JP2001079365A - Washing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the damage of a porous membrane and to maintain high membrane filtration flow rate over a long period of time by forcibly feeding ozone-containing water to the opposite direction to a filtering direction and also rocking the porous membrane by introducing bubbles to the raw water side of the porous membrane in the case when the porous membrane used for filtering various kinds of raw water is washed. SOLUTION: The raw water 1 consisting of river surface running water, etc., is fed forcibly to a membrane module 4 by a raw water feed pump 3 from a circulation tank 2, and the filtered water obtained here is stored at a filtered water tank 5. At the time of back washing of the membrane module 4, the filtered water in the filtered water tank 5 is fed forcibly to an ejector 8 by a backwashing pump 8 and allowed to contact with the ozone generated at an ozonizer 9, then sent to the membrane module 4. On the other hand, the air generated at a compressor 7 is sent to the raw water side of the membrane module 4 to air-bubble the membrane module 4. In such a way, an adhered matter at the membrane module 4 is easily decomposed oxidatively and also surely peeled by air-bubbling.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to filtration of water supply and industrial water or sewage secondary treatment water, and sewage treatment.
The present invention relates to a method for cleaning a membrane used for filtering wastewater.

[0002]

2. Description of the Related Art Filtration membranes used for filtration of various raw waters are excellent in filtration accuracy, require a small installation space, and are easy to manage.
Used in various filtration devices. However, with the continuation of filtration, substances to be removed such as organic substances in raw water adhere to the membrane surface, and the pores on the surface are closed, so that the filtration performance gradually decreases,
Eventually, filtration becomes impossible.

[0003] Therefore, as a method of cleaning the membrane to maintain the filtration performance, backwashing is used, in which filtered water or clarified water is jetted from the direction opposite to the filtration direction of the membrane to remove deposits on the filtration surface of the membrane. ing. Alternatively, sodium hypochlorite is added to the backwash water to further enhance the effect thereof, or a backwash method using ozone water as disclosed in JP-A-4-310220, 60-58
A method of back washing with ozonized pressurized air disclosed in Japanese Patent Publication No. 222 is known. Further, JP-A-63-427
As disclosed in Japanese Patent Publication No. 03-203, there is known a method of injecting ozonized air into the raw water side of a hollow fiber membrane as air bubbles.

[0004]

However, in the conventional washing method using only backwashing, the backwashing water flows preferentially in a portion where pores are slightly clogged, that is, where there is little hydraulic resistance. Although the minor part is washed, the backwash water hardly flows into the completely closed hole, and is not washed. Therefore, there is a problem that the entire membrane surface cannot be washed, and the filtration performance cannot be sufficiently restored even after the backflow washing. In addition, when the amount of the adhered substance on the film surface is large, the thick adhered layer cannot be peeled off only by the pressure at the time of the backflow cleaning, and the cleaning recovery property is insufficient.

[0005] On the other hand, the method of introducing bubbles into the raw water side (air bubbling) is particularly effective in cleaning by removing bubbles on the film surface as the films swing due to the introduced bubbles and come into contact with each other. As it does, its effect is greater than backwashing. However, conversely, adhering substances are pushed into the pores due to rubbing between the membranes, and rather the pores are blocked or the membrane surface is rubbed.As a result, the pores are crushed when used for a long time, and the filtration performance is rather reduced. There was a problem of lowering. That is, there has not yet been found a method for effectively washing without damaging the membrane and maintaining a high filtration flux.

[0006]

The present invention solves the above-mentioned problems. That is, in the present invention, (1) in the method for cleaning a porous membrane, backflow cleaning is performed in a direction opposite to a filtration direction with pressurized ozone-containing water of 0.05 mg / liter or more, and air bubbles are introduced into the raw water side. (2) Backwashing with a pressurized 0.05 mg / L or more ozone-containing water in the opposite direction to the filtration method, The present invention relates to a method for cleaning a film, characterized by introducing an ozone gas to oscillate the film.

Hereinafter, the present invention will be described specifically.
The present invention provides backwashing with strong oxidizing ozone-containing water and, at the same time, oscillating the membrane with air or ozone gas, thereby preventing damage to the membrane and obtaining high washing recovery properties. It can be maintained. The raw water that is an object of the present invention is raw water such as river water, lake water, groundwater, and storage water or industrial water, factory wastewater, and sewage.

Conventionally, when the raw water as described above is filtered through a membrane,
Suspended substances contained in the raw water and organic substances having a size larger than the pore diameter of the membrane used are blocked by the membrane, so-called concentration polarization and a cake layer are formed, and at the same time, the organic substances in the raw water clog the membrane. Or adsorb to the network inside the membrane. As a result, the filtration flux of the membrane when the raw water is filtered is reduced from a fraction to a few tenths of that when the clear water is filtered. The flux gradually decreases.

[0009] Substances that block these films generally exist in raw water in a state where organic substances cover inorganic particles,
It adheres firmly to the film surface by the adhesive force of the organic matter on the surface.
For this reason, backflow washing using water pressure using membrane filtration water or clarified water that is usually performed cannot remove strongly adhered substances,
Its cleaning recovery effect is small. On the other hand, when backwashing is performed with ozone-containing water, organic substances on the surface of the substance adhering to the film are oxidatively decomposed or denatured by the oxidizing power of ozone, and are easily separated from the film surface. However, with only backwashing with ozone-containing water, as described above, a portion that is slightly clogged is sufficiently washed, but a portion that is completely clogged or a portion with a thick adhered layer still cannot be sufficiently washed. .

Therefore, simultaneously with the backwashing with the ozone-containing water, bubbles can be introduced into the raw water side to oscillate the film, so that the adhered substances can be more effectively removed. Alternatively, by using ozone gas as bubbles, the oxidative decomposition and denaturation of organic substances further progress, and the effect is enhanced. In addition, when cleaning is usually performed by introducing air bubbles into the raw water, the membranes rub against each other and the cleaning effect is high, but at the same time, the membrane surface may be damaged and the permeation flux may be reduced. However, when air bubbles are introduced into the raw water side of the membrane at the same time as the backwashing as in the present invention, the distance between the membranes is increased due to the water flow of the backwash water, thereby preventing excessive rubbing. At the same time, adhering substances that are easily clogged between the films and fall off from the film surface are also easily discharged.

As described above, backwashing with ozone-containing water in the direction opposite to the filtration direction is performed, and at the same time, bubbles are introduced into the raw water side, so that the membrane can be effectively washed without damaging the membrane. The direction opposite to the filtration direction refers to the direction from the filtrate side to the raw water side of the membrane. The porous membrane is not particularly limited as long as it is a filtration membrane that is not deteriorated by ozone. For example, polyvinylidene fluoride (PVDF)
Membrane, polytetrafluoroethylene (PTFE) membrane, ethylene-
An organic film such as a fluororesin film such as a tetrafluoroethylene copolymer (ETFE) film and a polyfluoroacrylate (PFA) film can be used. It is particularly preferable to use a polyvinylidene fluoride (PVDF) film.

Among such ozone-resistant porous membranes, those having a pore size range from ultrafiltration (UF) membrane to microfiltration (MF) membrane can be used, but basically have a high filtration flow rate. Preferably, a microfiltration (MF) membrane is used. For example, a membrane having an average pore diameter of 0.001 to 1 μm is preferable, and a membrane having an average pore diameter of 0.05 to 1 μm is more preferable. As the shape of the porous membrane, any shape such as a hollow fiber shape, a flat film shape, and a tubular shape can be used. However, a hollow fiber shape having a large membrane area per unit volume is preferable. As the shape of the hollow fiber membrane, a waved fiber can be used. Generally, filtration is performed using a module containing a membrane.

The filtration method may be either a full filtration method or a cross flow filtration method. In addition, a pressure filtration method or a negative pressure filtration method may be used, but the pressure filtration method is preferable because a higher filtration flux can be obtained. In addition, any of internal pressure filtration and external pressure filtration may be used, but the method of the present invention is preferably applied to washing of a membrane used for external pressure filtration. In the present invention, at the same time as backwashing with ozone-containing water, washing is performed by introducing bubbles into the raw water side. The washing operation is performed by interrupting the filtration.

The concentration of ozone contained in the backwash water is 0.1.
It is preferably from 05 mg / liter to 50 mg / liter. More preferably, it is 0.1 mg / liter or more and 10 mg / liter or less. If the ozone concentration is too low, the oxidation by ozone does not proceed sufficiently, so that a sufficient cleaning effect cannot be obtained. Further, if the ozone concentration is excessively high, the cost associated with the generation of ozone is increased and is not realistic. Ozone added to the backwash water may be ozone alone or ozonized air. The introduction of ozone into the backwash water may be performed via an air diffuser or the like provided at an appropriate position in the backwash tank. Alternatively, a U-tube type can be used.

Further, as another configuration for adding ozone,
Ozone may be added by an ejector method or a line mixing method in the middle of a pipe for guiding backwash water to the porous membrane. In addition, as a method for generating ozone, a raw material for generating ozone in the case of discharge may be air or oxygen. Further, ozone generated by electrolysis of water may be used.

In the introduction of bubbles, gas is sent to the membrane surface on the raw water side, and the membrane is washed by vibrating the membrane surface. In the present invention, since the organic matter adsorbed on the membrane surface is decomposed or denatured by ozone contained in the backwash water to become a non-adsorbable substance, the non-adsorbable substance (organic substance, inorganic substance) that closes the pores of the membrane is removed. In addition, the sieve is effectively removed by the introduction of air bubbles, and a large cleaning effect is obtained. The introduction amount of the bubbles is preferably 0.5 to 20 times the filtration flow rate per unit time, and more preferably 1 to 10 times the flow rate.

According to the present invention, when introducing bubbles, the washing effect is high if the washing is always performed simultaneously with the backwashing. However, only the backwashing may be performed prior to the introduction of the bubbles (simultaneously, the backwashing). Alternatively, only backflow cleaning may be performed after introducing bubbles (simultaneously backflow cleaning). In addition, bubbles may be introduced while simultaneously introducing raw water, and backwashing may be performed at the same time.
It may be performed without introducing raw water. Alternatively, these may be alternately combined. The time for introducing bubbles simultaneously with the backwashing may be appropriately determined in consideration of the recovery of the filtration flow rate and the recovery rate of the filtered water. Since the present invention is configured as described above,
With high membrane filtration flux, high quality treated water is obtained.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to embodiments.

[0019]

Example 1 Raw water 1 has a turbidity of 1 to 5 degrees and a water temperature of 1
River surface water at 2 ° C. was used. As shown in FIG.
Is pumped to the membrane module 4 by the raw water supply pump 3 via the circulation tank 2, and the obtained filtrate is stored in the filtrate tank 5. At the time of backwashing, the filtered water in the drainage tank 5 is sent to the ejector 8 by the backwashing pump 6, where it contacts the ozone generated by the ozonizer 9 and is sent to the membrane module 4.

The air bubbling is performed by supplying the air generated by the compressor 7 to the raw water side (primary side) of the membrane module 4. The membrane module 4 is disclosed in
The inner diameter produced based on -215535 is 0.7
External pressure type in which a hollow fiber microfiltration (MF) membrane made of PVDF (polyvinylidene fluoride) having an outer diameter of 1.25 mmφ and an average pore diameter of 0.1 μm is housed in a 1 m long, 3 inch diameter PVC (polyvinyl chloride) casing. Module. The membrane area of the module is 7.0 m ² , and the clarified water filtration flux at a module filtration pressure of 50 kPa is 1.8 m ³ per hour.

The filtration was carried out by a constant pressure filtration in which the raw water 1 was supplied to the membrane module 4 at a constant pressure, and the filtration was carried out by a cross flow system in which the ratio of the amount of the membrane filtered water to the circulating water was 1: 1. The operating conditions were as follows: After performing filtration for 20 minutes, the dissolved ozone concentration was 3 mg.
The operation of performing backwashing with ozone-containing filtered water of 20 g / l for 20 seconds is repeated, and backflow washing with ozone-containing filtered water having a dissolved ozone concentration of 3 mg / l and air bubbling with air of 2 Nm ³ per hour are performed simultaneously every hour.
Minutes.

After operating for 10 months under the above operating conditions, the amount of membrane filtered water was 5.2 m ³ / m ² / day. Further, the membrane module after the operation was disassembled, the hollow fiber membrane was washed with a predetermined chemical solution, and the pure water permeability was measured. The pure water permeability was 95% of the water permeability of the unused membrane. Was slightly scratched on the outer surface.

[0023]

Example 2 In Example 1, the operating conditions for membrane filtration were as follows:
After the filtration was performed for 60 minutes, the operation system was changed to an operation method in which backwashing with filtered water having a dissolved ozone concentration of 3 mg / liter and air bubbling using air were simultaneously performed for 2 minutes. After 10 months, the membrane filtration flow rate was 5.0 m ³ / m ² / day. Further, the membrane module after the operation was disassembled, and after the hollow fiber membrane was washed with a predetermined chemical, the pure water permeation amount was measured. The permeation amount was 93% of the water permeation amount of the unused membrane.

[0024]

Example 3 In Example 1, the compressor 7 was connected to an ozone generator using an air source, and the operation of the membrane filtration device was performed under the same conditions as in Example 1. At this time, backwashing with ozone-containing drainage having a dissolved ozone concentration of 3 mg / liter and bubbling with ozone gas on the primary side of the module are performed simultaneously. The ozone gas concentration was 20 g / m ³ . After 10 months, the membrane filtration flow rate was 5.6 m ³ / m ² / day. Further, the membrane module after the operation was disassembled, and after the hollow fiber membrane was washed with a predetermined chemical, the pure water permeation amount was measured. The permeation amount was 98% of the water permeation amount of the unused membrane.

[0025]

Comparative Example 1 The same experiment as in Example 1 was performed except that the ozonizer 9 was removed. After 10 months, the membrane filtration flow rate was 1.8 m ³ / m ² / day. Further, the membrane module after operation was disassembled, and the hollow fiber membrane was subjected to chemical cleaning, and the pure water permeability was measured.
%.

[0026]

[Comparative Example 2] In Example 1, the operation of removing the ozonizer 9, further changing the operating conditions, performing filtration for 20 minutes, and performing backwashing with membrane filtered water for 20 seconds was repeated every hour. Air bubbling with 2 Nm ³ air was performed for 2 minutes. The membrane filtration flow rate after 10 months is 1.
It was 5 m ³ / m ² / day. Further, the membrane module after operation was disassembled, the hollow fiber membrane was subjected to chemical washing, and the pure water permeation amount was measured. The permeation amount was 60% of the water permeation amount of the unused membrane.

[0027]

According to the present invention, cleaning can be performed effectively without damaging the membrane, and as a result, a high membrane filtration flow rate can be maintained over a long period of time.

[Brief description of the drawings]

FIG. 1 shows an example of a processing flow incorporating a method for cleaning a film of the present invention.

Continued on the front page F term (reference) 4D006 GA07 HA19 KA63 KC02 KC03 KC14 KC16 KD21 KE01R KE03P KE05P KE06P KE07Q KE11R KE12P KE16P KE24Q KE28Q MA01 MA22 MA34 MB11 MC28 MC29X MC30 PA01 PB04P

Claims (2)

[Claims] In a method for cleaning a porous membrane, backflow cleaning is performed with pressurized ozone-containing water of 0.05 mg / liter or more in a direction opposite to a filtration direction, and simultaneously, bubbles are introduced into a raw water side of the membrane. A method for cleaning a film, comprising oscillating the film. 2. The method for cleaning a membrane according to claim 1, wherein bubbles introduced into the raw water side of the membrane are ozone gas.