Method for biologically treating organic waste water

Abstract

This method for biologically treating organic waste water comprises: passing water to be treated containing organic waste water through a first biological treatment tank 1 for performing aerobic biological treatment to biologically treat the water with bacteria; introducing first biologically treated water containing dispersed bacteria from the first biological treatment tank into a second biological treatment tank 2; and allowing a minute animal to catch the bacteria. The method for biologically treating organic waste water is characterized in that the first biological treatment tank 1 is a fluidized bed, and when an increase of a prescribed value or more occurs in the organic loading in the first biological treatment tank 1, an adjustment is performed so that an average [floating sludge VSS] / [carrier adhered VSS] ratio in the first biological treatment tank 1 is 0.5-5.

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 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.

[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 Patent Application Publication No. 55-20649

[0005] The multi-stage activated sludge process, which utilizes the feeding action of microorganisms, can reduce the amount 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 processes 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 component of the treatment. This can lead to unstable conversion to dispersed bacteria (hereinafter referred to as "dispersed bacteria") and 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.

[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, consisting of organic wastewater, is passed through a first biological treatment tank that performs aerobic biological treatment, 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 upon by microscopic animals, 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 [amount of suspended sludge VSS] / [amount of carrier-adhered VSS] 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 flow rate of the water to be treated flowing into the first biological treatment tank, the organic matter concentration of the water to be treated flowing into the first biological treatment tank, or the product of the flow rate of the water to be treated and the organic matter concentration of the water to be treated.

[0011] [3] The biological treatment method for organic wastewater according to [1] or [2], wherein the adjustment treatment is performed when the organic matter load in the first biological treatment tank is 1.2 times or more the average value of the organic matter load in the first biological treatment tank over the most recent predetermined time period.

[0012] [4] The biological treatment method for organic wastewater according to any one of [1] to [3], wherein the adjustment treatment is to adjust the packing rate of the carrier.

[0013] [5] The biological treatment method for organic wastewater according to any one of [1] to [3], wherein DO is adjusted as the adjustment treatment.

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

[0015] [7] The 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 [amount of suspended sludge VSS] / [amount of carrier-adhered VSS] is 2 to 5.

[0016] The present invention appropriately adjusts the ratio of suspended sludge VSS to carrier-adhered VSS in the first biological treatment tank, and makes 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 [amount of suspended sludge VSS] / [amount of carrier-adhered VSS] is preferably one or more of the following: adjusting the carrier filling 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.

[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 conventional biological treatment method for organic wastewater. Fig. 4 is an explanatory diagram of a conventional biological treatment method for organic wastewater.

[0019] Fig. 1 is a schematic diagram of the basic flow of the present invention. Organic wastewater (raw water) is introduced as water to be treated into a first biological treatment tank 1, and oxygen-containing gas (preferably air) is passed through an aeration pipe 1b to aerate the water. Organic components (soluble BOD, soluble COD) are then removed by bacteria. Ｃｒ , soluble TOC) is oxidatively decomposed. 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 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 tank is large, multiple tanks may be connected in parallel or in series, as in 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. 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.

[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 more, COD Ｃｒ Volume load is 2 kg / m ３By setting the HRT at 48 h or less and the suction time at 48 h or more, it is possible to obtain first biologically treated water in which dispersed bacteria (which are easily preyed upon by microscopic animals) that contribute to sludge reduction are dominant.

[0023] 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 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 exist in a suspended state within 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 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 subsequent 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 to be treated flowing into the first biological treatment tank, the organic matter concentration of the water to be treated flowing into the first biological treatment tank, or the product of the flow rate of the water to be treated and the organic matter concentration of the water to be 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 when there is an increase in the load as described above, 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 desirable 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 [amount of suspended sludge VSS] / [amount of carrier-adhered VSS] 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 dominate can be maintained.

[0028] To measure the amount of VSS adhering to the carrier, the adhering matter may be removed from the carrier and measured in the same manner as the SS and VSS concentrations of the suspended sludge, or the protein may be dissolved from the carrier, the protein concentration may be measured, and the VSS amount may be calculated from the protein concentration. 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 suspended sludge VSS 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 [amount of suspended sludge VSS] / [amount of carrier-adhered VSS] is to appropriately set the carrier packing rate. The carrier packing rate is determined by conducting a water flow test in advance to determine the suspended sludge concentration (VSS) and the amount of carrier-adhered VSS when water is passed through at the design load, i.e., the expected water quality, water volume, and load, and then setting the carrier packing rate so that the ratio of [amount of suspended sludge VSS] / [amount of carrier-adhered VSS] is within the above range (0.5 to 5, preferably 2 to 5). The first biological treatment tank 1 preferably has a BOD volume load of 1 kg-BOD / m ３ / d or more (COD Ｃｒ 2kg-COD volumetric load Ｃｒ / m ３ / d or more, TOC volume load 0.7kg-TOC / m ３ / d or more), and more preferably a BOD volume load of 2 kg-BOD / m ３ / d or more (COD Ｃｒ Volume load: 4kg-COD Ｃｒ / m ３ / d or more, TOC volume load 1.4 kg-TOC / m ３ / d or more), and the carrier loading rate is set to 5 to 20%, preferably 5 to 10%.

[0030] A second method for adjusting the ratio of [suspended sludge VSS amount] to [carrier-adhered VSS amount] 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 of 1 to 5 μm become dominant. These dispersed bacteria of 1 to 5 μm are quickly consumed in the second biological treatment tank 2.

[0031] A third method for adjusting the [suspended sludge VSS amount] / [carrier-adhered VSS amount] ratio involves adjusting the flow rate of water to be treated into the first biological treatment tank to suppress excessive adhesion to the carrier and adjust the [suspended sludge VSS amount] / [carrier-adhered VSS amount] ratio. As shown in Figure 2, one example of this method involves bypassing the first biological treatment tank 1 and adding a portion of the diluted organic wastewater (raw water) from the concentrated organic wastewater and diluted organic wastewater (with an organic matter concentration of 20% or less of the raw water's organic matter concentration) directly to the second biological treatment tank 2, and then flowing the remaining concentrated organic wastewater into the first biological treatment tank 1 as the treated water. This allows the HRT of the first biological treatment tank 1 to be set to 2 hours or more, preferably 4 hours or more, without significantly reducing the organic matter load on the first biological treatment tank 1, thereby increasing the contribution of suspended sludge to organic matter removal. Examples of diluted organic wastewater include wastewater from the latter half of the cleaning process before the raw water tank is combined. 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 used to adjust the ratio of [amount of suspended sludge VSS] / [amount of carrier-adhered VSS].

[0033] In addition, 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 effect can be maintained by periodically measuring the particle size distribution of SS in the treated water (first biological treated water) in the first biological treatment tank 1, and when the relative amount of particles with a particle size of 5 μm or less falls to 20% or less, strictly controlling the ratio of [amount of suspended sludge VSS] / [amount of carrier-adhered VSS] 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 in size can be 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.

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

[0036] [Reference Example 1] As shown in Figure 3, a biological treatment device with the same flow as in Figure 1 was operated under the following conditions, except that a first biological treatment tank 1 to which no carrier 1a was added was installed. Ｃｒ is soluble COD Ｃｒ <Operating conditions> Volume of first biological treatment tank (without carrier) 1: 2.5 L Volume of second biological treatment tank 2: 10 L Raw water: Food manufacturing wastewater (COD Ｃｒ =2000mg / L (S.COD Ｃｒ = 2000 mg / L), BOD = 1000 mg / L) Raw water volume: 12.5 L / d Sludge concentration in 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 treated water SS of first biological treatment tank 1 was 35% (after passing through a sieve with 2 mm openings) Sludge concentration in second biological treatment tank 2: SS = 5500 mg / L (VSS = 5000 mg / L) Treated water: COD Ｃｒ = 30 mg / L, SS = 10 mg / L Sludge conversion rate: 0.1 g-VSS / g-removed COD Ｃｒ SRT: 21st

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

[0038] [Reference Example 2] As shown in Figure 4, a standard activated sludge process using a biological treatment device consisting of an aerobic biological treatment tank 1' without the addition of carriers and a settling tank 3 was operated under the following conditions. <Operating conditions> Volume of aerobic biological treatment tank 1': 10 L Raw water: Food manufacturing wastewater (COD Ｃｒ =2000mg / L (S.COD Ｃｒ = 2000 mg / L), BOD = 1000 mg / L), raw water volume: 10 L / 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: 0.2 g-VSS / g-removed COD Ｃｒ SRT=10.5 days

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

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

[0041] <Results> 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 = 50 mg / L, and SS = 30 mg / L. The relative amount of SS particles with a particle size of 5 μm or less in the treated water of the first biological treatment tank 1 was 19% (after passing through a sieve with 2 mm openings).

[0042] Example 1 The biological treatment device shown in Figure 1 was operated under the following conditions (with varying raw water volume). <Operating conditions> Volume of first biological treatment tank (with carrier) 1: 2.5 L Carrier filling rate: 5% (3.5 mm square polyurethane foam carrier) Volume of second biological treatment tank 2: 10 L Raw water: food manufacturing wastewater (COD Ｃｒ =2000mg / L (S.COD Ｃｒ = 2000 mg / L), BOD = 1000 mg / L) Raw water volume: 12.5 L / d or 7 L / d (water is passed at a volume of 7 L / d for 0-8 hours each day, then the volume is increased to 12.5 L / d between 6:00 and 12:00 and continues until midnight, then returned to 7 L / d, and this operation is repeated) Sludge concentration (suspended sludge) in first biological treatment tank 1: SS = 600 mg / L (VSS = 570 mg / L) Amount of sludge attached per carrier: 0.7 g - VSS DO in first biological treatment tank 1: 1.0 mg / L Average [amount of suspended sludge VSS] / [amount of VSS attached to carrier] during water passage, including fluctuations: 1.4 Sludge concentration in second biological treatment tank 2: SS = 5000 mg / L (VSS = 4600 mg / L) Average treated water when passing 12.5 L / d: COD Ｃｒ = 25 mg / L, SS < 10 mg / L Sludge conversion rate: 0.10 g - VSS / g - removed COD Ｃｒ

[0043] <Results> Despite some fluctuations compared to Comparative Example 1, the sludge conversion rate was 0.1 g-VSS / g-removed COD Ｃｒ The COD of the treated water is maintained at the same level. Ｃｒ= 25 mg / L and SS < 10 mg / L, maintaining an extremely good condition. The relative amount of SS particles with a particle size of 5 μm or less in the treated water of the first biological treatment tank 1 was 25% (after passing through a sieve with 2 mm 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 =2000mg / L (S.COD Ｃｒ = 2000 mg / L), BOD = 1000 mg / L) Raw water volume: 12.5 L / d or 7 L / d (water is passed at 7 L / d from midnight to 8 am, the volume is increased to 12.5 L / d from 6 am to 12 noon, water is passed until midnight, and then returned to 7 L / d, this operation is repeated) DO of first biological treatment tank 1: 0.5 mg / L Sludge concentration (suspended sludge) in first biological treatment tank 1: SS = 600 mg / L (VSS = 570 mg / L) Carrier filling rate 5% (3.5 mm square polyurethane foam carrier) Amount of attached sludge per carrier: 0.7 g-VSS Average [amount of suspended sludge VSS] / [amount of VSS attached to carrier] during water passage, including fluctuations: 2.04 Sludge concentration in the second biological treatment tank 2: SS = 4800 mg / L (VSS = 4500 mg / L) Average treated water at 12.5 L / d flow rate: COD Ｃｒ = 25 mg / L, SS < 10 mg / L Sludge conversion rate: 0.09 g-VSS / g-removed COD Ｃｒ

[0045] <Results> 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 = 25 mg / L and SS < 10 mg / L, maintaining an extremely good condition. The relative amount of SS particles with a particle size of 5 μm or less in the treated water of the first biological treatment tank 1 was 31% (after passing through a sieve with 2 mm openings).

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

[0047] <Results> Sludge conversion rate was 0.18g-VSS / g-removed COD Ｃｒ The COD of the treated water also worsened. Ｃｒ = 50 mg / L, SS > 30 mg / L, indicating that SS leakage had occurred. The relative amount of SS particles with a particle size of 5 μm or less in the treated water of the first biological treatment tank 1 was 5% (after passing through a sieve with 2 mm 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 Figure 2 (part of the diluted organic wastewater was directly added to the second biological treatment tank 2) under the following load fluctuation conditions. <Operating conditions> Raw water: food manufacturing wastewater (COD Ｃｒ =1000mg / L (S.COD Ｃｒ = 1000 mg / L), BOD = 500 mg / 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) Raw water volume: Raw water is fed to the first biological treatment tank 1 at 25L / d or 14L / d (14L / d is fed from 0:00 to 8:00, the volume is increased to 25L / d between 6:00 and 12:00, and fed until midnight, then the volume is returned to 14L / d again, and this operation is repeated). However, when feeding at 25L / d, the volume of diluted organic wastewater (CODＣｒ <50 mg / L) 7.5 L / d is bypassed and added directly to the second biological treatment tank 2. Therefore, the amount of water passed through the first biological treatment tank 1 is reduced by the amount of bypassed water, equivalent to 17.5 L / d. Sludge concentration (suspended sludge) in the first biological treatment tank 1: SS = 420 mg / L (VSS = 400 mg / L) Carrier filling rate: 5% (3.5 mm square polyurethane carrier) Amount of sludge attached per carrier: 0.5 g - VSS DO in the first biological treatment tank 1: 0.2 mg / L Average [amount of suspended sludge VSS] / [amount of VSS attached to carrier] during water flow, including fluctuations: 2.00 Sludge concentration in the second biological treatment tank 2: SS = 4800 mg / L (VSS = 4500 mg / L) Average treated water during water flow of 25 L / d: COD Ｃｒ = 15 mg / L, SS < 10 mg / L Sludge conversion rate: 0.08 g-VSS / g-removed COD Ｃｒ

[0049] <Results> Sludge conversion rate was 0.08g-VSS / g-removed COD Ｃｒ The COD of the treated water is very low. Ｃｒ = 15 mg / L and SS < 10 mg / L, maintaining an extremely good condition. The relative amount of SS particles with a particle size of 5 μm or less in the treated water of the first biological treatment tank 1 was 34% (after passing through a sieve with 2 mm openings).

[0050] 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. 2024-038252 filed on March 12, 2024, the entire contents of which are incorporated by reference.

[0051] 1, 1' First biological treatment tank 2 Second biological treatment tank 3 Sedimentation tank

Claims

1. 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 from the first biological treatment tank containing dispersed bacteria is introduced into a second biological treatment tank, where the bacteria are preyed upon by microscopic animals, 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 to ensure that the average ratio of [amount of suspended sludge VSS] to [amount of carrier-adhered VSS] in the first biological treatment tank is 0.5 to 5.

2. The biological treatment method for organic wastewater of claim 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 to be treated flowing into the first biological treatment tank, or the product of the amount of water flowing into the first biological treatment tank and the organic matter concentration in the water to be treated.

3. A biological treatment method for organic wastewater as claimed in claim 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 of time.

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

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

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

7. A biological treatment method for organic wastewater according to any one of claims 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 procedure is carried out so that the ratio of [amount of suspended sludge VSS] / [amount of carrier-adhered VSS] is 2 to 5.

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