JP2003088885A - Method and apparatus for treating organic waste water - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a simple method and apparatus for biologically treating organic waste water in which the volume of excess sludge produced is suppressed while the deterioration of quality of treated water is prevented. SOLUTION: In the biological treating method in which the organic waste water 1 is treated in biological treating tanks 8, 9 and solid-liquid separation of the resultant activated sludge is carried out with membrane separation 10, a part of the activated sludge in the biological treating tank 9 is taken out to be subjected to liquefying treatment 11 and the liquefying treated sludge 4 is circulated in the biological treating tank 8 and further a part of another sludge is taken out from the biological treating tank 8 to be subjected to solid/ liquid separation 12 and the sludge 6 obtained by the solid/liquid separation is circulated in the biological treating tank 9 and hardly biologically decomposable substance is removed 13 from the resultant separated liquid 5. The liquefying treatment can be carried out by ozone treatment or ultrasonic treatment and the removal of the hardly biologically decomposable substance can be carried out by coagulating sedimentation or oxidation decomposition with ozone and/or hydrogen peroxide.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to the treatment of organic wastewater, and more particularly to a biological treatment method and apparatus for biologically treating organic wastewater and performing solid-liquid separation by membrane separation.

[0002]

2. Description of the Related Art In the treatment of organic wastewater, the biological treatment method using activated sludge has become the mainstream of wastewater treatment methods because it can deal with various wastewater. As one of the biological treatment methods, as a method for removing nitrogen in addition to removing organic matter in wastewater,
The nitrification denitrification method is known. This is a proposal for removal of nitrogen in wastewater.The biological treatment tank is divided into a denitrification tank that only performs stirring and a nitrification tank that performs nitrification. The liquid is supplied to the nitrification tank, the nitrification tank effluent is caused to flow to the solid-liquid separation device, and at the same time, a part of the nitrification tank effluent is circulated to the denitrification tank. In the nitrification tank, ammonia nitrogen is oxidized to nitrate nitrogen by nitrifying bacteria in the activated sludge, in addition to oxidative decomposition of organic substances in wastewater by BOD oxidizing bacteria. The activated sludge mixed solution that flowed out from the nitrification tank was denitrified to remove the nitric nitrogen in the activated sludge mixed solution.
Denitrifying bacteria in activated sludge are reduced to nitrogen by using organic matter contained in raw water. Through the above steps, ammoniacal nitrogen in the wastewater is reduced to gaseous nitrogen and removed from the wastewater.

As described above, biological treatment with activated sludge is an excellent method, but on the other hand, there is a problem that excess sludge is generated due to microorganisms grown in the biological treatment tank and suspended substances flowing in. . In recent years, excess sludge treatment costs have been increasing, and technology for suppressing it has been drawing attention. By refining and solubilizing sludge and supplying it again to the biological treatment tank, liquefied sludge can be stored in the biological treatment tank. It has been proposed to perform mineralization with the activated sludge. Regarding the micronization and liquefaction process, methods of physically micronizing activated sludge, solubilizing sludge by heating and utilizing high heat bacteria, and technology of solubilizing sludge by applying ozone are known. There is. However, when sludge is refined and liquefied by these methods,
Although the amount of excess sludge generated was reduced, there was a problem that a part of the finely sludge sludge flowed out into the treated water and the quality of the treated water deteriorated. In addition, these miniaturization and liquefaction methods have a problem that the whole apparatus is complicated because a miniaturization step and a liquefaction step are added to the conventional biological treatment system. When solid-liquid separation is performed using a membrane separation method in the case of reducing the volume of sludge by miniaturization or liquefaction, it is possible to suppress the outflow of sludge to treated water. However, at the same time, there is a problem that finely sludge is accumulated in the system, the volume reduction efficiency is lowered, and the membrane is easily clogged.

[0004]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above-mentioned conventional techniques, and it is possible to reduce the amount of excess sludge generated in the process of biological treatment while preventing deterioration of the quality of treated water. An object of the present invention is to provide a simple biological treatment method and apparatus for organic wastewater, which can reduce the amount of the biodegradable substance accumulated in the biological treatment tank and remove the hardly biodegradable substance.

[0005]

In order to solve the above problems, the present invention provides a biological treatment method in which organic wastewater is treated in a biological treatment tank and the resulting activated sludge is subjected to solid-liquid separation by membrane separation. A part of the activated sludge in the biological treatment tank is extracted and liquefied, and the liquefied sludge is circulated to the biological treatment tank, and another part of the activated sludge is further extracted from the biological treatment tank to perform solid-liquid separation. The sludge obtained by the solid-liquid separation is circulated in the biological treatment tank, while the hardly biodegradable substance is removed from the obtained separated liquid. In the above-mentioned treatment method, the liquefaction treatment can be carried out by ozone treatment or ultrasonic treatment, and the hardly biodegradable substance can be removed by coagulation-precipitation by addition of a coagulant, or ozone and / or an excess of ozone. It can be due to oxidative decomposition with hydrogen oxide.

Further, according to the present invention, in a biological treatment apparatus having a biological treatment tank for biologically treating organic wastewater and a membrane separation device for solid-liquid separating the resulting activated sludge, a part of the activated sludge in the biological treatment tank is provided. A path for extracting the liquefaction treatment apparatus, a liquefaction apparatus connected to the path, and a path for circulating the liquefaction-treated sludge in the biological treatment tank, and another path for extracting a part of the activated sludge in the biological treatment tank, A solid-liquid separation device connected to the path, a path for circulating the solid-liquid separated sludge in the biological treatment tank, and a removal means for removing a hardly biodegradable substance from the separated liquid obtained by the solid-liquid separation. It is decided to have and.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, by providing a denitrification tank and a nitrification tank in the biological treatment tank, nitrification and denitrification can be performed in the biological treatment tank at the same time as the removal of organic substances. Further, in order to proceed the nitrification denitrification reaction, it is necessary to circulate a part of the activated sludge from the nitrification tank to the denitrification tank, but by providing a device for liquefying sludge in the middle of this circulation, It is possible to return the nitrate nitrogen generated in the nitrification tank to the denitrification tank while performing the sludge liquefaction process. Further, as the organic matter required for the denitrification reaction in the denitrification tank, it is possible to use the organic matter derived from liquefied sludge in addition to the organic matter in the wastewater. The present inventors have found that when the sludge volume reduction treatment is performed, the finely sludge accumulated in the biological treatment tank has a particle size of 0.1 to 1 μm and is hardly biodegradable. . Therefore, by applying the membrane separation method, it was possible to prevent deterioration of the treated water quality while suppressing the amount of excess sludge generated. Furthermore, by removing the hardly biodegradable substances accumulated in the biological treatment tank, it was possible to prevent membrane contamination and perform stable operation for a long period of time. The biodegradable substance in the present invention includes a humic substance, a fibrous material such as cellulose, an ignition residual substance derived from a metal oxide, an endocrine disrupting substance, and dioxins.

Next, the present invention will be described in detail with reference to the drawings. FIG. 1 is a flow configuration diagram of an example of treating sewage by the treatment method of the present invention. As shown in FIG. 1, the sewage 1 is supplied to the denitrification tank 8 and mixed with the activated sludge in the denitrification tank 8,
Further, it is sent to the nitrification tank 9. The concentration of activated sludge in the denitrification tank 8 and the nitrification tank 9 varies depending on the nature of the inflowing sewage,
It is preferably 15000 mg / L. A membrane separation device 10 is provided in the nitrification tank 9, and membrane filtration treated water 2 is obtained from the nitrification solution 3 by filtration. The membrane of the membrane separation device 10 is
The pore size is preferably 1 to 0.04 μm. The nitrification liquid 3 is caused to flow out from the nitrification tank 9, and the generated nitrate nitrogen is sent to the denitrification tank 8. The flow rate of the nitrification liquid 3 is determined according to the nitrogen removal amount, but is usually 1 to 4 times the inflowing sewage amount. A liquefaction processing device 11 is provided in the middle of this circulation,
The activated sludge contained in the nitrification liquid 3 is liquefied. As the liquefying means, it is desirable to add an oxidant such as ozone or to make it fine by ultrasonic waves.

When ozone is used for the liquefaction treatment, the amount of ozone supplied is 10 to the amount of sludge flowing into the liquefaction treatment tank.
60 mg-O ₃ / g-SS, but especially 10-20 m
it is desirable that the _{g-O 3 / g-SS} . Further, in the liquefaction treatment by ultrasonic treatment, it is desirable to irradiate the sludge flowing into the liquefaction treatment tank with 1000 to 50000 kJ / kg-SS. The liquefaction-treated sludge 4 flowing out from the liquefaction-treatment device 11 is supplied to the denitrification tank 8. In order to remove the hardly biodegradable substance accumulated in the biological treatment plant, a part of the nitrification liquid 3 is extracted from the nitrification tank 9 and supplied to the solid-liquid separation tank 12.
In the solid-liquid separation tank 12, a dynamic filtration method (a filter made of a water-permeable sheet such as a non-woven fabric or a woven cloth is dipped in a biological treatment tank, etc., and a dynamic head layer of sludge formed on the surface of the filter provides a low head of water. It is desirable to use a method of obtaining filtered water by pressure), a sedimentation basin method, a centrifugal separation method, a pressure flotation method, or a membrane filtration method (having a pore size of the membrane of 1 μm or more). Further, the amount of the activated sludge mixed liquid in the biological treatment tank supplied to the solid-liquid separation tank 12 can be arbitrarily set regardless of the amount of raw water, but 1 to 10% of the amount of raw water is preferable for rational treatment. .

In the solid-liquid separation tank 12, activated sludge is separated from the nitrification liquid 3 and returned to the nitrification tank 9 as concentrated sludge 6. The concentrated sludge 6 can be returned to the denitrification tank 8. The nitrification liquid supernatant 5 separated in the solid-liquid separation tank 12 is C
The COD removal device 13 removes the hardly biodegradable substance sent to the OD removal device 13 and accumulated in the biological treatment tank.
The COD removal treated water 7 is separated. COD removal treated water 7
Can be discharged out of the system as treated water.
It is also possible to supply the COD-removed water 7 to the denitrification tank 8 or the nitrification tank 9, filter it with the membrane separation device 10, and then discharge it. In the COD removal device 13, a method of removing a hardly biodegradable substance by adding an aggregating agent to coagulate and settle it, a method of injecting ozone to decompose and remove the hardly biodegradable substance by oxidative decomposition, ozone or hydrogen peroxide The method of decomposing the hardly biodegradable substance by the accelerated oxidation method by adding the above, the method of adsorbing and removing it by activated carbon, and the method of concentrating and separating by membrane separation are desirable.

[0011]

EXAMPLES Hereinafter, the present invention will be described more specifically by way of examples. Example 1 In this Example 1, the treatment of groundwater was performed according to the flow shown in FIG. The liquefaction treatment of the nitrification liquid was performed using ozone-containing oxygen gas. The group groundwater 1 was supplied to a biological treatment tank composed of a denitrification tank 8 and a nitrification tank 9, and the nitrification liquid 3 from the nitrification tank 9 was supplied to an ozone reaction tank 11 which is a liquefaction processing device for liquefaction treatment. As the membrane separation device 10, a hollow fiber membrane module was installed in a nitrification tank and filtration was performed. Table 1
The operating conditions of the biological treatment tank are shown in.

[0012]

[Table 1] As shown in Table 1, the volume of the biological treatment tank of the present embodiment, nitrification tank 2m ^3, a denitrification tank 1 m ^3, MLSS of about 3000
It was mg / L. Raw water inflow to biological treatment tank is 15m
³ / d, BOD load on the whole biological treatment tank is about 0.1
It became 3 kg / kg · d. The circulation flow rate of circulation from the nitrification tank to the denitrification tank via the ozone reaction tank was set to 30 m ³ / d.

The ozone reaction tank 11 contains 30 m ^{3 of} nitrification liquid.
/ D. Ozone gas was supplied at a rate of 300 g / d. Table 2 shows the treatment results in the ozone reaction tank 11. As shown in Table 2, the solubility COD, BOD, T
-N increased after the ozone treatment, but these were due to the increase of each soluble component due to the liquefaction of sludge.

[Table 2]

In the present embodiment, in order to remove the hardly biodegradable substances accumulated in the system, a part of the nitrification liquid is extracted from the nitrification tank 9 and supplied to the solid-liquid separation device 12, where the solid-liquid separation is performed and the sludge 6 Was returned to the nitrification tank 9. The nitrification solution supernatant 5 was supplied to a coagulation-sedimentation device 13 which is a COD removal device, and iron chloride (FeCl ₃ ) was added as a coagulant to coagulate and precipitate the hardly biodegradable substance. Iron chloride is 50 mg / based on the nitrification solution supernatant 5.
L was added to adjust the pH to 6.0. The separated liquid was discharged together with the membrane filtration treated water 2 as the coagulation sedimentation treated water 7 which was the COD removal treated water. In Table 3, nitrification solution supernatant 5
And shows the water quality of the coagulation sedimentation treated water 7. In the nitrification solution supernatant 5,
The biodegradable substance detected as COD was removed by the coagulation sedimentation treatment.

[0015]

[Table 3] In this embodiment, the pore size is 0.4 μm for membrane separation in the nitrification tank 9.
Using the hollow fiber membrane module of m, filtration operation was performed with 13-minute suction filtration and 2-minute stoppage as 1 cycle. At this time, the membrane permeation flux was about 0.4 m / d. Table 4 shows the average water quality of the group groundwater 1 and the membrane filtration treated water 2. From Table 4, the water quality of the membrane filtration treated water 2 is 5 mg / L or less for BOD and SS.
Was not detected, and COD was 6.5 mg / L, and good water quality could be obtained. In addition, total nitrogen (T-N) is 7.2m
It was g / L, and about 70% of the total inflowing nitrogen could be removed.

[0016]

[Table 4] FIG. 3 shows the progress of the amount of sludge in the system in this example. By partially liquefying the nitrification tank sludge, the amount of sludge in the system was stable at about 9 kg, and the operation could be performed for about 2 months without discharging sludge. Further, the transmembrane pressure difference in the membrane separation device is 4 to
It was stable at 6 kPa and could be operated continuously without chemical cleaning.

Comparative Example 1 In this comparative example, the groundwater treatment was performed by the flow shown in FIG. Unlike Example 1, no part for performing aggregation and precipitation is provided. The liquefaction treatment of the nitrification liquid was performed using ozone-containing oxygen gas. The group groundwater 1 is supplied to a biological treatment tank including a denitrification tank 8 and a nitrification tank 9,
The nitrification liquid 3 from the ozone reaction tank 11 is a liquefaction processing device.
And was liquefied. As the membrane separation device 10,
The hollow fiber membrane module was installed in the nitrification tank and filtration was performed. Table 5 shows the operating conditions of the biological treatment tank.

[0018]

[Table 5] As shown in Table 5, the volume of the biological treatment tank of the present comparative example, nitrification tank 2m ^3, a denitrification tank 1 m ^3, MLSS of about 3000
It was mg / L. Raw water inflow to biological treatment tank is 15m
³ / d, BOD load on the whole biological treatment tank is about 0.1
It became 3 kg / kg · d. The circulation flow rate of circulation from the nitrification tank to the denitrification tank via the ozone reaction tank was set to 30 m ³ / d.

The ozone reaction tank 11 contains 30 m ^{3 of} nitrification liquid.
/ D. Ozone gas was supplied at a rate of 300 g / d. Table 6 shows the treatment results in the ozone reaction tank 11. As shown in Table 6, the solubility COD, BOD, T
-N increased after the ozone treatment, but these were due to the increase of each soluble component due to the liquefaction of sludge.

[Table 6]

In this comparative example, a hollow fiber membrane module having a pore diameter of 0.4 μm was used for membrane separation in the nitrification tank 9, and filtration operation was performed with 13-minute suction filtration and 2-minute stoppage as 1 cycle. At this time, the membrane permeation flux was about 0.4 m / d. Table 7 shows the average water quality of the group groundwater 1 and the membrane filtration treated water 2. From Table 7, the water quality of the membrane filtration treated water 2 is 5 m in BOD.
Although SS was not detected at g / L or less, COD was 10 mg / L and the water quality was worse than that in Example 1. Also,
The total nitrogen (TN) was 10.6 mg / L, and the water quality was also worse than in Example 1.

[0021]

[Table 7] FIG. 4 shows the progress of the amount of sludge in the system in this comparative example. The amount of sludge in the system increased although it was liquefied. Furthermore, the membrane separation apparatus reached a transmembrane pressure difference of 30 kPa in one month and performed chemical cleaning. As described above, based on the operation results of Example 1 and Comparative Example, it is possible to suppress the amount of surplus sludge generated while obtaining good treated water by separating and removing a part of the nitrification tank sludge by coagulation sedimentation. It was possible.

Implementation Example 2 In Example 2, the treatment of groundwater was carried out by the flow shown in FIG. The liquefaction treatment of the nitrification liquid was performed by an ultrasonic irradiation device. The group groundwater 1 is supplied to a biological treatment tank composed of a denitrification tank 8 and a nitrification tank 9, and the nitrification liquid 3 from the nitrification tank 9 is supplied to an ultrasonic treatment tank 11 which is a liquefaction processing device for liquefaction treatment. . As the membrane separation device 10, a hollow fiber membrane module was installed in a nitrification tank and filtration was performed. Table 8 shows the operating conditions of the biological treatment tank.

[Table 8]

As shown in Table 8, the volume of the biological treatment tank of the present embodiment, nitrification tank 2m ^3, a denitrification tank 1 m ^3, MLSS
Was about 3000 mg / L. The amount of raw water flowing into the biological treatment tank was 15 m ³ / d, and the BOD load on the entire biological treatment tank was about 0.13 kg / kg · d. The circulation flow rate from the nitrification tank to the denitrification tank through the ultrasonic treatment tank is 30 m ^3.
Set to / d. The ultrasonic treatment bath 11 contains 30 nitric acid.
Supplied in m ³ / d. Also, ultrasonic waves are 65
Irradiation was performed at a rate of 00 kJ / kg-SS. Table 9 shows the treatment results in the ultrasonic treatment bath 11. As shown in Table 9,
Solubility COD, BOD, and TN increased after ultrasonic treatment, but these are due to an increase in each soluble component due to liquefaction of sludge.

[0024]

[Table 9]

In Example 2, in order to remove the hardly biodegradable substance accumulated in the system, a part of the nitrification liquid was extracted from the nitrification tank 9 and supplied to the solid-liquid separation device 12, where the solid-liquid separation was performed to remove sludge. It was returned to the nitrification tank 9. The nitrification solution supernatant 5 was supplied to a coagulation-sedimentation device 13 which is a COD removal device, and iron chloride (FeCl ₃ ) was added as a coagulant to coagulate and precipitate the hardly biodegradable substance. Iron chloride was added to the nitrification solution supernatant 5 at 50 mg / L, and the pH was adjusted to 6.0. The separated liquid was discharged together with the membrane filtration treated water 2 as the coagulation sedimentation treated water 7 which was the COD removal treated water. Table 10 shows the water quality of the nitrification solution supernatant 5 and the coagulated sedimentation treated water 7. The nitrification solution supernatant 5 contains C
The biodegradable substance detected as OD was removed by the coagulation sedimentation treatment.

[Table 10]

In this example, a hollow fiber membrane module having a pore size of 0.4 μm was used for membrane separation in the nitrification tank 9, and filtration operation was performed with 13-minute suction filtration and 2-minute stoppage as one cycle. At this time, the membrane permeation flux was about 0.4 m / d. Table 11 shows the average water quality of the group groundwater 1 and the membrane filtration treated water 2. From Table 11, the water quality of the membrane filtration treated water 2 has a BOD of 5
mg / L or less, SS was not detected, and COD was 5.9 mg
/ L and good water quality could be obtained. Moreover, the total nitrogen (TN) was also 7.1 mg / L, and about 70% of the total inflowing nitrogen could be removed.

[0027]

[Table 11] FIG. 5 shows the progress of the amount of sludge in the system in this example. By partially liquefying the nitrification tank sludge, the amount of sludge in the system was stable at about 9 kg, and the operation could be performed for about 2 months without discharging sludge. Further, the transmembrane pressure difference in the membrane separation device is 4 to
It was stable at 6 kPa and could be operated continuously without chemical cleaning.

Example 3 In this Example 3, the groundwater was treated by the flow shown in FIG. The liquefaction treatment of the nitrification liquid was performed using ozone-containing oxygen gas. Further, a part of the nitrification liquid is subjected to solid-liquid separation in the solid-liquid separation tank 12, and the nitrification liquid supernatant 5 is subjected to COD.
It was supplied to the second ozone reaction tank 13, which is a removing device. Second
Ozone gas was injected into the ozone reaction tank 13 to oxidatively decompose humic substances and other hardly biodegradable substances remaining in the nitrification solution supernatant 5. Table 12 shows the water quality of the nitrification solution supernatant 5 and the treated water 7 of the second ozone reaction tank which is treated water for COD removal. In the nitrification solution supernatant 5, a biodegradable substance such as a humic substance detected as COD was removed by oxidative decomposition.

[0029]

[Table 12] Table 13 shows the average water quality of the group groundwater 1 and the membrane filtration treated water 2. From Table 13, as for the water quality of the membrane filtration treated water 2, BOD is 5 mg / L or less, SS is not detected, and COD is 5.9 m.
Good water quality of g / L could be obtained. Moreover, the total nitrogen (TN) was also 7.2 mg / L, and about 70% of the total nitrogen that had flowed in could be removed.

[0030]

[Table 13] By partially liquefying the nitrification tank sludge, the amount of sludge in the system was stable at about 9 kg, and the operation could be performed for about 2 months without discharging sludge. Further, since the humic substance and the like are decomposed in the second ozone reaction tank 13, the humic substance and the like do not accumulate in the nitrification tank 9, and the transmembrane pressure difference in the membrane separation device is 4 to 6
It was stable at kPa and could be operated continuously without chemical cleaning.

[0031]

EFFECTS OF THE INVENTION According to the present invention, it is possible to reduce the amount of surplus sludge generated in the process of treating organisms and to remove the hardly biodegradable substances that accumulate in the biological treatment tank, thus deteriorating the quality of treated water. It is possible to suppress the amount of excess sludge generated while preventing it,
It was possible to provide a simpler processing flow than before.

[Brief description of drawings]

FIG. 1 is a flow configuration diagram showing an example of a processing method of the present invention.

FIG. 2 is a flow configuration diagram showing a processing method used in Comparative Example 1.

FIG. 3 is a graph showing the progress of the amount of sludge according to the number of operating days in Example 1.

FIG. 4 is a graph showing the progress of sludge amount according to the number of operating days in Comparative Example 1.

FIG. 5 is a graph showing the progress of the amount of sludge according to the number of operating days in Example 2.

[Explanation of symbols]

1: Sewage, 2: Membrane filtration treated water, 3: Nitrification liquid, 4: Liquefaction treated sludge, 5: Nitrification liquid supernatant, 6: Concentrated sludge, 7: COD removal treated water, 8: Denitrification tank, 9: Nitrification tank 10: Membrane separation device, 11: Liquefaction processing device, 12: Solid-liquid separation tank, 13: C
OD removal device

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C02F 3/34 101 C02F 3/34 101A 4D059 101B 11/00 11/00 Z 11/06 11/06 A ( 72) Inventor Katsuyuki Kataoka 11-1 Haneda Asahi-cho, Ota-ku, Tokyo Inside the EBARA CORPORATION (72) Inventor Toshihiro Tanaka 11-11 Haneda-Asahi-cho, Ota-ku, Tokyo F-term inside the EBARA CORPORATION (reference) 4D006 GA02 HA01 KA01 KA71 KB13 KB14 KB21 PB20 PC62 4D015 BA19 BB05 CA02 DA13 EA32 FA02 FA03 FA24 FA26 4D028 BB07 BC17 BD17 BE01 BE02 BE04 BE08 4D040 BB02 BB24 BB54 4D050 AA20 BB02 BB09 BD06 42112 A05

Claims (4)

[Claims] 1. A biological treatment method in which organic wastewater is treated in a biological treatment tank, and the resulting activated sludge is subjected to solid-liquid separation by membrane separation. A part of the activated sludge in the biological treatment tank is extracted and liquefied. Then, the liquefied sludge is circulated to the biological treatment tank, and another part of the activated sludge is extracted from the biological treatment tank to perform solid-liquid separation, and the sludge obtained by the solid-liquid separation is used as the biological treatment tank. A method for treating organic wastewater, characterized in that the biodegradable substance is circulated in the same manner, while the hardly biodegradable substance is removed from the obtained separated liquid. 2. The method for treating organic wastewater according to claim 1, wherein the liquefaction treatment is performed by ozone treatment or ultrasonic treatment. 3. The method according to claim 1, wherein the biodegradable substance is removed by coagulation precipitation by adding a coagulant or by oxidative decomposition by ozone and / or hydrogen peroxide. Method of treating organic wastewater. 4. A biological treatment apparatus having a biological treatment tank for biologically treating organic wastewater and a membrane separation device for solid-liquid separation of the resulting activated sludge, and a path for extracting a part of the activated sludge from the biological treatment tank. , A liquefaction apparatus connected to the path, a path for circulating the liquefied sludge in a biological treatment tank, and another path for extracting a part of the activated sludge in the biological processing tank, and the path A solid-liquid separation device, a path for circulating the solid-liquid separated sludge in the biological treatment tank, and a removing unit for removing a hardly biodegradable substance from the separated liquid obtained by the solid-liquid separation. An organic wastewater treatment device characterized by: