JPH10216793A - Water treatment plant - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate the circulation of nitrifying liquid and simplify a plant by providing the plant with a nitrification tank filled with a biological carrier formed of immobilized nitrifying bacteria, also providing the nitrification tank with a phosphorus removal tank, an aerobic tank and a precipitation basin successively on the downstream side thereof and also forming a return piping for returning activated sludge in the precipitation basin to the nitrification tank. SOLUTION: A water treatment plant is provided with a nitrification tank 1 filled with a biological carrier 7 formed of ammonia oxidation bacteria (nitrifying bacteria) such as nitrate bacteria, nitrite bacteria or the like contained in activated sludge or the like. A phosphorus removal tank (anaerobic tank) 2 is disposed on the downstream side of the nitrification tank 1, and an aerobic tank (aeration tank) 3 is fitted in the phosphorus removal tank 2. A precipitation basin 4 is disposed on the downstream side of the aerobic tank (aeration tank) 3, and a return pipe 5a for returning the active sludge in the precipitation basin 4 to the phosphorus removal tank 2 is formed. Raw water flows all through inside the water treatment plant by the arrangement, and as the circulation of nitrifying liquid is not in existence, the necessity of the nitrifying liquid circulating pump is eliminated to reduce the power to be used, simplify the water treatment plant and make the building construction easy.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a water treatment apparatus for biologically removing nitrogen in sewage.

[0002]

2. Description of the Related Art Nitrogen compounds and phosphorus compounds in wastewater discharged to natural water systems have various effects on the water environment. For example, it promotes eutrophication, and when such a water system is used as a tap water source, depending on the form of nitrogen as well as the odor, the chlorine sterilization effect is weakened, which leads to a situation unfavorable for hygiene.

[0003] From such a background, in the treatment of municipal sewage containing much nitrogen, the removal of nitrogen is extremely effective in preventing eutrophication and water pollution. Nitrogen removal techniques include physicochemical removal and biological nitrogen removal, but biological nitrogen removal is the mainstream today. This is partly because physicochemical removal cannot cope with all forms of nitrogen removal, and there are problems such as the use of chemicals and the need for post-treatment. This is because all forms of nitrogen can be dealt with, and the final form of the treatment is nitrogen gas, so that it is reasonable in terms of material circulation and there is no risk of secondary pollution. Furthermore, the biological nitrogen removal method draws out the functions of nitrifying bacteria and denitrifying bacteria that control the nitrogen removal reaction by changing the operating conditions of the conventional activated sludge treatment process for the purpose of removing organic substances. Nitrogen can be effectively removed in parallel with the removal.

[0004] In the development of the biological nitrogen removal method, various processes have been proposed from the standpoint of ensuring the alkalinity required for the nitrification reaction, the rational securing of the organic carbon source required for the dechambering reaction, and improving the nitrogen removal rate. It has been. Therefore, as a system for biologically removing nitrogen using activated sludge or the like, conventionally, a modified activated sludge circulation method shown in FIG. 4 and an endogenous denitrification method shown in FIG. 5 have been mainly used.

[0005] The modified activated sludge circulation method, as shown in FIG.
The denitrification tank 10 provided with the stirrer 16, the aerobic tank 11 provided with the nitrification unit 12, and the sedimentation basin 13 are connected via a pipe, and the nitrate nitrogen and nitrite nitrogen in the nitrification unit 12 are converted into a nitrification solution. The waste water is returned to the denitrification tank (anaerobic tank) 10 by the circulation pump 15, and the activated sludge from the sedimentation basin 13 is returned to the
It is plumbed to return to.

In this modified activated sludge circulation method, organic nitrogen and ammonia nitrogen in sewage are oxidized to nitrate nitrogen or nitrite nitrogen in the nitrification section 12 of the aerobic tank 11 located at the last stage. Further, the nitrate nitrogen and the nitrite nitrogen are returned to the denitrification tank 10 located in front of the aerobic tank 11 together with the activated sludge using the nitrification liquid circulation pump 15,
It is reduced to nitrogen gas under the organic matter mixed in the raw water, and is removed from the raw water as a gaseous substance.

[0007] Of the aerobic bacteria generally contained in activated sludge, nitrifying bacteria that oxidize ammonia nitrogen and the like have a slower growth rate than bacterial groups that remove (oxidize) organic substances. The sludge retention time in the aerobic tank 11 (hydrodynamic residence time: HR
T) needs to be long. Therefore, as a result, the oxidation of ammonia nitrogen or the like proceeds after the oxidation and removal of the organic matter is completed.

Therefore, in the aerobic tank 11, there is no organic matter necessary for biologically reducing nitrate nitrogen or nitrite nitrogen generated by oxidation of ammonia nitrogen or the like (very much). To a trace). That is, in order to reduce and remove these nitrate nitrogen and nitrite nitrogen, the activated sludge mixed liquid (nitrification liquid) containing this nitrate nitrogen or nitrite nitrogen is placed in the pre-stage of the nitrification tank (aerobic tank 11). The contact tank is called a denitrification tank 10, and this is the principle of the modified activated sludge circulation method.

In the endogenous denitrification method, as shown in FIG. 5, a front aerobic tank 20, a denitrification tank 21 provided with a stirrer 25, a post-aerobic tank 22, and a sedimentation basin 14 are connected via a pipe. Contact, Settlement Pond 2
The activated sludge of No. 3 is arranged so as to return to the front aerobic tank 20 by the return pump 24. In this endogenous denitrification method, nitrification is performed in the pre-aerobic tank 20 at the beginning of the system. Normally, when nitrification progresses, organic matter in water, especially soluble organic matter used in denitrification, is almost eliminated, but if the concentration of organic matter in raw water is high, or the sedimentation basin should be omitted first, it should be taken here When organic matter is supplied as raw water, untreated organic matter is taken into the activated sludge without decomposition.

Therefore, the activated sludge is placed in a denitrification tank 21 under anaerobic conditions, so that the denitrifying bacteria in the activated sludge use the incorporated organic substances to denitrify nitrate nitrogen and nitrite nitrogen generated by nitrification. I do. Further, in the final stage, a finishing treatment for removing undecomposed organic substances is performed again in the post-aerobic tank 22. The above is the basic principle of endogenous denitrification.

[0011]

The modified activated sludge circulation method comprises:
As shown in FIG. 4, the amount corresponding to the amount of the inflowing raw water is not subjected to denitrification but becomes treated water via the sedimentation basin 13, so that part of the nitrate nitrogen always remains in the treated water. As for this concentration, assuming that the amount of raw water flowing in is 1 and the ratio of the amount of circulation returned to the denitrification tank 10 is R (times), the total amount of nitrification (1 + R)
Of these, only R is subjected to de-chambering, so the theoretical nitrogen removal rate is R / (1 + R). In general, the amount of circulation is often about twice the amount of raw water. In this case, the final nitrogen removal rate is about 0.67 (66.7%) according to this equation, and theoretically it is higher than this. Not be.

That is, the nitrogen removal by the modified activated sludge circulation method is determined by the ratio of the raw water to the circulating liquid, and the nitrogen removal performance increases by increasing the circulating liquid ratio. In this case, nitrification for sending the circulating liquid is performed. Large power is consumed in the liquid circulation pump 15. The biggest drawback of the activated sludge circulation method lies here.

Further, in the activated sludge circulation modified method, it is necessary to maintain a sufficient amount (concentration) of nitrifying bacteria in the activated sludge, so that the HRT becomes longer. Therefore, the reaction tank (aerobic tank 1
Since the capacity of 1) becomes large, the system itself must be large-scale. Further, it is necessary to increase the activated sludge concentration, and the sludge age must be long because the degree of blocking of the nitrifying bacteria is small, so that the activated sludge itself becomes fine and the sedimentation property in the sedimentation basin 13 tends to deteriorate. In addition, the solids concentration and the visibility of the treated water often deteriorated.

On the other hand, in the endogenous denitrification method, it is necessary to keep untreated organic matter in the sludge, so that the load of the organic matter increases, and as a result, the treatment time (HRT) of the reaction tank becomes longer. The system becomes larger. Further, in the endogenous denitrification method, as shown in FIG. 5, excessive aeration in the pre-aerobic tank 20 increases the carry-in of dissolved oxygen to the subsequent denitrification tank 21, which adversely affects denitrification. . As described above, it is often difficult to control aeration and the like in the denitrification tank (aerobic tank) 21.

Further, there is a drawback caused by increasing the sludge concentration itself as in the modified activated sludge circulation method. The present invention has been made to solve such a conventional problem, the purpose of which is to first eliminate the circulation of nitrification liquid by providing a nitrification tank filled with a biological carrier immobilized nitrifying bacteria. Another object of the present invention is to provide a simplified and easy-to-install water treatment apparatus.

[0016]

Means for Solving the Problems A first aspect of the present invention is a nitrification tank filled with a biological carrier on which nitrifying bacteria are immobilized, a denitrification tank provided downstream of the nitrification tank, and a denitrification tank. An aerobic tank provided on the downstream side, a sedimentation pond provided on the downstream side of the aerobic tank, and a return pipe for returning activated sludge of the sedimentation tank to the denitrification tank are provided.

According to a second aspect of the present invention, there is provided the water treatment apparatus according to the first aspect, wherein the biological carrier has immobilized ammonia-oxidizing bacteria that oxidize ammoniacal nitrogen and convert it to nitrite. is there. According to a third aspect of the present invention, in the water treatment apparatus according to the first aspect, the biological carrier has immobilized nitrifying bacteria that convert nitrite nitrogen into nitrate.

According to a fourth aspect of the present invention, there is provided the water treatment apparatus according to the first aspect, wherein the biological carrier includes an ammonia oxidizing bacterium that oxidizes ammonia nitrogen and converts it into nitrite and a nitrifying bacterium that converts nitrite nitrogen into nitrate. It is characterized by being fixed. According to a fifth aspect of the present invention, in the water treatment apparatus according to the first aspect, the nitrification tank is provided with a germ growth suppression device.

According to a sixth aspect of the present invention, in the water treatment apparatus of the fifth aspect, the apparatus for suppressing the growth of various bacteria is a carrier cleaning apparatus. According to a seventh aspect of the present invention, in the water treatment apparatus according to the first aspect, a solid matter separation device is provided at a stage preceding the nitrification tank.

(Action) In the first to seventh aspects, in the nitrification tank, an ammonia-oxidizing bacterium (nitrite bacterium, for example, Nitrosomonas) that oxidizes autotrophic ammonium nitrogen to nitrite under aerobic conditions. Alternatively, a nitrifying bacterium (a nitrate bacterium, for example, mainly Nitrosbacter) that converts nitrite nitrogen to nitrate obtains energy for growth by oxidizing inorganic nitrogen.

When these reactions are expressed in stoichiometry, the following chemical formulas (1) and (2) are obtained.

Embedded image

Embedded image Therefore, the nitrification reaction by nitrifying bacteria is (1)
By combining the expressions (2), the expression can be generally expressed as the following expression (3).

NH ₄ ⁺ + 2O ₂ → NO ₃ ⁻ + H ₂ O + 2H ⁺ (3) This reaction provides energy for the growth of nitrifying bacteria. In the nitrification tank located at the beginning, since activated sludge such as returned sludge does not flow, only nitrification occurs and organic matter is not removed. Next, when the nitrification liquid containing nitrate nitrogen and nitrite nitrogen generated by these reactions flows into the denitrification tank, under the anaerobic conditions in the denitrification tank, there is a passivation of the activated sludge that has flowed in as returned sludge. Denitrifying bacteria that are anaerobic bacteria (Pseudomonas, Micrococc
Us, Achromobacter, Bacillus, etc.) perform nitrate respiration or nitrite respiration, and nitrate nitrogen and nitrite nitrogen are reduced to nitrogen gas.

The respiratory reaction in the anaerobic state is as follows (4)
To (6). (NO ₃ ⁻ → NO ₂ ⁻ ) 2NO ₃ ⁻ +2 (H ₂ ) → 2NO ₂ ⁻ + 2H ₂ O (4) (NO ₂ ⁻ → N ₂ ) 2NO ₂ ⁻ + 3 (H ₂ ) → N ₂ + 2OH ⁻ + 2H ₂ O (5) (NO ₃ ⁻ → N ₂ ) 2NO ₃ ⁻ +5 (H ₂ ) → N ₂ + 2NO ₂ ⁻ + 4H ₂ O (6) (H ₂ ) in these reactions is provided by a hydrogen donor.

The nitrate nitrogen and nitrite nitrogen generated in the nitrification tank are reduced and removed in the denitrification tank. Then, the final stage aerobic tank removes (oxidizes) organic substances and the like in the final treatment.

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings.

FIG. 1 is an explanatory view showing one embodiment of the water treatment apparatus according to claims 1 to 4. The water treatment device according to the present embodiment includes a nitrification tank 1 filled with a biological carrier 7 on which ammonia-oxidizing bacteria (nitrifying bacteria) such as nitrifying bacteria and nitrifying bacteria contained in activated sludge are immobilized, A denitrification tank (anaerobic tank) 2 provided downstream of the nitrification tank 1, an aerobic tank (aeration tank) 3 provided in the denitrification tank 2, and a downstream of the aerobic tank (aeration tank) 3 And a return pipe 5a for returning the activated sludge from the settling tank 4 to the denitrification tank 2.

The nitrification tank 1 is an aerobic biological reaction tank having a known suitable aeration (aeration) device 1a. The nitrification tank 1 is filled with the following or a biological carrier 7 on which nitrifying bacteria are immobilized (for example, about 10% in terms of the empty tower volume ratio). Ammonia-oxidizing bacteria that oxidize ammonia nitrogen to nitrite (eg, mainly Nitrosomonas) Mixture with nitrifying bacteria (eg, mainly Nitrosbacter) that convert nitrite nitrogen to nitric acid The strain to be retained can be changed. At this time, the amount of aerated air can be reduced by using only the ammonia-oxidizing bacteria as the retained cells.

That is, in normal nitrification by activated sludge, nitrification of ammonia and organic nitrogen proceeds to final nitrate nitrogen. This is because the activated sludge
Since the seed nitrifying bacteria are present with the same level of performance, it is difficult to control the amount of normal explosive air or the oxidation-reduction potential indicating the oxidation characteristics of water (liquid). At this time, if nitrification can be kept until nitrite conversion of ammonia as in the present embodiment, oxygen for oxidizing nitrite to nitric acid becomes unnecessary, and as a result, the amount of aerated air , Power can be reduced. As for denitrification, denitrification from nitrous acid and denitrification from nitric acid proceed to the same extent, so there is no adverse effect on denitrification.

In the nitrification tank 1, the inflowing raw water flows into the denitrification tank 2 due to overflow or the like. Therefore, a well-known method for preventing the filled biological carrier 7 from moving to the denitrification tank 2. Appropriate solid-liquid separator (not shown)
Is added to the nitrification tank 1.

The HRT (hydrodynamic residence time) of the raw water in the nitrification tank 1 is about 2 to 3 hours. The function of the nitrification tank 1 is to convert the ammonia nitrogen in the raw water flowing into the biological carrier 7.
In the former case, it is oxidized to nitrite nitrogen, and in the latter case, it is oxidized to nitrate nitrogen by the ammonia oxidizing bacteria or nitrifying bacteria immobilized on the surface. In the nitrification tank 1,
Like the aerobic tank (aeration tank) in the conventional activated sludge method,
The concentration is not maintained by returning sludge. Therefore, in the nitrification tank 1, there is almost no activated sludge (microorganism group) for mainly removing organic substances (such as BOD).
This removal of organic matter is not performed.

Microorganisms present in the activated sludge can utilize nitrogen components other than ammonia nitrogen in raw water, so-called organic nitrogen, for the growth of the living body, or convert them into nitrate nitrogen or ammonia nitrogen. It has the ability to oxidize to these levels, but in the nitrification tank 1, these microbial groups are almost nonexistent.
This organic nitrogen passes here at the same concentration without undergoing any treatment. This amount varies depending on the raw water, but it is considered to be about 25% of the total nitrogen (TN) in normal sewage.

The denitrification tank 2 is an anaerobic tank having a suitable stirrer 6. Overflow water from the preceding nitrification tank 1 and return sludge, which is activated sludge sent by the return pump 5, flow into the denitrification tank 2 via the return pipe 5a. In the denitrification tank 2,
In the presence of organic matter (BOD) contained in the raw water, the denitrifying bacteria present in the activated sludge in the returned sludge are converted to nitrogen gas by nitrogen gas in the presence of organic matter (BOD) contained in the raw water. As a result, nitrogen is removed from the liquid.

The organic matter (BOD) in the raw water is mainly dissolved and consumed (removed) at this time. The aerobic tank 3
This is an aerobic biological reaction tank having a known suitable aeration (aeration) device 3a.

In the overflow water from the denitrification tank 2 flowing into the aerobic tank 3, untreated organic substances and organic nitrogen remain, which are removed here by the same treatment mechanism as in the ordinary activated sludge method. You. Of these, organic nitrogen, which is ultimately used for the growth of living organisms as activated sludge, is removed from the liquid, but other than that, nitric acid is mainly produced by nitrifying bacteria present in activated sludge. It is oxidized to nitrogen or nitrite nitrogen. However, since there is no mechanism for removing the nitrate nitrogen or nitrite nitrogen generated in this way after the aerobic tank 3, this nitrate nitrogen or nitrite nitrogen passes through the sedimentation tank 4. And discharged as treated water.

The activated sludge mixture flows from the aerobic tank 3 into the sedimentation basin 4, and the activated sludge in the mixture is settled and separated into concentrated activated sludge and a supernatant. The supernatant liquid is discharged as treated water from the sedimentation basin 4 as overflow water, and a part of the concentrated activated sludge is sent to the denitrification tank 2 as return sludge by the return pump 5. According to the present embodiment configured as described above, when biologically removing nitrogen, organic nitrogen and ammonia nitrogen contained in raw water (sewage) are basically replaced with nitric acid contained in activated sludge and the like. Ammonia-oxidizing bacteria (nitrifying bacteria) such as nitrifying bacteria and nitrifying bacteria
Oxidizes to nitrate nitrogen or nitrite nitrogen. In this case, oxygen in the air that is aerobically aerated is used as the oxygen source.

Next, the nitrate nitrogen and nitrite nitrogen thus generated are reduced to nitrogen gas by the denitrifying bacteria contained in the activated sludge, etc. under the organic matter (BOD) existing in the raw water. I do. As a result, nitrogen becomes a gaseous substance, and as a result, is removed from the raw water. In the present embodiment, the nitrification tank 1 located at the beginning is filled with the biological carrier 7 holding the nitrifying bacteria group that oxidizes ammonia nitrogen, and the activated sludge such as returned sludge does not flow. No removal occurs.

Further, after this, the denitrification tank 2 is placed, and the activated sludge is flowed as return sludge through the return pipe 5a, thereby reducing and removing the nitrate nitrogen and the nitrite nitrogen generated in the nitrification tank 1. . And aerobic tank 3 at the last stage
To remove (oxidize) the organic substances and the like in the final treatment. As described above, in the present embodiment, the raw water flows transiently in the water treatment apparatus, and there is no circulation of the nitrification liquid as in the modified activated sludge circulation method. Not only is it reduced, but the water treatment device is simplified and construction is easier.

In this embodiment, since the nitrifying bacteria can be maintained at a high concentration by immobilizing the nitrifying bacteria, the aerobic part (the nitrification tank 1) can be maintained.
+ The capacity of the aerobic tank 3) can be made equal to or less than that of the conventional standard activated sludge. In addition, since the HRT is short and the sludge concentration is about the same as that of ordinary standard activated sludge, it is possible to avoid the deterioration of the treated water quality, which tends to be caused by nitrification.

Further, in this embodiment, the first nitrification tank 1
Is purely nitrification of ammoniacal nitrogen,
It is only necessary to perform aeration as long as the dissolved oxygen concentration exceeds a certain level.
Control is easy. In addition, since the organic matter load and the like are almost the same as those of ordinary activated sludge, the scale of the apparatus does not increase.

Nitrogen removal performance and HRT in this embodiment
The relationship with (hydraulic residence time) is as shown in Table 1.

[Table 1] FIG. 2 is an explanatory view showing an embodiment of the water treatment apparatus according to claims 1 to 6. In the water treatment apparatus according to the present embodiment, a cleaning device 8 composed of an air lift device is disposed as a device for suppressing the growth of various bacteria in the nitrification tank 1 of the water treatment device shown in FIG. 1 (claims 5 and 6). Here, the germ propagation suppression device is for preventing a decrease in nitrification performance.

Cleaning device 8 comprising this air lift device
Is an air lift pipe 8a vertically arranged in the nitrification tank 1.
And a diffusion-preventing cylindrical body 8b arranged to cover the upper opening 8a1 side of the air lift pipe 8a, and an air lift air pipe 8c attached to the lower opening 8a2 side of the air lift pipe 8a. ing. In the cleaning device 8 including the air lift device, the air lift pipe 8
When air is supplied to a through the air-lifting air pipe 8c, the carrier 7 deposited on the bottom of the nitrification tank 1 is pumped up, and at the same time, the liquid in the nitrification tank 1 is pumped up simultaneously with the air in the air-lifting pipe 8a. Accordingly, the contaminants and microorganisms attached to the surface of the carrier 7 are separated, and the carrier 7 from which the contaminants and microorganisms are removed is returned into the nitrification tank 1 through the upper opening 8a1 of the air lift tube 8a.

As described above, by preventing pollutants and microorganisms from adhering to the surface of the carrier 7, it is possible to efficiently supply oxygen, ammonia, and the like required by the bacteria to the nitrifying bacteria and the like living inside the carrier 7. Can be. In particular, when microorganisms that take up (decompose) organic substances adhere to the surface of the carrier 7,
These microorganisms take up oxygen in the same manner as nitrifying bacteria in order to decompose organic matter, and as a result, competition between the nitrifying bacteria in the carrier 7 and oxygen uptake occurs. Not only is the required amount of oxygen or more oxygen necessary, but especially because these microorganisms adhere to the surface of the carrier 7, deprive the oxygen to be supplied to the inside of the carrier 7, oxygen is not supplied to the nitrifying bacteria, The efficiency of the ammonia oxidation itself deteriorates.

When the removal of the organic portion occurs in the nitrification tank 1, the amount of organic substances supplied to the anaerobic tank 11 located at the subsequent stage decreases, so that the denitrification in the anaerobic tank 11 does not occur efficiently. I will. In this embodiment, the cleaning device 8 including the air lift device is used, but the invention is not limited to the air lift, and a draft tube may be used.

FIG. 3 is an explanatory view showing one embodiment of the water treatment apparatus according to the first to fourth and seventh aspects. In the water treatment apparatus according to the present embodiment, a solid matter separation device 9 is arranged in front of the nitrification tank 1 of the water treatment apparatus shown in FIG. 1 (claim 7).

This solid matter separation device 9 removes solid organic matter and solid nitrogen from the inflowing raw water. In this water treatment device, the solid matter separation device 9 is disposed in front of the nitrification tank 1 to remove organic matter other than the solubility in the raw water, thereby consequently removing the organic matter applied to the water treatment device. The load is reduced, and the water treatment device becomes smaller.

Further, since the solid organic matter and the organic nitrogen derived from the solid matter are removed in the solid matter separation device 9, the amount of nitrate nitrogen produced in the aerobic tank 3 is reduced. As a result, the nitrogen removal rate of the entire water treatment apparatus is improved. Of course, the organic matter removal rate of the entire processing apparatus is also improved. In this embodiment, the solid matter separation device 9 is arranged in front of the nitrification tank 1 of the water treatment apparatus shown in FIG. 1, but the solid matter separation apparatus 9 is arranged in front of the nitrification tank 1 of the water treatment apparatus shown in FIG. May be arranged.

Although the present invention has been described with reference to sewage treatment, it can be applied to advanced treatment of domestic wastewater such as human waste / combined septic tank treatment.

[0048]

As described above, according to the first to seventh aspects of the present invention, since there is no nitrification liquid circulation unlike the modified activated sludge circulation method, a nitrification liquid circulation pump is not required and power is reduced. In addition, the water treatment device is simplified, and the construction is easier.

Further, since the concentration can be maintained at a high level by immobilizing the nitrifying bacteria, the capacity of the aerobic part (nitrification tank + aerobic tank) can be made equal to or less than that of the conventional standard activated sludge. Furthermore, H
Since the RT is short and the sludge concentration is almost the same as that of the standard activated sludge, it is possible to avoid the deterioration of the treated water quality, which is liable to be caused by nitrification. Furthermore, since the first nitrification tank is purely nitrification of ammonia nitrogen, it is only necessary to perform aeration to obtain a dissolved oxygen concentration of a certain level or more, and control is easy. Further, since the organic matter load and the like are almost the same as those of ordinary activated sludge, the scale of the apparatus does not increase.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing one embodiment of a water treatment apparatus according to claims 1 to 4.

FIG. 2 is an explanatory view showing one embodiment of the water treatment apparatus according to claims 1 to 6;

FIG. 3 is an explanatory view showing one embodiment of a water treatment apparatus according to claims 1 to 4 and 7;

FIG. 4 is an explanatory view showing a conventional activated sludge circulation modified method.

FIG. 5 is an explanatory view showing a conventional endogenous denitrification method.

[Explanation of symbols]

 Reference Signs List 1 nitrification tank 2 denitrification tank 3 aerobic tank (aeration tank) 4 sedimentation basin 5 return pump 5a return pipe 6 stirrer 7 biological carrier 8 washing device 9 solids separation device

Claims (7)

[Claims] A nitrification tank filled with a biological carrier on which nitrifying bacteria are immobilized; a denitrification tank provided downstream of the nitrification tank; an aerobic tank provided downstream of the denitrification tank; A water treatment apparatus comprising: a sedimentation pond provided downstream of an aerobic tank; and a return pipe for returning activated sludge of the sedimentation pond to a denitrification tank. 2. The water treatment apparatus according to claim 1, wherein the biological carrier has immobilized ammonia-oxidizing bacteria that oxidize ammonia nitrogen and convert it into nitrite. 3. The water treatment apparatus according to claim 1, wherein the biological carrier has immobilized nitrifying bacteria that convert nitrite nitrogen into nitric acid. 4. The water treatment apparatus according to claim 1, wherein the biological carrier has immobilized ammonium oxidizing bacteria for oxidizing ammonia nitrogen and converting it to nitrite and nitrifying bacteria for converting nitrite nitrogen to nitric acid. A water treatment apparatus characterized by the above-mentioned. 5. The water treatment apparatus according to claim 1, wherein the nitrification tank is provided with a germ growth suppression device. 6. The water treatment apparatus according to claim 5, wherein the germ growth suppression device is a carrier washing device. 7. The water treatment apparatus according to claim 1, wherein a solid separation device is provided at a stage preceding the nitrification tank.