JP2001070763A - Membrane cleaning method - Google Patents

Abstract

(57) [Summary] [PROBLEMS] The present invention relates to river water, lake water, groundwater, storage water,
It is an object of the present invention to effectively perform washing and maintain a high filtration flow rate when a secondary treatment water, factory wastewater, sewage, or the like is used as raw water and filtered through a porous membrane. SOLUTION: In the method for cleaning a porous membrane, backwashing is performed with water containing an acid in the direction opposite to the filtration direction, and then backwashing with water containing an oxidizing agent. Alternatively, in the opposite direction to the filtration direction, backwashing is performed with water containing an oxidizing agent, followed by backwashing with water containing an acid. Further, at the same time as the backwashing, bubbles are introduced into the raw water to oscillate the membrane.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for filtering a water supply system, a water source thereof, a water source for industrial water, a filtration process for sewage secondary treatment water, and a filtration process for sewage and drainage water. It is.

[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, small installation space, and easy operation management. ing. However, with the continuation of filtration, substances to be removed such as organic substances in raw water adhere to the membrane surface,
Since the pores on the surface are closed, the filtration performance gradually decreases, and 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 its effect, or a backwash method using ozone water as disclosed in Japanese Patent Application Laid-Open No. 4-310220 is disclosed. 60-582
A method of backwashing with ozonized pressurized air disclosed in Japanese Patent Publication No. 22 is known. Furthermore, a method of introducing air into the raw water side as air bubbles and a method of injecting ozonized air as air bubbles into the raw water side of the hollow fiber membrane as disclosed in JP-A-63-42703 are known. I have.

[0004]

Generally, substances that clog the membrane when natural water or waste water is subjected to membrane filtration are composed of organic substances and inorganic substances. Backwashing with conventionally known sodium hypochlorite added water or ozone water removes clogging by oxidizing organic substances, that is, it has a cleaning effect on organic substances, but removes inorganic substances that cannot be dissolved. Thus, there is no washing effect, so that there is a problem that the amount of filtered water gradually decreases when used for a long time.

On the other hand, the method of introducing air bubbles into the raw water side (air bubbling) is based on the fact that water and films in the module are oscillated by the introduced air bubbles, and substances adhering to the film surface are scraped by the films coming into contact with each other. Since the cleaning is performed by being dropped, the cleaning effect is high. However, on the contrary, the adhered substance is pushed into the pores of the membrane by rubbing between the membranes, and the pores are rather closed or the membrane surface is rubbed, so that the pores are crushed, and the filtration performance is reduced. There was a problem.

However, if filtration is continued for a long period of time even after performing these washings, the filtration eventually becomes impossible. Therefore, the membrane is periodically removed, and the washing operation is performed using chemicals for a long time. Needed. During this time, a spare membrane filtration device is required, which is economically disadvantageous. That is, a method for effectively cleaning the porous membrane and maintaining a high filtration flux has not been found yet.

[0007]

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 is characterized in that (1) in the method for cleaning a porous membrane, in the direction opposite to the filtration direction, backwashing with water containing an acid, followed by backwashing with water containing an oxidizing agent. (2) The method for cleaning a porous membrane, wherein the backwashing is performed with water containing an oxidizing agent in the direction opposite to the filtration direction, and then the backwashing is performed with water containing an acid. (3) The membrane cleaning method according to the above (1) or (2), wherein bubbles are introduced into the raw water side of the membrane at the same time as the backwashing to swing the membrane. The present invention relates to the membrane washing method according to any one of (1) to (3), wherein a hollow fiber filtration membrane is used as the porous membrane.

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 contained in the raw water and substances having a size larger than the pore diameter of the membrane to be used are blocked by the membrane, so-called concentration polarization and a cake layer are formed. ,
The particles 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.

[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. In contrast, backwashing with water containing oxidizing agents such as sodium hypochlorite and ozone,
Due to the oxidizing power, organic substances on the surface of the substance attached to the film are oxidatively decomposed or denatured, and are easily separated from the film surface.
However, backwashing only with oxidizing agent-containing water cannot completely remove deposits derived from inorganic substances such as iron, manganese, and calcium, as described above, and cannot maintain high water permeability for a long period of time. .

The present invention is a method for performing backwashing using backwash water to which an acid capable of dissolving inorganic substances has been added, in addition to backwash using water containing an oxidizing agent. This has made it possible to remove deposits derived from inorganic substances.
In addition, when washing is usually performed by introducing air bubbles into the raw water, the membranes are rubbed against each other and the washing effect is high, but at the same time, the membrane surface may be damaged and the permeation flux may be reduced. In such a case, it is preferable to introduce air bubbles into the raw water side of the membrane simultaneously with the backwashing. Since the distance between the membranes is increased by the water flow of the backwash water, it is possible to prevent excessive rubbing, and at the same time, it is easy to discharge adhered substances which are easily clogged between the membranes and fall off from the membrane surface.

As described above, backwashing with an oxidizing agent,
By carrying out backwashing with acid in the direction opposite to the filtration direction, it is possible to effectively remove deposits on the membrane surface, and by introducing bubbles to the raw water side simultaneously with backwashing, the membrane may be damaged. It is possible to effectively clean without causing a problem. This substantially eliminates the need for chemical cleaning, which is conventionally performed by periodically removing the membrane. The direction opposite to the filtration direction is the direction from the filtrate side to the stock solution side of the membrane. Further, the cleaning method of the present invention is preferably applied to cleaning of a membrane for performing external pressure filtration.

Hereinafter, each will be described in detail. As a porous membrane,
The filtration membrane is not particularly limited as long as it is not deteriorated by an acid and an oxidizing agent. For example, polyolefins such as polyethylene, polypropylene and polybutene;
Perfluoroalkyl vinyl ether copolymer (PF
A), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-hexafluoropropylene-perfluoroalkylvinyl ether copolymer (EPE), tetrafluoroethylene-ethylene copolymer (ETFE), polychloro Fluorinated resins such as trifluoroethylene (PCTFE), chlorotrifluoroethylene-ethylene copolymer (ECTFE) and polyvinylidene fluoride (PVDF); polysulfone, polyether sulfone, polyether ketone, polyether ether ketone, polyphenylene sulfide, etc. Super engineering plastics; celluloses such as cellulose acetate and ethyl cellulose; polyacrylonitrile; polyvinyl alcohol alone and mixtures thereof.

Further, when a strong oxidizing agent such as ozone is used together, an inorganic film such as ceramic, a polyvinylidene fluoride (PVDF) film, a polytetrafluoroethylene (PTFE)
Membrane, ethylene-tetrafluoroethylene copolymer (ET
An organic film such as a fluororesin film such as a FE) film and a polyfluoroacrylate (PFA) film can be used. It is particularly preferable to use a polyvinylidene fluoride (PVDF) film. Among such porous membranes, those whose pore size region is from an ultrafiltration (UF) membrane to a microfiltration (MF) membrane can be used, but a microfiltration (MF) membrane having a basically high filtration flow rate can be used. It is preferred to use. 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, a wavy hollow fiber shape, a flat film shape, a pleated shape, a spiral shape, and a tubular shape can be used, but the membrane area per unit volume can be used. It is preferably a hollow fiber shape capable of taking a large amount. Generally, filtration is performed using a module containing a membrane. The filtration method may be 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 the case of a hollow fiber membrane, either internal pressure filtration or external pressure filtration may be used, but external pressure filtration, which allows a large filtration area, is preferable.

The type of acid to be contained in the backwash water is iron,
There is no particular limitation as long as inorganic substances such as manganese, calcium or oxides thereof can be dissolved. For example, hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, oxalic acid, citric acid, acetic acid, or a mixture thereof can be used. The concentration of the acid contained in the backwash water is preferably 0.01% or more. Below this, the dissolution of the inorganic substance does not proceed sufficiently, so that a sufficient cleaning effect cannot be obtained.

The acid may be added to the backwashing water in a solid or liquid state directly into a drainage tank or the like, or as an aqueous solution in the middle of a pipe from the drainage tank to the porous membrane in an ejector. Or using a line mixer. The type of the oxidizing agent contained in the backwash water is not particularly limited as long as it can oxidize organic substances. Examples include sodium hypochlorite, ozone, chlorine dioxide, hydrogen peroxide, caustic soda, and mixtures thereof. In the case of sodium hypochlorite, chlorine dioxide, hydrogen peroxide, and caustic soda, the concentration of the oxidizing agent contained in the backwash water is preferably 0.05 mg / liter to 50% by weight, more preferably 0.1 mg / liter. More than 20% by weight
Or less, more preferably 10% by weight or less.
Below this, the dissolution of organic substances does not proceed sufficiently, so that a sufficient cleaning effect cannot be obtained. If the concentration is too high, a sufficient cleaning effect can be obtained, but the cost of chemicals is high and it is not economical.

The backwash time may be appropriately determined in consideration of the recovery of the filtration flow rate and the recovery rate of the filtered water. The flow rate of the backwash water is preferably 0.5 to 5 times the filtration flow rate per unit time, from the viewpoint of a balance between filtration water recovery and washing recovery.
A flow rate of 1 to 3 times is particularly preferred. Regarding the method of adding the oxidizing agent to the backwash water, a solid or liquid state may be directly added to a drainage tank or the like, or as an aqueous solution, an ejector or a line may be provided in the middle of the piping from the drainage tank to the porous membrane. You may add using a mixer etc. When ozone or chlorine dioxide is introduced in a gaseous state, the ozone or chlorine dioxide may be introduced via a diffuser tube provided at an appropriate position in the backwash tank. Alternatively, a U-tube type can be used. Also, as another configuration,
Ozone or chlorine dioxide 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 material for generating ozone in the case of discharge as an ozone generating method may be air or oxygen. Further, ozone generated by electrolysis of water may be used. The order of backwashing with an acid and backwashing with an oxidizing agent may be performed in any order, and can be appropriately selected depending on the quality of raw water. When the substance that clogs the membrane is mainly an inorganic substance, backwashing with an acid is usually performed, and then backwashing with an acid is performed at an appropriate frequency, and then backwashing with an oxidizing agent is performed. When the substance that clogs the film is mainly an organic substance, backwashing with an oxidizing agent is usually performed, backwashing with an oxidizing agent is performed at an appropriate frequency, and then backwashing with an acid may be performed.

In the case of backwashing with an oxidizing agent following backwashing with an acid, or in the case of performing backwashing with an acid following backwashing with an oxidizing agent, there is a possibility that the acid and the oxidizing agent are mixed in the module. is there. If mixing of the acid and the oxidizing agent in the module is not preferable, rinsing may be performed between backwashing. Rinsing may be carried out by backwashing with no added drainage or by introducing raw water. The introduction of bubbles is to clean the membrane by sending gas to the membrane surface on the raw water side and swinging the membrane surface. In the present invention, since the inorganic substance adsorbed on the membrane surface is dissolved or denatured by the acid contained in the backwash water to become a non-adsorbable substance, the substance that has closed the pores of the membrane is effectively removed by introducing bubbles. It is sieved and a great cleaning effect is obtained.

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 bubbles (simultaneously, 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 type of air bubble is not particularly limited as long as it causes the film to oscillate, and ozone gas or the like may be used in addition to air. 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. Since the present invention is configured as described above, it is possible to maintain a high membrane filtration flux over a long period of time.

[0022]

Embodiments of the present invention will be described below.

[0023]

Example 1 As raw water 1, river surface water having a turbidity of 5 to 20 ° C. and a water temperature of 20 ° C. was used. As shown in FIG. 1, raw water 1 is pumped through a circulation tank 2 by a raw water supply pump 3 to a membrane module 4, and the obtained filtered water is stored in a 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 to perform backwashing. Here, the acid tank 7 is provided in the middle of the pipe from the backwashing pump 6 to the membrane module 4. Is added to the backwash water by the acid feed pump 8. The oxidizing agent in the oxidizing agent tank 9 is added to the backwash water by the oxidizing agent feed pump 10. Air bubbling is performed by the compressor 11
The compressed air is supplied to the raw water 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 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 filtration water to the amount of circulation water was 1: 1. The operation conditions were as follows: after performing filtration for 20 minutes, performing backwashing with a 5% nitric acid solution for 15 seconds, and subsequently performing backwashing with a 10 mg / liter aqueous sodium hypochlorite solution for 20 seconds, Air bubbling with air of 2 Nm ³ per hour was performed for 2 minutes every hour.

The amount of membrane filtered water after operating for 6 months under the above operating conditions was 3.0 m ³ / m ² / day. Further, the membrane module after the operation was disassembled, the single yarn was washed with a predetermined chemical solution, and the pure water permeability was measured. The water permeability was 92% of the water permeability of the unused membrane. As a result of observing the film surface, the outer surface of the film was slightly damaged.

[0027]

Comparative Example 1 A membrane filtration operation was performed in the same manner as in Example 1 except that the supply of the acid from the acid supply pump was stopped and the supply of the oxidant from the oxidant supply pump was stopped. Was carried out. After 6 months, the membrane filtration flow rate was 1.2 m ³ / m ² / day. Further, the membrane module after the operation was disassembled, the single yarn was washed with a predetermined chemical solution, and the pure water permeation amount was measured. As a result, the water permeation amount was equivalent to 75% of the water permeation amount of the unused membrane.

[0028]

Example 2 In Example 1, the operating conditions were changed, filtration was performed for 20 minutes, backwashing with a 5% citric acid solution was performed for 15 seconds, and then backwashing with a 5 mg / liter concentration of chlorine dioxide solution. Is repeated for 15 seconds, and at the time of the ^third backwashing with citric acid and the backwashing with chlorine dioxide, air at a rate of 2 Nm ^{3 /} h is introduced simultaneously from the lower part of the module to perform air bubbling.
This operation was repeated for minutes. After 6 months, the membrane filtration flow rate was 3.3 m ³ / m ² / day. Further, the membrane module after the operation was disassembled, the single yarn 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. As a result of observing the film surface, the outer surface of the film was slightly damaged.

[0029]

Example 3 In Example 1, the operating conditions were changed, filtration was performed for 20 minutes, and then backwashing was performed with sodium hypochlorite water having a concentration of 10 mg / liter for 20 seconds, followed by oxalic acid having a concentration of 2%. The operation of performing backwashing with the solution for 20 seconds is repeated, and at the time of the ^third backwashing with sodium hypochlorite and the backwashing with oxalic acid, air at a rate of 2 Nm ^{3 /} h is simultaneously introduced from the lower part of the module to perform air bubbling for 2 minutes. Was repeated. After 6 months, the membrane filtration flow rate was 3.5 m ³ / m ² / day. Also,
After the operation, the membrane module was disassembled, the single yarn was washed with a predetermined chemical solution, and the pure water permeability was measured. The water permeability was 94% of the water permeability of the unused membrane. Was slightly injured.

[0030]

Example 4 In Example 1, the operating conditions were changed and filtration was performed for 20 minutes. Then, ozone generated by an ozone generator using an oxygen source was injected using an ejector, and a dissolved ozone concentration of 1 was obtained. The operation of performing backwashing with 0.5 mg / liter ozone-containing water for 20 seconds, and then performing backwashing with a mixed solution of a 2% citric acid solution and a 2% nitric acid solution for 20 seconds is repeated every hour. Then, air bubbling with air of 2 Nm ^{3 /} h was performed for 2 minutes. The membrane filtration flow rate after 6 months was 4.0 m ³ / m ² / day. Further, the membrane module after operation was disassembled, the single yarn was washed with a predetermined chemical solution, and the pure water permeation amount was measured. The permeation amount was 96% of the water permeation amount of an unused membrane. The outer surface was slightly scratched.

[0031]

According to the present invention, washing can be performed effectively, 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.

Continued on front page F-term (reference) 4D006 GA07 HA19 KA43 KA63 KC02 KC03 KC14 KC16 KD01 KD11 KD12 KD14 KD15 KD16 KD21 KD22 KD24 KD30 KE01R KE03P KE05P KE06P KE07Q KE11R 29 MC22 MC23 MC33 MC39 MC47 MC61 MC62 MC63 PA01 PB04 PB05 PB08

Claims (3)

[Claims] 1. A method for cleaning a porous membrane, comprising: backwashing with water containing an acid in a direction opposite to a filtration direction, followed by backwashing with water containing an oxidizing agent. Method. 2. A method for cleaning a porous membrane, comprising: backwashing with water containing an oxidizing agent in a direction opposite to a filtration direction, followed by backwashing with water containing an acid. Method. 3. The method for cleaning a membrane according to claim 1, wherein bubbles are introduced into the raw water side of the membrane at the same time as the backwashing to swing the membrane.