JPH04330993A - Treatment of waste water - Google Patents

Abstract

PURPOSE:To increase the flux permeated through a membrane and to improve efficiency by keeping the dissolved oxygen concn. in an aeration tank at a specified value in the waste water treatment by a combination of biological treatment and membrane separation. CONSTITUTION:An aerobic treating tank and an anaerobic biological treating tank are jointly used as a biological treating tank 1. In this case, the amt. of air to be injected is adjusted so that the dissolved oxygen concn. (Do) is controlled to >=1.0mg/1 in at least one tank among the aerobic biological treating tanks. Namely, the amt. of air is adjusted so that the Do in the biological treating tank 1 or in a circulating water tank 2 is adjusted to >=1mg/1 about 10min after the finish of the aerobic time zone.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wastewater treatment method, and more particularly to a wastewater treatment method that combines biological treatment and membrane separation treatment and is capable of obtaining a large membrane permeation flux.

0002

[Prior Art] Conventionally, biological treatment equipment for wastewater has used a sedimentation tank as a solid-liquid separation means, but in recent years, biological treatment equipment that uses a membrane separation device as a solid-liquid separation means has been used to increase sludge concentration. It is attracting attention as a compact wastewater treatment device that can be maintained and operated at high loads.

0003

[Problem to be solved by the invention] However, the management indicators for biological treatment using membranes are those of devices that combine conventional sedimentation tanks, and compatibility with membrane separation is not taken into account. Ta. As a result, due to some variation in processing conditions, the filtration performance of the membrane becomes very poor and the permeation flux often becomes very small. This phenomenon cannot be recovered even if the membrane is cleaned or replaced, and countermeasures have been desired.

The present invention solves the above-mentioned conventional problems and provides efficient wastewater treatment by maintaining high membrane filtration performance and permeation flux in a wastewater treatment method that combines biological treatment and membrane separation treatment. The purpose is to provide a method that can be used.

[0005]

[Means for Solving the Problems] The wastewater treatment method of the present invention is a wastewater treatment method that combines biological treatment and membrane separation treatment, in which the dissolved oxygen concentration in the aerobic tank is maintained at 1.0 mg/l or more. Features.

That is, in a wastewater treatment method that combines biological treatment and membrane separation treatment, the cause of the decrease in membrane filtration performance and permeation flux is mainly the dissolved substances in the sludge in the biological treatment tank. Cleaning or replacing the membrane does not provide any improvement. As a result of research focusing on dissolved substances in sludge in an aerobic biological treatment tank, the present inventors found that by maintaining the dissolved oxygen concentration (DO) in an aerobic biological treatment tank at a high concentration, the above-mentioned It was discovered that this phenomenon could be avoided, and the present invention was achieved.

The present invention will be explained in detail below. In the present invention, when an aerobic biological treatment tank and an anaerobic biological treatment tank are used together as biological treatment tanks, at least one of the aerobic biological treatment tanks, which is an aerobic tank, has a DO of 1.0 mg/
Adjust the amount of air blown so that it is at least 1 liter. Also,
When anaerobic and aerobic operations are repeated intermittently in a single tank, the DO should be 1.0 mg/l or more for about 10 minutes before the end of the aerobic period. In this case, even if the DO does not exceed 1 mg/l during the aerobic period, a water tank is provided in the system up to the membrane separator, and that tank is used as an aerobic tank, and the DO is reduced to 1 m
It is sufficient if it is not less than g/l. For example, in a circulation tank, DO
What is necessary is to make it 1 mg/l or more.

The present invention will now be described in more detail with reference to the drawings. FIG. 1, FIG. 2, and FIG. 3 are system diagrams showing a wastewater treatment apparatus suitable for carrying out the present invention. In each figure, 1 is a biological treatment tank, 1A is an anaerobic biological treatment tank, 1B is an aerobic biological treatment tank, 2 is a circulation tank, 3 is a membrane separation device, 3A is a separation membrane, 4 and 4A are pumps, and 5 is a showing the valve; 11;
11A, 12, 13, 14, 15, 15a, 15b, 1
6 and 17 indicate piping.

In the apparatus shown in FIG. 1, raw water is supplied to a biological treatment tank 1 through a pipe 11, and the biological treatment liquid is extracted from a pipe 12, and then passed through a circulation tank 2 and a pipe 13 having a pump 4 to a membrane separation device 3. The concentrated water is returned to the biological treatment apparatus 1 via the pipes 15 and 15a, and to the circulation tank 2 via the pipes 15 and 15b. On the other hand, the permeated water is taken out from the pipe 14 as treated water. The sludge in the circulation tank 2 is transferred to the pipe 16 as necessary.
Extract more. In such a device, when performing intermittent aeration to repeatedly perform anaerobic and aerobic treatment in the biological treatment device 1, as described above, at least 10 minutes before the end of the aerobic period, The amount of air blown is adjusted so that the DO in the biological treatment tank 1 or the circulating water tank 2 becomes 1 mg/l or more.

In the apparatus shown in FIG. 2, raw water is first supplied to the anaerobic biological treatment tank 1A through the pipe 11, and then the raw water is supplied to the anaerobic biological treatment tank 1A through the pipe 11.
1A to the aerobic biological treatment tank 1B, and the treated liquid is supplied to the membrane separation device 3 via piping 13 having a pump 4,
Concentrated water is passed through pipes 15 and 15a to anaerobic biological treatment tank 1A.
In addition, the aerobic biological treatment tank 1 is connected via the pipes 15 and 15b.
Return it to B. On the other hand, the permeated water is taken out from the pipe 14 as treated water. Note that the sludge in the aerobic biological treatment tank 1B is extracted from the pipe 16 as necessary, and a portion is returned to the anaerobic biological treatment tank 1A through the pipe 17 having the pump 4A. In such a device, the DO in the aerobic biological treatment tank 1B
Adjust the amount of air blown so that it is 1 mg/l or more.

In the apparatus shown in FIG. 3, raw water is supplied to the aerobic biological treatment tank 1B through a pipe 11, treated liquid is supplied to the membrane separation device 3 through a pipe 13 having a pump 4, and concentrated water is supplied to the membrane separation device 3 through a pipe 15. It is returned to the aerobic biological treatment tank 1B. On the other hand, the permeated water is taken out from the pipe 14 as treated water. Even in such a device, the amount of air blown is adjusted so that the DO in the aerobic biological treatment tank 1B is 1 mg/l or more.

[0012] In the present invention, as the separation membrane, M
Any of F (microfiltration) membranes, UF (ultrafiltration) membranes, RO (reverse osmosis) membranes, etc. can be used. Also in the embodiment shown in FIG. 3, sludge is extracted from the pipe 16 as needed.

[0013]

[Operation] In the conventional method using a settling tank, BOD decomposition proceeds if the activated sludge is kept aerobically, but dissolved substances constantly flow out of the settling tank and are not concentrated in the reaction tank. However, when combining membranes, even if a membrane with a large pore diameter is used, such as an MF membrane, a gel-like substance will be formed on the membrane surface and high molecular weight dissolved substances will be concentrated.

On the other hand, according to the present invention, by maintaining the DO in the aerobic tank at 1.0 mg/l or more, it is possible to suppress the formation of particularly high molecular weight substances among the dissolved substances generated in the tank. and in combination with membranes,
Accumulation of polymeric substances in the reaction tank can be prevented.

That is, the relationship between dissolved COD and changes in DO concentration in the aerobic tank is as shown in FIGS. is 1
Dissolved COD increases when it decreases below mg/l (5/1
7-6/16), then increasing DO increases dissolved COD
It is clear that the amount decreases (6/16 to 7/16). In addition, changes in the molecular weight distribution of dissolved substances with respect to DO in the biological treatment tank are shown in Figures 6 and 7.
If it is 1mg/l or more (Figure 6), DO0.5mg/l
It is clear that the accumulation of polymer solutes is suppressed compared to the case of about 1 (FIG. 7).

For this reason, according to the method of the present invention, the development of a layer of gel-like substance on the membrane surface is suppressed during membrane separation treatment, and the filtration performance of the membrane and the permeation flux of the membrane are maintained high. can do.

[0017]

[Examples] The present invention will be explained in more detail with reference to Examples and Comparative Examples below. Example 1 Synthetic sewage (peptone, glucose substrate, BOD: 1000 mg
/l, CODMn: 800mg/l). Treatment conditions Aerobic biological treatment tank: Capacity = 250 l Water temperature = 30°C MLSS = 10000 mg/l DO = 1 mg/l or more Circulation tank: Capacity = 100 l Membrane separation device: Effective membrane area = 2 m2 Membrane type = Fraction 5 Ten thousand polysulfone UF membrane water flow rate: 1
m/s Synthetic sewage: Water volume = 1 m3 / day As described above, as a result of operation by adjusting the amount of blown air so that the DO concentration in the biological treatment tank was 1 mg/l or more,
Membrane permeation flux is 1.2m3/m2・day (average operating pressure:
1.2 kgf/cm2), and the soluble CODMn concentration in the biological treatment tank is 4/27 in Figures 4 and 5.
~5/17, it was 100 to 300 mg/l. Comparative Example 1 In Example 1, the DO concentration in the biological treatment tank was 1.0 mg.
When the air volume was adjusted to be less than /l, although the COD and BOD of the membrane permeated water were the same as the results of Example 1, the membrane permeation flux decreased and the water flow rate and water temperature decreased. Under the same conditions, the membrane permeation flux was 0.5 m3/m2·day (average operating pressure 1.2 kg/cm2). In addition,
Regarding the soluble components in the biological treatment tank, GPC (gel permeation chromatography) fractionation (TSK Gel G
-4000 PW), as shown in FIG. 7, an increase in the peaks located at high molecular weights was observed compared to the case of Example 1 (FIG. 6). Furthermore, after the experiment in Comparative Example 1, when the conditions were returned to those in Example 1, the soluble CODMn rapidly decreased (Figs. 4 and 5).
6/16 to 7/16 in the figure). However, since no recovery in membrane permeation flux was observed, chemical cleaning (NaClO: 500
mg/l-asCl2), the membrane permeation flux recovered to 1.2 m3/m2 ·day.

[0018]

[Effects of the Invention] As detailed above, according to the wastewater treatment method of the present invention, in a wastewater treatment method that combines aerobic biological treatment and membrane separation treatment, dissolved organic components, especially polymer components, in the biological treatment tank can be effectively treated. This makes it possible to suppress the generation and accumulation of , as well as to suppress the development of a gel layer on the separation membrane, thereby maintaining high membrane filtration performance and membrane permeation flux. Therefore, the efficiency of wastewater treatment is greatly improved, and it is possible to efficiently obtain treated water of high quality.

[Brief explanation of the drawing]

FIG. 1 is a system diagram showing an embodiment of a wastewater treatment device suitable for carrying out the present invention.

FIG. 2 is a system diagram showing another embodiment of a wastewater treatment apparatus suitable for carrying out the present invention.

FIG. 3 is a system diagram showing another embodiment of a wastewater treatment device suitable for carrying out the present invention.

FIG. 4 is a graph showing changes in DO concentration over time.

FIG. 5 is a graph showing changes in COD concentration over time.

FIG. 6 is a graph showing the molecular weight distribution of dissolved substances when DO≧1 mg/l.

FIG. 7 is a graph showing the molecular weight distribution of dissolved substances when DO≈0.5 mg/l.

[Explanation of symbols]

1 Biological treatment tank 1A Anaerobic biological treatment tank 1B Aerobic biological treatment tank 2 Circulation tank 3 Membrane separation device 3A Separation membrane

Claims (1)

[Claims] Claim 1: In a wastewater treatment method combining biological treatment and membrane separation treatment, the dissolved oxygen concentration in the aerobic tank is set to 1.
A wastewater treatment method characterized by maintaining the concentration at 0 mg/l or more.