WO2016148086A1

WO2016148086A1 - Water treatment method and water treatment device

Info

Publication number: WO2016148086A1
Application number: PCT/JP2016/057855
Authority: WO
Inventors: 正英鈴木; 葛　甬生; 昌次郎渡邊
Original assignee: 水ｉｎｇ株式会社
Priority date: 2015-03-17
Filing date: 2016-03-11
Publication date: 2016-09-22
Also published as: JP6749313B2; JPWO2016148086A1

Abstract

Provided are a water treatment method and a water treatment device, whereby it becomes possible to treat water with high efficiency and in a safe manner when considered in the whole of the device. A water treatment method comprising: mixing organic waste water that is raw water with excess sludge obtained by a biological treatment for a hydraulic retention time of 10 minutes or shorter; subjecting the mixed solution of the organic waste water and the excess sludge to solid/liquid separation to produce a separated solution and separated sludge; converting the separated sludge into the form of a fuel; subjecting the separated solution to a biological treatment; and returning sludge formed by the biological treatment as excess sludge.

Description

Water treatment method and water treatment apparatus

The present invention relates to a water treatment method and a water treatment apparatus, and particularly to a treatment facility that decomposes and recovers organic matter from organic wastewater by combining biological treatment and sludge energy (fuel) treatment such as anaerobic digestion treatment. The present invention relates to an available water treatment method and a water treatment apparatus.

As a general method for treating organic wastewater such as sewage, the solid matter in the influent wastewater is first roughened in a first solid-liquid separation tank such as a sedimentation basin, and the treated water is treated in a reaction tank where microorganisms exist. A method is known in which biological treatment is performed while oxygen is supplied, and a mixed liquid containing microorganisms after biological treatment is separated and treated in a second solid-liquid separation tank such as a final sedimentation basin. . The organic sludge, which is a solid separated in the first and second solid-liquid separation tanks, is methane-fermented in an anaerobic digestion tank and recovered as methane, or dried and carbonized and used as fuel as biomass. The

In such a treatment method, the first solid-liquid separation is a method for increasing the amount of methane recovered and the amount of fuel heat by recovering more organic matter from the organic wastewater before the organic wastewater is biologically treated. There is a method in which a preliminary aeration tank is provided in front of the tank, and in some cases, excess sludge is mixed and aerated for about 20 to 30 minutes (for example, see Non-Patent Document 1). However, this method requires a water tank having a residence time of 20 to 30 minutes as a preliminary aeration tank, and requires an aeration apparatus and power for providing a sufficient dissolved oxygen amount (DO).

As another method, there is also a method in which excess sludge is simply mixed in the first solid-liquid separation tank (for example, see Non-Patent Document 2). However, since there is no mixing device in this method, a sufficient effect cannot be obtained.

As yet another method, there is a method of providing a filtration function using a filter medium in order to improve the separation efficiency in the first solid-liquid separation tank. However, this method cannot remove even the soluble organic matter in the influent wastewater.

As another method, there is a method using a two-stage activated sludge method, but the first aeration tank requires a water tank having a residence time of about 20 to 30 minutes and a sludge return line. An aeration apparatus and power for providing sufficient DO are required.

By applying these treatment methods, not only the amount of recovered energy is increased, but also the amount of oxygen supplied to the reaction vessel is reduced by reducing the amount of organic matter to be biologically treated. There is also an effect of reducing the amount of energy required for processing.

However, any of the conventional methods still has room for study in view of the efficiency and processing stability seen from the whole apparatus.

In view of the above problems, the present invention provides a water treatment method and a water treatment apparatus capable of highly efficient and stable treatment throughout the apparatus.

As a result of intensive studies by the present inventors in order to solve the above-mentioned problems, before solid-liquid separation of solids in organic wastewater as raw water in a first sedimentation basin or the like, surplus sludge obtained by biological treatment and It was found that the biological treatment and sludge fueling treatment performed thereafter can be performed efficiently by mixing for a very short period of time and solid-liquid separation.

The present invention completed on the basis of the above knowledge is, in one aspect, mixing organic wastewater as raw water with surplus sludge obtained by biological treatment and hydraulic retention time of 10 minutes or less, and organic wastewater, The liquid mixture of excess sludge is solid-liquid separated to obtain the separated liquid and separated sludge, the separated sludge is converted into fuel, the separated liquid is biologically treated, and the biological treatment generates. A water treatment method is provided that includes returning the sludge as excess sludge.

In one embodiment of the water treatment method according to the present invention, the sludge generated by biological treatment is returned as surplus sludge, and the mixing ratio of surplus sludge to organic wastewater as raw water is 1.5 [kg-sludge. SS / m ³ -raw water] or less and / or 3.0 [kg-SS / kg-raw water CODcr] or less.

In another embodiment, the water treatment method according to the present invention further includes mixing organic waste water and excess sludge with an oxygen concentration DO of 0.1 mg / L or more.

In yet another embodiment, the water treatment method according to the present invention includes mixing organic waste water and excess sludge under anaerobic conditions.

In yet another embodiment, the water treatment method according to the present invention includes mixing the organic waste water and the excess sludge after aeration of the excess sludge.

In still another embodiment of the water treatment method according to the present invention, converting the separated sludge into fuel includes anaerobically digesting the separated sludge to obtain methane gas and digested sludge, and a part of the digested sludge is obtained. And further supplying the separation liquid into a reaction vessel for biological treatment.

In another aspect of the present invention, a mixing device that mixes organic wastewater as raw water with surplus sludge obtained by biological treatment in a hydraulic retention time of 10 minutes or less, and a mixture of organic wastewater and surplus sludge A solid-liquid separation to obtain a separated liquid and separated sludge, a first solid-liquid separation tank, a fueling device for converting the separated sludge into fuel, a reaction tank for biologically treating the separated liquid, and biological There is provided a water treatment device including return means for returning sludge generated in the treatment as surplus sludge.

In one embodiment of the water treatment apparatus according to the present invention, the mixing apparatus is disposed in the first solid-liquid separation tank.

According to the present invention, it is possible to provide a water treatment method and a water treatment apparatus capable of highly efficient and stable treatment throughout the apparatus.

It is the schematic showing an example of the water treatment apparatus which concerns on embodiment of this invention. It is the schematic showing an example of the water treatment apparatus which concerns on a 1st modification. It is the schematic showing an example of the water treatment apparatus which concerns on a 2nd modification. It is the schematic showing an example of the water treatment apparatus which concerns on a 3rd modification. It is the schematic showing an example of the water treatment apparatus which concerns on a 4th modification. It is the schematic showing an example of the water treatment apparatus which concerns on a 5th modification.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. The following embodiments exemplify apparatuses and methods for embodying the technical idea of the present invention, and the technical idea of the present invention specifies the structure, arrangement, etc. of components as follows. Not what you want.

As shown in FIG. 1, the water treatment apparatus according to the embodiment of the present invention is a mixing apparatus 1 that mixes organic wastewater as raw water with surplus sludge, and adsorbs organic matter in the organic wastewater to the surplus sludge; A first solid-liquid separation tank 2, a reaction tank 3, a second solid-liquid separation tank 4, a fuelizer 6, and a return means 7 are provided.

As the inflow raw water used in the water treatment apparatus according to the present embodiment, organic wastewater containing organic substances such as sewage, manure, and urine can be used. Typically, but not limited to, the quality of influent raw water is 100 to 1000 mg / L biochemical oxygen demand (BOD) and 200 to 3000 mg / L chemical oxygen demand (CODcr). Organic wastewater having a suspended substance (SS) of about 100 to 1000 mg / L is suitably supplied to the water treatment apparatus according to this embodiment.

As a result of intensive studies by the present inventors, it was found that even if the mixing time of the organic waste water and the excess sludge is increased, the organic matter adsorption effect on the excess sludge does not change much. Therefore, the hydraulic residence time (HRT) of the mixing apparatus 1 is less than 20 minutes, more preferably within 10 minutes, and even more preferably within 5 minutes. By shortening the HRT of the mixing apparatus 1, the apparatus size of the mixing apparatus 1 can be reduced, so that the space of the entire apparatus can be saved and the efficiency of the apparatus configuration can be improved.

For further space saving, the mixing apparatus 1 shown in FIG. 1 is disposed in the first solid-liquid separation tank 2 as shown in FIG. 2, and mixing of organic waste water and excess sludge is organic. It is also preferable to carry out in a solid-liquid separation tank for solid-liquid separation of the mixed liquid of effluent wastewater and excess sludge. Thereby, the water tank for mixing organic waste water and excess sludge can be omitted. Considering both the organic matter adsorption effect and the downsizing of the apparatus, the lower limit time of HRT is, for example, preferably 10 seconds or more, more preferably 30 seconds or more.

As the mixing method used in the mixing device 1, for example, mechanical stirring by a stirring device, aeration method using a diffuser plate using air or the like, an aeration method using an underwater aerator, a method using piping or a water channel, a baffle wall is used. The mixing method that was used is available. In consideration of space saving and simplification of the entire device, the mixing device 1 does not need to be provided with an aeration device for mixing organic waste water and excess sludge in principle, but an aeration device is provided. Therefore, it is preferable to adjust the amount of aeration in the mixing device 1 so that the DO in the mixing device 1 is 0.1 mg / L or more, more preferably 0.2 mg / L or more. Thereby, the excess sludge is activated and the organic matter removing ability is improved, and the discharge of phosphorus from the activated sludge can be suppressed.

On the other hand, in the mixing apparatus 1, it is also effective to mix organic waste water and excess sludge under anaerobic conditions. By mixing under anaerobic conditions, an additional device such as an aeration device is not necessary, so that the entire device can be further simplified. In addition, as shown in FIG. 3, an aeration device 8 is provided in the middle of the return means 7, and after the excess sludge is preaerated, the organic waste water and the excess sludge are mixed under anaerobic conditions or aerobic conditions. Good. It is possible to suppress the discharge of phosphorus by aeration of excess sludge in advance with the aeration device 8.

1st solid-liquid separation tank 2 is an apparatus which carries out solid-liquid separation of the liquid mixture obtained with the mixing apparatus 1, for example, a first sedimentation tank etc. are utilized suitably. As means for solid-liquid separation, gravity sedimentation separation, centrifugation, flotation separation, agglomeration separation, membrane separation and the like can be used. In order to improve the separation efficiency, an inclined plate or a sludge blanket layer may be used, or a flocculant may be used. The separation liquid obtained in the first solid-liquid separation tank 2 is sent to the reaction tank 3 through a pipe. The separated sludge obtained in the first solid-liquid separation tank 2 is sent to the fuelizer 6 via a pipe.

The fueling device 6 is a device for decomposing the separated sludge separated in the first solid-liquid separation tank and converting it into fuel gas or sludge fuel. The fueling device 6 is not particularly limited as long as it is a device installed for the purpose of decomposing the separated sludge and converting it into fuel gas or sludge fuel. For example, an anaerobic digester that generates methane gas by anaerobic digestion of the separated sludge, or a dryer or a carbonizer that dries or carbonizes the separated sludge to produce fuel is also preferably used. When an anaerobic digester is used as the fueling device 6, a concentrated sludge having a TS concentration of 1 to 12 wt%, more typically 2 to 8 wt%, more typically 3 to 6 wt% is stored in a hydraulic environment. It is preferable to arrange an anaerobic digester that can be processed within 40 days, more typically within 30 days, and with a methane conversion of 40% or more, more typically 45% or more.

The pretreatment device 5 may be disposed in the front stage of the fueling device 6 as necessary. Although the pretreatment device 5 is not specified, a device for reducing the sludge moisture content of the separated sludge from the first solid-liquid separation tank 2 such as a concentrating device or a dewatering device, or a device for improving fuelization such as alkali treatment or ozone treatment May be used alone or in combination.

The reaction tank 3 is an apparatus for performing biological treatment on the separated liquid separated in the first solid-liquid separation tank. Examples of biological treatments include activated sludge methods (membrane separation activated sludge method, batch activated sludge method), biofilm treatment methods (fixed bed type biofilm method, fluidized bed type biofilm method) and the like. There is a temperate biological treatment. When using the activated sludge method, the reaction tank 3 is an apparatus using a conventional activated sludge method, an anaerobic aerobic method, a circulating nitrification denitrification method, a step inflow multistage nitrification denitrification method, an A2O method, or the like. Can be arranged. The reaction tank 3 may be connected to the return means 7 so that the excess sludge generated in the reaction tank 3 can be returned to the mixing device 1 as necessary.

The second solid-liquid separation tank 4 is an apparatus for solid-liquid separation of the treated water obtained in the reaction tank 3, and for example, a final sedimentation basin or the like is preferably used. As means for solid-liquid separation, the same means as in the first solid-liquid separation tank 2 can be employed. The treated water obtained in the reaction tank 3 is solid-liquid separated into treated water and separated sludge that can be discharged to the outside by sterilization or the like in the second solid-liquid separation tank. A return means 7 for extracting separated sludge is connected to the bottom of the second solid-liquid separation tank 4. Although not shown, a bypass route for sending excess sludge to the sludge fueling device 6 may be provided in preparation for not returning excess sludge to the mixing device 1.

The return means 7 is a device for returning the sludge obtained in the second solid-liquid separation tank 4 as surplus sludge to be supplied to the mixing device 1, and is constituted by piping or the like.

The amount of surplus sludge supplied to the mixing device 1 via the return means 7 varies depending on the components of the organic waste water. However, when processing general sewage, the surplus sludge is mixed with the organic waste water as raw water. The ratio is 1.5 [kg-sludge SS / m ³ -raw water] or less, more preferably 1.0 [kg-sludge SS / m ³ -raw water] or less, more preferably 0.51 [kg-sludge SS / s m ³ -raw water] It is preferable to return the excess sludge so that the following is satisfied. It is preferable to add so that the mixing ratio of surplus sludge to the organic waste water as raw water is 0.01 [kg-sludge SS / m ³ -raw water] or more, more preferably 0.05 [kg-sludge. SS / m ³ -raw water] is preferably returned so as to be equal to or higher than that.

The supply of surplus sludge by the return means 7 may be performed continuously, or the surplus sludge may be sent temporarily only during a time when the sludge load is high. When adding excess sludge continuously and constantly, the mixing ratio of excess sludge to organic wastewater as raw water is 0.01 to 0.6 [kg-sludge SS / m ³ -raw water], more preferably 0. It is preferable to add it so that it becomes 0.05 to 0.2 [kg-sludge SS / m ³ -raw water]. Thereby, in the whole apparatus which combined the biological treatment and the energy treatment of sludge, a process can be advanced more stably and efficiently. On the other hand, when surplus sludge is returned only during periods of high sludge load, the mixing ratio of surplus sludge to organic wastewater as raw water is 0.1 to 1.5 [kg-sludge SS / m ³ -raw water] More preferably, it is added so as to be 0.2 to 1.0 [kg-sludge SS / m ³ -raw water].

Alternatively, when the amount of excess sludge is evaluated by CODcr in the raw water, it is 3.0 [kg-SS / kg-raw water CODcr] or less, more preferably 1.0 [kg-SS / kg-raw water CODcr] or less, More preferably, it is returned to 0.4 [kg-SS / kg-raw water CODcr] or less. When the excess sludge amount is evaluated by CODcr in the raw water, the mixing ratio of the excess sludge amount is 0.02 [kg-SS / kg-raw water CODcr] or more, more preferably 0.1 [kg-SS / kg-raw water. CODcr] is preferably returned.

Even if the mixing ratio of excess sludge is increased too much, the organic matter removal effect is not obtained in proportion. On the other hand, since the amount of excess sludge generated in the entire apparatus is finite, the amount of excess sludge that can be added also varies depending on the processing conditions. ADVANTAGE OF THE INVENTION According to this invention, the water treatment method which can perform waste water treatment more efficiently by the whole apparatus can be provided by adjusting a mixing ratio to become the said range.

The return means 7 is connected to the bottom of the second solid-liquid separation tank 4, but the return means 7 is also connected to the reaction tank 3, and the second means is used to maintain the microorganism concentration in the reaction tank 3. The sludge generated in the solid-liquid separation tank 4 can be returned to the reaction tank 3 as return sludge, and a part of the return sludge can be returned to the mixing apparatus 1 as surplus sludge. Alternatively, as shown in FIG. 4, sludge may be drawn directly from the reaction tank 3 and supplied to the mixing device 1 as surplus sludge. In the water treatment apparatus shown in FIG. 4, the return means 7 is connected to the reaction tank 3, the sludge generated in the second solid-liquid separation tank 4 is returned to the reaction tank 3, and the sludge obtained in the reaction tank 3 Is directly returned to the mixing apparatus 1.

By using sludge sufficiently aerated by biological treatment as surplus sludge mixed with organic wastewater, the ability to remove organic substances from the organic wastewater in the mixing apparatus 1 can be improved. As the return means 7, return using a centrifugal pump is generally available, but an air lift pump may be used. By using an air lift pump, the power required for return can be reduced, and the entire apparatus can be made more efficient.

Or as shown in FIG. 5, the water treatment apparatus which concerns on the modification (4th modification) of embodiment of this invention is the digested sludge digested when using an anaerobic digester as the fueling apparatus 6 A digested sludge supply means 18 for supplying a part of the reaction tank 3 to the reaction tank 3 may be provided. Although illustration is omitted, in the case where the aeration apparatus 8 for aeration of excess sludge as shown in FIG. 3 is provided, a part of the digested sludge may be supplied to the aeration apparatus 8. By returning a part of the digested sludge to the reaction tank 3 or the aeration device 8, the digested sludge is reactivated and the amount of surplus sludge that contributes to organic matter removal can be increased, so that the efficiency of the entire process can be improved. In addition, since the amount of digested sludge that is finally aerobically decomposed and partially discharged is reduced, the labor required for digested sludge treatment can be further reduced, and the overall efficiency of the equipment can be reduced. It leads to improvement.

As shown in FIG. 6, as a water treatment device according to still another modification (fifth modification) of the embodiment of the present invention, a phosphorus recovery device 20, a dehydrator 30, and an anaerobic device are disposed downstream of the fuelizer 6. The control apparatus 10 which controls the supply amount of the excess sludge which the reductive ammonia oxidation processing apparatus 40 and the return means 7 return is supplied, and at least one part of the treated water processed with the anaerobic ammonia oxidation processing apparatus 40 is supplied to the mixing apparatus 1. May be. The control device 10 detects, for example, inflow water or sludge concentration (for example, MLSS concentration), supply flow rate, organic matter concentration (for example, CODcr concentration), ammonia nitrogen concentration, etc., disposed in various places in the water treatment device. Possible first detector 11, second detector 12, third detector 13, fourth detector 14, fifth detector 15, sixth detector 16, seventh detector 17 Based on at least one detection result of the eighth detector 18, the control device 10 can adjust the excess sludge supply amount via the excess sludge supply regulator 7a. According to the water treatment apparatus and method shown in FIG. 6, at least a part of the treated water treated by the phosphorus recovery apparatus 20 and the anaerobic ammonia oxidation treatment apparatus 40 is mixed with the mixing apparatus 1, thereby applying to the reaction tank 3. Nitrogen and phosphorus loads are reduced and the quality of treated water is improved.

(Other embodiments)
Although the present invention has been described according to the above-described embodiments, it should not be understood that the descriptions and drawings constituting a part of this disclosure limit the present invention. From this disclosure, various alternative embodiments and operational techniques will be apparent to those skilled in the art.

As the reaction tank 3, various known methods can be applied as long as the separation liquid separated in the first solid-liquid separation tank can be subjected to aerobic biological treatment. For example, when performing the process which applied the batch type activated sludge method in the reaction tank 3, discharge | emission of process water and extraction of sludge are performed in a batch type. Therefore, it is preferable to provide a sludge storage tank or a concentration tank (not shown) for storing sludge in the subsequent stage of the reaction tank 3 and supply excess sludge from the sludge storage tank or the concentration tank via the return means 7.

When using the membrane separation activated sludge method in the reaction tank 3, a tank integrated type in which the separation membrane is installed in the reaction tank, a tank separate type in which a membrane separation tank is provided at the subsequent stage of the reaction tank, or a casing without providing a water tank An outside tank type using a separation membrane housed in the inside is used, and such biological treatment is naturally applicable to the present water treatment method. Further, when the fixed bed type biofilm method is used in the reaction tank 3, the return sludge from the second solid-liquid separation tank 4 to the reaction tank 3 is not supplied. Of course, it is applicable.

As described above, the present invention includes various embodiments and the like which are not described herein, and the technical scope of the present invention is determined by the invention specifying matters according to the appropriate claims from the above description. Determined.

EXAMPLES Examples of the present invention will be described below together with comparative examples, but these examples are provided for better understanding of the present invention and its advantages, and are not intended to limit the invention.

In the case where the water treatment apparatus shown in FIG. 1 is used (Example) and the case where the mixing treatment by the mixing apparatus 1 in FIG. 1 is not performed (Comparative Example), the sewage is treated with raw water (organic) under the conditions shown in Table 1. Waste water was treated as waste water). That is, the raw water is a sewage with a BOD concentration of 180 mg / L, an SS of 150 mg / L, and the treated water is 600 L / d. The amount of sludge returned from the second solid-liquid separation tank to the reaction tank is 200 L. The treatment was performed as / d. In both the examples and comparative examples, an anaerobic digester was used as the fueling device. In the embodiment, surplus sludge is returned so that the mixing ratio of surplus sludge to raw water is 120 [mg-SS / L-raw water] and 0.3 [kg-SS / kg-raw water CODcr] in the case of CODcr concentration standard. did. The amount of methane gas generated was 22% higher in the example than in the comparative example.

Table 2 shows the results of the quality of treated water obtained from the second solid-liquid separation tank for the examples and comparative examples.

In the embodiment, in the mixing apparatus, a part of the SS in the organic waste water is adsorbed and aggregated in the activated sludge by mixing the organic waste water (raw water) and the activated sludge for a very short time (5 minutes). The removal rate of BOD and SS in the first solid-liquid separation tank (primary sedimentation) was improved. In addition, by improving the organic substance removal efficiency in the initial sedimentation, the BOD load of the separation liquid flowing into the reaction tank is reduced, the amount of aeration air necessary for biological treatment is greatly reduced, and the power can be reduced. . The treated water quality obtained in the reaction tank was also good. Furthermore, organic substances that have been aerobically decomposed in the conventional method can be recovered as solids in the first settling in this embodiment, and can be converted to methane gas in the fueling device, so that the amount of methane gas generated can be increased. It was. That is, according to the water treatment method according to the embodiment of the present invention, it is possible to provide a water treatment method capable of performing wastewater treatment more efficiently in the entire apparatus combining biological treatment and sludge energy conversion treatment. It was.

DESCRIPTION OF SYMBOLS 1 ... Mixing apparatus 2 ... 1st solid-liquid separation tank 3 ... Reaction tank 4 ... 2nd solid-liquid separation tank 5 ... Pretreatment apparatus 6 ... Fueling apparatus 7 ... Return means 7a ... Excess sludge supply regulator 8 ... Aeration Device 10 ... Control device 20 ... Phosphorus recovery device 30 ... Dehydrator 40 ... Anaerobic ammonia oxidation treatment device

Claims

Mixing organic wastewater as raw water with surplus sludge obtained by biological treatment in a hydraulic retention time of 10 minutes or less;
Solid-liquid separation of the mixed liquid of the organic waste water and the excess sludge to obtain a separated liquid and separated sludge;
Fueling the separated sludge;
Biologically treating the separation liquid;
Returning the sludge generated in the biological treatment as the surplus sludge;
Including water treatment method.
Returning the sludge generated by the biological treatment as the surplus sludge is such that the mixing ratio of the surplus sludge to the organic waste water as raw water is 1.5 [kg-sludge SS / m 3 -raw water] or less and / or 2. The water treatment method according to claim 1, further comprising returning it to 3.0 [kg-SS / kg-raw water CODcr] or less.
The water treatment method according to claim 1 or 2, further comprising mixing organic wastewater and excess sludge at an oxygen concentration DO of 0.1 mg / L or more.
The water treatment method according to claim 1 or 2, comprising mixing the organic waste water and the excess sludge under anaerobic conditions.
The water treatment method according to any one of claims 1 to 4, comprising aeration of the excess sludge and mixing of the organic waste water and the excess sludge.
Fueling the separated sludge includes anaerobically digesting the separated sludge to obtain methane gas and digested sludge,
The water treatment method according to any one of claims 1 to 5, further comprising supplying a part of the digested sludge into a reaction tank for performing biological treatment on the separated liquid.
A mixing device that mixes organic waste water as raw water with surplus sludge obtained by biological treatment in a hydraulic residence time of 10 minutes or less;
A first solid-liquid separation tank that obtains a separated liquid and separated sludge by solid-liquid separation of the mixed liquid of the organic waste water and the excess sludge;
A fueling device for fueling the separated sludge;
A reaction vessel for biologically treating the separated liquid;
A water treatment apparatus comprising return means for returning sludge generated in the biological treatment as the excess sludge.
The water treatment apparatus according to claim 7, wherein the mixing apparatus is disposed in the first solid-liquid separation tank.