JP4698274B2 - Filtration membrane cleaning method - Google Patents

Description

  The present invention is used for water treatment, industrial water, river water, lake water, ground water, stored water filtration, sewage secondary treatment water filtration, sewage, wastewater filtration, or separation or concentration of valuable materials. The present invention relates to a method for cleaning a filtration membrane.

  Membrane separation methods used for the filtration of various types of water to be treated are used in various types of filtration devices because of their excellent filtration accuracy, small installation space, and easy operation management. . However, with the continuation of filtration, suspended substances in the water to be treated adhere to the membrane surface and block the pores, so that the filtration performance gradually deteriorates and finally filtration becomes impossible. Therefore, in order to maintain the filtration performance, gas such as air is introduced into the treated water side of the filtration membrane as air bubbles (hereinafter referred to as “air washing”), and the filtrate is filtered from the filtrate side in the direction opposite to the filtration direction. Alternatively, back-pressure water cleaning (hereinafter referred to as back-washing) is generally performed in which a backwash medium such as clarified water is ejected to remove suspended substances deposited on the surface of the filtration membrane.

  In order to enhance the washing effect of air washing, a method of lowering the water level on the treated membrane side of the filtration membrane while jetting bubbles from below the membrane module (see, for example, Patent Documents 1 and 2) or ozonated air is used as a filtration membrane. There is known a method of injecting bubbles into the water to be treated (for example, see Patent Document 3). Furthermore, there is known a method for enhancing the cleaning effect of gas cleaning by allowing a floating solid for cleaning to exist around the membrane module and bringing the floating solid into contact with the membrane by air washing (for example, see Patent Documents 4 and 5). It has been. Furthermore, a method (for example, see Patent Document 6) is known in which cleaning bubbles are uniformly supplied to the membrane module and the adjacent membranes are positively brought into contact with each other to enhance the cleaning effect of the air washing. Further, a method of adding sodium hypochlorite having an oxidizing action to the backwash medium, a method of backwashing using ozone water (for example, refer to Patent Document 7), and a method of backwashing with ozonized pressurized air (for example, Patent Document 8) is known.

  The above-described backwashing method using an oxidizing agent such as sodium hypochlorite and ozone water, the air washing method of introducing air or ozonized air as bubbles to the treated water side of the filtration membrane, etc., increase the cleaning effect. Although effective, depending on conditions such as turbidity of the water to be treated, a sufficiently stable membrane filtration flux cannot always be obtained. In order to remove suspended substances on the membrane surface and maintain a high membrane filtration flux, it is effective to increase the gas flow rate at the time of air washing or lengthen the air washing time. However, these increase the vibration of the filtration membrane at the time of air washing, and an excessive load is applied to the filtration membrane, so that the life of the filtration membrane is shortened.

  In addition, the air washing methods shown in Patent Documents 1 and 2 are effective in enhancing the washing effect, while the method of using the bubble disappearance effect at the gas-liquid interface and the large fluctuation of the liquid level due to the bursting of the bubbles are effective. There is a problem that the cleaning effect due to the vibration of the filtration membrane due to the cleaning effect due to the cross flow flow accompanying the rise of the bubble and the elimination effect of the bubble volume is halved. Furthermore, since the water around the filtration membrane is eliminated by lowering the liquid level, the filtration membranes are in direct contact with each other due to the shaking of the bubbles, and the filtration membranes may be damaged or broken by rubbing against each other. In addition, suspended substances once separated from the membrane surface by air washing will adhere to the membrane surface again when the liquid level drops, so the concentrated suspended substance cannot be completely discharged out of the system. There is a problem that the effect is halved.

  Furthermore, the filtration membrane near the treated water intake of the membrane module has a short distance to the intake, and the pressure loss of the fluid (treated water) on the filtered membrane treated water side is small. Since the pressure difference on the water side, that is, the membrane differential pressure, becomes larger, the filtration membrane at the aforementioned site will filter a larger amount of treated water than the filtration membranes at other sites, and membrane contamination will proceed rapidly. The filtration performance is reduced. In other words, since the filter membrane is contaminated with respect to the longitudinal direction, it is necessary to sufficiently clean the entire filter membrane as much as possible, in particular, the filtration membrane of the aforementioned portion. Since most of the membrane is in contact with bubbles for a very short time, a sufficient cleaning effect cannot be obtained.

In addition, since the air washing methods shown in Patent Documents 4 and 5 use a floating solid, the floating solid is blocked in the membrane bundle in the membrane module, and the floating solid is in direct contact with the filtration membrane. The filter membrane is damaged or broken by rubbing, and the suspended solids separated by the air washing are discharged out of the system. As a result, floating solids also flow out, and the equipment for recovering them becomes excessive. There is a problem such as.
Japanese Patent Publication No. 6-71540 Japanese Patent No. 3351037 JP-A-63-42703 JP 2001-190937 A Japanese Patent Laid-Open No. 4-126528 JP-T-2001-510396 JP-A-4-310220 Japanese Unexamined Patent Publication No. 60-58222

  It is an object of the present invention to perform cleaning effectively while reducing the load on the filtration membrane.

As a result of intensive studies on the filtration membrane cleaning method, the present inventors have completed the following invention.
That is, the present invention is as follows.
(1) In a cleaning step of a membrane module composed of a large number of filtration membranes, by blowing a gas cleaning medium to the treated water side of the filtration membrane constituting the membrane module and bringing it into contact with the entire filtration membrane After the gas cleaning step A for removing suspended substances from the filtration membrane, the liquid level of the treated water in the membrane module is once lowered by filtration or discharge, and the treated water liquid level around the filtration membrane is lowered, The backwash medium is supplied from the treated water side of the filtration membrane, and the treated water side liquid level in the membrane module is removed from the filtration membrane by back pressure water washing that ejects the backwash medium that has passed through the membrane to the treated water side of the filtration membrane. A method for cleaning a filtration membrane, wherein the cleaning is performed by combining gas cleaning step B for removing suspended substances from the filtration membrane by performing gas cleaning while raising from below to above.

( 2 ) In the gas cleaning step B, as a means for raising the treated water side liquid level in the membrane module from below to above, the treated water in the membrane module is supplied by supplying the treated water to the treated water side of the filtration membrane. The method for cleaning a filtration membrane according to (1), wherein the liquid level of the treated water is raised.
( 3 ) The method for cleaning a filtration membrane according to (1) or (2), wherein the gas cleaning step B is performed before the gas cleaning step A.

  According to the present invention, it is possible to perform cleaning effectively while reducing the load on the filtration membrane. As a result, it is possible to maintain a high membrane filtration flux for a long period of time.

In the following, the present invention will be described in detail with respect to preferred embodiments.
The water to be treated that is the subject of the present invention is river water, lake water, ground water, stored water, secondary sewage treated water, factory effluent, or sewage. When the water to be treated as described above is filtered through a membrane, suspended substances contained in the water to be treated and substances having a size larger than the pore size of the membrane to be used are blocked by the membrane, and so-called concentration polarization and a cake layer are formed. At the same time, the membrane is clogged or adsorbed by the network inside the membrane. As a result, the membrane filtration flux when the water to be treated is filtered is reduced from a fraction to a few tenths compared with that when the clarified water is filtered, and the membrane is continuously filtered. The filtration flux gradually decreases.
In order to prevent this and maintain the membrane filtration flux, air washing is generally used in which a gas such as air is introduced as bubbles on the treated water side of the filtration membrane. However, as described above, the air washing conditions are not always sufficient because of the limitation of the durability of the filtration membrane.

  The filtration membrane cleaning method of the present invention comprises a gas cleaning step A for removing suspended substances from a filtration membrane by ejecting a gas cleaning medium to the treated water side of the filtration membrane and bringing it into contact with the entire filtration membrane, and a membrane The treated water in the module is temporarily lowered by filtering or discharging, and then the treated water liquid level around the filtration membrane is lowered, and then the treated water side liquid level is raised from below to lift from the filtration membrane. By washing in combination with the gas washing step B for removing turbid substances, (a) the cleaning effect by the cross flow flow accompanying the rise of the bubbles, (b) the liquid removal effect for the volume of the bubbles The cleaning effect by vibration of the filtration membrane, (c) the disappearance effect of bubbles at the gas-liquid interface and the cleaning effect by the large fluctuation of the liquid level due to the bursting of bubbles, and (d) once delamination from the membrane surface by gas cleaning Suspended material again on membrane surface Since it does not adhere, it becomes possible to completely discharge the suspended substances separated from the filtration membrane, and (e) allow sufficient time for the filtration membrane near the treated water intake of the membrane module to contact the bubbles. Therefore, the suspended substance can be efficiently separated from the surface of the filtration membrane as compared with the conventional air washing method.

  In the gas cleaning step B, the water level of the water surface to be treated when the water level in the membrane module is once lowered by filtration or discharge may be any water level in the vertical direction of the filtration membrane. In order to impart the five cleaning effects shown to the entire filtration membrane, it is preferable to lower it to the lowest end in the vertical direction of the filtration membrane. Further, in the gas cleaning step B, as a method of raising the treated water level around the filtration membrane from below to above, a method of raising the treated water level around the filtration membrane while performing backwashing or supplying treated water While there is a method of raising the surface of the water to be treated around the filtration membrane, a method of raising the surface of the water to be treated around the filtration membrane by backwashing is preferred. In addition, although backwashing (1) always performs simultaneously with the empty washing of the present invention, the cleaning effect is high, but (2) prior to the introduction of the backwashing, only the empty washing of the present invention may be performed. Or (3) You may perform only the air washing of this invention after introducing backwashing. Furthermore, (4) The backwashing may be introduced while introducing the water to be treated, and at the same time the air washing of the present invention may be performed, and furthermore, (1) to (4) may be combined alternately.

Here, in the cleaning process of the filtration membrane, the ratio of the process time of each of the gas cleaning process A and the gas cleaning process B during the cleaning process time is arbitrary, but is performed in the range of the ratio of 1:10 to 10: 1. Is preferred.
Furthermore, when performing washing while raising the liquid level, the state where there is no water around the filtration membrane is a short time, the filtration membranes are in direct contact with each other, and the filtration membranes are damaged, broken or dried. Can be prevented. Moreover, since an effective cleaning effect can be obtained compared to the conventional air cleaning method, it is possible to reduce the amount of gas cleaning medium to be used, so that the durability of the filtration membrane, membrane module, or energy efficiency can be reduced. This is also effective.

  The gas cleaning medium is used when air cleaning is performed by blowing the gas cleaning medium in the form of bubbles to the water to be treated of the filtration membrane while raising the liquid surface to be treated of the filtration membrane of the present invention from below to above. Using a gas containing at least one or more oxidizing agents such as chlorine, chlorine dioxide, hydrogen peroxide, ozone gas, etc., or using backwashing containing at least one or more of the above oxidizing agents together, a further cleaning effect can be obtained. Can do. The washing time may be appropriately determined in consideration of the recovery performance of the filtration pressure and the time availability of the filtration equipment.

  The filtration membrane of the present invention is not particularly limited. For example, polyolefins such as polyethylene, polypropylene, and polybutene; tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (FEP) Tetrafluoroethylene-hexafluoropropylene-perfluoroalkyl vinyl ether copolymer (EPE), tetrafluoroethylene-ethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), chlorotrifluoroethylene-ethylene copolymer Fluorine resins such as (ECTFE) and polyvinylidene fluoride (PVDF); polysulfone, polyethersulfone, polyetherketone, polyetheretherketone, polyphenylenesulfur Super engineering plastics such as I de; cellulose acetate, cellulose such as ethyl cellulose; polyacrylonitrile; alone and mixtures of these polyvinyl alcohol.

Furthermore, when a strong oxidizing agent such as ozone is used in combination, an inorganic film such as ceramic, a polyvinylidene fluoride (PVDF) film, a polytetrafluoroethylene (PTFE) film, an ethylene-tetrafluoroethylene copolymer (ETFE) An organic film such as a fluororesin film such as a film or a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA) film can be used.
Among such filtration membranes, those having a pore size region from a nanofiltration (NF) membrane to a microfiltration (MF) membrane can be used. In particular, MF having a fractional molecular weight of about 100 to MF having an average pore size of 10 μm or less is preferable.

  As the shape of the filtration membrane, any shape such as hollow fiber shape, waved hollow fiber shape, flat membrane shape, pleated shape, spiral shape, tubular shape, etc. can be used, but the membrane area per unit volume can be increased. A hollow fiber shape is more preferable. As the membrane module used in the present invention, the upper and lower ends of a membrane bundle composed of a large number of filtration membranes are bonded and fixed, and one or both ends are opened, and the ends to be bonded and fixed The cross-sectional shape may be a circle, a triangle, a quadrangle, a hexagon, an ellipse, etc., in particular, the cross-sectional shape of the end portion is circular, and the upper end portion has a membrane opening, A membrane module having a gas introduction hole for introducing gas to the surface of the filtration membrane at the lower end is preferred.

Furthermore, the installation method of the membrane module may be either a vertical direction or a horizontal direction with respect to the ground, but installation in the vertical direction is particularly preferable. The filtration method may be a whole amount filtration method or a cross flow filtration method. As a method for applying the filtration pressure, a pressure filtration method, a suction filtration method, or a water head difference method may be used. In the case of a hollow fiber membrane, either internal pressure filtration or external pressure filtration may be used.
Since the present invention is configured as described above, a high membrane filtration flux can be maintained over a long period of time.

  In the following, as membrane modules composed of a large number of filtration membranes, (1) a casing storage type membrane module in which a large number of filtration membranes are housed in a case and (2) a large number of filtration membranes are directly treated. Examples of the present invention are shown for each of the submerged membrane modules immersed in water.

FIG. 1 shows an example using a casing storage type membrane module. As the treated water 1, river surface water having a turbidity of 5 to 20 degrees was used. The treated water 1 is pumped to the membrane module 4 by the raw water supply pump 3 through the circulation tank 2, and the obtained filtrate is stored in a filtrate tank 5 that also serves as a backwash tank. At the time of backwashing, the filtrate in the drainage tank 5 is sent to the membrane module 4 by the backwashing pump 6 and backwashing is performed. Here, the oxidant tank is placed in the middle of the piping from the backwashing pump 6 to the membrane module 4. 7 oxidant can be added to the backwash water by the oxidant feed pump 8.
Further, the air washing for introducing air into the membrane module 4 is performed by supplying the air compressed by the compressor 9 to the treated water side of the membrane module 4.

  As a method of lowering the level of the water to be treated, immediately before the gas cleaning step B is performed, the water to be treated is discharged from the lower side of the membrane module 4 or the raw water supply by the raw water supply pump 3 is stopped. And a method of filtering the water to be treated inside the membrane module using the compressed air of the compressor 9 and discharging the water to be treated. After lowering the level of the water to be treated by these means, the raw water supply pump 3 performs raw washing while supplying the raw water to the membrane module 4, or by performing washing with backwashing and performing air washing. The gas cleaning effect can be obtained.

The operation in this example is performed in the steps of (1) filtration, (2) empty washing and backwashing of the present invention at the same time, and (3) discharging the suspended suspended matter. However, the operation process shown here is an example of the usage method of this invention, and the order of an operation process is not limited to this.
The membrane module 4 is made of a PVDF (polyvinylidene fluoride) hollow fiber microfiltration (MF) membrane having an inner diameter of 0.7 mmφ, an outer diameter of 1.3 mmφ, and an average pore size of 0.1 μm. It is an external pressure press filtration type casing housing type membrane module housed in a casing. The membrane area of the membrane module is 7 m ² .

  FIG. 2 shows an example using an immersion membrane module. As the treated water 1, river surface water having a turbidity of 5 to 20 degrees was used. The treated water 1 is fed by the raw water supply pump 3 to the immersion tank 11 in which the immersion membrane module is installed, and the filtrate obtained by the suction pump 12 is stored in the filtrate tank 5 that also serves as a backwash tank. At the time of backwashing, the filtrate in the drainage tank 5 is sent to the submerged membrane module by the backwashing pump 6 and backwashing is performed. Here, oxidation is performed in the middle of the piping from the backwashing pump 6 to the submerged membrane module. The oxidant in the oxidant tank 7 can be added to the backwash water by the oxidant feed pump 8.

  In addition, air washing for introducing air into the submerged membrane module is performed by supplying the air compressed by the compressor 9 to the treated water side of the submerged membrane module. As a method of lowering the liquid level of the water to be treated, immediately before performing the gas cleaning step B, in a method of discharging the water to be treated from below the immersion tank 11, or in a state where water supply by the raw water supply pump 3 is stopped, For example, the water to be treated is discharged by filtering the water to be treated in the immersion tank 11 using the suction pump 12. After lowering the liquid level of the water to be treated by these means, the raw water supply pump 3 is used to wash the raw water while supplying the raw water to the immersion tank 11, or by performing the air washing while backwashing. The gas cleaning effect can be obtained.

The operation in this example is performed in the steps of (1) filtration, (2) empty washing and backwashing of the present invention at the same time, and (3) discharging the suspended suspended matter. However, the operation process shown here is an example of the usage method of this invention, and the order of an operation process is not limited to this.
The submerged membrane module has an external pressure in which a PVDF (polyvinylidene fluoride) hollow fiber microfiltration (MF) membrane having an inner diameter of 0.7 mmφ, an outer diameter of 1.3 mmφ, and an average pore size of 0.1 μm is bundled in a 6-inch diameter equivalent. It is a suction filtration type immersion module. The membrane area of the membrane module is 50 m ² .

  The present invention will be described based on examples. The operating conditions of Example 2 and Comparative Example 2 are shown in FIG.

[Example 1]
The casing housing type membrane module was used as the membrane module. Filtration is a constant flow filtration that supplies the treated water 1 to the membrane module 4 at a constant flow rate (a flow rate that provides 3 m ³ of filtrate per 1 m ² of membrane area in a day with a membrane filtration flux of 3 m ³ / m ² / day). In addition, the cross-flow method was performed in which the ratio of the membrane filtered water amount and the circulating water amount was 5: 1. The operating conditions were as follows: filtration for 29 minutes, air washing (at the same time back washing) for 60 seconds, and discharge for 30 seconds. As the air washing gas, air compressed by a compressor was used. Further, immediately before the gas cleaning step B, the water to be treated was discharged from the membrane module and the liquid level of the water to be treated was lowered. Here, the surface of the water to be treated gradually rises due to the inflow of the backwash water. However, the surface of the water to be treated is gradually raised by the backwash water in the first 30 seconds within the 60 seconds of the air washing. Washing (gas washing step B) was carried out, and the remaining 30 seconds were subjected to air washing (gas washing step A) with the water to be treated in contact with the entire filtration membrane. The air flow rate used for air washing was 2 Nm ³ / hr. The filtrate recovery rate (the amount of filtrate obtained / the amount of treated water used) at this time was 93.7%. The membrane supply pressure after operating for about one month under the above operating conditions was 150 kPa.

[Comparative Example 1]
In Example 1, the gas washing step B was not performed, and the air washes (gas washing step A) was performed using the same amount of air at the same flow rate as in Example 1 (the air washing time was 60 seconds). Otherwise, the membrane filtration operation was carried out in the same manner as in Example 1. The filtrate recovery rate at this time was 93.7% as in Example 1. The membrane supply pressure after operating for about one month reached 300 kPa.

[Example 2]
The immersion type membrane module was used as the membrane module. Filtration supplies the water to be treated 1 to the immersion tank 11, and a filtered flow of ² m ³ per 1 m ² of membrane area is obtained per day with a constant flow rate (membrane filtration flux 2.0 m ³ / m ² / day) by a suction pump 12. Constant flow rate filtration). The operating conditions were as follows: filtration for 29 minutes, air washing (at the same time back washing) for 60 seconds, and discharge for 30 seconds. As the gas for gas cleaning, air compressed by a compressor was used. Moreover, the to-be-processed water was discharged | emitted from the immersion tank immediately before the gas washing process B, and the to-be-processed water liquid level was lowered | hung. Here, the surface of the water to be treated rises gradually due to the inflow of the backwash water, but in the 60 seconds of the air washing, the surface of the water to be treated is gradually raised by the backwash water in the first 30 seconds. (Gas washing process B) was performed, and the remaining 30 seconds were subjected to air washing (gas washing process A) in a state where the water to be treated was in contact with the entire filtration membrane.

The air flow rate used for air washing was 4 Nm ³ / hr. The filtrate recovery rate (the amount of filtrate obtained / the amount of treated water used) at this time was 95.6%. As shown in FIG. 3 (the vertical axis represents the transmembrane pressure difference and the horizontal axis represents the operation time), the rate of increase of the transmembrane pressure difference per unit time (the transmembrane pressure difference kPa increased during the operation of Example 2). Are divided by the operation time hr during the period) and are 0.027 kPa / hr (operation time 950 to 1000 hr in FIG. 3) and 0.0092 kPa / hr (operation time 1050 to 1075 hr in FIG. 3), respectively. Filtration operation was possible.

[Comparative Example 2]
In part of the continuous operation of Example 2, the gas cleaning step B was not performed, and the air washes (gas cleaning step A) was performed using the same amount of air at the same flow rate as in Example 2 (air cleaning time) Is 60 seconds). Otherwise, the membrane filtration operation was performed in the same manner as in Example 2. The filtrate recovery rate at this time was 95.6% as in Example 2. As shown in FIG. 3, the rate of increase of the transmembrane pressure difference per unit time (the value obtained by dividing the transmembrane pressure difference kPa increased during the operation of Comparative Example 2 by the operation time hr during the operation) is 0.135 kPa / hr. (The operation time is 1000 to 1050 hr in FIG. 3), and the increase rate of the transmembrane pressure difference is about 5 to 14 times higher than that of Example 2, indicating that stable filtration operation cannot be performed.

  In fields where river membranes, lake water, groundwater, stored water, secondary treated water from sewage, industrial effluent, sewage, etc. are used as treated water, or filter membranes are used for separation or concentration of valuable materials It can be suitably used.

The flowchart which showed an example of the processing flow incorporating the cleaning method of the film | membrane of this invention. The flowchart which showed an example of the processing flow incorporating the cleaning method of the film | membrane of this invention. The graph which compared the cleaning effect of Example 2 and Comparative Example 2.

Claims (3)

In the washing step of the membrane module composed of a large number of filtration membranes, a gas washing medium is ejected to the treated water side of the filtration membrane constituting the membrane module, and the membrane is brought into contact with the entire filtration membrane. A gas washing step A for removing suspended substances from the membrane, and once the water level in the membrane module is lowered by filtration or discharge, the liquid level around the filtration membrane is lowered, and then the backwash medium Is supplied from the treated water side of the filtration membrane, and the treated water side liquid level in the membrane module is raised from the lower side of the filtration membrane by back pressure water washing that ejects the backwash medium that has passed through the membrane to the treated water side of the filtration membrane. A method for cleaning a filtration membrane, comprising cleaning in combination with a gas cleaning step B for removing suspended substances from the filtration membrane by performing gas cleaning while being raised to a flow rate.   In the gas cleaning step B, the water to be treated in the membrane module is supplied by supplying the water to be treated to the treated water side of the filtration membrane as means for raising the liquid surface to be treated in the membrane module from below to above. 2. The filtration membrane cleaning method according to claim 1, wherein the surface is raised. The method of cleaning a filtration membrane according to claim 1 or 2 , wherein the gas cleaning step B is performed before the gas cleaning step A.

Images