JP2007090288A - Reverse osmosis membrane separation method and reverse osmosis membrane separation apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for stably operating an apparatus by preventing deterioration in membrane function in the case of separation using a reverse osmosis membrane for desalination of seawater. <P>SOLUTION: The reverse osmosis membrane separation method is for treating an acidic substance-containing object liquid to be treated with a membrane separation apparatus having a reverse osmosis membrane. The reverse osmosis membrane separation method includes addition of an organic compound or phosphorus compound having reducibility to the object liquid to be treated before treatment of the object liquid to be treated with the membrane separation apparatus. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a reverse osmosis membrane separation method and a reverse osmosis membrane separation device that separates impurities from various liquids containing impurities using a membrane separation device having a reverse osmosis membrane, and in particular, desalination of seawater and desalination of brine. The present invention relates to a reverse osmosis membrane separation method and a reverse osmosis membrane separation apparatus that perform ultrapure water production or wastewater treatment.

  Membrane separation technology is used in a wide range of fields such as desalination of seawater and brine, medical care, industrial pure water, ultrapure water production, and industrial wastewater treatment. In separation by these membranes, contamination of the separation device by microorganisms increases the fine particles in the liquid to be treated, TOC (total organic carbon), deteriorates the quality of the permeated water obtained, and microorganisms on the membrane surface. Proliferation or organic substances composed of microorganisms and their metabolites adhere to the membrane surface, causing problems such as reducing the permeability and separation of the membrane. Therefore, sterilization of the membrane separation apparatus is an important technique for performing membrane separation. Various sterilization methods for membrane separation apparatuses have been proposed so far, and generally, a method of adding a sterilizing agent to the supply liquid constantly or intermittently is employed.

  On the other hand, since the bactericidal agent causes oxidative degradation of the membrane, an agent for eliminating oxidization is added immediately before the bactericidal agent contacts the membrane to stabilize the operation.

  Chlorine-based disinfectants have been used for a long time as disinfectants, but recently a new disinfecting method that replaces chlorine such as chloramines, hydrogen peroxide, peracetic acid, sodium bisulfite, and sulfuric acid according to the membrane used. Has been proposed. However, chlorinated fungicides are currently used because of their price and ease of operation. Reverse osmosis membranes are chemically degraded by oxidizing substances such as chlorine due to the characteristics of the material. Recently, a film material having a relatively high oxidation resistance has been developed, but it cannot be said that its durability is sufficient (for example, see Patent Documents 1 to 4).

    For this reason, in general, the supply liquid is sterilized using a chlorine-based sterilizing agent, and the reducing agent is used before being supplied to the reverse osmosis membrane. After reducing free chlorine, the separation operation is performed. Yes. Here, sodium sulfite and sodium hydrogen sulfite are widely used as the reducing agent. These reducing agents are generally added in an amount 1 to 10 times that of the oxidizing substance. This is a concentration that takes into account that the reducing agent reacts with dissolved oxygen at the same time that the remaining fungicide is completely eliminated. However, when some are added excessively, certain bacteria propagate and biofouling may occur due to these bacterial cells and metabolites, leading to a decrease in membrane performance.

  In biofouling generated using sodium bisulfite as a reducing agent, it is known that many microorganisms (such as sulfur bacteria) that grow using sulfur-based chemical species as an energy source are deposited on the membrane surface. When measuring the concentration of sodium bisulfite in the concentrated water and adjusting the concentration of sodium bisulfite added to the feed solution, the added sodium bisulfite is consumed by the sulfur bacteria grown on the membrane surface, Until sodium bisulfite is detected in the concentrated water, the amount of sodium bisulfite added to the feed water is increased, which increases the amount of chemical used and leads to an increase in chemical cost.

  In addition, if heavy metal ions such as copper and cobalt are present in the feed liquid when operating the reverse osmosis membrane device, these heavy metals hardly pass through the membrane regardless of the type of membrane, so the concentration of concentrated heavy water By gradually increasing (concentration of copper ions etc. in the feed water), even if there is no oxidant in the reverse osmosis membrane feed water, this heavy metal acts as a catalyst on the membrane surface to reduce reducing agents such as sodium bisulfite. It is known that oxidizing substances such as perchlorate ions and chlorine are generated by reacting with chloride ions, which causes a reduction in the performance of the reverse osmosis membrane (for example, Patent Documents 5 to 9).

When a reducing agent such as sodium bisulfite is dissolved in water and stored, the closed vessel is filled with sulfurous acid gas, which is a decomposition product of sodium bisulfite, and is generated by oxidation of sodium bisulfite. Since the pH of the aqueous solution is greatly lowered due to the influence of sulfuric acid, special care must be taken during handling.
JP-A-1-180208 JP-A-5-96140 Japanese Patent Laid-Open No. 9-10666 JP 2003-144874 A JP-A-7-328931 JP 7-328932 A JP-A-9-57067 JP-A-9-57076 JP-A-7-308671

  That is, an object of the present invention is to perform reverse osmosis membrane separation having a step of sterilizing using a bactericide and removing the bactericide using a reducing agent before being processed by a membrane separation apparatus having a reverse osmosis membrane. It is an object of the present invention to provide a reverse osmosis membrane separation method and a reverse osmosis membrane separation device that can suppress a decrease in membrane performance, which has been a problem in the past, and stably operate the membrane separation device.

  In order to achieve the above object, the present invention basically has the following configuration. That is, “in a separation method in which a liquid to be treated containing an oxidizing substance is treated by a separation device having a reverse osmosis membrane, a reducing organic compound or a phosphorus compound is added immediately before the separation device. Reverse osmosis membrane separation method ", preferably," When treating the liquid to be treated with a reverse osmosis membrane, at least two kinds of reducing agents among reducing organic compounds or phosphorus compounds are used for a certain period of time. " A reverse osmosis membrane separation method characterized in that the kind of the two or more kinds of reducing agents is changed every time. "

  Further, the present invention provides a “reverse osmosis membrane separation apparatus having pretreatment means for pretreating raw water using an oxidizing substance and a membrane separation apparatus for reverse osmosis treatment of water to be treated after the pretreatment, A reverse osmosis membrane separation apparatus comprising an addition means for adding an organic compound or a phosphorus compound having a reducing property, which is subsequent to the pretreatment means and upstream of the membrane separation apparatus. " “At least two or more addition means are provided in the subsequent stage of the pretreatment means and in the previous stage of the membrane separation device so that at least two kinds of reducing agents of the reducing organic compound or phosphorus compound can be added. A reverse osmosis membrane separation device comprising control means for controlling to change the types of the two or more kinds of reducing agents to be added to the liquid to be treated every time. " That.

  According to the present invention, when water is purified using a membrane separator, a reducing method is used as a method for reducing an oxidizing substance such as a chlorine-based inorganic compound that sterilizes microorganisms present on or near the membrane surface. By using organic compounds or reducing phosphorus compounds (collectively referred to as “reducing agents”), biofouling on the membrane surface due to the growth of specific microorganisms is prevented, Performance can be maintained and stabilized, the period of use of the membrane can be extended, and the operating cost of the apparatus can be reduced. Furthermore, by using at least two or more of the reducing agents in combination, it changes the growth environment of specific microorganisms, further enhances the effect of inhibiting the growth of microorganisms on the membrane surface, and maintains and stabilizes performance. The effect that it can be further enhanced can be exhibited.

  The present invention is a reverse osmosis membrane separation method of treating a liquid to be treated containing an oxidizing substance with a membrane separation apparatus having a reverse osmosis membrane. Here, the membrane separation device refers to a device for supplying a liquid to be treated to a membrane module under pressure and separating it into a permeate and a concentrate for purposes such as fresh water, concentration and separation. An example of the membrane module used in the apparatus is a reverse osmosis membrane module.

  The reverse osmosis membrane used in the present invention is a semipermeable membrane that allows some components in the liquid mixture to be separated, for example, a solvent to permeate and does not allow other components to permeate. A nanofiltration membrane or a loose RO membrane is also included in the reverse osmosis membrane in a broad sense. As the material, polymer materials such as cellulose acetate polymer, polyamide, polyester, polyimide, and vinyl polymer are often used. The reverse osmosis membrane separation method of the present invention can be used regardless of the reverse osmosis membrane material, membrane structure and membrane form to be used, and both are effective. An asymmetric membrane that has a dense layer on at least one side and has fine pores with gradually increasing pore diameters from the dense layer to the inside of the membrane or the other side, and is formed of another material on the dense layer of the asymmetric membrane. A composite membrane having a thin active layer is preferably used, and hollow membranes and flat membranes are preferably used as the membrane form of the reverse osmosis membrane. The hollow fiber and the flat membrane preferably have a film thickness of 10 μm to 1 mm, the hollow fiber has an outer diameter of 50 μm to 4 mm, the flat membrane has an asymmetric membrane, the composite membrane has a woven fabric, a knitted fabric, It is preferably supported by a substrate such as a nonwoven fabric.

  Typical examples of such reverse osmosis membranes include a cellulose acetate asymmetric membrane, a polyamide asymmetric membrane, or a composite membrane having an active layer such as polyamide or polyurea. Among these, the method of the present invention is effective for cellulose acetate asymmetric membrane, polyamide asymmetric membrane, and polyamide composite membrane in terms of hydrolysis resistance, chemical resistance, and pressure resistance. In this respect, an aromatic polyamide composite membrane is preferable.

  The reverse osmosis membrane module constituting the membrane separation apparatus used in the present invention is formed in order to actually use the reverse osmosis membrane. When the membrane form of the reverse osmosis membrane is a flat membrane, the reverse osmosis membrane module is spiral. , Tubular and plate-and-frame modules, and when the reverse osmosis membrane is a hollow fiber, it can be used by being bundled and incorporated into the module. It does not depend on the form of the reverse osmosis membrane module.

  In addition, when the spiral reverse osmosis membrane module is used as the reverse osmosis membrane module, members such as a channel material for the supply water and a channel material for the permeated water are incorporated in the module. It is preferably used as a reverse osmosis membrane module designed for high pressure.

  In the present invention, it is essential that the liquid to be treated be pretreated with an oxidizing substance in advance before the treatment with the membrane separation apparatus. The oxidizing substance used here refers to a reverse osmosis membrane separation apparatus. When operating, it must be added in the pretreatment stage in order to prevent biofouling such as fungus and microorganism growth and adhesion in the supply liquid or in the pretreatment device and also in the membrane separation device. In view of the above, chlorine-based inorganic compounds, hydrogen peroxides, peracetic acids and the like can be suitably used. In general, it is preferable to use a chlorine-based inorganic compound from the viewpoint of cost, bactericidal power, ease of handling, and the like.

  Examples of such chlorine-based inorganic compounds include chlorine gas, white powder, sodium hypochlorite, chlorine dioxide, chloramine B, chloramine T, and the like. Among them, the strength of bactericidal power, In view of ease of handling and cost, it is preferable to use an aqueous sodium hypochlorite solution.

  When sodium hypochlorite is used as the chlorine-based inorganic compound, although depending on the quality of the water to be treated, the residual effective concentration after adding to the water to be treated is generally about 0.1 to 10 mg / L. It is desirable to reduce the amount of reducing agent added in a later step, and considering the effective concentration necessary for sterilization, the residual chlorine concentration should be about 0.1 to 1 mg / L. desirable. Here, residual chlorine means the sum of free chlorine and combined chlorine, and the concentration of residual chlorine can be easily measured by the orthotolidine method, diethyl-P-phenylenediamine method, amperometric method, iodine titration method, and the like. .

  In the reverse osmosis membrane separation method of the present invention, it is essential to add a reducing organic compound or a phosphorus compound to the treatment liquid before the treatment liquid is processed by the membrane separation apparatus.

Here, the organic compound having reducibility refers to a compound that can be oxidized and reduce the partner. For example, when sodium hypochlorite is reduced with sodium hydrogen sulfite, sulfite ions are oxidized to sulfate ions, and at the same time, hypochlorite ions are reduced to chloride ions according to the following reaction formula.
2ClO ⁻ + 2SO ₃ ²⁻ → 2Cl ⁻ + 2SO ₄ ²⁻ .

  As such reducing organic compounds, saccharides, aldehydes, and vitamins are preferably used in terms of cost and easy handling. Examples of the saccharide include glyceraldehyde, ribose, arabinose, xylose, glucose, mannose, galactose, maltose, lactose, and the like. Examples of the aldehyde include formaldehyde, acetaldehyde, formic acid, glutaraldehyde, and vitamins. As vitamin C (L-ascorbic acid) or a salt thereof.

  Further, the phosphorus compound having reducibility means a compound containing phosphorus that can be oxidized and reduce the partner, and among them, hypophosphorous acid or hypophosphorous acid that is a salt thereof. Sodium is preferably used in terms of reducing strength, cost, and ease of handling.

  Among these reducing organic compounds and phosphorus compounds having reducing properties, sodium hypophosphite, vitamins are considered in consideration of reactivity, ease of handling, and no influence on reverse osmosis membranes. C is preferred.

  These reducing agents are preferred in that they are easy to handle without producing harmful gases as in the case of storing sodium hydrogen sulfite or greatly reducing the pH in the reducing agent solution. Furthermore, when sodium hydrogen sulfite is added, although the amount is small, sulfate ions, which are the source of scale, are added to the feed solution, so that scale deposition in the reverse osmosis membrane device when a higher recovery rate is attempted. While these reducing agents contain the same site as the functional group contained in the scale inhibitor that forms a chelate with heavy metal ions and suppresses scale formation, Contrary to sodium bisulfite, it can be expected to have an effect of suppressing scale.

  The amount of these reducing agents to be added varies depending on the type of chemical used, the concentration of the oxidizing substance in the liquid to be treated, the liquid temperature, the permeation flow rate, etc., and is not particularly limited. It is preferable to add at a concentration equivalent to 1 to 10 times the concentration of the oxidizing substance.

  The frequency of adding the reducing agent to the liquid to be treated pretreated with the oxidizing substance is not particularly limited, but it may be added when the oxidizing substance is contained in the liquid to be treated. If an oxidizing substance is constantly added to the liquid, a reducing agent is also always added. If an oxidizing substance is intermittently added, the reducing agent may also be added intermittently.

  Furthermore, it is more preferable to change the kind of the reducing agent when the reducing agent is added to the liquid to be treated pretreated with the oxidizing substance. Dissolved oxygen in the liquid to be treated disappears with the addition of the reducing agent, and general aerobic bacteria do not grow.For example, when sodium bisulfite is added over a long period of time, reduction occurs. Similar to the growth of special bacteria and biofouling, the addition of other reducing agents over a long period of time may cause the growth of resistant bacteria in the environment. Therefore, by changing the type of reducing agent to be added, there is an effect of changing the environment in which the special bacteria grow, preventing the growth, and suppressing the occurrence of biofouling.

  The frequency of changing the type of reducing agent is not particularly limited as long as the amount of oxidant in the liquid to be treated is completely eliminated, but is not limited to any frequency. Depending on the degree, the type can be changed at intervals such as every day, every week, or every month.

  In the present invention, seawater is preferably used as raw water before pretreatment with an oxidizing substance. Unlike fresh water and brine, seawater contains many microorganisms and also contains a lot of carbon sources, nitrogen sources, and minerals that are necessary as nutrient sources. Since the risk of causing biofouling is increased, the effect is increased when the reverse osmosis membrane separation method of the present invention is adopted, which is preferable.

  The reverse osmosis membrane separation method of the present invention is performed using a reverse osmosis membrane separation device described below.

  The reverse osmosis membrane separation device of the present invention comprises pretreatment means for pretreatment using an oxidizing substance, and a membrane separation device for reverse osmosis treatment of the pretreated liquid. An addition means for adding an organic compound or a phosphorus compound having a reducing property is provided after the pretreatment means and before the membrane separation apparatus.

  As the pretreatment means, an oxidizing substance such as sodium hypochlorite described above can be added to the raw water, and the fungi and microorganisms in the supply liquid or in the pretreatment apparatus, and further in the membrane separation apparatus can be added. Anything that can prevent biofouling such as breeding and adhesion is acceptable. Use an oxidizing substance such as sodium hypochlorite as it is, or use pure water or permeated water from a membrane separator. Thus, it is desirable to be able to add a material that has been reduced to a predetermined concentration.

  The adding means for adding the reducing organic compound or phosphorus compound may be any means that can add the reducing agent described above to the liquid to be treated. The reducing agent may be pure water or permeated water from a membrane separator. What melt | dissolved solution is added and mixed so that it may become a predetermined concentration to a to-be-processed liquid, etc. are used suitably.

  As such reverse osmosis membrane separation, at least 2 addition means are provided so that at least two kinds of reducing agents of the reducing organic compound or phosphorus compound can be added after the pretreatment means and before the membrane separation apparatus. It is more preferable to provide two or more because the type of the reducing agent can be easily changed. Furthermore, it is more preferable to provide a control means for controlling the two or more kinds of reducing agents to be added to the liquid to be treated at regular intervals.

  In this way, by stopping the supply of chemical species serving as a source of microorganisms / or generating sources of oxidizing agents and using other compounds as reducing agents for removing oxidizing substances, the growth of microorganisms, By preventing the generation of oxidants and preventing deterioration of membrane performance, it can greatly contribute to stable plant operation.

  The present invention will be described below with reference to specific examples, but the present invention is not limited to these examples.

Example 1
Membrane separation using reverse osmosis membrane (SU-810 × 6 manufactured by Toray) made of polyamide under conditions of pressure 56 kg / cm ² , temperature 25 ° C., recovery rate 40%, pH 6.7, using seawater as supply water The apparatus was operated for reverse osmosis separation. A schematic diagram of the reverse osmosis membrane separation apparatus used in this example is shown in FIG. In the pretreatment process in the pretreatment device, 1 ppm of chlorine-based disinfectant (sodium hypochlorite) is continuously added to the raw water, and then 5 ppm of L-ascorbic acid is used as a dechlorinating agent in the reducing agent addition means. Continuous infusion. At this time, the film differential pressure hardly changed to 0.056 MPa after about one month with respect to the initial 0.05 MPa.

(Example 2)
Using the reverse osmosis membrane separation apparatus shown in FIG. 3, reverse osmosis separation is performed in the same manner as in Example 1 with the operating conditions of the membrane separation apparatus using reverse osmosis membranes made of polyamide (SU-810 × 6 manufactured by Toray). At the same time, in the pretreatment step in the pretreatment device, 1 ppm of chlorine-based disinfectant (sodium hypochlorite) is continuously added to the raw water, and then sodium hypophosphite as a dechlorinating agent in the reducing agent addition means. 5 ppm and 5 ppm of L-ascorbic acid were used, and the respective drugs were continuously infused under the control of changing alternately every 24 hours. At this time, the film differential pressure hardly changed to 0.053 MPa after about one month with respect to the initial 0.05 MPa.

(Comparative Example 1)
When reverse osmosis separation is performed under the same operating conditions of the membrane separation apparatus using reverse osmosis membranes made of polyamide (SU-810 × 6 manufactured by Toray) under the same conditions as in Example 1, a pretreatment step in the pretreatment apparatus Then, 1 ppm of a chlorine-based disinfectant (sodium hypochlorite) was continuously added to the raw water, and 6 ppm of sodium bisulfite alone was continuously injected as a dechlorinating agent. After about one month, the membrane differential pressure increased by about 0.03 MPa.

It is a schematic flowchart which shows the reverse osmosis membrane separation method which concerns on this invention. It is a general | schematic flowchart which shows one preferable embodiment of the reverse osmosis membrane separator which concerns on this invention. It is a general | schematic flowchart which shows the other preferable embodiment of the reverse osmosis membrane separator which concerns on this invention.

Claims (13)

In a reverse osmosis membrane separation method of treating a treatment liquid containing an oxidizing substance with a membrane separation apparatus having a reverse osmosis membrane, before treating the treatment liquid with the membrane separation apparatus, A reverse osmosis membrane separation method comprising adding a reducing organic compound or a phosphorus compound. The reverse osmosis membrane separation method according to claim 1, wherein the oxidizing substance is a chlorine-based inorganic compound. The reverse osmosis membrane separation method according to claim 1 or 2, wherein the reverse osmosis membrane is at least one selected from the group consisting of a cellulose acetate asymmetric membrane, a polyamide asymmetric membrane, and a polyamide composite membrane. The reverse osmosis separation method according to any one of claims 1 to 3, wherein the reducing organic compound is at least one selected from the group consisting of sugars, aldehydes, and vitamins. The reverse osmosis membrane separation method according to any one of claims 1 to 3, wherein the reducing phosphorus compound is hypophosphorous acid or a salt thereof. When treating the liquid to be treated with the membrane separation apparatus, at least two kinds of reducing agents are used among the reducing organic compounds or phosphorus compounds, and the kinds of the two or more kinds of reducing agents are used at regular intervals. The reverse osmosis membrane separation method according to claim 1, wherein the reverse osmosis membrane is added to the liquid to be treated. The reverse osmosis membrane separation method according to any one of claims 1 to 6, wherein seawater is used as raw water, and water obtained by treating the raw water with the oxidizing substance is used as the liquid to be treated. In a reverse osmosis membrane separation apparatus having a pretreatment means for pretreating raw water with an oxidizing substance and a membrane separation apparatus for reverse osmosis treatment of the pretreated water after the pretreatment, a post-stage of the pretreatment means And a reverse osmosis membrane separation device having an addition means for adding an organic compound or a phosphorus compound having a reducing property in a preceding stage of the membrane separation device. The reverse osmosis membrane separation device according to claim 8, wherein the oxidizing substance is a chlorine-based inorganic compound. The reverse osmosis membrane according to claim 8 or 9, wherein the reverse osmosis membrane used in the reverse osmosis membrane separation device is at least one selected from a cellulose acetate asymmetric membrane, a polyamide asymmetric membrane, and a polyamide composite membrane. Separation device. The reverse osmosis separation method according to any one of claims 8 to 10, wherein the reducing organic compound is at least one selected from the group consisting of sugars, aldehydes, and vitamins. The reverse osmosis membrane separation device according to any one of claims 8 to 10, wherein the reducing phosphorus compound is hypophosphorous acid or a salt thereof. At least two or more addition means are provided in the subsequent stage of the pretreatment means and in the previous stage of the membrane separation apparatus so that at least two kinds of reducing agents among the reducing organic compounds or phosphorus compounds can be added, and for a certain period of time. The reverse osmosis membrane separation according to any one of claims 8 to 12, further comprising a control unit that controls to change the types of the two or more kinds of reducing agents and add them to the liquid to be treated. apparatus.

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