JP5041698B2 - Sewage treatment method - Google Patents

Description

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

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.
1) Aggregation precipitation method: Phosphorus is removed by an aggregation precipitation method using an iron salt such as ferric chloride or ferric sulfate, or an aggregating agent such as aluminum sulfate or polyaluminum chloride. Although it is the most reliable method, the addition of a flocculant increases running costs such as treatment costs and chemical costs associated with an increase in excess sludge.
2) Biological phosphorus removal method: Some bacterial groups in activated sludge (aggregates of microorganisms) (hereinafter referred to as polyphosphate-accumulating bacteria) are subjected to anaerobic conditions (dissolved oxygen is also bound oxygen such as nitrate ions). When phosphorus is released in a state where there is no oxygen, excessive phosphorus is ingested under aerobic conditions (state where dissolved oxygen is present). Utilizing this property, phosphorus removal is performed. If such a system is adopted, the phosphorus concentration in the activated sludge of sewage is said to increase from 2 to 3% to about 5 to 6%. This method has been put into practical use in the field of urban sewage treatment.

  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. In the anaerobic tank, phosphorus is released from the polyphosphate-accumulating bacteria in the activated sludge under anaerobic conditions. Furthermore, in the aerobic tank, polyphosphate-accumulating bacteria are caused to ingest phosphorus in excess of the released amount. In the final sedimentation basin, activated sludge is settled and the supernatant is discharged. The concentrated activated sludge settled and separated in the final sedimentation basin is returned to the anaerobic tank as a return sludge by a return pump, and a part thereof is extracted as excess sludge. Since the excess sludge contains phosphorus in a high concentration, the phosphorus contained in the sewage is extracted out of the system in the form of excess sludge.

Biological phosphorus and nitrogen simultaneous removal process is a process that combines biological nitrogen removal process with biological phosphorus removal process, first settling tank, reaction tank (anaerobic tank, anaerobic tank, aerobic tank), It consists of the final sedimentation basin. The initial settling tank, anaerobic tank, aerobic tank and final settling tank are treated in the same manner as the biological phosphorus removal process. In the anaerobic tank, NO ₂ -N and NO ₃ -N in the nitrification liquid circulated and returned from the aerobic tank are converted into denitrifying bacteria and sewage in the activated sludge by making the oxygen-free state free of dissolved oxygen. Is removed as nitrogen air by denitrification reaction with organic matter.

In biological phosphorus removal process and biological phosphorus / nitrogen simultaneous removal process, treatment tends to become unstable, and establishment of appropriate operation management method for stable operation of biological phosphorus removal method is strongly desired. ing.
For example, when the organic substance concentration in the sewage is reduced due to mixing in sewage such as rainwater, the release of phosphorus in the anaerobic tank is suppressed. When the release phenomenon of phosphorus is suppressed in the anaerobic tank, the excessive uptake ability of phosphorus in the aerobic tank decreases. A large amount of organic matter is required in the sewage in order to perform phosphorus release well in the anaerobic tank, and the BOD / TP concentration ratio (BOD) / (TP) ratio is 20 to 25 as an index. If it is more than the above, release of phosphorus is said to be good. (See Non-Patent Document 1 and Non-Patent Document 2.) Where BOD: biochemical oxygen demand: mg / L
TP: Total phosphorus concentration: mg / L
Means.

  Furthermore, it has been pointed out that the excessive intake of phosphorus in the aerobic tank is related to the dissolved oxygen (DO) concentration. Insufficient dissolved oxygen concentration inhibits phosphorus intake, so it is generally desirable that the dissolved oxygen concentration at the end of the aerobic tank be 1.5 to 2.0 mg / L (see Non-Patent Document 1) or 1.5 to 3.0 mg / L. (See Non-Patent Document 2).

  However, even if the (BOD) / (TP) ratio of the anaerobic tank inflow water is 25 or more, if the release of phosphorus in the anaerobic tank is poor, a large amount of phosphorus remains in the treated water. There is.

The following factors can be considered as the cause.
1) Inflow of NOx-N (sum of NO ₂ -N and NO ₃ -N, the same shall apply hereinafter) and dissolved oxygen from sewage and return sludge mixed with rainwater into an anaerobic tank;
2) Backflow of NOx-N and dissolved oxygen from the nitrification liquid returned to the anaerobic tank to the anaerobic tank;
3) Entrainment of dissolved oxygen by excessive stirring in an anaerobic tank;
4) Shortening of HRT (hydraulic residence time) due to occurrence of short-circuit flow in an anaerobic tank.

The conditions of these anaerobic tanks vary greatly depending on the sewage treatment plant.For this reason, 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 good. Seems not limited.
For these reasons, it is important for phosphorus removal to simply monitor the release status of phosphorus in an anaerobic tank and develop a countermeasure.

  There are the following examples of reports on this issue. For example, as a method for easily monitoring the state of phosphorus release in an anaerobic tank, there is an example focusing on the ORP (oxidation-reduction potential, hereinafter the same) value of the silver / silver chloride electrode reference value.

  For example, the ORP value in an anaerobic tank is closely related to the phosphorus discharge phenomenon, and if the ORP value in the anaerobic tank in the biological dephosphorization process exceeds -270 mV, sewage containing a large amount of organic matter (the first sedimentation basin) Inflow water) or first settling basin sediment sludge is introduced into the anaerobic tank and the ORP value of the anaerobic tank is maintained at -270 mV or less (see Patent Document 1).

  In addition, 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 is varied according to this ORP value. A method for maintaining the ORP value of the anaerobic tank at −270 mV or less has also been proposed (see Patent Document 2).

  Furthermore, a method has also been proposed in which the organic matter concentration of the first settling basin effluent and the ORP value of the anaerobic tank are measured, and the anaerobic state is controlled based on these measured values (see Patent Document 3).

  However, the method using the first settling basin influent as in Patent Document 1 or Patent Document 2 not only has an unstable organic substance concentration in the first basin inflow itself, but also reduces the organic load and nitrogen load in the reaction tank. Due to the large fluctuations, it is quite difficult to control the removal of organic matter and phosphorus in the wastewater. Furthermore, as in Patent Document 3, in order to measure and control the organic matter concentration of raw water, it is necessary to continuously measure the organic acid. There are BOD (biochemical oxygen demand: mg / L), COD (chemical oxygen demand: mg / L), TOC (organic carbon quantity: mg / L) as indicators of organic acids. Even if it is an index, there is a problem that it is difficult to measure in real time. In addition, Patent Document 3 proposes replacing the organic acid by measuring the ultraviolet absorbance, but the measurement does not necessarily reflect the total organic acid so that an organic substance having a benzene ring cannot be measured by the ultraviolet absorbance. There is also a problem that results are not obtained.

Furthermore, the technology for simultaneous removal of biological phosphorus and nitrogen from sewage also has the above-mentioned problem of phosphorus removal, but there are many cases where the optimal conditions for nitrogen removal and the optimal conditions for phosphorus removal are often contradictory. difficult.
JP-A-3-278893 Japanese Patent Application No. 2001-252413 JP 2001-87793 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 The effect of organic compounds on biological phosphorus removal, Water Science Technology, Vol.23, No.4 / 6, p585-p591, 1991 Toa DKK Corporation Immersion detector with water jet cleaning JOC-711C Instruction Manual, P13

  That is, in the anaerobic tank (4), it is necessary to maintain a state in which there is neither dissolved oxygen nor conjugate oxygen. Therefore, there is a method using the brush cleaning shown in FIG. There are problems such as entanglement and the inside of the brush is contaminated with activated sludge, so that ORP is greatly reduced during cleaning and recovery takes time.

  FIG. 5 shows the change in the ORP value in the anaerobic tank measured with an ORP meter equipped with a brush cleaning device. The washing time was 30 seconds. As a result, it can be seen that the ORP value is reduced by about 100 mV during washing, and it takes about 90 minutes to recover the original ORP value. Until the ORP value recovers, the correct value of the anaerobic tank is not shown, and the anaerobic tank cannot be controlled.

On the other hand, as shown in FIG. 4, air cleaning, in which air is sprayed by a nozzle installed at the tip of the electrode, can be used for a long time without being affected by hair or the like, as in brush cleaning, and is cleaned in an anaerobic tank. it can.
However, as shown in Non-Patent Document 4, as a conventional technique, a device that measures the dissolved oxygen concentration with a DO meter (dissolved oxygen concentration measuring meter) and cleans the measurement site with air is disclosed, and exhibits a cleaning effect. Therefore, it is recommended that the cleaning time for each washing be 60 seconds. However, there is no one that has an air cleaning function in the electrode part of the ORP meter and defines the cleaning time. That is, the DO meter and the ORP meter are different from each other in measurement environment. The DO meter is originally used under aerobic conditions (the situation where dissolved oxygen is present), while the ORP meter is used also under anaerobic conditions (a situation where there is no dissolved oxygen). Therefore, when applied to ORP meter cleaning with a cleaning time of 60 seconds, the ORP indicated value has a large drop, and it takes time to recover, or by sending air into the anaerobic tank, the DO value in the anaerobic tank ( A problem arises in that the dissolved oxygen concentration is increased and the treatment state is deteriorated.

  Therefore, as a result of repeated investigations to solve the above problems, the present inventors stably remove phosphorus by measuring the ORP value and controlling the reaction state in the anaerobic tank by the following method. Succeeded.

The gist of the present invention is the following (1) to (4).
(1) A method for removing biological phosphorus from sewage comprising an initial sedimentation tank, an anaerobic tank, an aerobic tank and a final sedimentation tank, or an initial sedimentation tank, an anaerobic tank, an anaerobic tank, an aerobic tank and a final sedimentation tank in the electrode portion of the ORP (oxidation-reduction potential) of the anaerobic tank outlet, in 3 seconds or less in washed Kiyoshisu Rukoto with air at a pressure 0.05 to 0.5 MPa, and characterized in that suppress the fluctuation of the ORP value Said method.

(2) The organic acid is added to the anaerobic tank so that the ORP value (silver / silver chloride electrode reference value) at the anaerobic tank outlet is maintained in the range of −400 mV to −200 mV. 1) The method described.

(3) The method according to (2) above, wherein the organic acid added to the anaerobic tank is acetic acid or acetate.

(4) The method according to (3), wherein the residence time of the anaerobic tank is adjusted so that the ORP value at the outlet of the anaerobic tank is maintained in a range of −400 mV to −200 mV.

  By setting the air cleaning time per ORP meter to 3 seconds or less, it is possible to minimize the influence of the DO value in the anaerobic tank and the change in the ORP value accompanying the cleaning, and not affect the treatment.

  FIG. 6 shows changes in the ORP value when the cleaning interval is once an hour, the air pressure is 0.3 MPa, and the cleaning time is 0.3 seconds. Although the ORP value changes at the time of cleaning, it increases by about 5 to 10 mV, and the recovery of the ORP value is about 10 minutes, so the anaerobic tank is controlled even if cleaning is performed once an hour. There is no problem above. The DO value in the anaerobic tank immediately after cleaning is 0.0 mg / L, which is unchanged and does not affect phosphorus removal.

Table 1 shows an example of the fluctuation range of the ORP value when the air cleaning time per change is changed. If the cleaning time per time is 3 seconds or less, the increase rate of the ORP value is small and the recovery time is 20 minutes or less. Tend to affect. Therefore, the cleaning time is set to more than 0 to 3 seconds or less.

  The shorter the cleaning time, the smaller the range of change in ORP, so it is preferable to make it as short as possible. However, the time and interval are appropriately selected in consideration of the cleaning effect.

The pressure of the cleaning air may be 0.05 to 0.5 MPa shown in Non-Patent Document 4.
The inventors first maintain the ORP value of the anaerobic tank outlet (4) in the range of −400 mV or more and −200 mV or less (preferably, the cumulative frequency of the ORP value is continuously measured, and 50% or more It has been found that the removal of phosphorus is improved by maintaining it in the range of −350 mV to −250 mV. If it is less than -400mV, there will be no change in the amount of phosphorus released in the anaerobic tank, so the lower limit is set. Decreases and the phosphorus treatment deteriorates, so the upper limit was set to −200 mV or less.

  At this time, by setting the air cleaning condition to 3 seconds or less, it is possible to prevent fluctuations in the ORP value during cleaning from affecting the operation, and the ORP value can be continuously measured. The organic acid can be added so as to always maintain the range of −200 mV or less. Since organic acids and fermentation products in sewage are preferentially treated for release of phosphorus in the anaerobic tank, the treatment can be performed in the anaerobic tank by adding the organic acid. In particular, acetic acid is easily treated among organic acids.

  Furthermore, the residence time of an anaerobic tank can also be changed so that the conditions of an anaerobic tank exit may be maintained in the range of -400 mV or more and -200 mV or less. That is, in the biological phosphorus removal method of the present application, when the ORP value exceeds -200 mV, the water tank in the front row of the aerobic tank is an anaerobic tank and is changed from air agitation to mechanical agitation. In the biological nitrogen / phosphorus removal process, the anaerobic tank can be controlled to have the ORP value required for phosphorus release by changing the inflow position of the nitrification solution from the front tank of the anoxic tank to the second line. .

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

An example of the processing flow of the present invention is shown in FIGS.
FIG. 1 is a biological dephosphorization process of the present invention. The reaction tank consists of an anaerobic tank (4) and an aerobic tank (6).

FIG. 2 shows a biological dephosphorization / denitrogenation process having a denitrification function in addition to a dephosphorization function. The reaction tank consists of an anaerobic tank (4), an oxygen-free tank (5), and an aerobic tank (6).
The biological dephosphorization / denitrogenation process will be described with reference to the dephosphorization process of FIG. 1 and the biological dephosphorization / denitrogenation process of FIG.
First, coarse suspended solids (mainly sludge) contained in sewage (1) are settled and removed in the first sedimentation basin (2).
Thereafter, the first settling basin effluent (3) flows into the anaerobic tank (4).
The anaerobic tank (4) is managed as follows.

Polyphosphate-accumulating bacteria that perform biological phosphorus removal retain PO ₄ -P absorbed under aerobic conditions as granules of polyphosphate in the cells. In an anaerobic tank (4), The acid is hydrolyzed and released as PO ₄ -P, and organic substances in sewage, especially organic acids and fermentation products, are preferentially taken into cells. 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 large when an organic acid such as acetic acid is the substrate. Organic matter taken up into cells is stored in the form of glycogen and PHB (polyhydrobutyrate) polymer. When these intracellular substances are put under aerobic conditions again, they are oxidatively degraded and decrease, but polyphosphate-accumulating bacteria grow by using this substrate, and take up PO ₄ -P excessively, It is retained in the cell as polyphosphate granules.

  The important thing in promoting the reaction of polyphosphate-accumulating bacteria that perform such biological phosphorus removal is the presence of organic acids and fermentation products. These organic acids and fermentation products are one of the BOD components. However, even if the sewage BOD concentration is high, the concentration of these organic acids and fermentation products is not necessarily high. Therefore, indicators such as organic acid (mg / L) concentration in sewage are more effective than BOD. In the anaerobic tank (4), there are many studies on the types and amounts of organic substances necessary for phosphorus release, but there is a report that acetic acid utilization for phosphorus release is about 1.3 in molar ratio (for example, non-patent literature) 3). Therefore, the known amount of organic acid added to the anaerobic tank (4) may be referred to.

  However, in actual treatment equipment, NOx-N and DO often flow into the anaerobic tank (4) due to the effects of rainwater and return sludge, and NOx-N and DO exist in the anaerobic tank (4). Then, the organic acid is immediately decomposed. The reaction when acetic acid is used as the organic acid is shown below.

^{_{8NO 3 - + 5CH 3 COOH →}} 4N 2 + 10CO 2 + 6H 2 O + 8OH - (5)
8NO ₂ ⁻ + 3CH ₃ COOH → 4N ₂ + 6CO ₂ + 4H ₂ O + 8OH ⁻ (6)
2O ₂ + CH ₃ COOH → 2CO ₂ + 2H ₂ O (7)

From this, 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, if 1 mg / L of DO is present, acetic acid 0.9 mg / L is consumed accordingly. From this result, it can be seen that the influence of NO ₃ -N on the consumption of acetic acid is extremely large.

As a result of investigation, NO ₃ -N, NO ₂ -N, and DO are often present in sewage (1) and return sludge (8) mixed with rainwater. Depending on the conditions (high DO, high ORP operation), high concentration of NO ₃ -N may be present in the return sludge (8) returned from the final sedimentation tank (7) to the anaerobic tank (4). In this case, phosphorus is hardly released in the anaerobic tank (4). For example, even if the concentration of acetic acid in the influent sewage is 20 mg / L, if return sludge containing 10 mg / L of NO ₃ -N is mixed with 50 V / V% of the sewage, NO ₃ -N will contain 13.5 mg / L of acetic acid. Will be consumed.

  In addition, not only the consumption of organic acids by NOx-N and DO in sewage or return sludge, but also device characteristics (such as backflow from the denitrification tank) may affect. Therefore, it is difficult to judge the release of phosphorus in the anaerobic tank only by the organic acid concentration in the sewage, and the factors involved in the discharge of phosphorus in this anaerobic tank (organic acid concentration, NOx-N concentration, DO It is considered difficult to control the amount of organic acid added by grasping all of the concentration, device characteristics, etc.) in advance.

  Therefore, stable phosphorus removal can be achieved by adding organic acid (12) to the anaerobic tank (4) so that the ORP value of the anaerobic tank (4) is maintained at -200 mV or less and -400 mV or more. By measuring with an ORP meter with an air cleaning device, more continuous and stable treatment can be performed.

  When the ORP value of the anaerobic tank (4) becomes −250 mV or more, the organic acid (12) is added to the anaerobic tank (4) by the pump (13), and the ORP value of the anaerobic tank (4) becomes −260 mV. By stopping, operate so that the ORP value of the anaerobic tank (4) is within -200 mV to -400 mV.

  Or an anaerobic tank can also be controlled by changing the residence time of an anaerobic tank (4) so that the ORP value of an anaerobic tank (4) exit may be maintained in the range of -400 mV or more and -200 mV or less. In the biological phosphorus removal method, the aerobic tank (6) is divided into multiple water tanks, and the front row aerobic tank is equipped with both a mechanical stirrer and an air stirrer, and the ORP value of the anaerobic tank exceeds the control value. If this is the case, change the stirring method of the water tank in the front row of the aerobic tank (6) from air stirring to mechanical stirring to make an anaerobic tank (4). Also, in the biological nitrogen / phosphorus removal process, when the anaerobic tank (5) is divided into multiple water tanks and the ORP value of the anaerobic tank exceeds the control value, the inflow position of the nitrification solution is indicated by a dotted line In this way, the anaerobic tank (4) is controlled so as to have the ORP value necessary for phosphorus release by changing the water tank in the front row of the anaerobic tank (5) to the second row.

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

Example 1
The method of the present invention was applied to municipal sewage treatment.
As shown in Figure 2, after removing suspended matter from the sewage (1) in the first sedimentation basin (2), the first sedimentation basin effluent (3) is removed from the anaerobic tank (4), followed by the anoxic tank (5), In this process, an aerobic tank (6) is placed and a final sedimentation tank (7) is placed. The nitrification liquid (17) was returned to the 150V / V% anoxic tank (5) with respect to the amount of sewage (1) by the circulation pump (18). Return sludge (8) was returned to the anaerobic tank (4) by the return sludge pump (9) under the condition of 50V / V% of the sewage volume (1). MLSS (activated sludge suspended solids) in each reaction tank was maintained at 2500 to 3000 mg / L. In addition, A-SRT (sludge residence time in the aerobic tank) was managed in 12-13 days.

The water quality of the first sedimentation basin effluent (3) is as follows: BOD average 100 mg / L, TN (total nitrogen) average 30 mg / L, TP (total phosphorus) 4.5 mg / L, PO ₄ -P About 3.0 mg / L.

  First, using the ORP value of the anaerobic tank (5) as an index, an organic acid (formic acid, acetic acid, propionic acid, acetate, etc.) is added when the ORP value becomes -250 mV or more. In this example, acetic acid (12) is added to the anaerobic tank (4) at a flow rate of 30 mg / L as a representative organic acid so that the ORP value of the anaerobic tank (4) is -200 mV or less and -400 mV or more. The water quality was compared between when the ORP meter with a brush cleaning device was used and when the ORP meter with an air cleaning device was used.

  The cleaning frequency of the ORP meter with a brush cleaning device was once every 4 hours, and the cleaning time was 30 seconds. In addition, the cleaning frequency of the ORP meter with an air cleaning device was once per hour, and the cleaning time was 0.3 seconds with an air pressure of 0.3 MPa.

Table 2 shows the results. When controlled with an ORP meter with a brush cleaning device, the raw water PO ₄ -P concentration is 3.0 mg / L, whereas the treated water is 0.9 mg / L, whereas when controlled with an ORP meter with an air cleaning device, the raw water The treated water was as good as 0.2 mg / L for PO ₄ -P concentration of 3.3 mg / L.

Example 2
Next, when the ORP value of the anaerobic tank (5) is used as an index, and the ORP value of the anaerobic tank (4) becomes -250 mV or more, the residence time of the anaerobic tank (4) starts from 2 hours based on the sewage flow rate. The water quality was compared when the time was changed to 3 hours and when the residence time was fixed at 2 hours. Here, changing the residence time from 2 hours to 3 hours means changing the stirring method of the water tank in the front row of the aerobic tank (6) from air stirring to mechanical stirring to make the anaerobic tank (4) Means.

The ORP meter was cleaned with air, the cleaning frequency was once per hour, and the cleaning time was 0.3 seconds with an air pressure of 0.3 MPa.
Table 3 shows the results. When the residence time in the anaerobic tank is fixed, the raw water PO ₄ -P concentration is 2.9 mg / L, while the treated water is 1.2 mg / L. When implemented, the treated water was 0.1 mg / L against the raw water PO ₄ -P concentration of 2.8 mg / L, and a very good treated water quality was obtained.

Figure 2 is a biological dephosphorization process from sewage according to the present invention. 2 is a biological dephosphorization and denitrification process from sewage according to the present invention. It is the figure which showed an example of the brush washing | cleaning apparatus. It is the figure which showed an example of the air cleaning apparatus. It is the figure which showed the time change of the ORP value by a brush washing | cleaning apparatus. It is the figure which showed the time change of the ORP value by air washing.

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 Mechanical stirrer
12 Organic acid tank
13 Injection pump
14 ORP meter
15 Jet nozzle
16 DO total
17 Nitrification solution
18 Circulation pump
19 Treated water (Final sedimentation basin effluent)

Claims (4)

Primary sedimentation, anaerobic tank, an aerobic tank and settling tank, or primary sedimentation, anaerobic tank, anoxic tank, the biological method for removing phosphorus from sewage consisting aerobic tank and settling tank, the the electrode portion of the ORP (oxidation-reduction potential) of the anaerobic tank outlet, in washing Kiyoshisu Rukoto in 3 seconds or less in air pressure 0.05 to 0.5 MPa, characterized in that suppress the fluctuation of the ORP value, the Method.   The organic acid is added to the anaerobic tank so that the ORP value at the outlet of the anaerobic tank is maintained in a range of -400 mV or more and -200 mV or less based on a silver / silver chloride electrode. Method.   The method according to claim 2, wherein the organic acid added to the anaerobic tank is acetic acid or acetate.   The method according to claim 3, wherein the residence time of the anaerobic tank is adjusted so that the ORP value at the outlet of the anaerobic tank is maintained in the range of -400 mV or more and -200 mV or less.

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