JP3150506B2 - Wastewater treatment method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wastewater treatment method suitable for removing eutrophic components, particularly nitrogen components, from municipal sewage and organic wastewater.

[0002]

2. Description of the Related Art In recent years, removal of eutrophic components such as nitrogen and phosphorus has become a major issue in the treatment of municipal sewage and organic wastewater in order to prevent water pollution in public waters such as rivers, lakes and marshes, and the ocean. ing.

Conventionally, the removal of nitrogen is generally carried out by a biological removal process, and the nitrification of ammonia under aerobic conditions and the denitrification of nitric acid and nitrite under anaerobic conditions Reaction (nitrate respiration and nitrite respiration)
It was common.

[0004] As typical processes for performing this biological nitrogen removal, a nitrification liquid circulation system in which a nitrification tank and a denitrification tank are combined as shown in a flow sheet of FIG.
An intermittent aeration system in which nitrification and denitrification are performed in one complete mixing tank as shown in the flow sheet of FIG. 3 is known.

[0005]

In the case of performing nitrogen removal by the nitrification liquid circulation system, if the amount of residual nitrogen in the water to be treated is to be denitrified with an efficiency of 80% or more, the amount of the circulating liquid will be 4%. Since the energy consumption is more than twice, there is a problem that the energy consumption cannot be ignored.

[0006] On the other hand, in the intermittent aeration method, it is difficult to balance the nitrification time and the denitrification time. Since the anaerobic state was hardly formed, the denitrification did not proceed sufficiently, and the nitrogen concentration in the treated water was not sufficiently reduced. In addition, if an attempt is made to combine this with membrane separation that can completely prevent the loss of SS, the aeration is stopped in the denitrification step, so that the membrane does not oscillate, and it becomes difficult to continue filtration, and the filtration efficiency is reduced. The problem has arisen.

An object of the present invention is to provide a wastewater treatment method capable of denitrification efficiently without increasing the amount of circulating liquid.

Another object of the present invention is to provide a wastewater treatment method capable of reliably progressing a denitrification reaction even when there is a load change, and also having an easy operation management.

Still another object of the present invention is to provide a wastewater treatment method which enables continuous filtration and has high filtration efficiency.

[0010]

That is, according to the present invention, there is provided a process in which water to be treated is introduced into a first treatment tank and nitrification denitrification is performed by intermittent aeration, and water used in the first treatment tank is separated. Guiding the membrane to a second treatment tank provided with the membrane and continuously performing aeration to complete the nitrification reaction, performing suction filtration through a separation membrane, and discharging membrane-permeated water out of the system; And returning a part of the water in the treatment tank to the first treatment tank.

[0011]

According to the wastewater treatment method of the present invention, 55 to 95% of the nitrification reaction is carried out in the first treatment tank, and the remaining 5 to 45% is supplemented in the second treatment tank, as compared with the nitrification liquid circulation method. Therefore, the amount of liquid returned from the second processing tank to the first processing tank is reduced. In addition, compared with the intermittent aeration method, since there is no need to allow time for nitrification in the first treatment tank, formation of an anaerobic state in the first treatment tank is guaranteed, and denitrification is reliably performed. In the second treatment tank, the separation membrane for performing suction filtration by continuously releasing air bubbles from the bottom of the tank is swung for washing, so that continuous filtration is possible and the filtration efficiency is increased.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described more specifically based on the flow sheet shown in FIG.

The water to be treated is guided to the raw water tank 2 through the screen 1 and stored therein, where the quantitative and qualitative fluctuations of the inflowing water to be treated are absorbed. The water to be treated in the raw water tank is quantitatively sent to the first processing tank 4 by the raw water pump 3, and a dephosphorizing agent is added on the way if desired. Further, a part of the water to be treated, which has exited the raw water pump, may be returned to the raw water tank to perform the water stirring. When the inside of the first treatment tank is in an anaerobic state, in order to effectively carry out denitrification, it is preferable that the water to be treated supplied to the first treatment tank is also low in dissolved oxygen. It is preferable to perform the mixing of the water to be treated by water agitation.

In the first processing tank 4, an intermittent aeration process is performed. Here, the intermittent aeration process is a process of releasing an oxygen-containing gas such as air or oxygen-enriched air sent from the blower 5 as air bubbles by the air diffusing means 6 disposed at the bottom of the tank to create an aerobic state. And the step of stopping the release of bubbles to create an anaerobic state is performed alternately. Focusing on the nitrogen component, the nitrification reaction of ammonia to nitrate and nitrite proceeds under an aerobic condition, and the reductive decomposition reaction of nitric acid and nitrite into nitrogen gas and water proceeds under an anaerobic condition. The aeration state and the aeration stop state are normally set in a cycle of 10 to 180 minutes of aeration and 10 to 180 minutes of stop. The cycle time may be fixed or may be varied according to the load.
In the method of the present invention, the nitrification reaction is also performed in the second processing tank, so that the aeration in the first processing tank may be insufficient.
Rather, it is necessary to prevent the anaerobic state from being formed without the dissolved oxygen concentration being lowered even after the stop of the aeration due to overaeration. Therefore, it can be said that the degree of freedom of operation is greater than that of the conventional intermittent aeration method in which appropriate nitrification must be completed in one tank. The residence time in the first treatment tank is 5
It is appropriate to set it to 30 hours. In order to prevent a decrease in pH in the tank due to a nitrification reaction and a decrease in bacterial activity, an aqueous solution of sodium hydroxide or the like is appropriately added for pH adjustment. Mixing of the water in the tank is performed by air bubbles during aeration, but it is preferable to rotate the stirrer 7 when aeration is stopped.

The water that has been intermittently aerated in the first treatment tank is then led to the second treatment tank 8. A separation membrane 9 is immersed in the second processing tank. The separation membrane may be a homogeneous membrane or a porous membrane. Also, the form is not particularly limited, and may be any of a flat membrane, a tubular membrane, a hollow fiber membrane, and the like. In the second treatment tank, continuous aeration treatment is performed. The aeration treatment is performed by releasing oxygen-containing gas sent from the blower 10 as air bubbles from the aeration means 11 at the bottom of the tank, as in the case of the first treatment tank. It is. By this aeration, the nitrification reaction in the first treatment tank is complemented, and the bubbles are vigorously applied to the separation membrane on which suction filtration is performed, thereby swinging the membrane.
Prevents a decrease in the effective membrane area due to the adhesion of sludge and the like and clogging of the membrane. Since the water in the second treatment tank is continuously aerated, the tank is in a completely mixed state, and the sludge concentration is uniform throughout the tank.

Since only the permeated water from the filtration by suction through the separation membrane is discharged out of the system as treatment water through the suction pump 12, the MLSS in the second treatment tank is reduced from 7000 to 22%.
Even when operated under a considerably high concentration condition of 000, no leakage of sludge occurs. Also, since solid-liquid separation is performed at the membrane,
Washout of nitrifying bacteria that grow slowly does not have to be a problem. In addition, since the sludge concentration is high, excess sludge can be directly passed to waste treatment as needed without passing through a sludge concentration tank, which is required when treating in a sedimentation tank. When the filtration flow rate of the separation membrane is reduced after long-term continuous operation, it is effective to use the backwashing compressor 15 to backflow the treated water to the separation membrane for washing. It is appropriate that the residence time of the water in the second treatment tank is 1 to 5 hours.

A part of the water in the second treatment tank is returned to the first treatment tank via a return line 16 for denitrification and return of sludge from the nitrified water in the second treatment tank. Circulate to 4. The amount of returned water is usually about 70 to 200% of the supplied water to be treated, and even with this degree, denitrification (target water quality T-N 10 mg / l or less) with an efficiency of 80% or more is achieved. The return water may be returned to the first treatment tank as a shower during aeration for defoaming, but it is preferable to return air as little as possible to reduce dissolved oxygen when the aeration is stopped in order to reduce dissolved oxygen.

[0018]

The wastewater treatment method of the present invention has the following effects. (1) Since the nitrification reaction is carried out while being complemented in the two treatment tanks having different characteristics, the denitrification reaction in the first treatment tank can be effectively promoted, and the operation of the process is easy. Further, the amount of circulating water to the first treatment tank can be significantly reduced as compared with the conventional nitrification liquid circulation system. (2) Nitrifying bacteria are likely to be washed out of the system due to the slow growth rate.However, there is no need to worry about the sedimentation of sludge because only filtration water is discharged by suction filtration using a separation membrane. It is possible to operate the microorganisms in a dispersed state at a high concentration without reducing the concentration. In addition, since the concentration of microorganisms in the treatment tank is high, reduction in treatment time and improvement in treated water quality can be expected. (3) Since the inside of the second processing tank is continuously aerated, the separation membrane is constantly scrubbed, and continuous suction filtration can be performed for a long time. (4) Since the inside of the second treatment tank is in a completely mixed state and the sludge concentration is uniform, the concentration in the system can be easily controlled by adjusting the amount of water returned to the first treatment tank and the amount of waste sludge. (4) Since the sludge concentration in the second treatment tank is high, there is no need to prepare a sludge concentration tank when discarding excess sludge.

[Brief description of the drawings]

FIG. 1 is a flow sheet showing an example of a wastewater treatment method of the present invention.

FIG. 2 is a flow sheet showing a conventional nitrification liquid circulation system.

FIG. 3 is a flow sheet showing a conventional intermittent aeration method.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 screen 2 raw water tank 3 raw water pump 4 first treatment tank 5, 10 blower 6, 11 air diffuser 7 stirrer 8 second treatment tank 9 separation membrane 12 suction pump 13 dephosphorizer tank 14 pH adjusting liquid tank 15 reverse Washing compressor 16 Return line

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI C02F 9/00 504 C02F 9/00 504E (72) Inventor Yasuji Yamamoto 670-10 Kuzumotocho, Kashihara-shi, Nara (56) References JP-A-3-229697 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C02F 3/30

Claims (1)

(57) [Claims] 1. A process in which water to be treated is guided to a first treatment tank and subjected to intermittent aeration treatment to perform nitrification and denitrification, and a step of separating water used in the first treatment tank with a separation membrane. (2) a process for introducing nitrification by continuous aeration and nitrification, filtering by suction through a separation membrane, and discharging the permeated water out of the system; Returning to the first treatment tank.