JP2006136820A - Method for removing phosphorus and/or nitrogen from sewage - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To remove nitrogen and phosphorus efficiently and stably from sewage. <P>SOLUTION: In a process for removing biological phosphorus and/or nitrogen by using an anaerobic tank, an oxygen-free tank and an aerobic tank, an organic acid is loaded into the anaerobic tank by using the value of ORP in the anaerobic tank as an indicator and nitrogen and phosphorus are stably removed. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for stably and efficiently removing phosphorus and / or nitrogen contained in sewage.

  First, the prior art regarding phosphorus removal will be described.

  The total phosphorus concentration in city sewage is usually about 3 to 5 mg / L, and the following methods are known as methods for removing phosphorus in sewage.

  (1) Coagulation precipitation method: Coagulation precipitation methods using iron salts such as ferric chloride and ferric sulfate, and coagulants such as aluminum sulfate and polyaluminum chloride are widely known. Although it is the most reliable method, there is a problem that running costs such as treatment costs and chemical costs accompanying an increase in the amount of excess sludge increase due to the addition of the flocculant.

  (2) Biological phosphorus removal process: Some bacterial groups in activated sludge (aggregates of microorganisms) (hereinafter referred to as polyphosphate-accumulating bacteria) are under anaerobic conditions (dissolved oxygen is also bound by nitrate ions, etc.) When phosphorus is released in the absence of oxygen), it tends to overtake phosphorus under aerobic conditions. Utilizing this property, phosphorus removal is achieved. When such a method is adopted, the concentration of phosphorus in the activated sludge of the sewage is about 2 to 3 to 6% as the polyphosphate-accumulating bacteria ingest phosphorus. It is said that the phosphorus concentration in the sewage will decrease. The biological phosphorus removal process is in practical use in the field of municipal sewage treatment.

  Hereinafter, a conventional method for removing biological phosphorus and nitrogen related to the present invention will be described.

  The biological phosphorus removal process generally consists of an initial settling tank, a reaction tank (anaerobic tank and an aerobic tank), and a final settling tank. The first sedimentation basin removes coarse suspended solids in the sewage and reduces the organic load on the reaction tank. The reaction tank is composed of an anaerobic tank and an aerobic tank. The anaerobic tank is subjected to anaerobic conditions to release phosphorus from polyphosphate-accumulating bacteria in the activated sludge. Furthermore, in the aerobic tank, polyphosphate-accumulating bacteria are caused to ingest phosphorus in excess of the released amount. In the final sedimentation basin, the activated sludge is settled and the supernatant liquid is discharged. A part of the concentrated activated sludge separated and separated in the final sedimentation basin is pulled off as excess sludge and returned to the anaerobic tank by a return pump as return sludge. Since excess sludge contains phosphorus in a high concentration due to polyphosphate-accumulating bacteria, phosphorus contained in sewage is extracted out of the system in the form of excess sludge.

  The biological phosphorus removal process has the advantage of lower running costs compared to the above-described coagulation precipitation method.

  Next, a conventional technique related to nitrogen removal will be described.

  As a method for removing nitrogen from sewage, the following biological nitrification-nitrogen removal process is inexpensive and widely used. This method consists of biological oxidation reaction (nitrification reaction) by nitrifying bacteria such as Nitrobomoter, which is an absolute aerobic autotrophic bacterium, and denitrifying bacteria such as Pseudomonas, which is a facultative anaerobic heterotrophic bacterium. It consists of a combination with a chemical reduction reaction (denitrification reaction).

First, the nitrification reaction will be described. In the nitrification reaction, ammonia nitrogen (hereinafter referred to as NH ₄ -N) is converted into nitrite nitrogen (hereinafter referred to as NO ₂ -N) using nitrifying bacteria in the presence of dissolved oxygen, that is, under aerobic conditions. Or nitrate nitrogen (hereinafter referred to as NO ₃ -N). The nitrification reaction consists of a two-stage reaction represented by the following chemical formula 1 and chemical formula 2, and the types of nitrifying bacteria involved are different. That is, the reaction shown in Chemical Formula 1 is brought about by ammonia oxidizing bacteria having Nitrosomonas as a representative species, and the reaction shown in Chemical Formula 2 is brought about by nitrite oxidizing bacteria having Nitrobacter as a representative species.

  In order to remove nitrogen, it is extremely important to grow nitrifying bacteria in large quantities in activated sludge. However, nitrifying bacteria have a low growth rate compared to heterotrophic bacteria that decompose organic substances, and are susceptible to activity inhibition due to component fluctuations in sewage and wastewater. The growth of nitrifying bacteria is usually managed by using activated sludge residence time (SRT) and dissolved oxygen concentration (DO: Dissolved Oxygen) in an aerobic tank.

Next, the denitrification reaction will be described. NO ₂ —N and NO ₃ —N produced by the nitrification reaction are reduced by denitrifying bacteria in the activated sludge in the presence of oxygen-free and organic substances as shown in the following chemical formula 3 and chemical formula 4, and oxidized. Nitrogen gas (N ₂ O) or nitrogen gas (N ₂ ) is released into the atmosphere.

  In order to perform denitrification stably, the following two points are extremely important.

(1) No dissolved oxygen (anoxic condition): Denitrifying bacteria preferentially use dissolved oxygen when dissolved oxygen is present. Therefore, _NO 2 -N and _NO 3 -N is apt to remain.

  (2) A sufficient organic substance (hydrogen donor) is present: a sufficient hydrogen donor is necessary for denitrification. As a hydrogen donor, in municipal sewage and the like, organic matter (hereinafter also referred to as BOD) component in sewage is used as it is, and in wastewater not containing organic matter, methanol or the like is often added from the outside. In the case of BOD, the BOD concentration (mg / L) is required to have a BOD / N ratio of 3 or more (N is a nitrogen element) with respect to the nitrogen concentration (mg / L). Also, the type of organic matter is important and greatly affects the denitrification rate.

The biological nitrification-nitrogen removal process consists of an initial settling tank, a reaction tank (anoxic tank, and an aerobic tank), and a final settling tank. The first settling basin removes coarse suspended solids in the sewage and reduces the organic load in the reactor. The reaction tank consists of an anoxic tank and an aerobic tank. A part of the nitrification liquid (treated water in the aerobic tank) is circulated and returned from the aerobic tank to the anoxic tank (the state without dissolved oxygen). NO ₂ —N and NO ₃ —N in the nitrification solution are removed as nitrogen gas by a denitrification bacterium in the activated sludge and a denitrification reaction with organic matter in the sewage. In the aerobic tank, NH ₄ -N is oxidized (nitrification) by nitrifying bacteria in the activated sludge, and a part of the nitrifying liquid is circulated to the anoxic tank. In the final sedimentation basin, the activated sludge is settled and the supernatant liquid is discharged. The concentrated activated sludge separated and separated is pulled back as excess sludge and returned to the anoxic tank by a pump as return sludge.

The biological phosphorus / nitrogen removal process is a process in which the biological phosphorus removal process and the biological nitrification-nitrogen removal process described above are combined, and the first settling tank, reaction tank (anaerobic tank, anoxic tank, and Aerobic tank), and final sedimentation basin. The first settling basin removes coarse suspended solids in the sewage and reduces the organic load in the reactor. The reaction tank consists of an anaerobic tank, an oxygen-free tank, and an aerobic tank. In the anaerobic tank, as described above, the polyphosphate-accumulating bacteria in the activated sludge discharges phosphorus. A part of the nitrification liquid is circulated back to the anaerobic tank from the aerobic tank. In the anoxic tank, NO ₂ —N and NO ₃ —N in the nitrification liquid are removed as nitrogen gas by denitrification bacteria with activated organic sludge and denitrification reaction with organic matter in sewage.

On the other hand, in the aerobic tank, NH ₄ -N is oxidized (nitrification) by nitrifying bacteria in the activated sludge, and a part of the nitrifying liquid is circulated to the anaerobic tank. In addition, polyphosphate-accumulating bacteria in activated sludge ingest phosphorus excessively. In the final sedimentation basin, activated sludge is settled and the supernatant is discharged. The concentrated activated sludge separated and separated is pulled back as excess sludge and returned to the anaerobic tank by a return pump as return sludge. Since the excess activated sludge contains phosphorus in a high concentration due to the polyphosphate-accumulating bacteria, phosphorus contained in the sewage is extracted out of the system in the form of excess sludge.

  The biggest challenge of the biological phosphorus removal process is instability. For example, contamination with sewage such as rainwater reduces the concentration of organic matter in sewage and increases DO, and anaerobic conditions are not achieved in the anaerobic tank, and phosphorus release is suppressed. Once the release phenomenon of phosphorus is suppressed in the anaerobic tank, the ability of excessive intake of phosphorus in the aerobic tank is reduced. A large amount of BOD is required in the sewage in order to successfully release phosphorus in the anaerobic tank, and the BOD (mg / L) / TP (total phosphorus: mg / L) concentration ratio of the anaerobic tank inflow water is If it is 20-25 or more, it is said that the release | release of phosphorus is favorable (nonpatent literature 1, nonpatent literature 2).

  Furthermore, regarding the excessive intake of phosphorus in the aerobic tank, many relations with DO have been pointed out. Since phosphorus intake is inhibited when DO is insufficient, the DO at the end of the aerobic tank is generally preferably 1.5 to 2.0 mg / L (Non-patent Document 1) or 1.5 to 3.0 mg / L. (Non-Patent Document 2).

However, even if the BOD / TP ratio of the anaerobic tank inflow water is 25 or more, there is a case where phosphorus release in the anaerobic tank is poor and a large amount of phosphorus remains in the treated water. The following factors can be considered as the cause. (I) _NO 2 -N and _NO 3 -N from sewage and return sludge rainwater is mixed (hereinafter, described as NOx-N) or flowing into the anaerobic tank dissolved oxygen. (II) Backflow of NOx-N or dissolved oxygen from the nitrification solution returned to the anoxic tank to the anaerobic tank. (III) Entrainment of dissolved oxygen by excessive stirring in an anaerobic tank. (IV) The hydraulic residence time (henceforth HRT) shortening by generation | occurrence | production of the short circuit flow in an anaerobic tank etc. are mentioned.

  The conditions of these anaerobic tanks vary greatly depending on the sewage treatment plant. Therefore, even if the BOD / TP ratio of the anaerobic tank inflow water is 25 or more, the release of phosphorus in the anaerobic tank is not always good. It seems not.

  For these reasons, it is important to remove phosphorus by simply monitoring the release status of phosphorus in an anaerobic tank and developing a countermeasure. There are the following examples of reports on this issue. For example, as a method for simply monitoring the state of phosphorus release in an anaerobic tank, there is an example focusing on a redox potential (hereinafter referred to as ORP, silver / silver chloride electrode reference) value. For example, the ORP value of the anaerobic tank is closely related to the phosphorus discharge phenomenon, and if the ORP value of the anaerobic tank of the biological phosphorus removal process exceeds -270 mV, sewage containing a large amount of organic matter (first sedimentation tank inflow) Water) or first sedimentation basin sediment sludge is allowed to flow into the anaerobic tank and the ORP value of the anaerobic tank is maintained at −270 mV or less (Patent Document 1).

  Also, the ORP value of the sewage (first sedimentation basin inflow water) and the first sedimentation basin effluent is measured, and the mixing ratio of the sewage (first sedimentation basin inflow water) and the first sedimentation basin effluent according to the measured value of this ORP value. And a method of maintaining the ORP value of the anaerobic tank at −270 mV or less has been proposed (Patent Document 2).

  However, the method using the first sedimentation basin inflow water as in Patent Document 1 not only makes the organic matter concentration of the first sedimentation basin inflow water itself unstable, but also greatly fluctuates the organic matter load and nitrogen load in the reaction tank. It is quite difficult to control the removal of BOD components and nitrogen in wastewater.

Next, the problem of the biological nitrification-nitrogen removal process will be described. For nitrogen removal, oxygen consumption of 4.57 kgO ₂ / kgNH ₄ —N occurs from Chemical Formula 1 and Chemical Formula 2 with the nitrification reaction. Usually, in order to promote the nitrification reaction in the aerobic tank, it is usually required and managed that 1.5 mg / L of dissolved oxygen remains (Non-Patent Document 1 and Non-Patent Document 2). For this reason, since the amount of aeration is increased and the amount of power consumed by the blower is large, there is a strong demand for measures to reduce the energy consumption within a range where the nitrification reaction can be maintained.

  Further, regarding the biological nitrification-nitrogen removal process, the denitrification reaction may be difficult to proceed due to the lack of organic matter, as in the case of phosphorus removal. In order to facilitate the denitrification reaction, the BOD (mg / L) / TN (total nitrogen, mg / L) concentration ratio of sewage is desirably about 3 or more. When the organic matter in the sewage is insufficient and the denitrification reaction does not proceed, for example, a method of adding methyl alcohol as an organic matter to the anoxic tank is widely known (Non-patent Document 1). Even if the L) / TN (mg / L) concentration ratio is 3 or more, there are cases in which denitrification in the anoxic tank is poor and nitrogen remains in the treated water. For example, there are causes such as inhibition of denitrification due to excessive DO of the nitrifying liquid returned to the anoxic tank, excessive circulation amount, shortening of HRT due to occurrence of a short circuit flow in the anoxic tank, and the like. Therefore, even if the sewage BOD (mg / L) / TN (mg / L) concentration ratio is generally 3 or more, the removal of nitrogen in the anoxic tank is not always good.

Therefore, it is important how to easily monitor the denitrification situation in the anoxic tank and develop a countermeasure.
JP-A-3-278893 JP 2003-053384 A Advanced treatment facility design manual, p225-252, Japan Sewerage Association, 1994 Anaerobic-anoxic-aerobic operation management manual (draft), Tokyo Sewerage Service, March 1997, p21-p53

  To summarize the above problems, the biological phosphorus removal process consists of a reduction in organic matter concentration in sewage, a decrease in DO at the end of the aerobic tank, a decrease in the BOD / TP concentration ratio of the anaerobic tank inflow water, an anaerobic tank The removal of phosphorus may be hindered by a number of unstable factors such as an increase in NOx-N concentration, an increase in DO in the anaerobic tank, and a short circuit of HRT in the anaerobic tank. In addition, it is difficult to monitor many unstable elements.

  On the other hand, nitrogen removal is a reduction in organic matter concentration in sewage, a decrease in DO at the end of the aerobic layer, a decrease in the BOD / TN concentration ratio of sewage, an increase in DO in the anaerobic layer, and Nitrogen removal may be hindered by many unstable factors such as short circuits and increased circulation in anoxic tanks. In addition, it is difficult to monitor many unstable elements. Furthermore, there is a problem that power consumption by the blower increases.

  The biological phosphorus / nitrogen removal process from sewage has both the above-mentioned problems of phosphorus removal and nitrogen removal, but there are many cases where the optimal conditions for nitrogen removal and the optimal conditions for phosphorus removal are often contradictory. Is more difficult.

  The present invention provides a biological removal method capable of stably removing phosphorus and / or nitrogen from sewage by simply monitoring phosphorus removal in an anaerobic tank and denitrification in an anaerobic tank. The purpose is to provide.

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

  (1) Each sewage treatment tank of the first settling basin, anaerobic tank, aerobic tub and final basin, or each sewage treatment tank of the first settling basin, anaerobic tub, anaerobic tank, aerobic tub and final basin was used. In the biological sewage treatment method, the organic acid is added to the anaerobic tank so that the ORP value of the anaerobic tank is maintained in the range of −400 mV to −200 mV.

  (2) The organic acid is added to the anaerobic tank so that 50% or more of the cumulative frequency of the ORP value of the anaerobic tank is maintained in the range of −350 mV to −250 mV. Is a sewage treatment method.

  (3) Use each sewage treatment tank of the first sedimentation basin, anoxic tank, aerobic tank and final sedimentation basin, or each sewage treatment tank of the first sedimentation basin, anaerobic tank, anaerobic tank, aerobic tank and final sedimentation basin In the biological sewage treatment method, an organic acid is added to the anaerobic tank so that the ORP value of the anaerobic tank is maintained in the range of −200 mV to −100 mV. At least one selected from the group consisting of adjusting the amount of nitrating liquid circulating to the oxygen tank, adjusting the ORP value of the aerobic tank, and adjusting the DO of the aerobic tank is performed. This is a method for treating sewage.

  (4) adding the second organic acid to the anaerobic tank so that 50% or more of the cumulative frequency of the ORP value of the anoxic tank is maintained in a range of −200 mV to −150 mV, the aerobic At least selected from the group consisting of adjusting the circulation rate of the nitrification liquid from the tank to the anaerobic tank, adjusting the ORP value of the aerobic tank, and adjusting the DO of the aerobic tank The sewage treatment method according to 3), wherein one type is performed.

(5) The amount of the organic acid added to the anaerobic tank is adjusted so that the PO ₄ -P concentration in the anaerobic tank is 10 mg / L, and NOx− in the sewage and return sludge flowing into the anaerobic tank The amount of the organic acid consumed per unit time by N concentration and DO is calculated, and the amount of the organic acid added to the anaerobic tank is increased, (1) to (4), Sewage treatment method.

  (6) The aeration amount by the blower of the aerobic tank is adjusted so that the ORP value of the aerobic tank becomes +70 mV or more and +100 mV or less. The sewage according to any one of (1) to (5) This is a sewage treatment method.

  (7) Any of (1) to (6), wherein the aeration amount by the blower of the aerobic tank is adjusted so that DO of the aerobic tank is 0.5 mg / L or more and 1.5 mg / L or less. This is a sewage treatment method described in the above.

  (8) The sewage treatment method according to any one of (1) to (7), further including a second anaerobic tank and a second aerobic tank in order after the aerobic tank.

  (9) The sewage treatment method according to any one of (1) to (8), wherein the microorganism-immobilized carrier is introduced into one or more of an anaerobic tank, an oxygen-free tank, and an aerobic tank. is there.

  (10) The sewage treatment method according to any one of (1) to (9), wherein the organic acid added to at least one of the anaerobic tank and the anaerobic tank is acetic acid and / or acetate. is there.

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

  An example of the processing flow of the present invention is shown in FIGS. FIG. 1 is the biological phosphorus removal process of the present invention. The reaction tank is composed of an anaerobic tank 4 and an aerobic tank 6. FIG. 2 is the biological nitrification-nitrogen removal process of the present invention. The reaction tank is composed of an oxygen-free tank 5 and an aerobic tank 6. FIG. 3 shows the biological phosphorus / nitrogen removal process of the present invention having a denitrification function in addition to a phosphorus removal function. The reaction tank includes an anaerobic tank 4, an oxygen-free tank 5, and an aerobic tank 6.

  The biological phosphorus / nitrogen removal process includes the functions of the biological phosphorus removal process of FIG. 1 and the biological nitrification-nitrogen removal process of FIG. The embodiment of the present invention will be described using a nitrogen removal process as an example.

  First, coarse suspended matters (mainly sludge) contained in the sewage 1 are settled and removed in the first settling basin 2. Thereafter, the first settling basin effluent 3 flows into the anaerobic tank 4.

(Anaerobic tank)
The anaerobic tank 4 is managed as follows. First, the mechanism of biological phosphorus removal will be briefly described. The polyphosphate-accumulating bacterium that performs biological phosphorus removal retains PO ₄ -P absorbed under aerobic conditions as granules of polyphosphate in cells, and in the anaerobic tank 4, the polyphosphate of the granules is retained. It was hydrolyzed with release as PO 4 _-P, organic matter in sewage 1, especially ingested organic acid 20 and fermentation product preferentially to the cell. The release rate of PO ₄ -P varies greatly depending on the type and concentration of the substrate, and it is said that the release rate of PO ₄ -P is high when the organic acid 20 such as acetic acid is the substrate. Organic substances taken into cells are stored in the form of polymer substances such as glycogen and PHB (polyhydrobutyrate). These intracellular substances are reduced by oxidative degradation when the polyphosphate-accumulating bacteria are put under aerobic conditions, but the polyphosphate-accumulating bacteria grow by utilizing this substrate. In addition, since the polyphosphate-accumulating bacteria excessively take up PO ₄ -P more than the amount released in the anaerobic tank 4 and hold it as granules of polyphosphate in the cells, the PO ₄ -P in the sewage 1 decreases.

What is important in promoting the reaction of polyphosphate-accumulating bacteria that perform such biological phosphorus removal is the presence of organic acid 20 and fermentation products. These organic acids 20 and fermentation products contain BOD components. However, just because the sewage 1 has a high BOD concentration, the organic acid 20 and the fermentation product concentration are not necessarily high. Therefore, it is more effective to use the concentration (mg / L) of the organic acid in the sewage 1 as an index than the BOD concentration. In the anaerobic tank 4, there are many research examples regarding the types and required amounts of organic substances required for phosphorus release. When the desired release amount of phosphorus is 1, the amount of organic acid 20 to be added is in molar ratio. There is a report that it is preferably about 1.3 (for example, The effects of organic compounds on biological phosphorous removal, Water Science Technology, 1991, Vol. 23, No. 4/6, p585-p591). Therefore, the number of the organic acid 20 added to the anaerobic tank 4 can be referred to this known number. For example, when the molar ratio of acetic acid utilization to phosphorus release is 1.3, in order to promote 10 mg / L PO ₄ -P release in the anaerobic tank 4, the amount of acetic acid required is 25 mg / L per anaerobic tank volume. It becomes. However, in actual treatment equipment, NOx-N and dissolved oxygen often flow into the anaerobic tank 4 due to the influence of rainwater and return sludge 8, and NOx-N and dissolved oxygen exist in the anaerobic tank 4. The organic acid 20 is immediately decomposed. The reaction when acetic acid is used as the organic acid 20 is shown below.

From Chemical Formulas 5 to 7, for example, when NO ₃ -N is present at 1 mg / L, acetic acid 2.7 mg / L is consumed accordingly. Further, when NO ₂ —N is present at 1 mg / L, acetic acid 1.6 mg / L is consumed accordingly. On the other hand, when 1 mg / L of dissolved oxygen is present, acetic acid 0.9 mg / L is consumed accordingly. From this result, it can be seen that the influence of the presence of NO ₃ -N particularly on the consumption of acetic acid is extremely large.

As a result of investigation, NO ₃ -N, NO ₂ -N, and dissolved oxygen often exist in the sewage 1 and the return sludge 8 mixed with rainwater, but in particular, depending on the operating conditions of the aerobic tank 6. It was found that high concentration of NO ₃ -N exists in the return sludge 8 returned from the final sedimentation tank 7 to the anaerobic tank 4. In this case, there is a possibility that release of phosphorus is extremely difficult to occur in the anaerobic tank 4. For example, even if the concentration of acetic acid flowed come sewage was 20 mg / L, the return sludge 8 containing _NO 3 -N 10mg / L is mixed 50 V / V% relative to sewage 1, _NO 3 -N 13 Consumes 5 mg / L of acetic acid.

  Moreover, not only the consumption of the organic acid 20 by NOx-N or dissolved oxygen in the sewage 1 or the returned sludge 8 but also apparatus characteristics such as backflow from the anoxic tank may be affected. Therefore, it is difficult to judge the release of phosphorus in the anaerobic tank 4 only by the concentration of the organic acid 20 in the sewage 1, and the factors (organic acid 20, NOx, which are involved in the discharge of phosphorus in the anaerobic tank 4). -N concentration, DO, device characteristics, etc.) are all known in advance, and it is considered difficult to control the amount of organic acid 20 added.

  Therefore, the inventors have devised a method of adding the organic acid 20 while monitoring the anaerobic tank 4 with the ORP meter 13, and maintain the ORP value of the anaerobic tank 4 at −200 mV or lower and −400 mV or higher, preferably the ORP value. The method of adding the organic acid 20 to the anaerobic tank 4 was developed so that 50% or more of the measured cumulative frequency was maintained at −250 mV or less and −350 mV or more.

  An example of a specific method for adding the organic acid 20 by controlling the ORP value is as follows. When the ORP value of the anaerobic tank 4 is −250 mV or more, the organic acid 20 is anaerobically removed from the organic acid tank 11 by the chemical injection pump 12. By adding to the tank 4 and stopping when the ORP value of the anaerobic tank 4 reaches a target value, the ORP value of the anaerobic tank 4 is operated so as to be within −200 mV or less and −400 mV or more.

  More specifically, the cumulative frequency of the ORP value in the anaerobic tank 4 is measured for a certain period, and monitoring is performed by the ORP meter 13 in the anaerobic tank 4, and the organic acid 20 is adjusted so that the ORP value is −200 mV or less and −400 mV or more. Added. Furthermore, it is preferable to add the organic acid 20 so that 50% or more of the cumulative frequency of the ORP value is maintained in the range of −350 mV to −250 mV. As for the collection frequency of the cumulative frequency, data is collected every 1 to 10 minutes every day, this is analyzed, and the cumulative frequency of ORP values per fixed period is calculated. A detailed example of the calculation of the cumulative frequency will show an automatic data collection device (for example, Keyence Corporation) at an arbitrary time interval set between 1 minute and 10 minutes with the ORP value measured by the ORP meter 13 of the anaerobic tank 4 Model GR3500), and the ORP value of the raw data belonging to a predetermined period for which the cumulative frequency is to be analyzed among the recorded ORP measurement data is expressed as an ORP value for each arbitrary data section selected in the range of 1 mV to 50 mV, for example, Microsoft Aggregate using spreadsheet software such as Microsoft Excel, Inc., and calculate the cumulative frequency based on the frequency of appearance. For example, when the pattern of the water quality time fluctuation of the inflowing sewage 1 is substantially constant, the determination may be made based on the cumulative frequency of the ORP values per day. If there is a sudden change in the quality of the sewage 1 due to the influence of rainfall or the like, the determination may be made based on the cumulative frequency of ORP values during the period in which the influence of the rain is recognized.

FIG. 6 shows the relationship between the ORP value of the anaerobic tank 4 and the PO ₄ -P concentration in the anaerobic tank 4 examined by the inventors. In FIG. 6, monitoring is performed by the ORP meter 13, and acetic acid is added as an organic acid 20 to the anaerobic tank 4 at a rate of 30 mg / L per sewage inflow to add the organic acid 20 of the present invention in which the ORP value is controlled (FIG. 6). 6 shows the case of the invention method) and the conventional method without addition of the organic acid 20 (shown as the conventional method in FIG. 6). In the conventional method, the ORP value of the anaerobic tank 4 varies greatly from −100 to −300 mV, and the amount of released phosphorus is small, but in the invention method, 50% or more of the measured cumulative frequency of the ORP value of the anaerobic tank 4 is −250 mV or less. In addition, the amount of phosphorus released increased. When the organic acid 20 is added so as to control the ORP value in this way, the ORP value of the anaerobic tank 4 can be kept low, and the amount of phosphorus released increases.

  Considering the cost of chemicals, the ORP value is preferably maintained at −350 mV or higher.

  The timing of adding and stopping the organic acid 20 can be appropriately determined according to the capacity of the reaction vessel, HRT, the concentration of the organic acid 20, and the flow rate of the organic acid 20.

  FIG. 7 shows the relationship between the ORP value in the anaerobic tank 4 and the NOx-N concentration in the anaerobic tank 4. In the case of the addition method of organic acid 20 of the present invention (described as “invention method” in FIG. 7) in which acetic acid is added to the anaerobic tank 4 at 30 mg / L per sewage inflow while monitoring with the ORP meter 13 as the organic acid 20 And a conventional method in which the organic acid 20 is not added (referred to as a conventional method in FIG. 7). From this result, it can be seen that if NOx-N is present in the anaerobic tank 4, the ORP value is likely to increase. NOx-N flows into the anaerobic tank 4 from rainwater or return sludge 8. When acetic acid is added from the outside, NOx-N is easily consumed by the denitrifying bacteria in the activated sludge in the anaerobic tank 4 by the denitrification reaction, and as a result, the ORP value is also reduced to -300 to -200 mV.

8 shows the anaerobic tank 4 in the case of the method of adding the organic acid 20 of the present invention (described as the invention method in FIG. 8) and the conventional method in which the organic acid 20 is not added (described in the conventional method in FIG. 8). shows the relationship between _PO 4 -P concentration of _PO 4 -P and settling tank effluent 19. From this result, if _PO 4 -P concentration of the anaerobic tank 4 is 10 mg / L or more, _PO 4 -P concentration of the final sedimentation effluent 19 is seen to be a less 0.5 mg / L. Moreover, from this result, the standard of the PO ₄ -P release concentration in the anaerobic tank 4 is considered to be about 10 mg / L.

  As described above, when the ORP value in the anaerobic tank 4 is continuously monitored, the phosphorus in the anaerobic tank 4 can be obtained without continuously measuring the sewage 1, the return sludge 8 and the NOx-N or the organic acid 20 in the anaerobic tank 4. Can be estimated in real time. Further, the addition of the organic acid 20 for controlling the ORP value of the anaerobic tank 4 is extremely effective for stabilizing phosphorus removal, which was unstable in the prior art.

  Furthermore, the amount of the organic acid 20 added to the anaerobic tank 4 is appropriately added so that the ORP value of the anaerobic tank 4 falls within a predetermined range while monitoring the ORP value of the anaerobic tank 4 with the ORP meter 13. The addition flow rate can also be determined as appropriate. However, if the amount of the organic acid 20 added is too small, there may be a time lag from the ORP value range until the ORP value actually changes after the organic acid 20 is added. For this reason, it is desirable to predict the amount of organic acid 20 added to some extent in advance so that the ORP value can be kept within a predetermined control range.

For example, as described above, the amount of organic acid added is preferably about 1.3 in terms of molar ratio for the use of acetic acid for phosphorus release. Therefore, it is possible to refer to these known numbers for the amount of organic acid added to the anaerobic tank. For example, when the molar ratio is 1.3, the amount of acetic acid required to promote the release of 10 mg / L PO ₄ -P in the anaerobic tank is 25 mg / L.

  However, when it is clear that dissolved oxygen and NOx-N are present in the anaerobic tank 4 due to the sewage 1 and the returned sludge 8, and the organic acid 20 is consumed greatly, the following method may be used. it can. That is, the NOx-N concentration and DO in the first sedimentation basin effluent 3 and the return sludge 8 are periodically measured, and the consumed organic acid 20 is determined from the mass of these NOx-N and dissolved oxygen flowing into the anaerobic tank 4. It is preferable to further increase the amount of the organic acid 20 added to the anaerobic tank 4 by the value calculated by calculation.

  If the ORP value cannot be maintained within the predetermined range due to the structural problem of the anaerobic tank 4 even by the above method, the amount of the organic acid 20 added is appropriately increased while monitoring the ORP value with the ORP meter 13. It doesn't matter.

  In addition, when the concentration of the organic acid 20 in the first settling basin effluent 3 is always low, the ORP value of the anaerobic tank 4 is high, and external addition of the organic acid 20 is always required, in addition to the addition of the organic acid 20 Then, a part (for example, about 10%) of the sewage 1 may be directly flowed into the anaerobic tank 4 so that the ORP value of the anaerobic tank 4 becomes −250 mV or less.

(Anoxic tank)
Next, an operation method of the anoxic tank 5 will be described.

In the anaerobic tank 5, the nitrification liquid 15 containing NO ₂ —N and NO ₃ —N generated in the aerobic tank 6 is circulated to the anoxic tank 5 using the circulation pump 16, NO ₂ —N and NO ₃ —N in the nitrification solution 15 are reduced to nitrogen gas using BOD. When there is an anaerobic tank 4 in the previous stage, a part of the BOD component in the sewage 1, for example, the organic acid 20, decreases due to the uptake by the polyphosphate-accumulating bacteria, so the denitrification reaction is performed using the residual BOD component. The measurement of this residual BOD component is quite difficult because there are some adsorbed on activated sludge.

  The circulating amount of the nitrifying solution 15 is preferably 100 to 200 V / V% with respect to the inflow amount of the sewage 1. In this case, the theoretical nitrogen removal rate E is expressed by the following formula 1.

  In Equation 1, E is the theoretical nitrogen removal rate (%), and R is the nitrification liquid circulation ratio (−).

  However, in practice, the nitrogen removal rate is improved up to 200 V / V% as the circulation amount of the nitrification liquid 15 with respect to the inflow amount of the sewage 1, but it is difficult to improve the nitrogen removal rate even if it is increased further. This is because even if the circulation amount of the nitrification solution 15 is increased, the organic matter / nitrogen ratio in the anaerobic tank 5 is reduced, or the amount of dissolved oxygen brought into the nitrification solution 15 is increased, which tends to cause denitrification inhibition. is there.

  FIG. 9 shows the method of adding the organic acid 20 according to the present invention (described as the inventive method in FIG. 9) and the conventional method in which the organic acid 20 is not added (described as the conventional method in FIG. 9). The relationship between the ORP value and the NOx-N concentration in the oxygen-free tank 5 is shown. When the denitrification reaction of the anaerobic tank 5 decreases and NOx-N remains, the ORP value increases. For example, when the ORP value is −150 mV or less, the NOx—N concentration is 0.5 mg / L or less, but when the ORP value exceeds −100 mV, the NOx—N concentration exceeds 2.0 mg / L. From this result, it is desirable that the ORP value of the oxygen-free tank 5 is −200 mV or more and −100 mV or less. In addition, the cumulative frequency of the ORP value in the anaerobic tank 5 is measured for a certain period, and the ORP value in the anaerobic tank 5 is −200 mV to −100 mV and 50% or more of the cumulative frequency is −200 mV to −150 mV. It is more preferable that it is maintained.

  As for the collection frequency of the cumulative frequency, data is collected every 1 to 10 minutes every day, this is analyzed, and the cumulative frequency of ORP values per fixed period is calculated. When the pattern of fluctuations in the quality of the sewage water flowing in is almost constant, it may be determined by the cumulative frequency of ORP values per day. If there is a sudden change in the quality of the sewage 1 due to the influence of rainfall or the like, the determination may be made based on the cumulative frequency of ORP values during the period in which the influence of the rain is recognized.

  When the ORP value rises to over −100 mV in the anaerobic tank 5, or when 50% or more of the cumulative frequency of the ORP value in the anaerobic tank 5 is not maintained in the range of −200 mV to −150 mV An organic substance may be further added to the oxygen-free tank 5 using the ORP value of the tank 5 as an index. Among organic substances, the organic acid 20 has a high reaction rate and is also used for dephosphorization. Therefore, it is desirable to add it to the oxygen-free tank 5. Also, the ORP value of the anaerobic tank 5 may be reduced using the ORP value of the anaerobic tank 5 as an index to reduce the ORP value of the anoxic tank 5. Further, the ORP value and / or DO of the aerobic tank 6 may be lowered and the ORP value of the anaerobic tank 5 may be lowered within a range where the nitrification reaction is not inhibited.

On the other hand, when the ORP value of the oxygen-free tank 5 is reduced to less than −200 mV, it is estimated that although the NOx—N disappears, the organic matter in the sewage 1 remains excessively. The remaining organic matter flows into the aerobic tank 6 at the subsequent stage and consumes dissolved oxygen preferentially over NH ₄ -N, so that inhibition of the nitrification reaction tends to occur. For this reason, in such a case, while monitoring with the ORP meter 13 installed in the oxygen-free tank 5, the oxygen-free tank 5 is aerated to remove excessive residual organic substances, and the ORP value is set to -200 mV or more. It is desirable to maintain.

(Aerobic tank)
Next, a method for operating the aerobic tank 6 will be described.

In the aerobic tank 6, aeration is always performed by the blower 10, and NH ₄ -N is oxidized to NO ₂ -N by ammonia oxidizing bacteria in the presence of dissolved oxygen. Subsequently, NO ₂ -N is oxidized to NO ₃ -N by nitrite-oxidizing bacteria. The DO monitoring is performed by the DO meter 14.

  When the nitrification reaction proceeds smoothly, NOx-N accumulates and the ORP value of the aerobic tank 6 gradually increases. Normally, the ORP value of the aerobic tank 6 is based on the ORP value near the outlet of the aerobic tank 6, and the aeration amount by the blower 10 is adjusted by the ORP value.

  From the viewpoint of promoting nitrification reaction, it is usually desirable that DO at the aerobic tank 6 end is 1.5 to 3.0 mg / L, or ORP value at the aerobic tank 6 end is +100 to 125 mV or more. (Non-patent document 1, Research on high-efficiency treatment of sewage COD, phosphorus and nitrogen, Journal of Sewerage Society, 1993, Vol. 30, No. 364, p96-p106).

  However, this time, from the viewpoint of energy saving, the inventors maintained the ORP value at the end of the aerobic tank at +70 mV or more and +100 mV or less, and the DO at the end of the aerobic tank at 0.5 to 1.5 mg / L. The amount of aeration was controlled as indicated. As a result, if the ORP value at the end of the aerobic tank 6 is about +70 mV as shown in FIG. Found more than 90% progress. That is, from the viewpoint of energy saving, the ORP value at the end of the aerobic tank 6 is preferably in the range of +70 mV to +100 mV.

  Further, from the viewpoint of promoting denitrification, the nitrification solution 15 is fed from the aerobic tank 6 to the anaerobic tank 5, but if the DO in the aerobic tank 6 is too high, the denitrification in the anaerobic tank 5 is performed. Nitrogen reaction is inhibited. Also from this viewpoint, it is extremely desirable to set the DO of the aerobic tank 6 to 1.5 mg / L or less.

  Furthermore, in the aerobic tank 6, excessive intake of phosphorus by polyphosphate-accumulating bacteria is also performed, and many relations with DO have been pointed out regarding excessive intake of phosphorus in the aerobic tank 6. In general, the DO at the end of the aerobic tank 6 is preferably 1.5 to 3.0 mg / L. However, the inventors have found that if the ORP value of the aerobic tank 6 is +70 mV or higher, there will be no problem in phosphorus incorporation even if DO is 0.5 to 1.5 mg / L. Therefore, from the viewpoint of phosphorus incorporation, if the ORP value of the aerobic tank 6 is +70 mV or more, it is preferable to set DO to 0.5 to 1.5 mg / L.

  In the case of a single-stage combination of the anaerobic tank 5 and the aerobic tank 6, no matter how much the nitrification liquid 15 is circulated as described above, the nitrogen removal rate is limited in principle, and the final sedimentation tank outflow There is a possibility that NOx-N remains in the water 19. In the case of the sewage 1 having a high nitrogen concentration, the inflow of NOx-N to the anaerobic tank 4 tends to increase, and the phosphorus removal tends to deteriorate. If the nitrogen removal rate is improved to nearly 100%, such a problem will not occur. In order to cope with this, it is preferable to install a second anaerobic tank 17 and a second aerobic tank 18 in which the ORP meter 13 is newly incorporated in the subsequent stage of the aerobic tank 6. An example of the processing flow of the present invention is shown in FIG.

  In this case, since the inflow water to the second anaerobic tank 17 contains almost no organic matter, in order to efficiently perform denitrification in the second anaerobic tank 17, an organic matter such as an organic acid 20 is externally provided. Need to be added. Denitrification is basically possible even without addition of organic substances (known as endogenous denitrification), but in this case, considerable HRT is required. The ORP value of the second anaerobic tank 17 is preferably −200 mV or more and −150 mV or less, but the ORP value of the second anaerobic tank 17 is measured by measuring the cumulative frequency of the ORP value of the anoxic tank 5 for a certain period. Is −200 mV or more and −100 mV or less, and if the cumulative frequency of 50% or more is maintained in the range of −200 mV or more and −150 mV or less, there is no significant influence. As for the collection frequency of the cumulative frequency, data is collected every 1 to 10 minutes every day, this is analyzed, and the cumulative frequency of ORP values per fixed period is calculated. When the pattern of fluctuations in the quality of the sewage water flowing in is almost constant, it may be determined by the cumulative frequency of ORP values per day. If there is a sudden change in the quality of the sewage 1 due to the influence of rainfall or the like, the determination may be made based on the cumulative frequency of ORP values during the period in which the influence of the rain is recognized.

  In the second aerobic tank 18, the organic acid 20 is added as a hydrogen donor, and the main purpose is to oxidize the organic matter remaining in the outlet water of the second anoxic tank 17. Therefore, the ORP value and DO of the second aerobic tank 18 may be lower than those of the aerobic tank 6 for the purpose of promoting the nitrification reaction.

  Even if the ORP value of the second aerobic tank 18 is +50 to +70 mV and the DO is 0.5 to 1.0 mg / L, there is no problem in organic matter decomposition.

  In this method, since the ORP value and DO of the second aerobic tank 18 can be kept lower than in the single-stage method, energy saving is easy, and nitrogen can be removed nearly 100%. Moreover, since almost NOx-N does not exist in the return sludge 8, the discharge of phosphorus in the anaerobic tank 4 can be further stabilized.

  As in a small-scale sewage treatment plant, when the property fluctuation or water quantity fluctuation of the sewage 1 is large, the treatment tends to become unstable. Therefore, the anoxic tank 5 and the aerobic tank 6 are continuously connected as described above. It is desirable to use two or more stages in combination.

  Furthermore, in the case of a sewage treatment plant where the denitrification reaction and nitrification reaction are likely to be reduced due to fluctuations in the properties of the sewage 1 and fluctuations in the amount of water, the microorganism immobilization carrier (plastics, ceramics, slag, gel, etc.) May be added to each reaction tank to increase the concentration of microorganisms. By increasing the concentration of microorganisms, the processing speed increases, leading to stabilization of processing and high-efficiency processing. The amount of the microorganism-immobilized carrier charged into each tank is about 5 to 20 V / V% per reaction tank volume in the apparent volume of the carrier.

  As the organic acid added to the anaerobic tank, the oxygen-free tank, or the second oxygen-free tank, acetic acid and / or acetate is preferable.

  The ORP meter 13 is desirably installed in the vicinity of the outlet of each tank where the reaction is completed when the reaction tank is extruded flow or close to the extrusion flow. In the case of complete mixing, it is not particularly limited as in the case of an extruded flow, but it is desirable to install from the central part of the reaction tank to the vicinity of the outlet in order to avoid the influence of the inflowing water.

The final sedimentation basin 7 performs sedimentation separation of activated sludge. Usually, it is designed with a water area load of about 20 to 25 m ³ / m ² · day and an effective water depth of 3.5 to 4.0 m, but can be determined as appropriate.

  Examples of the present invention will be described below. In addition, this invention is not limited to a present Example.

Example 1
As shown in FIG. 3, the sewage 1 was treated using the first sedimentation tank 1, the anaerobic tank 4, the anoxic tank 5, the aerobic tank 6, and the final sedimentation tank 7. The nitrification solution 15 was returned to the anoxic tank 5 by the circulation pump 16 under the condition of 150 V / V% with respect to the sewage 1. The return sludge 8 was returned to the anaerobic tank 4 by the return sludge pump 9 under the condition of 50 V / V% with respect to the sewage 1. The MLSS (sludge concentration) of each reaction tank was maintained at 2500 to 3000 mg / L. Moreover, A-SRT (sludge residence time in the aerobic tank 6) was managed in 12 to 13 days.

The water quality of the first settling basin effluent 3 is about 100 mg / L for BOD, about 30 mg / L for TN, about ₄ mg / L for TP, and about 2.4 mg / L for PO ₄ -P.

  Using the ORP meter 13 of the anaerobic tank 4 (model HBM-312 manufactured by Toa DKK Co., Ltd.) as an index, the organic acid 20 was added to the anaerobic tank 4 on the basis of the ORP value of −250 mV, and the ORP of the anaerobic tank 4 The value was controlled such that 50% was from −400 mV to −200 mV, and the cumulative frequency was −250 mV or less.

Moreover, the addition density | concentration of the organic acid 20 to the anaerobic tank 4 was determined as follows. Assuming that the target phosphorus release concentration in the anaerobic tank 4 is 10 mg / L, as described above, the use of acetic acid for phosphorus release is about 0.6 to 1.3 in molar ratio, so the molar ratio is 1 .3, the acetic acid concentration required in the anaerobic tank 4 is 25 mg / L per 4 volumes of the anaerobic tank. Return sludge 8 to the anaerobic tank 4 flows, but return sludge 8, the _NO 3 -N contained 4 mg / L. Since the amount of the return sludge 8 was 50 V / V% with respect to the inflow amount of the sewage 1, the inflow concentration of NO ₃ -N in the anaerobic tank 4 is 2 mg / L. Therefore, acid consumption due to _NO 3 -N in the anaerobic tank 4 was estimated to be 5.4 mg / L. Incidentally, _NO 2 -N, and DO is almost 0, was negligible.

  When this result and the previous 25 mg / L are added together, the acetic acid concentration required in the anaerobic tank 4 is 30 mg / L per 4 volumes of the anaerobic tank. If all the acetic acid is to be supplemented with sewage 1, 45 mg / L-sewage acetic acid is required in terms of the sewage flow rate in consideration of the amount of returned sludge 8. However, since the sewage 1 contained only an average of 15 mg / L-sewage (0-30 mg / L) of acetic acid, the amount of acetic acid added per sewage flow rate was 30 mg / L-sewage.

Using the ORP value of the aerobic tank 6 as an index, the amount of aeration was increased or decreased at +90 mV, and the ORP value was changed in the range of +80 to +100 mV. At the same time, the DO in the aerobic tank 6 is measured with a DO meter 14 (model DDIC-7 manufactured by Toa DK Corporation), and the DO changes within the range of 0.5 to 1.0 mg / L under the control of the ORP value. Thus, the amount of aeration air used for controlling the ORP value was adjusted. The nitrification liquid 15 was returned to the 150 V / V% anoxic tank 5 with respect to the sewage 1 by the circulation pump 16. The return sludge 8 was returned to the anaerobic tank 4 by 50 V / V% with respect to the sewage 1 by the return sludge pump 9. The circulating amount of the nitrification solution 15 was 150 V / V% for both methods. (Comparative Example 1)
The ORP value of the anaerobic tank was not controlled.

  Control of the ORP value of the aerobic tank 6 was not performed, and DO management (DO management value = 2-3 mg / L) was performed. Except for the above, the same operation as in Example 1 was performed.

  FIG. 4 shows the cumulative frequency of ORP values in the anaerobic tank 4 of Example 1 and Comparative Example 1. As shown in FIG. 4, in Example 1, the ORP value of the anaerobic tank 4 varied greatly from 0 mV to −400 mV, and −200 mV or more accounted for 40% in cumulative frequency. On the other hand, in Comparative Example 1, the ORP value of the anaerobic tank 4 was maintained in the range of −200 mV to −400 mV, and −250 mV or less accounted for 50% in cumulative frequency. Thus, Example 1 was able to maintain the ORP value of the anaerobic tank 4 at a lower level than that of Comparative Example 1.

  In Example 1, even if DO is 0.5 to 1.0 mg / L in the aerobic tank 6, if the ORP value is maintained at +90 mV, the nitrification reaction proceeds almost completely, and The denitrification reaction was also superior to that of Comparative Example 1. In addition, phosphorus was taken in sufficiently.

  Table 1 shows a comparison of average water quality between Example 1 and Comparative Example 1.

In Comparative Example 1, the ORP value in the anaerobic tank 4 was high, and the discharge of phosphorus was not stable. As a result, the phosphorus concentration of the treated water 19 was not stable. On the other hand, in the method of the invention, the discharge of phosphorus in the anaerobic tank 4 occurred remarkably, the phosphorus uptake in the aerobic tank 6 was no problem, and the phosphorus concentration in the treated water 19 was stable. In the invention method, the PO ₄ -P concentration in the treated water 19 was also 1 mg / L or less, which is the treatment development target value.

  FIG. 5 shows the cumulative frequency of ORP values in the oxygen-free tank 5 of Example 1 and Comparative Example 1. As shown in FIG. 5, in Comparative Example 1, the ORP value of the oxygen-free tank 5 changed from 0 mV to -200 mV, and NOx-N was likely to remain. On the other hand, in the method of controlling the ORP value and DO of the aerobic tank 6, the ORP value of the anaerobic tank 5 is maintained in the range of −100 to −200 mV, and the cumulative frequency per day is −150 mV or less. Accounted for 50%. Thus, Example 1 was able to maintain the ORP value of the oxygen-free tank 5 at a lower level than that of Comparative Example 1. As a result, in Example 1, the denitrification performance was improved as compared with Comparative Example 1.

Table 2 shows a comparison of average water quality. In Example 1, NH ₄ —N remained slightly at about 0.5 mg / L, but the denitrification performance was improved by 2 mg / L, and as a result, the nitrogen removal performance was improved by 1.5 mg / L.

FIG. 6 shows the relationship between the ORP value in the anaerobic tank and the NOx-N concentration in the anaerobic tank. FIG. 7 shows the relationship between the ORP value in the anaerobic tank and the PO ₄ -P concentration in the anaerobic tank. FIG. 8 shows the relationship between the PO ₄ -P concentration in the anaerobic tank and the PO ₄ -P concentration in the final sedimentation basin effluent. FIG. 9 shows the relationship between the ORP value of the oxygen-free tank and the NO ₃ —N concentration of the oxygen-free tank. FIG. 10 shows the relationship between the ORP value of the aerobic tank and the progress of the nitrification reaction. 6 to 10 are as described above.

Figure 2 is a biological phosphorus removal process from sewage incorporating the control of ORP values according to the present invention. Figure 2 is a biological nitrification-nitrogen removal process from sewage incorporating the control of ORP values according to the present invention. Figure 2 is a biological phosphorus / nitrogen removal process from sewage incorporating control of the ORP value according to the present invention. It is a figure which shows the accumulation frequency of the ORP value of an anaerobic tank. It is a figure which shows the cumulative frequency of the ORP value of an anaerobic tank. It is a figure which shows the relationship between the ORP value of an anaerobic tank, and the NOx-N density | concentration of an anaerobic tank. Is a diagram showing a relationship between PO 4 _-P concentration of ORP value and anaerobic tank of the anaerobic tank. It is a diagram showing a relationship between _PO 4 -P concentration of the final sedimentation tank effluent _PO 4 -P concentration of anaerobic tank. It is a diagram showing a relationship between NO 3 _-N concentrations of ORP value and the anoxic tank in the anoxic tank. It is a figure which shows the relationship between the ORP value of an aerobic tank, and the progress degree of nitrification reaction. Figure 2 is a biological phosphorus / nitrogen removal process from sewage incorporating control of the ORP value according to the present invention.

Explanation of symbols

1 Sewage,
2 First sedimentation basin,
3 First settling basin effluent,
4 Anaerobic tank,
5 Anoxic tank,
6 Aerobic tank,
7 Final sedimentation basin,
8 Return sludge,
9 Return sludge pump,
10 Blower,
11 Organic acid tank,
12 Medication pump,
13 ORP meter,
14 DO meter,
15 Nitrification solution,
16 Circulation pump,
17 Second oxygen-free tank,
18 Second aerobic tank,
19 Final sedimentation basin effluent,
20 Organic acid.

Claims (10)

Biologicals using each sewage treatment tank of the first sedimentation tank, anaerobic tank, aerobic tank, and final sedimentation tank, or each sewage treatment tank of the first sedimentation tank, anaerobic tank, anoxic tank, aerobic tank, and final sedimentation tank Sewage treatment methods
An organic acid is added to the anaerobic tank so that the ORP value of the anaerobic tank is maintained in a range of −400 mV to −200 mV.   The organic acid is added to the anaerobic tank so that 50% or more of the cumulative frequency of the ORP value of the anaerobic tank is maintained in a range of -350 mV to -250 mV. Sewage treatment method. Biology using each sewage treatment tank of the first sedimentation tank, anoxic tank, aerobic tank, and final sedimentation tank, or each sewage treatment tank of the first sedimentation tank, anaerobic tank, anoxic tank, aerobic tank, and final sedimentation tank In a typical sewage treatment method,
In order to maintain the ORP value of the anaerobic tank in the range of −200 mV to −100 mV,
Adding an organic acid to the anaerobic tank, adjusting a circulation amount of the nitrification liquid from the aerobic tank to the anoxic tank, adjusting an ORP value of the aerobic tank, and the aerobic tank A sewage treatment method characterized by performing at least one selected from the group consisting of: 50% or more of the cumulative frequency of ORP values in the anaerobic tank is maintained in the range of −200 mV to −150 mV,
Adding the organic acid to the anaerobic tank, adjusting the amount of nitrification liquid circulating from the aerobic tank to the anaerobic tank, adjusting the ORP value of the aerobic tank, and the aerobic The sewage treatment method according to claim 3, wherein at least one selected from the group consisting of adjusting DO of the tank is performed. Adjusting the amount of the organic acid added to the anaerobic tank so that the PO ₄ -P concentration in the anaerobic tank is 10 mg / L;
Furthermore, calculating the amount of organic acid consumed per unit time by the NOx-N concentration and DO in the sewage and return sludge flowing into the anaerobic tank, and increasing the amount of the organic acid added to the anaerobic tank The sewage treatment method according to any one of claims 1 to 4, wherein:   The sewage treatment method according to any one of claims 1 to 5, wherein an aeration amount by a blower of the aerobic tank is adjusted so that an ORP value of the aerobic tank is +70 mV or more and +100 mV or less. .   The aeration amount by the blower of the aerobic tank is adjusted so that DO of the aerobic tank is 0.5 mg / L or more and 1.5 mg / L or less. A sewage treatment method described in 1.   The sewage treatment method according to any one of claims 1 to 7, further comprising a second anaerobic tank and a second aerobic tank in order after the aerobic tank.   The sewage treatment method according to any one of claims 1 to 8, wherein a microorganism-immobilized carrier is introduced into one or more of the anaerobic tank, the anoxic tank, and the aerobic tank.   The sewage treatment method according to any one of claims 1 to 9, wherein the organic acid added to at least one of the anaerobic tank and the oxygen-free tank is acetic acid and / or acetate.