Biological treatment method for organic wastewater

Abstract

To provide a biological treatment method for organic wastewater that has a stable effect of reducing a sludge volume.SOLUTION: There is provided a biological treatment method for organic wastewater, the method including: passing water to be treated, which is organic wastewater, through a first biological treatment tank (1) for performing aerobic biological treatment; biologically treating the water with bacteria; introducing the first biologically treated water containing dispersed bacteria from the first biological treatment tank into a second biological treatment tank (2); and allowing microorganisms to feed on the bacteria. The first biological treatment tank 1 is a fluidized bed, and when an organic matter load in the first biological treatment tank 1 increases by more than a predetermined amount, adjustment measures are taken so that an average floating sludge VSS amount / carrier-adhered VSS amount ratio in the first biological treatment tank 1 becomes 0.5 to 5.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[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 flow 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 the present invention relates to a treatment method that can improve treatment efficiency without deteriorating the quality of the treated water and reduce the amount of excess sludge generated. [Background technology]

[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, so a large site area is required. In addition, because 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 in a second biological treatment tank, the cells are preyed upon and removed by filter-feeding microorganisms, thereby reducing the volume of excess sludge. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 55-20649 Summary of the Invention [Problem to be solved by the invention]

[0005] The multi-stage activated sludge process, which utilizes the feeding action of microorganisms, can reduce the volume of sludge generated by approximately 50%, depending on the wastewater being treated. However, this sludge reduction effect is unstable. This is because the first biological treatment tank is heavily loaded and primarily treats suspended sludge. Load fluctuations, particularly fluctuations in water volume, can cause fluctuations in the amount of suspended sludge retained (VSS concentration), which is the main treatment target. This instability in the conversion of suspended sludge to dispersed bacteria (hereinafter referred to as "dispersed bacteria") leads to fluctuations in the sludge reduction effect. Furthermore, the "filter-feeding microorganisms" involved in sludge reduction in the multi-stage activated sludge process absorb and feed on bacteria, preferentially feeding on dispersed bacteria. However, if the resulting bacteria are larger than the diameter of the microorganisms, they cannot be fed, resulting in a low sludge reduction effect.

[0006] As a countermeasure against load fluctuations, adding carriers to the first biological treatment tank (maintaining carrier-adhered bacteria within the system) can strengthen the stability of treatment. However, if too much carrier is added, the contribution of carrier-adhered bacteria to treatment increases, and clumps of sludge detached from the carrier become the majority of the first biological treatment tank SS, reducing the production rate of dispersed bacteria that can be preyed on by filter-feeding microorganisms, and reducing the sludge volume reduction effect.

[0007] An object of the present invention is to provide a biological treatment method for organic wastewater in which the effect of reducing sludge volume by filter-feeding microorganisms is stable. [Means for solving the problem]

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

[0009] [1] A biological treatment method for organic wastewater, comprising: passing water to be treated, which is organic wastewater, through a first biological treatment tank that performs aerobic biological treatment, biologically treating the water with bacteria; introducing the first biologically treated water containing dispersed bacteria from the first biological treatment tank into a second biological treatment tank; and allowing microorganisms to feed on the bacteria; The method for biological treatment of organic wastewater is characterized in that the first biological treatment tank is a fluidized bed, and when the organic matter load in the first biological treatment tank increases by more than a predetermined amount, adjustment measures are taken so that the average ratio of suspended sludge VSS amount to carrier-adhered VSS amount in the first biological treatment tank is 0.5 to 5.

[0010] [2] The biological treatment method for organic wastewater according to [1], wherein the organic matter load is the amount of water flowing into the first biological treatment tank, the organic matter concentration in the water flowing into the first biological treatment tank, or the product of the amount of water flowing and the organic matter concentration in the water.

[0011] [3] The biological treatment method for organic wastewater according to [1], wherein the adjustment procedure is carried out when the organic matter load in the first biological treatment tank is 1.2 times or more the average organic matter load in the first biological treatment tank over the most recent specified period.

[0012] [4] The biological treatment method for organic wastewater according to [1], wherein the adjustment step is to adjust the packing rate of the carrier.

[0013] [5] The biological treatment method for organic wastewater according to [1], wherein DO is adjusted as the adjustment treatment.

[0014] [6] The method for biological treatment of organic wastewater according to [1], wherein the adjustment step is to adjust the flow rate of the water to be treated flowing into the first biological treatment tank.

[0015] [7] A biological treatment method for organic wastewater according to any one of [1] to [6], wherein the particle size distribution of SS in the treated water in the first biological treatment tank is measured, and when the relative amount of particles with a particle size of 5 μm or less is less than 20%, the adjustment treatment is carried out so that the ratio of the amount of VSS in suspended sludge to the amount of VSS attached to the carrier is 2 to 5. [Effects of the Invention]

[0016] The present invention appropriately adjusts the ratio of suspended sludge VSS to carrier-attached VSS in the first biological treatment tank, and turns the bacteria produced in the first biological treatment tank into dispersed bacteria that are easily preyed on by microscopic animals, even when the load in the first biological treatment tank fluctuates. This stabilizes the sludge reduction effect of filter-feeding microscopic animals.

[0017] The method for adjusting the ratio of suspended sludge VSS to carrier-adhered VSS is preferably one or more of the following: adjusting the carrier packing rate, adjusting DO (dissolved oxygen concentration) to reduce the amount of carrier adhesion, or adjusting the water flow rate to suppress excessive adhesion to the carrier. This method makes it possible to stably maintain good treated water quality even if the load on the first biological treatment tank fluctuates, and also reduces the amount of sludge. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is an explanatory diagram of a biological treatment method for organic wastewater according to an embodiment. [Figure 2] FIG. 1 is an explanatory diagram of a biological treatment method for organic wastewater according to an embodiment. [Figure 3] FIG. 1 is an explanatory diagram of a conventional biological treatment method for organic wastewater. [Figure 4] FIG. 1 is an explanatory diagram of a conventional biological treatment method for organic wastewater. DETAILED DESCRIPTION OF THE INVENTION

[0019] Fig. 1 is a schematic diagram of the basic flow of the present invention. Organic wastewater (raw water) is introduced into the first biological treatment tank 1 as water to be treated, and oxygen-containing gas (preferably air) is passed through the air diffuser 1b to aerate (aerate) the organic components (soluble BOD, soluble COD) by bacteria. Cr , soluble TOC) is decomposed by oxidation. 70% or more, preferably 80% or more, and even more preferably 90% or more. The first biological treatment tank 1 is a transient type, and sludge is not returned from the subsequent stage except during start-up or when treatment deteriorates. If the capacity of the first biological treatment tank 1 is large, multiple tanks may be connected in parallel or in series.

[0020] The 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 also introduced through the 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 tank is large, multiple tanks may be connected in parallel or in series, as with the first biological treatment tank. In Figure 1, three tanks are connected in series.

[0021] 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, and the supernatant water is taken out 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.

[0022] The pH of the first biological treatment tank 1 is preferably 6 to 8. However, if the raw water contains a large amount of oil, the pH may be set to 8.0 or higher. The BOD volume load on the first biological treatment tank 1 is 1 kg / m 3 / d or higher, COD Cr Volumetric load is 2kg / m 3 By setting the HRT at 48h or less and the HRT at 48h or more, it is possible to obtain primary biologically treated water in which dispersed bacteria (which are easily preyed upon by microscopic animals) predominate, which contributes to sludge reduction.

[0023] In this 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 biologically treated 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 20% or more, preferably 30% or more.

[0024] Even when dispersed bacteria dominate, they remain suspended in the system. Therefore, even within the design conditions, if the load temporarily increases due to load fluctuations (specifically, if the organic load in the first biological treatment tank fluctuates from a certain time to a value that is 1.2 times or more (e.g., 1.2 to 2 times) the average organic load in the first biological treatment tank over a specified period of time (e.g., within 6 hours)), this can lead to washout of dispersed bacteria and leakage of organic matter to downstream stages, resulting in a deterioration in treated water quality and sludge reduction. In this invention, the organic load refers to the flow rate (water load) of the water being treated flowing into the first biological treatment tank, the organic matter concentration of the water being treated flowing into the first biological treatment tank, or the product of the flow rate of the water being treated and the organic matter concentration of the water being treated.

[0025] Therefore, when there is a load fluctuation, the first biological treatment tank 1 is configured as a fluidized bed biological treatment tank with a fluidized bed of carriers 1a, as shown in Figure 1. This makes it possible to maintain 90% or more of the original organic matter removal capacity (removal rate) even with the above-mentioned load increase, and to maintain stable treated water quality and sludge reduction effects.

[0026] The shape of the carrier 1a may be any shape, such as a sphere, pellet, hollow cylinder, thread, plate, or square, and the size (diameter, length of one side) is about 0.1 to 10 mm. The material of the carrier may be any material, such as a natural material, inorganic material, or polymer material, and a gel-like substance may also be used. A preferred carrier is a polyurethane foam with a square shape and a side of 5 mm or less.

[0027] If an excessive amount of carriers is added to the first biological treatment tank 1, bacteria that have proliferated on or via the carriers 1a, rather than dispersed bacteria, will become dominant in the first biological treatment tank 1. Therefore, by adjusting the ratio of the suspended sludge VSS amount to the carrier-adhered VSS amount in the first biological treatment tank 1 to 0.5 to 5, preferably 2 to 5, the state in which dispersed bacteria with a particle size of 5 μm or less dominates can be maintained.

[0028] To measure the amount of VSS adhering to the carrier, the adhering matter can be removed from the carrier and measured in the same way as the SS and VSS concentrations of the suspended sludge, or the protein can be dissolved from the carrier, the protein concentration measured, and then converted from protein to VSS. The amount of VSS adhering to the carrier is calculated by multiplying the amount of adhering per carrier (mg-VSS / carrier) measured in this way by the number of carriers in the first biological treatment tank. The amount of VSS adhering to the carrier is calculated by multiplying the VSS concentration (mg / L) in the first biological treatment tank by the capacity of the first biological treatment tank.

[0029] The first method for adjusting the ratio of suspended sludge VSS amount to carrier-adhered VSS amount is to appropriately set the carrier filling rate. The carrier filling rate is determined by conducting a water flow test in advance to determine the suspended sludge concentration (VSS) and carrier-adhered VSS amount when water is passed through at the design load, i.e., the expected water quality, water volume, and load, and then setting the carrier filling rate so that the ratio of suspended sludge VSS amount to carrier-adhered VSS amount is within the above range (0.5 to 5, preferably 2 to 5). The first biological treatment tank 1 preferably has a BOD volumetric load of 1 kg-BOD / m 3 / d or more (COD Cr 2kg-COD volumetric load Cr / m 3 / d or more, 0.7kg-TOC / m at TOC volumetric load 3 / d or more), and more preferably a BOD volumetric load of 2 kg-BOD / m 3 / d or more (COD Cr 4kg-COD volumetric load Cr / m 3 / d or more, TOC volumetric loading of 1.4 kg-TOC / m 3 / d or more), and the carrier loading rate at that time is 5 to 20%, preferably 5 to 10%.

[0030] A second method for adjusting the ratio of suspended sludge VSS to carrier-adhered VSS is to adjust the aeration rate to adjust DO and reduce the carrier-adhered amount, thereby keeping the ratio within a predetermined range. In this method, by controlling the DO in the first biological treatment tank 1 to 0.5 mg / L or less, preferably 0.1 mg / L or less, and more preferably 0.05 mg / L or less, dispersed bacteria with a size of 1 to 5 μm become dominant. These dispersed bacteria with a size of 1 to 5 μm are quickly consumed in the second biological treatment tank 2.

[0031] A third method for adjusting the ratio of suspended sludge VSS to carrier-attached VSS is to adjust the flow rate of treated water to the first biological treatment tank to suppress excessive adhesion to the carriers and adjust the ratio. One example of this method, as shown in Figure 2, involves bypassing the first biological treatment tank and adding a portion of the diluted organic wastewater (raw water) directly to the second biological treatment tank 2, bypassing the first biological treatment tank, while allowing the remaining diluted organic wastewater to flow into the first biological treatment tank as treated water. This allows the HRT of the first biological treatment tank to be set at 2 hours or more, preferably 4 hours or more, without significantly reducing the organic load on the first biological treatment tank, thereby increasing the contribution of suspended sludge to organic matter removal. An example of diluted organic wastewater is wastewater from the latter half of the washing process before the raw water is combined with the first biological treatment tank. It is desirable to bypass a part of the dilute organic wastewater before the concentrated organic wastewater and the dilute organic wastewater are mixed to form organic wastewater (raw water).

[0032] Two or more of these methods may be applied to adjust the ratio of the amount of VSS in suspended sludge to the amount of VSS attached to the carrier.

[0033] At industrial wastewater treatment sites for food, beverage, etc., the water quality, water volume, and load often differ from initial expectations, and load fluctuations can reduce the sludge reduction effect after actual operation begins. In such facilities, good treated water quality and sludge reduction effects can be maintained by periodically measuring the SS particle size distribution of the treated water from the first biological treatment tank 1 (first biological treated water), and when the relative amount of particles with a particle size of 5 μm or less falls below 20%, strictly controlling the ratio of suspended sludge VSS amount to carrier-adhered VSS amount to be in the range of 2 to 5.

[0034] By biologically treating raw water using the above method, the dominance of dispersed bacteria of 1 to 5 μm size is maintained in the first biological treatment tank 1 even under load fluctuations, and good treated water quality and a sludge volume reduction effect of 50% or more compared to the standard activated sludge process can be consistently obtained. [Example]

[0035] Reference examples, comparative examples and working examples will be described below.

[0036] [Reference example 1] As shown in FIG. 3, a biological treatment device with the same flow as that shown in FIG. 1, except that the carrier 1a was not added, was operated under the following conditions. <Operating conditions> First biological treatment tank (without carrier): 2.5L Second biological treatment tank: 10L Raw water: Food manufacturing wastewater (COD) Cr = 2000 mg / L (S.COD Cr =2000mg / L), BOD=1000mg / L) Raw water amount: 12.5L / d Sludge concentration in the first biological treatment tank 1: SS = 1000 mg / L (VSS = 950 mg / L) The relative amount of particles with a particle size of 5 μm or less in the treated water from the first biological treatment tank is 35% (after passing through a sieve with 2 mm openings). Sludge concentration in the second biological treatment tank: SS = 5500 mg / L (VSS = 5000 mg / L) Treated water: COD Cr = 30 mg / L, SS = 10 mg / L Sludge conversion rate: 0.1g-VSS / g-removed COD Cr SRT:21 days

[0037] <Result> Sludge conversion rate is 0.1g-VSS / g-removed COD Cr The COD of the treated water is low. Cr = 30mg / L and SS = 10mg / L, which was a good quality of treated water.

[0038] [Reference example 2] As shown in Figure 4, a standard activated sludge process using a biological treatment device consisting of a biological treatment tank 1' without the addition of carriers and a settling tank 3 was operated under the following conditions. <Operating conditions> Aerobic biological treatment tank: 10L Raw water: Food manufacturing wastewater (COD) Cr = 2000 mg / L (S.COD Cr =2000mg / L), BOD=1000mg / L), Raw water amount: 10L / d Sludge concentration in biological treatment tank 1': SS = 5500 mg / L (VSS = 5000 mg / L) Treated water: COD Cr = 30 mg / L, SS = 10 mg / L Sludge conversion rate: 0.2g-VSS / g-removed COD Cr SRT=10.5 days

[0039] <Result> Treated water is COD Cr Although good treated water quality was obtained with VSS = 30mg / L and SS = 10mg / L, the sludge conversion rate was 0.2g-VSS / g-removed COD Cr was higher than that of Reference Example 1.

[0040] [Comparative Example 1] The biological treatment device shown in FIG. 3 (the same as that used in Reference Example 1) was operated with "raw water volume fluctuations" as follows. <Operating conditions> First biological treatment tank (without carrier): 2.5L Second biological treatment tank: 10L Raw water: Food manufacturing wastewater (COD) Cr = 2000 mg / L (S.COD Cr =2000mg / L), BOD=1000mg / L) Raw water volume: 12.5L / d or 7L / d (Water is passed at a daily equivalent volume of 7L / d from midnight to 8am, then the volume is increased to 12.5L / d from 6am to 12pm, and continues until midnight, then returns to 7L / d, and this cycle is repeated) Sludge concentration in the first biological treatment tank 1: SS = 1000 mg / L (VSS = 950 mg / L) Sludge concentration in the second biological treatment tank 2: SS = 5500 mg / L (VSS = 5000 mg / L) Average treated water COD at 12.5L / d Cr = 50 mg / L, SS = 30 mg / L Sludge conversion rate: 0.16g-VSS / g-removed COD Cr

[0041] <Result> Compared to Reference Example 1, the sludge conversion rate was 0.16 g-VSS / g-removed COD. Cr The COD of the treated water is high. Cr = 50mg / L, SS = 30mg / L. The relative amount of particles with a particle size of 5μm or less in the SS of the first biological treatment effluent was 19% (after passing through a sieve with 2mm openings).

[0042] [Example 1] The biological treatment device shown in Figure 1 was operated under the following conditions (with fluctuations in the amount of raw water). <Operating conditions> First biological treatment tank (with carrier): 2.5L Carrier filling rate: 5% (3.5mm square polyurethane foam carrier) Second biological treatment tank: 10L Raw water: Food manufacturing wastewater (COD) Cr = 2000 mg / L (S.COD Cr =2000mg / L), BOD=1000mg / L) Raw water volume: 12.5L / d or 7L / d (7L / d is fed from 0-8 hours of the day, then the volume is increased to 12.5L / d between 6-12 noon and continues until midnight, then returned to 7L / d, and this cycle is repeated) Sludge concentration in the first biological treatment tank 1 (suspended sludge): SS = 600 mg / L (VSS = 570 mg / L) Amount of sludge attached per carrier: 0.7g-VSS DO of first biological treatment tank 1: 1.0 mg / L Average suspended sludge VSS amount / carrier-attached VSS amount during water flow including fluctuations: 1.4 Sludge concentration in the second biological treatment tank: SS = 5000 mg / L (VSS = 4600 mg / L) Average treated water COD at 12.5L / d Cr =25mg / L, SS<10mg / L Sludge conversion rate: 0.10g-VSS / g-removed COD Cr

[0043] <Result> Although there was some variation compared to Comparative Example 1, the sludge conversion rate was 0.1g-VSS / g-removed COD Cr The COD of the treated water is maintained at the same level. Cr = 25mg / L and SS < 10mg / L, maintaining an excellent condition. The relative amount of particles with a particle size of 5μm or less in the SS of the first biological treatment tank treated water was 25% (after passing through a sieve with 2mm openings).

[0044] [Example 2] The same biological treatment device as in Example 1 was operated under the following conditions. <Operating conditions> Raw water: Food manufacturing wastewater (COD) Cr = 2000 mg / L (S.COD Cr =2000mg / L), BOD=1000mg / L) Water volume: 12.5L / d or 7L / d (7L / d is used from midnight to 8am, then increased to 12.5L / d between 6am and 12pm, and continues until midnight, then returned to 7L / d, repeating this cycle) DO of the first biological treatment tank: 0.5 mg / L Sludge concentration in the first biological treatment tank (suspended sludge): SS = 600 mg / L (VSS = 570 mg / L) Carrier filling rate: 5% (3.5mm square polyurethane foam carrier) Amount of sludge attached per carrier: 0.7g-VSS Average suspended sludge VSS amount / carrier-attached VSS amount during water flow including fluctuations: 2.04 Sludge concentration in the second biological treatment tank: SS = 4800 mg / L (VSS = 4500 mg / L) Average treated water COD at 12.5L / d Cr =25mg / L, SS<10mg / L Sludge conversion rate: 0.09g-VSS / g-removed COD Cr

[0045] <Result> Despite the load fluctuations, the sludge conversion rate was 0.09 g-VSS / g-removed COD Cr was significantly lower than that of Comparative Example 1, and the COD of the treated water was also Cr = 25mg / L and SS < 10mg / L, maintaining an excellent condition. The relative amount of particles with a particle size of 5μm or less in the SS of the first biological treatment tank treated water was 31% (after passing through a sieve with 2mm openings).

[0046] Comparative Example 2 <Operating conditions> The same biological treatment device as in Example 1 was operated under the following load fluctuation conditions. Raw water: Food manufacturing wastewater (COD) Cr = 1000 mg / L (S.COD Cr =1000mg / L), BOD=500mg / L) Water volume: 25L / d or 14L / d (14L / d is fed from midnight to 8am, then increased to 25L / d between 6am and 12pm, and continued until midnight, then returned to 14L / d, and this cycle is repeated.) Sludge concentration in the first biological treatment tank (suspended sludge): SS = 750 mg / L (VSS = 275 mg / L) Carrier filling rate: 5% (3.5mm square polyurethane carrier) Amount of sludge attached per carrier: 1.4g-VSS DO of the first biological treatment tank: 1.0 mg / L Average suspended sludge VSS amount / carrier-attached VSS amount during water flow including fluctuations: 0.49 Sludge concentration in the second biological treatment tank: SS = 4800 mg / L (VSS = 4500 mg / L) Average treated water COD when 25L / d water flow rate Cr =50mg / L, SS>30mg / L Sludge conversion rate: 0.18g-VSS / g-removed COD Cr

[0047] <Result> Sludge conversion rate is 0.18g-VSS / g-removed COD Cr The COD of the treated water also deteriorated. Cr = 50mg / L, SS > 30mg / L, indicating that there was an SS leak. The relative amount of particles with a particle size of 5μm or less in the SS of the treated water from the first biological treatment tank was 5% (after passing through a sieve with 2mm openings), and filamentous bacteria were dominant.

[0048] [Example 3] The same biological treatment device as in Example 1 was operated under the flow chart of FIG. 2 (part of the diluted organic wastewater was directly added to the second biological treatment tank) with the following load fluctuations. <Operating conditions> Raw water: Food manufacturing wastewater (COD) Cr = 1000 mg / L (S.COD Cr =1000mg / L), BOD=500mg / L) Dilute organic wastewater: Wastewater from the latter half of the washing process (COD Cr <50mg / L(S.COD Cr <50mg / L), BOD<30mg / L) Water volume: Raw water is fed to the first biological treatment tank at 25L / d or 14L / d (14L / d is fed from 0-8am, the volume is increased to 25L / d between 6-12pm, and fed until midnight, then the volume is returned to 14L / d, and this operation is repeated). However, when feeding 25L / d, the volume of diluted organic wastewater (COD Cr<50 mg / L) is bypassed and added directly to the second biological treatment tank 2. Therefore, the amount of water to be treated passing through the first biological treatment tank is reduced by the amount of water bypassed, to the equivalent of 17.5 L / d. Sludge concentration in the first biological treatment tank (suspended sludge): SS = 420 mg / L (VSS = 400 mg / L) Carrier filling rate: 5% (3.5mm square polyurethane carrier) Amount of sludge attached per carrier: 0.5g-VSS DO of the first biological treatment tank: 0.2 mg / L Average suspended sludge VSS amount / carrier-attached VSS amount during water flow including fluctuations: 2.00 Sludge concentration in the second biological treatment tank: SS = 4800 mg / L (VSS = 4500 mg / L) Average treated water COD when 25L / d water flow rate Cr =15mg / L, SS<10mg / L Sludge conversion rate: 0.08g-VSS / g-removed COD Cr

[0049] <Result> Sludge conversion rate is 0.08g-VSS / g-removed COD Cr The COD of the treated water is also very low. Cr = 15mg / L and SS < 10mg / L, maintaining very good conditions. The relative amount of particles with a particle size of 5μm or less in the SS of the treated water from the first biological treatment tank was 34% (after passing through a sieve with 2mm openings). [Explanation of symbols]

[0050] 1. First biological treatment tank 2. Second biological treatment tank 3 Sedimentation tank

Claims

1. A biological treatment method for organic wastewater, comprising: passing water to be treated, which is organic wastewater, through a first biological treatment tank that performs aerobic biological treatment, biologically treating the water with bacteria; introducing the first biologically treated water from the first biological treatment tank containing dispersed bacteria into a second biological treatment tank; and allowing microorganisms to feed on the bacteria; The method for biological treatment of organic wastewater is characterized in that the first biological treatment tank is a fluidized bed of carriers, and when an increase in the organic matter load in the first biological treatment tank exceeds a predetermined level, adjustment measures are taken to adjust the ratio of the average suspended sludge VSS amount in the first biological treatment tank to the average suspended sludge VSS amount attached to the carriers to be 0.5 to 5 (including adjusting the carrier filling rate in the first biological treatment tank, adjusting the DO in the first biological treatment tank, or adjusting the flow rate of water to be treated flowing into the first biological treatment tank).

2. 2. The biological treatment method for organic wastewater according to claim 1, wherein the organic matter load is the amount of water flowing into the first biological treatment tank, the organic matter concentration of the water flowing into the first biological treatment tank, or the product of the amount of water flowing and the organic matter concentration of the water.

3. 2. The biological treatment method for organic wastewater according to claim 1, wherein the adjustment procedure is performed when the organic matter load in the first biological treatment tank is 1.2 times or more the average organic matter load in the first biological treatment tank over the most recent specified period.

4. 2. The method for biological treatment of organic wastewater according to claim 1, wherein the adjustment step comprises adjusting the packing rate of carriers in the first biological treatment tank.

5. 2. The method for biological treatment of organic wastewater according to claim 1, wherein the DO in the first biological treatment tank is adjusted as the adjustment treatment.

6. 2. The method for biological treatment of organic wastewater according to claim 1, wherein the adjustment step comprises adjusting the amount of water to be treated flowing into the first biological treatment tank.

7. 7. The biological treatment method for organic wastewater according to claim 1, wherein the particle size distribution of SS in the treated water in the first biological treatment tank is measured, and when the relative amount of particles with a particle size of 5 μm or less is less than 20%, the adjustment treatment is carried out so that the ratio of the amount of suspended sludge VSS to the amount of carrier-adhered VSS is 2 to 5.

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