JP2010069397A - Wastewater treatment apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wastewater treatment apparatus which achieves sufficient solubilization efficiency and in which instruments installed in a solubilization tank operate desirably. <P>SOLUTION: In the apparatus, a solid matter contained in wastewater is separated in a solid-liquid separation means 2. For the volume reduction of this solid-liquid separated sludge, the sludge is solubilized in a solubilization tank 5, and the solubilized liquid is returned to the solid-liquid separation means 2 or a stage preceding the solid-liquid separation means 2 through a solubilized-liquid return line L3. Meanwhile, the sludge is prevented from depositing on the bottom of the solubilization tank 5 by a sludge deposition preventing means 25, so that the effective capacity of the solubilization tank 5 is secured without forming a dead space on the bottom of the solubilization tank 5 and sufficient solubilization efficiency is obtained. At the same time, the sensors of the instruments such as a level meter and a thermometer are not covered with the sludge, and the instruments are allowed to operate desirably. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a wastewater treatment apparatus.

  Anaerobic treatment methods, which are energy-saving treatment methods, have become widespread as treatment methods for wastewater discharged from food factories and organic wastewater such as sewage or human waste. In particular, for wastewater with a high concentration of organic matter, the UASB (Upflow Anaerobic Sludge Blanket) method, which uses a granulated granulated anaerobic microorganism, or an improved EGSB ( The Expanded Glanular Sludge Bed method is applied.

  Here, among the food factories, the waste water discharged from the beer production factories contains a high concentration of organic solids. In order to treat such wastewater by an anaerobic treatment method using granules such as the UASB method, it is necessary to reduce the concentration of organic solids in advance using some means.

For example, in the wastewater treatment apparatus disclosed in Patent Document 1, an inorganic flocculant is added to organic wastewater, the coagulation sedimentation treatment is performed in the coagulation sedimentation tank, and the precipitated sludge separated by precipitation is solubilized in the solubilization tank. At the same time, the solubilized liquid is returned to the coagulation sedimentation tank, and the organic solid concentration is reduced by solubilization in the solubilization tank in this way, so that the anaerobic treatment in the subsequent stage is performed favorably.
JP 2005-125202 A

  However, in the wastewater treatment apparatus disclosed in Patent Document 1, sufficient solubilization efficiency may not be obtained in the solubilization tank. Moreover, there also existed a problem that the instruments provided in the solubilization tank did not operate | move as desired.

  Therefore, an object of the present invention is to provide a wastewater treatment apparatus in which sufficient solubilization efficiency can be obtained, and instruments provided in a solubilization tank operate as desired.

  Here, as a result of intensive studies, the present inventors have found that the cause of the decrease in solubilization efficiency and the malfunction of the instruments is the accumulation of sludge at the bottom of the solubilization tank. That is, if sludge accumulates at the bottom of the solubilization tank, a dead space is generated at the bottom of the solubilization tank, and the effective capacity of the solubilization tank is reduced, so that sufficient solubilization efficiency cannot be obtained. It has been found that the sensor unit of the instrument such as a meter is covered with the accumulated sludge and the instrument does not operate as desired.

  Therefore, the wastewater treatment apparatus of the present invention includes a solid-liquid separation means for introducing wastewater and separating solids in the wastewater, and a solubilization tank for introducing and solubilizing sludge separated by solid-liquid separation by the solid-liquid separation means. And the solubilized liquid return line for returning the solubilized liquid solubilized in the solubilizing tank to the previous stage from the solid-liquid separating means or the solid-liquid separating means, and the accumulation of sludge at the bottom of the solubilizing tank is prevented. And sludge accumulation prevention means.

  In such a wastewater treatment apparatus, solids contained in the wastewater are separated by solid-liquid separation means, and the sludge is solubilized in a solubilization tank to reduce the volume of the solid-liquid separated sludge. While the lysate is returned to the preceding stage from the solid-liquid separation means or the solid-liquid separation means via the lysate return line, the accumulation of sludge at the bottom of the solubilization tank is prevented by the sludge accumulation prevention means. For this reason, dead space does not occur at the bottom of the solubilization tank, the effective capacity of the solubilization tank is secured, sufficient solubilization efficiency is obtained, and the sensor part of instruments such as a level meter and a thermometer is covered with sludge. The instruments operate as desired.

  Moreover, the waste water treatment apparatus of the present invention introduces a first solid-liquid separation unit that introduces waste water and separates solids in the waste water, and a biological solution that introduces the separated liquid separated by the first solid-liquid separation unit. A biological treatment tank to be treated, a second solid-liquid separation means for separating the solid content in the treated water by introducing the treated water biologically treated in the biological treatment tank, and sludge separated by solid-liquid separation by the second solid-liquid separation means A solubilization tank for solubilizing by introducing a solubilizing liquid, a solubilizing liquid return line for returning the solubilized liquid solubilized in the solubilizing tank to the first stage from the first solid-liquid separation means or the first solid-liquid separation means, The sludge accumulation preventing means for preventing the sludge from accumulating at the bottom of the solubilization tank may be provided.

  In such a wastewater treatment apparatus, the solid matter contained in the wastewater is separated in the first solid-liquid separation means, the separated liquid after the solid-liquid separation is biologically treated, and the solid content in the biologically treated water is the second solid-liquid. In order to reduce the volume of the sludge separated in the separation means and solid-liquid separated in the second solid-liquid separation means, the sludge is solubilized in the solubilization tank, and this solubilized liquid is first fed through the solubilized liquid return line. While returning from the solid / liquid separation means or the first solid / liquid separation means to the preceding stage, the accumulation of sludge at the bottom of the solubilization tank is prevented by the sludge accumulation prevention means. For this reason, even when biologically treated sludge is solubilized, dead space does not occur at the bottom of the solubilization tank, the effective capacity of the solubilization tank is ensured, sufficient solubilization efficiency is obtained, The sensor unit of the instrument such as a thermometer is not covered with sludge, and the instrument operates as desired.

  Here, if the sludge accumulation preventing means is configured to supply a part of the solubilizing liquid toward the bottom of the solubilizing tank, the sludge is formed at the bottom of the solubilizing tank while reducing the cost by using the solubilizing liquid. Is sufficiently agitated to prevent sludge accumulation.

  The sludge accumulation prevention means is installed outside the solubilization tank and sucks a part of the solubilization liquid flowing through the solubilization liquid return line, and the solubilization liquid from the pump is directed to the bottom of the solubilization tank. It is preferable to provide a solubilizing liquid supply means. According to this, corrosion can be prevented by installing the pump outside the tank while reducing the cost with a simple configuration of using the pump.

  Moreover, as a structure which exhibits the said effect | action effectively, the structure by which multiple solubilizing liquid supply means are provided in an up-down direction is also mentioned.

  Furthermore, when the solubilizing liquid supply means on the upper side of the solubilizing liquid supplying means is configured to supply the solubilizing liquid from the pump toward the liquid surface of the solubilizing tank, a swirling circulation flow in the vertical direction is formed. Thus, the agitation effect is further enhanced, the sludge accumulation is further prevented, and the scum removal effect is obtained.

  Thus, according to the present invention, it is possible to provide a wastewater treatment apparatus in which sufficient solubilization efficiency can be obtained and instruments provided in the solubilization tank operate as desired.

  Hereinafter, a preferred embodiment of a wastewater treatment apparatus according to the present invention will be described with reference to the drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant descriptions are omitted.

(One embodiment)
FIG. 1 is a block diagram showing a wastewater treatment apparatus according to an embodiment of the present invention, and FIG. 2 is a schematic configuration diagram specifically showing the periphery of the solubilization tank in FIG. Is for treating organic wastewater containing organic solids at a high concentration, such as wastewater discharged from a beer manufacturing plant. In the following description, “drainage” means “organic drainage”.

  As shown in FIG. 1, the waste water treatment apparatus 1 includes a first sedimentation basin (solid-liquid separation means) 2 that solid-liquid separates solids in waste water flowing through a line L1 by precipitation separation, and the separated and separated liquid is acidified. An acid generation tank 3 for fermentation and an upflow anaerobic treatment tank 4 for methane fermentation of acid-fermented organic acids are provided in this order, and precipitation sludge that has been precipitated and separated in the first sedimentation tank 2 is introduced via line L2. And a solubilization tank 5 for solubilization, and a solubilization liquid return line L3 for returning the solubilized liquid solubilized in the solubilization tank 5 to the settling tank 2 for the first time.

  As shown in FIG. 2, the first sedimentation basin 2 is one that gravity settles the solid content in the introduced waste water and separates it into a precipitated sludge and a supernatant separation liquid. Collect and discharge at the bottom of the pond. In addition, it is preferable to provide the adjustment tank for adjusting the inflow amount of waste_water | drain in the front | former stage of the first settling basin 2, and to hold | maintain waste water once by this adjustment tank.

  Returning to FIG. 1, the acid generation tank 3 is a tank in which a microorganism that performs an acid fermentation reaction is accommodated in a floating state in the tank, and acid fermentation is performed by the microorganism. Fermented and decomposed into organic acids.

  The upward flow type anaerobic treatment tank 4 performs methane fermentation by so-called granule methane bacteria that are self-granulated anaerobic microorganisms that produce methanogens. The wastewater containing organic acid discharged from the acid generator 3 is introduced into the lower part of the granule layer and brought into contact with the granule methane bacteria while facing upward, and the organic components in the wastewater are fermented with methane. To do.

  The line L2 that connects the first sedimentation tank 2 and the solubilization tank 5 has a sludge extraction pump 12 as shown in FIG. The sludge extraction pump 12 sucks the precipitated sludge first separated in the settling tank 2 through a line L2 and supplies it to the solubilization tank 5. As shown in FIG. 1, another line is connected to the line L <b> 2 so that the precipitated sludge can be appropriately discharged to the outside of the waste water treatment apparatus 1.

  The solubilization tank 5 solubilizes the precipitated sludge initially separated in the settling basin 2 under a predetermined temperature and pH condition. As shown in FIG. To prevent it, it has an upper lid 5a. A steam supply line L6 for heating the liquid to be treated in the tank is connected to the upper lid 5a of the solubilization tank 5 so as to penetrate along with a line L2 for supplying the precipitated sludge. The tank 5 is placed inside the tank 5. Note that an exhaust line is connected to the upper lid 5a so as to discharge the odor gas generated in the solubilization tank 5 to a deodorizing apparatus at a subsequent stage.

  Moreover, in the solubilization tank 5, instruments for solubilizing under predetermined conditions are provided. A thermometer 18 for measuring the liquid temperature in the solubilization tank 5 and controlling the supply amount of steam is provided in the lower part of the solubilization tank 5. Similarly, in the lower part of the solubilization tank 5, a difference for controlling the sludge extraction pump 12 by measuring the liquid level in the solubilization tank 5 and controlling the supply amount of the precipitated sludge from the first settling tank 2. A pressure type level meter 19 is provided.

  A solubilization liquid return line L3 that connects the solubilization tank 5 and the line L1 is connected to the bottom of the solubilization tank 5 and has a solubilization liquid return pump 13. The solubilizing liquid return pump 13 sucks the solubilized liquid solubilized in the solubilizing tank 5 through the solubilizing liquid return line L3 and returns it to the first sedimentation tank 2 through the line L1. The lysate return line L3 is provided with a flow meter 14 for measuring the return flow rate to the settling basin 2 and controlling the lysate return pump 13 to control the return flow rate of the lysate. ing. In addition, as mentioned above, when an adjustment tank is provided in the front | former stage of the first settling basin 2, it is preferable to return solubilization liquid upstream from this adjustment tank or the adjustment tank of the line L1.

  Here, in the present embodiment, a sludge accumulation prevention device (sludge accumulation prevention means) 25 is attached to the solubilization tank 5. This sludge accumulation prevention device 25 is connected to a circulation line L4 for connecting the solubilized liquid return line L3 and the inside of the solubilized tank 5 to circulate between the solubilized liquid return line L3 and the inside of the solubilized tank 5, and the circulation line L4. A solubilizing tank agitation pump (pump) 15 provided, an injection nozzle (solubilizing liquid supply means) 17 constituting the tip of the circulation line L4, and a solubilization tank agitation pump 15 and the injection nozzle 17 in the circulation line L4. It is set as the structure provided with the automatic valve 16 provided in between.

  The solubilization tank agitation pump 15 is for sucking a part of the solubilization liquid flowing through the solubilization liquid return line L <b> 3, and is installed outside the solubilization tank 5. The solubilizing tank agitation pump 15 is preferably a pump having a crushing function for crushing solids contained in the solubilized liquid because sludge is finely crushed.

  The circulation line L4 is branched into four paths in the middle, and two of the paths are connected side by side to the side surface of one side (right side in the drawing) of the solubilization tank 5 and enter the inside, and further two other paths. Are connected to the side surface on the opposite side (the left side in the figure) of the solubilization tank 5 in the vertical direction so as to enter the inside.

  The injection nozzle 17 is disposed in the solubilization tank 5 and injects (supplies) the solubilized liquid from the solubilization tank agitation pump 15 toward the bottom of the solubilization tank 5 from the injection port. In particular, in this embodiment, as shown in FIG. 2, the injection nozzles 17a, 17b, 17c, and 17d of each path constituting the injection nozzle 17 are installed with the injection ports inclined at about 45 ° vertically downward. As shown in FIG. 3, it is installed at an angle of about 45 ° in the clockwise direction along the peripheral wall of the solubilization tank 5 when viewed from the top of the solubilization tank 5.

  The automatic valve 16 is for appropriately switching the above-described path (line) for injecting the solubilizing liquid in order to better exhibit the sludge accumulation prevention effect by the sludge accumulation prevention device 25, as shown in FIG. , Provided in each of the four paths constituting the circulation line L4.

  The solubilization tank 5 provided with such a sludge accumulation prevention device 25 is provided with an alkali addition device for adjusting the inside of the solubilization tank 5 to a predetermined pH condition. This alkali addition apparatus has a NaOH storage tank 20, a NaOH pump 21, and a pH meter 22. The NaOH pump 21 is for adjusting the pH of the liquid to be treated in the solubilization tank 5 to a predetermined value by introducing NaOH into the circulation line L4 from the NaOH storage tank 20, and the pH meter 22 is a solubilizing liquid. Provided in a line connecting the return line L3 and the circulation line L4, the pH of the solubilized liquid is measured, and the NaOH pump 21 is controlled based on the measured value to control the amount of NaOH added.

  Next, wastewater treatment by the wastewater treatment apparatus 1 will be described.

  As shown in FIG. 1, wastewater containing organic solids at a high concentration is supplied to the first sedimentation basin 2 through the line L1, and the organic solids are precipitated and separated in the first sedimentation basin 2, and are supernatant water. The separation liquid is subjected to an acid fermentation treatment in the acid generation tank 3 and the organic components in the separation liquid are decomposed into organic acids and the like, and this treated water is subjected to a methane fermentation treatment in the upward flow anaerobic treatment tank 4 and discharged. The organic components therein are decomposed into water, carbon dioxide, methane gas, and the like, and treated water satisfying a predetermined water quality is discharged to the subsequent stage. On the other hand, methane gas is recovered through desulfurization treatment and used for energy.

  Based on the measured value of the level meter 19, the precipitated sludge initially separated in the settling tank 2 is supplied to the solubilization tank 5 through the line L 2 by the sludge extraction pump 12. In the solubilization tank 5, the steam supply amount and the NaOH addition amount are determined based on the measurement results of the thermometer 18 and the pH meter 22, and the steam and NaOH are supplied and added based on the determined supply amount and the addition amount. Solubilization is performed under predetermined temperature conditions and pH conditions, and the SS concentration in the precipitated sludge is reduced.

  The solubilized liquid in the solubilization tank 5 is first returned to the settling basin 2 via the line L1 by the solubilizing liquid return pump 13, and the same processing is repeated.

  On the other hand, in the sludge accumulation preventing device 25, as shown in FIG. 3, a part of the solubilized liquid sucked by the solubilizing tank agitation pump 15 is sprayed from the spray nozzles 17 (17a to 17d), and the solubilizing tank 5 is used. A stirring stream X is formed therein. Thereby, accumulation of the sludge of the bottom part of the solubilization tank 5 is prevented.

  Here, in this embodiment, since the injection nozzle 17 is installed so as to be inclined vertically downward toward the bottom of the solubilization tank 5 and along the peripheral wall of the solubilization tank 5, the stirring flow X is solubilized. It is formed in a swirl shape toward the bottom of the tank 5, and the accumulation of sludge at the bottom of the solubilization tank 5 is further prevented.

  As described above, in the wastewater treatment apparatus 1 according to the present embodiment, the sludge accumulation preventing device 25 prevents sludge accumulation at the bottom of the solubilization tank 5, so that no dead space is generated at the bottom of the solubilization tank 5. The effective capacity of the solubilization tank 5 is ensured and sufficient solubilization efficiency is obtained, and the sensor units of the instruments such as the level gauge 19 and the thermometer 18 are not covered with sludge, and the instruments operate as desired.

  In addition, since a part of the solubilized liquid is sprayed toward the bottom of the solubilization tank 5 by the sludge accumulation preventing device 25, sludge is sufficiently produced at the bottom of the solubilization tank 5 while reducing the cost by using the solubilizing liquid. To prevent the accumulation of sludge.

  A solubilization tank agitation pump 15 is installed outside the solubilization tank 5, sucks a part of the solubilization liquid flowing through the solubilization liquid return line L 3, and the injection nozzle 17 allows the solubilization tank agitation pump 15 to Since the lysate is injected toward the bottom of the solubilization tank 5, the cost can be reduced with a simple configuration of using a pump, and corrosion can be prevented by installing the pump outside the tank.

  Moreover, since the injection nozzle 17 is provided with two or more in the up-down direction, the stirring effect of the sludge in the solubilization tank 5 bottom part is improved further.

  Further, since the accumulation of sludge at the bottom of the solubilization tank 5 is prevented and the differential pressure type level meter 19 operates as desired, the flow rate of the precipitated sludge supplied to the solubilization tank 5 is appropriately controlled by the inverter control of the sludge extraction pump 12. can do.

  FIG. 4 is a schematic configuration diagram showing another example of the sludge accumulation preventing apparatus. The sludge accumulation preventing device 30 shown in FIG. 4 is different from the sludge accumulation preventing device 25 in that the injection ports of the upper injection nozzles 27 c and 27 d are installed toward the liquid surface of the solubilization tank 5.

  With such a configuration, a vertical circulation flow is formed by the lower injection nozzle 17 (17a, 17b) and the upper injection nozzle 27 (27c, 27d), and the stirring effect is further enhanced. In addition, even when scum is generated in the vicinity of the liquid surface in the solubilization tank 5, the scum can be flowed and the scum can be removed by a scum removing device (not shown).

  In the case of the configuration shown in FIG. 4, the automatic valve 16 may be opened and closed by timer control, and the solubilized liquid may be alternately ejected from the ejection nozzles 17 and 27 in a staggered manner. That is, by alternately repeating the injection from the injection nozzles 17a and 27d and the injection from the injection nozzles 17b and 27c, the upper and lower stirring flow is more efficiently formed, further preventing sludge accumulation and removing scum. Can be done effectively.

(Other embodiments)
FIG. 5 is a block diagram showing a wastewater treatment apparatus according to another embodiment of the present invention.

  The waste water treatment apparatus 10 of the present embodiment shown in FIG. 5 is different from the waste water treatment apparatus 1 shown in FIG. 1 in that the aerobic treatment with activated sludge is performed downstream of the upward flow anaerobic treatment tank 4. The aerobic treatment tank 6 and a final sedimentation basin (second solid-liquid separation means) 7 for solid-liquid separation of the solid content in the aerobic treated water by precipitation separation were further separated by precipitation in the final sedimentation basin 7. This is a point provided with a line L5 for introducing the precipitated sludge into the solubilization tank 5. Here, the acid generation tank 3, the upward flow anaerobic treatment tank 4, and the aerobic treatment tank 6 correspond to a biological treatment tank.

  The aerobic treatment tank 6 performs aerobic treatment by aeration, and the final sedimentation basin 7 gravity settles the solids in the treated water discharged from the aerobic treatment tank 6 to remove excess sludge and supernatant purified water. It is for separating into and. Note that a line for appropriately discharging excess sludge to the outside is connected to the line L5.

  According to the wastewater treatment apparatus 10 having such a configuration, the wastewater that has been anaerobically treated in the upward flow anaerobic treatment tank 4 is further aerobically treated with activated sludge in the aerobic treatment tank 6, The concentration of the organic component is further reduced, the solid content in the activated sludge treated water is precipitated and separated in the final sedimentation basin 7, and the clearer treated water is discharged to the subsequent stage as supernatant water. On the other hand, surplus sludge separated in the final sedimentation basin 7 is mixed with the primary sedimentation sludge separated in the first sedimentation basin 2 and solubilized in the solubilization tank 5, so that the sludge can be further reduced. Consistency is achieved. Moreover, it cannot be overemphasized that the sludge accumulation | storage prevention apparatus 25 has the same effect as previous embodiment.

  In this embodiment, lines L2 and L5 for introducing the initial sedimentation sludge of the first sedimentation tank (first solid-liquid separation means) 2 and the excess sludge of the final sedimentation tank 7 to the solubilization tank 5 are provided. However, only the line L5 may be provided to solubilize only the sedimentation sludge in the final sedimentation tank 7.

  Although the preferred embodiment of the wastewater treatment apparatus according to the present invention has been described in detail above, the present invention is not limited to the above embodiment. For example, in the above-described embodiment, the sludge accumulation prevention device 25 including the solubilization tank agitation pump 15 and the injection nozzle 17 is provided. However, a mechanical agitator is provided in the solubilization tank 5 as the sludge accumulation prevention means. The sludge accumulation preventing device 25 and a mechanical stirrer may be used in combination.

  In the above embodiment, the injection nozzles 17 and 27 are provided as the solubilizing liquid supply means, but the invention is not limited to the injection nozzle as long as the solubilizing liquid can be supplied. In the above embodiment, as the solubilization condition in the solubilization tank 5, the temperature and pH are adjusted by steam and alkali. However, either one of steam and alkali is supplied or added, and either the temperature or pH is adjusted. One of them may be adjusted.

  In the above embodiment, the alkali addition amount is controlled based on the measurement value by the pH meter 22, but the solubilized liquid return flow rate (or the solubilization tank 5) from the solubilization tank 5 measured by the flow meter 14. The amount of precipitation sludge inflow) may be controlled based on the binary value of the measured pH value. In this way, sufficient solubilization efficiency can be obtained even when the viscosity of the liquid to be treated in the solubilization tank 5 is high.

  In the above embodiment, the lysate is first returned to the settling basin 2 via the line L1 through which drainage flows. However, the lysate may be returned directly to the first settling basin 2 without going through the line L1. When an adjustment tank or the like is provided in the previous stage of the settling tank 2, the adjustment tank or the adjustment tank of the line L1 may be returned to the upstream side. Moreover, in the said embodiment, although the first sedimentation basin 2 and the final sedimentation basin 7 were provided as a solid-liquid separation means, it is not limited to a sedimentation basin, For example, it is good also as providing a membrane separation apparatus.

It is a block diagram which shows the waste water treatment apparatus which concerns on one Embodiment of this invention. It is a schematic block diagram which shows specifically the first sedimentation tank, solubilization tank, solubilization liquid return line, and its periphery in FIG. It is a top view which shows the stirring flow in the solubilization tank, injection nozzle, and solubilization tank in FIG. It is a schematic block diagram which shows the other example of a sludge accumulation prevention apparatus. It is a block diagram which shows the waste water treatment apparatus which concerns on other embodiment of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1,10 ... Waste water treatment apparatus, 2 ... First sedimentation basin (solid-liquid separation means, 1st solid-liquid separation means), 3 ... Acid production tank (biological treatment tank), 4 ... Upward flow type anaerobic treatment tank (biological Treatment tank), 5 ... solubilization tank, 6 ... aerobic treatment tank (biological treatment tank), 7 ... final sedimentation tank (second solid-liquid separation means), 15 ... solubilization tank agitation pump (pump), 17, 27 ... injection nozzle (solubilization liquid supply means), 25, 30 ... sludge accumulation prevention device (sludge accumulation prevention means), L3 ... solubilization liquid return line.

Claims (6)

Solid-liquid separation means for separating the solid content in the wastewater by introducing the wastewater;
A solubilization tank for introducing and solubilizing sludge solid-liquid separated by the solid-liquid separation means;
A solubilized liquid return line for returning the solubilized liquid solubilized in the solubilization tank to the previous stage from the solid-liquid separation means or the solid-liquid separation means;
Sludge accumulation preventing means for preventing sludge from accumulating at the bottom of the solubilization tank;
A wastewater treatment apparatus comprising: First solid-liquid separation means for introducing waste water and separating solids in the waste water;
A biological treatment tank for biological treatment by introducing the separated liquid separated by the first solid-liquid separation means;
Second solid-liquid separation means for introducing treated water biologically treated in the biological treatment tank and separating solids in the treated water;
A solubilization tank for introducing and solubilizing sludge solid-liquid separated by the second solid-liquid separation means;
A solubilized liquid return line for returning the solubilized liquid solubilized in the solubilization tank to the first stage from the first solid-liquid separation means or the first solid-liquid separation means;
Sludge accumulation preventing means for preventing sludge from accumulating at the bottom of the solubilization tank;
A wastewater treatment apparatus comprising:   The wastewater treatment apparatus according to claim 1 or 2, wherein the sludge accumulation preventing means supplies a part of the solubilized liquid toward the bottom of the solubilization tank. The sludge accumulation preventing means is
A pump that is installed outside the solubilization tank and sucks a part of the solubilized liquid flowing through the solubilized liquid return line;
Solubilizing liquid supply means for supplying the solubilizing liquid from the pump toward the bottom of the solubilization tank;
The wastewater treatment apparatus according to claim 3, comprising:   The waste water treatment apparatus according to claim 4, wherein a plurality of the solubilizing liquid supply means are provided in the vertical direction.   6. The waste water treatment according to claim 5, wherein the upper solubilizing liquid supply means of the solubilizing liquid supplying means supplies the solubilizing liquid from the pump toward the liquid surface of the solubilizing tank. apparatus.

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