JP2001079366A - Method for washing membrane - Google Patents

Abstract

PROBLEM TO BE SOLVED: To surely prevent the damage of a membrane and to maintain high membrane filtration flow rate over a long period of time by backwashing the membrane while rocking the membrane by introducing gas to the raw water side of the membrane and similarly forcibly feeding the liquid containing sodium hypochlorite and the like or gas from the filtered liquid side. 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 backwashing of the membrane module 4, the filtered water in the filtered water tank 5 is fed forcibly to the membrane module 4 by a backwashing pump 6. At this time, the sodium hypochlorite aquatic solution stored in an oxidizing agent tank 8 is poured into the filtered water side of the membrane module 4 by an oxidizing agent feed pump 9. The air generated at a compressor 7 is fed simultaneously 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 surely decomposed oxidatively and also peeled.

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 a filtration treatment of wastewater, in which high cleaning recovery is obtained without causing damage to the membrane, and as a result, a high filtration flux can be maintained.

[0002]

2. Description of the Related Art Filtration membranes used for filtration of various raw waters are used in various filtration devices because of their excellent filtration accuracy. However, with the continuation of the filtration, substances to be removed such as organic substances in the raw water adhere to the membrane surface and block the pores on the surface, so that the filtration performance gradually decreases, and finally the filtration becomes impossible. 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 a direction opposite to the filtration direction of the membrane to remove deposits on the filtration surface of the membrane. Alternatively, sodium hypochlorite is added to the backwash water to further enhance its effect.
As shown in JP-A-310220, a method of backwashing with ozone water and a method of backwashing with ozonized pressurized air disclosed in JP-A-60-58222 are known. . Further, as disclosed in JP-A-63-42703, a method is known in which ozonized air is injected as bubbles into the raw water side of a hollow fiber membrane.

[0003]

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. Minor parts are washed, but are not washed because backwash water hardly flows into the completely closed holes. 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.

On the other hand, in the method of introducing bubbles into the raw water side, the film is swung by the introduced bubbles, and the membranes are brought into contact with each other so that the adhered substances on the surface of the film are scraped off. The effect is greater than backwashing. However, on the contrary, the adhered substance is pushed into the pores due to the rubbing between the membranes, and the pores are rather closed,
Alternatively, as a result of the rubbing of the membrane surface, the pores are crushed, resulting in a problem that the filtration performance is reduced. That is, there has not yet been found a method for effectively washing without damaging the membrane and maintaining a high filtration flux.

[0005]

Means for Solving the Problems As a result of intensive studies on a method for cleaning a film, the present inventors have completed the following invention. That is, the present invention, in the method for cleaning a porous membrane, while introducing a gas to the raw water side to rock the membrane,
This is a method for cleaning a membrane, comprising performing backflow cleaning from the filtrate side with a liquid or gas containing at least one of pressurized sodium hypochlorite, chlorine dioxide, and hydrogen peroxide.

The details of the present invention will be described below. The raw water that is an object of the present invention is river water, lake water, groundwater, storage water, sewage secondary treatment water, industrial wastewater, sewage, and the like. Conventionally, when raw water as described above is filtered through a membrane, suspended substances and organic substances having a size larger than the pore diameter of the membrane to be used contained in the raw water are blocked by the membrane, and a so-called concentration polarization or cake layer is formed. 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 filtering the raw water is reduced from a fraction to several tenths of that when the clear water is filtered. The bundle gradually decreases.

[0007] Substances that block these films are generally present in raw water in a state where inorganic substances are covered with organic substances.
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. In contrast, when backwashing is performed with a liquid or gas containing at least one of sodium hypochlorite, chlorine dioxide, and hydrogen peroxide, the oxidizing power causes oxidative decomposition or denaturation of organic substances on the surface of the substance attached to the film. And it is easy to peel off from the film surface. However, only backflow cleaning of these liquids and gases, as described above,
The portion where the occlusion is slight is sufficiently washed, but the portion which is completely occluded or where the adhesion layer is thick cannot be sufficiently washed.

Therefore, a gas is introduced into the raw water side to oscillate the membrane and, at the same time, backwashing is performed with a liquid or gas containing at least one of sodium hypochlorite, chlorine dioxide and hydrogen peroxide. The attached substance can be peeled off. Further, when washing is usually performed by introducing bubbles into the raw water side, although the membranes rub against each other and the washing effect is high, the membrane surface may be scratched at the same time, and on the contrary, the permeation flux may decrease. However, when gas is introduced at the same time as backwashing as in the present invention, the distance between the membranes is increased due to the water flow or airflow due to backwashing, and excessive rubbing is prevented. At the same time, the attached substances that are usually clogged between the films and fall off from the film surface are also easily discharged.

As described above, the gas is introduced into the raw water side at the same time as the backwashing with the liquid or gas containing at least one of sodium hypochlorite, chlorine dioxide and hydrogen peroxide, thereby causing damage to the membrane. It can be effectively cleaned without using. Each is described in detail below.

[Porous membrane] As the porous membrane, polyolefins such as polyethylene, polypropylene and polybutene; tetrafluoroethylene-perfluoroalkylvinyl ether copolymer (PFA) and tetrafluoroethylene-hexafluoropropylene copolymer Coalescence (FEP), tetrafluoroethylene-hexafluoropropylene-perfluoroalkylvinylether copolymer (EPE), tetrafluoroethylene-ethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), chlorotrifluoroethylene- Ethylene copolymer (ECTFE), polyvinylidene fluoride (P
VDF), fluororesins such as polytetrafluoroethylene (PTFE); super engineering plastics such as polysulfone, polyethersulfone, polyetherketone, polyetheretherketone, and polyphenylene sulfide; celluloses such as cellulose acetate and ethylcellulose; Acrylonitrile, polyvinyl alcohol alone and mixtures thereof, and inorganic films such as ceramics may be mentioned. In particular, a fluorine-based resinous film and an inorganic film are preferable because of their excellent oxidation resistance, and it is particularly preferable to use a polyvinylidene fluoride (PVDF) film.

Among such porous membranes, those whose pore size region is a reverse osmosis membrane, a nanofilter, an ultrafiltration (UF) membrane, or a microfiltration (MF) membrane can be used. It is preferable to use an ultrafiltration (UF) membrane or a microfiltration (MF) membrane having a flow rate, and particularly preferable to use a microfiltration (MF) membrane. For example, a membrane having an average pore size of 0.001 to 1 μm is preferable, and an average pore size of 0.01 μm.
Films of 5 to 1 μm are more preferred. As the shape of the porous membrane, any shape such as a hollow fiber shape and a flat film shape can be used, but a hollow fiber shape which can provide a large membrane area per unit volume is preferable. Examples of the shape of the hollow fiber membrane include a straight hollow fiber membrane and a wavy hollow fiber membrane, and a wavy hollow fiber membrane is preferable for reasons such as turbidity discharge. Generally, filtration is performed using a module containing a membrane.

[Membrane Module] The membrane module used in the present invention is used by inserting and arranging it in a tank having a tube sheet or an outer housing in addition to a hollow fiber membrane module connected via piping or the like. And a cartridge type membrane module.

Examples of the thermosetting resin used for bonding and fixing both end portions of the membrane module include epoxy resin, urethane resin and silicone rubber. Further, by mixing a filler such as silica, carbon black, or carbon fluoride into these resins, the strength of the resin partition wall may be improved and the curing shrinkage may be reduced. Examples of the material used for the module case of the membrane module include polyolefins such as polyethylene, polypropylene, and polybutene; and polytetrafluoroethylene (PTF).
E), PFA, FEP, EPE, ETFE, PCTF
Fluorine resins such as E, ECTFE and PVDF; chlorine resins such as polyvinyl chloride and polyvinylidene chloride; acrylic resins such as polysulfone resin, polyether sulfone resin, polyallyl sulfone resin, polyphenyl ether resin, PMMA, and acrylonitrile-butadiene. -Styrene copolymer resin (ABS), acrylonitrile-styrene copolymer resin, polyphenylene sulfide resin, polyamide resin, polycarbonate resin, polyetherketone resin, polyetheretherketone resin alone or a mixture thereof, and aluminum, stainless steel Metals such as steel are exemplified.

[Filtering method of porous membrane]
Either a full filtration method or a cross flow filtration method may be used. 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. Either internal pressure filtration or external pressure filtration may be used, but external pressure filtration is preferred because the effect of air scrubbing is greater.

[Method of Cleaning Porous Membrane] According to the present invention, a gas is introduced into the raw water side and, at the same time, a liquid or gas containing at least one of sodium hypochlorite, chlorine dioxide and hydrogen peroxide is used to clean the membrane. This is to wash back from the filtrate side. Backwashing with a mixture of liquid and gas is also included. The washing operation is performed by interrupting the filtration.

The method of adding an oxidizing agent such as sodium hypochlorite, chlorine dioxide, hydrogen peroxide or the like to the backwash water may be directly added to a drainage tank in a gaseous or liquid state,
Alternatively, it may be added as an aqueous solution using an ejector or a line mixer in the middle of the pipe from the drainage tank to the porous membrane. When the gas is introduced in a gaseous state, the gas may be introduced through an air diffuser or the like provided at an appropriate position in the backwash tank. Alternatively, a U-tube type can be used. As another configuration, a gas 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.

The concentration of the oxidizing agent in the backwash water is 0.05
The range is preferably from mg / liter to 50% by weight.
If the concentration is lower than this, oxidation does not proceed sufficiently, so that it is difficult to obtain a sufficient cleaning effect. On the other hand, if the concentration exceeds this range, the cost of the chemical becomes too high. The concentration range is preferably 0.1 mg / liter or more and 20% by weight.
Or less, more preferably 0.1 mg / liter or more and 10
% By weight or less. The respective times of filtration, air scrubbing, and backwashing can be appropriately selected, and may be appropriately determined in consideration of the recovery of the filtration flow rate and the recovery rate of filtered water. Usually, 1/1000 to 1/1/1 of the filtration time between washings.
It is preferable to assign the time of 5 to the time of air scrubbing and backwashing. If the frequency is less than 1 / 10,000, the washing effect is small, and if the frequency is more than 1/5, the recovery rate of drainage becomes poor, which is not preferable.

The introduction of gas is to clean the membrane by sending the gas to the raw water side of the membrane and vibrating the membrane. In the present invention, since the organic substance adsorbed on the membrane surface is decomposed or denatured by the oxidizing agent contained in the backwash water to become a non-adsorbable substance, a non-adsorbable substance (organic substance, inorganic substance) that closes the pores of the membrane. However, they are effectively sieved by the introduction of gas, and a great cleaning effect is obtained. The flow rate of the air supplied per unit time of the air scrubbing is 0.1 to 20 times the filtration flow rate per unit time in the standard state.
Preferably, the flow rate is doubled, more preferably 0.5 to 10 times. Below these flows,
The cleaning effect is low, and if the flow rate is more than these, drying of the film may occur.

The backwash flow rate of gas, liquid, gas and liquid per unit time in backwashing is set at 0.1% of the filtration flow rate per unit time in consideration of the recovery rate of drainage and prevention of membrane rubbing.
A flow rate of 01 to 10 times is preferable, and a flow rate of 0.1 to 3 times is particularly preferable. When the flow rate is lower than the backwash flow rate, the effect of preventing rubbing of the membrane is low, and the cleaning effect is also low. Absent.

According to the present invention, when introducing the gas to the raw water side, 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 gas (simultaneously, the backwashing). Alternatively, after introducing gas (backwashing at the same time), only backwashing may be performed. Further, the gas may be introduced while introducing the raw water at the same time, and the backwash may be performed at the same time, or the cleaning may be performed without introducing the raw water. Or,
These may be combined alternately. Prior to the introduction of gas (simultaneously backwashing), only the raw water may be introduced. Alternatively, only the raw water may be introduced after the introduction of the gas (backwashing at the same time). Since the present invention is configured as described above, high quality treated water can be obtained with a high membrane filtration flux.

[0021]

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

[0022]

Example 1 Raw water 1 has a turbidity of 1 to 5 degrees and a water temperature of 1
River surface water at 2 to 15 ° C was used. As shown in FIG.
The raw water 1 is pumped to the membrane module 4 by the raw water supply pump 3 via the circulation tank 2, and the obtained filtered water is stored in the filtered water tank 5. At the time of backwashing, the filtered water in the drainage tank 5 is sent to the membrane module 4 by the backwashing pump 6, and the aqueous solution of sodium hypochlorite stored in the oxidizing agent tank 8 is oxidized by piping along the way. Injection was performed using a line mixer using the liquid sending pump 9. The air bubbling was 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 Japanese Unexamined Patent Publication No.
A hollow fiber microfiltration (MF) membrane made of PVDF (polyvinylidene fluoride) having an inner diameter of 0.7 mmφ, an outer diameter of 1.25 mmφ, and an average pore diameter of 0.1 μm, which is manufactured based on No. 5, is 1 m long and 3 inches in diameter. External pressure type module housed in PVC (polyvinyl chloride) casing. The module has a membrane area of 7.0 m ² and a module filtration pressure of 50 kPa.
The clarified water filtration flux at this time is 1.8 m ^{3 /} h.

The filtration was 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 membrane filtered water to the amount of circulating water was 1: 1. The operation conditions were as follows: After performing filtration for 20 minutes, repeating the operation of performing backwashing with sodium hypochlorite-containing filtered water having a sodium hypochlorite concentration of 3 mg / liter for 20 seconds,
Backwashing with filtered water containing sodium hypochlorite having a sodium hypochlorite concentration of 3 mg / liter every hour and air scrubbing with air at 2 Nm ^{3 /} hour simultaneously.
Minutes.

After operating under the above operating conditions for one month, the amount of membrane filtered water was 4.5 m ³ / m ² / day. In addition, the membrane module after operation was disassembled, and the single yarn was chemically washed with a mixed solution of sodium hypochlorite and caustic soda, and a mixed solution of oxalic acid and nitric acid. The water permeation amount was 97% of the water permeation amount of the film, and the outer surface of the membrane was observed with a scanning electron microscope at a magnification of 5,000. As a result, the outer surface of the membrane was slightly damaged.

[0026]

Example 2 In Example 1, the operating conditions for membrane filtration were as follows:
After filtration for 60 minutes, sodium hypochlorite concentration 3
The operation method was changed to a method in which backwashing with filtered water containing mg / liter and air scrubbing using air were performed simultaneously for 2 minutes. The membrane filtration flow rate after 12 months is 4.3 m ³ / m ² /
It was a day. In addition, the membrane module after operation was disassembled, and the single yarn was chemically washed with a mixed solution of sodium hypochlorite and caustic soda, and a mixed solution of oxalic acid and nitric acid. Of the membrane was observed with a scanning electron microscope at a magnification of 5,000, and as a result, the outer surface of the membrane was slightly damaged.

[0027]

Example 3 In Example 1, the operating conditions for membrane filtration were as follows:
The same experiment as in Example 1 was performed using filtered water containing 3 mg / liter of chlorine dioxide instead of backwashing with filtered water containing 3 mg / liter of sodium hypochlorite. One month later, the membrane filtration flow rate was 4.5 m ³ / m ² / day. In addition, the membrane module after operation was disassembled, and the single yarn was chemically washed with a mixed solution of sodium hypochlorite and caustic soda, and a mixed solution of oxalic acid and nitric acid. The water permeation amount was 97% of the water permeation amount of the film, and the outer surface of the membrane was observed with a scanning electron microscope at a magnification of 5,000. As a result, the outer surface of the membrane was slightly damaged.

[0028]

Example 4 In Example 1, the operating conditions for membrane filtration were as follows:
The same experiment as in Example 1 was performed using filtered water containing 3 mg / l of hydrogen peroxide instead of backwashing with filtered water containing 3 mg / l of sodium hypochlorite. The membrane filtration flow rate after 12 months was 4.1 m ³ / m ² / day. In addition, the membrane module after operation was disassembled, and the single yarn was chemically washed with a mixed solution of sodium hypochlorite and caustic soda, and a mixed solution of oxalic acid and nitric acid. The water permeation amount was 97% of the water permeation amount of the film, and the outer surface of the membrane was observed with a scanning electron microscope at a magnification of 5,000. As a result, the outer surface of the membrane was slightly damaged.

[0029]

Comparative Example 1 In the same manner as in Example 2 except that backflow washing with filtered water containing sodium hypochlorite and air scrubbing using air were performed simultaneously, scrubbing using only air was performed. An experiment was performed. After 6 months, the membrane filtration flow rate was 2.5 m ³ / m ² / day. In addition, the membrane module after operation was disassembled, and the single yarn was chemically washed with a mixed solution of sodium hypochlorite and caustic soda, and a mixed solution of oxalic acid and nitric acid. Was equivalent to 80% of the amount of water permeated. Observation of the outer surface of the membrane with a scanning electron microscope at a magnification of 5,000 times revealed that the membrane surface was rough, and some of the openings on the membrane surface were closed, which was considered to be a cause of a decrease in the amount of permeated water.

[0030]

Comparative Example 2 In Example 2, instead of simultaneously performing backwashing with filtered water containing sodium hypochlorite and air scrubbing using air, backwashing with filtered water and air scrubbing using air were simultaneously performed. And conducted a similar experiment. After 3 months, the membrane filtration flow rate is 3.5 m ³
/ M ² / day. In addition, the membrane module after operation was disassembled, and the single yarn was chemically washed with a mixed solution of sodium hypochlorite and caustic soda, and a mixed solution of oxalic acid and nitric acid. Was 93% of the amount of water permeated. When the outer surface of the film was observed with a scanning electron microscope at a magnification of 5,000, the outer surface of the film was slightly damaged.

[0031]

According to the present invention, the membrane can be effectively cleaned without damaging the membrane, and as a result, a high membrane filtration flow rate can be maintained for 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.

Claims (1)

[Claims] In a method for cleaning a porous membrane, a gas is introduced into a raw water side to oscillate the membrane, and at the same time, at least one of pressurized sodium hypochlorite, chlorine dioxide, and hydrogen peroxide is supplied from a filtrate side. A method for cleaning a membrane, comprising backwashing with a liquid or gas containing the above.