JP2000288578A - Method and apparatus for treating water containing trace of organic matter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To ensure stable and efficient treatment over a long period of time by holding the concn. of dissolved oxygen of a biological treatment tank to a specific value in a method for the biological treatment by oligotrophic bacteria and membrane separation treatment of water containing TOC in specific concn. and a trace of org. matter. SOLUTION: Water containing 5 ppb-50 ppm of TOC and a very small amt. of org. matter is introduced into a biological reaction tank 2 by a pump P1 through a raw water tank 1 and biologically treated with oligotrophic bacteria to decompose and remove the org. matter. A part of this biologically treated water is returned to the raw water tank 1 as circulating water and the remainder thereof is filtered by a strainer 4 through a biologically treated water tank 3 and subjected to membrane separation treatment in a UF membrane treatment device 5 to separate bacterial cells and membrane permeated water is taken out of the system as treated water and membrane conc. water is circulated to the biologically treated water tank 3. Herein, the concn. of dissolved oxygen is controlled so that the concn. of dissolved oxygen in the biological reaction tank 2 becomes 2 ppm or more.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD [0001] The present invention relates to TOC5ppb ~
Water containing a trace amount of organic matter such as 50 ppm
The present invention relates to a method and an apparatus for performing a biological treatment with an oligotrophic bacterium and a membrane separation treatment.

[0002]

_2. Description of the Related Art In semiconductor and liquid crystal manufacturing factories, the specific resistance of theoretical pure water (water consisting only of H ₂ O) is 18.24 MΩ.
Ultrapure water having a specific resistance of 17-18 MΩ · cm, which is very close to cm, is used in large quantities as cleaning water.

Conventionally, washing wastewater used for this washing is:
After being collected and treated, it is further treated together with industrial water, well water, etc., and reused in the production of ultrapure water. As a treatment technique for the washing wastewater (hereinafter sometimes referred to as “recovered water”), TOC contained in the recovered water at a low concentration is removed by biological treatment with oligotrophic bacteria, which are oligotrophic bacteria. (Japanese Patent No. 21303)
No. 84, Patent No. 2887284). That is, the recovered water is water containing a trace amount of organic matter having an organic matter content of 5 ppm or less in TOC, and such trace amount of water containing organic matter cannot be treated by a general activated sludge method.
However, by using a microorganism that grows even under oligotrophic conditions, such as an oligotrophic bacterium, it is possible to treat a trace amount of organic substance-containing water to obtain treated water having a significantly reduced TOC.

[0004] In a treatment method in which biological treatment and membrane separation treatment are combined, the concentration of residual chlorine in the biological treatment water is set to 0.1.
A method of improving the membrane filtration efficiency of a subsequent membrane separation device by adding a chlorine agent so as to be 3 to 1.5 ppm has been proposed (JP-A-5-329777). Also, a method has been proposed in which the concentration of dissolved oxygen in the biological treatment tank is maintained at 1.0 mg / L or more to maintain the permeation flux of the subsequent membrane separation device high (Japanese Patent Application Laid-Open No. 4-330993).
No.).

[0005] Further, there has been proposed a fluidized-bed biological reaction tank having a carrier holding microorganisms, which is provided with a hydrogen peroxide supply means in order to replenish the microorganisms carried on the carrier with oxygen. (JP-A-9-271793).

[0006]

According to the study of the present inventors, when the recovered water is biologically treated with oligotrophic bacteria, and the biologically treated water is subjected to membrane separation, the permeation flux of the membrane decreases with time. There was found.

As described above, in a treatment combining biological treatment and membrane separation treatment, a method of adding a chlorine agent and a method of maintaining dissolved oxygen have been proposed in order to improve the treatment efficiency in a membrane separation device. I have. Of these, Japanese Unexamined Patent Publication No.
The method of adding a chlorinating agent described in JP-A No. 77 is to remove bacteria and their metabolites attached to the membrane by chlorine to improve membrane filtration efficiency, but this method removes free chlorine. There is a problem that the membrane is deteriorated by the oxidizing power, and there is a disadvantage that a large amount of chlorine remains in the membrane treatment water because chlorine is added at a high concentration.

Further, Japanese Patent Application Laid-Open No. Hei 4-330993 discloses that maintaining the dissolved oxygen concentration in a biological treatment tank at 1.0 ppm or more improves the filterability of the membrane when membrane-treating biologically treated water. It has been clarified that this method is applied to general wastewater having a BOD of about 1000 ppm, and the microflora used in this method, that is, inhabiting water having a BOD of about 1000 ppm to remove organic matter. Activated sludge flora assimilated and TOC concentration 10ppm
It is completely different from the bacterial flora of oligotrophic bacteria inhabiting water of a degree, and the concentration of cells contained in biologically treated water also differs in the order of two to three. There are differences in structure and driving method. Therefore, there has been a demand for a method of improving the membrane filtration property of treated water after biological treatment of trace organic substance-containing water in consideration of the difference between the two.

Further, the method described in Japanese Patent Application Laid-Open No. 9-271793 only supplies oxygen necessary for decomposing organic substances by adding hydrogen peroxide. Is consumed in the decomposition of organic matter,
The dissolved oxygen concentration of the biological treatment water may not be maintained at 2 ppm or more, and in such a case, the membrane filtration property is deteriorated.

The present invention has been made in view of the above-mentioned conventional circumstances, and treats water containing a trace amount of organic matter, such as 5 ppb to 50 ppm of TOC, by biological treatment with oligotrophic bacteria and membrane separation treatment. In view of the above, an object of the present invention is to provide a method and an apparatus for performing a stable and efficient treatment for a long period of time by preventing a decrease in permeation flux of a membrane.

[0011]

According to the present invention, there is provided a method for treating trace organic matter-containing water, which comprises treating trace organic matter-containing water containing TOC at a concentration of 5 ppb to 50 ppm by biological treatment with oligotrophic bacteria and membrane separation treatment. The method is characterized in that the concentration of dissolved oxygen in the biological treatment tank is maintained at 2 ppm or more.

[0012] The apparatus for treating trace organic matter-containing water of the present invention comprises:
A biological treatment tank for introducing water containing trace amounts of organic matter containing TOC at a concentration of 5 ppb to 50 ppm and decomposing contaminants in the water by oligotrophic bacteria, and a membrane separation device for membrane-separating the treated water in the biological treatment tank. Wherein the biological treatment tank is maintained at a dissolved oxygen concentration of 2 ppm or more.

That is, the present inventors have proposed that TOC5ppb-5
When water containing a trace amount of organic matter such as 0 ppm is treated by the biological treatment with oligotrophic bacteria and the membrane separation treatment, as a result of intensive studies on the cause of a decrease in the permeation flow rate of the membrane, the following results are obtained. Knowledge was obtained.

[0014] The permeation flux of the membrane in the biological treatment and the membrane separation treatment depends on the activity state of the microorganisms in the biological treatment tank, and the dissolved oxygen concentration in the biological treatment tank is less than 2 ppm. As time elapses, the permeation flux of the membrane for separating cells decreases. This is because in the operation of the biological treatment tank where the dissolved oxygen concentration is less than 2 ppm, the biological phase or the biological metabolite easily adheres to the membrane and changes to an unsuitable one for the membrane separation treatment.

Therefore, in the present invention, by maintaining the dissolved oxygen concentration in the biological treatment tank at 2 ppm or more,
Suppresses the generation of substances that easily adhere to the membrane, which causes a decrease in the permeation flux of the membrane, prevents the permeation flux of the membrane from lowering, and enables stable and efficient processing over a long period of time. And
Moreover, according to this method, the risk of causing deterioration of the film is small as compared with the method described in Japanese Patent Application Laid-Open No. 5-329777, which uses a chlorine agent at a high concentration.

Conventionally, in the operation of a biological treatment tank using an oligotrophic bacterium such as an oligotrophic bacterium, the dissolved oxygen concentration is not controlled, and the dissolved oxygen concentration in a normal state is about 1 ppm. Was. This is considered to be sufficient for decomposing TOC contained at a low concentration if the dissolved oxygen concentration in the biological treatment tank is about 1 ppm. Even if the dissolved oxygen concentration is maintained at a higher level, an improvement in the quality of treated water can be expected. This is because it was considered that increasing the dissolved oxygen concentration further than this was useless.

In the present invention, biological treatment with oligotrophic bacteria is difficult even if the TOC of the trace organic substance-containing water as raw water is less than 5 ppb or more than 50 ppm.

By the way, usually, in a biological treatment tank using oligotrophic bacteria such as oligotrophic bacteria, a fluidized bed biological treatment tank using a carrier is used. In this type of biological treatment tank, bacteria (carriers) flow out when aeration is performed from below. To increase the dissolved oxygen concentration by blowing oxygen, the biological treatment water is circulated and the water is circulated through the circulation system. Oxygen is supplied by blowing oxygen. However, it is difficult to increase the concentration of dissolved oxygen in this method, so it is preferable to maintain the concentration of dissolved oxygen at 2 ppm or more by adding hydrogen peroxide according to claims 2 and 4.

[0019]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a system diagram showing an embodiment of the present invention.

In FIG. 1, reference numeral 1 denotes a raw water tank, and 2 denotes a biological reaction tank. Inside the tank, a fluidized bed 2B made of a carrier such as granular or pelletized activated carbon carrying microorganisms on a support bed 2A.
Are formed. Reference numeral 3 is a biological treatment water tank, 4 is a strainer, 5 is a UF membrane separation device, and P ₁ and P ₂ are pumps.

Raw water (TOC of recovered water etc. is 5 ppb to 5 ppb)
Trace organic substance-containing water) containing at 0ppm is introduced into the bioreactor 2 by pump P ₁ through the raw water tank 1 is biologically treated by oligo Toro Fick bacteria, organic material contained is decomposed and removed. A part of this biologically treated water is returned to the raw water tank 1 as circulating water, and
After being filtered by the strainer 4 by the pump P ₂
The cells are separated by the membrane separation treatment in the UF membrane separation device 5, and the permeated water is taken out of the system as treated water. The membrane concentrated water is circulated to the biological treatment water tank 3. Part of this circulating water is discharged out of the system as wastewater.

In the present invention, in such treatment, the dissolved oxygen concentration is controlled so that the dissolved oxygen concentration in the biological reaction tank 2 becomes 2 ppm or more. When the dissolved oxygen concentration is lower than 2 ppm, and particularly when the dissolved oxygen concentration is lower than 2 ppm for more than 12 hours, viscous bacterial metabolites and the like unsuitable for membrane treatment are generated, and the permeation flux of the membrane is remarkably increased. Will decrease.

The TOC concentration of the water to be treated and the dissolved amount of oxygen are adjusted so that the dissolved oxygen concentration in the biological reaction tank 2 becomes 2 ppm or more. An excessively high dissolved oxygen concentration is not preferable because the cost is increased and, when degassing is performed in a subsequent step, the load increases. The dissolved oxygen concentration in the biological reaction tank 2 is particularly preferably maintained in the range of 2 to 8 ppm, and particularly preferably in the range of 2.0 to 3.5 ppm.

In general, oxygen aeration can be considered to increase the dissolved oxygen concentration. However, it is not preferable when the aeration may cause bacterial cells (carriers) to flow out.

Therefore, in the present invention, it is preferable to inject hydrogen peroxide into the biological reaction tank 2 to increase the concentration of dissolved oxygen by oxygen generated by the decomposition of hydrogen peroxide.

In this case, the concentration of hydrogen peroxide added is 1-2.
The content is preferably 0 ppm, more preferably 4 to 10 ppm. In other words, hydrogen peroxide is usually used for sterilization, but the cells are less than 10% against sudden contamination of hydrogen peroxide.
In the case of constant contamination, antibodies are formed and can withstand up to several tens of ppm. In particular, when activated carbon is used as a microorganism-carrying carrier, hydrogen peroxide is easily decomposed, so that the concentration of dissolved oxygen can be increased efficiently without a bactericidal action at about 20 ppm.

Although the means for injecting hydrogen peroxide is not shown in FIG. 1, the hydrogen peroxide may be directly mixed into the biological reaction tank 2, or a system for introducing recovered water into the raw water tank 1. A system for introducing raw water from the raw water tank 1 to the biological reaction tank 2;
It may be injected into any of the systems for returning the circulating water from the biological reaction tank 2 to the raw water tank 1. The means for injecting hydrogen peroxide may be automatically controlled and injected in conjunction with a dissolved oxygen meter provided in the biological reaction tank 2 or the biologically treated water tank. The injection of hydrogen peroxide may be continuous injection or intermittent injection.

In the present invention, there is no particular limitation on the type of the membrane separation device, the type of the membrane, and the like.
(Microfiltration) membrane, UF (ultrafiltration) membrane, RO (reverse osmosis)
Any of membranes and the like may be used, but those suitable for treating biologically treated water by oligotrophic bacteria,
For example, a corrugated spiral UF film or the like is used. Also,
There is no particular limitation on the material of the film, and in particular, in the present invention, a hydrophilic organic film such as cellulose acetate, which is weak against chlorine, can be used.

The pH of the biologically treated water used for this membrane separation treatment
Is not particularly limited, but preferably around neutral.

The method and apparatus of the present invention are particularly effectively applied to a process for treating recovered water incorporated in a system for producing ultrapure water.

[0032]

The present invention will be described more specifically below with reference to examples and comparative examples.

Example 1 Using the processing apparatus shown in FIG. 1, organic substances in trace amounts of organic substance-containing water were removed under the following processing conditions.

[Treatment conditions] Raw water: Semiconductor substrate cleaning wastewater TOC: 1.5 ppm Water temperature: 20 ° C. pH: 8.0 Biological treatment method: Biofilm filtration treatment Reaction time: 20 min Membrane separation treatment method: Type of membrane: corrugated Spiral UF membrane (material: cellulose acetate) Pore size: 0.02 μm Operating pressure: 2 kgf / cm ² By passing water under the above conditions and adding 35% hydrogen peroxide as required, as shown in Table 1. As a result of operating for one month while maintaining the dissolved oxygen concentration of the biological treatment water near the outlet of the biological reaction tank at 2 ppm or more,
As shown in Table 1, the permeation flux of the membrane for one month was about 1.5 (m
/ Day · kgf / cm ² ).

Comparative Example 1 The procedure of Example 1 was repeated except that the operation was performed without adjusting the dissolved oxygen concentration of the biological treatment apparatus. The dissolved oxygen concentration in the biological treatment tank is shown in Table 1. Street 2p
pm, and the permeation flux of the membrane also decreased as shown in Table 1.

[0036]

[Table 1]

Example 2, Comparative Examples 2 and 3 In Example 1, an apparatus for injecting hydrogen peroxide was provided in the biological treatment tank, and a hydrogen peroxide solution having a concentration of 35% was constantly added so as to have a concentration shown in Table 2. The treatment was performed for one month in the same manner except that the hydrogen peroxide solution was not added in Comparative Example 2. As a result, the dissolved oxygen concentration of the biological treatment water near the biological treatment tank outlet and the permeation flux of the membrane are as shown in Table 2,
It was confirmed that by injecting an appropriate amount of hydrogen peroxide, the dissolved oxygen concentration in the biological treatment tank could be maintained at 2 ppm or more, and as a result, the permeation flux of the membrane could be kept high for a long period of time.

[0038]

[Table 2]

[0039]

As described in detail above, according to the method and apparatus for treating water containing trace organic substances according to the present invention, TOC 5pp
In treating water containing a trace amount of organic substances such as b to 50 ppm by biological treatment with oligotrophic bacteria and membrane separation treatment, the membrane can be treated without deterioration and complicated post-treatment. A stable and efficient treatment can be performed over a long period of time by preventing a decrease in the permeation flux.

[Brief description of the drawings]

FIG. 1 is a system diagram showing an embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 1 raw water tank 2 biological reaction tank 3 biological treatment water tank 4 strainer 5 UF membrane separation device

Claims (4)

[Claims] 1. A method for treating water containing trace amounts of organic substances containing TOC at a concentration of 5 ppb to 50 ppm by biological treatment with oligotrophic bacteria and membrane separation treatment, wherein the dissolved oxygen concentration in the biological treatment tank is 2 ppm or more. A method for treating trace organic matter-containing water, characterized by maintaining. 2. The method according to claim 1, wherein hydrogen peroxide is added to the biological treatment tank to maintain the dissolved oxygen concentration at 2 ppm or more. 3. A biological treatment tank into which a trace amount of organic substance-containing water containing TOC at a concentration of 5 ppb to 50 ppm is introduced, and a contaminant in the water is decomposed by oligotrophic bacteria; An apparatus for treating trace organic matter-containing water, comprising: a membrane separation device that performs the treatment. The biological treatment tank is maintained at a dissolved oxygen concentration of 2 ppm or more. 4. An apparatus for treating water containing a trace amount of organic matter according to claim 3, further comprising means for supplying hydrogen peroxide to said biological treatment tank.