JP2007136298A - Removal method of nitrogen and phosphorus from sewage, and removal apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a new removal method of nitrogen and phosphorus, and a removal apparatus. <P>SOLUTION: The biological nitrogen and phosphorus removal process (A<SB>2</SB>O method) comprises a primary settling tank, an anaerobic tank, anoxic tanks (denitrification tank), an aerobic tank, and a final settling tank, and circulates active sludge of a terminal part of the aerobic tank to the anoxic tank (denitrification tank). A second anoxic tank is provided at a rear stage of the aerobic tank, and the activated sludge of the terminal part of the second anoxic tank is circulated to the preceding anoxic tank (denitrification tank), and circulation of the activated sludge from the aerobic tank to the preceding anoxic tank (denitrification tank) is stopped. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention provides a stable and efficient method for biologically removing nitrogen and phosphorus contained in sewage and wastewater, in particular, a method for removing biological nitrogen and phosphorus simultaneously comprising an anaerobic tank, an anaerobic tank, and an aerobic tank. The present invention relates to a method of operating automatically, and an apparatus therefor.

First, the principle of the prior art regarding phosphorus removal will be described.
The total phosphorus concentration in city sewage is about 5 to 10 mg / L, which is derived from manure, detergents, industrial chemicals and the like. As a method for removing phosphorus, the coagulation sedimentation method using an iron or aluminum coagulant is the most reliable, but has the disadvantages of increasing the amount of excess sludge and chemical costs due to the coagulant. For this reason, a phosphorus removal process using activated sludge has been widely used as a method for removing phosphorus from sewage.

  This is because certain microorganisms (hereinafter referred to as phosphorus-accumulating bacteria) in activated sludge tend to release phosphorus under anaerobic conditions, and then try to consume excess phosphorus under aerobic conditions. Practical use is progressing in the field of urban sewage treatment. If such a system is adopted, it is said that the phosphorus concentration in the activated sludge of sewage increases from 2-3% to about 5-6%.

Next, the principle of the prior art regarding nitrogen removal will be described.
Most nitrogen in city sewage is ammoniacal nitrogen (NH ₄ -N), usually at a concentration of about 20-50 mg / L. The following biological nitrification-denitrification methods are widely known as methods for removing NH ₄ —N from municipal sewage. Biological oxidation reaction (also called nitrification process) by absolute aerobic / autotrophic bacteria (Nitrosomonas, Nitrobacter, etc.) and biological reduction reaction by facultative anaerobic, heterotrophic bacteria (Pseudomonas, etc.) It consists of a combination (also called denitrification process).

  First, the nitrification process consists of the following two-stage reaction, and the types of nitrifying bacteria involved are different.

2NH ₄ ⁺ + 3O ₂ → 2NO ₂ ⁻ + 2H ₂ O + 4H ⁺ −−−−−−−− (1)
^{_{2NO 2 - + O 2 → 2NO}} 3 - --------- (2)

The reaction represented by the formula (1) is brought about by an ammonia oxidizing bacterium represented by Nitrosomonas, and the reaction represented by the formula (2) is brought about by a nitrite oxidizing bacterium represented by Nitrobacter.

Next, in the denitrification process, nitrite nitrogen (NO ₂ -N) and nitrate nitrogen (NO ₃ -N) produced by the above reaction are subjected to oxygen-free conditions using facultative anaerobic heterotrophic bacteria. Below, it is reduced as follows, and is converted into nitrogen oxide gas (N ₂ O) or nitrogen gas (N ₂ ) and diffused into the atmosphere.

2NO ₂ ⁻ + 6H ₂ → N ₂ + 2H ₂ O + 2OH ⁻ −−−−−−−−− (3)
2NO ₃ ⁻ + 10H ₂ → N ₂ + 4H ₂ O + 2OH ⁻ −−−−−−−−−− (4)

  Facultative anaerobic heterotrophic bacteria require a hydrogen donor, and organic matter is usually used as the hydrogen donor. In municipal sewage and the like, organic matter (BOD component) in sewage is used as it is, and in wastewater not containing organic matter, methyl alcohol or the like is often added from the outside.

  Finally, the simultaneous removal process of nitrogen and phosphorus from sewage that has been put into practical use in the field of sewage treatment will be described. In principle, this method combines a biological phosphorus removal process and a biological nitrogen removal process to remove phosphorus and nitrogen simultaneously.

Specifically, as shown in FIG. 1, the present process is composed of a first sedimentation tank, an anaerobic tank, an oxygen-free tank (denitrification tank), an aerobic tank (nitrification tank), and a final sedimentation tank. It is widely called as A ₂ O method (Anaerobic-Anoxic-Oxic Process).

  First, sewage removes coarse suspended solids in the sewage by gravity sedimentation in the first sedimentation basin. In the anaerobic tank, excessive phosphorus is released. An anaerobic state is defined as a state without dissolved oxygen (DO) and bound oxygen present as NOx-N. Thereafter, phosphorus is taken in order in an anaerobic tank and an aerobic tank. An anoxic state is defined as a state without DO. The aerobic state is defined as a state where DO exists.

NH ₄ —N in sewage is oxidized to NO ₃ —N by nitrifying bacteria in an aerobic tank. The activated sludge containing NO ₃ —N in the aerobic tank is circulated in the anoxic tank. In anoxic tank, NO ₃ -N is removed from the water is reduced to N ₂ by a number of types of denitrifying bacteria. The nitrogen removal rate is determined by the circulation rate, and the circulation rate (circulation water amount / first sedimentation basin outflow amount) is usually about 100 V / V% to 200 V / V%. In the final sedimentation basin, activated sludge is separated by gravity sedimentation, the supernatant is discharged as treated water, and the settled sludge is returned to the anaerobic tank. The return rate (sludge return amount / initial sedimentation basin effluent amount) is usually about 50 V / V%. Some sludge is extracted outside the system as surplus sludge. That is, the phosphorus contained in the sewage is extracted in the form of phosphorus contained in the sludge.

Thus, the A ₂ O method is an excellent method that skillfully combines the principles of biological nitrogen and phosphorus removal, and its practical application is progressing.
On the other hand, there are many cases where biological phosphorus removal and nitrogen removal are difficult to control and the processing performance is not stable.

  For example, regarding phosphorus removal, it is important to cause sufficient release of phosphorus in an anaerobic tank. Factors that inhibit phosphorus removal include the inflow of DO and NOx-N and the deficiency of organic substances (particularly organic acids). For example, it is known that when rainwater flows into sewage, such a phenomenon that the processing performance is not stable occurs. In addition, in the oxygen-free tank, sufficient denitrification reaction may not be obtained, or phosphorus may be released although denitrification proceeds. Factors that inhibit denitrification include excessive inflow of DO and NOx-N and lack of organic substances. On the contrary, it is conceivable that the organic substance remains excessively as a cause of the release of phosphorus.

The inventors considered that the influence of the operating method of the aerobic tank was particularly large as a factor of destabilization of the A ₂ O method. That is, it is usually considered that the DO in the vicinity of the aerobic tank outlet in the A ₂ O method is desirably 1.5 to 3.0 mg / L from the viewpoint of promoting nitrification (see Non-Patent Document 1).

  Furthermore, in practice, the aerobic tank DO is often found to be operated at higher DO values, for example 3-5 mg / L. This phenomenon is particularly common when the amount of sewage actually being treated is smaller than the planned amount of sewage or when the water quality is lower than the planned value. In addition, the DO in the aerobic tank tends to be higher in summer when the water temperature rises and the reaction rate increases. This is because when the nitrification reaction is completed in the middle of the aerobic tank, the oxygen demand by the activated sludge is drastically reduced, so that DO in the aerobic tank tends to remain excessively even if the aeration amount is considerably reduced. Further, when the aeration amount is extremely reduced, the activated sludge settles and accumulates, so there is a limit to the reduction of the aeration amount from the viewpoint of stirring the activated sludge. For this reason, the DO at the end of the aerobic tank tends to be high.

  Furthermore, if the actual amount of sewage and the quality of sewage are in line with the planned values, the nitrification reaction should be able to be completed completely without raising DO in the aerobic tank. However, even in such a case, there is no need to maintain the aerobic tank DO at 1.5 to 3.0 mg / L at all, but the DO of the aerobic tank is maintained high. There are many.

  Although the operation of maintaining a high aerobic tank DO has disadvantages in terms of energy consumption, it has various advantages such as prevention of phosphorus release due to sludge anaerobization in the final sedimentation basin and improved transparency. At first glance, it seems like a very safe management.

However, in practice, such operation management in the A ₂ O method is a cause of various harmful effects.
First, since the operation method of such an aerobic tank causes a large amount of DO to exist in the circulating water from the aerobic tank to the anoxic tank, the denitrification performance in the anaerobic tank is deteriorated.

Below, the reaction formula with DO and the reaction formula with NOx-N in the case of acetic acid are shown as an organic substance. In general, denitrifying bacteria in activated sludge are facultative bacteria, and both DO and NOx-N can be used. However, if they coexist, DO is preferentially used. Therefore, when DO flows in the oxygen-free tank, DO immediately reacts with NOx-N via organic matter in the sewage and denitrifying bacteria as shown in the equation (5). The reaction (6) of NOx-N and acetic acid starts after the disappearance of DO, but if organic substances are used up to that point, equation (6) does not proceed and NOx-N remains. As a result, the denitrification efficiency decreases. For example, if O ₂ is present in the range of 1.5 mg / L to 3.0 mg / L as shown in Equation (5), acetic acid is also consumed in the same amount of 1.5 mg / L to 3.0 mg / L.

2O ₂ + CH ₃ COOH → 2CO ₂ + 2H ₂ O ------ (5)
^{_{8NO 3 - + 5CH 3 COOH →}} 4N 2 + 10CO 2 + 6H 2 O + 8OH - (6)
Therefore, in the biological nitrogen and phosphorus removal process (A ₂ O method), which consists of a first settling tank, anaerobic tank, anaerobic tank, aerobic tank, and final settling tank, and circulates the sludge of the aerobic tank to the anoxic tank. The DO of sludge circulating water is preferably 0 mg / L if possible, but if DO is 0.5 mg / L or less, the influence of DO can be almost ignored.

Next, when DO in the aerobic tank is high, NOx-N tends to remain in the returned sludge. When the sludge settles and concentrates in the final sedimentation basin, DO is gradually consumed by endogenous respiration inside the sludge. When DO disappears, NOx-N is consumed by an endogenous denitrification reaction (reaction between NOx-N accumulated and adsorbed in the activated sludge). Furthermore, when NOx-N disappears, phosphorus release is observed. However, if DO is present in a large amount, it is difficult for the endogenous denitrification reaction as described above to occur, so NOx-N tends to remain in the return sludge as it is. As a result, NO _x -N easily flows into the anaerobic tank through the returned sludge. If NO _x -N is present in the anaerobic tank, the organic acid necessary for the release of phosphorus is immediately decomposed as shown in the equation (6). For example, when NO ₃ —N is present at 1 mg / L, acetic acid 2.7 mg / L is consumed accordingly. It can be seen that the presence of NO ₃ —N has an extremely large influence on the consumption of acetic acid. If acetic acid is deficient in the anaerobic tank, the release of phosphorus is continuously suppressed, resulting in a decrease in the phosphorus removal rate.

  Furthermore, it is considered that the presence of DO in a high aerobic tank does not necessarily mean the progress of the nitrification reaction. In other words, oxygen is of course required for the nitrification reaction to proceed, but it cannot be determined that the nitrification reaction is proceeding because oxygen is present. For example, when the activity of nitrifying bacteria is reduced or killed, the oxygen consumption is lost and the aerobic tank shows a high DO.

  Similarly, the presence of DO in a high aerobic tank is not considered to mean the progress of phosphorus uptake reaction. The phosphorus ingestion reaction is an organic matter decomposition by phosphorus accumulating bacteria and requires oxygen, but the presence of oxygen does not necessarily cause the phosphorus ingestion reaction to proceed. For example, if the activity of phosphorus accumulating bacteria is reduced or killed, oxygen consumption is lost and the aerobic tank will show high DO. Thus, even if the aerobic tank is managed with high DO, this does not necessarily indicate the stability of the processing performance.

From this point of view, it is considered that the operation of the A ₂ O method is difficult to remove nitrogen and phosphorus stably with the conventional idea.
Advanced treatment design manual, p225-252, Japan Sewerage Association, 1994

To summarize the above problems, the influence of the operating method of the aerobic tank is extremely large in the A ₂ O method, and the conventional operating method of the aerobic tank is not necessarily applicable to all sites. That is, in many cases, the operation method for maintaining the DO in the aerobic tank at a high level is a major factor in the instability of the processing. Maintaining a high DO in the aerobic tank is deeply linked to the suppression of phosphorus release in the anaerobic tank and the decrease in denitrification efficiency in the anaerobic tank, leading to instability of the process. From this point of view, the present invention aims to provide a method for stabilizing the quality of treated water by drastically changing the operation management method of the A ₂ O method.

  As a result of repeated studies to solve the above problems, the present inventors have succeeded in stably and efficiently treating sewage containing nitrogen and phosphorus by the following method. The gist of the present invention is the following (1) to (10).

(1) Biological cycle consisting of first sedimentation tank, anaerobic tank, anaerobic tank (denitrification tank), aerobic tank, and final sedimentation tank, and circulating activated sludge at the end of the aerobic tank to the anoxic tank (denitrification tank) In the nitrogen and phosphorus removal process (A ₂ O method), a second oxygen-free tank is provided after the aerobic tank, and the activated sludge at the end of the second oxygen-free tank is used as the oxygen-free tank (denitrification tank) in the previous stage. And removing the sludge circulation from the end portion of the aerobic tank to the preceding anaerobic tank (denitrification tank).

(2) The above characterized in that an underwater stirrer is installed at the bottom of the second anaerobic tank after the aerobic tank to stir the activated sludge and lower the DO in the activated sludge to 0.5 mg / L or less ( The method for removing nitrogen and phosphorus from sewage as described in 1).

(3) Biological cycle consisting of first sedimentation tank, anaerobic tank, anaerobic tank (denitrification tank), aerobic tank, and final sedimentation tank, and circulating activated sludge at the end of the aerobic tank to the anoxic tank (denitrification tank) In the nitrogen and phosphorus removal process (A ₂ O method), by reducing aeration at the end of the aerobic tank where the activated sludge is circulated, the DO in the sludge circulating water is 0.5 mg / L or less and the end of the aerobic tank A method for removing nitrogen and phosphorus from sewage, wherein an underwater stirrer is installed at the bottom of the section to stir the activated sludge.

(4) The above characterized in that the endogenous respiration of activated sludge is used as means for lowering the DO at the end of the second anaerobic tank or the aerobic tank to 0.5 mg / L or less, following the aerobic tank ( The removal method of nitrogen and phosphorus from the sewage in any one of 1)-(3).

(5) In the aerobic tank, an NH ₄ -N concentration measured continuously, by controlling the aeration of the aerobic tank by measurement of NH ₄ -N, the NH ₄ -N concentration of the sludge circulation water 0. The method for removing nitrogen and phosphorus from sewage according to any one of the above (1) to (4), wherein the method is reduced to 5 mg / L or less.

(6) Mix the returned sludge and the first settling basin effluent, reduce DO and NOx-N (the sum of NO ₂ -N and NO ₃ -N) to below the detection limit, and then flow this mixed water into the anaerobic tank A method for removing nitrogen and phosphorus from sewage according to any one of the above (1) to (5), wherein

(7) As a means for mixing the return sludge and the first settling basin effluent and reducing DO and NOx-N (the sum of NO ₂ -N and NO ₃ -N) to below the detection limit, The method for removing nitrogen and phosphorus from sewage as described in (6) above, wherein a denitrification tank) is provided in front of the anaerobic tank.

(8) At least one of an organic substance and a sulfur compound is added to a third oxygen-free tank provided in the front stage of the anaerobic tank, and DO and NOx-N (NO ₂ -N and NO ₃ -N in the anaerobic tank inflow water) (6) or (7) above, wherein nitrogen and phosphorus are removed from sewage.

(9) A first sedimentation basin, an anaerobic tank, an anaerobic tank (denitrification tank), an aerobic tank, a second anaerobic tank, and a final sedimentation tank are provided in this order, and a part of the activated sludge in the final sedimentation tank is further provided. Biology from sewage comprising: an apparatus for circulating to the anaerobic tank; and an apparatus for circulating a part of the activated sludge in the second anoxic tank to the anoxic tank (denitrification tank). Nitrogen and phosphorus removal equipment.

(10) A third oxygen-free tank is further provided between the first settling basin and the anaerobic tank, and the final sludge is replaced with a device that circulates part of the activated sludge in the final settling basin to the anaerobic tank. An apparatus for removing biological nitrogen and phosphorus from sewage, comprising an apparatus for circulating a part of activated sludge in a sedimentation basin to the third oxygen-free tank.

  According to the present invention, nitrogen and phosphorus can be stably removed from sewage containing nitrogen and phosphorus.

A flow of the conventional A ₂ O method is shown in FIG. An example of the processing flow of the present invention is shown in FIGS.

In other words, the present invention is a modification of the conventional A ₂ O method. It can also be called A ₂ OA method (anaerobic-anoxic-aerobic-anoxic) or A ₃ OA method (anoxic-anaerobic-anoxic-aerobic-anoxic).
That is, the A ₂ OA method of FIG. 2 includes a first sedimentation tank 2, an anaerobic tank 5, an oxygen-free tank (denitrification tank) 6, an aerobic tank 7, a second oxygen-free tank 8, and a final sedimentation tank 9. A device that circulates part of the activated sludge in the final sedimentation basin to the anaerobic tank (consisting of a pipe for sending the return sludge 15 and the return sludge pump 14), and a part of the activated sludge in the second anoxic tank Is connected to the anaerobic tank (denitrification tank) (consisting of a pipe for sending sludge circulating water 18 including activated sludge and a sludge circulation pump 13).

In addition, the A ₃ OA method of FIG. 3 includes the first sedimentation tank 2, the third oxygen-free tank (denitrification tank) 20, the anaerobic tank 5, the oxygen-free tank (denitrification tank) 6, the aerobic tank 7, and the second oxygen-free tank. 8. A final sedimentation basin 9 is provided, and a device for circulating a part of the sludge from the final sedimentation basin to the third oxygen-free tank (consisting of a pipe for sending the return sludge 15 and a return sludge pump 14), An apparatus (consisting of a pipe for sending the sludge circulating water 18 including the activated sludge and the sludge circulation pump 13) that circulates a part of the activated sludge in the second oxygen-free tank to the oxygen-free tank (denitrification tank).

2 and 3 circulate not the activated sludge in the aerobic tank 7 but the activated sludge in the second anoxic tank 8.
FIG. 4 is the same as the conventional method in that the activated sludge in the aerobic tank 7 is circulated to the anoxic tank 6 (denitrification tank), but an underwater agitator 11 is installed near the end of the aerobic tank 7 and the DO value 17 Is greatly different in that it is 0.5 mg / L or less.

The DO 17 at the outlet of the aerobic tank 7 in the conventional A ₂ O method shown in FIG. 1 is preferably 1.5 to 3.0 mg / L from the viewpoint of promoting nitrification, promoting phosphorus intake, and preventing phosphorus release in the final sedimentation basin. It was said. However, such an operation index has many problems. This problem is enumerated as follows.

1) Since the activated sludge at the end of the aerobic tank 7 holding high DO is sent to the anoxic tank 6, the denitrification reaction in the anoxic tank 6 is likely to be hindered.
2) Since the activated sludge at the end of the aerobic tank 7 holding high DO is sent to the final sedimentation tank 9, NOx-N tends to remain in the return sludge 15. This NOx-N inhibits phosphorus release in the anaerobic tank 5.
3) The presence of high DO at the end of the aerobic tank 7 does not mean the progress of the nitrification reaction. In other words, oxygen is necessary for the progress of the nitrification reaction, but just because oxygen is present does not mean that the nitrification reaction is progressing.
4) The presence of high DO at the end of the aerobic tank 7 does not mean the progress of the phosphorus intake reaction. In other words, oxygen is necessary for the progress of the phosphorus uptake reaction, but the presence of oxygen does not necessarily mean that the phosphorus uptake reaction is progressing.
5) Even if DO is 0 in the anaerobic tank 5 or the anaerobic tank 6, if NOx-N is present, the release of phosphorus in the anaerobic tank 5 is suppressed. Phosphorus release occurs after NOx-N disappears. Therefore, in order to promote or suppress the release of phosphorus, it is necessary to manage not only DO but also NOx-N. This viewpoint is missing from the management of the anaerobic tank 5, the oxygen-free tank (denitrification tank) 6, and the final sedimentation tank 9.

The inventors have arrived at the present invention in order to solve the problems of the conventional A ₂ O method. This specific means will be described in detail below.
First, it consists of first sedimentation basin 2, anaerobic tank 5, anoxic tank 6 (denitrification tank), aerobic tank 7 and final sedimentation tank 8. Activated sludge at the end of aerobic tank 7 is treated with anaerobic tank 6 (denitrification tank). In the biological nitrogen and phosphorus removal process (A ₂ O method) to be circulated, a second oxygen-free tank 8 is further provided after the aerobic tank 7 (see FIG. 2). Then, the activated sludge at the end of the second oxygen-free tank 8 is circulated to the preceding oxygen-free tank (denitrification tank) 6. The DO in the second oxygen-free tank 8 and the activated sludge circulating liquid 18 is desirably 0.5 mg / L or less, and should preferably be 0 mg / L. Sludge circulation from the aerobic tank 7 to the preceding anaerobic tank (denitrification tank) 6 is stopped. The purpose of the second oxygen-free tank 8 provided in the rear stage of the aerobic tank 7 is literally oxygen-free, that is, to set DO to 0, and not denitrification. Rather, it is important not to reduce NOx-N from the viewpoint of suppressing the release of phosphorus in the final sedimentation tank 9. An underwater stirrer 11 is installed at the bottom of the second oxygen-free tank 8 and is operated only with stirring. No agitation by aeration.

In order to reduce DO, it is desirable to use the endogenous respiration of activated sludge. Generally, the oxygen consumption rate in the endogenous breathing state is about 0.12 gO ₂ / MLSS · g / day. In the A ₂ O method, the activated sludge concentration (MLSS concentration) in the reaction tank is operated at about 2,000 to 3,000 mg / L. Therefore, the HRT of the second oxygen-free tank 8 can reduce the DO of 10 to 15 mg / L to 0 in 1 hour. If the DO in the aerobic tank 7 is maintained at 3 mg / L and the activated sludge concentration is maintained at about 3,000 mg / L, 3 mg / L in the outlet water of the aerobic tank 7 under the condition that the HRT is about 12 minutes. The calculation is such that the DO of L can be zero. In addition, although it repeats, the objective of the 2nd anoxic tank 8 provided in the back | latter stage of this aerobic tank 7 is to make DO into 0 to the last. It is not intended for denitrification.

  What should be noted in the second anoxic tank 8 provided at the rear stage of the aerobic tank 7 is suppression of release of phosphorus in the final sedimentation tank 9. For this reason, in this invention, the NOx-N density | concentration of the outlet water of the 2nd anaerobic tank 8 remains 5.0 mg / L or more and 10 mg / L or less. NOx-N is not completely removed in the second oxygen-free tank 8. This is because the achievement of complete denitrification, that is, complete disappearance of NOx-N, promotes the release of phosphorus in the final sedimentation tank 9. On the other hand, if the NOx-N concentration is 5.0 mg / L or more, release of phosphorus in the final sedimentation tank 9 can be prevented. Thus, in order to prevent the release of phosphorus in the final sedimentation tank 9, denitrification should not be promoted in the second anoxic tank 8. Further, by leaving the NOx-N concentration to be 5.0 mg / L or more, it is possible to prevent the release of phosphorus in the return sludge 15 from the final sedimentation tank 9 to the anaerobic tank 5.

When it is difficult to newly install or expand the second oxygen-free tank 8 at the rear stage of the aerobic tank 7, the following operation method (see FIG. 4) is used. That is, aeration by the blower at the end portion of the aerobic tank 7 where the HRT is about 10 to 30 minutes is reduced or stopped. Specifically, DO17 is measured, and the amount of aeration is reduced so that the value of DO17 is 0 mg / L or more and 0.5 mg / L or less. Thereby, DO in the sludge circulating water 18 can also be 0 mg / L or more and 0.5 mg / L or less. Since the end part of the aerobic tank 7 in which the aeration amount is reduced needs to be operated with stirring to such an extent that activated sludge does not settle, it is necessary to install an underwater stirrer 11 at the bottom and stir. In the conventional A ₂ O method, the DO at the end portion of the aerobic tank 7 is usually desirably 1.5 to 3.0 mg / L. However, if it can be confirmed that the nitrification reaction is completed in the aerobic tank 7, it is not always necessary to follow this. If the nitrification reaction has already been completed, the DO at the end of the aerobic tank 7 may be 0 mg / L or more and 0.5 mg / L or less at all. As described above, DO cannot be used as a method for confirming completion of the nitrification reaction. As a method for confirming the completion of the nitrification reaction in the aerobic tank 7, there are the following methods.

First, in the aerobic tank 7, the NH ₄ -N concentration measured continuously, by controlling the aeration amount of aerobic tank 7 by measurement of NH ₄ -N, in the aerobic tank 7 ends, NH ₄ -N Reduce the concentration to 0.5 mg / L or less. If the NH ₄ -N concentration is reduced to 0.5 mg / L or less, the nitrification rate is 97.5 to 99% when NH ₄ -N in the sewage (1) is about 20 mg / L to 50 mg / L. It can be judged that the nitrification reaction is almost completed.

  As another method for confirming the completion of the nitrification reaction, there is a method in which ORP is continuously measured in the aerobic tank 7 and the amount of aeration in the aerobic tank 7 is controlled by these measured values. ORP is closely related to the progress of the nitrification reaction, and it can be judged that the nitrification reaction is almost complete if ORP rises to +80 mV or higher (silver / silver chloride standard). Therefore, the aerobic tank 7 may be aerated using the ORP of the aerobic tank 7 as an index. It is not necessary to perform aeration until the ORP is +100 mV (silver / silver chloride standard) or more.

  Regarding the intake of phosphorus in the aerobic tank 7, many relations with the DO concentration in the aerobic tank 7 have been pointed out heretofore. That is, if DO is insufficient in the aerobic tank 7, the intake of phosphorus in the aerobic tank 7 may be hindered. Therefore, the DO concentration at the end of the aerobic tank 7 is generally 1.5 to 3.0 mg. / L is desirable (Non-Patent Document 1). However, the inventors' views are quite different. Oxygen is necessary for the intake of phosphorus, but it is doubtful that the intake of phosphorus by phosphorus-accumulating bacteria is considered good just because there is enough oxygen. In fact, in our experiments, if there are enough phosphorus-accumulating bacteria in the activated sludge, the DO concentration in the aerobic tank 7 is 0.5 mg / L or 2 mg / L. There was no clear difference in phosphorus intake (see FIGS. 5 and 6). Therefore, even if the DO of the aerobic tank 7 is low, it seems that there is no problem with the intake of phosphorus.

  The operation of the oxygen-free tank 6 (denitrification tank) that performs denitrification is performed as follows. In the present invention, since the DO value of the sludge circulating water 18 flowing into the oxygen-free tank 6 (denitrification tank) is 0 to 0.5 mg / L or less, the inhibition of the denitrification reaction by DO is extremely slight. Therefore, the NOx-N removal rate of the oxygen-free tank 6 can be determined by the absolute amount of organic matter because the presence of DO can be ignored. As one means for controlling the anoxic tank (denitrification tank) 6, the ORP of the anoxic tank 6 is controlled to be −150 mV to −200 mV. In order to set ORP to −200 mV or more, the oxygen-free tank 6 is aerated by the blower 12 to raise the ORP. Moreover, in order to make ORP into -150 mV or less, what is necessary is just to add an organic substance and / or a sulfur compound to the anoxic tank 6 (denitrification tank). What is necessary is just to select suitably from methanol, an acetic acid, sodium acetate, etc. as an organic substance. As the sulfur compound, application of sodium thiosulfate, elemental sulfur, etc. can be considered (see formula (7)).

8NO ₃ ⁻ + 5S ₂ O ₃ ²⁻ + H ₂ O → 4N ₂ + 10SO ₄ ²⁻ + 2H ^{+ −−} (7)

Sewage (1) containing organic matter and sulfur compounds may be used.
Next, points to be noted in the final sedimentation basin 9 will be described. The release of phosphorus in the final sedimentation tank 9 is strongly related to the presence of NOx-N. If NOx-N is present, the release of phosphorus does not occur. The inventors believe that the release of phosphorus from the final sedimentation tank 9 in the A ₂ O method is affected not only by the residual amount of DO in the treated water but also by the residual amount of NOx-N. That is, sludge accumulation occurs at the bottom of the final sedimentation basin 9, and due to the influence of sludge accumulation time of several hours (usually about 2 to 4 hours), DO (endogenous respiration) in the sludge, followed by NOx-N Disappears (endogenous denitrification), and as a result of the progress of the anaerobic state, phosphorus is released by autolysis. No matter how much DO is lost, phosphorus release does not occur if only NOx-N remains. In fact, if the NOx-N of the final sedimentation basin inflow water is 5 mg / L or more, the inventors left NOx-N in the return sludge 15 at 1 mg / L or more in the return sludge under the condition that the residence time of the final sedimentation basin 9 is 4 hours. Was. As a result, no release of phosphorus was observed in the final sedimentation basin 9 or in the return sludge 15 line.

  However, on the other hand, if NOx-N contained in the return sludge 15 from the final sedimentation basin 9 to the anaerobic tank 5 remains too much, the release of phosphorus in the anaerobic tank 5 is affected. As a simple means for reducing NOx-N in the return sludge 15, the return sludge 15 and the first settling basin effluent 3 are mixed in advance, and the NOx-N contained in the return sludge 15 is reduced by denitrification reaction. The return sludge 15 and the first settling basin effluent 3 may be flowed into the anaerobic tank 5. Furthermore, providing a third anoxic tank 20 (denitrification tank) that mixes the return sludge 15 and the first settling basin effluent 3 and performs denitrification in front of the anaerobic tank 5 is from the viewpoint of stabilizing the denitrification reaction. This is desirable (see FIG. 3). When the organic matter concentration contained in the first sedimentation basin effluent 3 decreases due to the influence of rainfall or the like and DO exists, the first sedimentation basin effluent 3 alone is effective in reducing NOx-N in the return sludge 15. Absent. In such a case, the organic substance and / or the sulfur compound may be added to the third anoxic tank 20 (denitrification tank) provided in the front stage of the anaerobic tank 5. The degree of progress of the denitrification reaction in the third anoxic tank 20 (denitrification tank) can also be determined using the ORP as described above. What is necessary is just to add the addition amount of an organic substance or a sulfur compound so that ORP of the 3rd oxygen-free tank 20 (denitrification tank) may be set to -150mV (silver / silver chloride electrode reference | standard) or less. If the ORP of the third oxygen-free tank 20 (denitrification tank) is −150 mV or less, it can be determined that NOx—N is almost removed. What is necessary is just to select suitably from methanol, an acetic acid, sodium acetate, etc. as an organic substance. As the sulfur compound, application of sodium thiosulfate, elemental sulfur, etc. can be considered. Sewage (1) containing organic matter and sulfur compounds may be used.

Examples of the present invention will be described below.
Example 1: Application to removal of nitrogen and phosphorus from sewage (A ₂ OA method)
The method of the present invention was applied to municipal sewage treatment, and the stabilization of treated water by the A ₂ OA method proposed this time was examined (see FIG. 2).
As shown in FIG. 2, the device proposed this time includes an initial sedimentation tank 2, an anaerobic tank 5, an oxygen-free tank (denitrification tank) 6, an aerobic tank 7, a second oxygen-free tank 8, and a final sedimentation tank 9. In addition, an apparatus for circulating a part of the sludge in the final sedimentation basin 9 to the anaerobic tank 5 and a part of the activated sludge in the second anaerobic tank 8 in the anaerobic tank (denitrification tank) And a device that circulates to 6. Compared with the conventional A ₂ O method of FIG. 1, the second oxygen-free tank 8 is newly installed, and a part of the activated sludge in the second oxygen-free tank 8 is replaced with the oxygen-free tank ( The point of circulation to the denitrification tank 6 is greatly different. In the conventional A ₂ O method, the DO 17 at the end of the aerobic tank 7 is regarded as important, and is normally operated with a DO control value of 1.5 to 3.0 mg / L. An oxygen-free tank 8 is newly provided, and the point that the DO management value is 0 to 0.5 mg / L is greatly different.

  The processing according to the present invention is performed as follows. First, after the suspended matter of the sewage 1 is removed in the first sedimentation basin 2, the first sedimentation basin effluent 3 includes an anaerobic tank 5, an oxygen-free tank (denitrification tank) 6, an aerobic tank 7, a second oxygen-free tank 8, It flows down the final sedimentation basin 9. Further, a part of the activated sludge in the second oxygen-free tank 8 is circulated to the oxygen-free tank (denitrification tank) 6 using the circulation pump 13. The circulation rate of the activated sludge is 150 V / V% with respect to the settling basin effluent 3 at first. The sludge precipitated in the final sedimentation basin 9 is returned to the anaerobic tank 5 by 50 V / V% with respect to the first sedimentation basin effluent 3 using the return sludge pump 14.

  The hydraulic residence time (HRT) of each tank is 2 hours for the anaerobic tank 5, 4 hours for the anaerobic tank (denitrification tank) 6, 8 hours for the aerobic tank 7, and 30 minutes for the second anaerobic tank 8. did. The HRT of the final sedimentation tank 9 was 4 hours.

An ORP meter 16 was installed in each of the anaerobic tank 5 and the anaerobic tank (denitrification tank) 6 to monitor the release of phosphorus and the progress of the denitrification reaction. In addition, an ORP meter 16 and an NH ₄ -N meter 10 were installed in the aerobic tank 7 to monitor the intake of phosphorus and the progress of nitrification reaction. Furthermore, a DO meter 17 was installed in the second oxygen-free tank 8 to monitor the DO reduction status.

  The water quality of the first sedimentation basin effluent 3 is 106 mg / L on average for BOD, 67 mg / L on average for SS, 32 mg / L on average for TN (total nitrogen) (mostly ammonia nitrogen), TP ( Total phosphorus) was about 4 mg / L on average. Hereinafter, the operation method of each tank is described in detail.

In order to promote the release of phosphorus in the anaerobic tank 5, when the ORP 16 in the anaerobic tank 5 exceeds −270 mV, the organic substance is added from the organic substance adding device 4 using the ORP 16 in the anaerobic tank 5 as an index (in this case, acetic acid). Was added to the anaerobic tank 5 so as to be 30 mg / L-sewage, and the ORP16 of the anaerobic tank 5 was reduced to −270 mV or less. As a result, the PO ₄ -P release concentration in the anaerobic tank 5 was stable at about 10 mg / L to 20 mg / L.

In the anoxic tank (denitrification tank) 6, NOx-N in the sludge circulating water 18 circulated from the second anoxic tank 8 at the subsequent stage is removed. In the conventional A ₂ O method, since the sludge circulating water 18 is normally circulated from the aerobic tank 7, 1.5 to 3.0 mg / L or more of DO is continuously supplied to the anoxic tank (denitrification tank) 6. Brought in. As a result, in the anoxic tank (denitrification tank) 6, organic substances displayed by BOD are preferentially consumed by DO, and as a result, the removal of NOx—N often deteriorates. However, in the present invention, the activated sludge in the oxygen-free tank 8 is circulated to the oxygen-free tank (denitrification tank) 6 at 150 V / V% with respect to the first settling basin effluent 3 by the circulation pump 13. Therefore, DO was brought into the anaerobic tank (denitrification tank) 6, and the phenomenon in which the removal of NOx-N due to the DO brought in did not deteriorate.

  However, in the anaerobic tank (denitrification tank) 6, there was a case where the NOx-N removal rate was reduced due to a shortage of organic substances or excessive organic substances remained, and the phosphorus release phenomenon was confirmed in the anoxic tank 6. Therefore, the following operation was performed using the ORP 16 of the anoxic tank (denitrification tank) 6 as an index. That is, when the ORP 18 of the anaerobic tank (denitrification tank) 6 is −100 mV or higher, in other words, when there is a shortage of organic substances, the organic substance (acetic acid in this case) is removed from the organic substance addition device 4. (Denitrification tank) 6 was added, and the ORP16 of the anoxic tank (denitrification tank) 6 was lowered to −100 mV or less. In addition, when the ORP 16 of the anaerobic tank (denitrification tank) 6 is −200 mV or less, in other words, when excess organic matter remains, the blower 12 is temporarily operated to remove the oxygen-free tank (denitrification tank) 6. The ORP18 was maintained at −200 mV or more. In this way, the ORP 16 of the oxygen-free tank (denitrification tank) 6 was maintained from −100 mV to −200 mV. As a result, the NOx-N concentration at the outlet of the anaerobic tank (denitrification tank) 6 changed from 0 mg / L to 0.5 mg / L, and no increase in phosphorus concentration was observed.

The aerobic tank 7 oxidizes NH ₄ —N to NO ₃ —N and ingests phosphorus. The aerobic tank 7 was provided with a plurality of blowers 12, and the aerobic tank 7 was operated under the following operating conditions. The aeration amount of the blower 12 was controlled from the front part to the middle part of the aerobic tank 7 so that the ORP 16 of the aerobic tank 7 was maintained at +50 mV or more and +70 mV or less. The rear part of the aerobic tank 7 was operated by the blower 12 so that NH ₄ —N in the aerobic tank 7 was maintained at 0.5 mg / L or less. As a result, the PO ₄ -P concentration and NH ₄ -N concentration in the outlet water of the aerobic tank 7 are 0.02 mg / L and 0.25 mg / L, respectively, and phosphorus absorption and nitrification reaction proceed sufficiently. I was able to confirm.

Furthermore, while installing the underwater stirrer 11 in the 2nd oxygen-free tank 8, the 2nd oxygen-free tank 8 was drive | operated only by stirring, measuring DO17 of the 2nd oxygen-free tank 8 continuously. Generally, the oxygen consumption rate in the endogenous breathing state is about 0.12 gO ₂ / MLSS · g / day. Since the activated sludge concentration (MLSS concentration) in each reaction tank is about 2,000 to 3,000 mg / L, it is possible to set the DO of 10 to 15 mg / L to 0 if the HRT is 1 hour. . Since the activated sludge concentration was maintained at about 3,000 mg / L this time, 3 mg / L DO in the aerobic tank 7 outlet water was reduced to 0 under the condition of HRT of 30 minutes. The purpose of the second anoxic tank 8 is to set DO to 0. It is not intended for denitrification. Rather, denitrification should not be promoted to prevent phosphorus release in the final sedimentation basin 9. By leaving the NOx-N concentration at 5 to 10 mg / L, the release of phosphorus in the final sedimentation basin 9 and the return sludge 14 can be prevented within a normal processing time. The actual DO concentration of the outlet water of the second oxygen-free tank 8 was 0 mg / L, and the NOx-N concentration was 5 to 10 mg / L. Note that the oxygen-free tank 6 and the oxygen-free tank 20 are intended for denitrification, and the purpose of the same oxygen-free tank is greatly different.

PO ₄ —P and NH ₄ —N contained in the outlet water of the second anaerobic tank 8 were 0.02 mg / L and 0.25 mg / L, respectively, which were not significantly different from the aerobic tank 7. This means that phosphorus was not eluted in the second oxygen-free tank 8. The DO in the second anoxic tank 8 was maintained at 0 mg / L to 0.5 mg / L or less, but no release of phosphorus was observed. Even if the DO of the second anoxic tank 8 is as low as 0 mg / L to 0.5 mg / L or less, NOx-N is present on average 8 mg / L (5 mg / L to 10 mg / L), It seems that the release of phosphorus could be suppressed due to the presence of NOx-N.

As a result of the operation of the A ₂ OA method as described above, the TN concentration and the TP concentration of the treated water 19 become 7.6 mg / L and 0.40 mg / L on average, respectively. It was possible to remove phosphorus stably.

Example 2: Application to removal of nitrogen and phosphorus from sewage (comparison between inventive method and conventional method)
The method of the present invention was applied to urban sewage treatment, which is likely to be mixed with rainwater, and the treated water quality of the inventive method (A ₃ OA method) and the conventional method (A ₂ O method) that were proposed this time were compared (Figs. 1 and ₂ ). 3).

As shown in FIG. 3, the device proposed this time includes an initial sedimentation basin 2, a third anaerobic tank (denitrification tank) 20, an anaerobic tank 5, an anaerobic tank (denitrification tank) 6, an aerobic tank 7, and a second no It consists of an oxygen tank 8 and a final sedimentation tank 9. Compared with the conventional A ₂ O method of FIG. 1, not only the second oxygen-free tank 8 but also a third oxygen-free tank is provided between the first settling tank 2 and the anaerobic tank 5, and the final It is greatly different in that a device for circulating part of the sludge in the final sedimentation basin 9 to the third oxygen-free tank 8 is provided instead of the device for circulating part of the sludge in the sedimentation basin 9 to the anaerobic tank 5. ing. In the conventional A ₂ O method, the DO 17 at the end of the aerobic tank 7 is regarded as important, and is normally operated with a DO control value of 1.5 to 3.0 mg / L. An oxygen-free tank 8 is newly provided, and the point that the DO management value is 0 to 0.5 mg / L is greatly different.

  The treatment of sewage according to the present invention is carried out as follows. First, after the suspended matter in the sewage 1 is first removed in the settling basin 2, the first settling basin effluent 3 is mixed with the return sludge 15 in the third oxygen-free tank (denitrification tank) 20. Thereafter, the mixed water of the first settling basin effluent 3 and the return sludge 15 flows down the anaerobic tank 5, the anaerobic tank (denitrification tank) 6, the aerobic tank 7, the second anoxic tank 8, and the final settling tank 9. Go. Further, a part of the activated sludge in the second oxygen-free tank 8 is circulated to the oxygen-free tank (denitrification tank) 6 using the circulation pump 13. The circulation rate of the activated sludge is 150 V / V% with respect to the settling basin effluent 3 at first. The sludge precipitated in the final sedimentation basin 9 is returned to the anaerobic tank (denitrification tank) 20 at 50 V / V% with respect to the first sedimentation basin effluent 3 using the return sludge pump 14. Return it.

  The hydraulic retention time (HRT) of each tank is 1 hour for the third anaerobic tank (denitrification tank) 20, 2 hours for the anaerobic tank 5, 4 hours for the anaerobic tank (denitrification tank) 6, and the aerobic tank 7. For 8 hours and the second anoxic tank 8 for 30 minutes. The HRT of the final sedimentation tank 9 was 4 hours.

In the present invention, an ORP meter 16 is installed in each of the anaerobic tank 5, the oxygen-free tank (denitrification tank) 6, and the third oxygen-free tank (denitrification tank) 20 to monitor the progress of phosphorus release and the denitrification reaction. In addition, an ORP meter 16 and an NH ₄ -N meter 10 were installed in the aerobic tank 7 to monitor the intake of phosphorus and the progress of nitrification reaction. Furthermore, a DO meter 17 was installed in the second oxygen-free tank 8 to monitor the DO reduction status. On the other hand, in the conventional method of FIG. 1, an ORP meter 16 was installed in each of the anaerobic tank 5 and the anaerobic tank (denitrification tank) 6, but no particular control was performed. In addition, a DO meter 17 is installed at the end of the aerobic tank 7, and an operation is performed to control the blower air flow using DO as a control index so that DO is 1.5 mg / L or more and 3.0 mg / L or less. It was.

  The water quality of the first sedimentation basin effluent 3 is as follows: BOD average 80 mg / L, SS average 47 mg / L, TN (total nitrogen) average 30 mg / L (mostly ammonia nitrogen), TP ( Total phosphorus) was about 4 mg / L on average. In the case of this sewage, the average BOD / TP was 20 or less, and it was estimated that the release of phosphorus in the anaerobic tank was difficult to occur.

Hereinafter, the operation state of each tank will be described in detail.
In the invention method, the return sludge 15 and the first settling basin effluent 3 were mixed in advance in the third oxygen-free tank 20, and DO and NOx-N were reduced to below the detection limit and supplied to the anaerobic tank 5. In the 3rd anoxic tank 20, ORP was made into the parameter | index, and organic substance was added at -270 mV or more. Acetic acid was used as the organic substance, and the addition amount was 30 mg / L-sewage. The ORP of the third oxygen-free tank 20 changed from −250 to −320 mV.

The ORP 16 in the anaerobic tank 5 changed from −270 mV to −350 mV throughout the year, and in the anaerobic tank 5, the necessity for adding the organic substance 4 hardly occurred. In addition, the PO ₄ -P release concentration in the anaerobic tank 5 was stable at about 10 mg / L to 20 mg / L.

On the other hand, in the conventional method, the ORP 16 of the anaerobic tank 5 changed in the range of −180 mV to −280 mV, and particularly increased during rainfall. As a result, the PO ₄ -P release concentration in the anaerobic tank 5 was about 3 mg / L to 15 mg / L, which was a considerably low value compared to the inventive method.

In the anaerobic tank (denitrification tank) 6, NOx-N in the sludge circulating water 18 circulated from the subsequent anaerobic tank 8 is removed. In the conventional A ₂ O method, since the sludge circulating water 18 is circulated from the aerobic tank 7, 1.5. ˜3.0 mg / L of DO was usually contained, and this DO was continuously brought into the anoxic tank (denitrification tank) 6. As a result, in the anaerobic tank (denitrification tank) 6, organic substances displayed by BOD are preferentially consumed by DO, and as a result, the ORP often rises to −100 mV or more, and the removal of NOx—N deteriorates. A case was seen.

  However, since the second oxygen-free tank 8 is installed in the present invention, the DO of the sludge circulating water 18 is 0 to 0.5 mg / L or less. Therefore, the ORP of the anaerobic tank (denitrification tank) 6 was maintained at about -150 mV to -200 mV, and the phenomenon that the removal of NOx-N by bringing DO in did not deteriorate.

The aerobic tank 7 oxidizes NH ₄ —N to NO ₃ —N and ingests phosphorus. The aerobic tank 7 is provided with a plurality of blowers 12 in both the invention method and the conventional method, and the aerobic tank 7 was operated under the following operating conditions.

In the invention method, the aeration amount of the blower 12 was controlled from the front part to the middle part of the aerobic tank 7 so that the ORP 16 of the aerobic tank 7 was maintained at +50 mV or more and +70 mV or less. Further, the aeration amount of the blower 12 is controlled by the blower 12 at the rear portion of the aerobic tank 7 so that the ORP16 at the end of the aerobic tank 7 is maintained at +80 mV or more and +100 mV or less, and NH ₄ -N is 0.5 mg / N. It was operated so as to be maintained below L. As a result, the PO ₄ -P concentration and the NH ₄ -N concentration in the outlet water of the aerobic tank 7 are 0.05 mg / L (0.01 to 0.2 mg / L) on average and 0.20 mg / L (0 on average), respectively. .1 to 0.5 mg / L), and it was confirmed that phosphorus absorption and nitrification were sufficiently progressing. On the other hand, in the conventional method of FIG. 1, the aeration of the blower 12 from the front part to the rear part of the aerobic tank 7 is performed so that the DO 17 at the end of the aerobic tank 7 is maintained at +1.5 mg / L to 3.0 mg / L. The amount was controlled. However, it was difficult to control DO only by controlling the amount of aeration during the rainy season, and DO17 was likely to rise, reaching 6 mg / L. In addition, the PO ₄ -P concentration in the outlet water of the aerobic tank 7 changed from 0.5 to 2 mg / L (average 0.8 mg / L), and an unstable result was obtained. It was estimated that this main cause was due to insufficient release of PO ₄ -P in the anaerobic tank 5. Furthermore, the NH ₄ —N concentration was 1.20 mg / L (0.5 to 2 mg / L) on average, and although the DO was high, there was a case where the nitrification reaction did not proceed sufficiently.

Furthermore, in the invention method, the second anaerobic tank 8 was installed in the second anaerobic tank 8 and the second anaerobic tank 8 was operated only with stirring while continuously measuring the DO 17 of the second anaerobic tank 8. Generally, the oxygen consumption rate in the endogenous breathing state is about 0.12 gO ₂ / MLSS · g / day. Since the activated sludge concentration (MLSS concentration) in each reaction tank is about 2,000 to 3,000 mg / L, it is possible to set the DO of 10 to 15 mg / L to 0 if the HRT is 1 hour. . Since the activated sludge concentration was maintained at about 3,000 mg / L this time, the DO of 1-2 mg / L in the aerobic tank 7 outlet water was reduced to 0 under the condition of HRT of 10 minutes. PO ₄ -P and NH ₄ -N contained in the outlet water of the second oxygen-free tank 8 were 0.05 mg / L and 0.80 mg / L, respectively, which were the same as the aerobic tank 7. The DO in the second anoxic tank 8 was maintained at 0 mg / L to 0.2 mg / L or less, but no release of phosphorus was observed. Even if the DO of the second oxygen-free tank 8 is as low as 0 mg / L to 0.2 mg / L or less, NOx-N is present on average 7.5 mg / L (from 6 mg / L to 9 mg / L). This is probably because the release of phosphorus could be suppressed due to the presence of NOx-N.

In the final sedimentation tank 9, the sludge that has flowed out from the second oxygen-free tank 8 is settled and separated from the treated water 19. The settled sludge is returned to the third oxygen-free tank (denitrification tank) 20 by the return sludge pump 14 as the return sludge 15. The NOx-N concentration in the return sludge 15 decreased due to the endogenous denitrification reaction in the final sedimentation basin 9, but remained up to an average of 4 mg / L (2 mg / L to 6 mg / L). Since NOx-N was never depleted, no release of PO ₄ -P was seen. Furthermore, NOx-N contained in the return sludge 15 was completely removed by the third anoxic tank (denitrification tank) 20, and the inflow to the anaerobic tank 5 could be ignored.

As described above, nitrogen and phosphorus can be stably removed as shown in Table 5 if the present invention method is applied even in an area where the conventional method (A ₂ O method) cannot be applied. .

Example 3 Examination of Relationship between DO and Phosphorus Intake in Aerobic Tank 7 From the anaerobic tank 5 of FIG. 2, 60 L of activated sludge was extracted and put into two 100 L tanks (not shown). In one series, the initial DO is 0.4 to 0.5 mg / L or less, and in the other one series, stirring and aeration are performed to maintain DO at 2.0 to 2.5 mg / L, and the water quality changes over time. Tracked. FIG. 5 shows the results of the series maintaining DO at 0.4 to 0.5 mg / L or less, and FIG. 6 shows the results of the series maintaining DO at 2.0 to 2.5 mg / L. The water temperature was 25 ° C. and MLSS was 1,500 mg / L. From this result, ingestion of PO ₄ -P in the aerobic tank was not inhibited even when DO was 0.4 to 0.5 mg / L. For this reason, it was thought that it was not necessary to dare to maintain DO of an aerobic tank at 1.5 mg / L or more like a conventional standard. Moreover, it took about 4 to 5 hours for the intake of phosphorus irrespective of DO.

We are a process flow of a conventional A ₂ O process. It is a process flow of the A ₂ O variant according to the invention (A ₂ OA method). It is a process flow of the A ₂ O variant according to the invention (A ₃ OA method). ₂ is a process flow of an A ₂ O modification according to the present invention. It is a time-dependent change of the water quality in an aerobic tank (initial DO = 0.4-0.5 mg / L control). It is a time-dependent change of the water quality in an aerobic tank (initial DO = 2.0-2.5 mg / L control).

Explanation of symbols

1 Sewage 2 First sedimentation basin 3 First sedimentation basin effluent 4 Organic substance addition device 5 Anaerobic tank 6 Anoxic tank (denitrification tank)
7 Aerobic tank 8 Second oxygen-free tank 9 Final sedimentation tank 10 NH ₄ -N concentration meter 11 Submerged agitator 12 Blower 13 Sludge circulation pump 14 Return sludge pump 15 Return sludge 16 ORP meter 17 DO meter 18 Sludge circulation water 19 Treated water 20 Third oxygen-free tank (denitrification tank)

Claims (10)

Biological nitrogen and phosphorus consisting of a first sedimentation tank, anaerobic tank, anaerobic tank (denitrification tank), aerobic tank, and final sedimentation tank that circulates activated sludge at the end of the aerobic tank to the anoxic tank (denitrification tank) In the removal process (A ₂ O method), a second oxygen-free tank is provided after the aerobic tank, and the activated sludge at the end of the second oxygen-free tank is circulated to the oxygen-free tank (denitrification tank) in the preceding stage. In addition, a method for removing nitrogen and phosphorus from sewage, wherein the sludge circulation from the end portion of the aerobic tank to the preceding oxygen-free tank (denitrification tank) is stopped.   An underwater stirrer is installed at the bottom of the second anaerobic tank after the aerobic tank to stir the activated sludge and reduce DO in the activated sludge to 0.5 mg / L or less. The method described. Biological nitrogen and phosphorus consisting of a first sedimentation tank, anaerobic tank, anaerobic tank (denitrification tank), aerobic tank, and final sedimentation tank that circulates activated sludge at the end of the aerobic tank to the anoxic tank (denitrification tank) In the removal process (A ₂ O method), by reducing aeration at the end of the aerobic tank through which the activated sludge is circulated, the DO in the sludge circulating water is 0.5 mg / L or less and the bottom of the end of the aerobic tank A method of removing nitrogen and phosphorus from sewage, characterized by installing an underwater stirrer and stirring activated sludge.   The endogenous respiration of activated sludge is used as means for lowering DO at the end of the second anaerobic tank or the aerobic tank to 0.5 mg / L or less after the aerobic tank. 4. The method according to any one of items 3. In aerobic tank, an NH ₄ -N concentration measured continuously, by controlling the aeration of the aerobic tank by measurement of NH ₄ -N, the NH ₄ -N concentration of the sludge circulation water 0.5 mg / L The method according to claim 1, wherein the method is reduced to: After mixing the return sludge and the first settling basin effluent and reducing DO and NOx-N (the sum of NO ₂ -N and NO ₃ -N) to below the detection limit, let this mixed water flow into the anaerobic tank. A method according to any one of claims 1 to 5, characterized. The return sludge and primary sedimentation effluent water were mixed, as a means of reducing the DO and NOx-N (sum of NO ₂ -N and NO ₃ -N) to below the detection limit, the third anoxic tank (denitrification tank) The method according to claim 6, wherein the step is provided in front of the anaerobic tank. DO and NOx-N (sum of NO ₂ -N and NO ₃ -N) in the anaerobic tank inflow water by adding at least one of organic substances and sulfur compounds to the third anaerobic tank provided in the front stage of the anaerobic tank The method according to claim 6 or 7, characterized in that is reduced to below the detection limit.   A first sedimentation tank, an anaerobic tank, an oxygen-free tank (denitrification tank), an aerobic tank, a second oxygen-free tank, and a final sedimentation tank are provided in this order, and a part of the activated sludge in the final sedimentation tank is further added to the anaerobic tank. A biological nitrogen from sewage, and a device for circulating a part of the activated sludge in the second oxygen-free tank to the oxygen-free tank (denitrification tank) Phosphorus removal equipment.   A third oxygen-free tank is further provided between the first settling basin and the anaerobic tank, and instead of a device that circulates part of the activated sludge in the final settling basin to the anaerobic tank, An apparatus for removing biological nitrogen and phosphorus from sewage, comprising an apparatus for circulating a part of activated sludge to the third oxygen-free tank.

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