WO2012098969A1

WO2012098969A1 - Method for cleaning membrane module, method of fresh water generation, and fresh water generator

Info

Publication number: WO2012098969A1
Application number: PCT/JP2012/050416
Authority: WO
Inventors: 智宏前田; 谷口　雅英
Original assignee: 東レ株式会社
Priority date: 2011-01-20
Filing date: 2012-01-12
Publication date: 2012-07-26
Also published as: JPWO2012098969A1; CL2013002072A1; CN103328079B; CN103328079A

Abstract

A method of fresh water generation by filtering raw water through a microfiltration/ultrafiltration membrane module and subsequently separating the filtrate into filtrate water and concentrated water with a semipermeable-membrane unit. For the purpose of preventing clogging due to scale deposition and accumulation on the surface and within the pores of the membrane of the microfiltration/ultrafiltration membrane module, the fresh water generation method includes temporarily stopping the filtration through the microfiltration/ultrafiltration membrane module, supplying at least some of the concentrated water as cleaning water from the downstream side of the microfiltration/ultrafiltration membrane module to conduct back-pressure washing of the microfiltration/ultrafiltration membrane module, and incorporating a scale inhibitor and an alkaline solution into the cleaning water.

Description

Membrane module cleaning method, fresh water generation method and fresh water generation device

In the present invention, raw water is filtered through a microfiltration membrane / ultrafiltration membrane (hereinafter referred to as MF / UF membrane) module, and the filtrate is provided with a reverse osmosis membrane (RO membrane) or a nanofiltration membrane (NF membrane). In addition, the present invention relates to a fresh water generation method and a fresh water generation apparatus for filtering with a semipermeable membrane unit. In particular, the present invention relates to a fresh water generation method characterized by a method for cleaning an MF / UF membrane module in the fresh water generation method, and a fresh water generation apparatus capable of suitably implementing it.

In recent years, in water treatment applications such as water and sewage and wastewater treatment, membrane filtration methods that separate and remove impurities in raw water by membranes and convert them into clear water have been spreading. The substance to be removed varies depending on the type of the film, but in the case of the MF / UF film, generally, suspended substances, bacteria, protozoa, colloidal substances, and the like can be mentioned. In the case of an RO membrane or an NF membrane (hereinafter collectively referred to as a semipermeable membrane), soluble organic substances, viruses, ionic substances and the like can be mentioned.

When raw water is filtered through an MF / UF membrane, as the filtration continues, the amount of humic substances, proteins, etc. on the surface of the MF / UF membrane and within the pore size of the membrane increases, and an increase in the filtration differential pressure becomes a problem. Come.

Therefore, air is introduced into the primary side (feed water side) of the MF / UF membrane cage, the membranes are vibrated, and the membranes are brought into contact with each other to remove the adhered substances on the membrane surface, or to the MF / UF membrane. On the other hand, physical washing such as back pressure washing is performed in which membrane filtration water or clarified water is flowed in pressure in the opposite direction to filtration to remove adhered substances adhering to the membrane surface and membrane pore diameter. In order to further enhance the cleaning effect, for example, Patent Document 1 describes that back pressure cleaning is performed by adding an oxidizing agent such as sodium hypochlorite to back pressure cleaning water. The oxidizing agent has an effect of decomposing / removing organic substances such as humic acid and microorganism-derived protein adhering to the membrane surface and membrane pores.

In addition, a method of producing clear water by filtering raw water with a MF / UF membrane and then treating the filtered water with a semipermeable membrane is called an IMS process (see Non-Patent Document 1). In the IMS process, in

Patent Documents

2 and 3, there is a method in which a part of the reverse osmosis membrane concentrated water originally discharged out of the system in order to increase the water recovery rate is used as the reverse pressure washing water of the MF membrane / UF membrane. Proposed. However, when backwashing with reverse osmosis membrane concentrated water without using an oxidant, microorganisms in the membrane module are killed by osmotic shock, but organic substances such as humic substances and microorganism-derived proteins cannot be decomposed or removed. Therefore, there was a problem that fouling progressed early.

Therefore, Patent Document 4 proposes a method in which reverse osmosis membrane concentrated water to which an oxidizing agent such as sodium hypochlorite is added is used as MF / UF membrane reverse pressure washing water. However, in the case of concentrated water obtained by treating seawater with a reverse osmosis membrane, high concentrations of calcium and magnesium are present in the concentrated water. Therefore, when sodium hypochlorite is added to seawater membrane filtered water or reverse osmosis membrane concentrated water, the pH locally increases and scale is generated in the backwash water. In addition, when back pressure cleaning is performed using such back pressure cleaning water, scale adheres and accumulates in the membrane pores, and inorganic fouling significantly proceeds. For this reason, it has been considered impractical to use an oxidizing agent such as sodium hypochlorite as concentrated water of the semipermeable membrane unit as the back pressure washing water of the MF membrane / UF membrane.

JP 2001-79366 A JP 2006-272136 A JP 2007-181822 A JP-A-9-220449

An object of the present invention is to provide a fresh water generation method in which raw water is filtered with an MF / UF membrane module, and the filtered water is filtered with a semipermeable membrane unit having an RO membrane or an NF membrane. An object of the present invention is to provide a cleaning method, a fresh water generating method, and a fresh water generating device for an MF / UF membrane module, which can prevent clogging due to scales adhering and accumulating in holes.

In order to solve the above problems, the present invention is characterized by one of the following configurations.
(1) Interruption of filtration in the microfiltration / ultrafiltration membrane module in the fresh water production method in which raw water is filtered with a microfiltration / ultrafiltration membrane module and then separated into permeate and concentrated water with a semipermeable membrane unit. A method for cleaning a membrane module that is sometimes performed, wherein at least a part of the concentrated water is used as cleaning water, and the cleaning water containing a scale inhibitor and an alkaline solution is added to the secondary of the microfiltration / ultrafiltration membrane module. A method for cleaning a membrane module, which is supplied from the side and performs reverse pressure cleaning of the microfiltration / ultrafiltration membrane module.
(2) The membrane module cleaning method according to (1), wherein a scale inhibitor is added to at least a part of the concentrated water so that the scale water contains a scale inhibitor.
(3) A scale inhibitor is added to at least a part of the filtered water obtained from the microfiltration / ultrafiltration membrane module and separated by the semipermeable membrane unit so that the washing water contains the scale inhibitor. The method for cleaning a membrane module according to (1) above.
(4) The method for cleaning a membrane module according to (3), further comprising adding a scale inhibitor to at least a part of the concentrated water so that the scale water contains a scale inhibitor.
(5) A fresh water generation method in which raw water is filtered with a microfiltration / ultrafiltration membrane module and then separated into permeate and concentrated water with a semipermeable membrane unit, temporarily, the microfiltration / ultrafiltration Filtration in the membrane module is interrupted, and at least a part of the concentrated water is supplied as wash water from the secondary side of the microfiltration / ultrafiltration membrane module to perform back pressure washing of the microfiltration / ultrafiltration membrane module. A method for producing fresh water, wherein the washing water contains a scale inhibitor and an alkaline solution.
(6) The fresh water generation method according to (5), wherein a scale inhibitor is added to at least a part of the concentrated water so that the wash water contains the scale inhibitor.
(7) A scale inhibitor is added to at least a part of the filtrate obtained from the microfiltration / ultrafiltration membrane module and separated by the semipermeable membrane unit so that the wash water contains the scale inhibitor. The fresh water generation method according to (5).
(8) The fresh water generation method according to (7), further comprising adding a scale inhibitor to at least a part of the concentrated water so that the wash water contains the scale inhibitor.
(9) a microfiltration / ultrafiltration membrane module, a semipermeable membrane unit that separates at least a portion of filtrate of the microfiltration / ultrafiltration membrane module into permeate and concentrated water, and at least the concentrated water A back pressure washing unit that supplies a part of the washing water from the secondary side of the microfiltration / ultrafiltration membrane module to backwash the microfiltration / ultrafiltration membrane module, and prevents scale in the washing water A fresh water generator comprising a scale inhibitor supply unit that contains an agent and an alkaline solution supply unit that contains an alkaline solution in the washing water.
(10) The alkaline solution includes a unit for supplying the scale inhibitor to a line for supplying at least a part of the concentrated water from the secondary side of the microfiltration / ultrafiltration membrane module as the scale inhibitor supply unit. The fresh water generator according to (9), comprising a unit that supplies an alkaline solution to a line that supplies at least a part of the concentrated water from the secondary side of the microfiltration / ultrafiltration membrane module as a supply unit.
(11) As the scale inhibitor supply unit, a unit for supplying a scale inhibitor to a line for supplying filtered water of the microfiltration / ultrafiltration membrane module to the semipermeable membrane unit, and as the alkaline solution supply unit, The fresh water generator according to (9), further comprising a unit that supplies an alkaline solution to a line that supplies at least a part of the concentrated water from a secondary side of the microfiltration / ultrafiltration membrane module.
(12) The scale inhibitor supply unit further includes a unit that supplies the scale inhibitor to a line that supplies at least a part of the concentrated water from the secondary side of the microfiltration / ultrafiltration membrane module. The fresh water generator described in (11).

In the present invention, “microfiltration / ultrafiltration membrane module” means at least one of a microfiltration membrane module and an ultrafiltration membrane module.

According to the present invention, in the fresh water generation method in which raw water is filtered with an MF / UF membrane module and then separated into permeated water and concentrated water with a semipermeable membrane unit, filtration in the MF / UF membrane module is temporarily performed. Interrupting, supplying at least a portion of the concentrated water as wash water from the secondary side of the MF / UF membrane module to perform back pressure washing of the MF / UF membrane module, and adding a scale inhibitor to the wash water Incorporate an alkaline solution. That is, the MF / UF membrane module is back-pressure washed with washing water to which an alkaline solution is added, and a scale inhibitor is added to the washing water in advance. Thereby, it can suppress that a scale is produced | generated by local pH rise in back pressure wash water, and can prevent clogging by a scale adhering and accumulating in a membrane surface and a pore.

It is a flowchart which shows one embodiment of the fresh water generator of this invention. It is a flowchart which shows another embodiment of the fresh water generator of this invention.

Hereinafter, the present invention will be described in more detail based on embodiments shown in the drawings. In addition, this invention is not limited to the following embodiment.

1 according to the present invention includes, for example, a raw water storage tank 1 that stores raw water, a raw water supply pump 2 that supplies raw water, a raw water supply valve 3 that opens when the raw water is supplied, and a raw water filter. A hollow fiber membrane module 4 (MF / UF membrane module), an air vent valve 5 that is opened when performing back-pressure washing or air washing, a filtration water valve 6 that is opened during filtration, and a hollow fiber membrane filtrate. The filtrate storage tank 7 to be stored, the semipermeable membrane unit 18 for separating the filtrate of the hollow fiber membrane module 4 into permeate and concentrated water, and the filtrate obtained by the hollow fiber membrane module 4 to the semipermeable membrane unit 18. A booster pump 19 that supplies the concentrated water, a concentrated water storage tank 20 that stores the concentrated water of the semipermeable membrane unit 18, and a concentrated water drain valve 22 that is opened when the concentrated water of the semipermeable membrane unit is drained out of the system. Concentrated water is supplied as washing water to hollow fiber membrane module A backwash pump 8 that backwashes the fluid 4, a backwash valve 9 that opens during backwashing, a backwash pipe 10 through which concentrated water from the concentrated water storage tank 20 passes during backwashing, and a hollow fiber. A drain valve 11 that is opened when water on the primary side of the membrane module 4 is drained, an air valve 12 that is opened when compressed air is supplied to the lower part of the hollow fiber membrane module 4 and air-washed, and compressed air An air supply unit including a compressor 13 as a supply source, an alkaline solution storage tank 14 for storing an alkaline solution, an alkaline solution supply pump 15 for supplying an alkaline solution to concentrated water as a reverse pressure washing water, and a scale inhibitor Scale storage tank 16, a first scale supply pump 17 for supplying scale inhibitor to concentrated water as back pressure washing water, and a scale inhibitor for water supplied to the semipermeable membrane unit 18. Second scale preventive agent supply pump 21 is provided for supplying.

Here, in the apparatus shown in FIG. 1, as the scale inhibitor supply unit, the first scale inhibitor supply pump 17 that supplies the scale inhibitor to the concentrated water of the semipermeable membrane unit 18 and the supply to the semipermeable membrane unit 18. Although the second scale inhibitor supply pump 21 for supplying the scale inhibitor to water is provided, either one may be used.

In the apparatus shown in FIG. 1, the filtrate of the hollow fiber membrane module 4 is temporarily stored in the filtrate storage tank 7 as an intermediate tank, and then supplied to the semipermeable membrane unit 18, as shown in FIG. Alternatively, the filtered water of the hollow fiber membrane module 4 may be supplied directly to the semipermeable membrane unit 18 and separated into permeated water and concentrated water without using an intermediate tank.

In the apparatus shown in FIG. 1, the concentrated water of the semipermeable membrane unit 18 stored in the concentrated water storage tank 20 is used as cleaning water, and back pressure cleaning is performed using the backwash pump 8. As described above, the high pressure concentrated water of the semipermeable membrane unit 18 may be supplied to the hollow fiber membrane module 4 as it is without using the concentrated water storage tank 20 and the backwash pump 8 to perform back pressure washing.

The hollow fiber membrane module 4 may be an immersion membrane module that is immersed in a membrane immersion tank containing raw water and suction filtered with a pump, siphon, or the like in addition to the pressure membrane module as shown in FIG. In the case of a pressurizing membrane module, an external pressure type or an internal pressure type may be used, but an external pressure type is preferred from the viewpoint of simplicity of pretreatment.

The material of the MF / UF membrane constituting the hollow fiber membrane module 4 is not particularly limited as long as it is a porous hollow fiber membrane, but it is not limited to inorganic materials such as ceramics, polyethylene, polypropylene, polyacrylonitrile, ethylene-tetrafluoroethylene copolymer. Polymer, polychlorotrifluoroethylene, polytetrafluoroethylene, polyvinyl fluoride, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, chlorotrifluoroethylene-ethylene copolymer , Preferably containing at least one selected from the group consisting of polyvinylidene fluoride, polysulfone, cellulose acetate, polyvinyl alcohol, polyethersulfone, and polyvinyl chloride, and further, film strength and chemical resistance More preferably polyvinylidene fluoride (PVDF) in terms of sex, polyacrylonitrile is more preferable from the viewpoint that a strong high stain resistance hydrophilic.

The membrane constituting the hollow fiber membrane module 4 may be an MF membrane or a UF membrane, and a membrane having a surface pore diameter in the range of 0.001 μm to 10 μm can be appropriately selected.

The MF / UF membrane module may be a module using a flat membrane, a tubular membrane, a monolith membrane, etc., in addition to the hollow fiber membrane module 4 shown in FIG.

The filtration method in the MF / UF membrane module may be either a full filtration method or a cross flow filtration method, but full filtration is preferred from the viewpoint of low energy consumption. Here, as a control method of the filtration flow rate in the MF / UF membrane module, there is no problem even if it is constant flow filtration or constant pressure filtration, but the constant flow rate from the viewpoint of easy control of the amount of filtered water produced. Filtration is preferred.

On the other hand, the semipermeable membrane in the semipermeable membrane unit 18 is a membrane having semi-permeability that does not allow some components in the liquid mixture to be separated, for example, a solvent to permeate and other components to permeate. Is included. As the material, polymer materials such as cellulose acetate polymer, polyamide, polyester, polyimide, vinyl polymer are often used. In addition, the membrane structure includes an asymmetric membrane having a dense layer on at least one side of the membrane, and gradually having fine pores with a large pore diameter from the dense layer to the inside of the membrane or the other side, or a dense layer of the asymmetric membrane. A composite membrane having a very thin separation functional layer formed of another material on the top can be used as appropriate. The membrane form includes a hollow fiber membrane and a 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.

The operating pressure of the semipermeable membrane unit 18 is usually preferably 0.1 MPa to 15 MPa, and can be properly used depending on the type of supply water, the operation method, and the like. It is used at a relatively low pressure when supplying low osmotic pressure water such as brine or ultrapure water, and at a relatively high pressure when desalinating seawater, treating wastewater, and recovering useful materials.

The semipermeable membrane unit 18 provided with the NF membrane or the RO membrane is not particularly limited, but in order to facilitate handling, a fluid separation element is provided by placing a hollow fiber membrane-like or flat membrane-like semipermeable membrane in a casing. It is preferable to use a pressure vessel filled with (element). When the fluid separation element is formed of a flat membrane, for example, generally a semipermeable membrane is wound in a cylindrical shape together with a flow path material (net) around a cylindrical central pipe having a large number of holes. Examples of commercially available products include reverse osmosis membrane elements TM700 series and TM800 series manufactured by Toray Industries, Inc. These fluid separation elements may form a semipermeable membrane unit by one, but it is also preferable to form a semipermeable membrane unit by connecting a plurality of fluid separation elements in series or in parallel.

In such a fresh water generator, in a normal filtration process, the raw water stored in the raw water storage tank 1 with the raw water supply valve 3 and the filtered water valve 6 open is turned into a hollow fiber membrane module by the raw water supply pump 2. 4 is supplied to the primary side of the pressure filter 4 and pressure filtered by the hollow fiber membrane module 4. The filtration time is preferably set as appropriate according to the raw water quality and the filtration flux, but the filtration may be continued until a predetermined filtration differential pressure is reached.

The filtrate of the hollow fiber membrane module 4 is temporarily stored in the filtrate storage tank 7, then pressurized by the booster pump 19 and supplied to the semipermeable membrane unit 18. In the semipermeable membrane unit 18, the supplied water is separated into permeated water from which solutes such as salt have been removed and concentrated water from which solutes such as salt have been concentrated.

In the present invention, for example, after performing filtration and separation as described above for a predetermined time, the filtration in the MF / UF membrane module is temporarily interrupted, and the concentrated water of the semipermeable membrane unit is used to concentrate the MF / UF membrane module. Backwashing is performed. That is, in the apparatus shown in FIG. 1, at least the filtration in the hollow fiber membrane module 4 is temporarily interrupted, and the concentration obtained from the semipermeable membrane unit 18 in the hollow fiber membrane module 4 from the direction opposite to the filtration direction. Perform back-pressure washing to back flow water. In the case of the apparatus shown in FIG. 1, this cleaning can be performed while the operation of the semipermeable membrane unit 18 is continued, but the semipermeable membrane unit 18 in the meantime is stored in the filtrate storage tank 7. The filtered water is used.

In the back pressure washing of the hollow fiber membrane module 4, the raw water supply pump 2 is stopped, the raw water supply valve 3 and the filtrate water valve 6 are closed, and the filtration process of the hollow fiber membrane module 4 is stopped. And the backwash valve 9 is opened and the backwash pump 8 is operated.

In addition, in order to decompose and remove organic substances such as humic substances and microorganism-derived proteins adhering to the MF / UF membrane during back pressure washing, an alkaline solution is contained in the concentrated water obtained from the semipermeable membrane unit. Wash water.

Here, the concentrated water is, for example, water containing a significant concentration of scale-generating components such as calcium ions, magnesium ions, bicarbonate ions, carbonate ions, sulfate ions, sodium ions, chloride ions, and the like. Concentrated water in the unit. Therefore, when an alkaline solution is added to the water and the pH is locally increased, scale is generated and clogging is likely to occur. According to the study of the present inventor, clogging tends to occur when the TDS (Total Dissolved Solids) concentration is 1,000 mg / L or more, and clogging is more likely to occur when the concentration exceeds 10,000 mg / L. It has been found that clogging is more likely to occur when it exceeds 000 mg / L. Therefore, in the present invention, the MF / UF membrane module is back-pressure washed with concentrated water containing an alkaline solution and an anti-scale material. In this way, the concentrated water that has conventionally been required to be disposed of can be effectively used.

In the form shown in FIG. 1, the alkaline solution in the alkaline solution storage tank 14 is supplied by the alkaline solution supply pump 15, and the scale inhibitor in the scale inhibitor storage tank 16 is supplied by the first scale inhibitor supply pump 17. In the concentrated water, an alkaline solution and a scale inhibitor are contained. Then, the concentrated water is used as washing water to perform back pressure washing of the hollow fiber membrane module 4.

The TDS concentration refers to the total soluble substance concentration, and examples of the soluble substance include sodium ions, calcium ions, magnesium ions, chloride ions, carbonate ions, sulfate ions, and the like. Since water with a high salt concentration, such as seawater, contains only a very small amount of other soluble substances with respect to the salinity, the TDS concentration can be replaced by the salt concentration.

The back pressure washing of the hollow fiber membrane module 4 is performed, for example, regularly during the course of membrane filtration, and the frequency is usually about once every 15 to 120 minutes.

Further, the back pressure washing using the washing water containing the alkaline solution and the scale inhibitor may be applied at the time of every back pressure washing, but it is not always necessary to carry out the whole back washing process. In order to reduce the chemical cost, it is preferable to carry out the treatment at a frequency of several times a day to once a week.

Time of two types of back pressure cleaning (back pressure cleaning using washing water containing alkaline solution and scale inhibitor and back pressure washing using washing water not containing alkaline solution and scale inhibitor) is particularly limited. However, it is preferable that both be within the range of 5 seconds to 120 seconds. When the back pressure washing time for one time is less than 5 seconds, a sufficient washing effect cannot be obtained, and when it exceeds 120 seconds, the operation efficiency of the hollow fiber membrane module 4 is lowered. The flux for backwashing is not particularly limited, but is preferably 0.5 times or more of the filtration flux. If the back pressure washing flux is less than 0.5 times the filtration flux, it is difficult to sufficiently remove the dirt deposited on the membrane surface and pores. A higher back-pressure cleaning flux is preferable because the membrane cleaning effect is higher, but it is appropriately set within a range in which damage such as breakage of the hollow fiber membrane module container and membrane cracking does not occur.

As the alkaline solution, a sodium hydroxide solution, a sodium hypochlorite solution, or the like can be used. Among these, a sodium hypochlorite solution is preferable because it is easy to use and has a high membrane cleaning effect. In the case of using a sodium hydroxide solution, a cleaning effect is hardly obtained when the pH is less than 10, and when the pH exceeds 12, there is a high possibility of deteriorating the membrane. Therefore, a pH range of 10 to 12 is preferable. When a sodium hypochlorite solution is used, it is preferably added so that the chlorine concentration in the wash water is in the range of several mg / L to several thousand mg / L. In particular, as will be described later, when the sodium hypochlorite solution is retained in the hollow fiber membrane module 4, it is preferable that the chlorine concentration in the washing water be 50 mg / L or more and 1000 mg / L or less. This is because if the chlorine concentration is too low, the sodium hypochlorite solution is completely consumed while being held in the hollow fiber membrane module 4, and a sufficient cleaning effect cannot be obtained, and the chlorine concentration is too high. This is because the cost of treating the wastewater becomes high.

The scale inhibitor is a substance that forms a complex with a metal, a metal ion, or the like in a solution and solubilizes the metal or metal salt, and an organic or inorganic ionic polymer or monomer can be used. Organic polymers include synthetic polymers such as polyacrylic acid, sulfonated polystyrene, polyacrylamide, and polyallylamine; natural polymers such as carboxymethylcellulose, chitosan, and alginic acid; and monomers such as ethylenediaminetetraacetic acid (EDTA). Can be used. Moreover, polyphosphate etc. can be used as an inorganic type scale inhibitor. Among these scale inhibitors, polyphosphate and ethylenediaminetetraacetic acid (EDTA) are particularly preferably used from the viewpoints of availability, ease of operation such as solubility, and cost. The polyphosphate refers to a polymerized inorganic phosphate material having two or more phosphorus atoms in a molecule typified by sodium hexametaphosphate and bonded with an alkali metal, an alkaline earth metal and a phosphate atom. Typical polyphosphates include tetrasodium pyrophosphate, disodium pyrophosphate, sodium tripolyphosphate, sodium tetrapolyphosphate, sodium heptapolyphosphate, sodium decapolyphosphate, sodium metaphosphate, sodium hexametaphosphate, and potassium salts thereof. Etc. These scale inhibitors may be used alone, or a plurality of scale inhibitors may be mixed.

The addition concentration of the scale inhibitor is sufficient if at least the scale components in the wash water can be dissolved and removed. In consideration of operability such as cost and time required for dissolution, it is generally 0.01 ppm or more and 100 ppm or less, particularly 0.1 ppm or more and 50 ppm or less, more preferably 1 ppm in the case of a general seawater level. It is 20 ppm or less. When the addition concentration is lower than 0.01 ppm, the generation of scale may not be sufficiently suppressed, so that the performance of the semipermeable membrane may be deteriorated. On the other hand, if it exceeds 100 ppm, the scale inhibitor itself adheres to the surface of the semipermeable membrane to cause deterioration of water permeability or deteriorate the water quality. However, in the case of washing water containing a large amount of scale components and metals, it may be necessary to add tens to hundreds of ppm.

It is preferable to add the scale inhibitor to the washing water before adding the alkaline solution. If the alkaline solution is added to the washing water before the scale inhibitor is added, the possibility of scale formation in the washing water due to a local increase in pH increases.

The scale inhibitor need not be added to the concentrated water on the downstream side of the semipermeable membrane unit 18. The scale inhibitor is added to the water supplied to the semipermeable membrane unit 18 by the second scale inhibitor supply pump 21, that is, the filtered water of the hollow fiber membrane module 4, and the permeated water and concentrated water are added by the semipermeable membrane unit 18. May be separated. By doing in this way, the concentrated water used for washing | cleaning will eventually contain a scale inhibitor, and the concentrated water will already contain the scale inhibitor at the time of adding an alkaline solution. . In addition, scale treatment in the semipermeable membrane unit 18 can be prevented by adding a scale inhibitor to the water supplied to the semipermeable membrane unit 18 such as when processing seawater with a high hardness component as raw water. it can.

In this way, when the scale inhibitor is added using only the second scale inhibitor supply pump 21, the second scale prevention is performed in consideration of both the scale precipitation due to concentration in the semipermeable membrane unit 18 and the scale precipitation due to the addition of alkali. It is necessary to determine the addition amount by the agent supply pump 21.

Of course, the scale inhibitor is added to the water supplied to the semipermeable membrane unit 18 by the second scale inhibitor supply pump 21, and the concentrated water obtained from the semipermeable membrane unit 18 by the first scale inhibitor supply pump 17 is added. A scale inhibitor may be added.

Particularly, from the viewpoint of reducing chemical costs, it is preferable to add a scale inhibitor to the hollow fiber membrane filtrate of the hollow fiber membrane module 4 and further add a scale inhibitor to the concentrated water of the semipermeable membrane unit 18. At this time, the second scale inhibitor supply pump 21 adds an amount of scale inhibitor that can prevent scale precipitation due to concentration in the semipermeable membrane unit 18, and the first scale inhibitor supply pump 17 uses an alkaline solution. When performing the backwashing, it is preferable to add an amount of a scale inhibitor that can prevent scale precipitation due to the addition of the alkaline solution.

In the semipermeable membrane unit 18, it is effective to add a scale inhibitor to each semipermeable membrane unit supply water in order to prevent scale precipitation due to concentration. When the alkaline solution is added to the supply water of the semipermeable membrane unit 18 and the pH is adjusted to the alkali side, such as when boron is efficiently removed from seawater, the alkalinity is used so that the addition effect can be exhibited. It is preferable to add a scale inhibitor to the supply water of the semipermeable membrane unit 18 upstream of the addition of the solution.

In addition, it is also preferable to prevent an abrupt concentration or pH change in the vicinity of the addition port by providing an in-line mixer immediately after the addition of the chemical or by bringing the addition port into direct contact with the flow of the supply water.

In order to enhance the cleaning effect, following the reverse pressure cleaning using the cleaning water containing the alkaline solution and the scale inhibitor, the cleaning water containing the alkaline solution and the scale inhibitor is held in the hollow fiber membrane module 4 for a predetermined time. It is preferable to make it. The time for retaining the alkaline solution and the washing water containing the scale inhibitor in the hollow fiber membrane module 4 is preferably about 5 to 180 minutes, and more preferably about 10 to 30 minutes. This is because if the contact time is too short, the cleaning power is weak, and if it is too long, the time during which the apparatus is stopped becomes long and the operation efficiency of the apparatus decreases.

Further, when fouling substances are attached and accumulated on the membrane surface, the air valve 12 is opened, the compressed air of the compressor 13 is sent to the primary side of the hollow fiber membrane module 4, and air washing is performed to vibrate the membrane. It is preferable. The air cleaning is performed during the two types of back pressure cleaning described above, before and after the execution, or at least a part of the time during which the cleaning water containing the alkaline solution and the scale inhibitor is held in the hollow fiber membrane module 4. Is preferred. The water pushed out to the primary side of the hollow fiber membrane module 4 and the air supplied from the lower part of the hollow fiber membrane module 4 are discharged out of the system through the air vent valve 5. In this case, the higher the pressure of the compressed air, the higher the cleaning effect of the membrane, which is preferable. However, the pressure is appropriately set within a range in which the membrane is not damaged.

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

<Example 1>
The apparatus shown in FIG. 1 was prepared and fresh water was produced. The hollow fiber membrane module 4 includes a hollow fiber UF membrane made of polyvinylidene fluoride having a molecular weight cut off of 150,000 Da manufactured by Toray Industries, Inc., and a pressurized hollow fiber membrane module (HFU-2020) having a membrane area of 72 m ^2. One was used. For the semipermeable membrane unit 18, four reverse osmosis membrane elements (TM820-400) manufactured by Toray Industries, Inc. were used.

In the filtration step in the hollow fiber membrane module 4, the raw water supply valve 3 and the filtered water valve 6 are opened, the raw water supply pump 2 is operated, the turbidity is 4 degrees, and the TOC (Total Organic Carbon) concentration is 2 mg / L, the whole amount of seawater having a salt concentration of 3.5% was filtered at a filtration flux of 3 m / d. In the separation step in the semipermeable membrane unit 18, the second scale inhibitor is added so that the ethylenediaminetetraacetic acid (EDTA) in the scale inhibitor storage tank 16 is 10 ppm in the supply water of the semipermeable membrane unit 18. Membrane separation was performed at a membrane filtration flow rate of 60 m ³ / d, a concentrated water flow rate of 120 m ³ / d, and a recovery rate of 33% while being constantly added using the supply pump 21.

After 30 minutes of filtration with the hollow fiber membrane module 4, the raw water supply valve 3 and the filtrate water valve 6 are closed, the raw water supply pump 2 is stopped, and at the same time, the backwash valve 9, the air washing valve 12 and the air vent valve 5 are opened. The backwash pump 8 is operated, and backwashing with a flux of 3.3 m / d using concentrated water from the semipermeable membrane unit 18 and air washing for supplying air at 100 L / min from the lower side of the membrane module are simultaneously performed for 1 minute. Carried out. Thereafter, the backwash valve 9 and the air washing valve 12 were closed and the backwash pump 8 was stopped. At the same time, the drain valve 11 was opened, and the entire amount of water in the hollow fiber membrane module 4 was discharged out of the system. Thereafter, the raw water supply valve 3 is opened, the raw water supply pump 2 is operated, the raw water is supplied into the hollow fiber membrane module 4, the filtrate water valve 6 is opened, the air vent valve 5 is closed, and the process returns to the filtration step. The process was repeated.

In addition, once a day, back pressure cleaning was performed using cleaning water in which the sodium hypochlorite solution in the alkaline solution storage tank 14 was added to the concentrated water of the semipermeable membrane unit 18. Open the backwash valve 9, the air wash valve 12, and the air vent valve 5, operate the backwash pump 8 and the alkaline solution supply pump 15, and backwash with a flux of 3.3 m / d and below the hollow fiber membrane module 4. From the above, air cleaning for supplying air at 100 L / min was simultaneously performed for 1 minute. At this time, the amount of sodium hypochlorite solution added was appropriately adjusted by measuring with a pocket residual chlorine meter manufactured by Hack so that the chlorine concentration in the backwash water was 500 mg / L. Next, the backwash pump 8 and the alkaline solution supply pump 15 were stopped, and the inside of the hollow fiber membrane module 4 was held at a chlorine concentration of 500 mg / L for 20 minutes. Thereafter, the raw water supply valve 3 is opened, the raw water supply pump 2 is operated, the raw water is supplied into the hollow fiber membrane module 4, the filtrate water valve 6 is opened, the air vent valve 5 is closed, and the process returns to the filtration step. The process was repeated.

As a result, the filtration differential pressure of the hollow fiber membrane module 4 changed from 60 to 70 kPa for one month from the start of operation with respect to 60 kPa immediately after the start of operation, and stable operation was possible. The membrane filtration differential pressure of the semipermeable membrane unit 18 was 100 kPa immediately after the start of operation, whereas it was stable at about 120 kPa even after one month.

<Example 2>
Fresh water was produced in the same manner as in Example 1 except that the following points were changed. That is, EDTA was not added using the second scale inhibitor supply pump 21. Instead, EDTA was added using the first scale inhibitor supply pump 17 so that the concentration in the wash water was 20 ppm at the time of back pressure washing using the wash water to which the sodium hypochlorite solution was added. .

As a result, the filtration differential pressure of the hollow fiber membrane module 4 changed from 60 to 70 kPa for one month from the start of operation with respect to 60 kPa immediately after the start of operation, and stable operation was possible. On the other hand, the membrane filtration differential pressure of the semipermeable membrane unit 18 was 100 kPa immediately after the start of the operation, but increased to about 140 kPa after one month, but was relatively stable.

<Comparative Example 1>
Except for not adding ethylenediaminetetraacetic acid (EDTA) at all, it was exactly the same as Example 1.

As a result, the filtration differential pressure of the hollow fiber membrane module 4 became as high as 150 kPa after one month due to clogging due to scale components, compared with 60 kPa immediately after the start of operation, and it was forced to carry out chemical cleaning. The membrane filtration differential pressure of the semipermeable membrane unit 18 was 100 kPa immediately after the start of operation, whereas it became as high as 180 kPa after one month due to scale generation.

It is an object of the present invention to prevent clogging caused by scales adhering and accumulating on the membrane surface and pores in a membrane separation apparatus for membrane filtration of raw water with a membrane module, and effectively cleaning the membrane module. be able to.

1: Raw water storage tank 2: Raw water supply pump 3: Raw water supply valve 4: Hollow fiber membrane module 5: Air vent valve 6: Filtration water valve 7: Filtration water storage tank 8: Backwash pump 9: Backwash valve 10: Reverse Wash pipe 11: Drain valve 12: Air valve 13: Compressor 14: Alkaline solution reservoir 15: Alkaline solution supply pump 16: Scale inhibitor storage tank 17: First scale inhibitor supply pump 18: Semipermeable membrane unit 19: Booster Pump 20: Concentrated water storage tank 21: Second scale inhibitor supply pump 22: Semipermeable membrane concentrated water drain valve

Claims

The raw water is filtered through a microfiltration / ultrafiltration membrane module and then separated into permeated water and concentrated water by a semipermeable membrane unit. This is performed when the filtration in the microfiltration / ultrafiltration membrane module is interrupted. A cleaning method for a membrane module, wherein at least a part of the concentrated water is used as cleaning water, and the cleaning water containing a scale inhibitor and an alkaline solution is supplied from the secondary side of the microfiltration / ultrafiltration membrane module Then, the membrane module cleaning method of performing reverse pressure cleaning of the microfiltration / ultrafiltration membrane module.
The method for cleaning a membrane module according to claim 1, wherein a scale inhibitor is added to at least a part of the concentrated water so that the scale water contains a scale inhibitor.
A scale inhibitor is added to at least a part of the filtered water obtained from the microfiltration / ultrafiltration membrane module and separated by the semipermeable membrane unit, so that the washing water contains the scale inhibitor. 2. A method for cleaning a membrane module according to 1.
The method for cleaning a membrane module according to claim 3, further comprising adding a scale inhibitor to at least a part of the concentrated water so that the cleaning water contains a scale inhibitor.
A method for producing raw water by filtering raw water with a microfiltration / ultrafiltration membrane module, and then separating the permeated water and concentrated water with a semipermeable membrane unit, temporarily in the microfiltration / ultrafiltration membrane module The filtration is interrupted, and at least a part of the concentrated water is supplied as a wash water from the secondary side of the microfiltration / ultrafiltration membrane module to perform back pressure washing of the microfiltration / ultrafiltration membrane module, A water making method of adding a scale inhibitor and an alkaline solution to the washing water.
The desalinization method according to claim 5, wherein a scale inhibitor is added to at least a part of the concentrated water so as to contain the scale inhibitor in the washing water.
A scale inhibitor is added to at least a part of the filtered water obtained from the microfiltration / ultrafiltration membrane module and separated by the semipermeable membrane unit, so that the washing water contains the scale inhibitor. 5. The fresh water generation method according to 5.
Furthermore, the desalinating method according to claim 7, wherein a scale inhibitor is added to at least a part of the concentrated water so that the washing water contains the scale inhibitor.
A microfiltration / ultrafiltration membrane module; a semipermeable membrane unit that separates at least a portion of the filtered water of the microfiltration / ultrafiltration membrane module into permeated water and concentrated water; and at least a portion of the concentrated water. A back pressure cleaning unit for supplying back water from the secondary side of the microfiltration / ultrafiltration membrane module as a cleaning water to perform backpressure cleaning of the microfiltration / ultrafiltration membrane module, and a scale inhibitor in the cleaning water A fresh water generator comprising a scale inhibitor supply unit for squeezing and an alkaline solution supply unit for containing an alkaline solution in the washing water.
The scale inhibitor supply unit includes a unit that supplies scale inhibitor to a line that supplies at least a part of the concentrated water from the secondary side of the microfiltration / ultrafiltration membrane module, and the alkaline solution supply unit The fresh water generator of Claim 9 provided with the unit which supplies an alkaline solution to the line which supplies at least one part of the said concentrated water from the secondary side of the said microfiltration / ultrafiltration membrane module.
The scale inhibitor supply unit includes a unit for supplying a scale inhibitor to a line for supplying the filtrate of the microfiltration / ultrafiltration membrane module to the semipermeable membrane unit, and the concentrated water is used as the alkaline solution supply unit. The fresh water generator of Claim 9 provided with the unit which supplies an alkaline solution to the line which supplies at least one part of from the secondary side of the said microfiltration / ultrafiltration membrane module.
The scale inhibitor supply unit further includes a unit that supplies the scale inhibitor to a line that supplies at least a part of the concentrated water from the secondary side of the microfiltration / ultrafiltration membrane module. The fresh water generator described.