JP2001070764A - 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 without damaging a membrane and to maintain a high filtration flow rate when filtering a sewage secondary treatment water, factory effluent, sewage, or the like as raw water with a porous membrane. SOLUTION: Using a mixed solution containing 0.05 mg / liter or more of ozone and 0.01% by weight or more of an acid,
This is a method for cleaning a membrane, comprising performing backwashing in a direction opposite to a filtration direction.

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 Membrane filtration methods used for filtration of various raw waters are superior in filtration accuracy, require less installation space, and are easier 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 cleaning method using ozone-containing water as backwash water, ozone decomposes and alters organic substances in the deposits on the film surface.
Although the cleaning effect is higher than backwashing using clarified water or filtered water, the amount of filtered water will increase when used for a long time because deposits derived from inorganic substances such as iron, manganese, calcium, manganese, and silica are deposited on the membrane surface. There was a problem that it gradually decreased.

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. The cleaning effect is high because it is dropped and cleaned. However, on the contrary, the adhering substance is pushed into the pores due to the rubbing between the membranes, and the pores are closed or the membrane surface is rubbed, so that the pores are crushed, and the filtration performance is reduced. was there. 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, the present invention provides (1) a method for washing a porous membrane, in which a mixed solution containing 0.05 mg / liter or more of ozone and 0.01% by weight or more of an acid is used to wash in reverse direction to the filtration direction. (2) A method for cleaning a membrane, characterized in that bubbles are introduced into the raw water side of the module at the same time as the backwashing to oscillate the membrane.

Hereinafter, the present invention will be described specifically.
Raw water that is the subject of the present invention is river water, lake water, groundwater,
Raw water such as water for storage and industrial water, industrial wastewater, and sewage. Conventionally, if 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.

[0008] Substances that block these films generally exist in raw water with organic substances covering the 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, only backwashing with ozone-containing water cannot completely remove deposits derived from inorganic substances such as iron and manganese, as described above, and cannot maintain a high water permeability for a long period of time.

Therefore, by adding an acid capable of dissolving an inorganic substance such as iron or manganese to the backwash water in addition to ozone, it is possible to simultaneously deposit not only the organic substance but also the inorganic substance. It can be removed. In addition, when washing is usually performed by introducing bubbles to the raw water side, although the membranes rub against each other, 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, if bubbles are introduced at the same time as backwashing,
Since the distance between the membranes is increased by the flow of the backwash water, it is preferable because excessive rubbing can be prevented. Further, since the adhered substance on the film surface is easily peeled off by the ozone and the acid, it is also possible to prevent the film surface adhered matter from damaging the film when the films are rubbed. In addition, adhering substances that are easily clogged between the films and fall off from the film surface are also easily discharged.

As described above, the mixed solution of ozone and acid is
By applying pressure from the direction opposite to the filtration direction and permeating the membrane, it is possible to effectively remove deposits on the membrane surface. The term "from the opposite direction to the filtration direction" means the direction from the filtrate side to the raw water side of the membrane. In addition, by introducing bubbles to the raw water side at the same time as the backwashing using ozone and acid, it is possible to effectively wash without causing damage to the film. This method is preferably used as a method for cleaning a membrane used for external pressure filtration.

The porous membrane is not particularly limited as long as it is a filtration membrane that is not deteriorated by ozone. For example, an inorganic membrane such as an ozone-resistant ceramic, a polyvinylidene fluoride (PVDF) membrane, 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 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, a pleated shape, a spiral shape, and a tubular shape can be used, but a hollow fiber shape having a large membrane area per unit volume is preferable. The shape of the hollow fiber membrane may be a waved yarn. 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. Either internal pressure filtration or external pressure filtration may be used. The concentration of ozone contained in the backwash water is preferably 0.05 mg / L or more and 50 mg / L or less. 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. On the other hand, if the concentration of ozone is excessively high, the cost associated with the generation of ozone increases, which is not practical.

The 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. 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.

The source of ozone generation in the case of discharge by ozone generation may be air or oxygen. Further, ozone generated by electrolysis of water may be used. The type of the acid is not particularly limited as long as the inorganic substance can be dissolved. For example, hydrochloric acid, sulfuric acid, nitric acid,
Phosphoric acid, oxalic acid, citric acid, acetic acid and the like. Alternatively, a mixture thereof may be used.

The concentration of the acid is 0.01% by weight or more,
It is preferably 0% by weight or less. More preferably, the content is 0.1% by weight or more and 20% by weight or less. If the acid concentration is too low, the cleaning effect of the acid will decrease. On the other hand, if the concentration of the acid is excessively high, the cost increases, which is not practical. The acid may be added directly to the drainage tank or the like in a solid state, or may be added as an aqueous solution using an ejector or a line mixer in the middle of the piping from the drainage tank to the porous membrane. .

Ozone and acid may be constantly mixed and used as backwash water, or one of them may be intermittently injected. The mixing order of ozone and acid may be such that ozone is added to backwash water and then acid is added, or ozone is added after acid is added. Alternatively, both may be added simultaneously. The backwashing time may be appropriately determined in consideration of the recovery of the filtered water amount and the recovery rate of the filtered water.

The introduction of bubbles is to clean the membrane by sending gas to the membrane surface on the raw water side and oscillating the membrane surface. In the present invention, since the organic and inorganic substances adsorbed on the membrane surface become non-adsorbable substances due to ozone and acid contained in the backwash water, the non-adsorbable substances (organic and inorganic substances) that block the pores of the membrane are: The bubbles are effectively eliminated 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, the introduction of bubbles may be carried out after the backwashing, or the backwashing may be carried out after the introduction of bubbles. You may go. Alternatively, only backflow cleaning may be performed prior to introduction of bubbles (simultaneous backflow cleaning), or only backflow cleaning may be performed after introduction of bubbles (simultaneously backflow cleaning).

Further, the bubble 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. Alternatively, these may be alternately combined. The type of the bubble is not particularly limited as long as it swings the film, and ozone gas or the like can be used in addition to air. 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 As raw water 1, river surface water having a turbidity of 3 to 10 degrees and a water temperature of 10 ° 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 ejector 7 by the backwashing pump 6, where it contacts the ozone generated by the ozonizer 8,
Acid is added from the acid tank 10 by the liquid sending pump 9 and sent to the membrane module 4. Also, air bubbling is
The air generated by the compressor 11 is supplied to the membrane module 4
It is supplied to the raw water side.

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 operating conditions were as follows: After performing filtration for 20 minutes, the dissolved ozone concentration was 3 mg.
The operation of performing backwashing with a mixed solution containing 5% by weight of oxalic acid / liter for 20 seconds is repeated every hour for 2 hours.
Air bubbling with Nm ³ air was performed for 2 minutes.
The membrane filtration water amount after operating for 12 months under the above operating conditions is:
5.5 m ³ / m ² / day. Further, the membrane module after operation was disassembled, the hollow fiber membrane was washed with a predetermined chemical solution, and the pure water permeability was measured. The water permeability was 97% of the water permeability of the unused membrane. Was slightly scratched on the outer surface.

[0025]

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. The membrane filtration flow rate after 12 months is 3.5 m ³ / m ² /
It was a day. Further, the membrane module after operation was disassembled, the hollow fiber membrane was washed with a predetermined chemical solution, and the pure water permeability was measured. As a result, the water permeability was 75% of the water permeability of the unused membrane.

[0026]

Example 2 In Example 1, the operation conditions were changed, filtration was performed for 20 minutes, and then the operation of backwashing with a mixed solution of a dissolved ozone concentration of 3 mg / liter and oxalic acid of 5% by weight for 20 seconds was repeated. Backwashing with a mixed solution of a dissolved ozone concentration of 3 mg / liter and oxalic acid of 5% by weight every hour and air bubbling with air of 2 Nm ³ per hour were performed simultaneously for 2 minutes. The membrane filtration flow after 12 months is 6.0 m
³ / m ² / day. Further, the membrane module after operation was disassembled, the hollow fiber membrane 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 99% of the water permeation amount of the unused membrane.

[0027]

Example 3 In Example 1, the operation of changing the operating conditions, performing filtration for 20 minutes, and performing backwashing with ozone-containing water having a dissolved ozone concentration of 3 mg / liter for 20 seconds was repeated every hour. Backwashing with a mixed solution of a dissolved ozone concentration of 3 mg / liter and oxalic acid of 5% by weight and air bubbling with air of 2 Nm ³ per hour were simultaneously performed for 2 minutes. After 12 months, the membrane filtration flow rate was 5.0 m ³ / m ² / day. Further, the membrane module after operation was disassembled, the hollow fiber membrane was washed with a predetermined chemical solution, and the pure water permeability was measured. As a result, the pure water permeability was 96% of the unused membrane water permeability.

[0028]

Comparative Example 2 In Example 1, the operation conditions were changed and filtration was performed for 20 minutes, and then the operation of performing backwashing with ozone-containing water having a dissolved ozone concentration of 3 mg / liter for 20 seconds was repeated every hour. air bubbling due to air back washing and hour 2 Nm ³ with ozone-containing water of the dissolved ozone concentration 3 mg / liter was carried out at the same time 2 minutes. 1
After 2 months, the membrane filtration flow rate was 4.0 m ³ / m ² / day.
Further, the membrane module after operation was disassembled, the hollow fiber membrane 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 80% of the water permeation amount of the unused membrane.

[0029]

According to the present invention, effective cleaning can be performed 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.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI theme coat ゛ (reference) C11D 7/18 C11D 7/18 7/26 7/26 17/00 17/00 F term (reference) 3B201 AA46 AB53 BB87 BB96 BB98 BC05 CB01 CC21 4D006 GA07 HA18 HA19 KA43 KA63 KC03 KC14 KC16 KD11 KD12 KD14 KD15 KD16 KD21 KE01Q KE05P KE07Q KE11R KE12P KE16P KE24Q KE28Q02 MC03 MC01 MC02 MC03 MC03 MC03 EA31 EB07 EE03 FA15

Claims (2)

[Claims] 1. A method for cleaning a porous membrane, comprising:
A method for cleaning a membrane, comprising performing backwashing in a direction opposite to a filtration direction using a mixed solution containing o / g or more of ozone and 0.01% by weight or more of an acid. 2. The method for cleaning a membrane according to claim 1, wherein bubbles are introduced into the raw water side of the membrane and the membrane is rocked simultaneously with the backflow cleaning.