm is more preferable. When the average pore diameter is less than 0.001 μm, clogging is accelerated, and when it exceeds 10 μm, it becomes difficult to remove the pollutant.

  In the present invention, the reverse osmosis membrane used for the reverse osmosis membrane modules 3 and 5 is substantially reverse osmosis separation in which a part of components in the separated liquid mixture, for example, a solvent is permeated and other components are not permeated. It is a possible semi-permeable membrane, and a high molecular material such as cellulose acetate polymer, polyamide, polyester, polyimide, vinyl polymer is often used as the material. In addition, the membrane structure has a dense layer on at least one side of the membrane, an asymmetric membrane having fine pores gradually increasing from the dense layer to the inside of the membrane or the other side, and another layer on the dense layer of the asymmetric membrane. There are composite membranes having a very thin separation functional layer formed of a material. The membrane form includes hollow fiber and flat membrane. The present invention can be carried out regardless of the film material, film structure and film form, and any of them is effective, but as typical films, for example, cellulose acetate-based or polyamide-based asymmetric membranes and polyamide-based, There are composite membranes having a urea-based separation functional layer, and it is preferable to use a cellulose acetate-based asymmetric membrane and a polyamide-based composite membrane from the viewpoint of water production, durability, and salt rejection.

  In the present invention, the reverse osmosis membrane used for the first reverse osmosis membrane module 3 has a salt rejection rate of 90 when a saline solution at 25 ° C., pH 6.5, concentration 35,700 mg / L is supplied at 5.5 MPa. % Or more is preferable. The higher the salt rejection, the lower the chloride ion concentration in the permeated water, which is preferable. Therefore, the salt rejection is preferably 95% or more, and more preferably the salt rejection is 99% or more. . If the salt rejection is less than 90%, the amount of chlorine ions in the permeate increases, making it difficult to use the permeate as it is as drinking water or industrial water.

In the present invention, the reverse osmosis membrane used for the second reverse osmosis membrane module 5 has a permeation flux of 0 when a saline solution of 25 ° C., pH 6.5, and a concentration of 1500 mg / L is supplied at 1.5 MPa. It preferably has a performance of 8 m ³ / m ² · day or more. In order to secure a larger flow rate per unit element, the permeation flux is more preferably 1.0 m ³ / m ² · day or more. Further, the salt rejection when supplying 1.5 MPa of a saline solution at 25 ° C., pH 6.5 and 1,500 mg / L is 90% or more, further 98% or more, and 25 ° C., pH 6.5, It is preferable that the salt rejection rate is 90% or more, more preferably 98% or more when a magnesium sulfate aqueous solution having a concentration of 1500 mg / L is supplied at 1.5 MPa.

  A reverse osmosis membrane having such performance is incorporated into an element such as spiral, tubular, plate and frame for practical use, and hollow fibers are bundled and incorporated into the element. However, the present invention does not depend on the form of these reverse osmosis membrane elements.

  In the present invention, the reverse osmosis membrane modules 3 and 5 are not only modules in which the reverse osmosis membrane element is contained in one to several pressure vessels, but a plurality of these modules arranged in parallel. Is also included. Combination, number, and arrangement can be arbitrarily performed according to the purpose.

  Next, the flow of water in the membrane filtration device according to the present invention will be described.

In the membrane separation apparatus of the present invention, membrane filtration of raw water to obtain treated water, for example, as shown in FIG. 2, opens valves V ₁ and V ₂ and closes V ₄ to operate supply pump 6, The raw water is introduced into the microfiltration membrane module or the ultrafiltration membrane module 7. The treated water that has passed through the microfiltration membrane module or the ultrafiltration membrane module is stored in the pretreated treated water tank 1.

  The membrane filtration method may be a total amount membrane filtration method or a cross flow membrane filtration method. Moreover, although a pressurized membrane filtration system or a negative pressure membrane filtration system may be used, the pressurized membrane filtration system is preferable because a higher membrane filtration flux can be obtained. Moreover, although either an internal pressure membrane filtration or an external pressure membrane filtration may be sufficient, the external pressure membrane filtration is preferable because the effect of air scrubbing is large.

The membrane filtration flux is not particularly limited, but is preferably 0.1 to ⁵ m ³ / m ² · d. If the membrane filtration flux is less than 0.1 m ³ / m ² · d, the recovery rate of the product water is lowered, and if it exceeds 5 m ³ / m ² · d, the increase in the differential pressure of the membrane module may be increased.

  The membrane filtration time is not particularly limited, but it is preferably 5 to 60 minutes and then backwashing described later is performed. When the membrane filtration time is less than 5 minutes, the recovery rate of the production water is reduced, and when it exceeds 60 minutes, the increase in the differential pressure of the membrane module is increased.

  The treated water obtained by subjecting the raw water to membrane filtration is then pressurized by the high-pressure pump 2 and supplied to the first reverse osmosis membrane module 3. The supplied treated water is separated into a first permeated water from which solutes such as salt have been removed and a first concentrated water from which solutes such as salt have been concentrated, and the first concentrated water is discharged from the outlet. Is done. Then, the first permeate obtained by the first reverse osmosis membrane module 3 is further pressurized by the low pressure pump 4 and then supplied to the second reverse osmosis membrane module 5. The supplied first permeated water includes a second permeated water from which a solute such as a trace amount of salt is further removed, and a second concentrated water containing a low concentration of a salt by concentrating a solute such as a trace amount of salt. The second concentrated water is stored in the concentrated water tank 11. When the amount of the second concentrated water is larger than the amount necessary for backwashing the microfiltration membrane or the ultrafiltration membrane, the second concentrated water is preferably introduced into the pretreated treated water tank 1.

  Next, the back washing method of the microfiltration membrane or the ultrafiltration membrane in this membrane filtration apparatus is demonstrated.

In the backwashing of the microfiltration membrane or the ultrafiltration membrane, as shown in FIG. 2, the supply pump 6 is stopped, the valves V ₁ and V ₂ are closed, the valve V ₄ is opened, and the backwash pump 12 is turned on. The second concentrated water is operated from the concentrated water tank 11 and introduced from the treated water side of the microfiltration membrane or the ultrafiltration membrane 7. At this time, the oxidizing agent stored in the oxidizing agent tank 9 is added to the backwash water by operating the chemical injection pump 10. The oxidizing agent added to the backwash water is not limited to sodium hypochlorite, and chlorine dioxide, hydrogen peroxide, and the like are also preferable. In addition, it is desirable that the reverse osmosis membrane is continuously operated even when the microfiltration membrane or the ultrafiltration membrane is backwashed.

  The time for backwashing is not particularly limited, but is preferably in the range of 1 to 120 seconds. If the time of one backwash is less than 1 second, sufficient cleaning effect cannot be obtained, and if it exceeds 120 seconds, the operation efficiency of the microfiltration membrane module or the ultrafiltration membrane module is lowered.

The backwash flux is not particularly limited, but is preferably in the range of 0.1 to 10 m ³ / m ² · d. If the backwashing flux is less than 0.1 m ³ / m ² · d, it is difficult to sufficiently remove the organic pollutant adhered and deposited on the film surface, and if it exceeds 10 m ³ / m ² · d, It becomes easy to cause mechanical deterioration of the filtration membrane module or the ultrafiltration membrane module.

  Furthermore, when backwashing, the membrane is washed by sending gas to the raw water side of the microfiltration membrane module or ultrafiltration membrane module 7 and vibrating the membrane surface of the microfiltration membrane or ultrafiltration membrane. Is also preferable. Organic matter adsorbed on the membrane surface of the microfiltration membrane or ultrafiltration membrane is decomposed by the oxidizing agent contained in the backwash water to become a non-adsorbing substance, so that the pores of the microfiltration membrane or ultrafiltration membrane are blocked. The adsorptive substance is effectively screened out by introducing the gas, and the cleaning effect is enhanced.

  The following effects are obtained by washing with the second concentrated water obtained in the second reverse osmosis membrane module 5. In other words, in the conventional technology that backwashes with the treated water of the microfiltration membrane or ultrafiltration membrane, injection with a high concentration of oxidant such as hypochlorous acid causes a high pH and a scale is generated. Whereas it was not possible to use the second concentrated water obtained by the second reverse osmosis membrane module 5 as washing water, the salt concentration can be reduced to 1/10 or less, and scale is generated. As a result, oxidants such as hypochlorous acid can be injected at a high concentration. Moreover, since the 2nd concentrated water obtained with the 2nd reverse osmosis membrane module 5 is pressure | voltage-risen by the high pressure pump 2 or the low pressure pump 4, it is used for the treated water of the microfiltration membrane module or the ultrafiltration membrane module 7 In comparison, the water temperature has increased by 2 to 3 ° C. Since the reaction rate of oxidizing agents such as hypochlorous acid increases as the water temperature increases, the cleaning efficiency is higher than when the treated water of the microfiltration membrane module or ultrafiltration membrane module 7 is used as cleaning water. Also has.

  As described above, according to the present invention, while continuing the process of separating the raw water by reverse osmosis membrane, even if a high concentration oxidizing agent is injected into the microfiltration membrane or ultrafiltration membrane, it does not cause scale generation. It becomes possible to efficiently wash and remove the organic pollutant adhered and deposited on the surface of the microfiltration membrane or ultrafiltration membrane. Therefore, long-term stable operation of a membrane separation apparatus using a reverse osmosis membrane is possible.

  The present invention is suitably used for washing a microfiltration membrane or an ultrafiltration membrane used as a pretreatment application for an apparatus for separating raw water containing salt, such as seawater desalination or brine desalination, by reverse osmosis membrane separation.

It is a schematic flowchart which shows the filtration and backwashing process of the membrane separator which shows a prior art. It is a schematic flowchart which shows the filtration and backwashing process of the membrane separator which shows one Embodiment of this invention.

Explanation of symbols

1: Pretreated treated water tank 2: High pressure pump 3: First reverse osmosis membrane module 4: Low pressure pump 5: Second reverse osmosis membrane module 6: Supply pump 7: Microfiltration membrane module or ultrafiltration membrane Module 8: Backwash pump (1)
9: Oxidant tank 10: Chemical injection pump 11: Concentrated water tank 12: Backwash pump (2)

Claims (4)

The treated water obtained by treating raw water with a microfiltration membrane module or an ultrafiltration membrane module is subjected to reverse osmosis treatment with the first reverse osmosis membrane module and further subjected to reverse osmosis treatment with the first reverse osmosis membrane module. in membrane separation process for reverse osmosis treatment in the second reverse osmosis membrane module permeate was, only at least part of the concentrated water obtained by the second reverse osmosis membrane module, used for the microfiltration membrane module A membrane separation method comprising a washing step of washing water for an ultrafiltration membrane used in a microfiltration membrane or the ultrafiltration membrane module. And wherein only at least part of the concentrated water obtained in the second reverse osmosis membrane module, upon use as washing water of the microfiltration membrane or the ultrafiltration membrane, adding an oxidizing agent to the wash water The membrane separation method according to claim 1. A first reverse osmosis membrane module that includes a microfiltration membrane module or an ultrafiltration membrane module for treating raw water, and that performs reverse osmosis treatment of treated water obtained by the microfiltration membrane module or the ultrafiltration membrane module; Further, in the membrane separation apparatus having a second reverse osmosis membrane module for treating the permeate obtained by the first reverse osmosis membrane module with a reverse osmosis membrane, the concentrated water obtained by the second reverse osmosis membrane module A membrane separation apparatus comprising means for returning only at least a part of the microfiltration membrane used in the microfiltration membrane module or washing water for the ultrafiltration membrane used in the ultrafiltration membrane module. Further comprising an oxidizing agent tank for oxidizing agent reservoir, and, in the middle of the means for returning at least only a portion of the concentrated water as wash water, that the addition of the oxidizing agent outlet from the oxidizer tank is provided The membrane separator according to claim 3, wherein

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