JP2014057931A - Water production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a water production method in which raw water including an oil content is membrane-filtrated by a nano membrane filter or a reverse osmosis membrane to obtain membrane permeable water and membrane concentrated water and in which cost of equipment and an installation space of equipment can be reduced, and membrane clog derived from an oil content can be remarkably prevented.SOLUTION: There is provided a water production method in which raw water including an oil content is added with an alkali agent so that a pH may become at least 9, then is membrane-filtrated by an anion electric charge type nano membrane filter or a reverse osmosis membrane to obtain membrane permeable water and membrane concentrated water.

Description

  The present invention relates to a fresh water production method for obtaining membrane permeated water and membrane concentrated water by subjecting raw water containing oil to membrane separation with a nanofiltration membrane or a reverse osmosis membrane.

  Membrane separation methods have features such as energy saving, space saving, and improved filtered water quality, and therefore are widely used in various fields. For example, microfiltration membranes and ultrafiltration membranes can be applied to water purification processes that produce industrial water and tap water from river water, groundwater, industrial wastewater, and pretreatment in seawater desalination reverse osmosis membrane treatment processes. Can be mentioned. Nanofiltration membranes and reverse osmosis membranes can be applied to advanced water purification processes that remove mold odor, chromaticity, hardness components, etc. that cannot be removed with microfiltration membranes or ultrafiltration membranes, seawater desalination, brine water desalination, Application to pure water production etc. is mentioned (patent document 1).

  However, when separating waste water discharged from food factories and kitchens, etc., it often contains a large amount of animal and vegetable oils, which tends to cause clogging of the membrane. It was necessary to operate with a low membrane permeation flux. Therefore, as a pretreatment method for membrane separation, there is a method in which oil components in wastewater are floated and separated by a pressurized flotation method or adsorbed and removed by activated carbon (Patent Documents 2 and 3). However, the pressure levitation device requires high equipment costs and installation space, and there is a problem that requires labor related to maintenance such as cleaning of the device. Further, when the activated carbon has a high oil concentration in the wastewater, it is necessary to frequently replace the activated carbon, resulting in a problem of high replacement costs. Furthermore, the oil was not completely removed by the pretreatment method, and a part of the oil remained in the membrane feed water, so that the membrane was still clogged.

Japanese Patent Laid-Open No. 5-212252 JP-A-8-309393 JP-A-8-323350

  The present invention can reduce facility costs and installation space in a fresh water production method in which raw water containing oil is separated by a nanofiltration membrane or reverse osmosis membrane to obtain membrane permeated water and membrane concentrated water. It is an object of the present invention to provide a fresh water generation method capable of remarkably preventing clogging of a film derived from oil.

  In order to solve the above problems, the fresh water generation method of the present invention has the following characteristics.

  (1) After adding an alkali agent to the raw water containing oil until the pH becomes 9 or more, membrane separation is performed with an anion charge type nanofiltration membrane or reverse osmosis membrane to obtain membrane permeated water and membrane concentrated water. Fresh water generation method.

  (2) The fresh water generation method according to (1), wherein the alkaline agent contains sodium hydroxide or potassium hydroxide.

  (3) When the turbidity of the raw water after adding the alkaline agent until the pH becomes 9 or higher is 0.1 degree or higher, filter the membrane with a microfiltration membrane or ultrafiltration membrane after adding the alkaline agent The fresh water generation method according to (1) or (2), wherein pretreated membrane filtrate is obtained, and then the pretreated membrane filtrate is subjected to membrane separation with an anionically charged nanofiltration membrane or a reverse osmosis membrane.

  (4) After adding an alkaline agent until the pH is 9 or more, add an acid until the pH is less than 9, and then perform membrane separation with an anionically charged nanofiltration membrane or reverse osmosis membrane (1) or ( The fresh water generation method as described in 2).

  (5) When the turbidity of the raw water after adding the alkaline agent until the pH becomes 9 or higher is 0.1 degree or higher, after adding the alkaline agent, membrane filtration with a microfiltration membrane or ultrafiltration membrane, Subsequently, the water-producing method according to (4), wherein the acid is added until the pH of the pretreated membrane filtrate is less than 9, and then membrane separation is performed with an anionically charged nanofiltration membrane or a reverse osmosis membrane.

  (6) When the turbidity of the raw water after adding acid until the pH is less than 9 is 0.1 degree or more, pre-treatment by adding membrane with membrane filtration with ultrafiltration membrane or ultrafiltration membrane The fresh water generation method according to (4), wherein membrane filtration water is obtained, and then the pretreated membrane filtration water is subjected to membrane separation with an anion charged nanofiltration membrane or a reverse osmosis membrane.

  (7) The fresh water generation method according to any one of (1) to (6), wherein the membrane concentrated water is biologically treated.

  (8) The fresh water generation method according to (7), wherein the biological treatment is performed after adjusting the pH of the membrane concentrated water to 6 or more and 8 or less.

  According to the fresh water generation method of the present invention, in a fresh water generation method in which raw water containing oil is separated by an anion charged nanofiltration membrane or reverse osmosis membrane to obtain membrane permeated water and membrane concentrated water, the oil is alkaline. The fatty acid salt which is an anionic surfactant is produced | generated by reacting with an agent. Here, since the fatty acid salt repels on the surface of the anion-charged membrane, it is unlikely to cause clogging of the oil-derived membrane that has been generated conventionally, and it does not perform chemical cleaning of the membrane for a long time with a high membrane permeation flux. Stable operation is possible. Moreover, it is only necessary to add a facility for injecting the alkaline agent, and the facility cost and the installation space for the facility can be reduced.

It is an apparatus general | schematic flowchart which shows an example of the water treatment apparatus to which the fresh water generation method of this invention is applied.

  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 embodiments.

  The water treatment apparatus used in the fresh water generation method of the present invention includes, for example, as shown in FIG. 1, an alkaline agent storage tank 1 for storing an alkaline agent, an alkaline agent supply pump 2 for supplying an alkaline agent to raw water, Stirrer 3 that mixes and stirs raw water and an alkaline agent, alkali adjustment tank 4, alkaline water supply pump 5 that supplies raw water adjusted to alkali (hereinafter referred to as alkaline water), and alkaline water that is opened when alkaline water is supplied. Supply valve 6, microfiltration membrane / ultrafiltration membrane module 7 for membrane filtration of alkaline water, air vent valve 8 opened for back pressure washing or air washing, and filtered water opened for membrane filtration Supplying the filtrate 9 to the microfiltration membrane / ultrafiltration membrane module 7 and the filtration water storage tank 10 for storing the membrane filtration water obtained by the valve 9 and the microfiltration membrane / ultrafiltration membrane module 7 Pressure wash The backwash pump 11 that is operated at the same time, the backwash valve 12 that is opened when the backwashing is performed, the air blower 13 that is the air supply source of the air for the microfiltration membrane / ultrafiltration membrane module 7, and the air An air-washing valve 14 that opens when supplying air to the lower part of the filtration membrane / ultrafiltration membrane module 7 and air-washing, and an opening when discharging water on the membrane primary side of the microfiltration membrane / ultrafiltration membrane module 7 A drain valve 15, a booster pump 16 for supplying microfiltration membrane / ultrafiltration membrane filtration water, an anion charge type nanofiltration membrane / reverse osmosis membrane module 17, and nanofiltration membrane / reverse osmosis membrane concentrated water An activated sludge tank 18 for biological treatment, a microfiltration membrane / ultrafiltration membrane module 19 for filtering activated sludge, an air diffuser pipe 20 for supplying air bubbles into the activated sludge tank 18 and maintaining aerobicity , The activity for suction filtration of activated sludge Source and a suction filtering pump 21 is provided which.

  In the above-described water treatment apparatus, the alkaline agent stored in the alkaline agent storage tank 1 is supplied to the alkaline adjustment tank 4 by the alkaline agent supply pump 2 during the filtration step. The oil contained in the raw water reacts with the alkaline agent. When the oil is fat, it is hydrolyzed into glycerin and fatty acid salt. When the oil is a fatty acid, it becomes a fatty acid salt. By making the pH 9 or more, the reaction proceeds remarkably, so that it can be suitably used in the fresh water generation method of the present invention.

  In the present invention, oil refers to (a) saturated fatty acids such as lauric acid, myristic acid, palmitic acid, stearic acid, (b) unsaturated fatty acids such as oleic acid, linoleic acid, linolenic acid, and (c) fats and oils (saturated fatty acids). Alternatively, it is defined as containing at least one of unsaturated fatty acid triglycerides).

  As a method for measuring the oil concentration of raw water in the present invention, “n-hexane extract material” described in paragraph 24 of Japanese Industrial Standard JIS K 0102: 2008 is used. The purpose of this test is mainly to determine the amount of oil that is difficult to volatilize, and the amount of substance remaining after extracting the oil contained in water with hexane is expressed in mg / L. In this test, there are an extraction method, an extraction method using an extraction container, or a collection concentration / extraction method, any of which may be used.

  In the present invention, the hexane extract / residue is further dissolved in carbon tetrachloride, and the polar hexane extract / residue adsorbed using a Florisil column is defined as oil. On the other hand, non-polar hexane extraction / residue flowing out from the Florisil column becomes mineral oil.

  In the present invention, when the concentration of the oil in the raw water obtained by the above measurement is 0.5 mg / L or more, it is determined that the raw water contains the oil.

The raw water mixed and stirred with the alkaline agent by the stirrer 3 is supplied to the membrane primary side of the microfiltration membrane / ultrafiltration membrane module 7 by operating the alkaline water supply pump 5 and opening the alkaline water supply valve 6. Is done. Further, the filtration of the microfiltration membrane / ultrafiltration membrane module 7 is performed by opening the filtration water valve 9. The membrane filtrate is transferred from the membrane secondary side to the filtrate storage tank 10 through the filtrate valve 9. In the case of total filtration, the air vent valve 8, the backwash valve 12, the air wash valve 14, and the drain valve 15 are all closed. The filtration time is preferably set as appropriate according to the raw water quality, membrane permeation flux, etc., but in the case of constant flow filtration, the predetermined membrane filtration differential pressure or filtered water volume [m ³ ], in the case of constant pressure filtration, it is predetermined. The filtration time may be continued until the filtration flow rate [m ³ / hr] and the filtered water amount [m ³ ] are reached. The filtration flow rate is the amount of filtered water per unit time.

  Suspended when the turbidity of the raw water after adding and mixing and stirring until the pH reaches 9 or higher is 0.1 degree or higher and the membrane is separated directly by the anion charged nanofiltration membrane / reverse osmosis membrane module 17 Since the substance lowers the membrane permeation performance of the anion charge type nanofiltration membrane / reverse osmosis membrane module 17, as described above, after filtering with the microfiltration membrane / ultrafiltration membrane module 7, the anion charge type nanofiltration membrane / Membrane separation with reverse osmosis membrane module 17 is preferred.

  On the other hand, when the turbidity of the raw water after adding and mixing and stirring the alkaline agent until the pH becomes 9 or more is less than 0.1 degree, the anion is directly filtered without filtering through the microfiltration membrane / ultrafiltration membrane module 7. Membrane separation may be performed by the charged nanofiltration membrane / reverse osmosis membrane module 17. If the separation membrane of the nanofiltration membrane / reverse osmosis membrane module 17 deteriorates due to alkaline hydrolysis when the pH is 9 or higher, an alkaline agent is added and mixed until the pH reaches 9 or higher. After stirring, an acid may be added until the pH becomes less than 9, mixed and stirred, and membrane separation may be performed by the anionically charged nanofiltration membrane / reverse osmosis membrane module 17. Moreover, when the turbidity of the raw water after adding acid, mixing and stirring until the pH becomes less than 9 is 0.1 degree or more and membrane separation is performed directly with the anion charged nanofiltration membrane / reverse osmosis membrane module 17, Since the suspended substances deteriorate the membrane permeation performance of the anion charge type nanofiltration membrane / reverse osmosis membrane module 17, as described above, after filtering with the microfiltration membrane / ultrafiltration membrane module 7, the anion charge type nanofiltration It is preferable to perform membrane separation with the membrane / reverse osmosis membrane module 17.

  Turbidity is as described in paragraph 9 of Japanese Industrial Standards JIS K 0101: 1998. “Turbidity represents the degree of turbidity in water. Visual turbidity, transmitted light turbidity, scattered light turbidity. When measuring in comparison with kaolin standard solution, “degree (kaolin)” is used as a unit, and in comparing with formazin standard solution, “degree” (Formazin) "is expressed as a unit, and the turbidity of a solution containing 1 mg / L of each substance is defined as 1 degree. In general, formazine is more preferable than kaolin because it has more uniform particles, better dispersibility, and better reproducibility in light scattering properties. “NTU” (Nephelometric Turbidity Unit), which is a unit of turbidity, is based on a formazine standard solution, and is also described in the scattered light measurement method section of the “Water Supply Test Method” of the Japan Water Works Association. The turbidity of the raw water in the present invention is the scattered light turbidity, and the unit is defined as “degree (formazin)”.

  Since the fatty acid salt produced after adding the alkali agent and mixing and stirring is an anionic surfactant, the fatty acid salt is mostly formed from the separation membrane of the microfiltration membrane / ultrafiltration membrane module 7 without forming an oil emulsion. Pass through. The microfiltration membrane / ultrafiltration membrane filtrate containing the fatty acid salt is temporarily stored in the filtrate storage tank 10 and then pressurized by the booster pump 16 and supplied to the anion charged nanofiltration membrane / reverse osmosis membrane module 17. Is done. Microfiltration membrane / ultrafiltration membrane filtrate containing fatty acid salt includes membrane permeated water from which organic substances other than fatty acid salts and fatty acid salts and solutes such as salt are removed, and organic substances and salts other than fatty acid salts and fatty acid salts, etc. Is separated into concentrated membrane water. As described above, since the fatty acid salt is an anionic surfactant, it is repelled by the separation membrane of the anionically charged nanofiltration membrane / reverse osmosis membrane module 17 and easily concentrated. On the contrary, when the separation membrane of the nanofiltration membrane / reverse osmosis membrane module 17 is a cation charge type or nonion charge, the fatty acid salt is easily adsorbed and the permeation performance is remarkably lowered. The separation membrane of the reverse osmosis membrane module 17 is an anion charge type.

  The membrane permeated water of the nanofiltration membrane / reverse osmosis membrane module 17 can be used for drinking water, industrial water, agricultural water, landscape water, etc. depending on the water quality, but the membrane concentrated water is at least other than fatty acid salts and fatty acid salts. When organic matter is contained and the wastewater standard is exceeded, it is preferable to perform biological treatment. As an example, there is a membrane separation activated sludge method in which a microfiltration membrane / ultrafiltration membrane module 19 is immersed in the activated sludge tank 18 and biological treatment and membrane separation treatment are simultaneously performed.

  The membrane-separated activated sludge method keeps the biomass in the activated sludge tank 18 (generally expressed as MLSS (= Mixed Liquor Suspended Solids)), while maintaining a higher installation area than the normal activated sludge method. There is an advantage that it can be reduced, and further, separation of sludge and treated water is performed by membrane filtration regardless of gravity sedimentation, so that SS does not flow out into treated water, and clear treated water can be obtained. There are advantages.

  In the membrane separation activated sludge method, normally, a diffuser tube 20 is installed below the microfiltration membrane / ultrafiltration membrane module 19 to generate bubbles (aeration), and the membrane surface is cleaned with the rising flow of the bubbles. On the other hand, this aeration is also necessary for supplying oxygen to the activated sludge for biological treatment of the liquid to be treated. That is, in the membrane separation activated sludge method, aeration plays both roles of cleaning the membrane surface and supplying oxygen for biological treatment.

  The air diffuser 20 is not particularly limited as long as bubbles for cleaning the membrane surface can be generated, but an air diffuser having an air discharge hole in a vinyl chloride or stainless steel pipe is usually used. In addition, a porous rubber, ceramic, a diffuser tube using a membrane, and the like can be used. The bubbles generated from the air diffuser 20 may be fine bubbles or coarse bubbles. The size of the bubbles may be optimized according to conditions such as the type of separation membrane and the amount of air diffused.

  Instead of using the suction filtration pump 21 as the power of filtration, a siphon using a water level difference may be used. In addition, when the predetermined amount of water cannot be secured by the siphon, the suction filtration pump 21 may be used in combination.

  As biological treatment other than the membrane separation activated sludge method shown in the figure, a filler or a disk is installed in the water tank to significantly increase the surface area to which microorganisms adhere, and a biological film is formed to increase the frequency of contact with water. There is something. The Hanikome method using an assembly of plastic small tubes fixed in water, the rotating disk method using a rotating plate, the biological contact filtration method using a granular filter medium, and the like are used. Sand, anthracite, granular activated carbon, or the like may be used as a biological contact filtration type filter medium.

  When biologically treating the membrane concentrated water, it is preferable to adjust the pH of the membrane concentrated water to 6 or more and 8 or less in order to increase the growth rate of microorganisms and easily decompose organic substances.

  The alkaline agent stored in the alkaline agent storage tank 1 may be any one of alkali metal or alkaline earth metal hydroxides (salts), alkali metal carbonates and phosphates, and is a strong alkali sodium hydroxide. Or it is preferable that potassium hydroxide is included.

  The microfiltration membrane / ultrafiltration membrane module 7 may be an external pressure type or an internal pressure type. The membrane filtration method may be a total filtration module or a cross flow filtration module, but a complete filtration module is preferred from the viewpoint of low energy consumption. Further, it may be a pressurization type module or an immersion type module, but the pressurization type module is preferable from the viewpoint that a high flux is possible.

  The separation membrane used in the microfiltration membrane / ultrafiltration membrane module 7 is not particularly limited as long as it is porous, but depending on the desired quality and amount of treated water, a microfiltration membrane or an ultrafiltration membrane may be used. Or use both together. For example, if you want to remove turbid components, bacteria, etc., you can use either a microfiltration membrane or an ultrafiltration membrane, but if you want to remove viruses or high molecular organic substances, use an ultrafiltration membrane. Is preferred.

  Examples of the shape of the separation membrane include a hollow fiber membrane, a flat membrane, and a tubular membrane, and any of them may be used.

  The material of the separation membrane is polyethylene, polypropylene, polyacrylonitrile, ethylene-tetrafluoroethylene copolymer, polychlorotrifluoroethylene, polytetrafluoroethylene, polyvinyl fluoride, tetrafluoroethylene-hexafluoropropylene copolymer, tetra Including at least one selected from the group consisting of a fluoroethylene-perfluoroalkyl vinyl ether copolymer, and a chlorotrifluoroethylene-ethylene copolymer, polyvinylidene fluoride, polysulfone, cellulose acetate, polyvinyl alcohol, and polyethersulfone. Polyvinylidene fluoride (PVDF) is more preferable from the viewpoint of film strength and chemical resistance, and from the viewpoint of high hydrophilicity and strong stain resistance. Rironitoriru is more preferable.

  The control method for the filtration operation may be constant flow filtration or constant pressure filtration, but constant flow filtration is preferred from the viewpoint that a constant amount of treated water can be obtained and the overall control is easy.

  As a means for confirming that the separation membrane surface of the anion charge type nanofiltration membrane / reverse osmosis membrane module 17 has anion charge, measurement of zeta potential can be mentioned. The zeta potential is a measure of the net fixed charge on the surface of the separation membrane, and the zeta potential on the surface of the separation membrane of the present invention is calculated from the electric mobility according to the formula of Helmholtz-Smoluchowski shown in the following formula 1. Can be sought.

Zeta potential ζ = 4πηU / ε
(In the formula, U is the electric mobility, ε is the dielectric constant of the solution, and η is the viscosity of the solution.)
The principle of zeta potential measurement will be described. In the (water) solution in contact with the material, there exists a stationary layer that cannot flow in the vicinity of the surface due to the influence of the charge on the surface of the material. The zeta potential is the potential for the solution at the interface (slip surface) between the stationary and fluidized layers of the material.

Here, considering the aqueous solution in the quartz glass cell, since the quartz surface is normally negatively charged, positively charged ions and particles gather near the cell surface. On the other hand, negatively charged ions and particles increase in the center of the cell, and ion distribution occurs in the cell. When an electric field is applied in this state, the ion distribution is reflected in the cell, and ions move at different migration speeds at positions in the cell (referred to as electroosmotic flow). Since the migration speed reflects the charge on the cell surface, the charge (surface potential) on the cell surface can be evaluated by obtaining this migration speed distribution.

  The zeta potential on the surface of the separation membrane used in the anionically charged nanofiltration membrane / reverse osmosis membrane module 17 in the present invention is defined to be less than 0 mV at pH 7, but is preferably -10 mV or less, more preferably Is −20 mV or less.

  The desalination rate of the separation membrane used for the anionically charged nanofiltration membrane / reverse osmosis membrane module 17 is 93% or more (evaluation conditions NaCl concentration: 500 mg / L, operating pressure: 0.1 MPa). Alternatively, a nanofiltration membrane of 5% or more and less than 93% (evaluation conditions NaCl concentration: 500 mg / L, operating pressure: 0.1 MPa) can be selected and used. As the shape of the separation membrane, there are a hollow fiber and a flat membrane.

  As the material of the separation membrane, a polymer material such as cellulose acetate polymer, polyamide, polyester, polyimide, vinyl polymer can be used. The membrane structure has a dense layer on at least one surface of the membrane, an asymmetric membrane having fine pores with gradually increasing pore diameters from the dense layer to the inside of the membrane or the other surface, on the dense layer of the asymmetric membrane. A composite membrane having a very thin separation functional layer formed of another material can be used.

  In the present invention, it can be carried out regardless of these membrane materials, membrane structures and membrane forms, and any membrane can be used. For example, cellulose acetate-based or polyamide-based asymmetric membranes and polyamide-based, polyurea Examples include a composite membrane having a separation function layer, and a cellulose acetate-based asymmetric membrane and a polyamide-based composite membrane are preferably used from the viewpoint of water production, durability, and salt rejection. The separation functional layer made of cross-linked polyamide has amine ends and carboxyl group ends, but the amine ends are ionically dissociated in the low pH region to give the membrane cationic chargeability, and the carboxyl group ends are ionized in the high pH region. Dissociates into an anion to impart anion chargeability. Therefore, it is preferable that the number of carboxyl group ends is increased so that the zeta potential of the separation functional layer varies depending on the pH and the anion chargeability is increased in a high pH region.

  The nanofiltration membrane / reverse osmosis membrane having the above-mentioned performance is incorporated into elements such as spirals, tubulars, and plate-and-frames for practical use, and hollow fibers are bundled and incorporated into the elements. However, the present invention is not dependent on the form of these nanofiltration membrane / reverse osmosis membrane elements.

  In the present invention, the anionically charged nanofiltration membrane / reverse osmosis membrane module 17 is of course a module in which the nanofiltration membrane / reverse osmosis membrane element is contained in one to several pressure vessels. This includes modules in which multiple modules are arranged in parallel. Combination, number, and arrangement can be arbitrarily performed according to the purpose.

  The microfiltration membrane / ultrafiltration membrane module 19 may be an external pressure type or an internal pressure type. The membrane filtration method may be a total filtration module or a cross flow filtration module, but a complete filtration module is preferred from the viewpoint of low energy consumption. Further, it may be a pressurization type module or an immersion type module, but an immersion type module is preferred because it filters activated sludge. In the case of a pressurized module, install outside the activated sludge tank.

  The separation membrane used in the microfiltration membrane / ultrafiltration membrane module 19 is not particularly limited as long as it is porous. However, depending on the quality and amount of the desired treated water, a microfiltration membrane or an ultrafiltration membrane may be used. Or use both together. For example, if you want to remove turbid components, bacteria, etc., you can use either a microfiltration membrane or an ultrafiltration membrane, but if you want to remove viruses or high molecular organic substances, use an ultrafiltration membrane. Is preferred.

  Examples of the shape of the separation membrane include a hollow fiber membrane, a flat membrane, and a tubular membrane, and any of them may be used.

  The material of the separation membrane is polyethylene, polypropylene, polyacrylonitrile, ethylene-tetrafluoroethylene copolymer, polychlorotrifluoroethylene, polytetrafluoroethylene, polyvinyl fluoride, tetrafluoroethylene-hexafluoropropylene copolymer, tetra Including at least one selected from the group consisting of a fluoroethylene-perfluoroalkyl vinyl ether copolymer, and a chlorotrifluoroethylene-ethylene copolymer, polyvinylidene fluoride, polysulfone, cellulose acetate, polyvinyl alcohol, and polyethersulfone. Polyvinylidene fluoride (PVDF) is more preferable from the viewpoint of film strength and chemical resistance, and from the viewpoint of high hydrophilicity and strong stain resistance. Rironitoriru is more preferable.

  The control method for the filtration operation may be constant flow filtration or constant pressure filtration, but constant flow filtration is preferred from the viewpoint that a constant amount of treated water can be obtained and the overall control is easy.

  According to the fresh water generation method of the present invention as described above, the oil component reacts with the alkaline agent to produce a fatty acid salt that is an anionic surfactant. Therefore, the fatty acid salt repels on the surface of the separation membrane of the anionically charged nanofiltration membrane / reverse osmosis membrane module 17, so that it is difficult to cause clogging of the separation membrane derived from oil, which has been conventionally generated, and chemical cleaning of the separation membrane can be performed. Without implementation, stable operation over a long period of time is possible with a high membrane permeation flux. However, since a small amount of oil that did not react with the alkaline agent as a fatty acid salt, or organic matter other than oil, and scales of calcium, magnesium, etc., gradually precipitate on the surface of the separation membrane, the operating pressure is nanofiltration membrane / reverse osmosis membrane When reaching the pressure limit of the module 17, it is necessary to carry out high concentration chemical cleaning. Here, chemicals used for washing include sodium hydroxide, an alkaline solution containing an anionic surfactant, and an acidic solution containing hydrochloric acid, sulfuric acid, citric acid, etc., but the concentration and holding time are such that the separation membrane does not deteriorate. Is selected as appropriate. These chemical cleaning methods may be a method of cleaning each chemical individually or a method of cleaning using a plurality of chemicals alternately.

  In addition, when the microfiltration membrane / ultrafiltration membrane module 7 continued the membrane filtration, the membrane surface and the membrane pores could not react with the alkaline agent as a suspended substance or fatty acid salt with the amount of filtered water. As a result, the amount of adhering contaminants such as oil and organic substances other than oil increases, and a decrease in filtration flow rate or an increase in membrane filtration differential pressure becomes a problem.

  Therefore, the alkaline water supply pump 5 is stopped, the alkaline water supply valve 6 and the filtrate water valve 9 are closed, and the membrane filtration is once stopped. Then, the air blower 13 is operated, and the air bleeding valve 8 and the air washing valve are operated. 14 is opened, bubbles are introduced into the membrane primary side (raw water side) of the microfiltration membrane / ultrafiltration membrane module 7, the membranes are shaken, and the membranes are brought into contact with each other to scrape adhering substances on the membrane surface. Perform the air washing to drop or operate the backwash pump 11 and open the backwash valve 12 to move the membrane from the membrane secondary side (filtrated water side) to the membrane primary side in the direction opposite to the membrane filtration method. What is necessary is just to carry out the back pressure washing | cleaning which pushes in filtered water with a pressure and excludes the contaminant which adhered to the membrane surface and the membrane pore.

  Although not shown, in order to enhance the cleaning effect of back pressure cleaning, for example, chemicals such as sodium hypochlorite, hydrochloric acid, sodium hydroxide, etc. are added to membrane filtration water used for back pressure cleaning, or ozone is dissolved. It is more preferable to let them. Also, the drain valve 15 is opened, the water on the membrane primary side in the microfiltration membrane / ultrafiltration membrane module 7 is discharged, and the back pressure cleaning is performed in a state where the membrane primary side is in a gas state. It is preferable because contaminants are more easily peeled off from the surface of the membrane than the liquid state around the membrane primary side where water pressure is applied to the membrane primary side and is easily discharged out of the system.

  The air flow rate and time for air cleaning may be appropriately determined in consideration of cleaning efficiency, film rubbing, and prevention of film breakage. The backwash flow rate and time for backwashing may be appropriately determined in consideration of the washing efficiency, water recovery rate, and the like.

  As a procedure of air cleaning and back pressure cleaning, cleaning efficiency is good if performed simultaneously with air cleaning and back pressure cleaning, but only back pressure cleaning may be performed prior to air cleaning. Alternatively, only back pressure cleaning may be performed after air cleaning. Furthermore, air cleaning or back pressure cleaning may be performed while introducing raw water. Prior to introduction of gas (at the same time back pressure cleaning), only raw water may be introduced. Alternatively, only the raw water may be introduced after the introduction of gas (simultaneously backwashing). Alternatively, these may be combined alternately.

  By performing the air cleaning and the counter pressure cleaning described above, stable operation with a low membrane filtration differential pressure over a long period of time is possible. However, as a fatty acid salt, a slight amount of oil that cannot react with the alkali agent, organic substances other than oil, and inorganic substances such as iron, manganese, and aluminum gradually accumulate on the surface of the separation membrane. Therefore, when the membrane filtration differential pressure reaches near the pressure limit of the microfiltration membrane / ultrafiltration membrane module 7, it is preferable to carry out chemical cleaning at a high concentration.

  The chemical used for the cleaning can be selected after appropriately setting the concentration and holding time to such an extent that the separation membrane does not deteriorate. At least one of sodium hypochlorite, chlorine dioxide, hydrogen peroxide, ozone and the like can be selected. Is preferable because the cleaning effect on the organic matter is increased. In addition, it is preferable to contain at least one of hydrochloric acid, sulfuric acid, nitric acid, citric acid, oxalic acid and the like because the cleaning effect on iron, manganese, aluminum and the like is enhanced.

<Method for measuring zeta potential of separation membrane>
A separation membrane having a size of 20 mm × 30 mm was used, and standard particles for electrophoresis were those obtained by dispersing polystyrene particles (particle size: 520 nm) coated with hydroxypropylcellulose in a 10 mmol / l NaCl aqueous solution. The measuring device was an electrophoretic light scattering photometer ELS-800 manufactured by Otsuka Electronics, and the light source was a He-Ne laser.

Example 1
As shown in FIG. 1, the alkaline agent supply pump 2 is operated so that the pH of the factory effluent having a turbidity of 4 degrees as raw water, an oil concentration of 35 mg / l, and a pH of 6.5 is 9.5. The sodium hydroxide solution stored in the storage tank 1 was supplied to the alkali adjustment tank 4. For the microfiltration membrane / ultrafiltration membrane module 7, three pressure-type membrane modules having a membrane area of 72 m ² made of polyvinylidene fluoride hollow fiber ultrafiltration membrane with a molecular weight cut off of 150,000 Da manufactured by Toray Industries, Inc. were used. The alkaline water supply valve 6 and the filtration water valve 9 were opened, the alkaline water supply pump 5 was operated, and the entire amount was filtered with a membrane filtration flux of 1.5 m ³ / m ² / d. The anion charged nanofiltration membrane / reverse osmosis membrane module 17 is loaded with six reverse osmosis membrane elements (SUL-G20F, separation membrane zeta potential: -32 mV) manufactured by Toray Industries, Inc. in a pressure vessel. Was used for cross-flow operation at a membrane permeation flow rate of 240 m ³ / d, a membrane concentrated water flow rate of 60 m ³ / d, and a water recovery rate of 80%. The membrane concentrated water of the nanofiltration membrane / reverse osmosis membrane module 17 flows into the activated sludge tank 18 with MLSS of 8,000 mg / L, and is a polyfluorocarbon having a nominal pore size of 0.08 μm and a membrane area of 1.4 m ² manufactured by Toray Industries, Inc. Filtration was performed with a microfiltration membrane / ultrafiltration membrane module 19 in which 100 flat membrane elements made of vinylidene fluoride were integrated. The hydraulic residence time (HRT) was 6 hours.

  After the microfiltration membrane / ultrafiltration membrane module 7 has been filtered for 30 minutes, the alkaline water supply valve 6 and the filtration water valve 9 are closed and the alkaline water supply pump 5 is stopped. The air washing valve 14 was opened, the back washing pump 11 was operated, and back washing with a flow rate of 1.5 m / d and air washing at 100 L / min for supplying air from below the membrane module were simultaneously performed for 1 min. Thereafter, the backwash valve 12 and the air washing valve 14 are closed and the backwash pump 11 is stopped. At the same time, the drain valve 15 is opened to discharge all the water in the microfiltration membrane / ultrafiltration membrane module 7 out of the system. . Thereafter, the drain valve 15 is closed, the alkaline water supply valve 6 is opened, the alkaline water supply pump 5 is operated, and the raw water whose pH is adjusted to 9.5 is put into the microfiltration membrane / ultrafiltration membrane module 7. After the supply, the filtered water valve 9 was opened, the air vent valve 8 was closed, the flow returned to the filtration step, and the above steps were repeated.

  As a result, the filtration differential pressure of the microfiltration membrane / ultrafiltration membrane module 7 was stable at 26 kPa after 2 months with respect to 18 kPa immediately after the start of operation, and the chemical solution was not washed. Moreover, the operating pressure of the nanofiltration membrane / reverse osmosis membrane module 17 was stable at 1.05 MPa even after 2 months with respect to 0.85 MPa immediately after the start of operation.

(Comparative Example 1)
The operation was performed in exactly the same manner as in Example 1 except that no sodium hydroxide solution was supplied to the raw water.

  As a result, the filtration differential pressure of the microfiltration membrane / ultrafiltration membrane module 7 reached 150 kPa after 12 days with respect to 18 kPa immediately after the start of operation, and had to be washed with chemicals. Moreover, the operating pressure of the nanofiltration membrane / reverse osmosis membrane module 17 reached 3.5 MPa after 20 days, compared with 0.85 MPa immediately after the start of operation, and had to be washed with chemicals.

(Comparative Example 2)
Except for supplying the sodium hydroxide solution to the raw water, not using the microfiltration membrane / ultrafiltration membrane module 7, and directly separating the raw water with the nanofiltration membrane / reverse osmosis membrane module 17, the same as in Example 1. Driving was done in the same way.

  As a result, the operating pressure of the nanofiltration membrane / reverse osmosis membrane module 17 reached 3.5 MPa after 4 days compared with 0.85 MPa immediately after the start of operation, and had to be washed with chemicals.

(Comparative Example 3)
The operation was performed in exactly the same manner as in Example 1 except that the sodium hydroxide solution was supplied so that the pH of the industrial wastewater at pH 6.5 was 8.

  As a result, the filtration differential pressure of the microfiltration membrane / ultrafiltration membrane module 7 reached 150 kPa after 25 days with respect to 18 kPa immediately after the start of operation, and had to be washed with chemicals. In addition, the operating pressure of the nanofiltration membrane / reverse osmosis membrane module 17 reached 3.5 MPa after 35 days against 0.85 MPa immediately after the start of operation, and had to be washed with chemicals.

(Comparative Example 4)
The sodium hydroxide solution is supplied so that the pH of the factory wastewater of pH 6.5 is 8, and the raw water is directly removed from the nanofiltration membrane / reverse osmosis membrane module 17 without using the microfiltration membrane / ultrafiltration membrane module 7. The operation was carried out in exactly the same manner as in Example 1 except that the membrane was separated.

  As a result, the operating pressure of the nanofiltration membrane / reverse osmosis membrane module 17 reached 3.5 MPa after 11 days, compared with 0.85 MPa immediately after the start of operation, and had to be washed with chemicals.

(Comparative Example 5)
Exactly the same method as in Example 1 except that 6 pressure-reversed reverse osmosis membrane elements (SU-220, zeta potential of the separation membrane: +7 mV) manufactured by Toray Industries, Inc. were loaded in the pressure vessel instead of the anion charge type. I drove in.

  As a result, the filtration differential pressure of the microfiltration membrane / ultrafiltration membrane module 7 was stable at 26 kPa after 2 months with respect to 18 kPa immediately after the start of operation, and the chemical solution was not washed. However, the operating pressure of the nanofiltration membrane / reverse osmosis membrane module 17 reached 3.5 MPa after 7 days, compared with 0.85 MPa immediately after the start of operation, and had to be washed with chemicals.

1: Alkaline agent storage tank 2: Alkaline agent supply pump 3: Stirrer 4: Alkali adjustment tank 5: Alkaline water supply pump 6: Alkaline water supply valve 7: Microfiltration membrane / ultrafiltration membrane module 8: Air vent valve 9: Filtration water valve 10: Filtration water storage tank 11: Back washing pump 12: Back washing valve 13: Air blower 14: Air washing valve 15: Drain valve 16: Booster pump 17: Anion charge type nano filtration membrane / reverse osmosis membrane module 18: Activated sludge tank 19: Microfiltration membrane / ultrafiltration membrane module 20: Air diffuser 21: Suction filtration pump

Claims (8)

  A fresh water producing method for obtaining membrane permeated water and membrane concentrated water by adding an alkaline agent to the raw water containing oil until the pH reaches 9 or more and then separating the membrane with an anionically charged nanofiltration membrane or reverse osmosis membrane .   The fresh water generation method according to claim 1, wherein the alkaline agent contains sodium hydroxide or potassium hydroxide.   When the turbidity of the raw water after adding the alkaline agent until the pH becomes 9 or higher is 0.1 degree or higher, the membrane is filtered through a microfiltration membrane or an ultrafiltration membrane after the alkaline agent is added. The fresh water generation method according to claim 1 or 2, wherein filtrated water is obtained, and the pretreated membrane filtrate is subsequently subjected to membrane separation with an anionically charged nanofiltration membrane or a reverse osmosis membrane.   The alkaline agent is added until the pH becomes 9 or more, then the acid is added until the pH becomes less than 9, and then the membrane is separated by an anionically charged nanofiltration membrane or reverse osmosis membrane. Fresh water generation method.   When the turbidity of the raw water after adding the alkaline agent until the pH becomes 9 or higher is 0.1 degree or higher, after adding the alkaline agent, membrane filtration with a microfiltration membrane or ultrafiltration membrane, followed by The fresh water generation method according to claim 4, wherein the acid is added until the pH of the treated membrane filtrate is less than 9, and then membrane separation is performed with an anion charge type nanofiltration membrane or a reverse osmosis membrane.   When the turbidity of the raw water after addition of the acid until the pH is less than 9 is 0.1 degree or more, the membrane is filtered through a microfiltration membrane or an ultrafiltration membrane after the acid is added, and pretreated membrane filtrate And subsequently separating the pretreated membrane filtrate with an anionically charged nanofiltration membrane or reverse osmosis membrane.   The fresh water generation method according to any one of claims 1 to 6, wherein the membrane concentrated water is biologically treated.   The fresh water generation method according to claim 7, wherein the biological treatment is performed after adjusting the pH of the membrane concentrated water to 6 or more and 8 or less.

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