JP5024158B2 - Membrane filtration method - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a backwashing method for performing effective backwashing by efficiently using membrane filtered water or fresh water in a membrane filtration method for obtaining membrane filtered water by treating raw water containing salt with a membrane filter module. It is.

  For the production of fresh water from seawater / brine, for example, a membrane separation device equipped with a reverse osmosis membrane module is used. As shown in FIG. 1, this type of membrane separation apparatus basically performs a pretreatment for removing turbid components from raw water (seawater / brine) and then a predetermined pressure (for example, And is supplied to the reverse osmosis membrane module 13 to obtain permeated water and concentrated water permeated by the reverse osmosis action.

  Examples of the pretreatment include sand filtration, activated carbon filtration, and membrane filtration, and membrane filtration is particularly preferably used. However, when this type of membrane filtration device continues the membrane filtration, removal objects such as turbidity, organic matter, and inorganic matter contained in the raw water accumulate on the membrane surface, resulting in clogging of the membrane. When the membrane is clogged, the membrane filtration resistance of the filtration membrane module increases, and it becomes impossible to continue membrane filtration. Therefore, in order to continue membrane filtration, it is necessary to wash the membrane. For membrane cleaning, reverse flow cleaning (hereinafter referred to as backwashing) in which membrane filtered water flows backward from the secondary side (membrane filtered water side) of the membrane to the primary side (raw water side), or gas on the primary side of the membrane There are air cleaning to remove the dirt of the membrane by supplying to the membrane, and chemical cleaning to wash the drug-containing water in contact with the membrane for a certain period of time. It is very important to perform these washings effectively in order to carry out the membrane filtration stably and continuously.

  The degree of clogging of the membrane depends on the quality of the raw water, but generally the most problematic is clogging with organic matter. In order to eliminate this clogging of organic matter, it is effective to backwash with water containing an oxidizing agent such as sodium hypochlorite. As the washing water used for backwashing, membrane filtered water is generally used. It is used.

  Therefore, in Patent Document 1, in a membrane separation method in which membrane filtrate obtained by membrane filtration of raw water containing salt such as seawater and brine is subjected to reverse osmosis treatment with a reverse osmosis membrane module to obtain permeated water, There are disclosed a method of backwashing a filtration membrane module using water in which an oxidizing agent such as sodium hypochlorite is added to filtered water, and a method of backwashing a filtration membrane module using permeated water. However, when sodium hypochlorite is added to the membrane filtrate as in this method, it reacts with high-concentration calcium contained in the membrane filtrate to generate calcium scale and the like, and the sodium hypochlorite addition pipe is Problems such as clogging and filtration membrane module clogging are likely to occur. Further, when backwashing is performed without adding an oxidizing agent to the permeated water, there is a problem that a sufficient cleaning effect cannot be obtained.

  On the other hand, in Patent Document 2, membrane filtration water obtained by membrane filtration of raw water containing salt is subjected to reverse osmosis treatment with a first reverse osmosis membrane module, and further reverse osmosis treatment with a second reverse osmosis membrane module. In the membrane separation method for obtaining permeated water, a method is disclosed in which an oxidant is added to the concentrated water obtained by the second reverse osmosis membrane module to backwash the filtration membrane module. However, this method can be applied only to equipment in which reverse osmosis membrane modules are installed in two stages. In facilities where only one stage of reverse osmosis membrane module is installed, the salinity of the concentrated water is too high, so the effect cannot be obtained even if backwashing with concentrated water. In addition, the membrane filtration water is not used for backwashing, and the filtration membrane module is backwashed using the concentrated water obtained by the second reverse osmosis membrane module, but the second reverse osmosis membrane module is concentrated. The water is generally refluxed as the supply water for the first reverse osmosis membrane module, and there is a problem that the recovery rate of the permeated water obtained is lower than that. Moreover, although the salt concentration contained in the concentrated water of the 2nd reverse osmosis membrane module is reduced compared with raw | natural water, since it was not low enough, there also existed a problem that the expected cleaning effect was not acquired. .

JP 2006-272136 A JP 2006-272135 A

  The present invention solves the above-mentioned problems of the prior art, and effectively cleans clogging cause substances such as turbidity, organic matter, and inorganic matter accumulated in the filtration membrane module using membrane filtration water and fresh water efficiently. It aims at providing the membrane filtration method for removing.

Membrane filtration method of the present invention for achieving the above object is characterized in the following configuration.
(1) In the membrane filtration method by salt concentration processes the above raw water 1,000 mg / L in the filtration membrane module obtaining a membrane filtration water backwash step of performing backwash of the filtration membrane module during the membrane filtration step In the backwashing step , the filtration membrane is obtained by using reverse osmosis membrane permeated water having a salt concentration of less than 500 mg / L as a backwash water. the module backwashing, the make possible to discharge the water in the raw water side of the filtration membrane module, subsequently, the reverse osmosis membrane permeated water an acid agent was contained at a concentration of at least 50 mg / L in the backwash water as was carried out oxidant containing fresh water backwash engineering using performs that subsequently, maintaining the 3 to 120 minutes while being held the reverse osmosis membrane permeated water containing an oxidizing agent in the filtration membrane module, And the frequency of the oxidant-containing fresh water backwashing step is 3 Membrane filtration method characterized in that a once during ~ 10 days.

  According to the membrane filtration method of the present invention, clogging-causing substances such as turbidity, organic matter, and inorganic matter accumulated in the filtration membrane module can be effectively washed and removed using membrane filtration water and fresh water efficiently. Become.

  FIG. 2 and FIG. 3 are process schematic diagrams respectively showing a preferred embodiment of the membrane filtration method of the present invention. In each drawing, the same reference numerals indicate the same or equivalent components.

  In FIG. 2 and FIG. 3, the raw water containing salt such as seawater / brine collected is appropriately pretreated and then supplied to the filtration membrane module 3 via the raw water pipe 2 by the filtration pump 1. A membrane-filtered water obtained by membrane filtration is obtained. The membrane filtrate is stored in the filtrate tank 5 via the filtrate pipe 4. In the present invention, the step of obtaining membrane filtrate is referred to as a membrane filtration step. The membrane filtrate stored in the filtrate tank 5 is supplied to the reverse osmosis membrane module 13 via the reverse osmosis membrane supply pipe 12 by the high-pressure pump 11, and the permeated water subjected to the reverse osmosis treatment and the concentrated water are obtained. The permeated water is taken out through the permeated water pipe 14, and the concentrated water is discharged out of the system through the concentrated water pipe 15. In the present invention, the process of obtaining the permeate by performing the reverse osmosis process is referred to as a reverse osmosis process. In FIG. 2, fresh water supplied from outside the system is stored in a fresh water tank 17 through a fresh water pipe 16. In FIG. 3, the permeated water treated by the reverse osmosis membrane module 13 is supplied as fresh water and stored in the fresh water tank 17.

  In order to wash away turbidity, organic matter, and inorganic matter accumulated on the membrane surface of the filtration membrane module 3, after performing membrane filtration for a predetermined time, the filtration pump 1 is stopped and the filtration membrane module 3 is backwashed. . When the membrane filtrate stored in the filtrate tank 5 is used as the backwash water, the membrane filtrate is supplied to the filtration membrane module 3 via the backwash water pipe 22 by the backwash pump 21 and the secondary side of the membrane. It is carried out by flowing back from the membrane filtration water side to the primary side of the membrane (raw water side). In the present invention, the washing water used for backwashing is referred to as backwashing water. The backwash water flows out to the raw water side of the filtration membrane module 3 and is then discharged out of the filtration module 3 via the backwash water drain pipe 26 and / or the drain drain pipe 27. In the present invention, the step of performing backwashing using membrane filtrate is usually referred to as a backwashing step. After a predetermined time has elapsed, the backwash pump 21 is stopped and membrane filtration is performed again.

  In addition, when fresh water is used as backwash water, fresh water stored in the fresh water tank 17 is supplied to the filtration membrane module 3 by the fresh water back wash pump 31 via the fresh water back wash pipe 32, and the secondary side of the membrane. It is carried out by flowing back from the membrane filtration water side to the primary side of the membrane (raw water side). At this time, when the oxidant chemical stored in the oxidant storage tank 23 is injected into the fresh water backwash pipe 32 by the oxidant injection pump 24 through the oxidant injection pipe 25, the oxidant-containing fresh water is supplied as backwash water. Is done. The backwash water flows out to the raw water side of the filtration membrane module 3 and is then discharged out of the filtration module 3 via the backwash water drain pipe 26 and / or the drain drain pipe 27. After a predetermined time has elapsed, the fresh water backwash pump 31 and the oxidant injection pump 24 are stopped, and membrane filtration is performed again. In the present invention, the step of backwashing using fresh water containing an oxidizing agent as backwashing water is referred to as an oxidizing agent-containing freshwater backwashing step.

  Here, fresh water refers to water from which a considerable amount of ions have been removed and having drinking water quality. For example, the evaporation residue concentration (salt concentration) is less than 500 mg / L, and the chloride ion concentration is less than 200 mg / L. Although it is a certain water, drinking water, distilled water, reverse osmosis treated permeated water or the like can be used. However, if permeated water is used, it is easily transferred to the fresh water aquarium 17 via the permeated water return pipe 18 as shown in FIG. Since it can introduce | transduce, it is preferable.

  By the way, in raw water containing a high concentration of salt, such as seawater and brine, turbidity, organic matter, inorganic matter, etc. contained in the raw water are likely to precipitate and precipitate by salting out, and these are the membrane surfaces of the filtration membrane module. The film tends to clog. According to the study of the present inventor, when the relationship between the salinity of raw water and the likelihood of membrane clogging is less than 500 mg / L, clogging is likely to occur at 1,000 mg / L or more. It has been found that clogging is more likely to occur when exceeding 10,000 mg / L, and clogging is more likely to occur when exceeding 30,000 mg / L.

  Moreover, when seawater / brine is used as raw water, the obtained membrane filtered water is free from turbidity, organic matter, inorganic matter, and other removal objects contained in the raw water, but still contains salt. Therefore, the object to be removed, such as turbidity, organic matter, and inorganic matter, which has been deposited and precipitated by salting out and accumulated on the membrane surface, causing membrane clogging, is backwashed using membrane filtered water as backwash water. In this case, it is necessary to remove the film from the film surface by physical action. On the other hand, when backwashing is performed using fresh water as backwash water, objects to be removed such as turbidity, organic matter, and inorganic matter that have precipitated and precipitated due to the salting out phenomenon due to the salt concentration contained in the raw water are not suitable for fresh water with low salt concentration. Then, it is easy to dissolve, and the removal effect by the effect of peeling from the film surface by a physical action and the effect of dissolving and removing in fresh water are exhibited. For this reason, it is preferable to use fresh water for backwashing water because it has a high cleaning effect.

  At this time, the greater the difference in salt concentration between the raw water and fresh water, the more turbidity, organic matter, inorganic matter, etc., precipitated and precipitated on the membrane surface by salting out in the membrane filtration step, dissolved by backwashing using freshwater as backwash water. It becomes easy to remove. Here, since fresh water has an evaporation residue concentration of less than 500 mg / L, the salt concentration of raw water should be at least 1,000 mg / L or more in order to make the salt concentration difference between raw water and fresh water significant. Is more preferably 10,000 mg / L or more, and more preferably 30,000 mg / L or more because the difference in salt concentration between the raw water and fresh water is sufficiently large.

  In order to perform backwashing using fresh water, it is necessary to prepare fresh water in advance, and when fresh water is used for backwashing frequently performed during the membrane filtration step, the running cost of the membrane filtration method according to the present invention There is a demerit that becomes higher. In particular, in the case of using the membrane filtration method according to the present invention as a pretreatment for generating permeated water that is fresh water from seawater and brine, most of the generated permeated water will be consumed as backwash water, There is a demerit that lowers the permeate recovery rate. Therefore, backwashing with fresh water is difficult to perform frequently, although the backwashing effect is high.

  Here, the recovery rate is the ratio of the treated water flow rate to the supplied water amount. For example, in the membrane filtration step, the amount used as backwash water with respect to the pretreated raw water amount. It is the ratio of the amount of membrane filtration water subtracted. In the reverse osmosis treatment step, it is the ratio of the amount of permeated water obtained by subtracting the amount used as backwash water to the amount of membrane filtrate supplied to the reverse osmosis membrane module 13. Moreover, in the seawater desalination system combining the membrane filtration step and the reverse osmosis treatment step as shown in FIG. 2, the permeated water amount relative to the raw water amount supplied to the filtration membrane module 3 (excluding the portion used as backwash water). ).

  Furthermore, it is preferable to include an oxidizing agent in the backwash water because the cleaning effect of the film by the oxidizing agent can be enhanced. As the oxidizing agent, sodium hypochlorite, chlorine dioxide, hydrogen peroxide, chloramine and the like can be used, but sodium hypochlorite is preferable from the viewpoint of ease of use, cost, and cleaning effect. Usually, the backwashing of the filtration membrane module is periodically performed while continuing the membrane filtration, and the frequency thereof is about once every 15 to 120 minutes. Normally, membrane filtration water is used as backwash water, and backwashing is performed about once every 15 to 120 minutes to clean the substances causing clogging of the membrane such as turbidity, organic matter, and inorganic matter accumulated in the membrane. Remove. When membrane filtered water is used for backwashing water, the cleaning effect is smaller than when using freshwater or fresh water containing an oxidizer for backwashing water. Substances that cause clogging are gradually accumulated. Therefore, the backwashing using the oxidant-containing fresh water as the backwash water (the oxidant-containing freshwater backwash process) is performed once every 3 hours or about once every 10 days. In this way, accumulated substances such as turbidity, organic matter, and inorganic matter are washed and removed. As described above, the normal backwashing step and the oxidant-containing freshwater backwashing are carried out in combination to effectively remove the substances causing the clogging of the membrane, and at the same time, the freshwater used for the backwashing of the filtration membrane module. The amount can be minimized. By reducing the amount of fresh water used, the recovery rate in the seawater desalination system can be maintained high.

  Moreover, if the frequency of backwashing using oxidant-containing fresh water is increased, the cost of the oxidant contained in the backwash water and the cost of the chemical solution for neutralizing the oxidant in the washing wastewater containing backwash water are reduced. Get higher. Conversely, if the frequency is too low, the filtration membrane module is not effectively cleaned. Taking these into consideration, backwashing using fresh water containing an oxidizing agent is carried out at a frequency of about once every 3 hours to about once every 10 days. It is preferable to carry out the method once every 6 hours to once every 3 days.

  When backwashing using oxidant-containing fresh water, it is preferable to provide a chemical solution holding step for maintaining the state in which the oxidant-containing fresh water is held in the filtration membrane module following the backwashing with the oxidant-containing fresh water. By providing the chemical solution holding step, the contact time between the membrane of the filtration membrane module and the oxidizing agent can be increased, and the membrane cleaning effect can be further enhanced.

  What is necessary is just to implement a chemical | medical solution holding process with the following concrete methods. While supplying the fresh water stored in the fresh water tank 17 to the filtration membrane module 3 through the fresh water back washing pipe 32 by the fresh water back washing pump 31, the oxidizing chemical stored in the oxidizing agent storage tank 23 is used as the oxidizing agent. It is injected through an oxidant injection pipe 25 by an injection pump 24, an oxidant is added to fresh water, and this oxidant-containing fresh water is fed from the secondary side (membrane filtered water side) of the membrane in the filtration membrane module 3 to the membrane. When the chemical liquid back washing process is performed by flowing back to the primary side (raw water side), water on the primary side of the membrane of the filtration membrane module 3 is discharged out of the filtration membrane module via the drain drain pipe 27, The oxidant-containing fresh water is in a state where only the secondary side of the membrane of the filtration membrane module 3 is filled. When the fresh water backwash pump 31 and the oxidant injection pump 24 are stopped in this state, the state in which the membrane of the filtration membrane module 3 and the oxidant are in contact can be maintained.

  As another specific implementation method for performing the chemical solution holding step, the oxidant containing fresh water is obtained by adding the oxidant chemical solution to the fresh water stored in the fresh water tank 17 in the middle of the fresh water backwash pipe 32. Is supplied into the filtration membrane module 3, and when draining the chemical solution backflow from the membrane secondary side (membrane filtered water side) back to the membrane primary side (raw water side), the drain drain pipe 27 is closed. Both the secondary side of the membrane of the filtration membrane module 3 and the primary side of the membrane are filled with oxidant-containing fresh water. If the fresh water backwash pump 31 and the oxidant injection pump 24 are stopped in this state, the state where the membrane of the filtration membrane module 3 and the oxidant are in contact can be maintained.

  The time for this chemical solution holding step is preferably about 3 to 120 minutes, and more preferably about 10 to 30 minutes. If the time of the chemical solution holding process is too short, the contact time between the membrane and the oxidant is insufficient, and a high cleaning effect cannot be obtained. Conversely, if the time of the chemical solution holding process is too long, the stop time of the apparatus is This is because the operating rate decreases as the length increases.

  In addition, when a chemical solution holding step is provided, the reaction between the turbidity, organic matter, inorganic matter, etc. accumulated in the film during the chemical solution holding step and the oxidizing agent proceeds and the oxidizing agent is consumed. If it is too low, all of the oxidizing agent is consumed during the chemical solution holding step, and a sufficient cleaning effect may not be obtained.

  The concentration of the oxidizing agent contained in the backwash water is usually in the range of 0.1 mg / L to 10,000 mg / L, but is 50 mg / L or more in the present invention. In order to obtain a sufficient cleaning effect when the chemical solution holding step is provided, it is preferable to increase the concentration in consideration of a decrease in consumption of the oxidizing agent. For these reasons, it is preferable to set it to 200 mg / L or more. Further, if the concentration is higher than 1,000 mg / L, the cost of the oxidizing agent contained in the backwashing water and the cost of the chemical solution for neutralizing the oxidizing agent in the washing wastewater containing the backwashing water are increased. In view of economy, it is preferably 1,000 mg / L or less at the highest.

  In addition, when performing the oxidant-containing fresh water backwashing step, the backwashing is performed using fresh water not containing the oxidant as the backwash water, and then backwashing is performed using the fresh water containing the oxidant as the backwash water. Preferably it is done. In the membrane filtration step, since raw water containing salt is filtered, the primary side of the membrane and the secondary side of the membrane are filled with raw water or membrane filtered water containing salt. In this state, even if oxidant-containing fresh water is used as backwash water and backwashing is started, raw water or membrane filtrate containing salt remaining on the primary side of the membrane of the filtration membrane module and the secondary side of the membrane This is because the salt concentration of the oxidant-containing fresh water supplied as the backwash water becomes high by mixing with the water, and it is difficult to obtain a high cleaning effect.

  Therefore, the raw water remaining in the filtration membrane module and the membrane filtrate water are replaced with fresh water by performing backwashing using fresh water that does not contain an oxidizer as the backwash water in advance, and subsequently containing the oxidizer. When backwashing is performed using fresh water as backwashing water, a high cleaning effect can be obtained. When an external pressure type filtration membrane module is used, since the primary side of the membrane has a larger capacity than the secondary side of the membrane, a large amount of fresh water is required to replace the raw water on the primary side of the membrane with fresh water. Is required. Therefore, before performing the backwash using the fresh water not containing the oxidizer for the backwash water, the fresh water not containing the oxidizer is used for the backwash water during the backwash, or the oxidizer is contained. The amount of fresh water required to replace the raw water on the primary side of the membrane with fresh water is to discharge the water on the primary side of the membrane from the drain drain pipe after back washing using unwashed fresh water as backwash water Can be reduced, which is preferable.

  Furthermore, before backwashing using oxidant-containing fresh water for backwashing water, backwashing is performed using freshwater not containing oxidant for backwashing water, and turbidity and organic matter accumulated in the membrane of the filtration membrane module. If inorganic substances are removed in advance, it is possible to reduce the consumption of oxidant that should react with turbidity, organic matter, inorganic substances, etc. when removed by using oxidant-containing fresh water. Therefore, it is preferable because a high cleaning effect can be obtained and the cost of the oxidizing agent can be reduced.

  Here, examples of the pretreatment to be performed before the salt-containing raw water such as seawater / brine taken is subjected to membrane filtration by the filtration membrane module 3 include aggregation, precipitation, sand filtration, pressurized flotation, and activated carbon filtration. Biological treatment, pH adjustment, addition of bactericide, oxidation, etc. can be employed, but may be selected as appropriate.

  Here, the filtration membrane module 3 may be an external pressure type or an internal pressure type, but is preferably an external pressure type from the viewpoint of simplicity of pretreatment. The membrane filtration method may be a total filtration module or a cross flow filtration module, but a full 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 preferred from the viewpoint that a high flow rate operation is possible.

  Moreover, as a form of the filtration membrane module 3, a hollow fiber type, a flat membrane type, a spiral type, or a tubular type can be used, but the hollow fiber type is preferable from the viewpoint of cost reduction.

  Here, the membrane constituting the filtration membrane module 3 is not particularly limited as long as it is a porous membrane, but it is not limited to inorganic materials such as ceramics, polyethylene, polypropylene, polyacrylonitrile, ethylene-tetrafluoroethylene copolymer, polychloro Trifluoroethylene, polytetrafluoroethylene, polyvinyl fluoride, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, chlorotrifluoroethylene-ethylene copolymer, polyvinylidene fluoride, A membrane made of at least one selected from the group consisting of polysulfone, cellulose acetate, polyvinyl alcohol, polyethersulfone and vinyl chloride is preferred. Furthermore, from the viewpoint of film strength and chemical resistance, a film made of polyvinylidene fluoride (PVDF) is more preferable. In addition, a film made of polyacrylonitrile is more preferable from the viewpoint of high hydrophilicity and strong stain resistance. The pore diameter on the membrane surface is not particularly limited and may be a microfiltration membrane or an ultrafiltration membrane, but can be conveniently selected within a range of 0.001 μm to 10 μm.

  Moreover, when using a hollow fiber type filtration membrane module, the outer diameter of the hollow fiber membrane is not particularly limited, but the vibration property of the hollow fiber membrane at the time of washing is high and the washing property is excellent, so that it is within the range of 250 μm to 2000 μm. Preferably there is.

  Here, as a control method of the filtration flow rate in the membrane filtration step, there is no problem whether it is constant flow filtration or constant pressure filtration, but constant flow filtration is used from the viewpoint of easy control of the production amount of filtrate water. Some are preferred.

  For the reverse osmosis membrane constituting the reverse osmosis membrane module 13, for example, as a material, polyamide-based, polypiperazine amide-based, polyester amide-based, or a water-soluble vinyl polymer crosslinked can be used, The membrane structure has a dense layer on at least one side of the membrane, and has fine pores with gradually increasing pore diameters from the dense layer to the inside of the membrane or the membrane on the other side (asymmetric membrane), or such an asymmetric membrane. A material having a very thin separation functional layer (composite film) formed of another material on the dense layer can be used. However, a composite membrane is preferred for high water production during low-pressure operation, and among them, polyamide composite membranes and piperazine polyamide composite membranes are more preferable from the viewpoints of the amount of permeated water and chemical resistance.

  The reverse osmosis membrane module 13 is a spiral type element in which a flat membrane membrane is wound around a water collecting pipe, or a plate type support plate with flat membranes stretched on both sides at regular intervals via spacers. One or more plate-and-frame elements stacked and modularized, tubular elements using tubular membranes, and hollow fiber membrane elements bundled with hollow fiber membranes and housed in a case It is constructed by connecting in series. The element may be in any form, but it is preferable to use a spiral element from the viewpoint of operability and compatibility. The number of elements can be arbitrarily set according to the membrane performance. When spiral type elements are used, the number of elements to be loaded in one module is preferably arranged in a series of about 1 to 8 elements. A plurality of reverse osmosis membrane modules 13 may be arranged in parallel.

<Example 1>
One external pressure type PVDF hollow fiber microfiltration membrane module HFS-2020 (manufactured by Toray Industries, Inc.) is used as the filtration membrane module 3, and six spiral reverse osmosis membrane elements SU-810 (manufactured by Toray Industries, Inc.) are loaded. Using the reverse osmosis membrane module 13, the experiment was conducted under the following conditions in the flow shown in FIG.

Seawater is used as raw water, and the membrane filtration process is performed for 30 minutes using a filtration flux of 2.0 m ³ / (m ² · d), a total filtration method and a constant flow filtration method, and then the membrane filtration water is backwashed. The normal backwashing process used for 30 seconds is performed, air cleaning is performed, the raw water side in the filtration membrane module is completely discharged, the raw water side in the filtration membrane module is filled with raw water, and the membrane is washed again. The operation of returning to the filtration step was repeated. Furthermore, instead of performing the normal backwashing process once a day, the oxidizing agent-containing fresh water backwashing process using reverse osmosis membrane permeated water is performed for 120 seconds, the chemical solution holding process is performed for 20 minutes, and the oxidizing agent A fresh water backwash process (drug rinsing process) using reverse osmosis membrane permeated water that does not contain water for 120 seconds, discharges all of the water on the raw water side in the filtration membrane module, and the raw water side in the filtration membrane module Were backwashed with oxidant-containing fresh water in the order of filling the raw water with water, and the operation to return to the membrane filtration step was performed.

  In the oxidizing agent-containing fresh water backwashing step, an aqueous sodium hypochlorite solution (12%) was injected so that the oxidizing agent concentration was 300 mg / L. In the reverse osmosis treatment step, the experiment was conducted with the ratio of the reverse osmosis membrane permeated water to the reverse osmosis membrane feed water being 45%.

  The film differential pressure at the initial stage of operation was 20 kPa at 25 ° C. temperature corrected differential pressure, and the film differential pressure after one month operation was 40 kPa at 25 ° C. temperature corrected differential pressure. Moreover, the ratio of the amount of permeate which subtracted the part used as backwash water with respect to the amount of raw water was 41.6%. In addition, the average salt concentration of seawater, which is the raw water during the experiment period, was 34,000 mg / L, the average salt concentration of membrane filtration water normally used for backwashing was 34,000 mg / L, and used for backwashing with fresh water containing oxidizing agent. The average salt concentration of reverse osmosis membrane permeated water was 200 mg / L.

<Comparative Example 1>
Experiments were conducted in the same manner as in Example 1 except that the membrane filtration method was changed to the flow of FIG. 1 and the backwashing with the oxidizing agent-containing water was changed to the following conditions.
In backwashing with oxidant-containing water performed once a day, the oxidant-containing filtered water backwashing process using membrane filtered water containing oxidant as backwash water is performed for 120 seconds, and the chemical solution is retained. The process is performed for 20 minutes, a normal backwashing process (medicine rinsing process) using membrane filtered water containing no oxidizing agent for backwashing water is performed for 120 seconds, and the raw water side of the filtration membrane module is completely discharged and filtered. The raw water side in the membrane module was filled with raw water in order, and the operation to return to the membrane filtration step was performed.

  In the oxidizing agent-containing filtered water backwashing step, an aqueous sodium hypochlorite solution (12%) was injected so that the oxidizing agent concentration was 300 mg / L. In the reverse osmosis treatment step, the experiment was conducted with the ratio of the reverse osmosis membrane permeated water to the reverse osmosis membrane feed water being 45%.

  The membrane differential pressure at the initial stage of operation was 20 kPa at 25 ° C. temperature correction differential pressure, and the membrane differential pressure after one month operation was 60 kPa at 25 ° C. temperature correction differential pressure, and the cleaning effect was insufficient. Moreover, the ratio of the amount of permeate which subtracted the part used as backwash water with respect to the amount of raw water was 41.8%. In addition, the average salt concentration of seawater which is raw water during the experiment period and the average salt concentration of membrane filtration water used for backwash water were the same as those in Example 1.

<Comparative example 2>
Experiments were conducted in the same manner as in Example 1 except that the membrane filtration method was changed to the flow of FIG. 6 and the normal backwashing step performed between the membrane filtration step and the membrane filtration step was changed to the following conditions.
After performing the membrane filtration process for 30 minutes, subsequently, the backwash process using reverse osmosis membrane permeated water as backwash water is performed for 30 seconds, air washing is performed, and the water on the raw water side in the filtration membrane module is completely discharged. Then, washing was performed in the order of filling the raw water side in the filtration membrane module with the raw water, and the operation of returning to the membrane filtration step was repeated.

  The membrane differential pressure at the initial stage of operation was 20 kPa at 25 ° C. temperature correction differential pressure, and the membrane differential pressure after one month operation was 37 kPa at 25 ° C. temperature correction differential pressure. Moreover, the ratio of the amount of permeated water which subtracted the part used as backwash water with respect to the amount of raw water fell with 40.6%. In addition, the average salt concentration of the seawater which is the raw water during the experiment period and the average salt concentration of the reverse osmosis membrane permeated water used for the backwash water were the same as those in Example 1.

<Example 2>
One immersion type PVDF hollow fiber ultrafiltration membrane module CFU-1015V (manufactured by Toray Industries, Inc.) is used as the filtration membrane module 3, and six spiral reverse osmosis membrane elements SU-810 (manufactured by Toray Industries, Inc.) are used. Using the loaded reverse osmosis membrane module 13, the experiment was performed under the following conditions in the flow shown in FIG.

Seawater is used as raw water, and the membrane filtration process is performed for 30 minutes using a filtration flux of 1.7 m ³ / (m ² · d), a total filtration method and a constant flow filtration method, and then the membrane filtration water is backwashed. The normal backwashing process used for 30 seconds is performed, air cleaning is performed, the raw water side in the filtration membrane module is discharged in its entirety, and the raw water side in the filtration membrane module is fully filled with raw water for cleaning. The operation of returning to the membrane filtration step was repeated. Furthermore, instead of performing the normal backwashing process once a day, the oxidizing agent-containing fresh water backwashing process using reverse osmosis membrane permeated water is performed for 120 seconds, the chemical solution holding process is performed for 20 minutes, and the oxidizing agent A fresh water backwash process (drug rinsing process) using reverse osmosis membrane permeated water that does not contain water for 120 seconds, discharges all of the water on the raw water side in the filtration membrane module, and the raw water side in the filtration membrane module Were backwashed with oxidant-containing fresh water in the order of filling the raw water with water, and the operation to return to the membrane filtration step was performed.

  In the oxidizing agent-containing fresh water backwashing step, an aqueous sodium hypochlorite solution (12%) was injected so that the oxidizing agent concentration was 300 mg / L. In the reverse osmosis treatment step, the experiment was conducted with the ratio of the reverse osmosis membrane permeated water to the reverse osmosis membrane feed water being 45%.

  The film differential pressure at the initial stage of operation was 18 kPa at 25 ° C. temperature correction differential pressure, and the film differential pressure after one month operation was 35 kPa at 25 ° C. temperature correction differential pressure. Moreover, the ratio of the amount of permeate which subtracted the part used as backwash water with respect to the amount of raw water was 42.9%. The average salt concentration of seawater, which is the raw water during the experimental period, the average salt concentration of membrane filtration water used for normal backwashing, and the average salt concentration of reverse osmosis membrane permeated water used for oxidant-containing backwashing are shown in Examples. It was the same as the case of 1.

  From the above examples and comparative examples, according to the membrane filtration method of the present invention, by performing backwashing using fresh water containing oxidant in backwashing water from time to time, it is possible to suppress an increase in membrane differential pressure, Furthermore, in normal backwashing, the rate of water recovery can be maintained high by using membrane filtrate for backwashing water.

It is a process flow figure showing one embodiment of the conventional general membrane filtration method. It is a process flow figure showing one embodiment of a membrane filtration method concerning the present invention method. It is a process flow figure showing another embodiment of a membrane filtration method concerning the present invention method. It is a process flow figure showing an embodiment of a membrane filtration method concerning Example 1. It is a process flow figure showing an embodiment of a membrane filtration method concerning Example 2. It is a process flow figure showing an embodiment of a membrane filtration method concerning comparative example 2.

Explanation of symbols

1: Filtration pump 2: Raw water pipe 3: Filtration membrane module 4: Filtration water pipe 5: Filtration water tank 6: Immersion tank 7: Raw water supply pump 11: High pressure pump 12: Reverse osmosis membrane supply pipe 13: Reverse osmosis membrane module 14: Permeated water pipe 15: Concentrated water pipe 16: Fresh water pipe 17: Fresh water tank 18: Permeated water return pipe 21: Backwash pump 22: Backwash water pipe 23: Oxidant storage tank 24: Oxidant injection pump 25: Oxidant injection pipe 26: Backwash water drain pipe 27: Drain drain pipe 31: Fresh water back wash pump 32: Fresh water back wash pipe 33: Membrane filtrate backwash valve 34: Fresh water backwash valve

Claims (1)

In membrane filtration method by salt concentration processes the above raw water 1,000 mg / L in the filtration membrane module obtaining a membrane filtration water has a backwash step of performing backwash of the filtration membrane module during the membrane filtration step In the backwashing step , the membrane filter water is reversely osmosis treated using reverse osmosis membrane permeated water having a salt concentration of less than 500 mg / L, which contains no oxidant, washing, and the filtration membrane subjected to discharge water of the raw water side of the module, subsequently, using the reverse osmosis membrane permeated water an acid agent was contained at a concentration of at least 50 mg / L in the backwash water oxidation agent performed as containing fresh water backwash Engineering, subsequently, performed in that the reverse osmosis membrane permeated water containing the oxidizing agent to maintain the filter membrane state of being held in the module 3 to 120 minutes, and the The frequency of the oxidant-containing fresh water backwash process is 3 hours ~ Membrane filtration method characterized in that a one to zero days.

Images