JP2011152505A - Biological treatment vessel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a biological treatment vessel preventing sludge from floating in a final settling pond. <P>SOLUTION: In the biological treatment vessel 1, liquid 2 to be treated is caused to flow into the vessel from an influent overflow weir 4, is subjected to biological treatment and is caused to flow out to the outside of the vessel from an effluent overflow weir 5. In the vessel, an aeration means 7 and an agitation means 9 for agitating sludge are disposed, wherein the agitation means 9 is located at a position shallower than the aeration means 7, a deaeration region 10 for releasing fine air bubbles attached to the sludge is formed above the agitation means 9, and the effluent overflow weir 5 is communicated with the deaeration region 10. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a biological treatment tank for treating activated wastewater such as sewage with activated sludge.

  Conventionally, as this type of biological treatment tank, for example, as shown in FIG. 14, an aeration means 62 is installed at the bottom of a standard aeration tank 61 (biological treatment tank) having a water depth of about 4 to 5 m. BOD, nitrogen, phosphorus, etc. in treated water 63 (organic waste water) such as sewage are treated by aeration from 62.

  In the aerobic treatment as described above, in order to improve the sewage treatment capacity within the limited site, as shown by the solid line in FIG. 15, the water depth is deeper than that of the standard aeration tank 61 (see FIG. 14). A deep tank aeration tank 65 is used. The inside of the deep tank aeration tank 65 is divided into two by a baffle plate 66 provided at the center, and there is one in which an aeration means 62 is provided in one middle layer portion (a portion having a water depth of about 5 m). According to this, a swirl flow 67 is generated by aeration from the aeration means 62. Such a deep tank aeration tank is described, for example, in Patent Document 1 below.

  In the deep tank aeration tank 65 as described above, in order to reduce the amount of air necessary for biological treatment, as shown by the phantom line in FIG. There is one that increases the time during which the generated bubbles stay in the treated water 63.

JP-A-60-220192

  However, in the deep tank aeration tank 65 shown by the phantom line in FIG. 15, since the installation water depth of the aeration means 62 is deepened, a large amount of nitrogen in the air released from the aeration means 62 is contained in the treated water 63. Although dissolved, in the upper layer portion of the deep tank aeration tank 65, there is a problem that the dissolved nitrogen is vaporized and microbubbles are generated as the water pressure decreases, and the microbubbles adhere to the sludge. When the microbubbles adhere to the sludge as described above, the sludge floats up in the final sedimentation basin provided downstream of the deep tank aeration tank 65, and the quality of the treated water is deteriorated.

  An object of the present invention is to provide a biological treatment tank capable of preventing sludge from rising in a final sedimentation basin.

In order to achieve the above object, the first invention is a biological treatment tank in which a liquid to be treated flows into a tank from an inflow part to be biologically treated and flows out of the tank from an outflow part,
In the tank, aeration means and stirring means for stirring sludge are provided,
The stirring means is at a position shallower than the aeration means,
A deaeration region is formed above the stirring means to release the microbubbles adhering to the sludge,
The outflow portion communicates with the deaeration region.

  According to this, the liquid to be treated containing sludge flows into the tank from the inflow part, is aerated by the air released from the aeration means, is treated with activated sludge, passes through the deaeration region, and then flows out of the tank from the outflow part. To the final settling basin.

At this time, by increasing the installation water depth of the aeration means in the tank, the residence time during which bubbles released from the aeration means stay in the water to be treated increases.
On the other hand, when the installation depth of the aeration means is increased, in the upper layer of the biological treatment tank, as the water pressure decreases, nitrogen dissolved in the liquid to be treated is vaporized to generate microbubbles, and this microbubble is generated. Air bubbles adhere to the sludge. However, the sludge to which microbubbles adhere as described above is stirred by the stirring means in the deaeration region. By this stirring, the fine bubbles are separated from the sludge and float (degas) on the liquid surface in the degassing region. Thereby, it is possible to prevent the sludge to which the micro bubbles are attached from flowing out of the tank from the outflow portion, and the sludge can be flowed out of the tank from the outflow portion in a state where the micro bubbles are separated from the sludge. Thereby, it is possible to prevent the sludge from floating in the final settling basin on the downstream side of the rear stage, and it is possible to prevent the quality of the treated water from deteriorating.

Moreover, since the deaeration region is provided in the biological treatment tank, a new space for installing the deaeration equipment becomes unnecessary.
In the biological treatment tank according to the second invention, the stirring means is an aeration means.

  According to this, the sludge to which the microbubbles are attached is agitated by the air released from the aeration means in the deaeration region. By this stirring, the fine bubbles are separated from the sludge and float on the liquid surface in the deaeration region to be deaerated.

The biological treatment tank in the third invention is provided with a partition wall below the stirring means and above the aeration means,
The deaeration region and the region below the deaeration region are partitioned by a partition wall,
Returning means for returning the sludge in the region below the partition wall from the outflow portion side to the inside of the deaeration region or the upstream side of the deaeration region is formed.

  According to this, by aeration with the aeration means, sludge in the area below the partition wall is returned to the deaeration area or upstream of the deaeration area by the return means, and then stirred by the agitation means in the deaeration area. Then, it is deaerated and flows out of the tank through the outflow part from the deaeration region.

  Since the deaeration region and the region below the deaeration region are partitioned by the partition wall in this way, the sludge in the region below the partition wall flows out of the tank from the outflow part without passing through the deaeration region. (Short pass) can be prevented.

  In addition, the sludge in the area below the partition wall is returned to the degassing area or the upstream side of the degassing area by the return means without staying, so the dead space where the sludge stays below the partition wall (dead water retention) Part) can be prevented from being formed.

In the biological treatment tank according to the fourth invention, the return means comprises a return flow path,
The return flow path has a return inlet at one end and a return outlet at the other end,
The return inlet opens at the bottom of the bulkhead,
The return outlet opens in the deaeration region or upstream of the deaeration region.

  According to this, by performing aeration with the aeration means, the bubbles released from the aeration means to the region below the partition wall are temporarily blocked from floating by the partition wall and then introduced into the return flow path from the return inlet. Therefore, a powerful air lift effect occurs in the return flow path. As a result, the sludge in the area below the partition wall flows from the return inlet into the return flow path, and is returned from the return outlet of the return flow path to the inside of the deaeration area or upstream of the deaeration area, and then stirred in the deaeration area. It is agitated by the means and deaerated, and flows out of the tank through the outflow part from the deaeration region. In this way, sludge is returned to the inside of the deaeration region or upstream of the deaeration region by utilizing the air lift effect generated by aeration of the aeration means, so that a sludge return pump or the like is not required, and energy is consumed. Consumption can be reduced.

In the biological treatment tank of the fifth invention, the aeration means aerates the entire bottom of the tank.
According to this, since the generation of the swirling flow in the vertical direction is suppressed by aeration of the entire bottom of the tank with the aeration means, the residence time in which the bubbles released from the aeration means stay in the liquid to be treated is lengthened. This can improve oxygen transfer efficiency.

  As described above, according to the present invention, it is possible to prevent sludge from floating in the final sedimentation basin. Moreover, it can prevent that the sludge of the area | region below a partition flows out of a tank through an outflow part, without passing through a deaeration area | region. Furthermore, it is possible to prevent the formation of a dead space where sludge stays below the partition wall.

It is a longitudinal cross-sectional view of the deep tank aeration tank in the 1st Embodiment of this invention. It is a partially notched top view of the same deep tank aeration tank. It is a XX arrow line view in FIG. It is a YY arrow line view in FIG. It is a longitudinal cross-sectional view of the deep tank aeration tank in the 2nd Embodiment of this invention. It is a partially notched top view of the same deep tank aeration tank. It is a front view of the baffle board provided in the same deep tank aeration tank. It is a longitudinal cross-sectional view of the deep tank aeration tank in the 3rd Embodiment of this invention. It is a partially notched top view of the same deep tank aeration tank. It is a XX arrow line view in FIG. It is a top view of the deep tank aeration tank in the 4th Embodiment of this invention. It is a XX arrow line view in FIG. It is a YY arrow line view in FIG. It is a longitudinal cross-sectional view of the conventional standard aeration tank. It is a longitudinal cross-sectional view of the conventional deep tank aeration tank.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(First embodiment)
First, a first embodiment will be described below with reference to FIGS.

  Reference numeral 1 denotes a deep tank aeration tank (an example of a biological treatment tank) that performs activated sludge treatment (aerobic treatment) on treated water 2 that is organic wastewater such as sewage. At the upstream end of the tank body 3 of the deep tank aeration tank 1, an inflow overflow weir 4 (an example of an inflow part) that allows the treated water 2 to flow is provided, and at the downstream end of the tank body 3, An outflow overflow weir 5 (an example of an outflow portion) is provided to discharge the biologically treated water 2 to be treated. The inflow overflow weir 4 and the outflow overflow weir 5 are generally configured such that the treated water 2 flows in and out by overflow.

  The water depth of the tank body 3 is, for example, about 10 m, and a plurality of aeration means 7 for aeration of the entire surface are arranged at the bottom of the tank body 3. These aeration means 7 discharge air bubbles over the entire bottom of the tank body 3, and these aeration means 7 are connected to an aeration blower 8 (blower) installed outside the tank.

  In addition, a plurality of aeration means 9 (an example of agitation means) for agitating sludge are provided inside the tank body 3. These aeration means 9 are provided at a shallower (higher) position than the aeration means 7 (for example, near a water depth of about 1 to 5 m). Above the air diffuser 9, a deaeration region 10 is formed for releasing fine bubbles adhering to the sludge. The air diffuser 9 discharges air bubbles to the deaeration region 10, and the air diffuser 9 is connected to an air diffuser 11 (blower) installed outside the tank.

  Inside the tank body 3, a partition wall 13 is provided below the air diffuser 9 and above the aerator 7. Thereby, the deaeration region 10 and the region 14 below the deaeration region 10 (hereinafter referred to as the lower region 14) are partitioned by the partition wall 13.

  The tank body 3 is formed with a return flow path 16 (an example of a return means) for returning the sludge in the lower region 14 of the partition wall 13 from the outflow overflow weir 5 side to the upstream side of the deaeration region 10. The return flow path 16 includes an upper horizontal frame 17 and a vertical frame 18, and has a return inlet 18a at one end and a return outlet 17a at the other end. As shown in FIG. 3, the upper horizontal frame 17 is a member having a concave shape in the vertical cross section with its upper and both ends open, and has a horizontal return flow path 19 formed above the deaeration region 10. Is provided in the horizontal direction.

  As shown in FIG. 4, the vertical frame 18 is a tubular member having a rectangular cross section, and is provided in the vertical direction along the downstream wall surface in the tank body 3, and downstream of the deaeration region 10. It has a vertical return channel 20 formed on the side. The upper end of the vertical frame 18 communicates from below with one end portion facing the downstream side of the upper horizontal frame 17 (that is, the outflow overflow weir 5 side) and opens to the water surface in the tank. The return inlet 18 a is formed at the lower end of the vertical frame 18, and communicates from above with the other end of the partition wall 13 facing the downstream side (that is, the outflow overflow weir 5 side) and opens to the lower surface of the partition wall 13. is doing.

  One end of the upper horizontal frame 17 communicates with the upper end of the vertical frame 18. The return outlet 17a is formed at the other end of the upper horizontal frame 17, and opens to the upstream side of the deaeration region 10 (that is, the inflow overflow dam 4 side). As a result, the lower region 14 of the partition wall 13 and the upstream water surface of the deaeration region 10 communicate with each other via the return flow path 16.

  As shown in FIG. 1, the partition wall 13 is inclined so that the downstream end (that is, the vertical frame 18 side) is higher and the opposite upstream end is lower. The inflow overflow weir 4 is located above the aeration means 7, and the deaeration region 10 communicates adjacent to the outflow overflow weir 5 as shown in FIG. 2. A final sedimentation basin (not shown) is provided downstream of the overflow overflow weir 5 for precipitating solids and obtaining clear treated water.

Hereinafter, the operation of the above configuration will be described.
The treated water 2 flows into the tank body 3 from the inflow overflow weir 4 and is aerated by the entire surface by the air discharged from the aeration means 7 and is treated with activated sludge. 5 is discharged out of the tank and supplied to the final sedimentation basin.

  In this case, by increasing the installation water depth of the aeration means 7 in the tank, the residence time during which bubbles released from the aeration means 7 stay in the treated water 2 increases, so that the oxygen transfer efficiency is improved. The aeration amount necessary for biological treatment (aeration amount of the aeration means 7) is reduced.

  In addition, since aeration of the swirling flow in the vertical direction is suppressed by aeration on the entire surface by the aeration means 7, the residence time in which bubbles released from the aeration means 7 stay in the treated water 2 can be further increased. it can. Thereby, oxygen transfer efficiency improves.

  On the other hand, when the installation water depth of the aeration means 7 is increased, in the upper layer portion of the deep tank aeration tank 1, nitrogen dissolved in the water to be treated 2 is vaporized as the water pressure decreases, and microbubbles are generated. (At this time, oxygen is mostly consumed by aerobic microorganisms), and these microbubbles adhere to the sludge. However, the sludge to which microbubbles adhere as described above is agitated by the air discharged from the air diffuser 9 in the deaeration region 10.

  By this stirring, the fine bubbles are separated from the sludge and float on the water surface of the deaeration region 10 to be deaerated. As a result, it is possible to prevent the sludge with attached microbubbles from flowing out of the overflow overflow weir 5 to the outside of the tank, and with the microbubbles separated from the sludge, the sludge is discharged from the overflow overflow weir 5 to the outside of the tank. Can be drained. For this reason, it is possible to prevent sludge from floating in the final settling basin on the downstream side of the rear stage, and to prevent deterioration of treated water quality.

  As described above, by using the aeration means 9 as an example of the agitation means, the deaeration region 10 can be agitated and oxygen necessary for biological treatment can be supplied to the deaeration region 10. Thereby, also in the deaeration area | region 10, a biological treatment can be performed without trouble. Since the aeration means 9 is installed at a shallower position than the aeration means 7, the amount of microbubbles such as nitrogen accompanying the decrease in water pressure is very small compared to the aeration means 7, and can be ignored. is there.

Moreover, since the deaeration area | region 10 is provided in the deep tank aeration tank 1, the new space for installing deaeration equipment becomes unnecessary.
Further, by performing aeration on the entire surface with the aeration means 7 as described above, an air lift effect is generated on the entire inner surface of the tank body 3. As a result, sludge in the lower region 14 of the partition wall 13 flows from the return inlet 18a into the vertical return flow path 20 in the vertical frame 18, and flows from the vertical return flow path 20 to the horizontal return flow path 19 of the upper horizontal frame 17 for return. It is returned to the upstream water surface of the deaeration region 10 from the outlet 17a, and then agitated and deaerated by the air flowing into the deaeration region 10 and released from the air diffuser 9, and the outflow overflow from the deaeration region 10 It flows out of the tank through the weir 5.

  Thus, since the deaeration region 10 and the lower region 14 are partitioned by the partition wall 13, the sludge in the lower region 14 of the partition wall 13 passes through the outflow overflow weir 5 without passing through the deaeration region 10. It is possible to prevent outflow (short pass) out of the tank.

  In addition, the sludge in the lower region 14 of the partition wall 13 is returned to the upstream side of the deaeration region 10 through the return flow path 16 without staying, so that a dead space (sludge stays below the partition wall 13 ( It is possible to prevent the water retention part) from being formed.

  Further, the bubbles released from the aeration means 7 to the lower region 14 of the partition wall 13 are temporarily blocked from floating by the partition wall 13, moved along the inclination of the partition wall 13, and narrow vertical return flow paths 20 in the vertical frame 18. Therefore, a strong air lift effect is generated in the vertical return flow path 20, thereby reliably returning the sludge in the lower region 14 of the partition wall 13 to the upstream side of the deaeration region 10. be able to. In this way, since sludge is returned to the upstream side of the deaeration region 10 by utilizing the air lift effect generated by aeration of the aeration means 7, a dedicated pump for sludge return is unnecessary, and energy consumption is reduced. Can be achieved.

(Second Embodiment)
Next, a second embodiment will be described below with reference to FIGS.
A baffle plate 22 is provided in the upstream portion inside the tank body 3. The baffle plate 22 disperses the flow from the inflow overflow weir 4 to the outflow overflow weir 5 at a location closer to the upstream side. As shown in FIG. 7, the baffle plate 22 is formed with a plurality of flow holes 23 penetrating upstream (on the inflow overflow weir 4 side) and downstream (on the outflow overflow weir 5 side).

Hereinafter, the operation of the above configuration will be described.
The treated water 2 containing sludge flows into the tank body 3 from the inflow overflow weir 4 and tends to flow toward the outflow overflow weir 5, but this flow is upstream of the baffle plate 22. In this case, it is dispersed along this surface and flows toward the outflow overflow weir 5 on the downstream side through each flow hole 23. Thereby, the to-be-processed water 2 which flowed in from the inflow overflow dam 4 is reliably guide | induced to the deep part (bottom part) in the tank main body 3, and is wide from the shallow part in the tank main body 3 to the deep part, and from upstream to downstream. Therefore, the activated sludge treatment can be performed by effectively using the entire inside of the deep tank aeration tank 1.

(Third embodiment)
Next, a third embodiment will be described below with reference to FIGS.
In the first embodiment, assuming that the air diffuser 9 provided in the deaeration region 10 is the first air diffuser 9, in the third embodiment, as shown in FIGS. In addition to the first air diffuser 9, the second air diffuser 26 is provided in the upstream portion inside the tank body 3. The second air diffuser 26 is provided at a shallower (higher) position than the aerator 7 and at the same water depth (height) as the first air diffuser 9. The first air diffuser 9 and the second air diffuser 26 are connected to a common air diffuser 11.

  The upstream portion in the tank body 3 is divided by the baffle plate 28 into one processing unit 29a and the other processing unit 29b. Both processing parts 29a, 29b communicate with each other above and below the baffle plate 28. The second air diffuser 26 is provided in one processing unit 29a.

Hereinafter, the operation of the above configuration will be described.
As shown in FIG. 10, an air lift effect is generated by the air released from the second air diffuser 26, and the swirl flow in the vertical direction swirling through the upper and lower portions of the baffle plate 28 in the upstream portion in the tank body 3. 27 occurs. The treated water 2 containing sludge flows into the tank body 3 from the inflow overflow weir 4 and tends to flow toward the outflow overflow weir 5. This flow hits the swirl flow 27 and is dispersed vertically. Then, it flows toward the outflow overflow weir 5 on the downstream side. Thereby, the to-be-processed water 2 is reliably guide | induced to the deep part in the tank main body 3, and can generate the flow which goes to the downstream from the upstream in the wide range ranging from the shallow part in the tank main body 3 to the deep part. The activated sludge treatment can be performed by effectively using the entire inside of the deep tank aeration tank 1.

  Since the first air diffuser 9 and the second air diffuser 26 are provided at the same water depth, the first air diffuser 9 and the second air diffuser 26 have the same water pressure. As a result, the amount of air diffused from the first air diffuser 9 (air discharge amount) and the amount of air diffused from the second air diffuser 26 can be made equal.

  In the third embodiment, the second aeration means 26 is provided at a position shallower than the aeration means 7 and at the same water depth as the first aeration means 9, but at a position shallower than the aeration means 7. And it may be provided at a different water depth from the first air diffuser 9.

  In the first to third embodiments, sludge is returned to the upstream side of the deaeration region 10 by utilizing the air lift effect generated by aeration of the aeration means 7, but the sludge is deaerated using a pump. It may be returned to the upstream side of the area 10.

  In the first to third embodiments, the sludge in the lower region 14 of the partition wall 13 is returned to the upstream side of the deaeration region 10 in the return flow path 16, but the sludge in the lower region 14 of the partition wall 13 is returned. You may return in the deaeration area | region 10.

(Fourth embodiment)
Next, a fourth embodiment will be described below with reference to FIGS.
41 is an existing deep tank aeration tank (an example of a biological treatment tank), and an inflow overflow weir 4 for allowing the treated water 2 to flow into is provided at the upstream end of the tank body 42 of the deep tank aeration tank 41. An outflow overflow weir 5 is provided at the downstream side end of the tank body 42 to allow the biologically treated water 2 to flow out. A baffle plate 43 is provided between the inflow overflow weir 4 and the outflow overflow weir 5 in the center of the tank body 42. The inside of the tank body 42 is divided into one processing unit 44 and the other processing unit 45 by a baffle plate 43. The upper parts and the lower parts of both the processing units 44 and 45 communicate with each other.

  Aeration means 7 is arranged at the bottoms of both the processing units 44 and 45 in the tank body 42. Further, an air diffuser 9 (an example of an agitator) for agitating sludge is provided on the downstream side (outflow overflow weir 5 side) of one processing unit 44. The air diffusion means 9 is provided at a shallower (higher) position than the aeration means 7. Above the air diffuser 9, a deaeration region 10 is formed for releasing fine bubbles adhering to the sludge. The air diffuser 9 discharges air bubbles to the deaeration region 10. In addition, a non-aeration area 47 in which no aeration means 9 is provided is formed on the downstream side (outflow overflow weir 5 side) of the other processing unit 45. The inflow overflow weir 4 is located above the aeration means 7, and the deaeration region 10 is adjacent to the outflow overflow weir 5.

Hereinafter, the operation of the above configuration will be described.
The treated water 2 flows into the tank body 42 from the inflow overflow weir 4, is aerated by the air discharged from the aeration means 7, is treated with activated sludge, passes through the deaeration region 10, and then flows into the outflow overflow weir 5. Is discharged from the tank to the final settling tank.

  At this time, as shown in FIG. 13, the air released from the air diffuser 9 raises the deaeration region 10 and flows from one processing unit 44 to the other processing unit 45 across the baffle plate 43. Then, a swirl flow 46 is generated that descends the non-aeration region 47 and crosses the lower part of the baffle plate 43 and returns to one processing unit 44.

By increasing the installation water depth of the aeration means 7 in the tank, the residence time during which bubbles released from the aeration means 7 stay in the treated water 2 increases.
On the other hand, when the installation water depth of the aeration means 7 is deepened, in the upper layer part of the tank body 42, with the decrease of the water pressure, nitrogen dissolved in the water to be treated 2 is vaporized to generate micro bubbles, These micro bubbles adhere to the sludge. However, the sludge to which such microbubbles are attached is agitated by the swirling flow 46 generated by the air diffused from the air diffuser 9.

  By this stirring, microbubbles are separated from the sludge and float on the water surface to be deaerated. As a result, it is possible to prevent the sludge with attached microbubbles from flowing out of the overflow overflow weir 5 to the outside of the tank, and with the microbubbles separated from the sludge, the sludge is discharged from the overflow overflow weir 5 to the outside of the tank. Can be drained. Thereby, it is possible to prevent the sludge from floating in the final settling basin on the downstream side of the rear stage, and it is possible to prevent the quality of the treated water from deteriorating.

  In each of the above embodiments, the air diffuser 9 is used as an example of the agitator. However, the air diffuser 9 is not limited to the air diffuser 9, and for example, the rotating blade may be rotated by a driving device. Moreover, the numerical values such as the water depth shown in each of the above embodiments are examples, and are not limited to these numerical values.

  In each of the above embodiments, the inflow overflow weir 4 is provided as an example of the inflow portion, and the outflow overflow weir 5 is provided as an example of the outflow portion, but is not limited to such an overflow weir, For example, a pipe may be provided instead of the overflow weir.

1 Deep tank aeration tank (biological treatment tank)
2 Water to be treated (liquid to be treated)
4 Inflow section 5 Outflow section 7 Aeration means 9 Aeration means (stirring means)
DESCRIPTION OF SYMBOLS 10 Deaeration area | region 13 Partition 14 Area | region 16 below a partition 16 Return flow path (return means)
17a Return outlet 18a Return inlet 19, 20 Return flow path

Claims (5)

A biological treatment tank in which the liquid to be treated flows into the tank from the inflow part and is biologically treated, and flows out of the tank from the outflow part,
In the tank, aeration means and stirring means for stirring sludge are provided,
The stirring means is at a position shallower than the aeration means,
A deaeration region is formed above the stirring means to release the microbubbles adhering to the sludge,
A biological treatment tank characterized in that the outflow part communicates with the deaeration region. The biological treatment tank according to claim 1, wherein the stirring means is an aeration means. A partition is provided below the stirring means and above the aeration means,
The deaeration region and the region below the deaeration region are partitioned by a partition wall,
The biological treatment tank according to claim 1 or 2, wherein a return means for returning the sludge in the region below the partition wall from the outflow portion side to the inside of the deaeration region or the upstream side of the deaeration region is formed. . The return means consists of a return flow path,
The return flow path has a return inlet at one end and a return outlet at the other end,
The return inlet opens at the bottom of the bulkhead,
The biological treatment tank according to claim 3, wherein the return outlet opens in the deaeration region or upstream of the deaeration region. The biological treatment tank according to any one of claims 1 to 4, wherein the aeration means aerates the entire bottom surface of the tank.

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