JP2018075512A - Sewage treatment system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sewage treatment system capable of supplying aerobic bacteria with a sufficient amount of oxygen while steeply decreasing the amount of air being used.SOLUTION: The sewage treatment system comprises a biological reaction tank equipped with a liquid feed pipe for sending water to be treated coming from an upstream sedimentation pond or from an upstream anaerobic treatment tank into the biological reaction tank, or equipped with a liquid feed pipe for sending a supernatant liquid present in a downstream sedimentation pond into the biological reaction tank, which liquid feed pipe comprises a gas-liquid mixer such as a static mixer, with an air supply pipe connected thereto on a position upstream of the gas-liquid mixer so as to produce water mixed with bubbles in which oxygen (air) is dissolved by the gas-liquid mixer and to send the resulting water mixed with bubbles into the biological reaction tank.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a sewage treatment system in which sewage is subjected to aerobic biological treatment in a biological reaction tank, then subjected to sedimentation separation treatment, and a part of sludge is returned to the biological reaction tank.

The systems disclosed in Patent Documents 1 to 4 and the like are common as sewage treatment systems that have been practiced conventionally.
In the conventional sewage treatment system, as shown in FIG. 21, after the sewage that has entered the sewage treatment plant is removed of dust with a screen or the like, the head uses a drop (water head pressure difference) from the sand basin to the first sedimentation basin. The raw water is sent to the biological reaction tank while the dirt that tends to settle in the first sedimentation basin is collected by the rotary scraper installed at the bottom of the first sedimentation basin and sent to the storage room at the bottom.

  In biological reaction tanks, organic substances are adsorbed, ingested, and digested and decomposed by high-concentration microorganisms (aerobic bacteria). The raw water from the biological reaction tank is sent to the final sedimentation basin by a drop. In the final sedimentation basin, the activated sludge is slowly settled and the supernatant is poured, and the activated sludge in the final sedimentation basin is sent to the biological reaction tank by the pump and reacts again.

  Since sufficient oxygen is required to activate these aerobic microorganisms, a device called an air diffuser, which is usually equipped with an infinite number of fine ventilation holes, is appropriately installed at the bottom of the bioreactor containing raw water. It is installed. When a blower (blower) connected to the diffuser pipe is operated and ventilated, countless fine bubbles are injected into the raw water from the diffuser pipe holes (aeration method), and oxygen in the bubbles is dissolved with dissolved oxygen in the raw water. It supplies oxygen to microorganisms.

JP 2007-83153 A JP 2006-150316 A JP 2005-279351 A JP 2005-262140 A

  Almost all conventional aeration methods employ a diffuser system. In the case of this air diffuser system, since it is a technical means that blows only air to the air diffuser and diffuses fine bubbles to the target water tank in order to efficiently dissolve oxygen in the air in the water, it is always a biological reaction tank. It is installed at the bottom.

  Even if a diffuser tube is installed at the bottom, the bubbles reach the water surface in a short time, so that the dissolved oxygen becomes insufficient. For this reason, the biological reaction tank is deepened to increase the residence time of bubbles in water and the dissolution is promoted by the hydraulic effect, but there is a disadvantage that the apparatus becomes large.

  If the number of bubbles is increased by increasing the number of bubbles in water, the contact area with water is increased and the residence time is increased, so that the amount of dissolved oxygen can be increased. Therefore, at present, the average hole diameter of the diffusing tube is about 300 μm, the depth of the aeration tank is about 4 to 5 m, and the residence time is 6 to 8 hours. However, even this does not have a sufficient amount of dissolved oxygen. In addition, the power cost of the blower that blows air to the air diffuser cannot be ignored.

  In order to solve the above problems, the sewage treatment system according to the first aspect of the present invention is a biological reaction tank in which treated water sent from an upstream sedimentation basin or anaerobic treatment tank is aerobically treated and sent to a downstream sedimentation basin. The biological reaction tank has an upstream sedimentation basin or a liquid feed pipe that feeds treated water from an upstream anaerobic treatment tank into the biological reaction tank, or the downstream sedimentation basin A liquid feed pipe for feeding the supernatant liquid into the biological reaction tank is provided, and this liquid feed pipe is provided with a gas-liquid mixing device such as a static mixer, and in the upstream side of the gas-liquid mixing device of the liquid feed pipe An air supply pipe was connected, and the gas-liquid mixing apparatus generated bubble mixed water in which oxygen (air) was dissolved in a large amount in the water to be treated, and the bubble mixed water was fed into the biological reaction tank.

  In order to solve the above-mentioned problem, the sewage treatment system according to the second aspect of the present invention aerobically treats the treated water sent from the upstream sedimentation basin or the upstream anaerobic treatment tank and sends it to the downstream sedimentation basin. A sewage treatment system including a biological reaction tank, wherein a gas-liquid mixing device is provided in the middle of a sludge return pipe for sending sludge from the downstream sedimentation basin to the biological reaction tank, and the gas-liquid mixing of the sludge return pipe An air supply pipe is connected to the upstream side of the apparatus, and oxygen (air) is dissolved into the treated water that is returned together with sludge by the gas-liquid mixing apparatus to generate high-concentration bubble mixed water. Bubble mixed water was fed into the biological reaction tank.

  In the first invention, the liquid feeding pipe is connected to an inlet of a header pipe disposed in the upper part of the biological reaction tank, the header pipe is branched into a plurality of parts, and a gas-liquid mixing device is provided in each branch pipe. It can be set as the structure by which the air supply piping was connected to the upstream rather than the gas-liquid mixing apparatus of a branch pipe.

  The outlet of the gas-liquid mixing device is connected to the inlet of a header pipe, and this header pipe can be branched into a plurality of nozzles at each branch pipe outlet.

  In addition, the header pipe may be configured to penetrate the side wall of the biological reaction tank and have a jet outlet located in the biological reaction tank. Furthermore, at least one branch pipe branched from the header pipe may pass through a portion in the vicinity of the bottom of the side wall of the bioreactor tank, and a jet outlet may be located in the bioreactor tank.

  Further, a part of the branch pipe of the header pipe extends to the vicinity of the bottom of the biological reaction tank, and the bubble mixed water is jetted upward, and the other branch pipe extends to the middle of the biological reaction tank, The bubble mixed water may be ejected toward the head.

  Furthermore, a part of the branch pipe extends to the vicinity of the bottom of the biological reaction tank, and the other branch pipe extends to the middle of the biological reaction tank, and a gas-liquid mixing device and an air supply pipe are provided in each branch pipe. It is good also as a structure.

  For example, if a static mixer is used to forcibly mix both water and air as a gas-liquid mixing device, the nanobubbles, microvalves, etc. are not dissolved at the mixer outlet within a very short time when the total amount of added air passes through the mixer. However, it can be considered that a sufficient amount of bubble mixed water is generated.

Therefore, there is no problem of bubbles rising even when sprayed from the upper part of the biological reaction tank into the water tank.
As described above, a large amount of undissolved nanobubbles are mixed in the bubble mixed water, but these nanobubbles flow along with the water flow of the bubble mixed water. Since it is further reduced and oxygen dissolution is promoted, it is more reasonable to inject the bubble mixed water from the upper part of the water tank toward the bottom of the water tank.

Also, when oxygen-dissolved water is sprayed toward the bottom of the water tank, this water flow can obtain the effect of gravitational acceleration, and depending on the depth of the biological reaction tank (aeration tank), it collides with the tank bottom and the bottom of the tank Inverted flow and turbulent flow can be prevented, and sludge sedimentation and flocation can be prevented. In particular, in the past, the normal aeration layer liquid residence time is 6 to 8 hours as standard, but the liquid flow is very slow. However, when a static mixer is used as a gas-liquid mixing device, oxygen is dissolved while instantaneously deflocking in the static mixer. Compared to the above, the overall average air addition amount is about 1/6, which is very efficient.
Therefore, even if there is a problem of air bubbles rising from the upper part of the bioreactor into the water tank, oxygen water with a practical concentration (about 4 ppm) or more has already been secured at the mixer outlet. Absent.
In the case of the diffuser system, the device is installed at the bottom of the reaction tank, so the air from the diffuser system is microbubbles (300 micrometer on average) generated through the eye holes and jets from the ground toward the sky. Not only that, but the buoyancy effect of the bubbles rises rapidly to the liquid level, so the contact time of the bubbles with the liquid is extremely short and the dissolved effect due to pressurization is inversely proportional, so it does not seem reasonable .
By the way, the surface area simple comparison example of nanobubble vs. microbubble is as follows.
・ Bubble size is 1 micrometer or less, nanobubbles The maximum common value of nanobubbles is approximately 0.99 micrometers ・ Surface area = (π / 6) * D ³ (D: sphere diameter)
When the surface area of the bubbles is simply compared from the above formula, it is proportional to D ³ .
∴ Maximum common value Nano bubble surface area = (0.99) ³
= 0.97 micron m ≒ 1 micrometer 300 micrometer bubble surface area
= (300) ³ = 27,000,000 micrometer From this result, the surface area of the microbubbles is 27,000,000. Double the size.
By the way, when measuring nanobubbles by nitrogen gas, 200 nanometer meter bubbles are measured as a representative value of 100 million / ml, and 200 nano = 0.2 micron. 375,000,000. (33.77 billion times) will be smaller.
Furthermore, in order to effectively use the water pressure of the biological reaction tank, the bubble mixed water discharged from the gas-liquid mixing device can be discharged from the upper layer portion or the middle portion of the water surface to the lower layer portion, and the bubbles can be condensed finely. .

In the experiment using tap water, the static mixer owned by the applicant has a water temperature of 19 ° C., an air addition amount of 5 to 6 L / min per 10 L / min of water, and a mixer pressure loss of 300 kPa. G to 400 kPa. It has been confirmed by experiments that a gas mixture of 1.1 ppm at the inlet of the mixer produces high-concentration dissolved oxygen (8.6 to 8.8 ppm) close to a saturation concentration at the mixer outlet.
Furthermore, in order to eliminate the effect at the water depth, it is a comparison using tap water (room temperature). Sample water of 0.1 ppm low-concentration oxygen water is prepared with sodium sulfite as the tap water, and the membrane method (300 nominal) In a sample water one-pass experiment at a water depth of 6 cm with a static mixer owned by the applicant and a static mixer owned by the applicant, the membrane type is 2.9 ppm with an air addition amount three times the amount of water, and the static mixer type is 50% water ( 0.5 times), the amount of air added is 4.7 ppm. In comparison of the amount of air added to the amount of water, the static mixer has a higher oxygen value of about 1.9 ppm even if the amount of air added is reduced by 1/6. It has been confirmed that.

As an example, in the example of a wastewater treatment plant managed by the administration in the Kanto region, the membrane aeration tube aeration type biological reaction tank raw water treatment amount,
Raw water treatment amount: 100,000 m ³
Air consumption: 327,000m ³
Raw water to air consumption ratio = 327,000 / 100,000 = 3.27 times In an experiment with a static mixer using tap water,
Water amount: 10 L / min Air addition amount: 6 L / min Water to air consumption ratio = 6/10 = 0.6 times When simply comparing both, (0.6 / 3.27) × 100≈18%
Thus, the static mixer method can reduce air consumption by 82% compared to the membrane method.

In the case of the preceding paragraph, if the increase in power cost due to the mixer installation is calculated as an example, the mixer inlet flow rate is 2.0 m / sec, the air addition amount is 5 L / min, the mixer pressure loss (differential pressure) is 30 kPa. When the mixer is operated at G, it corresponds to 1/4 of the air usage of a normal diffuser tube type or membrane type aeration device, or less electricity cost reduction. As a result, air corresponding to at least 2/4 (half) of the amount of air used in the conventional diffuser system can be saved.
Since the amount of air used is almost proportional to the power cost associated with blower air generation, a significant reduction in power cost can be expected. At present, the conventional aeration equipment used for sewage treatment consumes a large amount of air. Therefore, reducing the electricity cost of the blower is a major social theme, but it is an effective means for solving this problem. .
By the way, the outline of the operating conditions of the static mixer that satisfies the above-described aspects are as follows: mixer inlet flow velocity: 1.5 to 2.5 m / sec, fluid mixer passing time: 0.1 to 1.0 sec, mixer operating differential pressure (Pressure loss): 20 to 50 kPa. G, the amount of air added to the amount of water flow: 0.2 to 1.0 times.

Overall view of the sewage treatment system according to the first embodiment Overall view of a sewage treatment system according to a modification of the first embodiment Overall view of the sewage treatment system according to the second embodiment Overall view of a sewage treatment system according to a modification of the second embodiment Overall view of the sewage treatment system according to the third embodiment Overall view of a sewage treatment system according to a modification of the third embodiment Overall view of the sewage treatment system according to the fourth embodiment Overall view of a sewage treatment system according to a modification of the fourth embodiment Overall view of the sewage treatment system according to the fifth embodiment Overall view of a sewage treatment system according to a modification of the fifth embodiment Overall view of the sewage treatment system according to the sixth embodiment Overall view of a sewage treatment system according to a modification of the sixth embodiment Overall view of the sewage treatment system according to the seventh embodiment Overall view of a sewage treatment system according to a modification of the seventh embodiment Overall view of the sewage treatment system according to the eighth embodiment Overall view of a sewage treatment system according to a modification of the eighth embodiment Overall view of the sewage treatment system according to the ninth embodiment Overall view of a sewage treatment system according to a modification of the ninth embodiment Overall view of a sewage treatment system according to a modification of the ninth embodiment Overall view of a sewage treatment system according to a modification of the ninth embodiment Diagram explaining a conventional sewage treatment system

  The sewage treatment system according to the present invention includes an initial sedimentation tank 1, a biological reaction tank (aeration tank) 2 for performing an aerobic treatment, and a final sedimentation tank 3 from the upstream side toward the downstream side. The sewage treatment system according to the present invention includes one in which an anaerobic treatment tank is initially disposed between the settling tank 1 and the biological reaction tank 2.

  A rotary scraper is disposed in the lower part of the initial sedimentation tank 1, and the sediment is collected by the scraper and taken out from the bottom of the initial sedimentation tank 1.

  First, the water to be treated in the sedimentation tank 1 is sent to the biological reaction tank 2 through a liquid feed pipe 4 equipped with a pump. A static mixer 5 as an example of a gas-liquid mixing device is provided in the middle of the liquid feeding pipe 4, and an air supply pipe 6 is connected to the liquid feeding pipe 4 upstream of the static mixer 5.

  In addition, a header pipe 7 arranged in advance on the biological reaction tank 2 is connected to the outlet of the static mixer 5, and the header pipe 7 is divided into a plurality of (three in the figure) branch pipes 7a. A downward nozzle 8 is attached to the lower end.

  A sludge return pipe 9 is provided from the bottom of the final sedimentation tank 3, and a part of the sludge accumulated in the final sedimentation tank 3 is returned to the biological reaction tank 2 using the sludge return pipe 9.

  Thus, in the static mixer 5, water and air are forcibly mixed, and almost all of the added air is mixed in the water to be treated within a very short time passing through the static mixer. Is generated. The bubble mixed water is ejected downward from the nozzle 8 into the biological reaction tank 2 to form a stirring flow in the biological reaction tank 2. Sufficient oxygen is supplied to the aerobic microorganisms in the biological reaction tank 2 by this stirring flow.

  FIG. 2 shows an overall view of a sewage treatment system according to a modification of the first embodiment. In this modification, a liquid feed pipe 10 is provided for returning the supernatant of the downstream final sedimentation tank 3 to the biological reaction tank 2. The liquid feed pipe 10 is provided with a static mixer 5 and an intake air supply pipe 6 is connected upstream thereof.

  In the case of the modification shown in FIG. 2, as in the case of FIG. 1, the bubble mixed water is ejected from the lower end of the branch pipe 7a constituting the header pipe 7 to form a circulating flow in the biological reaction tank 2, Sufficient oxygen is supplied to the aerial bacteria.

FIG. 3 shows an overall view of the sewage treatment system according to the second embodiment. In this embodiment,
A header pipe 7 is connected to the liquid feeding pipe 4, a static mixer 5 is provided in each branch pipe 7a constituting the header pipe 7, and an air supply pipe 6 is connected to each branch pipe 7a. In the case of this embodiment, the direction of the stirring flow can be controlled by arranging a large number of static mixers 5.

  FIG. 4 shows an overall view of a sewage treatment system according to a modification of the second embodiment. In this modification, as in the modification of FIG. 2, the supernatant of the downstream final sedimentation tank 3 is used as a biological reaction tank. 2 is provided, and the static mixer 5 is provided in the liquid supply tube 10.

FIG. 5 shows an overall view of the sewage treatment system according to the third embodiment. In this embodiment, the branch pipe 7a constituting the header pipe 7 passes through the upper side wall of the bioreaction tank 2 and enters the bioreaction tank 2. To face.
Thus, by setting it as the structure which penetrates the side wall of the biological reaction tank 2, it becomes easy to apply this system to the existing system.

  FIG. 6 shows an overall view of a sewage treatment system according to a modified example of the third embodiment. In this modified example, as in the modified example of FIG. 2 is provided, and the static mixer 5 is provided in the liquid supply tube 10.

  FIG. 7 shows an overall view of the sewage treatment system according to the fourth embodiment. In this embodiment, the branch pipe 7a constituting the header pipe 7 passes through the upper side wall of the biological reaction tank 2 and enters the biological reaction tank 2. Further, a static mixer 5 is provided in each branch pipe 7a.

  FIG. 8 shows an overall view of a sewage treatment system according to a modification of the fourth embodiment. In this modification, the supernatant of the downstream final sedimentation tank 3 is used as a biological reaction tank, as in the modification of FIG. 2 is provided, and the static mixer 5 is provided in the liquid supply tube 10.

FIG. 9 shows an overall view of the sewage treatment system according to the fifth embodiment. In this embodiment, the branch pipe 7a constituting the header pipe 7 passes through the lower part of the side wall of the biological reaction tank 2 and enters the biological reaction tank 2. The bubble mixed water is ejected along the bottom of the biological reaction tank 2.
In this way, by feeding the bubble mixed water along the bottom of the biological reaction tank 2, a sufficient amount of oxygen can be supplied to the aerobic bacteria.

  FIG. 10 shows an overall view of a sewage treatment system according to a modified example of the fifth embodiment. In this modified example, as in the modified example of FIG. 2 is provided, and the static mixer 5 is provided in the liquid supply tube 10.

  FIG. 11 shows an overall view of the sewage treatment system according to the sixth embodiment. In this embodiment, the static mixer 5 is provided in the liquid feeding pipe 4 in the fifth embodiment, but each branch pipe 7a has a static mixer. 5 is provided.

  FIG. 12 shows an overall view of a sewage treatment system according to a modified example of the sixth embodiment. In this modified example, the supernatant of the downstream final sedimentation tank 3 is used as a biological reaction tank, as in the modified example of FIG. 2 is provided, and the static mixer 5 is provided in the liquid supply tube 10.

  FIG. 13 shows an overall view of the sewage treatment system according to the seventh embodiment. In this embodiment, the branch pipe 7a constituting the header pipe 7 is divided into a first group of branch pipes and a second group of branch pipes, The height of the first group branch pipe and the second group branch pipe in the biological reaction tank 2 is differentiated, and the bubble mixed water is jetted downward from the upper branch pipe, and the lower branch pipe. The bubble mixed water is ejected downward from the water, and the ejected bubble mixed water collides with the biological reaction tank 2. By adopting such a configuration, the differential lock effect is enhanced.

  FIG. 14 shows an overall view of a sewage treatment system according to a modified example of the seventh embodiment. In this modified example, as in the modified example of FIG. 2 is provided, and the static mixer 5 is provided in the liquid supply tube 10.

  FIG. 15 shows an overall view of the sewage treatment system according to the eighth embodiment. In the seventh embodiment, the static mixer 5 is provided in the liquid feeding pipe 4, but in this embodiment, the static mixer 5 is provided in each branch pipe 7a. It is set as the structure which provided.

  FIG. 16 shows an overall view of a sewage treatment system according to a modification of the eighth embodiment. In this modification, the supernatant of the downstream final sedimentation tank 3 is used as a biological reaction tank, as in the modification of FIG. 2 is provided, and the static mixer 5 is provided in the liquid supply tube 10.

FIG. 17 shows an overall view of the sewage treatment system according to the ninth embodiment. In this embodiment, the sludge return pipe 9 branches from the bottom of the final sedimentation tank 3 on the downstream side, and the branch pipe is statically A mixer 5 is provided, and an air supply pipe 6 is connected to a portion upstream of the static mixer 5 of the sludge return pipe 9, and oxygen (air) is mixed by the static mixer 5 into the treated water to be returned together with the sludge. The bubble mixed water is generated and the bubble mixed water is fed into the biological reaction tank 2.
By adopting such a configuration, the diff lock is more effectively performed than in the past.

  FIG. 18 shows an overall view of a sewage treatment system according to a modification of the ninth embodiment. In this modification, the sludge return pipe 9 is provided with a static mixer 5 and, similarly to the modification of FIG. A liquid feeding pipe 10 for returning the supernatant of the final sedimentation tank 3 to the biological reaction tank 2 is provided, and a static mixer 5 is provided in the liquid feeding pipe 10.

  FIG. 19 shows an overall view of a sewage treatment system according to a modified example of the ninth embodiment. In this modified example, the sludge return pipe 9 penetrates the side surface of the biological reaction tank 2 and faces the biological reaction tank 2. 20, the downstream side of the sludge return pipe 9 from the static mixer 5 is divided into two upper and lower stages, and each pipe penetrates the side surface of the biological reaction tank 2 for biological reaction. It is configured to face the tank 2. By setting it as such a structure, the stirring effect improves.

  The sewage treatment system according to the present invention is not limited to the case of newly constructing a sewage treatment system, but can be applied to an existing sewage treatment system as it is.

  DESCRIPTION OF SYMBOLS 1 ... First sedimentation tank, 2 ... Biological reaction tank which performs aerobic treatment, 3 ... Final sedimentation tank, 4 ... Liquid feeding pipe, 5 ... Static mixer, 6 ... Air supply piping, 7 ... Header pipe, 7a ... Branch pipe 8 ... Nozzle, 9 ... Sludge return pipe, 10 ... Liquid feed pipe.

Claims (9)

  In a sewage treatment system including a biological reaction tank that aerobically treats treated water sent from an upstream sedimentation basin or an anaerobic treatment tank and sends the treated water to a downstream sedimentation basin, the biological reaction tank has an upstream side There is a liquid feed pipe that feeds water to be treated from the sedimentation basin or upstream anaerobic treatment tank to the biological reaction tank, or a liquid feed pipe that feeds the supernatant liquid of the downstream sedimentation basin to the biological reaction tank. The liquid pipe is provided with a gas-liquid mixing device such as a static mixer, and an air supply pipe is connected to the upstream side of the liquid-feeding tube with respect to the gas-liquid mixing device. Produces a mixed water of bubbles dissolved in the water to be treated, and sends the mixed water of bubbles into the biological reaction tank.   2. The sewage treatment system according to claim 1, wherein the liquid feeding pipe is connected to an inlet of a header pipe disposed at an upper part of the biological reaction tank, the header pipe is branched into a plurality of parts, and a static mixer is provided in each branch pipe. A sewage treatment system, wherein an air supply pipe is connected upstream of the gas-liquid mixing device of each branch pipe.   2. The sewage treatment system according to claim 1, wherein an outlet of the gas-liquid mixing device is connected to an inlet of a header pipe, the header pipe is branched into a plurality, and a nozzle is provided at each branch pipe outlet. Sewage treatment system.   4. The sewage treatment system according to claim 3, wherein the header pipe passes through a side wall of the biological reaction tank, and an outlet is located in the biological reaction tank.   5. The sewage treatment system according to claim 4, wherein at least one of the branched pipes of the header pipe passes through a portion near the bottom of the side wall of the biological reaction tank, and an ejection port is located in the biological reaction tank. A featured sewage treatment system.   4. The sewage treatment system according to claim 3, wherein a part of the branch pipe of the header pipe extends to the vicinity of the bottom of the biological reaction tank, and the bubble mixed water is jetted upward, and the other branch pipe is the biological pipe. A sewage treatment system that extends to the middle of the reaction tank and ejects the bubble mixed water downward.   3. The sewage treatment system according to claim 2, wherein a part of the branch pipe extends to the vicinity of the bottom of the biological reaction tank, and the other branch pipe extends partway through the biological reaction tank. A sewage treatment system comprising an apparatus and an air supply pipe.   In a sewage treatment system comprising a biological reaction tank that aerobically treats treated water sent from an upstream sedimentation basin or upstream anaerobic treatment tank and sends it to a downstream sedimentation basin, the downstream sedimentation A gas-liquid mixing device is provided in the middle of the sludge return pipe for sending sludge from the pond to the biological reaction tank, and an air supply pipe is connected to the upstream side of the gas-liquid mixing device of the sludge return pipe. A sewage treatment system characterized in that oxygen (air) is dissolved in the water to be treated which is returned together with sludge by a liquid mixing device, and the bubble mixed water is fed into the biological reaction tank.   In a sewage treatment system comprising a biological reaction tank that aerobically treats treated water sent from an upstream sedimentation basin or an upstream anaerobic treatment tank and sends the treated water to a downstream sedimentation basin, In order to effectively use the water pressure, the sewage treatment system is characterized in that the bubble mixed water discharged from the gas-liquid mixing device is discharged from the upper surface portion or the middle portion of the water surface to the lower layer portion to condense the bubbles finely.