JP2007136378A - Septic tank - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a septic tank which can promote generation of scum, and stably store the scum without destroying it. <P>SOLUTION: The septic tank comprises an anaerobic treatment tank having an inflow port and an outflow port, and a sludge storage tank for storing sludge pumped from the anaerobic treatment tank. The sludge storage tank has a sewage passage for returning sewage separated in the sludge storage tank to the anaerobic treatment tank. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a septic tank for treating sewage such as merged effluent and disposer effluent, and more particularly to a septic tank for concentrating and storing sludge generated in connection with sewage treatment.

  Until now, various things have been proposed as a septic tank (more specifically, a septic tank used in the previous stage of the aerobic treatment) for storing sludge generated in the treatment of sewage. The most common ones include precipitation separation tanks and anaerobic filter bed tanks.

  In the sedimentation separation tank, the solid matter that has flowed into the tank is allowed to settle and stored at the bottom of the tank, and a part of the solid that floats up and forms a scum near the liquid level is stored at the top of the tank. On the other hand, the anaerobic filter bed tank is provided with a microorganism adhesion material called an anaerobic filter bed in the tank, and sludge can be stored in this filter bed in addition to the tank bottom and the tank upper part (scum). According to the survey results so far, the sludge concentration (TSS concentration) that can be stored in each part is about 50,000 to 100,000 mg / L for scum, about 10,000 to 20,000 mg / L for the filter bed, and about 2 for the tank bottom. It is about 10,000 to 40,000 mg / L, and it is known that forming scum is most effective for concentrating and storing sludge. For this reason, various sludge scumming promotion methods have been proposed for the purpose of reducing the sludge storage capacity.

An example of this will be described with reference to FIG. 7. The septic tank includes a contaminant removal tank 41 that performs anaerobic treatment with microorganisms and an aeration tank 42 that performs an aerobic treatment. Means 44 for injecting nitrate nitrogen solution is provided. In this septic tank, when a nitrate nitrogen-containing liquid is injected into the bottom of the contaminant removal tank 41, nitrate nitrogen is reduced to nitrogen gas by the action of microorganisms present in the tank. This nitrogen gas is caused to adhere to the sludge accumulated at the bottom of the tank and float, and the sludge is concentrated and stored as scum S1. (See Patent Document 1)
JP 2001-25793 A

  However, since the conventional septic tank has a structure in which sewage directly flows into the scum reservoir 43, for example, when wastewater having a large flow rate per unit time such as washing wastewater or bathtub wastewater flows in, It is considered that the liquid level is disturbed by the water flow and the scum is easily broken, and it is difficult to stably form a large amount of scum.

  In view of the above problems, an object of the present invention is to provide a septic tank that promotes the formation of scum and can be stably stored without being destroyed.

In order to achieve the above object, the present invention has the following configuration.
(1) The present invention includes an anaerobic treatment tank having an inlet and an outlet, and a sludge storage tank that transfers and stores sludge in the anaerobic treatment tank with a pump. It is a septic tank having a sewage flow path for returning separated sewage to the anaerobic treatment tank.
(2) The septic tank according to item (1), wherein the sewage flow path has one end opened to an intermediate height in the sludge storage tank and the other end opened to the upper part of the anaerobic treatment tank.
(3) The septic tank in which the sludge transferred to the sludge storage tank is pulled out from the bottom part of the anaerobic treatment tank and flows into the intermediate height in the sludge storage tank in the item (1) or (2).
(4) The septic tank according to any one of items (1) to (3), wherein the anaerobic treatment tank and the sludge storage tank are adjacent to each other, and the sewage is returned from the sludge storage tank to the anaerobic treatment tank by natural flow.
(5) The septic tank according to any one of items (1) to (4), wherein the anaerobic treatment tank is provided with a flow rate adjusting unit at an upper part thereof.
(6) The septic tank according to any one of items (1) to (5), wherein the sludge storage tank has scumming promotion means in the tank.
(7) In any one of (1) to (6), the anaerobic treatment tank has a filter bed.
(8) The septic tank according to any one of items (1) to (7), wherein the anaerobic treatment tank has an aerobic treatment tank at a subsequent stage.

  According to the septic tank of the present invention, since the scum can be formed stably, the sludge storage capacity can be reduced, and as a result, the capacity of the entire septic tank can be made compact.

First, with reference to FIG. 1, the structure of an example of the septic tank of this invention is demonstrated.
The septic tank has an anaerobic treatment tank 1 and a sludge storage tank 2. The anaerobic treatment tank 1 includes a sewage inlet 3, an outlet 4, and a sludge transfer pump 5, and the sludge storage tank 2 includes a sludge inflow baffle 6 that receives the sludge transferred from the anaerobic treatment tank 1. And a sewage flow path 7 for returning the sewage from which the sludge has been separated to the anaerobic treatment tank 1.

The shape of the anaerobic treatment tank 1 can be selected as appropriate in a plan view, such as a substantially rectangular shape, a circular shape, an elliptical shape, etc., but a substantially rectangular shape is preferable in terms of ease of molding.
The position of the inlet 3 provided in the anaerobic treatment tank 1 is preferably provided at a position higher than the liquid level of the sludge storage tank 2. This is because if the inlet 3 is provided at a position lower than the water level of the sludge storage tank 2, the sewage returned from the sludge storage tank 2 may flow back to the inlet 3.
The position of the outlet 4 of the anaerobic treatment tank 1 is preferably provided at a position lower than the water level of the sludge storage tank 2. Then, even if a large amount of sewage flows in temporarily and the liquid level of the anaerobic treatment tank 1 is disturbed, the liquid level of the sludge storage tank 2 is not disturbed, so that it is difficult to destroy the scum. Can do. In addition, an advection baffle can be provided on the wall surface on which the outlet 4 is provided in order to prevent a short circuit of the sewage that has flowed in.

As the sludge transfer pump 5, an air lift pump type, an intermittent quantitative type by pneumatic feeding of a sealed container, an electric pump, or the like can be used, but an inexpensive air lift pump type is preferably used.
The suction port 8 of the sludge transfer pump 5 is preferably provided near the bottom of the anaerobic treatment tank 1, and more preferably provided near the bottom of the anaerobic treatment tank 1 on the outlet 4 side. This is because the sludge concentrated to a higher concentration can be efficiently transferred to the sludge storage tank 2 by providing the suction port 8 of the sludge transfer pump 5 near the bottom. Moreover, if the suction port 8 of the sludge transfer pump 5 is provided in the vicinity of the outlet 4, the sludge in the anaerobic treatment tank 1 can be made difficult to accumulate in the vicinity of the outlet 4, so that the sludge in the tank becomes full. Even when sewage flows in when approaching, sludge at the bottom can be made difficult to transfer to the next tank along the sewage flow.

The sludge storage tank 2 is provided with a sludge inflow baffle 6 that receives the sludge in the anaerobic treatment tank 1 and a sewage flow path 7 that returns the sewage after separating the sludge that has flowed into the anaerobic treatment tank 1.
The sludge inflow baffle 6 is arranged such that the upper end is at a position above the liquid level of the sludge storage tank 2 and the lower end is at an intermediate height in the sludge storage tank 2.
Here, the intermediate height is different from the meaning of the middle position in the water depth direction, and refers to the space between the scum S1 formed at the top of the sludge storage tank 2 and the sludge S2 deposited at the tank bottom. Specifically, it is preferable that the lower end position of the sludge inflow baffle 6 is approximately 1/3 to 2/3 of the water depth from the bottom of the sludge storage tank 2. When the lower end of the sludge inflow baffle 6 is provided at this position, the scum S1 formed at the top of the sludge storage tank 2 and the sludge S2 deposited at the bottom are disturbed by the water flow generated when the sludge is transferred from the anaerobic treatment tank 1. Can be difficult.

One end of the sewage flow path 7 is preferably provided at the upper part of the anaerobic treatment tank 1 and the other end is provided at an intermediate height of the sludge storage tank 2. Here, the upper part of the anaerobic treatment tank 1 indicates a position higher than the highest water level (HWL) of the anaerobic treatment tank 1, and is preferably a position lower than the inlet 3. In this way, when sewage flows into the anaerobic treatment tank 1, it is possible to prevent the sewage or the effluent from the anaerobic treatment tank 1 from flowing back into the sludge storage tank 2.
On the other hand, when the other end is provided at an intermediate height, when the sludge flows into the sludge storage tank 2, the scum S1 formed on the top of the sludge storage tank 2 or the sludge S2 deposited on the bottom is added to the anaerobic treatment tank 1. Advection can be made difficult. Further, the other end is a baffle plate or the like having an inclination toward the center of the sewage channel 7 in plan view in order to prevent the scum S1 floating in the mud storage tank 2 from entering the sewage channel 7 Is preferably provided.

  It is preferable to provide the sludge inflow baffle 6 and the sewage flow path 7 provided in the sludge storage tank 2 at positions separated from each other in plan view. When the sludge inflow baffle 6 and the sewage flow path 7 are close to each other, the sludge transferred from the anaerobic treatment tank 1 to the sludge inflow baffle 6 flows into the sewage flow path 7 before being separated into sludge and sewage. As a result, it becomes difficult to store sludge in the sludge storage tank 2.

Next, an example of another septic tank of the present invention will be described with reference to FIG.
In FIG. 2, the anaerobic treatment tank 1 and the sludge storage tank 2 are formed adjacent to each other. In this case, the sludge storage tank side open end of the sewage flow path 7 is disposed so as to be connected to the opening 30 provided on the wall surface where the anaerobic treatment tank 1 and the sludge storage tank 2 are in contact. The position of the opening 30 is preferably higher than the highest water level of the anaerobic treatment tank 1 and lower than the inlet 3.

Next, an example of another septic tank of the present invention will be described with reference to FIG.
In this embodiment, a flow rate adjusting unit 9 is formed in the upper part of the anaerobic treatment tank 1. On the wall side where the outlet 4 of the anaerobic treatment tank 1 is provided, a pumping pump 10 is provided for pumping the liquid in the tank from the liquid level of the lowest water level (LWL). In addition, a flow rate adjusting device that adjusts the amount of water transferred to the outlet of the liquid pumped up by the pump can be provided as necessary. It is preferable to provide an advection baffle 12 around the pumping pump 10 in order to prevent inflow of impurities contained in the inflowing sewage and a short circuit of the sewage.

  The suction port 31 of the pumping pump 10 is provided at the lowest water level (LWL). Then, the liquid level of the anaerobic treatment tank 1 depends on whether the amount of inflow of sewage is larger or smaller than the amount of liquid transferred from the flow control device, the lowest water level (LWL) and the highest water level (HW). .L). However, since the maximum water level (HWL) is set in advance for safety, the inflowing sewage does not overflow beyond the water level (HWL). In addition, although the pumping pump 10 has shown the air lift pump type | mold which used the air blower 35 in FIG. 3, it can replace with an air lift pump type | formula and can also use the intermittent metering pump by an air pressure supply of an airtight container, an electric pump, etc.

Next, an example of another septic tank of the present invention will be described with reference to FIG.
In FIG. 4, scumming promotion means 13 for promoting the formation of scum is provided in the sludge storage tank 2.
The scumming promotion means 13 is not particularly limited, and various types can be used. In this embodiment, a pressure dissolution type fine bubble generator is used. The pressure dissolution method is a method in which a gas is mixed in a liquid and dissolved under pressure, and then returned to normal pressure to precipitate fine bubbles, and bubbles having a diameter of about several μm to several tens of μm are generated. be able to. If such fine bubbles adhere to the sludge accumulated in the sludge storage tank 2, the specific gravity of the sludge becomes light and can be floated on the top of the tank and stored as scum.
In addition, when using this pressure dissolution type fine bubble generator, the processed liquid of the anaerobic treatment tank 1 may be used as the liquid for dissolving the gas, or an aerobic treatment tank is provided in the subsequent stage. In some cases, an aerobic treated solution can be used.
In addition, tap water, well water, middle water, rain water, or the like can be used in order to further reduce the risk of clogging with foreign substances in the microbubble generator.

  As a means for generating fine bubbles, besides the pressure-dissolution method, a method in which a mechanical shearing force is applied to a mixed fluid of liquid and gas to make the bubbles finer, the pressure inside the tube using a Venturi-shaped tube Any method such as a method of destroying and miniaturizing bubbles by change, a method using ultrasonic waves, or a method of generating bubbles by electrolysis can be used as appropriate.

The scumming promotion means 13 may be operated continuously, but is preferably operated intermittently, for example, once or several times a day. This is because the scum formed by the scumming promotion means 13 is less likely to collapse in a short period of time, so that the scum can be stably formed even during intermittent operation.
Further, as the scumming promotion means 13, a method of adding nitrate to generate nitrogen gas by the action of microorganisms living in the sludge storage tank 2, or adding carbonate and adjusting the pH to the acidic side to carbonate A method of generating bubbles in a biological or chemical manner such as a method of generating gas can also be used.
Furthermore, heating means such as a heater may be provided in the sludge storage tank 2 to activate the action of microorganisms and promote scumming.

  In the case where an aerobic treatment tank is provided after the septic tank of this embodiment, ammonia nitrogen contained in waste water in the aerobic treatment tank is nitrified to nitrate nitrogen, and the aerobic treatment containing this nitrate nitrogen has been completed. A part of the liquid can be returned into the sludge storage tank 2. If it does in this way, the scum-ization of the sludge in the sludge storage tank 2 can be further accelerated | stimulated using the effect | action of microorganisms, without using a special chemical | medical agent and an apparatus.

Next, an example of another septic tank of the present invention will be described with reference to FIG.
In the anaerobic treatment tank 1, a filter bed 14 is provided at a position below the liquid level of the lowest water level (LWL). When the filter bed 14 is provided, the concentration of anaerobic microorganisms in the anaerobic treatment tank 1 can be increased, so that the anaerobic digestion of the sludge can be promoted in the anaerobic treatment tank 1, resulting in a small amount of sludge generated as a whole. can do.

  The carrier shape of the filter bed 14 provided in the anaerobic treatment tank 1 is not particularly limited, and plate-like members such as loofah-like, corrugated and porous, honeycomb-like (honeycomb core) members, etc. Preferably used. Skeletal spherical, mesh-like cylindrical members and the like can also be used.

  FIG. 6 is an example of another septic tank of the present invention, and an aerobic treatment tank 17 having a biological reaction chamber 15 and a filtration chamber 16 is provided in the subsequent stage of the anaerobic treatment tank 1. In the present embodiment, the filtration chamber 16 is arranged in the upper part of the aerobic treatment tank 17 and the rest is used as the biological reaction chamber 15. However, the present invention is not limited to this, and the biological reaction chamber 15, the filtration chamber 16, Can be arranged vertically or juxtaposed on the left and right.

  In the biological reaction chamber 15, a reaction aeration member (aeration tube) 18 for aeration from the bottom is disposed, and air from the blower 35 is discharged. The biological reaction chamber 15 is formed with a biological reaction bed 15a filled with a carrier (a microbial carrier, a microorganism adhesion material, and a contact material), and sewage is aerobically treated in the biological reaction bed 15a. The biological reaction bed 15a may be a fluidized bed or a fixed bed.

  The shape of the carrier can be various shapes such as a plate shape, a net plate shape, a loofah shape, a porous shape, a cylindrical shape, a rod shape, a skeletal sphere shape, a string shape, a granular shape, an irregular lump shape, a cubic shape, and a fiber lump shape. What was processed can be used. Examples of the base material include synthetic resin processed products such as polyvinyl chloride, polyester, polyvinylidene chloride, polyvinyl formal, polyurethane, and melamine resin, inorganic processed products such as ceramics and silica sand, and fossils such as anthracite. A processed product, activated carbon or the like having a specific gravity of about 1 or 1 or more, a polyolefin resin such as polyethylene or polypropylene, or a specific gravity of about 1 or 1 or less such as polystyrene can be used.

  The liquid processed in the biological reaction chamber 15 is transferred to the upper portion of the filtration chamber 16 via the advection tube 19. In the vicinity of the bottom of the advection tube 19, it is preferable to provide a mesh or slit-shaped portion so that only the treated liquid can pass through without allowing the carrier in the biological reaction chamber 15 to flow out. In addition, the upper part of the advection pipe 19 is opened, and the advection port 32 to the filtration chamber 16 is provided in the part which becomes the water level of the aerobic treatment tank 17.

The advection pipe 19 is provided with a circulation pump 20 that returns a part of the aerobic treated water treated in the biological reaction chamber 15 to the anaerobic treatment tank 1. The circulation pump 20 and the anaerobic treatment tank 1 are connected by a circulation pipe 21. The reason why a part of the aerobic treated water is returned to the anaerobic treatment tank 1 is that the SS floating in the aerobic treatment tank 17 or the SS that tends to settle can be extracted, and there is nitrate nitrogen. This is because denitrification can be performed by biological action in the anaerobic treatment tank 1.
The returned water can be returned to the sludge storage tank 2. By returning the return water containing nitrate nitrogen to the sludge storage tank 2, the nitrogen gas generated when denitrified is attached to the sludge accumulated at the bottom of the tank, and the scumification of the sludge can be further promoted.

The return of the aerobic treated water from the circulation pump 20 may be continuous or intermittent.
In the present embodiment, the circulation pump 20 is an air lift pump that supplies air from the blower 35. However, an intermittent metering pump, an electric pump, or the like that pumps air into a sealed container can also be used.

The filtration chamber 16 is provided with a filtration bed 16a filled with a filter medium for capturing SS (floating matter) present or remaining in the aerobic treated water, and a filtration bed 16a is provided below or below the filtration bed 16a. A cleaning air diffusing member 23 for cleaning the floor 16a is disposed.
Further, at the bottom of the filtration chamber 16, a cleaning drainage pump 24 for extracting the cleaning drainage during cleaning and transferring it to the sludge storage tank 2 is disposed. The washing waste water can also be transferred to the anaerobic treatment tank 1.
In the filtration chamber 16, aerobic biological treatment also proceeds due to the presence of dissolved oxygen carried over from the biological reaction chamber 15.

  As the filter medium filled in the filtration chamber 16, one that floats in the liquid can be used, but a sedimentation filter medium is preferable. Examples of sedimentary filter media include synthetic resin processed products such as polyvinyl chloride, polyester, polyvinylidene chloride, polyvinyl formal, polyurethane, and melamine resin, inorganic processed products such as ceramics and quartz sand, and fossil processing such as anthracite. Products, activated carbon, etc., with specific gravity of about 1 or more, or polyolefin resin such as polyethylene, polypropylene, etc., and those with a specific gravity of about 1 or more adjusted by adding fillers to polystyrene. It may be formed and processed into a lump shape, a cylindrical shape, a net shape, a rod shape, a fiber lump shape, or a porous shape.

  In order to effectively clean the filter bed 16a, the cleaning air diffuser member 23 is preferably arranged so that the bubbles discharged from the cleaning air diffuser member 24 are in contact with the entire filter bed 16a. Visually, I-shaped, H-shaped, Japanese-shaped, eye-shaped, etc., or similar shapes can be used.

The cleaning air diffuser 23 and the cleaning drainage pump 24 are connected to the blower 35 via the flow path switching device 27 by the air pipe 26. The flow path switching device 27 is a device that switches so that the air sent from the blower 35 is sent to one of the air pipes 25 and 26, and includes, for example, a timer and a switching valve. During normal operation, the air from the blower 35 is set to be sent to the air pipe 25. For example, when the time set by a timer (during the washing operation) is reached, the flow path is switched to the air pipe 26 to wash the filtration bed 16a. I do.
Note that a dedicated blower for sending air to the cleaning air diffuser 23 and the cleaning drainage pump 24 may be provided, and a timer may be provided so that the blower is activated at a set time (during the cleaning operation). .

  In this embodiment, the cleaning wastewater is transferred to the sludge storage tank 2. When the nitrification reaction is proceeding in the biological reaction chamber 15, nitrate nitrogen is also present in the washing waste water of the filtration chamber 16, and therefore, by transferring this liquid to the sludge storage tank 2, the sludge storage tank 2 Scumification can be further promoted.

In the present embodiment, a system having the biological reaction chamber 15 and the filtration chamber 16 is used as the aerobic treatment tank 17, but the present invention is not limited to this, and various systems can be used. For example, a membrane utilization method may be used as the treatment method of the aerobic treatment tank 17. In this case, the suction (or pressurization) pump for obtaining treated water through the membrane can be shared with the pump used for the fine bubble generating device as the scumming means. As an example of the operation method, it is used as a membrane suction (or pressurization) pump during normal operation. When the fine bubble generator is operated (once or several times a day), the suction side flow path is switched to the membrane treated water storage section and the supply side flow path is switched to the fine bubble generation apparatus to store sludge. Fine bubbles are generated in the tank 2 to promote scumming. Thus, the cost concerning a septic tank can be reduced by sharing two functions with one pump.
Although not specifically described in the present embodiment, a treatment water tank for temporarily storing a liquid to be treated that has been aerobically treated in the aerobic treatment tank 17 and / or a treatment target after the aerobic treatment tank 17. A disinfection tank for disinfecting the liquid can also be provided.

  Hereinafter, the treatment of sewage will be described. The sewage enters the anaerobic treatment tank 1 from the inflow port 3, and the solid matter in the influent sewage is separated and anaerobic treatment is performed. Here, the separated solid (sludge) is transferred to the sludge storage tank 2 by the sludge transfer pump 5, scummed by the scumming promotion means 13, and stored at a high concentration in the upper part of the sludge storage tank 2. The advection liquid from the anaerobic treatment tank 1 is transferred from the pumping pump 10 to the aerobic treatment tank 17. In the aerobic treatment tank 17, the organic matter in the advection liquid from the anaerobic treatment tank 1 is aerobically biodegraded in the biological reaction chamber 15, and captured and filtered in the filtration chamber 16 together with SS generated by the biological treatment.

  Since the SS accumulates and clogs over time, the filter bed 16a is periodically or appropriately (reversely) washed. In this cleaning, the air of the blower 35 is discharged from the cleaning air diffuser member 23 to bubble the filter bed 16a, and a part of the air of the blower 35 is caused to flow through the air pipe 26 to be washed and drained withdrawing pump (air lift pump) 24. Also supply. The peeled SS becomes washing wastewater together with the liquid in the tank, descends the filtration bed 16a, returns to the sludge storage tank 2 through the washing drainage discharge member 24a by the washing drainage pump 24. Cleaning of the filter bed 16a is usually preferably performed at midnight when sewage is rarely discharged in a general household.

It is a schematic sectional drawing of an example of the septic tank which is an Example of this invention. It is a schematic sectional drawing which shows an example of the septic tank which is the other Example of this invention. It is a schematic sectional drawing which shows an example of the septic tank which is the other Example of this invention. It is a schematic sectional drawing which shows an example of the septic tank which is the other Example of this invention. It is a schematic sectional drawing which shows an example of the septic tank which is the other Example of this invention. It is a schematic sectional drawing which shows an example of the septic tank which is the other Example of this invention. It is a schematic sectional drawing which shows an example of the septic tank of a prior art example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Anaerobic treatment tank, 2 ... Sludge storage tank, 3 ... Inlet, 4 ... Outlet, 5 ... Sludge transfer pump, 6 ... Sludge inflow baffle, 7 ... Sewage flow path, 8 ... Suction port, 9 ... Flow rate adjustment part DESCRIPTION OF SYMBOLS 10 ... Pumping pump, 11 ... Flow control apparatus, 12 ... Advection baffle, 13 ... Scumming means, 14 ... Filter bed, 15 ... Biological reaction chamber, 15a ... Biological reaction bed, 16 ... Filtration chamber, 16a ... Filtration bed, 17 ... Aerobic treatment tank, 18 ... Aeration member for reaction, 19 ... Advection pipe, 20 ... Circulation pump, 21 ... Circulation pipe, 23 ... Aeration member for washing, 24 ... Cleaning drainage pump, 25 ... Air pipe, 26 ... Air piping, 27 ... Flow path switching device, 30 ... Opening part, 31 ... Suction port, 32 ... Advection port, 35 ... Blower, 41 ... Contaminant removal tank, 42 ... Aeration tank, 43 ... Scum storage part, S1 ... scum, S2 ... bottom sludge.

Claims (8)

  An anaerobic treatment tank having an inlet and an outlet and a sludge storage tank that transfers and stores sludge in the anaerobic treatment tank with a pump, and the sludge storage tank removes the sewage separated in the tank from the above anaerobic treatment. A septic tank that has a sewage flow path to return to the tank.   The septic tank according to claim 1, wherein the sewage flow path has one end opened to an intermediate height in the sludge storage tank and the other end opened to the upper part of the anaerobic treatment tank.   The septic tank according to claim 1 or 2, wherein the sludge transferred to the sludge storage tank is extracted from the bottom portion of the anaerobic treatment tank and flows into an intermediate height in the sludge storage tank.   4. The septic tank according to any one of claims 1 to 3, wherein the anaerobic treatment tank and the sludge storage tank are adjacent to each other, and the sewage is returned from the sludge storage tank to the anaerobic treatment tank by natural flow.   The septic tank according to any one of claims 1 to 4, wherein the anaerobic treatment tank is provided with a flow rate adjusting unit at an upper part thereof.   The septic tank according to any one of claims 1 to 5, wherein the sludge storage tank has scumming promotion means in the tank.   The septic tank in any one of Claim 1 thru | or 6 in which an anaerobic processing tank has a filter bed. The septic tank according to claim 1, wherein the anaerobic treatment tank has an aerobic treatment tank at a subsequent stage.

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