JP3591086B2 - Biological treatment of organic wastewater - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
TECHNICAL FIELD The present invention relates to a biological treatment method for organic wastewater in which organic wastewater is biologically treated and sludge generated is reduced in volume by solubilization treatment.
[0002]
[Prior art]
In the method of aerobic biological treatment of organic effluent in the presence of activated sludge, a large amount of hardly dewaterable surplus activated sludge is generated. Also, a method of anaerobically treating in the presence of anaerobic sludge generates a large amount of excess digested sludge. In order to reduce the volume of such excess sludge, a method of aerobically or anaerobically digesting excess sludge has been used. In the aerobic digestion, surplus sludge is simply aerated in a digestion tank to digest it, the aerated sludge is separated into solid and liquid, and the separated sludge is returned to the digestion tank. In anaerobic digestion, excess sludge is put into a digestion tank and digested by the action of anaerobic bacteria.
[0003]
Such a digestion method digests by utilizing the action of aerobic or anaerobic organisms.However, since the excess sludge itself is biologically stable through biological treatment, it can be used for sludge volume reduction. Is limited, and usually only 30-40% of the excess sludge is reduced.
[0004]
In order to improve such a point, Japanese Patent Publication No. 5-61994 describes a method for treating organic wastewater in which excess sludge is solubilized at a pH of 2.5 or lower and a temperature of 50 ° C or higher and then returned to an aeration tank. ing.
JP-A-1-224100 describes a method for treating organic sludge in which anaerobic digested sludge is solubilized at 100 to 180 ° C., and the solubilized sludge is returned to the anaerobic digestion tank. .
[0005]
[Problems to be solved by the invention]
However, although the volume of sludge can be reduced by such conventional methods, the conditions for solubilization are severe at high temperatures, so that non-biodegradable organic substances are generated and the chromaticity and COD of the processing solution increase. As a result, there is a problem that the quality of the treated water is deteriorated, a device having high heat resistance is required, the energy consumption is large, and the cost is high.
[0006]
An object of the present invention is to provide a biological treatment method for an organic wastewater capable of suppressing the deterioration of treated water quality and reducing the volume of sludge at low cost by heating at a low temperature in order to solve the above problems. It is to propose.
[0007]
[Means for Solving the Problems]
The present invention is a method of biologically treating an organic wastewater in a biological treatment tank in the presence of biological sludge containing aerobic or anaerobic microorganisms,
A biological treatment step of introducing an organic effluent into a biological treatment tank and performing aerobic or anaerobic biological treatment in the presence of biological sludge containing aerobic or anaerobic microorganisms;
Drawing out the mixed liquid or concentrated sludge in the biological treatment tank, heating the extracted sludge in the presence of a surfactant, solubilizing the sludge, and then transferring the sludge to the biological treatment tank. This is a biological treatment method for organic wastewater.
[0008]
The organic effluent to be treated in the present invention is an effluent or sludge containing an organic substance to be treated by biological treatment, but may contain a hardly biodegradable organic or inorganic substance. Such organic effluents include sewage, human waste, food factory effluents, other industrial effluents, and sludge such as excess sludge generated when these effluents are treated.
[0009]
The biological treatment step of biologically treating such organic effluent may be an aerobic biological treatment or an anaerobic biological treatment. The aerobic biological treatment includes an activated sludge method and a biofilm method. The activated sludge method is an aerobic biological treatment of organic effluent in the presence of activated sludge.The organic effluent is mixed with activated sludge in an aeration tank and aerated, and the mixture is concentrated by a concentrator. In general, a standard activated sludge method in which a part of the concentrated sludge is returned to the aeration tank is used, but another modified treatment method may be used. The biofilm method is a process in which a biofilm is formed on a carrier and brought into contact with the drainage under aerobic conditions. The anaerobic treatment includes an anaerobic digestion method and a high-load anaerobic treatment method.
[0010]
The processing conditions for the aerobic biological treatment and the anaerobic biological treatment are not particularly limited, and ordinary aerobic biological treatment or anaerobic biological treatment conditions can be employed. For example, in the case of aerobic biological treatment, the digestion tank (biological treatment tank) residence time can be 1 day or more, usually 3 to 15 days in digestion of sludge, and the sludge load is in the activated sludge treatment of organic wastewater. Operation is possible at 0.1-0.5 kg-BOD / MLSS / day. In the case of anaerobic treatment, in digestion of sludge, the residence time of the digestion tank (biological treatment tank) can be 2.5 days or more, usually 5 to 30 days.
[0011]
In the present invention, a part of the biological sludge is withdrawn from the treatment system in such biological treatment, and the extracted sludge is solubilized. When extracting biological sludge, the concentrated sludge that has been concentrated by the concentrator may be extracted, or may be extracted in the form of a mixed solution from the biological treatment tank, but the solubilization treatment may be performed in a small-capacity treatment tank. The former is preferable because it can be performed and the heating energy may be small. Known devices such as a precipitation device, a centrifugal separator, and a membrane separation device can be used as the concentration device. Among these, a centrifugal separator and a membrane separator are preferable because sludge can be highly concentrated.
[0012]
The concentrated sludge can be solubilized by extracting a part or the whole as a drawn sludge. In the former case, a part of the concentrated sludge can be solubilized, the other part can be returned to the biological treatment tank as return sludge, and the remainder can be discharged as surplus sludge or digested sludge. Active microorganisms are present in the biological treatment tank, and the organic matter in the liquid to be treated and the solubilized sludge grow as a substrate.Thus, it is not necessary to return the sludge to the biological treatment tank. Is preferably returned to the biological treatment tank. In the present invention, in addition to excess sludge, it is preferable to further extract and solubilize a part of the sludge returned to the biological treatment tank as return sludge, in which case the amount of excess sludge can be reduced. Depending on conditions, the amount of surplus sludge can be reduced to zero. This will be described in detail later.
[0013]
In the present invention, the extracted sludge is heated and solubilized in the presence of a surfactant.
Examples of the surfactant used in the present invention include nonionic surfactants such as polyoxyethylene nonylphenyl ether and polyoxyethylene alkyl ether; anionic surfactants such as sodium laurylbenzenesulfonate and sodium dodecyl sulfate; And cationic surfactants such as tetradecylamine and cetyltrimethylammonium chloride. Among these, nonionic surfactants which have high biodegradability and do not inhibit the digestive activity of microorganisms are preferred.
[0014]
The amount of the surfactant to be added is desirably 10 to 100,000 mg / kg, preferably 1,000 to 20,000 mg / kg, based on the MLVSS to be solubilized. By adding the surfactant in such an amount, even if the solubilized sludge is introduced into the biological treatment tank and biologically treated, the digestive activity of microorganisms is not inhibited, and the efficiency of the entire treatment is affected. The effect of adding the surfactant can be obtained without causing a new load that causes the solubilization, and the solubilization treatment can be performed under milder conditions as compared with the conventional method.
The surfactant is preferably added to the solubilization tank (heat treatment tank) or the drawn sludge supplied to the solubilization tank.
[0015]
The heating temperature is desirably 50 to 150 ° C, preferably 60 to 90 ° C. The heating time is desirably 15 minutes to 6 hours, preferably 30 minutes to 2 hours as a residence time in the case of the continuous method, and 10 minutes to 6 hours, preferably 30 minutes to 1 hour in the case of the batch method.
[0016]
By performing the solubilization treatment under such conditions, 90% or more of the organic sludge is reformed into easily biodegradable substances. Then, by subjecting the solubilized sludge to aerobic or anaerobic biological treatment, organic matter is decomposed by microorganisms, and sludge volume is reduced. Depending on the conditions, one solubilization treatment reduces the volume of 1/3 to 1/4 of the extracted sludge subjected to the treatment. The biological treatment of the solubilized sludge can be performed by returning (circulating) the sludge to the biological treatment tank of the treatment system from which the sludge has been extracted, or can be introduced into a biological treatment tank of another treatment system.
[0017]
In the present invention, the larger the amount of sludge to be solubilized, the higher the sludge volume reduction rate. However, sludge multiplies when biodegrading the organic matter in the solubilized sludge.Thus, simply treating the excess sludge with solubilization does not reduce the excess sludge to zero. When the excess sludge is withdrawn and solubilized so as to be zero, the amount of excess sludge generated from the entire system can be reduced to zero. For example, when sludge is digested, it is circulated to the solubilization tank during the residence time of the digestion tank (biological treatment tank), and considering the number of times of the solubilization treatment, about 1/3 to 1 / l of the inflow sludge amount. The amount corresponding to 6 may be solubilized. In addition, when the amount of the sludge to be solubilized increases, the biological treatment performance may be reduced.In such a case, a carrier for supporting the sludge is provided in the biological treatment tank to maintain a fixed amount of the sludge. Thereby, the biological treatment performance can be maintained high.
[0018]
The principle of sludge volume reduction in the present invention will be described with reference to the drawings.
FIG. 1 is a schematic diagram for explaining the principle of sludge volume reduction. In the figure, 1 is a biological treatment system, and 2 is a solubilization treatment system. The biological treatment system 1 is a treatment system in which organic wastewater is brought into contact with biological sludge to decompose aerobically or anaerobically, and a biological treatment tank and a concentrator are separately provided. Is shown in FIG. The solubilization treatment system 2 is illustrated as a treatment system for solubilizing drawn sludge that is drawn in a state of a mixed solution or a concentrated solution. Sludge is hydrolyzed by the solubilization treatment and becomes a BOD source.
[0019]
The biological treatment system 1 of FIG. 1 holds a certain amount of biological sludge 3a for performing biological treatment. When the liquid 4 to be treated is introduced into the biological treatment system 1 and biological treatment is performed, the BOD contained in the liquid 4 to be treated is assimilated into biological sludge 3a, and the sludge 3b is newly generated by the proliferation. On the other hand, the biological sludge 3a in the system is self-decomposed, and the self-decomposed component 3c is lost. Therefore, in a steady state, the difference between the generated sludge 3b and the self-decomposed component 3c multiplies as multiplication sludge 3d.
[0020]
The case where the sludge 3d is treated as surplus sludge in the solubilization treatment system 2 is shown by a broken line 5 in FIG. 1, but when the sludge 3d is solubilized and returned to the biological treatment system 1, the sludge treatment causes The generated BOD is converted into sludge, and another generated sludge 3e is generated, which becomes a substantial sludge multiplication and must be discharged as surplus sludge. On the other hand, a larger amount of the extracted sludge 3f than the multiplication sludge 3d is withdrawn from the biological treatment system 1 and solubilized in the solubilization treatment system 2 to be converted to BOD, and the solubilized sludge 6 is converted to the biological treatment system 1. By returning, another 3 g of generated sludge is generated from the BOD generated by the solubilization treatment. In this case, the difference between the extracted sludge 3f and the generated sludge 3g is the mineralized portion 3h.
[0021]
Here, by solubilizing a larger amount of the extracted sludge 3f than the proliferating sludge 3d and converting it to BOD, the mineralized portion is increased and the sludge volume is reduced as compared with the case where only the proliferating sludge 3d is solubilized. The rate will be higher. When the amount of the extracted sludge 3f is determined so that the propagation sludge 3d is equal to the mineralized portion 3h, the surplus sludge becomes substantially zero. When the amount of the proliferating sludge 3d is larger than that of the mineralized portion 3h, the difference becomes a substantial increase portion 3i, and is discharged out of the system as surplus sludge 7. Reference numeral 8 denotes a treatment liquid of the biological treatment system 1.
[0022]
The biological treatment capacity in the biological treatment system 1 is V, the biological sludge concentration is X, the sludge yield is Y, the flow rate of the liquid to be treated (the flow rate of the treatment liquid) is Q, the organic matter concentration of the liquid to be treated is Ci, and the organic matter of the treatment liquid is Ci. The concentration is Ce, the biologically treated organic matter concentration is (Ci-Ce), the sludge self-decomposition constant is Kd, the surplus sludge discharge amount is q, the withdrawal amount to the solubilization treatment system is Q ', and the solubilized sludge is Assuming that the rate of conversion into biological sludge is k, the material balance is expressed by the following equation [1].
[0023]
(Equation 1)
VdX / dt = YQ (Ci-Ce) -VKdX-qX-Q'X + kQ'X [1]
In the formula [1], VdX / dt is the amount of change in the biological sludge 3a in the biological treatment system 1, YQ (Ci-Ce) is the amount of the generated sludge 3b, VKdX is the amount of the autolysis portion 3c, qX Indicates the amount of surplus sludge 7 discharged, Q'X indicates the amount of the extracted sludge 3f, and kQ'X indicates the amount of the generated sludge 3g.
[0024]
Here, Q (Ci-Ce) / V = LV (tank load), q / v = 1 / SRT (surplus sludge residence time ratio), Q '/ V = θ (circulation ratio of biological sludge to the solubilization treatment system) ), (1−k) = δ (inorganization ratio), in a steady state, the equation [1] is simplified as the following equation [2].
(Equation 2)
Y LV / X = Kd + 1 / SRT + δθ [2]
[0025]
In a normal biological treatment system in which the solubilization treatment system 2 does not exist, there is no third term (δθ) in the equation [2], so that when the sludge load is fixed, the excess sludge (X / SRT) is determined in the second term. Is done. On the other hand, in the treatment system combined with the solubilization treatment, the excess sludge is reduced by the value of the third term as is apparent from the equation (2). Under conditions where the value of the third term is comparable to the value of the second term, the sludge load can be set to a normal value without discharging excess sludge (1 / SRT = 0). is there.
[0026]
In the method of the present invention, by performing the solubilization treatment in the presence of a surfactant, to obtain a volume reduction ratio comparable to the conventional one, a lower temperature than before or a short time when the treatment is performed at the same temperature. Can be solubilized. Therefore, it is possible to suppress the production of the biodegradable component, and to use a device having a lower heat resistance than before, to reduce the energy consumption and to suppress the odor. it can.
[0027]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, an embodiment of the present invention will be described with reference to the drawings.
2 to 4 are system diagrams each showing a biological treatment apparatus according to another embodiment. FIG. 2 shows an example in which a concentrated sludge obtained by concentrating an aerobic treatment liquid by a concentration apparatus is solubilized, and FIG. 3 is an aeration tank. FIG. 4 shows an example in which the mixed solution in the anaerobic biological treatment tank is solubilized.
In FIG. 2, 1 is an aerobic biological treatment system, 2 is a solubilization treatment system, 11 is an aeration tank, 12 is a sedimentation device as a concentrator, 21 is a solubilization treatment tank, 23 is a surfactant supply channel, and 24 Is a heater.
[0028]
In the aerobic biological treatment method for organic waste liquid by the processing apparatus of FIG. 2, the organic waste liquid or sludge is introduced into the aeration tank 11 from the liquid passage 13 to be treated, and returned sludge returned through the return sludge passage 14 and It mixes with the activated sludge in the aeration tank 11 and diffuses the air supplied from the air supply path 16 from the diffuser 15 to perform aerobic biological treatment. Thereby, the organic matter in the liquid to be treated is decomposed by the biological oxidation reaction.
[0029]
A part of the mixed liquid (reaction liquid) in the aeration tank 11 is introduced into the sedimentation device 12 through the communication path 17 and separated into a separated liquid and a separated sludge (concentrated sludge) by sedimentation. The separated liquid is discharged as a processing liquid from the processing liquid path 18 to the outside of the system, the separated sludge is taken out from the separated sludge extraction path 19, a part of the separated sludge is returned to the aeration tank 11 from the returned sludge path 14 as returned sludge, and A part or the whole is introduced into a solubilization tank 21 through a sludge extraction path 22 to perform a solubilization treatment.
[0030]
In the solubilization tank 21, a surfactant is added from the surfactant supply channel 23, heated by the heater 24, and the extracted sludge is solubilized while being slowly stirred by the stirrer 25. Thereby, the sludge is hydrolyzed to BOD. The solubilized sludge is continuously returned from the solubilized sludge passage 26 to the aeration tank 11 and subjected to aerobic biological treatment. Thereby, the BOD component converted by the solubilization treatment is decomposed and removed, and the volume of excess sludge generated from the aerobic biological treatment system 1 is reduced. When excess sludge is generated, the excess sludge is discharged from the excess sludge discharge passage 20 to the outside of the system.
[0031]
In the treatment method using the apparatus shown in FIG. 3, a part of the mixed solution in the aeration tank 11 is withdrawn from the sludge extraction passage 22 as extracted sludge, and a surfactant is added to the extracted sludge from the surfactant supply passage 23. It is introduced into the solubilization tank 21 for solubilization. Other operations are the same as those in FIG.
[0032]
2 and 3, the sedimentation device 12 is employed as the concentration device, but other concentration devices such as a membrane separation device and a centrifugal separation device may be employed. Anaerobic treatment can also be performed using an anaerobic treatment tank instead of the aeration tank 11. The solubilized sludge can also be transferred to an aerobic or anaerobic biological treatment system other than the biological treatment system 1 from which the sludge has been extracted, and subjected to biological treatment.
[0033]
In FIG. 4, 1 is an anaerobic biological treatment system, 2 is a solubilization treatment system, 31 is an anaerobic treatment tank, and 36 is a membrane separation device.
In the processing method using the processing apparatus shown in FIG. 4, the organic wastewater or sludge is introduced into the anaerobic treatment tank 31 from the liquid passage 13 to be treated, and the returned sludge returned through the return sludge path 14 and the organism in the anaerobic treatment tank 31 are returned. The mixture is mixed with the sludge, and the anaerobic biological treatment is performed while the mixture is gently stirred by the stirrer 32. Thereby, the organic matter in the liquid to be treated is decomposed by the acid-producing bacteria and the methane-fermenting bacteria. The digestion gas including the generated methane gas is discharged from the exhaust gas passage 33.
[0034]
A part of the mixed liquid (reaction liquid) in the anaerobic treatment tank 31 is taken out from the communication path 17, pressurized by a pump 34, guided to a membrane separation device 36, and separated by a separation membrane 37. This separates into a permeate 38 and a concentrate 39. The permeated liquid 38 is discharged as a processing liquid from the processing liquid path 18 to the outside of the system, and the concentrated liquid 39 is removed from the concentrated liquid discharge path 19a and returned as returned sludge from the returned sludge path 14 to the anaerobic treatment tank 31. If it occurs, it is discharged from the excess sludge discharge passage 20 to the outside of the system.
[0035]
In the solubilization treatment system 2, a part of the mixed solution in the anaerobic treatment tank 31 is withdrawn from the sludge extraction passage 22 as extracted sludge and introduced into the solubilization treatment tank 21 to perform the solubilization treatment. Other operations are the same as those in FIG.
[0036]
In FIG. 4, the membrane separation device 36 is used as the concentration device, but other concentration devices such as a precipitation device and a centrifugal separation device can be employed. Also, aerobic treatment can be performed using an aeration tank instead of the anaerobic treatment tank 31.
[0037]
【Example】
Comparative Example 1
The organic effluent was subjected to aerobic treatment using the apparatus shown in FIG. 3 except that the solubilization treatment in the solubilization treatment system 2 was omitted. That is, excess sewage sludge (MLSS = 6000 mg / l) was continuously treated in a 20 liter aeration tank 11 and a 10 liter settling apparatus 12. Sludge was continuously fed in an amount of 4 liters / day. One liter of the mixed solution in the aeration tank 11 was withdrawn per day. As a result, the MLSS in the aeration tank 11 became 10,600 mg / l two months later.
[0038]
Comparative Example 2
In the above operation, the mixed solution in the aeration tank 11 was withdrawn 1.5 liters per day, introduced into the solubilization tank 21, heated at 60 ° C. for 2 hours, solubilized, and returned to the aeration tank 11. Solubilization was performed daily. When the system was operated while maintaining the MLSS at 10,000 mg / l, 4.7 g of surplus sludge was produced in a dry weight per day after 60 days. The COD _{Cr of} this treated water was 310 to 380 mg / l. In addition, the COD _Cr of the treated water was measured on the supernatant liquid centrifuged at 15,000 rpm for 5 minutes.
[0039]
Comparative Example 3
In the above operation, the mixed solution in the aeration tank 11 was withdrawn one liter per day, introduced into the solubilization tank 21, heated at 120 ° C. for 2 hours, solubilized, and returned to the aeration tank 11. Solubilization was performed daily. When the entire amount of the settled sludge was returned to the aeration tank 11 without discharging the excess sludge at all, the MLSS gradually increased. After 2 months, the MLSS stabilized around 16,000 mg / l. The COD _{Cr of} this treated water was 730 to 840 mg / l.
[0040]
Example 1
In parallel with the operation of Comparative Example 2, the mixed solution in the aeration tank 11 was withdrawn one liter per day and introduced into the solubilization tank 21 for solubilization. The solubilization treatment was performed by adding 2% of polyoxyethylene nonylphenyl ether (HLB = 14) based on the amount of the heat-treated sludge and conducting the treatment at 60 ° C. for 2 hours. The solubilization treatment was performed every day, and the solubilized sludge was returned to the aeration tank 11. When the excess sludge was not discharged at all and the entire amount of the settled sludge was returned to the aeration tank 11, the MLSS gradually increased, but was stabilized at 12,000 to 14,000 mg / l after about one month. The COD _{Cr of} this treated water was 360 to 540 mg / l.
[0041]
Example 2
Following Example 1, the mixture in the aeration tank 11 was withdrawn one liter per day and introduced into the solubilization tank 21 for solubilization. The solubilization treatment was performed by adding sodium laurylbenzenesulfonate (anionic surfactant) in an amount of 1% based on the amount of the heat-treated sludge and performing the treatment at 60 ° C. for 2 hours. The solubilization treatment was performed every day, and the solubilized sludge was returned to the aeration tank 11. As a result, the MLSS was stable at 13,000 to 15,000 mg / l, and there was no need to discharge excess sludge (the entire settled sludge was returned to the aeration tank 11). The COD _Cr of the treated water was 430 to 570 mg / l.
[0042]
From the above results, it can be seen that by adding the surfactant, the sludge volume reduction rate is high even when the solubilization treatment is performed at a low temperature. That is, the volume reduction effect when the solubilization treatment was performed at 60 ° C. using the surfactant (Examples 1 and 2) was different from that when the solubilization treatment was performed at 120 ° C. without adding the surfactant (Comparative Example 2). Equal or better results were obtained. Moreover, the COD _Cr of Examples 1 and 2 is lower than that of Comparative Example 2, and it can be seen that the deterioration of the treated water can be suppressed.
[0043]
【The invention's effect】
In the biological treatment method of the organic wastewater of the present invention, the mixed liquid or the concentrated sludge in the biological treatment tank is drawn out, and the extracted sludge is heated and solubilized in the presence of a surfactant, and then is passed to the biological treatment tank. Since it includes the solubilization treatment step of transferring, it is possible to suppress the deterioration of treated water quality and to reduce the volume of sludge at low cost by using a device having low heat resistance.
[Brief description of the drawings]
FIG. 1 is a schematic diagram for explaining the principle of sludge volume reduction.
FIG. 2 is a system diagram illustrating a biological treatment apparatus according to an embodiment of the present invention.
FIG. 3 is a system diagram showing a biological treatment apparatus according to another embodiment of the present invention.
FIG. 4 is a system diagram showing a biological treatment apparatus according to another embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Biological treatment system 2 Solubilization treatment system 3a Biological sludge 3b, 3e, 3g Generated sludge 3c Self-decomposition part 3d Propagation sludge 3f Extraction sludge 3h Mineralization part 3i Increasing part 4 Liquid to be treated 6 Solubilization treatment sludge 7 Surplus sludge 8 Treatment Liquid 11 Aeration tank 12 Precipitation device 13 Liquid passage to be treated 14 Return sludge passage 15 Air diffuser 16 Air supply passage 17 Communication passage 18 Treatment liquid passage 19 Separate sludge discharge passage 19a Concentrated liquid discharge passage 20 Excess sludge discharge passage 21 Solubilization treatment Tank 22 Sludge extraction path 23 Surfactant supply path 24 Heater 25, 32 Stirrer 26 Solubilization treatment sludge path 31 Anaerobic treatment tank 33 Exhaust gas path 34 Pump 36 Membrane separation device 37 Separation membrane 38 Permeate 39 Concentrate

Claims (1)

A method for biologically treating an organic wastewater in a biological treatment tank in the presence of biological sludge containing aerobic or anaerobic microorganisms,
A biological treatment step of introducing an organic effluent into a biological treatment tank and performing aerobic or anaerobic biological treatment in the presence of biological sludge containing aerobic or anaerobic microorganisms;
Drawing out the mixed liquid or concentrated sludge in the biological treatment tank, heating the extracted sludge in the presence of a surfactant, solubilizing the sludge, and then transferring the sludge to the biological treatment tank. Biological treatment of organic wastewater.

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