JP2008012409A - Waste water treatment apparatus and waste water treatment method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a waste water treatment apparatus capable of dominating bacillus bacteria satisfactorily and efficiently at a low cost and capable of reducing both of the malodor caused upon biological treatment of organic waste water and the volume of excess sludge, and to provide a waste water treatment method. <P>SOLUTION: The waste water treatment apparatus 10 is provided with: a pretreatment means 12 of separating and removing foreign matter and suspended matter included in the organic waste water; an aerobic treatment means 14 of biologically treating the organic waste water under aerobic conditions; a solid-liquid separation means 16 of performing solid-liquid separation of aerobically treated liquid to obtain separated sludge and separated liquid; an anaerobic treatment means 32 of biologically treating at least a part of the separated sludge under anaerobic conditions; and digested sludge return paths L10, L10a, L10b that return at least a part of digested sludge to both of the pretreatment means 12 and the aerobic treatment means 14 or either one of them. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a wastewater treatment apparatus and a wastewater treatment method for biologically treating organic wastewater.

  In organic wastewater treatment facilities, it is necessary to take control measures against odors generated from treatment tanks and pipelines. Odor substances that cause malodor are mainly hydrogen sulfide, methylcaptan, methyl sulfide, methyl disulfide, and ammonia. These odorous substances are generated by decomposing organic substances containing sulfur and nitrogen in an anaerobic state.

  As one of the control measures against bad odor, a method using Bacillus bacteria is known. Bacillus bacteria are a kind of soil bacteria present in activated sludge used for biological treatment of organic wastewater, and have the effect of suppressing the generation of odorous substances. This is thought to be because Bacillus bacteria have the property of suppressing the action of sulfate-reducing bacteria that produce odorous substances.

As a wastewater treatment apparatus using Bacillus bacteria having such properties, for example, in Patent Document 1, a biota having Bacillus bacteria as a dominant species is formed in activated sludge, and this is used to biologically treat wastewater. An apparatus for performing is described.
JP 2005-329301 A

  By the way, when Bacillus genus bacteria become dominant species, the generation of odorous substances is suppressed, and surplus sludge generated by biological treatment of organic wastewater can be reduced. This is thought to be because sludge outflow from the biological treatment tank is suppressed and efficient biological treatment becomes possible due to the excellent sedimentation of Bacillus bacteria. Moreover, when a Bacillus genus bacterium becomes a dominant species, an increase in the amount of sludge derived from bacterial growth can be suppressed. This is thought to be because Bacillus bacteria have a higher rate of consuming the energy obtained by decomposing the BOD component for metabolism and a lower rate of consuming the same energy for growth compared to other bacteria.

  As described above, also from the viewpoint of volume reduction of excess sludge, a means for efficiently predominating Bacillus bacteria is required. In the wastewater treatment apparatus described in Patent Literature 1, an oxidizing agent is added to the water to be treated in order to make Bacillus bacteria dominant. In this case, since it is necessary to control the amount of the additive agent such as the oxidizing agent according to the change in the properties of the water to be treated, it can be said that it is difficult to perform the waste water treatment sufficiently efficiently. In addition, the use of additive chemicals increases the running cost of the wastewater treatment apparatus.

  The present invention has been made in view of such circumstances, and can dominate Bacillus bacteria sufficiently efficiently and at low cost, and occurs when biologically treating organic wastewater. An object of the present invention is to provide a wastewater treatment apparatus and a wastewater treatment method capable of achieving both reduction of malodor and volume reduction of excess sludge.

  The wastewater treatment apparatus of the present invention is a pretreatment means for separating and removing foreign substances and suspended solids contained in organic wastewater, and sludge containing Bacillus bacteria under aerobic conditions for organic wastewater from the pretreatment means. An aerobic treatment means for biological treatment, a solid-liquid separation means for separating the aerobic treatment liquid from the aerobic treatment means to obtain a separated sludge and a separated liquid, and at least a part of the separated sludge under anaerobic conditions Anaerobic treatment means for biological treatment, and a digested sludge return path for returning at least a part of the digested sludge from the anaerobic treatment means to either or both of the pretreatment means and the aerobic treatment means. And

  According to the wastewater treatment apparatus of the present invention, anaerobic treatment (sludge digestion treatment) by anaerobic treatment means can be performed on a part of the separated sludge. When the separated sludge is anaerobically treated, obligate aerobic bacteria that live under aerobic conditions are killed. In contrast, Bacillus bacteria have the property of forming spores, and this property allows them to survive without being killed even under anaerobic conditions.

  On the other hand, when Bacillus genus bacteria are exposed to an aerobic state, they are activated, and other bacteria are dissolved and proliferated while being predated as BOD. Furthermore, since the Bacillus bacterium has a property of releasing a substance that hinders the growth of other bacteria, the Bacillus bacterium is more likely to grow as the number of the Bacillus bacterium increases. Also, under an aerobic condition, some anaerobic bacteria that cannot live under the condition where dissolved oxygen exists are killed.

  The wastewater treatment apparatus of the present invention includes a digested sludge return path for returning digested sludge that has undergone anaerobic treatment by anaerobic treatment means on the upstream side of the apparatus. The returned digested sludge is subjected to the aerobic treatment by the aerobic treatment means again. That is, according to the wastewater treatment apparatus of the present invention, anaerobic treatment and aerobic treatment can be repeatedly performed on sludge containing Bacillus bacteria.

  Therefore, as the wastewater treatment apparatus is operated to treat organic wastewater, Bacillus bacteria that can survive under both anaerobic and aerobic conditions can be selectively grown. Thereby, the Bacillus genus bacteria which have the effect | action of reduction of malodor and volume reduction of an excess sludge can be made into a dominant species, without using an additive chemical | medical agent. The “dominant species” here means the species with the largest number in the biota living in the sludge. Also, “dominance” means making certain types of bacteria dominant in the target biota.

  Further, according to the anaerobic treatment means, the organic compound contained in the separated sludge can be decomposed by the action of the anaerobic bacteria, so that the volume of excess sludge can be reduced. Anaerobic treatment does not require power for blowing air and has an advantage that it can be performed at a low running cost. Furthermore, useful products such as methane gas generated by anaerobic bacteria decomposing organic compounds can be recovered.

  The wastewater treatment apparatus according to the present invention further includes a sludge concentration means for concentrating the separated sludge between the solid-liquid separation means and the anaerobic treatment means, and the concentrated sludge from the sludge concentration means is introduced into the anaerobic treatment means. It is preferable that it is the structure. The concentrated sludge from the sludge concentrating means contains a solid content (for example, bacterial cells, solid organic compounds) at a higher concentration than the separated sludge from the solid-liquid separating means. For this reason, more efficient volume reduction of sludge is implement | achieved by supplying concentrated sludge to an anaerobic treatment means.

  The wastewater treatment method according to the present invention includes a pretreatment step for separating and removing foreign substances and suspended solids contained in organic wastewater, and sludge containing Bacillus bacteria under aerobic conditions for the organic wastewater that has undergone the pretreatment step. An aerobic treatment step for biological treatment in a solid, a solid-liquid separation step for separating the aerobic treatment liquid obtained in the aerobic treatment step into solid sludge and a separated liquid, and at least a part of the separated sludge The sludge digestion process that biologically treats under conditions and the organic wastewater that is returned to at least a part of the digested sludge obtained in the sludge digestion process and used for the pretreatment process and the organic wastewater used for the aerobic process or And a digested sludge returning step to be added to either one of the above.

  The wastewater treatment method of the present invention includes a digested sludge return step for returning digested sludge that has undergone the sludge digestion step. By adding the returned digested sludge to the organic waste water before the aerobic treatment step, the digested sludge containing Bacillus bacteria can be aerobically treated again. That is, according to the wastewater treatment method of the present invention, anaerobic treatment and aerobic treatment can be repeatedly performed on sludge containing Bacillus bacteria.

  Therefore, as treatment by the wastewater treatment method of the present invention is performed, Bacillus bacteria that can survive both under anaerobic conditions and aerobic conditions can be selectively grown. Thereby, the Bacillus genus bacteria which have the effect | action of reduction of malodor and volume reduction of an excess sludge can be made into a dominant species, without using an additive chemical | medical agent.

  Moreover, in the sludge digestion step of the wastewater treatment method of the present invention, the organic compound contained in the separated sludge can be decomposed by the action of anaerobic bacteria, and the volume of excess sludge can be reduced.

  According to the present invention, it is possible to dominate Bacillus bacteria sufficiently efficiently and at a low cost, and it is possible to reduce bad odors and reduce excess sludge generated when biologically treating organic wastewater. A wastewater treatment apparatus and a wastewater treatment method that can achieve both can be provided.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. In the following description, the same reference numerals are used for the same elements, and duplicate descriptions are omitted.

(First embodiment)
FIG. 1 is a schematic configuration diagram of a wastewater treatment apparatus according to the first embodiment. The wastewater treatment apparatus 10 is an apparatus for biologically treating organic wastewater containing organic matter using activated sludge containing Bacillus bacteria.

  The wastewater treatment apparatus 10 includes a sand settling tank (pretreatment means) 12, an aeration tank (aerobic treatment means) 14, a settling tank (solid-liquid separation means) 16, a sludge concentration tank (sludge concentration means) 20, and a sludge digestion tank (anaerobic). Processing means) 22.

  The sand settling tank 12 is for separating and removing foreign substances and suspended solids contained in the organic wastewater introduced through the line L1. The sand settling tank 12 is provided with a net (not shown) at the inflow portion of the organic waste water, and foreign matters larger than the mesh are separated and removed. Moreover, what was settled out of the suspended solids contained in the organic wastewater which flowed into the sand settling tank 12 is discharged | emitted from the line L2. The organic wastewater treated in the sand settling tank 12 is introduced into the aeration tank 14 through the line L3. In the present specification, “line” means a pipeline.

  The aeration tank 14 is for biologically treating organic wastewater with activated sludge containing aerobic bacteria. Although not shown, the aeration tank 14 includes an aeration device for aeration of air or oxygen. In this embodiment, the organic matter contained in the organic wastewater is mainly decomposed by Bacillus bacteria which are aerobic bacteria. A line L4 is connected to the aeration tank 14. The aeration liquid (aerobic treatment liquid) discharged from the aeration tank 14 is introduced into the precipitation tank 16 through the line L4.

  The settling tank 16 is for separating the aerated liquid from the aerated tank 14 into separated sludge and separated liquid. The separation liquid is a so-called supernatant liquid, and the content of solid content (for example, activated sludge, solid organic compound) is sufficiently reduced. On the other hand, the separated sludge contains solids at a high concentration and also contains components that have not been decomposed by the aerobic treatment in the aeration tank 14. The activated sludge contains a plurality of bacteria including Bacillus bacteria.

  A line L5 and a line L6 are connected to the settling tank 16. The line L5 is a line for transferring the separation liquid to the waste water purification facility. Line L6 is a line for discharging separated sludge.

  A separated sludge return path L7 for returning a part of the separated sludge to the aeration tank 14 is connected to the line L6. The sludge returned through the separated sludge return path L7 is referred to as return sludge. On the other hand, the separated sludge (excess sludge) that is not returned through the separated sludge return path L7 is supplied to the sludge concentration tank 20 through the line L6.

  The sludge concentration tank 20 is for separating the separated sludge not returned through the separated sludge return path L7 into concentrated sludge and sludge concentration tank treatment liquid having different specific gravities. A line L8 and a line L9 are connected to the sludge concentration tank 20. The concentrated sludge is supplied to the sludge digestion tank 22 through the line L8. On the other hand, the sludge concentration tank treatment liquid is transferred to the waste water purification equipment through the line L9.

  The sludge digestion tank 22 is for anaerobically treating the concentrated sludge supplied through the line L8. The sludge digestion tank 22 is equipped with a stirring means (not shown) so that the concentrated sludge in the tank can be stirred. A line L10 and a line L11 are connected to the sludge digestion tank 22. At least a part of the digested sludge is returned to the upstream side of the wastewater treatment apparatus 10 through the line L10. On the other hand, line L11 is for discharging digested sludge that is not returned to the outside of the system, and is connected to a sludge treatment facility.

  A line L10a and a line L10b are connected to the line L10. Each of the line L10a and the line L10b is a line for introducing the digested sludge into the separated sludge return path L7 and the sand settling tank 12. The lines L10, L10a, and L10b function as a digested sludge return path for returning digested sludge to the upstream side of the wastewater treatment apparatus 10.

  The line L10a is connected to the separated sludge return path L7, but the line L10a may be connected to the line L3 or the aeration tank 14. Moreover, the wastewater treatment apparatus 10 does not necessarily include all of the lines L10, L10a, and L10b, and the digested sludge return path may be configured by the lines L10 and L10a, and is configured by the lines L10 and L10b. It may be.

  Next, a wastewater treatment method for organic wastewater using the wastewater treatment apparatus 10 will be described.

  First, organic wastewater is introduced into the sand settling tank 12 through the line L1. Here, foreign substances and suspended solids contained in the organic wastewater raw water are separated and removed.

  Organic wastewater is introduced into the aeration tank 14 through the line L3. In the aeration tank 14, the organic matter contained in the organic wastewater is biologically treated with activated sludge containing Bacillus bacteria. The oxidation-reduction potential (ORP) in the aeration tank 14 is preferably 50 mV or more, and more preferably 100 to 400 mV. If ORP is less than 50 mV, decomposition of suspended solids contained in organic wastewater and activation of Bacillus bacteria tend to be insufficient.

  The aeration liquid discharged from the aeration tank 14 through the line L4 is introduced into the precipitation tank 16. In the sedimentation tank 16, the sludge contained in the aerated liquid is precipitated and separated into separated sludge and separated liquid. The separated liquid which is the supernatant liquid is transferred to the waste water purification facility through the line L5. In the wastewater purification equipment, processing such as disinfection of the dehydrated separation liquid and high coagulation treatment is performed.

  On the other hand, the separated sludge from the settling tank 16 is discharged from the line L6. At least a part of the separated sludge discharged from the line L6 is returned to the aeration tank 14 through the separated sludge return path L7. The separated sludge that is not returned to the aeration tank 14 is introduced into the sludge concentration tank 20 through the line L6.

The amount of separated sludge introduced into the sludge concentration tank 20 without being returned through the separated sludge return path L7 may be the same amount or more than the amount of surplus sludge generated calculated by the following equation (1). More specifically, it is preferably 1.0 to 5.0 times the surplus sludge generation amount.
Excess sludge generation amount = (a ₁ × A ₁ + b ₁ × B ₁ ) − (c ₁ × C ₁ ) (1)
Where a ₁ is the sludge conversion rate relative to the soluble BOD unit mass;
Soluble BOD volume A ₁ is flowing into the aeration tank (g / day);
b ₁ is sludge conversion rate per unit mass of suspended solids (SS);
B ₁ is the amount of SS flowing into the aeration tank (g / day);
c ₁ is the autolysis rate (% / day) of activated sludge by endogenous respiration;
C ₁ is the amount of activated sludge suspended matter (MLSS) in the aeration tank (g);
Respectively.

Each parameter in Formula (1) can be measured by a conventionally known method. Incidentally, _{a 1} is usually in the range of 0.4 to 0.6, _{b 1} is usually in the range of 0.9 to 1.0, _{c 1} is usually in the range of 0.01 to 0.05.

  The separated sludge introduced through the line L6 is concentrated in the sludge concentration tank 20 and separated into the concentrated sludge and the sludge concentration tank treatment liquid. The sludge concentration tank treatment liquid is transferred to the waste water purification facility through the line L9. On the other hand, the concentrated sludge is introduced into the sludge digestion tank 22 through the line L8.

  In the sludge digestion tank 22, the concentrated sludge from the sludge concentration tank 20 is anaerobically treated. The anaerobic treatment is performed while stirring the concentrated sludge, and the ORP in the sludge digestion tank 22 is preferably −100 mV or less, and more preferably −500 to −150 mV. When ORP exceeds -100 mV, the volume of obligate aerobic bacteria and excess sludge tends to be insufficient.

  The residence time of the concentrated sludge in the sludge digestion tank 22 is preferably 6 to 100 hours, and more preferably 10 to 72 hours. If the residence time is less than 6 hours, the tendency of obligate aerobic bacteria tends to be insufficient. On the other hand, in order to ensure a sufficient amount of concentrated sludge to be anaerobically treated per unit time and to make the residence time exceed 100 hours, the equipment of the sludge digestion tank 22 tends to be excessive. The “residence time” herein means a value calculated by dividing the internal volume of the sludge digestion tank 22 by the volume of concentrated sludge supplied to the sludge digestion tank 22 per unit time.

  For the purpose of reducing the volume of sludge, the residence time in the case of digesting sludge under anaerobic conditions is 20 to 30 days at a temperature of 30 to 40 ° C, and 10 to 15 days at a temperature of 50 to 55 ° C. It is common to do. On the other hand, in the sludge digestion tank 22 of the present embodiment, the main purpose is dredging such as obligate aerobic bacteria. For this reason, as above-mentioned, the residence time of the separation sludge in the sludge digestion tank 22 should just be 6 to 100 hours. That is, in the sludge digestion tank 22, it is only necessary to reduce the volume of sludge for the amount corresponding to the organic compound decomposed during this residence time.

  The digested sludge that has been anaerobically treated in the sludge digestion tank 22 is added to the returned sludge in the separated sludge return path L7 through the line L10 and the line L10a. Moreover, digested sludge is added to the sand settling tank 12 through the line L10 and the line L10b. On the other hand, the digested sludge that is not returned is transferred to the sludge treatment facility through the line L11. In the sludge treatment facility, digested sludge is dehydrated and composted, carbonized, or incinerated.

The digestibility of the concentrated sludge in the sludge digestion tank 22 can be calculated by the following formula (2).
Digestibility = ((1-a ₂ ) × A ₂ − (1-b ₂ ) × B ₂ ) / (c ₂ × A ₂ × (1-a ₂ )) (2)
Where a ₂ is the moisture content of the concentrated sludge (based on the total mass of the concentrated sludge);
Concentrated sludge supply amount of A ₂ is the sludge digestion tank (g / day);
b ₂ is the water content of the digested sludge (total weight of the digested sludge);
B ₂ is the amount of digested sludge generated (concentrated sludge after sludge digestion) (g / day);
c ₂ is the organic content of the concentrated sludge (based on the total mass of the concentrated sludge);
Respectively.

  Depending on the nature of the organic wastewater to be treated by the wastewater treatment apparatus 10, the total amount of digested sludge treated anaerobically in the sludge digestion treatment tank 22 may be returned through the line L10. However, from the viewpoint of preventing accumulation of phosphorus and accumulation of inorganic substances in the system of the wastewater treatment apparatus 10, it is preferable that part of the digested sludge is discharged out of the system through the line L11. In this case, the amount of digested sludge discharged through the line L11 is preferably the same as or larger than the volume after the sludge digestion of the excess sludge generation amount calculated by the above formula (1). Specifically, it is preferable that the amount of excess sludge generated is 1.0 to 1.2 times the volume after the sludge digestion treatment.

  It is not always necessary to add digested sludge to both the separated sludge return path L7 and the sand settling tank 12, and either one may be used. However, it is preferable to supply digested sludge to both the separated sludge return path L7 and the sand settling tank 12 from the viewpoint of allowing Bacillus bacteria to exist over a wide area in the system of the wastewater treatment apparatus 10.

  Examples of the effects obtained by the wastewater treatment apparatus having the above configuration and the treatment method using the same include the following. That is, according to the wastewater treatment apparatus 10, the anaerobic treatment and the aerobic treatment of sludge circulating in the system can be repeatedly performed by operating the apparatus and treating the organic wastewater. For this reason, it is possible to predominate Bacillus bacteria without using an additive. By subjecting the sludge to anaerobic treatment in the sludge digestion tank 22, the volume of excess sludge can be reduced by the digestive action of anaerobic bacteria.

  Moreover, since the Bacillus genus bacteria have good sedimentation property, if the amount of the Bacillus genus bacteria contained in the activated sludge increases, the sedimentation property of the activated sludge also improves. Therefore, since activated sludge hardly flows out from the aeration tank 14, when the amount of Bacillus genus bacteria increases, excess sludge is likely to be reduced or reduced in volume. In addition, the Bacillus genus bacteria are derived from the growth of bacteria because the proportion of the energy obtained by the decomposition of the BOD component is high for metabolism and the proportion of the same energy for proliferation is low compared to other bacteria. Increase in excess sludge can be suppressed.

  Furthermore, since the sedimentation property of the activated sludge containing Bacillus bacteria is good, the activated sludge is efficiently separated from the organic wastewater by solid-liquid separation in the sedimentation tank 16. And since some activated sludge is returned to the upstream containing the aeration tank 14, the activated sludge density | concentration in the aeration tank 14 becomes deep. Therefore, the decomposition efficiency of the organic matter contained in the organic wastewater is improved. Thereby, the quality of the treated water contained in the organic wastewater biologically treated in the aeration tank 14 is improved.

  For example, in the case of biological treatment with activated sludge that is not the dominant species of the genus Bacillus, if the SS concentration in the aeration tank 14 is about 6000 mg / L, the precipitation rate of the solid matter for 30 minutes is usually 90-100. On the other hand, by setting Bacillus genus bacteria as the dominant species, the sedimentation rate of the solid matter for 30 minutes becomes about 20-40. This indicates that more organic substances contained in the organic wastewater are decomposed by Bacillus bacteria. In addition, Bacillus bacteria are gonococci, but the shape changes to filaments or spores, and filamentous bacteria promote aggregation.

  Furthermore, since the sedimentation property of activated sludge containing Bacillus bacteria is good, the concentration of activated sludge is easily stabilized. Therefore, operation management of the wastewater treatment apparatus 10 is facilitated.

Since Bacillus bacteria are aerobic bacteria and have a property of decomposing organic matter, organic matters in organic wastewater are efficiently decomposed by predominating Bacillus bacteria. Thereby, for example, when the number of cells of the genus Bacillus is about 10 ³ to 10 ⁸ / mL, the mold odor of the sand settling tank 12 and the aeration tank 14, the septic odor of sludge, and the like are reduced.

  If the organic wastewater contains sulfides and the organic wastewater becomes anaerobic, hydrogen sulfide may be formed and a bad odor may be produced, or corrosion may occur in each part of the wastewater treatment apparatus 10. However, Bacillus bacteria also have the property of hardly generating hydrogen sulfide. Therefore, the corrosion of the wastewater treatment apparatus 10 is suppressed and the corrosion odor is also reduced.

  As described above, in the wastewater treatment method for organic wastewater using the wastewater treatment apparatus 10, the bad odor and excess sludge from the wastewater treatment apparatus 10 are reduced by the action of the Bacillus bacteria while ensuring excellent treated water quality. Is done. In this embodiment, these effects can be obtained without using an additive. Moreover, since the additive chemical is not used, the operation management of the waste water treatment apparatus is easy, and the running cost can be reduced as compared with the case where the additive chemical is used.

  As mentioned above, although preferred embodiment of this invention was described, it cannot be overemphasized that this invention is not limited to the said embodiment.

  For example, although the wastewater treatment apparatus 10 according to the above-described embodiment includes the sludge concentration tank 20, the wastewater treatment apparatus according to the present invention may be configured without using the sludge concentration tank 20. In this case, what is necessary is just to connect the sedimentation tank 16 and the sludge digestion tank 22 with the line L6.

  In addition, returning and adding digested sludge to the sand settling tank 12 means not only adding directly to the sand settling tank 12, but also adding to the line L1 for allowing the organic wastewater to flow into the sand settling tank 12. . In addition, when the digested sludge is returned and added to the aeration tank 14, it is not only added directly to the aeration tank 14, but also added to the line L3 or the separated sludge return path L7 for flowing the organic waste water into the aeration tank 14. It is meant to include cases.

  Although the sand settling tank 12 was illustrated as a pre-processing means, it is not restricted to this. In addition, for example, a sand basin or a screen may be employed. Although the aeration tank 14 was illustrated as an aerobic processing means, it is not restricted to this. In addition, for example, as the aerobic treatment tank, one used for the rotary aeration method, one used for the contact oxidation method, one used for the biofilm method, and one used for the oxidation ditch method may be used. Moreover, although the precipitation tank 16 was illustrated as a solid-liquid separation means, for example, a centrifuge or a membrane separation device may be used. Moreover, although the sludge concentration tank 20 using specific gravity difference was illustrated as a sludge concentration means, for example, a centrifugal concentrator etc. may be sufficient.

(Example 1)
Human waste treatment was performed using a treatment apparatus having the same configuration as the wastewater treatment apparatus 10 shown in FIG. The processing conditions are summarized in Table 1. Although the physical properties of the liquid to be treated after each treatment step were fluctuated due to a temperature change or the like, the average values are summarized in Table 2. The digested sludge from the sludge digestion tank was returned to both the settling tank and the aeration tank. Digested sludge discharged out of the system from the sludge digestion tank was transferred to a sludge treatment facility, and after dehydration in the sludge treatment facility, incinerated.

(Comparative Example 1)
Using a wastewater treatment apparatus that does not have a sludge concentration tank and a sludge digestion tank, the liquid to be treated having the same physical properties as in Example 1 was treated. Table 3 shows the processing conditions. The separated sludge that was not returned to the aeration tank was transferred to the sludge treatment facility as it was, and after dehydration in the sludge treatment facility, it was incinerated.

  The results of Example 1 and Comparative Example 1 are shown in Table 4.

It is a schematic block diagram which shows suitable embodiment of the waste water treatment apparatus which concerns on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Waste water treatment apparatus, 12 ... Sand settling tank (pretreatment means), 14 ... Aeration tank (aerobic treatment means), 16 ... Settling tank (solid-liquid separation means), 20 ... Sludge concentration tank (sludge concentration means), 22 ... sludge digestion tank (anaerobic treatment means), L7 ... separated sludge return path, L10, L10a, L10b ... digested sludge return path (return path).

Claims (3)

Pretreatment means for separating and removing foreign substances and suspended solids contained in organic wastewater;
Aerobic treatment means for biologically treating the organic wastewater from the pretreatment means with sludge containing Bacillus bacteria under aerobic conditions;
Solid-liquid separation means for separating the aerobic treatment liquid from the aerobic treatment means to obtain a separated sludge and a separated liquid;
Anaerobic treatment means for biologically treating at least a part of the separated sludge under anaerobic conditions;
A digested sludge return path for returning at least a part of the digested sludge from the anaerobic treatment means to either or both of the pretreatment means and the aerobic treatment means;
A wastewater treatment apparatus comprising:   A sludge concentration means for concentrating the separated sludge is further provided between the solid-liquid separation means and the anaerobic treatment means, and the concentrated sludge from the sludge concentration means is configured to be introduced into the anaerobic treatment means. The wastewater treatment apparatus according to claim 1, wherein A pretreatment process for separating and removing foreign substances and suspended solids contained in organic wastewater;
The aerobic treatment step of biologically treating the organic wastewater that has undergone the pretreatment step with sludge containing Bacillus bacteria under aerobic conditions;
A solid-liquid separation step in which the aerobic treatment liquid obtained in the aerobic treatment step is subjected to solid-liquid separation to obtain a separated sludge and a separated liquid;
A sludge digestion process for biologically treating at least a portion of the separated sludge under anaerobic conditions;
Returning at least a part of the digested sludge obtained in the sludge digestion treatment step and returning the digested sludge to be added to the organic wastewater to be used in the pretreatment step and / or to the organic wastewater to be used in the aerobic treatment step Process,
A wastewater treatment method comprising:

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