JPH0663589A - Sewage treatment device - Google Patents

Abstract

PURPOSE:To provide the sewage treatment device with which treated water having high water quality is obtd. by suppressing the propagation of filamentous bacteria to prevent the generation of filamentous bulking and efficiently removing BOD and nitrogen to prevent the floating of sludge in a tank for sepn. of liquid from solid. CONSTITUTION:Raw water 1 is introduced into this device and is mixed with a return liquid mixture 2a from a denitrification tank 12 or aeration tank 13. The above device is constituted of a BOD absorption tank 11 which absorbs the BOD in the raw water in the sludge, the denitrification tank 12 which mixes the liquid mixture 15 of the BOD absorption tank 11, the return liquid mixture 2b from the aeration tank 13 and the return sludge 5 from the tank 7 for the sepn. of liquid from solid and anaerobically executes denitrification, the aeration tank 13 into which the liquid mixture 19 of the denitrification tank 12 is introduced and which aerobically oxidizes and decomposes the BOD and the tank 7 for the sepn. of liquid from solid which subjects the liquid mixture 21 of the aeration tank 13 to the sepn. of liquid from solid and returns the sludge.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sewage treatment apparatus for biologically removing BOD and nitrogen in sewage, and more particularly to a sewage treatment apparatus capable of preventing filamentous bulking.

[0002]

2. Description of the Related Art When biologically aerobic oxidation of BOD in sewage is carried out by an activated sludge method, filamentous bacteria grow and filamentous bulking occurs due to the properties of raw water, treatment conditions, etc. Separability deteriorates, and normal processing cannot be continued.

As an aerobic treatment method for preventing such filamentous bulking, a first aeration tank and a second aeration tank are provided, the first aeration tank is used as a floc-like bacterial growth portion, and the second aeration tank is used for oxidation treatment. There is a method of performing it (for example, Japanese Patent Publication No. 54-2202).
5 and 61-20356).

FIG. 2 is a flow chart showing such a conventional sewage treatment apparatus. In this apparatus, the raw water 1 enters the first aeration tank 3 together with the returned mixed liquid 2, and the air sent from the blower 4 causes the floc-like bacteria to proliferate and undergoes aeration treatment under the condition that BOD is oxidatively decomposed. The mixed liquid in the first aeration tank 3 enters the second aeration tank 6 together with the returned sludge 5, and is mixed and aerated by the air sent from the blower 4, and BOD is biologically oxidatively decomposed and removed. The mixed liquid in the second aeration tank 6 is solid-liquid separated in the solid-liquid separation tank 7, and the separated liquid is treated water 8
As a result, the sludge is returned to the second aeration tank by the sludge return pump 9. The mixed liquid in the second aeration tank 6 is returned to the first aeration tank 3 by the mixed liquid return pump 10.

[0005]

In such a conventional sewage treatment apparatus, aeration treatment is performed under the condition that floc-like bacteria grow in the first aeration tank. However, depending on the nature of raw water and other conditions. It is difficult to maintain the conditions under which floc-like bacteria grow, and depending on the treatment conditions, filamentous bacteria may grow and filamentous bulking may occur.

Further, when the raw water contains a large amount of nitrogen, nitrification proceeds in the second aeration tank, a denitrification reaction occurs while the aeration liquid stays in the solid-liquid separation tank, the separated sludge floats, and the treated water quality is improved. Getting worse. To prevent such nitrification, the second
There is a problem that it is necessary to strictly control the DO and MLSS concentrations in the aeration tank.

The object of the present invention is to solve the problems of the prior art by preventing the occurrence of filamentous bulking and efficiently removing BOD and nitrogen to prevent the sludge from floating in a solid-liquid separation tank. An object of the present invention is to provide a sewage treatment apparatus that can obtain treated water of high water quality.

[0008]

The present invention is directed to a BOD absorption tank in which raw water is introduced and mixed with a return mixed solution from a denitrification tank or an aeration tank to absorb BOD in the raw water into sludge, and a BOD absorption tank. A denitrification tank that mixes the mixed solution in the tank and the sludge returned from the solid-liquid separation tank to denitrify under anaerobic conditions, and the mixed solution in this denitrification tank is introduced and aerated to aerobically oxidize and decompose BOD. An aeration tank, a solid-liquid separation tank for solid-liquid separation of the mixed liquid of this aeration tank, a sludge returning device for returning the sludge separated by this solid-liquid separation tank to the denitrification tank, and a mixture of the denitrification tank or the aeration tank A sewage treatment apparatus comprising: a first mixed solution returning device for returning the liquid to the BOD absorption tank; and a second mixed liquid returning device for returning the mixed solution of the aeration tank to the denitrification tank. is there.

[0009] Examples of sewage to be treated by the sewage treatment apparatus of the present invention include sewage containing BOD and nitrogen, for example, domestic sewage, food processing wastewater, wastewater from a place, petrochemical wastewater and the like.

[0010]

In the sewage treatment apparatus of the present invention, first, BOD
The raw water and the returned mixed solution are introduced into the absorption tank and mixed, and BOD in the raw water is absorbed by the sludge in the returned mixed solution at a rate faster than the oxidative decomposition rate. This suppresses the growth of filamentous bacteria that grow at a rate commensurate with the oxidative decomposition rate. BOD
It is only necessary that the sludge can absorb the BOD component in the absorption tank, and it is not necessary to grow the floc-like bacteria or oxidatively decompose the BOD. Therefore, in the BOD absorption tank, it is preferable to perform aeration so that the sludge can easily absorb BOD, but a low degree of aeration is sufficient and DO may be zero.

In the denitrification tank, the mixed solution in the BOD absorption tank, the returned sludge from the solid-liquid separation tank, and, if necessary, the returned mixed solution from the aeration tank are introduced and mixed. Nitrous bacteria grow. The denitrifying bacteria assimilate the BOD absorbed in the BOD absorption tank, and in the process, reduce nitric acid or nitrite nitrogen contained in the return mixture to nitrogen gas to denitrify. Filamentous bacteria do not grow in anaerobic conditions.
On the other hand, sludge that has absorbed BOD maintains the BOD absorption state.

In the aeration tank, BOD-degrading bacteria in the sludge assimilate the absorbed BOD and grow by introducing the mixed solution of the denitrification tank and aerating, and the BOD is aerobically oxidatively decomposed. Most of the BOD in this mixed solution is absorbed in the sludge, and since the solution has a very low concentration, filamentous bacteria do not grow and bulking does not occur. Aeration is performed to such an extent that nitrogen in the liquid is converted to nitric acid or nitrite nitrogen.

In the solid-liquid separation tank, the mixed liquid of the aeration tank is introduced to perform solid-liquid separation, the separated liquid is sent to the next step as treated water, and the separated sludge is partially returned to the denitrification tank by the sludge returning device, The rest is discharged as excess sludge. The mixed solution in the denitrification tank or the aeration tank is returned to the BOD absorption tank by the first mixed solution returning device. The mixed solution in the aeration tank is returned to the denitrification tank by the second mixed solution returning device as needed, that is, when the nitrogen removal rate is increased.

In the sewage treatment apparatus of the present invention, BOD is absorbed into sludge faster than the oxidative decomposition rate of BOD in the BOD absorption tank, so that the denitrification tank maintains an anaerobic state, and the aeration tank contains BOD in the liquid. Since filamentous bacteria do not exist, filamentous bacteria do not grow in each tank and bulking does not occur.

Further, in the aeration tank, since excessive aeration can be performed to the extent that nitrification proceeds, the sludge is activated, and when it is returned to the BOD absorption tank directly or through the denitrification tank, the BO
The D absorption rate increases, and the BOD removal effect and the filamentous bacterium suppression effect increase. And part of nitric acid or nitrite nitrogen generated by nitrification is removed in the denitrification tank,
Floating of sludge in the solid-liquid separation tank is prevented, and treated water of high quality is obtained.

By returning the separated sludge in the solid-liquid separation tank to the denitrification tank, the denitrifying bacteria in the returned sludge can be effectively used for denitrification. Further, by returning the mixed liquid of the denitrification tank or the aeration tank to the BOD absorption tank, it is possible to return the sludge with a relatively stable sludge concentration and facilitate the adjustment of the BOD load on the MLSS.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a flow chart showing the sewage treatment apparatus of the embodiment. In the figure, 11 is a BOD absorption tank, 12 is a denitrification tank,
13 is an aeration tank. The BOD absorption tank 11 is provided with an air diffuser 14 for diffusing the air sent from the blower 4, and the raw water 1 and the return mixed liquid 2a from the denitrification tank 12 are introduced and mixed, and aerated to agitate the BOD in the raw water to sludge. It is designed to be absorbed by.

The denitrification tank 12 has a stirrer 16 and is maintained in an anaerobic state. The mixed solution 15 in the BOD absorption tank 11 and the aeration tank 1 are maintained.
The return mixed liquid 2b from the No. 3 and the return sludge 5 from the solid-liquid separation tank 7 are introduced and mixed under anaerobic condition to perform denitrification. The denitrification tank 12 is provided with a mixed solution return pump 17 as a first mixed solution return device, and returns the returned mixed solution 2a to the BOD absorption tank 11.

The aeration tank 13 has an air diffuser 18 for diffusing the air sent from the blower 4, and the mixed liquid 19 in the denitrification tank 12
Is introduced and aerated to aerobically oxidatively decompose BOD. The aeration tank 13 is provided with a mixed solution return pump 20 as a second mixed solution return device.
b is returned to the denitrification tank 12.

The solid-liquid separation tank 7 is the mixed liquid 21 of the aeration tank 13.
Is introduced into the denitrification tank 12 as a return sludge 5 by the sludge return pump 9 and the rest is surplus. The sludge is discharged as 22.

In the above wastewater treatment equipment, first, BO
Introduce the raw water 1 and the returned mixed liquid 2a into the D absorption tank 11,
Air is sent from the blower 4 to be diffused from the diffuser 14 for aeration and mixing, and the BOD in the raw water is absorbed by the sludge in the returned mixed liquid at a rate higher than the oxidative decomposition rate. This suppresses the growth of filamentous bacteria that grow at a rate commensurate with the oxidative decomposition rate. In the BOD absorption tank 11, it is sufficient that sludge can absorb BOD components, and it is not necessary to grow floc-like bacteria or oxidatively decompose BOD.

In the BOD absorption tank 11, it is preferable to perform aeration so that sludge can easily absorb BOD, but DO is 0.
However, it is suitable that the sludge absorbs BOD by performing low-level aeration so that the DO becomes 0 to 0.5 ppm. The mixing ratio of the raw water 1 and the returned mixed solution is 0.5 to
When mixed so as to be 1.5 kg-BOD / kg SS · d, BOD is efficiently absorbed. BOD concentration /
Sludge concentration is 0.05-0.1kg-BOD / kg-ML
It is preferably SS. Appropriate residence time in the BOD absorption tank is 0.5 to 2 hours.

In the BOD absorption device, since BOD is absorbed into sludge at a rate faster than the oxidative decomposition rate of BOD, filamentous bacteria do not grow and bulking does not occur.

In the denitrification tank 12, the mixed solution 15 in the BOD absorption tank 11, the returned mixed solution 2b from the aeration tank 13 and the returned sludge 5 are introduced and mixed, and agitated by an agitator 16 in an anaerobic state where DO is 0. Then, the denitrifying bacteria grow. This denitrifying bacterium utilizes the BOD absorbed in the BOD absorption tank 11,
In the process, nitric acid or nitrite nitrogen contained in the returned mixed solution is reduced to nitrogen gas to denitrify. A suitable residence time in the denitrification tank 12 is 1 to 4 hours. The capacity of the denitrification tank 12 is
It is preferable to make the determination based on the total amount of the denitrification load and the amount equivalent to the time lag of about 10 minutes with respect to the amount of raw water flowing in.

Since the denitrification tank 12 is maintained in an anaerobic state, filamentous bacteria do not grow and the cause of bulking in the subsequent step is removed. On the other hand, the sludge that absorbed BOD
Maintain the absorption state of BOD.

The mixed liquid in the denitrification tank 12 is the mixed liquid return pump 1
Then, it is returned to the BOD absorption tank 11 as the return mixed solution 2a. The return amount of the return mixed liquid 2a is suitably 5 to 30% by volume of the raw water flow rate. BO the mixture of the denitrification tank 12
By returning the sludge to the D absorption tank 11, the sludge can be returned with a relatively stable sludge concentration, and the adjustment of the BOD load on the MLSS can be facilitated.

In the aeration tank 13, the mixed liquid 19 in the denitrification tank 12
By introducing the air from the blower 4 and aerating the air from the air diffuser 18, the BOD-decomposing bacteria in the sludge assimilate the absorbed BOD and proliferate, and the BOD is aerobically oxidatively decomposed. It Most of BOD in this mixed solution is absorbed in the sludge and does not exist in the solution, so that filamentous bacteria do not grow and bulking does not occur. Aeration is performed to such an extent that nitrogen in the liquid is converted to nitric acid or nitrite nitrogen.

DO in the aeration tank 13 is 0.5 to 1 mg
/ L, MLSS is 3000-5000 mg / l, BOD
The load is preferably 0.1 to 0.3 kg / kg-MLSS, and the residence time is preferably 8 hours or more. The mixed solution in the aeration tank 13 is returned to the denitrification tank 12 as the returned mixed solution 2b by the mixed solution return pump 20. The returning amount of the returning mixed liquid 2b is preferably 50 to 100% by volume of the raw water flow rate.

In the aeration tank 13, since excess aeration can be performed to the extent that nitrification proceeds, sludge is activated, and the BOD when returned to the BOD absorption tank 11 through the denitrification tank 12
The absorption rate is increased, and the BOD removing effect and the filamentous bacterium suppressing effect are increased.

In the solid-liquid separation tank 7, the mixed liquid 2 in the aeration tank 13
1 is introduced to perform solid-liquid separation, and the separated liquid is sent to the next step as treated water 8. Part of the separated sludge is sent by the sludge return pump 9.
The returned sludge 5 is returned to the anaerobic tank 12, and the rest is discharged as excess sludge 22. Since nitric acid or nitrite nitrogen generated by nitrification in the aeration tank 13 is removed in the denitrification tank 12, floating of sludge due to the generated gas in the solid-liquid separation tank 7 is prevented and high-quality treated water is obtained. The retention time in the solid-liquid separation tank 7 is preferably 3 to 9 hours.

The amount of the returned sludge returned from the solid-liquid separation tank 7 is preferably 50 to 100% by volume of the raw water flow rate. By returning the separated sludge in the solid-liquid separation tank 7 to the denitrification tank 12, the denitrifying bacteria in the returned sludge can be effectively used for denitrification.

Test Example 1 In the sewage treatment apparatus shown in FIG. 1, DO in the BOD absorption tank 11 for food processing wastewater of BOD 700 mg / l.
Is 0.2 mg / l, the residence time is 1 hour, the ratio of the returned mixed liquid 2a to the raw water 1 is 30% by volume, and the BOD load is 1.2.
-Kg / kg VSS · d, DO in denitrification tank 12 is 0
mg / l, residence time 1.5 hours, ratio of the returned mixed liquid 2b to raw water 1 is 100% by volume, ratio of returned sludge to raw water 1 is 100% by volume, and DO in the aeration tank 13 is 1
mg / l, residence time 8 hours, MLSS treatment results, BOD removal rate 98%, nitrogen removal rate 72%, treated water BOD 14 mg / l, N 5 mg / l, SS 10 m
Treated water of g / l was obtained, sludge did not float in the solid-liquid separation tank 7, and bulking did not occur.

Test Example 2 For comparison, the sewage treatment apparatus of FIG. 2 was used for treatment without the denitrification tank of Test Example 1, and as a result, the BOD removal rate was 96%, the nitrogen removal rate was 15%, and the treated water BOD was removed. 30 mg /
1, N 15 mg / l, SS 30 mg / l, and sludge floated in the solid-liquid separation tank. The BO of the aeration tank 3
When treated with a D load of 1.8 kg / kg VSS,
It became SVI 220 after 6 days.

[0034]

According to the present invention, since the treatment is carried out by combining the BOD absorption tank, the denitrification tank and the aeration tank, the growth of filamentous bacteria is suppressed to prevent the filamentous bulking from occurring, and the BOD is eliminated. It is possible to efficiently remove nitrogen and nitrogen, prevent the sludge from floating in the solid-liquid separation tank, and obtain treated water of high water quality.

[Brief description of drawings]

FIG. 1 is a flow diagram of a sewage treatment apparatus according to an embodiment.

FIG. 2 is a flow diagram of a conventional sewage treatment apparatus.

[Explanation of symbols]

 1 Raw Water 2a, 2b Returned Mixture 4 Blower 5 Returned Sludge 7 Solid-Liquid Separation Tank 8 Treated Water 9 Sludge Return Pump 11 BOD Absorption Tank 12 Denitrification Tank 13 Aeration Tank 14, 18 Diffuser 15, 19 and 21 Mixture 16 Stirrer 17,20 Mixed liquid return pump

Claims (1)

[Claims] 1. A BOD absorption tank in which raw water is introduced and mixed with a return mixed solution from a denitrification tank or an aeration tank to absorb BOD in the raw water into sludge, a mixed solution of the BOD absorption tank, and a solid liquid. A denitrification tank that mixes the returned sludge from the separation tank to denitrify under anaerobic conditions, an aeration tank that introduces the mixed solution of this denitrification tank and aerates it to aerobically oxidize and decompose BOD, and the aeration tank of this aeration tank. A solid-liquid separation tank for solid-liquid separation of the mixed liquid, a sludge returning device for returning the sludge separated by the solid-liquid separation tank to the denitrification tank, and a mixed liquid for the denitrification tank or aeration tank to the BOD absorption tank A sewage treatment apparatus comprising: a first mixed liquid returning device; and a second mixed liquid returning device that returns the mixed liquid in the aeration tank to the denitrification tank.