JP2006055680A - Operation method for water cleaning system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an operation method for a water cleaning system capable of performing stable filtering operation over a long period of time. <P>SOLUTION: In the operation method for the water cleaning system including a hollow fiber membrane module, filtering operation is performed by crossflow filtration, and back pressure washing is performed at a predetermined interval during the filtering operation. The back pressure washing is performed by combining high frequency back pressure washing using sodium hypochlorite of a low concentration with low frequency back pressure washing using sodium hypochlorite of a high concentration. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an operation method of a water purification system using a hollow fiber membrane module.

  As a water purification system to which membrane separation technology is applied, cross flow filtration using a hollow fiber membrane module is performed. Cross flow filtration means that raw water is supplied to one of the membrane surfaces of the hollow fiber membrane (the separation membrane surface on the raw water supply side), and the permeated water that has permeated through the hollow fiber membrane is converted to the other membrane surface (on the permeate water side). When collecting from the separation membrane surface), by filtering the raw water flowing parallel to the separation membrane surface on the raw water supply side, turbid substances contained in the raw water adhering to the hollow fiber membrane surface are removed from the membrane surface. A filtration method having an effect of peeling off.

  In such a water purification system, cellulose acetate membranes and the like are frequently used as separation membranes because the binding of turbid substances contained in river water is relatively weak and recovery of the water permeation rate by backwashing is large.

  However, the river water used as raw water contains bacteria that decompose cellulose acetate and the like together with general bacteria, which adheres to the raw water supply separation membrane surface of the hollow fiber membrane and impairs the quality of the treated water, or the hollow fiber membrane The product itself may undergo degradation due to microbial degradation.

Therefore, in order to remove attached bacteria and prevent clogging caused by turbid substances in the raw water, back pressure washing (hereinafter referred to as “back washing”) is generally performed. Backwashing is a membrane cleaning method that supplies permeated water to the filtration membrane module in the opposite direction to that when permeated water is generated, peels off turbid substances on the surface of the separation membrane, and discharges the turbid substances out of the system. .
JP 2000-117069 A JP 11-179163 A

  To prevent bacterial growth on the membrane surface, there is a method of backwashing with a drug. However, if the drug concentration is increased, the membrane itself may deteriorate and the membrane life may be shortened. When the number of backwashes is increased by decreasing the value, the filtration operation efficiency (permeated water amount per unit time) is lowered. Conventionally, since the frequency of backwashing using a chemical was increased, the continuous operation time was shortened, and the filtration operation efficiency was reduced.

  Further, Patent Document 2 describes a system for injecting a bactericide into a circulation pipe in crossflow filtration, but this method not only makes the system complicated, but also makes it efficient to use suspended substances in the hollow fiber membrane. It cannot be removed well.

  Even when purifying raw water containing bacteria that decompose cellulose acetate and the like, such as river water, the present invention prevents membrane deterioration and performs stable filtration for a long period of time. It is an object of the present invention to provide a method for operating a water purification system capable of performing the above.

The present invention is a method for operating a water purification system including a hollow fiber membrane module, performing a filtration operation by cross-flow filtration as a means for solving the problem, and performing back pressure washing at a predetermined interval during the filtration operation.
Provided is a method for operating a water purification system in which high-frequency backwashing with a low concentration of sodium hypochlorite and low-frequency backwashing with a high concentration of sodium hypochlorite are combined when backpressure cleaning is performed.

  According to the operation method of the present invention, clogging of the filtration membrane module can be prevented by using both low concentration and high frequency backwashing and high concentration and low frequency backwashing, and stable continuous operation can be performed over a long period of time.

  The water purification system used in the operation method of the present invention only needs to use a hollow fiber membrane module. For example, together with the hollow fiber membrane module, a raw water tank, a sedimentation tank, a coagulation tank, a coagulant tank, an activated carbon treatment tower, a reverse What was chosen from a washing tank, a chemical | medical agent tank, a permeated water tank, etc. can be combined suitably, and what connected each component part with the pipe etc. can be used. As such a water purification system, for example, a publicly known system described in FIG. 1 of Japanese Patent Application Laid-Open No. 2002-301339, FIG. 1 of Japanese Patent Application Laid-Open No. 2003-94058, FIG. Mention may be made of water treatment systems.

  As the hollow fiber membrane module included in the water purification system, a known one in which a desired number of hollow fiber membranes (hollow fiber membrane bundles) are filled in a housing (case) having one or more liquid inlets / outlets is used. it can.

  As the hollow fiber membrane, a hollow fiber type ultrafiltration membrane or a microfiltration membrane is used, but the membrane material, the molecular weight cut off, the membrane pore diameter and the like are not particularly limited. As the membrane material, cellulose-based, polyacrylonitrile-based, polysulfone-based, polyolefin-based and the like can be used, but a cellulose-based material which is a hydrophilic material, which is said to be less likely to clog the membrane, is more preferable. Preferred is cellulose acetate. Since the permeation flux can be maintained high, the molecular weight cut off or the membrane pore diameter is preferably 30,000 to 500,000 daltons and a membrane pore diameter of 0.01 to 0.5 μm.

  In the operation method of the present invention, cross flow filtration is applied during the filtration operation. As the cross flow filtration, any of an internal pressure type cross flow filtration that flows raw water inside the hollow fiber membrane and an external pressure type cross flow filtration that flows outside the hollow fiber membrane can be applied. Is preferred.

  Filtration operation conditions (indicated by the amount of permeate and permeate flux) in the operation method of the present invention are appropriately selected according to the type of raw water, the form of the hollow fiber membrane module membrane, the required filtration performance, etc. It is.

  In the operation method of the present invention, when back-pressure washing is performed at predetermined intervals during the filtration operation, high-frequency backwash (low concentration high-frequency backwash) with low concentration sodium hypochlorite and high concentration hypochlorous acid. Combined with low-frequency backwashing with high acid sodium (high concentration low-frequency backwashing).

  The low concentration and high frequency backwashing is performed at a rate of once every 10 to 90 minutes, preferably 20 to 80 minutes, more preferably 30 to 70 minutes.

  Low concentration high frequency backwash uses 0.5 to 8.0 mg / liter, preferably 1.0 to 5.0 mg / liter, more preferably 2.0 to 5.0 mg / liter sodium hypochlorite. 0.5 to 3 minutes, preferably 1 to 3 minutes, more preferably 1 to 2 minutes.

  The high-concentration low-frequency backwashing is performed at a rate of once a day to 1 month, preferably 3 days to 20 days, more preferably 5 days to 14 days.

  High concentration low frequency backwash uses 15-100 mg / liter, preferably 20-80 mg / liter, more preferably 20-60 mg / liter sodium hypochlorite for 1-15 minutes, preferably 2-12 minutes. More preferably, it is performed for 3 to 10 minutes.

  Examples of the water used as the backwash water include membrane permeated water, tap water, ion-exchanged water, and ultrapure water. However, using membrane permeated water is preferable because the equipment is simple and the cost is low.

  In low-concentration high-frequency backwashing, the backwash water flux is preferably higher than the permeation flux, and in high-concentration low-frequency backwashing, the backwash water flux may be lower than the permeation flux. preferable.

  Further, a drug other than sodium hypochlorite can be used in combination, but the sodium hypochlorite concentration in the drug is set to 5% by mass or more. Examples of other chemicals include oxidizing agents such as peracetic acid and hydrogen peroxide, acids such as acetic acid, and stabilizers such as sodium pyrophosphate, phosphoric acid, citric acid, and oxalic acid.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to these.

Example 1
Using a water purification system including a hollow fiber membrane module, permeate generation by cross-flow filtration of river water, and low concentration high frequency backwash and high concentration low frequency backwash to prevent clogging of the hollow fiber membrane module Carried out.

As a hollow fiber membrane module, a cellulose acetate membrane (MOLSEP F50, manufactured by Daicel Chemical Industries, Ltd.) having an effective membrane area of 50 m ² was used. The filtration operation was performed at a permeated water amount of 6 m ³ / hr. Backwashing is performed once every 60 minutes with 3.0 mg / liter sodium hypochlorite for 60 seconds (low concentration high frequency backwashing) once every two weeks and 50 mg / liter of hypochlorous acid. Back washing with sodium chlorate was performed for 5 minutes (high concentration low frequency back washing).

  After 5 months of filtration operation under these conditions, the hollow fiber membrane was taken out and the membrane surface was observed. No change was observed in the membrane surface (no difference compared to an unused membrane). The amount of permeated water later was almost the same as the amount of permeated water at the beginning of operation.

Comparative Example 1
The water purification system was operated under the same operating conditions as in Example 1 except that high concentration and low frequency backwashing was not performed. When operated under these conditions, the hollow fiber membrane was clogged in about 3 months, and the amount of permeated water decreased.

Claims (4)

It is an operation method of a water purification system including a hollow fiber membrane module that performs a filtration operation by cross-flow filtration and performs back pressure washing at predetermined intervals during the filtration operation.
A method for operating a water purification system in which high-pressure back-pressure washing with a low concentration of sodium hypochlorite and low-frequency back-pressure washing with a high concentration of sodium hypochlorite are combined when performing back-pressure washing.   The operation method of the water purification system according to claim 1, wherein the high frequency back pressure cleaning is performed once every 10 to 90 minutes, and the low frequency back pressure cleaning is performed once a day to once a month.   High frequency back pressure wash with 0.5 to 8.0 mg / liter sodium hypochlorite for 0.5 to 3 minutes, low frequency back pressure wash with 15 to 100 mg / liter sodium hypochlorite 1 The operation method of the water purification system according to claim 1 or 2, wherein the operation time is -15 minutes. The operation method of the water purification system according to any one of claims 1 to 3, wherein the membrane material used in the hollow fiber membrane module is cellulose acetate.