Method for biologically treating organic waste water

Abstract

This method for biologically treating organic waste water includes: passing water to be treated composed of organic waste water through a first biological treatment tank in which aerobic biological treatment is performed; performing biological treatment with bacteria; introducing first biologically treated water containing bacteria in a dispersed state from the first biological treatment tank into a second biological treatment tank; and feeding the bacteria to minute animals. Said method is characterized by: the first biological treatment tank being a once-through type tank (excess sludge on the downstream side is not returned); employing a MABR system in which an oxygen permeable membrane is installed; and having a CODCr volume load from 4 to 20 kg / m3.

Description

Biological treatment method for organic wastewater

[0001] The present invention relates to a biological treatment method for organic wastewater using an activated sludge method, and in particular to a biological treatment method for organic wastewater that utilizes the predatory action of microorganisms. One aspect of the present invention relates to a treatment method for organic wastewater that can be used to treat organic wastewater of a wide range of concentrations, including domestic wastewater, sewage, food factories, and pulp factories, and that can improve treatment efficiency without deteriorating the quality of the treated water and reduce the amount of excess sludge generated.

[0002] The activated sludge process, which is used for biological treatment of organic wastewater, has advantages such as good treated water quality and easy maintenance, and is therefore widely used in sewage treatment and industrial wastewater treatment. However, the BOD volume load required for operation is 0.5 to 0.8 kg / m. ３ / d, a large site area is required. In addition, since 20% of the decomposed BOD is converted into bacterial cells, i.e., sludge, the treatment of large amounts of excess sludge becomes a problem.

[0003] Patent Document 1 describes a multi-stage activated sludge process in which organic wastewater is first aerobically treated with bacteria in a first biological treatment tank to oxidize and decompose the organic matter contained in the wastewater, converting it into non-aggregating bacterial cells, and then the organic matter is preyed upon and removed by filter-feeding microorganisms in a second biological treatment tank, thereby reducing the volume of excess sludge.

[0004] Japanese Unexamined Patent Publication No. 55-20649

[0005] The multi-stage activated sludge process, which utilizes the feeding action of microscopic animals, can reduce the amount of sludge generated by approximately 50%, depending on the wastewater being treated. However, this sludge reduction effect is not stable. This is because the first biological treatment tank is heavily loaded and mainly treats suspended sludge, which creates issues such as foaming, odor, VOC (volatile organic compounds) evaporation, and aeration energy, making it impossible to operate at the load originally intended.

[0006] Furthermore, if ideal operating conditions cannot be maintained, it becomes difficult to convert to the ideal non-aggregating bacteria, and the sludge reduction effect and treatment performance also fluctuate. This is because the "filter-feeding microorganisms" involved in sludge reduction in the multi-stage activated sludge process suck up and prey on bacteria, and therefore preferentially prey on dispersed bacteria (hereinafter sometimes referred to as dispersed bacteria). However, if the generated bacteria are larger than the diameter of the microorganisms, they cannot be preyed on, and the sludge reduction effect also decreases.

[0007] The present invention aims to provide a biological treatment method for organic wastewater that has a stable sludge reduction effect using filter-feeding microorganisms and low aeration costs. One aspect of the present invention aims to provide a biological treatment method for organic wastewater that is almost free of odor and foaming.

[0008] The gist of the present invention is as follows.

[0009] [1] A biological treatment method for organic wastewater, in which water to be treated, which is organic wastewater, is passed through a first biological treatment tank that performs aerobic biological treatment, and biologically treated with bacteria, and the first biologically treated water containing dispersed bacteria from the first biological treatment tank is introduced into a second biological treatment tank, and the bacteria are fed to microscopic animals, the first biological treatment tank is a pass-through type (excess sludge from the downstream side is not returned), is an MABR type with an oxygen-permeable membrane installed, and COD Ｃｒ Volume load: 4 to 20 kg / m ３ / d.

[0010] [2] A biological treatment method for organic wastewater, in which water to be treated, consisting of organic wastewater, is passed through a first biological treatment tank that performs aerobic biological treatment, and biologically treated with bacteria, and the first biologically treated water containing dispersed bacteria from the first biological treatment tank is introduced into a second biological treatment tank, and the bacteria are fed to microscopic animals, the first biological treatment tank is a pass-through type (excess sludge from the downstream side is not returned), is an MABR type with an oxygen-permeable membrane installed, and the COD of the raw water passing through is Ｃｒ The biological treatment method for organic wastewater is characterized in that the concentration of nitrite is 800 to 20,000 mg / L.

[0011] [3] A biological treatment method for organic wastewater, in which water to be treated, which is organic wastewater, is passed through a first biological treatment tank that performs aerobic biological treatment, the water is biologically treated with bacteria, the first biologically treated water from the first biological treatment tank containing dispersed bacteria is introduced into a second biological treatment tank, and the bacteria are fed to microscopic animals, characterized in that the first biological treatment tank is a pass-through type (excess sludge from the downstream side is not returned), is an MABR type tank equipped with an oxygen-permeable membrane, and has an HRT of 4 to 120 hours.

[0012] [4] The biological treatment method for organic wastewater according to any one of [1] to [3], wherein the ratio of suspended sludge (SS) to SS attached to the oxygen-permeable membrane in the first biological treatment tank is 2:1 or more.

[0013] [5] The biological treatment method for organic wastewater according to any one of [1] to [3], wherein the DO of the first biological treatment tank is 1.0 mg / L or less, or the ORP is −150 to 100 mV.

[0014] [6] The biological treatment method for organic wastewater according to any one of [1] to [3], wherein the particle size distribution of SS in the treated water of the first biological treatment tank is measured, and the operating conditions are adjusted so that the relative amount of particles with a particle size of 5 μm or less is 40% or more.

[0015] [7] The COD of the first biological treatment tank Ｃｒ Sludge load 1-20kg / kg-VSS / d, removal of COD Ｃｒ The biological treatment method for organic wastewater according to any one of [1] to [3], wherein the sludge load is 0.5 to 10 kg / kg-VSS / d.

[0016] In this biological treatment method for organic wastewater, water to be treated, consisting of organic wastewater, is passed through a first biological treatment tank where aerobic biological treatment is performed, where it is biologically treated using bacteria. The first biologically treated water from the first biological treatment tank, containing dispersed bacteria, is then introduced into a second biological treatment tank, where the bacteria are fed to microorganisms. In this method, oxygen is supplied to the first biological treatment tank using an oxygen-permeable membrane to suppress odor, organic matter volatilization, and foaming in the first biological treatment tank. This prevents the generation of diffused air bubbles, thereby virtually eliminating odor, foaming, and other problems. Furthermore, aeration costs are significantly reduced, and good treated water quality can be stably maintained.

[0017] In the present invention, the load of the first biological treatment tank, the raw water COD Ｃｒ By adjusting the retention time (HRT), adhesion of biofilms to the oxygen permeable membrane surface can be suppressed. Also, microscopic bacteria can be made dominant in the first biological treatment tank, reducing the amount of sludge.

[0018] Fig. 1 is an explanatory diagram of a biological treatment method for organic wastewater according to an embodiment. Fig. 2 is an explanatory diagram of a biological treatment method for organic wastewater according to an embodiment. Fig. 3 is an explanatory diagram of a biological treatment method for organic wastewater according to an embodiment. Fig. 4 is an explanatory diagram of a biological treatment method for organic wastewater according to an embodiment. Fig. 5 is an explanatory diagram of a biological treatment method for organic wastewater of a comparative example.

[0019] FIG. 1 is a flow chart showing a first embodiment of the present invention.

[0020] Organic wastewater (raw water) is introduced into the first biological treatment tank 1 as water to be treated, oxygen is supplied through the oxygen permeable membrane 1 m, and organic components (soluble BOD, soluble COD) are removed by bacteria. Ｃｒ , soluble TOC) is oxidatively decomposed. 70% or more, preferably 80% or more, and even more preferably 90% or more. The first biological treatment tank 1 is a pass-through type, and sludge is not returned from the subsequent stage except during start-up or when treatment deteriorates. If the volume of the first biological treatment tank 1 is large, multiple tanks may be connected in parallel.

[0021] By using the MABR method with an oxygen-permeable membrane 1m to supply oxygen to the first biological treatment tank 1, problems that arise when using a conventional aeration method in high-load treatment, such as odor, foaming, and evaporation of volatile organic compounds into the atmosphere, can be prevented. Furthermore, the aeration power required is reduced.

[0022] Since the oxygen-permeable membrane 1m alone is insufficient to agitate the inside of the tank, a circulation pump, a stirrer, or aeration may be used. Furthermore, since excessive adhesion of microorganisms to the oxygen-permeable membrane 1m reduces the oxygen supply capacity, an aeration mechanism such as an aeration pipe 1b for cleaning may be provided below the oxygen-permeable membrane unit, allowing for continuous or intermittent aeration. However, the amount of aeration per day for cleaning and agitation should be no more than half, preferably no more than one-third, and even more preferably no more than one-quarter of the amount of aeration required without the oxygen-permeable membrane (when the oxygen required for the reaction is supplemented by aeration).

[0023] The oxygen-permeable membrane may be in the form of a hollow fiber, flat membrane, or spiral membrane, but hollow fiber is preferable from the viewpoint of generating dispersed bacteria. The material may be silicon, polyethylene, polyurethane, or a combination of multiple materials, and may be either a non-porous or microporous membrane.

[0024] Treated water from the first biological treatment tank 1 (first biological treatment water) is introduced into the second biological treatment tank 2. In the second biological treatment tank 2, oxygen-containing gas is introduced through an aeration pipe 2b to biologically treat organic components, autolyze bacteria (mainly dispersed bacteria) produced in the first biological treatment tank, and reduce the volume of excess sludge through predation by microscopic animals. The second biological treatment tank 2 may be any of the activated sludge, membrane activated sludge, fluidized bed, etc. If the second biological treatment tank is large, multiple tanks may be connected in parallel or in series. In Figure 1, three tanks are connected in series.

[0025] The treated water from the second biological treatment tank 2 is introduced into the settling tank 3, where it is separated into supernatant water and settled sludge. The supernatant water is removed as treated water. A portion of the settled sludge is returned to the second biological treatment tank 2, and the remainder is discharged outside the system as excess sludge.

[0026] In one embodiment of the present invention, the pH of the first biological treatment tank 1 is preferably 6 or higher, particularly 6 to 8. However, if the raw water contains a large amount of oil, the pH may be 8.0 or higher.

[0027] In one embodiment of the present invention, the COD Ｃｒ Volume load: 4 kg / m ３ / d or more (BOD 2.0 kg / m ３ / d or more, TOC 1.5 kg / m ３ / d or more), preferably 4 to 20 kg / m ３ / d, particularly preferably 4 to 15 kg / m ３ / d.

[0028] In one aspect of the present invention, the COD of raw water passing through the Ｃｒ is 800 to 20,000 mg / L, preferably 2,000 to 15,000 mg / L, more preferably 5,000 to 15,000 mg / L.

[0029] In one embodiment of the present invention, the HRT in the first biological treatment tank is preferably 4 hours or more, particularly 6 hours or more (desirably 120 hours or less, particularly 90 hours or less).

[0030] This makes it possible to obtain treated water in which dispersed bacteria (which are easily preyed upon by microscopic animals) predominate, which contributes to sludge reduction.

[0031] In the present invention, a state in which dispersed bacteria that are easily preyed upon by filter-feeding microorganisms are dominant refers to a state in which the first biological treatment water is passed through a sieve with a mesh size of approximately 1 to 5 mm, large debris is removed, and then the particle size distribution is measured, and the relative amount of particles with a particle size of 5 μm or less is 40% or more, preferably 50% or more.

[0032] Even if dispersed bacteria become dominant in the first biological treatment tank 1, if excessive sludge adheres to the oxygen-permeable membrane 1m, the dispersed bacteria will become filamentous and flocculated, making them difficult for microscopic animals to ingest. Therefore, it is preferable that the ratio of floating sludge VSS to VSS attached to the oxygen-permeable membrane be 2:1 or more, and more preferably 5:1 or more.

[0033] Since it is difficult to measure this ratio in an actual system (it is necessary to lift up the MABR membrane), it is possible to substitute this by "operating a bench-top test-scale device in parallel and periodically checking the floating sludge / adhered sludge ratio," or "confirming in advance the cleaning aeration volume and frequency that will achieve the above floating sludge / adhered sludge ratio, and managing the ratio by adjusting the cleaning aeration volume and frequency."

[0034] The sludge conversion rate, excluding the amount of sludge that has grown on the membrane surface, is (treated water VSS - raw water VSS) / (removed S.COD Ｃｒ By controlling this ratio, it is possible to determine whether excessive sludge is growing on the membrane surface. Ｃｒ By controlling the ratio to be 0.05 to 0.6, particularly 0.06 to 0.3, it is possible to maintain a state in which excessive adhesion to the MABR membrane does not occur (a state in which the produced bacteria are peeled off and released into the treated water).

[0035] To achieve this, DO and ORP may be adjusted (adjusting the aeration rate of the oxygen-permeable membrane or the amount of cleaning air diffused) to reduce the amount of adhesion, thereby keeping the amount within a predetermined range. In this case, the DO of the first biological treatment tank is 1.0 mg / L or less, particularly 0.5 mg / L or less, and especially 0.2 mg / L or less, and 0.05 mg / L or more is preferred, and the ORP is -150 to 100 mV, particularly -100 to 50 mV. By controlling in this way, dispersed bacteria of 1 to 5 μm become dominant in the first biological treatment tank, and these are quickly consumed in the second biological treatment tank.

[0036] In addition, the COD Ｃｒ Sludge load 1-20kg / kg-VSS / d, removal of COD Ｃｒ Sludge load is 0.5 to 10 kg / kg-VSS / d (0.2 to 5.0 kg-BOD / kg-VSS / d), preferably COD Ｃｒ Sludge load 2-12 kg / kg-VSS / d, removal of COD Ｃｒ By setting the sludge load at 1 to 6 kg / kg-VSS / d (0.5 to 3.0 kg-BOD / kg-VSS / d), a high load treatment state can be maintained, enabling more stable generation of dispersed bacteria. Ｃｒ ) - (treated water COD Ｃｒ ) is calculated, so the COD of the treated water Ｃｒ The BOD also includes bacterial SS. The means for adjusting the sludge load include adjusting the water temperature, adjusting the amount of water passing through the first biological treatment tank (if the load is too high or the removal load is too low, the raw water is bypassed to the second biological treatment tank), adjusting the amount of air passing through the oxygen-permeable membrane, etc.

[0037] The method of the present invention is suitable for treating wastewater with a relatively high organic matter concentration. Ｃｒ is 800 mg / L or more (BOD>400 mg / L, TOC>300 mg / L), especially 2000 mg / L or more (BOD>1000 mg / L, TOC>800 mg / L), especially 5000 mg / L or more (BOD>2500 mg / L, TOC>2000 mg / L), and treatment without dilution is possible even in concentration ranges where dilution is required in conventional aerobic treatment. However, raw water COD ＣｒIt is desirable that the concentration is 20,000 mg / L or less, and particularly 15,000 mg / L or less.

[0038] In Figure 1, the second biological treatment tank 2 is of an aeration type, but as mentioned above, it may also be of the MABR type or the like. An example is shown in Figure 2. In Figure 2, the second biological treatment tank 2 consists of three tanks connected in series, and an oxygen-permeable membrane 2m is installed in the final tank to adopt the MABR type. Note that oxygen-permeable membranes may also be installed in more second biological treatment tanks.

[0039] 1 and 2, a thickener-type settling tank 3 is used, but a membrane separation tank 4 equipped with a submerged membrane 4m may also be used as shown in Fig. 3. 4b denotes an aeration pipe.

[0040] In the present invention, the second biological treatment tank 2 may also be a pass-through type without sludge return, as shown in Figure 4. While the second biological treatment tank in Figure 2 consists of two tanks, it may also consist of one or three or more tanks. Furthermore, a screen may be provided in the second biological treatment tank in Figure 4 and a fluidized bed carrier may be added to increase the load on the second biological treatment tank. The fluidized bed carrier may have any shape, such as a sphere, pellet, hollow cylinder, filament, plate, or square, and its size (diameter, side) is approximately 0.1 to 10 mm. The carrier material may be any material, such as a natural material, inorganic material, or polymer material. A gel-like substance may also be used, but the preferred material is a polyurethane foam with a square shape with a side length of 5 mm or less.

[0041] Other configurations in FIGS. 2 to 4 are the same as those in FIG. 1, and the same reference numerals denote the same parts.

[0042] Comparative Examples and Examples will be described below.

[0043] [Comparative Example 1] As shown in Figure 5, a biological treatment device with the same flow as that shown in Figure 1 was operated under the following conditions, except that no oxygen-permeable membrane 1m was installed in the first biological treatment tank 1 and instead a carrier 1a was added. <Operating conditions> First biological treatment tank: 2.5 L Second biological treatment tank: 10 L Sedimentation tank: A portion was returned to the second biological treatment tank as returned sludge, and the remainder was withdrawn as excess sludge Raw water: Simulated wastewater from food manufacturing. COD ｃｒ =5000mg / L (S.COD Ｃｒ= 5000 mg / L), BOD = 2500 mg / L Raw water volume: 6 L / d Aeration air volume in first biological treatment tank: 5 L / min Sludge concentration in first biological treatment tank (suspended sludge): SS = 1200 mg / L (VSS = 1000 mg / L) Carrier filling rate 5% (3.5 mm square polyurethane carrier) S.COD in first biological treatment tank Ｃｒ Removal rate: 80% Sludge concentration in the second biological treatment tank: SS = 5000 mg / L (VSS = 4600 mg / L) Urea, phosphate, and sulfate were added as nutrients to achieve a BOD:N:P:S = 100:5:1:0.1 Minerals: Ca, Mg, and K were added to achieve 1 mg / L or more per raw water Trace metals: Fe was added to achieve 0.1 mg / L or more per raw water, and Cu, Zn, Mo, Mn, Ni, and Co were added to achieve 0.005 mg / L or more per raw water Total alkalinity in the first biological treatment tank: 80 mg / L pH: Adjusted to 7.0 in each tank Water temperature: Adjusted to 25°C in each tank First biological treatment tank DO: 0.5 to 1.0 mg / L First biological treatment tank HRT = 10 h

[0044] <Results> The results of operation of the first biological treatment tank (referred to as the dispersed bacteria tank in Table 1) are shown in Table 1. As shown in Table 1, the relative amount of particles with a particle size of 5 μm or less in the first biological treatment effluent SS was 25%, and the sludge conversion rate was 0.12 g-VSS / g-removed COD Ｃｒ This is a 50% reduction from the standard activated sludge method. Ｃｒ = 25 mg / L, and SS < 10 mg / L, which were very good results. However, there was a lot of foaming, and it was necessary to add 10 mg / L of a silicone-based antifoaming agent to the raw water.

[0045] [Example 1] As shown in Figure 1, a biological treatment device was installed with an oxygen-permeable membrane 1m and an aeration pipe 1b and no carrier added. The biological treatment device consisted of a first biological treatment tank 1, a second biological treatment tank 2 without an oxygen-permeable membrane or carrier, and a settling tank 3. The biological treatment device was operated under the following conditions. <Operating conditions> First biological treatment tank (MABR type): 2.5 L Second biological treatment tank: 10 L Settling tank: A portion was returned to the second biological treatment tank as returned sludge, and the remainder was withdrawn as excess sludge. Raw water: Same as Comparative Example 1. Raw water volume: 6 L / d (same as Comparative Example 1). Sludge concentration in the first biological treatment tank (suspended sludge): SS = 1300 mg / L (VSS = 1100 mg / L). Oxygen-permeable membrane-attached VSS: 200 mg / L per tank. Oxygen-permeable membrane: hollow fiber silicon membrane. Membrane area: 0.004 m ２ Aeration rate from the aeration pipe 1b: Aeration is performed for 2 minutes at intervals of 8 minutes. The aeration rate during aeration is 2.5 L / min. S. COD in the first biological treatment tank Ｃｒ Removal rate: 80% Sludge concentration in second biological treatment tank: SS = 5000 mg / L (VSS = 4600 mg / L) Urea, phosphate, and sulfate were added as nutrients to achieve a BOD:N:P:S = 100:5:1:0.1 Minerals: Ca, Mg, and K were added to achieve 1 mg / L or more per raw water Trace metals: Fe was added to achieve 0.1 mg / L or more per raw water, and Cu, Zn, Mo, Mn, Ni, and Co were added to achieve 0.005 mg / L or more per raw water Total alkalinity in first biological treatment tank: 550 mg / L pH: Adjusted to 7.0 in each tank Water temperature: Adjusted to 25°C in each tank First biological treatment tank DO: 0.2 to 0.5 mg / L, ORP: -70 to 20 mV First biological treatment tank HRT = 10 h First biological treatment tank T.COD Ｃｒ Removal rate: 40% COD in the first biological treatment tank Ｃｒ Sludge load: 9.2 kg / kg-VSS / d Second biological treatment tank COD Ｃｒ Sludge load: 3.7kg / kg-VSS / d

[0046] <Results> The results of operation of the first biological treatment tank are shown in Table 1. As shown in Table 1, the relative amount of particles with a particle size of 5 μm or less in the first biological treatment effluent SS was 55%, and compared to Comparative Example 1, the dominance of dispersed bacteria progressed, and the sludge conversion rate was 0.06 g-VSS / g-removed COD ＣｒThe COD of the treated water is very low. Ｃｒ = 25 mg / L, and SS < 10 mg / L, which were very good. Note that the amount of air diffused into the tank by the air diffuser 1b and for cleaning was low and intermittent, so there was almost no foaming and it was not necessary to add a defoaming agent.

[0047] [Example 2] A test was carried out in the same manner as in Example 1, except that the raw water flow rate was set to 3 L / d. Table 1 shows the operation results of the first biological treatment tank.

[0048] [Examples 3 to 4, Comparative Examples 2 to 4] The following raw water was used, and tests were carried out in the same manner as in Example 1, except that the raw water flow rate was changed as shown in Table 1. The raw water used in Examples 3 to 4 and Comparative Examples 2 to 4 was the same simulated wastewater from food production as in Comparative Example 1, diluted with pure water to adjust the concentration as follows. Example 3: COD Ｃｒ : 2300 mg / L Example 4: COD Ｃｒ :800mg / L Comparative example 2: COD Ｃｒ :1500mg / L Comparative example 3: COD Ｃｒ :4500mg / L Comparative example 4: COD Ｃｒ : 450 mg / L

[0049] The operation results of the first biological treatment tank are shown in Table 1.

[0050]

[0051] From a comparison of Example 4 with Comparative Examples 3 and 4, COD Ｃｒ Volume load 4kg / m ３ It was found that when the particle size is 1 / d or more, the ratio of SS with particle sizes that are easily preyed upon increases, resulting in a good sludge conversion rate.

[0052] From a comparison of Examples 1, 2, and 4 and Comparative Example 4, the raw water COD Ｃｒ When the concentration is 800 mg / L or more, the proportion of SS with particle sizes that are easily preyed upon increases, and the sludge conversion rate improves.It was found that this effect is particularly pronounced when the concentration is 5000 mg / L or more.

[0053] Comparing Examples 3 and 4 with Comparative Examples 2 and 4, it was found that when the HRT of the first biological treatment tank is set to 4 hours or more, the proportion of SS with particle sizes that are easily preyed upon increases in the treated water SS, resulting in a better sludge conversion rate.

[0054] Although the present invention has been described in detail using specific embodiments, it will be apparent to those skilled in the art that various modifications are possible within the scope of the effects of the invention. This application is based on Japanese Patent Application No. 2024-074889 filed on May 2, 2024, the entire contents of which are incorporated by reference.

[0055] 1 First biological treatment tank 1m Oxygen permeable membrane 2 Second biological treatment tank 3 Sedimentation tank 4 Membrane separation tank 4m Submerged membrane

Claims

1. In a biological treatment method for organic wastewater, water to be treated, which is organic wastewater, is passed through a first biological treatment tank that performs aerobic biological treatment, where it is biologically treated by bacteria, and the first biologically treated water containing dispersed bacteria from the first biological treatment tank is introduced into a second biological treatment tank, where the bacteria are preyed on by microorganisms. The first biological treatment tank is a pass-through type (excess sludge from the downstream side is not returned), is an MABR type with an oxygen-permeable membrane installed, and has a COD Ｃｒ Volume load: 4 to 20 kg / m ３ / d.

2. In a biological treatment method for organic wastewater, water to be treated, which is organic wastewater, is passed through a first biological treatment tank that performs aerobic biological treatment, where it is biologically treated with bacteria, and the first biologically treated water containing dispersed bacteria from the first biological treatment tank is introduced into a second biological treatment tank, where the bacteria are preyed on by microorganisms. The first biological treatment tank is a one-way type (excess sludge from the downstream side is not returned), and is an MABR type with an oxygen-permeable membrane installed, and the COD of the raw water passing through is Ｃｒ The biological treatment method for organic wastewater is characterized in that the concentration of nitrite is 800 to 20,000 mg / L.

3. A biological treatment method for organic wastewater, in which water to be treated, consisting of organic wastewater, is passed through a first biological treatment tank that performs aerobic biological treatment, where it is biologically treated with bacteria, and the first biologically treated water containing dispersed bacteria from the first biological treatment tank is introduced into a second biological treatment tank, where the bacteria are fed to microscopic animals, characterized in that the first biological treatment tank is a pass-through type (wherein excess sludge from the downstream side is not returned), is of the MABR type equipped with an oxygen-permeable membrane, and has an HRT of 4 to 120 hours.

4. A biological treatment method for organic wastewater according to any one of claims 1 to 3, wherein the ratio of suspended sludge SS to SS attached to the oxygen-permeable membrane in the first biological treatment tank is 2:1 or more.

5. The method for biological treatment of organic wastewater according to any one of claims 1 to 3, wherein the DO in the first biological treatment tank is 1.0 mg / L or less, or the ORP is -150 to 100 mV.

6. A biological treatment method for organic wastewater according to any one of claims 1 to 3, wherein the particle size distribution of SS in the treated water of the first biological treatment tank is measured, and the operating conditions are adjusted so that the relative amount of particles with a particle size of 5 μm or less is 40% or more.

7. COD of the first biological treatment tank Ｃｒ Sludge load 1-20kg / kg-VSS / d, removal of COD Ｃｒ 4. The method for biological treatment of organic wastewater according to claim 1, wherein the sludge load is set to 0.5 to 10 kg / kg-VSS / d.

Images